NEW U-PB CALCITE AGES FROM DEFORMATION-RELATED VEINS IN THE CHICOMUSELO FOLD BELT - INSIGHTS INTO THE CLOSURE AND EARLY FRAGMENTATION OF PANGEA IN SOUTHERNMOST MEXICO

The Chicomuselo Fold Belt (CFB) located in the southern Maya Block in SE Mexico is one of the oldest and least-studied Paleozoic contractional orogens of southern North America and formed during the final stages of the assembly of Pangea. It involves three deformed sedimentary successions: the Carboniferous Santa Rosa Fm., a slightly metamorphosed package of black shale and fine-grained siliciclastic sandstone; the early Permian Grupera Fm, consisting of thinly-bedded siliciclastic sandstone interbedded with carbonate layers at the top; and the middle Permian Paso Hondo Fm. (PH), a thickly-bedded platformal limestone and dolostone. The CFB is a more than 40 km wide, NW-SE oriented anticlinorium, with a pervasive axial-plane foliation in fine-grained rocks and metric folds in limestone layers that are consistent with tectonic transport towards the NE. The belt is unconformably overlain by Middle Jurassic redbeds and cut by normal and strike-slip faults formed or reactivated in Jurassic and/or Neogene times.

In order to elucidate the timing and style of deformation, calcite veins associated with well-exposed metric-scale folds and faults in the PH were selected for a detailed structural and textural characterization and calcite U-Pb dating by LA-ICP-MS. For calcite U-Pb benchmarking, we also dated well-preserved fusulinids, yielding ages that are in good agreement with independent biostratigraphic age constraints (283-268.8 Ma). We analyzed around 800 spots in 14 thin sections and obtained >70 reliable U-Pb calcite ages. These absolute ages generally follow relative structural cross-cutting relations and fall into five age groups: (1) 280-260 Ma, deposition and early diagenesis; (2) 255-220 Ma, timing of shortening in the CFB; (3) 210-180 Ma, early extensional fragmentation of western Pangea; (4) a subordinate peak at 170-130 Ma, potentially related to the opening of the Gulf of Mexico; and (5), 16-8 Ma, faulting associated with the Chiapanecan orogeny and/or Polochic fault.

These preliminary calcite U-Pb ages are in good agreement with recent illite Ar ages dating deformation in the CFB and with ages of magmatism and metamorphism in the Chiapas massif (~250 Ma) and extensional faulting (170-180 Ma). This method, however, allows for a better temporal discrimination of different deformation episodes when integrated with detailed structural analysis of veins and their cements.