Dynamic Features of Angular Unconformity Formation——Extensional and Compressional Angular Unconformities

Angular unconformity is one of the most direct and strongest evidences for approving the tectonic movements of the earth's crust. Its dynamic genesis process has been understood to be mainly related to the compressional setting for a long time. Especially, in a detailed structural analysis for a specific region, when an angular unconformity is discovered people would regard it as the result of orogenic movements of a certain period or a certain episode and neglect the extensional facts. Based on a dialectical point of view of extension-compression, this paper has proved that angular unconformities can be formed not only in compressional settings, but also in extensional ones. Further more, their geological features are compared and he possible genetic mechanisms for angular unconformity under different dynamic settings are studied.

Exploration Significance of Unconformity Structure on Subtle Pools

Vertical structure of an unconformity can be divided into three layers:basal conglomerate or transgressive sand,weathered clay layer and leached rock.When the weathered clay layer has a little thickness or limited distribution,the overlying and underlying strata will contact directly,and the lithology is often different.This lithologic difference causes different fluid transporting capacity,and it also

A STRATIGRAPHIC UNCONFORMITY SURFACE BETWEEN THE NEOGENE AND THE PALEOGENE IN THE JIYANG DEPRESSION,BOHAI BAY BASIN, EASTERN CHINA:GEOLOGIC CHARACTERISTICS, FORMATIONPROCESS AND GEODYNAMIC ORIGIN

Regional stratigraphic unconformity surfaces are commonly observed in both orogenic beltsand sedimentary basins. They have superficially simple features, but in fact contain abundant geo-logic and geodynamic in formation on basin evolution, which was rarely systematically studied.Based on the elaborate research on geologic characteristics of the stratigraphic unconformity sur-face between the Neogene and the Paleogene in the Jiyang depression, Bohai Bay basin, EasternChina, this paper reconstructed its formation process and paleotopography in the late Oligocene. ltis the most interesting that this stratigraphic uncon formity surface represents the transiton of theJiyang depression evolution from rifting extension to thermal down-warping and also implies an u-plift process during the transition. This paper thought it worthy to study the origin of thisstratigraphic uncohformity surface and its relation to the stage transition of basin evolution. ThenpossibIe influences of regional stress field and deep dynamic process variations on the stratigraphicunconforrnity surface formation were anaIyzed. The stratigraphic unconformity surface betweenthe Neogene and the Paleogene in the Jiyang depression should be the direct result of extremestretching of llthosphere and diapirism of hot upper mantle, which can also possibly explain the co-incidence of the unconformity surface formation with the stage transition of basin evolution. Fur-thermore. the influences of the stratigraphic unconformity surface formation on oil and gas accu-mulation, the erosion amount calculation, and the elaborate time structure recorded by thestratigraphic unconformity surface were discussed.

The role of the Mid-Cimmerian Unconformity on the quality of the underlying Skagerrak sandstone reservoir in the Kittiwake Field,central North Sea

The central aim of this paper is to address the role of unconformities in affecting reservoir quality.Do they facilitate diagenesis that leads to either enhanced or reduced porosity through dissolution or cementation?Or,do they have little effect?We have investigated the Late Triassic Skagerrak sandstone reservoir underlying the Mid-Cimmerian Unconformity in the Kittiwake Field,central North Sea.There is strong evidence for the development of secondary porosity through the dissolution of unstable silicate minerals,primarily feldspars.This includes the presence of oversized pores,partial dissolution of framework grains,and complete dissolution of grains leaving remnant grain margins and partially filled cores.This dissolution as a late-stage event is demonstrated by the complete lack of compaction effects on the secondary pores and diagenetic products despite present burial depths in excess of 3000 m.These observations,coupled with an absence of systematic trends linked to the unconformity surface in respects of reservoir porosity,feldspar amount and dissolution,and kaolinization,lead to the conclusion that there has been no effect of the Mid-Cimmerian Unconformity on reservoir quality in the Kittiwake Field.There is no evidence for leaching or cementation linked to meteoric water influx either shortly after deposition or following the uplift and exposure,which led to development of the Mid-Cimmerian Unconformity.Instead,we propose that the late-stage dissolution of feldspar and generation of secondary porosity are most likely related to the influx of organic acids and carbon dioxide generated either from thermogenic maturation of the source rock or from biodegradation of oil within the reservoir near the oil-water contact(OWC)transition zone.

Is there a great unconformity between Xiamaling and Longshan formations in the North China Craton?

The existence of a 0.3 Ga"great unconformity"between the Mesoproterozoic"Undefined System"Xiamaling Formation and the"Neoproterozoic"Longshan Formation has long been controversial.In this study,stratigraphy,sedimentology,detrital zircon dating,lithium isotope,and major and trace elements were applied to systematically analyze their relationship.Results demonstrate that coarse to fine sandstone-mudstone normal cycles with different grades and thicknesses exist from the Xiamaling to the Longshan formations.Sedimentary succession formed from a restricted platform with lagoon-shallow marine shelf-tidal flat-beach and tidal shoal developing in ascending order.The pebbly sandstone at the bottom of the Long shan Formation does not have the properties of basal conglomerate with tectonic significance.The youngest detrital zircon is older than 1.6 Ga for the Long shan Formation and overlying Jing'eryu Formation sandstones.In addition,no(zircon)provenance has been found from the large igneous province of 1.32-1.2 Ga in Xiamaling Formation.Theδ^(7)Li isotope values exhibit a gradually increasing trend.The Li content and CIA(Chemical index of alteration)gradually decreases from the top of the Xiamaling Formation to the lower part of the Long shan Formation.Simultaneously,as K_(2)O/Al_(2)O_(3)and FeO/Fe_(2)O_(3)decreases,Na_(2)O/Al_(2)O_(3)increases,and Li/Al,V/Cr,and V/(V+Ni)fluctuates slightly.At the boundary,the Ce/Ce^(*)and Eu/Eu^(*)show positive and negative anomalies,respectively.Further,the Rb/Sr and Al/Si values decrease,and U/Th values increase.This indicates that the geochemistry of the weathering crust at the interface of the Xiamaling Formation-Long shan Formation is not characteristic.However,it indicates a weakening of continental weathering intensity from the bottom to the top of the interface.These results do not support the existence of a large unconformity between the Xiamaling and the Longshan formations.Therefore,it is suggested that the Long shan Formation-Jing'eryu Formation should be moved to the lower level in the stratigraphic chart,where it should form part of the Mesoproterozoic"Undefined System"along with the Xiamaling Formation.On this basis,the paleogeography of the Xiamaling Formation-Longshan Formation-Jing'eryu Formation sedimentary period in the Yanliao area of North China has been reclassified.

The unconformity in Miocene sequence of western Qaidam Basin and its tectonic significance

An angular unconformity has been identified between the upper and the lower Youshashan Formation based on the study of the regional geology, drillhole, and geophysical data of the Altyn Tagh Mountain foreland, western Qaidam Basin. The distribution of the unconformity, the contacts between the upper and the lower Youshashan Formation, and its relation to the characteristics of the paleo-structure belts suggest a close relationship between the unconformities in the southern flank of the Altyn Tagh Mountain in the west of the Qaidam Basin and the activity of the Altyn Tagh Fault. It was the same time when the Atlyn Tagh Fault Zone was transformed from regional uplifting to rapid-strike-slipping movement during the Miocene, resulting in the cessation of the regional uplift and subsequent massive regional transgression during the deposition of the upper Youshashan Formation, which unconformably overlies (onlaps) the underlying strata of different ages.

Unconformity structures controlling stratigraphic reservoirs in the north-west margin of Junggar basin, North-west China

Tectonic movements formed several unconfor- mities in the north-west margin of the Junggar basin. Based on data of outcrop, core, and samples, the unconformity is a structural body whose formation associates with weath- ering, leaching, and onlap. At the same time, the structural body may be divided into three layers, including upper layer, mid layer, and lower layer. The upper layer with good primary porosity serves as the hydrocarbon migration system, and also accumulates the hydrocarbon. The mid layer with compactness and ductility can play a role as cap rock, the strength of which increases with depth. The lower layer with good secondary porosity due to weathering and leaching can form the stratigraphic truncation traps. A typical stratigraphie reservoir lying in the unconformity between the Jurassic and Triassic in the north-west margin of the Junggar basin was meticulously analyzed in order to reveal the key controlling factors. The results showed that the hydrocarbon distribution in the stratigraphic onlap reservoirs was controlled by the onlap line, the hydro- carbon distribution in the stratigraphic truncation reser- voirs was confined by the truncation line, and the mid layer acted as the key sealing rock. So a conclusion was drawn that ＂two lines （onlap line and truncation line） and a body （unconformity structural body）＂ control the formation and distribution of stratigraphic reservoirs.

Discovery of Neoarchean unconformity and its implication for continental cratonization of North China craton

An ancient unconformity event is identified within the granulite-facies province of North China, which is marked by the boundary between the overlying khondalite series (KS) and the basement grey gneiss complex (BGGC). REE patterns of KS suggest that their protoliths aM mature sediments. As documented by isotopic ages and metamorphic P-T-t paths, KS is originally deposited after the basement complex formed. Furthermore, the BGGC was overprinted by the metamorphic event associated with the KS. The binary structure of KS-BGGC is well preserved in the western part of North China craton (NCC), and it is structurally truncated and overlain by the Palaeoproterozoic Wntai rift, which suggests that the unconformity occurred in the Neoarchean, and represents the major tectono-thermal episodes of the Neoarchean continental cratonization of NCC.

Tectonic features,genetic mechanisms and basin evolution of the eastern Doseo Basin,Chad

The features of the unconformity,fault and tectonic inversion in the eastern Doseo Basin,Chad,were analyzed,and the genetic mechanisms and basin evolution were discussed using seismic and drilling data.The following results are obtained.First,four stratigraphic unconformities,i.e.basement(Tg),Mangara Group(T10),lower Upper Cretaceous(T5)and Cretaceous(T4),four faulting stages,i.e.Barremian extensional faults,Aptian–Coniacian strike-slip faults,Campanian strike-slip faults,and Eocene strike-slip faults,and two tectonic inversions,i.e.Santonian and end of Cretaceous,were developed in the Doseo Basin.Second,the Doseo Basin was an early failed intracontinental passive rift basin transformed by the strike-slip movement and tectonic inversion.The initial rifting between the African and South American plates induced the nearly N-S stretching of the Doseo Basin,giving rise to the formation of the embryonic Doseo rift basin.The nearly E-W strike-slip movement of Borogop(F1)in the western section of the Central African Shear Zone resulted in the gradual cease of the near north-south rifting and long-term strike-slip transformation,forming a dextral transtension fault system with inherited activity but gradually weakened in intensity(interrupted by two tectonic inversions).This fault system was composed of the main shear(F1),R-type shear(F2-F3)and P-type shear(F4-F5)faults,with the strike-slip associated faults as branches.The strike-slip movements of F1 in Cretaceous and Eocene were controlled by the dextral shear opening of the equatorial south Atlantic and rapid expanding of the Indian Ocean,respectively.The combined function of the strike-slip movement of F1 and the convergence between Africa and Eurasia made the Doseo Basin underwent the Santonian dextral transpressional inversion characterized by intensive folding deformation leading to the echelon NE-SW and NNE-SSW nose-shaped uplifts and unconformity(T5)on high parts of the uplifts.The convergence between Africa and Eurasia caused the intensive tectonic inversion of Doseo Basin at the end of Cretaceous manifesting as intensive uplift,denudation and folding deformation,forming the regional unconformity(T4)and superposing a nearly E-W structural configuration on the Santonian structures.Third,the Doseo Basin experienced four evolutional stages with the features of short rifting and long depression,i.e.Barremian rifting,Aptian rifting–depression transition,Albian–Late Cretaceous depression,and Cenozoic extinction,under the control of the tectonic movements between Africa and its peripheral plates.

Regional unconformities and their controls on hydrocarbon accumulation in Sichuan Basin, sW China

Based on outcrop,seismic and drilling data,the main regional unconformities in the Sichuan Basin and their controls on hydrocarbon accumulation were systematically studied.Three findings are obtained.First,six regional stratigraphic unconformities are mainly developed in the Sichuan Basin,from the bottom up which are between pre-Sinian and Sinian,between Sinian and Cambrian,between pre-Permian and Permian,between middle and upper Permian,between middle and upper Triassic,and between Triassic and Jurassic.Especially,16 of 21l conventional(and tight)gas fields discovered are believed to have formed in relation to regional unconformities.Second,regional unconformity mainly controls hydrocarbon accumulation from five aspects:(1)The porosity and permeability of reservoirs under the unconformity are improved through weathering crust karstification to form large-scale karst reservoirs;(2)Good source-reservoir-caprock assemblage can form near the unconformity,which provides a basis for forming large gas field;(3)Regional unconformity may lead to stratigraphic pinch-out and rugged ancient landform,giving rise to a large area of stratigraphic and lithologic trap groups;(4)Regional unconformity provides a dominant channel for lateral migration of oil and gas;and(5)Regional unconformity is conducive to large-scale accumulation of oil and gas.Third,the areas related to regional unconformities are the exploration focus of large gas fields in the Sichuan Basin.The pre-Sinian is found with source rocks,reservoir rocks and other favorable conditions for the formation of large gas fields,and presents a large exploration potential.Thus,it is expected to be an important strategic replacement.

Distribution, evolution and structural properties of Wushenqi paleo-uplift in Ordos Basin, NW China

This paper depicts the distribution of the Wushenqi paleo-uplift in the Ordos Basin by using the latest drilling and seismic data, and analyzes the tectonic evolution of the paleo-uplift with the support of Bischke curve and balanced section. The compressional Wushenqi paleo-uplift which developed in the Early Caledonian orogeny(Huaiyuan orogeny) is approximately a ellipse extending in S-N direction. Its long axis is about 194 km and short axis is about 55-94 km in nearly W-E direction. The denudation thickness and area of the Cambrian in the core are 170-196 m and 11 298 km^(2), respectively. It was mainly formed during the Huaiyuan orogeny according to the chronostratigraphic framework. It was in the embryonic stage in the Middle-Late Cambrian, denuded after developed obviously at the end of Late Cambrian. The paleo-uplift of the 3rd member of the Ordovician Majiagou Formation was reactivated and reduced in area. In the sedimentary period of the Ma 4 Member-pre-Carboniferous, the paleo-uplift experienced non-uniform uplift and denudation. It entered the stable period of burial and preservation in the Carboniferous and later period. The Wushenqi paleo-uplift was formed on the weak area of the basement and tectonic belts, into an compressional structure with irregular morphology, under the control of the non-coaxial compression in the south and north and the stress transmitted by the uplift in the basin. The Wushenqi paleo-uplift has a controlling effect on the sedimentary reservoirs and hydrocarbon accumulation.

Early Jurassic Soft-Sediment Deformation Interpreted as Seismites in the Wuqia Pull-Apart Basin and the Strike-Slip Talas-Ferghana Fault, Xinjiang, China

The early Jurassic soft-sediment deformation occurring within lacustrine sandstone is distributed mainly in the Wuqia region of SW Tianshan Mountains, Xinjiang, western China. Triggered by earthquakes, such deformation was found to occur in three beds overlying the lower Jurassic Kangsu Formation. The main styles of deformation structures comprise load cast, ball-and- pillow, droplet, cusps, homogeneous layer, and liquefied unconformity. The deformation layers reflect a series of three strong earthquakes at the end of early Jurassic in the Wuqia region. The differences of deformation mechanisms undergone might represent the varying magnitudes of the earthquake events. During the early Jurassic, the Wuqia region was located in a pull-apart basin controlled by the significant Talas-Ferghana strike-slip fault in central Asia, which initiated the soft-sediment deformation induced by earthquakes. Our research suggests that the paleoseismic magnitudes could have ranged from Ms 6.5 to 7.

Types and Characteristics of Volcanostratigraphic Boundaries and Their Oil-Gas Reservoir Significance

Just like in sedimentary stratigraphy, the factor for constructing volcanostratigraphic volcanostratigraphic boundary is an important framework. The fundamental factor of volcanostratigraphic boundaries is to classify the types and define their characteristics. Based on field investigation and cross-wells section analysis of Mesozoic volcanostratigraphy in NE China, 5 types of volcanostratigraphic boundaries have been recognized, namely eruptive conformity boundary （ECB）, eruptive unconformity boundary （EUB）, eruptive interval unconformity boundary （EIUB）, tectonic unconformity boundary （TUB） and intrusive contacts boundary （ICB）. Except ICB, the unconformity boundaries can be divided into angular unconformity and paraconformity. The time spans and signs of these boundaries are analyzed by using age data of some volcanic fields that have been published. The time spans of ECB and EUB are from several minutes to years. In lava flows, cooling crust is distributed above and below ECB and EUB; in pyroclastic flows, airfalls and lahars, a fine layer below these boundaries has no discernable erosion at every part of the boundary. EUB may be curved or cross curved and jagged. The scale of ECB/EUB is dependent on the scale of lava flow or pyroclastic flows. The time span of EIUB is from decades to thousands of years. There is also weathered crust under EIUB and sedimentary rock beds overlie EIUB. In most instances, weathered crust and thin sedimentary beds are associated with each other laterally. The boundary is a smooth curved plane. The scale of EIUB is dependent on the scale of the volcano or volcano groups. The characteristics of TUB are similar to EIUB＇s. The time interval of TUB is from tens of thousands to millions of years. The scale of TUB depends on the scale of the basin or volcanic field. Both the lab data and logging data of wells in the Songliao Basin reveal that the porosity is greatly related to the boundaries in the lava flows. There is a high-porosity belt below ECB, EUB or EIUB, and the porosity decreases when it is apart from the boundary. The high-porosity belt below ECB and EUB is mainly contributed by primary porosity, such as vesicles. The high-porosity belt below EIUB is mainly contributed by primary and secondary porosity, such as association of vesicles and spongy pores, so the area near the boundary in lava flows is a very important target for reservoirs.

Structural Characteristics and their Significance for Hydrocarbon Accumulation in the Northern Slope of the Central Sichuan Paleo-uplift

Based on 2D and 3D seismic data,the latest drilling data and field outcrop data of the northern slope of the Central Sichuan paleo-uplift,the structural analysis method is used to analyze unconformity development characteristics and fault characteristics during the key structural transformation period,discussing the influence of the structural characteristics on the hydrocarbon accumulation of deep carbonate rocks.The results show that:(1)The two key unconformities of the Tongwan and Caledonian periods were primarily developed in deep carbonate rocks.Firstly,Tongwan’s unconformities are characterized by regional disconformities between the second and third members of the Dengying Formation,the top formation of the Sinian and the lower Cambrian,strips of which zigzag through the north and south sides of the study area.Secondly,the Caledonian unconformity is characterized by a regional unconformable contact between the lower Permian and the ower Paleozoic strata.From NE to SW,the age of the strata,which were subject to erosion,changes from new to old,the denudation distribution showing as a nose-shaped structure which inclines towards the ENE.(2)Boundary fault and transtensional strike-slip faults developed in the Sinian to Paleozoic strata.In profile,there are three types of structural styles:steep and erect,flower structures,’Y’and reversed’Y’type faults.In plane view,the Sinian developed extensional boundary faults extending in an almost NS direction,strike-slip faults developing and extending linearly in approximately EW,WNW and NE strikes in the Cambrian,with characteristically more in the south and less in the north.(3)The faults in the northern slope show obvious zonal deformations in transverse view as well as significant stages and stratified activity in a longitudinal direction.Among them,the activity of faults in the Sinian was the strongest,followed by the activity in the Cambrian period,the activity intensity of faults in the Permian period being the weakest.This fault activity can be divided into four periods:Sinian,Cambrian-Permian,the early Indosinian period and the late Indosinian-Himalayan period,the transtensional strikeslip faults being the products of oblique extensions of pre-existing weak zones in the Xingkai and Emei taphrogenesis,with a particular inheritance in the main faults.(4)Combined with hydrocarbon accumulation factors,it is considered that the epigenetic karstification of the Tongwan and Caledonian unconformities in the northern slope controlled the formation and distribution of carbonate karst reservoirs over a large area,also acting as a good pathway for oil and gas migration.The extensional faults developed at the margin of the NS trending rift,controlling the sag-platform sedimentary pattern in the Dengying Formation of the Sinian.Strike-slip faults in NE,WNW and ENE directions may control the microgeomorphological pattern inside the platform and intensify the differential distribution of grain beach facies.The multi-stage hereditary activity of strike-slip faults not only improved the porosity and permeability of the reservoirs,but also acted as the main channel of oil and gas migration,providing favorable conditions for the development of the current multi-layer gasbearing scenario in the northern slope of the Central Sichuan Basin.

Estimation of the amount of erosion at unconformities in the last stage of the Eocene Sanduo period in the Subei Basin,China

Strata erosion is a widespread phenomenon in sedimentary basins. The generation, migration, and accumulation of hydrocarbon is influenced by the scale of erosion, so estimating the amount of erosion is essential in the analysis of oil and gas bearing basins. According to the geological features in the Subei Basin and the actual data, using the integrated method, we estimated the level of erosion at the unconformities caused by the Sanduo event. By using the mudstone interval transit time method and the vitrinite reflectance method on data from typical wells, it can be concluded that the Gaoyou, Jinhu, and Hongze depressions suffered strong strata erosion from the late Eocene to Oligocene, and the total strata erosion thickness was 300–1,100 m. Different tectonic units in the same depression have extremely uneven erosion intensity: the low convex regions have the maximum erosion thickness, amounting to 800–1,100 m; the slope regions have an erosion thickness of generally 600–800 m; the erosion thickness of the slope-hollow transition zone is 300–500 m. For the whole basin, we used the strata thickness trend analysis method combined with the interval transit time and vitrinite reflectance methods to estimate the erosion thickness in the Sanduo period. The results show that the most severe erosion of the Sanduo event in the Subei Basin is between 1,000 m to 1,200 m, mainly located in depressions around the Jianhu Uplift; the deep hollow area has the least erosion, generally about 300–600 m, and the erosion in the slope area is about 600–900 m. Compared with the northern part, the southern part has relatively little erosion. It is also proved that the Sanduo movement has heterogeneous intensity, and the western region has greater intensity than the eastern region.

Kinematics of syn-tectonic unconformities and implications for the tectonic evolution of the Hala'alat Mountains at the northwestern margin of the Junggar Basin,Central Asian Orogenic Belt

The Hala＇alat Mountains are located at the transition between the West Junggar and the Junggar Basin. In this area, rocks are Carboniferous, with younger strata above them that have been identified through well data and high-resolution 3D seismic profiles. Among these strata, seven unconformities are observed and distributed at the bases of： the Permian Jiamuhe Formation, the Permian Fengcheng Formation, the Triassic Baikouquan Formation, the Jurassic Badaowan Formation, the Jurassic Xishanyao Formation, the Cretaceous Tugulu Group and the Paleogene. On the basis of balanced sections, these unconformities are determined to have been formed by erosion of uplifts or rotated fault blocks primarily during the Mesozoic and Cenozoic. In conjunction with the currently understood tectonic background of the sur- rounding areas, the following conclusions are proposed： the unconformities at the bases of the Permian Jiamuhe and Fengcheng formations are most likely related to the subduction and closure of the Junggar Ocean during the late Carboniferous-early Permian; the unconformities at the bases of the Triassic Baikoucluan and Jurassic Badaowan formations are closely related to the late Permian Triassic Durbut sinistral slip fault; the unconformities at the bases of the middle Jurassic Xisbanyao Formation and Cretaceous Tugulu Group may be related to reactivation of the Durbut dextral slip fault in the late Jurassic -early Cretaceous, and the unconformity that gives rise to the widely observed absence of the upper Cretaceous in the northern Junggar Basin may be closely related to large scale uplift. All of these geological phenomena indicate that the West Junggar was not calm in the Mesozoic and Cenozoic and that it experienced at least four periods of tectonic movement.

Buried channels provide keys to infer Quaternary stratigraphic and paleo-environmental changes:A case study from the west coast of India

High resolution shallow seismic data was acquired from inner continental shelf of Goa,west coast of India to map underlying stratigraphic and buried geomorphic features of shelf strata.Seismic data revealed characteristic channel incisions beneath 4-15 m thick sediment layer and corresponds to multi cycle incisions.Stratigraphic analysis of these incision signatures reveals three prominent subaerial unconformities S6,S7 and S9.These unconformities were exposed during the last glacial,penultimate glacial(MIS-6)and prior to penultimate glacial(MIS-8)periods.On the basis of interpreted age of subaerial unconformities and differences in their morphological features,observed channel incisions have been divided grossly into three phases of incision.Phase-1 incisions are older than^330 kyr BP,whereas,incisions of Phase-2 and Phase-3 correspond to^320-125 kyr BP and^115-10 kyr BP respectively.Plan form of these incisions varied from a straight channel type to ingrown meander and then to anastomosing channel types.These channels meet at the confluence of present-day Mandovi and Zuari rivers.The confluence point has varied in due course of time because of cyclic incision and burial with repeated sea level fluctuations.The preserved main channel width varies from^100 m to 1000 m.and maximum channel depth reaches up to^35 m.Comparison of quantitative and qualitative morphologic results of different phases of incisions suggest that Phase-2 channels had^33%more mean bank full discharge than that of the Phase-3 channels.Phase-2 incisions had been carved in higher hydraulic energy condition as compared to Phase-3 incisions implying that the Indian summer monsoon was better during formative stages of Phase-2 incisions.

THE EXTENSIONAL MOVEMENT AND ACTION IN THE SOUTHERN TIBET SINCE HERCYNIAN

Based on the study of the location of extensional action in the tectonic background and the state of regional tectonic stress, the extensional action can be distinguished into active and passive types. The active extensional movement is mainly composed of a set of deposition formed under extensional stress and syntectonic and syndepositional deformational structure. The passive extentional movement is expressed as regional extensional action induced under the compressional elevation. Extensional movement and action are well\|developed in the Tibet\|Tethys domain. By the analysis of tectonic deformation, extensional unconformity, depositional facies and paleogeographical rebuilding in the South Tibet, it is recognized that the extensional movement consists of active extension of Hercynian, Indo\|China and Early Yanshan epochs, and the passive extension of Late Yanshan and Himalayan epochs. Some of the characteristics of extensional movement and structures are discussed in this paper(Table.1).

Tectonic Setting of the Kadiri Schist Belt, Andhra Pradesh, India

Plate tectonic activity has played a critical role in the development of petrotectonic associations in the Kadiri schist belt. The calc alkaline association of basalt, andesite, dacite and rhyolite （BADR） is the signature volcanic rock suite of the convergent margin. The N-S belt has gone below the unconformity plane of Cuddapah sediments. In the northern part geochemical and structural attributes of the Kadiri greenstone belt is studied along with microscopic observations of selected samples. Harker diagram plots of major elements generally indicate a liquid line of descent from a common source, such that BADR rocks are derived from a common parent magma of basaltic to andesitic composition. These calc-alkaline volcanic rocks are formed at convergent margins where more silicic rocks represent more highly fractionated melt. All the litho-units of this greenstone belt indicate crush and strain effects. The stretched pebbles in the deformed volcanic matrix with tectonite development along with associated greenschist facies metamorphism, alteration and hydration is remarkable. Flow foliation plane with N-S strike and very low angle （5~ to 10~） easterly dip and N-S axial planar schistosity formed due to later phase isoclinal folding can be clearly identified in the field. Basic intrusives are quite common in the surrounding area. All the observations including the field setting and geochemistry clearly demonstrate ocean-continent subduction as the tectonic environment of the study area.

Rectifying the Neoproterozoic Stratigraphic Framework of Eastern Jiangnan Orogen, Southeast China

Studies on the metamorphic and related magmatic rocks within the Jiangnan Orogen,southern China,are important to understand the formation and evolution of this Neoproterozoic orogen because they can provide evidence for revealing the tectonic evolution of the South China Block.Following on from earlier attempts at stratigraphic correlation and creating a framework for the low-grade metamorphosed basement,new first-hand information on the composition and deformation of the East Jiangnan Orogen basement indicates that the sedimentary rocks in the north are composed of the sequential Shangxi Group(Gp)and the overlying Likou Gp,with a clear regional unconformity in between.The Likou Gp includes the Zhentou Formation(Fm.)and the Dengjia Fm.,with the previously named Puling Fm.only basalt interbeds within the Dengjia Fm.The Xucun granodiorite pluton of^830 Ma intrudes the Shangxi Gp,thereby resulting in a 100 mwide hornfels zone within the wall rocks,indicating that the formation of the group occurred earlier than that of the pluton.By contrast,the southern metamorphic rocks of the Xikou Gp and the overlying Jingtan Fm.are flaky,disordered,and strongly deformed.The Jingtan Fm.is roughly equivalent to the Heshangzhen Gp and includes the Zhoujiacun conglomerates in the lower part and a rhyolite in the upper part with interbedded basalt.This assessment of Neoproterozoic stratigraphic sequences and magmatic rocks in the east of the Jiangnan Orogen differs from and rectifies previous studies,emphasizing that explanation of analytical results,particularly geochronology,should be consistent with facts gathered in the field.