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ABSTRACT: Laramide Thrusting and Tertiary Deformation in Tierra Caliente, Michoacan and Guerrero States, Southwestern Mexico

Christopher A. Johnson, Harold Lang, J. Antonio Barros, Enrique Cabral-Cano, Christopher G. A. Harrison

Field investigations and detailed interpretations of Landsat Thematic Mapper images are in progress to improve understanding of regional structure and tectonics of the southernmost extension of the North American cordillera. Two areas have been selected within the Ciudad Altamirano 1:250,000 topographical sheet for geologic mapping and structural interpretation at 1:50,000 scale. Our results to date require modification of previous ideas concerning the style and timing of deformations, the role and timing of terrane accretion in the overall tectonic history of the region, and the importance of southern Mexico to investigations of the tectonic evolution of the plates in the region.

The relative sequence of deformation in the area correlates well with variations in relative motion between North America and plates in the Pacific. Post-Campanian thrusts and generally eastward-verging folds deformed the Mesozoic sequence during the (Laramide equivalent) Hidalgoan orogeny, associated with high-velocity east-west convergence with the Farallon plate that began about 70 Ma. The resulting unconformity was covered by the Tertiary Balsas Formation, a thick sequence of mostly continental clastics. The Tertiary stratigraphy is regionally and sometimes locally variable, but it can be divided into two members. The lower member is relatively volcanic poor and more deformed, and it lies below a regionally significant mid-Tertiary unconformity, which may mark a change to northeas -directed convergence with the Farallon plate sometime prior to 40 Ma. Continued mid-Tertiary deformation in southern Mexico may be related to eastward movement of the Chortis block and the resulting truncation of the Pacific margin of Mexico.

We also suggest a tentative correlation between the volcaniclastic member of the Lower Cretaceous San Lucas Formation and the protolith of the Roca Verde metamorphics to the east. Alternatively, the San Lucas Formation may represent an overlap sequence eroded from the Roca Verde during Early Cretaceous uplift. More work is necessary to refine these correlations, but either interpretation would imply that accretion of the Guerrero terrane occurred early in or prior to the Early Cretaceous. On the other hand, it could mean that the eastern boundary of that Guerrero terrane has been misidentified, or that the concept of suspect terranes in southern Mexico may require reexamination.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990