GEOLOGIJA 45/1, 277–280, Ljubljana 2002 Predhodna objava rezultatov strukturnega profiliranja Kra{kega roba in Istre (AC Kozina – Srmin, Se~ovlje) Preliminary results of structural profiling of the Kras edge and Istria (Kozina – Srmin Motorway, Se~ovlje) Ladislav PLACER Geolo{ki zavod Slovenije, Dimi~eva 14, 1000 Ljubljana, Slovenija Klju~ne besede: Tektonika, meja Jadransko predgorje – Zunanji Dinaridi, Istra Key words: Tectonics, boundary Adriatic foreland – External Dinarides, Istria Kratka vsebina Nova avtocesta Ljubljana – Koper poteka na odseku med Kozino in Srminom preko Kra{kega roba, ki ga v geolo{ki terminologiji imenujemo Kra{ki narivni rob. Ta v naj{ir{em smislu zajema prostor med robom Tr‘a{ko-Komenske planote in Savudrijskim grebenom ter tvori mejo med Jadranskim predgorjem in Zunanjimi Dinaridi. Detajlno geolo{ko kartiranje odseka avtoceste Kozina – Srmin je pokazalo, da Kra{ki narivni rob ni monofazna tektonska struktura, temve~ se je izoblikoval od konca eocena do danes preko ve~ razli~nih deformacijskih faz, ki vklju~ujejo poleg manj{ih, odzive treh pomembnej{ih dogodkov; dinarskega krovnega narivanja, premike ob zmi~nih prelomih smeri NW-SE in podrivanje Istre proti NE. Slednje dogajanje je poru{ilo prvotno simetrijo jugozahodne meje Zunanjih Dinaridov med Ju‘nimi Alpami in velebitskim lokom. Abstract On the section Kozina-Srmin the new motorway Ljubljana-Koper is crossing the Kras edge, which in geologic terminology is refered to as the Kras thrust edge. In the widest sense it comprises the area between the edge of the Trieste-Komen plateau and the Savudrija ridge, creating the boundary between the Adriatic foreland and the External Dinarides. Detailed geologic mapping of the motorway section Kozina-Srmin slowed that the Kras thrust edge is not a monophase tectonic structure, but has been formed through several different deformation phases from the Eocene until today. Besides smaller ones these phases include responses of three significant events; the Dinaric nappe thrusting, displacements along the strice-slipe faults with NW-SE trending and underthrusting of the Istria toward the NE. The latter event destroyed the primary of the SW boundary of the External Dinarides between Southern Alps and the Velebit arc. V okviru programa geolo{ke naravne de- Spremljava zajema strukturne (Geolo{ki za-di{~ine, ki ga financira Dru‘ba za avtoceste vod Slovenije), stratigrafsko-paleontolo{ke v Republiki Sloveniji (DARS), se sistema- (Paleontolo{ki in{titut ZRC SAZU) in spe-ti~no geolo{ko spremlja zemeljska dela na leolo{ke (In{titut za raziskav krasa ZRC SA-novi avtocesti med Kozino in Srminom. ZU) fenomene. Ker poteka trasa preko 22-277-280.p65 277 17. 09. 02, 15:00 Black 278 celotnega Kra{kega roba, ki je ena od krajinskih znamenitosti slovenskega ozemlja, je strukturno-geolo{ka spremljava zastavljena kot poglobljen {tudij njegove celotne nagubane in naluskane zgradbe, ki zajema v geo-lo{kem smislu {ir{e obmo~je od samega roba in sega nekako od Kozine do Savudrijskega tektonskega hrbta. Imenujemo ga Kra{ki na-rivni rob, ki predstavlja mejo med Zunanjimi Dinaridi in Jadranskim predgorjem. Zato se vzporedno obdeluje tudi nagubane plasti v Strunjanu, arhivirane podatke rudnika premoga v Se~ovljah in hidrogeolo{ke vrtine na obmo~ju Tr‘a{ko-Koprske sinklinale. Dosedanje védenje o zgradbi Kra{kega narivnega roba je izhajalo iz podatkov Osnovne geolo{ke karte, po kateri je rob Kra{ke planote naluskan v sistem narinjenih gub ob bolj ali manj strmih reverznih prelomih, ki se v jugozahodnem krilu ^i-~arijske antiklinale raztezajo v dinarski smeri proti jugovzhodu. Na obmo~ju U~ke pa naj bi nastopali tudi krovni narivi. Na podlagi analize premikov Trnovskega in Hru{i-{kega pokrova ter Sne‘ni{ke narivne grude je bila postavljena hipoteza o eksponencialni zakonitosti narivanja Zunanjih Dinaridov tega dela Slovenije (Placer 1981) po kateri je bila dolo~ena skupna dol‘ina luskanja Kra-{kega narivnega roba v profilu Hru{ica – Tr‘a{ki zaliv okoli 4 km. Raziskave mezo-zojskih sedimentov vzhodne Furlanije pa so pokazale na mo‘nost, da bi jurski in kredni pelagi~ni sedimenti s tega obmo~ja lahko prvotno tvorili enoten sedimentacijski bazen z enakimi sedimenti srednjega in ju‘nega Jadrana (Cati et al. 1989). Danes naj bi bil ta bazen prekrit z narivi Kra{kega nariv-nega roba. Tako zastavljena hipoteza terja izdatnej{i premik Istre nasproti Zunanjim Dinaridom in ima oporo v interpretacijah {tevilnih geologov, ki so razlagali zamik Istre nasproti Kvarnerskim otokom z desnim premikom ob prelomu vzdol‘ vzhodne istrske obale ali pa brez preloma, le s povijanjem nagubanih in narinjenih struktur. Vzporedno s sedimentnimi so potekale tudi strati-grafsko-paleontolo{ke raziskave in nakazovale razliko v razvoju posameznih strukturnih enot Istre nasproti Zunanjim Dinaridom (Drobne , 2000). Na{teta dejstva se ujemajo s hipotezo Romandi}a (1990) o rotaciji Istre v nasprotni smeri urinega kazalca in njenim pomikom proti severovzhodu. Vse to terja ovrednotenje na{tetih dom- 22-277-280.p65 278 Black Ladislav Placer nev, zato je strukturna spremljava avtocestnega odseka Kozina – Srmin prvovrstna raziskovalna prilo‘nost. Na podlagi dosedanjih ugotovitev je mogo~e skleniti, da vklju~uje Kra{ki narivni rob ve~ faz premikov od katerih predstavlja luskanje v zvezi z ekspo-nencialno odvisnostjo krovnih narivov Trnovskega cikla le eno, najstarej{o fazo v nizu deformacij, ki poleg luskanja vklju~ujejo {e obse‘no podrivanje in zmikanje. Ti premiki niso lokalnega zna~aja, temve~ so odraz spreminjajo~ih se odnosov med Jadranskim predgorjem in Zunanjimi Dinaridi od konca eocena do danes. Poleg tega predstavlja Istra izpostavljeno ogli{~e rotirajo~ega bloka ve~-jih dimenzij, ki je poru{il prvotno simetrijo narivnega roba Zunanjih Dinaridov med Ju‘nimi Alpami in velebitskim lokom. Navidezno iregularne deformacije v sistemu dinarsko usmerjenih struktur so posledica tega dogajanja. Strukturna spremljava avtoceste je podrejena standardom kartiranja v merilu 1 : 5000 in ve~jemu merilu, ~e je to potrebno, zajema pa tudi {ir{i pas ob avtocesti. Cilj kartiranja je dolo~iti tip deformacij in smer ter velikost premikov, njihovo eno ali ve~-faznost in podobno. Vodilne strukture se {te-vil~ijo od prvega preloma na severovzhodu, ki ga je mogo~e {teti za deformacijo povezano s Kra{kim narivnim robom (Skadanski prelom po vasi Skadan{~ina pri Kozini) proti jugozahodu. Za vsak tip pomembnej{ih deformacij (normalni prelom; reverzni prelom; zmi~ni prelom; polo‘ni nariv; prelom z ve~-faznimi premiki n.pr. 1. zmi~ni premik, 2. vertikalni premik; prelom nedolo~ljivega zna~aja in podobno) je {tevil~enje vodeno posebej. Od Kozine peko Srmina, Izole, Stru-njana in Se~ovelj do Savudrijskega tektonskega hrbta izdelujemo referen~ni profil, ki bo podlaga za konstrukcijo profilov severozahodno od tod preko globokih vrtin v vzhodni Furlaniji in jugovzhodno od tod preko tektonskih oken in krp v okolici U~ke. Na obmo~ju Istre je na voljo nekaj deset hidrogeolo{kih vrtin, ki jih vklju~ujemo v interpretacijo. Iz profilov in zaporedja deformacij bo mogo~e izdelati model geneze Kra-{kega narivnega roba od konca eocena do dana{njih dni in dolo~iti njegovo vlogo v recentni dinamiki tega prostora. Ker nastanek Kra{kega narivnega roba ni izoliran fenomen, ugotovitve vplivajo na interpretacijo {ir{ega prostora. 17. 09. 02, 15:00 Predhodna objava rezultatov strukturnega profiliranja Kra{kega roba in Istre 279 Preliminary results of structural profiling of the Kras edge and Istria (Kozina – Srmin Motorway, Se~ovlje) In the frame of the Natural Heritage Program financed by the Motorway Construction Authority of Republic Slovenia (DARS), a systematic geologic accompanying of excavation works at construction of the new motorway between Kozina and Srmin is being conducted. Since the motorway trace passes across the entire Kras edge that is one of important natural features of the Slovenian territory, the structural geologic accompanying of the construction site was planned as a careful study of the entire folded and thrust faulted structure. In geologic sense it comprises a wider area of the edge and extends from about Kozina to the Sa-vudrija horst. The structure is called the Kras thrust edge, which represents the boundary between northwestern part of the External Dinarides and Adriatic foreland. Therefore parallel to this also the folded beds at Stru-njan are studied as well as the archive data of the Se~ovlje colliery and hydrogeologic boreholes in the area of the Trieste-Koper syncline. The previous understanding of the structure of the Kras thrust edge has been derived from data of the Basic geologic map 1: 100.000 according to which the Kras plateau edge was thrusted into a system of thrust folds along more or less steep reverse faults that extend in the southwest limb of the ^i~arija anticline in the Dinaric direction toward southeast. In the Mt. U~ke area also owerthrusts are believed to exist. Based on an analysis of shifts of the Trnovo and Hru-{ica nappe and the Sne‘nik thrust sheet the hypothesis of the exponential nature of over-thrusting of External Dinarides of this part of Slovenia was proposed (Placer 1981) according to which the total length of thrusting of the Kras thrust edge in the Hru{ica – Trieste gulf area was estimated to be about 4 km. Studies of the Mesozoic beds of the eastern Friuli indicated in addition the possibility of an originally uniform sedimentation basin for the Jurassic and Cretaceous pelagic sediments of this area and similar sediments of the central and southern Adriatic (Cati et al. 1989). At present this basin should be covered by thrusts of the Kras thrust edge. Such a hypothesis requires a considerable movement of Istria towards the External Dinarides, and it is supported by interpretations of numerous geologists who explained the movement of Istria with respect to Kvarner islands by a dextral shift along a fault following the eastern Istrian shore, or without a fault, only by bending of the folded and thrust structures. Parallel with sedimentation studies also stratigrap-hic-paleontologic studies were performed. They indicated a difference in the development of individual structural units of Istria with respect to External Dinarides (Drob -n e , 2000). The listed facts are consistent with Romandi}’s (1990) hypothesis on anti-clockwise rotation of Istria and its shift towards northeast. All this requires to elaborate evaluation of the listed hypotheses. Now, with the possibility of structural accompanying of the motorway Kozina – Srmin construction a unique research opportunity is being offered. The results established hitherto permit to make inference about several phases of movements in the Kras thrust edge. In them the reverse faulting associated with the exponential dependence of the nappe structure of the Trnovo cycle represents only a single, the oldest phase in the series of deformations. Besides reverse faulting the latter include also wrench faulting in NW-SE direction and extensive underthrusting of the Istria towards NE. These movements are not of local character, but are an expression of changing relations between the Adriatic foreland and the External Dinarides from end of Eocene to the present. In addition to this, Istria represents the exposed edge of a rotating block of larger size that destroyed the original symetry of the overthrust edge of the External Dinarides between the Southern Alps and the Velebit arc. The seemingly irregular deformations in the system of Dinaric trending structures are the result of this process. The structural accompanying of the motorway construction is being performed according to standards for mapping at the 1 : 5000 and larger scales, if necessary, and 22-277-280.p65 17. 09. 02, 15:00 279 Black 280 Ladislav Placer it covers a broader belt of terrain along the motorway trace. The goal of mapping is to determine the type of deformation, and direction as well as extent of shifts, their uni-or polyphase nature and the like. The leading structures are numbered from the first fault in southeast that could be considered a deformation associated with the Kras thrust edge (the Skadan{~ina fault named after the village of Skadan{~ina near Kozina) towards southeast. The numbering was carried out separately for every type of more important deformations (normal fault; reverse fault; strike-slip fault; low-angle fault; fault with polyphase shifts, e.g. 1. strike-slip shift, 2. vertical shift; fault of undefined nature, and similar). From Kozina across Srmin, Izola, Strunjan and Se~ovlje to the Savudrija horst a reference profile is being compiled that shall serve as a base for constructing profiles to northwest from there across the deep boreholes in eastern Friuli, and to southeast of there across tectonic windows and klipps around Mt. U~ka. In the Istria region several tens of hydrogeologic boreholes are available, and they are also being included into interpretation. From profiles and succesion of deformations a genetic model of Kras thurst edge from end of Eocene to the present can be set up, and its role in recent dynamics of this area assessed. Since the genesis of the Kras thrust edge is not an isolated phenomenon, the expected results will indubitably contribute also to the structural interpretation of the wider region. Literatura – References Cati, A., Sartorio, D. & Venturini, S . 1989: Carbonate platforms in the subsurface. – Mem. Soc. Geol. It. 40 (1987), 295-308, Roma. Drobne, K. 2000: The North-Western Part of the Adria Carbonate Platform from the K/Tc Boundary to the Flysch Deposition during Pale-ocene-Eocene. – Riassunti delle comunicazioni orali e dei poster, 80a Riunione estiva, 231-232, Trieste. P l a c e r , L . 1981: Geolo{ka zgradba jugozahodne Slovenije ( Geologic structure of the suth-western Slovenia, Abstract). – Gologija 24/1, 27-60, Ljubljana. Romandi}, S. 1990: Analiza podataka du-bokog geoelektri~kog sondiranja na podru~ju za-padnog dijela Dinarida (Deep geoelectrical sounding data analysis for the western part of the Dinarides). – Nafta 41/78, 301-372, Zagreb. 22-277-280.p65 17. 09. 02, 15:00 280 Black