Naslov
Kinematic analysis of outcrop-scale joint and fault systems in the Mt. Velebit – implication to tectogenesis and active seismo-tectonics

Autori
Tomljenović, Bruno ; Balling, Phillip ; Matoš, Bojan ; Vlahović, Igor ; Herak, Marijan ; Herak, Davorka ; Blažok, Lovro ; Posarić, Dino ; Širol, Andre ; Schmid, Stefan ; Ustaszewski, Kamil

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, prošireni sažetak, znanstveni

Izvornik
5th Reg. Mtg. Quaternary Geology dedicated to Geohazards & Final Conf. LoLADRIA project / - Zagreb : Cratian Academy of Sciences and Arts, 2017, 69-70

Skup
5th Reg. Mtg. Quaternary Geology dedicated to Geohazards & Final Conf. LoLADRIA project

Mjesto i datum
Starigrad, Hrvatska, 09.11.2017. - 10.11.2017

Vrsta sudjelovanja
Predavanje

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
External Dinarides ; Mt. Velebit ; kinematic analysis ; deformation stages ; paleostress fields ; forward modelling ; balanced cross sections ; passive roof duplex model

Sažetak
Mt. Velebit lies in the central part of external Dinarides formed by thrusting along the eastern margin of the Adriatic plate during Cenozoic convergence between the Adriatic and European plates. At present, the Adria–Europe convergence between Adriatic offshore area and Pannonian Basin amounts to 2–5 mm/yr and induces prevailingly a compressional stress regime across the external Dinarides, with predominantly N–S to NE–SW trending compression in the northwestern and southeastern parts, respectively. This stress partitioning is confirmed by the seismicity that is not uniformly distributed across the fold-thrust belt, being also variable in intensity and in kinematics of seismogenic sources as indicated by focal mechanisms of instrumentally recorded earthquakes. In the NW part of external Dinarides in Slovenia and in the Bay of Kvarner the ongoing seismo-tectonic activity is dominantly released along NW-striking Plio-Pleistocene dextral and dextral-reverse faults, which cut across the Eocene/Oligocene thrust faults (e.g. Moulin et al., 2016). On the other hand, in the Adriatic offshore and external Dinarides SE of Zadar, the ongoing seismicity is controlled by an orogen perpendicular NE–SW trending compression mostly accommodated by SW-verging Dinaridic thrust faults, only locally associated with strike-slip motions. The aforementioned parts of the external Dinarides are separated by Mt. Velebit, the most prominent geomorphological structure, which extends in NW–SE direction for ca. 140 km along the Adriatic coastline. Surprisingly, the mountain and its hinterland are characterized by conspicuously low instrumental seismicity, despite of a hypothetical NE-dipping Velebit thrust fault that is supposed to extend all along the SW front of the mountain (Herak and Bahun, 1979 ; Tari Kovačić and Mrinjek, 1994), and which is considered as a composite seismogenic source capable to generate earthquakes of M 6.5 (Kastelic et al. 2013). In order to characterise the ongoing seismicity and potential seismogenic sources of the Mt. Velebit area in more detail, a multidisciplinary seismological and tectonic project is undertaken (“VELEBIT: from Top to Bottom” ; http://www.pmf.unizg.hr/geof/znanost/seizmologija/velebit), among others addressing the questions about structural architecture, tectonic history and the present-day stress regime in this area. Here we present preliminary results of kinematic analysis of outcrop-scale shear joint/fault-slip data collected on more than 1400 locations in the Mt. Velebit area, aimed at the reconstruction of brittle deformation stages and related paleostress fields in the tectonic history of this area. Until now we have processed data collected in the NW part of the mountain (the Senj-Prizna section) measured in the Velebit breccia, a peculiar lithostratigraphic unit of post-Mid-Eocene age that crops out all along the SW mountain front. Based on orientation and kinematic compatibility criteria, we have separated six groups composed of conjugate fault/shear joint sets used for calculation of paleostress fields. Relative chronology between analysed sets and separated groups is only partly defined, and hence they are described here based on their kinematics and calculated paleostress fields. Two less frequent groups comprise fault/shear sets with prevailing dip-slip and reverse motions: a first one is composed of antithetic NNW-striking sets with prevalence of WSW-dipping planes and top-to-ENE motion that indicate the ENE–WSW trending compression ; a second one comprises WNW-dipping reverse fault/shear joint set which define WNW-ESE trending compression. Structures of both these groups strike parallel and slightly oblique to the structural grain and the mountain front in the NW part of the Velebit and thus are supposed to form during the major compressional tectonic phase in this part of external Dinarides. However, by far the most frequent fault-slip data are fault/shear joint structures indicating a normal sense of slip, separated so far into three groups based on their orientation: the most numerous group is composed of NW-striking antithetic fault/shear joint sets that indicate NE-SW trending extension ; a second group comprises NNE-striking antithetic fault/shear joint sets, which indicate the WNW–ESE trending extension, while a third group of normal fault/shear joints includes W-striking antithetic sets that define the N–S trending extension. However, although separated into three kinematically homogeneous groups, these structures possibly resulted from one and the same state of stress characterised by radial extension that according to extremely good preservation and frequency of occurrence of these structures might play an important role in the formation of the Velebit breccia. The remaining group of fault/shear joint structures comprises a subvertical conjugate pair composed of NW-striking dextral and NE-striking sinistral sets that indicate N–S trending compression. Based on overprinting outcrop- to map-scale relationships, this group of strike-slip faults is considered to be the youngest brittle deformation structures in the study area. In addition to this, we performed a detailed outcrop-scale analysis along the Lika and Brušane-Oštarije fault zones aimed at the collection of shear-sense indicators used for the reconstruction of the kinematic history of these faults. Our analysis shows the prevalence of dip-slip and top-to-NE motions along both faults, that together with the SW dipping direction defined by field observations and map-scale relationships shown on the 1:100’000 scale sheets of Basic Geological Map of Yugoslavia (Sokač et al., 1974 ; Šušnjar et al., 1973), led us to propose the working hypothesis, which presumes these are NE-verging thrusts/reverse faults and not the NE-dipping normal faults as previously thought. Consequently, we also challenge earlier tectonic models that interpret the Mt. Velebit structure as a SW-vergent antiformal stack or thrust duplex formed above the major NE-dipping thrust system. Rather we apply the concept of a passive roof duplex that was recently tested by Balling et al. (2017) by the construction of forward modelled balanced cross sections. The passive roof duplex model can explain, at least partly, the aseismic behaviour of the SW-dipping Velebit thrusts since they ride passively over the underlying duplex wedge envisaged at a depth between 10 to 15 km.

Izvorni jezik
Engleski

Znanstvena područja
Geologija, Geofizika

POVEZANOST RADA