High-resolution seismic imaging of the sub-bottom Quaternary deposits revealed an active fault in the Vinodol-Velebit channel (Kvarner, Croatia)

Kvarner area belongs to the External Dinarides fold-and-thrust belt that is characterized by intensive tectonic deformations of a few kilometers thick sedimentary cover of the central part of the Adriatic microplate. The main deformations occurred predominantly during Eocene to Oligocene thin-skinned tectonics, while late-orogenic thick-skinned deformations and wrenching resulted in the exhumation of the orogenic belt. The latter tectonic mechanism is supposed to be still active, but there is no reported evidence of active faults on the surface of predominantly karstic terrain. Nevertheless, seismological data reveal subsurface activity along various fault plane solutions, and crucial evidence of possible active deformations on the surface is expected within stratified superficial deposits in the area. However, stratified Quaternary sediments are rare onshore and on the Kvarner islands but are widespread on the bottom of the surrounding Adriatic Sea. During the targeted high-resolution sub-bottom geoacoustic seismic survey we focused on the zones that are characterized by earthquakes and on the previously arbitrarily recognized regional seismogenic sources. However, only shallow seismic profiles along and across the Vinodol and the NW part of the Velebit channel revealed clear evidence of fault-related deformations of the youngest Quaternary sediments. The fault zone is up to hundreds of meters wide, limited by parallel sub-vertical fault planes, and characterized by deformations of the strata between the planes either in a positive (uplifted) or a negative (downthrown) manner along the strike, which are typical for strike-slip faults. Besides, the disturbed layering of the uppermost well-stratified unit (Late Pleistocene) resembles fluid/sediment escape structures that could be related to strong shaking during prehistorical earthquakes. The fault zone is also tentatively recognized in the onshore bedrock along the strike of the submerged fault, where it appears as an indistinct fractured zone that is more corroded than surrounding bedrock carbonates. Therefore, sub-bottom profiling has been proven to be a useful tool for identifying active faults and should be used as a key method in future seismotectonic research of submerged seismogenic zones.