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From Transition Zone to Core Complex: Deep Reflection Seismic Imaging of the Basin and Range, Southeast Arizona
Four oil industry reflection seismic lines in the Basin and Range Province near the Colorado Plateau Transition Zone of southeast Arizona were reprocessed by extended correlation down to 14 s to provide deep crustal information between the southern margin of the Transition Zone and the Pinaleno Mountains core complex. These data show a relatively non-reflective upper crust beneath highly reflective basin fill sediments of the Safford basin suggesting that subhorizontal mafic intrusions similar to the Bagdad reflection sequence farther west do not occur within the study area.
Moderately reflective middle crust beneath the eastern end of the seismic data along the southern margin of the Transition Zone occurs between about 4.4 s and 7.0 s. This reflective middle crust thickens westward to between 2.0 s and 8.0 s near the core complex. Thickening and creation of the reflectivity is likely due to latest Oligocene through early Miocene mid-crustal flow from beneath the Transition Zone into the developing core complex, combined with magmatic inflation beneath the core complex. Higher amplitude more continuous reflections near the top of the middle crust indicate increased shearing and mylonitization beneath the Eagle Pass detachment fault as it flattened into a regional brittle-ductile transition beneath the Transition Zone.
High amplitude high continuity lower crustal reflections occur from about 6.5 s to 9.6 s beneath the east end of the seismic data, and 8.0 s to 10.5 s beneath the west end. A narrow band of weaker reflectivity occurs in the bottom part of the lower crust near the Transition Zone, but merges into high to moderately reflective lower crust closer to the core complex. The base of the crust is indicated by an increase in reflectivity at the Moho or an abrupt downward decrease in moderately reflective lower crust. Lower crustal reflectivity is interpreted as late Oligocene through middle Miocene subhorizontal sheet-like mafic intrusions surrounded by more felsic to intermediate rocks. Crustal melting induced by heat from the intrusions or magmatic differentiation is interpreted to have caused late Oligocene to early Miocene andesitic to rhyolitic volcanic and plutonic activity, and may have been responsible for weakening the crust, thus allowing rapid early Miocene crustal extension, detachment faulting, mid-crustal flow, and core complex uplift. Middle Miocene reduction in the rate of mafic intrusive activity and extension likely caused a change to basaltic volcanism, cessation of mid-crustal flow, and formation of the southwest-dipping Safford basin.
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