High Regional Heat Flow Research Articles

Lateral variations in the upper mantle velocity structure in the northwestern pacific margin

The upper mantle velocity structure in various island arc-marginal sea regions of the northwestern Pacific has been studied in detail to a depth of about 640 km from the analysis of P- and S-wave travel times data of 363 earthquakes with focal depths varying from 35 to 640 km. Wave velocities were obtained at the depths of foci by using Kaila's (1969) analytical method in several units of the northwestern Pacific margin: 1.(1) Honshu-Izu Bonin-Japan Sea,2.(2) Kyushu-Shikoku,3.(3) Hokkaido,4.(4) Okhotsk Sea,5.(5) southern Kurile Islands,6.(6) northern Kurile Islands,7.(7) Kamchatka,8.(8) Ryukyu Islands, and9.(9) Taiwan-Luzon. There is substantial evidence for large lateral variations, of the order of 8–10%, in both P- and S-wave velocity structure to about 250 km depth within the subducting lithospheric slab in the northwestern Pacific margin. The P velocity determined at 40 km depth, is found to be about 8.20 km/sec in the southern Kurile Islands 7.90 km/sec in the northern Kurile Islands, and the Japanese Is., 7.80 km/sec in Kamchatka and 8.05 km/sec in the Ryukyu Islands, and Taiwan-Luzon regions. The S velocity determined at 40 km depth, in the southern Kurile Is., is also found to be relatively higher, about 4.60 km/sec, compared to all other regions in the northwestern Pacific margin where it is varying from 4.30 to 4.45 km/sec. P and S velocities determined to about 185 km depth in the southern Kurile Islands region are found to be 3–5% higher than those obtained at comparable depths in the Hokkaido island, northern Kurile Islands and Kamchatka. P. velocities, determined to a depth of about 255 km, in the Ryukyu and Taiwan-Luzon regions are quite similar and are about 5% higher on the average, than the nearly constant P velocity of 7.88 km/sec found in the Kyushu and Shikoku islands of the southwest Japan. In the southwest Japan region, both P and S wave velocities are found to remain constant to almost 255 km depth. This might be due to the presence of a large magma chamber resulting also in the extensive volcanic activity observed in southwest Japan (e.g. Sakurazima volcano).The P- and S-wave velocity-depth functions, determined in the depth range of 290–640 km, in the Okhotsk Sea region, reveal a sharp first-order velocity discontinuity at 390 km depth. The observed velocity jumps are 9.2% for P and 5.6% for S waves in this region. In the central Japan region also, comprising the Honshu and the Japan Sea, there is a sharp first-order velocity discontinuity at depths of 365 km for P and 345 km for S waves with associated velocity jumps of 8.6% for P and 4.8% for S waves. The S velocities below this discontinuity to about 640 km depth in the central Japan and the Okhotsk sea regions are found to be considerably lower. The entire area, comprising the high heat-flow regions of the marginal seas of Japan and Okhotsk, may be a high temperature/high attenuation zone extending to 640 km depth. There is no evidence for the presence of a significant low-velocity layer in the northwestern Pacific margin.

Curie isotherm surfaces inferred from high‐altitude magnetic anomaly data

Traditional methods of Curie depth estimation involving spectral analysis of low‐level aeromagnetic data require that depth to the bottom of a magnetic crustal layer be large relative to depth to top, so that spectral separation can be achieved. An alternate method for high‐altitude data, where depth to magnetic layer top and bottom are virtually indistinguishable, is described. Apparent magnetization variation in a thin equivalent layer is obtained by inversion of the anomaly data. The information in such a model is essentially the vertical integral of magnetization in the real layer to some resolution limit and within some ambiguity in level. This can be transformed to a model of thickness variation in a layer of constant magnetization, given some minimal constraints. The thickness variations may reflect Curie isotherm undulations in regions of high crustal heat flow; Curie depth models then provide a constraint on crustal thermal models. The method was first applied to a magnetization model for the western United States derived from Magsat data. A Curie surface model was obtained and used to constrain simple thermal models along two crustal sections. Associated surface heat flow is in agreement with observations. The method was then applied to upward continued aircraft data from the Oregon Cascades. Results are in reasonably good agreement with previous results based on spectral analysis and with measured heat flow.

PETROLEUM GEOLOGY OF THE MERLINLEIGH SUB-BASIN, WESTERN AUSTRALIA

Esso's recent drilling program in the Merlinleigh Sub-basin, onshore Carnarvon Basin, represents the culmination of the first phase of concerted exploration activity in the area since the WAPET era of the 1960s. The region is unusual among Australian petroleum provinces in having excellent exposures of reservoir, source and seal rocks of Palaeozoic age. While both Esso wells (Burna 1 and Gascoyne 1) failed to encounter hydrocarbons in the primary Wooramel Group play, encouraging potential still exists. The reservoir in the Wooramel Group play is the Early Permian Moogooloo Sandstone, a fluviodeltaic to nearshore sheet-sand facies with porosities to 23 per cent and permeabilities in excess of 100 millidarcys. Likely hydrocarbon sources are siltstones in the overlying Byro Group, with total organic carbon contents averaging 3 per cent, and calcilutites in the subjacent Callytharra Formation with similar organic content. Locally, the Jimba Jimba Calcarenite Member (Billidee Formation) and the Cordalia Sandstone also provide rich source units. The least certain aspects of the Early Permian play are fault and top seal, and reservoir quality at depth. Notwithstanding the relatively shallow depths to source strata in the area, vitrinite reflectance analyses from drill cores indicate that maturation is attained as shallow as 900 m on the folded and faulted western margin of the sub-basin, and at an approximate depth of 1200 m in the depocentre beneath the Kennedy Range. This can be related to high regional heat flow, and to erosion of some 1500-2000 m of sediments prior to the regional Early Cretaceous transgression.Older plays which have been identified in the area remain to be adequately evaluated. Potential reservoir sands are present in the Silurian Tumblagooda Sandstone, the Middle and Late Devonian Nannyarra and Munabia Sandstones, and the Early Carboniferous Williambury Formation. Possible source rocks include carbonates of Middle Devonian and Early Carboniferous age. One of the objects of current research has been to locate areas where seal, provided by the glacigene Lyons Formation of Late Carboniferous-Early Permian age, is sufficiently thin to permit economic drilling.

The pattern of anomalous geomagnetic variation fields over the midcontinent gravity high

Magnetometer array data, collected by H. Porath and co‐workers from the University of Texas at Dallas in the midwest United States during the fall of 1969, have been reanalyzed to further define the nature of the reported geomagnetic variation anomaly in the region of the midcontinent gravity high. Variation events with a wide range of horizontal source field polarization and frequency have been digitized from the original film records. These data have been used in a vertical‐field response arrow (induction vector) analysis. The results indicate that there is no significant overall conductivity anomaly associated with the geologic structure causing the gravity feature. Rather, the pattern of anomalous variation fields is one of isolated anomalies at high frequency. These anomalies are coincident with postulated fault structures perpendicular to the gravity high lineament and with localized regions of high heat flow.

Exploration and Development of the Cerro Prieto Geothermal Field

Summary A multidisciplinary effort to locate, delineate, and characterize the geothermal system at Cerro Prieto field, Baja California, Mexico, began in the late 1950's. It led to the identification of an important high-temperature, liquid-dominated geothermal system which went into production in 1973. This paper summarizes and discusses the exploration and monitoring studies related to this field. Introduction The Cerro Prieto geothermal field is located in the Mexicali Valley, Baja California, Mexico, about 30 km [19 miles] south of the U.S. border (Fig. 1). It has been in production since 1973, being the first liquid-dominated geothermal system in North America to produce significant electrical power. The general geologic similarity between Cerro Prieto and the geothermal fields of the neighboring Imperial Valley (southern California), and the experience gained by the Comisión Federal de Electricidad of Mexico (CFE) in locating and developing the resource were the main factors that led to the signing, in 1977, of a 5-year agreement between CFE and the U.S. Energy R&D Admin., now the U.S. DOE, to conduct a cooperative study of Cerro Prieto.1 Lawrence Berkeley Laboratory (LBL) coordinated U.S. technical activities carried out under the agreement. Because of the success of this cooperative program, discussions are being held to sign a new DOE/CFE agreement to study Cerro Prieto and other geothermal areas in Mexico. Exploration for geothermal energy in the Cerro Prieto area began in the late 1950's, and the first deep exploration wells were drilled in 1960–61 over the thermal anomaly. By early 1983, about 120 deep wells (up to 3550 m [11,647 ft] in depth) had been drilled, delineating a good portion of the geothermal reservoir (Fig. 2). Presently, 180 MW of electricity are being generated; CFE, which manages and operates Cerro Prieto, is building two new power plants that will increase the output to 620 MWe (megawatts of electric power) before the end of 1985. A vast amount of data on the subsurface of the area and on the characteristics of the producing wells has been gathered. Cerro Prieto is one of the more thoroughly documented and best understood geothermal systems in the world. This paper reviews the exploration effort which first led to the discovery of the field, then to the delineation of the resource, and finally to the definition of the subsurface fluid and heat circulation. Other studies carried out on Cerro Prieto are summarized in Ref. 2. Geologic Setting of the Area Cerro Prieto is situated in the southern portion of the Salton trough, an actively developing structural depression filled with sediments. It is the landward continuation of the Gulf of California (Fig. 1) and a region of high seismicity and high heat flow. The Salton trough is the result of tectonic activity, which has created a series of spreading centers and transform faults that link the East Pacific Rise, an oceanic ridge, with the San Andreas fault system, a transform boundary (Fig. 3).3,4 The Cerro Prieto field is located on a tensional spreading center at the ends of the right-lateral strike-slip Imperial and Cerro Prieto faults.3,4 Although the mechanics for crustal deformation and heating and for magma generation and movement into the shallow crust are not completely known, it is believed that Cerro Prieto is an area where gabbroic magma rising from a deeper chamber is creating a volcanic oceanic-type crust.5,6 Rapid sedimentation into this pull-apart basin has provided the reservoir and caprock for the geothermal system, and the thick sediments obscure the presence of the heating and plutonic activity. An area of seismicity connects the Cerro Prieto and Imperial faults (Fig. 4), providing evidence for crustal extension and magma ascent via dikes. There probably exists a plexus of dikes and sills7,8 and perhaps some deeper plutons. Basaltic dikes have been intersected in wells drilled to depths of 3 km [2 miles] and more in the eastern part of the field.9,10 The only significant volume of young volcanics to breach the surface are the two rhyodacitic cones ( 0.11 million years before present) that comprise the Cerro Prieto volcano. The composition of these rocks indicates that crustal heating by the gabbroic intrusions has been sufficient to partially melt and mobilize the silicic crust.

Pericollisional Strike-Slip Basins in Western Cordillera, Canada: ABSTRACT

The late Mesozoic-Paleogene evolution of the Canadian Cordillera was dominated by accretion of elongate crustal blocks against the North American craton. Geologic and paleomagnetic evidence suggest that these exotic terranes dispersed from volcanic arcs and oceanic platforms and approached North America along anastomosing right-lateral faults with great cumulative displacement. Obduction of oceanic allochthons was followed by transpressive thickening and regional metamorphism of the cratonic margin in the mid-Jurassic. Strike-slip motion and emplacement of plutonic rocks continued near relict sutures and reactivated deep faults. Sedimentary basins related to strike-slip faults formed by elongation of accreted terranes (Stikinia and Wrangellia) and by shear within the eformed cratonic margin zone (Rocky Mountain Trench). Subsidence is reflected by northwest-southeast stretching along pull-apart structures, and by massive influx of turbidites from incipient collision zones and relict arc relief. It was interrupted and outlived by rotation of blocks, folding of basin sediments, and vigorous progradation of deltaic-fluvial clastics from rising collision belts. Transition from predominant transtension to prevailing transpression is diachronous from basin to basin. Near the Stikine-Wrangellia collision zone (Bowser basin), it occurred in the Late Jurassic; along the Stikine-Wrangellia border it occurred in the mid to Late Cretaceous. Only small nonmarine basins developed in the Rocky Mountain Trench system, which, in its southernmost part, was closed com letely during Paleogene thrust faulting. The strike-slip basins of the western Canadian Cordillera were subject to high regional heat flow and also suffered from widespread intrusion of Paleogene granitoids. Therefore, they are generally poor oil and gas prospects. End_of_Article - Last_Page 473------------

Experience with the EM-60 electromagnetic system for geothermal exploration in Nevada

Lawrence Berkeley Laboratory (LBL) conducted controlled‐source electromagnetic (EM) surveys at three geothermal prospects in northern Nevada to test and demonstrate the applicability of LBL’s EM-60 system to geothermal exploration. During the late summer and early fall of 1979, over 40 soundings were made in Panther Canyon (Grass Valley), near Winnemucca; Soda Lakes, near Fallon; and McCoy, west of Austin. The EM-60 is a frequency‐domain system using three‐component magnetic detection. For the surveys reported here, we applied ±65 A to a 100-m diameter four‐turn horizontal loop, generating a dipole moment [Formula: see text] over the frequency range [Formula: see text] to [Formula: see text]. With such a source loop and a remote magnetic reference for noise cancellation, we made soundings at transmitter‐receiver separations of up to 4 km over the frequency range of 0.05 to 500 Hz. This has yielded a maximum depth of exploration to 2 km or more, adequate for the purposes of the geothermal investigations. Recorded spectra are interpreted by means of simple apparent‐resistivity calculations made in the field and by layered‐model inversions computed in the laboratory. The EM interpretations are then compared with other available geologic/geophysical data sets for the purpose of combined interpretation and method evaluation. Experience with the EM-60 system in Nevada has shown it to be an efficient and possibly more cost‐effective alternative to dc resistivity and magnetotellurics for geothermal exploration. An average of two soundings per field day for depths of exploration up to 2 km was obtained routinely. Results from EM-60 work at Panther Canyon compare very favorably with earlier dipole‐dipole resistivity surveys. Both methods outlined an irregularly shaped, buried conductive body associated with a region of high heat flow, but the feature was delineated by means of the EM-60 in just over half the field time required for the dipole‐dipole resistivity survey. At Soda Lakes, 13 high‐quality EM soundings were obtained from two transmitters in six field days under ideal field conditions. With the EM-60 data, we were able to map the depth to and inclination of a buried conductive body associated with an area of high subsurface temperatures. In this case, the EM results confirmed an earlier MT survey interpretation and gave additional detailed near‐surface information. At the remote and mountainous McCoy site, data interpretation was complicated because of the rugged terrain. By modifying existing interpretive software, we were able to calculate the effects of tilted‐source dipoles and elevation differences on soundings and thus interpret data. The EM soundings detected a conductive zone at a depth of 200 m at the south end of the prospect, where a nearby drillhole had encountered water at 100°C at the same depth. In addition, EM soundings at McCoy provided information on a deep conductor below 2 km which has yet to be drilled.

Origin of the lithospheric tension causing basin formation

Most fault-controlled basin formation within plate interiors occurs by normal faulting in response to horizontal deviatoric tension in the continental crust. It is suggested that the tension originates either from the plate boundary forces acting at trenches or as a result of isostatically compensated uplifted regions such as East Africa. The tension produced by both mechanisms is greatest in high heat-flow regions where the upper elastic part of the lithosphere is thinned and weakened. Particularly widespread tension in the continental lithosphere occurs when subduction takes place on opposite sides of a large continental mass, such as Pangaea in the early Mesozoic, where it led to widespread graben formation and, in association with hot spot activity, to continental splitting.

Jurassic volcanic rocks of the northern North Sea

The lavas and minor intrusive rocks from the Jurassic volcanic province of the Forties-Piper area of the North Sea can be subdivided into three distinct series on the basis of petrography, whole-rock chemistry and mineral chemistry. These series, which probably evolved from one or more alkali olivine basalt parent magmas are: ( i ) an ankaramite-basaltic hawaiite-hawaiite series, ( ii ) an alkali olivine basalt-hawaiite series, and ( iii ) an intrusive hawaiite-intrusive mugearite series. Variations within series are due mainly to differential distribution of mafic phenocryst phases but liquid immiscibility and magmatic fractionation are also believed to have contributed. The Middle Jurassic volcanism in the North Sea area is thought to be related to the tensional tectonic regime which existed in NW Europe during the Jurassic or to be due to a region of high heat flow extending northeastwards from the contemporary spreading axis between Iberia and Newfoundland and possibly to a combination of the two.

Philippine Geothermal Energy Resource: An Available Indigenous Alternative: ABSTRACT

The Philippines is indeed fortunate to be located within the West Pacific Island Arc dotted by Neogene volcanic centers. The multistage development of volcanic-plutonic events in this western part of the Circum-Pacific basin has generated regions of high heat flow where known potential geothermal resources are located. With the increasing power demands reflecting a favorable growth of the country's economy, the Philippine government embarked on an accelerated program to harness the country's geothermal energy for power utilization at the start of the energy crisis in early 70s. For a period of 10 years (1972-81), the Philippines has successfully launched a systematic and continuing program of assessing, exploring, developing, and exploiting its vast potential geothermal resources. Of the several potential areas scattered all over the archipelago, four geothermal fields have already contributed some 501 megawatts equivalent to almost 12% of the total electrical power supply of the Philippines. This paper deals with the geothermal resource development of End_Page 963------------------------------ the Philippines, a major achievement of a developing country of the Third World in the use of new and renewable sources of energy. End_of_Article - Last_Page 964------------

Deep seismic surveys off southern Honshu, Japan, and earthquake prediction

The densely populated Tokai (southern Honshu) region of Japan expects a major earthquake in the near future. Recent studies indicate that partial melting of wet peridotite at hypocentral depths in regions of high heat-flow is possible and this phenomenon may be a significant trigger in creating such earthquakes. Changes in seismic shock wave velocities caused by partial melting can be calculated and the monitoring of such velocity changes may assist in the prediction of major earthquakes in the vicinity of Japan.

Principal Features of Epithermal Lode Gold Deposits of Circum-Pacific: ABSTRACT

Production from epithermal primary lode gold deposits along the Circum-Pacific rim exceeds one million ounces annually, from around 20 countries. Distribution of this important deposit type coincides with, and is genetically related to, convergent plate boundaries and the chain of associated volcanic-plutonic activity framing the ocean basin. Deposits are emplaced as clusters along two subparallel belts: the ocean margin transition zone and the zone within the continental framework boundary. Ocean margin deposits are associated with island arc-type intermediate to mafic igneous activity, active geothermal phenomena, and subduction-related regional fracture systems. Deposits inside the continental margin are associated with intermediate to felsic volcanic-plutonic belts an exhibit block faulting or volcano-tectonic fracture control. They feature strong Pb-Zn-Ag and local S-W metal associations. Within both belts, a continuum exists between near surface hot spring deposits, local disseminated replacement (Carlin-type) deposits, and deeper bonanza systems. Hot spring and replacement deposits feature relatively high Au:Ag ratios, contain micron-sized gold particles, are enriched in Hg-As-Sb-Ba, and are hosted by hydrofracted quartz-pyrite stockworks or fine-grained carbonaceous limestones. Bonanza deposits are characterized by polymetallic veins and stockworks. Economic concentrations require initial high gold solubility, unrestricted recharge of meteoric water into a region of steady high heat flow, fracture-controlled fluid focusing, and either host-rock r activity or episodic self-sealing, with explosive pressure release in the zone of deposition. Boiling, temperature decrease, solution oxidation and local ground-water mixing are the primary processes of ore deposition. Modern-day analogs of such ore forming systems exist in New Zealand, Japan, western USA, Indonesia, and the Philippines, among others. End_of_Article - Last_Page 968------------

Migration in relation to possible tectonic and regional controls in eastern Australian volcanism

The Wellman-McDougall model for southward migration of central volcano activity in eastern Australia is extended to the basaltic lava provinces. Latitude-age plots of volcanic episodes are related to trails initiated from regions of active volcanism at the commencement of northward drift of Australia (53 m.y. B.P.), from Southern Ocean spreading. These trails intersect at least 75%, and possibly up to 95%, of basaltic episodes and suggest a migratory control. The migration of central volcano felsic activity, however, consistently exceeds sea-floor spreading rates with a relative southward motion of 4–10 mm/yr. The trails give a mean migration direction of 24 ± 9°W of S before 29–30 m.y., but 8–12°W of S after 29 m.y. Extrapolation of the central volcano activity back in time, places it across the Coral Sea—Louisiade spreading system and New Guinea collision zone at the beginning of drift. This suggests that these structures may provide residual thermo-tectonic anomalies in the mantle with potential to initiate volcanism under the moving Australian plate. Kinematic reconstruction of Australia's drift relative to these structures shows that the western felsic and K-rich ultramafic activity falls on progressive overlaps of the Coral Sea — Louisiade anomalies. The basaltic volcanism generally overlies the central regions of the anomalies. The young basalt fields of Victoria—South Australia correspond with positions of the Coral Sea—Louisiade anomalies over the last 5 m.y. The regions of youngest volcanism, conductivity anomaly and high heat flow lie near the present projected position of the anomalies. Some volcanic episodes are not obviously related to migration trails. The young basaltic activity in northern Queensland may result from movement of the Australian Plate towards unstable regions of volcanism and uplift in Papua New Guinea. Central volcano province spacings appear to match expected lithospheric thickness. Spacings of basaltic episodes suggest that other factors, such as zonal rotational flow in the asthenosphere, may also be involved. Cessation of much volcanism over the eastern Australian highlands about 10 m.y. ago is attributed to imposition of the present regional compressive stress field, probably as a result of changes in the sea-floor spreading patterns. An apparent change in migration direction of volcanism 29–30 m.y. ago may be related to changes in east-west spreading and continental motions adjacent to the Australian plate. Differences between migration directions of volcanism and absolute motions determined for the Australian Plate may reflect effects of the earth's rotation and migration of spreading ridges. The migratory model developed here predicts future Australian volcanism mostly in or south of the present regions of younger volcanics and high heat flow. It would be basaltic, but may be accompanied by felsic activity in Tasmania.

Global distribution of long-period P-wave attenuation and its tectonic implications.

A large number of WWSSN long-period P-wave records from 41 inter- mediate- and deep-focus earthquakes have been analyzed aiming at investigating global variations of attenuative properties in the upper mantle, with statistical treatments using a damped least squares method. The present results reveal significant regional variations of attenuation over the world. High Q is observed along the Pacific coastal region and the eastern part of the United States, India, Indochina, and Australia, most of which are located in stable shield region. On the other hand, low Q is observed in the western United States and a part of Europe, which are in tectonically active region. There appears to be some correlation between the obtained differential attenuation and heat flows, suggesting that high heat flow regions are characterized by low Q and vice versa. However, there is no clear relation between travel-time residuals and attenuation. A large number of the data used in this study generally give support to the SL8 model as an average Q model in the earth's mantle.

Overview of Geothermal Exploration on Western Slopes of Mt. Hood, Oregon: ABSTRACT

Since 1977, Northwest Geothermal Corp., the U.S. Geological Survey, the Oregon Dept. of Geology and Mineral Industries, and the U.S. Dept. of Energy have cooperatively explored the western slopes of Mt. Hood, Oregon, for geothermal water suitable for direct utilization. The high regional heat flow of the Cascade Mountains provided the impetus for the exploration. A possible magma chamber associated with Mt. Hood enhanced the prospect. The anticipated resource is deep circulating meteoric waters. The stratigraphic history of the western Mt. Hood area is one of andesitic volcanism from late Oligocene to Holocene. The Miocene basalts of the Columbia River Group interrupt and locally interfinger with this sequence. A large quartz diorite stock of questionable age is exposed immediately southwest of Mt. Hood. The regional structural setting has been interpreted to be a right lateral wrench tectonic system resulting from north-south compression. Major northwest-trending right lateral faults with some vertical component, and northeast-trending antithetic faults were mapped. Low amplitude en-echelon west-northwest-trending folds are present in the younger rocks. An older northeast-trending fold pattern, coupled with thrusting, was mapped in the basalts of the Columbia River Group. The exploratory drilling program was designed to develop stratigraphic, structural, and hydrologic information, as well as to establish the geothermal gradient. The initial 564-m observation well in the Old Maid Flat area of Mt. Hood yielded a conductive gradient of 67°C/km. Gradients on the western side range from 20 to 83°C/km. Two deep tests (1,220 m and 1,837 m), funded by DOE, have been drilled. Both wells, while finding no fluids, have conductive gradients to total depth. End_of_Article - Last_Page 958------------

Comparisons of magnetic anomalies of lithospheric origin measured by satellite and airborne magnetometers over western Canada

Crustal magnetic anomaly data from the Orbiting Geophysical Observatories 2, 4, and 6 (Pogo) satellites are compared with upward-continued aeromagnetic data between 50–85°N latitude and 220–260°E longitude. Agreement is good, both in anomaly location and in amplitude, giving confidence that it is possible to proceed with the derivation and interpretation of satellite anomaly maps in all parts of the globe. The data contain a magnetic high over the Alpha ridge suggesting continental composition and a magnetic low over the southern Canada basin and northern Canadian Arctic Islands (Sverdrup basin). The low in the Sverdrup basin corresponds to a region of high heat flow, suggesting a shallow Curie isotherm. A ridge of high field, with two distinct peaks in amplitude, is found over the northern portion of the platform deposits and a relative high is located in the central portion of the Churchill Province. No features are present to indicate a magnetic boundary between Slave and Bear Provinces, but a trend change is evident between Slave and Churchill Provinces. South of 60° latitude a broad magnetic low is located over very thick (40–50 km) crust, interpreted to be a region of low magnetization.

Heat-Flow and Heat-Production Studies in North Dakota: ABSTRACT

End_Page 780------------------------------Thirty-one new heat-flow determinations made in both oil and water wells in North Dakota range from 0.6 to 1.9 HFU. Heat-production data from basement rocks used with nearby heat-flow values indicate that only two of six sites may be considered to be similar to Basin and Range values for heat flow. One site occurs in southwestern North Dakota in a region west of 103°W where no heat-flow value is less than 1.5 HFU. The other site in north-central North Dakota is problematic, but an eastern United States interpretation is favored. The remaining four sites are interpreted as eastern U.S. values. A transition in heat-flow character from eastern United States values to Basin and Range values occurs west of 103°W long. in southwestern North Dakota. The narrow width (28 km) of the transition zone between heat-flow provinces implies a shallow depth to partly molten lower crust or upper mantle. The heat-flow results coincide with a zone of anomalous electrical conductivity. When used with experimental petrologic data for peridotite in the presence of excess water, temperature calculations suggest that a partial melt zone begins approximately at 45 to 55 km, which coincides with crustal thicknesses from seismic refraction data for southwestern North Dakota. For southwestern North Dakota, uplift of the badlands appears to be a result of partial melting. If the high heat-flow region extends northward into northwestern North Dakota, the generation of petroleum was controlled by the zone of high heat flow. This interpretation implies that source rocks are depleted in their mobile petroleum components within the region of high heat flow. End_of_Article - Last_Page 781------------

Heat flow in the Basin and Range province and thermal effects of tectonic extension

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

Thrust faulting and crust--upper mantle structure in East Australia

Summary. The mid-crustal earthquake of 1973 March 9 (mb= 5.5, h≤ 20 km) located 60 km south-west of Sydney, Australia, provides unambiguous evidence of contemporary thrust faulting in South-eastern Australia — a region of high heat flow and Cenozoic basaltic volcanism. Aftershock locations suggest a steeply dipping fault in the depth range from 8 to 24 km with a lateral extent of about 8 km. The mechanism solution is consistent with a tectonic stress field that is dominated by east—west horizontal compression. A seismic moment of 5.7 ± 1023± 20 per cent dyne-cm was computed from surface-wave amplitudes. Minimum values of slip and stress drop, 2 cm and 1 bar respectively, were estimated from the moment and a fault size taken’ from aftershock locations. Refinement modelling by a controlled Monte Carlo technique was used to provide unbiased models directly from multimode group velocities. The dispersion of fundamental and higher mode surface waves recorded at the digital high-gain station at Charters Towers, Queensland, and the WWSSN station at Adelaide, South Australia, is satisfied by crust- and upper-mantle models which have neither pronounced S-wave low-velocity zones nor thick high-velocity lids within 140 km of the Earth's surface. These models have subcrustal shear velocities of 4.20–4.32 km/s which are 0.4–0.5 km/s slower than Canadian shield shear velocities (CANSD).

The effects of alteration on the natural remanent magnetization of three ophiolite complexes: Possible implications for the oceanic crust.

Magnetic properties are compared for the pillow basalts and sheeted dike complexes for the following three ophiolite suites: Macquarie Island (-27 million years), Troodos Massif, Cyprus (∼middle Cretaceous), and Smartville complex, California (∼Jurassic). The magnetic properties of the pillow basalts strongly depend on their degree of alteration. From the least altered, low-temperatureoxidized, upper pillow basalts of Macquarie Island and the pillow basalts of the Troodos Massif (zeolite facies metamorphism) to the most altered pillow basalts of the Smartville complex (greenschist facies metamorphism) the average intensity of the natural remanent magnetization (NRM) decreases from a maximum of 80×10-4 to 0.50×10-4 gauss; the Koenigsberger ratio (Q) decreases from 6 to 0.3; the contribution due to viscous remanent magnetization (VRM) increases, and the NRM stability with respect to alternating fields decreases. For the most altered pillow basalts of the Smartville complex the NRM is probably predominantly a VRM, acquired during the present polarity epoch. Thermomagnetic analyses for the least altered pillow basalts exhibit the characteristic behavior of low-temperature-oxidized cation deficient titanomagnetites, while the more altered pillow basalts have reversible thermomagnetic curves with magnetite-like Curie points, probably caused in situ by thermally induced unmixing of the low-temperature-oxidized titanomagnetites. The sheeted dike complexes have NRM intensities of about 10×10-4 gauss and Q values of about 0.8, and most of their magnetic properties do not vary much between the three ophiolite complexes. All the sheeted dike samples have reversible thermomagnetic curves with magnetite-like Curie points. If these ophiolite complexes represent material which was originally formed at spreading centers and if their alteration represents ocean floor metamorphism, then with progressive alteration the pillow basalts lose their magnetic recording qualities and become poorer sources for the marine magnetic anomalies, and the underlying sheeted dike complexes become relatively more important. The most probable site for the occurrence of the sea floor metamorphism is in the relatively high temperature, high heat flow regions near spreading centers.