STRIKE-SLIP FAULTS IN THE WESTERN COLUMBIA RIVER FLOOD BASALT PROVINCE, OREGON AND WASHINGTON

Northwest-trending high angle faults now number in the hundreds in the 340 km-wide interval from the Willamette Valley eastward to Umatilla (123-119^o W. Long.). Many of these have been confirmed to be right-lateral strike-slip (RLSS) faults and, in terms of kinematic linkage, it is probable that most of the rest are as well. Some faults are only a few kilometers long whereas others are regional in extent. Eight RLSS faults are currently known to have mapped lengths greater than 100 km; conjugate NE trending left-lateral strike-slip faults have also been identified but are comparatively uncommon. Stress fields vary locally due to changes in fault trends with respect to a NNW-SSE directed regional compressive stress field. Fault segments at a high angle to the regional stress field experience “transpression” while those at a low angle experience “transtension.” Structures formed under transpression include: en echelon sets of asymmetric domical folds (some with thrusts); en echelon faults and tension cracks. Structures formed under transtension include: grabens and horsts; and en echelon faults. Where younger units of the Columbia River Basalt Group (CRBG) are present, nearer to the center of the Columbia Plateau, the fault zones tend to be composed of discontinuous en echelon folds and faults indicating an immature strain pattern lacking interconnection. Where exposed the surface structures root downward into a single through-going zone. Where Yakima fold (YF) structures are intersected, profound changes in fault and fold patterns occur on either side of the RLSS fault indicating that the intersecting fault is functioning as an accommodation structure. In some cases there are vergence reversals, such as at the intersection of the Laurel fault and the Columbia Hills anticline. Such reversals are extremely rare in thrust belts around the world. Eruptive vents in the Simcoe (SMV) back-arc volcanic field and Boring Lavas (BL) are often aligned along RLSS faults; also, recent seismicity with compatible focal mechanisms has occurred along some faults. Syndepostional movement of RLSS faults occurred during CRBG emplacement (e.g. Laurel Fault & Portland Hills) and after the period of SMV & BL emplacement; Pleistocene deposits are known to have been cut by both the Luna Butte Portland Hills Faults.