GEOPHYSICAL DOMAINS OF THE OREGON ARC AND BACKARC AND THEIR TECTONIC SIGNIFICANCE

Regional gravity data from the Pacific Northwest show a northeast-trending crustal fabric also reflected in seismicity, faulting, Quaternary volcanism, the distribution of Mesozoic accreted terranes, and Cretaceous plutonism. These associations suggest that pre-existing structures played a crucial role in determining the tectonic response of large crustal domains. The southern Cascade arc marks the trailing edge of the coherent westward migrating Oregon forearc rotating clockwise with respect to stable North America (Wells and Simpson, 2000). Motion of the forearc arises from its unique position with respect to the subducting Juan de Fuca plate and impingement of the northwestward migrating Sierra Nevada block. Regional gravity data from this area show broad crustal features that reveal the internal fabric left in the wake of the rotating forearc. Based on these features, we have identified a region of crust that we refer to as the Harney Basin deformation zone (HBDZ) that appears to be accommodating strain between rigid blocks of the Sierra Nevada to the south and the Oregon forearc to the north. The HBDZ is characterized by a series of northeast-striking elongate gravity lows spatially tied to Cascade-arc volcanism and crustal seismicity, suggesting that these are zones of weakened crust that focused magma transport and faulting and aided partitioning of strain associated with east-west basin-and-range extension and right lateral shear ultimately related to North American-Pacific plate interactions (Blakely et al., 1997). The southern edge of this deforming zone coincides with a young physiographic depression. It also marks a major terrane boundary, juxtaposes distinct Tertiary volcanic stratigraphy, coincides with an active tectonic boundary, and parallels a belt of Cretaceous plutonism. Taken together, these associations suggest that the southern edge of the HBDZ represents a significant crustal discontinuity reactivated during multiple episodes of magmatic and tectonic activity since terrane accretion. Likewise, 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 Jurra-Cretaceous time that itself forms a northeast-elongate block.