A tentative 2D thermal model of central India across the Narmada-Son Lineament (NSL)

This work deals with 2D thermal modeling in order to delineate the crustal thermal structure of central India along two Deep Seismic Sounding (DSS) profiles, namely Khajuriakalan–Pulgaon and Ujjan–Mahan, traversing the Narmada-Son-Lineament (NSL) in an almost north–south direction. Knowledge of the crustal structure and P-wave velocity distribution up to the Moho, obtained from DSS studies, has been used for the development of the thermal model. Numerical results reveal that the Moho temperature in this region of central India varies between 500 and 580 °C. The estimated heat flow density value is found to vary between 46 and 49 mW/m². The Curie depth varies between 40 and 42 km and is in close agreement with the Curie depth (40±4 km) estimated from the analysis of MAGSAT data. Based on the present work and previous work, it is suggested that the major part of peninsular India consisting of the Wardha–Pranhita Godavari graben/basin, Bastar craton and the adjoining region of the Narmada Son Lineament between profiles I and III towards the north and northwest of the Bastar craton are characterized with a similar mantle heat flow density value equal to 23 mW/m². Variation in surface heat flow density values in these regions are caused by variation in the radioactive heat production and fluid circulation in the upper crustal layer.     

A tentative 2D thermal model of central India across the Narmada-Son Lineament (NSL)

This work deals with 2D thermal modeling in order to delineate the crustal thermal structure of central India along two Deep Seismic Sounding (DSS) profiles, namely Khajuriakalan–Pulgaon and Ujjan–Mahan, traversing the Narmada-Son-Lineament (NSL) in an almost north–south direction. Knowledge of the crustal structure and P-wave velocity distribution up to the Moho, obtained from DSS studies, has been used for the development of the thermal model. Numerical results reveal that the Moho temperature in this region of central India varies between 500 and 580 °C. The estimated heat flow density value is found to vary between 46 and 49 mW/m². The Curie depth varies between 40 and 42 km and is in close agreement with the Curie depth (40±4 km) estimated from the analysis of MAGSAT data. Based on the present work and previous work, it is suggested that the major part of peninsular India consisting of the Wardha–Pranhita Godavari graben/basin, Bastar craton and the adjoining region of the Narmada Son Lineament between profiles I and III towards the north and northwest of the Bastar craton are characterized with a similar mantle heat flow density value equal to ∼23 mW/m². Variation in surface heat flow density values in these regions are caused by variation in the radioactive heat production and fluid circulation in the upper crustal layer.     

Structure,tectonics and thermal state of the Lithosphere beneath intraplate seismic region of Latur,central India: An appraisal

Recent surge in intraplate seismicity has led to detailed geological and geophysical investigations, covering different continental segments of India including seismogenic region of Latur. A synthesis of such data sets to understand the prevailing tectonic and thermal state of the Lithosphere beneath Latur region, that witnessed a large scale human loss due to 1993 seismic activity, has revealed shallow surfacing of denser deeper crustal segments which may have resulted due to ongoing active subsurface tectonic activity like uplift and erosion since geological past. Below this region, Moho temperature exceeds 500°C, heat flow input from the mantle is quite high (29–35 mW/m²) and the asthenosphere is shallow (∼100±10 km). It is suggested that stress generated by ongoing upliftment and related subcrustal thermal anomaly is concentrating in this denser and stronger mafic crust within which earthquakes tend to nucleate. In all likelihood, the seismic activity witnessed in the region may stem from the deep crustal/lithospheric dynamics rather than the role of fluids at the hypocentral depth.     

Heat flow and lithospheric thermal regime in the Northeast German Basin

New values of surface heat flow are reported for 13 deep borehole locations in the Northeast German Basin (NEGB) ranging from 68 to 91 mW m^− 2 with a mean of 77 ± 3 mW m^− 2. The values are derived from continuous temperature logs, measured thermal conductivity, and log-derived radiogenic heat production. The heat-flow values are supposed free of effects from surface palaeoclimatic temperature variations, from regional as well as local fluid flow and from thermal refraction in the vicinity of salt structures and thus represent unperturbed crustal heat flow. Two-D numerical lithospheric thermal models are developed for a 500 km section along the DEKORP-BASIN 9601 deep seismic line across the basin with a north-eastward extension across the Tornquist Zone. A detailed conceptual model of crustal structure and composition, thermal conductivity, and heat production distribution is developed. Different boundary conditions for the thickness of thermal lithosphere were used to fit surface heat flow. The best fit is achieved with a thickness of thermal lithosphere of about 75 km beneath the NEGB. This estimate is corroborated by seismological studies and somewhat less than typical for stabilized Phanerozoic lithosphere. Modelled Moho temperatures in the basin are about 800 °C; heat flow from the mantle is about 35 to 40 mW m^− 2. In the southernmost part of the section, beneath the Harz Mountains, higher Moho temperatures up to 900 to 1000 °C are shown. While the relatively high level of surface heat flow in the NEGB obviously is of longer wave length and related to lithosphere thickness, changes in crustal structure and composition are responsible for short-wave-length anomalies.     

2-D Crustal thermal structure along Thuadara-Sindad DSS profile across Narmada-Son lineament,central India

Central India is traversed by a WSW-ENE trending Narmada-Son lineament (NSL) which is characterized by the presence of numerous hot springs, feeder dykes for Deccan Traps and seismicity all along its length. It is divided in two parts by the Barwani-Sukta Fault (BSF). To the west of this fault a graben exists, whereas to the east the basement is uplifted between Narmada North Fault (NNF) and Narmada South Fault (NSF). The present work deals with the 2-D thermal modeling to delineate the crustal thermal structure of the western part of NSL region along the Thuadara-Sindad Deep Seismic Sounding (DSS) profile which runs almost in the N-S direction across the NSL. Numerical results of the model reveal that the conductive surface heat flow value in the region under consideration varies between 45 and 47mW/m². Out of which 23mW/m² is the contribution from the mantle heat flow and the remaining from within the crust. The Curie depth is found to vary between 46 and 47 km and is in close agreement with the earlier reported Curie depth estimated from the analysis of MAGSAT data. The Moho temperature varies between 470 and 500°C. This study suggests that this western part of central Indian region is characterized by low mantle heat flow which in turn makes the lower crust brittle and amenable to the occurrence of deep focused earthquakes such as Satpura (1938) earthquake.     

Temperature-depth profiles in Czechoslovakia and some adjacent areas derived from heat-flow measurements, deep seismic sounding and other geophysical data

The results of seismic measurements along three deep seismic sounding (DSS) profiles on the territory of Czechoslovakia and in adjacent countries have provided sufficient material about the crustal structure and the depth of the Moho discontinuity. These data, together with gravity and aeromagnetic data and the determinations of heat-flow values, were used to select several locations where the temperature—depth profiles were calculated. The Moho temperature of about 500 C beneath the Bohemian Massif increases to 800–1000 C and even more beneath the inner Neogene depressions of the Carpathian system. The regional differences in mantle heat-flow contribution between both these provinces may reach 1 μcal. cm⁻² sec⁻¹; such a variation in energy inflow may then be the driving force for the geological evolution. The geophysical implications of different thermal structure of the crust are discussed. Because of high subsurface temperatures in the Hungarian basin, partial melting at a depth of about 30 km may not be excluded.     

Evidence for Continent-Continent Collision Zone in the South Indian Shield Region

With a view towards understanding the evolutionary history of the complex South Indian shield, several geological and geophysical studies have been carried out. Recent geophysical studies include magnetotelluric (MT), deep seismic sounding (DSS), gravity, magnetic and deep resistivity soundings (DRS). In the present study, MT results along 140 km Andiyur-Turaiyur east-west profile is presented. The data are subjected to Groom-Bailey decomposition and static shift correction before deriving a 2-D model. The 2-D modeling results have shown that the upper crust (up to about 15 km) towards western part of the profile have exhibited high resistive character of about 40, 000 ohm-m as compared to the eastern part (less than 5, 000 ohm-m). The mid-lower crust has shown a decrease in resistivity in western part of the profile, the order of resistivity being 2, 000 ohm-m. An anomalous steep conductive feature (less than 100 ohm-m) is observed near Sankari at mid-lower crustal depths (>20 km) towards middle part of the profile. This feature is spatially correlatable with the well-known Moyar-Bhavani Shear Zone (MBSZ). The features obtained in the present study are consistent with earlier MT studies in this region and correlatable with other geophysical studies. DSS studies near the study region gave an evidence for differing crustal structure on either side of MBSZ. Variation in geoelectric character along the profile both in the upper crust and mid-lower crust indicate a block structure in the SGT with shear zones acting as boundaries. The new evidence in the form of distinct geoelectric structure and also variation in seismic structure indicate a continent-continent collision zone in this region and plays an important role for the Gondwana reconstruction models of South Indian shield.     

Magma plumbing beneath Anak Krakatau volcano, Indonesia: evidence for multiple magma storage regions

Understanding magma plumbing is essential for predicting the behaviour of explosive volcanoes. We investigate magma plumbing at the highly active Anak Krakatau volcano (Indonesia), situated on the rim of the 1883 Krakatau caldera by employing a suite of thermobarometric models. These include clinopyroxene-melt thermobarometry, plagioclase-melt thermobarometry, clinopyroxene composition barometry and olivine-melt thermometry. Petrological studies have previously identified shallow magma storage in the region of 2–8 km beneath Krakatau, while existing seismic evidence points towards mid- to deep-crustal storage zone(s), at 9 and 22 km, respectively. Our results show that clinopyroxene in Anak Krakatau lavas crystallized at a depth of 7–12 km, while plagioclase records both shallow crustal (3–7 km) and sub-Moho (23–28 km) levels of crystallization. These magma storage regions coincide with well-constrained major lithological boundaries in the crust, implying that magma ascent and storage at Anak Krakatau is strongly controlled by crustal properties. A tandem seismic tomography survey independently identified a separate upper crustal (<7 km) and a lower to mid-crustal magma storage region (>7 km). Both petrological and seismic methods are sensitive in detecting magma bodies in the crust, but suffer from various limitations. Combined geophysical and petrological surveys, in turn, offer increased potential for a comprehensive characterization of magma plumbing at active volcanic complexes.     

Seismic evidence for thick and underplated late Archaean crust of eastern Dharwar craton

The deep crustal structure of eastern Dharwar craton has been investigated through τ-p extremal inversion of P-wave travel times from a network of seismographs recording quarry blasts. Travel times have been observed in the distance range 30–250 km in a laterally homogeneous lithospheric segment Main features of the inferred velocity-depth relationship include: (a) 29 km thick combined upper and middle crust velocity varying from 6 km/s to 7 km/s, with no observable velocity discontinuity in this depth range; (b) a lower crust (∼ 29–41 km) with velocity increasing from 7.0 to 7.3 km/s; (c) an average upper mantle velocity of 8.1 km/s; and (d) presence of a 12 km thick high velocity crustal layer (7.4 – 7.8 km/s) in the depth range 41–53 km, with a distinct velocity gradient marking a velocity increase of 0.4 km/s. The anomalous 53 km thick crust is viewed as a consequence of magmatic underplating at the base of the crust in the process of cratonization of the eastern Dharwar craton during late Archaean. The underplated material reflects here with the velocity of 7–3 to 7–8 km/s below the depth of 40 km. Our proposition of magmatic underplating is also supported by the presence of large scale I-granitoid, a product of partial melting of the upper mantle material.     

Magmatic underplating beneath the Rajmahal Traps: Gravity signature and derived 3-D configuration

The early Cretaceous thermal perturbation beneath the eastern continental margin of the Indian shield resulted in the eruption of the Rajmahal Traps. To understand the impact of the magmatic process that originated in the deep mantle on the lower crustal level of the eastern Indian shield and adjoining Bengal basin the conspicuous gravity anomalies observed over the region have been modelled integrating with available geophysical information. The 3-D gravity modelling has delineated 10–15 km thick high-density (ρ = 3.02 g/cm³) accreted igneous layer at the base of the crust beneath the Rajmahal Traps. Thickness of this layer varies from 16 km to the west of the Rajmahal towards north to about 12 km near Kharagpur towards south and about 18 km to the east of the Raniganj in the central part of the region. The greater thickness of the magmatic body beneath the central part of the region presents itself as the locus of the potential feeder channel for the Rajmahal Traps. It is suggested that the crustal accretion is the imprint of the mantle thermal perturbation, over which the eastern margin of the eastern Indian shield opened around 117 Ma ago. The nosing of the crustal accretion in the down south suggests the possible imprint of the subsequent magmatic intrusion along the plume path.     

Deep structure of the Earth's crust in the contact zone of the Palaeozoic and Precambrian Platforms in Poland (Tornquist-Teisseyre zone)

One of the major tectonic problems in Europe concerns the southwest margin of the East European Platform in the region of the so-called Polish-Danish trough. In general, this margin is assumed to be the Tornquist-Teisseyre (T-T) Line, running approximately from northwest to southeast in this part of Europe. Determination of deep crustal structure of the contact zone between the Precambrian Platform and the Palaeozoic Platform was the main aim of the deep seismic sounding (DSS) programme in Poland in 1965–1982.Deep seismic soundings of the Earth's crust have been made in the T-T Line zone along nine profiles with a total length of about 2600 km. The results of deep seismic soundings have shown that the crust in the marginal zone of the East European Platform has highly anomalous properties. The width of this zone ranges from 50 km in northwest Poland to about 90 km in southeast Poland. The crustal thickness of the Palaeozoic Platform in Poland is 30–35 km, and of the Precambrian Platform 42–47 km, while in the T-T tectonic zone it varies from 50 to 55 km. Above the Moho boundary, in the T-T zone, at a depth of 40–45 km, there is a seismic discontinuity with P-wave velocities of 7.5–7.7 km/s. Boundary velocities, mean velocities and stratification of the Earth's crust vary distinctly along the T-T zone. There are also observed high gravimetric and magnetic anomalies in the T-T zone. The T-T tectonic zone determined in this manner is a deep tectonic trough with rift properties.The deep fractures delineating the T-T tectonic zone are of fundamental importance for the localization of the plate edge of the Precambrian Platform of eastern Europe. In the light of DSS results, the northeastern margin of the T-T tectonic zone is a former plate boundary of the East European Platform.     

渤海盆地现今岩石圈热结构及新生代构造-热演化史

依据236口井共2 706组的静温数据以及25口井的系统测温数据,分析计算了渤海盆地地温梯度及大地热流;建立地壳分层结构模型,利用回剥法计算现今地幔热流、深部温度以及岩石圈厚度;在此基础上,利用地球动力学方法恢复本区热流演化史。结果表明：渤海盆地背景地温梯度为322 ℃/km,热流值为648 mW/m2;盆地现今热岩石圈厚度在61~69 km之间,地幔热流占地表热流的比例在60%左右,属于“热幔冷壳”型岩石圈热结构,盆地地壳底部或莫霍面温度变动在548~749 ℃之间;热流演化的特征与盆地的构造演化背景吻合,新生代以来盆地经历了3期岩石圈减薄并加热的过程,在东营组沉积末期热流达到最高(70~83 mW/m2）,这期间盆地内产出多期碱性玄武岩,表明盆地经历了波及地幔的裂谷过程,随后进入热沉降期,热流逐渐降低,盆地向坳陷型转变。     

Heat flow and crustal thermal structure in the Late Archaean Closepet Granite batholith,south India

The Late Archaean Closepet Granite batholith in south India is exposed at different crustal levels grading from greenschist facies in the north through amphibolite and granulite facies in the south along a ∼400 km long segment in the Dharwar craton. Two areas, Pavagada and Magadi, located in the Main Mass of the batholith, best represent the granitoid of the greenschist and amphibolite facies crustal levels respectively. Heat flow estimates of 38 mW m⁻² from Pavagada and 25 mW m⁻² from Magadi have been obtained through measurements in deep (430 and 445 m) and carefully sited boreholes. Measurements made in four boreholes of opportunity in Pavagada area yield a mean heat flow of 39 ± 4 (s.d.) mW m⁻², which is in good agreement with the estimate from deep borehole. The study, therefore, demonstrates a clear-cut heat flow variation concomitant with the crustal levels exposed in the two areas. The mean heat production estimates for the greenschist facies and amphibolite facies layers constituting the Main Mass of the batholith are 2.9 and 1.8 μW m⁻³, respectively. The enhanced heat flow in the Pavagada area is consistent with the occurrence of a radioelement-enriched 2-km-thick greenschist facies layer granitoid overlying the granitoid of the amphibolite facies layer which is twice as thick as represented in the Magadi area. The crustal heat production models indicate similar mantle heat flow estimates in the range 12–14 mW m⁻², consistent with the other parts of the greenstone-granite-gneiss terrain of the Dharwar craton.     

Palaeozoic Intraplate Crustal Anatexis in the Mount Painter Province, South Australia: Timing, Thermal Budgets and the Role of Crustal Heat Production

The effect of radiogenic heat production within the crust onthermal processes such as crustal anatexis is generally disregardedas bulk geochemical models suggest that crustal heat generationrates are too low to effect significant heating. However, theMount Painter Province in northern South Australia is characterizedby a total crustal contribution to surface heat flow of morethan twice the global average. The province is composed dominantlyof Proterozoic granites and granite gneisses with an area averageheat production of 16·1 µW/m³; individual lithologieshave heat production >60 µW/m³. These Proterozoic rocksare intruded by the British Empire Granite, a younger intrusivewhose origin has remained enigmatic. Isotope geochemistry suggestscrustal sources for the melt and it has a crystallization ageof 440–450 Ma, which places the setting >750 km inboardof the nearest active plate boundary zone at this time. Phaseequilibria calculations suggest that temperatures of at least720–750°C are required to produce the granite butthe intensity of crustal thickening during Palaeozoic deformation(12%) cannot account for these conditions. Here we describea model for the generation of the British Empire Granite inwhich the primary thermal perturbation for mid-crustal anatexiswas provided by the burial of the high heat-producing MountPainter basement rocks beneath the known thickness of Neoproterozoiccover sediments. The high heat-producing rocks at Mount Painterimply that the natural range and variability of crustal heatproduction is much greater than previously believed, with importantconsequences for our understanding of temperature-dependentcrustal processes including the exploitation of geothermal energyresources. KEY WORDS: geothermal energy; low-pressure anatexis; thermal conductivity; thermal regime     

Crustal studies of the Koyna region using explosion data from deep seismic soundings

This study is based on the seismic data collected as a result of explosions carried out during the 1976 and 1978 Deep Seismic Sounding (DSS) field operations in the Koyna region. These shots were exploded from twelve shot points by the National Geophysical Research Institute along the Guhagar-Chorochi and Kelsi-Loni profiles.Refraction studies of the records reveal a two-layered crust. The top layer consists 17 km of granite and the second layer 19 km of basalt, giving the average depth of the Moho as 36 km in the region. The velocities of the phases P_g, P* and P_n have been computed as 5.82 ± 0.01, 6.61 ± 0.05 and 8.23 ± 0.05 km/sec respectively and those of S_g, S* and S_n as 3.41 ± 0.00, 4.09 ± 0.07 and 4.60 ± 0.08 km/sec respectively. The shear wave velocity in the basement rock has been found to be lower in comparison with other regions of the peninsular India.In some cases reflections were recorded both from the Moho as well as from the intermediate layer. These reveal a crustal thickness of 39 km with 19 km of granitic and 20 km of basaltic layers.Coda signal durations from DSS explosions recorded by microearthquake seismographs indicate a lateral heterogeneity in the crust on either side of Karad in an east-west direction.     

One- and two-dimensional thermal modelling of orogenic crustal extension in the Tormes Gneissic Dome, NW Iberian Massif, Spain

 Situated in the inner zone of the Variscan Iberian Massif, the Tormes Gneissic Dome offers a good opportunity for thermal modelling of orogenic crustal extension, because the P–T–t loops are well constrained by an extensive set of thermobarometric, structural and geochronological data. As an example of feedback between forward and inverse methods, the aim of this study was to establish one- and two-dimensional thermal models that reproduce the contrasting petrological P–T paths of two structural units separated by an extensional tectonic contact in the metamorphic complex, and to explain the spatial and temporary development of the low-pressure metamorphism in the rocks located just above this contact. In one dimension, the syn-extension path of the lower unit resulting from modelling is characterized by an isothermal decompression phase, followed by near isobaric cooling, which is typical of exhumed rocks. The upper unit path records a syn-extension near isobaric heating, more important in rocks just above the tectonic contact. Condensed isograds of low-pressure/high-temperature metamorphism in the basal upper unit are thus interpreted as a consequence of advective crustal extension and conductive upward heat transfer. In two dimensions, the delaminated simple shear geometric model of crustal extension explains the observed temperature rise in excess of 500  °C in the basal upper unit and is consistent with the spatial distribution of M2 low-pressure/high-temperature isograds. This demonstrates the important role of extensional structures produced during the collapse of the thickened crust in the thermal evolution. The heating phase, well explained with intermediate dip angle for extensional fault in the upper crust (45°) and finite extension of 75 km, is followed by cooling, thus reflecting normal erosional process. Received: 1 September 1998 / Accepted: 29 June 1999     

The post-collisional stabilization of thickened crust in the Southern Urals

The Urals are characterized by a depression of the Moho to a depth of 57 km. This structure is interpreted as a relic orogenic root, which has been conserved because no significant post-collisional processes occurred. However, there is evidence that voluminous post-collisional magmatism affected the lower crust. In this paper, we use thermal finite element models to quantify the influence of the post-collisional magmatism on the stabilization of the root. We show that at least 70% of the heat producing elements migrated in granitic melts from the lower crust to the upper crust. As a result the crustal heat flow reduced and the lithosphere could stabilize at a thickness of 180 km. Furthermore, we propose that a granulite metamorphic event during the thermal relaxation of the collision zone prevented the 57 km thick crust from delamination. These results strongly indicate that post-collisional processes were necessary for the stabilization of the Uralian crust and lithosphere.     

Crustal structure of the Indian subcontinent

Studies carried out by various investigators up to 1971 to delineate the Indian crustal structure using body wave travel times, surface wave dispersion and gravity methods are summarised and reviewed. The average crustal thickness is found to be 35–40 km in the Indian peninsular shield, 30–35 km in the Indo Gangetic plains and 60–80 km in the Himalayas and the Tibetan plateau region. The limitations of the various methods used and the errors in the estimation of crustal thickness by them are discussed. As no deep refraction work for crustal studies has been carried out so far in India, this topic is not covered in this study.     

Young orogenic gold mineralisation in active collisional mountains, Taiwan

Gold-bearing vein systems in the high mountains of Taiwan are part of the youngest tectonic-hydrothermal system on Earth. Tectonic collision initiated in the Pliocene has stacked Eocene–Miocene marine sedimentary rocks to form steep mountains nearly 4 km high. Thinner portions of the sedimentary pile (∼5 km) are currently producing hydrocarbons in a fold and thrust belt, and orogenic gold occurs in quartz veins in thicker parts of the pile (∼10 km) in the Slate Belt that underlies the mountains. Metamorphic fluids (2–5 wt.% NaCl equivalent) are rising from the active greenschist facies metamorphic zone and transporting gold released during rock recrystallisation. Metamorphic fluid flow at the Pingfengshan historic gold mine was focussed in well-defined (4 km³) fracture zones with networks of quartz veins, whereas large surrounding volumes of rock are largely unveined. Gold and arsenopyrite occur in several superimposed vein generations, with ankeritic alteration of host rocks superimposed on chlorite–calcite alteration zones as fluids cooled and became out of equilibrium with the host rocks. Mineralising fluids had δ¹⁸O near +10‰, δ¹³C was between −1‰ and −6‰ and these fluids were in isotopic equilibrium with host rocks at ∼350°C. Ankeritic veins were emplaced in extensional sites in kink fold axial surfaces, formed as the rock mass was transported laterally from compressional to extensional regimes in the orogen. Rapid exhumation (>2 mm/year) of the Slate Belt is causing a widespread shallow conductive thermal anomaly without igneous intrusions. Meteoric water is penetrating into the conductive thermal anomaly to contribute to crustal fluid flow and generate shallow boiling fluids (∼250°C) with fluid temperature greater than rock temperature. The meteoric-hydrothermal system impinges on, but causes only minor dilution of, the gold mineralisation system at depth.