The Pinal Schist, southeast Arizona, USA: contraction of a Palaeoproterozoic rift basin

Abstract

The Palaeoproterozoic Pinal Schist of southeast Arizona comprises mainly greenschist facies turbiditic metasedimentary rocks with associated volcanic rocks. The deformation history of the Pinal Schist includes foliation and elongation-lineation development within sedimentary protoliths during early basin contraction, followed by gentle warping of the metamorphosed sediments into upright, regional-scale horizontal folds with 10 km wavelengths. Continued contraction caused development of shear zones approximately parallel to anticlinal axial surfaces, and syn-kinematic granitic rocks intruded into anticlinal cores. Such shear zones occur throughout southeast Arizona in the turbiditic rocks of the Pinal Schist as well as in rocks of more continental character associated with the Tonto Basin Supergroup in the Mazatzal Mountains. It is proposed that shear zone distribution in both the Pinal Schist and the adjacent Tonto Basin Supergroup, reflects control by a pre-existing rift margin. The Pinal Schist and Tonto Basin Supergroup therefore represent distal and proximal rift-margin facies, respectively. Reactivation of early extensional faults as reverse shear zones during margin contraction controls shear zone distribution in southeast Arizona with deformation character changing from ductile to brittle with shallowing crustal level. This model explains shear zone and lithological distribution and deformational style in southeast Arizona, and it also implies a continuum of sediment deposition and deformation throughout this part of the Mazatzal orogeny.