Cascadia’s Mid-Life Crisis: Miocene Changes in the Forearc due to Rotation and Subduction Dynamics

The Cascadia forearc is unique among global subduction zones because of the accretion of a large igneous province (Siletzia) and continuous clockwise rotation of the margin due to oblique subduction of the Juan de Fuca plate. We reconstruct the stratigraphic architecture and sediment accumulation history in the forearc by investigating multiple, along-strike forearc basins. Integrating potential-field models, 2D seismic reflections, and deep borehole data, our 3D mapping allows us to track basin depocenters through space and time, revealing a fundamental structural shift in the Miocene that significantly reshaped the forearc.

During the Paleogene, the Cascadia forearc was a broad, marine basin characterized by high sediment accumulation rates and unrestricted accommodation space. Our results show that during the Miocene in the central forearc (Portland-Tualatin Basin), what was once a single continuous basin was subdivided as transpressional stress structurally inverted older Paleogene normal faults. Similarly, in the Chehalis Basin to the north, clockwise rotation reoriented fault systems relative to the regional stress field, pushing the basin depocenter northward as deformation shifted from northwest-striking to west-striking faults. This structural transition occurred as the outer-arc high (Coast Range) emerged, causing depositional environments to shift from marine to terrestrial. It is also coincided with a steep drop in sediment accumulation rates: a 7-fold decrease in the Chehalis Basin (196 to 27 m/Myr) and a 10-fold decrease in the Portland-Tualatin basins (305 to 29 m/Myr) to the south.

We propose that along-strike variations in subduction geometry also impact basin evolution. In the southern and central forearc, a relatively steep subduction angle and clockwise rotation pushed the outer-arc high close to the magmatic arc, leaving minimal accommodation space. In contrast, shallower subduction to the north near the Seattle Basin maintained a wider separation, allowing high sediment accumulation rates to persist through the Neogene (211 m/Myr).

By integrating basin analysis with regional tectonics, we constrain along-strike variation in Cascadia forearc geometry through the Cenozoic. We find that sustained rotation and the influence of Siletzia basement, not just sedimentary accretion, have controlled the evolution of fault systems and Cascadia forearc deposition. Accordingly, this work provides a framework for understanding the evolution of forearc basins where long-term rotation and strain-partitioning dominate.