Investigating Relationships Between the Yakima Fold and Thrust Belt and Basement Structure in Washington State Using Matched Filtering of Gravity and High-Resolution Aeromagnetic Data

Abstract:

The Yakima Fold and Thrust Belt (YFTB), located in the Cascadia backarc of south-central Washington, deforms Miocene flood basalts of the Columbia River Basalt Group (CRBG). A key element in assessing the earthquake hazard of YFTB structures is the degree to which faults penetrate through the CRBG into underlying basement rocks. To help make this evaluation, we systematically applied matched filtering to high-resolution aeromagnetic and gravity data throughout the YFTB to determine pre-Tertiary basement depths beneath CRBG. The depths to the top of basement thus determined range from 0 to ~4 km and are in general agreement with CRBG isopachs derived from a sparse (~7) number of deep wells. Our analysis shows a complex basement topography that is correlated with many YFTB structures. Deep magnetic sources illuminated by matched filters indicate that YFTB faults continue through the CRBG and into basement, with the more southerly faults extending to depths of up to 11 km. Gravity data show changes in strike orientation of YFTB faults as they traverse a ~60 km-wide N-S gravity high caused by an older structure beneath the CRBG. Our results show that the majority of YFTB structures are deep-seated but differ in character north and south of the Olympic-Wallowa Lineament (OWL), a regional WNW alignment of structures that bisects the YFTB and continues northwestward to the Puget Lowland. The magnetic anomalies of the southerly faults (Columbia Hills, Horse Heaven Hills, Toppenish Ridge, and Ahtanum Ridge) have higher amplitudes, have deeper sources, and are more continuous than the northerly faults (Umtanum Ridge, Saddle Mountain, and Frenchman Hills). The southerly faults strike W to WSW sub parallel to the Columbia River and potentially extend farther west to intersect with the Mount Saint Helens seismic zone and the Willamette Valley, while northerly faults strike NW sub parallel to the OWL with the Umtanum Ridge fault being the southern boundary. We suggest that the marked change in strike and character between northerly and southerly YFTB faults manifests a fundamental crustal boundary that is aligned WNW with the OWL and White River-Naches fault zone. The alignment of this crustal boundary supports suggestions by others that the White River-Naches fault zone may be a kinematic link between Cascadia’s backarc and forearc.