THE SUBDUCTION-COLLISION TRANSITION IN THE ARABIA-EURASIA COLLISION AND FORMATION OF THE BASAL CAUCASUS DETACHMENT IN THE AJINOHUR AREA, AZERBAIJAN

Reconstructing the transition from oceanic subduction to continental collision is essential for understanding the formation of continental megathrusts and the earthquake hazards they pose. Between the Black and Caspian Seas, the Greater Caucasus Mountains define the northern margin of the Arabia-Eurasia collision, where they developed from Cenozoic closure of a back-arc basin and now accommodate most orogen-perpendicular convergence. Here we argue that post-Pliocene shortening within the southern foreland of the Greater Caucasus links with a previously documented zone of active rock uplift under the topographic crest of the range via a gently north-dipping, crustal-scale basal thrust herein named the Basal Caucasus Detachment. Our 1:100,000-scale structural mapping and 3 measured stratigraphic sections in the Ajinohur area (~ 47°E) of Azerbaijan document significant top-to-the south shortening within a fold-thrust belt ~50 km wide across strike. Active folding is indicated by tilting of Holocene(?) alluvium on the south margin of this belt. Deformation initiated by at least late Pleistocene time (0.88 Ma), as indicated by deformation of Baku strata, and was likely underway by mid-Pleistocene time (~1.5 Ma), as suggested by evidence in Apsheron deposits of possible growth strata, topographic partitioning of the basin, and the formation of an offlap unconformity. The orogenic structure we describe is broadly analogous to that of the Himalaya, with active faulting localized in a foreland-fold thrust belt above a gently north-dipping basal thrust that roots to the north beneath the main range. Active rock uplift under the topographic crest of the range results from passive translation of the hanging wall of the Basal Caucasus Detachment over an underlying ramp or blind duplex along the sole fault. A discrete band of crustal earthquakes under this same part of the range is likely produced by this same blind structure. We infer that the Basal Caucasus Detachment evolved from a pre-collisional subduction megathrust by propagation of deformation into the lower plate as it entered the trench. The seismic hazard from this potential continental megathrust remains unknown but is potentially significant, considering the extent of this shallowly dipping fault within the seismogenic zone.