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

• Published in: Journal of structural geology Vol. 20; no. 7; pp. 883 - 904
• Main Authors: Klepeis, Keith A., Crawford, Maria Luisa, Gehrels, George
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.07.1998
• ISSN: 0191-8141; 1873-1201
• DOI: 10.1016/S0191-8141(98)00020-0

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 CSZ 3 and D CSZ 4) 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 CSZ 3) produced a moderately to gently, north-northeast-dipping foliation and north-east-plunging mineral lineations. D CSZ 3 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 D CSZ 3, deformation ( D WTB 1–2) 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 ( D CSZ 4) produced a narrow, 1–2 km wide, zone comprised of a steeply-dipping to subvertical foliation that overprints and transposes all D WTB 1–2 and D CSZ 3 structures. D CSZ 4 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.