THE POST 100 MA ARCHITECTURE OF THE COAST MOUNTAINS, BRITISH COLUMBIA AND SOUTHEAST ALASKA

Quantitative pressure, temperature, and time (P-T-t) data for metamorphism combined with structural analysis are required for evaluating tectonic and magmatic models for the Coast Mountains. We present new P-T-t data for the Stikine area and combine with existing data for a more complete picture. The most complete datasets from the Mt. Waddington and Skeena River areas differ. In the Mt. Waddington area, the orogen is flanked by an east-vergent Late Cretaceous-Paleogene fold and thrust belt which inverts c. 65 Ma mid to upper amphibolite facies metamorphic rocks. These structures are likely continuous northward at least to Bella Coola. Rocks west of the Coast Mountains batholith (CMB) comprise greenschist facies < ~4 kbar rocks lacking evidence for Cretaceous thrusting. In the Skeena River area, the Skeena thrust belt, is similar to the Waddington belt, but likely was inactive by Late Cretaceous and there is no significant metamorphic inversion, instead upper crustal rocks were down dropped along the Eastside detachment c. 55-50 Ma. West of the CMB, Cretaceous greenschist facies rocks were transported westward on a series of thrust faults, the easternmost transported c. 108-95 Ma ~9 kbar regional metamorphic rocks. This fold-thrust belt is juxtaposed with the CMB core along the steep Cretaceous to Paleocene Coast shear zone (CSZ). In the Stikine River area, no east vergent Cretaceous thrust belt has been mapped, instead a narrow belt of c. 57 Ma low pressure andalusite & sillimanite zone rocks flank the core of the CMB. On the west, the western thrust belt includes ~9 kbar rocks and was active synchronous with c. 91 Ma plutonism. The western belt and CSZ continue northward to Holkham Bay where the CSZ includes 75-60 Ma sillimanite zone rocks. Near the Juneau, the western belt and CSZ are truncated by the Chatham Strait – Lynn Canal fault. Our data, are compatible with c. 90 Ma contractional metamorphic belts on the west from Juneau to south of the Skeena River and a 90? to c. 65 Ma belt on the eastern side of the CMB from the Skeena R. southward. New P-T paths for the eastern CMB core along the Stikine R. transect indicate limited loading during garnet growth c. 57 Ma. Therefore, any extension along an eastside detachment at this latitude would likely be younger.