MAGMATISM, DEFORMATION, AND LONGITUDINAL DISPLACEMENT WITHIN THE CRETACEOUS COAST MOUNTAINS BATHOLITH (CMB), BRITISH COLUMBIA

The Coast Mountains batholith (CMB) is the locus of magmatism, deformation, and metamorphism for >1500 km in British Columbia and SE Alaska. Commonly the CMB is considered a static weld across previously accreted terranes after ~120 Ma which hinders reconciliation of paleomagnetic and geologic data (Baja BC controversy) and may distort models of magmatic evolution. Our research in the southern batholith reveals a more dynamic structural evolution linked to magmatism and reflecting shifting plate motions. Arc magmatism migrated northeastward from ~120 Ma to its demise at ~45 Ma, punctuated by spatially distinct high magmatic flux events (HFE) at 114-102, 85-70, and 61-46 Ma. During each HFE, deformation was focused in the area of active magmatism. The oldest and westernmost HFE was accompanied by sinistral transpression from ca 114 to 100 Ma. At 85 – 70 Ma, the locus of magmatism and deformation stepped NE and changed to contractional deformation from ~87 Ma – 72 Ma. In contrast, magmatism and crustal shortening virtually ceased at ~85 Ma in the southernmost CMB. At 72 Ma the areal extent of deformation narrowed to the subvertical Coast shear zone (CSZ). This shift was regional and disrupted magmatic patterns for >700 km along the CMB. At this time the trailing edge of magmatism stepped eastward to the CSZ and subsequent magmatism occurred only east of the CSZ. Dextral slip on the CSZ is inferred from geologic contrasts and kinematics of the northern CSZ. Speculative restoration of features in the southern CMB suggests a minimum of ~ 225 km of dextral slip on the CSZ but does not restore magmatic patterns. Following establishment of the CSZ, the youngest and farthest NE HFE was accompanied by minor crustal shortening. Although deformation and magmatism are spatially and temporally linked, the primitive character of the magmas indicates that supracrustal contamination during crustal thickening did not drive magmatism. Instead, HFE likely facilitated and crustal deformation and shifting plate motions controlled the style of deformation within the active arc region. Our results highlight key events which may signal late Cretaceous plate reorganization: segmentation of the arc at ~85 Ma and initiation of dextral slip on the CSZ and accommodation of Salinian-style repetition of the Intermontane terrane in the southernmost arc at 72 Ma.