Tectonic Evolution Of The Eastern Black Sea And Caucasus

Author : Charles Cashman Trexler
Publisher :
ISBN 13 : 9780438628380
Total Pages : pages
Book Rating : 4.6/5 (283 download)

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Download or read book Structural Investigations of the Tectonic History of the Western Greater Caucasus Mountains, Republic of Georgia written by Charles Cashman Trexler and published by . This book was released on 2018 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The Greater Caucasus Mountains, located between the Black and Caspian Seas at the northern margin of the Arabia-Eurasia collision zone, presently accommodate most (70%) of the orogen-perpendicular shortening within this sector of the collision zone. However, the structural geometry and tectonic evolution of this range remain poorly constrained. The core of the Greater Caucasus is predominantly made up of marine sedimentary rocks that represent a Mesozoic-age basin that has been tectonically inverted over the last ~35 Ma. The width of this marine basin, the timing of its closure, and the total amount of shortening accommodated during its closure are debated. Multiple models have been proposed for the tectonic evolution of the Greater Caucasus Mountains, including pure-shear thickening of primarily continental crust, inversion of rift-related extensional structures to close a back-arc basin, and marine basin closure through subduction ultimately concluding with continental collision, subduction termination, and slab breakoff. Specifically, patterns in exhumation, shortening rate, and subcrustal seismicity along strike of the range support the model that the modern range may record ongoing subduction termination and slab breakoff, with an along-strike transition from an attached subducted slab in the east to a detached slab in the west. However, the structural architecture of the range and shortening rates across the orogen are not well constrained. The competing models explaining Greater Caucasus evolution make specific predictions for the geology exposed within the range and. For example, the subduction and slab breakoff model for the evolution of the Greater Caucasus requires that the back-arc basin was at least several hundred kilometers wide. Investigation of the stratigraphy and structural geology exposed in the core of the range is therefore critical to evaluating the slab breakoff model against alternative models of Greater Caucasus evolution. We combine 1:100k structural mapping along two orogen-perpendicular transects with stratigraphic and structural analyses to constrain the magnitude of shortening accommodated within the Greater Caucasus, and thus the minimum width of the Greater Caucasus ocean basin prior to basin closure and continental collision. Using these data, we find that balanced geologic cross sections of the western and central Greater Caucasus (along the Enguri and Aragvi Rivers, respectively) indicate a minimum of ~200 km of total shortening since exhumation of the orogen began. This estimate is a minimum, because it does not account for non-accretionary underthrusting. These magnitudes of shortening are similar to those expected for subduction/ slab breakoff, and exceed those expected for buoyancy-driven uplift. In addition, they are comparable to estimates of total shortening within the Greater Caucasus derived from paleomagnetic data. The subduction termination and slab breakoff model also predicts a change in shortening rate through time, as the subducting slab detaches, the marine basin closes, and the convergent margin transitions into a continental collision. We estimate the long-term (last ~30 My) shortening rate for the western Greater Caucasus by combining total shortening from balanced cross sections within the orogen and timing of initiation of exhumation and uplift as, and calculate a minimum shortening rate of 6-8 mm/yr. We compare this long-term geologic shortening rate with the shortening rate across active structures in the modern Greater Caucasus. Deformed river terraces along the antecedent Enguri River in the western Greater Caucasus allow us to quantify Quaternary deformation across the Rioni Basin fold/thrust belt (RFTB). We integrate surficial and bedrock geologic mapping and topographic surveys to build a kinematic model describing both terrace deformation and the finite deformation recorded by underlying folded late Neogene strata within the Dzumi fold. Luminescence dates from 3 samples in a loess cap on a folded terrace give a maximum age for terrace abandonment of 94.1 ± 12.8 ky. Monte Carlo simulations accounting for uncertainties in fold geometry indicate 151 +65/-27 m of displacement since formation of the folded terrace, implying an average shortening rate of 1.6 +1.5/-0.6 mm/yr over the past ~100 ky. The magnitudes of shortening and rates we find in the RFTB are consistent with modern geodetic shortening rates across the western Greater Caucasus, which range from 0-4 mm/yr. This apparent discrepancy between short- and long-term shortening rates can be explained by a change in shortening rate over time. A decrease in shortening rate over time has been predicted as a result of the subduction and slab breakoff model of Greater Caucasus evolution. The Greater Caucasus may thus offer insights into the geologically ephemeral response to the transition from subduction to collision, and how that transition is recorded in the surface geologic record.