EVOLUTION OF THE CONTINENTAL MARGIN EXAMINED THOUGH BATHOLITH-SCALE PATTERNS OF MAGMATISM AND DEFORMATION, COAST MOUNTAINS, BRITISH COLUMBIA

The Coast Mountains batholith (CMB) formed during postulated significant latitudinal translation of large terranes along the margin of North America in the Late Cretaceous and Paleogene. The tectonomagmatic history of the CMB thus augments kinematic models based on plate reconstructions. Past work shows that the tempo of magmatism varies significantly along > 800 km of the batholith in British Columbia and occurred during spatially and temporally variable deformation. This complex record is simplified by integrating batholith-scale deformation with the spatial distribution of magmatic ages, rather than magmatic tempo. Spatiotemporal analysis of published geochronologic and structural results delineates four batholith-wide tectonomagmatic stages: 120 – 105 Ma: diffuse magmatism with segmented sinistral shear zones in active magmatic centers; 110 – ca 72 Ma: NE-migrating magmatic arc with well defined bands of magmatic ages >800 km long and NE-younging crustal contraction; 72 Ma: SW retreat of magmatism to Coast shear zone (CSZ) for >800 km along strike, and cessation of arc migration and orogen-scale shortening. 72 Ma – 45 Ma: Widespread magmatism and complex patterns of deformation east of CSZ.

These batholith-scale events reflect shifting plate kinematics and support published models invoking Early Cretaceous sinistral oblique subduction, orthogonal Late Cretaceous subduction, and Paleogene dextral convergence. Within this broad framework, our results show that for ~30 m.y., the CMB developed by migration of well-defined age bands toward the continent, compatible with eastward-dipping subduction. This plate configuration changed at 72 Ma, when the well-organized arc abruptly shifted to non-migratory magmatism and complex deformation east of the CSZ. Subsequent deformation and magmatism are interpreted as the signal of a ~150 km wide dextral shear zone in the eastern batholith which marks the onset of a significant dextral component to the plate boundary. The broad shear zone encompasses ductile and brittle dextral faults (including the Yalakom fault and CSZ), extensional complexes, and synplutonic shortening. Displacement on this dextral system exceeded ~400 km between ca 70- 45 Ma. This displacement in the CMB began ~ 15 m.y. later than in the western US, raising questions of strain compatibility and plate configurations along the margin.