HR: 11:50h
AN: U32A-07    [Abstracts]
TI: Paleomagnetic Rotations in the Northwest U.S. - Setting the Stage for Contemporary Deformation Models of the Cascadia Convergent Margin
AU: * Wells, R E
EM: rwells@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd. MS 973, Menlo Park, CA 94025 United States
AB: In 1957 Allan Cox reported in Nature that the paleopole for the Eocene Siletz River Volcanics (SRV) of the Oregon Coast Range lay far to the east of the geocentric axial dipole, an oddity that defied adequate explanation for nearly two decades. Subsequent paleomagnetic studies by Myrl Beck, Allan Cox, their armies of students, and colleagues from the USGS, GSC, UCSC, and other institutions demonstrated that much of the central Cascadia forearc and arc had undergone large clockwise rotations during Cenozoic time. Between 1976 and 1989, paleomagnetists assembled a remarkable data set from more than 30 studies comprising approximately 1500 sites, that showed that rotation was largest in the Oregon Coast Range (OCR), and decreased northward into Canada, southward into California, and eastward into the backarc. Three competing models were proposed to explain the rotations, in part influenced by the oceanic basaltic origin of the SRV: 1) rotation during accretion of an allochthonous oceanic terrane; 2) rotation during Basin and Range extension; and 3) rotation due to dextral shear between N. America (NA) and northward-moving oceanic plates to the west. Onlap relations determined from geologic mapping of the rotated terranes showed that rotation during accretion was insignificant, and that the argument between those preferring microplate rotation (e.g., Willamette plate of Magill et al.) and those preferring distributed shear (e.g., Columbia River area, S. Sheriff) depended on the presence or absence of accommodating strike slip faults. In 1988, members of the Northwest paleomagnetic community attended the NATO meeting on Paleomagnetic Rotations and Continental Deformation, where McKenzie, Kiessel, Laj, and others argued that in the Aegean region, block rotations could be linked to contemporary deformation and seismicity. Subsequently England and Wells successfully modeled Cascadia's westward increase in rotation along the Columbia River as due to deformation of a thin, quasi-viscous lithospheric sheet by oblique subduction. At about the same time, Beck and Christiansen noted that regions of high moment release in the great Rat Islands earthquake in the western Aleutians (Mw 8.7) coincided with the centers of large rotating fore-arc blocks. Recognizing that block rotations could be related to seismicity in Cascadia, Wells, Weaver and Blakely linked the rotation of the OCR to the geodetically determined motion of the Sierra Nevada block (SN) to calculate a pole of rotation and velocity field for the OCR. The model combined plate-like behavior for the SN, OCR, and stable NA with intervening zones of internal deformation in the Klamath Mts., northern Cascadia forearc, and the arc. The rotation pole lies nearby in northeastern Oregon. Calculated forearc velocities are consistent with the rate and direction of extension in the Basin and Range province, the change from extension in the southern Cascade arc to compression in the northern arc, and the northward shortening and crustal seismicity in western Washington as it is compressed against the Canadian Coast Mountains buttress. The model provides an independent measure of the secular motion of crustal blocks that is useful for comparison with new GPS-derived models incorporating elastic deformation of the subduction zone.
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
SC: Union [U]
MN: 2004 AGU Fall Meeting