The Sacramento Mountains, a metamorphic core complex that lies in the Colorado River extensional corridor of southeastern California, contains complex lithologic and structural relationships. Detailed mapping and petrographic analysis of lower plate rocks from the central part of the range show that three deformation events have been recorded. An amphibolite facies-grade foliation (Sg), which represents the oldest deformational event (D1), is the predominant fabric in the quartzofeldspathic country rock (grey gneiss). The second deformational event (D2) produced a variably developed greenschist facies-grade fabric (Sm) in the grey gneiss and in two post-D1 plutonic suites. Both of these plutons are thought to be Cretaceous or Tertiary in age. Variations in mylonitic development throughout the lower plate can be attributed to the ratio of strong minerals to weak minerals. Those with high strong-to-weak ratios (eg. granodiorite, diorite, tonalite, quartz diorite) do not form foliations as easily as rocks with low strong-to-weak ratios (eg. granite). This results in a diffuse distribution of mylonitic fabrics and many small scale shear zone boundaries. There is no evidence, however, for a major shear zone boundary (mylonitic front) in the Sacramento Mountains. Northeastward-directed Tertiary extension (D3) resulted in development of multiple high-angle and low-angle faults within and adjacent to the lower plate. Chloritic breccia is found along many of these faults and always cross-cuts the mylonitic fabric. Stage 1 of chloritic breccia evolution was the development of low-angle brittle shear zones and oblique fractures, and was accompanied by deposition of cryptocrystalline epidote. Stage 2 was the formation of conjugate fracture sets at a high-angle to the brittle shear zone accompanied by precipitation of chlorite as well as epidote and other propylitic minerals. Evidence for the importance of both high- and low-angle faulting during the structural evolution of the Sacramento Mountains is found in the many faults both within and adjacent to the lower plate. The upper and lower plates are separated by the Sacramento fault system that includes gently, moderately, and steeply dipping, dip-slip to oblique-slip fault segments. Recent models of core complex development hint at the complexity of fault systems and recognize the importance of high-angle faulting in upper plate rocks. However, most lower plate structures are attributed to low-angle faulting and folding of the fault surface. In the Sacramento Mountains, the lack of evidence for folding of the lower plate plus the development of multiple subvertical faults, both within and between plates, suggests that high-angle faulting was important throughout the evolution of the core complex, and not just in the final stages. / Ph. D.