© Author(s) 2021. CC Atribution 4.0 License Tectonics and gravitational phenomena (Nanos, Slovenia) Tektonika in gravitacijski pojavi (Nanos, Slovenija) Ladislav PLACER 1 , Andrej MIHEVC 2 & Igor RIŽNAR 3 1 Geološki zavod Slovenije, Dimičeva ul. 14, SI-1000 Ljubljana, Slovenija; e-mail: ladislav.placer@telemach.net 2 Inštitut za raziskovanje krasa, Titov trg 2, SI-6230 Postojna, Slovenija; e-mail: andrej.mihevc@zrc-sazu.si 3 Geološke ekspertize Igor Rižnar s. p., SI-1000 Ljubljana, Slovenija; e-mail: igor.riznar@telemach.net Prejeto / Received 22.1. 2021; Sprejeto / Accepted 1. 7. 2021; Objavljeno na spletu / Published online 19. 7. 2021 Key words: gravitational phenomena, geomorphology, Istra Pushed Area, Nanos, Hrušica, razdol, Adria Ključne besede: gravitacijski pojavi, geomorfologija, istrsko potisno območje, Nanos, Hrušica, razdol, Adria Abstract The Istra Pushed Area is a specifically deformed territory of the northwestern part of the External Dinarides. It formed due to the movement of the Istra block as part of the Adriatic Microplate (Adria) towards the Dinarides since the middle Miocene. The movement of the Istra block caused hereditary shifts along the old dislocations dating back to the early formation stage of the formation of the Dinarides at the end of the Eocene and their deformation. These deformations are reflected also in certain extreme gravitational phenomena along the boundary between the External Dinaric Imbricated Belt and the External Dinaric Thrust Belt, where Mesozoic carbonates are thrusted upon the Cenozoic flysch. The boundary zone between these two belts connects the Trnovo, Hrušica and Snežnik Thrust Fronts. Four specific gravitational phenomena that occurred in this boundary zone are presented here, as they are remarkable in terms of their size: Črna griža (Trnovo Nappe), Suhi vrh (Hrušica Nappe), Petelinje mlake and Ilirska Bistrica (both from the Snežnik Nappe). The phenomena at Suhi vrh is described in detail herein. Izvleček Istrsko potisno območje je specifično deformirano ozemlje severozahodnega dela Zunanjih Dinaridov. Nastalo je zaradi pomikanja istrskega bloka, ki je del Jadranske mikroplošče (Adria), proti Dinaridom. To se dogaja od srede miocena naprej. Pomikanje istrskega bloka je povzročilo nasledstvene premike po starih dislokacijah iz zaključka prvega obdobja nastajanja Dinaridov konec eocena in njihovo deformacijo. Te deformacije se odražajo tudi v ekstremnih gravitacijskih pojavih na meji med Zunanjedinarskim naluskanim pasom in Zunanjedinarskim narivnim pasom, kjer so mezozojski karbonati narinjeni na kenozojski fliš. Omenjeni mejni pas povezuje čela krovnih narivov Trnovskega, Hrušiškega in Snežniškega pokrova. V članku so prikazani štirje specifični gravitacijski pojavi, ki so izjemni po velikosti: Črni školj (Trnovski pokrov), Suhi vrh (Hrušiški pokrov) ter Petelinje mlake in Ilirska Bistrica (oba Snežniški pokrov). Natančneje je opisan pojav Suhi vrh. GEOLOGIJA 64/1 , 35-63, Ljubljana 2021 https://doi.org/10.5474/geologija.2021.003 Introduction A formal boundary between the Adriatic Mi- croplate (Adria) and the Dinarides in the NW Di- narides after Schmidt et al. (2008) runs along the External Dinarides thrust boundary in the Istra and Gulf of Trieste hinterland (Fig. 1). As the Adria rotates in a counterclockwise direction obliquely to the Dinarides (Weber et al., 2006) the move- ments against the Dinarides are released along two components: orthogonally (pushing, and con- Uvod Formalna meja med Jadransko mikroploščo (Adria) in Dinaridi poteka po Schmid-u in so- delavcih (2008) po narivni meji Zunanjih Dina- ridov, ki se v severozahodnih Dinaridih nahaja v zaledju Istre in Tržaškega zaliva (sl. 1). Adria rotira v nasprotni smeri urinega kazalca pošev- no na Dinaride (Weber et al., 2006), zato se njeni pomiki nasproti Dinaridom sproščajo po dveh komponentah, pravokotno nanje (potiskanje in 36 Ladislav PLACER, Andrej MIHEVC & Igor RIŽNAR sequential folding and underthrusting), and par- allel to the Dinarides (via right lateral strike slip along subvertical faults) (Placer et al., 2010). Istra is a structural block within the Adria and moves against the Dinarides faster than the blocks SE of it. As a consequence, the External Dinarides in the Istra hinterland between the Southern Alps and the Velebit are cambered (bent) towards the NE. This bent part of the External Dinarides is called the Istra Pushed Area (Placer, 2010). Formally, the assumed movement of the Istra against the Dinarides is evidenced by local block rotation in the Čičarija in the Istra hinterland by palaeomagnetic analysis of the cave sediments there (Vrabec et al., 2018). Using said analysis, the theory of the existence of the Istra Pushed Area posledično gubanje ter podrivanje) in vzporedno z njimi (desno zmikanje ob subvertikalnih pre- lomih) (Placer et al., 2010). Istra je blok v okviru Adrie, ki se proti Dinaridom premika hitreje od blokov jugovzhodno od tod, zato so Zunanji Di- naridi v njenem zaledju med Južnimi Alpami in Velebitom usločeni proti severovzhodu. Ta pre- del Zunanjih Dinaridov se v strukturnem smis- lu imenuje istrsko potisno območje (Placer et al., 2010). Domneva o premikanju Istre proti Dinaridom je bila formalno dokazana z lokalno rotacijo blo- kov v Čičariji v zaledju Istre, dokaz pa temelji na analizi starosti in paleomagnetizma jamskih se- dimentov (Vrabec et al., 2018). S tem je bila po- trjena teorija o obstoju istrskega potisnega ob- Fig. 1. The Adria (sensu stricto) and the northwestern Dinarides boundary zone. Belt of large gravitational phenomena. Sl. 1. Mejno območje Adrie (sensu stricto) in severozahodnih Dinaridov. Pas velikih gravitacijskih pojavov. 1 External Dinarides boundary / meja Zunanjih Dinaridov 2 External Dinaric Thrust Belt boundary / meja Zunanjedinarskega narivnega pasu 3 Nappe boundary / meja pokrova 4 Belt of large gravitational phenomena / pas velikih gravitacijskih pojavov 5 Outstanding gravitaitional phenomena / izjemni gravitacijski pojavi: a – Črni školj, b – Suhi vrh, c – Petelinje mlake, d – Ilirska Bistrica 6 T – Trnovo Nappe / Trnovski pokrov, H – Hrušica Nappe / Hrušiški pokrov, S – Snežnik Nappe / Snežniški pokrov 37 Tectonics and gravitational phenomena (Nanos, Slovenia) was confirmed and forms the basis of the study of the structures within the area. The principal fea- ture within the Istra Pushed Area is the deforma- tion of older brittle and ductile deformation. Block rotation analysis revealed a correlation between the local rotation and bending of the Čičarija Im- bricate Structure. Regional data shows that other Dinaric structures within the Istra Pushed Area such as the Trieste–Komen Anticlinorium and the Vipava Synclinorium are also bent, so correlation between the surface structure and the local block rotation is expected there as well. Istra’s movement against the Dinarides is an uneven but continuous process that has been tak- ing place since at least 5 Ma, which is the oldest age of the cave sediments analyzed. However, this age reflects the method range rather than the ab- solute duration of the described motion (Vrabec et al., 2018). The process started already in the Middle or even Lower Miocene; [and regarding the GPS measurements (Weber et al., 2006) takes place even nowadays], which is best reflected in the gravitational processes at work on the mor- phologically exposed boundary of the External Dinaric nappe thrusts, formally in the fore part of the External Dinaric Thrust Belt. Belt (zone) of large gravitational phenomena The boundary zone along the External Dinaric Thrust Belt comprised of the Trnovo, Hrušica and Snežnik Thrust Fronts, is particularly out- standing in the Istra Pushed Area (Fig. 1). The instability of the thrust fronts due to Mz carbon- ates thrust upon the siliciclastic flysch best char - acterise this belt. Local post-tectonic uplift and the denuding of the flysch footwalls have also played an important part in said development. These processes led to a significant geomorpho- logic step with a highly elevated risk of landslide (Komac & Ribičič, 2008); and at the same time came to constitute a highly attractive subject of research (Popit, 2016; 2017; Verbovšek et al., 2017; 2019). The thrust front zone is referred to as a “belt of large gravitational phenomena” due to the intense and diverse gravitational activity there. Its width is determined by the extent of these phenomena, where a wide variety of mass wasting forms is recognized, such as translation- al (planar) and rotational slides, rockfalls, rock- slides and mass flows. Longer-term tectonic and denudational activity is reflected in the overlap- ping of multiple superimposed landslides follow- ing each other in different temporal increments. Peculiar “gravitational duplexes” (as they could be called) are the most interesting among these močja in postavljeno je bilo stvarno izhodišče za študij deformacij v njem. Glavna značilnost istrskega potisnega obmo- čja so deformacije starejših plikativnih in disjun- ktivnih struktur. Analiza rotacije blokov v Čiča- riji je pokazala na soglasje med lokalno rotacijo in bočno usločenostjo Čičarijskega antiklinorija. Regionalni podatki kažejo, da so bočno usločene tudi druge dinarske strukture istrskega potisne- ga območja, npr. Tržaško-Komenski antiklinorij in Vipavski sinklinorij, zato tudi tu pričakujemo soglasje med površinsko strukturo in lokalno ro- tacijo blokov. Premikanje Istre proti Dinaridom je ne- enakomeren, vendar kontinuiran proces, ki je starejši od 5 milijonov let. Na to kaže najvišja starost analiziranih jamskih sedimentov v Či- čariji (Vrabec et al., 2018). Pričetek tega procesa pa sega v srednji ali celo v spodnji miocen in je glede na meritve GPS (Weber et al., 2010), de- javen še danes. Slednje se najlepše kaže v gra- vitacijskih procesih na morfološko izpostavljeni meji Zunanjedinarskih krovnih narivov, for- malno v čelnem delu Zunanjedinarskega nariv- nega pasu. Pas velikih gravitacijskih pojavov Znotraj istrskega potisnega območja pose- bej izstopa mejni del Zunanjedinarskega na- rivnega pasu, ki ga sestavljajo čela Trnovske - ga, Hrušiškega in Snežniškega pokrova (prej Snežniška narivna gruda) (sl. 1). Glavna zna- čilnost tega pasu je nestabilnost čela krovnih enot, ki je posledica nariva mezozojskih karbo- natov na siliciklastične flišne plasti. Poleg tega je imelo pomembno vlogo še krajevno omejeno postnarivno tektonsko dviganje in denuda- cija fliša talninskih krovnih enot. Ti procesi so pripeljali do izoblikovanja geomorfološke stopnje, ki predstavlja izrazito območje tvega- nja za proženje zemeljskih plazov (Komac & Ribičič, 2008) in hkrati privlačno območje nji- hovega proučevanja (Popit, 2016, 2017; Verbov- šek et al., 2017, 2019). Zaradi raznolike in in- tenzivne gravitacijske dejavnosti, smo čelni rob opisanih krovnih enot poimenovali »pas velikih gravitacijskih pojavov«. Njegovo širino določa vplivno območje zdrsov. Tu je nastala cela vrsta gravitacijskih pojavov kot so planarni in ro- tacijski plazovi, odlomi, blokovni zdrsi in masni tokovi. Na daljšo tektonsko in denu- dacijsko aktivnost kaže sožitje fosilnih in re- centnih plazov, od katerih so najbolj zanimi- vi svojevrstni »gravitacijski dupleksi« kot bi lahko imenovali plazove v nadstropjih, ki so 38 Ladislav PLACER, Andrej MIHEVC & Igor RIŽNAR “multistorey” landslides. Individual superim- posed mass wasting events (phenomena) may also differ from among each other in terms of their various genetic types, like the rotational fossil Mala Gora landslide superimposed by an active translational Slano blato landslide (Placer et al., 2008), and a rotational fossil Reševnik landslide superimposed by an active translational Razdrto landslide (Placer, 2006). The gravitational phenomena here are divid- ed into two distinctive areas according to the prevailing mechanisms at work: the Trnovo and Hrušica Nappes Thrust Fronts on one side, and the Snežnik Nappe Thrust Front on the other. In the first case, large gravitational phenomena oc- cur between Vitovlje above Šempas in the Vipava valley, and at Razdrto, in the western part of the Postojna basin. Two slopes stand out here, run- ning parallel to the NNE–SSW trending regional fractures and hence close to the NW–SE trend- ing thrust plane. The two areas share identical geomorphologic characteristics: the eastern slope of the Čaven high karst plane (plateau) between Mt. Črni školj (1180 m a.s.l.), Mt. Mali Modrasovec (1306 m a.s.l.) and Mt. Mala Gora (1032 m a.s.l.) nastajali zaporedoma v krajših ali daljših časov- nih presledkih. Ti so lahko genetsko istorodni ali raznorodni; taka večja pojava sta npr. rota- cijski fosilni plaz Mala Gora na katerem leži ak- tivni planarni zemeljski plaz Slano blato (Placer et al., 2008) in rotacijski fosilni plaz Reševnik na katerem leži aktivni planarni zemeljski plaz Razdrto (Placer, 2006). Gravitacijski pojavi obravnavanega pasu se po prevladujočem mehanizmu delijo na dve območji, na eni strani sta čeli Trnovskega in Hrušiškega pokrova, na drugi čelo Snežniškega pokrova. V prvem primeru nastopajo veliki gra- vitacijski pojavi med Vitovljami nad Šempasom v Vipavski dolini in Razdrtim v zahodnem delu Postojnske kotline. Tu izstopata dve pobočji, ki ležita v smeri prevladujočih regionalnih razpok SSW-NNE, torej blizu smeri narivanja SW-NE, v katerih je določljiva lega narivne ploskve. Območji imata identične geomorfološke zna- čilnosti, pri Trnovskem pokrovu je to vzhodno pobočje visoke kraške planote Čaven med Črnim školjem (1080 m), Malim Modrasovcem (1306 m) in Malo Goro (1032 m) (sl. 1, a; sl. 2), pri Hru- šiškem pokrovu jugovzhodno pobočje visoke Fig. 2. Črni školj landslide area. Sl. 2. Plazišče Črni školj. 1 C – Trnovo Nappe carbonates / karbonati Trnovskega pokrova 2 F – Flysch of the External Dinaric Imbricated Belt / fliš Zunanjedinarskega naluskanega pasu 3 Trnovo Nappe boundary /meja Trnovskega pokrova 4 Podgora (former Predjama) fault / Podgorski (prej Predjamski) prelom 5 Mala Gora rotational landslide / rotacijski plaz Mala Gora 6 Črni školj landslide area / plazišče Črni školj Fig. 3. Suhi vrh gravitational phenomena. Sl. 3. Gravitacijski pojav Suhi vrh. 1 C – Hrušica Nappe carbonates / karbonati Hrušiškega pokrova 2 F – Flysch of the Snežnik Nappe and of the External Dinaric Imbricated Belt / fliš Snežniškega pokrova in Zunanjedinarskega naluskanega pasu 3 Hrušica Nappe boundary / meja Hrušiškega pokrova 4 Fault / prelom 5 Suhi vrh gravitational area / gravitacijsko območje Suhi vrh 39 Tectonics and gravitational phenomena (Nanos, Slovenia) (Fig. 1, a; Fig. 2) in the case of the Trnovo Nappe, and the SE slope of the Nanos high karst plane (plateau) between Mt. Pleša (1262 m a.s.l.) and Mt. V rtu (1108 m a.s.l.) (Fig. 1, b; Fig. 3). In the thrust front of the Snežnik Nappe, gravitational phenomena comparable to those in the Trnovo and Hrušica Nappes Thrust Fronts are only found SE of Ilirska Bistrica. Two out- standing phenomena are present in the interme- diate area between Razdrto and Ilirska Bistrica: an inferred fossil rotational landslide at Petelinje mlake (Fig. 1, c; Fig. 4), and the Ilirska Bistrica fossil rotational landslide (Fig. 1, d; Fig. 5), both formed under particularly specific conditions. Except for these two phenomena, the Snežnik Nappe Thrust Front does not exhibit any other large gravitational features, as the Snežnik Nappe flysch is thrust upon the flysch of the External Dinaric Imbricated Belt in the eastern part of the Postojna basin, where the relief between the two nappes is levelled due to the absence of car- bonates there. Due south-east, between Hruševje and Šembije, at the edge of the Ilirska Bistrica fossil landslide, the geomorphological step is not yet high enough to pose a gravitational risk. A brief, informative introduction of the tec- tonic environment for the four outstanding cases present in the belt of the large gravitational phe- nomena is necessary in order to understand the described differences in the geomorphology and kraške planote Nanos med Plešo (1262 m) in vr- hom V rtu (1108 m) (sl. 1, b; sl. 3). V čelu Snežniškega pokrova nastopajo gravi- tacijski pojavi, ki jih je mogoče primerjati s tis- timi v Trnovskem in Hrušiškem pokrovu, šele jugovzhodno od Ilirske Bistrice. Na vmesnem prostoru med Razdrtim in Ilirsko Bistrico na- stopata dva izjemna pojava, domnevni fosilni rotacijski plaz Petelinje mlake (sl. 1, c; sl. 4) in ilirskobistriški fosilni rotacijski plaz (sl. 1, d; sl. 5), ki sta nastala pri specifičnih pogojih. Izven teh dveh pojavov so razmere umirjene, tu je na zahodni strani Postojnske kotline fliš Snežni- škega pokrova narinjen na fliš Zunanjedinar- skega naluskanega pasu, zaradi česar je površje obeh krovnih enot uravnano na skupni nivo, na jugovzhodu med Hruševjem in Šembijami na robu velikega ilirskobistriškega fosilnega pla- zu, pa geomorfološki prag v čelu Snežniškega pokrova še ni tako visok, da bi bil gravitacijsko ogrožen. Za razumevanje opisanih razlik v geomor- fologiji in gravitacijskih pojavih, je potrebno vsaj informativno predstaviti tektonsko okolje štirih izstopajočih primerov v pasu velikih gra- vitacijskih pojavov: Črni školj (a) v Trnovskem pokrovu, Suhi vrh (b) v Hrušiškem pokrovu ter Petelinje mlake (c) in Ilirska Bistrica (d) v Snež- niškem pokrovu (sl. 1). Na območju Črnega ško- Fig. 4. Petelinje mlake rotational landslide. Sl. 4. Rotacijski plaz Petelinje mlake. 1 C – Snežnik Nappe carbonates / karbonati Snežniškega pokrova 2 Juršče fault / Jurški prelom 3 Landslide mass / masa plazu Fig. 5. Ilirska Bistrica rotational landslide. Sl. 5. Rotacijski plaz Ilirska Bistrica. 1 CS – Snežnik Nappe carbonates / karbonati Snežniškega pokrova 2 CC – Carbonates of the External Dinaric Imbricated Belt / karbonati Zunanjedinarskga naluskanega pasu 3 F – Flysch of the External Dinaric Imbricated Belt / fliš Zunanjedinarskega naluskanega pasu 4 Snežnik Nappe boundary / meja Snežniškega pokrova 5 Tectonized boundary of the inverse beds within the External Dinaric Imbricated Belt / tektonizirana meja inverznih plasti znotraj Zunanjedinarskega naluskanega pasu 6 Landslide mass / masa plazu 40 Ladislav PLACER, Andrej MIHEVC & Igor RIŽNAR the gravitational phenomena: Črni školj (a) in the Trnovo Nappe Thrust Front, Suhi vrh (b) in the Hrušica Nappe Thrust Front, Petelinje mlake (c), and Ilirska Bistrica (d) in the Snežnik Nappe Thrust Front (Fig. 1). The Trnovo Nappe thrust plane lies at approximately 650 m a.s.l. in the Mala Gora fossil rotational landslide in the Črni školj area (Fig. 2). The thrust plane rises along the Mala Gora rotational landslide rupture surface at the Mt. Mali Modrasovec slope some 100 m to a height of 750 m a.s.l., and to 770 m a.s.l. along the road below Črni školj. Generally speaking, the thrust plane simply rises gently upward. A large area with slided blocks referred to as the Črni školj landslide area is described in detail by Koc- jančič et al. (2019). The landslide area was formed as the result of the uplifted carbonate block with flysch in its base, and the denudational lowering of the Vipava valley. The Mala Gora fossil rota- tional slide is younger and dissects (cuts) the Črni školj landslide area. The thrust plane on the SE slope of Mt. Nanos (Fig. 3) rises from 750 m a.s.l. south of Pleša (1262 m a.s.l.), up to 1000 m a.s.l. below Suhi vrh (1313 m a.s.l.) and descends down to 770 m a.s.l. beneath Mt. Tisovec (911 m a.s.l.). The specific structure of the Suhi vrh area – the result of extremely uplifted thrusted carbonate block with flysch in its base – is a consequence of gravitational processes. Howev- er, the structure differs from classical slides be- neath Mt. Pleša and the V rtu peak (1108 m a.s.l.), due to the erosion and denudation of this part of the Postojna basin. Namely, the thrust plane of the Trnovo and Hrušica Nappes are uplifted and con- vexly bent behind the thrust front, but far less so in the Hrušica than in the Trnovo Nappe. Unfortunately, we do not have knowledge of the structural base that would help us better un- derstand the landslide at Petelinje mlake (Fig. 4), as the area hasn’t been mapped in detail. Its po- sition, two kilometres behind the thrust front is remarkable, reflecting significant post-thrusting modifications and a specific type of formation. The Juršče fault is surprising, as it seems to have been formed after the triggering of the Ilirska Bistri- ca landslide. The Ilirska Bistrica landslide (Fig. 5) was formed by the slipping of the carbonate block belonging to the Snežnik Nappe along the flysch basement. There is no evidence of the convex thrust plane bending, but the erosional deepening of the Reka river is certain. A unique rotational landslide formed owing to the close proximity of the Raša right lateral strike-slip fault, and the opening up of the pull-apart basin within the fault zone. The pull-apart basin was situated just beneath the lja (sl. 2) se narivna ploskev Trnovskega pokrova v fosilnem rotacijskem plazu Mala Gora nahaja približno na koti 650 m. V pobočju Malega Mod- rasovca se ob drsni ploskvi plazu Mala Gora dvigne za okoli 100 m na 750 m, ob cesti pod Črnim školjem pa sega do višine okoli 770 m. V splošnem lahko rečemo, da se rahlo dviga. Pod pobočjem med Črnim školjem, Malim Modra- sovcem in Malo Goro je obsežno območje zdrse- lih blokov, ki ga imenujemo plazišče Črni školj. Podrobno so ga opisali Kocjančič in sodelavci (2019). Nastalo je zaradi dviga narinjenega kar- bonatnega bloka s flišem v podlagi in denuda- cijskega nižanja nivoja Vipavske doline. Fosilni rotacijski plaz Mala Gora je mlajšega datuma in seka plazišče Črni školj. V jugovzhodnem pobočju Nanosa (sl. 3) se na - rivna ploskev od kote 750 južno od Pleše (1262 m) dvigne do 1000 m pod Suhim vrhom (1313 m), nakar se spusti na 770 m pod Tisovcem (911 m). Območje Suhega vrha ima zaradi ekstremnega dviga narinjenega karbonatnega bloka s flišem v podlagi specifično zgradbo, ki je posledica gravitacije, vendar se razlikuje od klasičnih pobočnih zdrsov pod stenami med Plešo in vr- hom V rtu (1108 m), ki jih je povzročila erozija in denudacija tega dela Postojnske kotline. Pri Trnovskem in Hrušiškem pokrovu se je narivna ploskev za čelom pokrova dvignila in konveksno usločila, vendar pri Hrušiškem bistveno bolj kot pri Trnovskem. Pri Petelinjih mlakah (sl. 4) strukturna os- nova za nastanek plazu ni jasna, ker območje ni natančneje kartirano. Izjemna je njegova lega dva kilometra za čelom Snežniškega pokrova, ki kaže na velike postnarivne spremembe in na specifične pogoje nastanka. Preseneča Jurški prelom, ki je moral nastati po sprožitvi plazu. Ilirskobistriški fosilni plaz (sl. 5) je nastal tako, da je karbonatni blok Snežniškega pokrova zdrsnil po flišni podlagi. Tu nimamo dokazov, da bi se narivna ploskev konveksno izbočila, za- gotovo pa se je erozijsko poglobila dolina reke Reke. Do nastanka edinstvenega rotacijskega plazu je prišlo zaradi bližine desnozmičnega Raškega preloma in razprtja bazena tipa pull-a- part znotraj njegove širše prelomne cone. Bazen je ležal tik pod pobočjem narivnega čela, tako da je to ob razprtju izgubilo oporo in zdrsnilo navzdol (Placer & Jamšek, 2011). Pojav je moral biti hipen, kar ga povezuje s seizmičnim dogod- kom. Bistriški pul-apartski bazen je pokrivWal osrednji del Bistriške kotline pod plazom. 41 Tectonics and gravitational phenomena (Nanos, Slovenia) thrust front slope, lost stability, and collapsed at the opening of the basin (Placer & Jamšek, 2011). The collapse had to be instantaneous which brings it in relation with a seismic event. The Ilirska Bis- trica pull-apart basin covered the central part of the Ilirska Bistrica basin beneath the landslide. Gravitational phenomena on the SE slope of Mt. Nanos The SE slope of Mt. Nanos reveals some in- sight into the structure of the fore and rear parts of the Hrušica Nappe Thrust Front, as well as the morphology of its thrust plane, which was de- formed in post-thrust processes. The gravitation- al phenomena at Suhi vrh reflects these deforma - tion (processes). Structure of Mt. Nanos and Mt. Hrušica Mt. Nanos is part of a vast and extensive Hrušica Nappe Thrust Front composed of Juras- sic and Cretaceous carbonate rocks and uncon- formably deposited Eocene flysch (Fig. 6). At the NW part a large NW-plunging anticline in the NW turns into a recumbent one thrusted upon the Snežnik Nappe Eocene flysch (in the central and eastern parts of the Postojna basin), and the External Dinaric Imbricated Belt, also referred to as parautochton (Rebrnice and W part of the Postojna basin). Also, part of the External Di- naric Imbricated Belt is the Jurassic carbon- ate Šmihel klippe (Čar & Juren, 1980). Jurassic and Cretaceous carbonate rocks, as part of the Trnovo Nappe, are thrust upon the Eocene flysch belonging to the Hrušica Nappe (Srednja gora, 1275 m a.s.l., Streliški vrh, 1266 m a.s.l.) (Fig. 6.). The Snežnik Nappe thrust plane crosses the western part of the Postojna basin. The thrust plane is identifiable in the Hruševje area, where the carbonate rocks of the Snežnik Nappe are thrust upon the External Dinaric Imbricated Belt, but NW of Hrušica the flysches of both nappes meet, hence the trace is not identified there. De- tailed sedimentological and paleontological map- ping would be required to determine the Snežnik Nappe thrust plane trace there. A thrust plane trace is only provisionally shown in Figure 6. The structure of the lower part of the Hrušica Nappe between Strane, Predjama, and Studeno differs from the upper part; however, there is not enough data on the Buser’s (1967) geologic map to construct the correct structural interpretation. The unconformity between the Upper Cretaceous limestone and marl (scaglia), and the Eocene fly- sch between Strane and Studeno represents the main problem in the map. Following the bound- Gravitacijski pojavi na jugovzhodnem pobočju Nanosa Jugovzhodno pobočje Nanosa ponuja vpog- led v notranjo zgradbo čelnega in začelnega dela krovne enote Hrušiškega pokrova in morfologijo njegove narivne ploskve. Ta je bila deformirana v postnarivnih procesih. Gravitacijski pojav Suhi vrh je odraz teh deformacij. Zgradba Nanosa in Hrušice Nanos leži v čelu obsežnega Hrušiškega po - krova, zgrajen je iz jurskih in krednih kar- bonatnih kamnin ter diskordantno odložene- ga eocenskega fliša (sl. 6). Plasti krovne enote tvorijo na severozahodu veliko proti SZ tone- čo antiklinalo, na jugovzhodu pa veliko poleg- lo antiklinalo, ki sta narinjeni na eocenski fliš Snežniškega pokrova (osrednji in vzhodni del Postojnske kotline) in Zunanjedinarskega na- luskanega pasu ali paravtohtona (Rebrnice, za- hodni del Postojnske kotline). Del Hrušiškega pokrova je tudi Šmihelska tektonska krpa iz jurskih plasti (Čar & Juren, 1980). Severno od Nanoške antiklinale so na eocenski fliš Hruši- škega pokrova narinjeni jurski in kredni karbo- nati Trnovskega pokrova (Srednja gora 1275 m, Streliški vrh 1266 m) (sl. 6). Preko zahodnega dela Postojnske kotline po- teka narivnica Snežniškega pokrova, ki je do - ločljiva na območju Hruševja, kjer so karbonati Snežniškega pokrova narinjeni na fliš Zuna- njedinarskega naluskanega pasu. Od Hruševja proti severozahodu potek narivnice ni določen, ker se tu stikata fliša obeh krovnih enot. Za nje - no določitev bi bilo potrebno izvesti detajlno se- dimentološko in paleontološko profiliranje. Na sliki 6 je potek narivnice le nakazan. Spodnji ustroj Hrušiškega pokrova med Stranami, Predjamo in naprej do Studenega je zgrajen drugače kot zgornji del, vendar na OGK, list Postojna (Buser et al., 1967) ne najdemo do- volj podatkov za korektno strukturno rešitev tega vprašanja. Glavni problem na karti pred- stavlja diskordantna meja med zgornjekrednim apnencem in laporjem (scaglia) ter eocenskim flišem med Stranami in Studenim, ki bi gle- de na vpad plasti in potek na površju (pravi- lo V) morala biti inverzna. Ker pa razvoj plasti nad diskordanco kaže na poševni rez, je Placer (1981) to mejo interpretiral kot krovno narivno ploskev, zgornjekredne in spodnjekredne plasti v krovni grudi pa razvrstil v tri vmesne krovne luske. Taka razdelitev bi po analogiji ustreza- la spodnjemu ustroju Trnovskega pokrova na območju Idrije (Mlakar, 1969). Čar & Šebela 42 Ladislav PLACER, Andrej MIHEVC & Igor RIŽNAR Fig. 6. Geological - geomorphological sketch of Mt. Nanos a part of Mt. Hrušica. Sl. 6. Geološko-geomorfološka skica Nanosa in dela Hrušice. 1 Mesozoic carbonates / mezozojski karbonati 2 Eocene flysch / eocenski fliš 3 Mezozoic and Paleogene carbonates / mezozojski in paleogenski karbonati 4 Landslide areas / plazišča: Rebrnice, A – »Ubeljska stena«, B – Votla stena, C – Rjava stena 5 Strike-slip fault / zmični prelom: BF – Belsko fault / Belski prelom, CF – Črnjavska dolina fault / Črnjavski prelom, PF – Predjama fault / Predjamski prelom 6 Normal fault / normalni prelom: RoF – Roček fault and inferred faults of the similar mechanism / RoF – Ročkov prelom in domnevni prelomi enakega mehanizma 7 Normal fault with hereditary gravitational slide / normalni prelom z nasledstvenim gravitacijskim zdrsom: ReF – Reševnik fault / Reševniški prelom 43 Tectonics and gravitational phenomena (Nanos, Slovenia) ary course on the surface (according to the V-rule) it should be in inverse position. However, Plac- er, (1981) interpreted the boundary as a low-an- gle thrust fault (a sole thrust), since the structure above the unconformity indicates an oblique cut. The Upper and Lower Cretaceous beds in the hanging wall are interpreted as three intermedi- ate duplexes. Such an interpretation is analogous to the structure of the lower part of the Trnovo Nappe in the Idrija area (Mlakar, 1969). Mapping the Predjama area (Čar & Šebela, 2001) confirmed a tectonic boundary between the Eocene flysch and the Cretaceous beds and, consequentially, Placer’s (1981) interpretation. The upper level of the Hruši- ca Nappe structure between Studeno and Strane is simple: Upper Cretaceous and Jurassic carbon- ates are thrust upon the Upper and Lower Creta- ceous carbonates of the lower level. In general, the described structure confirms the interpretation with intermediate duplexes, only they are fewer in number, with only one or two east of Tisovec and probably none between Tisovec and Strane. The duplexes are here included in the Hrušica Nappe, as these issues are not of particular importance for our purposes herein. After the thrusting, a large recumbent Nanos anticline (Limanowski, 1910; Buser, 1967; Placer, 1981) in the thrust front of the Hrušica Nappe was regionally folded into the Nanos anticline and Hrušica syncline. The principal structural elements of the Hruši- ca Nappe are presented in Figure 6. The recum- bent Nanos anticline in the Hrušica Nappe Thrust Front is defined by the position of the northward dipping recumbent fold’s axial plane, dividing the normal from the inverse strata. The Nanos anticline and Hrušica syncline axes plunge in the NW direction. The Hrušica Syncline is cut by the Predja- ma fault, a dislocation named by Buser (1976), who defined the fault according to stratigraphic (2001) sta pri Predjami s kartiranjem potrdila tektonsko mejo med zgornjekrednim apnencem in flišem kar je v splošnem potrjevalo Placerje- vo domnevo. Zgradba zgornje etaže Hrušiškega pokrova je drugačna toda enostavna, na obrav- navanem odseku jo sestavljajo zgornjekredni in jurski karbonati, ki so narinjeni na spodnje in zgornjekredne karbonate spodnjega ustroja. Vse to v splošnem potrjuje interpretacijo s krov- nimi vmesnimi luskami, le da je njihovo število manjše; med Stranami in Tisovcem verjetno ni nobene, vzhodno od Tisovca pa ena ali dve. V tem članku to vprašanje ni toliko pomembno, zato so vmesne luske vključene v Hrušiški po - krov. Velika polegla nanoška čelna antiklinala (Limanowski, 1910; Buser, 1967; Placer, 1981) je bila z ostalim delom Hrušiškega pokrova v postnarivnem obdobju regionalno nagubana, tedaj sta nastali Nanoška antiklinala in Hruši- ška sinklinala. Na sliki 6 so podani glavni strukturni ele- menti Hrušiškega pokrova. Polegla krovna antiklinala je določena z lego osne ravnine, ki ločuje inverzne in normalne plasti in vpada ge - neralno proti severu. Osi Nanoške antiklinale in Hrušiške sinklinale toneta proti severozaho- du. Hrušiško sinklinalo seka dislokacija, ki jo je Buser poimenoval Predjamski prelom; po strati- grafskih kriterijih ga je povlekel mimo Predja- me in ga proti severozahodu povezal s prelomom po dolini Bele, nato s prelomom mimo Predmeje in naprej s prelomom Avče-Dol. Jugovzhodno od Predjame ga je podaljšal v Snežniško hribovje (Buser, 1976), na OGK (Buser et al., 1967) pa tega preloma JV od Predjame ni vrisal. Na lidarju je razvidno, da se glavna prelomna ploskev, ki po- teka po dolini Bele, nadaljuje v smeri ESE proti Belskemu, kjer močno premakne narivnico Hru - šiškega pokrova. Prelom, ki poteka mimo Predja- 8 Regional sub-vertical fractures (strike angle 20° - 40°) / regionalne subvertikalne razpoke smeri 20° do 40° 9 Deformational curve of the regional fractures with 20° to 40° strike angle due to right lateral offset along the Belsko fault / deformacijska krivulja regionalnih razpok 20° do 40° zaradi desnega premika ob Belskem prelomu 10 Hrušica Nappe boundary / meja Hrušiškega pokrova 11 Snežnik Nappe boundary / meja Snežniškega pokrova 12 Tectonic klippe / tektonska krpa: 1 – Streliški vrh t. k. / Streliška t. k., 2 – Šmihel t. k. / Šmihelska t. k. 13 Bedding: normal, inverse / plasti: normalne, inverzne 14 Axial plane of the recumbent frontal anticline / osna ravnina polegle krovne antiklinale 15 Mt. Nanos anticline axis / os Nanoške antiklinale 16 Hrušica syncline axis / os Hrušiške sinklinale 17 Blind valley rim / rob slepe doline 18 Spring, sinkhole / izvir, ponor 19 Motorway / avtocesta 20 Position of geological cross-sections 1a, 1b (Fig. 11) and 2 (Fig. 16) / lega profilov 1a, 1b (sl. 11) in 2 (sl. 16) 21 Geological boundary / geološka meja 44 Ladislav PLACER, Andrej MIHEVC & Igor RIŽNAR criteria. Buser drew the fault trace along Predja- ma and connected it in the NW direction with a fault along the Bela valley, further due NW with a fault passing Predmeja, and still further with the Avče - Dol fault. Southeast of Predjama, Buser (1976) extended the fault into the Snežnik hills, but this fault segment is not yet drawn on the Buser’s geological map (Buser et. al., 1967). It is clear from the lidar image that the main fault plane from the Bela valley continues due ESE toward Belsko, where it considerably offsets the Hrušica Nappe thrust plane. The fault passing Predjama is only a secondary fault (branch) detached from the main fault plane. We suggest therefore, naming the Bela fault that fault that runs along the Bela valley and passes Belsko, while the splay (secondary fault) passing Predjama (30/75) should retain the name Predjama fault. The latter dies out due SE with- in a relatively short distance, much like the oth- er Bela fault’s secondary branches (splays). There are many secondary faults leaning on the Bela fault, and all host geomorphologically responsive sinkholes: the Črnjavska dolina fault hosts a Pre- povedanci sinkhole, the Predjama fault, and the Bukovje fault (Čar & Šebela, 2001). Apart from the aforementioned right lateral strike-slip sec- ondary faults, other N-NW trending sub-vertical faults with subsided NW blocks are also import- ant. They too lean on the Bela fault, but only the Roček fault (70/80) among them has been named (after the Roček hot spring). The multiphase evolution of the Bela fault and both sets of described secondary faults leaning on it (the ones with a strike-slip, as well as those of nor- mal character, is reflected in various geomorpho - logic effects. Significant vertical offsets are obvious along the normal faults (e.g., Roček fault), while no vertical offsets are observed along the strike-slip faults. However, discussing the genesis of these fault systems is not the purpose of this article. We propose the name Predmeja fault for the northern part of the Predjama fault (Buser, 1976), as the fault segment between Predmeja and Col reflects the highest geomorphic response. A NNE–SSW (roughly 20°) trending system of subvertical fractures stands out over the entire Hrušica Nappe area. Due to the dextral strike- slip motion of the Bela fault these fractures are sigmoidally bent and trend in the SW–NE (rough- ly 40°) direction in their most deformed part. The Reševnik fault (210/60) is exposed just behind the Hrušica Nappe Thrust Front above Razdrto. Its principal and parallel secondary fault planes later adopted the role of gravitation- ally generated slip planes. The Reševnik fault is me je le sekundarni odcep omenjene glavne pre- lomne ploskve. Zato predlagamo, da se prelom, ki poteka po dolini Bele in gre na Belsko imenuje Belski prelom, Predjamski prelom pa naj se ime- nuje le sekundarni krak, ki poteka mimo Pred- jame (30/75) in proti jugovzhodu, tako kot drugi sekundarni prelomi Belskega preloma, kmalu zamre. Sekundarnih krakov, ki se naslanjajo na Belski prelom je več, ob vseh so nastali geomor- fološko odzivni ponori. To so Črnjavski prelom (Črnjavska dolina), ob katerem je nastal ponor na Prepovedancih, Predjamski prelom in Buko- vski prelom (Čar & Šebela, 2001) mimo Bukov- ja. Poleg omenjenih sekundarnih desnozmičnih prelomov so pomembni še subvertikalni prelomi v smeri NNW-SSE, ob katerih je severovzhodno krilo ugreznjeno. Tudi ti se naslanjajo na Belski prelom; od teh je poimenovan le Ročkov prelom (70/80). Belski prelom z obema snopoma sekundarnih prelomov, zmičnih in subvertikalnih z vertikalno komponento premika, kaže na večfazni razvoj z različnimi geomorfološkimi učinki. Ob prelomih zmičnega snopa ni v reliefu vertikalnih skokov, ob prelomih tipa Ročkov prelom, pa je opazna iz- datna stopnja v reliefu. Geneza teh sistemov ni predmet tega članka. Za segment Buserjevega Predjamskega prelo- ma med Predmejo in Colom (Buser, 1976) predla- gamo, da se preimenuje v Predmejski prelom, ker je ta odsek na celotni trasi geomorfološko najbolj odziven. Na celotnem območju Hrušiške krovne eno- te izstopa sistem regionalnih subvertikalnih razpok v smeri SSW-NNE (okoli 20°). Te so za- radi desnega zmika ob Belskem prelomu v sme- ri WNW-ESE regionalno sigmoidalno usločene, tako da imajo v najbolj zasukanem delu smer SW-NE (okoli 40°). V boku Hrušiškega pokrova je takoj za njego- vim čelom nad Razdrtim viden Reševniški prelom (210/60), katerega glavna in vzporedne prelomne ploskve so pozneje prevzele vlogo gravitacijskih drsnih ploskev. Reševniški prelom je viden le v jugovzhodnem pobočju Nanosa, na površju proti severozahodu pa še ni bil sleden. Pobočni zdrsi so prisotni povsod, kjer so kar- bonati na flišu ekstremno dvignjeni nad krajino. Na slikah 6 in 8 so razdeljeni na tiste pod čelom nariva (Rebrnice) in na tiste pod jugovzhodnim pobočjem Nanosa pod »Ubeljsko«, Votlo in Rja- vo steno. Ime »Ubeljska stena« ne obstaja v geo- grafski ali ljudski terminologiji, uvedli smo ga iz praktičnih razlogov in zajema strmo pobočje med Razdrtim in Votlo steno. 45 Tectonics and gravitational phenomena (Nanos, Slovenia) only observable on the SE slope of Mt. Nanos and has not yet been mapped due NW. Landslides are present wherever carbonates lying on flysch are uplifted high above the sur- rounding landscape. Landslides are divided into those beneath the thrust front (Rebrnice) and those on the SE slope of Mt. Nanos (beneath the Ubeljska stena, Votla stena and Rjava stena) (Figs. 6 and 8). An “Ubeljska stena” toponym does not exist in geographical or other terminology. We introduced the term for practical purposes in order to refer to the steep slope between Razdrto and Votla stena. The structure of the Hrušica Nappe front exposed on the SE slope of Mt. Nanos between Razdrto and Roček spring (Fig. 8) is presented in the panoramic photo (Fig. 7). Geologic cross-sec- tions are constructed across and parallel to the view in Figure 7. The most important part of the geological structure there is a recumbent anti- cline reconstructed from its visible core in the Votla stena, just beneath Suhi vrh. The inverse strata are present from the Ubeljska stena, across Votla and Rjava stena. Its axial plane rises to- wards the thrust front and should lean on the thrust plane in the opposite direction. However, this point (where the recumbent fold’s axial plane meets the thrust plane) is not determinable due to scarce structural data on the geological map (Buser et al., 1967). An inverse limb more than 4 km long is surprising. Zgradba čelnega in začelnega dela krove eno - te Hrušiškega pokrova je vidna v jugovzhodnem pobočju Nanosa med Razdrtim in izvirom Ro- ček (sl. 8). Predstavljena je na panoramskem po- snetku na katerem je skiciran strukturni profil (sl. 7). Tu je najpomembnejši element strukture rekonstruirana polegla krovna antiklinala, ki je izvedena iz njenega vidnega jedra v Votli steni tik pod Suhim vrhom. Inverzne plasti se razte- zajo preko Ubeljske, Votle in Rjave stene. Osna ravnina polegle antiklinale se proti čelu pokro- va dviguje, v nasprotno smer pa bi se morala naslanjati na narivno ploskev, vendar te točke ni mogoče določiti zaradi skopih strukturnih po- datkov na OGK, list Postojna (Buser et al., 1967). V profilu preseneča izdatna dolžina inverznega krila, ki presega 4 kilometre. Narivna ploskev v profilu na sliki 7 je do- ločena po morfoloških znakih v pobočju, kjer poteka pod pregibom med strmo karbonatno steno v krovnini in flišnim pobočjem v tal - nini, ki je položnejše in prekrito s pobočnim gruščem in plazovi. Razen nad Sv. Brikcijem ni fliša videti nikjer. V steni nad Razdrtim je videti Reševniški prelom, kjer se narivna plo- skev od 750 m dvigne za nekaj deset metrov na 800 m, proti Sv. Brikciju pa se vedno bolj str- mo dviga do 1000 m. Pod Votlo steno se spu- sti do višine 850 m nad Stranami, od koder se polagoma spušča do Ročkovega preloma, kjer doseže višino okoli 770 m. Na drugi strani pre- Fig. 7. Panoramic photography of the SE slope of Mt. Nanos and transverse geological cross-section of the Hrušica Nappe front part. Sl. 7. Panorama jugovzhodnega pobočja Nanosa in prečni geološki profil čelnega dela Hrušiškega pokrova. 1 Hrušica Nappe / Hrušiški pokrov: J – Jurassic carbonates / jurski karbonati, K – Cretaceous carbonates / kredni karbonati 2 1E – Eocene flysch of the Snežnik Nappe / eocenski fliš Snežniškega pokrova 3 2E – Eocene flysch of the External Dinaric Imbricated Belt / eocenski fliš Zunanjedinarskega naluskanega pasu 4 Bedding: normal, inverse / plasti: normalne, inverzne 5 Hrušica Nappe boundary / meja Hrušiškega pokrova 6 Hrušica Nappe boundary projection in the Ubeljska stena cross-section / projekcija meje Hrušiškega pokrova v ravnini profila Ubeljske stene 7 Normal fault / normalni prelom: ReF – Reševnik fault / Reševniški prelom, RoF – Roček fault / Ročkov prelom 8 Axial plane of the recumbent frontal anticline / osna ravnina polegle krovne antiklinale 46 Ladislav PLACER, Andrej MIHEVC & Igor RIŽNAR Fig. 8. Geological - geomorphological sketch of the SE part of Mt. Nanos. Sl. 8. Geološko - geomorfološka skica jugovzhodnega dela Nanosa. 1 Mesozoic carbonates / mezozojski karbonati 2 Eocene flysch / eocenski fliš 3 Alluvium / aluvij 4 Landslide areas / plazišča: Rebrnice, A – »Ubeljska stena«, B – Votla stena, C – Rjava stena 5 Strike-slip fault / zmični prelom: CF – Črnjavska dolina fault / Črnjavski prelom 6 Normal fault / normalni prelom: RoF – Roček fault 70/80 / Ročkov prelom 70/80 7 Normal fault with hereditary gravitational slide / normalni prelom z nasledstvenim gravitacijskim zdrsom: ReF – Reševnik fault 210/60 / Reševniški prelom 210/60 8 Nappe boundary / meja pokrova 9 Tectonized zone within the recumbent frontal anticline core / tektonizirana cona v jedru prevrnjene krovne antiklinale 10 Shear razdol / strižni razdol: a, b, c, d, e 11 Divergent razdol / razmični razdol: f 12 Marginal trench / robni jarek: g – probable / verjetni, h – inferred / domnevni 13 Gravitational block / gravitacijski blok: E, G 14 Spring, sinkhole / izvir, ponor 15 Gravel pit, reversely rotated bedding (Fig. 10) / občasni kop gramoza, povratno rotirane plasti (sl. 10) 16 Motorway / avtocesta 17 Position of geological cross-sections 1a, 1b (Fig. 11) and 2 (Fig. 16) / lega profilov 1a, 1b (sl. 11) in 2 (sl. 16) 47 Tectonics and gravitational phenomena (Nanos, Slovenia) The thrust plane in the cross section in Figure 7 is defined by the morphologic features on the slope that appear a few metres below the inflection point, between a steep carbonate slope belonging to the hanging wall and a more gradual flysch footwall slope covered by the slope scree and landslides. Flysch is exposed only above Sv. Brikcij (St. Bric- tius). The Hrušica Nappe thrust plane is located at 750 m a.s.l. in the thrust front above Razdrto. On the other side of the Reševnik fault it rises to 800 m a.s.l. and rises ever more steeply up to 1000 m a.s.l. at Sv. Brikcij. Beneath the Votla stena the thrust plane drops down to 850 m a.s.l. above Strane and descends down to 770 m a.s.l. at the Roček Fault. Across the fault, it drops to 730 m a.s.l. and gradu- ally descends down to the lowest point (on this sec- tion) at 500 m a.s.l. at Predjama. The cross-section geometry is distorted, as the Rjava stena and Ubel- jska stena don’t lie on the same plane, but lie closer to the observer for a distance roughly the length of the Votla stena (about 1500 m) (Fig. 6). An inferred course of the Hrušica Nappe thrust plane in an ex- tension of the Ubeljska stena is presented as a dot- ted line in Figure 7 and illustrates the apex of the convex bulge beneath Suhi vrh (1313 m a.s.l.), the highest geographic point in the area (of Mt. Nanos). However, the bulge and the highest peak do not coincide with the axis of the Nanos anticline. An explanation of this phenomena would go beyond the aim of this paper; however, let us summarize by stating that both the bulge and the Nanos an- ticline arose simultaneously, and that the reason for this peculiarity lies in the internal structure of the Hrušica Nappe. Here we are confronted by two different structural features formed in the same folding phase. The (convex) bulge in the flysch base has an anticlinal form, hence the Nanos flysch an - tiform. Not only is the flysch in the base cambered, but the entire nappe, so we can generally refer to this feature as the Nanos antiform. The Nanos antiform, recumbent Nanos anti- cline and Hrušica syncline reflect the regional im- portance of the post-thrusting folding. A Hruši- ca synform can be deduced from the existence of the Nanos antiform, but unfortunately, we can- not directly prove it, as the Hrušica Nappe thrust plane isn’t exposed in the NE block of the Idrija fault. We do, however, find a hint (in the form of circumstantial evidence) in the post-thrust Trnovo Nappe structure that the Hrušica thrust plane, at some depth, continues also to the NE. The Križna gora synform structure at the SE margin of Trnovo Nappe north of Srednja gora (1275 m a.s.l.) (Fig. 6) formed in the core of the Hrušica syncline and con- tinues under the central part of the Trnovski gozd loma naglo preskoči na okoli 730 m in se potem polagoma spušča do Predjame, kjer leži nekaj pod 500 m. Tu je najbolj spuščena na tem od - seku meje Hrušiškega pokrova. Prerez nariv- ne ploskve je popačen, ker Rjava stena ne leži v ravnini Ubeljske stene, temveč je za dolžino Votle stene, okoli 1500 m, pomaknjena proti opazovalcu (sl. 6). Lega narivne ploskve v na- mišljenem podaljšanem prerezu Ubeljske stene je narisana pikčasto (sl. 7) in lepo nakazuje vrh konveksne izbokline. Ta se nahaja pod Suhim vrhom (1313 m), ki je najvišja točka Nanosa in se ne pokriva z osjo Nanoške antiklinale. Gre za strukturno posebnost katere razlaga prese- ga okvir tega članka, za sedaj pa je dovolj če vemo, da sta obe izbočeni strukturi nastali istočasno in da tiči izvor anomalije v notranji zgradbi krovne enote. Pred seboj imamo dva različna strukturna objekta, ki pa sta nastala v isti fazi gubanja. Konveksna izboklina flišne podlage ima obliko antiforme in jo imenujemo nanoška flišna antiforma. Izbočen ni samo fliš v podlagi temveč tudi narivna enota, zato govo - rimo v splošnem o nanoški antiformi. Regionalni pomen postnarivnega gubanja Hrušiškega pokrova se kaže v obstoju nanoške antiforme, Nanoške antiklinale in Hrušiške sinklinale. Iz obstoja nanoške antiforme skle- pamo tudi na obstoj hrušiške sinforme, ki pa je ne moremo neposredno dokazati, ker na se- verovzhodni strani Idrijskega preloma nariv- nica Hrušiškega pokrova nikjer ne izdanja. Da se gubanje narivne ploskve nadaljuje v globini verjetno tudi proti severovzhodu, obstaja po- sredni namig v postnarivni zgradbi Trnovske- ga pokrova. V jedru Hrušiške sinklinale je raz- vita sinformna struktura Križne gore, ki leži na skrajnem JV robu Trnovskega pokrova, se- verno od Srednje gore (1275 m) (sl. 6). Sinforma se nadaljuje pod osrednji del Trnovskega gozda proti severozahodu (Placer & Čar, 1974), proti severovzhodu se previje v antiformno struktu- ro Idrijskega tektonskega polokna, ta v sinfor- mno strukturo Idrijsko-Žirovskega ozemlja in ta v antiformno strukturo Poljansko-Vrhniških nizov. Omenjene sinforme in antiforme ima- jo dinarsko smer. Lega sinformne strukture Križne gore v osi Hrušiške sinklinale pomeni, da sta nastali v isti fazi gubanja. 48 Ladislav PLACER, Andrej MIHEVC & Igor RIŽNAR due NW (Placer & Čar, 1974). To the NE it is cam- bered in an antiform structure of the Idrija tectonic half-window, and the Idrija tectonic half-window transits into the Žiri - Idrija synform which contin- ues into the Poljane - Vrhnika hills antiform fur- ther to the northeast. These synforms share a NW (Dinaric) trend. The position of the Križna gora synform on the axis of the Hrušica syncline means that they both formed in the same folding phase. Structure of the southwestern part of Mt. Nanos Two unusual relief structures, the result of gravitational processes, surprise on the SE slope of Mt. Nanos, the steephead valley closed by the Ubeljska and Votla stena called the Ubeljsko steephead, and geological structure of the block behind the Votla and Rjava stena called the Suhi vrh gravitational structure. There, other classical slope features are present, as well in the Rebrnice slope and in the landslide areas (zones) named after the cliffs above them – the Ubeljska stena landslide area, the Votla stena landslide area, and the Rjava stena landslide area (Fig. 8). Zgradba jugovzhodnega dela Nanosa V jugozahodnem pobočju Nanosa preseneča- ta dve neobičajni reliefni strukturi, ki sta posle - dici gravitacijskih procesov, prva je zatrep, ki ga zapirata Ubeljska in Votla stena, imenujemo ga ubeljski zatrep, druga je zgradba bloka v za- ledju Votle in Rjave stene, imenujemo jo gravi- tacijska struktura Suhega vrha. Poleg tega so tu tudi klasični pobočni pojavi na plazišču Rebr- nice in na plaziščih, ki jih poimenujemo po ste - nah nad njimi; plazišče Ubeljska stena, plazišče Votla stena in plazišče Rjava stena (sl. 8). Plazišče Rebrnice se nahaja pod jugozaho- dnim pobočjem Nanosa, vendar prispeva po- znavanje razmer na tem prostoru pomemben delež k razumevanju zgradbe celotnega obmo- čja. Za Rebrnice so značilni strukturni, rota- cijski in zemeljski planarni plazovi ter podori, kot tudi (blatno) drobirsko plazenje in tečenje (Popit, 2016). Pomemben nasledstveni dejavnik nestabilnosti je tu Reševniški prelom (sl. 9), za katerega domnevamo, da je njegova prelom- na ploskev v sedanjem stadiju denudacije čela Fig. 9. Gravitational slide along the fault planes in the Reševnik fault zone. Hrušica Nappe front above Razdrto village. Sl. 9. Gravitacijski zdrs po prelomnih ploskvah v coni Reševniškega preloma. Čelo Hrušiškega pokrova nad Razdrtim. 1 C – Hrušica Nappe carbonates / karbonati Hrušiškega pokrova 2 F – Flysch of the External Dinaric Imbricated Belt / fliš Zunanjedinarskega naluskanega pasu 3 Main fault plane 210/60 / glavna prelomna ploskev 210/60 4 Parallel fault plane 210/50 and other hereditary gravitational rupture planes / vzporedna prelomna ploskev 210/50 in ostale kot nasledstvene gravitacijske zdrsne ploskve 5 Inverse bedding / inverzne plasti 6 Hrušica Nappe boundary / meja Hrušiškega pokrova 49 Tectonics and gravitational phenomena (Nanos, Slovenia) The Rebrnice landslide area is situated beneath the SW slope of Mt. Nanos, and its revealed struc- ture contributes significantly to our understanding of the structure of the entire area. Structural and rotational landslides, translational earth slumps, rockfalls, as well as debris flows (Popit, 2016) are characteristic features of the Rebrnice. The Reševnik fault (Fig. 9) is an important hereditary factor of slope instability there, and it is inferred that its fault plane took over the role of the gravita- tional slip plane in the present denudation stage of the Hrušica Nappe Thrust Front. (The arguments for the interpretation are provided further into the text herein.) Slope scree and minor rockfalls pre- vail on the surface of the Ubeljska stena and Votla stena landslide areas. More important, however, is the Rjava stena landslide area, where large block slides and rockfalls prevail in the northern part, and rotational landslides in the southern part, re- spectively. A rotated dip (335/20) in the displaced material of a fossil rotational rubble slide above the village of Strane is presented in Figure 10. Popit (2017) found a similar rotated bedding in the Rebr- nice area. There is an active landslide in the central part of the Strane village itself, but the landslide mechanism has not yet been investigated. Let us take a look at the longitudinal cross-sec- tions 1a and 1b (Fig. 11) before we describe the gravitational structure of Mt. Suhi vrh. The thrust plane is constr ucted according to the flysch outcrops Hrušiškega pokrova, prevzela vlogo gravita- cijske zdrsne ploskve. Argumenti za tako in- terpretacijo bodo podani kasneje. V plaziščih Ubeljska stena in Votla stena na površju pre- vladujejo pobočni grušč in manjši skalni podo - ri. Pomembnejše pa je plazišče Rjava stena, kjer v severnem delu prevladujejo veliki blokovni zdrsi in podori, v južnem delu pa rotacijski pla- zovi. Za ilustracijo je na sliki 10 prikazan po- vratni vpad plasti (335/20) fosilnega gruščnate- ga rotacijskega plazu nad vasjo Strane. Podobne povratne vpade plasti litificiranih meliščnih zaplat ugotavlja Popit (2017) v zaledju plazov Šumljak na območju Rebrnic. V sami vasi Stra- ne pa obstoja delujoči plaz, ki zajema osrednji del naselja. Mehanizem tega plazu ni raziskan. Preden opišemo gravitacijsko strukturo Su- hega vrha, si moramo ogledati vzdolžna profila Nanosa 1a in 1b na sliki 11. V profilu 1a je nariv- na ploskev konstruirana po izdankih fliša nad Sv. Brikcijem, ki stoji na višini 960 m, fliš pod Suhim vrhom torej presega višino 1000 m in se nato spušča proti dnu Rjave stene, kjer dosega višino okoli 850 m. Pri interpretaciji profila so bili odločilni trije elementi zgradbe, na prvem mestu je dejstvo, da v strukturi Votle stene ni videti stopničastega zaporedja normalnih pre- lomov ali velikih gravitacijskih drsnih ploskev, ki bi pojasnjevale nastanek dolin a, b in c (sl. 8), na drugem mestu je podatek, da ležijo omenjene Fig. 10. Reversely rotated bedding of the rotational landslide in the gravel pit in the Rjava stena landslide area (Figs. 8 and 11, 1a). Sl. 10. Povratno zasukane plasti rotacijskega plazu v občasnem kopu gramoza na plazišču Rjava stena (sl. 8 in 11, 1a). 50 Ladislav PLACER, Andrej MIHEVC & Igor RIŽNAR Fig. 11. Longitudinal structural cross-sections of the Mt. Nanos and a kinematic sketch. Profile 1a: Nanos (village) – Mt. Suhi vrh (1313 m) – Rjava stena – Strane (village). Profile 1b: Nanos (village) – »Ubeljska stena« – Ograde (near village Strane). Kinematic sketch of differential offsets of the carbonate micro and macrolithons along regional sub-vertical fractures (strike angle 20° - 40°) above the convexly cambered (bent) thrust plane in the Nanos antiform. Sl. 11. Vzdolžna strukturna profila Nanosa in kinematska skica. Profil 1a: Nanos – Suhi vrh (1313 m) – Rjava stena – Strane. Profil 1b: Nanos – »Ubeljska stena« – Ograde pri Stranah. Kinematska skica diferencialnih premikov karbonatnih mikro in makrolitonov ob regionalnih subvertikalnih razpokah (od 20° do 40°) nad konveksno usločeno krovno narivno ploskvijo na - noške antiforme. 51 Tectonics and gravitational phenomena (Nanos, Slovenia) above Sv. Bikcij (St. Brictius) (960 m a.s.l.) in the 1a cross-section. The flysch beneath Mt Suhi vrh is therefore higher than 1000 m a.s.l. and descends to- wards the base of the Rjava stena, where it is found at approx. 850 m a.s.l. Three structural elements are decisive for the cross-section interpretation. The first is the fact that there is no succession of step- like normal faults in the Votla stena structure, nor any large gravitational slip planes to explain the formation of the a, b and c valleys in Figure 8. The second lies in the fact that these valleys are paral- lel to the regional NNE – SSW-oriented subvertical fractures rotated in a NW - SE direction; and final - ly, the fact that the geomorphological responsive- ness of this regional fracture system is consistent with the thrust plane morphology, which takes the shape of a convex bulge called the Nanos antiform. The fractures’ geomorphologic responsiveness to the thrust plane morphology is reflected in the Li - dar-derived image as individual furrows and more or less obvious strings of dolines (the s fracture lines) that deviate from other doline strings, like those formed along the bedding traces (Fig. 11). The geomorphic response to these fractures is stron- ger, but less obvious in the area where the Hrušica Nappe thrust plane is steeper, between the village of Nanos and Suhi vrh (1313 m a.s.l.). The geomor- phologic response to the bedding of the Lower Cre- taceous limestones in the form of step-like valleys is stronger there and thus prevails. The only inter- ruption of individual ridges between these asym- metric valleys seems to represent a geomorphologic response to the NNE - SSW oriented fractures (the ss lines) there (Fig. 11). In the block above the Rjava stena, however, relatively deep, straight symmetric valleys are formed (the sss lines). For the latter, a new term razdol is proposed here. The abbreviated Slovene terms razpoka (Slovene term for fracture) and dol = dolina (valley) are combined into the pro- posed term razdol. Only razdols have a character- istic shape in the Mt. Nanos area, while more or less doline v smeri sistema regionalnih subvertikal- nih razpok SSW-NNE, ki so tu rotirane v smer SW-NE, in nazadnje dejstvo, da je geomorfolo- ška odzivnost omenjenega sistema regionalnih razpok usklajena z morfologijo narivne ploskve, ki ima obliko konveksne izbokline imenova- ne nanoška antiforma. Geomorfološka odziv- nost teh razpok na morfologijo narivne ploskve se kaže tako, da se na širšem območju zaselka Nanos razpoke na lidarju vidne kot posamezne brazde in bolj ali manj očitni nizi vrtač (razpok - linske linije s), ki po usmerjenosti odstopajo od ostalih, npr. razvitih vzdolž plastnatosti (sl. 11). Na območju, kjer je Hrušiška narivna ploskev strmejša, med zaselkom Nanos in Suhim vrhom (1313 m) je geomorfološki odziv teh razpok moč - neje izražen vendar slabše viden, ker je zakrit z veliko močnejšim geomorfološkim odzivom na plastnatost (medplastni zdrsi) spodnjekrednih apnencev v smeri NNW, zato se odraža le kot prekinitev grebenov asimetričnih dolin vzpore- dnih plastnatosti (linije - ss) (sl. 11). V bloku nad Rjavo steno pa se odziv na te razpoke kaže kot simetrične razpoklinsko-korozijske doline (sss) za katere uvajamo novo ime, ki združuje termi - na razpoka (skrajšano raz) in dolina (skrajšano dol) v nov termin razdol. Na območju Nanosa imajo vzorčno geomorfološko obliko le razdoli, medtem ko so bolj ali manj razviti razpoklinski nizi vrtač slabše vidni in neprimerni za temelj - no predstavitev. Profil 1b kaže razmere v zaledju Ubeljske stene, kjer poteka preko zdrselega bloka G (sl. 8). Razlaga temelji na interpretaciji senče - nega modela reliefa izdelanega iz lidarskih po- datkov (eVode, 2016). 1 Mesozoic carbonates / mezozojski karbonati 2 Eocene flysch / eocenski fliš 3 Slope scree, rotational landslide / pobočni grušč, rotacijski plaz 4 Bedding: normal, inverse / plasti: normalne, inverzne 5 Fault: RoF – Roček fault, ReF – Reševnik fault / prelom: RoF – Ročkov prelom, ReF – Reševniški prelom 6 Hrušica Nappe thrust plane / narivna ploskev Hrušiškega pokrova 7 Tectonized zone in the core of the recumbent frontal anticline / tektonizirana cona v jedru polegle krovne antiklinale 8 Regional subvertical fractures with 20° - 40° strike angle: s – fracture lines without geomorphological response: ss – fractu- ral furrows: sss – razdols / regionalne subvertikalne razpoke smeri 20° do 40°: s – razpoklinske linije s slabim geomorfološ- kim odzivom: ss – razpoklinske linije z močnejšim geomorfološkim odzivom: sss – razdoli 9 Shear razdol / strižni razdol: a, b, c 10 Divergent razdol /razmični razdol: f 11 Marginal trench / robni jarek: g – probable / verjetni, h – inferred / domnevni 12 Divergence area / območje razmikanja 13 Hypothetical gravitational heredital slide along the Reševnik fault in the Rebrnice slope / hipotetični gravitacijski nasledstveni zdrs po Reševniškem prelomu na Rebrnicah 52 Ladislav PLACER, Andrej MIHEVC & Igor RIŽNAR developed doline strings along other fracture sys- tems are less pronounced and hence not suitable for comprehensive presentation. The geological cross-section 1b across a grav- itationally slided G block (Fig. 8) illustrates the geological structure behind the Ubeljska stena. The explanation is based on the interpretation of a shaded relief model constructed from lidar im- ages (eVode, 2016). A comparison of the two cross sections (1a and 1b) shows they are identical, except for the missing carbonate cover in the 1b cross-section that is still present in the 1a cross-section behind the Votla and Rjava stena. The missing carbonate cover E of Ubeljska stena gradually disintegrated into slid- ed and collapsed blocks and corroded relatively quickly due to the presence of water. A simplified kinematic sketch illustrating the relations between the convex shape of the flysch basement beneath the thrust fault and the structur- al blocks (macrolithons) in the carbonate hanging block is given in Fig. 11. Offsets in the rigid media Na podlagi primerjave med profiloma 1a in 1b je moč ugotoviti, da sta identična, razlika je le v tem, da v drugem profilu ni več karbonatnega pokrova, ki še obstaja v zaledju Votle in Rjave stene. Tu je postopoma razpadel v plazišče in za - radi prisotnosti vode hitreje korodiral. Na sliki 11 je pridana poenostavljena kine- matska skica, ki ponazarja odnos med konve- ksno obliko flišne podlage pod narivno ploskvi- jo in bloki v karbonatni krovni enoti. Premiki v trdnem mediju so se sproščali na več načinov, ali z zdrsi po posameznih razpokah med mikrolito- ni, po snopih razpok med makrolitoni in redke- je po eni zbirni razpoki, ki je postala prelom in prevzela vlogo snopa med makrolitoni. Premiki ob razpokah so povzročili tektonizacijo kamni- ne zaradi česar je postala korozijsko oslabljena. Razdalja med snopi razpok in zbirnimi razpo- kami je pogosto sistemska, ker je povezana z različnimi mehanskimi in prostorskimi pogoji, zato je tudi razdalja med razdoli pogosto enaka (sl. 12). Fig. 12. A. Panoramic view of the Votla stena, B. Kinematic sketch. Sl. 12. A. Panorama Votle stene, B. Kinematska skica. 1 C – Hrušica Nappe carbonates / karbonati Hrušiškega pokrova 2 F – Flysch of the Snežnik Nappe / fliš Snežniškega pokrova 3 Bedding: normal, inverse / plasti: normalne, inverzne 4 Axial plane of the recumbent frontal anticline / osna ravnina polegle krovne antiklinale 5 Area of intense differential offsets along regional fractures / območje intenzivnejših diferencialnih premikov po regional- nih razpokah 6 Direction of the regional fractures along regional fractures / smer diferencialnih premikov po regionalnih razpokah 7 Direction of intense differential movements along fractures / razpoke z intenzivnejšim diferencialnim premikom 8 Hrušica Nappe boundary / meja Hrušiškega pokrova 53 Tectonics and gravitational phenomena (Nanos, Slovenia) (limestone) are realized in several ways – either by slips along individual fractures between microli- thons, along fracture sheafs between macrolithons, or less frequently along one collective (cumulative) fracture that became a fault and took over the frac- ture sheaf role between the macrolithons. The offsets along fractures caused a tectonization of the rock and consequently accelerated corrosion and hence Na panoramskem posnetku Votle stene (sl. 12) je na levi videti jedro polegle gube, od koder se inverzne plasti vlečejo povprek cele stene, nato razdola a in b, subvertikalne razpoke in snop re- aktiviranih razpok v severozahodnem pobočju vsakega od obeh razdolov. Razdoli a, b in c so nastali nad jugovzhodnim pobočjem nanoške antiforme, mehanizem na- stanka je lepo viden pri razdolih a in b. Njihova smer je enaka smeri razpok in so zaradi tega so- razmerno ravni (sl. 13), v ozkem dnu razdola pa je običajno niz ponorov (sl. 14). Pred seboj ima- mo strižno razpoklinsko-korozivno dolino ali strižni razdol. Drugačna je morfološka podoba razdola f, ki se nahaja na vrhu nanoške antiforme (sl. 11, 1a); tu se niso toliko uveljavili gravitacijski diferen- cialni zdrsi temveč razmikanje in kot posledica ugrezanje makrolitonov, podobno kot v prime- ru ugrezanja temena antiklinale. Temu ustre- za tudi morfologija tega razdola; njegovo dno je ravno in predstavlja površino ugreznjenega makrolitona, po nastanku enak malemu tek- tonskemu jarku. Razdol, ki je nastal z ugreza- njem makrolitona (strukturnega bloka) imenu- jemo razmično razpoklinsko-korozivna dolina ali razmični razdol (sl. 15). Na sliki 8 je prikazana razporeditev razdolov a, b, c, d in e, ki so nastali v razpoklinskih siste- mih po diferencialnih strižnih premikih in raz - dol f, ki je razmičnega (divergentnega) nastan- ka. Poleg tega obstajata še dve, dolini podobni Fig. 13. Razdols morphology. Sl. 13. Morfologija razdolov. 1 C – Hrušica Nappe carbonates / karbonati Hrušiškega pokrova 2 F – Flysch of the Snežnik Nappe / fliš Snežniškega pokrova 3 Bedding: normal, inverse / plasti: normalne, inverzne 4 Nappe boundary / meja pokrova 5 Shear razdol / strižni razdol: a, b, c, d 6 Divergent razol / razmični razdol: f Fig. 14. Shear razdol a. A valley formed in a course of successive shear offsets in a fracture system, enhanced by corrosion. Sl. 14. Strižni razdol a. Razpoklinsko-korozivna dolina nastala pri strižnih sukcesivnih premikih po razpoklinskem sistemu. 54 Ladislav PLACER, Andrej MIHEVC & Igor RIŽNAR tvorbi h in g, ki kažeta na gravitacijska zdrsa iz - ven regionalnega sistema razpok (NNE - SSW), v prvem primeru kaže na to njena polkrožna ob - lika, ki domnevno predstavlja robni jarek in v zaledju drsno ploskev rotacijskega plazu. Blok jugovzhodno od domnevnega robnega jarka h leži okoli 50 do 100 m nižje od tistega na severo- zahodu poleg tega je jugovzhodni blok značilno prizadet, tu se lega plasti večkrat spremeni, kar kaže na obstoj različnih manjših blokov, med- tem ko imajo plasti severozahodno od tod enotno smer. V primeru domnevnega robnega jarka g je zdrs nakazan z izstopajočo lego bloka G. Prvi primer (g) je domneven, drugi (h) pa verjeten. Poleg bloka G izstopa tudi blok E, kjer izgleda, da je prišlo najprej do diferencialnih premikov po sistemu razpok po katerih je nastal razdol e, weakened the rock between offset macrolithons. The distance between the fracture sheafs and col- lective fractures is often systemic, as it is related to various mechanical and spatial conditions, and thus frequently equidistant from each other (Fig. 12). The core of a recumbent fold with its inverse strata along the entire wall, together with two razdols are visible in the Votla stena panoramic photograph (Fig. 12), along with the subvertical fractures and finally a sheaf of tectonized frac- tures on the NW slope of both razdols. Razdols a, b and c were formed above the SW slope of the Nanos antiform, and the formation mechanism is obvious in the a and b razdols, as their orientation is parallel to the fractures which is why they are relatively straight (Fig. 13) and usually host a string of sinkholes along the nar- row bottom (Fig. 14). This type of razdol formed along either a single (leading) or multiple shear fractures, hence the term shear razdol. The morphology of razdol f is somewhat differ- ent (Fig. 11, 1a), as it was formed on the crest of the Nanos antiform. Divergent offsets and the conse- quential subsidence of the macrolithons prevail in this type of razdol, much like a crestal collapse or a miniature tectonic graben. The base of the shear razdol is therefore flat, as it represents the subsid - ed microlithon upper surface. This type of razdol is referred to as a divergent razdol (Fig. 15). A disposition of the a, b, c, d, and e shear razdols formed in the fracture systems by subsidence along the shears between the microlithons and the diver- gent razdol f is presented in Figure 8. Features sim- ilar to valleys h and g formed outside the regional (NNE - SSW) fracture system exhibit signs of mass movement. The h valley’s circular shape resembles a valley between a head scarp with a shear plane in the hinterland and the reverse slope of the rotated slump blocks. The structural block south-east of the inferred head scarp (h) is 50 to 100 m lower from the block that lies to the north-west and character- istically deformed. The bedding geometry changes several times across the presumably slumped block resembling multiple rotated slump blocks, while north-west of the inferred head the scarp bedding is uniform. In the case of the g valley, a gravitation- al offset of the G block is evident from its protrud- ing position. The first case (g) is inferred, the other (h) is probable. Not only the G block protrudes, but the E block also protrudes from the Ubeljska stena scarp. It seems that differential offsets along the fracture system in which the e valley formed took place first, and later a more pronounced gravita - tional offset proceeded along one of the fractures or the fracture sheaf (Fig. 8). Fig. 15. Divergent razdol f. A valley formed due to divergent offsets in a fracture system enhanced by corrosion. A flat valley bottom and steep side walls suggest subsidence of the intermediate block (macrolithon). Sl. 15. Razmični razdol f. Razpoklinsko-krozivna dolina nastala zaradi razpiranja po razpoklinskem sistemu. Ravno dno in bočne stene nakazujejo, da se je vmesni blok (makro- liton) ugreznil. 55 Tectonics and gravitational phenomena (Nanos, Slovenia) Razdols d and e and a trench (valley) g beneath the head scarp are remnants of the gravitational structures, much like those behind the Rjava and Votla stena. Geological conditions in the Ubeljsko steephead were identical to those behind the Rjava and Votla stena before the carbonate cover disap- peared due to gravitational, corrosive and denu- dational processes, which were the same as those observed at the Rjava stena landslide area. Gravitational phenomena comprising a to f razdols and both g and h marginal trenches belong to the Suhi vrh gravitational structure. Razdols aren’t formed due to classical slope processes, but as part of processes related to gravity. Let us look at transverse cross-section 2 in Fig. 16 in order to complete the image of the struc- ture of the SE part of Mt. Nanos. The thrust plane position is taken from the 1a and 1b cross-sections; however, difficulties arise in the interpretation – related primarily to its dip rather than elevation above the village of Žvanuti. There is no (karstic) spring to drain the karstic water from Mt. Nanos between Razdrto and Vipava. This fact is formally supported by the 45/20 thrust plane dip in the out- crop at 540 m a.s.l. in the roadcut of the 4 th serpentine (from above) of the road to Mt. Nanos, about 3 km NW of cross-section 2. No other relevant data ex- ists on the area between the described outcrop and Razdrto. The absence of karstic (there are springs and streamlets on the SW slope of Mt. Nanos, but these only drain the meteoric water from the slope scree) springs on the SW slope of Mt. Nanos does not correspond to the Nanos antiform beneath Suhi vrh (1313 m a.s.l.); as a result, the only available solution lies in the existence of the 210/60 Reševnik fault above Razdrto (Figs. 8 and 9). The subsid- ed SW block makes it a seemingly normal fault. pozneje pa tudi do močnejšega gravitacijskega zdrsa po eni od razpok ali snopu razpok (sl. 8). Razdola d in e ter »robni jarek« g so ostan- ki gravitacijskih struktur, ki so enake tistim v bloku za Votlo in Rjavo steno. Pred nastankom Ubeljskega zatrepa so tu obstajale enake razme- re na celotni površini zatrepa, karbonatni po- krov je tu izginil zaradi gravitacijskih, koroziv- nih in denudacijskih procesov, ki so bili taki kot jih danes opazujemo na plazišču Rjava stena. Gravitacijske strukture, ki zajemajo razdole od a do f in oba »robna jarka« g in h pripadajo gravitacijski strukturi Suhega vrha. Razdoli niso nastali zaradi klasičnih pobočnih procesov, gre pa za pojave, ki so povezani z gravitacijo. Da bi zaokrožili predstavo o zgradbi jugovzho - dnega dela Nanosa, si oglejmo še prečni profil 2 na sl. 16, Lega krovne narivne ploskve je povzeta po profilih 1a in 1b, težave pa nastopijo pri interpre- taciji njene lege nad Žvanuti na območju Rebrnic. Vprašljiva ni njena višinska kota temveč vpad; v čelu Hrušiškega pokrova od Razdrtega do izvira Vipave ni nikjer izvira, ki bi odvajal kraško vodo iz Nanosa, formalno je ta podatek podprt z vpa- dom narivne ploskve v golici na četrti serpentini nanoške ceste od zgoraj navzdol, ki znaša 45/20 in se nahaja okoli 3 km severozahodno od profila 2. Od omenjene golice do Razdrtega ni ustreznega podatka. Odsotnost kraških izvirov se ne ujema z antiformo pod Suhim vrhom, v pobočju Rebnic so namreč le potočki, ki odvajajo meteorno vodo. V tem trenutku vidimo rešitev problema v obstoju Reševniškega preloma 210/60 nad Razdrtim (sl. 8 in 9), ob katerem je jugozahodno krilo spuščeno, zaradi česar daje videz normalnega preloma. Ven - dar ponujajo razmere na območju Razdrtega tudi drugačno razlago, zato postavljamo domnevo, Rebrnice Fig. 16. Transverse structural cross-section 2: Žvanuti – Debeli vrh – Tisovec – Griže. Key in Fig. 11. Sl. 16. Prečni strukturni profil 2: Žvanuti – Debeli vrh – Tisovec – Griže. Legenda na sl. 11. 56 Ladislav PLACER, Andrej MIHEVC & Igor RIŽNAR However, the geological conditions of the Razdrto area offer an alternative explanation, in which the Reševnik fault planes took over the rupture sur- face role in the sub-recent erosional-denudational environment. This assumption is supported by a 210/50 fault plane within the Reševnik fault zone. According to the spatial criterion this fault plane could act as a rupture surface of a large Reševnik fossil rotational landslide superimposed by the re- cent Razdrto translational landslide (Placer, 2006). The Reševnik rotational landslide is confirmed by the reversely tilted flysch beds. As the Reševnik fault has not been mapped and followed on the surface due NW, only a hypotheti- cal solution is provided in cross-section 2 (Fig. 16). Here, the Reševnik fault is only extrapolated some 3.5 km due NW. A normal fault could not have caused a backtilting of the thrust plane, but only a rotational landslide could have done so. Only spe- cifically targeted research could provide the an - swer as to whether one or more fossil gravitational phenomena occurred in the Rebrnice slope. A Nanos antiform Knowledge of the Hrušica thrust plane mor- phology is important in order to gain an under- standing of the structural, geomorphologic, and hydrologic issues of the area considered here and in the wider area as well. The interpretation of its morphology – structural map of the Nanos anti- form (Fig. 17) – is based on the course of its bound- aries and structural cross-sections (1a, 1b and 2). The internal structure of the structural block behind the Votla and Rjava stena is reflected in the Votla stena structure (cross-section 1a in Figs. 11 and 12). It has been established that the Hrušica Nappe thrust plane inclination towards the Posto- jna basin (SE) is not a consequence of rotational or translational mass movements. It is a differential uplift of the Nanos antiform that triggered differen- tial offsets along the discontinuous SW - NE (SSW - NNE) fracture system. The interpretation is reliable only near the nappe boundary. Beneath the carbon- ate nappe, the interpretation is based on the coinci- dence of the anomaly apex and Mt. Suhi vrh (1313 m a.s.l.), the highest peak of Mt. Nanos. Based on this coincidence, it is inferred that the most uplifted parts of Mt. Nanos between Mt. Suhi vrh (1313 m a.s.l.) and Mt. Debeli hrib (1209 m a.s.l.) along the 330° course may be related to this deformation as well. This trend is subtly predented with the Nanos antiform longitudinal axis trend seen in Figure 16. The isolines course in the Rebrnice slope is based on the cross-section 2 interpretation (Fig. 16). Con- struction of the 700 m, 800 m and the 900 m isolines da je prelomna ploskev tega preloma v subre- centnih erozijsko-denudacijskih pogojih postala ploskev gravitacijskega zdrsa. To domnevo ute- meljujemo s prelomno ploskvijo znotraj cone Re- ševniškega preloma 210/50, ki bi po prostorskem kriteriju lahko bila nosilka velikega fosilnega rotacijskega plazu Reševnik na katerem leži re- centni planarni plaz Razdrto (Placer, 2006). Na rotacijski plaz Reševnik kažejo povratno zasuka - ne flišne plasti. Reševniški prelom ni bil kartiran in sleden na površju proti severozahodu, zato je mogoče po- dati le hipotetično rešitev, kot je prikazana v pro - filu 2 (sl. 16). Tu je Reševniški prelom ekstrapo- liran okoli 3,5 km proti severozahodu, normalni premik ob prelomu ne bi mogel zasukati narivne ploskve v nasprotno smer, to se je lahko dogodilo le zaradi fosilnega rotacijskega plazu. Ali obstoja na Rebrnicah eden ali več fosilnih gravitacijskih pojavov, ni mogoče ugotoviti brez usmerjene raz- iskave. Nanoška antiforma Poznavanje morfologije narivne ploskve Hru- šiškega pokrova je pomembno za razumeva- nje strukturnih, geomorfoloških in hidroloških vprašanj obravnavanega in širšega prostora. In- terpretacija morfologije narivne ploskve sloni na poteku meje pokrova in na strukturnih profilih 1a, 1b in 2, rezultat je strukturna karta nanoške flišne antiforme (sl. 17). Notranja zgradba strukturnega bloka za Votlo in Rjavo steno se kaže v zgradbi Votle stene (pro- fil 1a na sl. 11 in 12). Ugotovljeno je, da vpad na- rivne ploskve proti Postojnski kotlini ni posledi- ca rotacijskega ali drugačnega plazenja, temveč usločenja nanoške antiforme, kar je povzročilo nastanek diferencialnih premikov po diskonti- nuitetah razpoklinskega sistema SW-NE (SSW- -NNE). Interpretacija je zanesljivejša le blizu meje pokrova, od tu pa je razdeljena na del pod karbonatnim pokrovom in na del nad sedanjim površjem zahodnega dela Postojnske kotline. In- terpretacija poteka narivnice pod karbonatnim pokrovom temelji na sovpadanju vrha anomalije in Suhega vrha (1313 m), ki je najvišji vrh Nano- sa. Na podlagi tega sklepamo, da so najvišji deli Nanosa proti Debelemu hribu (1209 m), v smeri 330°, lahko povezani s to deformacijo. Ta smer je rahlo nakazana z obliko antiforme. Potek izoli- nij na Rebrnicah sloni na interpretaciji profila 2 (sl. 16). Izolinije 900, 800 in 700 nad zahodnim delom Postojnske kotline sledijo razmeram na območju Votle in Rjave stene, kjer vpadajo proti kotlini. 57 Tectonics and gravitational phenomena (Nanos, Slovenia) Fig. 17. Structural sketch of the Nanos antiform. Sl. 17. Strukturna skica nanoške antiforme. 1 Mesozoic carbonates of the Hrušica Nappe / mezozojski karbonati Hrušiškega pokrova 2 Eocene flysch of the External Dinaric Imbricated Belt and of the Snežnik Nappe / eocenski fliš Zunanjedinarskega naluska- nega pasu in Snežniškega pokrova 3 Mezozoic and Paleogene carbonates of the External Dinaric Imbricated Belt / mezozojski in paleogenski karbonati Zunanjedinarskega naluskanega pasu 4 Hrušica Nappe boundary / meja Hrušiškega pokrova 5 Thrust plane isoline beneath the nappe / izolinija narivne ploskve pod krovno enoto 6 Thrust plane isoline above the surface / izolinija narivne ploskve nad sedanjim površjem 7 Nanos antiform / nanoška antiforma: A – antiform apex (St. Brictius ridge) / vrh antiforme (hrbet Sv. Brikcija), B – Šmihel ridge / šmihelski hrbet 8 Current Adriatic - Black Sea watershed divide / sedanja razvodnica med jadranskim in črnomorskim povodjem : a – secti- on under the Hrušica Nappe / odsek pod Hrušiškim pokrovom, b – section in the Šmihel (village) area / odsek na območju Šmihela, c – section east of Razdrto (village) / odsek vzhodno od Razdrtega 9 Geological boundary / geološka meja 58 Ladislav PLACER, Andrej MIHEVC & Igor RIŽNAR above the western part of the Postojna basin fol- lows the conditions in the Votla and Rjava stena, where the thrust plane dips towards the basin. In the Šmihel area NE of the Roček fault (70/80) the interpretation is based on the distribution of ponors (sinkholes) W of Predjama and the Šmihel (tectonic) klippe. The thrust plane lies partly be- low and partly above the 600 m isoline, while the 700 m isoline definitively appears near the ridge between Prepovedanci and Praprotniki, just above the 700 m a.s.l. The described features indicate the existence of a ridge – here called the Šmihel ridge – as part of the Nanos antiform (Fig. 17, B). When introducing the term Šmihel ridge we must also name the main part of the Nanos an- tiform for purposes of clearer communication, which is called the Sv. Brikcij ridge or Bric’s ridge (Fig. 17, A). The Nanos antiform determines the Adriatic - Black Sea watershed divide in the west- ern part of the Postojna basin (Fig. 17, 8a, 8b). The watershed divide course in the southern part of the Postojna basin east of the village of Razdrto was established as a result of deformations young- er than the Nanos antiform S and SE of the Hruši- ca Nappe (Fig. 17, 8c). Relief and hydrographic network evolution The formation and degradation (erosion) of both ridges in the Nanos antiform had a fundamental impact on the morphology of the hydrographic network in the western part of the Postojna basin (Fig. 17). The oldest landforms that are still recog- nizable arose when the Hrušica Nappe still cov- ered the western part of the Postojna basin and its southern boundary extended to the Razdrto - Hruševje line. Meteoric water was drained verti- cally through a fractured and karstified limestone nappe to reach a southward-tilted thrust plane. The water formed carstic caves just above the imper- meable flysch basement and flowed due south. The springs were aligned along the nappe boundary be- tween Razdrto and Hruševje. After a short course over flysch the water carved two blind valleys due south, the Biščevci and Sajevče valleys. The surface above the blind valleys stands at 650 to 700 m a.s.l., and the valley bottoms out at about 550 m a.s.l. or 20 m above today’s Nanoščica Creek valley. The Biščevci blind valley is 1500 m long and up to 500 m wide. The valley bottom is covered by fluvial depos - its several meters thick of flysch provenance. Today, the Biščevci valley is a relict blind valley, as the hy - drological conditions south-eastward in the karst have changed radically. After rainfall, the water springs from the Biščevci valley and flows due north as a heavy stream into the Nanoščica. When Na območju Šmihela na severovzhodni strani Ročkovega preloma (70/80), sloni interpretacija na podatkih meje pokrova v pasu ponikalnic in Šmihelske tektonske krpe. Ta leži nekaj pod in nekaj nad izohipso 600, izohipsa 700 pa vsekakor poteka okoli grebena med Prepovedanci in Prap- rotniki, ki je nekaj višji od 700 m. To kaže na obstoj hrbta, ki je del nanoške antiforme. Imenu- jemo ga šmihelski hrbet (sl. 17, B). Ob uvedbi termina šmihelski hrbet je zaradi lažjega sporazumevanja potrebno poimenova- ti tudi glavni del nanoške antiforme (sl. 17, A), imenujemo ga hrbet Sv. Brikcija ali bricov hrbet. Razvodnico med jadranskim in črnomorskim povodjem v zahodnem delu Postojnske kotline določa nanoška antiforma (sl. 17, 8a, 8b). Potek razvodnice na jugu kotline, od Razdrtega pro- ti vzhodu, pa se je oblikoval zaradi deformacij ozemlja južno in jugozahodno od pokrova. Te so mlajše od nanoške antiforme (sl. 17, 8c). Razvoj reliefa in hidrografske mreže Nastanek in razgradnja obeh hrbtov nanoške flišne antiforme sta vplivali na današnjo obliko- vanost hidrografske mreže in reliefa v zahod- nem delu Postojnske kotline (sl. 17). Najstarej- še danes še zaznavne reliefne oblike so nastale, ko je rob Hrušiškega pokrova v zahodnem delu kotline segal še več kilometrov južneje, nekako do črte Razdrto - Hruševje. Padavinska voda je skozi razpokan in zakrasel apnenčasti po- krov vertikalno odtekala do nagnjene narivne ploskve. Tik nad njo je v apnencih oblikovala jame, ter nad nepropustno flišno podlago od- tekala proti jugu. Izviri so bili razporejeni ob robu pokrova na črti med Razdrtim in Hrušev- jem. Izvirna in površinska voda s fliša je nato po kratkem toku po flišu oblikovala dve proti jugu usmerjeni slepi dolini, Biščevce in Sajevško do- lino. Dolini sta vrezani v površje, ki je na viši- nah med 650 in 750 m. Dna slepih dolin sta na nadmorski višini okrog 550 m, oziroma 20 m nad sedanjo dolino Nanoščice. Slepa dolina Biščevci je dolga 1500 m in do 500 m široka. Dno doline pokrivajo več m debele plasti fluvialnih sedi- mentov, ki jih je vanjo naplavila ponikalnica s fliša. Danes so Biščevci reliktna slepa dolina, saj so se hidrološke razmere v krasu jugozahodno od tod povsem spremenile. V dolini po dežju iz- vira voda, ki teče proti severu in se kot močan potok izliva v Nanoščico, ob nizkem vodostaju pa se voda skozi apnenec kraško preceja proti Sajevški slepi dolini. Sajevško slepo dolino je po dimenzijah sodeč oblikovala večja voda, verje- tno Šmihelski potok. Danes pa v njej ponika le 59 Tectonics and gravitational phenomena (Nanos, Slovenia) water levels are low, however, it seeps through the karstified limestone towards the Sajevče blind val - ley. In view of its size, the Sajevče valley must have been formed by a relatively large stream, proba- bly the Šmihel creek. Today, however, only a small streamlet drains the surface water from the flysch and sinks into the valley. The streamlet sinks in the cave of Markov spodmol, part of the Vodna jama v Lozi water cave that stretches 6 km. The Biščevci and Sajevče blind valleys are the largest blind valleys in the Postojna basin. Their position suggests the primal drainage direction of the western part of the Postojna basin due south. According to their size and shape the southward directed stream courses with a low gradient in the karstic part lasted a relatively long time. The lev- elled Slavenski ravnik and a preserved unroofed cave there at 600 m a.s.l. also support this inter- pretation. The Hrušica Nappe southern front withdrew northward rather quickly due to rockfalls and landslides, and underground drainage contrib- uted greatly to the withdrawal as well. The car- bonate cover was undermined and disintegrat- ed rather quickly, as karstic caves were formed along the thrust plane and underground rivers eroded the flysch in the base. Heavily disinte- grated and karstified limestone cover there ac- celerated karstic denudation, as denudation is far more effective in the fractured and tectonized limestone, rockfalls and slope screes. The under- ground drainage pattern formed along the thrust plane was reproduced down into the flysch and established the courses of the present Globočnjak, Žabovec and Šmihel creek valleys due south. Only a very small amount of water is drained from the carbonate cover into the Ubeljsko steep- head, hence the indistinct fluvial relief and far slower degradation of the thrust margin above. The Hrušica Nappe margin within the Bric’s ridge already lies north of its highest part. The surfi - cial watershed divide runs between the Nanošči- ca creek tributaries and the streams that flow due north and sink under the nappe margin west of Predjama; the creeks are as follows: Stranske pon - ikve (S.p. in Fig. 17), Šmihelske ponikve creek (Š.p. in Fig. 17), and Lokva, all part of the Vipava River catchment area. The elevations of their sinkholes, as determined by the thrust plane are: 608 m a.s.l, 600 m a.s.l., and 462 m a.s.l., respectively. The for- mation of the valleys was also influenced by the shape and inclination of the thrust plane. The flu - vial relief is more pronounced here because it is younger, due to the steeper thrust plane, and the high gradient in the karst. majhen potok, ki krajevno zbira vodo na flišu. Potok ponika v jami Markov spodmol, ki je del 6 km dolge Vodne jame v Lozi. To sta največji slepi dolini v Postojnski ko- tlini. Kažeta na prvotno smer odtekanja zahod - nega dela kotline proti jugu. Po velikosti ter ob- liki dolin je trajal odtok v to smer dolgo časa, gradient v krasu pa je bil majhen. O tem priča uravnava Slavenskega ravnika južno od tod in v njem ohranjena 3 km dolga brezstropa jama na višini okrog 600 m. Južni rob Hrušiškega pokrova se je zaradi po - dorov in plazov relativno hitro umikal proti se- veru. K umikanju je močno prispevala podzem- na drenaža. Ob narivni ploskvi so se oblikovale jame, jamske reke pa so erodirale fliš v podlagi. To je spodjedalo in destabiliziralo karbonatni pokrov, ki se je zaradi tega hitreje podiral, po- speševalo pa je tudi kraško denudacijo, ki je ve- liko hitrejša na pretrtih in porušenih apnencih, podorih in meliščih. Vzorec jamske podzemne drenaže, ki se je oblikovala ob narivni ploskvi pa se je reproduciral navzdol v flišne kamnine in zasnoval potek današnjih dolin Globočjaka, Ža- bovca in Šmihelskega potoka proti jugu. Ker sega Ubeljski zatrep skoraj do najvišjega dela bricovega hrbta nanoške antiforme, se da- nes proti njemu ob narivni ploskvi izceja le malo vode iz apnencev. Zato je fluvialni relief na po- bočjih neizrazit, upočasnilo pa se je tudi rušenje apnenčastega narivnega roba. Rob hrušiškega nariva v območju šmihelske- ga hrbta leži že severno od njegovega najvišje- ga dela. Tu poteka površinska razvodnica med pritoki Nanoščice in potoki, ki tečejo proti se- veru in ponikajo pod robom nariva zahodno od Predjame. To so potoki Stranske ponikve (ozna- ka S.p. na sl. 17), Šmihelske ponikve in Lokva, ki pripadajo povodju Vipave. Višine ponorov, ki jih določa narivna ploskev, so 608 m, 600 m in 462 m. Tudi na oblikovanje dolin potokov je vpli- vala oblika in pa vpad narivne ploskve. Fluvial- ni relief je tu bolj izrazit zaradi manjše starosti, večjega naklona narivnice in velikega gradienta v krasu. Nastajanje obeh hrbtov nanoške flišne anti- forme je potekalo sočasno z oblikovanjem relie- fa v zahodnem delu Postojnske kotline. Po tek- tonskem dvigu območja slepih dolin Biščevci in Sajevške doline, se je Nanoščica preusmerila proti vzhodu v ponorno cono pri Postojnski jami, kjer so ponori na višini 510 m. Odtok proti severu ji je preprečil šmihelski hrbet, ki ima smer za- hod-vzhod. 60 Ladislav PLACER, Andrej MIHEVC & Igor RIŽNAR Both ridges in the Nanos antiform formed si- multaneously with the formation of the relief in the western part of the Postojna basin. After the tectonic uplift of the Biščevci and Sajevče blind valleys area the Nanoščica creek shifted its course to the west into a ponor (sinkhole) zone at Posto- jnska jama cave at 510 m a.s.l. as the E - W trend- ing Šmihel ridge restrained its course due north. The three-dimensional elevation model of the Nanos southeastern slope in Figure 18 is somewhat distorted, nevertheless the geomorphologic fea- tures are still well expressed. Ubeljsko steephead in the first plan is bounded by the Ubeljska stena and Votla stena. Razdols a and b are well exposed parallel to the Rjava stena, behind the Votla stena. The difference between the landslide area below the Ubeljska stena and Votla stena covered by the slope scree differ significantly from the landslide area below the Rjava stena composed of multiple slided rotational blocks. Šmihel ridge, hosting the Black Sea – Adriatic watershed is visible in the north-eastern corner of the elevation model. Streams on its northern Tridimenzionalni višinski model jugovzhod- nega pobočja Nanosa na sliki 18 je nekoliko po - pačen, vendar so geomorfološke značilnosti lepo vidne. V ospredju izstopa Ubeljski zatrep, ki ga zapirata Ubeljska in Votla stena. Za Votlo steno in vzporedno z Rjavo steno, sta lepo vidna raz- dola a in b. Opazna je razlika med plaziščem pod Ubeljsko in Votlo steno, na katerem je pretežno pobočni grušč in plaziščem pod Rjavo steno, ki ga sestavlja več rotacijskih blokovnih zdrsov. V severnem kotu modela je viden šmihelski hrbet po katerem teče razvodnica med jadran- skim in črnomorskim povodjem. Potoki na njegovi severni strani ponikajo pod Hrušiški pokrov, kar je pogojeno s hrušiško sinformo, nasprotno pa ni v območju nanoške antiforme nobenega ponora, tu vse vode odtekajo stran od njenega vrha, ki se nahaja za dnom Ubeljskega zatrepa. Popačenje modela je krivo, da ta značil - nost ni opazna. Fig. 18. Three-dimensional view on the southeastern part of Mt. Nanos and of the eastern part of the Postojna basin. Sl. 18. Tridimenzionalni pogled na jugovzhodno pobočje Nanosa in vzhodni del Postojnske kotline. 61 Tectonics and gravitational phenomena (Nanos, Slovenia) side are sinking under the Hrušica thrust due to Hrušica synform and there is no sinkhole in the Nanos antiform area and all the waters run away from its summit behind the base of the Ubeljsko steephead. The described morphologic feature is obscured by the distortion of the elevation model. Conclusion Gravitational phenomena along the SE margin of Mt. Nanos are understood as the degradation of the lateral Hrušica Nappe boundary and its retreat due NE and N. The Suhi vrh gravitational zone in the southeastern flank of the Hrušica Nappe (Fig. 1, b) differs in its mechanism from the landslides in the zone of large gravitational phenomena in the Trnovo Nappe south-eastern block (Fig. 1, a), and the Trnovo and Hrušica Nappe Thrust Fronts (Fig. 1). The Suhi vrh gravitational area (Fig. 1, b) formed due to an antiform uplift in the frontal part of the Hrušica Nappe and its flysch basement with it. Differential gravitational offsets along a system of sub-vertical regional fractures of the SSW - NNE trend manifested as a result of this antiform uplift. These offset kinematics correspond to the inter- nal rotation of blocks and differ from the classical gravitational mass movements in the landslide ar- eas beneath the Ubeljska, Votla and Rjava stena. The Ubeljsko steephead was formed due to the degradation of the Suhi vrh gravitational struc- tures by classical rockfalls, landslides and denu- dational processes. A similar scenario takes place in the preserved part of the Suhi vrh structure be- hind the Votla and Rjava stena, where mass move- ments undermine the Rjava stena cliff. Here, the denudational process started later, as the nappe thrust front retreats due NE. The formation of the differential offsets de- scribed in the Nanos antiform (Fig. 11) is inhib- ited in the Črni školj and Mali Modrasovec area (Fig. 1, a; Fig. 2) despite a well-developed system of regional subvertical NNE - SSW trending frac- tures, as the bulge of a flysch antiform behind the Trnovo Nappe Thrust Front is barely noticeable. Only classical mass movement types are present there. From the regional point of view it is import- ant, however, that the antiform considered here lies NW of the Nanos antiform, so we can conclude that it represents its structural continuation or its distant spatial repetition. At this point it is reason- able to introduce the term “Čaven antiform” in or- der to distinguish it from the Nanos antiform. Large fossil rotational landslides in the Snežnik Nappe Thrust Front, the inferred landslide at Pe- telinje mlake (Fig. 1, c) and the confirmed Ilirska Sklep Gravitacijske pojave ob jugovzhodnem robu Nanosa in Hrušice razumemo kot rušenje boč- ne meje Hrušiškega pokrova in njeno pomikanje proti severovzhodu in severu. Gravitacijsko ob- močje Suhega vrha v jugovzhodnem boku Hruši- škega pokrova (sl. 1, b) se po mehanizmu razlikuje od pobočnih zdrsov v pasu velikih gravitacijskih pojavov v jugovzhodnem boku Trnovskega po- krova (sl. 1, a) ter v čelih Trnovskega, Hrušiškega in Snežniškega pokrova (sl. 1). Gravitacijsko območje Suhega vrha (sl. 1, b) je nastalo zaradi dviga antiforme v čelnem delu Hrušiškega pokrova in s tem tudi njene flišne podlage. Posledica tega so bili gravitacijski di- ferencialni premiki po sistemu subvertikalnih regionalnih razpok SSW-NNE. Kinematika teh premikov ustreza interni rotaciji in se razlikuje od mehanizma klasičnih pobočnih gravitacijskih pojavov v plaziščih pod Ubeljsko, Votlo in Rjavo steno. Ubeljski zatrep je nastal zaradi razpada gra- vitacijskih struktur Suhega vrha na območju zatrepa, ki so jih degradirali klasični pobočni zdrsi in denudacijski procesi. Podobno usodo do- življa ohranjena gravitacijska struktura Suhega vrha za Votlo in Rjavo steno, ki ju izpodjedajo pobočni zdrsi plazišča Rjava stena. Proces de- nudacije se je tu pričel pozneje, ker se čelni rob Hrušiškega pokrova generalno umika proti se- verovzhodu. Na območju Črnega školja in Malega Modra- sovca (sl. 1, a; sl. 2) je izboklina flišne antiforme v čelnem delu in začelju Trnovskega pokrova ko- maj opazna, zato tu ni pogojev za nastanek di- ferencialnih premikov kot smo jih opisali v pri- meru nanoške antiforme (sl. 11), čeprav je tudi tu lepo razvit sistem regionalnih subvertikalnih razpok SSW-NNE. Obstajajo le klasični pobočni zdrsi plazišča Črni školj. V regionalnem smislu pa je pomembno, da leži obravnavana antifor- ma severozahodno od nanoške antiforme, zato je mogoče sklepati, da predstavlja njeno strukturno nadaljevanje ali pa prostorsko odmaknjeno po- novitev. Zaradi razlikovanja je smiselno uvesti pojem čavenska antiforma. Velika rotacijska fosilna plazova v čelu Snež- niškega pokrova; domnevni Petelinje mlake (sl. 1, c) in dokazani Ilirska Bistrica (sl. 1, d), kažeta na drugačne pogoje postnarivnega deformiranja kot v Hrušiškem in Trnovskem pokrovu. Skupna značilnost vseh štirih izjemnih gra- vitacijskih pojavov je, da so povezani z deforma- cijami istrskega potisnega območja, ki so nastale po fazi dinarskega narivanja. Potisno območje 62 Ladislav PLACER, Andrej MIHEVC & Igor RIŽNAR Bistrica landslide (Fig. 1, d) indicate conditions of post-thrust deformation different from those in the Hrušica and Trnovo Nappes. A common feature of all four exceptional gravitational phe- nomena is their causal link to the deformations of the Istra Pushed Area that occurred after the Dinaric thrust phase. The Istra Pushed Area is a consequence of the Adriatic Microplate (Adria) movement towards the Dinarides – in our case, the movement of Istra and the offshore (and sea bed) of the Trieste Gulf. The Istran block is mov- ing more intensely than the neighbouring Adria blocks SE of Istra. The process started in the Mid- dle Miocene, and its activity (also recent) is man- ifested in many different ways. Differences in the types of gravitational phe- nomena in the Hrušica and Trnovo Nappes Thrust Fronts on the one hand, and the Snežnik Nappe on the other, are also reflected in the type of post- thrust deformation typical across the entire Istra Pushed Area, with particular differences in geo- morphologic development. 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