TECTONO-MAGMATIC EVOLUTION OF THE EASTERN HARRISON LAKE REGION, BRITISH COLUMBIA, REFLECTS MID-CRETACEOUS CHANGE IN PLATE KINEMATICS

A mid-Cretaceous shift in plate kinematics of Farallon plate from 110-85 Ma relative to the western margin of North American, followed by the birth of the Kula plate, is recorded in plutons and structures in the northwest Cascades in Washington and southern Coast Mountains in British Columbia. Throughout much of the Coast belt the Cretaceous structural history that records this important change in plate kinematics is largely obscured by the emplacement of younger plutons that are part of the Coast plutonic complex and the overprint of younger Paleogene dextral strike-slip structures. However, the region east of the Harrison Lake within the southern Coast Mountains preserves a history that reflects the transition from sinistral to dextral transpression along the plate boundary. New U-Pb data refine the timing for the emplacement of plutons that were affected by earlier sinistral transpression (pre-96 Ma) and later dextral transpression (post-89 Ma) related to the mid- to Late Cretaceous shift in plate kinematics along the plate margin. Early Cretaceous and older plutons (e.g. Breakenridge complex) capture the earliest recorded northwest-directed, orogen parallel structures resulting from the Farallon plate subducting southeastward relative to the North American plate, causing transpressional stress with a sinistral component. These structures are overprinted by orogen-normal, east-directed folding (94-84 Ma) and oblique thrusts with a dextral component (Breakenridge fault) that are interpreted to reflect a mid- to Late Cretaceous change in plate convergence. In the mid-Cretaceous, subduction of the Farallon plate became more orthogonal relative to North America. This was followed by the Late Cretaceous birth of the Kula plate, which subducted northeastward relative to the North American plate, resulting in transpression with a dextral component. We suggest that peak crustal thickening of at least 35 km (10 kbar) recorded in rocks along the east side of Harrison Lake from ca. 96-86 Ma culminated during this period of transition from sinistral to dextral transpression. The transition to Paleogene transtension created a series of dextral strike-slip structures throughout the region (e.g. Fraser River fault, Harrison Lake fault), some of which likely reactivated many of the older sinistral structures.