NEOGENE BASALTIFICATION OF THE CRUST AT THE NORTHWESTERN MARGIN OF THE BASIN AND RANGE PROVINCE: MAGMATIC ACCOMMODATION OF EXTENSION?

The High Lava Plains make up a volcanic plateau that formed since about 12 Ma and that coincides with the structural termination of the northwest Basin and Range Province, expressed as the Brothers Fault Zone. The degree of extension in the northwestern Basin and Range Province declines northward based on decreases in scarp height and dip of bedding, while Neogene volcanic rocks become more prominent. This combination suggests that some extension is accommodated magmatically leading to basaltification of the continental crust in the process.

The High Lava Plains (HLP) plateau encompasses at least 30,000 km2 and is underlain by subequal extents of basalt lavas and rhyolite ignimbrites, intercalated with volcaniclastic deposits. Volcanic vents alignments and dikes are commonly parallel to the northwesterly fault fabric of the Brothers Fault Zone. Basaltification of the crust underlying the plateau is indicated by decreasing tilts with younger age among volcanic deposits exposed on the northern margin of the High Lava Plain. At Hampton Butte, in the western HLP, 10.2-Ma low-potassium tholeiites, typical of the HLP, dip about 20 degrees to the south. 6.8-Ma ignimbrite banks onto the lavas and is itself tilted about 10 degrees south and 2-Ma basalt are flat. We interpret the low relief relative to the Blue Mountains to the north and to the Basin and Range to the south to reflect injection of basalt into the crust and consequent crustal loading.

Basaltification of the crust is further supported by petrologic evidence that successive rhyolitic crustal melts are increasingly primitive, that is similar to ambient basalts, in isotopic character. In the central HLP, rhyolites decrease in 87Sr/86 Sri from 0.706 at 8 Ma to 0.7042 at 7 Ma and 0.7038 at 6 Ma, increasingly approaching values for regional basalt. Assimilation signatures in sparse andesitic rocks are consistent with an increasingly mafic crustal contaminant.