EOCENE TRANSTENSION AND STRAIN PATTERNS FROM DIKE SWARMS AND STRIKE-SLIP FAULTS IN THE WASHINGTON CASCADES

The Washington Cascades preserve an exceptional shallow to mid-crustal record of Eocene (ca. 49-45 Ma) transtension marked by dextral strike-slip faulting, intrusion of dikes and plutons, rapid non-marine sedimentation, and ductile flow and rapid cooling of crystalline rocks in parts of the North Cascades core. Transtension occurred shortly after collision of the Siletzia oceanic plateau (ca. 50 Ma) and slab rollback and break-off. In the shallow crust, dike swarms and dextral strike-slip faults provide important constraints on the regional strain field. Our measurements of more than 1,000 Eocene dikes indicate a complex strain pattern. The southernmost and most voluminous of the dike swarms is the mafic Teanaway swarm. These dikes (>400) strike NE (average ~030°), thicknesses average ~15 m, and extension reaches as high as 40% in a >7-km-long, across-strike transect. To the north, rhyolitic to basaltic dikes overlap spatially with 49-45 Ma plutons, but in several cases dikes predate the nearby plutons by ca. 500 kyr. NE-striking dikes are widespread, but NW striking ones intrude some domains. The NE-striking dikes are compatible with the dextral motion on the NW-striking (~350-320°) regional faults. Dikes that intruded into, or near these faults, and strike NW may have been controlled by fault-zone anisotropy. Other NW-striking dikes do not fit either of these scenarios and may reflect a different regional strain field. Sparse high-precision (TIMS) U-Pb zircon ages suggest that NE-striking dikes predate many of the NW-trending ones. WNW-ESE extension recorded by the Teanaway dikes and those near the Eocene Cooper Mountain, Duncan Hill, and Golden Horn intrusions is oblique to sub-parallel to the strike of the North Cascades orogen and counter-clockwise to Eocene stretching lineations in the mid-crustal Skagit gneisses. This difference may indicate decoupling across an unknown shear zone. In one scenario, NE-striking dikes intruded during an interval of accelerated strike-slip faulting after plate reorganization and NW-striking dikes record a switch to more arc-normal extension. East of the Cascades, Eocene dikes adjacent to coeval core complexes record E-W extension, suggesting strain partitioning between strike-slip along the plate boundary and E-W extension inboard.