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-3(CSZ) and D-4(CSZ) 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-3(CSZ)) produced a moderately to gently, north-northeast-dipping foliation and north-east-plunging mineral lineations. D-3(CSZ) involved dominantly east-side-up, top-to-the-southwest displacements during and 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 degrees C occurred at this time. Prior to D-3(CSZ), deformation (D-1-2(WTB))now preserved west of the Coast shear zone resulted in tectonic imbrication of lithologically distinctive crustal fragments at 8-9 kbars, and west- to southwest-vergent ductile thrust faults before similar to 92 Ma. From similar to 57 Ma to 55 Ma, deformation in the western Coast shear zone (D-4(CSZ)) produced a narrow, 1-2 km wide, zone comprised of a steeply-dipping to subvertical foliation that overprints and transposes all D-1-2(WTB) and D-3(CSZ) structures. D-4(CSZ) involved bulk eastside-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. (C) 1998 Elsevier Science Ltd. All rights reserved.