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Title:
The Transition Between N-S and NE-SW Directed Crustal Shortening in the Central and Northern Puget Lowland: New Thoughts on the Southern Whidbey Island Fault
Authors:
Brocher, T. M.; Blakely, R. J.; Wells, R. E.; Sherrod, B. L.; Ramachandran, K.
Affiliation:
AA(U.S. Geological Survey, 345 Middlefield Rd., MS 977, Menlo Park, CA 94025 United States ; ), AB(U.S. Geological Survey, 345 Middlefield Rd., MS 977, Menlo Park, CA 94025 United States ; ), AC(U.S. Geological Survey, 345 Middlefield Rd., MS 977, Menlo Park, CA 94025 United States ; ), AD(U.S. Geological Survey, Dept. Earth & Space Science Box 351310 University of Washington, Seattle, WA 98195 United States ; ), AE(Pacific Geoscience Centre, Geological Survey of Canada 9860 W. Saanich Road, Box 6000, Sidney, BC V8L 4BC Canada ; )
Publication:
American Geophysical Union, Fall Meeting 2005, abstract #S54A-06
Publication Date:
12/2005
Origin:
AGU
AGU Keywords:
8000 STRUCTURAL GEOLOGY, 8002 Continental neotectonics (8107), 8036 Paleoseismology (7221), 8038 Regional crustal structure, 8108 Continental tectonics: compressional
Abstract Copyright:
(c) 2005: American Geophysical Union
Bibliographic Code:
2005AGUFM.S54A..06B

Abstract

We hypothesize that the southern Whidbey Island fault (SWIF) is a NW-SE oriented fold and thrust belt accommodating NE-directed crustal shortening. The SWIF has been considered a dextral strike-slip fault based largely on two interpretations: (1) its northwest orientation in a region believed to be undergoing dominantly N-S compression, and (2) interpretation of industry seismic-reflection data across the SWIF as a flower structure, suggestive of transpressional faulting. Both interpretations require reconsideration based on evidence outlined below. Recent GPS studies (e.g., Miller et al., 2001) have shown that the Puget Lowland is a zone of transition between N-directed compression to the south and NE-SW directed compression (parallel to the plate-convergence vector) to the north. While N-S compression provides an adequate explanation for the E-trending Seattle and Tacoma thrust faults to the south, recent paleoseismic and geophysical studies suggest that NE-SW compression producing NE-directed tectonic wedging (passive roof duplexing) dominates at the SWIF. Evidence for a SW-dipping floor thrust forming the base of the tectonic wedge is provided by gravity and seismic tomography models demonstrating higher structural relief of basement rocks to the south of the SWIF than to its north. Aeromagnetic anomalies, lidar studies, and paleoseismic evidence indicate a broader (about 25 km wide) zone of abundant NE-side-up shallow reverse faults parallel to the SWIF than previously recognized. We interpret these faults as evidence for a zone of NW-oriented, NE-dipping splay faults soling into a shallow (3 to 4 km deep), NE-dipping detachment surface forming the top of the tectonic wedge. We re-examined oil industry seismic-reflection profiles across the SWIF, previously seen as evidence for transpressional faults, and find them more compatible with shallow thrust folds associated with shallow (upper 3 to 4 km) splay faults. In sum, these observations are consistent with a blind NE-vergent wedge tip, accompanied by shallow, NE-dipping roof and splay thrusts of late Holocene age. Available data do not rule out a small component of transpressional faulting along the SWIF. Indeed, paleoseismic excavations at the Utsulady fault, which lies subparallel to and north of the SWIF, reveal late Holocene sinistral slip. Thus, the SWIF may represent the southernmost crustal fault in the central Puget Lowland that responds to plate convergence rather than northward migration of the forearc. To the north, along southern Vancouver Island, other fold and thrust belts (e.g., the Cowichan Fold and Thrust Belt) are clearly oriented perpendicular to NE-directed plate convergence as measured by GPS studies.
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