The Snake Range DÉCOLLEMENT: AN Exhumed Mid-Tertiary Ductile-Brittle Transition

The Snake Range décollement (SRD) in east-central Nevada separates supracrustal rocks extended by normal faulting from ductilely deformed igneous and metamorphic rocks. A well-known stratigraphy unaffected by earlier faulting permits analysis of both upper and lower plate strain leading to a better understanding of how vastly different rock types and deformational styles are juxtaposed along low-angle faults in metamorphic core complexes. Middle Cambrian to Permian upper plate rocks are cut by two generations of NE trending, east directed normal faults. Both generations were initiated as high-angle (60°) planar faults that flattened abruptly into the SRD and rotated domino style to low angles, yielding a total rotation of bedding of about 80-90°. Faulting is Tertiary in age as 35-m.y.-old volcanic rocks are involved and resulted in about 450-500% extension in a N55W-;S55E direction. The SRD developed as a subhorizontal surface 6-7 km deep at the top of the Cambrian Pioche Shale. Lower plate granitic rocks and their late Precambrian-Cambrian metamorphic country rocks were involved in progressive ductile to brittle extension at low greenschist grade, forming a penetrative subhorizontal foliation and N55-70W lineation that increases in intensity eastward and upward toward the SRD. Stretching and thinning in the lower plate is coaxial and comparable in magnitude to upper plate extension, and is interpreted as synchronous. K-Ar ages ranging from 20 to 40 m.y. in the lower plate suggests the N. Snake Range represents a Tertiary thermal anomaly. We conclude that the SRD developed as a ductile-brittle transition zone at 6-7 km depth. Gravity data suggests that the gently domed SRD is cut by younger Basin and Range faults, but the geology of adjacent ranges suggests that the SRD does not continue more than 60 km in any given direction. The lack of stratigraphic omission across the SRD rules out large amounts of movement on a surface that originally cut downsection, and we suggest that extensional detachment faults such as the SRD can be developed locally as boundaries between brittlely extended rocks and underlying ductile extension and intrusion.