Locally thin crust and high crustal VP/VS ratio beneath the Armenian volcanic highland of the Lesser Caucasus: A case for delamination

This region links the Iranian and Anatolian plateaus to the east and west, respectively, and is the heart of the Caucasus-Iran-Anatolia (CIA) orogenic belt. The CIA orogenic belt is a wide volcanic province that includes the Greater Caucasus, Lesser Caucasus, Eastern Pontides, NE Anatolian Plateau, and NW Iranian Plateau, associated with the collision between Eurasia and Arabia starting in the Oligocene-Miocene. The two major collision events have left two main suture zones in the CIA: the Sevan-Akera Suture (SAS) for the Late Cretaceous-Early Eocene collision and the Bitlis-Zagros Suture (BZS) for the Late Eocene-Early Miocene collision. The former is interpreted as the closure of a back-arc basin in the northern Neotethys, and the latter is the evidence of the Jurassic-Eocene northward subduction of the southern Neotethys. Based on plate motion reconstructions, the convergence between Eurasia and Arabia is still actively ongoing, with a fairly steady rate of approximately 20-25 mm/year since collision. Research in West Asia has focused on the lithospheric structure and magmatism of the East Anatolian Plateau caused by the Eurasia-Arabia collision. The postcollisional magmatism became active in the Late Miocene to Quaternary since the uplift of the East Anatolian Accretionary Complex (EAAC). The lithosphere beneath the EAAC is exceptionally thin, with a thickness of 60-80 km, according to the low velocities in the upper mantle. The mechanisms inferred to explain the large-scale lithospheric thinning include the steepening and break-off of the southern Neotethyan slab and/or the whole-scale removal of the thickened lithosphere. The geographical distribution of Pleistocene and Holocene volcanoes generally exhibits two linear diagonal patterns from the BZS to the SAS in the Lesser Caucasus. Based on radiometric ages in the Eastern Anatolian Plateau, it was proposed that the postcollisional volcanism initiated in the Erzurum-Kars Region in the mid-Miocene at ~11 Ma and migrated toward the BZS and its border with Armenia where volcanic activities are about 3 Ma old. With a more comprehensive data set for the CIA, Lin et al. (2020) differentiated the activities into two phases: (1) slab break-off related magmatism starting ~17 Ma in SE Anatolia followed by a propagation toward NE Anatolia from 9 to 5 Ma and (2) extensive eruptions in NE Anatolia after 6.5 Ma and a migration toward Armenia and NW Iran. A new local array in Armenia provides a unique chance to compare structures under two distinct types of volcano clusters, the Early-Middle Pleistocene stratovolcano Aragats and the Gegham volcanic ridge, which was active during the Pleistocene to Holocene. The crustal structure beneath the Armenian Highland and the Lesser Caucasus remains uncertain due to sparse sampling. The study reveals a thick crust (up to ~52 km thick) beneath the Central Greater Caucasus and an unusually thin crust (32-35 km thick) beneath the northwestern part of Armenia near the Aragats stratovolcano and Gegham volcanic ridge. The average crustal VP/VS ratio in the Armenian Highland is anomalously high (≥1.9), with the highest value approaching 2.1 under the Gegham Ridge.