Faulting in a propagating continental rift: Insight from the late Miocene structural development of the Abert Rim fault, southern Oregon, USA

New geological mapping and 40Ar–39Ar ages reveal a temporal progression of faulting along a major extensional fault (the Abert Rim fault) at the active margin of the Basin and Range Province in the western U.S. The onset of extensional deformation near Lake Abert coincided with widespread basaltic volcanism. Fault cross-cutting relationships and tilt of volcanic layers demonstrate that NW-striking faults formed between ∼ 8.9 and 7.5 Ma and were subsequently cut by the NNE-striking Abert Rim fault. Sequential restoration of cross-sections indicate that the Abert Rim fault south of 42°40′ N latitude had > 250 m of stratigraphic separation prior to deposition of the Rattlesnake Tuff at 7 Ma. Low values of net extension (∼ 4%) and long-term rates of geologic deformation (≪ 1 mm/yr) suggest that regional extension since ∼ 10 Ma has occurred primarily by diking. We model province wide along- and across-strike expansion within the margin of the Basin and Range Province as regional dilation accompanying northward propagation of the Walker Lane transform. The growth of this extensional province is superimposed on a previously unrecognized early Miocene volcanic landscape marked by volcanism between ∼ 21.4 and 22.3 Ma. Dike orientations within one of these early Miocene volcanoes and evidence for the existence of as much as 500 m of paleo-topography prior to widespread deposition of the Steens basalts at ∼ 16 Ma suggests that late Miocene faults near Lake Abert may reactivate an older structural fabric.