The Chaparral shear zone: Deformation partitioning and heterogeneous bulk crustal shortening during Proterozoic orogeny in central Arizona

Abstract

The Proterozoic Chaparral shear zone of central Arizona is one of a network of subvertical, northeast-striking shear zones that divide the Proterozoic orogenic belt of Arizona into tectonic blocks. The zone is several kilometers wide and contains variably developed mylonitic foliation that is subparallel to the regional subvertical foliation. Stretching lineations plunge shallowly northeast, and displacement across the zone was dominantly right-lateral. Several lines of evidence constrain displacement across the zone to be greater than 5 km, but probably less than tens of kilometers: (1) stratigraphic and plutonic rocks can be correlated across the zone, (2) structural and metamorphic histories of tectonic blocks on opposite sides of the zone are similar, and (3) integration of shear strain (estimated by deflection of earlier fabric) suggests greater than 5 km of strike slip. More important than scale of movement across the zone, structural studies have clarified several aspects of the assembly history of the orogen. Early north- to northwest-striking S 1 foliation, on both sides of the Chaparral shear zone, records one or more deformational events that took place between 1.75 and 1.7 Ga. Early fabrics record northeast-southwest or east-west shortening and west-verging thrusting that we interpret to have formed during development of a primitive arc complex. In contrast, intense deformation in the Chaparral shear zone took place during regional northwest-southeast shortening, D 2 , that produced the dominant subvertical northeast-striking foliation and the present block architecture of the orogen. D 2 is interpreted to have been in part synchronous with emplacement of 1.70 Ga granitoids and to record assembly of volcanic belts to North America. Of general interest is the character of D 2 deformation partitioning in and across the shear zone. Two types of high-strain domains were generated during shortening. One type, represented by rocks southeast of the shear zone, involved extreme shortening and transposition by folding. The other type, represented by the shear zone, involved simple shear deformation in a complicated array of anastomosing and conjugate shear zones. Overprinting relationships suggest that the second type may have nucleated on the first, implying an important component of deformation partitioning in time, as well as space. Mylonitic fabric in the shear zone developed by progressive heterogeneous simple shear followed by brittle fracturing. Conjugate shear bands suggest a shallow southeast-plunging σ 1 late during strike-slip deformation. This is consistent with regional D 2 shortening but not with the incremental shortening direction expected during right-lateral simple shear. This supports a regional kinematic model in which the Chaparral shear zone (right-lateral) and a temporally related left-lateral shear zone were regional-scale conjugate shear zones that accommodated heterogeneous D 2 northwest-southeast orogenic shortening via escape-block tectonics. Thus, orogen-parallel strike-slip displacements in the Chaparral shear zone were a response to partitioned shortening in the late stages of accretion rather than transpressional accretion during oblique subduction.