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Regional and local stratigraphic, metamorphic, and structural constraints permit reconstruction of the southern Snake Range extensional deformational system in east central Nevada. The dominant structure of the range, the southern Snake Range décollement (SSRD), operated during Oligocene and Miocene extensional deformation to exhume a footwall of multiply deformed metasedimentary and plutonic rocks. Intrusion of three plutons (∼160 Ma, 79.1 ± 0.5 Ma, and 36 ± 1 Ma, respectively) and development of two cleavages preceded the onset of extensional deformation. Plastic deformation of lower plate metasedimentary rocks accompanied the early phases of regional extension and produced bedding-parallel grain shape foliations and WNW trending stretching lineations. These fabrics parallel the SSRD even in low-strain domains, suggesting that a significant component of pure shear strain probably accompanied noncoaxial deformation associated with motion on the SSRD, consistent with other lines of evidence.

Meanwhile, hanging wall rocks were greatly extended by at least two generations of tilt block-style normal faults soling into the SSRD, with the earlier faults antithetic to the SSRD and the later faults dipping in the same direction as the SSRD. A retrodeformed regional cross-section sequence illustrates plausible alternative schemes for reconstructing the southern Snake Range extensional system.

In one scheme, the SSRD forms as a crustal scale stretching shear zone separating an upper plate that extends on steeply inclined normal faults from a lower plate that stretches by penetrative flow. In the other, lower plate deformation incorporates a component of coaxial stretching, but the SSRD also functions as a conventional shear zone accommodating through-going displacement between opposing plates. In either case, as tectonic unroofing proceeds, differential isostatic unloading induces the SSRD to rotate to steeper dips as it migrates into the frictional sliding regime, thus enabling it to remain active as a brittle normal fault until it finally rotates to its present shallow inclination. In either scenario, cross-section constraints suggest that total extension accommodated by the SSRD was probably between 8 km and 24 km.

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John Wiley & Sons





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