J. H. Sass

Heat flow from Cajon Pass, fault strength, and tectonic implications

Measured heat flow at Cajon Pass is consistent with predictions based on local site conditions and regional heat flow. With observations now ranging to a depth of 3½ km, there is still no evidence for significant frictional heating anywhere on the San Andreas fault. The result supports the view, long suggested from heat flow studies, that the fault is weak in spite of estimates based on Byerlees law, isotropic strength, and hydrostatic fluid pressure that suggest a strength several times larger. Recent evidence (Zoback et al., 1987; Mount and Suppe, 1987) that the maximum principal stress might be almost normal to the San Andreas fault would support the weak-fault model and add constraints over and above those imposed by heat flow; e.g., local friction coefficients μ ≲ 0.1 or fluid pressures along the fault greater than lithostatic (λ > 1), compared to μ ≲ 0.2 or fluid pressure greater than twice hydrostatic (λ > 0.74) for the heat flow constraint alone. These constraints are a challenge to existing models of faulting, and they are stimulating promising new points of view. The balance of plate boundary forces around a weak fault depends on the basal traction coupling the seismic layer to the rest of the system; heat flow limits the coupling force across the fault to an insignificant ∼ 1011 N/m. The weak fault also precludes significant near-field basal driving tractions, but it permits a large basal drag force which could result in a highly stressed seismic layer offering appreciable resistance to plate motion through its base. Such tractions could develop progressively if the fault surface weakens as it evolves; if they exist, they should cause an observable reduction in shear stress resolved in the fault direction and a rotation of principal axes as the fault is approached; if they do not exist, the seismic layer rides passively on the lower crust. Heat flow measurements should detect whether such basal tractions might be associated with basal decoupling and flow. Coupling at the base of the seismic layer is controlled by the rheological profile, the usual representation of which raises three questions in applications to the San Andreas fault zone. First, the linear frictional portion through the seismic layer implies a resisting force on the fault much greater than the heat flow limit permits. Second, the large stresses implied for the temperature-sensitive ductile layer might be unsustainable; they could lead to shear heating and weakening at plate boundary strain rates. Third, in the ductile layer the stress is sensitive to whether deformation is concentrated in narrow vertical mylonite zones, as sometimes assumed in models of the earthquake cycle, or more broadly distributed by bulk flow in a deep-crustal “asthenosphere.” Horizontal basal shear stresses are of the same order as vertical strike-slip stresses near the base of the seismic layer; they could result in bulk flow or horizontal detachment leading to a different pattern of long-term stress, strain rate, and dissipation and a requirement for decoupling and basal drag on the seismic layer in the near field. Results from the San Andreas fault taken with long-standing speculation about the orthogonal relation between oceanic transform faults and extensional spreading centers suggest that strike-slip transform faults might be anomalously weak in both continental and oceanic settings.

Characterization of rock thermal conductivity by high-resolution optical scanning

Abstract We compared three laboratory methods for thermal conductivity measurements: divided-bar line-source and optical scanning These methods are widely used in geothermal and petrophysical studies particularly as applied to research on cores from deep scientific boreholes The relatively new optical scanning method has recently been perfected and applied to geophysical problems A comparison among these methods for determining the thermal conductivity tensor for anisotropic rocks is based on a representative collection of 80 crystalline rock samples from the KTB continental deep borehole (Germany) Despite substantial thermal inhomogeneity of rock thermal conductivity (up to 40–50% variation) and high anisotropy (with ratios of principal values attaining 2 and more) the results of measurements agree very well among the different methods The discrepancy for measurements along the foliation is negligible (

Heat flow from a scientific research well at Cajon Pass, California

The long-standing “stress/heat flow paradox” was the primary scientific motivation for the Cajon Pass borehole. For nearly two decades, the absence of a fault-centered heat flow anomaly from measurements to relatively shallow (∼200 m) depths had indicated low average shear stresses (≤20 MPa) on the San Andreas fault, while laboratory data on rock strength and in situ stress determinations to about a kilometer had indicated high stress (∼100 MPa). Initial results from an unsuccessful 1.7-km-deep oil well at the site gave a high heat flow (∼90 mW m−2) consistent with a strong San Andreas fault; however, the late Cenozoic geologic history of the Cajon Pass area suggested that the anomalous heat flow was the transient effect of rapid erosion. Theoretical studies predicted that the ∼30% surface anomaly would be substantially reduced at depths of 3–5 km. The research borehole reached a total depth of 3.5 km. Below a superficial covering of Tertiary sedimentary rocks, it penetrated gneissic rocks with composition ranging from gabbroic to granodioritic. Core recovery amounted to only about 3% of the total depth, necessitating the use of drill cuttings to characterize thermal conductivity. This, in turn, resulted in much higher uncertainties in average conductivity (±10–15%) than would have occurred with a continuously cored hole (±3–5%). From a time series of temperature logs, equilibrium temperature gradients were established over selected intervals of 250–500 m to within 95% confidence limits of 2%. These gradients were combined with harmonic mean thermal conductivities having larger uncertainties to give interval heat flows, which vary systematically from 100 ± 5 mW m−2 in the uppermost 400 m to 75 ± 3 mW m−2 in the lowermost 300 m. Thus, at the Cajon Pass site, heat flow is decreasing with depth at a mean rate of more than 7 mW m−2 per kilometer, consistent with a frictionless fault and with theoretical predictions based on local erosional history.

Heat flow and subsurface temperature as evidence for basin-scale ground-water flow, north slope of Alaska

In conjunction with the U.S. Geological Surveys exploration program in the National Petroleum Reserve, Alaska (NPRA) several high-resolution temperature logs were made in each of 21 drillholes between 1977 and 1984. These time-series of shallow (average 600-m depth) temperature profiles were extrapolated to infinite time to yield equilibrium temperature profiles (±0.1 °C). Thermal gradients are inversely correlated with elevation, and vary from 22 °C/km in the foothills of the Brooks Range to as high as 53 °C/km on the coastal plain to the north. Shallow temperature data were supplemented with 24 equilibrium temperatures (±3-5 °C) estimated from series of bottom-hole temperatures (BHTs) measured near the bottom of petroleum exploration wells. A total of 601 thermal conductivity measurements were made on drill cuttings and cores. Near-surface heat flow (±20%) is inversely correlated with elevation and ranges from a low of 27 mW/m2 in the foothills of the Brooks Range in the south, to a high of 90 mW/m2 near the north coast. Subsurface temperatures and thermal gradients estimated from corrected BHTs are similarly much higher on the coastal plain than in the foothills province to the south. Significant east-west variation in heat flow and subsurface temperature is also observed; higher heat flow and temperature coincide with higher basement topography. The observed thermal pattern is consistent with forced convection by a topographically driven ground-water flow system; alternative explanations are largely unsatisfactory. Average ground-water (Darcy) velocity in the postulated flow system is estimated to be of the order of 0.1 m/yr; the effective basin-scale permeability is estimated to be of the order of 10-14 m2. Organic maturation data collected in other studies indicate that systematic variations in thermal state may have persisted for tens of millions of years. The ground-water flow system thought to be responsible for present heat-flow variations conceivably has existed for the same period of time, possibly providing the driving mechanism for petroleum migration and accumulation at Prudhoe Bay.

Thermal regime of the continental lithosphere

Abstract Thermally, the lithosphere may be defined as that outer portion of the earth in which heat is transferred primarily by conduction. It generally includes the crust and part of the mantle. The thermal regime of continental lithosphere is determined by many factors including heat flow from the asthenosphere, the vertical and lateral variation of both thermal conductivity and radiogenic heat production, tectonic history, and such superficial processes as climatic history and the shallow hydrothermal regime. From studies of the global heat flow data set, two generalizations regarding continental lithosphere have arisen, namely that: 1) there is a negative correlation between heat flow and tectonic age of continental lithosphere; and 2) the thermal evolution of continental lithosphere is similar to that of ocean basins with the result that the “stable geotherm” is similar in both environments. When continental heat-flow data are studied from a regional rather than a global point of view, considerable doubt arises as to the general applicability of either statement. R. U. M. Rao and his associates have demonstrated that while Precambrian terranes do have demonstrably lower heat flows than, say, Tertiary terranes, the data are not normally distributed and it is not possible to establish a negative correlation between heat flow and age in any rigorous statistical way. The scatter in the relation may be explained in terms of the variations in the duration, intensity and even the sign of continental thermotectonic events in contrast to the simple situation (creation of new oceanic lithosphere at mid-ocean ridges) which prevails in the oceans. The scatter also is partially attributable to the large and laterally variable radiogenic component of heat flow on continents. For a province for which a heat flow-heat production relation has been established, much of the scatter in surface heat flow due to crustal radiogenic heat production versus age is eliminated by determining reduced heat flow (surface heat flow minus radiogenic component) as a function of tectonic age, but much scatter remains, and it is still not possible to establish a heat flux-age relation in a rigorous way. Primarily because of the spatial variability in radiogenic heat production, no single geotherm can be used to characterize the thermal regime of a stable continental terrane. Thus, while some sites on stable continental blocks may have a geotherm fortuitously similar to that for old ocean basins, there is no reason to expect that this will be true generally, and many stable continental terranes will be characterized by geotherms markedly different from the geotherm for old ocean basins.

Thermal conductivity determinations on solid rock: a comparison between a steady-state divided-bar apparatus and a commercial transient line-source device

Abstract Two apparatuses were used to measure thermal conductivities on pairs of contiguous samples from 17 specimens of solid rock: the USGS divided-bar apparatus, a steadystate comparative method, and the Shotherm “Quick Thermal Meter” (QTM), which employs a transient strip heat source. Both devices were calibrated relative to fused silica. Both devices have a reproducibility of ±5% or better depending, to some extent, on the physical nature of the specimen being tested. For solid rocks, specimen preparation for the divided bar is much more tedious and expensive than for the QTM, which seems insensitive to minor surface roughness. The QTM does, however, require quite large specimens (30 mm × 60 mm × 100 mm as a minimum for rocks) with even larger specimens (50 mm × 100 mm × 100 mm) required for higher conductivity material (3.5 W m−1 K−1 and greater). Experimental times are comparable; however, the QTM is a self-contained unit that can be transported easily and set up quickly and requires no more space than a standard desk top. From a formal statistical comparison, it appears that, over a large range of conductivities (1.4 to ∼5 W m−1 K−1) and rock types, the two instruments will yield the same value of thermal conductivity for isotropic rocks.

Thermal regime of the southern Basin and Range Provincec 1. Heat flow data from Arizona and the Mojave Desert of California and Nevada

With about 150 new heat flow values, more than 200 values of heat flow are now available from the crystalline terranes of southern California, the Basin and Range Province of Arizona, and Paleozoic sedimentary rocks of the southwestern Colorado Plateau lCPr. Heat flow ranges from about 5 mW m−2 on the CP near Flagstaff, Arizona, to more than 150 mW m−2 in the crystalline rocks bordering the Salton Trough in SE California. The heat flow pattern within this region is complex, although it correlates with regional physiographic and tectonic features. Unlike the adjacent Sierra Nevada Batholith where heat flow is a linear function of nearhsurface radiogenic heat production, no statistically significant correlation exists between heat flow and heat production in the study area, possibly because of its complex tectonic history, involving lateral movement of basement terranes, and relatively young heat sources and sinks of different strengths, ages, and durations. Contemporary and Neogene extensional tectonism appears to be responsible for the very high heat flow l>100 mW m−2r associated with the Salton Trough and its neighboring ranges, the Death Valley fault zone and its southward extension along the eastern boundary of the Mojave block, and zones of shallow depth l<10 kmr to the Curie isotherm las inferred from aerornagnetic datar in west central Arizona. Low l<60 mW m−2r heat flow in the Peninsular Ranges and eastern Transverse Ranges of California may be caused by downward advection associated with subduction and compressional tectonics. Relatively low heat flow l67±4 mW m−2r is also associated with the main trend of metamorphic core complexes in Arizona, and the outcropping rocks in the core complexes have a low radioactive heat production l1.3±0.3 μW m−3r compared to the other crystalline rocks in the region l2.1±0.2 μW m−3r.

Determination of thermal conductivity for deep boreholes

Two methods for thermal conductivity determinations on rock cores and fragments were tested on a suite of samples from the Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland (KTB) superdeep drill hole in Germany. They were also compared with estimates of thermal conductivity using the mineral composition of the rock and physical well logs and with in situ thermal conductivity measurements. Laboratory methods provide reasonably precise determinations of the thermal conductivities of both solid core (±5%) and drill cuttings (±10%) at room temperature and pressure. The most common methods presently used for crystalline rocks are the steady state divided-bar (DB) technique and the transient half-space line source (LS). Sample preparation and measurement times are comparable for the DB and LS, with sample preparation being more time consuming on average. For isotropic rocks there is little to choose from between the two methods, which both give reliable values of conductivity in the vertical direction. The LS is easier to set up and use in field laboratory situations, which renders it the preferred method for field reconnaissance. The gneissic crystalline rocks penetrated by the KTB boreholes typically have anisotropy of the order of 10-20%. The DB provides unambiguous values of conductivity in a given direction, so its use is preferable for obtaining both principal conductivities and the vertical component. Anisotropy can be estimated using LS measurements in many different directions, but the potential for large random errors is much greater than with the more straightforward DB approach. For deep research wells the difficulties of extrapolating laboratory results to in situ conditions (particularly temperature) present additional obstacles to determining heat flow. Laboratory measurements of water-saturated samples under in situ conditions, combined with in situ measurements and judicious use of calculations based on mineralogy and well log derived physical properties, can aid in the accurate characterization of thermal conductivity in deep wells. The application of different methods helped to link variations of heat flow with depth in the KTB hole to the anisotropy of thermal conductivity or thermal refraction and thus allowed the calculation of background heat flux in this geologically complex area.

Thermal conductivity of water‐saturated rocks from the KTB Pilot Hole at temperatures of 25 to 300°C

The conductivitites of selected gneiss (two) and amphibolite (one) core samples have been measured under conditions of elevated temperature and pressure with a needle-probe. Water-saturated thermal conductivity measurements spanning temperatures from 25 to 300°C and hydrostatic pressures of 0.1 and 34 MPa confirm the general decrease in conductivity with increasing temperature but deviate significantly from results reported from measurements on dry samples over the same temperature range. The thermal conductivity of water-saturated amphibolite decreases with temperature at a rate approximately 40% less than the rate for dry amphibolite, and the conductivity of water-saturated gneiss decreases at a rate approximately 20% less than the rate for dry gneiss. The available evidence points to thermal cracking as the primary cause of the more rapid decrease in dry thermal conductivity with temperature. The effects of thermal cracking were also observed in the water-saturated samples but resulted in a net decrease in room-temperature conductivity of less than 3%. These results highlight the importance of duplicating in-situ conditions when determining thermal conductivity for the deep crust.

Fluid flow in the resurgent dome of Long Valley Caldera: implications from thermal data and deep electrical sounding

Abstract Temperatures of 100°C are measured at 3 km depth in a well located on the resurgent dome in the center of Long Valley Caldera, California, despite an assumed >800°C magma chamber at 6–8 km depth. Local downflow of cold meteoric water as a process for cooling the resurgent dome is ruled out by a Peclet-number analysis of temperature logs. These analyses reveal zones with fluid circulation at the upper and lower boundaries of the Bishop Tuff, and an upflow zone in the metasedimentary rocks. Vertical Darcy velocities range from 10 to 70 cm a −1 . A 21-km-long geoelectrical profile across the caldera provides resistivity values to the order of 10 0 to >10 3 Ωm down to a depth of 6 km, as well as variations of self-potential. Interpretation of the electrical data with respect to hydrothermal fluid movement confirms that there is no downflow beneath the resurgent dome. To explain the unexpectedly low temperatures in the resurgent dome, we challenge the common view that the caldera as a whole is a regime of high temperatures and the resurgent dome is a local cold anomaly. Instead, we suggest that the caldera was cooled to normal thermal conditions by vigorous hydrothermal activity in the past, and that a present-day hot water flow system is responsible for local hot anomalies, such as Hot Creek and the area of the Casa Diablo geothermal power plant. The source of hot water has been associated with recent shallow intrusions into the West Moat. The focus of planning for future power plants should be to locate this present-day flow system instead of relying on heat from the old magma chamber.