THE NORTHWESTERN BOUNDARY OF THE BASIN AND RANGE PROVINCE: INSIGHTS AND SURPRISES REGARDING ITS EVOLUTION BASED ON EARTHSCOPE REFRACTION, TELESEISMIC, POTENTIAL FIELD AND GEOLOGIC DATA
High Vp/Vs at the western margin of the province suggests the middle and lower parts of the 37-km thick crust beneath the Modoc Plateau are particularly mafic. At the NW boundary of the BRP, rock units are progressively tilted (by bending) approaching the seismically imaged Surprise Valley Fault which dips only 35°. To the east the crust thins to 31 km across a ≈70-km wide region of unusually low upper-crustal Vp, Vs and density, and low Vp/Vs throughout the crust that we interpret as the continuation of the Sierra Nevada batholith through the NW BRP. Farther east, beneath the Santa Rosa Range, crustal thickness is similar but Vp/Vs increases to values characteristic of more intermediate compositions.
Geochronology and geologic mapping provides the "4D" (temporal) context for our geophysical data. Reconstructions accounting for measured extension (0 to 25% across our transect), magmatic under- and intra-plating estimated from volcanic thicknesses and chemistry (0.4 to 4.4 km) and estimated erosion (1 to 7 km) since emplacement of Sierra Nevada plutons imply that at ≈85 Ma, the present-day NW margin of the BRP was a transition from normal crustal thickness of ≈40 km in the east, to thin (≈34 km) crust in the west. A high plateau underlain by over-thickened crust was not a precursor to Basin-Range extension in this region.