Snake River Plain Yellowstone Volcanic Province PDF Download

The Yellowstone caldera, part of the late Quaternary Yellowstone volcanic field, exhibits contrasting deformation styles with the adjacent Hebgen Lake fault, site of a 1959 M(L) 7.5 earthquake, and the eastern Snake River Plain, the 16 Ma-old bimodal rhyolite-basalt volcanic field that was created as the North American continent passed over the Yellowstone hotspot. The Yellowstone volcanic field is at the eastern boundary of the Basin and Range tectonic province and experiences both regional extension and volcanic activity. 1987-1995 GPS measurements by the University of Utah, along with 1 973-1987 U.S. Geological Survey trilateration data for the Hebgen Lake fault and 1990 National Geodetic Survey GPS data for the Snake River Plain, were combined to evaluate the contemporary deformation of this volcanic system. Data from these sources indicate that the overall deformation is extensional with local variations in the Yellowstone caldera, the Hebgen Lake fault zone, and the Snake River Plain. The extension rate is about 5 mm/yr in a NE-SW direction across the Yellowstone -Hebgen Lake region; when the Snake River Plain is included, extension rates increase to 6.7 mm/yr across the entire Yellowstone-Snake River Plain volcanic system. The local variations in the regional pattern include: 1) horizontal compression of 3.6 mm/yr and subsidence up to 15 mm/yr within the caldera, 2) northeast extension of 2.4 mm/yr across the Hebgen Lake fault, and 3) extension rates of 1.9 mm/yr in the NE-SW direction in the central part of the Snake River Plain. In the caldera, the principal horizontal deformation is radially compressive. Caldera subsidence was observed to be slowing from 1993 to 1995, with a possible return to uplift in 1995. The Yellowstone caldera had a more heterogeneous strain field than the nearby fault, which is likely due to local deformation from magmatic and hydrothermal processes. At the Hebgen Lake fault, only 10 km from the contracting caldera, the principal deformation is extensional. Extension has slowed from 4.0 to 2.4 mm/yr between 1973 and 1995, indicating that viscoelastic processes may control strain release following the 1959 earthquake. The different deformation styles imply a sharp stress transition between the Yellowstone caldera and Hebgen Lake fault zone where stress orientations rotate and magnitudes change sign over a distance of only a few kilometers while strain rates remain small. This transition also corresponds to the area of greatest seismicity in the entire Yellowstone-Hebgen Lake region. To compare seismic and aseismic deformation, earthquake magnitudes from the Yellowstone seismic catalog were converted to scalar seismic moment rates, and the GPS-derived strain rates were converted into geodetic moment rates. Seismic moment rates reflect slip on faults and geodetic moment rates reflect total deformation, so the difference between the two types of moment rates gives an estimate of the amount of aseismic deformation. Most of the scalar seismic moment in Yellowstone is released north of the caldera, while most geodetic moment is released in the Hebgen Lake fault zone or caldera. The data show that the Hebgen Lake fault and Yellowstone caldera are undergoing primarily aseismic deformation. In the case of the Hebgen Lake faulty, this is probably the result of the regional extensional stress combined with viscoelastic effects following the 1959 earthquake. For the caldera, deformation is likely due to magmatic and hydrothermal sources. The high seismicity and deformation rates of the Yellowstone-Hebgen Lake region are contrasted with the seismically quiescent eastern Snake River Plain, where strain rates are an order of magnitude less than the Hebgen Lake fault zone or Yellowstone caldera. The Snake River Plain is part of the eastern Basin and Range, and the extension data are consistent with sllip directions and extension rates inferred for large, north and south of the plain. This suggests that Basin-Range extension probably controls Snake River Plain deformation, although the absence of surface faulting and historic earthquakes indicates that seismic slip is not the mechanism by which extension is accommodated.