Late Cretaceous-Paleocene stratigraphic and structural evolution of the central Mexican fold and thrust belt, from detrital zircon (U-Th)/(He-Pb) ages

Abstract

Combined U-Pb and (U-Th)/He ages on a small sample set of Cretaceous foreland-basin sandstone from a transect of the fold-thrust belt in east-central Mexico reveal correspondence between episodes of thrust-belt exhumation with previously postulated deformational events and with at least one wedge of synorogenic sediment in the adjacent Tampico-Misantla basin. Detrital zircon U-Pb ages, sandstone petrography and multidimensional scaling analysis of the Cenomanian-Turonian Mineral de Pozos sandstone, the Coniacian Soyatal and Campanian San Felipe formations, and the Paleocene Chicontepec Formation indicate sediment sources in the Mexican Cordilleran magmatic arc of western Mexico, the accreted Guerrero arc terrane, and Mesozoic sedimentary rocks of central and western Mexico. (U-Th)/He cooling ages define into three age clusters: (1) ~136-120 Ma, (2) ~99-80 Ma, and (3) ~66-56 Ma. Cooling ages are younger toward the transect's western part, which represents the interior part of the fold-thrust belt. Dominant Early Cretaceous cooling ages in the San Felipe Formation indicate lack of, or partial, resetting of the He system. Late Cretaceous cooling ages are dominant in the Soyatal Formation and are partially equivalent to their maximum depositional age (88 ± 1 Ma), suggesting derivation of grains from adjacent rapidly exhumed thrust sheets. Alternatively, these grains were partial reset. Youngest cooling ages dominate the Mineral de Pozos sandstone samples, indicating Paleocene exhumation and erosion of interior thrust sheets. The Paleocene exhumation event is equivalent to stratal ages of the lower part of the Chicontepec Formation, and sandstone composition of that unit indicates dominant sediment sources in carbonate rocks of the frontal part of the fold-thrust belt. Combined cooling ages and sandstone petrography thus indicate coeval exhumation of the entire width of the orogen in the Paleogene, rather than out-of-sequence thrust-sheet uplift and exhumation of only the interior part of the thrust wedge. Orogen-wide exhumation is corroborated by a previously published early Paleocene illite 40Ar/39Ar age of 64 ± 2 Ma for thrust deformation at the eastern flank of the Valles-San Luis Potosí carbonate platform, which formed the latest Cretaceous-Paleocene boundary between the fold-thrust belt and foreland basin in which the Chicontepec Formation was deposited.