paleoproterozoic granitoids in the basement of bangladesh a piece of the indian shield or an exotic fragment of the gondwana jigsaw abstract we present the first precise age for precambrian basement rocks in bangladesh these lie within the dinajpur block located between the indian craton to the southwest and the shillong massif to the east there are no surface outcrops and the rocks were intersected by drill holes they consist of a suite of tonalitic and granodioritic rocks variously deformed to granitic gneiss and intruded by younger monzogranite a tonalite obtained at a depth of 227 48 m in drill hole bh 2 at maddhapara in northwestern bangladesh records a shrimp zircon 207pb 206pb magmatic age of 1722 6 ma paleoproterozoic rocks with similar magmatic ages are unknown in the adjacent indian blocks of the chotanagpur plateau indian craton or shillong massif this lack of comparable ages may be due to the paucity of precise radiometric ages from the indian terrains or more likely because there are real age differences with the buried rocks at maddhapara representing a separate and discrete microcontinental fragment the dinajpur block that was trapped by the northward migration of india during gondwana dispersal

mid archaean evolution of the eastern indian craton geochemical and isotopic evidence from the bonai pluton abstract the archaean eastern indian craton consists of several granitoid batholiths intruding greenstone like sequences the bonai pluton one such body in the central sector of the craton consists of a dominant porphyritic and a subordinate equigranular variety ranging in composition from trondhjemite to granite xenoliths of massive trondhjemite migmatite and quartzite occur in both phases the granitoid xenoliths represent remnants of a sialic crust and are the oldest recognizable element in the bonai pluton these are high al 2 o 3 trondhjemites with a highly fractionated rare earth element ree pattern a pb pb isochron age of 3369 57 ma has been obtained from a few spatially related xenoliths the porphyritic and equigranular granitoids show similar petrographic characteristics and most of the samples show a calc alkaline trondhjemitic association a few granite samples however belong to a normal calc alkaline association the trondhjemitic samples are of the low al 2 o 3 type with fractionated light ree lree flat heavy ree hree and a marked negative eu anomaly granites of the calc alkaline association have an identical ree pattern pb isotopic data for all samples together yield a pb pb isochron age of 3163 126 ma the ree characteristics of the two trondhjemite suites can be generated by partial melting of a mafic source with variable residual phases at different depths on the other hand the normal calc alkaline granite may be generated by partial melting of trondhjemite similar in composition to that represented by the 3369 ma xenoliths isotopic ages and the chemical nature of the bonai pluton are similar to those identified in the adjacent singhbhum granite complex it is suggested that the entire eastern indian craton has a common mid archaean history the earliest sialic crust formed at 3400 ma and was followed by trondhjemitic addition at 3300 ma during this phase restricted reworking of the older sialic crust produced calc alkaline granites

petrology geochemistry and zircon u pb lu hf isotopes of paleoproterozoic granite gneiss from bomdila in the western arunachal himalaya ne india

proto tethyan remnants in northwest iran geochemistry of the gneisses and metapelitic rocks abstract the iran continental crust was metamorphosed intruded by granitoid magmas folded and faulted during the late precambrian by the pan african orogeny the basement complex in the takab complex northwest iran consists of gneisses amphibolites pelitic schists meta ultramafic and calc silicate rocks geochemically the protoliths of the takab gneisses are slightly peraluminous and medium to high potassic with calc alkaline affinity these gneisses may have been emplaced in volcanic arc tectonic setting furthermore the metapelitic protolith is shale deposited in an active continental margin setting all these characteristics and presence of paleo suture zone and ophiolitic rocks i e serpentines meta mafic and meta ultramafic rocks around the high grade metamorphic rocks suggest that a continental margin magmatic arc andean type formed the takab precambrian basement the basement complexes are extensively overprinted by the pan african orogeny and younger igneous events this supports the inference that early cambrian orogenesis in the takab complex region of northwest iran marks one of the fundamental lithospheric boundaries within gondwana which belonged to a greater late neoproterozoic early paleozoic orogenic system that was active along the proto tethyan margin of the gondwana supercontinent extending at least from its arabian margin to the himalayan margin of the indian subcontinent

provenance of the greater himalayan sequence evidence from mafic granulites and amphibolites in nw bhutan abstract mafic granulites and amphibolites in the masang kang area of nw bhutan himalaya have been investigated for their geochemical and isotopic characteristics in order to determine their protolith history bulk rock major and trace element geochemistry indicate that the rocks were originally tholeiitic and alkali basalts with minor ultramafics u pb zircon sims data suggest an age of 1742 39 ma for mafic magmatism the age corrected e nd 1742 values of the rocks are highly variable ranging from high positive 8 4 to negative 3 3 the positive value suggests a primitive magma source similar to that of rift related tholeiites we suggest that the rocks of the masang kang suite were produced during a major late paleoproterozoic thermal event that caused the mobilization and enrichment of the sub continental lithospheric mantle beneath the north indian margin the geochemical signature of these rift related metabasic rocks may have been produced during an earlier episode of oceanic underplating or subduction from which the fluid required to mobilize and enrich the overlying sub lithospheric mantle may have been derived though their occurrence is rare paleoproterozoic igneous rocks within the greater himalayan sequence ghs in addition to sources identified throughout the lhs may have contributed to the detrital zircon population that form the 1 7 1 9 ga peak in the age spectra of the lesser himalayan sequence lhs in addition the coeval paleoproterozoic magmatism in both lhs and ghs suggests that the two lithotectonic units may have belonged to the same continental plate at that time period

core complex geometries and regional scale flow in the lower crust a simple flexural model that treats the lithosphere as a thin elastic plate overlying a fluid asthenosphere produces an excellent fit to the domal detachment fault geometries observed in typical core complexes of the basin and range province however this mode of isostatic compensation requires an unacceptably large reduction in local topographic elevation 3 5 km above core complex structures because topographic lows of this magnitude are not observed above core complexes today it requires mountain ranges 3 5 km higher than the surrounding areas to have existed at the site of each future core complex and requires these mountain ranges to have had large crustal roots that were subsequently uplifted to form a flat moho as the core complexes developed an alternate mode of isostatic compensation whereby compensation occurs primarily by regional scale flow of material within the lower crust beneath an elastic midcrust provides an equally good fit to the observed detachment fault geometries while requiring little reduction in local topographic elevation above developing core complex structures and providing an obvious means of maintaining a flat moho beneath these zones of upper crustal doming if this interpretation is correct then the effective elastic plate thickness below the basin and range province probably cannot exceed a maximum of about 4 km with a best guess estimate of about 0 5 to 1 km computation of the temperature structure associated with crustal flow during doming shows that an increase in temperature at the moho of about 100 c follows doming by several million years perhaps explaining the observed coincidence of core complexes with crustally derived igneous rocks subsequent cooling of the mid to lower crust beneath the core complex could result in freezing of the initially ductile lower crust within the core of the dome effectively locking the domal shape into the crust even after the detachment surface is cut by younger faults

tertiary extension and contraction of lower plate rocks in the central mojave metamorphic core complex southern california several authors recently have proposed the rolling hinge model for the formation of extensional detachment faults in metamorphic core complexes in this model isostatic uplift of the denuded footwall of a major normal fault causes the near surface part of the fault to rotate from a higher to a lower dip for the rolling hinge model to be acceptable the footwalls of detachment faults must have undergone the strain pattern dictated by the rolling hinge in this paper the structural development of the footwall of the central mojave metamorphic core complex is used as a field test tertiary structures in the lower plate of the core complex record ne directed ductile extensional shear followed by cataclastic ne side up shear with a significant component of contraction along the previous extension direction this strain history agrees with the rolling hinge model and particularly with a version of the model in which footwall uplift occurs by a combination of elastic flexure and flexurally driven upper crustal failure similar late stage layer parallel contraction appears to be present in some other tectonically denuded terranes of the basin and range suggesting that the flexural failure rolling hinge model for extensional terranes may be widely applicable

atlantic meridional overturning circulation and the southern hemisphere supergyre 1 the ocean s role in climate manifests itself through its high heat capacity its own rich internal dynamics and its ability to transport vast quantities of heat and freshwater of particular interest is the global ocean circulation associated with the atlantic meridional overturning amoc because observations are sparse the detailed global structure of the amoc remains poorly understood particularly the pathways along which water returns to the atlantic to compensate the export of north atlantic deep water nadw here we provide the first quantitative 3 dimensional global view of the amoc using a lagrangian reconstruction which integrates hundred of thousands water particle trajectories in an ocean model the resulting pattern elucidates the role of the wind in structuring the pathways of the amoc in particular the lagrangian analysis reveals a strong link with the three subtropical gyres of the southern hemisphere sh which merge into a supergyre spanning the three ocean basins the coupling between the upper ocean wind driven circulation and the overturning in the model suggests that changes in the sh winds can alter the pathways of the amoc and therefore the water properties and the associated heat and freshwater transports

u pb zircon geochronology of major lithologic units in the eastern himalaya implications for the origin and assembly of himalayan rocks models for the origin and deformation of himalayan rocks are dependent upon geometric and age relationships between major units we present field mapping and u pb dating of igneous and detrital zircons that establish the lithostratigraphic architecture of the eastern himalaya revealing that 1 the south tibet detachment along the bhutan china border is a top to the north ductile shear zone 2 late triassic and early cretaceous sedimentary samples from the northern indian margin show a similar age range of detrital zircons from ca 3500 ma to ca 200 ma but the late triassic rocks are distinguished by a significant age cluster between ca 280 and ca 220 ma and a well defined age peak at ca 570 ma 3 an augen gneiss in the south tibet detachment shear zone in southeast tibet has a cambrian ordovician crystallization age 4 main central thrust hanging wall paragneiss and footwall quartzites from the far western arunachal himalaya share similar provenance and late proterozoic maximum depositional ages and 5 main central thrust footwall metagraywacke from the central western arunachal himalaya has a paleoproterozoic maximum depositional age indicated by a single prominent age peak of ca 1780 ma recent work in the eastern himalaya demonstrates that in the early middle miocene the himalayan crystalline core here was emplaced southward between two subhorizontal shear zones that merge to the south a proposed subsequent middle miocene brittle low angle normal fault accomplishing exhumation of these rocks along the range crest can be precluded because new and existing mapping demonstrates only a ductile shear zone here the ca 280 220 ma detrital zircons of the late triassic strata are derived from an arc developed along the northern margin of the lhasa terrane detritus from this arc was deposited on the northern margin of india during india lhasa rifting along strike heterogeneity in main central thrust footwall chronostratigraphy is indicated by detrital zircon age spectrum differences from central western to far western arunachal nonetheless the late proterozoic rocks in the main central thrust hanging wall and footwall in far western arunachal can be correlated to each other and to previously analyzed rocks in the south tibet detachment hanging wall to the west and in the indian craton to the south these findings are synthesized in a reconstruction showing late triassic india lhasa rifting and cenozoic eastern himalayan construction via in situ thrusting of basement and cover sequences along the north indian margin

geochronological constraints on evolution of singhbhum mobile belt and associated basic volcanics of eastern indian shield abstract the singhbhum mobile belt smb of the eastern indian shield represents a roughly east west trending arcuate belt of folded supracrustals overlying the granite greenstone basement of the singhbhum orissa craton along its northern eastern and western margins and is bounded by the chotanagpur gneissic complex to further north the radiometric ages of the basement singhbhum and equivalent granites and the intrusive anorogenic mayurbhanj granite pluton constrain the time of evolution of this mobile belt between 3 12 and 3 09 ga hence the smb supracrustals also known as singhbhum group is late mesoarchaean in age and not proterozoic as thought earlier the evolution of the smb was followed by emplacement of some major basic igneous rocks within or adjacent to the supracrustals these include simlipal volcanics at 3 09 ga on the smb mayurbhanj gabbro along with mayurbhanj granite at 3 09 ga along the marginal part of the craton near the smb and the dalma volcanics on the smb along with the dhanjori volcanics adjacent to smb at 2 80 ga the 2 80 ga old basic volcanics is also associated with emplacement of some small granite plutons occurring along the marginal part of the craton one of them the tamperkola granite intrudes the smb the 3 09 ga onward igneous activities along the marginal part of singhbhum orissa craton took place essentially under anorogenic tectonic setting before being affected by a major metamorphism at 2 50 ga which is recorded on the dalma volcanics and on some small granite pluton occurs along the marginal part of the craton the jagannathpur and stratigraphically equivalent malangtoli volcanics occurring within the singhbhum orissa craton at the west were erupted at 2 25 ga the boundary between the smb supracrustals and the singhbhum orissa craton is demarked by a prominent shear zone known as the singhbhum shear zone which shows multiple reactivation the oldest being at 3 09 ga followed by subsequent reactivation during palaeo and mesoproterozoic periods at 2 2 1 8 1 6 1 5 1 4 and 1 0 ga respectively the singhbhum group and the adjacent chotanagpur gneissic complex appear to have evolved from a near shore syn rift and a distal post rift stable shelf sedimentary assemblages respectively which were deposited without any stratigraphic break in a marine basin existed in the present north of the singhbhum orissa craton both of these assemblages were deformed and metamorphosed together during proterozoic at 2 5 to 2 3 ga 1 6 ga and 1 0 ga

mesoscale and submesoscale processes in the southeast atlantic and their impact on the regional thermohaline structure the turbulent processes in the cape basin the southeasternmost gate of the atlantic ocean play a key role in the transport and mixing of upper to intermediate water masses entering the area from the indian ocean making them especially relevant for the indo atlantic transfer of heat and salt in this paper two numerical simulations at different horizontal resolutions are used to study meso and submesoscale dynamics their phenomenology their evolution and their impact on the local water masses submesoscale processes seasonally affect both the upper and intermediate layers but there are clear dynamical differences between the two layers several types of instabilities underline this spatial and temporal variability near the surface mixed layer instabilities occur during winter while mesoscale driven instabilities as the symmetric type prevail in summer the connection between these two seasonal regimes is ensured in anticyclonic eddies and within the mixed layers by charney baroclinic instabilities involved in the local formation and subduction of mode water that we have dubbed as agulhas rings mode water intermediate depths are instead characterized by mesoscale mechanisms of density compensation and lateral stirring of the tracer variance triggering a significant filamentogenesis whose vertical scales are comparable to those mentioned in previous studies this leads to a particularly efficient mixing of antarctic intermediate waters of indian and atlantic origins lagrangian estimates highlight the new and significant role of fine scale structures in setting the water masses properties of upper and lower thermocline waters materializing the indo atlantic exchange and therefore potentially affecting the global ocean circulation

dynamics of agulhas retroflection and ring formation in a numerical model part ii energetics and ring formation abstract an energetics analysis of several numerical experiments on an idealized south atlantic indian ocean basin is presented the model used in the experiments is the quasi isopycnic coordinate model of bleck and boundra forced by wind and configured with two or three layers the region of focus is the most dynamically active one the agulhas current retroflection south of africa and the dynamical mechanisms associated with formation of agulhas rings are given special attention whether rings form in the model and their frequency depend on two primary factors the shape of africa and southward inertia baroclinicity in the overshooting agulhas the boundary condition on africa no slip free slip and horizontal resolution are also important experiments in which rings form exhibit considerably larger values of km to ke transfer than those in which no rings form in three of the experiments ring formation is studied in detail with the help of instantaneous top and bottom layer flow patterns and time se

characteristics energetics and origins of agulhas current meanders and their limited influence on ring shedding abstractthe agulhas current intermittently undergoes dramatic offshore excursions from its mean path because of the downstream passage of mesoscale solitary meanders or natal pulses new observations and analyses are presented of the variability of the current and its meanders using mooring observations from the agulhas current time series experiment act near 34 s using a new rotary eof method mesoscale meanders and smaller scale meanders are differentiated and each captured in a single mode of variance during mesoscale meanders an onshore cyclonic circulation and an offshore anticyclonic circulation act together to displace the jet offshore leading to sudden and strong positive conversion of kinetic energy from the mean flow to the meander via nonlinear interactions smaller meanders are principally represented by a single cyclonic circulation spanning the entire jet that acts to displace the jet without extracting kinetic energy from the mean flow synthesizing in situ observations with altimeter

the retroflection paradox abstract the classical question of what happens when a warm western boundary current such as the north brazil current nbc retroflects is addressed analytically using a reduced gravity nonlinear model the traditional view is that the northwestward flowing current separates from the wall turns to the right looking offshore and forms a zonal boundary current that flows eastward integration of the steady inviscid momentum equation along the boundary gives the longshore momentum flux or flow force and shows that such a scenario leads to a paradox to resolve the paradox the separated current must constantly shed anticyclones which propagate to the northwest due to and an interaction with the boundary this new eddy shedding mechanism which is not related to the traditional instability of a zonal jet may explain why the nbc must produce rings a nonlinear analytical solution to the problem is constructed with the aid of a powerful theoretical approach based on the idea that nonlinear periodic

postmylonitic deformation in the raft river metamorphic core complex northwestern utah evidence of a rolling hinge much of the recent debate over low angle normal faults exposed in metamorphic core complexes has centered on the rolling hinge model the model predicts tilting of seismogenic high angle normal faults to lower dips by footwall deformation in response to isostatic forces caused by footwall exhumation this shallow brittle deformation should visibly overprint the mylonitic fabric in the footwall of a metamorphic core complex the predicted style and magnitude of rolling hinge strain depends upon the macroscopic mechanism by which the footwall deforms two end members have been proposed subvertical simple shear and flexural failure each mechanism should generate a distinctive pattern of structures that strike perpendicular to the regional extension direction subvertical simple shear svss should generate subvertical faults and kink bands with a shear sense antithetic to the detachment for an svss hinge the hinge related strain magnitude should depend only on initial fault dip rolling hinge structures should shorten the mylonitic foliation by 13 for an initial fault dip of 30 in flexural failure the footwall behaves as a flexed elastic beam that partially fails in response to bending stresses resulting structures include conjugate faults and kink bands that both extend and contract the mylonitic foliation extensional sets could predominate as a result of superposition of far field and flexural stresses strain magnitudes do not depend on fault dip but depend on the thickness and radius of curvature of the flexed footwall beam and vary with location within that beam postmylonitic structures were examined in the footwall of the raft river metamorphic core complex in northwestern utah to test these predictions observed structures strike perpendicular to the regional extension direction and include joints normal faults tension gash arrays and both extensional and contractional kink bands aside from the subvertical joints the extensional structures dip moderately to steeply and are mainly either synthetic to the detachment or form conjugate sets range wide the extensional structures accomplish about 4 elongation of the mylonitic foliation contractional structures dip steeply mainly record shear antithetic to the detachment and accomplish 1 contraction of the foliation these observations are consistent with the presence of a rolling hinge in the raft river mountains but a rolling hinge that reoriented a high angle normal fault by svss is excluded the pattern and magnitudes of strain favor hinge related deformation mainly by flexural failure with a subordinate component of svss

fluid inclusion constraints on the kinematics of footwall uplift beneath the brenner line normal fault eastern alps dynamic models of isostatic footwall uplift in response to normal faulting can be divided into those in which uplift is accomplished by flexural failure and those in which uplift occurs via subvertical simple shear each class of model predicts a different incremental strain history that should be recorded in the footwall in the tauern window eastern alps postmylonitic structures in the footwall of the brenner line normal shear zone predominantly consist of closely spaced steep west down and east down microfaults formation of the west down faults before and at greater depths than the east down faults would be consistent with unroofing via subvertical simple shear in contrast formation of the two fault types as a conjugate set would be more indicative of unroofing via elastic processes the field data alone do not provide a sufficient test of the two hypotheses because crosscutting relations are only rarely observed and there is no control on the depth at which the structures formed however both depth and timing constraints on the formation of the late structures can be obtained by correlating the orientations of fluid inclusion lined microfaults with the macroscopic west down and east down faults obtaining density data for the inclusions and correlating these data with previously obtained geochronologic data the results indicate that the west down structures formed at depths of 10 20 km and temperatures 450 c in the mid to late oligocene and that the east down structures formed at 2 to 10 km depth and temperatures of 300 50 c in the mid miocene these data support the hypothesis that a rolling hinge was present in the footwall of the brenner line and that isostatically driven footwall deformation was accomplished predominantly by subvertical simple shear the depths at which west down and east down faulting occurred coupled with the angle of dip of the brenner line yield a minimum lateral displacement on the fault of 15 26 km approximately coeval ductile shearing and brittle faulting at depths of 15 20 km and temperatures in excess of 400 c may reflect local variations in strain rate as the footwall rocks entered the zone of rolling hinge deformation

isostatic rebound due to tectonic denudation a viscous flow model of a layered lithosphere a four layer model of the upper 150 km of the earth is used to calculate the viscous response of continental crust and the underlying mantle to tectonic denudation the model comprises a strong upper crustal layer a weak lower crustal layer a very strong mantle lithosphere layer and a weak mantle asthenosphere layer which is in accord with experimental constraints on strength depth profiles for continental lithosphere the strength of each layer is represented by its effective viscosity flow in the crust and mantle is driven by buoyancy forces which arise from the unloading of an allochthon along a detachment fault by a series of instantaneous displacements earthquakes or rapid creep events numerical solutions obtained by using a finite element technique predict footwall uplift moho deflection and surface topography that are consistent with observations from the basin and range province of the western united states the calculated curvature of the footwall uplift is also similar to that observed and is sensitive to the geometry of the detachment fault such bending need not be elastically controlled hence the curvatures of footwall domes do not clearly place limits on the effective elastic thickness of the extending crust the upward deflection of the moho and the surface topography are sensitive to the viscosity structure and enable us to bound the range of the various viscosities by matching observations from the basin and range province which indicate no moho deflection and low magnitude of surface topography 3 5 km we estimate the upper crustal lower crustal and mantle lithospheric viscosities in the ranges 1021 1023 pa s 1019 1021 pa s and 1021 1023 pa s respectively

break up of australia india madagascar block opening of the indian ocean and continental accretion in southeast asia with special reference to the characteristics of the peri indian collision zones abstract linear belts of gondwana basins developed in the indian continent since late palaeozoic along favoured sites of precambrian weak zones like cratonic sutures and reactivated mobile belts the tibetan and sibumasu west yunnan continental blocks that were located adjacent to proto himalayan part of the indian continent rifted and drifted from the northern margin of the east gondwanic indo australian continent during late palaeozoic when the said northern margin was under glacial or cool climatic condition and rift drift tectonic setting the indo burma andaman iba sikule lolotoi blocks were also rifted and drifted from the same northern margin during late jurassic this was followed by the break up of the australia india madagascar continental block during the cretaceous the activity was associated with hot spot related volcanism and opening up of the indian ocean the late cretaceous and tertiary phases of opening of the arabian sea succeeded the early cretaceous phase of opening of the bay of bengal part of the indian ocean the palaeo and neo tethyan sutures in tibet yunnan laos thailand and vietnam reveal the complex opening and closing history of the tethys the iba block rotated clockwise from its initial e w orientation because of 90 e and adjacent dextral transcurrent fault movements caused due to faster northward movement of the indian plate relative to that of australia the india tibet terminal collision during early middle eocene initiated himalayan orogenesis and contemporaneously there was foreland basin development that was accompanied with sporadic but laterally extensive continental flood basalt cfb type and related volcanism the paleogene rocks of the himalayan foreland basin are involved in tectonism and are mostly concealed under older rocks the mesozoic early eocene ophiolite terrane on iba does not represent the eastern suture of the indian plate but occurs as klippe on iba caused due to oblique collision between sibumasu and iba during late oligocene post collisional indentation of y shaped indian continent into the asian collage produced himalayan syntaxes clockwise rotation of the sibumasu block which was then sutured to the tibetan and se asian blocks and tectonic extrusion of the indochina block along the ailao shan red river asrr shear zone highly potassic magmatic rocks were emplaced during late palaeogene at the oroclinally flexed marginal parts of the south china continental lithosphere these magmatic bodies were dislocated by the asrr left lateral shear zone soon afterwards petrogenetic and tectonic processes that generated the eocene cfb volcanics at the himalayan foreland basin may have also produced late palaeogene magmatism from outer parts of the namche barwa syntaxis their site specific location and time sequence suggest them to be genetically related to the india asia collision process and indian continent s indentation induced syntaxial buckling deep mantle reaching fractures were apparently produced during india asia terminal collision at the strongly flexed leading brittle edge of the indian continental lithosphere and possibly later in time at the outer oroclinally bent marginal parts of the rigid south china continental lithosphere generating typical magma the subduction zone that developed along the western margin of iba due to oblique convergence between the iba and the indian plate is still active the northern end of iba ultimately collided with the ne prolongation of the indian continent and was accreted to it during mio pliocene the shillong massif was uplifted and overthrust over the bengal basin located over its passive margin to the south whereas the eocene distal shelf sediments of iba were overthrust over the tertiary shelf of the indian continent

petrogenetic interpretation of granitoid rock series using multicationic parameters abstract granitoid rock compositions from a range of tectonic environments are plotted on a multicationic diagram devised by de la roche and his coworkers this shows that there is a systematic change through an orogenic cycle which leads progressively to the ultimate development of alkaline magmas possible source materials and mechanisms of magma generation are considered from analysis of mineral compositional vectors these suggest that most granitoid series result from a two stage process first fractional crystallisation of clinopyroxene olivine and calcic plagioclase from a basic source with tholeiitic affinities produces a magma of intermediate composition this magma then undergoes periodic mixing with a felsic magma followed by in situ fractionation to generate individual intrusions within granitoid series

sedimentation pattern and tectonic evolution of the proterozoic singhbhum basin in the eastern indian shield abstract the proterozoic singhbhum basin psb located on the flank of an archaean platform ca 3400 ma is a distinct lithotectonic unit of the singhbhum crustal province of the eastern indian shield the present contribution traces the evolution of this basin filled by volcanics dominated submarine supracrustals and sedimentary formations the proterozoic singhbhum basin has been identified to be a marginal basin evolved in a back arc setting an axial volcanic belt manifesting mor like basaltic eruptions divided the basin into two domains with parallel but distinct sedimentation patterns an asymmetry in sedimentation pattern and source polarity are documented in litho as semblages and ree distribution patterns of cover rocks in the basinal domain flanking the southern platform prevailing tectonics strongly controlled the mechanism of sedimentation which coupled with the volcanic history of the basin indicates its riftogenic character progressive lithospheric stretching successively produced picritic volcanics and mor type basalts along the axial zone of the basin being related to a stretching factor of 2 continental margin rifting ultimately resulted in an oceanic transition as supported also by available geophysical data the evolution of the proterozoic singhbhum basin in its broad perspective was controlled by subduction related processes

area balance depth to detachment and strain in extension a new lost area cross section construction and validation technique is based on the area balance of a graben system formed above a detachment area balance requires that the area displaced below the original regional level of each bed in a graben the lost area be balanced by the displaced area at the boundary of the system given by the product of the displacement times the depth to detachment this relationship is a straight line on a plot of lost area versus depth the slope of the line is the displacement and the depth intercept is the depth of the detachment in general beds within the graben system undergo both visible and homogeneous layer parallel extension the homogeneous component can be determined from the width of the graben system the lost area and the depth to detachment conversely the detachment depth may be calculated from the total layer parallel extension and the lost area the techniques are illustrated by application to models and to very well constrained natural examples an earthquake induced slope failure in alaska and kilometric sized horsts and grabens in the black warrior foreland basin of alabama a half graben without footwall uplift and a full graben are the same on a lost area diagram a half graben with footwall uplift linked to the hanging wall extension requires that both the hanging wall and footwall areas be included to determine the detachment depth and displacement all the models show layer parallel extension e in the upper levels that are less than the crustal scale strain

agulhas ring formation as a barotropic instability of the retroflection 1 agulhas leakage is an important link in the global ocean circulation as it transfers a significant volume of relatively warm and salty water from the indian ocean to the atlantic ocean the main route of this transfer is through the shedding of large agulhas rings from the agulhas retroflection in this paper we study the dynamics of the ring formation process by analyzing the stability of the indian atlantic supergyre in a reduced gravity model we show that the ring shedding process results from a barotropic instability of the steady circulation in the agulhas retroflection region the destabilizing mode appears to be linked to a rossby basin mode of the combined south indian atlantic basin which is localized in the retroflection region by the background flow

geologic correlation of the himalayan orogen and indian craton part 2 structural geology geochronology and tectonic evolution of the eastern himalaya despite being the largest active collisional orogen on earth the growth mechanism of the himalaya remains uncertain current debate has focused on the role of dynamic interaction between tectonics and climate and mass exchanges between the himalayan and tibetan crust during cenozoic india asia collision a major uncertainty in the debate comes from the lack of geologic information on the eastern segment of the himalayas from 91 e to 97 e which makes up about one quarter of the mountain belt to address this issue we conducted detailed field mapping u pb zircon age dating and 40ar 39ar thermochronology along two geologic traverses at longitudes of 92 e and 94 e across the eastern himalaya our dating indicates the region experienced magmatic events at 1745 1760 ma 825 878 ma 480 520 ma and 28 20 ma the first three events also occurred in the northeastern indian craton while the last is unique to the himalaya correlation of magmatic events and age equivalent lithologic units suggests that the eastern segment of the himalaya was constructed in situ by basement involved thrusting which is inconsistent with the hypothesis of high grade himalaya rocks derived from tibet via channel flow the main central thrust in the eastern himalaya forms the roof of a major thrust duplex its northern part was initiated at ca 13 ma while the southern part was initiated at ca 10 ma as indicated by 40ar 39ar thermochronometry crustal thickening of the main central thrust hanging wall was expressed by discrete ductile thrusting between 12 ma and 7 ma overlapping in time with motion on the main central thrust below restoration of two possible geologic cross sections from one of our geologic traverses where one assumes the existence of pre cenozoic deformation below the himalaya and the other assumes flat lying strata prior to the india asia collision leads to estimated shortening of 775 km 76 strain and 515 km 70 strain respectively we favor the presence of significant basement topography below the eastern himalaya based on projections of early paleozoic structures from the shillong plateau i e the central shillong thrust located 50 km south of our study area since northeastern india and possibly the eastern himalaya both experienced early paleozoic contraction the estimated shortening from this study may have resulted from a combined effect of early paleozoic and cenozoic deformation

pre breakup continental extension in east gondwanaland and the early opening of the eastern indian ocean abstract extension of continental crust by up to 360 km on a north northeast azimuth in the great australian bight occurred prior to the middle cretaceous 96 ma onset of seafloor spreading between australia and antarctica this large amount of continental extension is constrained to the late jurassic 160 ma middle cretaceous interval about a pole estimated to lie about 40 southeastward of southeastern australia using bathymetric data combined with seismic and magnetic determinations of the continent ocean boundaries off australia india and antarctica we determine a revised fit of east gondwanaland prior to continental extension and at various stages thereafter in the first stage from 160 to 132 5 ma m11 india australia rotated from antarctica with continental crustal extension between australia and antarctica and largely transform motion between the coromandel coast margin of india and the kron prins olav kyst margin of antarctica in the second stage from 132 5 to 96 ma india rotated northwestward away from australia antarctica producing m10 and younger magnetic anomalies along the western margin of australia continental extension was greatest between australia and antarctica during this stage the newly determined 132 5 96 ma rotation between india and australia provides an improved fit between the orientation of observed m10 m0 magnetic anomalies and the southward decrease in spreading rate off the western australian margin together with bathymetry the new rotation parameters lead to identification of the abandoned 96 ma spreading ridge along the lost dutchmen and dirck hartog ridges the naturaliste fracture zone is found to lie nearly parallel to the early cretaceous transform direction between india and antarctica suggesting that slow seafloor spreading required between australia and antarctica to accommodate the continental extension further east occurred between naturaliste fracture zone and the m4 m0 anomalies adjacent to the naturaliste plateau the revised fit of india in east gondwanaland reduces the eastward extent of greater india to 95 e but does not change its extent as far north as the cape range fracture zone the new pole determined for the early cretaceous rotation between india and australia antarctica lies within 15 of the southern tip of india significantly closer than in previous determinations thereby reducing the amount of seafloor spreading generated between africa and antarctica by the india antarctica rotation and thus offering a possible solution to the perceived spreading rate problems between m11 and m0 in the somali mozambique and antarctic basins

flexural rotation of normal faults a conceptual model is proposed for the generation of low angle normal faults in metamorphic core complexes the model is based on three assumptions 1 the isostatic response to normal fault motion is of regional extent 2 when a fault segment is significantly rotated from the optimum angle of slip relative to the crustal stress field it is replaced by a new planar fault oriented in the optimum direction and 3 the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust the stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations active low angle normal faults are difficult to reconcile with rock mechanics theories earthquake focal mechanism studies and geochronologic results indicating rapid cooling of core complex rocks the model does not require active fault slip on low angle faults the flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation the anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust this can explain the observed short wavelength of topographic response to normal fault loads the model results in 1 a nearly flat lying abandoned normal fault or detachment below slices of upper plate rocks and sedimentary infill 2 a strong contrast in metamorphic grade across the abandoned detachment 3 rapid movement of lower plate rocks from midcrustal depths to shallower depths these results are qualitatively in agreement with geologic observations for core complexes

the narmada son brahmaputra transform a mesozoic fracture zone in gondwanic india abstract a transcurrent fault zone with about 600 km dextral slip is postulated to cut across the indian sub continent from the arabian sea to the himalayan burmese syntaxis the transcurrent movement was most active during the early to late cretaceous when india moved from the kerguelen hotspot rajmahal abor volcanic episode to the reunion hotspot deccan volcanic episode

ultra high temperature metamorphism and its significance in the central indian tectonic zone abstract in the present study from the southern margin of the central indian tectonic zone it is demonstrated how the metamorphic p t path of ultrahigh temperature granulite terranes can be reconstructed using the metamorphic transition in corundum granulites from early biotite melting to later fmas solid solid reaction the extreme metamorphism in these rocks caused two stage biotite melting resulting in initial porphyroblastic garnet 1 and later sapphirine spinel 1 incongruent solid mineral assemblages during this process the leucocratic and melanocratic layers in the corundum granulites evolved from an initial silica oversaturated to a later silica undersaturated domain in the melanocratic layer this allowed localized concentration of sapphirine spinel 1 and residual sillimanite 1 producing an extremely restitic assemblage at the culmination of peak metamorphism bm 1 bm 1 is constrained at 1000 c at relatively deep crustal levels p 9 kbar from the stability of ferroaugite in a co metamorphosed iron formation granulite during subsequent metamorphism bm 2 the reaction path and history in the corundum granulites shifted to the restitic domain allowing reacting sapphirine spinel 1 and sillimanite to produce coronal garnet 2 corundum assemblage via a fmas univariant reaction in the final stages of reaction history biotite 2 sillimanite 2 spinel 2 assemblage was produced after garnet 2 corundum due to localized melt crystal interaction the metamorphic sequence when interpreted with the help of a newly constructed qualitative kfmash petrogenetic grid reveals successive stages of heating increasing pressure and cooling around the kfmash invariant point opx crd which is consistent with a counterclockwise metamorphic p t path the near isobaric nature of post peak cooling t 250 300 c is also evident from multistage pyroxene exsolution and by the appearance of lamellar and coronal garnets in the iron formation granulites this study provides the first tight constraint for ultrahigh t metamorphism along a counter clockwise p t trajectory in the central indian tectonic zone and has important bearing for terrane correlations in this part of east gondwanaland in addition the new kfmash grid allows evaluation of metamorphic phase relations in ultrahigh t corundum bearing and corundum absent aluminous granulites

crustal structure of bengal basin and shillong plateau extension of eastern ghat and satpura mobile belts to himalayan fronts and seismotectonics abstract gravity data from the east india and the bangladesh are processed and compiled together to provide a composite complete bouguer anomaly map of this region modelling of gravity profiles across the east indian shield chhotanagpur granite gneiss complex cggc the shillong plateau and the bengal basin in bangladesh constrained from seismic studies suggest crustal thickness of 37 38 km and thrusted high density rocks in the middle crust under the east indian shield reducing to 30 32 km with thick sediments of 12 14 km under bangladesh reduced crustal thickness under bangladesh is attributed to oceanic type of crust which is connected to indian continent along hinge zone characterized by linear gravity highs connecting east coast of india to the shillong plateau thrusted high density rocks under the cggc and the shillong plateau are in comparison to those under the eastern ghat and the satpura mobile belts egmb smb respectively suggest the north ward and east ward extensions of these orogenies margins of the thrusted block under shillong plateau when projected on surface coincide approximately with the south dipping dauki and the brahmaputra faults which coincides with central bulge due to lithospheric flexure indicating their surface expositions

on the role of isostasy in the evolution of normal fault systems the footwalls of west dipping normal faults that separate the west central colorado plateau from the basin and range province record at least 5 7 km and perhaps as much as 15 20 km of west side up neogene uplift with an axis just 10 20 km west of undeformed plateau strata the uplift is expressed as folding and steep faulting in pre tertiary cratonic and disconformably overlying neogene strata forming a basement cored anticline and coincident topographic high on the western margin of the plateau the authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin profound isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains calling into question several assumptions fundamental to existing models of the evolution of normal fault systems

field tests of rolling hinges existence mechanical types and implications for extensional tectonics in rolling hinge models a locus of isostatic footwall uplift migrates together with the withdrawing hanging wall of a large displacement tens of kilometers normal fault a fault bend fold that migrates through the footwall results causing progressive deactivation of the updip part of the fault as it is tilted to a lower dip rolling hinge migration should leave a discernible geologic record but compelling tests are difficult to design because predictions of rolling hinge models are nonunique and alternative models commonly can explain individual data sets field tests based on thermochronology paleomagnetism and structural analysis are typically applied to footwall rocks whereas sedimentology geochronology of volcanic rocks and structural analysis are typically applied to upper plate rocks we review existing theoretical models and case studies in 14 detachment terranes some negative results are reasonably compelling but positive tests as yet tend to permit rather than demand a rolling hinge locally both positive and negative conclusions have been reached about the passage of a hinge within the same terrane such differences in interpretation may depend more on the scale of observation or on the specific model considered than on the geological development of the area subvertical simple shear and flexural failure mechanisms with or without significant induced flow of lower crustal material are thought to dominate hinge related footwall deformation in individual sites although rolling hinge models offer an alternative to low initial fault dips hinge related deformation does not demand a steep initial fault dip few areas in which rolling hinge tectonics are permissible had initially steep faults 35 rather where the evidence most strongly favors a rolling hinge detachment faults had initial dips 35 in the future conclusive results are most likely to be obtained if diverse tests are applied to both the upper and lower plates of detachment terranes where other geological sources of ambiguity are minimal at best the rolling hinge mechanism does not universally explain the observation that earthquakes are rare on low angle normal faults nor solve the mechanical problems of frictional slip on such faults

petrology geochemistry and la icp ms u pb geochronology of paleoproterozoic basement rocks in bangladesh an evaluation of calc alkaline magmatism and implication for columbia supercontinent amalgamation abstract the paleoproterozoic 1 7 ga basement rocks from maddhapara bangladesh show a large range of chemical variations e g sio2 50 7 74 7 and include diorite quartz diorite monzodiorite quartz monzonite and granite the pluton overall displays metaluminous calc alkaline orogenic suite mostly i type suites formed within subduction related magmatism the observed major elements show general trends for fractional crystallization trace element contents also indicate the possibility of a fractionation or assimilation explain the entire variation from diorite to monzonite even granite the pluton may have evolved the unique chemical features by a process that included partial melting of calc alkaline lithologies and mixing of mantle derived magmas followed by fractional crystallization and by assimilation of country rocks the pluton shows evidence of crystal fractionation involving largely plagioclase amphibole and possibly biotite some of the fractionated magmas may have mixed with more potassic melts from distinct parts of the continental lithosphere to produce granites and or pegmatites new geochronological results of granitic pegmatite 1722 10 ma are indisputably consistent with diorite and tonalite and those data showing credible geochronological sequence i e diorite tonalite granitic pegmatite identical paleoproterozoic age 1 7 ga with distinctive magmatism of the maddhapara basement rocks have agreeable relationship with the citz india the consistent magmatism is also common in the transamazonian of south america trans hudson orogeny in north america bohemian massif and the svecofennian poland have identified the sequential growth of the continent through the amalgamation of juvenile terrains succeeded by a major collisional orogeny such paleoproterozoic subduction related orogens in australia have similar counterparts in antarctica and other part of the world these types of paleoproterozoic magmatism dominantly contributed to assemble amalgamation and breakup of the enormous columbia supercontinent

role of tectonic denudation in warping and uplift of low angle normal faults large displacement on a low angle normal fault results in isostatic uplift of the lower plate in response to tectonic denudation simple models of the denudation process predict warping of the lower plate into a broad antiform or antiform synform pair with axes perpendicular to the direction of extension the amount of warping is strongly influenced by initial fault geometry surface topography the amount of extension and the distribution of extension within the upper plate late stage processes that augment antiform growth include one sided denudation of the antiform reverse faulting due to concave upward flexure and wholesale detachment and reverse drag folding of the antiform antiformal uplifts now exhumed by erosion form mountain ranges in the southwestern united states where conditions were favorable for warping during mid tertiary crustal extension four major structural domains are apparent in transects across these uplifts and adjacent areas 1 unextended area 2 synformal upper plate 3 antiformal uplift and 4 wedge shaped upper plate each domain is separated from adjacent domains by the traces of a master low angle normal fault

tectonics and surface effects of the supercontinent columbia abstract assembly of the supercontinent columbia at about 1 85 1 90 ga coincided with several events that affected the entire earth the oldest worldwide network of orogenic belts formed at the same time although some granite granodiorite gg suites had formed earlier the gg suites became common in the 1 8 1 9 ga orogenic belts these suites succeeded the older tonalite trondhjemite granodiorite ttg suites which were not produced after 1 8 ga changes on the earth s surface at 1 8 1 9 ga include rapid increase in the concentration of oxygen in the atmosphere and oceans and probably the evolution of eukaryotes all of these surface changes occurred as columbia accreted and the assembly of columbia may have contributed to the drastic changes in the earth s surface environment as well as to the evolution of primitive life forms

mesoarchaean crustal history of the eastern indian craton smnd and upb isotopic evidence abstract a variety of xenoliths representing high al trondhjemites and supracrustal lithologies occur within mesoarchaean granitoid rocks of the eastern indian craton a trondhjemitic xenolith from the bonai pluton yields a u pb age of 3380 6 4 ma for magmatic zircon and a minimum age of 3448 ma for xenocrystic zircon grains this sample also shows an nd value of 4 2 and t dm of 3 9 ga whereas four other xenoliths yield nd values ranging from 0 3 to 1 8 and t dm of 3 54 to 3 37 which are consistent with published data from the region and suggest a derivation from protoliths with relatively short lived crustal residence times

orthogonal folding of extensional detachments structure and origin of the sierra nevada elongated dome betics se spain 1 a close relationship of the kinematics and timing between low angle extensional faulting and upright folding is established for the miocene detachment systems in the hinterland of the betics in southeastern spain folding accompanied tectonic denudation developing elongated domes with fold axes both parallel and perpendicular to the direction of extension the geometry kinematics and tectonic evolution of two major sequentially developed extensional fault systems have been characterized in several e w elongated mountain ranges of the central betics on the basis of new cartographic and structural data and a comprehensive revision of other available geological and geophysical observations the extensional systems have an average wsw direction of extension and led to exhumation of the two lower metamorphic complexes of the betics the nevado filabride and the alpujarride during the middle and upper miocene serravallian to tortonian the extended domain contains a core complex with distal and proximal antiformal hinges separated by around 60 km and with a fold amplitude of 6 km measured parallel to the direction of extension the total amount of extension across the core complex is about 109 116 km corresponding to a stretching factor of 3 5 3 9 estimated using the distance between fold axial surfaces and a geometrical model accomplishing footwall deformation during tectonic denudation via subvertical simple shear the elongated domal and basinal geometry of the detachments and their respective footwalls is due to the interference between two sets of orthogonal large scale open folds trending n s and e w longitudinal n s trending folds are interpreted as isostatic folds i e folds that developed in response to differential unloading of the extensional detachment footwalls inducing ductile flow in the middle crust these folds formed in a rolling hinge anticline in which rotation migrates westward through the footwall as it is progressively unroofed in contrast e w trending folds being subparallel to the direction of extension have a contractional origin and progressively affect to the west the isostatically readjusted segments of the detachments once they are inactive orthogonal folding occurred since the serravallian to lower pliocene extension is still active from the rolling hinge anticline in sierra nevada to the west the middle miocene to pliocene tectonic evolution of the sierra nevada elongated dome at the core of the betic hinterland is another example of the coexistence of extension and contraction during continued overall convergence and mountain building

a numerical study of the agulhas retroflection the role of bottom topography abstract the oceanic circulation near the eastern margin of the african continent is dominated by the poleward flow of the agulhas current the agulhas current separates from the african coast near the southern tip of the continent makes an abrupt turn to the east known as the agulhas retroflection and flows into the indian ocean as a meandering jet called the agulhas return current the object of this study is to investigate possible causes for the agulhas retroflection a series of process oriented experiments are conducted with the princeton ocean model a primitive equation numerical model in sigma coordinates the model is started from rest forced at its surface with different wind stress distributions and run until dynamical equilibria are reached after which the steady states of the model are analysed in these experiments the effects of inertia wind stress distribution and realistic bottom topography on the agulhas retroflection are analyzed the results of these experiments indicate tha

structural expression of a rolling hinge in the footwall of the brenner line normal fault eastern alps the kinematic and temporal sequence of structures observed to overprint mylonites along the brenner line low angle normal fault may record passage of the footwall through two rolling hinges at the top and bottom of a ramp in the shear zone the structures comprise west down brittle and brittle ductile structures and east down brittle structures pt conditions of formation 250 to 400 c and 2 23 km depth obtained from analysis of oriented fluid inclusion planes indicate that west down structures were formed at greater depths and temperatures and therefore earlier than the east down structures these data suggest that the brittle structures formed under conditions that permit crystal plastic deformation at long term geologic strain rates and therefore probably reflect transient rapid strain rates and or high fluid pressure structures inferred to have formed at a lower hinge are consistent with viscous flow models of rolling hinge deformation and support the concept of a crustal asthenosphere such high temperatures at shallow crustal depth also suggest significant upward advection of heat by extensional unroofing of warm rocks which may have reduced the flexural rigidity of the footwall and thus affected mechanical behavior at the upper rolling hinge exposed mylonitic foliation within a few hundred meters of the brenner line and on top of the east west trending anticlines in the footwall dips 15 west our data favor a ramp dip of 25 but permit a dip as great as 45 fluid inclusion data suggest that structures related to the hinge at the base of the ramp formed at depths of 12 25 km if the average dip of the brenner shear zone to those depths was 20 intermediate between the favored ramp dip and the dip of exposed foliation then the horizontal component of slip could be as high as 33 63 km the two discrete sets of structures with opposite shear senses formed in the temporal sequence indicated by pt data are consistent with subvertical simple shear models of rolling hinge strain this kinematic pattern is not predicted by the flexural failure model for rolling hinges however the predominance of normal slip at the upper hinge which extends rather than shortens the mylonitic foliation fails to match the subvertical simple shear model which predicts shortening of the foliation there one possible solution is that superposition of regional extension upon hinge related stresses modified the rolling hinge kinematics such a modified subvertical shear model can account for the observed small foliation parallel extensional strains if the foliation was bent 5 10 passing through the upper hinge if more bending than that occurred the data suggest rolling hinge kinematics in which deformation is achieved by uniform sense simple shear across the shear zone as in the subvertical simple shear model but in which material lines parallel to the shear zone foliation and the detachment fault undergo very small length changes presumably indicating that footwall rocks retained significant resistance to shear and underwent minimal permanent strain the mechanics that would generate such a rolling hinge are uncertain but may incorporate aspects of both subvertical simple shear and flexural failure an important kinematic consequence of such a rolling hinge is that all of the net slip across a normal fault not only its horizontal component is converted into horizontal extension this implies a significantly larger magnitude of crustal extension across dipping normal faults whose footwalls passed through a rolling hinge than for those that did not develop along with a hinge

low angle normal faults and seismicity a review although large low angle normal faults in the continental crust are widely recognized doubts persist that they either initiate or slip at shallow dips 30 because 1 global compilations of normal fault focal mechanisms show only a small fraction of events with either nodal plane dipping less than 30 and 2 andersonian fault mechanics predict that normal faults dipping less than 30 cannot slip geological reconstructions thermochronology paleomagnetic studies and seismic reflection profiles mainly published in the last 5 years reinforce the view that active low angle normal faulting in the brittle crust is widespread underscoring the paradox of the seismicity data for dip slip faults large enough to break the entire brittle layer during earthquakes m w 6 5 consideration of their surface area and efficiency in accommodating extension as a function of dip suggests average recurrence intervals of earthquakes r tan assuming stress drop rigidity modulus and thickness of the seismogenic layer do not vary systematically with dip if the global distribution of fault dip normalized to total fault length is uniform the global recurrence of earthquakes as a function of dip is shown to be r tan sin this relationship predicts that the frequency of earthquakes with nodal planes dipping between 30 and 60 will exceed those with planes shallower than 30 by a factor of 10 in good agreement with continental seismicity assuming major normal faults dipping more than 60 are relatively uncommon revision of andersonian fault mechanics to include rotation of the stress axes with depth perhaps as a result of deep crustal shear against the brittle layer would explain both the common occurrence of low angle faults and the lack of large faults dipping more than 60 if correct this resolution of the paradox may indicate significant seismic hazard from large low angle normal faults

fault and bed rotation during continental extension block rotation or vertical shear abstract for almost a century the view has existed that the tilting of blocks between closely spaced planar normal faults is rigid body rotation this interpretation requires only simple geometry and has consequently found widespread application however it is not consistent with the deformation expected around normal faults given the present knowledge of stress fields and rheology in basement in the brittle upper crust which is better regarded instead as distributed vertical simple shear rigid body rotation and vertical shear involve different relations between fault and bed tilting and thus predict different initial fault dips for particular present day dips of faults and beds these two schemes also predict different amounts of extension and it is consequently important to establish which is correct with this aim in mind we examine normal faults associated with neogene extension in western turkey and the western united states and with mesozoic extension in the north sea except where extension and the associated tilting are minimal rigid body rotation predicts unrealistically steep initial fault dips some extensional basins also exhibit reversals of normal fault polarity and tilt polarity of beds which are incompatible with rigid body rotation we therefore conclude that the general cause of the tilting is vertical shear not rigid body rotation this has three main observational consequences first the heave on any normal fault equals the amount of extension across it second no feature near a normal fault can rotate through the vertical a normal fault thus cannot rotate through the vertical and appear as a reverse fault third any initially vertical feature near a normal fault will remain vertical a vertical dyke in the tilted surroundings of a normal fault is thus not necessarily younger than the extension that caused the tilting

geology and depositional history of cenozoic sediments of the bengal basin of bangladesh abstract the bengal basin lies on the eastern side of the indian subcontinent and occupies the northern part of the bengal geosyncline sedimentation started with the breakup of gondwanaland cretaceous fluvial and deltaic sedimentation was slowly taken over by open marine and deep sea fan sedimentation by the mid eocene the basin was witnessing the maximum marine transgression and most of the stable shelf was in carbonate regime as part of the extended tethyan carbonate shelf the deeper part of the basin was dominated by deep sea fan sedimentation fed by clastic turbidites from the northeast with limited carbonate turbidites from the shelf the collision of the indian plate with the tibetan plate and with the burmese plate in the miocene resulted in a rapid switch in sedimentation pattern in the bengal basin the basin structure and sedimentation were both strongly influenced by the collision pattern of the plates and by the uplift of the himalayas with the miocene collision of the indian plate there was a change from flysch to molasse sedimentation the present day basin configuration with the ganges bramaputra delta on the north and the bengal deep sea fan on the south was established in the pliocene the stratigraphic and tectonic framework of the bengal basin suggest that the deposition of hydrocarbon source rocks was associated with the flysch sedimentation whereas formation of traps and deposition of most of the reservoir rocks in the basin was associated with mid pliocene tectonic movements and with the molasse sedimentation the pre mid miocene sediments now deeply buried in the deeper part of the bengal basin and in the eastern folded belt are the major source of gas of the eastern folded belt of the basin

the nature of mesoproterozoic central indian tectonic zone with exhumed and reworked older granulites abstract the ese wnw trending central indian tectonic zone citz divides the indian subcontinent into southern and northern crustal provinces its narrow northern belt consists of closed early proterozoic mahakoshal rift zone which is presently confined between two moho reaching faults that characterize the son narmada sona lineament the mesoproterozoic sausar mobile belt smb the chhotanagpur granite gneiss complex cggc and the gneissic complex of northeast india represent the wider southern belt the mahakoshal rift belt has a thick pile of mafic ultramafic rocks and possibly oceanic components the basin closed due to a south directed subduction creating a magmatic arc that was sutured to its southern boundary during 1 8 1 7 ga a south dipping ductile shear zone known as the son narmada south fault affects both its supracrustals and intrusive granitoids the son narmada south fault generally delimits the northern boundary of the gondwana basins whereas the son narmada north fault delineates the northern boundary of the mahakoshal mobile belt and the southern boundary of the meso neoproterozoic vindhyan basin the smb representing the southern belt of the citz in the central sector comprises strongly folded and metamorphosed non volcanic and mn rich sausar sediments intermixed with reworked basement gneisses migmatites and granulites representing the pre sausar assemblage the sausar orogeny closed during terminal mesoproterozoic ca 1 0 ga on the other hand metamorphism in the basement gneiss migmatites and northern and southern granulite belts pre dates the sausar orogeny the earliest structure of the smb truncates and rotates the n s oriented structures of the southward located sakoli and the dongargarh belts of neoarchaean palaeoproterozoic age the northern granulite belt of the smb and the cggc and the high grade supracrustals from the sonapahar area in ne india all indicate very high p t conditions during 1 7 1 5 ga peak metamorphism which was followed by decompressional cooling the setting resembles continental collision which was followed by tectonic exhumation and denudation on the other hand the southern granulite belt from the smb documents a late archaean 2 6 ga event and a later 1 4 ga event the dalma volcanic belt located close to the southern tectonized boundary of the cggc belt and possibly the assemblage of copious mafic enclaves within the ca 550 ma granite plutons from ne india represent back arc like setting during 1 6 1 5 ga the setting is caused due to the southward subduction of the north indian block beneath the southern block leading to continental collision the metasedimentary belt located further to the south of the dalma volcanic belt and flanking the archaean singbhum craton to the north on the other hand is palaeoproterozoic in age but has been metamorphosed during ca 1 6 ga because of the northern collision related orogeny reworking and incorporation of basement gneisses and granulites from the mid crust level to the younger sausar cover sediments denote renewed spell of thrust tectonics that remobilized the collision zone rocks of the earlier orogeny the tectonic transport was from the north to the south with north dipping thrusts thus there was a reversal in the direction of convergence from the preceding orogeny the sausar orogeny closed during ca 1 0 ga it caused anatectic event producing granitoid plutons migmatites and charnockites in the cggc belt and migmatites in the gneissic complex in ne india

indian derived sediments deposited in australia during gondwana assembly abstract sedimentary exchanges across continents during the collisional assembly and lifespan of supercontinents provide a powerful way of testing the assembling process and configuration of supercontinents the ord basin in north western australia contains mid cambrian shallow marine and deltaic sedimentary successions that post date the ca 650 520 ma paterson petermann orogeny ppo but are synchronous with the ca 530 470 ma kurgiakh orogeny in north india we report new u pb ages and hf isotopes of detrital zircons from the mid cambrian ord basin that suggest indian derived sedimentary input to australian cambrian basins compilations of published ediacaran ordovician samples from the centralian superbasin indicate that during the ca 650 520 ma ppo the 1200 1100 ma musgrave province in central australia was exhumed and fed sediments to the syn ppo centralian superbasin soon after the ppo termination the musgrave province likely remained a topographic high and continued to supply detritus to the amadeus and officer basins that were fragmented from the centralian superbasin supplemented by the 630 ma granites of the paterson orogen conversely we conclude that the mid cambrian ord basin did not record similar sedimentary provenance from the musgrave province rather indian derived detritus travelled along the northeastern gondwanan continental margin via longshore currents and in turn incorporated paterson granitic detritus during the assembly of gondwana including the kurgiakh orogeny in north india a broad provenance comparison between potential eastern gondwanan source regions and relevant asian neighbours suggests that south china and south qiangtang are affiliated with northern india but lhasa appears to have had a mixed affiliation between india and australia

two cases of continental collisions and related tectonics during the proterozoic period in india insights from gravity modelling constrained by seismic and magnetotelluric studies abstract the aravalli delhi mobile belt admb in nw india includes the proterozoic aravalli and delhi rifts and fold belts a seismic profile and gravity modelling across the admb shows several dipping reflections and suggests a thick crust 45 46 km and a high density 3 04 g cm 3 dome shaped body in the lower crust this high density body in the lower crust may represent underplated lower crust caused by extension during evolution of the proterozoic rifts part of the gravity high observed over the admb is attributed to the upthrust blocks of high density lower crustal granulite rocks the gravity low along the eastern margin of the admb is modelled as being due to a westerly dipping 45 low density body in the crust in accordance with seismic reflectors which may represent remanents of the subducted crustal rocks the hindoli group of rocks above this body is characterized by high conductivity indicating a high fluid content and a fractured nature which may be related to the collisional tectonism of this region a sharp gravity high along the western margin of the admb is attributed to the basic rocks of an ophiolite melange the gravity low and high west of the admb are attributed to plutonic activity and bimodal calc alkaline volcanics in linear basins and are related to arc and back arc rift magmatism respectively the admb therefore represents a proterozoic collision zone between the bundelkhand craton towards the east and the rajasthan craton towards the west with a plausible subduction towards the west along the eastern margin of the admb the thrusting is mainly confined to the obducted block towards the west although some thrusting is inferred even in the subducted block the central indian suture cis is located at the southern margin of the satpura mobile belt smb in central india and separates high grade metasediments and granulite towards the north from low grade proterozoic volcanogenic successions towards the south a seismic profile and gravity modelling across the cis show a thick crust with a thicker crust north of the cis compared to the south and a contact in the lower crust whose projection on the surface coincides with the cis the cis at the surface coincides with a low density 2 65 g cm 3 and conductive body that is interpreted to be fluid filled and fractured in nature the high density 2 81 g cm 3 body modelled in the upper crust north of the cis coinciding approximately with a highly conductive body and dome shaped seismic reflectors may represent an upthrust block from the lower crust as high density granulite rocks are exposed in this region short wavelength gravity highs and lows south of the cis represent exposed metavolcanics and granite intrusions which represent tholeiitic series rocks to calc alkaline series rocks associated with island arcs and back arc magmatism therefore the large wavelength gravity low south of the cis is modelled as a low density body below the moho which conforms with seismic reflectors and may represent crustal rocks subducted towards the south thrusting in this case is indicated in the subducted block towards the north of the cis the bipolar gravity anomaly across the cis therefore indicates a collision zone between the bundelkhand craton towards the north and bhandara deccan cratons towards the south

geothermobarometry and fluid inclusions of dioritic rocks in bangladesh implications for emplacement depth and exhumation rate abstract application of hornblende thermobarometry and fluid inclusion studies to the palaeoproterozoic 1 7 ga basement rocks from maddhapara nw bangladesh provide information on the pressure and temperature p t conditions of crystallization the emplacement depth and composition of magmatic fluid the basement rocks are predominantly diorite or quartz diorite with a mineral assemblage of plagioclase hornblende biotite quartz k feldspar titanite and secondary epidote and chlorite the calculated p t conditions of the dioritic rocks are 680 725 c and 4 9 6 4 kbar which probably correspond to crystallization conditions fluid inclusion studies suggest that low to medium salinity 0 6 4 wt nacl eq h 2 o rich fluids are trapped during the crystallization of quartz and plagioclase the isochore range calculated for primary aqueous inclusions is consistent with the p t condition obtained by geothermobarometry the basement rocks likely crystallized at a depth of 17 22 km with a minimum average exhumation rate of 12 15 m ma during palaeoproterozoic to lopingian time such slow exhumation indicates low relief continental shield surface during this period

documenting basin scale geometry and provenance through detrital geochemical data lessons from the neoproterozoic to ordovician lesser greater and tethyan himalayan strata of bhutan abstract detrital zircon dz ages augmented with end 0 and 13 c isotopic values from 18 new and 22 published samples collected from lesser himalayan lh greater himalayan gh and tethyan himalayan th rocks in bhutan support deposition of 7 km of sedimentary rock in late cambrian ordovician time and provide a stratigraphic framework for the pre collisional indian margin youngest gh dz grains become younger upsection from 900 ma to 477 ma youngest dz grains in th samples are 490 460 ma both the lh jaishidanda formation fm and the lh baxa group overlie paleoproterozoic lh rocks the jaishidanda fm exhibits distinct populations of youngest dz peaks 475 550 ma and 800 1000 ma the baxa group manas pangsari and phuntsholing formations contains youngest dz peaks at both 500 525 ma and 0 9 1 0 ga however most samples from the baxa group in western bhutan contain no grains younger than 1 8 ga samples from the lh paro fm which sits directly under the mct in western bhutan have youngest dz peaks at 0 5 0 8 1 0 1 7 1 8 ga end values generally match dz spectra with samples that contain old youngest grain populations corresponding to more negative end signatures the paro fm is an exception where end 0 values from quartzite samples are quite negative 19 to 24 whereas the end 0 values from interbedded schist contain younger detritus 12 to 17 13 c values from the jaishidanda paro and manas formations have 13 c values 1 8 to 6 suggestive of deposition over late neoproterozoic to ordovician time 13 c values from the pangsari fm vary from 2 8 to 1 8 compatible with deposition in the early to middle neoproterozoic the young latest cambrian ordovician grains preserved in th gh and lh rocks suggest that the late cambrian ordovician orogeny documented in gh rocks throughout the orogen served as a significant sediment source in bhutan

u pb geochronology and hf nd isotopic geochemistry of the badu complex southeastern china implications for the precambrian crustal evolution and paleogeography of the cathaysia block abstract the oldest rocks of the cathaysia block south china comprise the badu complex and paleoproterozoic granites in the wuyishan area southern zhejiang and northwestern fujian provinces new zircon u pb ages hf isotopes and trace elements for metamorphic rocks from the badu complex and bulk nd isotope compositions of these rocks and granites in the wuyishan area provide important constraints on the precambrian crustal evolution of the cathaysia block inherited cores of zircon grains from the metamorphic rocks are of magmatic origin predominantly formed at ca 2 5 ga while overgrowth rims reflect two episodes of granulite facies metamorphism related to collisional orogeny at 1 89 1 88 ga and 252 234 ma the unimodal age distribution 2 5 ga of detrital zircons and the positive hf t of most neoarchean zircons suggest that the detritus of these sedimentary protoliths of the badu complex came from a proximal volcanic arc and that they were deposited in an arc basin synchronously with 2 5 ga volcanism zircon u pb ages and hf isotopes with whole rock nd isotopes suggest that the juvenile crust of the eastern cathaysia block was generated mainly at 2 5 ga and 2 8 ga and minor at 3 5 3 3 ga with some evidence for the generation at 3 7 3 6 ga and 4 0 ga a strong 1 9 ga orogeny and the 3 3 3 0 ga thermal event only involved the reworking of older crust material paleoproterozoic 1 89 1 86 ga granitic magmatic activity and high grade metamorphism in the eastern cathaysia block were synchronous with the assembly of the columbia supercontinent using integrated geochronological hf nd isotopic and petrologic data as a barcode we compare the cathaysia block with other paleoproterozoic orogens worldwide and argue that its closest affinity is with the south korean peninsula and the lesser himalaya of nw india consequently the eastern cathaysia block and the south korean massif as the united cathay south korea terrane was close to the lesser himalaya terrane in the paleoproterozoic configuration of the columbia supercontinent the spatial linkage was maintained for ca 1 ga until the fragmentation of the rodinia supercontinent during neoproterozoic time

on sverdrup discontinuities and vortices in the southwest indian ocean abstract the southwest indian ocean is distinguished by discontinuities in the wind driven sverdrup circulation these connect the northern and southern tips of madagascar with africa and the southern tip of africa with south america in an analytical barotropic model with a flat bottom the discontinuities produce intense westward jets those off the northern tip of madagascar and the southern tip of africa are always present while the strength of that off southern madagascar depends on the position of the zero curl line in the indian ocean the jet is strong if the line intersects madagascar but weak if the line is north of the island all three jets are barotropically unstable by the rayleigh kuo criterion the authors studied the development of the instability using a primitive equation model with a flat bottom and realistic coastlines the model produced westward jets at the three sites and these became unstable after several weeks generating 200 300 km scale eddies the eddies generated west of

comment on tertiary extension and contraction of lower plate rocks in the central mojave metamorphic core complex southern california by john m bartley john m fletcher and allen f glazner

geochronology and tectonic evolution of karimnagar and bhopalpatnam granulite belts central india abstract the karimnagar granulite belt kgb and the bhopalpatnam granulite belt bgb occur along both flanks of the pranhita godavari pg rift basin we present a state of the art overview on the geochronological and tectonic aspects of these belts and surrounding geologic domains and report new age data on zircon monazite and uraninite recovered from granulite facies assemblages from kgb and bgb based on electron microprobe analyses epma zircons from kgb charnockites show core ages of up to 3 1 ga mantled by rims of 2 6 ga zircons from bgb have 1 9 ga cores mantled by 1 7 ga rims zircons with core ages of 1 6 to 1 7 ga in bgb rocks suggest new growth at this time monazites and uranitite from kgb show clear peaks with well defined ages in the narrow range between 2 42 0 08 ga and 2 47 0 03 ga rims of monazite show mean age of 2 21 0 08 ga monazites from bgb define sharp linear trend in pbo vs tho2 diagram delineating a clear isochron with age of 1 59 0 03 ga age data from kgb and bgb presented in this report negate current models linking these terrains to godavari granulite belt and considering them as single and contemporaneous entity the mid archaean to early palaeoproterozoic signature recognized from kgb is totally missing in bgb on the other hand kgb rocks do not record any evidence for major mesoproterozoic thermal regime the two granulite belts shouldering the pg rift basin have therefore evolved in different times under distinct p t conditions and thermal regimes our results have important implications in evaluating models of supercontinent assemblies particularly the older assemblies of ur columbia and rodinia while tectonothermal events in kgb broadly match with those of east dharwar we propose that bgb represents a 1 6 ga collisional mobile belt between the bastar and the dharwar cratons the 1 6 ga collisional mobile belt at the southern margin of the bastar craton was superposed by rift activity along the pg basin at 1 5 ga this sequence of events goes against the existence of a 3 0 ga old contiguous assembly of ur but closely matches with the history of accretion and break up of the columbia further parts of the pg basin located away from the influence of the eastern ghats mobile belt neither recorded any grenville ages 1 0 ga corresponding to the rodinia accretion nor late pan african ages ca 550 ma relating to the gondwana amalgamation indicating that the region did not witness any of these younger tectonic events

interannual eddy kinetic energy modulations in the agulhas return current interannual variability in the mesoscale eddy field in the agulhas return current arc of 32 42 degrees s and 15 35 degrees e is investigated based on satellite altimeter observations and state estimate from the estimating the circulation and climate of the ocean phase ii from 1993 to 2016 it is found that the interannual modulation of eddy kinetic energy in the arc region is externally mediated by the wind stress forcing that generates the westward propagating sea surface height anomalies across the south indian ocean subtropical gyre the wind forced sea surface height anomalies influence the upstream agulhas current volume transports by modulating the intensity of barotropic instability of the arc mean flow centered around the retroflection region the agulhas current inflow variability leads to the downstream interannual eddy kinetic energy fluctuations in the arc region

40ar 39ar ages of precambrian manganese ore minerals from sweden india and morocco in order to test their chronometric potential 40ar 39ar stepheating and 4he analyses have been carried out on five manganese ore minerals of the hollandite cryptomelane series from three precambrian manganese deposits ultevis sweden sitapar india bachkoun morocco samples from the metamorphic occurrences ultevis and sitapar yielded ar ages of 1 8 ga and 0 95 ga interpreted as the age of postmetamorphic cooling hollandites ultevis and of an early k introducing alteration process subsequent to amphibolite facies metamorphism cryptomelanes sitapar both data are consistent with known chronologies of the svecokarelian and satpura orogenic cycles a date of 670 ma obtained for a hollandite from a volcanogenic vein deposit bachkoun however contrasts with published extrusion ages of 580 560 ma for the volcanic host rocks ouarzazate series probably due to incorporation of excess argon the use of the 40ar 39ar technique together with multiple isotope systematics made it nevertheless possible to establish a reasonable estimate of a mineralization age close to 580 ma measurement of fractional ar losses during vacuum step heating 500 1600 c although indicating good ar retentivities failed to define model diffusion parameters because of non linear arrhenius arrays helium diffusion results 200 1200 c indicated retention of radiogenic 4he by the samples corroborated by u he mineral dates between 0 96 and 0 31 ga potassium bearing manganese oxides are therefore able to retain argon possibly also 4he through geological times and may thus provide ages of ore forming processes and perhaps later cooling and alteration stages

styles of footwall uplift along the simplon and brenner normal fault systems central and eastern alps the simplon and brenner extensional shear zones of the central and eastern alps respectively are low angle detachments that accommodated orogen parallel extension and unroofed midcrustal rocks in their footwalls an integrated field and fluid inclusion study of the simplon low angle shear zone reveals several postmylonitic structure types that are consistent with passage of a complex rolling hinge through the simplon footwall including synthetic west down and antithetic east down semibrittle and brittle structures and strike slip extensional and contractional structures all of which are consistent with southwest directed extension along the main detachment zone fluid inclusion analysis of kinematically referenced inclusion arrays indicates that west down semibrittle and brittle shears were active at conditions above 300 450 c and 14 26 km antithetic deformation along brittle faults occurred at much shallower conditions of 300 c and 3 7 km deformation along these structures accommodated footwall uplift by a subvertical simple shear mechanism in the southern part of the simplon region these results are strikingly similar to those from the brenner shear zone in contrast with the brenner region however the northern simplon footwall shows strike slip dominated postmylonitic deformation related to slip along the rhone line the footwalls of these detachment systems define a unique class of core complex characterized by the development of a midcrustal synthetic lower hinge that acts to steepen the detachment system and accommodate footwall uplift by a subvertical simple shear mechanism both systems have poorly developed upper hinges that flatten the middle to upper crustal ramp by only a few degrees of dip which is in contrast to detachment fault systems of the western united states where evidence of a deep hinge is rarely observed and the upper hinge acts to deactivate the flattened updip part of the detachment our results emphasize the role of tectonic setting buoyancy forces and synkinematic fluids in controlling the observed style and mechanisms of large magnitude extension during collision between continental masses