LATE MIOCENE ACCELERATION OF BASIN AND RANGE EXTENSION ADJACENT TO THE NORTHERN WALKER LANE

The progressive transition of western North America from an Andean type margin to a dextral transform boundary is linked to cessation of arc magmatism, extensional collapse of orogenically thickened crust, and northward propagation of inland dextral deformation in the Walker Lane. The Shawave, Selenite, Lake, and Fox Ranges in northwestern Nevada are positioned in the transitional region between the relatively rigid central Basin and Range and the more rapidly deforming Walker Lane, which provides an ideal location to investigate the interaction between these geodynamic processes. New ⁴⁰Ar/³⁹Ar ages, fault kinematic data, geologic mapping, restoration of cross sections through the region, and compilation of regional data illustrate: 1) Cessation of intermediate arc volcanism, transition to clastic basin sedimentation, and onset of Basin and Range extension began at ~13 Ma at this latitude (~40°N), well before the development of dextral Walker Lane faulting in the region (~3-9 Ma), 2) total extension estimated from restored cross sections in the area is ~8.6-14.5 km (21-23%), with long term extension rates between 0.66-1.1 mm/yr, and 3) preserved fault surfaces east of the dextral Pyramid Lake fault record WNW directed extension in a normal faulting regime, which is consistent with the modern stress field recorded by geodetic and focal mechanism data. Together, these observations show that cessation of major magmatism and initiation of Basin and Range deformation at ~40°N began earlier than predicted. Geodynamic models suggest these processes should closely track with northward migration of the Mendocino Triple Junction, though intermediate volcanism continued through ~9 Ma in regions directly to the south of this transect. A normal faulting regime has persisted in this area since 13 Ma, and has not been altered by development of dextral strike-slip faults in the adjacent Walker Lane. However, deformation has not been steady through time given long-term extension rates are lower than Quaternary and geodetic estimates of >1 and >1-2 mm/yr (respectively). Extension likely accelerated as dextral deformation propagated northward into the Pyramid Lake domain in the late Miocene to early Pliocene. This is consistent with data showing recent rejuvenation after a lull in deformation between ~12-7 Ma at 39°N.