Arthur H. Lachenbruch

Changing climate: geothermal evidence from permafrost in the Alaskan Arctic

Temperature profiles measured in permafrost in northernmost Alaska usually have anomalous curvature in the upper 100 meters or so. When analyzed by heat-conduction theory, the profiles indicate a variable but widespread secular warming of the permafrost surface, generally in the range of 2 to 4 Celsius degrees during the last few decades to a century. Although details of the climatic change cannot be resolved with existing data, there is little doubt of its general magnitude and timing; alternative explanations are limited by the fact that heat transfer in cold permafrost is exclusively by conduction. Since models of greenhouse warming predict climatic change will be greatest in the Arctic and might already be in progress, it is prudent to attempt to understand the rapidly changing thermal regime in this region.

Continental extension, magmatism and elevation; formal relations and rules of thumb

To investigate simplified relations between elevation and the extensional, magmatic and thermal processes that influence lithosphere buoyancy, we assume that the lithosphere floats on an asthenosphere of uniform density and has no flexural strength. A simple graph relating elevation to lithosphere density and thickness provides an overview of expectable conditions around the earth and a simple test for consistancy of continental and oceanic lithosphere models. The mass-balance relations yield simple general rules for estimating elevation changes caused by various tectonic, magmatic and thermal processes without referring to detailed models. The rules are general because they depend principally on buoyancy, which under our assumptions is specified by elevation, a known quantity; they do not generally require a knowledge of lithosphere thickness and density. The elevation of an extended terrain contains important information on its tectonic and magmatic history. In the Great Basin where Cenozoic extension is estimated to be 100%, the present high mean elevation ( ~ 1.75 km) probably requires substantial low-density magmatic contributions to the extending lithosphere. The elevation cannot be reasonably explained solely as the buoyant residue of a very high initial terrane, or of a lithosphere that was initially very thick and subsequently delaminated and heated. Even models with a high initial elevation typically call for 10 km or so of accumulated magmatic material of near-crustal density. To understand the evolution of the Great Basin, it is important to determine whether such intruded material is present; some could replenish the stretching crust by underplating and crustal intrusion and some might reside in the upper mantle. The elevation maintained or approached by an intruded extending lithosphere depends on the ratio B of how fast magma is supplied from the asthenosphere ( b km/Ma) to how fast the lithosphere spreads the magma out by extension (γ Ma−1). For a surface maintained 212km below sea level (e.g., an ocean ridge) B is about 5 km; for continental extension the ratio may be much greater. The frequent association of volcanism with continental extension, the high elevation (and buoyancy) of some appreciably extended terrains, and the oceanic spreading analog all suggest that magmatism may play an important role in continental extension. Better estimates of total extension and elevation change in extended regions can help to identify that role.

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.

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 in the Basin and Range Province and Thermal Effects of Tectonic Extension

In regions of tectonic extension, vertical convective transport of heat in the lithosphere is inevitable. The resulting departure of lithosphere temperature and thickness from conduction-model estimates depends upon the mechanical mode of extension and upon how rapidly extension is (and has been) taking place. Present knowledge of these processes is insufficient to provide adequate constraints on thermal models. The high and variable regional heat flow and the intense local heat discharge at volcanic centers in the Basin and Range province of the United States could be accounted for by regional and local variations in extensional strain rate without invoking anomalous conductive heat flow from the asthenosphere. Anomalous surface heat flow typical of the province could be generated by distributed extension at average rates of about 1/2 to 1%/m.y., similar to rates estimated from structural evidence. To account for higher heat flow in subregions like the Battle mountain High, these rates would be increased by a factor of about 3, and locally at active bimodal volcanic centers, by an order of magnitude more.

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 evolution of the Sierra Nevada: Tectonic implications of new heat flow data

Eight new heat flow measurements in the southern Sierra Nevada constrain models of thermal evolution and lithospheric structure. Low reduced heat flows (18 to 21 mW/m²) in the southwest Sierra Nevada are consistent with previous results from the northwestern and central Sierra Nevada and extend the known region of linear heat flow-heat production correlation an additional 150 km to the south. Isostatic residual gravity and measured rock densities are also linearly correlated in the central Sierra Nevada, suggesting a general association between the upper crustal distribution of density and heat production. The linear residual gravity-density relation implies that isostatic residual gravity anomalies have upper crustal sources and therefore is evidence against a flexural model of Cenozoic Sierra Nevada uplift. New reduced heat flows measured in the southeast Sierra Nevada are relatively high (32 to 57 mW/m²) and correlate spatially with a region of high seismicity that includes extensional earthquake swarms; this correlation supports the view that Basin and Range extensional tectonics and associated magmatic processes are encroaching on the eastern Sierra Nevada. In the stable southwest Sierra Nevada, as in the central and northwestern Sierra Nevada, the persistence of low reduced heat flow at the surface is consistent with a thermal origin for the Cenozoic uplift of the Sierra Nevada, provided the conductive lithosphere is at least 60 to 90 km thick. Simple order-of-magnitude calculations show that thermal uplift models combining mechanisms such as advective warming and thinning of the mantle lithosphere, advection of basaltic magma and heat into the crust, simple thermal expansion, and eclogite-to-basalt phase conversion can account for the timing and amount of Cenozoic Sierra Nevada uplift. Thermal models do not need the assumptions of unusual lithospheric strength and crustal buoyancy required by mechanical models for Cenozoic uplift driven by a Mesozoic crustal root.

Oceanic ridges and transform faults: Their intersection angles and resistance to plate motion

Abstract The persistent near-orthogonal pattern formed by oceanic ridges and transform faults defies explanation in terms of rigid plates because it probably depends on the energy associated with deformation. For passive spreading, it is likely that the ridges and transforms adjust to a configuration offering minimum resistance to plate separation. This leads to a simple geometric model which yields conditions for the occurrence of transform faults and an aid to interpretation of structural patterns in the sea floor. Under reasonable assumptions, it is much more difficult for diverging plates to spread a kilometer of ridge than to slip a kilometer of transform fault, and the patterns observed at spreading centers might extend to lithospheric depths. Under these conditions, the resisting force at spreading centers could play a significant role in the dynamics of plate-tectonic systems.

THERMAL EFFECTS OF THE OCEAN ON PERMAFROST

In high latitudes the large difference between the mean annual temperature at the ground surface and in the unfrozen sediments beneath bodies of water can affect ground temperatures to depths of several hundred feet. The effect is of particular interest near the edge of the ocean where it depends upon the magnitude of the temperature difference between the land surface and ocean bottom, the thermal properties of the ground materials, and past changes in climate and/or shore-line configuration. Theoretical considerations suggest that, except where there are transgressing shore lines, permafrost to depths greater than about 100 feet beneath the ocean bottom is not to be expected at points farther than a few thousand feet offshore. Similar considerations indicate that geothermal installations along the Arctic coast can give information regarding post-Pleistocene shore-line changes. The geothermal effects of bodies of water offer an explanation for the anomalously large outward earth-heat flow recently reported by A. D. Misener for Resolute Bay, Cornwallis Island, N. W. T., Canada.

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.