The Western U.S. Lithosphere Blues
Dr. Rob Porritt, University of Southern California
The Western United States has long been studied as the typically atypical plate boundary zone. Recent deformation is observed from the Pacific-North America coast to the Rocky Mountains. Along the coast and south of the Mendocino Triple Junction in Northern California, the San Andreas Fault Zone accommodates translational motion between the Pacific and North American plates through a series of en echelon strike-slip faults. North of the triple junction, the Juan de Fuca-North America plate boundary zone is a prolifically aseismic subduction zone with paleoseismic records of megathrust events, but insufficient events in the modern age to map the subducting plate deeper than 100km depth. Moving inboard from the coast, extension through the Basin and Range Province is pushing apart the Sierra Nevada block and the Colorado Plateau. Major igneous features include the Tertiary Ignimbrite Flare-up, the 17Ma Yellowstone and Newberry hotspot tracks, and the active Cascades Arc.
Using teleseismic and ambient noise data from the Earthscope USArray, regional seismic networks, and the US backbone seismic network, we image a generally low seismic wave-speed upper mantle with discrete high wave-speed features throughout the region. We interpret these features to be primarily fragments of the Farallon Plate caught in the upper mantle. Alternatively, many of these features have been interpreted as lithospheric instabilities as an explanation for the anomalously thin lithosphere and regional mantle anisotropy inconsistent with simple plate motion. In this webinar, I present the argument that many, if not all, of the observed high wave-speed anomalies are pieces of the Farallon plate. This argument does not rule out the important role played by lithospheric instabilities in long-term continental growth, but rather forces the discussion toward the expanded role of oceanic plate subduction in continental evolution.
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