QUANTIFYING VARIATION IN FAULT SLIP AND DISTRIBUTED OFF-FAULT DEFORMATION, SANTA CRUZ MOUNTAINS, CENTRAL CALIFORNIA
To evaluate fault slip history, we use a synthesis of the slip histories of five important regional faults. The synthesis demonstrates considerable complexity of the slip history on the SAF itself. Major events include both abandonment of the Pilarcitos segment of the SAF at ~1.6 Ma and variation in geologic slip rate between ~6 mm/yr and ~20 mm/yr since ~5.4 Ma. The slip synthesis provides less detail about secondary faults, but two general points are clear. First, fault activity is highly variable in space, time, and sense of slip. Second, fault activity at any one time is highly localized; one fault may be active while a nearby structure is inactive.
To evaluate distributed off-fault strain history, we use a dataset derived from shortening measurements of five major unconformities on more than 60 cross sections. To estimate strain magnitude recorded by older surfaces, we progressively subtract shortening magnitude of young markers from older markers. The analysis demonstrates that strain magnitude is consistently largest within about 5 km of the SAF, providing evidence of long-term deformation partitioning near this major structure. Observed negative elongation magnitudes are roughly twice as large near the SAF compared to rocks of similar age farther away, with a gradual transition between the regions. This pattern of partitioning near faults is also apparent but less pronounced near the secondary faults SW of the SAF.
Variation in strain between different fault-bounded crustal blocks provides evidence of regional deformation partitioning. Blocks comprised of shallow basement with a veneer of sedimentary rock record less strain than blocks with thicker cover. Relative strain magnitudes for rocks of similar age suggest strain rate variations of 2-10 times between different blocks. This long-term deformation partitioning has produced strain magnitude variation of 2-5 times between crustal blocks.