Paleomagnetism of the Ocotillo Badlands, southern California, and implications for slip transfer through an antidilational fault jog

Details Paleomagnetism of the Ocotillo Badlands, southern California, and implications for slip transfer through an antidilational fault jog Paleomagnetism of the Ocotillo Badlands, southern California, and implications for slip transfer through an antidilational fault jog

Mar 1991

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991e%26psl.102..277b&link_type=abstract

Earth and Planetary Science Letters, Volume 102, Issue 3-4, p. 277-288.

Computer Science

2

Scientific paper

The right-lateral Coyote Creek fault is the southernmost segment of the San Jacinto fault zone in California. At the Ocotillo Badlands, an anticlinorium of locally updomed material is exposed at a step-over between echelon segments of the fault. Here, uplift and deformation most probably result from multiple slip increments transferred across the antidilational fault jog, under the assumption that material away from the jog region deforms by rigid body translation. Within the 2 km wide fault jog, contraction has occurred by folding of the Quaternary sedimentary strata about east-west trending hinge lines. Structural analysis of this deformation shows that folding has accommodated ~ 800 m of fault slip transferred through the antidilational jog. By comparison, total right slip on the Coyote Creek fault is 2.5 km, measured 25 km to the northwest at Coyote Ridge. A magnetostratigraphic study of the deformed strata was undertaken to determine the longevity of this fault discontinuity at the Ocotillo Badlands.
The eroded core of the updomed material at the Ocotillo Badlands exposes a 325 m thick sequence of sedimentary rock, made up by the lacustrine Borrego Formation (~ 200 m) and the overlying conglomeratic Ocotillo Formation (~ 125 m). Stepwise thermal demagnetization of samples from the exposed strata suggests that the formation boundary also marks a magnetic polarity reversal, of reversed field to normal field as one travels upwards through the composite section. The inferred primary magnetization is probably produced by a detrital remanent magnetization (DRM) and is often overprinted with a weak, viscous present field component. Comparison with magnetostratigraphy of the Borrego Badlands 10 km to the northwest indicates that the reversal sampled is the onset of the Jaramillo event (0.97 Ma).
The presence of a 20° angular unconformity within the upper portion of the exposed stratigraphy shows that deformation within the Ocotillo Badlands began during deposition of the Ocotillo Formation, shortly after the time of the field reversal. If total slip on the Coyote Creek fault at the Ocotillo Badlands is greater than 800 m, it appears that the fault jog has been a transitory feature within the fault zone, with slip alternately bypassing it or being transferred across it. Such switchyard behavior of slip transfer through the fault jog has important implications for understanding structural controls on earthquake rupture.

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