MESOZOIC TO CENOZOIC TECTONIC AND PALEOGEOGRAPHIC EVOLUTION OF THE RUBY-EAST HUMBOLDT-WOOD HILLS METAMORPHIC CORE COMPLEX REGION, NEVADA, AND THE ROLE OF TECTONIC INHERITANCE IN ITS DEVELOPMENT

The Ruby-East Humboldt-Wood Hills extensional terrain in NE Nevada straddles a bight in the Paleozoic continental shelf-slope margin that localized large-scale crustal thickening during the late Mesozoic. Tectonic burial along this bight peaked by ~84 Ma with metamorphism of strata as young as Mississippian to pressures up to 10 kb due to emplacement of a SE-tapering thrust wedge of Archean to Paleozoic strata in the Windermere allochthon (WA). Four phases of extension exhumed the footwall of the WA to produce the metamorphic core complex. The first two phases resulted in partial exhumation of the WA footwall by translation of the allochthon on NW-dipping normal faults that may record gravitational spreading of the crustal welt. The Pequop normal fault, bracketed between 84 and 41 Ma, accommodated the first phase of extension, and was probably in part coincident with sphene and mica cooling ages in the Wood Hills and hornblende cooling ages in the East Humboldt Range. The lack of syntectonic sediment implies that the Pequop fault basin was probably externally drained. The second major phase of extension probably began in the Late Eocene or Oligocene and evolved diachronously from ESE to WNW on the Mary's River normal fault system. Remnants of the early Mary's River basin fill are preserved in fault-bounded fragments of inferred Oligocene strata in the Windermere Hills, northern Wood Hills, and East Humboldt Range. The third phase of extension was manifested by the N-dipping Holborn normal fault (Mueller et al. 1999), which transected the early Mary's River basin fill, accommodating differential uplift between the metamorphic core complex to the south and lower grade rocks to the north. The last phase of extension produced widespread N-striking, E- and W-dipping normal faults that progressively exhumed and dismembered the WA footwall. This phase involved major filling of synextensional basins by the upper Humboldt Formation from >15.5 to < 7.5 Ma. Finally, post-7 Ma extension resulted in emergence of modern fault systems and ranges. According to this synthesis, tectonic inheritance played a crucial role in localizing crustal thickening and the early phases of extension along the bight, but probably had less influence on the latest phase of extension.