Emplacement kinematics of nepheline syenites from the Terrane Boundary Shear Zone of the Eastern Ghats Mobile Belt,west of Khariar,NW Orissa: Evidence from meso- and microstructures

Nepheline syenite plutons emplaced within the Terrane Boundary Shear Zone of the Eastern Ghats Mobile Belt west of Khariar in northwestern Orissa are marked by a well-developed magmatic fabric including magmatic foliation, mineral lineations, folds and S-C fabrics. The minerals in the plutons, namely microcline, orthoclase, albite, nepheline, hornblende, biotite and aegirine show, by and large, well-developed crystal faces and lack undulose extinction and dynamic recrystallization, suggesting a magmatic origin. The magmatic fabric of the plutons is concordant with a solid-state strain fabric of the surrounding mylonites that developed due to noncoaxial strain along the Terrane Boundary Shear Zone during thrusting of the Eastern Ghats Mobile Belt over the Bastar Craton. However, a small fraction of the minerals, more commonly from the periphery of the plutons, is overprinted by a solid state strain fabric similar to that of the host rock. This fabric is manifested by discrete shear fractures, along which the feldspars are deformed into ribbons, have undergone dynamic recrystallization and show undulose extinction and myrmekitic growth. The shear fractures and the magmatic foliations are mutually parallel to the C-fabric of the host mylonites. Coexistence of concordant solid state strain fabric and magmatic fabric has been interpreted as a transitional feature from magmatic state to subsolidus deformation of the plutons, while the nepheline syenite magma was solidifying from a crystal-melt mush state under a noncoaxial strain. This suggests the emplacement of the plutons synkinematic to thrusting along the Terrane Boundary Shear Zone. The isotopic data by earlier workers suggest emplacement of nepheline syenite at 1500 +3/−4Ma, lending support for thrusting of the mobile belt over the craton around that time.     

The tectonic generation of the Limpopo Mobile Belt,and a definition of its western extremity

A new mechanism is suggested for the generation of the interference fold pattern which characterizes the Limpopo Mobile Belt. The mechanism is directly related to shear movement along the Tuli-Sabi Shear Zone, renamed the Tuli-Sabi Straightening Zone. The mobile belt is regarded as a taphrogenic lineament (McConnell, 1974) and its generation is compatible with the tectonic environment active in Proterozoic times according to Sutton and Watson (1974). Field evidence shows that the Tuli-Sabi Straightening Zone dies out in Botswana at Moshakabela, and it is reasoned that the mobile belt as a whole also disappears in this vicinity. It does not extend into central and western Botswana beneath the Karroo and Kalahari cover. Detailed examination of ERTS-1 imagery of northeastern Botswana strengthens these deductions. The Tuli-Sabi Straightening Zone and the characteristic fold patterns of the mobile belt can be seen quite clearly on the satellite imagery. Furthermore, the Tuli-Sabi Straightening Zone appears to be displaced southwards at the international boundary between Botswana and Rhodesia. The existence of a fold belt trending about N150° superimposed on the Limpopo Mobile Belt in the west of the area is postulated which is not the Shashe Mobile Belt (Crockett, 1967).     

Tectonic Implication of the Apatite Fission-track Analysis of the Mylonites from the Terrane Boundary Shear Zone of the Eastern Ghats Mobile Belt Around Lakhna,Orissa

Apatite fission-track analysis of the rocks within and adjoining the Terrane Boundary Shear Zone of the Eastern Ghats Mobile Belt, India, yield apparent ages ranging from 340246 to 268234 Ma. They are interpreted to be the result of slow (Ordovician to Recent) coolinglexhumation. Erosion rates are calculated at approximately 0.5–0.25″ C/My. Genetic algorithm modeling suggests the possibility of a minor heating event at approximately 120 Ma; this is the time when the region was passing over Kerguelen hot spot as the Indian plate separated from Antarctica. The rocks within and outside the shear zone do not show any difference in age suggesting that there has been no movement (reactivation) along the shear zone during this drifting. Based on this assumption, the slightly higher rates of cooling, following this event, are attributed to continued slow denudation as well as thermal relaxation of the continent subsequent to hot spot influence.     

Structural fabric of the Southern Indian shield as defined by gravity trends

Southern Indian shield represents a mosaic comprised of several smaller structural domains separated by discrete shear zones. Here we present a horizontal Bouguer gravity gradient map of the Indian shield, south of 14 °N, to define a continental mosaic of gravity trends domains akin to structural domains. The gravity gradient image is based on 7862 newly collected observations merged with 6359 old gravity data. This combined dataset delineates structural boundaries of the five gravity domains related to the Eastern Dharwar Craton, the Eastern Ghats Mobile Belt, the extended Eastern Ghats Mobile Belt, the Southern Granulite Terrain, and the Western Dharwar Craton. Other belts of significant gravity gradients are found associated with the Eastern and the Western coasts. The loci of Closepet granite and Kolar schist belts do not manifest themselves as boundary zones between two distinct gravity domains of the Eastern Dharwar Craton. Lack of a gravity gradient across Karur–Oddanchatram–Kodaikanal and Karur–Kambam–Painavu–Trichur Shear Zones may be attributed to a lack of gravity measurements caused by difficulties in collecting data in topographically difficult terrain. The subdued gravity gradient across the Palghat–Cauvery Shear Zone and a weak gradient across the Achankovil Shear Zone indicates a lithological and/or morphological boundary rather than a terrane boundary. Alternatively, structural domains encompassing Palghat–Cauvery and Achankovil Shear Zones may have been in a neighbouring position during the Gondwana assembly, when Pan-African thermal perturbation reactivated the structures and reworked partly or totally obliterating earlier crustal fabric.     

THRUST PACKAGES OF 1.68 Ga INDIAN SUPRA-CRUSTAL ROCKS IN THE MIOCENE SIWALIK BELT,CENTRAL NEPAL HIMALAYAS

THRUST PACKAGES OF 1.68 Ga INDIAN SUPRA-CRUSTAL ROCKS IN THE MIOCENE SIWALIK BELT,CENTRAL NEPAL HIMALAYAS     

Differential Tectonic Deformation of the Longmen Mountain Thrust Belt,Western Sichuan Basin,China

Field investigation and seismic section explanation showed that the Longmen Mountain Thrust Belt has obvious differential deformation: zonation, segmentation and stratification. Zonation means that, from NW to NE, the Longmen Mountain Thrust Belt can be divided into the Songpan-Garzê Tectonic Belt, ductile deformation belt, base involved thrust belt, frontal fold-thrust belt, and foreland depression. Segmentation means that it can be divided into five segments from north to south: the northern segment, the Anxian Transfer Zone, the center segment, the Guanxian Transfer Zone and the southern segment. Stratification means that the detachment layers partition the structural styles in profile. The detachment layers in the Longmen Mountain Thrust Belt can be classified into three categories: the deep-level detachment layers, including the crust-mantle system detachment layer, intracrustal detachment layer, and Presinian system basal detachment layer; the middle-level detachment layers, including Cambrian-Ordovician detachment layer, Silurian detachment layer, etc.; and shallow-level detachment layers, including Upper Triassic Xujiahe Formation detachment layer and the Jurassic detachment layers. The multi-level detachment layers have a very important effect on the shaping and evolution of Longmen Mountain Thrust Belt.     

Structure of the Nagavali Shear Zone,Eastern Ghats Mobile Belt,India: Correlation in the East Gondwana Reconstruction

Interpretation of satellite data in combination with regional field traverses, delineating the major structural features such as the Nagavali and Vamsadhara Shear Zones and associated fold patterns, provides a synoptic picture of the regional tectonic framework of the central part of the Eastern Ghats Mobile Belt. The complex geology of the study area can broadly be grouped into three distinct deformational events. D₁ fabrics represented by near flat-lying gneissic foliations, paralleling the lithological banding are best preserved in low strain domains and are related to Middle to late Archaean thrusting (3000-2600 Ma). The second deformational event D₂ is characterized by the development of shear zones and associated mylonitic fabrics and magmatism probably during 1450-850 Ma. The Pan-African thermal (500-550 Ma) overprint is restricted to shear zones in the form of reworking. Regionally, the central part of the Eastern Ghats Mobile Belt can be divided into five distinct structural domains based on structural geometry of folds, foliations and lineations. A three-dimensional block diagram of the Nagavali and Vamsadhara Shear Zones involving fold-thrust tectonics associated with westward thrusting is presented here. A correlation of Pan-African Shear Zones in adjacent continents wrapping around the Archaean Dharwar Craton in the reconstruction of Rodinia and East Gondwana supercontinent suggests an east-west convergence.     

Transpressive Structures in the Ghadir Shear Belt,Eastern Desert,Egypt: Evidence for Partitioning of Oblique Convergence in the Arabian‐Nubian Shield during Gondwana Agglutination

Transpressional deformation has played an important role in the late Neoproterozoic evolution of the ArabianNubian Shield including the Central Eastern Desert of Egypt. The Ghadir Shear Belt is a 35 km-long, NW-oriented brittleductile shear zone that underwent overall sinistral transpression during the Late Neoproterozoic. Within this shear belt, strain is highly partitioned into shortening, oblique, extensional and strike-slip structures at multiple scales. Moreover, strain partitioning is heterogeneous along-strike giving rise to three distinct structural domains. In the East Ghadir and Ambaut shear belts, the strain is pure-shear dominated whereas the narrow sectors parallel to the shear walls in the West Ghadir Shear Zone are simple-shear dominated. These domains are comparable to splay-dominated and thrust-dominated strike-slip shear zones. The kinematic transition along the Ghadir shear belt is consistent with separate strike-slip and thrustsense shear zones. The earlier fabric(S1), is locally recognized in low strain areas and SW-ward thrusts. S2 is associated with a shallowly plunging stretching lineation(L2), and defines ～NW-SE major upright macroscopic folds in the East Ghadir shear belt. F2 folds are superimposed by ～NNW–SSE tight-minor and major F3 folds that are kinematically compatible with sinistral transpressional deformation along the West Ghadir Shear Zone and may represent strain partitioning during deformation. F2 and F3 folds are superimposed by ENE–WSW gentle F4 folds in the Ambaut shear belt. The sub-parallelism of F3 and F4 fold axes with the shear zones may have resulted from strain partitioning associated with simple shear deformation along narrow mylonite zones and pure shear-dominant deformation in fold zones. Dextral ENEstriking shear zones were subsequently active at ca. 595 Ma, coeval with sinistral shearing along NW-to NNW-striking shear zones. The occurrence of upright folds and folds with vertical axes suggests that transpression plays a significant role in the tectonic evolution of the Ghadir shear belt. Oblique convergence may have been provoked by the buckling of the Hafafit gneiss-cored domes and relative rotations between its segments. Upright folds, fold with vertical axes and sinistral strike-slip shear zones developed in response to strain partitioning. The West Ghadir Shear Zone contains thrusts and strikeslip shear zones that resulted from lateral escape tectonics associated with lateral imbrication and transpression in response to oblique squeezing of the Arabian-Nubian Shield during agglutination of East and West Gondwana.     

Tectonic setting of the Balaram-Kui-Surpagla-Kengora granulites of the South Delhi Terrane of the Aravalli Mobile Belt,NW India and its implication on correlation with the East African Orogen in the Gondwana assembly

Granulites are developed in various tectonic settings and during different geological periods, and have been used for continental correlation within supercontinent models. In this context the Balaram-Kui-Surpagla-Kengora granulites of the South Delhi Terrane of the Aravalli Mobile Belt of northwestern India are significant. The granulites occur as shear zone bounded lensoidal bodies within low-grade rocks of the South Delhi Terrane and comprise pelitic and calcareous granulites, a gabbro-norite-basic granulite suite and multiple phases of granites of the Ambaji suite. The granulites have undergone three major phases of folding and shearing. The F₁ and F₂ folds are coaxial along NE-SW axis, and F₃ folds are developed across the former along NW-SE axis. Thus, various types of interference patterns are produced. The granulite facies metamorphism is marked by a spinel–cordierite–garnet–sillimanite–quartz assemblage with melt phase and is synkinematic to the F₁ phase of folding. The peak thermobarometric condition is set at ≥850 °C and 5.5–6.8 kb. The granulites have been exhumed through thrusting along multiple ductile shear zones during syn- to post-F₂ folding. Late-stage shearing has produced cataclasites and pseudotachylites. Sensitive High Resolution Ion MicroProbe (SHRIMP) U–Pb dating of zircon from pelitic granulites and synkinematically emplaced granites indicate that: (1) the sedimentary succession of the South Delhi Terrane was deposited between 1240 and 860 Ma with detritus derived from magmatic sources with ages between 1620 and 1240 Ma; (2) folding and granulite metamorphism have taken place between ca. 860 and 800 Ma, and exhumation at around ca. 800–760 Ma; and (3) the last phase of granitic activity occurred at ca. 759 Ma. This shows, for the first time, that the granulites of the South Delhi Terrane are much younger than those of the Sandmata Granulite Complex of the northern part of the Aravalli Mobile Belt, the Saussar granulites of the Central India Mobile Belt and the Eastern Ghats Mobile Belt. Instead, they show similarities to the Neoproterozoic granulites of the Circum Indian Orogens that include the East African Orogen (East Africa and Madagascar), the Southern Granulite Terrane of India and much of Sri Lanka. We suggest that the South Delhi Basin probably marks a trace of the proto-Mozambique Ocean in NW India within Gondwana, that closed when the Marwar Craton, arc fragments (Bemarivo Belt in Madagascar and the Seychelles) and components of the Arabian-Nubian Shield collided with the Aravalli-Bundelkhand Protocontinent at ca. 850–750 Ma.     

Structural architecture of the northern composite terrane, the Eastern Ghats Mobile Belt, India: Implications for Gondwana tectonics

New data from structural mapping and tectonic evaluation in the northern parts of the Eastern Ghats Mobile Belt (EGMB-north) involving the interpretation of satellite images, field traverses, critical outcrop mapping and kinematic studies of macro- as well as microstructures of the shear zone rocks together with the geometry and disposition of Gondwana basins led to, for the first time, the elucidation of post-Grenvillian structural architecture of the terrane. This helps in assessing the sequence of successive tectonothermal events that were responsible for the origin and progressive evolution of the Permo-Carboniferous coal bearing sediments along the Mahanadi rift that forms significant in the reconstruction models of east Gondwana.The composite terrane of high-grade metamorphic rocks (EGMB-north), strikes E–W in contrast to the regional NE–SW trend of the EGMB. The structural architecture obtained from this study is controlled by the boundary shear zones and associated link shear zones. The dextral kinematic displacements along the Northern Boundary Shear Zone (NBSZ) as well as the Mahanadi Shear Zone (MSZ) and Koraput–Sonapur–Rairakhol Shear Zone (KSRSZ) were derived from multi-scale field based structural observations. A N–S structural cross-section presents a crustal-scale ‘flower structure’ across the composite terrane exposing different domains displaying distinctive internal structures with widely varying different geological evolution history and strain partitioning, separated by crustal-scale shear zones. Deep seismic imaging and gravity signatures support ‘flower structure’ model. The pervasive first formed gneissic fabrics were continuously reworked and partitioned into a series of E–W, crustal-scale shear zones.The Neoproterozoic regional dextral transpressional tectonics along the shear zones and their repeated reactivation could be responsible for initiation and successive evolution of Gondwana basins and different episodes of sedimentation. Available geochronological data shows that the structural architecture presented here is post-Grenvillian, which has been repeatedly reactivated through long-lived transpressional tectonics. The composite terrane is characterized by all the typical features of an oblique convergent orogen with transpressional kinematics in the middle to lower crust. The kinematic changes from transpression to transtensional stresses were found to be associated with global geodynamics related to the transformation from Rodinia to Gondwana configuration.     

Deformation Pattern in the Kurnool and Nallarnalai Groups in the Northeastern Part (Palnad Area) of the Cuddapah Basin, South India and its Implication on Rodinia/Gondwana Tectonics

The Proterozoic basins of India adjoining the Eastern Ghats Granulite Belt (EGGB) in eastern and southern India contain both Mesproterozoic and Neoproterozoic successions. The intracratonic set-up and contractional deformation fo the Neoproterozoc successions in the Paland sub-basin in the northeastern part of Cuddapah basin and similar crustal shortening in contemporaneous successions lying west of the EGGB and Nellore Schist Belt (NSB) are considered in relation to the proposed geodynamic evolution of the the Rodinia and Gondwana supercontinents. Tectonic shortening in the Palnad sub-basin (northeast Cuddapah), partitioned into top-to-westnorthwest thrust shear, flexural folds and cleavage development under overall E-W contraction, suggests foreland style continental shortening within an intracratonic set-up. A thrust sheet containing the Nallamalai rocks and overlying the Kurnool rocks in the northeastern part of Palnad sub-basin exhibits early tight to isoclinal folds and slaty (phylllitic) cleavage, which can be correlated with early Mesoproterozoic deformation structures in the nothern Nallamalai Fold Belt (NFB). NNE-SSW trending folds and cleavage affect the Kurnool Group and overprint earlier structures in the thrust sheet. Thrusting of the Nallamalai rocks and the later structures may have been related to convergence of the Eastern Ghats terrane and the East-Dharwar-Bastar craton during Early Neoproterozoic (Greenvillian) and/or later rejuvenation related to Pan-African amalgamation of East and West Gondwana.     

Lachlan Fold Belt in New South Wales

The Lachlan Fold Belt is a Middle Palaeozoic orogenic belt in which terminal tectogenesis occurred during the Early Carboniferous (Kanimblan Orogeny). This fold belt went through a complicated tectonic history and developed from the stratotectonic Lachlan Marginal Mobile Zone (or geosyncline of other authors). The Lachlan Fold Belt can be divided into structural zones which are characterized by varying tectonic styles. Zones of intensive deformation alternate with less deformed zones.The formation of the Lachlan Fold Belt may be viewed in terms of a series of tensional and compressional deformational events with the major compressional or tensional stress maintaining an approximate east—west orientation (relative to the grain of the fold belt) for the life of the Lachlan Marginal Mobile Zone.     

雪峰山早中生代构造演化:构造学和年代学分析木

雪峰山主体地处湖南省境内,位于华南板块的中心区域,是一条典型的陆内造山带.通过详细的野外地质观察,我们将其分为3个构造单元:西部外区,主要以大型箱状褶皱为主;中部区,与西部区以主逆冲断层相分隔,劈理发育呈扇状,是雪峰山构造带的核心区域,也是变质级别最深、变形最强的区域;东部区,变形集中在脆韧性区域之上,以极性北西构造为...     

Structural architecture of the northern composite terrane,the Eastern Ghats Mobile Belt,India: Implications for Gondwana tectonics

New data from structural mapping and tectonic evaluation in the northern parts of the Eastern Ghats Mobile Belt (EGMB-north) involving the interpretation of satellite images, field traverses, critical outcrop mapping and kinematic studies of macro- as well as microstructures of the shear zone rocks together with the geometry and disposition of Gondwana basins led to, for the first time, the elucidation of post-Grenvillian structural architecture of the terrane. This helps in assessing the sequence of successive tectonothermal events that were responsible for the origin and progressive evolution of the Permo-Carboniferous coal bearing sediments along the Mahanadi rift that forms significant in the reconstruction models of east Gondwana.The composite terrane of high-grade metamorphic rocks (EGMB-north), strikes E–W in contrast to the regional NE–SW trend of the EGMB. The structural architecture obtained from this study is controlled by the boundary shear zones and associated link shear zones. The dextral kinematic displacements along the Northern Boundary Shear Zone (NBSZ) as well as the Mahanadi Shear Zone (MSZ) and Koraput–Sonapur–Rairakhol Shear Zone (KSRSZ) were derived from multi-scale field based structural observations. A N–S structural cross-section presents a crustal-scale ‘flower structure’ across the composite terrane exposing different domains displaying distinctive internal structures with widely varying different geological evolution history and strain partitioning, separated by crustal-scale shear zones. Deep seismic imaging and gravity signatures support ‘flower structure’ model. The pervasive first formed gneissic fabrics were continuously reworked and partitioned into a series of E–W, crustal-scale shear zones.The Neoproterozoic regional dextral transpressional tectonics along the shear zones and their repeated reactivation could be responsible for initiation and successive evolution of Gondwana basins and different episodes of sedimentation. Available geochronological data shows that the structural architecture presented here is post-Grenvillian, which has been repeatedly reactivated through long-lived transpressional tectonics. The composite terrane is characterized by all the typical features of an oblique convergent orogen with transpressional kinematics in the middle to lower crust. The kinematic changes from transpression to transtensional stresses were found to be associated with global geodynamics related to the transformation from Rodinia to Gondwana configuration.     

Shear-zone controlled basins in the Blouberg area, Northern Province, South Africa: syn- and post-tectonic sedimentation relating to ca. 2.0 Ga reactivation of the Limpopo Belt

The extent of the deposition and of the preservation of the Blouberg Formation and Waterberg Group was at least partially controlled by brittle reactivation along the Palala Shear Zone. The Palala Shear Zone in the Blouberg area (Northern Province, South Africa) is characterised by granulite-grade gneiss, and formed by sinistral transpressional collision between the Southern Marginal Zone (Kaapvaal Craton) and the Central Zone of the Limpopo Belt. The Limpopo collision is thought to have occurred either at 2.0 Ga or at 2.7 Ga with reactivation at 2.0 Ga. Deposition of the Blouberg Formation was characterised by syn-sedimentary tectonism, which is reflected by a sudden upward coarsening in sedimentary rocks, and by the presence of a strongly folded and thrusted lower member. Bedding orientations and slickenside lineation orientations suggest that vergence was towards the south, and such a tectonism can be inferred to have produced a highland area to the north, bound on the southern margin by the southern strand of the Melinda Fault. The presence of an inferred northerly upland area is supported by palaeocurrent directions and the preservational extent of the Setlaole and Makgabeng Formations of the Waterberg Group (post-Blouberg Formation). The extent and stratigraphy of the overlying Mogalakwena Formation suggests that these strata onlapped northwards over the denuding highlands. Younger Sibasa basalts of the Soutpansberg Group have been dated at ca. 1.85 Ga. Blouberg and Waterberg strata can therefore be interpreted as syn- and post-tectonic sedimentary rocks, respectively, following a ca. 2.0 reactivation event along the Palala Shear Zone. It is difficult to reconcile the succession of geological events at Blouberg with a ca. 2.0 Ga Limpopo orogeny, and thus sedimentary strata in the study area support a 2.7 Ga date for Limpopo collision, with syn-Blouberg tectonism relating to ca. 2.0 reactivation within the previously assembled Limpopo Belt.     

Quantifying 3D post-accretionary tectonic strain in the Arabian–Nubian Shield: Superimposition of the Oko Shear Zone on the Nakasib Suture,Red Sea Hills,Sudan

Deformed conglomeratic clasts exposed along the Neoproterozoic Nakasib Suture and the Oko Shear Zone are used to calculate three-dimensional (3D) tectonic strain associated with the latter to quantify strain associated with post-accretionary deformational belts in the Arabian–Nubian Shield. The Nakasib Suture is a NE-trending fold and thrust belt that is sinistrally offset (∼10 km) by the cross-cutting NNW- to NW-trending strike-slip faults of the Oko Shear Zone. The Nakasib Suture was formed as a result of collision between the Haya terrane and the Gebeit terrane at ∼750 Ma ago. The Oko Shear Zone was subsequently formed as a result of an E–W directed shortening of the Arabian–Nubian Shield due to collision between East and West Gondwana at ∼670–610 Ma ago. This analysis indicates the following: (1) The Nakasib Suture is dominated by flattening strain with the flattening plane of the associated strain ellipsoid oriented at 21°/77°SE. This flattening deformation is interpreted to be associated with nappe emplacement from north to south. (2) Some regions along the Nakasib Suture are characterized by constriction strain that might be due to refolding of the early nappes about NE-trending axes. (3) The Oko Shear Zone is characterized by constriction strain, with the XY plane of the strain ellipsoid oriented at 171°/68°E. The strain ellipsoid associated with the Oko Shear Zone manifests superimposition of E–W shortening on the NE-trending fold and thrust belt associated with the Nakasib Suture. (4) The tectonic strain of the Oko Shear Zone, superimposed over the structures of the Nakasib Suture, is characterized by a strain ellipsoid whose flattening plane is oriented at 21°/49°W. The strain ellipsoid of the tectonic strain has a major axis with a quadratic elongation of 3.6 and an orientation of 357°/25°, an intermediate axis with a quadratic elongation of 1.2 and an orientation of 231°/30°, and a minor axis with a quadratic elongation of 0.25 and an orientation of 115°/18°. This suggests that the post-accretionary deformation of the Arabian–Nubian Shield was superimposed as a NW–SE directed shortening that created early N–S shortening zones and late NW-trending sinistral strike-slip faults.     

Development of the Khao Khwang Fold and Thrust Belt: Implications for the geodynamic setting of Thailand and Cambodia during the Indosinian Orogeny

The Indosinian Orogeny in Thailand is often viewed as having developed between strongly linear terranes, which today trend approximately N–S. The terranes were subsequently disrupted by later tectonics, particularly NW–SE trending Cenozoic strike-slip faults. The ENE–WSW to NE–SW striking thrusts and folds in the Khao Khwang Platform area of the Saraburi Group on the SW margin of the Indochina Terrane are not easily explained in the context of this traditional view. Reversal of the clockwise rotation shown to have affected the block north of the Mae Ping Fault zone only enhances the E–W orientation of structures in the fold and thrust belt, and moves the belt further east towards Cambodia. One solution for the trend that fits better with regional understanding from hydrocarbon exploration of the Khorat Plateau is that the Indochina Terrane was actually a series of continental blocks, separated by Permian rifting. During the Early Triassic the early stages of collision (South China-Cathaysian Terrane collision with Vietnam Indochina) resulted in the amalgamation of disparate blocks that now form the Indochina Terrane by closure along the rifts. At the same time or following on from the collision there was closure of the back-arc area between Indochina and the Sukhothai zone. The rift basins, were thrusted and inverted during the early stages of the Indosinian orogeny, and only underwent minor reactivated when later Sibumasu collided with Sukhothai Zone-Indochina Terrane margin during the Late Triassic. The scenario described above requires the presence of a (minor) E–W trending suture in NW Cambodia. Evidence for this suture is suggested by the presence of Permo-Triassic calc-alkaline volcanism.     

Geochronology and Tectonic Evolution of Karimnagar and Bhopalpatnam Granulite Belts, Central India

The Karimnagar Granulite Belt (KGB) and the Bhopalpatnam Granulite Belt (BGB) occur along both flanks of the Pranhita-Godavari (PG) rift basin. We present a state-of-the-art overview on the geochronological and tectonic aspects of these belts and surrounding geologic domains, and report new age data on zircon, monazite and uraninite recovered from granulite facies assemblages from KGB and BGB based on electron microprobe analyses (EPMA). Zircons from KGB charnockites show core ages of up to 3.1 Ga mantled by rims of 2.6 Ga. Zircons from BGB have 1.9 Ga cores mantled by 1.7 Ga rims. Zircons with core ages of 1.6 to 1.7 Ga in BGB rocks suggest new growth at this time. Monazites and uranitite from KGB show clear peaks with well-defined ages in the narrow range between 2.42±0.08 Ga and 2.47°0.03 Ga. Rims of monazite show mean age of 2.21±0.08 Ga. Monazites from BGB define sharp linear trend in PbO vs. ThO₂* diagram delineating a clear isochron with age of 1.59±0.03 Ga. Age data from KGB and BGB presented in this report negate current models linking these terrains to "Godavari Granulite Belt" and considering them as single and contemporaneous entity. The mid-Archaean to early Palaeoproterozoic signature recognized from KGB is totally missing in BGB. On the other hand, KGB rocks do not record any evidence for major Mesoproterozoic thermal regime. The two granulite belts shouldering the PG rift basin have therefore evolved in different times under distinct P-T conditions and thermal regimes. Our results have important implications in evaluating models of supercontinent assemblies, particularly the older assemblies of Ur, Columbia and Rodinia. While tectonothermal events in KGB broadly match with those of East Dharwar, we propose that BGB represents a 1.6 Ga collisional mobile belt between the Bastar and the Dharwar cratons. The 1.6 Ga collisional mobile belt at the southern margin of the Bastar craton was superposed by rift activity along the PG basin at 1.5 Ga. This sequence of events goes against the existence of a 3.0 Ga old contiguous assembly of Ur but closely matches with the history of accretion and break-up of the Columbia. Further, parts of the PG basin located away from the influence of the Eastern Ghats Mobile Belt, neither recorded any Grenville ages (1.0 Ga) corresponding to the Rodinia accretion nor late Pan-African ages (ca. 550 Ma) relating to the Gondwana amalgamation, indicating that the region did not witness any of these younger tectonic events.     

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.     

Structural Characteristics and Formation Mechanism in the Micangshan Foreland,South China

Lying at the junction of the Dabashan, Longmenshan and Qinling mountains, the Micangshan Orogenic Belt coupled with a basin is a duplex structure and back-thrust triangular belt with little horizontal displacement, small thrust faults and continuous sedimentary cover. On the basis of 3D seismic data, and through sedimentary and structural research, the Micangshan foreland can be divided into five subbelts, which from north to south are: basement thrust, frontal thrust, foreland depression-back-thrust triangle, foreland fold belt or anticline belt, and the Tongjiang Depression. Along the direction of strike from west to east, the arcuate structural belt of Micangshan can be divided into west, middle and east segments. During the collision between the Qinling and Yangtze plates, the Micangshan Orogenic Belt was subjected to the interaction of three rigid terranes: Bikou, Foping, and Fenghuangshan (a.k.a. Ziyang) terranes. The collision processes of rigid terranes controlled the structural development of the Micangshan foreland, which are: (a) the former collision between the Micangshan-Hannan and Bikou terranes forming the earlier rudiments of the structure; and (b) the later collision forming the main body of the structural belt. The formation processes of the Micangshan Orogenic Belt can be divided into four stages: (1) in the early stage of the Indosinian movement, the Micangshan-Hannan Rigid Terrane was jointed to the Qinling Plate by the clockwise subduction of the Yangtze Plate toward the Qinling Plate; (2) since the late Triassic, the earlier rudiments of the Tongnanba and Jiulongshan anticlines and corresponding syncline were formed by compression from different directions of the Bikou, Foping and Micangshan-Hannan terranes; (3) in the early stage of the Himalayan movement, the Micangshan-Hannan Terrane formed the Micangshan Nappe torwards the foreland basin and the compression stresses were mainly concentrated along both its flanks, whereas the Micangshan-Hannan Terrane wedged into the Qinling Orogenic Belt with force; (4) in the late stage of the Himalayan movement, the main collision of the Qinling Plate made the old basement rocks of the terrane uplift quickly, to form the Micangshan Orogenic Belt. The Micangshan foreland arcuate structure was formed due to the non-homogeneity of terrane movement.