THE COLUMBIA PLATEAU CENTRAL GRAVITY HIGH AND ITS RELATION TO THE YAKIMA FOLD AND THRUST BELT, WASHINGTON

The Yakima Fold and Thrust Belt (YFTB), located in south-central Washington, includes a series of northwest- to southeast-striking active reverse faults and folds in Miocene flood basalts of the Columbia River Basalt Group (CRBG). The deformed and highly magnetic basalt produces striking aeromagnetic anomalies along the YFTB structures. The strike of the folds and faults form a fan-shaped array, except where they cross a ~70 km wide, north-striking Bouguer gravity anomaly – the Central Gravity High (CGH), where the folds abruptly trend E-W.

Here we investigate the structures causing the YFTB and CGH anomalies by using matched filtering and joint forward-modeling of the high-resolution aeromagnetic and gravity data, constrained by seismic and well log data. The gravity high is interpreted to be an underlying basement high separating north-trending gravity lows over Eocene sedimentary basins. Our results illuminate deeper aspects of the YFTB, showing that the folds and faults penetrate into the sedimentary and basement rocks beneath the CRBG. The majority of deep-seated YFTB structures 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. We suggest that the marked change in strike and character between northerly and southerly YFTB faults reveals 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 could be an expression of a larger kinematic link between the Cascadia fore-arc and back-arc.