CRUSTAL STRUCTURE IN EASTERN REGION OF QINGHAI—TIBET PLATEAU

CRUSTAL STRUCTURE IN EASTERN REGION OF QINGHAI—TIBET PLATEAU     

Deep structure of the Longling–Ruili fault underneath Ruili basin near the eastern Himalayan syntaxis: insights from magnetotelluric imaging

Magnetotelluric (MT) geophysical profiling has been applied to the determination of the deep structure of the Longling–Ruili fault (LRF), part of a convergent strike-slip fault system, underneath thick Caenozoic cover in Ruili basin in southwestern Yunnan, China. The recorded MT data have been inverted using a two-dimensional (2-D) nonlinear conjugate gradients scheme with a variety of smooth starting models, and the resulting models show common subsurface conductivity structures that are deemed geological significant. The models show the presence of a conductive (5–60 Ω m) cover sequence that is thickest (1–1.5 km) in the centre of the basin and rapidly pinches out towards the margins. A half-graben structure is interpreted for the Ruili basin. This is underlain by about 7–10 km thick upper crustal layer of high resistivity (>200–4000 Ω m) that is dissected by steep faults, which we interpret to flatten at depth and root into an underlying mid-crustal conductive layer at about 10 km depth. The mid-crustal layer does not appear to have been severely affected by faulting; we interpret it as a zone of partial melt or intracrustal detachment. The MT models suggest SE directed thrusting of basement rocks in the area. The Longling–Ruili fault is interpreted as a NW-dipping feature bounding one of the identified upper crustal fragments underneath Ruili city. We suggest that MT imaging is a potent tool for deep subsurface mapping in this terrain.     

Conductive structure of Ukrainian Carpathians from EM observations

Geomagnetic variation observations in the Carpathian region gave the data for tracing the axis of a 1200 km long Carpatian electrical conductivity anomaly (CA) and estimation of its integral longitudinal conductivity (~2 × 10⁸ S × m). We made also 35 magnetotelluric soundings (MTS) in the south-east part of the Ukrainian Carpathians. The shape of MTS curves regularly changes from south-west to north-east forming 6 zones of identical behaviour. Most interesting MTS curves are above the CA. The longitudinal curves define the CA at a depth of 10 km; the transverse ones are not sensitive to crustal CA but they define a mantle conductor at a depth of 100–200 km with conductance ~5000 S which can be identified with the asthenosphere. The principal crustal conductors manifested by MTS data in Carpatians are CA subducting in south-west direction from moderately conductive sediments and a conductive zone of Transcarpathian deep fault. Correlation of electrical conductivity structure with seismicity is discussed.     

The Timing of Crustal Extension,Diking, and Eruption of the Deccan Flood Basalts

A randomly oriented dike swarm in the Western Ghats region has been postulated to be the feeder dike swarm of the ～2 km thick sequence exposed in that region of the Deccan province, and interpreted as evidence for the lack of crustal extension before this major flood basalt event. An enormous, central shield volcano has also been postulated in the same region based on flow stratigraphic studies and the randomly oriented dikes. These interpretations are subject to numerous objections and the lack of crustal extension before Deccan volcanism is not supported by presently available data. Rift zones of the province and the western Indian continental margin remain highly probable source areas for large volumes of the Deccan lavas.     

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.     

Implications for the lithospheric structure of Cambay rift zone,western India:Inferences from a magnetotelluric study

Broad-band and long-period magnetotelluric(MT) data were acquired along an east-west trending traverse of nearly 200 km across the Kachchh,Cambay rift basins,and Aravalli-Delhi fold belt(ADFB),western India.The regional strike analysis of MT data indicated an approximate N59°E geoelectric strike direction under the traverse and it is in fair agreement with the predominant geological strike in the study area.The decomposed transverse electric(TE)-and transverse magnetic(TM)-data modes were inverted using a nonlinear conjugate gradient algorithm to image the electrical lithospheric structure across the Cambay rift basin and its surrounding regions.These studies show a thick(～1-5 km) layer of conductive Tertiary-Mesozoic sediments beneath the Kachchh and Cambay rift basins.The resistive blocks indicate presence of basic/ultrabasic volcanic intrusives,depleted mantle lithosphere,and different Precambrian structural units.The crustal conductor delineated within the ADFB indicates the presence of fluids within the fault zones,sulfide mineralization within polyphase metamorphic rocks,and/or Aravalli-Delhi sediments/metasediments.The observed conductive anomalies beneath the Cambay rift basin indicate the presence of basaltic underplating,volatile(CO_2,H_2 O) enriched melts and channelization of melt fractions/fluids into crustal depths that occurred due to plume-lithosphere interactions.The variations in electrical resistivity observed across the profile indicate that the impact of Reunion plume on lithospheric structures of the Cambay rift basin is more dominant at western continental margin of India(WCMI) and thus support the hypothesis proposed by Campbell Griffiths about the plume-lithosphere interactions.     

Magnetotelluric images of the crustal structure of Chyulu Hills volcanic field, Kenya

Electromagnetic experiments were conducted in 1995 as part of a multidisciplinary research project to investigate the deep structure of the Chyulu Hills volcanic chain on the eastern flank of the Kenya Rift in East Africa. Transient electromagnetic (TEM) and broadband (120–0.0001 Hz) magnetotelluric (MT) soundings were made at eight stations along a seismic survey line and the data were processed using standard techniques. The TEM data provided effective correction for static shifts in MT data. The MT data were inverted for the structure in the upper 20 km of the crust using a 2-D inversion scheme and a variety of starting models. The resulting 2-D models show interesting features but the wide spacing between the MT stations limited model resolution to a large extent. These models suggest that there are significant differences in the physical state of the crust between the northern and southern parts of the Chyulu Hills volcanic field. North of the Chyulu Hills, the resistivity structure consists of a 10–12-km-thick resistive (up to 4000 Ω m) upper crustal layer, ca. 10-km-thick mid-crustal layer of moderate resistivity (50 Ω m), and a conductive substratum. The resistive upper crustal unit is considerably thinner over the main ridge (where it is ca. 2 km thick) and further south (where it may be up to 5 km thick). Below this cover unit, steep zones of low resistivity (0.01–10 Ω m) occur underneath the main ridge and at its NW and SE margins (near survey positions 100 and 150–210 km on seismic line F of Novak et al. [Novak, O., Prodehl, C., Jacob, A.W.B., Okoth, W., 1997. Crustal structure of the southern flank of the Kenya Rift deduced from wide-angle P-wave data. In: Fuchs, K., Altherr, R., Muller, B., Prodehl, C. (Eds.), Structure and Dynamic Processes in the Lithosphere of the Afro-Arabian Rift System. Tectonophysics, vol. 278, 171–186]). These conductors appear to be best developed in upper crustal (1–8 km) and middle crustal (9–18 km) zones in the areas affected by volcanism. The low-resistivity anomalies are interpreted as possible magmatic features and may be related to the low-velocity zones recently detected at greater depth in the same geographic locations. The MT results, thus, provide a necessary upper crustal constraint on the anomalous zone in Chyulu Hills, and we suggest that MT is a logical compliment to seismics for the exploration of the deep crust in this volcanic-covered basement terrain. A detailed 3-D field study is recommended to gain a better understanding of the deep structure of the volcanic field.     

Magnetotelluric investigation of lithospheric electrical structure beneath the Dharwar Craton in south India:Evidence for mantle suture and plume-continental interaction

Broad-band and long period magnetotelluric measurements made at 63 locations along ～500 km long Chikmagalur-Kavali profile,that cut across the Dharwar craton(DC)and Eastern Ghat Mobile Belt(EGMB)in south India,is modelled to examine the lithosphere architecture of the cratonic domain and define tectonic boundaries.The 2-D resistivity model shows moderately conductive features that intersperse a highly resistive background of crystalline rocks and spatially connect to the exposed schist belts or granitic intrusions in the DC.These features are therefore interpreted as images of fossil pathways of the volcanic emplacements associated with the greenstone belt and granite suite formation exposed in the region.A near vertical conductive feature in the upper mantle under the Chitradurga Shear Zone represents the Archean suture between the western and eastern blocks of DC.Although thick(～200 km)cratonic(highly resistive)lithosphere is preserved,significant part of the cratonic lithosphere below the western DC is modified due to plume-continental lithosphere interactions during the Cretaceous—Tertiary period.A west-verging moderately conductive feature imaged beneath EGMB lithosphere is interpreted as the remnant of the Proterozoic collision process between the Indian land mass and East Antarctica.Thin(～120 km)lithosphere is seen below the EGMB,which form the exterior margin of the India shield subsequent to its separation from East Antarctica through rifting and opening of the Indian Ocean in the Cretaceous.     

Research of the Conductive Structure of Crust and the Upper Mantle beneath the South-Central Tibetan Plateau

With the super-wide band magnetotelluric sounding data of the Jilong (吉隆)-Cuoqin (措勤) profile (named line 800) which was completed in 2001 and the Dingri (定日)-Cuomai (措迈) profile (named line 900) which was completed in 2004,we obtained the strike direction of each MT station by strike analysis,then traced profiles that were perpendicular to the main strike direction,and finally obtained the resistivity model of each profile by nonlinear conjugate gradients (NLCG) inversion. With these two models,we described the resistivity structure features of the crust and the upper mantle of the center-southern Tibetan plateau and its relationship with Yalung Tsangpo suture: the upper crust of the research area is a resistive layer with resistivity value range of 200-3 000 ?·m. The depth of its bottom surface is about 15-20 km generally,but the bottom surface of resistive layer is deeper in the middle of these two profiles. At line 900,it is about 30 km deep,and even at line 800,it is about 38 km deep. There is a gradient belt of resistivity at the depth of 15-45 km,and a conductive layer is beneath it with resistivity even less than 5 ?·m. This conductive layer is composed of individual conductive bodies,and at the south of the Yalung Tsangpo suture,the conductive bodies are smaller with thickness about 10 km and lean to the north slightly. However,at the north of the Yalung Tsangpo suture,the conductive bodies are larger with thickness about 30 km and also lean to the north slightly. Relatively,the conductive bodies of line 900 are thinner than those of line 800,and the depth of the bottom surface of line 900 is also shallower. At last,after analyzing the effect factors to the resistivity of rocks,it was concluded that the very conductive layer was caused by partial melt or connective water in rocks. It suggests that the middle and lower crust of the center-southern Tibetan plateau is very thick,hot,flabby,and waxy.     

The lithospheric transition between the Delamerian and Lachlan orogens in western Victoria: new insights from 3D magnetotelluric imaging

The magnetotelluric (MT) method was used to image the crust and upper mantle beneath the Delamerian and Lachlan orogens in western Victoria, Australia. During the Cambrian time period, this region changed from being the extended passive margin of Proterozoic Australia into an Andean-style convergent margin that progressively began to accrete younger oceanic terranes. Several broadband MT transects, which were collected in stages along coincident deep (full crust imaging) seismic reflection lines, have now been combined to create a continuous 500 km east–west transect over the Delamerian–Lachlan transition region in the Stawell Zone. We present the electrical resistivity structure of the lithosphere using both 3D and 2D inversion methods. Additionally, 1D inversions of long-period AusLAMP (Australian Lithospheric Architecture Magnetotelluric Project) MT data on a 55 km regionally spaced grid were used to provide starting constraints for the 3D inversion of the 2D profile. The Delamerian to Lachlan Orogen transition region coincides with the Mortlake Discontinuity, which marks an isotopic discontinuity in Cenozoic basalts, with higher strontium isotope enrichment ratios in the Lachlan Orogen relative to the Delamerian Orogen. Phase tensor ellipses of the MT data reveal a distinct change in electrical resistivity structure near the location of the Mortlake Discontinuity, and results of 3D and 2D inversions along the MT profile image a more conductive lower crust and upper mantle beneath the Lachlan Orogen than the Delamerian Orogen. Increased conductivity is commonly ascribed to mantle enrichment and thus supports the notion that the isotope enrichment of the Cenozoic basalts at least partially reflects an enriched mantle source rather than crustal contamination. Fault slivers of the lower crust from the more conductive Lachlan region expose Cambrian boninites and island arc andesites indicative of subduction, a process that can enrich the mantle isotopically, and also electrically, by introducing carbon (graphite) and water (hydrogen).     

Crustal Electrical Conductivity Structure of Southern Tibet from Magnetotelluric Survey

INTRODUCTIONAsatypicalintercontinent collisionarea ,Tibetanplateauisapresentlypopularregionnoticedbygeoscientistsallovertheworld ,sinceitisanidealandnaturallabforthestudyofthegeologicalevolutionoftheinnerlandmountain creatingzoneandoftheprocessofthecrust mantledeepdy namicsaswellasoftheinteractionbetweenthesurfaceandthecrustmovement.ThestudyoftheTibetanplateauevolutionisimportantnotonlyinsolvingsometheoreticalproblemsaboutcontinentaldynamicsandglobalchangesingeosciences ;butalsoinexplorin…     

青藏高原东南部地壳导电性结构与断裂构造特征——下察隅-昌都剖面大地电磁探测结果

根据2004年在青藏高原东南部完成的下察隅—昌都(1000线)宽频带大地电磁探测剖面数据研究高原东南部地壳导电性结构及断裂构造特征,这有助于推进印度与亚洲岩石圈碰撞、俯冲构造模式的研究。研究结果表明,沿剖面上地壳大范围分布的是规模不等的高阻体,电阻率大约在90～3000Ω.m,厚度由南向北增加,底界面的深度大约在5～30km变化。高阻层之下发现由不连续高导体构成的中地壳低阻层,其电阻率小于10Ω.m;其结构与青藏高原中、西部的壳幔高导体相似,但规模小得多,底面埋深也浅得多。沿剖面的上地壳存在多组规模不等、产状不同的横向电性梯度带或畸变带,它们反映了沿剖面地区地壳的断裂分布。通过与该区高精度重力资料对比,在重要的电性梯度带上,均存在布格重力低异常和负重力均衡异常。结合区域地质资料分析推断了嘉黎—然乌、班公—怒江和甲桑卡—赤布张错等主要断裂构造带的空间格局。     

准噶尔中东部地区深部电性结构电磁探测

为了解准噶尔盆地深部构造特征,综合利用“深部探测技术与试验研究(SinoProbe)”项目在准噶尔盆地45°N 88°E处建立的大地电磁标准点实测资料,应用非线性共轭梯度法(NLCG)对该测站两条短剖面进行二维反演,结合新疆准噶尔盆地区域地质资料,对该地区地层电性结构进行了初步分析,发现准噶尔盆地中东部地区地下结构具有很好的电性分层.与现有地质资料相结合,分析发现其电性分层与地壳分层具有较好的对应.根据岩石层电导性推断:研究区域莫霍面埋深在46 km附近,岩石圈厚度在100 km左右.研究结果对准噶尔中东部地区深部地壳结构的认识具有一定的参考价值.      

The crustal structure of the western Arabian platform from the spectral analysis of long-period P-wave amplitude ratios

The crustal structure of the western Arabian platform is derived using the spectral analysis of long-period P-wave amplitude ratios. The ratio of the vertical to the horizontal component is used to obtain the crustal transfer function based on thickness variations, crustal velocities, densities and the angle of emergence at the lower crust and upper mantle interface. Eleven well-defined earthquakes recorded at the long-period RYD station during the period from 1985 to 1994 were selected for analysis based on the following criteria: focal depths with a range between 7 and 89 km, body-wave magnitudes greater than 4.7, epicentral distances with a range from 8.8° to 26.5°, and back azimuthal coverage from 196° to 340°. Spectral analysis calculations were based on the comparison of the observed spectral ratios with those computed from theoretical P-wave motion obtained using the Thomson–Haskell matrix formulation for horizontally layered crustal models. The selection of the most suitable model was based on the identification of the theoretical model which exhibits the highest cross-correlation coefficient with the observed transfer function ratio. By comparing the spectral peak positions of the observed and theoretical values, the thickness and velocity can be resolved within 3 km and 1 km/s, respectively, of the observed values. The spectral analysis of long-period P-waves can detect a thin layer near the surface of about 1.6 km thick and a velocity contrast of about 10% with that of the underlying layer. A strong velocity gradient of about 0.05 km/s per km was found in the upper crust and 0.02 km/s per km in the lower crust. The derived crustal model is not unique due to the theoretical assumptions (horizontal layering, constant densities and velocities in each layer), quality of the data and complexities of the crustal structure. The crustal model suggests that the crust consists of five distinct layers. The upper crustal layer has a P-wave velocity of about 5.6 km/s and is about 1.6 km thick. The second layer has a velocity of about 6.2 km/s and is 10.2 km thick. The third layer shows a velocity of 6.6 km/s and is 6.8 km thick. The fourth layer has a velocity of about 6.8 km/s and is 12.3 km thick. The lower crustal layer has a velocity of about 7.5 km/s and is 9.3 km thick. The Mohorovicic discontinuity beneath the western Arabian platform indicates a velocity of 8.2 km/s of the upper mantle and 42 km depth.     

Spectral analysis and Euler deconvolution technique of gravity data to decipher the basement depth in the Dehradun-Badrinath area

The stratigraphic and tectonic setting in the northwest part of Himalayan belt is complex and thrusted due to the collision of Indian plate and the Eurasian plate. During the past, the Himalaya is divided into four parts; these are Outer Himalaya, Lesser Himalaya, Greater or Higher Himalaya and Tethys Himalaya. The appearance of basement rocks played a significant role in the Himalayan periphery for stratigraphic, structural and tectonic movement. The deformation pattern of the crustal rocks causing the rise of basement rocks which constitutes an integral part of crustal configuration during the evolutionary stages of the Himalaya. In this study, an attempt has been made to estimate the basement depth configuration using spectral analysis and Euler deconvolution technique of gravity data in the northwest Himalaya region. The elevation increases continuously from 500 m to 5100 m in SW to NE direction, however, Bouguer gravity anomaly decreases continuously from ?130 mGal to ?390 mGal in SW to NE direction due to the isostaic adjustment. Gravity anomaly is very low near Harsil, Badrinath and Joshimath area and observed higher elevation due to the deep rooted basement. However, there are extrusion of crystalline basement in and around the Badrinath area due to the resettlement of geologic process which are overlaid to the top surface of the sedimentary layers. Euler deconvolution technique has been applied to detect the direct basement depth and results show a good correlation with the average depth of the spectral analysis and other works carried by different authors. Three gravity profiles are selected in appropriate places orienting SW-NE direction with a profile length of 160 km, 150 km and 140 km respectively in the study area for calculating the average depth of the basement rock. The average basement depth calculated is around 11.27 km using the spectral analysis technique and results are well correlated with the results of various workers. Euler deconvolution studies along the three selected profiles have been interpreted. It has been observed that there are more number of cluster points falling between depth ranges of 10 to 15 km, dipping in SW to NE direction. Euler’s study shows deep rooted connection near Main Frontal Thrust (MFT), Main Boundary Thrust (MBT), Main Central Thrust (MCT), Bearing Thrust (BT) and Vaikrita Thrust (VT) locations as per profile study. Based on these studies three geological models have been prepared along the profiles showing different tectonic resettlement and depth of crystalline basement. Crystalline rocks exposed at the surface may be due to uplift along the shear in the MCT zone by kinetic flow basically, Munsiayari Thrust (MT) and VT in the of NW-Himalaya region.     

纳如松多矿集区电性结构特征及对成矿作用的约束

为了研究西藏纳如松多矿集区的电性结构特征和对成矿作用的约束,对覆盖矿集区的大地电磁测深数据进行全面的数据处理分析,得到了可靠的二维电性结构模型.研究结果表明,分别在深度为40~50 km,20~30 km和10 km处见高导体,推测这些高导体可能为部分熔融和水流体共同所致.由于纳如松多矿集区内矿床为岩浆-热液型,深部岩浆的上涌在成矿作用中起到关键作用,所以壳内高导体可能为与成矿有关岩浆房的电性痕迹,将这些高导体连起来可能代表着深部热液向上运移的古通道.电性结构主要体现了壳内高导体与区域成矿动力作用的关系,向上运移的富矿岩浆也可能通过局部的隐伏构造运移到Pb-Zn和Fe-Cu矿床的位置,再演化形成矿体.     

<Superscript>40</Superscript>Ar-<Superscript>39</Superscript>Ar age of a lava flow from the Bhimashankar Formation,Giravali Ghat,Deccan Traps

We report here a⁴⁰Ar-³⁹Ar age of 66.0 ± 0.9 Ma (2σ) for a reversely magnetised tholeiitic lava flow from the Bhimashankar Formation (Fm.), Giravali Ghat, western Deccan province, India. This age is consistent with the view that the 1.8–2 km thick bottom part of the exposed basalt flow sequence in the Western Ghats was extruded very close to 67.4 Ma.     

Detection of a collision zone in south Indian shield region from magnetotelluric studies

Magnetotelluric (MT) investigations were carried out along a profile in the greenschist–granulite transition zone within the south Indian shield region (SISR). The profile runs over a length of 110 km from Kuppam in the north to Bommidi in the south. It covers the transition zone with 12 MT stations using a wide-band (1 kHz–1 ks) data acquisition system. The Mettur shear zone (MTSZ) forms the NE extension of Moyar–Bhavani shear zone that traverses along the transition zone. The regional geoelectric strike direction of N40°E identified from the present study is consistent with the strike direction of the MTSZ in the center of the profile. The 2-D conductivity model derived from the data display distinct high electrical resistivity character (10,000 Ω m) below the Archaean Dharwar craton and less resistive (< 3000 Ω m) under the southern granulite terrain located south of the MTSZ. The MTSZ separating the two regions is characterized by steep anomalous high conductive feature at lower crustal depths. The deep seismic sounding (DSS) study carried out along the profile shows dipping signatures on either side of the shear zone. The variation of deep electrical resistivity together with the dipping signature of reflectors indicate two distinct terrains, namely, the Archaean Dharwar Craton in the north and the Proterozoic granulite terrain towards south. They got accreted along the MTSZ, which could represent a possible collision boundary.     

长江中下游成矿带岩浆-成矿系统深部背景和过程的电性结构约束

长江中下游成矿带是我国重要的铁铜金属"仓库",然而,关于矿体密集分布成因的深部背景研究仍存较多争议,前人相继提出了碰撞变形、俯冲变形、构造机制转换等多种解释模型.本文基于成矿系统概念和前人对成矿系统各组成要素的研究成果,分析长江中下游成矿带内宁芜、庐枞、南陵-宣城和安庆-贵池矿集区的深部电性结构,并发现成矿带地壳深部普...     

Deep Crustal Electrical Signatures of Eastern Dharwar Craton, India

Wide band magnetotelluric (MT) investigations were carried out along a profile from Kavali in the east to Anantapur towards west across the Eastern Ghat Granulite Terrain (EGGT), Eastern Dhanvar Craton (EDC) and a Proterozoic Cuddapah Basin. This 300 km long profile was covered with 20 stations at an interval of 12–18 km. The MT data is subjected to robust processing, decomposition and static shift correction before deriving a 2-D model. The model shows a resistive crust (−10,000–30,000 ohm-m) to a depth of 8–10 km towards west of the Cuddapah basin. The mid crust is less resistive (about 500 ohm-m) and the lower crust with a slight increase in resistivity (about 1,500 ohm-m) in the depth range of 20–22 km. The resistivity picture to the east of the Cuddapah basin also showed a different deep crustal structure. The resistivity of upper crust is about 5,000 ohm-m and about 200 ohm-m for mid and lower crust. The sediment resistivity of Cuddapah basin is of the order of 15–20 ohm-m. MT model has shown good correlation with results from other geophysical studies like deep seismic sounding (DSS), gravity and magnetics. The results indicate that the lower crustal layers are of intermediate type showing hydrous composition in Eastern Dhanvar Craton.