Late Cretaceous - Tertiary Structural Evolution of Southeastern Arizona

This project forms the basis of Ross Waldrip's M.S. research and involves geologic mapping, detailed structural analysis, and geo/thermochronologic studies in the southern Galiuro Mountains of southeastern Arizona where contractional structures of Laramide age are exposed in the footwalls of mid-Tertiary normal faults.

The North American Cordilleran fold-thrust belt (CFTB) from southern Canada in the north to the latitude of Nevada in the south is reasonably well understood and characterized by an overall eastward younging in E-directed contractional deformation from Late Jurassic to Eocene time (e.g., DeCelles et al., 2004). However, to the south in eastern California and Arizona, the kinematic evolution of Cretaceous - early Tertiary contraction, and its relationship to the CFTB to the north and southeast, remain enigmatic. For example, the Late Cretaceous Maria fold-thrust belt of eastern California and west-central Arizona trends ~E-W at a high angle to the CFTB (Fig. 1) and exhibits S-directed vergence (Reynolds et al., 1986; Laubach et al., 1989; Richard et al., 1990). Our knowledge of the contractional history Arizona diminishes even further eastward, where in southeastern Arizona there are few constraints on the style or vergence of regional contraction, let alone on individual thrust fault exposures. This project has the potential to determine if scattered thrust fault exposures exhibiting variable dip directions but similar stratigraphic separations represent relatively small-displacement thrust ramps of opposing vergence or a single large-displacement thrust flat with unidirectional vergence.

A second outstanding issue concerns the structural evolution of metamorphic core complexes and the presently low-angle normal fault systems which bound them. While the architecture of these systems are well constrained, unambiguous field tests of proposed models of structural evolution are scarce. The most popular models invoke a breakaway zone, along which extension is initially accommodated in the upper crust. It is characterized by a listric or low-angle normal fault system in the brittle upper crust and which feeds slip into a subhorizontal or low-angle shear zone at depth. Younger listric/low-angle normal fault systems break in the direction of transport. Together with isostatic doming of the footwall due to unloading, these younger faults result in abandonment and folding/back-tilting of the original breakaway zone and exhumation of a portion of the ductile shear zone. The models predict breakaway zone normal faults to have slipped at low-angle in the brittle upper crust and during the earliest stages of core complex development. Furthermore, it has been proposed that extension-parallel corrugations exhibited by metamorphic core complexes were molded into this shape as they were exhumed in the footwalls of curviplanar brittle normal faults (Spencer, 1999). This "continuous casting" model predicts that the axes of the corrugations should correlate with salients and recesses in the breakaway zone. Except for a few unique localities, such as our map area, the important structural and timing relations necessary to test the models described above are buried beneath younger rift-basin fill. This project is constraining the relative timing, initial geometry, and kinematics of normal faults exposed along the western flank of the southern Galiuro Mountains. It is hoped that these data will provide strong field-based evidence for or against proposed models of core complex development.