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Date Permissions Signed
Date of Award
Master of Science (MS)
Brown, Edwin H.
Babcock, R. Scott (Randall Scott)
Christman, Robert A. (Robert Adam), 1924-
The structural geology of the Mt. Shuksan area is dominated by the Shuksan thrust fault. This fault juxtaposes the mid to late Paleozoic Chilliwack Group with the structurally overlying, early Cretaceous Shuksan Metamorphic Suite. The Shuksan thrust fault is a complex imbricate zone, approximately 1.5 km wide in map view, in which rocks of the Shuksan Suite and Chilliwack Group are imbricated with lesser amounts of exotic tectonic slices. Mesoscopic structures are chaotic and mostly disordered although a dominant shear fabric is pervasive. The macroscopic structure of the fault zone is relatively simple: it forms a shallowly-dipping plane with a general attitude of N30°-35°W, 25°-35°NE. The low-angle structures are slightly modified by Tertiary(?) high-angle faults and shear zones which exhibit small displacements. No evidence of a "root zone", as postulated by Misch (1966), was discovered in this study.
The imbricate zone is composed of tectonic slices and fragments of exotic and autoclastic rocks, commonly in a sheared pelitic matrix or in other sheared rock of the Chilliwack Group. Exotic slices present include: serpentinite, ultramafic tectonite (forsterite-tremolite-talc rock), schist of the Vedder Complex, blueschist termed the Baker Lake blueschist in this study, and meta-hornblende gabbro of uncertain affinity. Forsterite-tremolite-talc rock contains a mineral assemblage indicative of the upper amphibolite facies of regional metamorphism. It, and the meta-hornblende gabbro, are the only rock units of the tectonic slices that occur north of Day Creek. The other tectonic slices occur in the south of the study area in association with a possible large tectonic fragment of titanaugite-bearing meta-basalt and meta-chert which may be correlative with the Haystack Mountain unit of Cruver (1983).
Radiometric dating of albite-muscovite schist similar to that on Vedder Mountain yields a K/Ar date of 274 ± 9 Ma and a Rb/Sr date of 273 ± 6 Ma. These ages are within the range of dates for the Vedder Complex (Armstrong and others, 1983) and suggest correlation of this schist with the Vedder Complex. In addition, petrographic, chemical, and mineralogic data suggest that other barroisitic-amphibole-bearing coarse schists from the study area are also correlative with the Vedder Complex.
A blueschist fragment, the Baker Lake blueschist, contains the distinctive mineral assemblage: crossite + lawsonite + chloromelanite. This key assemblage suggests metamorphism at ~250°C at ~6.5 kbars. Chemical, mineralogical, and petrographic characteristics of this blueschist suggest that it is not correlative with other known high pressure rock units of the western North Cascades.
Shuksan Thrust Fault
Western Washington University
Subject – LCSH
Petrology--Washington (State)--Shuksan, Mount, region; Geology, Structural
Shuksan, Mount, region (Wash.)
Copying of this document in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this thesis for commercial purposes, or for financial gain, shall not be allowed without the author's written permission.
Leiggi, Peter A., "Structure and Petrology along a Segment of the Shuksan Thrust Fault, Mount Shuksan Area, Washington" (1986). WWU Graduate School Collection. 837.
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Plate III A
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Plate III B
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Plate III C
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Plate III D