Eocene Tectonics and Active Deformation in Cascadia

Andrew Miner
March 2002


A distinct middle Eocene mountain-building event has long been recognized in the Pacific Northwest (e.g., Misch, 1966). This orogeny included strong folding, widespread thrust faulting, and approximately 100-200 km of dextral slip on the Fraser-Straight Creek fault system, which is discordant to the preexisting structural fabric. The end of the short-lived event is marked by a pulse of extension in the northern Basin and Range, deep erosionto a profound regional unconformity, and establishment of the Cascade arc along a newnorth-south axis (Hammond, 1979).

In this review I argue that collision of an oceanic plateau, Siletzia, triggered the mid-Eocene orogeny in the Pacific Northwest (Figure 1). This idea is not strictly new: various scenarios for the formation and accretion of Siletzia and its effect on the Eocene landscapeof the region have been offered and debated for many years (Heller et al., 1987; Simpsonand Cox, 1977; Wells et al., 1984). Only recently, however, has the true nature of Siletzia become evident. At the same time, detailed structural and geochronological studies have largely disproved the paradigm that transtension dominated the entire Eocene history of the region. These developments are not widely appreciated, but they require reassessment of regional tectonic models.

This study began as an attempt to better understand the complex pattern of active structures in the region. The tectonics of Washington have previously been interpreted as resulting from strike-slip fault interactions: mostly transtensive in the middle Eocene andtranspressive at present. Resolving the variously fault linkages, however, becomes awkward:roles of individual faults must be assigned on an ad hoc basis and are not regionally consistent (Johnson et al., 1994; Stanley et al., 1996). Both middle Eocene and neotectonic features, however, are consistent with distributed contraction across an oblique margin.