Lower Crustal Detachment in the Cordillera and Other Hot Backarcs

English Abstract:

Lower crust horizontal detachment is widespread in hot backarcs like the Cordillera, based on four lines of evidence. (1) Temperature-depth estimates (~800 C at Moho) and laboratory rheology data indicate the lower crust in backarcs is likely very weak. In contrast, craton crusts are cold and strong, 400-500 C at Moho. Although the crust is more mafic with depth so stronger at the same temperature, the strength generally decreases downward. However, the lowest part of crust may be in dry granulite conditions, >730 C, so stronger than just above in amphibolite conditions (~700C). The uppermost mantle is stronger than the crust because of the rock type. The effective elastic thicknesses, Te, provides a check on the depth to the weak lower crust. (2) Regional lower crust horizontal seismic reflectors are interpreted to indicate current or past horizontal shear detachment; e.g., SW British Columbia, western US, UK, Spain, Germany, Japan. The top of the reflectors is commonly at 15-20 km, ~450-500 C; the base at ~30 km, ~730 C. The base of the reflectors is often ~5 km above Moho, consistent with underlying strong dry granulite rocks. (3) Structural and tectonic evidence for lower crustal detachment include: long distance crustal strain transfer, i.e., Yakatat collision northern BC, southwestern BC overthrusting of craton in Rocky Mtns, Andes thrusting in foreland. Also, many long-distance terrane motions and associated horizontal deformation in oroclines are best explained by motion of only the upper crust. (4) Extensive outcrop sections of former lower crust that exhibit horizontal fabric shearing at 750-800 C and 30 km., e.g., Athabasca granulite terrane. The strong evidence of horizontal detachment in the lower crust means that geological structural and tectonic interpretations need to allow for common 15-20 km thick upper crust transport and deformation, independent of the mantle.