Pressure, Temperature, and Structural Evolution of West-Central New Hampshire: Hot Thrusts over Cold Basement

Pressure-temperature (P-T) paths have been calculated from pelitesand amphibolites of several major Acadian structures in west-centralNew Hampshire by using both inclusion thermobarometry and differentialthermodynamics (the Gibbs method). P-T paths calculated forrocks exposed in the Orfordville and Bronson Hill anticlinoriaare ‘clockwise’ and show 1–2.5 kb of exhumationwith 30–100 C of heating. Because this type of path ischaracteristic of the lower plate of overthrust terranes, theserocks are interpreted to be (para)autochthonous. P-T paths forrocks exposed in an intervening synclinorium (the Hardscrabblesynclinorium) show isothermal loading of 1–3 kb followedby possible isobaric cooling. This behavior is characteristicof rocks occupying a middle-plate structural position withina multiple thrust package, and so these rocks are interpretedto be allochthonous. The interpretation that the Hardscrabblerocks are allochthonous differs from previous models, but betterexplains the petrologic data and is consistent with the stratigraphicand structural data on which other models have been based. Correlation of the P-T paths with deformational events throughkinematic and textural analysis indicates that during nappestage deformation, the synclinorial rocks were transported westward,and that the anticlinorial and synclinorial rocks were buriedto depths of 25–30 and 20–25 km respectively. Theexhumation with heating recorded by the anticlinorial samplesoccurred during the dome stage of deformation, and differentiallyuplifted the anticlinorial rocks relative to the synclinorialrocks; this differential uplift may have been accommodated throughreactivation of early thrust faults with normal movement sense.P-T paths of the Hardscrabble synclinorium rocks are suggestiveof a relatively elevated initial geothermal gradient for theirpre-nappe source terrane, which is interpreted to have beenbetween the Kearsarge-Central Maine basin and the Bronson Hillparautochthon.