IMPLICATIONS OF HIGH RESOLUTION SEISMIC TOMOGRAPHY FOR THE STRUCTURE AND EVOLUTION OF THE PUGET BASINS

New high resolution tomographic images of the Puget lowland region based on earthquake and active source seismic observations show details that provide constraints on models of the basin tectonics and evolution. The basins are underlain by a thick basement sequence of tectonically “strong” Crescent formation. The most recent sequence of basins appears to be superimposed on an older sequence of early Tertiary sedimentary units whose structure is largely uncorrelated with latest phase of basin construction. The perhaps coincidental superposition of older basin structure with the most recent phase of deformation on the Seattle fault may contribute to the extreme geophysical signature of the Seattle fault and basin. The Seattle fault zone exhibits clear evidence of a south dipping thrust displacing the Crescent basement to mid-crustal depths. Evidence of a west dipping thrust bounding the northeast side of the high velocity region south of the Seattle fault also exists in the tomographic images, suggesting that eastward or northeastward convergence also plays an important role in the basin tectonics.

The Puget basins form a semi-circular ring, roughly concentric about the eastern Olympic Mts. A purely compressional model for the origin of the Seattle basin and the Seattle fault is flawed because, in order to isostatically compensate for the large mass deficiency of the Seattle basin, the hanging wall of the Seattle fault (south side) should exhibit significant mountain building which is not observed. A tectonic model of latest-phase basin formation that is consistent with gravity, seismic and geological observations, and that also explains the observed radial distribution of basins with respect to the Olympics, is elastic flexural down-warp in response to uplift and subsequent erosion of the Crescent from the central Olympics. The combination of arc-parallel compression with flexural subsidence of the Seattle basin explains the occurrence of thrust faulting on the Seattle fault in the absence of isostatic loading of the hanging wall.