Pressure, temperature, and structural evolution of west-central New Hampshire: Hot thrusts over cold basement

Pressure-temperature (P-T) paths have been calculated from pelites and amphibolites of several major Acadian structures in west-central New Hampshire by using both inclusion thermobarometry and differential thermodynamics (the Gibbs method). P-T paths calculated for rocks exposed in the Orfordville and Bronson Hill anticlinoria are 'clockwise' and show 1-2.5 kb of exhumation with 30-100 °C of heating. Because this type of path is characteristic of the lower plate of overthrust terranes, these rocks are interpreted to be (para)autochthonous. P-T paths for rocks exposed in an intervening synclinorium (the Hardscrabble synclinorium) show isothermal loading of 1-3 kb followed by possible isobaric cooling. This behavior is characteristic of rocks occupying a middle-plate structural position within a multiple thrust package, and so these rocks are interpreted to be allochthonous. The interpretation that the Hardscrabble rocks are allochthonous differs from previous models, but better explains the petrologic data and is consistent with the stratigraphic and structural data on which other models have been based.Correlation of the P-T paths with deformational events through kinematic and textural analysis indicates that during nappe stage deformation, the synclinorial rocks were transported westward, and that the anticlinorial and synclinorial rocks were buried to depths of 25-30 and 20-25 km respectively. The exhumation with heating recorded by the anticlinorial samples occurred during the dome stage of deformation, and differentially uplifted the anticlinorial rocks relative to the synclinorial rocks; this differential uplift may have been accommodated through reactivation of early thrust faults with normal movement sense. P-T paths of the Hardscrabble synclinorium rocks are suggestive of a relatively elevated initial geothermal gradient for their pre-nappe source terrane, which is interpreted to have been between the Kearsarge-Central Maine basin and the Bronson Hill parautochthon.