Crustal structure of the northwestern Basin and Range Province and its transition to unextended volcanic plateaus

Abstract

[1] The northwestern margin of the Basin and Range Province is characterized by a transition from lowmagnitude ($20%) extension in northwestern Nevada to relatively unextended volcanic plateaus in northeastern California. Seismic-velocity and potential-field modeling provides new control on the Mesozoic-to-present tectonic evolution of this poorly understood portion of the U.S. Cordillera. We document $20% crustal thinning associated with Basin and Range extension from a crustal thickness of $37 km under northeastern California to $31 km under northwestern Nevada that is consistent with the amount of extension recorded in the upper crust in northwestern Nevada, suggesting the crustal response to extension was relatively homogeneous over the entire crustal column. Our modeling also shows a welldefined, 80-km-wide zone of unusually low upper-crustal velocities ($5.9-6.1 km/s) that coincide with the surface location of sparse Cretaceous granites, locating the elusive northern extension of the Sierra Nevada batholith through northwestern Nevada for the first time in the subsurface. Combining geological and geophysical data, we reconstruct the late Cretaceous-to-present crustal evolution of this region, documenting an interplay between magmatic addition to the crust, erosional exhumation, sedimentation, and extension that has reversed the direction of crustal thinning from a west-facing continental margin to an east-facing interior basin margin over this time interval. Finally, we find no evidence in northwestern Nevada for unusually thick crust (>40 km) prior to Basin and Range extension.

Early Tertiary paleogeologic map of the northern Sierra Nevada batholith and the northwestern Basin and Range

“…Petrologic and geochronologic data indicate that the granitic rocks crossing the northwestern Basin and Range correlate with the ca. 120-85 Ma plutonic belts of the Sierra Nevada batholith (Smith et al, 1971;Barton et al, 1988;Lerch et al, 2007;Van Buer, 2008). The magnitude of Tertiary extension in the northwestern Basin and Range is well constrained (Colgan et al, 2006) and is insuffi cient to account for the eastward bend in the Sierra Nevada batholith (Figs.…”

“…1D) is imaged geophysically as a boundary between lower (to the east) and higher (to the west) seismic velocities in the upper 15 km of the crust (Fig. 1B; Lerch et al, 2007). Petrologic and geochronologic data indicate that the granitic rocks crossing the northwestern Basin and Range correlate with the ca.…”

“…Extension in the northern Basin and Range over the past 50 m.y. has been episodic and heterogeneous, with different regions extending at different times (and/or multiple times) and by different amounts (e.g., Miller et al, 1999;Stockli, 2000;Surpless et al, 2002;McQuarrie and Wernicke, 2005;Colgan et al, 2006 (Lerch et al, 2007) colored by inferred basement at 17 Ma in central Nevada, 12 Ma in northern Nevada, and is believed to be younger than 8 Ma along the eastern Sierra Nevada (Fig. 1C).…”

“…Estimates of batholith and backarc crustal thickness based on restoration of regional cross sections (e.g., Chase et al, 1998;DeCelles et al, 1995), mineral thermobarometry (e.g., Camilleri et al, 1997;Lewis et al, 1999), and paleotopography using oxygen isotopes (e.g., Mulch et al, 2006) all suggest that thick crust (50-65 km) and high elevations (2-4 km) persisted to the beginning of Tertiary extension. Conversely, restoration of known amounts of Tertiary extension together with geophysical data on Moho depths in northwestern Nevada suggest normal (~38 km) thickness crust prior to extension (e.g., Colgan et al, 2006;Lerch et al, 2007). Analysis of our early Tertiary paleogeologic map represents an independent approach to addressing the controversial paleogeology and paleotopog raphy across the region.…”

Understanding a critical basinal link in Cretaceous Cordilleran paleogeography: Detailed provenance of the Hornbrook Formation, Oregon and California

“…9). The nowburied northwestern edge of the Cretaceous batholith has been imaged seismically in northwest Nevada (Lerch et al, 2007). Precambrian ages obtained from quartzite cobbles in the Klamath River Conglomerate (Fig.…”