HR: 1340h
AN: T13B-0460    [Abstracts]
TI: Flow and Chemistry Pulsations, Monterey: Implications for Stress Transient Modulations of Hydrologic and Geochemical Systems in the Greater San Andreas Fault Zone
AU: Brown, K M
EM: kmbrown@ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, La Jolla, San Diego, CA 92093 United States
AU: Fueri, E
EM: efuri@ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, La Jolla, San Diego, CA 92093 United States
AU: * Hilton, D R
EM: drhilton@ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, La Jolla, San Diego, CA 92093 United States
AB: Submarine fluid venting at continental shelf and slope regions has been recognized over the past ten years as an important, yet under-studied process in marine science. Seeps are now known to be a general feature of the hydrogeology of many tectonically active continental margins. The eastern Pacific margin is characterized by a variety of tectonic settings (i.e. convergent and strike-slip) where active venting of fluids and gases has been documented. Reports include vents off Alaska, Costa Rica, Monterey Bay, Eel River basin, and Heceta Bay, OR. Indications of seismic tremor, linked to hydrologic transience in the offshore regions of subduction zones have recently been published elsewhere (see Brown et al, EPSL 2005). We now address here the varying nature of submarine fluid discharges in a San Andreas strike-slip setting. A key element of the proposed work is the combined multidisciplinary measurement of fluid flow, seep temperatures, and dissolved noble gases and chemistry of the Monterey seep sites at Extrovert Cliff. The seeps are situated close to several active strike-slip faults including the Monterey and San Gregorio fault zones. Initial results of 2 week deployments in 2004 of flow meters at Extravert Cliff indicated high flow rates and elevated seep temperatures that vary by as much as a factor of 2 on diurnal time scales with subtle changes over longer periods (>2 weeks). There are also indicative chemical signals of deeply sourced fluids that vary widely with time that show the following signals: 1) Elevated abundances of both mantle derived Helium (3He) as well as 4He and 40Ar of radiogenic crustal relevant trace element components; 2) Altered fluid chemistry (including, Ca Mg, Li and B); 3) The fluid temperature, flow rates, and gas chemistry, in particular, vary with time. We have both long-term and sub-diurnal variations in flow and temperature as well as the 3He/4He ratios, helium concentration, CO2 concentration and d13C values perhaps influenced by tidally (and tectonically?) induced subsurface mixing processes. As a result of the sensitivity to the fluid source regions, recently, attention has turned to assessing the behavior of He in regions of amagmatic seismic activity and extensive faulting. A key study in this respect is that deep pore fluids from the San Andreas Fault (SAF) have elevated 3He/4He ratios (i.e. greater than crustal production values). Other recent studies in the western US have shown elevated ratios in groundwater basins in close proximity to regions of active faulting outside of the SAF. There are well-documented indications that fluid chemistry can change prior and in response to seismic activity. Based on our initial analysis at Monterey, the composition of some seeps has been found to vary between crustal and near mantle end member He and CO2 compositions over short periods of time. This is partly because B and tidal pore pressure changes DP in the subsurface aquifers supplying the seeps vary in size and timing with depth. We put forward a conceptual poroelastic model showing how over diurnal time periods tidal loading, and loading efficiency (B) may interact with the three different fluid sources to generate short period variability in seep flow, temperature and geochemistry out put. Given the seeps outputs' sensitivity to tidal loading, we hypothesize that such seeps should be highly sensitive indicators of subsurface tectonic stress changes of both a aseismic and seismic nature and that subtle precursory effects in out put chemistry, flow rates, and temperature should not be unexpected.
UR: http://daneel.ucsd.edu/Brown2005.pdf
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1207 Transient deformation (6924, 7230, 7240)
DE: 1222 Ocean monitoring with geodetic techniques (1225, 1641, 3010, 4532, 4556, 4560, 6959)
DE: 1242 Seismic cycle related deformations (6924, 7209, 7223, 7230)
DE: 3021 Marine hydrogeology
SC: Tectonophysics [T]
MN: Fall Meeting 2005