Regions Of Low Heat Flow Research Articles

Kimberlites and lamproites: Criteria for similarity and differences

Based on original data on the East European and Siberian platforms and materials on the best studied foreign objects, a comparative analysis of kimberlites and lamproites was conducted and the criteria of their differences were formulated. Among most significant differences are the following: (1) the high-Mg potassic rocks (kimberlites and lamproites) show major-component variations, which are significantly wider in lamproites as compared to kimberlites. Kimberlites differ from lamproites not only in the content of SiO2, but also in alkalis, volatiles, and some trace elements. Kimberlites are characterized by CO2-dominated regime, whereas formation of lamproites was assisted by essentially H2O fluid; (2) Kimberlites are localized within ancient cratons, while within-plate lamproites are restricted to adjacent Proterozoic belts. Kimberlites are produced in the low-heat flow regions, whereas lamproites occur in the high-heat flow regions; (3) Kimberlites and lamproites were formed in different time; in particular, most productive kimberlitic magmatism was observed in the EEP and SP in the Devonian; (4) Kimberlite and lamproite bodies have different morphology: lamproites compose small subvolcanic bodies with lava flows, while kimberlites form volcanic pipes with no lavas; (5) Kimberlites contain highly silica-undersaturated minerals, while ultrabasic lamproites—silica-undersaturated ones; priderite and wadeite, the characteristic accessory minerals of lamproites, are not observed in kimberlites; (6) The primary melts of kimberlites and lamproites were derived from different types of mantle. The moderate and low-Ti kimberlites were generated from BSE or EMI type mantle. Precisely these types of kimberlites host diamond deposits, including economic grade objects in EEP. The lamproite sources were localized only in the enriched mantle (EMI and EMII). At the same time, these rocks share some similarities, primarily, with respect to their genesis and classification. Diamonds are common accessory minerals of kimberlites (low-Ti and some other types), but are observed only in only lamproite variety—olivine lamproites.

The present-day thermal state of Mars

The present-day thermal state of the martian interior is a very important issue for understanding the internal evolution of the planet. Here, in order to obtain an improved upper limit for the heat flow at the north polar region, we use the lower limit of the effective elastic thickness of the lithosphere loaded by the north polar cap, crustal heat-producing elements (HPE) abundances based on martian geochemistry, and a temperature-dependent thermal conductivity for the upper mantle. We also perform similar calculations for the south polar region, although uncertainties in lithospheric flexure make the results less robust. Our results show that the present-day surface and sublithospheric heat flows cannot be higher than 19 and 12 mW m −2, respectively, in the north polar region, and similar values might be representative of the south polar region (although with a somewhat higher surface heat flow due to the radioactive contribution from a thicker crust). These values, if representative of martian averages, do not necessarily imply sub-chondritic HPE bulk abundances for Mars (as previously suggested), since (1) chondritic composition models produce a present-day total heat power equivalent to an average surface heat flow of 14–22 mW m −2 and (2) some convective models obtain similar heat flows for the present time. Regions of low heat flow may even have existed during the last billions of years, in accordance with several surface heat flow estimates of ∼20 mW m −2 or less for terrains loaded during Hesperian or Amazonian times. On the other hand, there are some evidences suggesting the current existence of regions of enhanced heat flow, and therefore average heat flows could be higher than those obtained for the north (and maybe the south) polar region.

Tectonic Framework and Deep Structure of South China and Their Constraint to Oil‐Gas Field Distribution

Abstract:South China could be divided into one stable craton, the Yangtze Craton (YzC), and several orogenic belts in the surrounding region, that is the Triassic Qinling‐Dabie Orogenic Belt (QDOB) in the north, the Songpan‐Garzê Orogenic Belt (SGOB) in the northwest, the Mesozoic‐Cenozoic Three‐river Orogenic Belt (TOB) in the west, the Youjiang Orogenic Belt (YOB) in the southwest, the Middle Paleozoic Huanan Orogenic Belt (HOB) in the southeast, and the Mesozoic‐Cenozoic Maritime Orogenic Belt (MOB) along the coast. Seismic tomographic images reveal that the Moho depth is deeper than 40 km and the lithosphere is about 210 km thick beneath the YzC. The SGOB is characterized by thick crust (>40 km) and thin lithosphere (<150 km). The HOB, YOB and MOB have a thin crust (<40 km) and thin lithosphere (<150 km). Terrestrial heat flow survey revealed a distribution pattern with a low heat flow region in the eastern YzC and western HOB and two high heat flow regions in the TOB and MOB respectively. Such a “high‐low‐high” heat flow distribution pattern could have resulted from Cenozoic asthenosphere upwelling. All oil‐gas fields are concentrated in the central part of the YzC. Remnant oil pools have been discovered along the southern margin of the YzC and its adjacent orogenic belts. From a viewpoint of geological and geophysical structure, regions in South China with thick lithosphere and low heat flow value, as well as weak deformation, might be the ideal region for further petroleum exploration.

Heat flow, thermal regime, and elastic thickness of the lithosphere in the Trans-Hudson Orogen

Heat flow studies on the exposed part of the Trans-Hudson Orogen (THO) in northern Manitoba and Saskatchewan allow constraints on crustal composition and lithosphere structure. The average of all heat flow values in the THO is the same as in other geological provinces of the Canadian Shield. However, where juvenile crust is exposed, heat flow is on average lower than in the Superior and Grenville provinces (37 vs. 41 mW m–2). Heat flow increases towards the surrounding Archean provinces, Rae–Hearne to the west, Sask to the south, and Superior to the east. There are strong differences in heat flow within and between the belts of the THO. The poor correlation between heat flow and heat production in the rocks exposed at the surface implies that these differences involve a large fraction of the crustal column. One new heat flow determination confirms the existence of a ``cold spot'' around the town of Lynn Lake in the northern part of the THO. Heat flow data in the Kisseynew and Glennie domains remain sparse, but they indicate that this low heat flow region extends as far south as the Flin Flon – Snow Lake Belt. The Lynn Lake Belt is underlain by poorly radiogenic rocks, possibly Kisseynew-type crust with oceanic basement. Northward increase in heat flow along the Thompson Belt is consistent with the view that the belt is thrust over Kisseynew-type basement only in the south. Heat flow increases southward in the Paleozoic basin because of higher heat production in basement rocks, probably from the Sask craton. We used the low heat flow regions to obtain an upper bound of 15 mW m–2 for the mantle heat flow in the THO. The effective elastic thickness of the lithosphere can be determined from the coherence between the topography and the Bouguer gravity. The effective elastic thickness is high (>40 km) thoughout the Canadian Shield and is highest in the central part of the shield, in particular in the Lynn Lake region. There seems to be a negative correlation between elastic thickness and heat flow in the central and western Canadian Shield. This indicates that, even in stable continents, the elastic thickness is largely controlled by the lithospheric temperatures that depend strongly on crustal heat generation and hence crustal structure.

Seismic observations of the 22/11/1995 Gulf of Aqaba earthquake sequence

We analyzed the aftershock sequence of the Gulf of Aqaba earthquake recorded by short period and broad band stations between November 1995 and December 1996. The seismicity pattern reveals significant activity often confined to known surface traces of active faults or within local grabens. Most of the aftershock activity is concentrated north of the main shock in the Aragonese and Eilat basins, while the activity is diffused south of the main shock in the Dakar and Tiran basins. Most of the moderate to strong aftershocks of the whole aftershock sequence occurred in the Aragonese basin, mainly in the first 100 days following the main shock. The Eilat basin is characterized by a larger number of aftershocks composed mainly of small-magnitude implying a slower decay rate. Assuming a detection level of M L=3.2 we obtain b-values of 1.01, 1.15, and 0.89 for the whole Gulf of Aqaba, and Eilat and Aragonese basins, respectively, in good agreement with recent studies of the Dead Sea fault. Following Omori's law, we obtained a decay parameter of 0.94, 0.88, and 0.96 for the aftershock sequences in the whole Gulf of Aqaba, and the Eilat and Aragonese basins, respectively, correlative with regions of low heat flow. For 1630 events in the local magnitude range of 0.6≤ M L≤6.2 we found seismic moment estimates, M 0, of 5×10 18≤ M 0≤7.7×10 26 dyn cm, and Brune stress drop estimates, Δ σ, between 1 and 200 bars, with a characteristic value of 90 bars in the Eilat and Aragonese basins for M 0>5×10 21 dyn cm. These characteristics are comparable to those for events occurring on the main Dead Sea–Jordan fault. Most of the moment release in the basins occurred shortly after the occurrence of the main shock. The total moment release in the Eilat and Aragonese basins is only a fraction of the moment release of the main shock suggesting an almost complete stress release.

Influence of the earthquake cycle and lithospheric rheology on the dynamics of the Eastern California Shear Zone

The Eastern California Shear Zone is bounded by the high heat flow region of the Basin and Range province and the low heat flow region of the Sierra Nevada block. This difference in thermal state influences the rheology of the lower crust/upper mantle, resulting in a viscosity contrast between the two regions. We analyze the effect of such a contrast on the kinematics and dynamics of the shear zone with numerical models. This viscosity contrast drives asymmetric strain accumulation in the upper crust, producing an asymmetric surface velocity field. An additional consequence of this strain pattern is the potential for asymmetric co‐seismic displacement during an earthquake.

Paleogeothermal influence on organic metamorphism in the neotectonics of the Japanese Islands

The paleogeothermal influence on the lowest grade metamorphic and diagenetic alteration of young sediments is discussed with special reference to the present conditions of the island arc system around Japan. Vitrinite reflectance ( R 0) is adopted to detect the thermal influence. The regional variation of R 0 in certain horizons, the stratigraphic variation in steeply dipping successions and the depth variation in boreholes are investigated in Cenozoic sediments of specific tectonic situations of fore-arc and back-arc depressions in the northeastern and the southwestern Japan arcs. The results have revealed a zonal distribution of a contrasting pattern of organic metamorphism that underwent either lower temperature and/or higher pressure conditions or higher temperature and/or lower pressure conditions. The distribution of organic metamorphism seems to correspond to the present heat flow distribution; i.e., the former (low temperature/high pressure) corresponds to the present low heat flow region and the latter (high temperature/low pressure) to the present high heat flow region.

Multiple ScS attenuation and travel times beneath western North America

Variations in attenuation and travel time of multiple ScS whole-mantle reverberations indicate very strong lateral gradients in upper-mantle properties beneath the tectonically active province of western North America. Whole-mantle attenuation estimates (Qscs) obtained by spectral stacking of phases sampling discrete subregions span the entire range previously determined using a similar procedure for a global set of continental and oceanic Qscs measurements. The relationship between travel time and attenuation anomalies, which has been used to argue for predominantly thermal control on the variations in oceanic environments, is poorly defined within the continental regime. However, a correlation between surface heat flow and Qscs variations in the United States indicates that thermal variations do control attenuation heterogeneity. For paths beneath Mexico, stacking 13 pairs of ScS, phases from the 24 October 1980 Huajuapan, Mexico and the 19 June 1982 El Salvador events yields Qscs - 142 _+ 10 in the frequency band 7.8 to 46.9 mHz. Time-domain measurements for the same phases yield a similar estimate of Qs~s = 148 __. 9. The average SCSn+l -- ScS n JB (Jeffreys-Bullen) travel-time residual (htscs) is +0.9 sec (slower than JB). All of the differential travel-time estimates are corrected for dispersion effects in the passband of the observations, and are appropriate for a reference period of about 30 sec. Multiple ScS arrivals from the 29 April 1965 Puget Sound earthquake recorded by WWSSN, CSN, LRSM and Pasadena stations provide the first determination of whole-mantle attenuation beneath the westernmost United States. Pasadena recordings from three instruments provide ScS, paths traversing the Cascade and Sierra Nevada ranges. Stacking 9 ScS, pairs yields Qscs = 208 __. 39 (7.8 to 54.7 mHz) and the same phases give htscs = +2.5 sec. Six pairs of ScS, reverberations under the Pacific Northwest give Qs¢s = 344 - 88 (15.6 to 54.7 mHz) and Atscs = +5.5 sec. A combined estimate for the relatively low heat flow region of the Pacific borderlands gives Qscs = 257 __. 20 (7.8 to 46.9 mHz), comparable to the high Qscs estimates previously determined for South America. On the other hand, 12 SCSn pairs sampling the northern Basin and Range, which has high heat flow, yield a very low Qs~s = 95 -_. 4 (7.8 to 31.3 mHz) with htscs = +5.1 sec. The latter Qscs value is even lower than estimates for young oceanic regions, suggesting extensive partial melting in the low-velocity zone under the northern Basin and Range.

A detailed study of the distribution of heat flow and radioactivity in New Hampshire (U.S.A.)

We present a set of 39 new determinations of heat flow and radiogenic heat production for several different geological environments in the State of New Hampshire (U.S.A.). With the extensive data set now available for the state, the linear relation of heat flow and heat production appears as a very useful generalization for the study of the heat flow field of a geological province. Our measurements indicate that the vertical distribution of radiogenic heat production is similar in plutonic and metasedimentary rocks. Our data are compatible with the linear relationship established earlier by F. Birch and his co-workers in 1968. Young granites are markedly enriched in radioactive elements and those which do not outcrop are revealed by anomalies in the general relation of heat flow versus radioactivity. Heat flow is high for plutons by low elsewhere. The mean heat flow through metasedimentary formations is 1.15 μcal/cm 2 s (48 mW/m 2), a value near the mean heat flow for old cratons. The lowest heat flow measured is 0.76 μcal/cm 2 s (32 mW/m 2) for a unit poor in radioactivity. The heat flow field grades smoothly into the low heat flow regions of the Canadian Shield. The New Hampshire region is in thermal equilibrium: its heat flow is in secular equilibrium with the heat generated by crustal sources and that supplied from the mantle. In this area, the thermal perturbations due to orogenic events decrease below the detection level in less than 200–275 Ma. The thickness of the layer which is thermally affected during continent-continent collision-type orogenies cannot be greater than about 190 km.

On the regional variation of heat flow, geotherms, and lithospheric thickness

Geotherm families in which surface heat flow is the principal independent variable have been constructed for continental and oceanic lithospheres. The continental model is characterized by geotherms in which surface heat flow is in equilibrium with heat flowing into the lithosphere at its base plus heat generated by radioactive decay within the lithosphere. The model accommodates the regional variation of the surface heat flow with proportional variations in the radioactivity of the surficial enriched zone and in the deeper heat flow. The proportionality is dictated by a new and general linear relationship between reduced heat flow and mean heat flow for a region ( q ∗ ≅ 0.6 q ̄ 0 ), which enables both q ∗ and the mean heat production of the enriched zone to be estimated from knowledge of the mean surface heat flow of a province. The oceanic model is characterized by the transient cooling of a semi-infinite medium with an initial temperature gradient and some near-surface radiogenic heat production. The model yields a heat flow in satisfactory agreement with observations in the oldest ocean basins. The depth at which both the oceanic and continental geotherms reach ~0.85 of the melting temperature is shown to be a consistent estimator of the depth to the top of the low-velocity channel, or the thickness of the high-velocity lid overlying the channel. We identify the lid as synonymous with the lithosphere, and produce a global map of lithospheric thickness based on the regional variation of surface heat flow. The lithosphere is less than 100 km thick over most of the globe, but thickens appreciably and becomes more viscous beneath the Precambrian shields and platforms, regions of low heat flow. These characteristics of shields are consistent with recently reported models of the driving mechanisms of the plate system, which require greater retarding forces beneath plates with large continental areas.

Effect of a Region of Low Viscosity on Thermal Convection in the Mantle

GEOPHYSICAL and geological studies have revealed a world-wide pattern of tectonic features and have provided increasing evidence of crustal mobility. These have recently been related through the remarkable work of Vine and Matthews and others1,2 on sea floor spreading. There is some indication3 that the rifts which mark the bifurcation line of sea floor spreading are also regions of high heat flow. Regions of low heat flow appear to parallel the rifts at distances of several thousand kilometres.

Terrestrial heat flow in Lake Malawi, Africa

Twenty measurements of heat flow through the floor of deep Lake Malawi (Central Africa) gave values ranging from 0.15 to 2.88 μcal/cm² sec. A systematic areal distribution of the values indicated regions of low heat flow at the northern and southern ends of the lake (average = 0.54 and 0.70 μcal/cm² sec, respectively), separated by a central region of high heat flow (average = 2.30 μcal/cm² sec.). The variability implies local heat sources within a few tens of kilometers of the earth's surface and/or disturbances from the local environment.

Some geophysical implications from gravity and heat flow data

The negative correlation coefficient, −0.82, between Izsak's satellite geoid and Lee and MacDonald's heat flow distribution suggests a correlation between the geoidal undulations and the highs and lows of heat flow, in the sense that depressions on the geoid correlate with regions of high heat flow while rises on the geoid correlate with regions of low heat flow. This implies that the depressions on the geoid are related to hotter and lighter material in the interior of the earth and the rises on the geoid are related to colder and heavier material. Assuming that the density anomaly Δρ, which is responsible for the geoidal undulations, is related to a temperature perturbation ΔT by Δρ/ρ = −αΔT, where α is the coefficient of thermal expansion, and that ΔT is caused by an inhomogeneous distribution of radiogenic heat sources, the location of this distribution of heat sources is found within the outer 100 to 200 km of the mantle, and the corresponding temperature variation has a maximum amplitude of about 100°C. The corresponding fluctuation of radiogenic heat is about ±1 to ±2×10−14 cal/cm3 sec, which implies that the upper mantle contains much stronger heat sources than does peridotite or dunite. An eclogitic upper mantle seems, in this respect, more likely.

Terrestrial heat flow in Japan

Terrestrial heat flow, Q=K×ΔT/ΔZ cal/cm2 sec has been determined at 51 localities (39 on land and 12 in the sea) in and around the Japanese Islands. The average values of observed heat flow in land and sea are 1.53µ cal/cm2sec and 1.48µcal/cm2sec respectively. These value do not differ greatly from the world’s averages. The outstanding features of the heat flow distribution are as follows:a) High heat flow region (Q>2.0µcal/cm2sec) exists in the Inner Zone of the Honshu Arc. This region of high heat flow is more distinct in the northeastern Japan than in the southwestern Japan.b) The High heat flow region seems to extend, through the Fossa Magna area, down to the Izu-Mariana Arc.c) It is also probable that a similar high heat flow zone exists in the inner side of the Kurile Arc.d) These zones of high heat flow precisely coincide with the zones of the Cenozoic orogeny in the area concerned.e) Far off the coast of the northeastern Japan, the area at about 150° E may be a high heat flow region.f) Low heat flow (Q<1.0µcal/cm2sec) prevails in the Pacific coast side of the northeastern Japan and in the oceanic area directly east of it, including the area of the Japan Trench.g) The region bounded by the above mentioned high and low heat flow regions has heat flow which is more or less normal. Based on these measurements, a « steady state ” temperature distribution in the crust has been calculated for each of the above regions of high, low and intermediate heat flow, and it was found that there is a large temperature differences between the bottom of the crust of the high and low heat flow regions: the temperature at the Moho boundary in the high heat flow regions should be as high as some 800∼1000°C (d=27 km), whereas that under the low heat flow region should be only about 200°C (d=23 km). The high general temperature at the Moho under the high heat flow region seems to favor a production of magma in the upper mantle. Calculated Moho temperatures disfavor the hypothesis that the Moho boundary is due to phase transition.

Terrestrial Heat Flow in Japan

Results of terrestrial heat flow measurements so far made in Japan are briefly reviewed. As seen in the figure 1 in the text, it has been established that the heat flow distribution is quite distinct in and around Japan. On the whole, the high heat flow region coincides with the Cenozoic volcanic region in the area, i. e. the Japan Sea side of the Honshu arc and the low heat flow regions lie on the Pacific Ocean side. The pattern of heat flow distribution seems to have important bearings with many other geological and geophysical aspects of the Japanese islands. A tentative picture of the state under Japanese area is presented based on the mantle connection hypothesis.