The influence of climate variability on records of Holocene slip rate: Insights from the Pearblossom site, Mojave San Andreas

Monday, October 26, 2020 - 12:20pm
U-Mass Geosciences

Slip rates on the Mojave San Andreas fault (MSAF) are not well understood, but are useful for evaluating seismic hazard and for understanding how regional fault systems develop and interact.  Here I calculate a minimum slip rate at the Pearblossom site of 17 mm/yr and a maximum slip rate of 76 mm/yr by reconstructing a beheaded channel that has been offset by  34 ± 2 to 81 ± 2 m since between 1.44 ± 0.43 ka and 1.27 ± 0.18 ka, as determined by feldspar infrared stimulated luminescence dating.  A pulse of aggradation at ~0.6 ka both upstream and downstream of the fault at Pearblossom suggests that in addition to fault slip, climate has influenced deposition at the site.  Comparison of charcoal ages from slip rate sites, paleoseismic sites, and lake records with records of Southern California climate from sedimentological features of lake cores indicates that changes in precipitation drive sediment deposition and channel incision events on the MSAF.  This relationship has two important implications for slip rate studies.  First, markers of offset may not form during dry periods.  Second, pulses of aggradation during wet periods may complicate site reconstructions and produce offsets that appear to be caused by fault slip but are instead caused by incision or deposition in a deflected configuration.  Channel deflection morphology and upstream channel position can provide an indication of whether a site is likely to record useful information about fault slip.  A deflected channel that is narrow relative to the width of its deflection is likely to have incised in a deflected configuration.  For a right-lateral fault, a deflected channel incised on the left side of an alluvial fan gives a minimum offset measurement, while a deflected channel incised on the right side gives a maximum offset measurement.

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