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Documenting the geometry, magnitude and kinematics of ductile deformation provides important insights into the structural and rheological evolution of continental lithosphere. The Northern Snake Range metamorphic core complex in eastern Nevada provides an exceptional opportunity to investigate the geometry and magnitude of ductile strain during high-magnitude continental extension. Decades of mapping-based research has provided exceptional stratigraphic context for the footwall of the low-angle, top-down-to-ESE, normal-sense Northern Snake Range dècollement (NSRD). In the northern part of the range, Middle-Late Cambrian marble units in the NSRD footwall, which have a cumulative stratigraphic thickness of 1107 ± 107 m in adjacent ranges, were ductilely thinned during Late Eocene-Late Oligocene extension. From west to east across the range, these rocks have been thinned from 869-935-m-thick (15-21% structural thinning) to 54-88 m-thick (92-95% structural thinning) across a 12 km lineation-parallel distance. Ductile extensional strain was accompanied by the development of pervasive linear-planar fabrics and produced megaboudins of calcareous schist units that are ~100-500-m-long, ~15-25-m-thick, and separated by as much as ~1000 m. The magnitude of subhorizontal, ESE-directed, lineation-parallel ductile extension increases eastward across the range from 24 ± 21% to 1226 ± 256%, and total ductile extension across the range is 12.1 ± 2.2 km (167 ± 31%). Quartz recrystallization microstructures and published calcite-dolomite thermometry indicate deformation temperatures of ~400-550 °C during initial Late Eocene-Late Oligocene ductile extensional shearing. NSRD footwall rocks in the eastern part of the range experienced a longer ductile extensional strain history and a prolonged residence time at higher temperatures compared to the western part of the range. This was facilitated by the progressive eastward migration of denudation-related cooling and was likely enhanced by shear heating that scaled eastward with strain magnitude, and/or a possible eastward increase in burial depth. These factors promoted the development of the extreme ductile strain gradient in the NSRD footwall across the Northern Snake Range. 

The geometry and magnitude of finite strain in the ductile footwalls of metamorphic core complexes are important parameters for testing the predictions of models of extension, yet are often difficult to quantify due to the rare preservation of deformed markers. The footwall of the Northern Snake Range core complex in eastern Nevada preserves a coherent stratigraphy of ductilely thinned Neoproterozoic-Cambrian metasedimentary rocks that are exposed over a 30 km transport-parallel distance, and thus provides an important opportunity to quantify footwall strain. We measured strain ellipsoids from stretched detrital quartz grains and ribbons in 45 samples that span the full exposed distance of the ductilely sheared footwall of the master detachment fault (the Northern Snake Range décollement), and we combined our data with 11 published strain ellipsoids. On the eastern side of the range, where recrystallization limits the preservation of detrital quartz grains, we estimated finite strain by comparing the attenuated thicknesses of Neoproterozoic-Cambrian rock units to their regional stratigraphic thicknesses. Our data demonstrate a dramatic gradient in ductile strain in the transport direction, from 39% subhorizontal extension and 32% subvertical thinning at the western flank of the Northern Snake Range to 450-1440% extension and 81-94% thinning at the eastern flank. The footwall underwent 18-20 km of cumulative ductile extension, which is equivalent to 38-43% of the 47 km of total extension accommodated on brittle structures. Kinematic vorticity estimates from published quartz petrofabrics define an eastward-increasing component of top-to-the-ESE simple shear. Our data are compatible with a rolling hinge model of extension, where displacement on a low-angle, upper-crustal, brittle detachment fault system was fed downward to a zone of distributed, simple shear-dominant, top-down-to-ESE ductile shearing beneath the quartz crystal-plastic transition. The progressive eastward translation and brittle thinning of the hanging wall resulted in the eastward migration of exhumation of footwall rocks. Migrating exhumation may in part be responsible for the eastward-increasing finite strain gradient, as footwall rocks in the eastern part of the range experienced a longer strain history.