Metamorphism, structure and stratigraphy in the Southern Park Ranges, British Columbia

Abstract

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 Chancellor Group.Two phases of folding and metamorphism are recognized. The first phase of folding (F1) occurred before and during the first phase of metamorphism (M1), whereas the second phase of folding (F2) was associated with a second (very minor) phase of metamorphism (M2). Both F1 and F2 structures are apparently associated with the rise of the Porcupine Creek Anticlinorium, a major fold complex trending northwest–southeast across the eastern part of the study area. Associated with F2 folds are several west-dipping thrust faults, some of which terminate upwards in the cores of F2 anticlines. All structures are cut by late-stage, west-dipping normal faults.The first phase of metamorphism (M1) resulted in a Barrovian sequence of isograds such that chlorite 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) at a pressure of approximately 5 kbar (5 × 105 kPa).The M1 isograds in the Park Ranges 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 underwent temperatures approximately 40 °C higher than the study area (for equivalent metamorphic zones) and a pressure between 2 and 3 kbar (2 × 105 and 3 × 105 kPa) 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 the Purcell Fault, due to a higher ductility in the western terrane, is interpreted as a result of the difference in depth during the deformation of the two terranes.