| Summary: | 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.]
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