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Interpretation of the Seattle uplift, Washington, as a passive-roof duplex

Bulletin of the Seismological Society of America

By:
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DOI: 10.1785/012003190

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Abstract

We interpret seismic lines and a wide variety of other geological and geophysical data to suggest that the Seattle uplift is a passive-roof duplex. A passive-roof duplex is bounded top and bottom by thrust faults with opposite senses of vergence that form a triangle zone at the leading edge of the advancing thrust sheet. In passive-roof duplexes the roof thrust slips only when the floor thrust ruptures. The Seattle fault is a south-dipping reverse fault forming the leading edge of the Seattle uplift, a 40-km-wide fold-and-thrust belt. The recently discovered, north-dipping Tacoma reverse fault is interpreted as a back thrust on the trailing edge of the belt, making the belt doubly vergent. Floor thrusts in the Seattle and Tacoma fault zones, imaged as discontinuous reflections, are interpreted as blind faults that flatten updip into bedding plane thrusts. Shallow monoclines in both the Seattle and Tacoma basins are interpreted to overlie the leading edges of thrust-bounded wedge tips advancing into the basins. Across the Seattle uplift, seismic lines image several shallow, short-wavelength folds exhibiting Quaternary or late Quaternary growth. From reflector truncation, several north-dipping thrust faults (splay thrusts) are inferred to core these shallow folds and to splay upward from a shallow roof thrust. Some of these shallow splay thrusts ruptured to the surface in the late Holocene. Ages from offset soils in trenches across the fault scarps and from abruptly raised shorelines indicate that the splay, roof, and floor thrusts of the Seattle and Tacoma faults ruptured about 1100 years ago.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Interpretation of the Seattle uplift, Washington, as a passive-roof duplex
Series title:
Bulletin of the Seismological Society of America
DOI:
10.1785/012003190
Volume
94
Issue:
4
Year Published:
2004
Language:
English
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Bulletin of the Seismological Society of America
First page:
1379
Last page:
1401
Number of Pages:
23