Reduced Heat Flow Research Articles

Terrestrial heat flow and radiogenic heat production in Finland, the central Baltic Shield

Heat flow density and radiogenic heat production of the bedrock were studied in Finland in part of the central Baltic (Fennoscandian) Shield. Heat flow data were collected from 35 holes 270–1080 m deep. The heat production values at the sites were determined from the drill core samples by gamma ray spectrometry. The areal variation in heat production was studied with the aid of K, U and Th analyses of 1054 glacial till samples collected for the “Geochemical Atlas of Finland”. A heat production map constructed from this data set revealed a strong areal variation that can be attributed to the known lithological, geochemical and tectonic features of the bedrock. In general, heat production seems to increase with decreasing geological age. The presented data strongly suggest that the southern and western parts of Finland are geothermally anomalous (apparent heat flow density 38–68 mW/m2, surface heat production > 2.0 μW/m3) in contrast to the other parts of the country (< 42 mW/m2, < 2.0 μW/m3). The heat production-heat flow density plots were constructed from apparent and palaeoclimatically corrected heat flow densities; the heat production values were determined from both drill core samples and till samples. The plot constructed from the till data and apparent heat flow densities, which yielded a scale depth of 10.8 ± 2.6 km and a reduced heat flow density of 15.8 ± 4.9 mW/m2, was considered the most reliable. The results are discussed in terms of crustal fracturing and deep groundwater flow. If the crystalline bedrock is approached as a medium consisting of blocks internally conductive but bounded by fracture zones in which heat is transported by groundwater flow, an alternative hypothesis can be constructed for interpreting the linear heat production-heat flow density plots. According to this hypothesis, the scale depth and reduced heat flow densities refer only to the geothermal properties of the uppermost bedrock block. In the present study, this concept is supported by results from deep electromagnetic and seismic soundings.

Selection and realization of methods of reducing heat flow into cryogenic vessels with liquid nitrogen

The effect of different design and service factors on heat flow into cryogenic vessels is examined. Results are presented from tests of cryogenic vessels equipped with new containers with vacuum-shield thermal insulation.

New heat flow density and radiogenic heat production data in the Canadian Shield and the Quebec Appalachians

New heat flow density (HFD) measurements were performed at 10 sites in Quebec. For five of the sites located in the Superior Province, the heat flow density varies between 24 and 35 mW/m2 (26 and 37 mW/m2 after adjustment for Pleistocene climatic variations). In the Grenville Province, the values obtained range between 25 and 28 mW/m2 (29 and 31 mW/m2 after adjustment). For two nearby sites in the Gaspé region (Appalachians), the heat flow density is 47 mW/m2 (48 mW/m2 after adjustment). Radiogenic heat production was also measured. At the sites located in the meta-volcano-sedimentary sequences of the Superior Province, the heat production is low (0.1–0.6 μW/m3) and it does not always correlate with the surface heat flow. In the Grenville Province, the HFD is close to (slightly higher than) the reduced heat flow of the Superior. The higher HFD in the Appalachians is partly explained by the higher crustal heat production, and partly by higher deep heat flow.

Thermal structure of the lithosphere in Fujian, China

The principal thermal parameters of the lithosphere (average reduced heat flow, heat production, and heat production depth parameter) have been determined from an analysis of silica heat flow estimates in Fujian, China. Silica data indicate that relative to western Fujian, heat flow is higher in eastern Fujian, with thinner lithosphere, a shallower depth to the Curie isotherm (about 27 km), and slightly higher geothermal activity. However, even in eastern Fujian, the thermal anomaly is small, no anomalous heat source is indicated, and conditions are not thought to be favorable for geothermal resources related to magma chambers, hydrothermal reservoirs, or hot dry rock. Widespread thermal springs in this region are heated by deep circulation of groundwater along faults in association with a slightly elevated geothermal gradient.

Significance of past and recent heat-flow and radioactivity studies in the Southern Rocky Mountains region

As reported in The Geological Society of America Bulletin of 1950, Francis Birch's innovative heat-flow research in the Colorado Front Range introduced frequently used terrain correction and thermal-conductivity measurement methods. That report also presented the first empirical evidence for a positive correlation between above-normal flux and radioactive roots in an isostatically compensated mountain area with a thick crust, an observation that strongly influenced continental heat-flow studies for the next 18 years. Birch's Front Range study clearly showed that reliable continental heat-flow research requires knowledge of bedrock radioactivity. This concept reached an unprecedented level of acceptance in 1968, when linear relations between heat flow and radiogenic heat production were discovered for three contrasting provinces in the United States. Subsequently, such lines have been central to virtually all heat-flow and radioactivity research on land. Heat-flow (Q) data for 139 boreholes provide new detail on thermal regimes in the Southern Rocky Mountains and bordering areas, as do radiogenic heat-production (A) data for 60 locales. Interpretations also emphasize reduced and residual heat-flow values; reduced values correspond to the intercepts of regional Q-A lines, whereas a residual value is the flux at a locale after the probable effect of near-surface heat production has been subtracted. The observed Q-A lines demonstrate that the Front Range and other easterly frontal ranges of the Southern Rocky Mountains in Colorado and northern New Mexico are characterized by a reduced flux (54-58 mWm-2) that is dramatically higher than that (∼27 mWm-2) in the Wyoming Basin-Southern Rocky Mountains area in southeastern Wyoming. Because the transitions between these provinces are narrow (≤50-60 km), sources in the upper crust must explain some of the contrasting reduced heat-flow values. In southeastern Wyoming, normal heat flow in Archean and early Proterozoic basement terranes probably reflects deep erosion that produced a thin (∼7 km) near-surface granitic layer that overlies a low-radioactivity lower crust. In the Colorado Front Range, Proterozoic, Mesozoic, and Cenozoic silicic rocks with relatively enriched radiogenic heat could comprise a 20- to 25-km-thick granitic layer in the upper crust that produces a large part of the above-normal reduced and residual flux. Here, partial melting of deep protolithic rocks in late Mesozoic and Cenozoic times could have produced a lower crust with low radiogenic heat production. By these views, Birch's heat flow-radioactive mountain root model of the Colorado Front Range is confirmed if a large part of the topography is isostatically compensated by low-density pre-Miocene crystal-line masses in the upper crust. Background reduced heat flow in Colorado parts of the Southern Rocky Mountains and the eastern Colorado Plateau is high (54-68 mWm-2). Zones of unusually high residual flux (88-118 mWm-2) occur in the Rio Grande rift zone in the environs of the Colorado mineral belt in the Leadville-northern Sawatch Range region, eastern parts of the San Juan Mountains in southern Colorado, and in the Park Range-Mountain Parks area near the Colorado-Wyoming border. The flux in these areas implies unrealistically high equilibrium temperatures near the crustmantle boundary, and the 50- to 60-km-wide borders of the Leadville-northern Sawatch Range residual heat-flow anomaly must be caused by sources in the upper crust. There-fore, young (10- to 1-m.y. -old) intrusions in a late Cenozoic rhyolitic complex in the upper crust are preferred to explain gravity lows, late Cenozoic uplift and igneous activity, and the high residual flux in the Leadville-northern Sawatch Range area. Similar models may apply elsewhere in the northern rift zone. If this interpretation is correct, magmatic thickening of the crust, not extensional-subsidence mechanisms, probably explains late Cenozoic uplift and extension of the northern Rio Grande rift-Southern Rocky Mountains system. Because very high regional flux in the northern rift zone and the San Juan Mountains areas implies above-liquidus equilibrium temperatures in the lower crust and upper mantle, serious inconsistencies arise when steady-state thermal models are examined relative to volumes of late Tertiary volcanism, uniform crustal thickness, and the absence of unusually low P-wave velocities in the mantle in the Southern Rocky Mountains.

Heat flow, heat production and crustal structure in peri-Atlantic regions

A linear relationship is commonly observed between heat flow and heat generation. It provides information about the distribution of radioelements with depth. Over large regions, the linear relationship is no more tenable, but points still cluster within an elongated trend in which a regression analysis provides the bulk reduced heat flow and a gross estimate of the heat production distribution with depth over the whole considered region. When geological provinces share an identical heat flow/heat generation relation they form a geothermal province. Data measurements in Western Europe suggest that at least two provinces should be considered. One can be related to the Hercynian crust and the second to the older Caledonian crust. We suggest that the depth obtained from thermal data corresponds to the depth of the mafic granulite layer. Western France, South and Central England share identical geothermal parameters which can be correlated with the depth of the granulitic layer down to 17–18 km. Data from Scotland do not fit the relationship of South and Central U.K. and present a lower slope indicating that the radiogenic enriched layer is shallow (8–10 km). This correlates with the seismic structure of the Caledonian crust where the granulite layer is observed at this depth. Data from Scotland can be compared to those from Norway, Newfoundland, New Hampshire and eastern Appalachian crust where the same crustal composition is observed.

Helium isotopic evidence for recent subcrustal volcanism in eastern Australia

A component with ∼6% mantle helium (³He/4He=5‐9×10−7) has been identified in borewaters from the Great Artesian Basin, Australia that is isotopically distinct from previously identified in situ production (³He/4He = 1.6×10−8) and a crustal degassing flux (³He/4He = 6.75×10−8). The occurrence of this component is limited to three identified localities (∼20.5°S 144°E; ∼25.5°S 146°E; ∼27°S 144.5°E) all within the region of high reduced heat flow in eastern Australia. No evidence for a mantle component was apparent in central Australia. This component is most probably the result of continued intrusive volcanism (&lt;1 m.y.) associated with the Cainozoic lava flow fields in the region. Although the volcanic activity has no present‐day surface manifestation, its mantle helium was probably carried to the near‐surface by large‐scale fluid transport processes in the crust.

Heat flow provinces reconsidered

The concept of a heat flow province, an area in which there is apparently a linear relationship between near-surface heat flow and radiogenic heat generation, is considered. The relationship has been suggested for a wide variety of tectonic terrains, ranging in age from Archaean to Cenozoic. It is expressed as q = q 0 + A 0D in which q and A o are the measured heat flow and heat generation and q o and D are interpreted to be the uniform reduced heat flow from the mantle and lower crust, and D is a scale depth for the distribution of radiogenic elements. Reported values for D are in a remarkably narrow range, and are typically ∼ 10 km. However, there can be substantial uncertainties associated with the data. In particular, heat generation measurements from a limited number of sites may not adequately represent even a relatively uniform pluton. When the data are re-analysed with reasonable estimates of uncertainty associated with them, the calculated values of q o and D are, in most cases, significantly different from those calculated without regard to uncertainties in the data. Only the data from the Sierra Nevada batholith, a deep-seated plutonic complex, appear to satisfy the relationship even when their uncertainties are taken into account. With the re-analysis, D generally falls in the approximate range 10–20 km. This is also the range of depths to a commonly-observed discontinuity in the mid- to lower-crust, sometimes called the Conrad discontinuity, which suggests a possible correlation between the two. A model in which blocks of crust 10–20 km thick, depths being randomly arranged, are each assigned a uniform heat generation, randomly generated from a reasonable range, produces an apparent relationship between heat flow and heat generation that is at least as good as most of those deduced from real data. The model produces crustal temperatures that are generally considerably lower in the lower crust than those commonly calculated; this is consistent with the observations from seismic refraction that the depth to the Moho can vary widely even in stable shield environments. It is concluded that the concept of a heat flow province as originally expounded is invalid.

Radioactivity, Heat Flow, and Rifting of the Indian Continental Crust

The relationship between heat flow (Q) and heat production (A) in India is quite different for the northern and southern parts of the peninsula. In the north, locations in a variety of tectonic settings (including Gondwana rifts) show a linear relationship between Q and A with a common reduced heat flow (Q*) about $40 mWm^{-2}$. In southern India, the heat production of surface rocks is greater than is expected for the measured heat flow. Relative to other continental shields, g* is high in northern India and about average in the south. Apparently the Indian crust evolved with a content of radioactive elements higher than in most shields, and the southern part of the peninsula has undergone regional depletion in the lower crust and possibly upper mantle. This depletion may coincide with a regional granulite event about 2,500 m.y. ago. The high heat flow and heat production in northern India may explain several unique characteristics of the Indian peninsula. One feature is the continued reactivation of rif...

Heat production in an Archean crustal profile and implications for heat flow and mobilization of heat-producing elements

We have measured concentrations of heat producing elements (Th, U, and K) in 58 samples representative of the main lithologies in a 100 km transect of the Superior Province of the Canadian Shield, from the Michipicoten (Wawa) greenstone belt, near Wawa, Ontario, through a domal gneiss terrane of amphibolite grade, to the granulite belt of the Kapuskasing Structural Zone, near Foleyet. This transect has been interpreted as an oblique cross section through some 25 km of crust, uplifted along a major thrust fault, and thus provides an opportunity to examine in detail a continuous profile into deep continental crust of Archean age. Mean heat production values for these terranes, based on aereal distribution of major rock types and calculated from their Th, U, and K concentrations are: Michipicoten greenstone belt = 0.72 μW m −3; Wawa domal gneiss terrane (amphibolite grade) = 1.37 μW m −3; Kapuskasing granulites = 0.44 μW m −3. Among the silicic plutonic rocks (tonalites, granites, and their derivative gneisses), the relatively large variation in heat production correlates with modal abundances of accessory minerals including allanite, sphene, zircon, and apatite. We interpret these variations as primary (pre-metamorphic). The relatively high weighted mean heat production of the domal gneiss terrane can be accounted for by the larger proportion there of late-stage Th-, U-, and K-rich granitoid plutons. These may have been derived from the underlying Kapuskasing granulite terrane, leaving it slightly depleted in heat producing elements. Transport of Th, U, and K, therefore, could have taken place in silicate melts rather than in aqueous or carbonic metamorphic fluids. This conclusion is supported by the lack of a statistically significant difference in heat production between tonalites, tonalite gneisses and mafic rocks of amphibolite versus granulite grade. The pre-metamorphic radioactivity profile for this crustal section is likely to have been uniformly low, with a mean heat production value less than 1 μW m −3. This result is distinctly different from measured profiles in more silicic terranes, which show decreasing heat production with depth. This implies fundamental differences in crustal radioactivity distributions between granitic and more mafic terranes, and may be an important factor in selective reactivation of lithologically different terranes, possibly resulting in preferential stabilization of basic terranes in the geological record. Our results indicate that a previously determined apparently linear heat flow-heat production relationship for the Kapuskasing area does not relate to the distribution of heat production with depth. Low, but significant heat production, 0.4–0.5 μW m −3, continues to lower crustal depths with no correlation to the depth parameter from the linear relationship. This low heat production may be a minimum average granulite heat production and suggests that, in general, heat flow through the Moho is 8–10 mW m −2 lower than the reduced heat flow calculated from the heat flow-heat production regression.

AN AIR‐GAP INSULATED PISTON

The concept of an air‐gap insulated piston has been explored using bolted and welded/roll bonded designs. Pistons with bolted‐on crowns demonstrated the effectiveness of air gap insulation, but roll bonded and welded designs were found to be more robust and to provide the complete sealing of the air gap necessary for continued insulation. Evolution of the design to combine high insulation with adequate durability is discussed. Engine running times of up to 200 hours at full load have been achieved for an air gap piston which reduces heat flow to the crown by 33 per cent. An improved design giving 41 per cent reduction of heat flow has been tested for 78 hours at full engine load with no evident deterioration. Development is continuing to provide a fully durable piston achieving up to 50 per cent reduction in heat flow.

Possible thermal structure of the eastern part of the Central Alps

This is a survey of the trend of temperature in respect of depth in the lithosphere under the eastern part of the Central Alps between Innsbruck and Graz, using a geothermal model based on the reduced heat production along the vertical and on the thermal, conductivity dependence upon temperature and pressure. A linear relationship has been found between the reduced heat flow and the mean surface heat flow; it was thus possible to estimate the radiogenic heat flow produced by the sialic crust and the heat flow entering its base. The geotherm corresponding to the surface heat flow in combination with the melting point curves of granitic and gabbroic rocks and with the mantle solidus was used to explain the observed low-velocity zones in the crust and to determine the depth of the lithosphere-asthenosphere transition.

The vertical distribution of radiogenic heat production in the Precambrian crust of Norway and Sweden: Geothermal implications

The present geology of southern Scandinavia offers the unique opportunity to sample deep and intermediate levels from the same crustal section for both heat flow and heat production. In the central part of southern Norway, amphibolite facies terranes appear to lie on top of the same deeper crustal formations which crop out on their western and eastern margins. An extensive data set on the geochemistry of all types of rocks in the region culled from the literature is used to derive values for radiogenic heat production for each kind of crustal facies. Using constraints from heat flow data in the same area allows a reliable model of the distribution of crustal heat production. The average heat production of granulite facies terranes is 0.4 µW/m³, similar to values in other parts of the world. For amphibolite facies rocks, the value is 1.6 µW/m³. The present shield also includes heat producing element enriched granites formed in later events and the average heat production of presently exposed crust is 2.7 µ/m³. Using heat flow and radioactivity data from ten stations, the reduced heat flow in the area is 22 ± 2 mW/m². This corresponds to the heat flow at the top of 28 km of deep crustal facies, implying that the mantle heat flow is probably as low as 10 mW/m². Over the whole crustal thickness, the average amount of radiogenic heat is 31 mW/m².

Heat flow at the proposed Appalachian Ultradeep Core Hole (ADCOH) Site: Tectonic implications

The heat flow in northwestern South Carolina at the Appalachian Ultradeep Core Hole (ADCOH) site area is approximately 55 mW/m². This data supplements other data to the east in the Piedmont and Atlantic Coastal Plain provinces where heat flows &gt; 55 mW/m² are characteristic of post‐ and late‐synmetamorphic granitoids. Piedmont heat flow and heat generation data for granites, metagranites, and one Slate Belt site, in a zone approximately parallel to major structural Appalachian trends, define a linear relation. Tectonic truncation of heat‐producing crust at a depth of about 8 km (a depth equal to the slope of the heat flow‐heat production line) is proposed to explain the linear relation. Using the value of reduced heat flow estimated from this empirical relation, and assuming thicknesses of heat‐producing crust defined by new ADCOH seismic data, the heat flow and heat production at the ADCOH site are consistent with a depth to the base of the Inner Piedmont crystalline allochthon of about 5.5 km. Seismic data at the ADCOH site confirm that the Inner Piedmont is tectonically truncated at about 5.5 km by the Blue Ridge master decollement. Temperatures at 10 km at the ADCOH site are predicted to be less than 200 °C.

Heat flow and heat generation estimates for the churchill basement of the western canadian basin in Alberta, Canada

Heat flow through the sediments and temperatures of the Churchill province basement under the sedimentary cover are determined for 24 locations in the central part of the Prairies basin in Alberta where the vertical heat flux is approximately constant from the base of the sediments to the surface. The contribution to heat flow from heat generation in the sediments is also considered. The average heat flow through the sediments is found to be 71 mWm −2 ± 12 mWm −2 which is about 30 mWm −2 higher than in the neighbouring shield area of the Churchill province, and the contribution from heat generation in the sediments to the surface heat flow is only approximately 2.5 mWm −2. The relationship between basement heat generation and heat flow is investigated, and it is found that the platform heat flow/heat generation values are in general higher than those from the Churchill province of the shield found by Drury 1985. Although for the platform and shield data, the reduced heat flow is about 40 mWm −2 and the slope is about 8 km, it is apparent that the platform data alone are not good enough to establish a precise relationship.

Thermal gradients in the continental crust

Summary A set of preferred geotherms for the continental crust is calculated using assumptions of steady state conductive heat transfer. The calculations use observed heat flow as the principal constraint, temperature and pressure dependent thermal conductivity functions, and a radiogenic heat generation profile that is consistent with a generalized petrological model of the lithosphere. Properties of these geotherms include: (1) a nearly constant gradient through the upper crust because the decrease in thermal conductivity at higher temperatures for felsic rocks counteracts the decreasing heat flow caused by crustal radioactivity; (2) divergence of the geotherms amounting to temperature differences of more than 500 K between rifts and shields at Moho depths, and (3) convergence of geotherms below 250 km in the asthenosphere as convective heat transfer dominates. Emphasis is also placed on understanding the sensitivity of calculated temperatures to measureable properties. Computed geotherms are overall most sensitive to surface heat flow or reduced heat flow, and uncertainties in this quantity will ultimately limit the accuracy of temperature estimations for the deeper crust and lithosphere. Otherwise, the sensitivity to parameters such as heat production, thermal conductivity and its temperature dependence throughout the lithosphere and crustal thickness depends on depth and whether the region is characterized by high or low heat flow. Upper mantle heat production, pressure coefficients of thermal conductivity, and the position of the upper/lower crust boundary are relatively insensitive parameters for lower crustal temperature calculations. Dynamic events accompanying crust forming processes lead to significant perturbations of the steady state static temperature field described above. Pressure-temperature pairs from granulite terrains plot near high temperature limits for those steady state geotherms and imply either a transient P-T-t path or a magmatic heat input from the mantle accompanying crustal thickening.

Statistical analysis of the mean heat flow/reduced heat flow relationship for continents and its tectonothermal implications

Previous analysis of the relationship between mean heat flow and reduced heat flow qr indicated that , suggesting that the upper crustal radiogenic contribution to heat flow was coupled to changes in qr. Statistical reexamination of this relationship reveals that this empirical relationship results for several reasons and that other interpretations may apply in stable tectonic areas. Importantly, the slope of the line changes significantly when certain points are excluded from the regression with the strongest effect exerted by the Basin and Range and eastern Australia. Because these areas were tectonically active during the Cenozoic, they may be outliers, exhibiting disproportionate influence on the regression results. Because and qr exhibit large errors and are correlated, we used a computer simulation method to fit a straight line to correlated variables with errors. We find that the slope of the line is reduced and that the fit is sensitive to outliers. If we restrict the analysis to more stable tectonic regions, we determine a relationship with a small slope and large intercept. Inclusion of data from regions which have been tectonically active in the Cenozoic gives results closer to the relationship. We suggest that this previous result reflects the presence of data from thermally perturbed regions in the data set. This, in conjunction with the nearly horizontal line found for stable regimes, indicates that coupling between the upper crust and mantle‐lower crust system is not implied. Furthermore, for stable regions is about 27 mW/m2 with a small standard deviation (4 mW/m2). The average upper crustal radiogenic component is nearly equivalent but has a large standard deviation (14 mW/m2), suggesting that variations in for heat flow provinces in stable regions is largely controlled by upper crustal radiogenic variations.

A Technique for Measuring the Apparent Thermal Conductivity of Flat Insulations

A technique for measuring the apparent thermal conductivity, λ a, of flat insu lations (either batt or loose) is described. This technique consists of an elec trically heated flat Nichrome-screen wire sandwiched between a pair of samples. Results from a limited experimental test are described and compared with results from a pipe insulation tester. Thermal modeling of this system shows that accurate measurements with λ a down to 0.01 W/m·K** and insula tion thicknesses below 0.089 m (3½ in.) should be straightforward. Measure ment of λ a for material as thick as 0.305 m (12 in.) should also be possible with a lateral guard scheme to reduce heat flow along the screen wire heater.

Heat flux measurements in southwestern British Columbia: the thermal consequences of plate tectonics

Measured heat fluxes from previously published data and 34 additional boreholes outline the terrestrial heat flow field in southern British Columbia. Combined with heat generation representative of the crust at 10 sites in the Intermontane and Omineca belts, the data define a heat flow province with a reduced heat flow of 63 mW m−2 and a depth scale of 10 km. Such a linear relationship is not found or expected in the Insular Belt and the western half of the Coast Plutonic Complex where low heat fluxes are interpreted to be the result of recent subduction. The apparent boundary between low and high heat flux is a transition over a distance of 20 km, located in Jervis Inlet 20–40 km seaward of the Pleistocene Garibaldi Volcanic Belt.The warm, thin crust of the Intermontane and Omenica Crystalline belts is similar to that of areas of the Basin and Range Province where the youngest volcanics are more than 17 Ma in age. Processes 50 Ma ago that completely heated the crust and upper mantle could theoretically produce such high heat fluxes by conductive cooling of the lithosphere. But it is more likely that the asthenosphere flows towards the subduction zone, bringing heat to the base of the lithosphere. Since the reduced heat flow is high but constant, large differences in upper crustal temperatures within this heat flow province at present are caused by large variations in both crustal heat generation and near-surface thermal conductivity. The sharp transition in heat flux near the coast is the result of the combined effects of convective heating of the eastern Coast Plutonic Complex, pronounced differential uplift and erosion across a boundary within the Coast Plutonic Complex, and the subducting oceanic plate.

Heat flow in southern Mexico and the thermal effects of subduction

Twenty new heat flow measurements in southern Mexico are presented. These measurements document a very broad zone of low heat flow between the coastline and the Trans‐Mexico volcanic belt. At the Trans‐Mexico volcanic belt, heat flow increases to high values and remains high into central Mexico. Heat flow values in the Trans‐Mexico volcanic belt and northward are in excess of 80 mW m−2 (1.9 μcal/cm2 s) and typical of values observed in the high heat flow regions of the western United States. The average heat flow in the Sierra Madre del Sur is 26 mW m−2 (0.6 μcal/cm2 s), in the Sierra Madre Oriental is 89 mW m−2 (2.1 μcal/cm2 s), and in the Trans‐Mexico volcanic belt is 91 mW m−2 (2.2 μcal/cm2 s). A surface heat flow profile was constructed perpendicular to the trench, and the data were matched with a simple thermal subduction model. The resulting final model parameter set suggests that the angle of subduction is low and that convective heat transfer in the back arc region is probable. For data south of the Trans‐Mexico volcanic belt, reduced heat flow values (upper crustal radioactive effect removed) are less than 15 mW m−2 (0.4 μcal/cm2 s). Thus essentially the total mantle heat flow is being absorbed by the subducting block. The values are lower than the heat flow observed in the Sierra Nevada Mountains of California and support the hypothesis that the heat flow couple associated with the Sierra Nevada Basin and Range province is a relic subduction zone thermal pattern.