EVIDENCE FOR TWO CRETACEOUS SHORTENING EVENTS IN SOUTHERN NORTH AMERICA FROM THE STRATIGRAPHIC RECORD OF THE SIERRA DE LOS CUARZOS AREA, CENTRAL MEXICO

After the break-up of Pangea, the Pacific margin of North America was involved in different orogenic events that were triggered by distinct tectonic processes related to prolonged subduction (e.g. accretion of terranes and variation in the convergence rate among others). In Mexico, it is difficult to recognize the superposed effect of different tectonic processes because all the shortening structures developed between the Cretaceous and Eocene have been conventionally grouped into a single orogenic wedge that has been correlated with the Laramide and Sevier belts in western USA.

The stratigraphic record exposed in the Sierra de los Cuarzos area, central Mexico, offers the opportunity of deciphering with greater detail the complex tectonic evolution of the Pacific margin of Mexico. In fact, the Sierra de los Cuarzos area contains exposures of an Upper Jurassic-Cretaceous clastic succession, which is composed of three stratigraphic units that display a similar facies assemblage but significantly different compositions. Based on the sandstone provenance analysis and paleontologic determinations, we interpret the youngest two units exposed in the Sierra de los Cuarzos area as two different clastic wedges related to two distinct tectonic processes that are: 1) the late Aptian accretion of the Guerrero terrane arc assemblages, which produced a relatively narrow suture belt and the deposition of volcaniclastic slump, debris-flow, and mud-flow deposits along the Pacific continental margin of North America; 2) the Cenomanian-Eocene onset of the Mexican fold-thrust belt, which is a ~700 km-wide orogenic wedge that was probably driven by variations in the convergence rate along the North American Pacific trench, and that produced the deposition of thick calcareous turbidites across eastern Mexico.

Based on our new data, we critically discuss the previous scenarios proposed for the tectonic evolution of central Mexico, and suggest a new model that takes into account the two tectonic events indirectly recognized in the stratigraphy of the Sierra de los Cuarzos area.