| Decades of research has illustrated that the continental lithosphere, particularly the crust within it, is compositionally evolved and heterogeneous. The compositional heterogeneity is clearly associated with rheological layering, yet the precise rheological behavior of each layer has proven difficult to quantify. In particular, two important questions have remained unresolved for several decades. (1) What is the integrated and peak strength of the continental lithosphere and where does the peak strength reside? (2) To what degree is deformation localized into faults and shear zones at different depths? A variety of sub-disciplines in the earth sciences, including geology, geodesy, and geophysics have focused on these issues for both modern and ancient continental orogens. In this thesis, I address these two questions, both directly and indirectly, but from two quite separate sub-disciplines: active tectonics and structural geology. Chapter 2 focuses on whether discrepancies in geologic vs. geodetic slip rates are real, given the uncertainties in geologic slip rate estimates. In Chapters 3 and 4 the magnitude of stress within the middle and lower crust is quantified for two separate tectonic settings: the Whipple Mountains in eastern California (Chapter 3), representing post-collisional extension, and the Sierra Alhamilla in southern Spain (Chapter 4), representing syn-collisional extension. The stress profiles differ in detail, but lead to the same overarching conclusions: (1) the magnitude of stress at the brittle-ductile transition is high, consistent with the extrapolation of Byerlee's law at hydrostatic pore fluid pressures; and (2) the decreases in stress with depth and the associated strain rates are consistent with weak quartzite flow laws, but inconsistent with strong ones. In Chapter 5 I show that measurements of dynamic stress from pseudotachylites, are within error of measurements of static stress from brittle-to-ductile shear zones, suggesting that the formation of melt along the fault may have triggered seismic arrest, rather than lubrication. Chapter 6 is a methods chapter demonstrating that NIST 6XX glass standards can be used to quantify the Ti contents of quartz if the matrix effect between these glasses and nearly pure quartz can be corrected. |
