Structural trends in the Cuddapah basin from deep seismic soundings (DSS) and their tectonic implications

Structural trends in the upper Proterozoic Cuddapah basin, at the basement level and at the Moho level have been discussed based on Deep Seismic Sounding (DSS) studies. Results of DSS studies along the Alampur-Koniki profile (profile 2 of Fig. 1) crossing the northern part of the Cuddapah basin have been discussed in detail. These results, combined with the results of the Kavali-Paranpalle section of the Kavali-Udipi DSS profile (profile 1 of Fig. 1, Kaila et al., 1979) crossing the basin on its southern flank, along with geological data and earthquake epicentral locations, are used to explain the structural trends of the area. It has been shown that the Cuddapah basin was first created in its western part by downfaulting of the crustal block between faults 7 and 14 towards the west and fault 6 in the east (Fig. 1). Subsequently, the eastern part was downfaulted against fault 6 before the commencement of upper Cuddapah sedimentation. Further downfaulting towards the north along fault 5 created the Srisailam block. Minor-scale downfaulting between faults 7 and 13 in the west and fault 6 in the east and fault 8 in the north gave rise to the Kurnool sub-basin at a later stage. Similar downfaulting east of fault 9 and north of fault 5 gave rise to the Palnad sub-basin. Both these sub-basins received Kurnool sediments.After the close of Kurnool sedimentation, the blocks between faults 4 and 6 along profile II and between 11 and 6 along profile I were uplifted at the basement level, thus giving rise to the Nallamalai hills and Iswarakuppam dome (Fig. 1). The low-angle thrust fault 3 on the eastern margin of the Cuddapah basin might be a post-Cuddapah phenomenon. The low-angle thrust fault 2 probably occurred in the post-Dharwar period. Faults 1, 17 and 10 near the east coast of India seem to be comparatively younger probably of Mesozoic time, along which the coastal block is downfaulted giving rise to the sedimentary basins.     

Controlled Source Audio Magneto Telluric (CSAMT) studies for uranium exploration in Durgi area,Palnad sub-basin,Cuddapah basin,India

Cuddapah basin is known for hosting unconformity proximal uranium deposits viz., Lambapur, Peddagattu, Chitirial and Koppunuru along the northern margin of the basin. It is well known that these deposits are mostly associated with basement granitoids in Srisailam Sub-basin, and with cover sediments in Palnad subbasin where basement topography and fault/fracture system influence the fluid flow causing basement alteration and ore deposition. Geological setup, surface manifestation of uranium anomalies and association of the hydro-uranium anomalies near Durgi area in southern part of the Palnad sub-basin, have prompted detail investigation by geophysical methods to probe greater depths. Controlled Source Audio Magneto Telluric (CSAMT) survey conducted over five decades of frequency (0.1-9600 Hz) delineated the various lithounits of Kurnool and Nallamalai Groups along with their thicknesses as there exist an appreciable resistivity contrast. Interpretation of CSAMT sounding data are constrained by resistivity logs and litholog data obtained from the boreholes drilled within the basin indicated three to four layered structure. Sub-surface 2-D and 3-D geo-electrical models are simulated by stitching 1-D layered inverted resistivity earth models. Stitched 1-D inverted resistivity sections revealed the unconformity between the Kurnool Group and Nallamalai Group along with basement undulations. The faults/fractures delineated from the CSAMT data corroborated well with the results of gravity data acquired over the same area. Simulated 3-D voxel resistivity model helped in visualising the faults/fractures, their depth extent, thickness of the Banganapalle quartzite and basement configuration. Integrated interpretation of CSAMT, gravity and borehole data facilitated in delineating the unconformity and the structural features favourable for uranium mineralisation in deeper parts of the Palnad sub-basin.     

Intracontinental thrusts and inclined transpression along eastern margin of the East Dharwar craton, India

Recent works suggest Proterozoic plate convergence along the southeastern margin of India which led to amalgamation of the high grade Eastern Ghats belt (EGB) and adjoining fold-and-thrust belts to the East Dhrawar craton. Two major thrusts namely the Vellikonda thrust at the western margin of the Nellore Schist belt (NSB) and the Maidukuru thrust at the western margin of the Nallamalai fold belt (NFB) accommodate significant upper crustal shortening, which is indicated by juxtaposition of geological terranes with distinct tectonostratigraphy, varying deformation intensity, structural styles and metamorphic grade. Kinematic analysis of structures and fabric of the fault zone rocks in these intracontinental thrust zones and the hanging wall and footwall rocks suggest spatially heterogeneous partitioning of strain into various combinations of E-W shortening, top-to-west shear on stratum parallel subhorizontal detachments or on easterly dipping thrusts, and a strike slip component. Although relatively less prominent than the other two components of the strain triangle, non-orthogonal slickenfibres associated with flexural slip folds and mylonitic foliation-stretching lineation orientation geometry within the arcuate NSB and NFB indicate left lateral strike slip subparallel to the overall N-S trend. On the whole an inclined transpression is inferred to have controlled the spatially heterogeneous development of thrust related fabric in the terrane between the Eastern Ghats belt south of the Godavari graben and the East Dharwar craton.     

Inversion of calcite twin data,paleostress reconstruction and multiphase weak deformation in cratonic interior – Evidence from the Proterozoic Cuddapah basin,India

Paleostress orientations from mechanically twinned calcite in carbonate rocks and veins in the neighborhood of large faults were investigated to comment on the nature of weak upper crustal stresses affecting sedimentary successions within the Proterozoic Cuddapah basin, India. Application of Turner's P–B–T method and Spang's Numerical dynamic analysis on Cuddapah samples provided paleostress orientations comparable to those derived from fault-slip inversion. Results from the neighborhood of E–W faults cutting through the Paleoproterozoic Papaghni and Chitravati groups and the Neoproterozoic Kurnool Group in the western Cuddapah basin, reveal existence of multiple deformation events − (1) NE–SW σ₃ in strike-slip to extensional regime along with an additional event having NW–SE σ_3, for lower Cuddapah samples; (2) compressional/transpressional event with ESE–WNW or NNE–SSW σ₁ mainly from younger Kurnool samples.Integrating results from calcite twin data inversion, fault-slip analysis and regional geology we propose that late Mesoproterozoic crustal extension led to initial opening of the Kurnool sub-basin, subsequently influenced by weak compressional deformation. The dynamic analysis of calcite twins thus constrains the stress regimes influencing basin initiation in the southern Indian cratonic interior and subsequent basin inversion in relation to craton margin mobile belts and plausible global tectonic events in the Proterozoic.     

Tectonics in the Cuddapah fold-thrust belt in the Indian shield,Andhra Pradesh,India and its implication on the crustal amalgamation of India and Rayner craton of Antarctica during Neoproterozoic orogenesis

Neoproterozoic orogenesis in East Antarctica and India led to the amalgamation of northern Prince Charles Mountains-Rayner complex of Antarctica with the Krishna Province of India along the present eastern coast of India with the development of ~990–900 Ma old fold-thrust belt. The frontal part of the fold-thrust belt [henceforth called the Cuddapah fold-thrust belt (CFTB)], recognized in the intercratonic, Palaeoproterozoic–Neoproterozoic Cuddapah Basin, includes two frontal thrust sheets carried by the eastern Velikonda and the western Nallamalai thrusts, along with a part of the undeformed foreland, constituting frontal part of a larger fold-thrust belt now fragmented and separated in different continents of Gondwanaland. Therefore, the intercratonic deformation now preserved in the Palaeoproterozoic–Neoproterozoic Cuddapah Basin is related to the collision of the Indian shield to the Antarctic block during the amalgamation of the Rodinia Supercontinent. CFTB is dominated by quasi-plastic deformational structures, representing exhumed deeper level fault-propagation folding related to the Velikonda thrust, while the Nallamalai thrust represents the forelandward thrust of the CFTB dominated by elastico-frictional deformation structures.     

Crustal structure of the northern part of the Proterozoic Cuddapah basin of India from deep seismic soundings and gravity data

A crustal depth section was obtained from Deep Seismic Soundings (DSS) along the Alampur-Koniki-Ganapeshwaram profile, cutting across the northern part of the Proterozoic Cuddapah basin, India, running just south of latitude 16° N and between longitude 78° E and 81°E. The existence of a low-angle thrust fault at the eastern margin of the Cuddapah basin (Kaila et al., 1979) was confirmed along a second profile. Another low-angle thrust, along which charnockites with the granitic basement are upthrust against the Dharwars was delineated further east. The contact of the khondalites (lower Precambrian) with quaternary sediments near the east coast of India seems to be a fault boundary, which may be responsible for the thick sedimentary accumulation in the adjoining offshore region.The basement in the western part of the Cuddapah basin is very shallow and is gently downdipping eastward, to a depth of 1.7 km about 20 km west of Atmakur. It attains a depth of about 4.5 km in the deepest part of the Kurnool sub-basin, around Atmakur. Under the Nallamalai ranges its depth varies between 3.5 and 6.5 km, with an easterly dip. In the region north of the Iswarkuppam dome, the basement is at a depth of about 5.0 km, to about 6.8 km in the eastern part of the Cuddapah basin. Outside the eastern margin of the basin, the depth of the basement is about 1.8 km and further eastwards it is exposed. A fault at the contact of the khondalites with quaternary sediments near the east coast brings the basement down to a depth of approximately 1.3 km.In the Kurnool sub-basin the depth to the Moho discontinuity varies from 35 km under Atmakur to 39 km under the Nallamalai hills. In the region of the Iswarkuppam dome it is at a depth of about 36 km, deepening to about 39 km before rising to 37 km towards the east. Two-dimensional velocity modelling using ray-tracing techniques tends to confirm these results.Gravity modelling of the crustal structure, utilizing a four-layer crustal model in most parts along this profile, conforms to the observed gravity values. A weak zone in the eastern part of the profile where high-density material (density 3.05 g/cm³) has been found seems to be responsible for the gravity high in that part.     

Detrital mineral age,radiogenic isotopic stratigraphy and tectonic significance of the Cuddapah Basin,India

The Cuddapah Basin is one of a series of Proterozoic basins that overlie the cratons of India that, due to limited geochronological and provenance constraints, have remained subject to speculation as to their time of deposition, sediment source locations, and tectonic/geodynamic significance.Here we present 21 new, stratigraphically constrained, U–Pb detrital zircon samples from all the main depositional units within the Cuddapah Basin. These data are supported by Hf isotopic data from 12 of these samples, that also encompass the stratigraphic range, and detrital muscovite ⁴⁰Ar/³⁹Ar data from a sample of the Srisailam Formation. Taken together, the data demonstrate that the Papaghni and lower Chitravati Groups were sourced from the Dharwar Craton, in what is interpreted to be a rift basin that evolved into a passive margin. The Nallamalai Group is here constrained to be deposited between 1659 ± 22 Ma and ~ 1590 Ma. It was sourced from the coeval Krishna Orogen to the east, and was deposited in its foreland basin. Nallamalai Group detrital zircon U–Pb and Hf isotope values directly overlap with similar data from the Ongole Domain metasedimentary rocks. Depositional age constraints on the Srisailam Formation are permissive with it being coeval with the Nallamalai Group and it was possibly deposited within the same basin. The Kurnool Group saw a return to Dharwar Craton derived provenance and is constrained to being Neoproterozoic. It may represent deposition in a long-wavelength basin forelandward of the Tonian Eastern Ghats Orogeny. Detrital zircons from the Gandikota Formation, which is traditionally considered a part of the Chitravati Group, constrain it to being deposited after 1181 ± 29 Ma, more than 700 Ma after the lower Chitravati Group. It is possible that the Gandikota Formation is correlative with the Kurnool Group.The new data suggest that the Nallamalai Group correlates temporally and tectonically with the Somanpalli Group of the Pranhita–Godavari Valley Basin, which is tightly constrained to being deposited at ~ 1620 Ma. These syn-orogenic foreland basin deposits firmly link the SE India Proterozoic basins to their orogenic hinterland with their discovery filling a ‘missing-link’ in the tectonic development of the region.     

Deformation history of the Hampden Synform in the Eastern Fold Belt of the Mt Isa terrane

The moderately metamorphosed and deformed rocks exposed in the Hampden Synform, Eastern Fold Belt, in the Mt Isa terrane, underwent complex multiple deformations during the early Mesoproterozoic Isan Orogeny (ca 1590–1500 Ma). The earliest deformation elements preserved in the Hampden Synform are first‐generation tight to isoclinal folds and an associated axial‐planar slaty cleavage. Preservation of recumbent first‐generation folds in the hinge zones of second‐generation folds, and the approximately northeast‐southwest orientation of restored L¹ ₀ intersection lineation suggest recumbent folding occurred during east‐west to northwest‐southeast shortening. First‐generation folds are refolded by north‐south‐oriented upright non‐cylindrical tight to isoclinal second‐generation folds. A differentiated axial‐planar cleavage to the second‐generation fold is the dominant fabric in the study area. This fabric crenulates an earlier fabric in the hinge zones of second‐generation folds, but forms a composite cleavage on the fold limbs. Two weakly developed steeply dipping crenulation cleavages overprint the dominant composite cleavage at a relatively high angle (>45°). These deformations appear to have had little regional effect. The composite cleavage is also overprinted by a subhorizontal crenulation cleavage inferred to have developed during vertical shortening associated with late‐orogenic pluton emplacement. We interpret the sequence of deformation events in the Hampden Synform to reflect the progression from thin‐skinned crustal shortening during the development of first‐generation structures to thick‐skinned crustal shortening during subsequent events. The Hampden Synform is interpreted to occur within a progressively deformed thrust slice located in the hangingwall of the Overhang Shear.     

Occurrence of High Level Magma Intrusives and Associated Mineralization near Gani Inlier,Kurnool Sub-Basin,Cuddapah Basin,India

An ensemble of spatially associated high level magma intrusives comprising dykes, laccolith and vent have been identified near Gani inlier in the Kurnool sub-basin. Geological setting, morphology of igneous intrusives and their relationships with the sediment cover have been documented. Diabase in laccolith, dykes and vesicular basalt in vents are prominent rocks and have been subjected to extensive deuteric alterations viz. saussurtisation and uralitisation. It is envisioned that the basic magma was intruded as dykes in strike-slip fault domain, outpoured through the vents as vesicular basalts at the intersection zones of cross faults. The vertical ascending melt was deflected as a sheet due to fault control at a sub-surface level of ～700m along the lithological discontinuities in country rock sediments. The sheet to laccolith transition has resulted in the uplift of older Tadpatri Formation as an inlier amidst the Neoproterozoic Kurnool sediment cover. The localization of iron ore and copper mineralization adjacent to intrusives is attributed to late stage melt-fluid activity associated with the igneous activity. Based on field relationships it is suggested that the high level magmatic activity had occurred after the Kurnool sedimentation in this part of the Kurnool sub-basin.     

Structural pattern and kinematic framework of deformation in the southern Nallamalai fold–fault belt,Cuddapah district,Andhra Pradesh,Southern India

An association of westerly verging asymmetric folds, easterly dipping cleavages and contractional faults control the pattern and intensity of structures at different scales in the southern Nallamalai fold–fault belt, Cuddapah district of Andhra Pradesh, Southern India. Variation in structural geometry is manifested across the section by the occurrence of relatively low amplitude folds, sometimes only a monocline and by the near absence of contractional faults in the WSW, but tight to isoclinal folds with frequent fold–fault interactions through the central areas towards ENE.The relationships of structural elements in terms of orientation, style, sense of movement and general vergence indicate their development under a progressive contractional deformation. The structures are interpreted to result from a combination of bulk inhomogeneous shortening across the belt and a top-to-west, variable simple shear. Localized developments of crenulation cleavage, rotation of cleavage in the shorter limbs of some mesoscale asymmetric folds and general variation of structural elements in morphology and associations across the belt, indicate partitioning of deformation and a varying degree of non-coaxiality in discrete domains of the bulk deformation.     

印度南部元古代Cudappah盆地白云岩中的菱镁矿成矿作用及其构造-气候指示意义

Cudappah盆地是位于印度南部的一个元古代盆地，盆地中含有许多矿床，包括菱镁矿和滑石矿。该盆地发育于Chat活动带东部边缘，并发育沙质、粘土质和碳酸盐岩的多期重复沉积旋回。盆地可划分为四个次级盆地，即Papaghni、Nallamalai、Srisailam和Kurnool次级盆地。晶质菱镁矿床赋存于Papashni次级盆地的Vempalli组地层中。每个次级盆地很可能都沿着一系列裂谷中的断块发育，并且这些裂谷形成于中元古代的热事件。每个沉积旋回中发育有相似的沉积岩套表明，在盆地演化过程中具有相同的构造和气候环境。根据盆地中矿床(重晶石矿、菱镁矿和滑石矿)的产出，我们对每个次级盆地均提出了一个多期重复沉积旋回的演化模式，该旋回包括了从角砾岩到白云岩的变化。     

Collision tectonics in the late Precambrian Eastern Ghats Mobile Belt: mesoscopic to satellite-scale structural observations

Regional mapping of a section across the Eastern Ghats Mobile Belt (EGMB) north of the Godavari graben in Eastern Peninsular India by using Landsat Thematic Mapper data enables recognition of a number of shear zones, lineaments, and structural domes and basins. A conspicuous megashear occurs at the western boundary of the granulite facies rocks of the EGMB adjacent to the Archean granite-greenstone craton. The confinement of a suite of alkaline igneous rocks to this shear zone is a notable feature. The strike extensions of this shear belt extend through to the Elchuru alkaline complex, Prakasam District, Andhra Pradesh, and the syenite plutons of Koraput district, Orissa. The contrasting lithologies, metamorphism and structural history on either side of the shear zone suggests that it might be a Precambrian suture zone. The mesoscopic structural features in the EGMB include prominent foliation with moderate to steep dips, folds, faults/shears, S-C fabrics, pinch and swell structures and other linear fabric elements. These observations favour the consideration of drastic crustal shortening and thickening and a complex deformational sequence. The major rock units in this part of EGMB comprise garnetiferous sillimanite gneisses, quartzites and calc-granulites forming the khondalitic suite of rocks and a wide variety of charnockitic rocks. The contact of the two rock units is generally sheared and often migmatised. The structural fabric suggests two major tectonic events: an essentially horizontal tectonic regime resulting in thrust systems and associated structures, subsequently followed by strike-slip tectonics characterized by high shear strains. Features such as westward-verging thrusts, large-scale recumbent folds, major shear zones, structural domes and basins, indications of tectonic crustal shortening, extensive calc-alkali magmatism and widespread migmatization in the region are attributed to collisional processes during Proterozoic times. The spatial disposition of the EGMB and its linkage with the distribution of similar rock units during the late Precambrian time in a global tectonic scenario are discussed.     

Emplacement of Proterozoic massif-type anorthosite during regional shortening: evidence from the Bolangir anorthosite complex (Eastern Ghats Province, India)

A study of the 933±32-Ma-old Bolangir massif-type anorthosite complex (Eastern Ghats Province, India) yielded strong evidence for anorthosite emplacement during regional shortening, and thereby new insights in massif-type anorthosite formation. Several lines of evidence strongly suggest synchronism of plutonism and regional deformation. First, structures in the country rocks, which imply N–S-directed shortening accompanied by E–W extension, are mirrored by a E–W trending post-magmatic foliation and N–S trending shear zones in the anorthosite complex. Near the intrusion, the foliation in the country rocks becomes parallel to the contact and an internal marginal foliation, and foliation triple points occur in the country rocks. Second, synshortening dikes inside and outside the anorthosite complex are filled with pluton-related melts. Third, ferrodiorites, which are considered late-stage differentiates of the anorthositic pluton, concentrate in tectonic voids at the pluton margin. Some of these occurrences have been affected by the last increments of the regional deformation, but others transect the same structures. Ascent mechanism and significance of the adjacent terrane boundary of the Eastern Ghats Belt for ascent and emplacement of the Bolangir anorthosite complex are discussed. The results of this study imply that emplacement of Proterozoic massif-type anorthosite is not restricted to extensional settings.     

印度三个不同时代地洼区的铀成矿作用(英文)

在印度三个时代不同的地洼区中,出现重要的赋存于沉积岩中的铀矿化作用。这些地洼区是:a)印度南部的元古代库达帕洼地;b)印度东北部梅加拉亚的白垩纪Madadek盆地;c)印度北部的晚第三纪喜马拉雅前渊。库达帕洼地呈新月形,沿南印度克拉通东缘沉积,紧邻东加茨活动带,面积达44,500km~2。该洼地由数个上升、下落断块组成。在这些断块内,浅水砂质(库达帕亚组)和钙质(库尔努尔组)沉积物厚度超过12km,同时从2,000—600Ma,至少有四期粒玄岩墙侵入。铀矿化作用主要呈沥青铀矿(含硫化物)和次生铀矿物形式,沿库达帕洼地面南缘出现于磷质碳酸盐岩和共生物的帕帕格尼组砂质岩、砾岩中。如Tummalapalli、Ammasripall,在梅加拉亚,约200米厚的河流相、海相砂质、泥质沉积物分布于1.5km高的西隆高原南缘的Mahadek盆地。该盆地的铀矿化限于河流相、边缘海相早白垩世“石英粗砂碎屑岩/亚长石砂岩”型Mehadek砾岩中,如Comasahat,Pdensashakap、Domiasiat,并呈沥青铀矿、水硅铀矿、钛钠矿形式。这些矿物与还原剂(如碳、生物成因黄铁矿)紧密共生。在喜马拉雅前渊,铀矿化赋存于河流相锡瓦利克砂岩中,并主要产于锡瓦利克砂岩的下-中或中-上接触面上,如Thein、Morni、Hamirpur,Naugajiarao等地。矿化主要呈沥青铀矿、水硅铀矿(与硫化物共生)和大量次生铀矿物形式——这主要是由于持续至现在的反复的活化作用和沉淀作用。这三个地洼区铀成矿作用的共同特点是河流相沉积物围岩(主要为砂岩,并来源于丰富的酸性源区)、远成热液成矿作用和原始成矿作用时的强烈还原环境。这特征可作为在地洼区寻找赋存于沉积岩中的铀矿化的标志。     

Evolution of thrust system deformation in an intracratonic chain: Study of the Southern-Central Tunisian Atlas

The development of belt structures in intracratonic chains is guided by the convergence system. In the Southern-Central Tunisian Atlas, several parameters control the evolution of thrust folds during different tectonic phases. One of these phases is tectonic inheritance, which leads to the reactivation of pre-existing normal faults during compressive phases. The angle between the direction of these faults and the shortening axis (NNW-SSE) is the most important parameter for interpreting the mode of the evolution of thrust folds. Jebel Elkebar is an example of a structure developed on NW-SE-oriented faults that is perpendicular to the shortening axis. Based on the geometry of its folds, Jebel Elkebar is interpreted as a 'Fault Related Fold'. The E-W-oriented Orbata structure is oblique to the direction of the shortening axis and is interpreted through the model 'Fault Propagation Fold' with 'Breakthrough'. The Gafsa Fault, which is parallel to the shortening axis, is a transpressional fault interpreted through the 'Strain Partitioning' mode, which is associated with the oblique ramp fold. The development of various thrust folds requires the presence of a basal decollement level during the Triassic succession. In the Southern-Central Tunisian Atlas, the deformation is variable (geometry of fold closure) and is correlated with the depth of the decollement level; indeed, the intensity of deformation is proportional to the depth of the decollement level. Consequently, the most important deformation is in the higher successions and is a vertical migration of the decollement level associated with thin-skinned deformation.     

Structural analysis of the Lombard thrust sheet and adjacent areas in the Helena salient,southwest Montana,USA

The Helena salient is a prominent craton–convex curve in the Cordillera thrust belt of Montana, USA. The Lombard thrust sheet is the primary sheet in the salient. Structural analysis of fold trends, cleavage attitudes, and movement on minor faults is used to better understand both the geometry of the Lombard thrust and the kinematic development of the salient.Early W–E to WNW–ENE shortening directions in the Lombard sheet are indicated by fold trends in the center of the thrust sheet. The same narrow range of shortening directions is inferred from kinematic analysis of movement on minor faults and the orientations of unrotated cleavage planes along the southern lateral ramp boundary of the salient. As the salient developed, the amount and direction of shortening were locally modified as listric detachment faults rotated some tight folds to the NW, and as right-lateral simple shear, caused by lock-up and folding of the Jefferson Canyon fault above the lateral ramp, rotated other folds northeastward. Where the lateral ramp and frontal-oblique ramp intersect, folds were rotated back to the NW. Our interpretation of dominant W–E to WNW–ESE shortening in the Lombard sheet, later altered by local rotations, supports a model of salient formation by primary parallel transport modified by interactions with a lateral ramp.     

Fold-Thrust-Belt Structure of the Proterozoic Eastern Ghats Mobile Belt: A Proposed Correlation Between India and Antarctica in Gondwana

The Proterozoic Eastern Ghats Mobile Belt along the east coast of India shares a thrusted lower contact with the surrounding cratons. The thrust, known as the Terrane Boundary shear zone, is associated with two large lateral ramps resulting in a curved outline on the northwestern corner of the mobile belt. The Eastern Ghats Mobile Belt is divided into two lithotectonic units, the Lathore Group and the Turekela Group, based on their lithological assemblages and deformational history. On the basis of published data from a Deep Seismic Sounding (DSS) profile of the Eastern Ghats crust, the Terrane Boundary Shear Zone is considered to be listric in nature and acts as the sole thrust between craton and mobile belt. The Lathore and Turekela Groups are nappes. With this structural configuration the NW part is described as a fold thrust belt. However, the thrusting postdates folding and granulite metamorphism that occurred in the Eastern Ghats, as in the Caledonide type of fold thrust belt of NW Scotland. The Terrane Boundary Shear Zone is interpreted to be contiguous with the Rayner-Napier boundary of the Enderby Land in a Gondwana assembly.     

Foreland- and hinterland-verging structures in fold-and-thrust belt: an example from the Variscan foreland of Sardinia

In the Variscan foreland of SW-Sardinia (Western Mediterranean sea), close to the leading edge of the nappe zone, nappe emplacement caused folding and repetition of stratigraphic successions, km-scale offset of stratigraphic boundaries and an extensive brittle-ductile shear zone. Thrusts assumed a significant role, accommodating a progressive change of shortening direction and forming complicated thrust triangle zones. During thrust emplacement of the nappes, strong penetrative deformation affected rocks beneath the basal thrust of the nappe stack and produced coeval structures with both foreland-directed and hinterland-directed (backthrusting) shear sense. Cross-cutting and overprinting relationships clearly show that the shortening direction changed progressively from N–S to E–W, producing in sequence: (1) E–W trending open folds contemporaneous with early nappe emplacement in the nearby nappe zone; (2) recumbent, quasi-isoclinal folds with axial plane foliation and widespread, “top-towards-the-SW”, penetrative shearing; (3) N–S trending folds with axial plane foliation, contemporaneous with late nappe emplacement; (4) backthrusts and related asymmetrical folds developed during the final stages of shortening, postdating foreland-verging structures. Structures at (3) and (4) occurred during the same tectonic transport “top-towards-the-E” of the nappe zone over the foreland. The several generations of folds, thrusts, and foliations with different orientations developed, result in a complex finite structural architecture, not completely explicable by the theoretical model proposed up to date.     

Mesoproterozoic ophiolitic mélange from the SE periphery of the Indian plate: U–Pb zircon ages and tectonic implications

The Kanigiri mélange within the Proterozoic Nellore–Khammam schist belt in southern Peninsular India includes ophiolitic fragments that represent the remnants of an oceanic plate. The ophiolitic units were accreted along a NE-trending suture that juxtaposes the Proterozoic Eastern Ghats Granulite Belt (EGGB) against the Archean Nellore Schist Belt of the Dharwar craton. The ophiolite components in the Kanigiri mélange include plagiogranites and gabbros which show mutually intrusive relations indicating their coeval nature. We report laser ablation-ICP-MS age data and REE geochemistry of zircons from the gabbro and granite. The zircons from both gabbro and granite show high REE contents, prominent HREE enrichment and a conspicuous negative Eu anomaly, suggesting a common melt source. Zircon REE abundances and normalized patterns show little intersample and intrasample variations. U–Pb dating of the zircons reveals prominent Mesoproterozoic ages for the plagiogranite, with the ca.1.33 Ga age of the Kanigiri ophiolitic mélange offering important clues for arc–continent collision during the final stages of amalgamation of the Columbia-derived fragments within the Neoproterozoic supercontinent assembly.