Mesoscale and regional kinematics of the northwestern Yalakom fault system: Major Paleogene dextral faulting in British Columbia, Canada

The northwestern Yalakom fault system (YFS) lies on the eastern margin of the Coast Belt in southwestern British Columbia. The Yalakom fault is the most significant structure in the fault system, with at least 115 km of dextral slip, mostly of early Tertiary age. The NW YFS is up to 25 km wide and is bounded on the southwest by the Tchaikazan fault zone, which had 5 km or more of dextral slip. Kinematic and paleostress analysis of mesofaults and map‐scale structures shows that secondary faults within the NW YFS had both dextral and sinistral oblique slip. Three large synclines had increasing amounts of clockwise, vertical axis rotation from southeast to northwest. Block rotation between two synthetic strike‐slip faults with large offset (Yalakom) and moderate offset (Tchaikazan) is similar to current kinematics between the San Andreas and San Jacinto faults in southern California. The NW YFS was active from latest Cretaceous(?) to (mainly) Eocene time and postdates early Late Cretaceous thrust faulting to the southwest. Early stage mesofaults are compatible with the thrust belt and suggest that open folds developed in the foreland of the thrust belt before strike‐slip faulting commenced. The abrupt northwest termination of the YFS may be related to extension within the adjacent Tatla Lake metamorphic complex. Some dextral slip may have been transferred farther northwest through the Coast Belt, but these structures have not been documented in west central British Columbia. Most of the 120 km of dextral slip on the YFS was transferred to the north via a mosaic of normal and strike‐slip faults in central British Columbia. Thus the YFS forms the southwestern margin of a broad zone of transtensional structures that extends to the Tintina‐Rocky Mountain Trench fault zone; these structures accommodated northwestward displacement of the northwestern Cordillera in Paleogene time.