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Description
Commercial seismic reflection data in the Transition Zone of west -central Arizona show major low-angle undulating reflectors within the upper 10-15 km of the crust. These prominent, well-defined reflectors appear to depict an extensive Mid-Tertiary detachment-related deformation system in the region east of the Rawhide and northeast of the Buckskin and Harcuvar Mountains. Major detachment complexes are exposed in these ranges and tilt generally east and northeast where last exposed before the Colorado Plateau. Abundant, high-quality reflectors and reflection characteristics from the seismic data provide excellent constraints for a detailed description of the geometry and overall low-angle structural fabric of the region. The seismic records clearly show that the upper crust is primarily composed of multiple, stacked crustal lenses on the order of 2-3 km thick and up to 8-10 km long. Vertical stacking of crustal lens-shaped reflectors are very continuous and image exceedingly well by the seismic reflection method. The dips of these reflectors are remarkably low, particularly in the vicinity of the Colorado Plateau, with a regional dip gently to the northeast. The abundance of reflectors suggests that crustal layering due to lensoidal plutonic bodies, packages of metamorphic and mylonitic rocks, and Mesozoic thrust structures can be imaged. The individual identities of the many low-angle reflectors as detachment-related faults, plutonic-metamorphic boundaries, mylonitic zones, Mesozoic thrust faults, or a combination of these are interpreted from the seismic data. A synthetic seismogram generated from the Kirkland State A-1 well provides excellent constraints to identify and interpret major shallow reflection events. A coherent high-amplitude seismic reflector can be traced across the Transition Zone dipping gently to the northeast and ranging in depth from 1 km to 5 km. Well data suggests this reflector represents a major zone of motion along a low-angle northeast-dipping dislocation surface. The low-angle character of the reflectors suggests that detachment-related faulting, while accommodating a large percentage of crustal extension, does not cut into deeper portions of the crust in this region. The low-angle nature of the detachment-related system supports a sub-horizontal, thin-skinned simple shear model as opposed to a deep- rooted crustal shear model. Motion between anastomosing crustal lenses may be a major mechanism of “lower-plate" extension. Movement between crustal lenses may also be the primary cause for the development of the antiformal-synformal geometry of detachment-related faults and ranges within the detachment terrane. Horizontal reflectors beneath domed ranges suggest that the domed character of detachment ranges is not primarily due to isostatic uplift in response to tectonic denudation.
