Structural geology of the Columbia River fault zone at Revelstoke, British Columbia

McNeill, Paul Douglas

Structural geology of the Columbia River fault zone at Revelstoke, British Columbia

Date

1998

Authors

McNeill, Paul Douglas

Publisher

University of New Brunswick

Abstract

The Columbia River fault zone forms the eastern margin of the Shuswap Metamorphic complex in southern B1itish Columbia as pai1 of the Omineca Belt of the Cordillera Orogeny. This is also the eastern margin of the Monashee metamorphic core complex and broadly parallel with the Monashee decollement, an easterly directed thrust nappe. The Monashee decollement is considered responsible for eastward emplacement of the Selkirk allochthon. The Columbia River fault zone is characterized by brittle fractures which cut and disrupt the ductile Monashee decollement. It juxtaposes rocks of the Selkirk allochthon in the hanging wall with rocks of the Monashee complex in the foot wall across a 1 km thick zone of mylonite and brittle faults. The Monashee complex has several stages of deformational history. One of its most striking features is the S2 transposition foliation which occurs throughout the complex. This is the result of F / F2 isoclinal folding. The limbs form the S2 foliation parallel to the axial plane of F/F2 folds. F3 folds have highly va1iable styles and are commonly indistinguishable from F/ F2 folds. A large km scale F3 fold hinge is exposed in the western slopes of the Columbia River valley confirmed by a redistribution of S2 suggesting a fold hinge plunging gently east-southeast. The limbs of this fold extend into the valley, coincident with the shear zone. Across the valley in the Clachnacudainn slice of the Selkirk allochthon has only developed a single observable foliation which is not concordant with the Monashee complex. This is disrupted by S5 mylonite as ai•e structures of the Monashee complex. The S5 shear zone is characterized by mylonitic fabrics reactivating S2, a stretching lineation, shear bands, sheath folding and boudinage. These indicate a southeast, top-to-the-southeast displacement sense. The mylonite zone has reactivated S2 on the limb of the large F 3 structure. Two populations of brittle faults are present in the Columbia River fault zone. S2 parallel, shallowly eastward dipping faults with well developed gouge zones and steep, S2 disrupting, brecciated faults. Slip vectors indicate a dominant top-to-the-southeast normal sense displacement and a late tight-lateral strike slip displacement sense. An east west trending lineament crossing the valley is a fracture controlled feature with minimal offset but cannot be used to estimate st1ike slip displacement on the Columbia River fault. This data with geomorphological patterns indicate two normal en-echelon fractures bridged at Revelstoke explaining the bend in the Columbia River valley lineament and the Columbia River. The above evidence and other mapping suggests that uplift of the Monashee Complex was accommodated by both b1ittle and ductile processes.