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 (DCSZ3 and DCSZ4) 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 (DCSZ3) produced a moderately to gently, north-northeast-dipping foliation and north-east-plunging mineral lineations. DCSZ3 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°C occurred at this time. Prior to DCSZ3, deformation (DWTB1–2) 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 ∼92 Ma. From ∼57 Ma to 55 Ma, deformation in the western Coast shear zone (DCSZ4) produced a narrow, 1–2 km wide, zone comprised of a steeply-dipping to subvertical foliation that overprints and transposes all DWTB1–2 and DCSZ3 structures. DCSZ4 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.