Heat Flow and the Earthscope Initiative With an Emphasis on Heat Flow Studies in Support of the San Andreas Fault Observatory at Depth (SAFOD) Colin F. Williams and Arthur H. Lachenbruch, Earthquake Hazards Team, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025. E-mail: [email protected] John H. Sass, Volcano Hazards Team, U.S. Geological Survey, Flagstaff, AZ Introduction Two applications of heat flow in the geosciences are particularly relevant to Earthscope’s planned seismic and tectonic studies along the Pacific-North American plate boundary and elsewhere in the western U.S. First, because most physical and chemical processes relevant to rock mechanics and crustal rheology are dependent, at least in part, on temperature, an accurate estimate of in situ conditions is critical for providing the sort of “ground truth” necessary for evaluating the relevance of theoretical or laboratory-based models for geologic processes. Second, the vertical transport of rock and/or fluid disturbs the Earth’s thermal regime in ways that can be easily measured and modeled. The corresponding vertical transport phenomena, whether of the crust itself or of a fluid within the crust, usually reflect processes tied to active tectonics such as uplift and erosion, subsidence and sedimentation, or the possible redistribution of frictional heat on active faults due to ground-water flow. These basic aspects of thermal studies can be tied into a number of elements of the Earthscope initiative. For SAFOD, plans are well underway for detailed heat flow investigations in the proposed drillholes, and these plans are described in detail below. For PBO, the installation of approximately 175 strainmeters along the San Andreas fault system provides the opportunity to substantially augment the information available from more than 400 existing heat flow measurements in California. The scientific rationale for incorporating heat flow with the PBO strainmeter drilling is described in the abstract by Harris, Chapman and Williams (this volume). Finally, both USArray and InSAR offer opportunities to acquire complementary data to test competing interpretations of the observed variation of heat flow in the western U.S. For example, long-standing questions regarding the nature of regional thermal features of the Basin and Range could be resolved through detailed maps of crustal and lithospheric thickness generated by USArray, while the information provided by InSAR on the relationship of active deformation to fault-hosted hydrothermal systems could revolutionize our understanding of the nature and evolution of geothermal reservoirs. SAFOD Thermal Science Plan: As part of the USGS contribution to SAFOD, we plan to conduct a comprehensive series of thermal studies in the SAFOD pilot hole and subsequent fault-crossing hole at Parkfield in order to provide fundamental constraints on the processes and properties influencing the strength and behavior of the San Andreas fault zone. The primary objectives are to acquire precision temperature and thermal properties data, use these thermal data to derive vertical profiles of heat flow for each hole, and integrate the heat flow measurements with post-drilling temperature monitoring in an effort to discern any transient thermal signals associated with fault zone processes, including heat generation and mass transport. The planned studies address the overall scientific objectives of the drilling project in the following ways. (1) Fault strength: Near-surface heat flow measurements provided the first (and some of the most compelling) evidence that the San Andreas fault is substantially weaker than laboratory studies of frictional sliding would suggest (Brune et al., 1969; Henyey and Wasserburg, 1971; Lachenbruch and Sass, 1980; Williams et al., 2000). The conclusion that any frictional heat flow anomaly along the San Andreas fault is limited to less than 10 mW/m2 (equivalent to an average frictional strength of 20 MPa) is based on more than 100 heat flow measurements located along the San Andreas fault system. Data from shallow holes in the Parkfield area itself confirm the absence of a frictional heat flow anomaly, but the uncertainties associated with the individual measurements are greater than for many other locations along the fault (Sass et al., 1997). Consequently, a deep, highly accurate measurement of heat flow is critical for verifying the absence of a significant fault-centered heat flow anomaly at this location. Many of the uncertainties associated with shallow heat flow measurements (e.g. the thermal effects of topography, changing surface temperature, erosion, sedimentation, ground-water flow), are either negligible or more easily quantified by a vertical profile of heat flow to depths of 2 to 4 km. Evidence for fault weakness does not eliminate the possibility of fault-centered heat generation altogether. If the processes controlling heat generation and transport within the fault zone are primarily conductive, precise temperature and heat flow measurements across the fault at depth should provide the resolution needed to quantify the approximate level of frictional heating within the limiting values provided by the near-surface measurements. Further constraints on the level of fault-centered heat generation will come from long-term thermal monitoring as discussed in (4) below. Finally, the vertical and lateral extent of the thermal profile obtainable in the pilot hole will provide an assessment of the value of shallow heat flow measurements to estimate thermal conditions at seismogenic depths. (2) Deformation Mechanisms: Many of the deformation mechanisms thought to be active in both the shallow and seismogenic portions of the San Andreas fault zone are constrained by thermal stability fields (e.g., Morrow et al., 1992; Chester et al., 1993; Blanpied et al., 1995; Williams, 1996; Strehlau and Williams, 1998). Accurate equilibrium temperature measurements within the SAFOD holes will provide constraints on the temperature field at the depth of transition from aseismic creep to seismic slip. Correlation of in situ temperatures to the stability fields and reaction kinetics of mineral assemblages and deformation-related microstructures in the recovered core will provide investigators with quantitative constraints on the actual mechanisms determining the nature and strength of the fault zone at these depths. (3) Fluids and Fluid Transport: Many of the proposed mechanisms for fault weakness rely on the presence of fluids, particularly high pressure fluids, within the fault zone (e.g., Byerlee, 1990; Rice, 1992). There may also be a net flux of fluid within the plane of the fault or laterally either into or out of the plane of the fault. The vertical movement of fluids is a well known source of disturbance to conductive temperature gradients within the crust, and lateral fluid flow across the fault zone has also been suggested as a means of dissipating the frictional heat generated by fault slip (e.g., Williams and Narasimhan, 1989). Flow rates less than 0.01 m/yr can be distinguished in many circumstances, and a continuous thermal profile across the fault zone should be a particularly sensitive indicator of long- term fluid movement. The possible cyclic nature of permeability seals and fluid pressure suggests that fluid flow within the fault zone may also vary over time, either with the seismic cycle or over some other, as yet unknown, time scale. Because of the relatively slow rate of heat conduction in rocks, measurements within and immediately adjacent to the fault zone could reveal the timing and magnitude of these cyclic variations. For example, a 10 oC temperature anomaly on the fault plane varying at the approximate 22 year time scale of the characteristic Parkfield earthquake will follow a predictable and detectable path of amplitude decay and phase lag for many 10's of meters into the surrounding wall rock. Pressure, stress and permeability measurements within the fault zone will provide critical information on the present mechanical condition of the San Andreas, but the integration of thermal and chemical constraints on past fluid movement is very important to providing the context for the evolution of fluid pressure, stress and permeability over the seismic cycle and longer term. (4) Fault Zone Thermal Monitoring: The SAFOD holes provide a unique opportunity to look for thermal signatures associated with deformation over a time period of years to decades. As noted above, fault zone processes (and associated heat generation or fluid transport) may be episodic. Although detailed heat flow measurements may reveal the exact nature of heat generation within the fault zone, it is also possible that the potentially complex history of inter-related frictional heating, episodic fluid transients, and long-term fluid transport may not yield to analysis of a single fault- crossing heat flow profile. A well-designed program of temperature monitoring within the completed pilot is a necessary component of the search for signals related to fault strength and seismic processes. As noted by Lachenbruch (1986), fault slip under specified frictional conditions will generate a measurable transient heating anomaly. Both the amplitude and the lateral extent of the anomaly will evolve over time as the heat conducts into the surrounding country rock. At a fault crossing depth of 3.5 km, events with slip less than 0.1 m should yield measurable (>0.1 oC) temperature anomalies for more than 3 months after the event. It should