Regional Heat Flow Research Articles

Introduction to Special Section on the Geological, Geophysical, and Tectonic Setting of the Cascade Range

The Cascade Range is an active volcanic arc that is formed by the subduction of the Juan de Fuca plate system beneath North America [Riddihough, 1978]. Knowledge of the Cascade Range, its volcanic rocks, and its tectonic setting is not only of broad scientific interest but also bears directly upon the estimation of geothermal resources and the evaluation of volcanic and seismic hazards.An understanding of the regional aspects of the Cascade Range is at the present time particularly important to the assessment of geothermal resources. Although identified geothermal resources in the Cascade Range are modest [Brook et al., 1979], substantial undiscovered geothermal resources have been inferred [e.g., Brook et al., 1979; Black et al., 1983; Bloomquist et al., 1985], primarily from the favorable volcanic and tectonic setting but also from high regional heat flow along the Cascade Range [Blackwell et al., 1982]. Consequently, a multiyear, multidisciplinary program of studies to refine the geothermal resource estimates of the Cascade Range is a major objective of the U.S. Geological Survey (USGS) Geothermal Research Program [Muffler, 1987]. The smallest and easiest part of this effort is an updated inventory of known hydrothermal convection systems. A much larger and far more difficult task is the estimation of the undiscovered geothermal resources. Existing drill hole information in the Cascade Range is sparse, and under the foreseeable economic climate, direct evaluation of the thermal regime by numerous deep holes is unlikely. However, the magnitude of the undiscovered geothermal resources can be addressed indirectly by an analysis of the tectonic, geophysical, volcanic, and hydrologie setting of the Cascade Range, thus allowing extrapolation of available drill hole data.

Heat flow in the state of washington and thermal conditions in the Cascade Range

Heat flow data for the state of Washington are presented and discussed. The heat flow in the Okanogan Highland averages 75 mW m−2, and the gradient averages 25°C km−1. The heat flow in the Columbia Basin averages 62 mW m−2, and the mean gradient is 41°C km−1. Both of these provinces are interpreted to have a mantle heat flow of about 55–60 mW m−2, the same value as in the Basin and Range province to the south and the intermountain region of Canada to the north. These areas comprise the high heat flow, back arc region of the Cordillera. In the coastal provinces and the western part of the southern Washington Cascade Range the heat flow is below normal and averages 40 mW m−2 with an average gradient of 26°C km−1. This low heat flow is related to the absorption of heat by the subducting slab, part of the Juan de Fuca plate, that is beneath the Pacific Northwest. Thus the low heat flow area represents the outer arc part of the subduction zone. Within the volcanic arc, the Cascade Range, the heat flow pattern is complicated. The heat flow is best characterized in the southern Washington Cascade Range. The heat flow there averages 75 mW m−2 and the gradient averages 45°C km−1. The heat flow peaks at over 80 mW m−2 along the axial region that coincides with the Indian Heaven, Mount Adams, and Goat Rocks centers of Quaternary volcanism. As is the case in northern Oregon and southern British Columbia, the western edge of the region of high heat flow has a half width of 10 km, implying a heat source no deeper than about 10 km. In the northern Washington Cascade Range the data are too sparse to determine the average heat flow. There are two saddles in the heat flow pattern in Washington, along the Columbia River and in central Washington. The origin for the contrasting heat flow may be segmentation of the heat source or some more local effect. The heat flow of the Cascade Range is well characterized in several locations, and the pattern is similar at all localities. The most striking feature is the 10 km half width of the western side of the high heat flow region where it abuts the low heat flow outer arc region. The axial heat flow ranges from about 80 to greater than 100 mW m−2. The midcrustal temperatures are interpreted to range from about 400°C to 800°C. The source of these high temperatures is interpreted to be a long‐lived midcrustal zone of magma residence that is characteristic of the Cascade Range regardless of erustal type, rate of volcanism, or composition of volcanism. For example, in southern Washington the region of high heat flow spans the width of the Quaternary zone of volcanism with Mount St. Helens and Mount Adams at the west and east edges, respectively. On the other hand, most of the Oregon stratovolcanoes are near the center of anomalous region.

Source characteristics of earthquakes along the southern San Jacinto and Imperial fault zones (1937 to 1954)

Abstract Body waveform inversion techniques are used to study the source parameters of four earthquakes occurring between 1937 and 1954 along the southern San Jacinto and Imperial faults (1937 Buck Ridge, 1940 Imperial Valley, 1942 Borrego Mountain, and 1954 Salada Wash events). All earthquakes had simple rupture histories with the exception of the 1940 Imperial Valley main shock, which consisted of at least four subevents whose relative locations indicate unilateral rupture toward the southeast. Earthquakes in regions of high heat flow (>80 mW/m2) had focal depths near the base of the seismogenic zone (8 to 10 km). The 1937 Buck Ridge earthquake, located in a region of lower heat flow, however, appears to have occurred at a shallow (3 ± 2 km) depth. The location, mechanism, and aftershock distribution for the 1942 Borrego Mountain earthquake suggest it could have occurred along the Split Mountain fault, a recently identified northeast-trending cross fault located between the Elsinore and Coyote Creek faults or along an unnamed fault that parallels the trend of the Coyote Creek fault. Moment and rupture length estimates obtained from this study agree well with estimates obtained in previous studies that used different data sets.

Rheology of the lithosphere in the East African Rift System

SUMMARY rheological properties of the lithosphere in the East African Rift System are estimated from regional heat flow and seismic constraints. Heat flow data are used to infer average, maximum, and minimum geotherms for the Eastern Rift, the Western Rift, and the surrounding shield (having surface heat flow of 106f51, 68 f 47, and 53 f 19 mW m-*, respectively). Combining the geotherms with brittle and ductile deformation laws for a lithosphere of appropriate structure and composition yields rheological profiles, thickness of brittle layers, and thickness and total strength of the lithosphere. The thickness of the uppermost brittle layer varies from 10 f 2 km in the Eastern Rift, to 18:; km in the Western Rift, and 26+ km in the shield; seismicity is confined to the brittle layers. The rheological thickness of the lithosphere in Eastern Rift (23:; km), Western Rift, and shield is approximately in the the ratio 1:2.5:5, and matches the elastic flexural thickness. The total resistance to deformation is one order of magnitude larger in the shield than in the rifted regions (-10 N m-'), where the whole lithosphere is probably in a state of failure.

Heat flow, tectonics, and fluid circulation at the toe of the Barbados Ridge Accretionary Prism

Detailed patterns of near surface tectonics and heat flow were investigated at two locations at the toe of the Barbados Ridge accretionary prism. At the two locations, in the thickly sedimented southern portion of the prism near 12°20′N and in the northern area drilled during Ocean Drilling Program (ODP) leg 110 near 15°32′N, relative highs of heat flow were associated with zones of thrusting. We attribute them to upward advection of water rising from the décollement along faults through the sediments. In the ODP area, a significant heating plate effect from warm fluids flowing along the décollement and thrusts is required to account for the high regional heat flow. This effect implies high rates of fluid flow at the décollement, which can only be non‐steady state or would require massive lateral transport of fluids along strike.

Sediment thickness in the southern Canada Basin

Multichannel seismic reflection data are used, in conjunction with deep crustal seismic refraction data, to estimate the thickness of sediments in the southern Canada Basin of the Arctic Ocean north of Alaska. The sediments are interpreted to be of Hauterivian (mid-Early Cretaceous) to Holocene age. Comparison of the seismic reflection character of seismic reflections in the study area with that in other basins indicates that a base-of-sediment—top of oceanic layer 2 reflection is not present above the depth at which the water-bottom multiple obscures all deeper arrivals, which is in conflict with the conclusions drawn from aeromagnetic, refraction, and other reflection studies. Seismic velocity structure, determined from the reflection data, indicates that the reflections above the multiple are from sedimentary strata. In the absence of seismic reflection evidence for the top of layer 2 above the multiple, we estimate total sediment thickness by using the layer 3 refractions and subtracting an average assumed layer 2 thickness from the top of layer 3. Assuming that an average thickness of oceanic layer 2 (1.4 km) overlies layer 3 in the southern Canada Basin, sediment thickness in the study area is estimated to range between 6.5 km where water depth is 3.8 km to greater than 11 km where the water depth is 2 km. This is nearly double that of any previous estimates and should have a significant effect on calculations such as the age of Canada Basin, regional heat flow, and long-term sedimentation rates.

Constraints on thermal models of the basin and range province

Several mechanisms that have been proposed to explain the high heat flow in regions of tectonic extension are examined. The transient thermal regime is computed and the results are discussed. Conductive heating of the lithosphere from below requires a time of at least 75 Ma to affect the surface heat flow. Advective heating by the injection of magmas into the lithosphere does not produce steady-state conditions before 75 to 300 Ma (depending on boundary conditions). Only mechanical stretching of at least the shallow lithosphere can explain the increase in surface heat flow in a time of the order of 30 to 50 Ma. The heat flow data are compatible with several mechanisms for the extension of the lower lithosphere (underplating or dike injection) and other geologic and geophysical data are needed to constrain the models.

Deep Seismic and geochemical constraints on the nature of rift-induced magmatism during breakup of the North Atlantic

The deep crustal structure of the Norwegian and East Greenland conjugate passive margins has been investigated with two-ship multichannel reflection and refraction techniques in the region from the Iceland-Færøe Ridge to the Greenland Fracture Zone. The data demonstrate the presence of thick (15–20 km) igneous crust that was emplaced along the conjugate margins during the initiation of seafloor spreading. Seaward dipping units comprise the uppermost 3–6 km of the thick crust. The thickness of igneous crust diminishes seaward, with oceanic layer 2 approaching normal thickness nearer to the margin than does oceanic layer 3. Wide-angle reflection techniques, used in conjunction with expanded spread profiling, reveal deep Moho reflections beneath 20 km thick oceanic crust that continue seaward into the ocean basin.Margins that have experienced unusually prolific magmatic activity during breakup have been termed “ volcanic” to distinguish them from “ non-volcanic” margins that have evolved into accreting plate boundaries without excessive volcanism. The origin of volcanic margins cannot be determined from Seismically derived crustal thicknesses alone. However, thermal and chemical properties of the mantle source may be revealed in the compositions of magmatic products emplaced within volcanic passive margins. A combined investigation of crustal structure and associated basalt composition could better resolve processes that result in volcanic passive margins.On the Vøring Plateau of the Norwegian margin, Seismic lines coincide with DSDP Leg 38 drill sites. Whole rock major element abundances in the freshest basalts are compatible with moderate extents of partial melting of mantle much like that believed to underlie modern mid-ocean ridge systems and would be expected to result in the emplacement of oceanic crust about 9 (± 2) km thick. This is about the thickness of oceanic crust located seaward of the dipping basaltic units off the Hatton, Møre and Vøring Plateau margins and 10 km south of Iceland on the Reykjanes Ridge. These results, together with regional heat flow, depth, gravity and geoid anomalies, suggest that the mantle beneath this region has experienced a moderate, long-lived temperature increase compared to mantle beneath normal ocean basins.The much greater thicknesses of igneous crust adjacent to the margins do not appear to reflect an unusually hot flux associated with their construction, since this would lead to much greater extents of partial melting than are inferred from the basalt geochemistry. Evidently, the difference between volcanic and non-volcanic margins is not simply proximity to a hot spot. Instead, we suggest that the magmatism is closely related to the continental rifting, phase. Tectonism resulting in an abrupt transition from continental to oceanic lithosphere may induce small-scale convection that results in the circulation of a greater amount of mantle material into the pressure-temperature field where partial melting begins than would occur through passive upwelling alone. Initial crustal thicknesses are enhanced as a result of the convective partial melting. Crustal thickness is expected to diminish as this secondary convection slows, but the degree of partial melting of the source should not change dramatically.

Heat flow in the Great Plains of the United States

Anomalous high heat flow previously reported for the Great Plains is inconsistent with the tectonic setting and requires reexamination. Forty‐six new heat flow measurements, 12 revised heat flow values, and several hundred geothermal gradient measurements indicate extensive geothermal anomalies with heat flows ranging from 80 to 140 mW m−2 in the northern and central Great Plains. Heat flow in the Great Plains outside the geothermally anomalous regions ranges from 40 − 60 mW m−2. The heat flow anomalies result from the thermal effects of regional groundwater flow where it moves upward either within a dipping aquifer or by cross‐formational flow through fractures. The gravitational driving force for the groundwater flow derives from the eastward sloping surface of the Great Plains, and the locations of the geothermal amonalies are determined by the structures of the aquifers and the crystalline basement rocks. The most widespread and largest‐amplitude geothermal anomaly occurs in southern South Dakota and northern Nebraska. Another large anomaly occurs on the eastern flank of the Denver Basin, and small anomalies occur on structures such as the Billings and Nesson anticlines in the Williston Basin. Previous reports of high heat flow in the Great Plains generally are supported by the results of this study. However, the source of anomalous heat is shown to be nontectonic, and theoretical arguments for normal continental heat flow in the Great Plains are supported. Another difference from the results of previous heat flow studies is that the thermal conductivities of shales in the Mesozoic strata in the Great Plains are about 40% lower than the conductivities that commonly have been used for shales. This observation and recent studies which have suggested lower thermal conductivities for shales in the Great Plains are the reasons for revision of some previous heat flow calculations. A significant result of revising some of the previous heat flow values is that the high heat flow zone that previously has been shown to extend through North Dakota into Canada is a region of normal continental heat flow.

Heat Flow and Geothermal Gradients of Irian Jaya-Papua New Guinea: Implications for Regional Hydrocarbon Exploration: ABSTRACT

Compilation of published and unpublished bottom hole temperatures (corrected for circulation times) together with published oceanographic heat flow analyses from the surrounding seas allow an analysis of the regional heat flow and geothermal gradients of the New Guinea region. In two dimensions the thermal state may be described as a pervasive northwest striking trend paralleling the central Cordillera with basinal and intra-basinal anomalies. The heat flow averages for the Salawati, Bintuni, and Papua Basins are 1.98 ±0.76 HFU, 1.49 ±0.77 HFU, and 1.57 ±0.49 HFU, respectively. The average geothermal gradients for the three basins are 3.9 ± 1.48°C/100 m, 3.31 ± 1.5°C/100 m, and 2.61 ±0.8°C/100 m, respectively.

Thermal and Compaction Processes in a Young Rifted Basin Containing Evaporites: Gulf of Lions, France (1)

The Gulf of Lions (northwestern Mediterranean) is a young Oligocene-Miocene rifted basin characterized by the presence of a thick evaporitic sequence. Observed data (surface heat flow, well temperatures, geopressures below the evaporites, crustal thinning) are used to model (1) the basin-forming mechanisms, and (2) the intrasedimentary processes that influence the temperature history, and thus, the maturation of source rocks. A two-dimensional finite-difference basin model, Temispack, simulates the sedimentation, compaction, and overpressures related to the low permeability of the post-rift sequence, the heat transfer from the rifted lithosphere into the sediments, and the level of maturation by using a kinetic approach. The classic passive margin model, which relates the history of heat flow and the rate of subsidence to a uniform thinning factor, yields subsidence and heat flow predictions that agree with observations. The recently accelerated subsidence observed in the deep margin is probably an effect of recent regional compression rather than the consequence of alternative rifting mechanisms. The history of maturation of organic matter was influenced by the variation of the regional heat flow due to the rifting, the effect of sedimentation (or blanketing), and the variability of the lithologies and porosities. The undercompaction of sediments underlying the evaporites is accounted for by the model, which suggests that hydraulic fracturing is taking place at present in the overpressured sediments. Fluid flow associated with the compaction disequilibrium has negligible thermal effect. Undercompacted shales below the evaporites have lower thermal diffusivity and thus higher temperatures, which has a significant effect on maturation.

Topographic and volcanic asymmetry around the Red Sea: Constraints on rift models

The Red Sea rift is asymmetric, with the locus of uplift and Tertiary volcanism in Saudi Arabia, 200–400 km east of the present rift axis. One model for this asymmetry involves simple shear extension on an east dipping, low‐angle master fault that penetrates the lithosphere. Uplift and volcanism would mark the location where the fault enters the asthenosphere. However, observed seismicity, regional heat flow, and sedimentation are not in accord with this model's predictions. An alternate model involves interaction of upwelling asthenosphere in the early rifting stage with a nearby crustal weak zone such as a suture that controls rift location. In general, the location of the weak zone and subsequent rift will not coincide with the locus of mantle upwelling, leading to several asymmetric rift features. In this model, Tertiary volcanism in Saudi Arabia marks the location of initial upwelling, and uplift is due to crustal thickening associated with magmatic underplating and crustal intrusion. The incipient crustal rift and the locus of mantle upwelling will tend to align as rifting continues and stable seafloor spreading develops, implying relative migration of the lithosphere and asthenosphere. Absolute plate motion models for Africa and strongly asymmetric spreading in the Red Sea are consistent with northeast migration of the incipient rift and adjacent lithosphere with respect to a zone of asthenospheric upwelling.

A magnetotelluric study of the High Cascades graben in central Oregon

We have analyzed wideband magnetotelluric data from three east‐west transects across the Central Oregon High Cascades. The data were modelled using preliminary one‐dimensional inversions of rotationally invariant parameters followed by conventional two‐dimensional analysis. The resulting electrical profiles show offsets at the boundaries of the High Cascades that are consistent with downfaulting beneath the High Cascades Graben. Conductive and resistive layers in the upper crust are interpreted in terms of the known and inferred volcanotectonic history of the range. A pervasive conductor is present in the lower crust. It deepens eastward across the Western Cascades, High Cascades, and adjoining Deschutes Plateau and may indicate concomitant eastward crustal thickening. Regional heat flow and gravity studies have been interpreted as evidence of a transition from thinner crust on the west to thicker crust on the east. This transition may mark a “suture” between pre‐Tertiary continental crust and an accreted oceanic terrane that underlies western Oregon, or it may indicate the westward limit of the anomalous lithosphere that is thought to underlie the Basin and Range province.

Heat flow data and vertical groundwater movement, examples from southwestern Virginia

In the present study we show that estimates of vertical specific discharge are often obtainable from plots of conducted heat flow Q versus temperature T. Q‐T plots allow for variations in thermal conductivity across a zone of flow, whereas temperature gradient G versus T plots assume constant conductivity. It is demonstrated that the two different plots can provide quite different estimates of vertical specific discharge. By considering vertically convected heat flow across a zone, better estimates of regional (geologically representative) heat flow can be made. Agreement between corrected heat flow values, conducted heat flows calculated near fluid flow exit boundaries, and regional values suggests that the steady state flow model used is reasonably representative, although the fluid flow depth interval is typically difficult to estimate confidently. The effects of small continuous well bore flow can be removed if continuity in the temperature log results; otherwise, it is most difficult to estimate specific discharge as confirmed by unrepresentative corrected heat flow data.

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.

Heat flow, structure and evolution of the lithosphere of Mongolia

The geothermal characteristics of Mongolia are presented based on heat flow measurements taken in boreholes and at the bottom of Lake Khubsugul, and on heat flow values inferred from 3He/4He isotope ratios in spontaneous spring gases. Three types of regional heat flow distribution are recognized: a narrow linear anomaly in the Khubsugul segment of the Baikal rift zone, an area of regularly varying heat flow controlled by the age of the last tectonomagmatic activity in Central Mongolia, and a zone of anomalously low heat flow in the South Mongolian Hercynian belt. The radiogenic contribution to the terrestrial heat flow is determined, a contribution which proved to be different in the southern and northern megablocks of Mongolia. Variations in the thermal regime of the northern (central) and southern megablocks of Mongolia are accounted for by their tectonic evolution.

Interpretation of heat flow density of the Apennine chain, Italy

The Apennine chain is a consequence of the continental collision between Europe and Africa characterized by several compressive phases with events of intense shortening. During one of these phases, from Early Oligocene to Early Miocene, the continental crust broke into various segments which became packed one over the other, forming an Adriatic-verging structure of imbricated listric wedges. This event is thermally modeled in the Southern Apennines by the successive overthrust of two slabs of 7 km total thickness and explains the low heat flow density (HFD) (< 40 mW m−2) observed there. In the Northwestern Apennines, modeling is on the basis of the overthrusting of a slab 10 km thick, which reduced the HFD of the steady lithosphere (63 mW m−2) observed on the Iblean platform to 50.4 mW m−2. From the Late Miocene onwards, tensional tectonism developed in the internal areas of the chain. Application of McKenzie's “simple stretching” model shows that the high regional heat flow (> 100 mW m−2), obtaining by filtering the observed data, is explained by a stretching of the lithosphere of a factor β = 3.1.

The influence of groundwater flow on thermal regimes in mountainous terrain: A model study

Groundwater flow in high‐relief mountainous terrain is modeled to examine how geology, surface topography, climate, and regional heat flow influence the pattern and magnitude of an advective thermal disturbance. A numerical procedure is used to estimate the position of the water table within constraints provided by the available infiltration rate and the permeability of the mountain massif. Results show that where the water table is located at depth within the mountain massif, the rate of groundwater recharge, rather than permeability, is the appropriate factor to characterize the potential for an advective disturbance. Thermal disturbances should be assessed within the framework of the two‐dimensional character of the flow system because interpretations based on one‐dimensional models are prone to significant error. Modeling of site‐specific systems can exploit the existence of a permeability “window,” where temperatures in discharge areas and in springs discharging from fracture zones reach peak values. Temperatures of thermal springs reflect the complex interaction between flow within the mountain massif and flow through permeable fracture zones. Consequently, simple calculations relating the geothermal gradient, depth of maximum groundwater circulation, and discharge temperature improperly represent the physics of the process.

Characteristic nature of sedimentation and coalification in the Tertiary system of the Japanese Islands

The major coal-bearing formations in the Japanese Islands are Tertiary in age. Because the island-arc system is situated in the Cenozoic Alpine orogenic belt of the northwestern Pacific margin, the coal fields are smaller in size, but more strongly affected by sedimentological and geotectonic changes as compared with those in stable continents. Also, there is a notable difference in the mode of tectonic deformation from one tectonic province to another even within individual islands. Japanese coal is characterized by high volatile matter and high hydrogen contents. These properties are partly due to the unusual scarcity of inertinite macerals, except for a small amount of sclerotinite, and abundant degradinite. The degree of coalification ranges from lignite to anthracite. The bituminous coal exhibits extraordinarily high fluidity during carbonization. Geological investigations, as related to sedimentation and subsequent tectonic development of the Japanese coal basins with special reference to paleogeothermal history of the geotectonically characteristic provinces, have revealed the cause of the characteristic nature of Japanese coal. Scarcity of inertinite in the coal petrographical composition is ascribed to continual subsidence of the coal basins, which implies that the original peat was kept wet and buried quickly. During the diagenetic alteration from peat to high rank coal, the more hydrogenous reactive fraction was preserved in the chemical structure of the coal, which might be induced by deeper burial depths in a lower heat-flow region. The shorter duration of coalification reactions under warmer paleogeothermal conditions than those in the Euramerican Paleozoic coal fields has produced one of the characteristic features of Japanese coal.

Temporal changes in geothermal gradient, Franciscan Subduction Complex, northern California

The Franciscan Complex comprises several tectonostratigraphic terranes that were accreted to coastal northern California during late Mesozoic to Miocene time. Estimates of peak paleotemperature (based upon measurements of vitrinite reflectance) reveal spatial and temporal patterns which, when integrated with pressure‐depth limits imposed by stratigraphic relations and inorganic metamorphic phases, enable reconstruction of past geothermal gradients. Geothermal gradients during metamorphism of matrix to the Mesozoic melange (the Central Belt) may have been unusually small; this condition can be explained by the depression of isotherms during subduction of old oceanic crust. However, the gradients offshore northern California increased during the Paleogene period (the Yager and Coastal terranes). During the middle Miocene epoch a pulse of hydrothermal discharge produced an overprint across the entire King Range terrane, and high regional heat flow continued through the remainder of the Cenozoic. This change in thermal regime must have been influenced by the proximity of the Pacific‐Farallon ridge‐and‐transform system to the Franciscan‐Cascadia subduction front. The thermal structure of the subduction complex was modified because of conductive and advective heat flux from the subducting oceanic lithospheric into the overriding accretionary wedge.