KINEMATICS AND CHRONOLOGY OF THE LARAMIDE SHORTENING IN THE HUETAMO-ZIHUATANEJO AREA (SOUTHERN MEXICO): WAS IT REALLY RELATED TO THE ACCRETION OF THE GUERRERO TERRANE?

The mesozoic volcano-sedimentary successions exposed in southwestern Mexico, have been traditionally grouped into the Guerrero Terrane. Most authors interpreted these successions to represent the record of an intraoceanic arc or multi-arc system, separated from North American by one or more trenches. Following these authors, the Guerrero Terrane accreted during late Cretaceous-early Tertiary times along the continental margin of North America, originating the Laramide Orogeny in Mexico. Alternatively, other authors proposed that the mesozoic successions of the Guerrero Terrane formed part of North America since the Mesozoic. In this case, the intervention of another driving force is required to explain the Laramide shortening of the southern margin of the North American plate. We test these two different scenarios, using U-Pb geochronology and kinematics of the Laramide structures between Huetamo and Zihuatanejo areas (Michoacan and Guerrero states, Mexico). The structural analysis and the stratigraphic record, supported by previous data and new U-Pb ages, indicate that two major folding episodes occurred in this region. The first episode (D1) started at the beginning of the upper Cretaceous, and produced a regional uplift, marked by the end of marine sedimentation and the deposition of continental red beds. The second episode (D2) occurred during late Cretaceous-early Tertiary time, originating gentle folds in the red beds. The absence of refolded folds in the pre-D1 succession suggests that these two folding events were coaxial, and that folds of the study area are Type 0 interference structures of Ramsay (1967). Similarly to other regions of southern Mexico, we propose that the first episode marked the migration of the Laramide contractile front. In this case, the early upper Cretaceous age of the D1 folding supports the migration of the Laramide front from west to east across southern Mexico. The second folding episode occurred when the Laramide deformation front was located ~200 km east of the study area. Therefore we interpret the D2 contractile structures as out-of-sequence folds, developed in the hinterland during the eastward migration of the deformation front. The timing of the contractile deformation indicates that the Laramide orogeny in southern Mexico is incompatible with the accretion of the Guerrero terrane.