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SAN ANDREAS FAULT STUDIES: FLUIDS, STRENGTH, AND HEAT TRANSPORT

Heat Flow Studies | Pore Pressure and Fault Weakness | Lab Measurements | Papers

 

ADVECTIVE HEAT TRANSPORT & THE STRENGTH OF THE SAN ANDREAS FAULT

Along the San Andreas Fault in California, heat flow observations and inferred stress orientations have been interpreted to indicate that the fault is weak. Specifically, heat flow data along much of the San Andreas Fault's length show no indication of a fault-centered heat-flow high, as would be expected from frictional heating as the fault moves (red curves in the two plots shown below). Stress orientations, inferred from geological structures, earthquake focal mechanisms, and borehole breakouts, have also been interpreted to indicate that the San Andreas is weak in comparison to the surrounding crust. Taken together, the heat flow and stress observations have led to the conclusion that the San Andreas is a "weak fault", in that it slips under considerably smaller shear stresses than predicted by laboratory experiments on a wide range of rocks. This discrepancy has become known as the San Andreas “stress - heat flow paradox”.

Critics of the weak fault hypothesis have suggested that the fault actually does generate frictional heat, but that groundwater flow, driven by topographic relief, redistributes and effectively “hides” the heat. The idea of heat redistribution by topographically-driven groundwater flow is attractive because there is considerable topography associated with the fault itself. The two scenarios depicted below constitute end-member hypotheses that could explain observed heat flow (the pink curves). On the left-hand-side, a strong fault could generate frictional heat, which is then "smeared out" by groundwater flow. On the right-hand-side, a weak fault would generate only a small amount of heat. On the left, a strong fault generates heat, which is carried away by groundwater flowing from high elevation recharge areas to low elevation discharge areas. On the right, low strength (possibly caused by a combination of high pore pressures and weak fault gouge) results in a small amount of heating.

Our work has shown that topographically driven groundwater flow is an unlikely explanation for the lack of an observed heat flow anomaly. Numerical models of coupled fluid flow and heat transport predict decreased heat flow in areas of groundwater recharge (topographic highs), and elevated heat flow in areas of groundwater discharge (topographic lows); yet this is not observed (below, left). Our results show that the data are most consistent with a “weak fault”, even in the presence of substantial advective heat redistribution. Furthermore, the degree of scatter in observed heat flow is inconsistent with substantial advective heat transport (below, right).   Graduate students Patrick Fulton and Maggie Popek have worked with me to understand sources of scatter in the heat flow data near Parkfield, in order better constrain the signals from hydrologic processes and frictional heating.

 

 

 

PORE PRESSURE AND THE STRENGTH OF THE SAN ANDREAS FAULT

If the San Andreas Fault is indeed weak, elevated fluid pressures are one likely explanation. Hypothesized mechanisms for generating elevated fluid pressures along the San Andreas include dehydration of the Franciscan assemblage (right; after Irwin & Barnes, 1975) and upward flux of mantle fluids. We have rigorously tested these hypotheses, using numerical models of fluid flow that incorporate well-constrained and realistic ranges for the timing of fluid release, and consider a wide range of crustal permeability architectures.

 

Dehydration of the Franciscan assemblage generates fluids for only a limited time, between 0-5 Myr after formation of the San Andreas fault system (left, after Fulton et al., 2009). The figure below shows a transect parallel to the fault based on the “crustal conveyor” geodynamic model of Furlong & Guzofski (2002). The fluid sources are too small and too short-lived to generate or sustain fluid pressures high enough needed to explain the apparent weakness of the San Andreas (below) (Fulton et al., 2009; Fulton and Saffer, 2009).

 

Recent data from the SAFOD borehole suggest that the fault acts as a hydrologic barrier to depths of ~3 km (left). These observations include moderate fluid overpressures documented by mud weights, and a mantle helium signature in mud gases, both seen only to the NE of the fault (schematic showing these observations at top, geophysical logs at bottom). Some of our recent work has focused on evaluating the potential role of mantle fluids in generating excess pore pressures either throughout the crust or localized along the fault in a general sense. The results show that mantle derived fluids, possibly related to serpentinite dehydration, could impact the mechanics of the San Andreas (Fulton and Saffer, 2009).

 

 

 

 

LABORATORY STUDIES OF SAN ANDREAS MATERIALS: PERMEABILITY & STRENGTH

 

 

Above right: Map showing field sample locations for outcrop samples, taken to represent the main lithologies in the vicinity of the San Andreas Fault. Many of these lithologies were not penetrated by the SAFOD borehole. Above Left: Detailed cross section near the SAFOD borehole, showing interpreted rock units in the subsurface (courtesy of R. Arrowsmith and M. Thayer, Ariz. State Univ.). Below: Field sampling of lower Great Valley sedimentary rocks on the NE side of the fault.

 

 

We are currently working on a collaborative project to measure the frictional, permeability, and elastic properties of the active San Andreas Fault (SAF) zone sampled during SAFOD Phase 3 drilling, and outcrop samples of lithologies present within the 3-D crustal volume containing the fault. Our laboratory measurements will help to constrain: (1) the strength, sliding stability, and healing of major faults, and (2) the hydraulic behavior of faults - both locally as related to long-term and dynamic weakening mechanisms and regionally as elements within crustal scale fluid flow systems. The friction and permeability measurements are being carried out in the Penn State rock and sediment mechanics laboratory; elastic property measurements are being conducted by colleague Harold Tobin at the University of Wisconsin.

 

 

 

 

 

Relevant publications & meeting abstracts on this topic:

Fulton, P.M., and Saffer, D.M. (2009), Potential role of mantle-derived fluids in weakening the San Andreas Fault, J. Geophys. Res., 114, B07408, doi:10.1029/2008JB006087.

Fulton, P.M., Saffer, D.M., and Bekins, B.A. (2009), A critical evaluation of crustal dehydration as the cause of a weak and overpressured San Andreas Fault, Earth Planet Sci. Lett., 284, 447-454, doi:10.1016/j.epsl.2009.05.009.

Fulton, P.M., and Saffer, D.M. (2009), The Effect of Thermal Refraction on Heat Flow near the San Andreas Fault, Parkfield, CA., J. Geophys. Res., 114, B06408, doi:10.1029/2008JB005796.

Carpenter, B.M., Marone, C., and Saffer, D.M. (2009), Frictional Behavior of Materials in the 3D SAFOD Volume, Geophys. Res. Lett., 36, L05302, doi:10.1029/2008GL036660.

Fulton, P.M., Saffer, D.M., and Bekins, B.A., Debunking crustal dehydration as the cause of a weak and overpressured San Andreas Fault, submitted, Earth Planet Sci. Lett., Feb. 2008.

Fulton, P., Saffer, D.M., Harris, R.N., and Bekins, B.A., 2004, Re-evaluation of heat flow data near Parkfield, CA: Evidence for a weak San Andreas Fault, Geophys. Res. Lett., 31, L15S15, doi: 10.1029/2003GL019378.

Saffer, Demian M., Bekins, B.A., and Hickman, S.H., 2003, Topographically driven groundwater flow and the San Andreas heat flow paradox revisited, J. Geophys. Res., 108 (B5), doi:10.1029/2002JB001849.

Harris, R.N., D.S. Chapman, K.P. Furlong, D.M. Saffer, 2004, Thermal processes in the context of EarthScope, EOS, 85, 292.

Fulton, P.M., and Saffer, D.M., Mantle-derived fluids and their potential role in weakening the San Andreas Fault, AGU Fall meeting, 2007.

Fulton, P.M., and Saffer, D.M., The Effect of Thermal Refraction on Heat Flow Scatter near the San Andreas Fault, Parkfield, CA, Earthscope National meeting, 2007.

Fulton, P.M., and Saffer, D.M., The Effect of Thermal Refraction on Heat Flow Scatter near the San Andreas Fault, Parkfield, CA, AGU Fall meeting, 2006.

Fulton, P.M., Saffer, D.M., Harris, R.N., and Bekins, B.A., Thermal Processes and Their Signature in California Coast Range Heat Flow Measurements, 6th International Meeting, Heat flow and the structure of the lithosphere, June 5-10, 2006, Czech Republic.

Fulton, P.M., Saffer, D.M., Bekins, B.A., Crustal Dehydration and Overpressure Development on the San Andreas Fault, AGU Fall meeting, 2005.

Fulton, Patrick M., Saffer, D.M., and Bekins, B.A., 2005, Fluid overpressures on the San Andreas Fault following passage of the Mendocino Triple Junction, Earthscope National Meeting, Albuquerque, NM, March, 2005.

Fulton, P.M., Saffer, D.M., and Bekins, B.A., Fluid overpressures on the San Andreas Fault following the passage of the Mendocino Triple Junction, AGU Fall meeting, 2004.

Fulton, Patrick M., Saffer, D.M., Harris, R.N., and Bekins, B.A., 2004, 3-D terrain corrections, groundwater flow, and the SAF heat-flow paradox revisited, Earthscope workshop on thermal processes, March, 2004.

Fulton, P.M., Saffer, D.M., Bekins, B.A., and Harris, R.N., 3-D Terrain Corrections to Heat Flow Data, Topographically-Driven Groundwater Flow, and the Strength of the San Andreas Fault at Parkfield, CA, AGU Fall meeting, 2003.

 

 

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