Tectonostratigraphy and major structures of the Georgian Greater Caucasus: Implications for structural architecture, along-strike continuity, and orogen evolution

Abstract

Although the Greater Caucasus Mountains have played a central role in absorbing late Cenozoic convergence between the Arabian and Eurasian plates, the orogenic architecture and the ways in which it accommodates modern shortening remain debated. Here, we addressed this problem using geologic mapping along two transects across the southern half of the western Greater Caucasus to reveal a suite of regionally coherent stratigraphic packages that are juxtaposed across a series of thrust faults, which we call the North Georgia fault system. From south to north within this system, stratigraphically repeated ~5–10-km-thick thrust sheets show systematically increasing bedding dip angles (<30° in the south to subvertical in the core of the range). Likewise, exhumation depth increases toward the core of the range, based on low-temperature thermochronologic data and metamorphic grade of exposed rocks. In contrast, active shortening in the modern system is accommodated, at least in part, by thrust faults along the southern margin of the orogen. Facilitated by the North Georgia fault system, the western Greater Caucasus Mountains broadly behave as an in-sequence, southward-propagating imbricate thrust fan, with older faults within the range progressively abandoned and new structures forming to accommodate shortening as the thrust propagates southward. We suggest that the single-fault-centric “Main Caucasus thrust” paradigm is no longer appropriate, as it is a system of faults, the North Georgia fault system, that dominates the architecture of the western Greater Caucasus Mountains.

Building a Young Mountain Range: Insight Into the Growth of the Greater Caucasus Mountains From Detrital Zircon (U‐Th)/He Thermochronology and 10Be Erosion Rates

“…While the magnitudes of displacement across these structures are generally not well constrained (e.g., Trexler et al, 2022), it is ultimately not surprising that implied rates of exhumation or total amounts of exhumation differ between sites in the southern versus northern GC and across multiple different structures.…”

“…No clear evidence of a slab is observed in the western GC, possibly due to slab detachment beneath this portion of the range (Figure 1, Mumladze et al., 2015) or, alternatively, a fundamentally different pre‐existing basin architecture in the western GC (e.g., Adamia, Alania, et al., 2011) that did not result in formal subduction and where shortening was dominated by inversion of former high‐angle rift structures (Vincent et al., 2016). However, the dominance of inversion tectonics in the western GC is largely inconsistent with more detailed structural observations that instead highlight the accretionary nature of this portion of the range (Trexler et al., 2022), consistent with a variety of broadscale observations of the geology of the range (e.g., Dotduyev, 1986; Philip et al., 1989). While definitive evidence of a past subduction zone in the western GC remains elusive, the surface response to a hypothesized slab detachment has been invoked to explain apparently excess amounts and rates of exhumation in the western GC (Forte et al., 2016; Vincent et al., 2020).…”

“…Based on the locations of major structures from Trexler et al. (2022), three of our sample catchments, Katex – 82916‐3A, Kish – 90416‐1A, and Bum – 90516‐1A, cross faults, but for both the Katex and Bum, the vast majority of the catchments are within one fault block and thus we do not consider this to be a major source of uncertainty. The Kish catchment is bisected by a fault (Figure S2 in Supporting Information S1), but it is unclear the extent to which this influences our thermal modeling results for this sample.…”

“…One implicit assumption of the modeling approach used by QTQt is that all portions of the sampled catchment experienced the same form of thermal history, so does not allow for discontinuities, that is, active faults, within the catchment. Based on the locations of major structures from Trexler et al (2022), three of our sample catchments, Katex -82916-3A, Kish -90416-1A, and Bum -90516-1A, cross faults, but for both the Katex and Bum, the vast majority of the catchments are within one fault block and thus we do not consider this to be a major source of uncertainty. The Kish catchment is bisected by a fault (Figure S2 in Supporting Information S1), but it is unclear the extent to which this influences our thermal modeling results for this sample.…”

“…Tye (2019) working in the extreme eastern GC propose a similar structural model for the range, that is, an imbricate fan or accretionary prism style of deformation lacking a clear master structure, but do find that out‐of‐sequence propagation may be important, similar to prior results in this portion of the range within the KFTB and eastern GC (e.g., Forte et al., 2013, 2015). Critically, the detailed structural geometries within the core of the GC as revealed by recent work (Trexler et al., 2022; Tye, 2019) are largely inconsistent with a significant role for inversion of high‐angle rift structures as sometimes considered for the western GC (e.g., Vincent et al., 2016) and are more consistent with models of accretionary orogens (e.g., Willett et al., 1993) where high angle structures in the interior of the range reflect rotation of formerly low‐angle structures during accretion (e.g., Hoth et al., 2007).…”