Abstract

Two-dimensional seismic modeling of exposed rocks implicates subhorizontal, tabular mafic intrusions as the likely cause of the unusually prominent reflections that constitute the Bagdad Reflection Sequence (BRS). These basement reflections, from depths of ∼3-15 km, are observed on COCORP and other data to extend for some 100 km across the Basin and Range/Colorado Plateau Transition Zone in west-central Arizona. The nearby Buck Mountains fortuitously expose an upended upper-crustal block, which is used in this study as the basis for a 12 x 6 km seismic model. Ray tracing through this model produces synthetic reflections from the numerous Proterozoic diabase intrusions occurring in the Buck Mountains. These synthetic reflections approximate the scale, geometry, and character of reflections observed on the COCORP data, indicating that the intrusions in the Buck Mountains furnish a viable geologic analogy for the BRS. Several lines of evidence argue that the major competing hypothesis—that Tertiary detachment surfaces cause the BRS—is less satisfactory.

Although we cannot rule out the possibility that the BRS is related to hypothetical Tertiary intrusions of similar distribution and structure, geologic evidence more directly implies that these reflectors are Proterozoic in age. If so, the Bagdad reflections serve as strain markers and can be used to constrain subsequent deformation in the Arizona Transition Zone. Their relatively continuous nature thus argues against widespread disruption of the upper crust in the Transition Zone during Tertiary extension. Moreover, identification of these reflectors as subhorizontal mafic intrusions may have profound consequences for interpretation of similar reflections observed elsewhere by bolstering the contention that mafic intrusions are an important cause of basement reflections in the continental crust.

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