Abstract

Cenozoic extensional deformation in southern Arizona included (1) a Neogene phase of Basin and Range deformation recorded by high-angle normal faults and (2) an earlier phase of detachment faulting and brittle-ductile crustal shearing associated with tectonic denudation of metamorphic core complexes. In the Catalina-Rincon region, exposed fault zones produced at different crustal depths during successive extensional episodes display differing fault geometries and types of fault rocks formed during progressive crustal extension. Detachment faults are associated with both mylonites produced by ductile shear and cataclasites produced by brittle shear. Younger faults formed at shallower depths are associated with less intense cataclastic deformation and with brittle fracturing that includes transtensile phenomena at the shallowest crustal levels represented. Qualitative measures of net displacement along individual fault zones are provided by (1) the nature of contrasts among successively overprinted fabrics and internal structures in the footwall and (2) the degree of contrast between fabrics and structures of footwall and hanging-wall rocks. Footwalls of the oldest structures display varied brittle overprints of ductile fabrics and are juxtaposed across gouge zones along detachment surfaces with stratal successions cut by multiple brittle shear surfaces. Footwalls of younger structures formed at shallower depths display multiple generations of cataclastic features, including brecciation of variable intensity and cataclasite dikes, but are juxtaposed against hanging-wall strata that are only moderately deformed by subsidiary faults. The shallowest fault zones lack either structural overprints in their footwalls or any significant contrasts between footwall and hanging-wall deformation. Exposures of mid-crustal rocks within the core complexes reflect successive exhumation and uplift of fault footwalls during sequential episodes of deformation. The present high elevation of mylonitic rocks in the Catalina-Rincon metamorphic core complex reflects dip slip and isostatic footwall flexure during Basin and Range deformation as well as tectonic denudation during detachment faulting. Net uplift of core rocks resulted from multiple phases of deformation.

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