EARLY TERTIARY DEFORMATION IN SOUTHERN MEXICO AND ITS RELATION TO THE TRANSFER OF THE CHORTIS BLOCK FROM THE NORTH AMERICA TO THE CARIBBEAN PLATE

Deformation and magmatism in southern Mexico during late Cretaceous and Tertiary times and, particularly, their relation with the detachment of the Chortis Block and its motion to the east-southeast represent a stimulating matter of research. In fact, the timing of the transfer of the Chortis block from the North America to the Caribbean plate and the nature and spatial extent of the deformation associated with this developing plate boundary are not well understood. Our detailed field study focused on the Cretaceous to Tertiary Guerrero-Morelos Platform (GMP) located about 70 to 170 km to the north of the truncated margin of North America. In this area new and compiled 40Ar/39Ar ages define two major volcanic episodes: 1) a Maastrichtian – Early Paleocene episode aligned along the E-W course of the Balsas River, which marks the end of the Late Cretaceous Laramide shortening; and 2) a widespread Late Eocene – Oligocene episode that marks the end of an early Tertiary phase of deformation. The rock sequences within this time interval consist of continental sedimentary deposits and volcanic rocks (Tetelcingo, Balsas, Oapan, and other locally named formations), and fill basins bounded by ~N-S folds and thrusts formed during the previous east-directed shortening. These sequences show a previously unreported complex deformation that decreases gradually towards the top. Two of these basins in the north (Copalillo and Tuzantlan basins) are involved in NNE-SSW folds with deformation increasing toward, and parallel to, the Papalutla Fault. They also show NW-SE strike slip faults. In the central GMP (Balsas River basin) the early Tertiary succession reaches its maximum thickness (about 500 m) and deformation is characterized by NNW-SSE folds. In the southern part of the GMP (Chilpancingo and Chilapa), the early Tertiary succession is involved in an asymmetric NW-SE trending synclinorium and affected by NW-SE strike slip faults. The absence of normal faults in the entire study region is worth nothing. Contrary to previous regional studies that envisioned a generalized Tertiary transtension in southern Mexico our data indicate that Paleogene deformation in the GMP was essentially transpressive. They also indicate that the early stage of the Chortis Block detachment was accomplished through a several hundreds of km wide deformation zone.