Kathy Goetz Troost

The Cordilleran Ice Sheet

Publisher Summary This chapter discusses the advances in both global and regional understanding of Quaternary history, deposits, and geomorphic processes that have brought new information and new techniques for characterizing the growth, decay, and products of the Cordilleran Ice Sheet during the Pleistocene. The Cordilleran Ice Sheet, the smaller of two great continental ice sheets that covered North America during Quaternary glacial periods, extends from the mountains of coastal south and southeast Alaska, along the Coast Mountains of British Columbia and into northern Washington and northwestern Montana. Ice has advanced south into western Washington at least six times, but the marine-isotope record suggests that these are but a fraction of the total that entered the region in the past 2.5 million years. Reconstruction of the Puget lobe of the Cordilleran Ice Sheet during the last glacial maximum requires basal sliding at the rates of several hundred meters per year, with pore-water pressures nearly that of the ice overburden. Landforms produced during glaciation include an extensive low-gradient outwash plain in front of the advancing ice sheet, a prominent system of subparallel troughs deeply incised into that plain and carved mainly by subglacial meltwater, and widespread streamlined landforms.

Magnetostratigraphy, paleomagnetic correlation, and deformation of Pleistocene deposits in the south central Puget Lowland, Washington

[1] Paleomagnetic results from Pleistocene sedimentary deposits in the central Puget Lowland indicate that the region has experienced widespread deformation within the last 780 kyr. Three oriented samples were collected from unaltered fine-grained sediments mostly at sea level to determine the magnetostratigraphy at 83 sites. Of these, 47 have normal, 18 have reversed, and 18 have transitional (8 localities) polarities. Records of reversed- to normal-polarity transitions of the geomagnetic field were found in thick sections of silt near the eastern end of the Tacoma Narrows Bridge, and again at Wingehaven Park near the northern tip of Vashon Island. The transitional horizons, probably related to the Bruhnes-Matuyama reversal, apparently fall between previously dated Pleistocene sediments at the Puyallup Valley type section (all reversed-polarity) to the south and the Whidbey Island type section (all normal-polarity) to the north. The samples, in general, are of sufficient quality to record paleosecular variation (PSV) of the geomagnetic field, and a statistical technique is used to correlate horizons with significant agreement in their paleomagnetic directions. Our data are consistent with the broad structures of the Seattle uplift inferred at depth from seismic reflection, gravity, and aeromagnetic profiles, but the magnitude of vertical adjustments is greatly subdued in the Pleistocene deposits. INDEX TERMS: 1520 Geomagnetism and Paleomagnetism: Magnetostratigraphy; 1522 Geomagnetism and Paleomagnetism: Paleomagnetic secular variation; 1525 Geomagnetism and Paleomagnetism: Paleomagnetism applied to tectonics (regional, global); 1535 Geomagnetism and Paleomagnetism: Reversals (process, timescale, magnetostratigraphy); KEYWORDS: Puget Lowland, Pleistocene, magnetostratigraphy, paleosecular variation, tectonic deformation

Kinematics of shallow backthrusts in the Seattle fault zone, Washington State

Near-surface thrust fault splays and antithetic backthrusts at the tips of major thrust fault systems can distribute slip across multiple shallow fault strands, complicating earthquake hazard analyses based on studies of surface faulting. The shallow expression of the fault strands forming the Seattle fault zone of Washington State shows the structural relationships and interactions between such fault strands. Paleoseismic studies document an ∼7000 yr history of earthquakes on multiple faults within the Seattle fault zone, with some backthrusts inferred to rupture in small (M ∼5.5–6.0) earthquakes at times other than during earthquakes on the main thrust faults. We interpret seismic-reflection profiles to show three main thrust faults, one of which is a blind thrust fault directly beneath downtown Seattle, and four small backthrusts within the Seattle fault zone. We then model fault slip, constrained by shallow deformation, to show that the Seattle fault forms a fault propagation fold rather than the alternatively proposed roof thrust system. Fault slip modeling shows that back-thrust ruptures driven by moderate (M ∼6.5–6.7) earthquakes on the main thrust faults are consistent with the paleoseismic data. The results indicate that paleoseismic data from the back-thrust ruptures reveal the times of moderate earthquakes on the main fault system, rather than indicating smaller (M ∼5.5–6.0) earthquakes involving only the backthrusts. Estimates of cumulative shortening during known Seattle fault zone earthquakes support the inference that the Seattle fault has been the major seismic hazard in the northern Cascadia forearc in the late Holocene.