INFLUENCE OF THE MARIA FOLD AND THRUST BELT ON STYLES OF OLIGO-MIOCENE EXTENSION IN WESTERN ARIZONA: APPLICATION OF CRITICAL-TAPER THEORY

The Maria fold and thrust belt (MFTB) is an east-west trending zone of generally south-vergent thrusts and folds that developed within what had been cratonic North America. Siliciclastic strata of the latest Jurassic and Cretaceous McCoy Mountains Formation includes rift-related strata that predate the belt. This rift may have localized primarily latest Cretaceous to earliest Tertiary deformation that produced the MFTB and that accounts for its unusual east-west orientation. The Colorado River extensional corridor, a zone of severe NE-SW Oligo-Miocene extension from the southern tip of Nevada to about Interstate Highway 10 west of Phoenix, Arizona, crosses the MFTB. The fact that these two belts cross, rather than parallel each other, makes this area unique (in North America at least) for evaluating relationships between crustal thickening and later extension. North of the axis of the MFTB, highly effective denudational faulting exhumed the unusually large Harcuvar metamorphic core complex. In contrast, directly south of the MFTB, highly slivered and tilted fault blocks were produced by extreme extension, especially in the Vulture and Wickenburg Mountains. We propose that the MFTB formed an east-west trending mountain range that was present when extension began, and that contrasting styles of extension on opposite sides of the range reflect the mechanical consequences of different surface slope, as predicted by critical taper theory. The aerially extensive, top-northeast, Eagle Eye – Bullard – Buckskin – Rawhide – Whipple detachment system projects southeastward beneath the highly slivered Vulture and Wickenburg Mountains and the moderately faulted Big Horn Mountains. This fault system dips regionally northeastward, in approximately the same direction as inferred surface slope on the north flank and eastern end of the MFTB. In contrast, on the south side of the MFTB, surface slope was generally southward, roughly opposite to the northeast dip of the detachment fault system. We infer that because of these opposite surface slopes, the north side of the belt was well within the stable sliding field according to critical-taper theory, whereas the south side was within the unstable sliding field. We conclude that contrasting structural styles of extension are consistent with predictions of critical-taper theory.