Quaternary slip history of the Santa Susana fault, western Transverse Ranges: Insights from U-Pb detrital zircon geochronology

Jonathan Ingram, Reed J. Burgette, & Brian A. Hampton

Published August 14, 2018, SCEC Contribution #8540, 2018 SCEC Annual Meeting Poster #256

The Santa Susana fault (SSF) is the western extension of the Sierra Madre reverse fault system, an east-west trending reverse fault system in the western Transverse Ranges. The SSF has the highest possible slip rate of any Sierra Madre fault segment but a wide uncertainty in slip rate in the UCERF-3 model. This ranges from negligible to significant (0.5-10 mm/yr). Slip on the SSF and related faults was associated with growth of the Santa Susana Mountains, which form the southern boundary of the Ventura basin. The Saugus Fm. in the footwall of the SSF is comprised of poorly lithified Pleistocene strata and was cut off from source areas to the north by slip on the SSF and uplift of the mountains. The upper range of the UCERF-3 slip rate is derived from previous studies that estimated initiation of uplift between 2.3-0.7 Ma through paleomagnetic dating and upsection changes in clast provenance, yielding a geologic slip rate of 5.9 ±3.9 mm/yr. Our study utilizes detrital zircon geochronology and sandstone modal compositions to build upon previous cobble provenance studies to examine the provenance of the Saugus Fm. and refine the timing of uplift of the Santa Susana Mountains.

Samples were collected from the Saugus Formation along stratigraphic transects in the SSF footwall and from Neogene units exposed in the hanging wall. Sandstone modal compositions and U-Pb detrital zircon ages are consistent with sediment derived from crystalline basement in the San Gabriel Mtns. Distinctive upsection trends in the detrital zircon spectra of the samples reveal changes in provenance over time. Across the transition from the lower marine to the middle non-marine members of the Saugus Fm. there is a disappearance of the 1.2 Ga anorthosite complex signature from the north in the San Gabriel Mtns. and a shift to similar provenance with the upper Neogene units in the SSF hanging wall. This indicates that the SSF hanging wall was a local source of sediment by ~1.1 Ma based on sedimentation rates and sample locations.
Preliminary models for the structural growth of the SSF and balanced cross sections are used to estimate slip for two portions of the Quaternary that can be tied to provenance changes. The cross sections were balanced to the top of the Neogene Pico Fm., which we infer to pre-date the reverse activation of the SSF. This refined slip record for the SSF will be used to assess slip variation over the Quaternary history of the SSF.

Citation
Ingram, J., Burgette, R. J., & Hampton, B. A. (2018, 08). Quaternary slip history of the Santa Susana fault, western Transverse Ranges: Insights from U-Pb detrital zircon geochronology. Poster Presentation at 2018 SCEC Annual Meeting.

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