The Cascadia “string of pearls”—Forearc basins of the Salish lowland and implications for the tectonic evolution and earthquake hazards of the Pacific Northwest

Abstract:

The forearc of the Cascadia convergent margin hosts an alignment of sedimentary basins and intervening uplifts extending along the Salish lowland from the Canadian Coast Mountains to NW Oregon. The basins evolved after Eocene emplacement of Siletzia, a basaltic terrane of oceanic origin that now forms the basement, and they continue to evolve today, as evidenced by active crustal faults that bound their margins. The basins are best developed near the Canadian border, where they form a “string of pearls” around the Olympic Mountains and where gravity anomalies indicate that Siletzia is broken into discrete blocks, the result of northward motion of the forearc against slower moving Canada.

Although the Salish lowland and its basins are primarily related to subduction, the basins have been modified by forearc block motions that reveal contrasting tectonic histories. The Seattle basin in Washington forms the footwall of the E-striking Seattle fault, a N-verging thrust fault responsible for a M7 earthquake circa 900 AD. The Seattle basin is filled with Oligocene and younger sediments above Siletzia basement in contact with an accretionary prism, a combination that amplifies low-frequency ground motions during earthquakes. Inverse models based on gravity data and seismic tomography illuminate an asymmetric basin ~9 km deep at its deepest point. High-resolution aeromagnetic anomalies provide a proxy for geologic mapping of hanging wall geology, thereby illuminating active strands of the Seattle fault.

The Tualatin basin lies SW of Portland, Oregon, sandwiched between the active Gales Creek fault to the SW and the Oatfield and Portland Hills faults to the NE. Our three-dimensional model based on >3000 gravity measurements suggests that the Tualatin basin formed as a pull-apart basin between these overlapping strike-slip faults. The Tualatin basin is floored by Siletzia at 6 km depth and mantled with up to 1 km of deformed Miocene flood basalts of the Columbia River Basalt Group (CRBG), indicating that CRBG is underlain by up to 5 km of post-Eocene sediments. Thus, the Tualatin basin began its pull-apart evolution long before CRBG emplacement and continues to evolve today as the Gales Creek and other faults in the area accommodate modern deformation of CRBG and younger sediments in response to clockwise crustal rotation.