CENOZOIC PALEOSTRESS EVOLUTION OF THE CASCADE RANGE, WASHINGTON & OREGON

The present-day maximum horizontal compressive stress direction (MHC) in western Washington and Oregon (from earthquakes, well breakouts, Recent structures, etc.) is N14^oE +/- 24^o onshore and roughly east-west in the offshore accretionary complex. Fault-slip and dike trend data from 35 sites in the Cascade Range indicate that this part of the arc can be divided along its length into three segments which share distinct Cenozoic paleostress histories. Cenozoic clockwise tectonic rotations affected the central and southern segments of the arc (″western Oregon block″), but did not affect the northern segment. In the northern segment (47.2-49^oN), the stress field has remained essentially unchanged for the entire period from 0-47 Ma, with MHC averaging about N23^oE (+/-10^o) and maximum tensile stress averaging N67^oW. In the central segment (45-47.2^o N), more northwesterly MHC (N8^oW +/- 14^o, unrotated) was in effect both before and after one early Miocene episode of easterly MHC (N89^oE +/-14^o, unrotated). In the southern segment (43-45^o N), MHC was oriented easterly (N82^oE +/-11^o, unrotated) since at least 40 Ma, changing to the present northerly MHC sometime between 5 and 11 Ma.

For most of the Cenozoic, the Washington Cascade Range experienced northerly MHC, while the Oregon Cascade Range experienced east-west MHC until switching to northerly MHC in the late Miocene (11-5 Ma). The boundary between the two stress regimes moved northward from 45^oN to 47.2^oN during at least one early Miocene episode but retreated to its former position after that episode. These migrations of stress regime boundaries took place during the Cenozoic history of northward migration and clockwise rotation of the western Oregon block, driven by oblique convergence between the Juan de Fuca and North American plates. The northerly MHC in Washington has resulted from driving the western Oregon block northward against stable North America. Cenozoic changes in plate boundary conditions (decreasing convergence rate, age of subducted seafloor, and length of convergent plate boundary) and a late Miocene 45o clockwise rotation of MHC from NNW to NNE in the adjacent Basin and Range province may have resulted in the late Miocene retreat of the easterly MHC stress regime from the entire Oregon arc-forearc to its present offshore position.