Abstract

The central Mojave metamorphic core complex is defined by a belt of Miocene brittle-ductile extension and coeval magmatism. The brittle-ductile fault zone defines a basin-and-dome geometry that results from the interference of two orthogonal fold sets that we infer to have formed by mechanically independent processes. One fold set contains axes that lie parallel to the extension direction of the shear zone and has a maximum characteristic wavelength of about 10 km. The axial surfaces of these folds can be traced from the footwall mylonites, through the brittle detachment, and into hanging-wall strata. However, folds of mylonitic layering have smaller interlimb angles than those of the brittle detachment, suggesting that the folds began to form during ductile shearing and continued to amplify in the brittle regime, possibly after movement across the fault zone ceased. Mesoscopic fabrics related to the transport-parallel fold set indicate that the folds record true crustal shortening perpendicular to the extension direction. We interpret these folds to form in response to elevated horizontal compressive stress perpendicular to the extension direction and suggest that this stress regime may be a natural consequence of large-magnitude extension.

The other fold set has axes perpendicular to the extension direction and a characteristic maximum wavelength of about 50 km. Mesoscopic fabrics related to these folds include northwest-striking joints, kink bands, and en echelon tension-gash arrays. These fabrics formed after mylonitization and record both layer-parallel extension and northeast-side-up subvertical shear. The postmylonitic fabrics are kinematically compatible with rolling-hinge-style isostatic rebound of the footwall following tectonic denudation.

The relative timing of extension-related magma intrusion and ductile deformation varies through the central Mojave metamorphic core complex. On the scale of the small mountain ranges that make up the central Mojave metamorphic core complex, no correlation was observed between either shear zone thickness or intensity of ductile deformation and either the proximity or relative volume of extension-related igneous rocks. This suggests that models that invoke a single upper-crustal genetic relationship, such as magmatism triggering extension or vice versa, do not apply to the central Mojave metamorphic core complex.

Systematic variation in the relative timing of dike emplacement and mylonitization suggests that, at the time of dike emplacement, rocks in the Mitchel Range were below the brittle-ductile transition while those in the Hinkley Hills were above it. The Hinkley Hills and Mitchel Range are separated by ∼2 km in the dip direction of the fault zone, which suggests that the vertical thickness of the brittle-ductile transition probably was between 100 and 950 m.

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