Deformation of Continental Crust along the Queen Charlotte Transform Boundary, Coast Mountains, British Columbia and SE Alaska

New structural, paleomagnetic, and apatite (U-Th)/He results from a ~150 km transect across the continental margin inboard of the Queen Charlotte basin help define spatial and temporal patterns of deformation associated with the formation and evolution of this Tertiary transform boundary. In the northeastern part of the transect, ~200 – 275 km from the transform fault, north-striking dip-slip brittle faults and vertical axis rotation of large (10's of km) crustal domains occurred after ~50 Ma and before intrusion a swarm of northeast-striking mafic dikes at 20 Ma. The crustal rotation likely resulted from differential crustal extension of the orogen prior to or early in the formation of the transform margin, and was followed by dip-slip/normal faulting associated with the transform. Following dike intrusion, northwest-striking dextral faults were active in the core of the orogen, while dip-slip faulting produced large jumps in sea-level apatite (U-Th)/He ages in the islands east of the Queen Charlotte basin, ~150-200 km from the transform. This dip-slip faulting continued until ~10 Ma. The youngest deformation on the transect is local tilting around NNE-trending axes associated with rapid, glacially-driven, post-4 Ma exhumation in the core of the orogen.

Integrated with published work, these results show that: 1) varying amounts of crustal extension affected the continental margin more than 250 km from the transform fault for 25-40 m.y. after transform initiation at ~45 Ma; 2) the locus of extension migrated toward the transform with time, while dextral strike-slip faulting continued across the margin; 3) areas with the greatest and youngest extension roughly correspond to the part of the Eocene arc that was highly extended between 55-50 Ma, suggesting crustal warming and thinning associated with this event controlled subsequent transform-related extension.