Zone Of High Heat Flow Research Articles

Geological and Geophysical Manifestations of the Reunion Plume-Indian Lithosphere Interactions-Evidence from Northwest India

A number of geological and geophysical features have been recorded in western Rajasthan and in the Kachchh ( nee Kutch) region of northern Gujarat, northwest Indian Shield, which evolved during a short span across the K-T boundary. The features include, besides the extensive Deccan Trap related volcanicity, the formation of rift basins, the emplacement of magma plutonism of diverse petrochemical types, the development of the E-W and N-S trending fracture systems, the formation of structural domes due to magma diapirism, and the occurrence of isolated patches of positive Bouguer gravity anomalies and zones of high-heat flow. A critical analysis of all these features point towards a common origin resulting from the impingement of Reunion Plume at the base of the Indian lithosphere. The formation of these interrelated features across the K-T boundary is incidental to the fact that it was around this time that the plume-head remained focussed under this part of the Indian lithosphere. The importance of bolide impact at the K-T boundary lies only in the intensification of the existing plume activities.

Characteristics of surface heat flow in the South China Sea

A total of 592 heat flow measurements, ranging from 8 to 192 mW/m 2, 78 percent of which are between 50 and 100 mW/m 2, have been collected in the South China Sea (SCS). To overcome shortcomings such as the uneven distribution of the measurements and the occurrence of abnormal heat flow values, the tectonic evolution of different areas and their crustal thickness have been combined in analyzing geothermal characters. The results show that the oceanic basins, where they are floored by oceanic crust, the western part of the southern margin and the western fault system have high heat flow values. Heat flow along the northern margin of the SCS increases from 61 mW/m 2 on the shelf to 73–80 mW/m 2 in the slope area, and in the Xisha–Zhongsha area increases from about 70 mW/m 2 to about 85 mW/m 2 from the NW to the SE. Heat flow in the Nansha (Spratley) Islands is about 60 mW/m 2 or even higher, and decreases from the NW to SE; The average heat flow of the Mekong Basin is about 60 mW/m 2, which is similar to that of the Beibuwan Basin on the northern margin. Heat flow on the eastern margin and on the eastern part of the southern margin is lower, especially in the Luzon Trough, where the average heat flow is lower than 40 mW/m 2. Observed heat flow values in the SW subbasin are generally lower than predicted theoretically from the age of the ocean floor, unlike values in the eastern subbasin. A high heat flow zone in the lower slope area of the northern margin is recognized for the first time.

Heat flow models across the Trans-European Suture Zone in the area of the POLONAISE’97 seismic experiment

Heat flow data from the Polish basin show a sharp change in the transition from the East European Craton (EEC) and Teisseyre–Tornquist Zone (TTZ) in the north-east to the accreted terranes in the south west (Paleozoic Platform). The analysis of this data and numerical modelling of the crustal temperatures show evidence of extensive crustal–mantle warming in the area between the Sudetes to the south and the Trans-European Suture Zone to the north. The change in heat flow is 100% when compared with values for the EEC. Heat flow in the anomalous zone is also higher than in the Sudetes. The axis of the anomaly is aligned with the Dolsk Fault and Variscan deformation front. Low crustal/mantle temperatures derived from the relationship between temperature and P n velocities (more than 8.2 and as high as 8.4 km/s) are at odds with high crustal temperatures calculated from surface heat flow, seismic velocity based heat generation models and thermal conductivity. High heat flow (Variscan platform) and related high temperatures of the crust coincide with small crustal thickness (30–35 km). The opposite is the case for the low heat flow EEC (45–50 km). High heat flow above thin crust and low heat flow above thick crust with no major variation in elevation is supported by a simple isostatic balance model. Crustal heat generation explains part of the high heat flow within the zone with thick meta-sediments reaching down to 20 km depth, however, it is far from explaining high heat flow in Variscan crust and in the transition zone into a cold EEC. 2D numerical models of heat flow based on new seismic data require a contrast of 15 mW/m 2 in mantle heat flow. High mantle heat flow (35–40 mW/m 2) is likely to occur in the high heat flow zone while cold crust and cold and high-density mantle (mantle heat flow of 20–30 mW/m 2) is typical of the EEC. Thermal lithosphere thickness for the craton is 200 km while it is only 100 km in the accreted terranes to the southwest of the TTZ. The TTZ in Poland appears as a relatively cold area.

Estimates of geothermal gradients and heat flow from BSRs along the Western Continental Margin of India

Geothermal gradients and heat flow were estimated from the position of Bottom Simulating Reflectors (BSRs) identified on seismic reflection sections from the Western Continental Margin of India (WCMI). The estimated geothermal gradients along the WCMI range between 35 to 65 °C/km and heat flow varies from 50 to 130 mW/m². The geothermal gradient structure of WCMI inferred from the BSRs, shows a landward decrease in heat flow coupled with a high heat flow zone in the north and relatively low heat flow zone at the southern end. A decrease of sediment thickness proximal to the ocean/continent boundary could explain these high heat flow distribution trends west of the Laccadive ridge. The Bombay‐Saurashtra offshore region emerges as a zone of high heat flow and its proximity to the Deccan volcanic activity at 65 Ma needs further study.

The thermal history of the Bowen Basin, Queensland, Australia: vitrinite reflectance and clay mineralogy of Late Permian coal measures

The thermal history of the Bowen Basin (Queensland, Australia) has been investigated using vitrinite reflectance data and clay mineralogy. Vitrinite reflectance data combined with a study of clay mineral reactions indicates that the maximum temperatures which induced organic maturation of the Bowen Basin coals and extensive clay mineralisation are not related to deep burial metamorphism during the latest Middle Triassic–earliest Late Triassic as previously believed. The results of the present study indicate that the development of a zone of high heat flow in the latest Late Triassic had a major control on the thermal history of the Bowen Basin. High palaeogeothermal gradients estimated in the northern Bowen Basin are interpreted to result from convective heat transfer during a hydrothermal event. Variable heat distributions due to localised fracture-enhanced permeable zones acting as hot reservoirs in the deeper part of the basin may have been responsible for some significant local thermal anomalies in the lower coal measures. The estimated palaeogeothermal gradients in the southern Bowen Basin also indicate high heat flow in the lower sections of the stratigraphy. Sections in the southern Bowen Basin, however, are believed to reflect a rock dominated semi-closed system with low water/rock ratio, where rocks are impervious to circulating fluids and thus heat transfer may have occurred by conduction. The correlation between vitrinite reflectance and clay mineralogy shows a delay in illitisation reaction relative to organic maturity for many illite/smectite (I/S) mixed-layer clays in the northern Bowen Basin. This phenomenon can be explained as a result of insufficient time for the completion of mineral reactions and a variable potassium supply in relatively impermeable rocks. The relationship between I/S expandability and vitrinite reflectance for the Bowen Basin data compared to basins with known tectonic regimes suggests a thermal history in a rift setting for the Bowen Basin. The effect of thin igneous intrusions on clay mineral reactions is very limited. Intensive illitisation due to heating of intrusions can only be observed in narrow zones immediately adjacent to intrusive bodies. This further demonstrates that mineral reactions are too slow to record the effect of extremely short heating duration, in contrast to organic maturity indicators. These differences between mineral and organic parameters aid in the identification of local contact metamorphic effects.

Hydrothermal alteration within the basement of the sedimented ridge environment of Middle Valley, northern Juan de Fuca Ridge

The Middle Valley segment at the northern end of the Juan de Fuca Ridge is a deep extensional rift blanketed with 200–500 m of Pleistocene turbiditic sediment. Sites 857 and 858 were drilled during Ocean Drilling Program Leg 139 to determine whether these two sites were hydrologically linked end members of an active hydrothermal circulation system. Site 858 was placed in an area of active hydrothermal discharge with fluids up to 270 °C venting through anhydrite-bearing mounds on top of altered sediment. The shallow basement of fine-grained basalt that underlies the vents at Site 858 is interpreted as a seamount that was subsequently buried by turbidites. Site 857 was placed 1.6 km south of the Site 858 vents in a zone of high heat flow and numerous seismically imaged ridge-parallel faults. Drilling at Site 857 encountered sediments that are increasingly altered with depth and that overlie a series of mafic sills at depths of 460–940 m below sea floor. Sill margins and adjacent baked sediment are highly altered to magnesian chlorite and crosscut with veins filled with quartz, chlorite, sulfides, epidote, and wairakite. The sill interiors vary from slightly altered, with unaltered plagioclase and clinopyroxene in a mesostasis replaced by chlorite, to local zones of intense alteration and brecciation. In these latter zones, the sill interiors are pervasively replaced by chlorite, epidote, quartz, pyrite, titanite, and rare actinolite. The most complete replacement is associated with brecciated horizons with low recovery and slickensides on fracture surfaces, which we interpret as intersections between faults and the sills. Geochemically, the alteration of the sill complex is reflected in significant whole-rock depletions in Ca, Sr, and Na with corresponding enrichments in Mg, Al, and most metals. The latter results from the formation of conspicuous sulfide poikiloblasts. In contrast, metamorphism of the Site 858 seamount includes incomplete albitization of plagioclase phenocrysts and replacement of sparse mafic phenocrysts. Much of the basement alteration at Site 858 is confined to crosscutting veins except for a highly altered and veined horizon at the contact between basaltic basement and the overlying sediment. The sill complex at Site 857 is more highly depleted in 18 O (δ 18 O = 2.4‰ − 4.7‰) and more pervasively replaced by secondary minerals relative to the extrusives at Site 858 (δ 18 O = 4.5‰ – 5.5‰). There is no evidence of significant albitization of the plagioclase at Site 857, suggesting high Ca/Na in the pore fluids. Fluid-inclusion data from hydrothermal minerals in altered mafic rocks and veins at Sites 857 and 858 show a consistency of homogenization temperatures, varying from 245 to 270 °C, which is within the range of temperatures observed for the fluids venting at Site 858. The consistency of the fluid inclusion temperatures, the lack of albitization within the Site 857 sills, and the apparently low water/rock ratio collectively suggest that the sill complex at Site 857 is in thermal equilibrium and being altered by a highly evolved Ca-rich fluid similar to the fluids now venting at Site 858. The alteration evident in these two deep crustal drillsites is a result of the ongoing hydrothermal circulation and is consistent with downhole logging results, instrumented borehole results, and hydrothermal fluid chemistry. The pervasive alteration of the laterally extensive sill-sediment complex at Site 857 determines the chemistry of the fluids that are venting at Site 858. The limited alteration of the Site 858 lavas suggests that this basement edifice acts as a penetrator or ventilator for the regional hydrothermal reservoir with much of the flow focussed at the highly altered and veined sediment-basalt contact.

The alkali element and boron geochemistry of the Escanaba Trough sediment-hosted hydrothermal system

A suite of conjugate pore fluid and sediment samples were collected during Leg 169 of the ODP from within the clastic sedimentary sequences which host massive sulphides at Central Hill, Escanaba Trough (ODP Site 1038). We report the alkali element and boron, and Li and B isotope data for these samples. Relative to a reference site (Site 1037) located outside the zone of high heat flow, pore fluids from Site 1038 show a wide variation in Cl (300–800 mM), and have far higher concentrations of Li (up to 6.2 mM), B (up to 9.7 mM), Cs (up to 5.0 μM), and Rb (up to 97 μM). We show that the pore fluids are derived from hydrothermal circulation that has extended into the basement oceanic crust, with input of the alkali elements and B as the rising hydrothermal fluids interact geochemically with the overlying clastic sediments. There is, however, no marked depletion of these elements in the conjugate sediments, suggesting that there has been advective transport of fluids away from the primary hydrothermal reaction site. This is supported by modelling of the Li and B isotope systematics of the pore fluids, which shows that they record extensive formation of secondary minerals during cooling of the fluids from ∼350 to ∼20°C. Precipitation of metal-rich sulphides would have occurred prior to the formation of these minerals, thus, the pore fluid Li and B isotope data can place important constraints on the locus of sulphide deposition beneath the seafloor at Escanaba.

Present heat flow and paleo-geothermal regime in the Canadian Arctic margin: analysis of industrial thermal data and coalification gradients

Calculations of the present geothermal gradient and terrestrial heat flow were made on 156 deep wells of the Canadian Arctic Archipelago. Corrected bottom hole temperature (BHT) data and drill stem test (DST) temperatures were used to determine the thermal gradients for sites for which the quality of data was sufficient. Thermal gradients evaluated for depths below the base of permafrost for the onshore wells and below sea bottom for the offshore wells were combined with the estimates of effective thermal conductivity to approximate heat flow for these sites. The present geothermal gradient is in the 15–50 mK/m range (mean = 31 ± 7 mK/m). Present heat flow is mainly in the 35–90 mW/m 2 range (mean = 53 ± 12 mW/m 2). Maps of the present geothermal gradient and present heat flow have been constructed for the basin. The analysis of vitrinite reflectance profiles and the calculation of logarithmic coalification gradients for 101 boreholes in the Sverdrup Basin showed large variations related in many cases to regional variations of present terrestrial heat flow. Paleo-geothermal gradients estimated from these data are mostly in the range of 15–50 mK/m (mean = 28 ± 9 mK/m) and paleo-heat flow is in the 40–90 mW/m 2 range (mean = 57 ± 18 mW/m 2) related to the time of maximum burial in the Early Tertiary. Mean values of the present heat flow and paleo-heat flow for the Sverdrup Basin are almost identical considering the uncertainties of the methods used (53 ± 12 versus 57 ± 18 mW/m 2, respectively). Present geothermal gradients and paleo-geothermal gradients are also close when means are compared (31 ± 7 versus 28 ± 9 mK/m respectively). A zone of high present heat flow and a paleo-heat flow zone coincide in places with the northeastern-southwestern incipient rift landward of the Arctic margin first described by Balkwill and Fox (1982). Correlation between present heat flow and paleo-heat flow for the time of maximum burial in the earliest Tertiary suggests that the high heat flow zone has prevailed since that time.

A synmetamorphic lateral fluid flow model for gold mineralization in the Archean southern Kalgoorlie and Norseman terranes, Western Australia

Gold deposits are widely distributed in the Archean southern Kalgoorlie and Norseman greenstone terranes, Eastern Goldfields superterrane, Western Australia, which have collectively produced more than 1,850 metric tons (t) of gold. Gold deposits occur in a variety of host rocks and structural settings, but all are associated with quartz veins and/or shear zones that formed during the latest stages of regional compression. Available data indicate a similar H 2 O-CO 2 (-CH 4 ) ore fluid at each deposit. Alteration assemblages vary with metamorphic grade in a manner which indicates that late tectonic regional metamorphic gradients were still in place when gold was deposited from hydrothermal fluids. These late tectonic thermal gradients were related to differential uplift of deep hot crust, at the margin of the greenstone belt, accompanied by intrusion of voluminous monzogranite magma. At this time, the western margin of the greenstone belt was a zone of high heat flow, as were intrusive centers in the northern part of the southern Kalgoorlie terrane. These spatial and temporal relationships, in combination with the apparent absence of a viable local source for the ore fluid at most deposits, suggest that gold was deposited from a small number of large, synmetamorphic hydrothermal systems. Published thermal models predict up-temperature, lateral fluid flow toward zones of high heat flow, under these circumstances. A model involving up-temperature, lateral flow of metamorphic fluids overcomes some of the problems associated with deeply sourced and conventional metamorphic models for the ore fluids and is consistent with most of the features of gold deposits in the southern Kalgoorlie and Norseman terranes. Potassium metasomatism and radiogenic isotope signatures associated with gold mineralization are ascribed to a metamorphic fluid that was modified by exchange with biotite-bearing felsic rocks (granitoids, felsic volcanic and volcaniclastic rocks) in the course of lateral flow toward zones of high heat flow.

Paleomagnetic and 40Ar/39Ar age constraints on the chronologic evolution of the Rio Puerco volcanic necks and Mesa Prieta, west-central New Mexico: Implications for transition zone magmatism

The Rio Puerco volcanic necks and Mesa Prieta, located in the transition zone between the Rio Grande rift and southeastern Colorado Plateau, New Mexico, are part of the larger Mount Taylor volcanic field. The comparatively small volume, but numerous, eruptions represented by the volcanic necks provide geochronologic insight to the larger magmatic system at Mount Taylor and in a broader sense to regional late Cenozoic volcanism of the southeastern Colorado Plateau transition zone. Whole-rock 40 Ar/ 39 Ar ages from 17 basaltic plugs, flows, and dikes range in age from 4.49 to 2.05 Ma and overlap the 3.73 to 1.57 Ma volcanism observed at Mount Taylor. Volcanic eruptions in the Rio Puerco Valley were few and sporadic prior to 3.17 Ma; after this time, eruptions progressively increased to a zenith at ca. 2.70 Ma and ceased altogether after 2.56 Ma. Volcanism at Mesa Prieta (2.36–2.05 Ma) is significantly younger than that recognized for the volcanic necks and is considerably more voluminous and chemically evolved. Paleomagnetic data from the volcanic necks show that basalts carry reliable thermoremanent magnetizations; 9 of 13 sites exhibit well-grouped statistical characteristics. Polarity results provide an independent check of the 40 Ar/ 39 Ar analytical accuracy and are useful in resolving problems associated with excess Ar and/or Ar loss. The magmatic evolution of the Rio Puerco necks differs from the rest of the Mount Taylor volcanic field in duration of volcanism, erupted volume, and composition. These differences are believed to be related to the nonuniform character of the lithosphere9s structure and dynamic thermal construct across the Jemez zone in the vicinity of the Mount Taylor volcanic field. Our model proposes a zone of localized high heat flow and higher magma production beneath Mount Taylor volcano from 3.73 to 1.57 Ma to produce a suite of voluminous basalt-rhyolite compositions. However, east of Mount Taylor in the Rio Puerco Valley where the lithosphere is proposed to be cooler and more rigid throughout the same time interval, small-volume, xenolith-bearing, mafic eruptions are more typical. A cooler lithosphere is consistent with both low magma production rates and the observed small-volume eruptions of the Rio Puerco necks. A preexisting structural trend in the Rio Puerco Valley is believed to have assisted the ascent of volatile-rich, Rio Puerco magmas, particularly through the upper crust.

Heat flow in the Oregon Cascade Range and its correlation with regional gravity, Curie point depths, and geology

New heat flow data for the Oregon Cascade Range are presented and discussed. Heat flow measurements from several deep wells (up to 2500 m deep), as well as extensive new data from industry exploration efforts in the Breitenbush and the Santiam Pass‐Belknap/Foley areas are described. The regional heat flow pattern is similar to that discussed previously. The heat flow is about 100 mW m−2 in the High Cascade Range and at the eastern edge of the Western Cascade Range, It is about 40–50 mW m−2 to the west in the outer arc block of the subduction zone. In the high heat flow zone the heat flow is low at shallow depths in young volcanic rocks due to the high permeability of the rocks and the resultant rapid groundwater flow. Below a depth of 200–400 m much of the area appears to be dominated by conductive heat transfer at least to 2–2.5 km depth. There are perturbations to the regional heat flow in the vicinity of the hot springs where values are up to twice the background. The gravity field in the Cascade Range has characteristics that can be closely related to the heat flow pattern. The relationship may be causal, and to examine the relationship in more detail, earlier two‐dimensional modeling is extended to three dimensions. Consideration of the effects of a midcrustal density anomaly, such as might be associated with a region with at least areas of partial melt, has two major consequences. The first of these is that a high‐frequency gravity gradient near the Western Cascade Range/High Cascade Range boundary is explained. Second, the negative gravity anomaly associated with the north half of the High Cascade Range can be removed, and as a result, the prominent northeast/southwest striking regional Bouguer gravity anomaly associated with the north edge of the Blue Mountains becomes continuous across the Cascade Range with a similar feature along the north side of the Klamath Mountains. Apparently, this zone is a major crustal feature upon which the negative gravity anomaly coincident with the high heat flow is superimposed. The correlation, or lack thereof, of the heat flow, depth to Curie point, gravity field, crustal electrical resistivity, crustal seismic velocity, and geology in the High/Western Cascade Ranges is summarized. Many of the data show aspects that can be interpreted in relation to possible high temperatures in the midcrust of the Cascade Range. The High Cascade Range midcrust has unusually high temperatures and contains a zone of magma staging at 10±2 km depth that can also be identified in subdued form in the Cascade Range in Washington and British Columbia.

Marine heat flow observations on the Canadian Arctic continental shelf and slope

We present 36 new heat flow measurements, grouped within five separate sites on the Canadian Arctic continental shelf and slope, between 95°–115°W and 80°–82°N. Observations consist of temperature gradients and in-situ thermal conductivity using a modified, marine-type, multiple-penetration probe with a length of 4–6 m. Repeat measurements from nearby locations at different times of the year indicate that, for depths deeper than 200 m, the bottom water remains sufficiently stable to allow shallow measurements to characterize the deeper, geothermal flux. In this respect, the cold, ice-covered, deep-water Arctic margin is unique among the world's passive continental margins. Using these data, we construct a preliminary geothermal transect between the continental Sverdrup Basin to the south and the oceanic Canada Basin to the north. Values of 35–45 mW m −2 exist on the Axel Heiberg headland, Nansen Channel and on the continental slope, and indicate a broad-scale reduction from higher values of 55–60 mW m −2 within the Canadian Basin to values of 40–50 mW m −2 within the Sverdrup Basin. In contrast, there is a zone of high heat flow, with well-constrained values of 80 mW m −2 within Peary Channel. Low values in the northeastern sector of the margin suggest that the early Tertiary, Eurekean Orogen near Ellesmere Island has not thermally affected the margin. Based on calculations for pure-shear extension, the 15 mW m −2 reduction across the continental margin suggests that the Canada Basin proper may not have opened until Late Cretaceous, unless the pre-existing continental crust contains an extremely low amount of radiogenic material. Lithospheric models cannot account for the high values in Peary Channel which may indicate water movement within the sedimentary basin.

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.

Geophysical and petrological evidence for the presence of an “anomalous” upper mantle beneath the Sahara basins (Algeria)

North of 20°N, the northwest part of the stable African shield is characterized by an anomalous high heat flow zone (80–110 mW m −2). This east-west elongated zone is oblique to major Proterozoic structural units and cannot be related to differences in crustal heat production. Maximum heat flow occurs in the southern part of the Sahara basins (100–120 mW m −2). Only a mantle heat flow contribution of 60–70 mW m −2 can account for this very high surface heat flow. The estimated geotherms suggest that the temperature of upper mantle is anomalously high and partial melting may be present at shallow depths. This hypothesis is supported by the existence of a long-wavelength negative component of the gravity field that corresponds with the high heat flow axis and with the very low lithospheric S-wave velocities (4.2–4.4 km/s) down to 160 km obtained from surface wave studies. In addition, the chemistry of the scarce Cenozoic melilitite lavas (Illizi volcanic district), geographically associated with the thermal anomaly, as well as the mineralogy of their inclusions (phlogopitized garnet/spinel peridotite and alkali clinopyroxenite) show that the upper mantle beneath this region is locally highly metasomatised and partially melted. From these preliminary results, it appears that the northwest African upper mantle has been modified by recent thermal rejuvenation processes (including gas transfer) that are likely to be more extensive in the southern part of the Sahara basins.

Surface mercury geochemistry as a guide to volcanic vent structure and zones of high heat flow in the valley of ten thousand smokes, Katmai National Park, Alaska

A reconnaissance survey of Hg° was designed to model the 1912 Novarupta vent structure and delineate zones of near-surface high heat flow. Statistical analysis of 294 regolith samples collected at 127 sites from fossil/active fumaroles and relatively unaltered ash-flow tuff and air-fall ejecta indicates two Hg° sample populations; one associated with fumarole deposits and the other corresponding to the relatively unaltered regolith. Measured Hg° concentrations range from 12 to 6913 ppb. Sample threshold (upper limit of background Hg° concentrations; fumarole deposits: 351 ppb, unaltered regolith: 110 ppb) and contrast (sample value/threshold) determinations enable differentiation between relict and actively accumulating Hg° contents. All significant Hg° anomalies (contrast values > 1.5) occur within Novarupta Basin. High-magnitude, active Hg 2 enrichment was found on the Turtle, a dome-like feature northeast of the Novarupta extrusive dome. The Hg° data, coupled with available geologic and geophysical evidence, suggest a shallow intrusion beneath the Turtle. Results from two Novarupta Basin sample traverses do not support the presence of a near-surface magma feeder dike connecting the proposed intrusion beneath the Turtle with the Novarupta dome. Based on the Hg° data, the preferred vent model is one generated by collapse of supporting walls after the major eruptive phase into a cored-out explosive vent. Collapse of vent walls is initiated along a series of deep-extending faults which subsequently serve as migration pathways for Hg° to the surface. The overall vent morphology is funnel-like with subsidence concentrated in the narrow funnel center. Results of this study show that Hg° surveys can further constrain the morphology of the 1912 vent and aid in delineating zones of near-surface high heat flow in this region.

Heat flow measurements in the northern part of Honshu, northeast Japan, using shallow holes

Abstract One of the most notable features of heat flow distribution in the Japanese Islands is the existence of a low heat flow zone on the Pacific Ocean side of the Northeast Japan Island Arc and a high heat flow zone on the Japan Sea side. However, the detail of the transition between the low and high heat flow zones has not been apparent because of the lack of available deep holes. Heat flow measurements in the transition zone have been made using shallow holes. By the combined use of a new multisensor long-term underground temperature recording system and conventional techniques, four reliable heat flow values were obtained in the transition zone. The results show that the heat flow value increases steeply from the aseismic front (20–40 mW/m2) to the volcanic front (120–130 mW/m2). If the heat transfer in the crust is conductive, the temperatures beneath the aseismic front and the volcanic front at the depth of 30 km will be about 100°C and about 1000 °C, respectively.

Heat flow and heat production in Brittany (Western France)

New heat flow determinations in Brittany (Western France) allow mapping with a good coverage (1 point per 1000 km 2) over that region. A high heat flow zone, oblique to the present geological structures, trends from northern Central Brittany to the south, and then turns to the ESE towards the Massif Central. Heat production ( A) and heat flow ( Q) are tested against a linear relationship. The data do not fit a strict linear relationship, but the points are distributed within an elongated cluster in which a linear regression can be approximated, yielding the following equation: Q = 26.6(±3.5) mW/m 2 + 15.6(± 0.6) km × A. Several tests of significance corroborate the linear cluster and do not alter the computed parameters. The mapping of the residual values shows a geographical distribution which can be correlated with filtered gravimetric and magnetic data. Those residuals reflect local variations in heat production distribution or in the heat transport mechanism. The high value of the slope (15.6 km) computed during the linear regression reflects the influence of the upper to middle crust. This value is too elevated to represent the thickness of the granitic intrusions, which are known to be shallow rooted (4–6 km) from gravity data interpretation. Compared to adjacent Hercynian countries, the data lie within the range of those of central and southwestern England.

Regional trends in radiogenic heat generation in the Precambrian basement of the Western Canadian Basin

Radiogenic heat generation values for 381 basement samples from 229 sites in the western Canadian basin exhibit a lognormal frequency distribution. The mean value <A> = 2.06 (S.D. = 1.22) µWm−3 is larger than the radiogenic heat generation values reported for the shield in the Superior (ca. 1.2 µWm−3, Jessop and Lewis, 1978) and Churchill (ca. 0.7 µWm−3, Drury, 1985) provinces. When equal Log A contour intervals are used to map the basement heat generation, three large zones of relatively high heat generation are found. One coincides with the Peace River Arch basement structure and one with the Athabasca axis (Darnley, 1981). There is no apparent indication of increased heat flow through the Paleozoic formations associated with these two zones. The third zone, in southwestern Saskatchewan, coincides with a high heat flow zone in the Swift Current area. The lack of correlation between heat flow and heat generation in Alberta may be due to the disturbance to the heat flow in the Paleozoic formations by water motion, or may indicate that the heat is from uranium, thorium and potassium isotope enrichment near the basement surface rather than enrichment throughout the entire upper crust.

Heat flow maps and deep ground water circulation: Examples from Switzerland

The effect of regional and local ground water circulation systems on the Heat Flow Density (HFD) field is demonstrated by two examples from Switzerland, one near St. Gall in an area at the northern border of the Alps, and the other northwest of Zurich along the eastern end of the Jura mountains. Detailed HFD maps of both areas slow pronounced high heat flow zones which are attributed to discharge of subsurface water which has migrated laterally over several 10 km. Seepage velocities on the order of several mm/yr have been calculated. Geothermal information is not available about the infiltration zones where low HFD values are expected. Geochemical and isotopic analysis of water samples from springs and drillholes indicates the recharge zones and demonstrates the effect of extensive regional systems. These results indicate that in regions with significant topographic relief HFD mapping can be seriously biased if drillholes are positioned in valleys which correspond to discharge areas with relatively high HFD, whereas the low heat flow zones remain undetected.

Thermal Maturation and Burial History Processes in Lower Mesozoic Sediments of North Atlantic Georges Bank Basin: ABSTRACT

The time-temperature burial relationships that influence organic maturation processes in the Upper Triassic-Lower Jurassic rift basin sediments and the Middle Jurassic shelf-edge carbonates and sand-shale facies of the North Atlantic Georges Bank basin are believed to be regionally influenced by sediment loading and residual subsidence associated with the contractions and cooling of a thinned lithosphere and crust. Localized zones of high heat flow may have occurred during the rifting process. Relict heat flow from such sources may have influenced the principal zone of oil formation (PZOF) in the Georges Bank basin. Plots of specific temperature-sensitive, intercompound ratios of isomeric groups for light hydrocarbons that separate chromatographically in the cyclohexane t rough methylcyclohexane range and plots of the molecular ratio of n-hexane to methylcyclohexane were found to be consistent with the temperature-sensitive, saturated hydrocarbon to organic carbon ratios. These ratios suggest that the threshold of intense oil generation in the depocenter of the basin begins at a depth of about 3.2 km with the PZOF occurring between 3.9 and 4.9 km. Correlation of these temperature-sensitive maturation parameters with the time-temperature burial history models indicates that Middle to Lower Jurassic and probable Liassic to Upper Triassic units may be the most favorable exploration targets within the main Georges Bank basin. End_of_Article - Last_Page 1443------------