Kinematics, age of deformation, and regional significance of the Cayoosh Creek fault, Lillooet, British Columbia
Creators: name: Smith, Letitia Maile
Publisher: name: University of British Columbia
Year: 2009

description: The Cayoosh Creek fault, located in the eastern Coast Belt of the Canadian Cordillera, is a low-angle fault truncated by the Marshall Creek fault to the east and the Phair Creek fault to the west. The Marshall Creek and Phair Creek faults are associated with the regionally significant Yalakom and Fraser-Straight Creek fault systems. The upper plate of the Cayoosh Creek fault contains Mississippian to lower Middle Jurassic units of the Bridge River Complex which overlie Middle Jurassic to Lower Cretaceous rocks of the Cayoosh Assemblage. The hangingwall is typically prehnite-pumpellyite to lower greenschist facies, whereas footwall rocks record mid- to upper greenschist grade metamorphism. While the older-over-younger relationship implies contraction, the juxtaposition of low over high metamorphic grade suggests normal faulting. This study seeks to associate the kinematic history and age of deformation of the Cayoosh Creek fault with that of the regional fault systems. Three phases of deformation have been documented in the study area. D1 is represented by related southwest vergent isoclinal folds and foliation (S1). A shallow northeast-dipping mylonitic foliation (S2), and northwest-trending folds and lineations define D2. Peak metamorphism during D2 is constrained to 500°-550° C and 1.8-2.9 kilobars. Steep to upright axial planes of northwest-southeast trending open and gentle folds define a possible third deformational event (D3). Zircons from a post-kinematic dike, a D2 synkinematic dike, and locally deformed granite were dated using U-Pb radiogenic isotopes. Results yielded ages of 47.0 +/- 0.2 Ma, 47.8 +/- 0.1 Ma, and 48.8 +/- 0.1 Ma respectively. The D1 fabrics record mid-Cretaceous (?) southwest-directed contraction and emplacement of Bridge River Complex over Cayoosh Assemblage. The penetrative D2 fabrics are the result of 10-12 kilometers of top-towards the northwest extension on the Cayoosh Creek fault. D2 largely occurred between 48.8-47.0 Ma, but may have commenced earlier. D3 fabrics may have been related to progressive D2 deformation. The Cayoosh Creek fault is integrally related to the dextral strike-slip fault systems on the Coast Belt- Intermontane Belt boundary. Orogen-parallel extension and translation were the dominant means of accommodation during Early to Middle Eocene oblique-convergence on the plate margin. The Cayoosh Creek fault—as well as the Tatla Lake metamorphic complex, Mission Ridge area, and Ross Lake shear zone—lies within a belt of high metamorphic grade and low-angle ductile deformation that coincides with the axis of the Eocene magmatic arc. The likely cause of regional extension was gravitational collapse of an overthickened and thermally weakened crust, and a shift in relative plate motions. Significant deformation on the Cayoosh Creek fault had diminished by 47.0 Ma but regional strike-slip faulting in the arc-axial belt continued into the Oligocene. [Scientific formulae used in this abstract could not be reproduced.]