Structure and Petrology along a Segment of the Shuksan Thrust Fault, Mount Shuksan Area, Washington

Author

Peter A. Leiggi, Western Washington University

Date Permissions Signed

9-19-2018

Date of Award

Fall 1986

Document Type

Masters Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

Brown, Edwin H.

Second Advisor

Babcock, R. Scott (Randall Scott)

Third Advisor

Christman, Robert A. (Robert Adam), 1924-

Abstract

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.

Type

Text

Keywords

Shuksan Thrust Fault

Publisher

Western Washington University

OCLC Number

1083724534

Subject – LCSH

Petrology--Washington (State)--Shuksan, Mount, region; Geology, Structural

Geographic Coverage

Shuksan, Mount, region (Wash.)

Format

application/pdf

Genre/Form

masters theses

Language

English

Rights

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.

Recommended Citation

Leiggi, Peter A., "Structure and Petrology along a Segment of the Shuksan Thrust Fault, Mount Shuksan Area, Washington" (1986). WWU Graduate School Collection. 837.
https://cedar.wwu.edu/wwuet/837