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The northeastern Rincon Mountains record a superposed history of low-angle normal-slip shear strain. Moderate- to low-angle faults, mapped previously as Laramide thrust faults, are recognized as normal faults of Tertiary age. Two faults are predominant: a younger-overolder ductile fault forms the base of a metasedimentary carapace, a ductile shear zone (decollement zone) of southwest vergent slip, and an older-over-younger (locally younger-over-older) fault named herein as the San Pedro basal detachment fault forms a brittle shear surface of west-southwest slip. The decollement zone is characterized by passive-slip folding, flexural-flow folding, boudinage, stretched pebbles, and low-angle ductile normal faults. Structural analysis reveals southwest- ergent simple shear strain with a component of superimposed pure shear strain (vertical flattening). The San Pedro basal etachment fault underlies a faulted, distended allochthon. The internal structure of the allochthon is characterized by an imbricate shingling of tilted fault blocks against west-dipping normal faults, suggesting emplacement from the east by an extensional and/or gravitional mechanism. Detachment faulting involved Late Oligocene sedimentary rocks yet cuts ∼26 m.y. old dikes. Mid-Miocene (?) faults form north-trending fault blocks which have rotated rocks of the metamorphic basement and the allochthon eastward. High-angle normal faults of the Basin and Range disturbance form an eastern fault margin across which the northeastern Rincon Mountains have been uplifted. The deformation recorded in the northeastern Rincon Mountains is interpreted to reflect mid-Tertiary crustal extension. Early structural elements define a ductile shear zone which is either truncated or overprinted by a subsequently thinner zone of brittle shear. The shear zone descends stratigraphically westward across the Rincon Mountains. Reconstructions of its mid-Tertiary configuration show the shear zone to be a surface of normal-slip. Displacement near the surface is by brittle shear, but is progressively replaced by ductile shear down-dip. Evolution of the surface superimposes the region of brittle shear against the region of ductile shear. Late Cenozoic block faulting has segmented, tilted, and exhumed the surface.

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The Cornelia Pluton was previously found to have been faulted in two (Gilluly, 1946). The downthrown apex is host to a copper ore-body, and the upthrown block represents a vertical cross-section of the igneous system. Field work confirmed the character of the relationship between three texturally-distinct quartz monzonites described by Wadsworth (1968). Plagioclase crystals were chosen from all the phases, including previously undescribed hydrothermal rocks, and analyzed with a microprobe. Graphs of anorthite content versus radial distance revealed systematic patterns of compositional variation, analysis of which yielded a consistent, time-dependent model for the emplacement and crystallization of the system. The granodiorite resulted from an earlier intrusive event. The quartz monzonites crystallized initially downward from the system's roof, and, subsequently, upward from the floor. Saturation of the remaining pocket(s) of magma led to the development of an inner fine-grained phase, and possibly, the evolution of hydrothermal fluids. The genetic connection between these fluids and eventual mineralization of the cupola is suggested.

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The Pima mine, a 39,000 ton per day copper mine, is located 17 miles south of Tucson, Arizona. The mine is in a sequence of Paleozoic and Mesozoic sediments striking east-northeast and dipping southeasterly intruded by Tertiary quartz monzonite porphyry. The Paleozoic Permian(?) dolomites, limestones, and sandstones have been altered to calc-silicate skarn, marble and quartzite. The overlying Mesozoic Triassic(?) clastic sediments have undergone some recrystallization and hydrothermal alteration. The clastic metasediments are divided into three distinct lithologic units: lithic arenite, arkose, and interbedded and overlying black argillite. Three types of hydrothermal alteration in the clastic rocks are propylitic alteration (eqidote, chlorite, and quartz, with or without sericite), quartz-sericite alteration, and potassic alteration (K-feldspar and quartz, with or without sericite and chlorite). The porphyry has undergone potassic alteration. Dominant structures are an east-west post mineral fault in teh western part of the pit, and strong low angle shearing and faulting which truncates the ore body at depth. Two joint sets, one parallel to bedding and the other at right angles to bedding are prominent in the mine. Faulting is in two dominant directions, one striking northwest, dipping northeast and the other striking northeast, dipping northwest. Mineralization is predominantly disseminated pyrite and chalcopyrite, and molybdenite-quartz veinlets. There are less important amounts of magnetite, hematite, sphalerite, galena, tennantite, and bornite. Mineralization is believed to be both structurally and chemically controlled.

No description available.