Structure, Metamorphism, and Geochronology Along the Southern Margin of the Breakenridge Orthogneiss, Coast Range, Southern British Columbia

Author

John A. (John Andrew) Feltman, Western Washington University

Date Permissions Signed

7-4-2018

Date of Award

Winter 1997

Document Type

Masters Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

Brown, Edwin H.

Second Advisor

Talbot, James L.

Third Advisor

Schermer, Elizabeth, 1959-

Abstract

The Breakenridge orthogneiss is located at the southern end of the Coast Plutonic Complex, in the southwest Canadian Cordillera. It consists of sheeted orthogneiss sills and metamorphosed country rock folded into a tight, upright antiform. The deformational and metamorphic history along the southern margin of this structure is the focus of this study.

The orthogneiss is in original intrusive contact with enveloping metavolcanic rocks of the Jura-Cretaceous Slollicum Schist. A new U-Pb zircon age of 103.8 ± 0.5 Ma, together with a published age of 96 Ma (Parrish and Monger, 1992), establishes an episode of igneous intrusion and crystallization between 104-96 Ma.

Metamorphic grade ranges from greenschist facies (chlorite zone) on the western side of the area, to amphibolite facies (kyanite zone) in the core of the Breakenridge antiform. Metamorphic pressures range from 8 kb in the gneissic core of the antiform over a horizontal distance of 1.25 km, requiring post-metamorphic truncation of the metamorphic gradient. Isobars are roughly concentric about the antiform, suggesting that the structure exposes a window of highgrade metamorphic rocks. New ⁴⁰Ar/³⁹Ar dates on hornblende and micas indicate that cooling began by 87 Ma, providing a minimum age bracket for high-grade metamorphism.

Two deformation events, D₁ and D₂ are recorded in the study area. D₁ structures consist of penetrative foliation and lineation. Foliation strikes NW and dips steeply to the NE. An abrupt change between down-dip and strike-parallel lineations suggests opposing kinematic regimes of arc-normal shortening and arc-parallel strike-slip deformation. D₂ structures record folding of D₁ fabrics in the Breakenridge antiform. The main intrusive contact of the orthogneiss, marker units of politic schist, and D₁ fabrics are folded into a SE-plunging, reclined fold at the south end of the antiform. The doubly-plunging geometry of the antiform as a whole, accompanied by rare, small scale D₂ folds and downdip lineations, suggests post-D₁ shortening and differential upward movement of the high grade metamorphic core as a regional-scale incipient sheath fold.

A post-metamorphic fault, the Breakenridge fault, is inferred on the western side of the study area, based on the abrupt change in pressure, truncation of the oligoclase isograd, different lineation orientations, and the trend of lithologic contacts. The fault, which is best described as a distributed shear zone, may have been active during or after D_². The fault may have formed with the Breakenridge antiform as part of a fault-fold structure. The narrow temporal window in which down-dip and strike-parallel fabrics appear to have formed suggests that strain may have been partitioned in a transpressional tectonic setting. An orogen-scale flower structure model and a detachment partitioning model are proposed to explain the possible temporal and spatial relationship of orogen-normal thrusting and orogen-parallel strike-slip kinematics.

Type

Text

Keywords

Orthogneiss, Geochronology, Coast Plutonic Complex

Publisher

Western Washington University

OCLC Number

1061862440

Subject – LCSH

Geology, Structural--British Columbia--Coast Range; Metamorphism (Geology)--British Columbia--Coast Range; Geological time; Formations (Geology)--British Columbia--Coast Range; Geology, Stratigraphic

Geographic Coverage

Coast Range (B.C.)

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

Feltman, John A. (John Andrew), "Structure, Metamorphism, and Geochronology Along the Southern Margin of the Breakenridge Orthogneiss, Coast Range, Southern British Columbia" (1997). WWU Graduate School Collection. 795.
https://cedar.wwu.edu/wwuet/795