David D. Blackwell

Heat generation of plutonic rocks and continental heat flow provinces

Combined radioactivity and heat flow measurements in pluionie rocks at 38 localities in the United States define three heat flow provinces; the eastern United States, the Sierra Nevada, and a zone of high heat flow in the western United States which includes the Basin and Range province. In each of these provinces heat flow ( Q ) and heat production ( A ) are related by an equation of the form Q = a + bA . The simplest interpretation of this linear relation is that the radioactivity measured at the surface is constant from the surface to depth b but varies from place to place. Thus the fraction of heat flow from the lower crust and upper mantle, a , remains constant within a heat flow province while the variable upper crustal radioactivity generates the variable heat flow observed at the surface. In the eastern United States b = 7.5 km and a = 0.79 cal/cm 2 sec, in the Sierra Nevada b = 10.1 km and a = 0.40 cal/cm 2 sec, and in the Basin and Range province b = 9.4 km and a = 1.4 cal/cm 2 sec. The line characteristic of the eastern United States may have broad applicability to stable portions of continents and thus be considered the reference curve for normal continental heat flow. The similarity of all the slopes indicates that most local variability of heat flow is due to sources in the uppermost 7–11 km of the earths crust, and that the contribution from the lower crust and upper mantle is quite uniform over large regions. The intercept values can be used to infer the proportion of heat flow from the mantle and to map provinces with different mantle heat flow. These heat flow provinces correlate closely with surface geological provinces.

Neotectonics of North America

This volume is part of the Geological Society of America’s Continent-Scale Map (CSM) subset of the Decade of North American Geology (DNAG) publications. Twenty-eight chapters deal with regional discussions of seismicity, stress, and thermal aspects of North America. One chapter provides a regional overview of North American neotectonics, and another deals with late Quaternary isostatic recovery of North America and Greenland. Seventeen chapters discuss seismicity, four discuss regional stress fields, and five discuss regional patterns of the thermal aspect data. These chapters supplement the information portrayed on three 1:5,000,000-scale maps of North America in the DNAG CSM subset: the Seismicity Map, the Stress Map, and the Geothermal Map.

Thermal Conductivity of Sedimentary Rocks: Measurement and Significance

The thermal histories of sedimentary basins and their effect on organic maturation are topics of active study. The focus of these studies is on large-scale thermal events, such as an initial rifting event, that affect temperatures in a basin. Events of less global significance, however, are more important to the internal temperatures of a sedimentary basin. Such effects as internal thermal events (magma intrusion, diaparism), contrasts in heat production of U, Th, and K in the sediments and underlying basement, large- and small-scale flow of fluid, and thermal conductivity variations, both vertical and horizontal, can raise or lower temperatures much more than lithospheric-scale events. The nature and effect of such thermal effects are briefly discussed in this chapter. The most basic effect, but one of the least well known, is the thermal conductivity of the rocks in the basin. If the mean thermal conductivity cannot be accurately predicted, even the most sophisticated and appropriate modeling techniques for analyzing thermal histories and organic maturation levels may fail when applied to real basins. Temperature variations related to thermal conductivity variations are illustrated using precision temperature-gradient logs from various sedimentary basin settings. Different ways of determining the thermal conductivity of sedimentary rocks are discussed, including laboratory measurements on cuttings and core samples, in situ direct measurements, inference from well log measurements of travel time, gamma-ray activity and so forth, conversion of seismic reflection travel time to thermal resistance, and inversion of detailed temperature logs. Laboratory measurements are in some cases unreliable, especially for shales, one of the most abundant sedimentary lithologies. Actual shale thermal conductivities appear to be 25 to 50% lower than the literature values and do not appear to vary as a function of compaction in the expected way. Thus, some sort of in situ technique of thermal conductivity determination is needed. The use of precision temperature logs with spot sampling for laboratory comparison is favored and several examples of this technique from the Midcontinent, Gulf Coast, and Rocky Mountains are illustrated. The detailed temperature log from the Gulf Coast demonstrates high gradients in shale sections at 2 km depth because of the low thermal conductivity. The thermal properties of shale have implications for interpretation of the thermal effects of geopressuring.

Heat flow patterns of the North American continent: A discussion of the DNAG Geothermal Map of North America

The large and small-scale geothermal features of the North American continent and surrounding ocean areas illustrated on the new 1:5,000,000 DNAG Geothermal Map of North America are summarized. Sources for the data included on the map are given. The types of data included are heat flow sites coded by value, contours of heat flow with a color fill, areas of major groundwater effects on regional heat flow, the top-of-geopressure in the Gulf Coast region, temperature on the Dakota aquifer in the midcontinent, location of major hot springs and geothermal systems, and major center of Quaternary and Holocene volcanism. The large scale heat flow pattern that is well known for the conterminous United States and Canada of normal heat flow east of the Cordillera and generally high heat flow west of the front of the Cordillera dominates the continental portion of the map. However, details of the heat flow variations are also seen and are discussed briefly in this and the accompanying papers.

Thermal Constraints on Earthquake Depths in California

The high-quality databases for California seismicity (from the Southern and Northern California Earthquake Centers) and an extensive compilation of thermal measurements in California are used to quantify the concept of temperature as a fundamental parameter for determining the thickness of the seismogenic zone. The base of this zone, below which only a small percentage of crustal earthquakes occur, is termed the “cutout depth,” and it is at or near the brittle-ductile transition in the crust. Based on laboratory deformation studies, this transition should be temperature, strain rate, lithology, and stress-state dependent. In this study, qualitative comparisons between the heat flow in California and earthquake hypocentral distributions show first that, as expected, earthquake cutout depths are inversely related to heat flow. Second, the epicentral distributions tend to parallel thermal transitions. This correlation is probably related to stress concentrations in these locations. An ancillary observation is that the cooler western Mojave block in southern California appears to be behaving similarly to the Tibetan indenter block in Asia, as faults tend to go around it into areas of higher heat flow where the seismogenic zone is thinner and the crust may be weaker, and it is pushed toward (and under) the southern cold Sierra Nevada block. Third, to quantitatively compare the data sets, temperatures for the seismicity cutout depth in California were calculated using the steady-state, 1D heat conduction equation, with the variables based on published values for heat flow, heat generation, and thermal conductivity. The analysis was restricted to profiles along which the heat flow and earthquake databases were constrained, which allowed the error in the temperature calculation to be determined using Monte Carlo simulations. The results show that two distinct ranges of temperatures (dependent upon location) describe the cutout depth ( D 99%) of seismicity for California: 450 ± 50°C and 260 ± 40°C. The 450 ± 50°C cutout depth temperature is most widespread geographically, occurs within many provinces, and is higher than the frequently referenced temperature of 300 ± 50°C for the brittle-ductile transition. The lower temperature (260°C) seems to be restricted to provinces where the heat flow is near or below 50 mW m-2, such as the Mojave block and Sierra Nevada. The differences in these cutout temperatures suggest that additional factors, such as strain rate, stress regime, and/or lithology, may contribute to the seismicity cutout depth. Fourth, along a profile with significant seismic activity and a 450°C cutout temperature, the envelope for the maximum energy released by earthquakes falls at or below the 300°C isotherm. Detailed characterization of the heat flow and earthquake synergy in this manner furthers the understanding of the earthquake process and can aid in the estimation of the maximum depth of rupture for great earthquakes, particularly in areas of low seismicity, thus reducing uncertainties in hazard calculations. Manuscript received 26 February 2003.

Application of optical-fiber temperature logging - An example in a sedimentary environment

Continuous‐temperature depth logs, especially when recorded in boreholes under thermal equilibrium conditions, provide detailed information of the subsurface thermal structure, which is necessary for the determination of reliable heat‐flow and rock thermal properties. In conjunction with independent thermal‐conductivity determinations, thermal logging data also allow the separation of heat conduction effects from thermal convection effects by fluid flow driven by various pressure differences such as pore fluid pressure. The Earths thermal field is related intimately to geothermal resources and hydrocarbon resources. Therefore, the characterization of temperature in the subsurface and its relationship to lithology is of critical importance.

Regional implications of heat flow of the Snake River Plain, Northwestern United States

Abstract The Snake River Plain is a major topographic feature of the Northwestern United States. It marks the track of an upper mantle and crustal melting event that propagated across the area from southwest to northeast at a velocity of about 3.5 cm/yr. The melting event has the same energetics as a large oceanic hotspot or plume and so the area is the continental analog of an oceanic hotspot track such as the Hawaiian Island-Emperor Seamount chain. Thus, the unique features of the area reflect the response of a continental lithosphere to a very energetic hotspot. The crust is extensively modified by basalt magma emplacement into the crust and by the resulting massive rhyolite volcanism from melted crustal material, presently occurring at Yellowstone National Park. The volcanism is associated with little crustal extension. Heat flow values are high along the margins of the Eastern and Western Snake River Plains and there is abundant evidence for low-grade geothermal resources associated with regional groundwater systems. The regional heat flow pattern in the Western Snake River Plains reflects the influence of crustal-scale thermal refraction associated with the large sedimentary basin that has formed there. Heat flow values in shallow holes in the Eastern Snake River Plains are low due to the Snake River Plains aquifer, an extensive basalt aquifer where water flow rates approach 1 km/yr. Below the aquifer, conductive heat flow values are about 100 mW m −2 . Deep holes in the region suggest a systematic eastward increase in heat flow in the Snake River Plains from about 75–90 mW m −2 to 90–110 mW m −2 . Temperatures in the upper crust do not behave similarly because the thermal conductivity of the Plio-Pleistocene sedimentary rocks in the west is lower than that in the volcanic rocks characteristic of the Eastern Snake River Plains. Extremely high heat loss values (averaging 2500 mW m −2 ) and upper crustal temperatures are characteristic of the Yellowstone caldera.

14. Experimental Methods in Continental Heat Flow

Publisher Summary Heat flow measurements are made for a number of purposes and the techniques vary depending on the intended use. Because the internal processes of the earth are thermally driven, heat flow measurements are typically used to study tectonics. Regional heat flow values contain information on the thermal structure of the lithosphere and some aspects of the geochemistry of the crust. This chapter discusses the equipment for measurement of temperature, thermal conductivity, and heat production from radioactive decay. It describes the equipment associated with well testing and reservoir evaluation and/or determination of various water flow parameters. These methods are generally regarded as a part of reservoir engineering and/or hydrology. The chapter deals with temperature measuring equipment, with thermal conductivity measuring equipment, and with heat production measuring equipment.

Field comparison of conventional and new technology temperature logging systems

Abstract Field tests were conducted in the summer of 1995 on four state-of-the-art temperature logging systems: an analog, electric-line, system; two pressure and temperature recording memory tools (in-hole computer systems); and a Distributed optical fibre Temperature Sensing (DTS) system. The tools produced accurate, detailed, temperature versus depth and temperature gradient versus depth logs at depths to 2 km and temperatures to 200°C. Absolute temperature differences up to 0.4°C were noted between tools. The computer and electric-line tools have significantly better precision and resolution than the DTS, but the DTS has the advantage of being able to measure temperature instantaneously throughout the hole, and would be well suited for monitoring dynamic systems and gas-filled wells. The multiple independent logs demonstrate that most of the “noise” seen in gradient logs is due to convection cells, which may have dimensions several times the borehole diameter, and that these convection cells are currently the limiting factor in resolving wellbore temperatures in most settings.

Areal Distribution and Geophysical Significance of Heat Generation in the Idaho Batholith and Adjacent Intrusions in Eastern Oregon and Western Montana

Gamma-ray spectrometry has been used to measure potassium (K), uranium (U), thorium (Th), heat generation ( A ), Th/U, and ( A – Ak )/K ( Ak = heat from potassium) for over 600 samples in the Idaho batholith. Values of heat generation are remarkably consistent within a given pluton, but vary markedly and predictably between plutons of different composition and(or) different levels of emplacement. Values of ( A – Ak )/K are similar for suites of rocks that were emplaced at approximately the same depth during the same intrusive event, but are considerably different between suites belonging to different intrusive events emplaced at different depths. Thus, on the basis of their ( A – Ak )/K ratio and various geologic parameters, the rocks of the Idaho batholith can be divided into four intrusive groups. Table 1 lists these groups with their characteristics, weighted according to areal abundances of the constituent plutons. The remarkable uniformity of heat generation for a given igneous unit, even over large geographical areas, and the uniformity of ( A – Ak )/K. for rocks intruded during the same intrusive event suggest that these parameters are diagnostic properties of igneous rocks, and may be used by the geologist to map individual units, to correlate different rock types intruded during the same intrusive event, and to separate similar rock types intruded during different intrusive events.