Deformation from the Bottom up – a Case for Biaxial Extension

Regional crustal extension occurs following lithospheric delamination, slab breakoff, and subduction rollback, all of which provide a driving force at depth that leads to extension of the lower crust that propagates upwards to the surface. Detailed structural studies at all crustal levels suggests that biaxial extensional strain plays a significant but generally unrecognized role in formation of deep to shallow crustal structures.

Grenville-age gneisses in Western Australia show multiple periods of biaxial extension synchronous with granulite facies metamorphism, migmatization and magma generation. Extension parallel to the earlier contraction direction is more extensive, but synchronous perpendicular extension is also observed. Current models favor slab “breakoff” or delamination accompanied by asthenospheric upwelling. At upper crustal levels in the La Spezia region, Northern Apennines, Italy, brittle deformation during opening of the Tyrrhenian Sea resulted in synchronous biaxial extension with normal faults perpendicular to the opening direction showing most offset but with pronounced normal faults parallel to the opening direction. Again deformation was a direct result of asthenospheric upwelling during opening of an oceanic basin. Mid-crustal extension of theTertiary Catalina metamorphic core complex shows biaxial extension along the Tanque Verde corrugation near Tucson, AZ. Here regional extension that leads to unroofing is dominant, but perpendicular extension is documented throughout the entire transition from ductile flow to brittle faulting. Regardless of the favored model for core complex formation, nearly all indicate upwelling of the asthenosphere accompanied crustal thinning.

In sum, biaxial extension is observed at deeper crustal levels where magmas are being generated, at the mid- to upper crustal levels during the transition from ductile flow to brittle detachment faulting, and at upper crustal levels where purely brittle deformation affects unmetamorphosed sedimentary rocks. Together these examples suggest that synchronous extension in perpendicular directions is likely where deformation is driven by upwelling of the asthenosphere.