Early Mesozoic tectonic history of the boundary area, east-central Alaska

Thrust faulting in the Yukon‐Tanana Upland of east‐central Alaska has long been hypothesized, but few thrust faults have been definitely identified. In the Boundary area near the international border between Alaska and Canada metamorphic rocks of the amphibolite facies have been thrust over rocks of the greenschist facies. Subordinate thrust faults and imbrication occur within both groups of rocks. Brecciation, slickensides, slickenfibers, rootless, isoclinally folded quartz bands, and abrupt lithologic changes from gneiss and amphibolite to quartz‐white mica‐chlorite schist characterize the thrust zone. Ultramafic rocks occur locally on or near the thrust plane. Thrusting is postulated to have occurred when a terrane composed of the amphibolite facies rocks converged with rocks along the continental margin of North America in Late Triassic and Early Jurassic time.