The structural evolution of southeastern Arizona has been dominated by the differential vertical uplift of the Precambrian and Triassic-Jurassic granites. Most of the ranges are complex anticlines with Precambrian or Triassic-Jurassic granites in their core. Some uplifts exceed 25,000 feet. Many of the ranges began to rise in Triassic-Jurassic time and have continued to rise intermittently in essentially the same position at least through Miocene time. This conclusion is supported by: (1) truncation of Paleozoic strata on the flanks of the present ranges in pre-Lower Cretaceous time, (2) depositional thinning of Cretaceous strata down the flanks of the present ranges and the development of Cretaceous basins adjacent to and parallel with incipient ranges rising in Cretaceo s time, (3) high-angle "Laramide" reverse faults which define the flanks of present ranges and uplift the granite cores with respect to the schist which underlies the range flanks, and (4) Basin-and-Range type faulting which further developed uplifts already in existence.

Most of the previous investigators of the structural geology of southeastern Arizona have described intensive and extensive overthrusting. Overthrusting has probably been overemphasized, primarily because of a failure to discriminate between large overthrusts and detached blocks which have moved down the flanks of large anticlines under the influence of gravity. This conclusion is based on recognition of a possible source and an available declivity, and, in particular, on a study of the internal structures of the detached blocks which has often shown that the blocks moved down the mountain flank rather than out of the valley onto the uplifted mountain block.

Of interest to the petroleum geologist are the structural and stratigraphic variations along the flanks of the intermittently rising anticlines and the comparatively simple structure which may exist in the intervening valleys.

End_of_Article - Last_Page 413------------