Metamorphism, structure and stratigraphy of the Park Ranges (western Rocky Mountains), British Columbia
Creators: name: Craw, David
Publisher: name: University of Calgary
Year: 1977
Subjects: subject: QE 187 C685 1977 Microfiche
subject: Geology - British Columbia
subject: Geology, Structural
subject: Geology, Stratigraphic
subject: Metamorphism (Geology)

description: Bibliography: p. 129-133.
description: The Southern. Park Ranges lie immediately east of the Rocky Mountain Trench, near the Big Bend of the Columbia River, B.C. They are composed of quartzites, pelites and carbonates in an uninterrupted sequence from the Proterozoic Windermere Supergroup, through the Lower Cambrian Gog Group, to the Middle· Cambrian Tsar Creek and Kinbasket units. Two phases of folding and metamorphism are recognized. The first phase of folding (F1) occurred before and during the first phase of metamorphism (M1), while the second phase of folding (F2) was associated with a second (very minor) phase of metamorphism (M2). F1 is seen only as minor structures at the mesoscopic and microscopic scale, although there is some minor-fold vergence evidence for F1 macroscopic structures. F2 structures are seen on microscopic, mesoscopic, and macroscopic scales and are all apparently associated with the Porcupine Creek Anticlinorium, a major fold complex trending NW-SE across the eastern part of the study area. Commonly, F2 folds are nearly concentric with some thickening of incompetent units in the fold hinges. Associated with F2 folds are several west dipping thrust faults, some of which terminate upwards into the cores of F2 anticlines. All structures are cut by late-stage, west-dipping normal faults. Quartz c-axis orientation data show that a distinct postmetamorphic fabric has been imprinted on quartzites near thrust fault and normal fault zones, confirming that both of these fault types are post-metamorphic. The first phase of metamorphism resulted in a Barrovian sequence of .isograds such that chlori te zone rocks occur in the east and sillimanite zone rocks occur in the west. Phase equilibria studies suggest that the metamorphic temperatures in the area ranged from less than 480°c (east)to 570°c (west) _while under a pressure of approximately 5 kb. The pelitic rocks were studied to determine the nature and sequence of prograde metamorphic chemical reactions within the rocks, and the bearing of these reactions on observed textures (e.g., poikiloblastic kyanite). The reactions are depicted on a T-X (Fe-Mg) phase diagram similar to those proposed by Thompson (1976b). Calculations show that potassium released in the reactions almost certainly remained in the rock, causing potassium enrichment of K-bearing phases. The isograds outline a thermal antiform whose western limb is truncated by the Purcell Fault (a major Cordilleran structure) on the western slopes of the study area. A comparison of main metamorphic phases between the Park Ranges and the Selkirk and Monashee Mountains across the Purcell Fault, suggests that the western (Selkirk-Monashee) terrane was subjected to temperatures approximately 40°C higher than the study area (for equivalent metamorphic zones) and a pressure between 2 and 3 kb higher than the study area. This pressure difference suggests that there has been a minimum of 7 km post-metamorphic vertical motion on the Purcell Fault. A difference in structural style across t:;he Purcell Fault, due to a higher ductility in the western terrane, is considered to be a result of the difference in depth during the deformation of the two terranes.