Geology Faculty Publications

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Fault-Related Rocks: A Photographic Atlas


Photomicrographs 143A-143F record fabric variations in quartzite with increasing structural depth in a > 1 km thick, amphibolite-facies, normal-sense shear zone in the East Humboldt metamorphic core complex, Nevada (Figure 143.1). This shear zone and the overlying detachment system unroofed an infrastructure of high-grade, migmatitic gneiss during Oligocene to early Miocene extension (Dallmeyer and others, 1986; Wright and Snoke, 1993; McGrew and Snee, 1994). Thermobarometric constraints from near the base of the mylonitic zone record deformation conditions of 550°- 620°C and 300-400 MPa (Hurlow and others, 1991). Sample WBC6 (l43A and 143B) characterizes the mylonitic zone, whereas sample 8706-1 (143C and 143D) is transitional in nature, and sample 8727-3 (143E and 143F) represents the infrastructure (Figure 143.1). These three samples record a decreasing contribution from simple shear strain and an actual reversal in shear sense at the deepest structural levels. Inferred variations in quartz deformation mechanisms probably reflect decreasing strain rate and increasing temperature with depth beneath the detachment. We argue that the East Humboldt Range shear system represents a fundamental boundary between an upper crust deforming primarily by rigid block translation and a deeper crust deforming by regional-scale ductile flow.

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Princeton University Press

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Princeton, NJ

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