THE SIGNIFICANCE OF DUCTILE EXTENSIONAL STRAIN IN THE FOOTWALL OF A MAJOR DETACHMENT FAULT: INSIGHTS FROM THE NORTHERN SNAKE RANGE METAMORPHIC CORE COMPLEX, EASTERN NEVADA

Evaluating ductile strain in metamorphic core complexes improves understanding of processes that accommodate high-magnitude extension. Here, we investigate ductile strain in the Northern Snake Range core complex in Nevada, which preserves micro- and macro-scale strain markers. Neoproterozoic-Cambrian metasedimentary rocks in the footwall of the top-down-to-ESE Northern Snake Range décollement (NSRD) experienced low-strain Late Cretaceous shearing that generated NNW-trending (333° mean) lineations preserved in the western part of the range, which was overprinted by high-strain Eocene-Oligocene extensional shearing in the central and eastern parts of the range that generated ESE-trending (112° mean) lineations. To quantify ductile strain in the NSRD footwall, we utilized new (n=6) and published (n=8) strain ellipsoids obtained using the Rf-θ method on detrital quartz clasts in the Cambrian Prospect Mountain Quartzite (Cpm). We also measured macro-scale structural thicknesses of the Cpm using new cross sections (n=21) and a published seismic reflection profile (n=1), which we converted to % foliation-normal (Z) shortening using the 1248±43 m average regional stratigraphic thickness of the Cpm (n=22). Our data define 6 strain domains (SD1 to SD6 from west to east): SD1 (1150 m Cpm thickness, 8±3% Z shortening) and SD2 (average 2D strain ellipse of 1.7±0.1), which experienced only Late Cretaceous strain, and SD3 (552±21 m Cpm thickness, 56% Z shortening), SD4 (163±24 m Cpm thickness, 87±2% Z shortening), SD5 (95±5 m Cpm thickness, 92±1% Z shortening), and SD6 (46±7 m Cpm thickness, 96±1% Z shortening), which experienced both Late Cretaceous and Eocene-Oligocene strain. We utilized the 5.5±2.4% average lineation-normal extension measured in 28 published ellipsoids collected 3-7 km to the south to calculate % lineation-parallel (X) extension. This defines an eastward increase from 3% and 27% X extension in SD1 and SD2, to 93%, 647%, 1149% and 2577% X extension in SD3-6, respectively. Retro-deforming the 23.4 km modern length of SD3-6 to the Late Cretaceous strain magnitude of SD2 defines 18.6±0.7 km of total Eocene-Oligocene ductile extension (398±73%). This is equivalent to 34-64% of the 29-54 km published range of total brittle extension, which highlights that ductile extensional strain is a 1^st-order component of the strain field of metamorphic core complexes.