WHAT DROVE BASIN AND RANGE EXTENSION? (Invited Presentation)

Paleogene to Neogene extension of the western North American Cordillera produced the iconic Basin and Range Province. While the ultimate products of extension are clear (e.g., thinned crust and lithosphere; co-linear, normal fault-bounded mountain ranges), the physical drivers of and controls on style of extension are debated. Proposed drivers include, 1) gravitational potential energy (GPE) gradients, 2) shear tractions applied laterally at the Pacific-North American plate boundary, and 3) shear tractions applied basally to the North American lithosphere. Here, I evaluate these potential drivers using a detail tectonic reconstruction of the central Basin and Range. The reconstruction shows that Sierra Nevada-Colorado Plateau relative velocity generally decreased (from ~25 mm/yr to ~11 mm/yr) and rotated clockwise (from WSW to NW) since middle Miocene time (ca. 16 Ma). Slowing and clockwise rotation of intraplate extensional velocity coincided with acceleration of Pacific-North American relative motion, but not with major changes in direction. Clockwise rotation of Basin and Range extension toward parallelism with Pacific-North American motion supports that the Sierra Nevada-Great Valley microplate was progressively coupled to the Pacific plate. Coupling may have been driven by enhanced lateral traction during lengthening of the San Andreas fault zone, and/or basal traction due to capture by the Pacific Plate of stalled slabs attached to the base of the Sierra-Great Valley. Modeling of syn-extensional strain suggests inherent mechanical heterogeneity (e.g., batholithic vs. sedimentary cover) localized intraplate strain. Progressive delocalization and west-ward migration of extension was likely controlled by feedbacks including a) loss of GPE and b) mid-crustal strengthening due to exhumation of metamorphic core complexes. Lastly, the kinematic model resolves W- and NW-increasing slip gradients along the Garlock and Furnace Creek fault zones, as well as N-S syn-extensional contraction. These suggest tectonic wedging and west-ward extrusion in the central Basin and Range.