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Date Permissions Signed

10-14-2019

Date of Award

Spring 1991

Document Type

Masters Thesis

Department or Program Affiliation

Geology Department

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

Talbot, James L.

Second Advisor

Babcock, R. Scott (Randall Scott)

Third Advisor

Brown, Edwin H.

Abstract

The Holden area, in the Crystalline Core of the North Cascades, contains deformed and undeformed plutons ranging from Triassic to Eocene in age.

Two deformational styles have been identified in the Holden area. Contractional deformation is indicated by steeply-plunging, down-dip, mineral and stretching lineations in the syn-tectonic Seven Fingered Jack pluton (estimated midLate Cretaceous), and in the pre-tectonic Dumbell Mountain pluton (220 Ma), associated with apparent flattened fabrics. Strike-slip shear deformation is represented by gently-plunging to subhorizontal, strike-parallel mineral and stretching lineations, which are commonly associated with constricted fabrics. These fabrics occur prominently in the Dumbell Mountain pluton, and crosscut successive intrusive phases in the syn-tectonic root of the Duncan Hill pluton (47-45 Ma emplacement age). Thus, contractional and strike-slip styles of deformation may have been active from mid-Late Cretaceous to early Tertiary in the Holden area. Deformation of uncertain style was active in the mid-Late Cretaceous in the Cardinal Peale pluton (73 Ma), evidenced by: 1) an elongate map pattern; 2) xenoliths aligned parallel to pluton contacts; and 3) a northwest striking foliation defined by recrystallized textures, all of which are aligned parallel to the foliation in the country rock.

Metamorphism reached the amphibolite facies in the Holden area as indicated by the assemblage hornblende + oligoclase + garnet. A few chlorite + albite + epidote assemblages and local alteration of biotite, hornblende, and garnet to chlorite reflect overprinting by a pervasive low-grade metamorphic event. Steeply-plunging lineation and strike-parallel lineation are defined by amphibolite facies assemblages, indicating that medium-to high-grade metamorphism was coeval with deformation. Strike-parallel lineations defined by both ductile and brittle fabrics with associated chlorite are probably related to deformation associated with later low-grade metamorphic conditions.

Existing models for the structural development of the Cascades include a collisional model resulting in essentially SW-NE directed contraction (Brandon and Cowan, 1985), and a strike-slip or transpressional model resulting in NW-SE lineations with low plunge (Brown and Talbot, 1989). Data for the Holden and surrounding areas indicates that both contraction and strike-slip shear deformation were active at the same time. It is possible that oblique convergence from 80 Ma to 43 Ma was partitioned into orogen-normal and orogen-parallel components to produce coeval contractional and strike-slip shear structures. The development of two structural styles in the Holden area emphasizes the need to incorporate both strike-slip and contractional processes as important deformation mechanisms in the orogenesis of the Crystalline Core of the North Cascades.

Type

Text

Keywords

North Cascades, Deformed plutonic rocks, Undeformed plutonic rocks

Publisher

Western Washington University

OCLC Number

1140971842

Subject – LCSH

Geology, Structural--Washington (State)--Cascade Range; Rock deformation--Washington (State)--Cascade Range; Metamorphic rocks--Washington (State)--Cascade Range; Intrusions (Geology)--Washington (State)--Cascade Range

Geographic Coverage

Cascade Range; Washington (State)

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 document for commercial purposes, or for financial gain, shall not be allowed without the author’s written permission.

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