STRUCTURAL GEOLOGY AND TECTONIC EVOLUTION OF THE NORTHEASTERN RINCON MOUNTAINS, COCHISE AND PIMA COUNTIES, ARIZONA

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.