Distributed Ductile Thinning During Thrust Emplacement: A Commonly Overlooked Exhumation Mechanism

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Quantifying the processes that control exhumation is essential for understanding the evolution of mountain belts. In the Cordilleran orogen in Nevada (western United States), rocks exhumed in the Ruby–East Humboldt metamorphic core complex underwent 4 ± 2 kbar of decompression between 85 and 60 Ma, which has been interpreted as a consequence of synorogenic extension. However, evidence for significant normal faulting in this region prior to 45 Ma is lacking. Here, we present an alternative interpretation: that this decompression can be attributed to distributed ductile thinning (DDT) of mid-crustal metamorphic rocks above the basal Cordilleran décollement during eastward translation. Such a process has been documented within the Himalayan Main Central thrust sheet, which locally accommodated up to 15 km of DDT during Miocene translation. Other examples of DDT have been documented in the Alpine and Caledonian orogens (Europe), and the Sanbagawa belt (Japan). DDT may represent a widespread exhumation process that can account for a significant portion of the decompression path of deeply exhumed rocks. As a condition of strain compatibility, thrust-parallel stretching accompanying DDT is expected to enhance displacement magnitude in the transport direction, and is therefore an important component of the deformation field that must be considered for accurate assessment of mass balance in thrust systems.