Date of Award

2010

Level of Access

Campus-Only Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor

Scott E. Johnson

Second Committee Member

Christopher C. Gerbi

Third Committee Member

Edward S. Grew

Abstract

This thesis investigates the interaction of micro-scale chemical and mechanical processes that occur during localized deformation of middle to lower crustal rocks, and evaluates the potential effects of grain-scale weakening processes on orogenesis. Structural and petrologic data from two kilometer-scale ductile shear zone systems (and their adjacent wall rocks) from exhumed orogenic belts provide useful information on mechanisms associated with the weakening of discrete crustal volumes. Additionally, crustal-scale numerical models are presented that investigate how the structural and topographic evolution of a generic model orogen is affected by the presence of low-viscosity mid-crustal shear zones. The models also test the relative influence, and combined effect, of shear zone depth/geometry and surface erosion. Related petrological and geochronological investigations help constrain the tectono-metamorphic evolution of the investigated areas. A spatial and temporal relationship between localized shearing, fluid access, and metamorphic hydration reactions is recognized in all of the study areas. In each case, early deformation was mostly frictional (i.e. grain- to outcrop-scale fracturing), and became increasingly dominated by ductile creep processes as H2O availability and reaction progress increase. Along the strain and reaction progress gradients, samples generally show a transition from microstructures and mineral compositions characteristic of chemical disequilibrium within zones of low finite strain to fully recrystallized assemblages with more uniform structural fabrics and mineral compositions within high strain zones. Microstructural observations, grain scale chemical mapping, and crystallographic orientation data from shear zone samples indicate strong interactions between chemical and mechanical process associated with strain localization. Zircon U-Pb geochronology and oxygen isotopic ratios from a variety of sample types in one of the study areas confirm a genetic relationship among externally derived fluids and localized deformation within both outcrop-scale and kilometer-scale, litho-tectonic domain-bounding shear zones. Numerical models indicate that the presence of horizontal mid-crustal low-viscosity zones have only a minor effect on the internal velocity structure and topographic evolution of the model orogen when no erosion is applied. The addition of surface erosion strongly enhances the effect of the low-viscosity zones on almost all measured parameters and structural patterns, and tends to narrow the region of orogenic uplift and focus strain and vertical velocities into the sub-erosional area. Furthermore, the depth of the low-viscosity zones is also important in determining their effect on the model velocities, total displacements, and resulting topographic profiles. Shallower level zones (e.g. 20-22 km depth), have the strongest effect on vertical velocities and displacements, with higher maximum elevations and steeper inboard slopes. Deeper level zones (e.g. 28-30 km depth) have only a minor effect on the vertical dimensions of the orogen, but greatly enhance minimum and maximum horizontal velocities in the lower crust and lead to a much stronger decoupling of upper and lower crust.

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