Orogen-Parallel and -Normal Extension at the Dinarides-Hellenides Junction during Clockwise Rotation and Radial Expansion of the Retreating Hellenic Arc-Trench System

Abstract:

Normal faults at the junction of the Dinarides and Hellenides in northern Albania trend both perpendicular and parallel to thrusts and fold axes. Most prominent is the SSE-dipping Skutari-Pec Normal Fault (SPNF), one of a system of normal faults that accommodate NW-SE-directed, orogen-parallel extension. Extensional throw along the SPNF increases from zero in northwestern Albania to at least 2 km near the Albania-Kosovo border, near where the brittle arm of the SPNF bounds the Mio-Pliocene Kosovo Basin. This differential orogen-parallel extension along the SPNF defines a vertical rotational axis in northwestern Albania that is interpreted to have accommodated 20-30° of clockwise motion of internal units on the southeastern (Hellenic) side of the fault with respect to the northwestern (Dinaric) side. Such rotation is in agreement with previously published paleomagnetic data and plate motion studies. The footwall of the SPNF exposes lower units of the Dinaric nappe stack that underwent vertical shortening and non-coaxial shearing during extensional exhumation. In the hangingwall of the SPNF, a system of orogen-parallel trending normal faults accommodate orogen-normal displacement of up to several km. Both orogen-parallel and –perpendicular systems of normal faults cut Late Cretaceous to Oligocene thrusts and folds, and are associated with pronounced Miocene paleo-relief. Most of this normal faulting is Miocene to Pliocene based on syn-extensional deposits in the Kosovo Basins and on thermal modelling of ZHe, AFT and AHe data, which suggest accelerated cooling at 18 Ma and between 4-6 Ma. Both fault systems also cut Holocene deposits, indicating ongoing extension. This corroborates published GPS data and earthquake focal mechanisms indicating active extension of the crust south and southeast of the SPNF. It is proposed that extension and clockwise rotation in this area were coeval and accommodated southwestward retreat and radial expansion of the Hellenic arc during rollback subduction.