Relations Between Deformation, Metamorphism and Magmatism in the Presidential Range, New Hampshire

Siluro-Devonian metasedimentary rocks in the Presidential Range of eastern New Hampshire are polymetamorphic and were multiply deformed during the Acadian Orogeny. The two most ubiquitous metamorphic events are an early, regional andalusite-staurolite event (M2), and a later higher pressure, higher temperature event (M3) resulting in an exposed field gradient from staurolite to migmatite zone conditions. Of five proposed deformations, two are regionally extensive. The earliest deformation (D1) caused km-scale, recumbent isoclinal folding of stratigraphy and led to the development of a regionally-extensive axial-planar foliation (S1). The next regionally extensive deformation (D4) caused open to tight, generally upright to recumbent asymmetric folds of S1 on the meter to hundred meter scale, with a locally developed crenulation cleavage (S4). Prograde metamorphism caused local variations in rheology, which affected the style of deformation. During M2, large, modally abundant andalusite porphyroblasts nucleated in the pelitic portions of turbidite couplets, strengthening them relative to psammitic portions. During D1 deformation, this led to a reversal of cleavage refraction, whereby the angle between S1 and bedding is greater in pelitic layers than psammitic layers. During M3, these porphyroblasts were variably pseudomorphed by muscovite and sillimanite. This, combined with the thermal gradient associated with M3, apparently led to variability in D4 deformation style. At the lowest grades, D4 fold style is generally open and upright to slightly asymmetric. In these zones, the pseudomorph reactions have commonly not gone to completion and appear to be late relative to D4. As grade increased, the pseudomorph reactions went closer to completion, and appear to be early relative to D4. This difference in reaction completeness and timing corresponds to a change in D4 fold style from open, upright folds to tight or isoclinal recumbent folds. At the highest grade, in the migmatite zone, D4 folding is generally chaotic, possibly reflecting the weakening of the rock in the presence of a melt phase. The results of three-dimensional thermal modeling are used to partly constrain the time scale for deformation. The models explore the transient thermal evolution of the crust during the emplacement of multiple granitoid plutons, which is appropriate for this area given the regional geology. The time scale for the thermal perturbation to travel through the crust places a limit on the time scale allowable for syn-metamorphic deformation.