Search

Laramide reverse faults in southeastern Arizona commonly are obscured by midto late Cenozoic extension and subsequent cover, resulting in debate about their configuration and origin. A new mid-Cenozoic paleogeologic map depicts the structural configuration before extension, and new structural reconstructions characterize Laramide shortening in terms of structural style, magnitude, evolution, and timing. Reverse faults restore to moderate to high angles, are associated with fault-propagation folds, and involve significant basement and thus constitute thick-skinned deformation. The paleogeologic map suggests several major basement-cored block uplifts, many of which are newly identified. The largest uplifts may measure 150 km along strike, similar to those in the classic Laramide province of Wyoming and Colorado. Estimated shortening across the central study area is 14% or 23 km, whereas it is only 5% (9 km) to the north and 11% (12 km) to the south. Shortening by this mechanism is inadequate to explain previous estimates of crustal thickening in the region (~50-60 km). Therefore, magmatic underplating, lower-crustal flow, or underplating of trench sediments and lithospheric material also may have contributed to thickening. Shortening largely occurred from 86 Ma to 64 Ma and possibly as late as 53 Ma, with initiation being younger to the northeast or north. Integration with data from southwestern New Mexico implies complex geometry for the subducting flat slab. Finally, reverse faults generally do not appear to have reactivated older faults, as previously suggested, primarily because reverse faults have associated fault-propagation folds in rocks that predate supposed reactivated structures. [ABSTRACT FROM AUTHOR]

This study investigates the Late Cretaceous through mid-Cenozoic structural evolution of the Catalina core complex and adjacent areas by integrating new geologic mapping, structural analysis, and geochronologic data. Multiple generations of normal faults associated with mid-Cenozoic extensional deformation cut across older reverse faults that formed during the Laramide orogeny. A proposed stepwise, cross-sectional structural reconstruction of mid-Cenozoic extension satisfies surface geologic and reflection seismologic constraints, balances, and indicates that detachment faults played no role in the formation of the core complex and Laramide reverse faults represent major thick-skinned structures. The orientations of the oldest synextensional strata, pre-shortening normal faults, and pre-Cenozoic strata unaffected by Laramide compression indicate that rocks across most of the study area were steeply tilted east since the mid-Cenozoic. Crosscutting relations between faults and synextensional strata reveal that sequential generations of primarily down-to-the-west, mid-Cenozoic normal faults produced the net eastward tilting of ~60°. Restorations of the balanced cross section demonstrate that Cenozoic normal faults were originally steeply dipping and resulted in an estimated 59 km or 120% extension across the study area. Representative segments of those gently dipping faults are exposed at shallow, intermediate (~5-10 km), and deep structural levels (~10-20 km), as distinguished by the nature of deformation in the exhumed footwall, and these segments all restore to high angles, which indicates that they were not listric. Offset on major normal faults does not exceed 11 km, as opposed to tens of kilometers of offset commonly ascribed to "detachment" faults in most interpretations of this and other Cordilleran metamorphic core complexes. Once mid-Cenozoic extension is restored, reverse faults with moderate to steep original dips bound basement-cored uplifts that exhibit significant involvement of basement rocks. Net vertical uplift from all reverse faults is estimated to be 9.4 km, and estimated total shortening was 12 km or 20%. This magnitude of uplift is consistent with the vast exposure of metamorphosed and foliated cover strata in the northeastern and eastern Santa Catalina and Rincon Mountains and with the distribution of subsequently dismembered mid-Cenozoic erosion surfaces along the San Pedro Valley. New and existing geochronologic data constrain the timing of offset on local reverse faults to ca. 75-54 Ma. The thick-skinned style of Laramide shortening in the area is consistent with the structure of surrounding locales. Because detachment faults do not appear to have resulted in the formation of the Catalina core complex, other extensional systems that have been interpreted within the context of detachments may require further structural analyses including identification of crosscutting relations between generations of normal faults and palinspastic reconstructions. [ABSTRACT FROM AUTHOR]