Footwall dip of a core complex detachment fault: thermobarometric constraints from the northern Snake Range (Basin and Range, USA)

Low-angle detachment faults are common features in areas of large-scale continental extension and are typically associated with metamorphic core complexes, where they separate upper plate brittle extension from lower plate ductile stretching and metamorphism. In many core complexes, the footwall rocks have been exhumed from middle to lower crustal depths, leading to considerable debate about the relationship between hangingwall and footwall rocks, and the role that detachment faults play in footwall exhumation. Here, garnet-biotite thermometry and garnet-muscovite-biotite-plagioclase barometry results are presented, together with garnet and zircon geochronology data, from seven locations within metapelitic rocks in the footwall of the northern Snake Range decollement (NSRD). These locations lie both parallel and normal to the direction of footwall transport to constrain the pre-exhumation geometry of the footwall. To determine P-T gradients precisely within the footwall, the Delta PT method of Worley & Powell (2000) has been employed, which minimizes the contribution of systematic uncertainties to thermobarometric calculations. The results show that footwall rocks reached pressures of 6-8 kbar and temperatures of 500-650 degrees C, equivalent to burial depths of 23-30 km. Burial depth remains constant in the WNW-ESE direction of footwall transport, but increases from south to north. The lack of a burial gradient in the direction of footwall transport implies that the footwall rocks, which today define a sub-horizontal datum in the direction of fault transport, also defined a subhorizontal datum at depth in Late Cretaceous time. This suggests that the footwall was not tilted about the normal to the fault transport direction during exhumation, and hence that the NSRD did not form as a low-angle normal fault cutting down through the lower crust. Instead, the following evolution for the northern Snake Range footwall is proposed. (i) Mesozoic contraction caused substantial crustal thickening by duplication and folding of the miogeoclinal sequence, accompanied by upper greenschist to amphibolite facies metamorphism. (ii) About half of the total exhumation was accomplished by roughly coaxial stretching and thinning in Late Cretaceous to Early Tertiary time, accompanied by retrogression and mylonitic deformation. (iii) The footwall rocks were then 'captured' from the middle crust along a moderately dipping NSRD that soled into the middle crust with a rolling-hinge geometry at both upper and lower terminations.