HR: 0800h
AN: T41B-0590    [Abstracts]
TI: Long-Lived Crustal Structures and Their Role in the Tectonic Evolution of the Cascadia Backarc
AU: * Glen, J M
EM: jglen@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States
AU: Blakely, R J
EM: blakely@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States
AB: The backarc region of the Cascadia forearc in Oregon and northern California displays a northeast-trending crustal fabric evident in regional gravity data, seismicity, fault distribution, Cretaceous plutonism, Mesozoic accreted terranes, and Quaternary volcanism. These associations involve rocks of diverse ages and origins and indicate that long-lived crustal structures may have influenced backarc tectonic evolution and may be a natural consequence of clockwise rotation and westward migration of the southern Cascade forearc. A series of northeast-striking elongate gravity lows are aligned to Cascade-arc volcanism and crustal seismicity. We suggest that these lows indicate an internal fabric in the pre-Tertiary basement that has focused Neogene magma transport and faulting occurring in the wake of the migrating volcanic arc. In a broader sense, they aided partitioning of strain associated with east-west Basin-and-Range extension and right lateral shear ultimately related to North American-Pacific plate interactions. This wide region of deformed crust, which we refer to as the Harney Basin deformation zone (HBDZ), appears to be accommodating strain between rigid blocks of the Sierra Nevada to the south and the coherent Oregon forearc to the north. The southern edge of the HBDZ coincides with a young physiographic depression, which in turn marks a major terrane boundary, juxtaposes distinct Tertiary volcanic stratigraphy, coincides with an active tectonic boundary, and parallels a belt of Cretaceous plutonism. The northern boundary of the HBDZ is marked by the Klamath-Blue Mountains lineament that lies along the southern edge of the Blue Mountains province, an assemblage of terranes accreted in Jura-Cretaceous time that itself forms a northeast-elongate block. Taken together, these associations suggest that the HBDZ has an inherited crustal fabric of northeast trending structures that has influenced multiple episodes of magmatic and tectonic activity since terrane accretion, circa Eocene time. One possible model that describes the role these structures play in guiding backarc deformation indicates that the entire HBDZ may have undergone significant CW rotations similar to those observed in the Blue Mountains.
DE: 1219 Gravity anomalies and Earth structure (0920, 7205, 7240)
DE: 1517 Magnetic anomalies: modeling and interpretation
DE: 3001 Back-arc basin processes
DE: 8110 Continental tectonics: general (0905)
DE: 8178 Tectonics and magmatism
SC: Tectonophysics [T]
MN: 2007 Fall Meeting