High Resolution Seismic Imaging Of Shallow Sedimentary Volcanic Interfaces Beneath The Western Snake River Plain Near Boise Idaho

In 1978 a major seismic profiling experiment was conducted in the Yellowstone-eastern Snake River Plain region of Idaho and Wyoming. Fifteen shots were recorded that provided coverage to distances of 300 km. In this paper, travel time and synthetic seismogram modeling was used to evaluate an average P wave velocity and apparent Q structure of the crust from two seismic profiles (reversed) across the Yellowstone National Park region. This area includes the well-known hydrothermal features of Yellowstone National Park (geysers, fumeroles, etc.), a large collapse caldera, and extensive silicic volcanism of Quaternary age?features attributed to shallow crustal sources of magma. The averaged crustal structure for this region as interpreted from the seismic data consists of (1) a highly variable, near-surface layer approximately 2 km thick with variable velocities of 3.0 to 4.8 km/s and a low apparent Q of 30 that is interpreted to be composed of weathered rhyolites and sedimentary infill, (2) an upper crustal layer 3 to 4 km thick with variable velocities of 4.9 to 5.5 km/s and apparent Q of 50 to 200 that is thought to represent the accumulation of the Pleistocene-Quaternary rhyolite flows, ash flow tuffs, and possible Paleozoic and Precambrian metamorphic equivalents, (3) the crystalline, upper crust that is characterized by a laterally inhomogeneous layer that varies in velocity from 4.0 to 6.1 km/s, averaging 5 km thick with a Q of 300. This layer appears to be a cooling but still hot body of granitic composition beneath the Yellowstone caldera. It is thought to be a remnant of the magma chambers that produced the Quaternary silicic volcanic rocks of the Yellowstone Plateau and may still be a major contributor to the high heat flow, (4) a laterally homogeneous intermediate crustal layer 8 to 10 km thick with a velocity of 6.5 km/s and apparent Q of 100 to 300, (5) a homogeneous 25-km-thick lower crust with a velocity of 6.7 to 6.8 km/s and an apparent Q of 300, and (6) a total crustal thickness of ?43 km. The upper crustal layer, 5.5 to 6.0 km/s, is thought to be the thermally altered equivalent of the continental crystalline basement that is normally 15 to 20 km thick in the surrounding thermally undisturbed Archean crust. An interpretation from these results suggests that mafic melts from the mantle have penetrated the lower crust without significant variations in the velocity structure but produce the main source of heat that drives the volcanic and hydrothermal systems of Yellowstone. The high apparent attenuation and large lateral velocity variations in the upper crust are consistent with a model in which partial fractionation, partial melting, and metamorphism differentiate the original upper crust to produce silicic melts that were extruded as rhyolites and ash flow tuffs across the Yellowstone Plateau. This seismic model is consistent with the evidence for a systematic northeastward propogation of silicic volcanic centers along the eastern Snake River Plain to their present location beneath the Yellowstone hydrothermal system. While these findings do not bear directly on the origin and source of the heat, i.e., mantle lumes, lithospheric fractures, mantle radiogenetic heat, basal lithospheric shearing, etc., they provide a constraint on the configuration and lateral extent of crustal layers that reflect thermal and compositional boundaries.

Surface wave analysis of both earthquake and ambient noise seismic data from arrays of broadband seismic stations provides new high resolution images of shear wave velocity of the crust and upper mantle in western North America. In the Yellowstone Hotspot region, new constraints are shown on the high velocity midcrustal layer of the eastern Snake River Plain that represents approximately 10 km of magmatic thickening and subsequent forcing of lower crustal outflow. In the Coast Mountain Batholith area of western British Columbia, the lack of a significant region of high velocities in the lower crust indicates that the foundering of negatively buoyant eclogitic lower crust has been efficient. A high resolution shear velocity model of the Sierra Nevada batholith region finds crustal thickening beneath the batholith, sinking material beneath the central Sierras with adjacent upwelling asthenospheric mantle, and the new image suggests that the Isabella (San Joaquin Valley) anomaly has a quasi-planar NW-SE striking geometry perhaps more consistent with being a Monterey plate slab remnant than an eclogite dominated feature. In addition to the surface wave results, new constraints are found on the sharpness of the 410-km velocity discontinuity via the wave-field continuation approach applied to five regional earthquakes in western North America. The 410-km discontinuity gradient ranges from 7-25 km, indicating the presence of water atop the transition zone in the two regions with large 410 km discontinuity gradient widths.