Exploring Subduction, Slab Breakoff, and Upper-Plate Deformation in the Georgian Greater Caucasus: Shortening Estimates from Area- and Line-Balanced Crustal Scale Cross Sections

Abstract:

Between the Black and Caspian Seas, the Greater Caucasus Mountains (GC) delineate the northern margin of the Arabia-Eurasia collision zone. The role of subduction in formation of the GC is not widely recognized, despite patterns of subcrustal seismicity attesting to its importance at the E end of the range. The GC currently absorbs most orogen-perpendicular plate convergence (11 mm/yr, 70% of total), but its tectonic evolution relation to subduction remain unclear. Proposed models include 1) subduction of an ~400 km wide backarc ocean basin followed by slab breakoff under the W end of the range; 2) loss of a lithospheric root followed by buoyancy-driven uplift; and 3) closure of a small (~100 km?) basin with no/minimal subduction. Patterns of modern seismicity, exhumation, and shortening are most consistent with the subduction/slab breakoff model, suggesting the western GC may capture the surface expression of processes associated with recent slab breakoff.

Each model of the GC makes specific predictions for the amount of shortening within the orogen, with the subduction/breakoff model predicting large magnitudes of convergence. Here we estimate orogenic shortening by balancing mass along several, orogen-perpendicular, crustal-scale cross sections across the GC. First we estimate the original length of undeformed crust by comparing the modern deformed volume (determined from modern topography and moho depth and assuming no net erosional loss) with hypothesized end-member original crustal thicknesses (based on seismic data in the Eastern Black Sea and Scythian platform). These end-member assumptions allow shortening magnitudes as great as ~700 km (12 km-thick oceanic crust), and as small as ~100 km (39 km-thick Scythian margin). Second, we use line-length balanced crustal-scale geologic cross sections to estimate shortening in the western and central GC. We generated these sections using previously published 1:200k geologic maps and our own focused field observations. These sections contain ~230 km of total shortening. Based on area-balanced cross sections along the same transects, this magnitude of shortening falls within the expected range of the subduction/ slab breakoff model, and is greater than might be expected from either buoyancy driven uplift or closure of a small basin.