STRIKE-SLIP FAULTING DOMINATES THE RESPONSE OF THE LESSER CAUCASUS TO THE ARABIA-EURASIA CONTINENT COLLISION

Crustal heterogeneities may play a key role in controlling the localization of strain in complex collisional orogenic belts. The Lesser Caucasus are one such heterogeneity, a potential rheologic knot within the young Arabia-Eurasia collision zone. Approximately 2 of 17 mm/yr of convergence between the Arabian and Eurasian plates is accommodated in the Lesser Caucasus. The main structure in the range is the Pembak-Sevan strike-slip fault, which reactivates a Jurassic-aged tectonic suture that structurally divides the Lesser Caucasus into a Mesozoic intra-oceanic arc to the north and continental crust of the Iranian-Anatolian plateau to the south. Whether strain within the Lesser Caucasus accumulates across a broad zone of shortening and exhumation, or a narrow zone of strike-slip deformation remains unresolved. New apatite (U-Th)/He data, as well as existing apatite fission track analyses (Cavazza et al., 2015), record Eocene and older cooling ages across most of the northern Lesser Caucasus. These early Cenozoic cooling ages are spatially coincident with a high-elevation low-relief geomorphic surface that spans the width of the northern extent of the range. Together, the thermochronologic and geomorphic data preclude exhumation in excess of 2-3 km across the northern Lesser Caucasus since the initiation of Arabian-Eurasian collision. In contrast, apatite (U-Th)/He cooling ages from the footwall of a pull-apart basin along the Pembak-Sevan fault near Vanadzor, Armenia, record late Miocene exhumation, revealing an average cooling rate of ~9ºC/Myr since ~10 Ma and thus significant strike-slip motion on this fault. The timing of activation of the Pembak-Sevan fault correlates with a broadly observed Miocene reorganization of faulting in the Arabian-Eurasian collision zone. The localization of deformation along a pre-existing Jurassic suture suggests that the Lesser Caucasus arc behaves as a rheologically rigid block, localizing collision related stresses along a pre-existing Jurassic suture as well as transmitting them northward to the Greater Caucasus. The Caucasus collision zone thus provides a potential example of ongoing rheologically controlled strain localization that may be applicable to other extant and ancient orogens.