The Snake Range Décollement: An exhumed Mid‐Tertiary ductile‐brittle transition

Abstract

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.

History and tectonic implications of low‐angle detachment faults and orogen parallel extension, Picentini Mountains, Southern Apennines fold and thrust belt, Italy

“…The restorable deformation model, with listric and planar faults in the upper plate merging into a shallowly dipping detachment in the upper crust, is a commonly invoked but contentious model for largescale orogenic extension [e.g., Wernicke, 1992]. In the Picentini domain, given the level of exposure combined with the kinematic, structural, and stratigraphic control, it is clear that the shallow dips of the basal detachment faults cannot be accommodated by wholesale tilting of the decollement either by so-called domino faulting [Miller et al, 1983] or by the rollinghinge model [Buck, 1988] Furthermore, in the Picentini domain the extension was fundamentally asymmetric, and the associated structures are typical of overall simple shear conditions. The absence of significant lateral variation in the structure of extensional allochthons and faults across the extensional domain supports the conclusion based on kinematic data and field geometries that low-angle normal faulting was essentially plane strain.…”

Geologic, structural, and thermochronologic constraints on the tectonic evolution of the Sierra Mazatán core complex, Sonora, Mexico: New insights into metamorphic core complex formation

“…Endmember models of slip magnitudes at core complexes include the brittle-ductile transition model of Miller et al [1983] and Gans et al [1985] for the Snake Range where only minor displacement occurred across the fault (up to 10 km), to models where detachment faults have accommodated up to 40-60 km of slip, as interpreted for the Snake Range [Bartley and Wernicke, 1984], for the Central Mojave core complex [Glazner et al, 1989], and the Whipple Mountains [Lister and Davis, 1989]. Geologic data at several core complexes have been used to infer very high slip rates (8 -30 mm/a) on these faults [e.g., Lister and Davis, 1989].…”

Observations from the Basin and Range Province (western United States) pertinent to the interpretation of regional detachment faults

“…Armstrong 1972;Crittenden et al 1980;Wernicke 1981Wernicke , 1985Allmendinger et al 1983;Davis 1983;Miller et al 1983;Spencer 1984;Lister & Davis 1989;Spencer & Chase 1989;Axen et al 1990Axen et al , 1993John & Foster 1993;Livaccari et al 1993;Axen & Bartley 1997;Brady et al 2000a;Snow & Wernicke 2000;Livaccari & Geissman 2001;Axen 2004;Carney & Janecke 2005). The detachment concept is now widely applied in extensional and passive margin settings (e.g.…”