THE GUERRERO TERRANE SUTURE BELT AT GUANAJUATO AND ITS RELATION WITH THE MEXICAN FOLD-THRUST BELT

The Late Cretaceous-Paleogene Mexican Fold-Thrust Belt is an eastward-tapering orogenic wedge that is bounded by the west by the Guerrero terrane, which is the second largest juvenile terrane accreted to the North American Cordillera. The juxtaposition of such an allochthonous lithospheric block to the Mexican continental core has motivated many authors to propose a linkage between the accretion of the Guerrero terrane and the regional shortening in the Mexican Fold-Thrust Belt. In order to test this hypothesis, we studied the Guerrero terrane suture belt at Guanajuato, and try to understand its possible relation with the shortening structures of the Mexican Fold-Thrust Belt along the Peña de Bernal-Tamazunchale section. At Guanajuato, the Guerrero terrane suture is represented by a tectonic pile made up of kilometer- to decameter-scale thrust sheets, containing Tithonian-Early Aptian arc and back-arc successions that were isoclinally folded and sheared during the D1 deformation event. This tectonic pile is unconformably overlain by Albian neritic limestones of the La Perlita Formation, which post-date the accretion-related D1 deformation phase. Subsequently, the La Perlita Formation was gently folded together with the arc and back-arc successions during the D2 deformation event. These stratigraphic and structural data indicate that at least two major Cretaceous tectonic event are recorded in this part of the Mexican Cordillera: (1) the Late Aptian D1 shortening, produced by the accretion of the Guerrero terrane to nuclear Mexico, and (2) the Late Cretaceous D2 shortening, which is not studied in detail. Considering that the oldest ages of deformation along the Peña de Belnal-Tamazunchale section are in the range of 91-85 Ma, the accretion of the Guerrero terrane should have occurred at least 20 Ma before the development of the Mexican Fold-Thrust Belt. Thus, we discard preliminarily the hypothesis of the Guerrero terrane as the “bulldozer” for the Mexican Fold-Thrust Belt, and alternatively propose that this latter should be correlated with the Late Cretaceous D2 shortening structures documented at Guanajuato.