THE SNAKE RANGE METAMORPHIC CORE COMPLEX, NEVADA—A CLOSER LOOK

Metamorphic core complexes are still perplexing in terms of how and why they formed. The low-angle normal fault model was launched with the Snake Range as example (Wernicke 1981) and subsequently applied to extensional provinces worldwide. Ideas about the crustal-scale processes involved in core complex genesis have since proliferated and range from driven by partial melting, crustal flow and diapirism to simple shear zones that transect a relatively rigid crust. The age of core complex formation is crucial to our understanding of how and why the U.S. Cordillera transitioned from regional shortening to extension and the difficulty in dating metamorphism and mylonites together with determining P-T has led to a confusing plethora of ideas. Are lower plate structures collapse-related but synchronous with foreland shortening (e.g. Wells and Hoish, 2008)? Or are mylonites linked to Basin and Range extension, >30 Ma later? Finally, the cross-sectional geometry of core complexes (and how much extension is implied) make a big difference in how we restore the Basin and Range and estimate pre-extensional crustal thicknesses, etc.

Map relations, structural considerations, and an extensive geochronology/ thermochronology database for the broader Snake Range region permit a close look at the problems related to the genesis of this core complex. The main pulse of magmatism (~39-35 Ma) was accompanied by metamorphism and deformation at depth. However, evidence for supracrustal normal faulting at this time is limited, possibly taking place right above the evolving core complex. Most crustal extension occurred in the Miocene at about 17 Ma, concurrent with Basin and Range faulting, bringing rocks from beneath the brittle-ductile transition to the surface. The N. Snake Range is part of a 150 km long footwall to a Miocene fault system. Associated mylonites involve Tertiary dikes and are eroded into flanking basins at about 17 Ma. It is difficult to place part of this footwall (but not the rest of it) at 8 kb depths (e.g. Cooper et al., 2010) and furthermore, structure(s) responsible for the required ~4 km exhumation between Late K peak mm’sm and onset of Tertiary extension have not been identified.