Stratigraphic redefinition of the Zihuatanejo area, southwestern Mexico
Keywords: Guerrero terrane, La Unión-Zihuatanejo assemblage, Zihuatanejo, Mexico
AbstractThe Mesozoic volcano-sedimentary successions exposed in southwestern Mexico have been traditionally grouped into the Guerrero terrane, and interpreted to represent an allochthonous long-lived island arc or a complex multi-arc system accreted to the continental margin of North America. However, the nature of these arcs (intraoceanic versus continental), as well as the timing of the supposed accretion, are still debated. One of the key regions in this debate is that of Zihuatanejo, where a Cretaceous island arc (Zihuatanejo arc) and associated accretionary complex (Las Ollas Complex) have been defined in the past. In this paper we present a redefinition of the stratigraphy of the Zihuatanejo region, based on field mapping supported by new 40Ar/39Ar and U-Pb geochronology, which has significant implications for the Cretaceous-Paleogene evolution of the Guerrero terrane. The lowermost stratigraphic levels of the Zihuatanejo region are represented by the pre-Cretaceous polydeformed and partly metamorphosed rocks of the Las Ollas Complex and Lagunillas Formation. These rocks are unconformably overlain by the Early Cretaceous shallow marine Playa Hermosa arc assemblage. Volcanic rocks of this assemblage are restricted to a few meters of andesitic lavas interbedded with volcaniclastic sandstone and limestone, which suggests that the study area was in a peripheral position with respect to the main axis of a magmatic arc. The rocks of the Playa Hermosa assemblage were uplifted by folding, eroded, and unconformably overlain by Upper Cretaceous continental to shallow marine rocks of the La Unión-Zihuatanejo assemblage, which are only gently folded. Similar continental rocks (Cutzamala Formation) are exposed in the Huetamo area, ~80 km northeast of Zihuatanejo, and are interpreted as overlapping successions of the Guerrero terrane. Based on this correlation, we propose that the Upper Cretaceous La Unión-Zihuatanejo assemblage is not part of the tectono-stratigraphic Guerrero terrane but rather constitute an overlap succession that post-dates the possible accretion of this terrane to nuclear Mexico. Finally, the Mesozoic rocks of the Zihuatanejo area are intruded by 48–40 Ma granitoids, and unconformably overlain by 41–39 Ma horizontally bedded volcanic and volcaniclastic rocks, which post-date the final contractile deformation in the Zihuatanejo area.