LARGE-MAGNITUDE CONTINENTAL EXTENSION - AN EXAMPLE FROM THE CENTRAL MOJAVE METAMORPHIC CORE COMPLEX

The central Mojave metamorphic core complex is defined by a belt of Mi ocene brittle-ductile extension and coeval magmatism, The brittle-duct ile 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 tha t lie parallel to the extension direction of the shear zone and has a maximum characteristic wavelength of about 10 km. The axial surfaces o f these folds can be traced from the footwall mylonites, through the b rittle detachment, and into hanging-wall strata, However, folds of myl onitic layering have smaller interlimb angles than those of the brittl e detachment, suggesting that the folds began to form during ductile s hearing 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 cr ustal shortening perpendicular to the extension direction, We interpre t these folds to form in response to elevated horizontal compressive s tress perpendicular to the extension direction and suggest that this s tress 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 Irm. Mesoscopic f abrics 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 psotmylonitic fabrics are kinematically compatible with rolling-hinge-style isostatic rebound of the footwall following tectonic denudation, The relative timing of extension-relat ed magma intrusion and ductile deformation varies through the central Mojave metamorphic core complex, On the scale of the small mountain ra nges that make up the central Mojave metamorphic core complex, no corr elation 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 trigge ring extension or vice versa, do not apply to the central Mojave metam orphic core complex, Systematic variation in the relative timing of di ke emplacement and mylonitization suggests that, at the time of dike e mplacement, 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 similar to 2 km in the dip d irection of the fault zone, which suggests that the vertical thickness of the brittle-ductile transition probably was between 100 and 950 m.