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Description
The San Felipe Hills, first mapped by T. W. Dibblee, Jr., lie along strike of the San Felipe Hills fault. Underlying the hills are ~14,000 feet.(4.2 km) of alluvial fan, fan-delta, and lacustrine Tertiary to Quaternary sediments. Detailed mapping within the San Felipe Hills shows that Quaternary sediments are deformed into a set of EW trending northerly verging, doubly plunging anticlines and synclines with wavelengths on the order of ~90 to 380 m. lnterlimb angles vary from 28° to 155°, indicating a tight to gentle fold form. Cutting these folds is a set of conjugate right-lateral and left-lateral strike-slip faults, along with a set of normal faults. Statistical analysis shows that right-lateral strike-slip faults dip sub-vertically and strike N35W. In contrast, left-lateral strike-slip faults strike N37E and dip sub-vertically. Normal faults strike N09E with sub-vertical dips. Strike-slip movement along conjugate faults is clearly indicated by offset hinge lines, a relationship that also suggests that the set of folds formed prior to the onset of brittle faulting. These data imply that the San Felipe Hills were shortened in a NS direction, a relationship that is consistent with a NS orientation for the maximum principal stress direction. Deformation within the San Felipe Hills appears to be the result of a complex multiphase history associated with the development of a left-stepping branch of the San Felipe Hills fault. The first phase of deformation produced the ubiquitous EW trending folds mapped in the San Felipe Hills. Conjugate right-lateral and left-lateral faults developed during phase 2, offsetting hingelines of phase 1 folds. An ~5 km zone of high strain developed as a result of mass redistribution along the north side of the San Felipe Hills fault during phase 2. This phase of deformation may be related to the propagation of the left-stepping branch of the San Felipe Hills fault into the study area. Within the zone of high strain, folds are tighter, and the density of brittle faults is significantly greater. Escape of material from this high strain zone is reflected in a set NS normal faults and refolded phase 1 folds. The inferred rheological properties of the sedimentary section and crystalline basement suggest that during development of the structural fabric sediments behaved in a plastic fashion, whereas, basement rocks, though not exposed, likely behaved as elastic-brittle materials. This inferred contrast in rheological behavior implies that the crystalline basement is likely decoupled from the overlying younger Tertiary and Quaternary sediments. Hence, folds mapped at the surface in or near the zone of high strain are the likely products of fault-propagation folds lying above one or more decollemonts. In short, the results of work reported here suggest that the structural fabric in the San Felipe Hills is the characteristic response of a mechanically decoupled sedimentary cover to the termination of a significant strike-slip fault. In order to test ideas developed in this thesis, detailed mapping of other types of fault terminations in other areas should be completed, and future work should be focused on determining the seismic hazards associated with the development of high strain zones at such terminations.
