NEW FINITE-STRAIN MEASUREMENTS FROM THE OLYMPIC MOUNTAINS: IMPLICATIONS FOR LONG-TERM DEFORMATION IN THE CASCADIA FOREARC

There are two competing interpretations for long-term deformation in the Cascadia forearc. The “Oregon block” interpretation (e.g., Wang, 1996; Wells et al., 1998; McCaffrey et al., 2000) argues that forearc deformation is dominated by margin-parallel transport and clockwise rotation of the western Oregon forearc (aka “Oregon block”). Vancouver Island is considered to a “rigid” buttress. As a result, the northward motion of the Oregon block is thought to be resolved by N-S shortening in the Olympic Mountains (NW Washington State). The “wide wedge” interpretation (Brandon et al., 1998; Pazzaglia and Brandon, 2001; Batt et al., 2001) argues that the Cascadia forearc, from the trench to the east side of the forearc high (Coast Ranges), is underlain by a growing subduction wedge, which is actively shortening in the relative convergence direction.

We report new strain measurements from the accreted rocks in the Olympic Mountains (NW Washington State), which bear on this issue. The Cascadia subduction zone was formed at about 30 Ma. The core of the Olympics exposes trench sediments that were accreted into the Cascadia wedge between 30 and 15 Ma. Erosion there has produced the deepest exhumation in the Cascadia forearc high, reaching ~15 km and metamorphic temperatures of ~300 C. Rocks that have experienced temperatures >150 C show a well-developed, steeply dipping, margin-parallel, pressure-solution cleavage, with an associated down-dip extensional lineation. We have measured absolute finite strain associated with pressure-solution deformation in 58 sandstone samples. The intermediate strain direction, Y, is horizontal and parallel to the margin, and its strain magnitude is zero, indicating a plane-strain deformation. The maximum and minimum strain directions, X and Z respectively, lie in a vertical section that is parallel to the relative convergence direction, which has an azimuth of about 55 degrees. This geometry indicates that deformation was formed by relative convergence and accretion. We see no evidence for the N-S shortening predicted by the block model.