The rheologic evolution of the middle crust during prograde metamorphism: Implications for the geodynamic evolution of convergent orogens

The rheologic structure of the lithosphere is a fundamental control on the dynamics of convergent orogens. In order to better understand the process of orogenesis, we first need to obtain a better understanding of the temporal and spatial variability in lithosphere rheology. In modern orogens, we can use geophysical observations such as seismic surveys, heat flow measurements and geodetic analysis to study the spatial variability in lithosphere rheology. However, these studies only provide a snap shot in time of lithosphere rheology, and don't give us any constraints on the temporal variability of lithosphere rheology.; In order to address the temporal variability in lithosphere rheology, I undertook an integrated field and numerical modeling study of an exposed mid-crustal section in eastern New Hampshire that preserves evidence for changing relative strengths of deformed rocks as a function of metamorphic conditions. Field investigations were used to constrain the relative strengths of different rock types deformed in different metamorphic regimes. An early period of porphyroblast growth resulted in significant metamorphic strengthening in this mid-crustal section, and a later period of extensive partial melting led to significant metamorphic weakening and strain localization.; In order to better study the relationship between porphyroblast growth, rock strength and bed-scale strain localization, I completed a series of two-dimensional numerical models investigating how the above-mentioned phenomena change as a function of changing porphyroblast abundance. These models support the hypothesis that porphyroblast growth can lead to significant strengthening.; Finally, to investigate the effects metamorphic strengthening and weakening reactions have on the dynamics of convergent orogens I completed a series of three-dimensional, orogen-scale numerical models that have a zone of mid-crustal metamorphic strengthening and weakening in them. These models show that metamorphic strengthening leads to strain-rate partitioning around the strengthened zone and results in suppressed topographic uplift rates above the strong zone. Conversely, metamorphic weakening leads to strain-rate partitioning into the weakened zone and a zone of enhanced topographic uplift above the weakened zone. These models provide insight into the relationship between mid-crustal rheology, strain localization and topography.