Mohan L. Gupta

Heat flow and heat generation in the Archaean Dharwar cratons and implications for the Southern Indian Shield geotherm and lithospheric thickness

Abstract We report terrestrial heat flow density measurements from 11 new sites in the Archaean Eastern and Western Dharwar cratons of the Southern Indian Shield. The new values, together with already published data, have yielded a mean heat flow of 40 ± 3.4 (s.d.) mW m −2 ( n = 11) for the Eastern Dharwar craton, which is a predominantly gneiss-granite terrain. The mean heat flow in the Archaean supracrustal belts of Western Dharwar craton, which is a granite-greenstone terrain, is 31 ± 4.1 (s.d.) mW m −2 ( n = 4). The observed difference in surface heat flow between the two cratons is ascribed to differences in radiogenic heat production in upper crustal layers, inferred to be related to different crust-forming processes. It has been argued that low heat production appears to be an inherent characteristic of the crust similar to that of the Western Dharwar craton, which evolved during the early Archaean and suffered depletion of the lithophile elements of its lower crust prior to the late Archaean. Radiogenic heat production measurements were carried out on borehole core samples for one heat flow site (Sortur), and data have been compiled from the literature on three other sites and on a variety of rocks of the Southern Indian Shield. Heat flow and radiogenic heat production for Sortur, when combined with the published data, define a linear relation ( r = 0.97) with a slope of 11.5 km and an intercept of 23 mw m −2 . The thickness of the thermal lithosphere beneath the Dharwar cratons is over 200 km, thereby indicating, beneath the cratons, the existence of deep continental roots-a concept which is also supported by teleseismic P-wave travel time data. The geothermal data has clearly demonstrated that the Archaean Dharwar cratons of the Indian landmass are associated with low mean surface heat flow (37.7 mW m −2 ), reduced heat flow, Moho heat flux (12–17 mW m −2 ), and a cool and thick lithosphere—all characteristics of many Archaean-early Proterozoic terrains of other shields. It is inferred that the thermal structure of the South Indian lithosphere is identical with that of other cratons.

Surface heat flow and probable evolution of deccan volcanism

Abstract In the light of surface heat-flow observations, as well as other related geological and geophysical data, the origin of the Deccan basalts has been examined. The Indian lithosphere, after its detachment from Gondwanaland, apparently traversed a rising plume at La Reunion, which virtually bored through the lithosphere to emerge as the Deccan Trap volcanism on the surface. Subsequent volcanic and plutonic activity appears to have continued not only up to the Oligocene, as is indicated by the alkaline magmatic activity observed near the junction of the three prominent features — the West Coast faults, the Narmada-Son-Tapti lineament, and the Cambay Graben — but also up to the Mio-Pliocene, as indicated by the heat flow and gravity data over the Cambay Graben. The dyke-swarms and sills, which are mostly post-trappean, evolved from the lithosphere after the Indian Plate moved away from the hot spot.

Surface heat flow and igneous intrusion in the Cambay Basin, India

Abstract Heat flow values from some additional locations in the Cenozoic Cambay Basin have been determined. Together with the previously published data, they show that the heat flow is moderate (55–67 mW/m′) in the southern part of the basin towards Broach and Ankleswar, and that there is a clear trend of high heat flow (75–93 mW/m 2 ; range of average values for six different, widely separated, locations) in a part of the basin located north of the Mahisagan river between Cambay and Mehsana along a stretch of about 140 km. Conductive steady state geotherms, calculated using observed high surface heat flow values and appropriate models show, beneath the Cambay-Mehsana area, a large degree of melting in the lower crust and upper mantle, which is not suggested by the existing geodata. Considering this aspect and taking into account the existence of a normal crust about 37 km thick below the Cambay-Tarapur and Ahmedabad-Mehsana blocks (as obtained from deep seismic soundings), it has been inferred that the heat flow anomaly is due to transient thermal perturbations introduced from tectonic activity in the form of magmatic intrusions. A careful analysis of heat flow, gravity and other related geodata point out and support the possibility of a Miocene/Pliocene basic intrusive body at a depth of around 10 km under the Cambay-Mehsana area. Further, the consistent trend of the thermal and gravity fields indicates thinning of the postulated intrusive body from Cambay towards Mehsana.

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

Abstract 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 Earths crust and the melting near the surface developed at the very early stages of the Earths 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.

Geothermal studies in the Hyderabad granitic region and the crustal thermal structure of the Southern Indian Shield

Based on temperature measurements in three boreholes (one specially drilled for the purpose) and thermal conductivity determinations, heat flow density values were determined for three sites in the Archaean Hyderabad granitic batholith. A mean heat flow density value of 40± 1 (s.d.) mW m−2 has been obtained. The heat generation in its rocks (5.57 μW m−3) is significantly higher than in average crustal rocks. It has been proposed that the Hyderabad batholith has a layered structure with a thin ( ≈ 1 km) surface layer of high radioactivity. These results together with the already reported data have been used to estimate the conductive steady-state temperature within and at the base of the crust of the Southern Indian Shield, yielding values of the same order as found in the Western Australian Shield.

Heat flow in the Bastar Craton, central Indian Shield: implications for thermal characteristics of Proterozoic cratons

Measurements of surface heat flow density in various lithologies from the central parts of the Proterozoic Bastar Craton of the central Indian Shield have yielded heat flow values ranging from 51 to 64 mW m−2, with a mean value of 56 mW m−2 (standard deviation = ±6.1 mW m−2). These values are considerably higher than in the Archaean and Archaean to Early Proterozoic cratons of many Precambrian shields, but are compatible with the heat flow data from the Proterozoic Gawler Craton, central Australian Shield. It is inferred that (1) the contrast in surface heat flow between the Archaean and Proterozoic cratons is a consequence of the difference in the radiogenic heat production in their upper (10–20 km thick) crustal layers, and (2) the Proterozoic cratons, as these could also have enclaves of basic Archaean rocks, show a wide range of surface heat flow values controlled mainly by the varying amounts of upper crustal radiogenic heat source.

Heat flow studies in the Godavari Valley (India)

Abstract Heat flow at six places in the Godavari Gondwana Basin of India are presented in this paper. These are based on measurements in boreholes of depths of 190–310 m. Heat flow values at Bellampalli (1.06), in the centre of the valley and at Mailaram (1.10), which is in Archean gneisses and schists bordering the Gondwana sediments, are normal. Heat flow increases from Chelpur to Pasra from 1.25 to 2.01, at the southwestern edge of the central portion of the valley. Heat flow at Chintalapudi and Aswaraopet in the southern part of the valley is high (2.22 and 2.49). Heat flow at Sattupalli situated 30 km northwest of these places is 1.52. The high heat flux at Chintalapudi and Aswaraopet may be due to rising hot water from the deep interior along faults in the Precambrian basement.

Heat flow at Damua and Mohapani, Satpura Gondwana basin, India

Abstract Terrestrial heat flow has been evaluated from measurements in two bore holes upto 475 and 380 meters depth, at Damua and Mohapani, in Satpura Gondwana basin in the central part of the Peninsular shield of India. The values obtained are1.46 ± 0.2and1.18 ± 0.13 μcal/cm2 sec respectively. These values are higher than those reported earlier for the northwestern part of the Godavari valley which is a sedimentary trough filled with Gondwana sediments. The values are typical of continental platform regions and indicate normal conditions underlying the basin.

Is the Indian Shield hotter than other Gondwana shields

Abstract Geothermal data on various Precambrian terrains from the African, Australian, Indian and South American (Brazil only) Gondwana landmasses have been compiled, synthesised and statistically analysed. The results do not support the prevailing notion that the Indian Shield is hotter than other shields. The study clearly shows that the mean surface heat flow values from the various Precambrian cratons and mobile belts of the Indian landmass for which the data have become available are either equal to, or even lower in some cases, than that in similar terrains from other Gondwana continents. Further, on the basis of available data, it is found that the Moho and the reduced heat flow values and Moho temperatures in the South Indian, South African, Western Australian and Brazilian shields fall within a narrow range, thus indicating, within the error limits of the estimation, the similarity of these shields in terms of these characteristics. In conclusion it is shown that the Indian landmass is not hotter than the other Gondwana landmasses, including even the presently immobile African continent, and that the “super-mobility” of the Indian landmass does not appear to be associated with its thermal characteristics. The cause of the latter lies elsewhere.

Terrestrial heat flow and tectonics of the Cambay Basin, Gujarat State (India)☆

Abstract Heat flow values are presented for four sites in the Cenozoic Cambay Basin, Gujarat State, India, based on temperature measurements to a depth of 1,200 m in seven water filled wells, drilled for oil exploration. Values obtained from oilfields of Kathana, (22° 17N 72° 48E) Nawagam (22°50N 72° 30E) and Kalol (23° 16N 72° 30E), which are situated in different tectonic blocks along the northern part of the basin are varying and high (1.8 – 2.2 μcal / cm 2 ·sec). These heat flow measurements support the previously determined high heat flow (2.3) at Cambay gas field (22° 23N 72° 35E). Relatively low value of heat flux (1.62) has been obtained in the southern pa part of the basin at Anklesvar oilfield (21°35N 27°55E). The high heat flow values in the region extending from Cambay to Kalol can be explained as due to igneous intrusion in the crust underneath the basin during Pliocene-Miocene times. Also discussed are other observations based on various geophysical studies that have been carried out in the basin. An attempt is also made in this paper to correlate the heat flow values to a few tectonic features of the basin.