Distributed ductile thinning (DDT) during thrusting: an overlooked exhumation mechanism for mid-crustal rocks

Abstract:

Quantifying the relative contributions of exhumation processes is essential for understanding the evolution of mountain belts. Exhumation is commonly attributed to stream erosion that intensifies during crustal thickening, and/or to tectonic denudation of the footwalls of normal faults and detachment systems. Here, we argue for the importance of a third exhumation process: distributed ductile thinning (DDT) during contractional deformation, by highlighting case studies from the North American Cordillera and Himalaya. DDT exhumes deep-seated rocks by pervasively stretching and thinning thick sections of overlying rocks, thereby reducing overburden. In the Cordillera in Nevada, amphibolite-facies rocks in the Ruby-East Humboldt metamorphic core complex underwent 4 ± 2 kbar of decompression between 85 and 60 Ma, which has been interpreted to reflect a phase of extension during thrusting. However, evidence for significant normal faulting in this region prior to 45 Ma is lacking. Instead, based on pressures that define condensing of structural levels over this time interval, as well as extreme attenuation of stratigraphic units, we propose that DDT of mid-crustal rocks above the basal décollement during thrust translation best explains this early decompression. Similarly, within the Himalayan Main Central thrust sheet, upright, super-lithostatic (~2x) baric field gradients and fabrics that define thrust-normal thinning and transport-parallel stretching indicate that DDT locally accommodated up to 10-15 km of exhumation (3-4 kbar of decompression; as much as 33-50% of the total exhumation path). Partial melting enhanced the potential for ductile flow in these Cordilleran and Himalayan examples, promoting high-strain penetrative stretching and thinning. Other examples of DDT have been documented in the Alps, Caledonides, Sanbagawa Belt, and Betic Cordillera. DDT may represent a widespread exhumation process that can account for a significant portion of the decompression path of deeply exhumed metamorphic rocks. As a condition of strain compatibility, thrust-parallel stretching accompanying DDT enhances displacement 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.