METAMORPHISM, TRANSIENT MID-CRUSTAL RHEOLOGY AND THE EXHUMATION OF HIGH-GRADE METAMORPHIC ROCKS

Using a natural example in eastern New Hampshire, we constrain a series models illustrating the geodynamic effects of prograde metamorphism. In the White Mountains region of New Hampshire an early metamorphic strengthening associated with porphyroblast growth is overprinted by a metamorphic weakening associated with widespread partial melting. The magnitude of strengthening during the early metamorphism was investigated using foliation refraction angles, which are a proxy for the relative strengths of adjacent units. Weakening during the later metamorphism was qualitatively assessed by recognizing that the migmatite zone preserves deformation features that have no obvious correlative in non-migmatized rocks.

Two-dimensional numerical models are presented investigating the strengthening of metapelitic units relative to metapsammitic units during porphyroblast growth. The amount of strengthening in the metapelitic layer is a function of the porphyroblast abundance and strength, and the extent to which metapelite layers strengthen relative to metapsammitic layers is a function of the initial strength contrast between the layers. To investigate the larger-scale effects of metamorphic strengthening/weakening, we present a series of three-dimensional models illustrating the divergence of strain-rate distribution and topographic uplift of models with zones of metamorphic strengthening/weakening from a reference model. We conclude that, in general, metamorphic strengthening results in strain-rate partitioning around the zone of strengthening as well as suppressed rates of topographic uplift relative to a pre-strengthened state, which is the opposite of what occurs during metamorphic weakening. The magnitude of the divergence is largely a function of the strength contrast between the strengthened/weakened zone and the surrounding crust as well as the lateral position of the zone of strengthening/weakening. The models clearly demonstrate that changes in mid-crustal rheology associated with metamorphism have large-scale effects on the development of a collisional orogen.