FLEXURAL ISOSTATIC FOLDING DURING BRITTLE, LOW-ANGLE DETACHMENT FAULTING, GRANT RANGE, EASTERN NEVADA: A LONG-DURATION ‘FIXED HINGE’

Several aspects of the structural evolution of detachment systems remain controversial. One debate centers on the role of denudation-induced isostatic rebound in rotating detachment faults to shallower dip angles, with end-member models ranging from gentle doming of faults active at ≤30˚, to ‘rolling hinge’ models that invoke significant rotation of faults initially active at ~60˚. Here, field relations exposed within a system of brittle detachment faults in the Grant Range in eastern Nevada allow testing models for the style of syn-extensional isostatic folding. The faults deform a 10 km-thick section of Paleozoic-Paleogene sedimentary and volcanic rocks, and exhibit brecciation and stratigraphic cutoff angles of 5-15˚ at all structural levels. The fault system is folded across an anticlinal culmination, which grew during extension, as indicated by interlimb angles in the axial zone that increase from 130˚ to 160˚ in progressively younger faults. Retro-deformation of folding and minimal angularity across a Paleogene unconformity indicate the faults were initially active at ≤15˚ angles. Retro-deformation of offset indicates ≥49 km (98%) extension, and stratigraphic omission in the axial zone is as high as 5-6 km. We propose a model of stationary, sustained isostatic uplift and incision at the culmination axis (a ‘fixed hinge’), with back-rotation and excision up-dip of the axis producing bottom-to-top growth of an imbricate stack of faults, and excision down-dip of the axis yielding apparent thrust relationships. Culmination growth rotated faults from primary dip angles of ≤15˚W to as high as 20˚E, which is similar to the typical ≤30˚ of rotation documented in field-based studies of Cordilleran detachment systems. The lack of hinge migration in the down-transport direction was likely aided by excision down-dip of the culmination axis, which resulted in local stratigraphic repetition, and therefore less denudation than the axial zone. The Grant Range illustrates a unique example in which increased denudation and incision over the axial zone created a positive feedback process that continued to spatially-focus uplift, thereby favoring a fixed hinge. The end product is a low-wavelength culmination that comes to a sharp apex, which differs from the broad domes observed in many core complexes.