3D thermal structure of the continental lithosphere beneath China and adjacent regions

Based on the Crust2.0 model and the topography data of Chinese continent and its adjacent regions, a three-dimensional finite element model is constructed in terms of the spherical coordinate system. In our numerical model, the average annual ground temperature from 195 meteorological stations and temperature of upper mantle derived from the seismic velocities are adopted as the top and bottom boundary conditions, respectively. The observed thermal conductivity and heat production from P wave velocity based on empirical formula are employed in our numerical model as well. The comparison between the calculated and observed surface heat flow proved that our results are reliable. The temperature beneath the Precambrian cratons is lower than that of other areas for 100–300 °C also. The typical temperature rang at the Moho is estimated to be 800–1000 °C beneath the Tibetan plateau and 500–700 °C beneath the Precambrian cratons (such as Indian plate, Sichuan basin, South China, North China and Tarim), respectively. The thermal state in the eastern part of Sino-Korean craton at the depth deeper than 60 km indicates that it was destructed. The thermal structure in center of Tibetan plateau (especially beneath Qiangtang area) supports the proposed flow of lower crustal or upper mantle material to the east. Generally, the distribution of volcanoes in Chinese continent is consistent with the high temperature areas in the crust or upper mantle. There are many obvious thermal transition zones across the orogenic belts. The thermal transition zone between eastern and western parts in the crust of Chinese continent is consistent with the north–south seismic zone.     

Thermorheological model for the European Central Alps: brittle–ductile transition and lithospheric strength

A two‐dimensional thermorheological model of the Central Alps along a north–south transect is presented. Thermophysical and rheological parameters of the various lithological units are chosen from seismic and gravity information. The inferred temperature distribution matches surface heat flow and results in Moho temperatures between 500 and 800 °C. Both European and Adriatic lithospheres have a ‘jelly‐sandwich’ structure, with a 15–20 km thick brittle upper crust overlying a ductile lower crust and a mantle lid whose uppermost part is brittle. The total strength of the lithosphere is of the order of 0.5–1.0 × 10¹³ N m⁻¹ if the upper mantle is dry, or slightly less if the upper mantle is wet. In both cases, the higher values correspond to the Adriatic indenter.     

Petrological-geophysical models of the internal structure of the lithospheric mantle of the Siberian Craton

Based on the simultaneous inversion of unique ultralong-range seismic profiles Craton, Kimberlite, Meteorite, and Rift, sourced by peaceful nuclear and chemical explosions, and petrological and geochemical data on the composition of xenoliths of garnet peridotite and fertile primitive mantle material, the first reconstruction was obtained for the thermal state and density of the lithospheric mantle of the Siberian craton at depths of 100–300 km accounting for the effects of phase transformation, anharmonicity, and anelasticity. The upper mantle beneath Siberia is characterized by significant variations in seismic velocities, relief of seismic boundaries, degree of layering, and distribution of temperature and density. The mapping of the present-day lateral and vertical variations in the thermal state of the mantle showed that temperatures in the central part of the craton at depths of 100–200 km are somewhat lower than those at the periphery and 300–400°C lower than the mean temperature of tectonically younger mantle surrounding the craton. The temperature profiles derived from the seismic models lie between the 32.5 and 35 mW/m² conductive geotherms, and the mantle heat flow was estimated as 11–17 mW/m². The depth of the base of the cratonic thermal lithosphere (thermal boundary layer) is close to the 1450 ± 100°C isotherm at 300 ± 30 km, which is consistent with published heat flow, thermobarometry, and seismic tomography data. It was shown that the density distribution in the Siberian cratonic mantle cannot be described by a single homogeneous composition, either depleted or enriched. In addition to thermal anomalies, the mantle density heterogeneities must be related to variations in chemical composition with depth. This implies significant fertilization at depths greater than 180–200 km and is compatible with the existence of chemical stratification in the lithospheric mantle of the craton. In the asthenosphere-lithosphere transition zone, the craton root material is not very different in chemical composition, thermal regime, and density from the underlying asthenosphere. It was shown that minor variations in the chemical composition of the cratonic mantle and position of chemical (petrological) boundaries and the lithosphere-asthenosphere boundary cannot be reliably determined from the interpretation of seismic velocity models only.     

Crustal density structure from 3D gravity modeling beneath Himalaya and Lhasa blocks,Tibet

The Himalaya and Lhasa blocks act as the main belt of convergence and collision between the Indian and Eurasian plates. Their crustal structures can be used to understand the dynamic process of continent–continent collision. Herein, we present a 3D crustal density model beneath these two tectonic blocks constrained by a review of all available active seismic and passive seismological results on the velocity structure of crust and lower lithosphere. From our final crustal density model, we infer that the present subduction-angle of the Indian plate is small, but presents some variations along the west–east extension of the orogenic belt: The dip angle of the Moho interface is about 8–9° in the eastern and western part of the orogenic belt, and about 16° in the central part. Integrating crustal P-wave velocity distribution from wide-angle seismic profiling, geothermal data and our crustal density model, we infer a crustal composition model, which is composed of an upper crust with granite–granodiorite and granite gneiss beneath the Lhasa block; biotite gneiss and phyllite beneath the Himalaya, a middle crust with granulite facies and possible pelitic gneisses, and a lower crust with gabbro–norite–troctolite and mafic granulite beneath the Lhasa block. Our density structure (<3.2 g/cm³) and composition (no fitting to eclogite) in the lower crust do not be favor to the speculation of ecologitized lower crust beneath Himalaya and the southern of Lhasa block.     

Geothermal structure of the eastern Pontides orogenic belt： Implications oceanic lithosphere Black Sea basin and the eastern for subduction polarity of Tethys

The numerical results of thermal modeling studies indicate that the lithosphere is cold and strong beneath the Black Sea basin.The thermal lithospheric thickness increases southward from the eastern Pontides orogenic belt（49.4 km） to Black Sea basin（152.2 km）.The Moho temperature increases from 367℃in the trench to 978℃in the arc region.The heat flow values for the Moho surface change between 16.4 mW m^-2 in the Black Sea basin and 56.9 mW m^-2 in the eastern Pontides orogenic belt. Along the southern Black Sea coast,the trench region has a relatively low geothermal potential with respect to the arc and back-arc region.The numerical studies support the existence of southward subduction beneath the Pontides during the late Mesozoic-Cenozoic.     

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.     

Estimates of heat flow and heat production and a thermal model of the São Francisco craton

An updated analysis of geothermal data from the highland area of eastern Brazil has been carried out and the characteristics of regional variations in geothermal gradients and heat flow examined. The database employed includes results of geothermal measurements at 45 localities. The results indicate that the Salvador craton and the adjacent metamorphic fold belts northeastern parts of the study area are characterized by geothermal gradients in the range of 6–17°C/km. The estimated heat flow values fall in the range of 28–53 mW/m², with low values in the cratonic area relative to the fold belts. On the other hand, the São Francisco craton and the intracratonic São Francisco sedimentary basin in the southwestern parts are characterized by relatively higher gradient values, in the range of 14–42°C/km, with the corresponding heat flow values falling in the range of 36–89 mW/m². Maps of regional variations indicate that high heat flow anomaly in the São Francisco craton is limited to areas of sedimentary cover, to the west of the Espinhaço mountain belt. Crustal thermal models have been developed to examine the implications of the observed intracratonic variations in heat flow. The thermal models take into consideration variation of thermal conductivity with temperature as well as change of radiogenic heat generation with depth. Vertical distributions of seismic velocities were used in obtaining estimates of radiogenic heat production in crustal layers. Crustal temperatures are calculated based on a procedure that makes simultaneous use of the Kirchoff and Generalized Integral Transforms, providing thereby analytical solutions in 2D and 3D geometry. The results point to temperature variations of up to 300°C at the Moho depth, between the northern Salvador and southern São Francisco cratons. There are indications that differences in rheological properties, related to thermal field, are responsible for the contrasting styles of deformation patterns in the adjacent metamorphic fold belts.     

Heat flow in the rifted continental margin of the South China Sea near Taiwan and its tectonic implications

Temperature measurements carried out on 9 hydrocarbon exploration boreholes together with Bottom Simulating Reflectors (BSRs) from reflection seismic images are used in this study to derive geothermal gradients and heat flows in the northern margin of the South China Sea near Taiwan. The method of Horner plot is applied to obtain true formation temperatures from measured borehole temperatures, which are disturbed by drilling processes. Sub-seafloor depths of BSRs are used to calculate sub-bottom temperatures using theoretical pressure/temperature phase boundary that marks the base of gas hydrate stability zone. Our results show that the geothermal gradients and heat flows in the study area range from 28 to 128 °C/km and 40 to 159 mW/m², respectively. There is a marked difference in geothermal gradients and heat flow beneath the shelf and slope regions. It is cooler beneath the shelf with an average geothermal gradient of 34.5 °C/km, and 62.7 mW/m² heat flow. The continental slope shows a higher average geothermal gradient of 56.4 °C/km, and 70.9 mW/m² heat flow. Lower heat flow on the shelf is most likely caused by thicker sediments that have accumulated there compared to the sediment thickness beneath the slope. In addition, the continental crust is highly extended beneath the continental slope, yielding higher heat flow in this region. A half graben exists beneath the continental slope with a north-dipping graben-bounding fault. A high heat-flow anomaly coincides at the location of this graben-bounding fault at the Jiulong Ridge, indicating vigorous vertical fluid convection which may take place along this fault.     

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.     

Meso—Cenozoic lithospheric thermal structure in the Bohai Bay Basin,eastern North China Craton

The Bohai Bay Basin is a region where part of the North China Craton has been thinned and destroyed. It has experienced two periods of crustal thinning that occurred during the Cretaceous and Paleogene, but investigations of its Mesozoic and Cenozoic lithospheric thermal structure are limited. Therefore, in this study,the distributions of mantle heat flow, crustal heat flow, and Moho temperatures during the Meso-Cenozoic are calculated based on analyses of the thermal history of the Bohai Bay Basin. The results indicate that the ratio of mantle heat flow to surface heat flow peaked during the late stages of the early Cretaceous and during the middle to late Paleogene. The corresponding mantle heat flow was more than 65% of the surface heat flow. Moho temperatures reached three peaks: 900-1100℃ in the late stages of the early Cretaceous;820-900℃ in the middle to late Paleogene; and(in the Linqing Depression, Cangxian Uplift, and Jizhong Depression) 770-810℃ during the early Neogene. These results reveal that the Bohai Bay Basin experienced significant geological change during the Cretaceous, including the transformation of lithospheric thermal structure from "cold mantle and hot crust" before the Cretaceous to "hot mantle and cold crust" after the Cretaceous. The results also indicate that the basin experienced two large-scale rifting events.Therefore, this work may provide the thermal parameters for further investigations of the geodynamic evolution of eastern China.     

Thermal structure of lithosphere in Central Asian and Pacific belts and their adjacent cratons,from data of geoscience transects

We investigate the 2D thermal structure of lithosphere in the Central Asian and Pacific tectonic belts and adjacent cratonic areas of Siberia and North China using a synthesis of geothermal data from six geoscience transects covered by seismic, resistivity, and gravity surveys. The patterns of rock density, radiogenic heat production derived from U and Th abundances, thermal conductivity, temperatures, and respective heat flows reveal a layered structure. The model with layers distinguished according to density and thermal parameters includes well pronounced dome-shaped features in the crust which correlate with upwarps of the asthenospheric top. The domes are marked by high heat flows of 60–90 mW/m² with a mantle component higher than the crustal one (30–60 mW/m² against 20–30 mW/m²) and temperatures as high as 800–1100 °C at the Moho. Many of these features correspond to known and potential petroleum basins.     

Geothermal model of the earth's crust in the pannonian basin

A geothermal model of the hyperthermal zone of the Pannonian basin is constructed. On the basis of results of seismic measurements along five deep seismic sounding profiles on the territory of the basin and the surrounding areas and also of measurements of heat flow, heat production by radioactive elements and thermal conductivity of rocks, the variation of temperature with depth and maps of Mono-temperatures and heat flux through this surface are calculated and constructed, respectively. It is shown by numerical-model calculations that the heat anomaly of the Pannonian basin indicated by a number of surface measurements is mainly of mantle origin. Inhomogeneities of the heat-flow increase with depth down to the upper mantle and the temperature on the Moho-surface below the hyperthermal zone has values on average 400–500°C more than those in the surrounding areas. Heat flux through the Moho under the Pannonian basin is also higher by about 40–50 mW m⁻². On the basis of the present calculations, it can be suggested that the upper mantle is probably partially molten beneath the annonian basin. As a most reasonable source mechanism of formation of this heat anomaly, the frictional heating arising in areas of induced secondary convection that probably has proceeded also beneath the basin from the Triassic to the Miocene is suggested here.     

Crustal velocity structure beneath the western Andes of Colombian using receiver-function inversion

Analysis of teleseismic records obtained in two broadband seismic stations of three components located on the Andean region of Colombia is presented in this work. The two stations are located at the Western Cordillera (WC), station BOL, and at the Central Cordillera (CC), station PBLA. The analysis of seismograms was performed by inversion of the receiver functions (RF) in order to obtain the crustal velocity structure beneath the receivers. The receiver function is a spectral ratio obtained from teleseismic earthquakes recorded by broadband seismic stations, which allows the calculation of the velocity structure beneath the receiver by removing source effects in the horizontal components of the seismic traces. Data stacking was performed in order to improve signal to noise ratio and then the data was inverted by using two optimization algorithms: a genetic algorithm (GA), and a simulated annealing algorithm (SA). The present work calculates the receiver functions using teleseismic earthquakes at epicentral distances (Δ) ranging between 30° and 90° and recorded at the two stations within the years 2007 and 2009.Delay times between P and PS waves converted at the Moho boundary were used to constrain the velocity structure. The receiver functions at the stations were generated from seismic events within a broad range of back azimuth. Data from gravity and magnetism were also used during the geophysical survey. The depth of the Moho boundary was found to be at 40 km in the WC beneath station BOL and at 43 km in the CC beneath station PBLA. The upper crust, with a thickness of 5 km, is characterized by a shear wave velocity of about 3.0 km s⁻¹; the shallower layers, at approximately 1.0 km, have shear wave velocities between 2.2 and 2.6 km s⁻¹, which corresponds to sediments overlying the upper crust. These observations support the hypothesis of a thickness of the crust at the root of the mountain range to be between 32 and 50 km. The calculated receiver functions were compared with artificial ones generated from the inversion of 48000 models of horizontal layers for each station using a GA and an SA that allowed a satisfactory coverage of all the sample space in order to avoid non-unique solutions. Beneath station BOL a moderate low-velocity zone (LVZ) was found, which was caused by accretionary processes of the ophiolite complex in the WC.     

Tectonics and thermal structure of western Satpura,India

Increased seismicity and occurrences of hot springs having surface temperature of 36–58 °C are observed in the central part of India (74–81° E, 20–25° N), where the NE trending Middle Proterozoic Aravalli Mobile Belt meets the ENE trending Satpura Mobile Belt. Earlier Deep Seismic Sounding (DSS) studies along Thuadara-Sendhwa-Sindad profile in the area has showed Mesozoic Sediments up to around 4 km depth covered by Deccan Trap and the Moho depth with a boundary velocity (P_n) of 8.2 km/s. In the present study, surface heat flow of 48 ± 4 mW m^?2 has been estimated based on P_n velocity, which agrees with the value of heat flow of 52 ± 4 mW m^?2 based on Curie point isotherms estimates. The calculated temperature-depth profile shows temperature of 80–120 °C at the basement, which is equivalent to oil window temperature in Mesozoic sediments and around 570–635 °C at Moho depth of 38–43 km and the thermal lithosphere is about 110 km thick, which is comparatively higher than those of adjoining regions. The present study reveals the brittle–ductile transition zone at 14–41 km depth (temperature around 250–600 °C) where earthquake nucleation takes place.     

Structure of the thermal lithosphere and asthenosphere beneath oceans and continents

The lithosphere and asthenosphere make up a common geodynamic system but are characterized by different physical parameters. The former has a temperature of 1200–1300°C, a density of 3.3 g/cm³, and a viscosity of 10²² poise, while the latter has a density of 3.23 g/cm³, a viscosity in the range 10²¹-10^18–19 poise, and a temperature from 1200–1300°C to 1600–1700°C. The asthenosphere is distinguished by a great variability of its physical state in the lateral and vertical directions. This circumstance necessitates the recognition of the different types of the asthenosphere: seismic (LVZ zone), electrical, thermal, and seismological. The structure and the physical state of the thermal asthenosphere is considered in this paper on the basis of P-T parameters. Its state normally fits viscous Newtonian liquid beneath the continents and provides partial (5–20%) melting in spreading zones and along continental margins. No partial melting is detected beneath the main portion of the continents. The interaction between the asthenosphere and lithosphere is characterized by spatiotemporal migration of partial melting zones and asthenosphere upwelling, and such interaction determines the entire range of geodynamic processes from spreading and rifting to collision and vertical motions of different senses.     

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.     

Crustal thickening processes in the Central Andes and the different natures of the Moho-discontinuity

New seismic data from the Central Andes allow us to clarify the crustal structure of this mountain chain and to address the problem of crustal thickening. Evidence for the deep crustal root can be observed in both gravimetric and seismological data. Crustal structure and composition change significantly from east to west. In the eastern part of the backarc the Moho discontinuity is clearly recognisable. However only poor Moho arrivals are observed by active seismic measurements beneath the Altiplano and the Western Cordillera where broad-band seismology data indicate such a discontinuity. In the Precordillera, a pronounced discontinuity is detected at a depth of 70 km. Along the coast, the oceanic Moho is developed at a depth of 40 km. There are several processes which can change the petrological and petrophysical properties of the rocks forming the crust. Variations of the classical Moho discontinuity are presented which do not correspond to the petrological crust/mantle boundary. Tectonic shortening in the backarc is the dominant process contributing to at least 50–55% to the root formation along 21°S. In the forearc and arc, hydration of the mantle wedge produced ≈15–20% of crustal thickening. Magmatic thickening and tectonic erosion contributed only ≈5%. The other ≈25% is not yet explained.     

Mantle heat flow and geotherms for the main geologic domains in Morocco

 Thermal and deep seismic soundings data are used to study the dependence between the compressional P_n velocity and the surface heat flow or the temperature at the Moho discontinuity in Morocco. This correlation indicates a significant decrease in P_n velocity where high heat flow and Moho temperature are observed. This result is consistent with respect to other regions of the world. Crustal heat generation models and geotherms are constructed for the major Moroccan geological domains extending from the Precambrian units in the south to the Alpine units in the north. The crustal contribution in surface heat flow is on average 35 mWm^–2, with high values of 41–42 mWm^–2 in the western and eastern Meseta where Hercynian granite intrusions could enrich the crust in radioactive heat sources. High mantle heat flow values are obtained beneath the Alboran neogene basin (62 mWm^–2), the Rif (47 mWm^–2), the Middle Atlas (41 mWm^–2), and the south Atlantic margin (40 mWm^–2) where the crust is thinned by an extensional tectonic regime. Despite their similar formation context, the intra-continental belts of the Middle and the High Atlas show different geothermal field components. A lithospheric heating process in the Middle Atlas could be the result of a Plio-Quaternary basaltic volcanism. Finally, the Precambrian basement of the Anti-Atlas like all the West African shield is a stable domain showing the lowest subsurface temperatures. Received: 14 January 1998 / Accepted: 29 June 1999     

Origin of high-velocity anomalies beneath the Siberian craton: A fingerprint of multistage magma underplating since the Neoarchean

Despite the violent eruption of the Siberian Traps at ~ 250 Ma, the Siberian craton has an extremely low heat flow (18–25 mW/m²) and a very thick lithosphere (300–350 km), which makes it an ideal place to study the influence of mantle plumes on the long-term stability of cratons. Compared with seismic velocities of rocks, the lower crust of the Siberian craton is composed mainly of mafic granulites and could be rather heterogeneous in composition. The very high V_p (> 7.2 km/s) in the lowermost crust can be fit by a mixture of garnet granulites, two-pyroxene granulites, and garnet gabbro due to magma underplating. The high-velocity anomaly in the upper mantle (V_p = 8.3-8.6 km/s) can be interpreted by a mixture of eclogites and garnet peridotites. Combined with the study of lower crustal and mantle xenoliths, we recognized multistage magma underplating at the crust-mantle boundary beneath the Siberian craton, including the Neoarchean growth and Paleoproterozoic assembly of the Siberian craton beneath the Markha terrane, the Proterozoic collision along the Sayan-Taimyr suture zone, and the Triassic Siberian Trap event beneath the central Tunguska basin. The Moho becomes a metamorphism boundary of mafic rocks between granulite facies and eclogite facies rather than a chemical boundary that separates the mafic lower crust from the ultramafic upper mantle. Therefore, multistage magma underplating since the Neoarchean will result in a seismic Moho shallower than the petrologic Moho. Such magmatism-induced compositional change and dehydration will increase viscosity of the lithospheric mantle, and finally trigger lithospheric thickening after mantle plume activity. Hence, mantle plumes are not the key factor for craton destruction.     

Estimation of crustal discontinuities from reflected seismic waves recorded at Shillong and Mikir Hills Plateau,Northeast India

In this study, an attempt is made to determine seismic velocity structure of the crust and upper mantle beneath the Shillong-Mikir Hills Plateau in northeast India region. The principle of the technique is to relate seismic travel times with crustal thickness above the Conrad and Moho discontinuities. Broadband digital waveforms of the local earthquakes make a precise detection of the seismic phases possible that are reflected at these discontinuities. The results show that the Conrad discontinuity is at 18–20 (±0.5) km beneath the Shillong-Mikir Hills Plateau and the Moho discontinuity is at 30 ± 1.0 km beneath the Shillong Plateau and at 35 ± 1.0 km beneath the Mikir Hills.