3 research outputs found
Thrust tectonics and Cretaceous intracontinental shortening in Southeast Alaska
An imbricate thrust belt extending along strike for more than 2000 km overprints the
tectonic boundary between two of the largest allochthonous crustal fragments (Intermontane and
Insular superterranes) in the North American Cordillera, and affects rocks west of the Coast
Plutonic Complex in southeast Alaska and western British Columbia. Deformation was broadly
coeval with mid-Cretaceous magmatism and involved the emplacement of west-directed thrust
nappes over a structurally intact and relatively unmetamorphosed basement. The Palaeozoic and
lower Mesozoic Alexander terrane forms structural basement for much of the thrust belt along a
moderately northeast-dipping decollement.
There were two main episodes of mid-Cretaceous deformation, which were contemporaneous with
the emplacement of tabular plutonic bodies. Older structures record ductile southwest-vergent
folding and faulting, and regional metamorphism, associated with a well-developed axial-planar
foliation. Second-generation structures include southwest-directed thrust faults that juxtapose
contrasting metamorphic grades and refold earlier structures.
Structural, stratigraphical and geochronologic data suggest that regional-scale deformation in
southeast Alaska occurred between 113 Ma and 89 Ma. Deformation involved the imbrication of
marginal basin(s) and a magmatic arc, overprinting the older tectonic boundary between the
Insular superterrane and the late Mesozoic western margin of North America (i.e. the Intermontane
supertcrrane). Contractional deformation along the length of the thrust belt was broadly coeval
with arc magmatism, and thus records intra-arc tectonism. Late Palaeocene to Early Eocene
deformation and uplift may mark the transition from contractional to extensional tectonism, and
perhaps records the collapse of tectonically thickened crust