INVESTIGATING THE STRUCTURE OF THE ‘SAND HILLS FAULT’ THROUGH DIFFUSE CO2 DEGASSING MEASUREMENTS

Alexander Berne, & Joann M. Stock

Submitted August 16, 2021, SCEC Contribution #11569, 2021 SCEC Annual Meeting Poster #078

Carbon Dioxide (CO2 ) is continuously discharged from volcanic systems with shallow subsurface magma chambers (Irwin and Barnes, JGR,1980). In these regions, faulted rock exhibits a higher permeability than surrounding country rock and can act as a conduit to guide released CO2 to the surface. Past CO2 degassing surveys demonstrate a strong spatial dependence of diffuse CO2 emissions (e.g., soil degassing) on the position of mapped geologic faults elsewhere (Poret et al., JVGR, 2019; Sun et al., FES, 2021). Near the Salton Buttes, a volcanically active area in the Imperial Valley to the southeastern end of the Salton Sea in Southern California (USA), a suspected fault structure known as the Sand Hills Fault (Wister Mud Pot Lineament) correlates with the position of numerous ‘mud pots’ (e.g., structures which actively vent CO2) (Lynch and Hudnut, BSSA, 2008). The trace of the Sand Hills Fault coincides with the possible location of a now-abandoned portion of the Southern San Andreas Fault; confirmation of its existence would therefore constrain tectonic evolutionary models of the Salton Trough. However, despite its importance, only very limited geologic or geophysical evidence exists for a fault structure in the area (Elders et al., Science, 1972). This structure is not presently seismically active and lies east of the seismicity that has been recorded in the Brawley Seismic Zone.

We conduct a regional CO2 ground flux survey to identify the spatial pattern of diffuse degassing around the Sand Hills Fault. Measurements are conducted near known mudpot locations in the Wister Unit, a location at the southeast of the Salton Sea which coincides with the northwestern segment of the Sand Hills Fault. CO2 emitted from the ground is collected into a control volume housed with an Arduino-based NDIR CO2 sensor. Change in CO2 concentration in the control volume per unit time is used to determine ground flux at a particular location. Using collected field data, we generate 2D spatial maps of CO2 ground flux at two sampled sites. Ground flux values vary from 0-2 g m-2 day -1 (background) to ~1000 g m-2 day -1. Additionally, degassing structure azimuths are consistent with a regional southeastward extension of the San Andreas Fault. From these data, we determine that degassing structures are an expression of an underlying regional fault structure, and thereby provide additional evidence for the existence of the Sand Hills Fault.

Citation
Berne, A., & Stock, J. M. (2021, 08). INVESTIGATING THE STRUCTURE OF THE ‘SAND HILLS FAULT’ THROUGH DIFFUSE CO2 DEGASSING MEASUREMENTS. Poster Presentation at 2021 SCEC Annual Meeting.


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