Dr. Gene Humphreys

Presentation: Looking Deep for the Causes of Recent Tectonic and Magmatic Activity in the Western United States

Young and ongoing tectonism and magmatism occur across the western U.S. from the Great Plains to the Pacific Ocean. The broad distribution of tectonic and magmatic activity is noteworthy not simply because it is unusual, but also because these are the processes that make and restructure continents. Seismic imaging provides clues as to what processes are fundamental, and indicates that the lithosphere and asthenosphere are directly involved. Our current understanding has required a synthesis of lithospheric seismic imaging with geologic observations and geodynamic modeling. The presentation will address one or more of the following aspects of western U.S. Cenozoic history, by mutual agreement between Gene and the host institution.

1. Effects of Pre-Cenozoic inheritance. The Laramide orogeny modified North American lithosphere in a way that eventually resulted in a broad western U.S. uplift and, in what now is the Basin and Range Providence, remarkable ignimbrite flare-ups. How did this happen?

2. Early Cenozoic tectonics. Accretion of the oceanic Siletzia lithosphere about 50 million years ago marked the beginning of the end for the Laramide orogeny, and the start of ignimbrite flare-ups. It also created structure that has strongly influenced the Pacific Northwest ever since. What's the story?

3. Current-day tectonics. In a manner atypical of plate tectonics, deformation occurs over a wide area of the western U.S. This includes the extending continental interior, major zones of contraction along the western plate margin, and a transform-related shear zone of several hundred kilometers width. What plate and non-plate tectonic processes are at work, and is it plate strength or the forces that are unusual?

4. Yellowstone-related activity. Over the past 17 million years, Yellowstone-related magmatism created Earth's most recent flood basalt province, the Columbia Basin. It also magmatically (re)constructed large volumes of lithosphere, and created a prominent continental hotspot track. What is happening to the Pacific Northwest?

Dr. Robert B. Smith

Presentation: The Yellowstone Hotspot: Past, Present and Future

The Yellowstone hotspot resulted from interaction of a mantle plume with the overriding North American plate producing a ~800-km wide, ~300-m high topographic swell centered on Yellowstone National Park. It also produced the 800 km-long, 17 million year old Yellowstone-Snake River Plain (YSRP) volcanic field. The Yellowstone Plateau has extensive seismicity including the deadly M7.5 Hebgen Lake Earthquake in 1959. There are also extensive earthquake swarms; the latest in late 2008 and early 2009 produced a thousand events on the caldera edge that migrated at ~1,000 meters per day beginning with an explosive M4 earthquake. Heat flow is extraordinarily high, ~ 2,000 mWm2, indicative of the thermal energetics of this active volcano-tectonic system. Large-scale geophysical experiments have provided seismic and GPS images of the hotspot and data on its kinematic and dynamic properties. Tomography reveals a caldera-wide Yellowstone crustal magma reservoir, that is fed by an upper-mantle plume composed of melt blobs, 80 km to 650 km deep, titled 60 degrees NW. Deformation of Yellowstone is dominated by SW-extension at up to ~0.3 cm/yr, a fourth of the total Basin-Range opening rate, but with superimposed volcanic related uplift and subsidence at decadal scales, averaging ~2 cm/yr. An unprecedented episode of caldera uplift, up to 7 cm/yr from 2004-2008, was modeled as recharge of the crustal magma body at 10-km depth. Upper mantle convection models for Yellowstone are characterized by eastward flow beneath Yellowstone at 5 cm/yr. This suggests that the strong eastward mantle flow that deflects the ascending melt into a tilted configuration, i.e. "bending the plume in the mantle wind." Dynamic models reveal relatively low plume temperatures, up to 150 degrees K excess temperatures, consistent with a weak buoyancy flux of ~0.25 Mg/s, but strong enough to produce the Yellowstone swell. Kinematic and dynamic modeling suggest that the excess gravitational potential of the swell drives the SW motion of the YSRP lithosphere "downhill" where it becomes part of clockwise rotation of western U.S. intraplate blocks. Extrapolating the location of the Yellowstone mantle-source southwestward to an initial position at 17 million years ago beneath eastern Oregon and the southern Columbia Plateau basalt field, suggests a common mantle source for these features. We suggest that the original plume ascended vertically behind the subducting Juan de Fuca plate, becoming entrained in faster mantle flow beneath continental lithosphere and became tilted into its present configuration about 12 million years ago.

Dr. Emily E. Brodsky

Presentation: Earthquakes Triggered by Seismic Waves

Everyone knows that aftershocks follow large earthquakes. But why? Embarrassingly, the mechanism by which earthquakes trigger other earthquakes is still an unresolved question in seismology. This talk will discuss evidence that the ground shaking from seismic waves plays a key role in this process. Starting with clear observations of distant triggering from seismic waves, we will review the role of dynamic triggering in generating earthquakes. Using a large dataset of distant and local triggering, we will ultimately show that triggering at all distances can be well predicted based on the amplitude of the seismic waves from previous earthquakes.

Dr. Mark Zoback

Presentation: Structure, Composition and Properties of the San Andreas Fault in Central California: Results from SAFOD's First Five Years

The San Andreas Fault Observatory at Depth (SAFOD) was drilled to study the physical and chemical processes controlling faulting and earthquake generation along an active, plate-bounding fault at depth. SAFOD is located near Parkfield, California, and penetrates a section of the fault that is moving through a combination of repeating microearthquakes and fault creep. Geophysical logs define the San Andreas Fault Zone to be relatively broad (~200 m), containing several discrete zones only 2-3 m wide with extremely low P- and S-wave velocities and low resistivity. Two of these zones have progressively deformed the cemented casing at measured depths of 3194 m and 3301 m (corresponding to vertical depths of 2.6 - 2.7 km), indicating that they are actively creeping shear zones. The 3194 m casing deformation zone lies ~100 m above a cluster of repeating M2 earthquakes that form the southwestern boundary of the active fault zone. Talc and serpentine were discovered in drill cuttings associated with the deepest casing deformation zone, and these minerals may be responsible for the predominantly creeping behavior and anomalously low shear strength of the San Andreas Fault at this location. Core was obtained in 2007 across the active deformation zones at 3194 and 3301 m and from just outside the geologically defined San Andreas Fault Zone. Cores crossing the two deformation zones are composed of shales, siltstones and mudstones and contain 1-2 m of a highly foliated, relatively incohesive fault gouge. In both cases, this fault gouge exactly correlates in depth with casing deformation and, thus, represents the actively creeping traces of the San Andreas Fault at depth. This fault gouge has an anomalous mineralogy with respect to the adjacent country rock in that it contains veined serpentinite bodies, serpentinite porphyroclasts and an ordered chlorite/smectite phase. Pervasive shearing within the gouge is indicated by anastomozing, slickensided surfaces, with authigenic clay mineralogy and host-grain/clay microstructures suggestive of extensive fluid-rock interaction and dissolution-precipitation creep. These cores are now being tested in laboratories at a number of institutions around the world to study their composition, deformation mechanisms, physical properties and rheological behavior.

Dr. Ramon Arrowsmith

Presentation: High spatial resolution tectonic geomorphology of active fault zones of western North America

GeoEarthscope and related LiDAR topography efforts have illuminated thousands of kilometers of active fault traces at decimeter resolution. These new data provide a spectacular opportunity to characterize fault zone geometry, slip-rate variation over time, and the interaction of surface and tectonic processes in the development of tectonic landforms. Bridging the gap in measurements of lithospheric deformation-between the seconds to decades of earthquake seismology and geodesy, compared to the million-year time scale of geology and structural seismology-tectonic landforms and earthquake geology contribute information about the strain release at the 100 to 100,000 year time scale. These results include the slip distribution from recent earthquakes and the shapes and sizes of the semi-independently moving blocks that comprise major fault zones.