Final response on RC1

Abstract

We describe two previously unreported, 5–7 km long normal fault scarps (NFS) occurring atop fault-related anticlines in the coastal ranges of the Dinarides fold-and-thrust belt in southern Montenegro, a region under predominant contraction. Both NFS show well-exposed, 6–9 m high, striated and locally polished fault surfaces in limestones, documenting active faulting during the Holocene. Sharply delimited ribbons on free rock faces show different color, varying karstification and lichen growth and suggest stepwise footwall exhumation, typical of repeated normal faulting earthquake events. Displacements, surface rupture lengths and geometries of the outcropping fault planes imply paleoearthquakes with Mw ≈ 6 ± 0.5 and slip rates of c. 0.3–0.5 mm/yr since the Last Glacial Maximum. Slip rates based on cosmogenic 36Cl data from the scarps are significantly higher: modeling suggests 1.5 ± 0.1 mm/yr and 6–15 cm slip every c. 35–100 yrs, commencing c. 6 kyr ago. The total throw on both NFS – although poorly constrained – is estimated to max. 200 m, and offsets the basal thrust of a regionally important tectonic unit. Both NFS are incipient extensional structures that postdate growth of the fault-related anticlines on top of which they occur. Interestingly, the position of the extensional features agrees with recent geodetic data, suggesting that our study area is located exactly at the transition from NE-SW-directed shortening in the northwest to NE-SW-directed extension to the southeast. While the contraction reflects ongoing Adria-Europe convergence taken up along the frontal portions of the Dinarides, the incipient extensional structures might be induced by rollback of the Hellenic slab in the SE, whose effects on the upper plate appear to be migrating along-strike the Hellenides towards the northwest. The newly found NFS provide evidence for a kinematic change of a thrust belt segment over time. Alternatively, the NFS might be regarded as second-order features accommodating changes in dip of the underlying first-order thrust faults to which they are tied genetically.