Structural history of the crustal-scale Coast shear zone north of Portland Canal, southeast Alaska and British Columbia

Structural, metamorphic and U-Pb geochronologic data reveal how a steep, crustal-scale shear zone influenced the evolution of the Paleogene Coast Mountains batholith during and since its emplacement. We document two distinct stages of deformation (D^CSZ₃ and D^CSZ₄) that produced the Coast shear zone north of Portland Inlet. Between 65 Ma and 57 Ma, deformation now preserved within the eastern side of the Coast shear zone (D^CSZ₃) produced a moderately to gently, north-northeast-dipping foliation and north-east-plunging mineral lineations. D^CSZ₃ involved dominantly east-side-up, top-to-the-southwest displacements during the after the intrusion of tabular tonalite and granodiorite plutons. Widespread crustal thickening followed by rapid exhumation, east-side-up tilting of the batholith, and decompression of rocks equilibrating at 5.6±0.4 kbars, 710±30°C occurred at this time. Prior to D^CSZ₃, deformation (D^WTB_1-2) now preserved west of the Coast shear zone resulted in tectonic imbrication of lithologically distinctive crustal fragments at 809 kbars, and west- to southwest-vergent ductile thrust faults before ~92 Ma. From ~57 Ma to 55 Ma, deformation in the western Coast shear zone (D^CSZ₄) produced a narrow, 1-2 km wide, zone comprised of a steeply-dipping to subvertical folidation that overprints and transposes all D^WTB_1/2 and D^CSZ₃ structures. D^CSZ₄ involved bulk east-side-down displacements parallel to a steeply-plunging, down-dip sillimanite lineation and regional tilting of the batholith. This east-side-down displacement may reflect a final period of crustal readjustment and collapse following an earlier period of crustal thickening during batholith construction. The variable history of motion within the Coast shear zone appears to reflect a response to different periods of batholith development within a convergent to obliquely-convergent continental margin.