fabric evolution in salem attur shear zone south india and its implications on the kinematics abstract domains of opposing shear sense relative to the overall kinematic framework often add to the ambiguities in the interpretation of movement picture along major shear zones rheologic properties strain partitioning as well as rotation of fabric are the principal mechanisms that contribute to the evolution of fabric during shearing the relative dominance of these mechanisms varies with progressive strain leading to the development of domainal fabric departing from the regional stress strain vectors the salem attur shear zone exhibits imprints of multiphase shearing and domains of meso to micro fabrics indicating opposing shear sense resulting in conflicting interpretations of the regional kinematics since the sense of movement is a major constraint in matching the shear zones in gondwana fragments this paper focuses on understanding the movement picture of the shear zone and the mechanism of evolution of domainal fabric

structure and tectonics of the achankovil shear zone southern india an integrated approach to resolve the kinematics of the controversial achankovil shear zone aksz has been attempted involving remote sensing data shaded relief topo maps ground details of lithology and mesoscopic structures an excellent correlation of structural trends exists on all scales of observation the aksz is distinctly defined by nw se trending foliation fabrics with steep dips to southwest the adjacent madurai block and trivandrum block show contrasting lithological and structural characteristics as shown in structural cross sections the mesoscopic structural studies reveal the presence of sub horizontal stretching lineations asymmetric structures like s c fabrics porphyroclasts s shaped folds and shear bands confirming the strike slip component of shear along aksz the deformation undergone by the aksz could be described in terms of an initial dextral deformation d1 reactivated and superimposed by sinistral kinematics d2 which is also supported by megascopic structural interpretation of remote sensing data the megascopic structural interpretation of aksz displays en echelon pattern of lineaments with right overstepping arrangement which can be interpreted as an evidence of the latest sinistral transpressional deformation

the nilgiri enderbites south india nature and age constraints on protolith formation high grade metamorphism and cooling history abstract the trident shaped nilgiri highland massif represents the deepest level exposure 9 10 kbar c 35 km palaeo depth of granulite grade lower crust in southern peninsular india the crustal section is essentially composed of garnetiferous enderbitic granulites with minor kyanite bearing gneisses quartzites and banded magnetite quartzites its deeper northern part is intruded by intermediate and basic magmatic rocks now represented by non garnetiferous enderbites garnetiferous basic granulites gabbroic to anorthositic two pyroxene granulites and pyroxenites the garnetiferous enderbitic granulites are tonalitic in composition displaying a typical calc alkaline trend and exhibit chemical features of both i type as well as s type granitoids they show moderate enrichment of lree la n 35 70 no to weakly negative eu anomalies and fairly constant levels of hree yb n 6 5 13 0 and resemble the ree spectra of tonalitic trondhjemitic granitoids and greywackes of late archaean to palaeoproterozoic age sedimentary protoliths are indicated by negative df values ranging from 1 to 4 al 2 o 3 cao na 2 o k 2 o molar ratios 1 1 high 18 o whole rock values 10 12 which match the values reported for greywackes whole rock isotope data for garnetiferous enderbites define linear arrays in the rb sr and sm nd evolution diagrams rb sr system 2460 81 ma with sr i 0 7030 5 sm nd system 2506 70 ma with nd i 0 509363 60 implying a provenance of the sedimentary precursors that was dominated by felsic volcanic rocks or tonalitic trondhjemitic granitoids of uniform isotopic composition and early proterozoic age the u pb systematics of composite zircon grains with detrital cores and metamorphic overgrowths in accord with the sm nd and rb sr errorchron ages and sm nd garnet whole rock data suggest that formation of the provenance deposition of the sedimentary precursors and subsequent high grade tectono metamorphic processes must have occurred in the short time span between 2600 and 2500 ma based on the regional geological framework the petrological geochemical and isotopic data the nilgiri granulite terrain is interpreted as a metasedimentary allochthonous unit that was thrust onto the dharwar craton during palaeoproterozoic collisional tectonics

kinematics strain pattern and geochronology of the salem attur shear zone tectonic implications for the multiple sheared salem namakkal blocks of the southern granulite terrane india abstract structural mapping strain analysis and a variety of geochronological studies were carried out to determine the tectonothermal evolution of the salem attur shear zone in the southern granulite terrane of south india the salem namakkal blocks containing the shear zone consisted of quartzofeldspathic gneiss charnockite and mafic granulite and had undergone multiple phases of magmatism spanning over a period of 3 2 0 5 ga the rocks were deformed by four phases of deformation d1 d4 the d1 deformation was characterized by isoclinal and recumbent ne sw trending f1 fold with a pervasive subhorizontal axial planar granulitic fabric s1 and associated quartzofeldspathic leucosomes granulite metamorphism was dated at ca 2 5 2 3 ga the f1 fold and s1 fabric were coaxially refolded by tight to isoclinal upright to steeply inclined ne sw trending f2 folds during d2 deformation the d2 deformation was associated with f2 axial planar shear zones crenulations and leucosomes s2 fabric large scale d2 shear zones characterized by high temperature ductile shear fabric with a vertical flow host syntectonic syenite pluton which was dated at ca 2 5 2 4 ga a p t condition of 7 kb 600 c was inferred for the d2 deformation the d3 deformation was characterized by nw se to e w trending f3 folds and the salem attur shear zone the shear zone was a greenschist to amphibolite facies shear zone being characterized by mylonitic foliation and dominantly down dip stretching lineation defined by quartz biotite and hornblende minerals and dated at ca 2 0 ga it indicated n nne vergence of thrusting with the mean kinematic vorticity number wm as 0 7 suggesting general simple shear strain with 50 pure shear component the d4 deformation was manifested as nne ssw striking strike slip faults and nw se striking extensional normal faults pseudotachylite veins having an age of 1 9 ga injected during strike slip faulting and granite pegmatite veins showing age of 0 8 0 5 ga intruded during normal faulting the salem namakkal blocks thus recorded a long lived shearing history we suggest that the salem attur shear zone and other shear zones such as palghat cauvery moyar bhavani karur kambam painavu trichur and achankovil shear zones were paleoproterozoic intraterrane shear zones which were overprinted by meso neoproterozoic cambrian ductile and brittle deformations

palghat gap a dextral shear zone from the south indian granulite terrain abstract the origin of palghat gap the most prominent discontinuity in the western ghats is controversial the rocks of the gap hornblende biotite gneiss mylonitic augen gneiss charnockite and amphibolite are characterised by three groups of folds formed during two deformations shear indicators like augens pull aparts rootless folds sheath folds minor shears and quartz fabric strongly suggest ductile dextral shearing kinematic analysis of structural elements supports shearing in a n s compressional field with xy being subvertical features on both mesoscopic and microscopic scales strongly favour palghat gap being treated as a ductile shear zone the present gap topography is the product of shearing and erosion

age and tectonic evolution of neoproterozoic ductile shear zones in the southern granulite terrain of india with implications for gondwana studies 1 the high grade rocks of the southern granulite terrain sgt of peninsular india are bounded to the north by the archean dharwar craton another high grade terrane the mesoproterozoic eastern ghats occurs to the northeast of the sgt the tectonic relationship between these crustal domains is complex we present new geochronological and structural data that indicate a continuation of the dharwar craton into the southern granulite terrain as far south as a newly identified neoproterozoic shear zone here named the karur kamban painavu trichur shear zone kkptsz south of the kkptsz mesoproterozoic dates of the sgt are similar to those recorded in the eastern ghats and the two domains may have been conterminous thirty three new u pb th single zircon and monazite dates of samples from six structural transects across the regional shear zones indicate that the sgt has experienced at least seven thermo tectonic events at 2 5 ga 2 0 ga 1 6 ga 1 0 ga 800 ma 600 ma and 550 ma and two distinct episodes of metasomatism charnockitization between 2 50 2 53 and between 0 55 0 53 ga deformation along a number of major shear zones in the sgt is neoproterozoic to earliest paleozoic in age with an early phase d2 concentrated between 700 800 ma and a later phase d3 between 550 and 600 ma major charnockitization 530 550 ma post dates d3 and is in turn overprinted by granitization retrogression and uplift between 525 and 480 ma the kkptsz active between 560 and 570 ma is either a terrane boundary or a tectonized decollement between cover and archean basement rocks represented by predominantly paragneisses to the south and orthogneisses to the north respectively other regional neoproterozoic shear zones do not appear to separate allochthonous terranes as previously suggested on the basis of nd model ages and rb sr biotite whole rock dates the neoproterozoic cambrian tectonothermal history of the sgt and eastern ghats is similar to that recorded in parts of madagascar east africa and antarctica and is used to reconstruct parts of central gondwana here named the deccan continent with more robust confidence

crustal evolution in south india constraints from nd isotopes nd isotope analyses from a range of lithologies including charnockites metasediments and granitoids that comprise the high grade terrain of south india define contrasting age provinces north of the palghat cauvery shear zone the karnataka craton and the granulite blocks of nilgiri and madras are characterized by model nd ages from 3 4 to 2 4 ga with significant crustal growth during the late archean south of the palghat cauvery shear zone a pan african granulite facies event resulted in sm nd fractionation as observed in several incipient charnockite localities model nd age calculations that allow for sm nd fractionation at 550 ma yield ages in the range of 2 9 to 1 3 ga this range compares closely with published model nd ages from the high grade terrain of sri lanka mid proterozoic crustal growth 1 1 ga observed in the vijayan complex of sri lanka may have provided source material for metasediments characterized by young model nd ages in southern peninsular india the palghat cauvery shear z

bounded influence magnetotelluric response function estimation author posting blackwell 2004 this article is posted here by permission of blackwell for personal use not for redistribution the definitive version was published in geophysical journal international 157 2004 988 1006 doi 10 1111 j 1365 246x 2004 02203 x

on the development of zones of reverse shearing in mylonitic rocks abstract most shear zones contain a minor proportion of domains recognizable from thin section to outcrop scale which show an opposing shear sense relative to the overall kinematic framework in this paper we discuss how these universal features result from the evolving deformation strain partitioning that generally accompanies progressive general shear principally in low metamorphic grade shear zones because the rheologic properties and consequently the contributions of the different deformation mechanisms change continuously with progressive strain an individual domain with a predominant deformation mechanism much faster or slower than the average may depart from the general kinematics enhancing the viscosity contrast between adjacent layers and causing locally reverse shear in the domain boundaries care is needed therefore in interpreting opposing shear senses in the same shear zone as indicating superimposed deformation events

the generation of ductile and brittle shear bands in a low angle mylonite zone in the saint barthelemy massif french pyrenees a ductile thrust zone developed in gneisses during retrogression from lower amphibolite facies conditions to the upper greenschist facies the last major structures formed in the zone are isolated shear bands divided into three types anastomosing inhomogenous ultramylonitic shear bands type i are subparallel to the mylonitic foliation in the gneiss sg most of these bands developed by ductile deformation processes only planar homogeneous ultramylonite bands type ii are usually oblique to sg they generated as pseudotachylyte bands by brittle fracturing and underwent strong subsequent ductile deformation type iii shear bands are planar and oblique to sg they consist of pseudotachylyte weakly affected by ductile deformation type i ii and iii bands seem to represent progressively younger structures on a local scale linked to falling p t conditions the systematic variation in orientation of the different shear bands with respect to sg is interpreted as being due to a different response of brittle and ductile structures to the orientation of the kinematic frame and the rock anisotropy

robust smooth magnetotelluric transfer functions jimmy c larsen randall l mackie 2 adele man e l l a adolfo fiordelisi4 and shirley rieven2 pacific marine environmental laboratory national oceanic and atmospheric administration 7600 sand point way ne seattle wa 981 15 0070 usa wassachusetts institute of technology 42 carleton street cambridge m a 02142 usa cnr international institute for geothermal research piazza soljerino 2 pisa 56126 italy enel esplorazione mineraria via andrea pisano 120 pisa 56122 italy

scientific drilling in the great rift valley the 2005 lake malawi scientific drilling project an overview of the past 145 000years of climate variability in southern hemisphere east africa abstract the recovery of detailed and continuous paleoclimate records from the interior of the african continent has long been of interest for understanding climate dynamics of the tropics and also for constraining the environmental backdrop to the evolution and spread of early homo sapiens in 2005 an international team of scientists collected a series of scientific drill cores from lake malawi the first long and continuous high fidelity records of tropical climate change from interior east africa the paleoclimate records which include lithostratigraphic geochemical geophysical and paleobiological observations documented in this special issue of palaeo3 indicate an interval of high amplitude climate variability between 145 000 and 60 000 years ago when several severe arid intervals reduced lake malawi s volume by more than 95 these intervals of pronounced tropical african aridity in the early late pleistocene around lake malawi were much more severe than the last glacial maximum lgm a well documented period of drought in equatorial and northern hemisphere tropical east africa after 70 000 years ago climate shifted to more humid conditions and lake levels rose during this latter interval however wind patterns shifted rapidly and perhaps synchronously with high latitude shifts and changes in thermohaline circulation this transition to wetter more stable conditions coincided with diminished orbital eccentricity and a reduction in precession dominated climatic extremes the observed climate mode switch to decreased environmental variability is consistent with terrestrial and marine records from in and around tropical africa

stretching lineations in transpressional shear zones an example from the sierra nevada batholith california abstract ductile shear zones and associated stretching lineations are generally considered to be the result of two dimensional simple shear deformations with stretching lineations interpreted to rotate into parallelism with the direction of tectonic transport with increasing deformation however stretching lineations perpendicular to the inferred tectonic transport direction are displayed by some shear zones field studies in the sierra nevada batholith have revealed a single shear zone the rosy finch gem lake shear zone that contains both steeply plunging stretching lineations in older metamorphosed sedimentary and granitic rocks and shallowly plunging stretching lineations in syntectonic granitoids dextral sense of shear indicators are found in all these units and deformation occurred simultaneously along the length of the shear zone theoretical strain modeling indicates that stretching lineations can be either horizontal or vertical within transpressional shear zones to investigate the kinematics of three dimensional deformation we examine the role of stretching lineations which form as a result of finite strain including the possibility of partitioning a component of the strike slip movement onto shear bands comparing the field data with the strain modeling we interpret the along strike variation in lineation behavior within the rosy finch gem lake shear zone to be a result of the differences in finite strain recorded by different units the older units showing steeply plunging lineation have recorded more finite strain across a narrower shear zone than the syntectonic granites which record shallowly plunging lineations we conclude that the orientation of the lineation within transpressional shear zones does not necessarily correlate with the transport direction but may reflect along strike variations in finite strain and or strain partitioning

seasonal signals in lake malawi sediments the sedimentary record of lake malawi one of the large rift valley lakes of east africa contains thick sequences of biologically undisturbed finely laminated muds and silts laminated sequences were recovered in cores from the extensive diatom ooze and silty diatom clay facies of the lake basin that is permanently anoxic below 250 m the sequences consist of light dark laminae couplets with average thickness of 1 mm the light laminae are completely dominated by the diatom genus melosira with a relatively minor contribution from the genus stephanodiscus the dark laminae consist of 50 diatoms equal proportions of melosira and stephanodiscus few nitzschia and occasional littoral diatom genera and 50 terrestrial plant debris mineral grains and fine clay and organic material based on a 210pb and 14c calculated sedimentation rate of 1 mm yr 1 for lake malawi basin sediments and the well defined seasonal variations in fluvial input and diatom productivity levels our data suggest that the lamination couplets are annual varves representing biannual sedimentation sequences the light laminae result from diatom blooms occurring during the dry windy season april october when the upper 250 m of the lake is well mixed localized upwelling occurs and the epilimnion becomes enriched with nutrients the dark laminae represent sedimentation in the rainy low wind velocity season november march when runoff and allochthonous terrestrial input are at a maximum and primary productivity is at a minimum

the cauvery shear zone southern granulite terrain india a crustal scale flower structure abstract the cauvery shear zone csz is a crustal scale shear system within the southern granulite terrain along the southern margin of the archaean dharwar craton structural interpretation of satellite data and field observations reveal four major shear zones within the csz system they show dextral shear kinematics synchronous with a major neoproterozoic tectono metamorphic event d2 associated with intracrustal melting and migmatisation the disposition geometry and contemporaneity of shear fabrics of the csz system are modelled in terms of a crustal scale flower structure akin to transpressional and collisional orogens in the light of recent seismic evidence for a displaced moho structure and a mid to lower crustal low velocity zone the flower structure across the csz may extend to mantle depths

structure and strain variation at mid crustal levels in a transpressional orogen a review of kaoko belt structure and the character of west gondwana amalgamation and dispersal abstract the neoproterozoic cambrian kaoko belt is an orogen scale 800 180 km transpressional system important in the amalgamation of west gondwana mid crustal transpression at amphibolite to granulite facies conditions is dominated by two major 400 km exposed strike slip shear zones bounding a 20 40 km wide high grade orogen core to the east a deeply buried nappe dominated escape zone has inverted metamorphic sequence and verges outwards onto a platformal foreland to the west an arc like neoproterozoic coastal terrane was amalgamated and variably reworked during transpression the major purros and three palms mylonite zones have calculated shear displacements on the order of 120 180 km these shear zones are moderately to steeply dipping mylonite zones of 1 5 km width are arcuate and curvilinear in map view and show along strike variation in slip kinematics also highly curved in vertical section the shear zones define a flower to half flower geometry for the orogen core an oblique network of mylonitic shear zones akin to riedel shears links the major shear zones and defines regional scale shear lozenges internally deformed by tight upright folding and shear fabrics these shear zones create domains in the orogen core with varying dominance of pure shear in shear lozenges and simple shear in shear zones however absence of dip slip domains and the smoothly continuous traces of sub horizontal to shallow and acute oblique stretching lineations across all parts of the belt preclude marked kinematic partitioning and the internal part of the belt resembles large scale triclinic shear clast aspect ratios boudin train extension sheath fold aspect geometry degree of rotation of planes producing flanking folds composite s c foliations pressure fringes on pyrite and garnet porphyroclasts provide a semi quantitative measure of strain intensity average strain ratios are x z 40 1 for the major shear zones x z 12 1 for the orogen core x z 8 1 for the escape zone and x z 3 1 for the coastal terrane a more continuous pattern of strain intensity across the whole belt is mapped using a qualitative foliation intensity index foliation traces have a sigmoidal pattern in the orogen core swinging from sub parallel to the boundary shear zones to higher acute angles in the internal parts deformation character also varies from upright open folding in amphibolite facies domains in the north upright tight chevron folding in a low grade central domain to a high grade domain of tight to isoclinal inter folded basement and cover with inclination decreasing towards the south the kaoko belt is a well exposed sector of an extensive 3000 km long broad 400 km arcuate orogenic system adamastor orogen that amalgamated west gondwana bringing the south american sao francisco and rio de la plata cratons and african kalahari and congo cratons components together though a complex system most sectors involved oblique collision and accretion of magmatic arcs of 660 610 ma age followed by peak metamorphism and main phase transpressional orogenesis between 585 and 560 ma with shear zones remaining active until 530 ma this e w amalgamation immediately pre dates the final n s amalgamation of gondwana along the kuunga orogen between 535 and 510 ma the large scale adamastor orogen consisting of kaoko dom feliciano ribeira aracuai and west congo mobile belts also shows broadly similar and symmetric structural architecture throughout the high grade thermally softened core partitioned intense wrench dominated strains and networks of transcurrent shear zones that dip inwards with listric form either side of the internal zone containing amalgamated arcs and high grade core are nappe fold and thrust belts that rework attenuated passive margin basement and adamastor ocean sediments and structures verge outward at moderate to high angles onto both foreland margins the kaoko belt well illustrates the highly partitioned nature of transpressional systems in general and patterns in common throughout the greater adamastor orogen such as metamorphic zonation and heterogeneous distribution of deformation style flow regime and highly variable degrees of reworking strain and recrystallization this highly partitioned and steep structural grain localized lithospheric extension and rifting 415 ma later during breakup and dispersal of gondwana

age and sedimentary provenance of the southern granulites south india u th pb shrimp secondary ion mass spectrometry abstract southern india lies at a junction in the gondwana forming orogenic belts between the east african orogen and the kuunga orogen it contains voluminous high grade metasedimentary gneisses that make up an important component of the record of collision and amalgamation of gondwana here we present u pb secondary ion mass spectrometry sims isotopic data from detrital zircon cores from throughout southern india that demonstrate dominant neoarchaean to palaeoproterozoic age components that are incompatible with the known ages of potential southern and central indian source regions the original sediments to the trivandrum block gneisses were deposited between 1900 and 515 ma whereas a sample from the achancovil unit and possible also a sample from the madurai block were deposited in neoproterozoic times we speculate that these rocks broadly correlate with southern and western malagasy metasedimentary rocks including the itremo and molo groups and formed an extensive basin or basins that lay on the west side present orientation of the neoproterozoic continent azania in addition metamorphic zircon from four samples yielded an age of 513 6 ma that is interpreted as dating high grade metamorphism throughout much of the southern granulite terrane

structure and evolution of the cauvery shear zone system southern granulite terrain india evidence from deep seismic and other geophysical studies abstract the southern granulite terrain with exposed archean lower crustal rocks is studied using various geophysical tools the crustal structure derived from seismic reflection and refraction wide angle reflection studies is used to understand the tectonic evolution of the region deep seismic reflection section along the kolattur palani segment shows an oppositely dipping reflection fabric near the moyar bhavani shear zone which is interpreted as a signature of collision between the dharwar craton and another crustal block in the south the thickened crust due to collision was delaminated during the orogenic collapse and modified the central part covering the cauvery shear zone system located between the moyar bhavani and karur oddanchatram shear zones the delaminated lower crust is altered by magmatic underplating as evidenced by the high velocity layer just above the moho the velocity model of the region indicates crustal thickening at the boundary of the dharwar craton and moyar bhavani shear zone and thinning further south back scattered seismic wave field with negative moveout and the moho offset indicate the spatial location and strike slip nature of the shear zones present study suggests that the late archean collision and suturing of the dharwar craton with the southern crustal block at the moyar bhavani shear zone may be responsible for the evolution of late archean granulites late neoproterozoic rifting is observed along the paleo fault zones the seismic studies constrained by gravity magnetic and magnetotelluric data suggest that the moyar bhavani and karur oddanchatram shear zones of the cauvery shear zone system mark terrane boundaries suture zones

an outline of shear sense analysis in high grade rocks ductile shear zones are important in tectonic reconstructions as a source of information on the relative motion of large crustal blocks or plates in the geological past methods to interpret fabric in ductile shear zones were mostly developed for low grade rocks where overprinting relations are usually well preserved however high grade shear zones are common and dominate in many precambrian terrains high grade shear zones should be analysed in a different way from low grade zones the plane on which shear sense markers should be observed the vorticity profile plane is more difficult to find than in low grade shear zones the most reliable shear sense markers in high grade shear zones are shear bands mineral fish mantled porphyroclasts sigmoids and asymmetric boudins 2006 published by elsevier b v on behalf of international association for gondwana research

kinematics of rock flow and the interpretation of geological structures with particular reference to shear zones abstract current interpretations of structures are generally based on homogeneous and steady deformation models despite the fact that both the heterogeneity of rocks materially rheologically and geometrically and the time dependence of imposed geological conditions give rise to significant heterogeneous and non steady flow in concert with field observations we emphasize that the expectation of heterogeneity and non steadiness is the key to understanding natural deformation and that in order to carry out successful structural analysis and tectonic interpretation it is necessary to recognize the first order distinction between imposed boundary conditions typically used to define the tectonic regime e g transcurrent transpression and the response recorded by rocks within the zone structures and fabrics using s c fabric as an example it is demonstrated how flow with a non zero spinning component resulting from the rheological contrasts and or geologically realistic time dependent boundary displacement can drastically change the ideal geometric and kinematic relations between the fabric and the host zone in agreement with both theoretical analysis and field observation it is shown that natural flow regimes range from pure shear to pure rotation including super simple shear in consideration of the heterogeneity and non steadiness of natural deformation kinematic analysis is justifiable only within a homogeneous domain and steady period flow kinematics and mechanisms are interrelated in that firstly mechanisms provide internal constraints on kinematics ensuring that only certain flows are possible and secondly flow kinematics will favour development of certain mechanisms

dextral pan african shear along the southwestern edge of the achankovil shear belt south india constraints on gondwana reconstructions the achankovil shear belt of the southern indian peninsular shield is prominent on landsat images it coincides with the boundary between charnockites of the madurai block to the north and khondalites of the trivandrum block to the south and with a major change in the aeromagnetic pattern that can be traced across southern india field investigations reveal a major shear zone along the southwestern edge of the achankovil shear belt the tenmala shear zone rocks in the tenmala shear zone include charnockite garnet biotite gneiss garnet sillimanite gneiss cordierite gneiss and granite kinematic indicators include stretched and asymmetric garnet feldspar and quartz porphyroclasts shear bands asymmetric folds extensional and contractional composite structures hook folds on rotated and deformed gash veins and lineations kinematic analysis of these features along 60 km of the shear zone indicates primarily dextral shear with a minor component of reverse shear textures as well as mineral assembl

joint inversion of marine magnetotelluric and gravity data incorporating seismic constraints preliminary results of sub basalt imaging off the faroe shelf breakup of the north atlantic during the early tertiary was accompanied by widespread and massive magmatism resulting in the coverage of large areas of the north atlantic with flood basalts these flood basalts hamper seismic investigations of underlying sequences and thus the understanding of the rifting subsidence and evolution of the margin which in turn increases the risk for hydrocarbon exploration in this paper we present a methodology for the simultaneous joint inversion of diverse geophysical datasets i e free air gravity and magnetotelluric soundings mt using seismic a priori constraints the attraction of the joint inversion approach is that different geophysical measurements are sensitive to different properties of the sub surface so through joint inversion we significantly reduce the null space and produce a single model that fits all datasets within a predefined tolerance using sensitivity analysis of synthetic data we show how each data set contains complementary important information of the supra and sub basalt structure while separate inversions of individual datasets fail to image through the basalt layer our joint inversion approach leads to a much improved sub basalt structure application of the joint inversion algorithm to satellite gravity data and mt data acquired on the flare10 seismic line south west of faroe islands supports the existence of a 1 km to 2 km thick low velocity region that might be indicative of the existence of a sedimentary basin underneath the basalt layer though in this paper we demonstrate the use of joint inversion on a sub basalt target we believe it has wider applicability to other areas where conventional seismic imaging fails

ductile shearing in attur shear zone and its relation with moyar shear zone south india abstract in the eastern part of southern peninsular india the charnockitic hills of the madras block are cut across by the e w trending attur shear zone asz which is characterised by a thick 1 to 1 5 km phyllonite zone showing intense mylonitisation due to ductile shearing steeply plunging 70 80 stretching lineation on steeply dipping mylonitic foliation within this zone indicates a relative vertical upliftment of the adjacent blocks a dextral shearing event from west to east is envisaged from the kinematic analysis of shear sense indicators such as s c fabric asymmetric folds asymmetric augens and asymmetric porphyroclasts simultaneous development of these features related to vertical and horizontal movements may be explained by the mechanism of transpressional deformation the attur shear zone may be correlated with the moyar shear zone based on distinct lithological and structural similarities

late pleistocene temperature history of southeast africa a tex86 temperature record from lake malawi we present a tex86 derived surface water temperature record for lake malawi that provides the first continuous continental record of temperature variability in the continental tropics spanning the past similar to 74 kyr with millennial scale resolution average temperature during marine isotope stage mis 5a was 26 5 degrees c with a range from 25 7 to 27 3 degrees c comparable to holocene temperatures mis 4 was a relatively cold period with temperatures generally decreasing from 25 5 degrees c at 68 ka to a minimum of 20 degrees cat similar to 60 ka 1 5 2 degrees c colder than the last glacial maximum lgm termination of mis 4 is characterized by a rapid increase of 3 4 degrees c in only similar to 0 5 kyr temperatures were relatively stable throughout mis 3 at the resolution of this study with an average of 23 8 degrees c and a range from 25 1 to 22 9 degrees c the lack of millennial scale temperature variability during mis 3 suggests that lake malawi s documented response to the bipolar seesaw brown et al 2007 is not reflected in its thermal history our temperature estimates for the lgm and holocene are consistent with a previously published tex86 record from lake malawi with a temperature of similar to 22 6 degrees c for the lgm similar to 25 26 degrees c in the mid holocene and similar to 25 28 for the late holocene in general the present extended tex86 record indicates that temperature variability in tropical east africa during late mis 5 and mis 4 was as great as that associated with the deglaciation and holocene a decrease in southern hemisphere insolation between 70 and 60 ka may have played an important role in forcing temperatures during mis 4 but after 60 ka other factors such as the extent of the polar ice sheets or atmospheric co2 may have forced temperature in tropical africa to a greater extent than local summer insolation

climatic and volcanic forcing revealed in a 50 000 year diatom record from lake massoko tanzania the interactions between climatic and volcanic forcing on diatom communities contained in a 50 000 year sedimentary sequence from lake massoko tanzania were examined at the century scale 19 discrete tephra inputs to the lake isolated the sedimentary nutrient supply and shifted the diatom communities to those tolerant of low phosphorus levels whereas at the millennial scale diatom inferred shifts in precipitation evaporation based on conductivity optima and diatom life form ratios were broadly similar to lake level reconstructions from lake rukwa lake malawi and others in the region some fluctuations of lake massoko are consistent with the precession driven changes in insolation but the major climate shifts do not relate directly to orbital forcing of summer insolation south of the equator and show more consistency with records from the equatorial and northern tropics that receive rainfall from the passing of the intertropical convergence zone sea surface temperatures are strongly correlated to multimillennial scale climate patterns over this region of africa

partitioning of non coaxiality in deforming layered rock masses abstract partitioning of non coaxiality and other flow parameters in deforming layered rock masses with nearly incompressible newtonian rheology is examined in two dimensions by the finite element method the results show that layers with viscosity contrast in suitable orientation deform by different and even by a reverse sense of non coaxiality this remarkable partitioning of non coaxiality is due to a large rotation rate of instantaneous stretching axes in a heterogeneous body as a result of geometry changing through deformation rather than through rotation of material lines

multiple tectonothermal events in the granulite blocks of southern india revealed from epma dating implications on the history of supercontinents abstract we report age data on zircon monazite uraninite and huttonite from a suite of 29 samples covering four major granulite blocks in southern india using an electron microprobe technique the rocks analysed in this study cover all of the major lithounits in these terrains and include garnet bearing and garnet free charnockites garnet biotite gneisses khondalites calc silicate rocks and a suite of orthogneisses biotite gneiss biotite hornblende gneiss two pink metagranites representing the magmatic phase were also analysed zircons from the madras block yield well defined isochrons at 2 5 2 6 ga core to rim analyses of single zircon grains show age zoning with 2 6 2 9 ga igneous cores mantled by 2 4 2 5 ga rims detrital zircons show age up to ca 3 2 ga monazites in this block have cores and rims with 2 5 2 3 ga a suite of 19 samples from the madurai block brings out the multiple tectonothermal events in this terrain zircons from an orthogneiss yield well defined isochrons at 1 7 0 1 ga 0 82 0 05 ga and 0 58 0 04 ga from core inner rim and rim portions respectively zircon grains in other rocks preserve either core or secondary growth ages at 0 8 1 0 ga zircons in a pink metagranite from this block show sharply defined isochrons of 0 68 0 03 ga for the core and 0 57 0 01 ga for the secondary portion a late pan african overprint is observed throughout this block with zircon rims monazite uraninite and huttonite yielding age values in the range of 0 45 0 60 ga zircons from both the trivandrum and nagercoil blocks show a major tectonothermal event at 0 55 ga with faint indications of previous tectonothermal events during 0 8 1 0 and 1 7 2 0 ga monazite data from both the trivandrum and nagercoil blocks are essentially similar to those from the madurai block except for presence of relic monazite in the former our study confirms the notion that the palghat cauvery shear zone marks the major terrain boundary between an archean craton in the north and proterozoic terrains in the south it also strengthens the view of paleoproterozoic accretion and pan african reworking of blocks south of this shear zone the ages of production accretion and reworking in the terrains of southern india yield important information for the histories of columbia 1 8 1 5 ga rodinia 1 1 1 0 ga and gondwana 0 6 0 5 ga supercontinents the southern indian terrains formed part of a worldwide network of orogenic belts that is centered around 1 8 ga and outlines the configuration of columbia accretion of terrains in the madurai block to archean rocks north of the palghat cauvery shear zone at this time is consistent with the ages suggested to be the oldest metamorphic event in the eastern ghats orogen of eastern india and the rayner belt of coastal east antarctica our data confirm earlier evidence that all of southernmost india underwent resetting of isotopic systems during the final accretion of gondwana at the time of the pan african orogeny 0 5 ga the possibility that the trivandrum and nagercoil terrains accreted to the madurai block after 0 7 ga suggests that this pan african zone may also have involved accretion and closure of ocean basins if that happened then this zone may be the long sought suture between east and west gondwana

3d inversion of marine csem and mt data an approach to shallow water problem abstractthe application of the marine controlled source electromagnetic csem technique for shallow water hydrocarbon exploration is challenged by the effect of an airwave that masks the response from the reservoir at depth we applied gauss newton based 3d inversion method to synthetic csem and magnetotelluric mt data to demonstrate the capability of 3d inversion to recover thin resistive targets in shallow water environments the experiment on the model with water depth of 300 m shows that the target signals at 0 25 hz are significant enough to resolve the reservoir at a burial depth of 1 km for the given noise level 6 of the electric field amplitude for the 100 m water depth case the high resistivity zone recovered is located much deeper and thicker than the true reservoir suggesting that the airwave component obscures the target signal more seriously however inverting inline and broadside data together or adding inline data at a different frequency 0 1 hz is quite effective in improving t

geophysical evidence for marine prolongation of the palghat cauvery shear system into the offshore cauvery basin eastern continental margin of india abstract the precambrian basement shear zones played a major role in developing offshore structures that are zones of weakness along the eastern continental margin of india a compilation and analysis of various geophysical datasets particularly magnetic gravity seismicity and seismic reflection data support a clear geometric connection between the shear zones and offshore structures the presence of shear zones within the crystalline basement in the continental domain and oceanic domain is demarcated primarily by prominent magnetic lineation we identified three major magnetic lineaments in the offshore cauvery basin which correspond to the plaghat cauvery shear system pcss of southern granulite terrain sgt the faults interpreted in the seismic sections are correlated to the extension of pcss and indicate the location of reactivation of the shear zones which give rise to considerable intraplate seismicity the geophysical crustal model shows deep crustal scale faults associated with the marine prolongation of pcss the shear zone extended structures in the offshore might have formed at the breakup stage at the onset of rifting and might be extending up to the continent ocean transition zone cotz the pcss partly seems to accommodate the stress created by the northward push of indian plate against the asian plate it is also possible that the shear zone might have extended to the nearby land mass that marks it as one of the principal component in gondwana reconstruction

3d marine magnetotelluric inversion a hybrid impedance and direct field approach abstractthe measured electromagnetic field in magnetotellurics mt is composed of the natural source field and its subsurface response commonly the data are represented as impedances the complex ratio between the horizontal electric and magnetic fields this measure is independent of the source distribution because the impedance tensor estimation contains a deconvolution operator we have used a gauss newton type 3d mt inversion scheme to compare impedance data inversion with an inversion using the recorded electric field directly the use of the observed electric field is beneficial to the inversion algorithm because it simplifies the estimation of the sensitivities the direct field approach permits the use of the observed data without processing but it presumes knowledge of the source distribution a method to estimate the time variable strength and polarization of the incoming plane wave source is presented and tested on synthetic and real data examples the direct field inversion is successfully

precambrian tectonics and crustal evolution in south india about 3 4 ga ago voluminous calc alkaline magmas represented by the granitoid gneisses of southern india were emplaced into a now poorly preserved non continental crust unstable ensialic basins initiated at about 3 0 ga were filled with volcanic and sedimentary rocks up to about 2 6 ga this basement cover sequence was deformed at the close of the archaean first by northward accretion and thickening of several crustal slabs formerly separated by prisms of stable shelf sediments structures produced by this episode were refolded and dislocated by large n s strike slip shear belts to impart intense steep planar fabrics to large volumes of the crust fluids rich in co possibly derived from sedimentary material driven beneath the crustal slabs by the thickening mechanism purged the deep crust of h o to form granulites upward migration of hot h o rich fluids encouraged partial melting at intermediate crustal levels these late archaean tectonic and thermal events began soon after the close of basin f

strain cleavage and microstructure variations in sandstone implications for stiff layer behaviour in chevron folding sandstone layers within tightly chevron folded quartz rich turbidites of the lachlan fold belt southeastern australia show a fanning spaced cleavage overprinted by a non fanning grain alignment cleavage low internal strains recorded by quartz grain separations xz 1 4 1 8 and low fold flattening strains xz 1 8 angular relations between spaced cleavage and bedding in folds with varying tightness suggest that sandstone layers initially act as stress supporting members with spaced cleavage development due to pressure solution taking place at limb dips up to 15 during fold development sandstone layers undergo rigid body rotation involving interlayer slip without marked internal deformation except at fold hinges after fold lock up bedding spaced cleavage angles change due to inner arc collapse by pressure solution and subsequent flattening with formation of a grain alignment cleavage and mica beard growth internal deformation of sandstone is characterized by microstructures related to solution redeposition processes and growth of mica chevron folds form by homogeneous shortening and pressure solution due to layer parallel shortening tangential longitudinal strain due to buckling fold tightening by inner arc collapse due to pressure solution and fold flattening at metamorphic peak with growth of mica geometric modelling of internal strains and bedding cleavage angles requires a significant component of interlayer slip during chevron fold development under greenschist facies conditions

joint contrast source inversion of marine magnetotelluric and controlled source electromagnetic data abstractwe evaluated a joint contrast source inversion scheme for marine controlled source electromagnetic mcsem and magnetotelluric mt data based on a scattered field formulation the scheme considered only contrasts in electric conductivity and it allowed the medium to be transversely isotropic with a vertical symmetry axis the method was based on the integral equation formulation of electromagnetic field propagation and we demonstrated how the method solved the inverse problem of determining the conductivity structure of the subsurface the method did not consider mt impedances as data input to inversion but instead explicitly the field components and the consequences of this approach were discussed although there are challenges associated with source estimation and data noise we found it easier to make connections to csem and it simplified some computational issues three synthetic examples were considered to demonstrate the method a reservoir below an anisotropic overburden a salt diapi

米国南西部 コロラド川伸張地域 black山脈アコモデーション帯の背斜部分における中新世伸張と伸張しゅう曲運動 原標題は英語

the partitioning of deformation in flowing rock masses abstract most geological structures owe their development to heterogeneous and or non steady flow flow partitioning refers to the division of the instantaneous velocity field into components related to translation strain and rotation flow partitioning during the development of a geological structure usually changes with time and different histories of flow are involved at different points in the developing structure heterogeneity leads to the development of flow domains with different characteristics notably the degree of non coaxiality of deformation and different partitionings of the rotational component of flow into shear induced vorticity and spin during non steady flow there is often continual repartitioning of vorticity between the spin and shear induced components spin is an important aspect of flow leading to the development of structure this paper discusses the partitioning of deformation in a flowing rock mass coaxial flow tends to occur in isotropic materials in regions near weakly constrained boundaries non coaxial flow is favoured by suitably oriented material anisotropy and or the presence of well defined boundary constraints involving parallel relative displacements a developing anisotropy e g a crystallographic fabric in a shear zone can induce a change from coaxial to non coaxial flow other effects are related to the scale of approximation for example bulk shear induced vorticity can be locally converted to spin flow in the crust and mantle of the earth produces geological structures on all scales and involves many different types of rheological response the factors that affect flow partitioning e g spin versus non coaxial deformation are therefore of interest

analysis of airborne magnetic and gravity anomalies of peninsular shield india integrated with seismic and magnetotelluric results and gravity anomalies of madagascar sri lanka and east antarctica abstract modelling of gravity and airborne magnetic data integrated with seismic studies suggest that the linear gravity and magnetic anomalies associated with moyar bhavani shear zone mbsz and palghat cauvery shear zone pcsz are caused by high density and high susceptibility rocks in upper crust which may represent mafic lower crustal rocks this along with thick crust 44 45 km under the southern granulite terrain sgt indicates collision of dharwar craton towards north and sgt towards south with n s directed compression during 2 6 2 5 ga this collision may be related to contemporary collision northwards between eastern madagascar western dharwar craton wdc and eastern dharwar craton edc arcuate shaped n and s verging thrusts mbsz mettur shear and pcsz gangavalli shear respectively across cauvery shear zone system csz in sgt also suggest that the wdc edc and sgt might have collided almost simultaneously during 2 6 2 5 ga due to nw se directed compressional forces with csz as central core complex in plate tectonics paradigm preserving rocks of oceanic affinity gravity anomalies of schist belts of wdc suggest marginal and intra arc basin setting the gravity highs of egfb along east coast of india and regional gravity low over east antarctica are attributed to thrusted high density lower crustal upper mantle rocks at a depth of 5 6 km along w verging thrust which is supported by high seismic velocity and crustal thickening respectively it may represent a collision zone at about 1 0 ga between india and east antarctica paired gravity anomalies in the central part of sri lanka related to high density intrusives under western margin of highland complex and crustal thickening 40 km along eastern margin of highland complex with several arc type magmatic rocks of about 1 0 ga in vijayan complex towards the east may represent collision between them with w verging thrust as in case of egfb the gravity high of sri lanka in the central part falls in line with that of egfb in case it is fitted in gulf of mannar and may represent the extension of this orogeny in sri lanka

discrimination of iron ore deposits of granulite terrain of southern peninsular india using aster data abstract this work describes a new image processing technique for discriminating iron ores magnetite quartzite deposits and associated lithology in high grade granulite region of salem southern peninsular india using visible near infrared and short wave infrared reflectance data of advanced spaceborne thermal emission and reflection radiometer aster image spectra show that the magnetite quartzite and associated lithology of garnetiferrous pyroxene granulite hornblende biotite gneiss amphibolite dunite and pegmatite have absorption features around spectral bands 1 3 5 and 7 aster band ratios 1 3 2 3 5 4 5 7 6 in rgb are constructed by summing the bands representing the shoulders of absorption features as a numerator and the band located nearest the absorption feature as a denominator to map iron ores and band ratios 2 4 3 5 7 6 7 9 8 in rgb for associated lithology the results show that aster band ratios 1 3 2 3 5 4 5 7 6 in a red green blue rgb color combination identifies the iron ores much better than previously published aster band ratios analysis a principal component analysis pca is applied to reduce redundant information in highly correlated bands pca 3 2 and 1 for iron ores and 5 4 2 for granulite rock in rgb enabled the discrimination between the iron ores and garnetiferrous pyroxene granulite rock thus this image processing technique is very much suitable for discriminating the different types of rocks of granulite region as outcome of the present work the geology map of salem region is provided based on the interpretation of aster image results and field verification work it is recommended that the proposed methods have great potential for mapping of iron ores and associated lithology of granulite region with similar rock units of granulite regions of southern peninsular india this work also demonstrates the ability of aster s to provide information on iron ores which is valuable for mineral prospecting and exploration activities

a theory of finite strain variation through contrasting layers and its bearing on cleavage refraction abstract a theory of finite strain variation in contrasting viscous layers is presented the theory is applicable to layers which are oblique to two principal strains but parallel to the third and is not restricted to plane strain results may be obtained algebraically or by use of the mohr diagram for strain solutions are given for various examples of layering attitude and viscosity ratio measured with respect to a reference layer it is shown that the finite strain ellipsoid changes in shape and orientation across contrasting layers and in some cases the principal axes may be exchanged low strain is indicated in relatively more viscous layers and high strain with extension at a smaller angle to the layering in less viscous layers the latter may approximate simple shear parallel to layering if the strain is sufficiently high the geological implications of the theory are strong strain variations in layered rock sequences it is suggested that in general strain will be inhomogeneous from layer to layer both in orientation and amount the relationship of strain and cleavage are reviewed in the introduction to establish the validity of predicting cleavage patterns from strain data qualitative comparisons between the theoretical results of strain variation and natural cleavage refraction in layered rocks would appear to justify the assumption that cleavages of varied morphologies are sub parallel to the xy planes of strain with this assumption some three dimensional features of planar and linear fabric refraction in contrasting rocks are predicted

sm nd and rb sr ages and p t history of the archean sittampundi and bhavani layered meta anorthosite complexes in cauvery shear zone south india evidence for neoproterozoic reworking of archean crust sittampundi and bhavani archean layered meta anorthosite complexes occur as tectonic lenses within the cauvery shear zone csz a crustal scale shear dividing the precambrian granulite crust of south india into late archean 2 5 ga and proterozoic c 0 55 ga blocks they and their host supracrustal gneiss rocks record at least two stages of tectonometamorphic history the first is seen as regional scale refolded isoclinal folds and granulite metamorphism d1 m1 while the second stage is associated with dominantly e w dextral transcurrent shearing and metamorphic recrystallisation d2 mcsz whole rock sm nd isochrons for several comagmatic rocks of the layered complexes yield concordant ages sittampundi 2935 60 ma nd 1 85 0 16 and bhavani 2899 28 ma nd 2 18 0 14 2 errors our sm nd results suggest that 1 the magmatic protoliths of the sittampundi and bhavani layered complexes were extracted from similar uniform and lree depleted mantle sources 2 m1 metamorphism occurred soon after emplacement at c 3 0 ga ago p t estimates on garnet granulites from the sittampundi complex characterise the mcsz as a high p event with metamorphic peak conditions of c 11 8 kbar and 830 c minimum the mcsz is associated with significant isothermal decompression of the order of 4 5 3 5 kbar followed by static high temperature rehydration and retrogression around 600 c the timing of mcsz is inferred to be neoproterozoic at c 730 ma based on a whole rock garnet plagioclase hornblende sm nd isochron age for a garnet granulite from the sittampundi complex and its agreement with the 800 600 ma published age data on post kinematic plutonic rocks within the csz these results demonstrate that the cauvery shear zone is a zone of neoproterozoic reworking of archean crust broadly similar to the interface between the napier and rayner complexes of the east antarctic shield in a model proterozoic gondwana supercontinent

strain pattern and deformational history in the eastern part of the cauvery shear zone southern india abstract regional structural mapping guided by remote sensing interpretation in the eastern part of the cauvery shear zone csz southern granulite terrain reveals a structural framework of two major boundary shear zones interlinked by a set of sigmoidal shear belts the strain pattern and deformational history are manifest in terms of two major episodes d1 and d2 while d1 structures are well preserved in low strain areas the d2 fabrics are well reflected in high strain belts enveloping the low strain domains our multiscale structural observations can be modelled in terms of a plan view duplex showing a cross sectional flower structure related to regional transpression resulting from late neoproterozoic collisional processes in the eastern gondwana

southern granulite terrain south of the palghat cauvery shear zone implications for india madagascar connection abstract the present paper correlates the southern madgascar terrain south of the ranotsara shear with the granulite terrain of southern india occurring south of the palghat cauvery p c shear zone both the terrains have witnessed high temperature to ultra high temperature granulite metamorphism at 550 ma and are traversed by shear zones and deep crustal faults the 550 ma old granulite terrains of madagascar and southern india have similar lithologies in particular sapphirine bearing pelitic assemblages graphite deposits and gem occurrences are common to both these terrains the 550 ma old southern granulite terrain of southern india comprises of different blocks the madurai and the kerala khondalite belt but all the blocks have similar lithologies with pelite calc silicate rocks inter banded with two pyroxene granulite bodies these lithologies occur amidst an essentially charnockitic terrain the protolith ages of the southern granulite terrain south of the p c shear zone ranges between 2400 2100 ma the terrain as a whole has witnessed the 550 ma old granulite event the granulite metamorphism took place under temperatures of 800 1000 c and at pressures of 9 5 to 5 kbar the source of heat for the high temperature granulite event of the southern madagascar terrain has been linked to advective heat transfer along mantle deep faults the source for the high temperature granulite metamorphism for the southern granulite terrain may be attributed to high temperature carbonatite and alkaline intrusives in an extensional setting which followed an initial crustal thickening many workers have linked madagascar to southern india by connecting the ranotsara shear either to the p c shear zone or to the achankovil shear zone further south the important factor is the lithologies of the madagascar terrain south of ranotsara shear zone and the 550 ma old southern indian granulite terrain are similar in many aspects it will be more appropriate to link the ranotsara shear to the curvilinear lineament bounding the anaimalai kodaikanal ranges and which merges with the southern margin of the p c shear zone however north of the ranotsara shear fault the northern madagascar terrain comprises of a dominant itremo sequence

dextral pan african shear along the southwestern edge of the achankovil shear belt south india constraints on gondwana reconstructions a discussion achankovil shear zone acsz figure 1a intering charnockites garnet biotite gneiss biotite gneiss calc gneiss and granite as revealed in the preted as a major nw trending strike slip shear zone of probable proterozoic age drury et al 1984 lithological mapping carried out as part of this study figure 2 a preliminary structural assesshas prominently figured in gondwana correlative studies e g windley et al 1994 although it is ment reveals the presence of a distinctive assemblage of asymmetric structures in the gneissic exclear that there is a major lithologic change across acsz figure 1b detailed studies indicating the posures indicating a consistent sinistral sense of shear gneissosity forming the secondary compoprobable sense of shear are lacking across this 10 20 km wide and 100 km long shear zone resitional banding strikes wnw ese to nw se dipping moderately toward sw with relatively rare cently sacks et al 1997 reported field observations bearing on the probable sense of shear of the ne dip and is conformable in all the rock types of the area in conformity with the general nw se acsz after combining field criteria and kinematic analysis of asymmetric features along the sw trend of the acsz structurally the terrain has undergone four phases of deformation summarized in edge of the acsz known as the tenmala shear zone they concluded that the acsz indicated prirajesh and santosh 1997 open to isoclinal intrafolial folds encountered in marily dextral shear with a minor component of reverse shear the authors illustrate their discusthe study area are invariably asymmetric fold limbs are zones of transposed gneissosity that consion with examples of kinematic indicators including shear bands asymmetric folds stretched and tain indicators of a probable sinistral sense of shear and well developed mineral elongation lineation asymmetric garnet feldspar and quartz porphyroclasts hook folds and lineations thus they appardefined by quartz ribbons trails of xenoblastic garnet grains and the alignment of the long axes of kently question the correlation of the dextral achankovil shear zone rather tenmala shear zone with feldspar augen pinch and swell structures and boudins commonly show rotation with respect to the sinistral bongolava ranotsara shear zone brsz in the india madagascar gondwana enfoliation tight s asymmetric f2 folds and synkinematic pegmatite veins are seen in the western semble we focus the present discussion on the probable part of the study area the development of a refold structure by superposed buckling is illustrated in sense of shear of acsz in the light of new data our study area falls around punalur and its adjoinfigure 3a there are smaller asymmetric sand zfolds congruent with the larger f1 with one limb ing areas within the quilon district southern kerala south india and is located between longitudes striking e w while the other strikes n50 e showing negative extension during the development of 76 55 and 76 59 e and latitudes 9 1 and 9 3 n figure 2 represented in the survey of india topo f2 on the traces of preserved axial planar cleavage the simple shear is dextral on the left limb and sisheet no 58c 16 outcrops within the study area expose a strongly deformed suite of rocks comprisnistral on the right limb asymmetrically folded veins figure 3b the bulk asymmetry of which represents a sinistral sense of shear have been found 1 manuscript received may 14 1997 in a few locations elongate quartz lenticles and 2 centre for earth science studies pb 7250 trivandrum 695 031 india garnet and biotite aggregates define penetrative lin

late pleistocene paleohydrography and diatom paleoecology of the central basin of lake malawi africa analysis of sedimentary diatom assemblages 10 to 144 ka form the basis for a detailed reconstruction of the paleohydrography and diatom paleoecology of lake malawi lake level fluctuations on the order of hundreds of meters were in ferred from dramatic changes in the fossil and sedimentary archives many of the fossil diatom assemblages we observed have no analog in modern lake malawi cyclotelloid diatom species are a major component of fossil assemblages prior to 35 ka but are not found in significant abundances in the modern diatom communities in lake malawi salinity and alkalin ity tolerant plankton has not been reported in the modern lake system but frequently dominant fossil diatom assemblages prior to 85 ka large stephanodiscoid species that often dominate the plankton today are rarely present in the fossil record prior to 31 ka similarly prior to 31 ka common central basin aulacoseiroid species are replaced by species found in the shallow well mixed southern basin surprisingly tychoplankton and periphyton were not common throughout prolonged lowstands but tended to increase in relative abundance during periods of inferred deeper lake environments a high resolution lake level reconstruction was generated by a principle component analysis of fossil diatom and wetsieved fossil and mineralogical residue records prior to 70 ka fossil assemblages suggest that the central basin was periodically a much shallower more saline and or alkaline well mixed environment the most significant reconstructed lowstands are 600 m below the modern lake level and span thousands of years these conditions contrast starkly with the deep dilute dysaerobic environments of the modern central basin after 70 ka our reconstruction indicates sustained deeper water environments were common marked by a few brief but significant lowstands high amplitude lake level fluctuations appear related to changes in insolation seismic reflection data and additional sediment cores recovered from the northern basin of lake malawi provide evidence that supports our reconstruction