Diverse Deformation Mechanisms and Lithologic Controls in an Active Orogenic Wedge: Structural Geology and Thermochronometry of the Eastern Greater Caucasus

Abstract

Orogenic wedges are common at convergent plate margins and deform internally to maintain a self-similar geometry during growth. New structural mapping and thermochronometry data illustrate that the eastern Greater Caucasus mountain range of western Asia undergoes deformation via distinct mechanisms that correspond with contrasting lithologies of two sedimentary rock packages within the orogen. The orogen interior comprises a package of Mesozoic thin-bedded (<10 cm) sandstones and shales. These strata are deformed throughout by short-wavelength (<1 km) folds that are not fault-bend or fault-propagation folds. In contrast, a coeval package of thick-bedded (up to 5 m) volcaniclastic sandstone and carbonate, known as the Vandam Zone, has been accreted and is deformed via imbrication of coherent thrust sheets forming fault-related folds of 5–10 km wavelength. Structural reconstructions and thermochronometric data indicate that the Vandam Zone package was accreted between ca. 13 and 3 Ma. Following Vandam Zone accretion, thermal modeling of thermochronometric data indicates rapid exhumation (∼0.3–1 mm/yr) in the wedge interior beginning between ca. 6 and 3 Ma, and a novel thermochronometric paleo-rotation analysis suggests out-of-sequence folding of wedge-interior strata after ca. 3 Ma. Field relationships suggest that the Vandam Zone underwent syn-convergent extension following accretion. Together, the data record spatially and temporally variable deformation, dependent on both the mechanical properties of deforming lithologies and perturbations such as accretion of material from the down-going to the overriding plate. The diverse modes of deformation are consistent with the maintenance of critical taper.