Stable Continental Areas Research Articles

Seismic triggering in a stable continental area: The Lugo 1995–1997 seismic sequences (NW Spain)

The northwestern area of the Iberian Peninsula traditionally is considered a seismically stable area. However, since 1979 this region has experienced a significant increase in seismic activity. In 1995 and 1997, two earthquakes, with magnitude M w 4.2 and 5.1, respectively, caused significant damage in the region (intensity EMS: V). In this work, we analysed the focal mechanisms that had the most coherence with the local structures and tectonics. Modelling of the coseismic Coulomb static stress changes generated by the 1995 sequence was performed in order to find a possible genetic relationship with the 1997 sequence. Models using oblique (reverse strike-slip) movement on a fault with the same geometry and orientation as the Baralla fault, mapped at the surface, yield the best fit with the spatial distribution of the 1997 sequence. The existence of causal relationships between two close seismic sequences is important for the improvement of future seismic hazard assessments methodologies.

Active geodynamics of the central Mediterranean Sea: Tensional tectonic evidences in western Sicily from mantle‐derived helium

We report results on the measured high 3He/4He isotope ratio in western Sicily, interpreted together with the heat data. The study of this sector of the Europe‐Africa interaction is crucial to a better understanding of the tectonics and the geodynamical evolution of the central Mediterranean area. The estimated mantle‐derived helium fluxes in the investigated areas are up to 2–3 orders of magnitude greater than those of a stable continental area. The highest flux, found in the southernmost area near the Sicily Channel, where recent eruptions of the Ferdinandea Island occurred 20 miles out to sea off Sciacca, has been associated with a clear excess of heat flow. Our results indicate that there is an accumulation of magma below the continental crust of western Sicily that is possibly intruding and out‐gassing through roughly N‐S trending deep fault systems linked to the mantle, that have an extensional component. Although the identification of these faults is not sufficiently constrained by our data, they could possibly be linked to the pre‐existing faults that originated during the Mesozoic extensional‐transtensional tectonic phases.

Gas flux to the atmosphere from mud volcanoes in eastern Romania

AbstractGas flux measurements have for the first time been taken from vents and soil of eastern Romania mud volcanoes, the largest geological structures in Europe releasing methane into the atmosphere. In the quiescent phase, the methane emission from single vents is up to 28 t yr−1. Diffuse soil microseepage is of the order of 102−105 mg m−2 day−1. A total output of at least 1200 tonnes of CH4per year can be conservatively estimated over the area investigated alone (∼ 2.3 km2). Helium fluxes are up to five orders of magnitude higher than the average flux in a stable continental area, pointing to a close link between mud volcanoes and crustal degassing through faults crossing the deep hydrocarbon reservoirs. These data represent a key contribution towards refining global CH4‐emission estimates, which indicate mud volcanoes as a significant and unavoidable source of greenhouse gases for the atmosphere.

Thermal and compositional anomalies beneath the North American continent

The thermal and compositional structure of the upper mantle beneath the North American continent is investigated using a joint inversion of seismic velocities and density perturbations. The velocity data consist of a new regional shear wave velocity model of North America and the Caribbean region obtained by surface wave tomography. The density data are estimated using a relative density‐to‐shear velocity scaling factor computed for continents by combining regionally filtered seismic and gravity data. We express the mineralogical variations in the mantle in terms of the global volumic fraction of iron, the parameter which has the strongest influence on density and velocity. The inferred thermal and iron content anomalies are well constrained by the data and show an age dependence down to a depth of 230 ± 50 km. Below the North American craton, the mantle is colder than average and depleted in iron. Maximum values are found at 100 km with = −440 K and = −4%, relative to the average mantle. These chemical and thermal characteristics induce opposite buoyancy forces which could explain the longevity of cratonic lithosphere. In stable continental areas, the signal is of lower amplitudes ( = −280 K and = −2.5% at 100 km). Beneath the western Cordillera, a tectonically active region, we see no significant thermal or chemical anomaly.

Modelling of helium transport in groundwater along a section in the Pannonian basin

Underground water flow in sedimentary basins controls the distribution of dissolved salts and gases, and their concentrations may therefore be used as indicators of the flow direction. Recent measurements of the 4He concentration in deep waters of the Pannonian basin have great importance in this respect. This paper presents an example of the simultaneous computation of water flow and helium distribution along a section crossing the Great Hungarian Plain. The model consists of three permeable layers. The boundaries of the layers are prescribed using geologic sections constrained by ample borehole and seismic data. The results of the finite-difference calculations are fitted to the observed helium concentrations using a least-squares algorithm that varies the model parameters. The significance of the model is that it reconstructs the structure and flux of the groundwater flow and estimates the poorly known hydrogeological parameters of the flow regime such as hydraulic conductivities, conductivity anisotropy and dispersion coefficients. The statistical uncertainty of the estimated parameters is around half an order of magnitude. An estimate of the regional average of the incoming helium flux is also obtained. The total helium flux in the Great Hungarian Plain at the surface is within the range observed in old stable continental areas of the Earth.

Timing and exhumation of eclogite facies shear zones, Musgrave Block, central Australia

Timing constraints on shear zones can provide an insight into the kinematic and exhumation evolution of metamorphic belts. In the Musgrave Block, central Australia, granulite facies gneisses have been affected, to varying degrees, by mylonitic deformation, some of which attained eclogite facies. The Davenport Shear Zone is a dominant strike‐slip system that formed at eclogite facies conditions (T ≈650 °C and P≈12.0 kbar). Sm–Nd mineral isochrons obtained from equilibrated high‐pressure assemblages, as well as 40Ar–39Ar data, show that the eclogite and greenschist facies high‐strain overprints were coeval, at c. 550 Ma. Mylonitic processes do not appear to have reset the U–Pb system in zircon, but may have partially disturbed it. The thermal gradient in the Musgrave Block crust at c. 550 Ma was c. 16 °C km−1 and at c. 535 Ma was c. 18 °C km−1, based on P–T estimates of eclogite and greenschist facies shear zones, respectively. These estimates are similar to present‐day geothermal gradients in many stable continental shield areas, suggesting that the region did not undergo a significant transient perturbation of the geotherm. Therefore, in the Musgrave Block, cooling subsequent to eclogite facies metamorphism appears to have been controlled by exhumation, rather than by the removal of a heat source. Estimated exhumation rates in the range 0.2 to ≥1.5 mm year−1 are comparable with other orogenic belts, rather than cratonic areas elsewhere.

The 1993 Killari earthquake in central India: A new fault in Mesozoic basalt flows?

The Mw = 6.1 1993 Killari earthquake in central India was one of the deadliest earthquakes to occur in a stable continental area. It had a centroid 2.6 km deep and ruptured to the surface. A good fit of teleseismic waveforms is obtained with a simple 4.2‐s‐long pulse that releases a moment Mo = 1.8 × 1018 nm from a reverse fault rupture with strike, dip, and rake of 126°, 46°, and 100°, respectively. A ≈10‐km‐wide meizoseismal area is tightly defined by relative casualty count; in the center of this area, aftershock hypocenters (Baumbach et al., 1994) outline a southwest dipping rupture that extends 5.5–8 km along strike and from the surface 8–10 km along dip. These dimensions correspond with an average displacement of 1.5–0.8 m and a stress drop of 10–4.5 MPa, respectively. We mapped a 1‐km‐long zone of compressional scarps which are spatially correlated with the rupture outlined by hypocenter. In the central portion of this zone, the scarps are multiple and face in opposite directions. Deformation features indicate north–northeast directed shortening of at least 0.5 m. A leveling profile of an irrigation canal about 3 km northwest of the scarps displays a one‐wavelength warp, about 1 km long and 1.3 m in peak‐to‐peak amplitude, that may reflect deformation associated with the rupture. Two trenches across the scarps exposed faults that offset the soil‐rock interface as much as 50 cm. We found no convincing evidence suggesting that these faults existed prior to the earthquake. In the basalt, these faults reactivated exfoliation fractures in shear but were not associated with a zone of preexisting breccia and mineralization. The hypothesis that the 1993 rupture is a new fault in the Deccan Traps is consistent with lack of geomorphic evidence of prior faulting and with the lack of accumulated deformation or tilting in basalt layers exposed as close as few tens of meters from the scarp. No convincing clues about a seismogenic fault have yet been detected from either historic seismicity or geology in the 1993 source area, but a burst of seismicity 1 year before the mainshock and 2.5 years after the first impounding of a nearby reservoir was interpreted by some as a possible precursor to a larger earthquake before the 1993 mainshock. Precursory seismicity and artificial effects that cause significant mechanical changes in the crust may provide an indication of future potentially damaging earthquakes in stable continental areas.

Determination of Site-Dependent ScatteringQFromP-Wave Coda Analysis With an Energy-Flux Model

The seismic coda of deep events is mainly generated by near-receiver scattering in the lithosphere and therefore contains information about its small-scale structure. A theoretical model which describes coda excitation and decay rate in terms of scattering and anelastic attenuation of the medium is the energy-flux model. Here, a version of the model appropriate for a wavefront propagating through a scattering layer above a homogeneous half space is used to interpret short period data from several stations of the Global Digital Seismic Network which are available on CD-ROMs, Data from events with depths greater than 300 km and epicentral distances less than 60° allow P-coda observations for about 70 s after the first onset undisturbed by later arrivals. At nine stations in the circumpacific area estimates of scattering Q (Qs) are obtained for the frequency range 0.5 Hz to 4 Hz. Qs differs significantly between sites: strong scattering (Qs= 100–200) was found under stations MAJO, TATO, GUMO and API. Relatively weak scattering (Qs 500) occurs at NWAO and CTAO, which are located on stable continental areas. the variation of Qs with frequency qualitatively agrees with Qs obtained from single-scattering theory for von Karman type autocorrelation functions of the heterogeneities. At several sites correlation distances a and rms velocity fluctuations e could be determined. At other sites only the ratio ε/av was obtained, where v is the order of the von Karman autocorrelation function. Anelastic Q (Qi) could only be estimated at TATO, GUMO and SNZO. However, resolution is poor, because the coda decay is dominated by the leaking of scattered energy into the mantle and not by anelastic effects. There is an indication that scattering (except at TATO) is limited mainly to the uppermost 30 to 50 km.

Propagation and attenuation characteristics of the crustal phaseLg

TheLg wave consists of the superposition ofS waves supercritically reflected, and thus trapped, in the crust. This mode of propagation explains the strong amplitude of this phase and the large distance range in which it is observed. The numerical simulation leads to successful comparison between observed seismograms in stable continental areas and synthetics computed for simple standard crustal models. In regions with strong lateral variations, the influence of large-scale heterogeneities on theLg amplitude is not yet clearly established in terms of the geometrical characteristics of the crustal structure.

Earthquakes in Stable Continental Crust

Earthquakes can strike even in stable crust, well away from the familiar earthquake zones at the edges of tectonic plates, but their mere occurrence is both a source of concern in planning critical facilities such as nuclear power plants. The authors sought answers to two major questions: Just how much seismic activity does take place within the stable parts of continents And are there specific geologic features that make some areas of stable crust particularly susceptible to earthquakes They began by studying North America alone, but it soon became clear that the fairly short record of these rare events on a single continent would not provide enough data for reliable analysis. Hence, they decided to substitute space for time--to survey earthquake frequency and distribution in stable continental areas worldwide. This paper discusses their findings.

The 1984 southern Yellow Sea earthquake of Eastern China: Source properties and seismotectonic implications for a stable continental area

On 21 May 1984, a strong shock ( MS = 6.3, mb = 5.7) occurred in the southern Yellow Sea which shook the densely populated coastal area of Eastern China. About 1 min before the main shock, a foreshock of mb 5.4 occurred. Because of interference from the foreshock, no reliable P -wave first motions or focal mechanisms have been obtained for the main shock from local seismic data. In this paper, long-period teleseismic data have been used to investigate the source mechanism. Fault plane solutions determined from first motions as well as P and SH waveforms show a predominant strike-slip motion with a small dip-slip component on a steeply dipping plane. The tectonic environment of this event supports the observation that margins of former rifts tend to be sites of midplate seismic source regions. The Yellow Sea developed in a cratonic environment due to early Cenozoic rifting. The major graben faults strike NNE-SSW to NE-SW and are intersected by second-order faults trending WNW-ESE. Aftershock distribution suggests that the fault which generated the main shock probably strikes WNW-ESE. The mechanism determined in this paper strikes 120°, dips 88°, and slips 28°. This solution suggests that the earthquake ruptured along a secondary fault near the intersection of a more major NNE-SSW trending fault. The epicentral area is subject to to an ENE-WSW compression and a NNW-SSE extension. The seismic moment and focal depth are determined to be 1.09 × 1025 dyne-cm and 12 ± 4 km, respectively. Relative hypocentral locations indicate that the focal depth of the foreshock is similar to that of the main shock, and the hypocenter of the foreshock is located very close to the WNW-ESE striking nodal plane of the main shock mechanism and is about 20 km away from the main shock hypocenter. Source properties of the southern Yellow Sea earthquake have been compared with those of three major earthquakes of North China. The stress drop of the Yellow Sea earthquake is determined to be about 42 bars. This value is much lower than the stress drops computed for the 1966 Hsingtai and the 1975 Haicheng earthquakes of North China. The southern Yellow Sea is characterized by short recurrence intervals, while the Hsingtai and Haicheng areas have very long recurrence intervals. The short recurrence intervals and low stress drop may reflect a lower material strength at the source region in the Yellow Sea.

Thermal state under the Tohoko arc with consideration of crustal heat generation

The thermal structure beneath subduction zones plays a key role in aiding understanding of tectonic activities such as seismicity, volcanism, arc formation and genesis of back-arc basins around subduction zones. Many simulations have been made to estimate the thermal process in the mantle wedge which causes the surface activities. In the Tohoku Arc, many heat flow measurements have been made and the heat flow profile from the trench to the back-arc has been constructed. In this study, the thermal structure in the crust of Northeast Japan is investigated to account for the surface heat flow data. To estimate the contribution of the heat generation in the crust to the surface heat flow, the heat production rate was obtained on granitic and metamorphic rocks collected from the Hidaka metamorphic zone (Hokkaido, Japan) where the vertical crustal section of an island arc is understood to be exposed at the surface. Instrumentation neutron activation analysis and gamma-ray spectrometry were adopted to analyze the concentration of heat-producing elements. The obtained rate of heat generation and its decrease with increasing depth for the upper part of the crust were found to be similar to those estimated for stable continental areas but the heat generation rate was almost constant in the lower part of the crust. The vertical distribution of the heat production rate in the crust of the Northeastern Japan arc was estimated from the present results and previous estimates of heat production rates in various rocks, using the crustal structure determined by explosion seismology. The total heat generation in the crust thus estimated is about 30 mW/m 2 . With the surface heat flow data and estimated heat generation distribution in the crust, the thermal structure in the crust was simulated numerically. Modeling calculations were made on a cross section along the line from the Japan Trench to the Japan Sea, a direction parallel to the direction of convergence of the Pacific and Eurasian plates. The models were two-dimensional and included frictional heating at the upper surface of the slab. The calculated thermal structure in the crust shows that the temperature at the Moho is about 850°C and almost about 250°C lower than previous estimates. The magnitude of the interplate stress was derived by considering the rate of frictional heating at the interface between the plates. In order to explain the surface heat flow between the trench and aseismic front, the interplate stress should be less than 20 MPa at depth in the crust.

Crustal heat production and mantle heat flow in Central and Eastern Europe

The conversion of seismic velocity ( υ p ) into radiogenic heat production ( A) enables the distribution of crustal heat sources to be estimated. This technique was used to assess the mantle (Moho) heat flow for 49 seismic velocity-depth profiles located along five continental traverses in Central and Eastern Europe. The effect of pressure and temperature on υ p was taken into account, as was the age dependence of A. The uppermost part of the crust is dominated by microcracks that may have allowed a certain redistribution of radioactive elements by deep groundwater migration. Furthermore, owing to the highly variable value of the pressure derivative of v p in the uppermost crust, the use of a A−υ p relationship is problematic within this depth range. This radioactivity enriched zone, the thickness of which can be determined from the parameters of the heat flow-heat production relationship, was therefore treated separately. According to the model adopted for the upper crust, the Moho heat flow ranges from 14 to 26 mW m −2 in the stable continental areas such as the shields or ancient platforms. Younger terrains are characterized by elevated Moho heat flows of 20 to 40 mW m −2 and the Moho heat flow may eventually attain values over 50 mW m −2 in regions characterized by very high surface heat flow, such as the Pannonian Basin for example. The present data were compared with crustal models proposed by other authors. Moho heat flow estimates based on a simple exponential distribution of heat sources valid for the whole crust might be too high if applied to stable continental crust. Our data confirm a considerable variability in the mantle heat flow corresponding to the large-scale tectonothermal evolution, with a generally lower Moho heat flow typical of consolidated stable continental crust. The discrepancy in relation to the above estimates can be accounted for by a heat generation hump beneath an upper zone of exponential decline.

Evidence for aligned cracks in the Earth's crust

Three-component records of shear waves from small earthquakes recorded within the shear-wave window at the surface and shear waves recorded below the surface in three-component VSPs almost routinely display shear-wave splitting indicating some form of effective anisotropy. This splitting appears to be the result of liquid-filled cracks throughout at least the brittle, top 10-20 km of the crust, where the cracks are aligned with respect to the direction of the principal axes of stress. Liquid-filled cracks with small aspect ratios have very little effect on P-wave propagation, but may have major effects on the polarisations of shear-waves. Fluids are a very common constituent of the crustal rock mass (Fyfe, Price and Thompson 1978). Water is contained in the pore space of sedimentary rocks when they are first deposited, and every prograde metamorphic process releases chemically-bound water into microcracks of the otherwise intact rock mass. Deep wells (Kozlovsky 1984) find water-filled fractures deep in the crust, and even upper mantle xenoliths are pervaded by fluid-filled microcracks, where the fluid is largely CO2 (Andersen, O'Reilly and Griffin 1984). We shall usually refer to such cracks in the top half of the crust as 'waterfilled' but recognise that they may be oil-filled in hydrocarbon reservoirs. The major difficulty in accepting the proposition that such cracks are common is that seismic investigations, which are the most direct way we have of evaluating the properties of rocks in situ, have previously displayed little evidence for widespread cracking. The reason most seismic investigations in the past, including the detailed surveys by the hydrocarbon industry, have not found evidence for widespread cracking is because these studies have usually been confined to P-waves, and water-filled cracks have little effect on P-wave propagation. In contrast, the behaviour of shear-wave particle displacements (polarisations) is very sensitive to crack distributions whenever the cracks display preferred orientations. This paper reports recent observations of shear waves in seismic zones by my colleagues and myself at the British Geological Survey (Crampin et al. 1980; Booth et al. 1985; Crampin and Booth 1985; Crampin et al. 1985b) and in stable continental areas by the hydrocarbon industry, that display a number of phenomena which suggest that distributions of aligned water-filled cracks are widespread in (at least) the brittle, top 10 to 20 km of the crust. Monitoring the behaviour of such extensive dilatancy anisotropy (EDA) by the analysis of shearwave polarisations has been suggested as a way of monitoring the build-up of stress before earthquakes (Crampin et al. 1984).

Electrical conductivity models for the continental crust based on laboratory measurements on high-grade metamorphic rocks

Summary. Deep electrical conductivity soundings are increasingly being used as an additional source of information regarding the nature of the lower continental crust. However, a lack of relevant laboratory-based conductivity measurements makes interpretation of such soundings difficult. Laboratory measurements have been made on saturated and unsaturated samples of possible lower crustal rock types subjected to confining pressures up to 0.4 GPa, temperatures up to 300°C, and with variable pore fluid pressure up to the confining pressure. Extrapolation of these results suggests that the surprisingly high conductivities deduced for depths of approximately 20 km in certain stable continental areas may result from a combination of basic rock type and high pore fluid pressures, for whereas the conductivities measured in acid rock types can be explained in terms of conduction through the pore fluid alone, the conductivities measured in basic rock types imply enhanced conduction presumably through the matrix or along grain boundaries. The lower conductivities deduced from field experiments for the upper crust may be due to more acid rock types and/or lower pore fluid pressures, perhaps due to hydration reactions. In areas where the high conductivity layer is coincident with a low velocity layer an explanation in terms of changing pore fluid pressure, i.e. low to high, with increasing depth seems more likely.

Heat flow in the Indian Peninsula—its geological and geophysical implications

The distribution of surface heat flow in the various geological units of the Indian Peninsula is presented and discussed in the light of other related geodata. The heat flow data show a large variation (26 to 107 mW m −2), but indicate, in general, a thermal regime characteristic of stable continental areas. The Dharwar schist belts—greenstone belts, and greenstone-like geosynclinal piles—arc characterised by low heat flow. It appears very probable; (a) that favourable conditions for the generation of the Earth's crust and the melting near the surface developed at the very early stages of the Earth's evolution; and (b) that true greenstone belts, which represent the relics of the primitive basic ultrabasic crust, may have been more or less continuous once. It is our contention that their association with low surface heat flow is primarily due to their initial evolution and growth. The low heat flow values in green stone belts show a general increase in Archaean-Proterozoic granitic and gneisic terrains. The average heat flow in Archaean areas is significantly lower than in Proterozoic parts of the Indian Shield. In general, the average heat flow shows an inverse relation with the age of Precambrian orogeny. Further, the Proterozoic geosynclinal belts, in which sedimentation and mobility continued up to Upper Proterozoic, have been found to mark the highest heat flow areas of Precambrian India. Surprisingly, the Deccan Traps, the Cretaceous-Paleocene basic effusives, are associated with low heat flow values (40 mW m −2). The observed high heat flow in the northern part of a basin, in which a full sequence of Tertiary to Recent sediments overlie the Deccan Traps, has been ascribed to a shallow basic crustal Miocene-Pliocene intrusive body. The Gondwanas, which occur in intracratonic small, isolated, elongate trough-like, and semi-elliptical depressions, show a contrasting geothermal character with many zones of high surface heat flow, for which there is no clear explanation at present. Most of the Indian Peninsula is comprised of three cratons-the Dharwar, the Aravallis, and the Singhbhum. The Dharwar craton appears to be associated with the lowest surface heat flow, and the heat flow from the mantle and the lower crust is q r≈ 23 mW m −2 . This is followed by q r≈ 30 mW m −2 for the Aravalli craton and about 38 mW m −2 for the Singhbhum craton. Thus for the same surface radioactivity and heat flow, crustal temperatures are lowest in the Dharwar craton. For this craton, the temperatures at the base of the crust are similar to those in the western Australian shield, and in the Superior Province of the Canadian shield. We further suggest that U, Th and K in the top radioactive layer, probably do not follow the same concentration-depth pattern i.e., the characteristic depth, D, for these heat producing radioactive elements is most likely different and is the main cause of a wide scatter in the values of q r for various heat flow-heat production pairs within a heat flow province. We demonstrate that it is not worthwhile to study the correlation of surface heat flow to the gravity anomaly in the Indian Peninsula without removing the effects of the pronounced Indian Ocean gravity low from the gravity anomalies. Surface heat flow, q s , and crustal thickness. Zm, as obtained from DSS and earthquake data, show a positive (slightly exponential) correlation, which is compatible with crustal properties and dynamics. It is demonstrated that the correlation between q s and Zm in Precambrian shields can be positive or negative or even negligible, depending upon their past geological history.

Crustal structure, geophysical models and contemporary tectonism of the colorado plateau

A regional analysis of the crust and upper mantle of the Colorado Plateau is presented, using existing geophysical and geological data combined with new surface wave dispersion and groundwater geothermometry data; the tectonic implications of these models are also investigated. Surface wave and seismic refraction data indicate that the crust of the interior of the Colorado Plateau is 44 + or - 3 km thick, and its crustal structure is typical of stable continental areas. Pn velocities, however, appear to be lower (7.8 km/s) than would be expected in a stable region, while silica geothermometry indicates that the average heat flow for the plateau is 55 mW per sq m (1.3 HFU).

Geoid anomalies and the thickness of the lithospere

Over broad regions of the earth, such as stable continental interiors and deep ocean basins, isostatic equilibrium is a good approximation. The isostatic requirement relates the density distributions in the continental and oceanic lithospheres. These density distributions are the result of differences in both composition and temperature. In regions of isostatic equilibrium the dipole moment of the near‐surface density distribution is proportional to the local geoid anomaly. By using accepted crustal density distributions and either error function or linear temperature distributions the difference in geoid height between stable continental areas and deep ocean basins has been determined as a function of the continental lithospheric thickness. If the continental lithosphere were greater than 200 km thick, the geoid anomaly over the continents would be systematically negative in relation to that over the ocean basins. By using the GEM 9 satellite geoid the mean geoid anomalies over ocean basins and stable continental areas have been obtained. No systematic difference between continental and oceanic geoids is observed. We conclude from our geoid studies that the thickness of the continental lithosphere is near 180 km. This is in good agreement with various interpretations of the surface heat flow observations.

Electrical Conductivity Models of the Crust

Compositional crustal models (continental, subcontinental, oceanic) are used as a basis for a construction of the electrical conductivity distribution within the crust. In addition, the procedure incorporates other parameters, such as pressure, porosity, pore fluid salinity, and resistivity. Calculation is made in three steps. First, 'dry' models based on mineral conduction alone are constructed. Second, electrolytic conduction through the pore fluids is estimated from the Archie's Law. Both results are then combined to produce the final, 'wet' models. These appear to agree with generalized conductivity profiles for the stable continental areas, as well as with the published values for the oceanic crust. Magnetotelluric response curves calculated for the final models give, for each crustal type, a band of possible responses which agree well with theoretical and in situ results.

Heat-flow — heat-generation studies in Norway

Fifteen heat-flow determinations based on data from 34 drill holes throughout central and southern Norway are presented. Five combined heat-flow — heat-generation measurements from homogeneous Precambrian and Permian crystalline rocks from southern Norway confirm a linear relation between heat flow and heat generation of the form Q = Q 0 + bA, where Q is surface heat flow (1hfu = 10 −6 cal cm −2 sec −1), A is surface heat generation (1hgu = 10 −13 cal cm −3 sec −1), and b and Q 0 are constants. The slope of the line ( b = 8.4 km) is in good agreement with results obtained from other stable continental areas, but the intercept ( Q 0 = 0.48 hfu) is considerably lower, suggesting the presence of a zone of low heat flow in southern Norway. Nine heat-flow determinations are from the Paleozoic, Caledonian orogenic belt. These values range from 1.09 to 1.29 hfu with an average value of 1.18, are consistent with model data from other Paleozoic orogenic areas including the Appalachian system of North America, and do not appear to reflect the low heat flow observed in southern Norway.