Terrane accretion, variations of the convergence rate, and interaction with the Caribbean plate have been proposed as the causes of the Mexican Laramide orogeny. However, the mechanism that triggered this regional deformation event remains unknown. Based on available data, some authors supported the notion that the Laramide shortening migrated from the present-day Pacific coast to the Mexican mainland. However, such migration has been inferred based on paleontologic and isotopic data from the central and eastern parts of the fold-and-thrust belt, without considering the western part of the orogen. The identification of a chronologic pattern of the deformation is crucial to understand the cause of the Laramide orogeny, because it is a direct consequence of the tectonic process that triggered this regional shortening.
Here we present the first structural study of the Zihuatanejo area, which is located in southwestern Mexico, within the interior of the Guerrero composite terrane. Our data document that this region underwent progressive shortening during the Late Cretaceous, which resulted in regional uplift and unconformable deposition of continental red beds over a Lower Cretaceous marine arc succession. We interpret the continental rocks as the infill of a piggyback basin related to the early evolution of the Laramide orogeny. According to this scenario, the ∼94 Ma age obtained for the oldest continental strata constrains the beginning of the Laramide shortening in the Zihuatanejo area, which was thus the first area of the Laramide belt to be deformed. Considering that during terrane accretion the deformation propagates from the suture zone to the continent and terrane interiors, the timing of the Laramide deformation in southern Mexico cannot be explained as a direct consequence of the accretion of the Guerrero terrane. In fact, we document that the Laramide shortening started at the Cenomanian–Turonian boundary within the interior of the terrane, and migrated progressively eastward involving the suture belt and the continental mainland.
Based on the chronologic pattern documented for the Late Cretaceous–Paleogene shortening in southern Mexico, the Laramide deformation front can be envisioned as a tectonic wave that propagates from the present-day Pacific coast to the continental mainland, probably as the result of the increasing subduction rate or collision of a Caribbean terrane along the Mexican Pacific trench.
- Received 3 August 2011.
- Accepted 4 August 2011.
- © 2011 Geological Society of America