WAS MID-MIOCENE EXTENSION IN THE NORTHERN BASIN AND RANGE A PRODUCT OF PLUME IMPINGEMENT, PLATE REORGANIZATION, OR BOTH?

Mid-Miocene crustal extension that began across the northern Basin and Range (NBR) at ~16-17 Ma has often been attributed to a sudden change in plate-boundary conditions, perhaps related to rapid growth of the transform plate boundary. This abrupt onset of crustal stretching, however, was also contemporaneous with two magmatic events more commonly attributed to arrival of the Yellowstone mantle plume: (1) initiation of Columbia River flood-basalt (CRB) volcanism at ~16.7 Ma, and (2) the onset of rhyolitic eruptions along the western edge of the Snake River Plain hotspot track at ~16.5 Ma. The unlikely coincidence that plate reorganization and plume impingement began at the same time has led some workers to suggest that mid-Miocene volcanism was itself a consequence of tectonic reorganization. An alternative model is that crustal extension was partly or wholly a consequence of plume impingement. This hypothesis is supported by recent seismic data resolving a plume tail beneath Yellowstone to a depth of ~900 km, and by geologic evidence that back-tracks its mid-Miocene location into the NBR at 16.7 Ma.

During the main eruption phase of the CRBs (~16.7-15.6 Ma), the greatest volume of lava was generated in the area of least extension (NE Oregon) and the smallest volume was generated in the area of most extension (NBR). This inverse relationship is difficult to reconcile with a passive-mantle origin that invokes crustal extension as the root cause for flood-basalt volcanism, but it is more consistent with an active-mantle origin, as reflected in the geochemical evidence for a mantle-plume source. The surface manifestation of plume emplacement was largely controlled by the basement architecture, with the greater flood-basalt volume generated above thin oceanic lithosphere of accreted terranes, and the much smaller volume of coeval intrusions and volcanic rocks in the NBR generated above a thicker basement with continental affinities. The NNW orientation of the northern Nevada rift and related aeromagnetic anomalies was largely controlled by the mid-Miocene stress regime. However, a lithospheric weak zone appears to have played a role in the westernmost region of extension and magmatism that lies along a 210-km-long belt extending from the Smoke Creek dike swarm and into the High Rock caldera complex to the NNE.