the thermal structure of mantle plumes axisymmetric or triple junction summary a mantle plume is probably a complex 3 d thermal structure that possesses approximate axisymmetry as it approaches the base of the lithosphere from below but followed down towards the base of the layer probably consists of a triple junction or quadruple junction of connected hot sheets a relatively weak hot sheet rising only part way through the layer probably connects two neighbouring mantle plumes these conclusions are suggested by numerical experiments on a 3 d constant viscosity plane layer with stress free boundaries which detail the gradational change in the planform of a convecting layer from the top of the layer to its base the planform of a convecting layer is a map in the horizontal plane of the principal thermal anomalies in the layer these anomalies are the main sources of positive for hot fluid or negative for cold fluid buoyancy and therefore they drive the convective flow they may appear in cross section as structures with either axial symmetry columns planar symmetry sheets or some complex asymmetric form when convection is driven at least partially by basal heating the planform near the top of the layer may be described as a network of cold sinking sheets and isolated hot columns while near the base of the layer it appears as a network of hot rising sheets and isolated cold columns the hot columns near the upper surface arise from the vertices or nodes of the network of hot sheets on the lower surface and similarly the cold columns at the base of the layer form below the vertices of the network of cold sheets near the upper surface near the upper surface the apparent planform of this experiment is analogous to that of mantle convection the cold sheets compared to subduction zones and the hot columns compared to mantle plumes the hot plumes impinging on the upper surface produce approximately axisymmetric temperature anomalies surface uplift and extensional stress fields however the relatively minor deviations from axisymmetry of surface observables reflect the deep structure of the mantle plume formed by the junction of three or four hot sheets on the base of the layer it seems likely that the commonly occurring triple junction form of continental rifts may reflect an underlying structure that is implicit in the convective circulation of the mantle beneath

an oscillating trend in phanerozoic non skeletal carbonate mineralogy textural and compositional data on ancient ooids and cements suggest the existence of an oscillating phanerozoic temporal trend in non skeletal carbonate mineralogy the trend shows good correlation with eustatic climatic oscillations proposed by others the most likely mechanism appears to be plate tectonically influenced oscillations in pco2 the vapour pressure of co2 rather than change in oceanic mg ca ratio

slow ridge hotspot interactions from global gravity seismic tomography and 87sr 86sr isotope data summary among the mantle hotspots present under oceanic areas a large number are located on or close to active oceanic ridges this is especially true in the slow spreading atlantic and indian oceans the recent availability of worldwide gravity grids and the increasing coverage of geochemical data sets along active spreading centres allow a fruitful comparison of these data with global geoid and seismic tomography models and allow one to study interactions between mantle plumes and active slow spreading ridges the observed correlations allow us to draw preliminary conclusions on the general links between surficial processes which shape the detailed morphology of the ridge axes and deeper processes active in the upper mantle below the ridge axial domains as a whole the interactions are first studied at the scale of the atlantic the mid atlantic ridge from iceland to bouvet island from the correlation between the zero age free air gravity anomaly which reflects the zero age depth of the ridge axis and sr isotopic ratios of ridge axis basalts the study is then extended to a more global scale the slow ridges from iceland to the gulf of aden by including geoid and uppermantle tomography models the interactions appear complex ranging from the evect of large and very productive plumes almost totally overprinting the long wavelength segmentation pattern of the ridge to that of weaker hotspots barely marking some of the observables in the ridge axial domain intermediate cases are observed in which hotspots of medium activity or whose activity has gradually decreased located at some distance from the ridge axis produce geophysical or geochemical signals whose variation along the axis can be correlated with the geometry of the plume head in the upper mantle such observations tend to preclude the use of a single hotspot ridge interaction model and stress the need for additional observations in various plume ridge configurations

chaotic subduction like downflows in a spherical model of convection in the earth s mantle numerical models of three dimensional nonlinear thermal convection of very viscous fluids have recently been developed in rectangular1 4 and spherical5 12 geometries as analogues of convection in the earth s mantle here we describe model calculations for a compressible fluid in a three dimensional spherical shell with 80 of the surface heat flow generated within the model mantle in agreement with estimates for the earth s mantle13 the rayleigh number and the numerical resolution for these calculations are greater than those of our previous studies9 10 of internally heated convection in a spherical shell our numerical solutions are strongly chaotic with surface planforms dominated by long curvilinear downflows reminiscent of the descending slabs in the earth s mantle although analogy to the earth s mantle is necessarily imperfect owing to for example the absence of variable viscosity and rheology and hence of lithospheric plates our results suggest that descending slabs play an important part in driving mantle convection and that their chaotic evolution may influence the spatial and temporal behaviour of plates and thus the dispersal and aggregation of continents

the proximity of hotspots to convergent and divergent plate boundaries an analysis of four different hotspot distributions ranging from morgan s 1972 original list of 19 to vogt s 1981 list of 117 reveals that the hotspots are preferentially located near divergent plate boundaries the probability of this proximity occurring by chance alone is quite remote less than 0 01 for all four hotspot distributions the same analysis also reveals that the hotspots are preferentially excluded from regions near convergent plate boundaries the probability of this exclusion occurring by chance alone is 0 1 or less for three out of the four distributions examined we interpret this behavior as being a consequence of the effects of large scale convective circulation on ascending mantle plumes mantle thermal plumes the most probable source of hotspots arise from instabilities in a basal thermal boundary layer plumes are suppressed from regions beneath convergent boundaries by descending flow and are entrained into the upwelling flow beneath spreading centers plate scale convective circulation driven by subduction may also advect mantle thermal plumes toward spreading centers

paleomagnetic data bearing on style of miocene deformation in the lake mead area southern nevada abstract paleomagnetic and structural data from intermediate to mafic composition lava flows and related dikes in all major blocks of the late miocene hamblin cleopatra volcano which was structurally dismembered during the development of the lake mead fault system lmfs provide limits on the magnitude and sense of tilting and vertical axis rotation of crust during extension of this part of the basin and range province sinistral separation along the fault system dissected the volcano into three major blocks the eastern cleopatra lobe of the volcano is structurally the most intact section of the volcano normal and reverse polarity data from paleomagnetic sites collected along traverses in the cleopatra lobe yield an in situ grand mean of declination d 339 inclination i 54 95 3 1 k 27 2 n 81 sites the rocks of the central core of the volcano yield an in situ grand mean of d 3 i 59 95 6 8 k 42 5 n 11 sites six normal five reverse polarity sites collected within the western hamblin lobe of the volcano are exclusively of reverse polarity and yield an overall in situ mean of d 168 i 58 95 6 5 k 28 9 n 18 sites interpretation of the paleomagnetic data in the context of the structural history of the volcano and surrounding area considers the possibility of two different types of structural corrections a stratigraphic tilt correction involves restoring flows to the horizontal using the present strike this correction assumes no initial possibly radial dip of flows of the volcano and is considered invalid a structural tilt correction to the data assumes that dikes of the radiating swarm associated with the volcano were originally vertical and results in block mean directions of d 9 i 53 95 3 1 k 27 2 and d 58 i 78 95 6 8 k 42 5 for the cleopatra lobe and the central intrusive core respectively the data from the cleopatra lobe are slightly discordant in a clockwise sense from expected middle to late miocene field directions the data from the volcano are not consistent with a proposed structural model of uniform moderate magnitude statistically significant counter clockwise vertical axis rotation of fault bounded blocks during overall sinsitral displacement along the lmfs we also analyzed dikes of the northernmost part of the miocene wilson ridge hypabyssal igneous complex strata of the triassic chinle formation and basalt flows of the miocene west end wash callville mesa volcanic centers dikes in the wilson ridge pluton and the triassic strata yield magnetizations with directions suggestive of statistically significant clockwise vertical axis rotations consistent with local large magnitude shear of crustal fragments near some of the faults of the lmfs late cenozoic deformation of the hamblin cleopatra volcano area appears to have been non uniform in scale and magnitude and no single structural model involving strictly strike slip faulting can account for the observed paleomagnetic data

carbonate platform development in a paleoproterozoic extensional basin vempalle formation cuddapah basin india abstract sedimentological investigation of the late paleoproterozoic orosirian vempalle formation of the cuddapah basin dharwar craton india reveals three facies association that range from supratidal to deep subtidal sedimentary rocks of this succession are dominated by heterolithic carbonate mudstone intraformational carbonate conglomerate and a variety of columnar domal and stratiform microbialite facies deposition occurred in an extensional regime during development of a low gradient ramp where the distribution of microbialite facies is distinctly depth partitioned a gradual increase in synoptic relief of columnar stromatolites through the section and the upward transition from stratiform to columnar microbialites record a prolonged marine transgression with little or no influx of terrigenous detritus siliciclastic influx along the northeastern side of the shelf reflects the redistribution of topographic highs concomitant with large scale volcanic activity redistribution of topographic highs eventually led to progradation of peritidal facies and shutting down of the carbonate factory earthquake induced ground shaking and voluminous volcanism experienced by this platform point to the reactivation of a deep seated mantle plume that resulted in thermal doming of the dharwar crust prior to the onset of cuddapah deposition isotopic and elemental chemistry of a selection of vempalle formation carbonate rocks record elevated mn2 and fe2 concentrations and depleted carbon isotope values in inner ramp lagoonal facies relative to more open marine stromatolitic facies patterns of isotopic and elemental variation suggest the presence of geochemically distinct water masses either within the water column or within substrate pore fluids that resulted from a combination of globally low marine oxygenation and restricted oceanographic circulation in inner ramp environments these data suggest that even in the aftermath of early paleoproterozoic oxygenation that ocean chemistry was heterogeneous and strongly affected by local basin conditions

evidence for paleoproterozoic cap carbonates in north america the early paleoproterozoic snowy pass supergroup of the medicine bow mountains and sierra madre wyoming usa and huronian supergroup ontario canada were deposited along the present day southern flank of the wyoming and superior cratons whereas three discrete levels of glacial diamictite are developed in both successions carbonate strata are known only directly above the middle diamictite vagner and bottle creek formations in medicine bow mountains and sierra madre respectively and espanola formation in southern ontario in these thick correlative siliciclastics dominated strata the carbonates from each succession record negative 13 c values 4 0 to 0 8 v pdb and attenuated carbon isotopic difference between organic and inorganic phases oxygen in carbonates is strongly depleted in 18 o suggesting exchange with hot fluids which is consistent with pervasive recrystallization of carbonates and remobilization of elements however the stratigraphic coherence of carbon isotopic compositions and the general lack of correlation between 13 c and either 18 o values or trace element concentrations supports a primary origin for 13 c depleted carbonates which are interpreted here to reflect anomalous oceanic compositions the intimate association of thick carbonate units containing abundant carbonate debris flows with immediately underlying glacial strata indicates that chemical precipitation resulted from a rapid flux of carbonate alkalinity onto ocean margins during post glacial transgression although these early paleoproterozoic carbonates are similar to neoproterozoic cap dolomites in stratigraphic position and carbon isotopic compositions the older post glacial accumulations begin with limestone and lack many of the sedimentary structures typical of neoproterozoic deposits furthermore it is not understood why carbonates only occur above the middle of the three glacial horizons whereas these deposits are ubiquitous above neoproterozoic diamictites the differences might reflect lower overall carbonate saturation in early paleoproterozoic oceans which contrasts sharply with archean and later paleoproterozoic intervals and higher siliciclastic inputs in rift environments which shut down carbonate deposition geological and geochemical indicators suggest a stepwise increase in atmospheric oxygen across the paleoproterozoic glacial epoch the tempo and mode of atmospheric oxygen rise has significant consequences for the abundance of the important greenhouse gases ch4 and co2 and hence for oceanic acidity if we accept that atmospheric oxidation of methane to carbon

interaction of rising and sinking mantle plumes the frequency of plume formation in fully developed thermal convection is determined experimentally because the fluid has a temperature dependent viscosity the cold and hot thermal boundary layers have different thicknesses and viscosities as a result plumes are released from these layers with different frequencies there also appears to be a direct temporal interaction between the cold and hot boundary layers in particular whereas there is a single frequency for cold plume formation hot plumes form with multiple frequencies one of which is similar to the frequency of cold plumes the measurements thus suggest that cold sinking plumes may induce hot rising plumes

1891 1883 ma southern bastar cuddapah mafic igneous events india a newly recognized large igneous province abstract a newly recognized remnant of a paleoproterozoic large igneous province has been identified in the southern bastar craton and nearby cuddapah basin from the adjacent dharwar craton india high precision u pb dates of 1891 1 0 9 ma baddeleyite and 1883 0 1 4 ma baddeleyite and zircon for two se trending mafic dykes from the bd2 dyke swarm southern bastar craton and 1885 4 3 1 ma baddeleyite for a mafic sill from the cuddapah basin indicate the existence of 1891 1883 ma mafic magmatism that spans an area of at least 90 000 km2 in the south indian shield this record of 1 9 ga mafic ultramafic magmatism associated with concomitant intracontinental rifting and basin development preserved along much of the south eastern margin of the south indian shield is a widespread geologic phenomenon on earth similar periods of intraplate mafic ultramafic magmatism occur along the margin of the superior craton in north america 1 88 ga molson large igneous province and in southern africa along the northern margin of the kaapvaal craton 1 88 1 87 ga dolerite sills intruding the waterberg group existing paleomagnetic data for the molson and waterberg 1 88 ga large igneous provinces indicate that the superior and kalahari cratons were at similar paleolatitudes at 1 88 ga but a paleocontinental reconstruction at this time involving these cratons is impeded by the lack of a robust geological pin such as a limpopo like 2 0 ga deformation zone in the superior province the widespread occurrence of 1 88 ga intraplate and plate margin mafic magmatism and basin development in numerous archean cratons worldwide likely reflects a period of global scale mantle upwelling or enhanced mantle plume activity at this time

the fluid dynamics of plume ridge and plume plate interactions an experimental investigation abstract a number of hotspots occur at or near mid ocean ridges e g iceland causing elevated bathymetry gravity anomalies and geochemical enrichment for hundreds of kilometers along otherwise normal ridge axes this is commonly presumed to be the result of hot plume material from the deep mantle flowing outward from a narrow plume conduit here we explore the dynmics of plume ridge and plume plate interaction using laboratory experiments we model chemically buoyant plumes as opposed to thermally buoyant plumes so we ignore the effects of thermal diffusion while concentrating on the strictly fluid dynamical aspects of the interaction we find that the spreading of a continuously fed plume beneath a fixed plate is described by a simple expression derived from a balance between plume buoyancy and viscous resistance to spreading for plumes spreading beneath ridges we find two remarkably distinct phases of flow at first the plume spreads approximately as it would beneath a fixed plate then a rapid transition occurs to a steady state condition in which the plume width along the ridge waist is constant this width is found to be proportional to a natural length scale that increases with the square root of the plume volumetric flux and the ratio of the mantle plume viscosities and decreases with the square root of the plate spreading velocity although our experimental conditions with uniform mantle viscosity and no thermal diffusion are perhaps oversimplified compared to the real earth the results scale reasonably well to observation our model suggests that sufficient time has elapsed 60 myr for the iceland plume to reach steady state in its interaction with the mid atlantic ridge using independent estimates for the plume volumetric flux at the iceland hotspot and assuming a viscosity ratio of one the model predicts a minimum waist width of about 850 km in close agreement with the observed value of about 920 km data from other hotspots jan mayen azores galapagos are also consistent with the experimental results for average mantle plume viscosity ratios of 1 3 using estimated plume fluxes with a simple conservation of mass equation yields reasonable estimates of plume thickness ranging from 70 to 180 km for these hotspots thus the experimental model appears to be reasonably consistent with observation given that thermal diffusion has been disregarded

melting dehydration and the dynamics of off axis plume ridge interaction 1 we present results from a series of two dimensional numerical experiments in which a thermally buoyant off axis mantle plume interacts with a nearby ridge axis these experiments incorporate melting and a number of related dynamical feedbacks including energy loss to latent heating and fe depletion buoyancy as well as viscosity increases due to dehydration during melting results indicate that dehydration which acts to increase the viscosity of the residual mantle during melting profoundly impacts the morphology of flow between plume and ridge axis as upwelling plume material begins to melt it becomes significantly more viscous and readily accretes to the overlying lithosphere this creates a viscous plug that grows downward from the base of the lithosphere directly above the plume conduit plume material traveling to the ridge axis is then deflected horizontally at subsolidus depths by this plug allowing pristine unmelted plume material to reach the ridge axis experiments without dehydration show that the loss of thermal energy to latent heating during melting marginally decreases the flow of plume material along the base of the lithosphere to the ridge axis alternatively extraction of fe from the solid during melting slightly increases plume flow to the ridge these results also demonstrate that the flow of plume material to the ridge axis is sensitive to the slope of the base of the rheological lithosphere consistent with previous studies of off axis plume ridge interaction

cyclicity and paleoenvironmental dynamics rocknest platform northwest canada the superbly exposed rocknest formation northwest territories canada contains 140 to 160 shallowing upward cycles 1 to 15 m thick that contain distinctive marker beds and can be correlated for 200 km parallel to strike and 120 km perpendicular to strike cycles are grouped according to cycle base lithology which reflects paleogeographic position on the platform shale based cycles of the distal inner shelf pass westward into dolosiltite based cycles then into tufa based cycles of the shoal complex and then into grainstone based cycles of the outer shelf cycles are bounded by erosional surfaces that become less pronounced away from the shoal complex cycles formed in response to small asymmetric rapid rise slow fall eustatic oscillations in sea level 10 m during sea level rise rapid transgression and submergence of tidal flats occurred along the eastern margin of the shoal complex followed by eastward expansion of the shoal complex and slow progradation of tidal flats over lagoonal facies during sea level fall a glacio eustatic model for cyclicity accounts for the data favoring asymmetric eustatic sea level oscillations subaerial exposure and vadose diagenesis of platform during sea level falls arrested progradation and incomplete shoaling of many cycles and cycle periods of 20 000 to 100 000 yr 18 000 to 30 000 yr for rocknest inner shelf slopes ranged from 8 cm km average at high sea level stand to 2 cm km average at low sea level stand tidal flat progradation rates were 7 to 9 km 1 000 yr interaction of inner shelf slope sea level fall rate and sedimentation rate generates complete condensed or expanded cycle profiles one dimensional computer modeling reveals that sedimentation rates of shoal complex facies were 250 cm 1 000 yr that lag times in sedimentation following transgression may have ranged from zero on the shoal complex to 2 000 yr on the distal inner shelf and that subtidal sedimentation rates may have ranged from 80 cm 1 000 yr for the proximal inner shelf to 20 cm 1 000 yr for the distal inner shelf

multiscale seismic tomography and mantle dynamics abstract in this article we first introduce the methodology of multiscale seismic tomography and the way to interpret the obtained tomographic images and then review the significant recent results of multiscale seismic tomography with emphasis on mantle plumes and subducting slabs global and regional tomography shows that most of the slab materials under western pacific and east asia are stagnant in the mantle transition zone before finally collapsing down to the core mantle boundary as a result of large gravitational instability from phase transitions local and teleseismic tomography studies have imaged clearly the subducting slabs and arc magma chambers in the upper mantle wedge indicating that geodynamic systems associated with arc magmatism and back arc spreading are related to deep processes such as convective circulation in the mantle wedge and dehydration reactions of the subducting slab because most hotspots are located in poorly instrumented continental and oceanic regions 3 d crust and upper mantle structure is determined for only a few hotspots such as iceland yellowstone and eifel which are covered by seismic networks and plume like slow anomalies are revealed under those hotspots global tomography has revealed deep mantle plumes under the major hotspots such as hawaii iceland kerguelen south pacific and africa strong lateral heterogeneities are revealed at the bottom of the mantle which are associated with the deeply subducted slabs and the birth of mantle plumes a thorough understanding of the deep earth structure will only be achieved by a combination of more effective seismic imaging techniques and dense coverage of global seismic networks particularly in the oceans

thermal variations in the mantle inferred from 660 km discontinuity topography and tomographic wave speed variations summary variations in mantle temperatures cause changes in seismic wave speeds and changes in the pressure at which mineralogical reactions occur changing the depth at which a seismic discontinuity occurs as an experiment in using global seismological data sets in a statistical way to draw inferences concerning the earth using angular correlation functions we analyse wave speed variations in p and s wave tomographic studies and perturbations to the depth of the 660 km discontinuity with the goal of inferring lateral temperature variations in the lower mantle on account of discrepancies between the temperatures estimated using various data sets the results are not particularly encouraging we find variations in temperatures of 60 or 90 120 k at the top of the lower mantle depending on the data set used 60 k in the middle of the lower mantle and potentially 60 or 300 k in the core mantle boundary region based on 660 km depth perturbations plumes might be 160 k warmer than the ambient lower mantle future improvements in angular correlation analyses could be obtained through finer gridding of the global data sets

vertical axis rotations in the las vegas valley shear zone southern nevada paleomagnetic constraints on kinematics and dynamics of block rotations paleomagnetic study of vertical axis rotations at 23 localities along the right lateral las vegas valley shear zone lvvsz in southern nevada indicates that clockwise rotations generally increase with proximity to the lvvsz reaching 100 at the closest localities rates of rotation determined at four localities range from l m y to 12 m y the maximum characteristic size of rotating blocks is 2 4 km considerably smaller than the dimensions of the zone of deformation associated with the shear zone 100 km in length 15 km in half width thus the deformation does not accord with kinematic models of block rotations that assume uniform rotations within domains having dimensions of the order of the width of the deforming zone instead the deformation appears to be quasi continuous when considered on a scale of 10 km we suggest that it is consistent with the deformation of a highly plastic layer of upper crustal material mechanically decoupled from deeper parts of the crust and that the across strike distribution of rotations is controlled by the rheological properties of the upper crust the length of the shear zone and the total amount of offset across the shear zone

significance of subtidal microbial deposits in shark bay australia abstract microbial deposits at shark bay australia constitute one of the largest and most diverse modern occurrences around the world the microbial carbonate system has developed in response to environmental change from near open marine into restricted embayment with hypersalinity and high evaporation bacterial sediments started depositing at about 2000 years bp after the holocene maximum flooding of sea level about 6800 years bp due to increasing salinity as the sea level fell by about 2 5 m within the holocene highstand system tract the extent and types of subtidal microbial substrate in this world heritage area have not previously been fully systematically mapped and described here we document the importance of subtidal microbial deposits occupying as much as 10 times the occurrence area of the intertidal microbial mats and stromatolites and demonstrate that deeper structures have a cerebroid external morphology with a non laminated internal fabric subtidal structures contain consistently different internal fabrics with carbonate grains and fragments interbedded with aragonitic micrite arranged in millimetric laminae or sub spherical patches which display fabrics such as well laminated smooth coarse laminated wavy colloform sub laminar to irregular with voids and biofragments of bivalves microbial pavement irregular clotted pustular and non laminated cerebroid constituting carbonate sediment with stromatolitic facies smooth and colloform thrombolitic facies pustular and cryptomicrobial non laminated cerebroid and microbial pavement each microbial structure grows in specific settings according to water depth msl with cerebroid build ups commonly occurring between 1 and 2 5 m and microbial pavement extending as deep as 6 m because of the late holocene sea level fall microbial structures often record in their internal features a shallowing up carbonate cycle these newly documented extensive subtidal microbial structures with distinctive morphologies and fabrics have importance as modern occurrences and provide potential analogues for understanding ancient microbial systems and environments

microbenthic distribution of proterozoic tidal flats environmental and taphonomic considerations silicified carbonates of the late mesoproterozoic to early neoproterozoic society cliffs formation baffin island contain distinctive microfabrics and microbenthic assemblages whose paleo environmental distribution within the formation parallels the distribution of these elements through proterozoic time in the society cliffs formation restricted carbonates including microdigitate stromatolites laminated tufa and tufted microbial mats consist predominantly of synsedimentary cements these facies and the cyanobacterial fossils they contain are common in paleoproterozoic successions but rare in neoproterozoic and younger rocks less restricted tidal flat facies in the formation are composed of laminated microbialites dominated by micritic carbonate lithified early yet demonstrably after compaction these strata contain cyanobacteria that are characteristic in neoproterozoic rocks within the formation the facies dependent distribution of microbial populations reflects both the style and timing of carbonate deposition because of the strong substrate specificity of benthic cyanobacteria a reasonable conclusion is that secular changes in microbenthic assemblages through proterozoic time reflect a decrease in the overall representation of rapidly lithified carbonate substrates in younger peritidal environments as well as concomitant changes in the taphonomic window of silicification through which early life is observed

reconstructing sea level change from the internal architecture of stromatolite reefs an example from the mesoproterozoic sulky formation dismal lakes group arctic canada the mesoproterozoic dismal lakes group arctic canada contains a relatively thin yet regionally extensive stromatolitic reef complex that developed subtidally during a major transgression shoaled to sea level and was overlain by intertidal to supratidal carbonate and evaporite strata the september lake reef complex exhibits a complex internal architecture that records the interaction between stromatolite growth and changes in accommodation space derived from both higher order 4th or 5th order parasequence scale changes in sea level and the variable bathymetry of the sea floor reef growth which was initiated during three sea level cycles records progressive marine transgression over depositional lows that were formed during pre reef subaerial exposure and erosion of the underlying strata a fourth sea level cycle represented by spectacular coniform stromatolites with 10 m of synoptic relief marks a more dramatic rise in sea level and establishment of the main reef complex aggradation and ev

rates and timing of vertical axis block rotations across the central sierra nevada walker lane transition in the bodie hills california nevada 1 we use paleomagnetic data from tertiary volcanic rocks to address the rates and timing of vertical axis block rotations across the central sierra nevada walker lane transition in the bodie hills california nevada samples from the upper miocene 9 ma eureka valley tuff suggest clockwise vertical axis block rotations between ne striking left lateral faults in the bridgeport and mono basins results in the bodie hills suggest clockwise rotations r r 95 confidence limits of 74 8 since early to middle miocene 12 20 ma 42 11 since late miocene 8 9 ma and 14 10 since pliocene 3 ma time with no detectable northward translation the data are compatible with a relatively steady rotation rate of 5 2 ma 1 2 since the middle miocene over the three examined timescales the average rotation rates have probably not varied by more than a factor of two over time spans equal to half of the total time interval our paleomagnetic data suggest that block rotations in the region of the mina deflection began prior to late miocene time 9 ma and perhaps since the middle miocene if rotation rates were relatively constant block rotation in the bodie hills is similar in age and long term average rate to rotations in the transverse ranges of southern california associated with early transtensional dextral shear deformation we speculate that the age of rotations in the bodie hills indicates dextral shear and strain accommodation within the central walker lane belt resulting from coupling of the pacific and north america plates

a palaeoproterozoic dolomite vempalle formation cuddapah basin india showing phanerozoic type dolomitisation abstract the palaeoproterozoic vempalle formation of the cuddapah basin india significantly adds to our understanding of the evolution of precambrian marine carbonate systems and the redox state of the earth s early oceans a facies microfacies diagenetic geochemical examination of samples from a 1000 m long exposure in a freshly cut canal section shows that 10 15 of precursor limestone is still preserved in the vempalle formation in the form of remnant patches of calcimicrite and ooids with calcite spar cement the ooids preserving primary radial and concentric fabrics and radial fractures are considered to have been originally precipitated as calcite which may have been low mg in places the preserved calcite spar that is partially replaced by fabric destructive dolomite shows type i calcite twin lamellae petrographic observations demonstrate that vempalle formation dolomite formed through very early precipitation which in stromatolites preserved microbial filaments as well as through fabric destructive dolomitization during shallow to moderate burial vempalle formation dolomite is characterized by micritic dolomite crystals which suggest rapid early dolomitization of lime mud and micritic calcite from a supersaturated mg ca rich solution probably near surface or during shallow burial depletion of na and sr contents of vempalle formation dolomite along with negative 18o values indicate dolomite recrystallisation during burial and further replacement dolomite 13c values of 0 5 to 2 are likely inherited original marine values geochemical proxies trace elements and rare earths imply that cuddapah basin seawater and dolomitizing fluids were anoxic and ferruginous but not euxinic geochemical analyses also indicate that the burial diagenetic fluids evolved from eu enriched seawater that probably resulted from continental rifting around 1 9 2 0 ga this probable ocean chemistry is in contrast with the anoxic ferruginous and extremely high mg ca dolomite oceans that prevailed during proterozoic time the vempalle dolomite shows more similarities with dolomitised phanerozoic platform carbonates than typical precambrian dolomite with its well preserved textures and burial dolospar cements

plume capture by divergent plate motions implications for the distribution of hotspots geochemistry of mid ocean ridge basalts and estimates of the heat flux at the core mantle boundary abstract the coexistence of stationary mantle plumes with plate scale flow is problematic in geodynamics we present results from laboratory experiments aimed at understanding the effects of an imposed large scale circulation on thermal convection at high rayleigh number 106 ra 109 in a fluid with a temperature dependent viscosity in a large tank a layer of corn syrup is heated from below while being stirred by large scale flow due to the opposing motions of a pair of conveyor belts immersed in the syrup at the top of the tank three regimes are observed depending on the ratio v of the imposed horizontal flow velocity to the rise velocity of plumes ascending from the hot boundary and on the ratio of the viscosity of the interior fluid to the viscosity of the hottest fluid in contact with the bottom boundary when v 1 and 1 large scale circulation has a negligible effect on convection and the heat flux is due to the formation and rise of randomly spaced plumes when v 10 and 100 plume formation is suppressed entirely and the heat flux is carried by a sheet like upwelling located in the center of the tank at intermediate v and depending on established plume conduits are advected along the bottom boundary and ascending plumes are focused towards the central upwelling heat transfer across the layer occurs through a combination of ascending plumes and large scale flow scaling analyses show that the bottom boundary layer thickness and in turn the basal heat flux q depend on the peclet number pe and when 10 q pe1 2 and when 1 q pe 1 3 consistent with classical scalings when applied to the earth our results suggest that plate driven mantle flow focuses ascending plumes towards upwellings in the central pacific and africa as well as into mid ocean ridges furthermore plumes may be captured by strong upwelling flow beneath fast spreading ridges this behavior may explain why hotspots are more abundant near slow spreading ridges than fast spreading ridges and may also explain some observed variations of mid ocean ridge basalt morb geochemistry with spreading rate moreover our results suggest that a potentially significant fraction of the core heat flux is due to plumes that are drawn into upwelling flows beneath ridges and not observed as hotspots

characteristics distribution and morphogenesis of subtidal microbial systems in shark bay australia abstract the distribution nature and extent of microbial deposits in hamelin pool shark bay have been investigated and mapped with emphasis on the occurrence external morphologies internal fabrics constructional mechanisms microbial communities growth rates and sediment associations in the intertidal and previously little researched subtidal zone detailed georeferenced substrate mapping revealed extensive subtidal microbial deposits occupying approximately 300 km 2 of the total holocene 1400 km 2 area of hamelin pool the microbial pavement covers 227 km 2 of the subtidal substrate that together with columnar structures reveals a subtidal microbial habitat which occupies an area 10 times larger than the area of the intertidal deposits microbial carbonate is composed of aragonite 80 98 that reveals high positive values of 13 c 4 46 to 5 88 and 18 o 3 06 to 3 88 as a characteristic of the highly evaporative environment with extensive microbial activity oldest dated heads are 1915 and 1680 14 c years bp and the overall system was deposited in two stages the first between 2000 and 1200 and the last from 900 years bp to the present slow growth rates vary from less than 0 1 mm year to 0 5 mm year different internal fabrics were constructed according to their position in relation to the littoral zone by distinct microbial communities and lateral fabric relations have been established evidence of shallowing upward fabric sequences of microbial origin reflects relative falling sea levels during the late holocene and is likely useful in ancient environmental interpretation a sequence of events and mechanisms are described emphasizing differences between the stromatolitic thrombolitic and cryptomicrobial deposits in shark bay the new substrate map and depositional history for this distinctive and peculiar microbial habitat establish the significance of subtidal structures and emphasize the geoscientific importance of hamelin pool especially with respect to early life studies and ancient analogues for understanding microbial activity deposit characteristics fenestral fabrics and distribution

bifurcation of the yellowstone plume driven by subduction induced mantle flow in the northwestern united states two neighbouring volcanic chains the snake river plain and high lava plains decrease in age in opposing directions laboratory experiments show that diverging volcanic chains can form where vigorous mantle circulation beneath a subduction zone causes an upwelling mantle plume to split into two

peritidal carbonate platform growth and cyclicity in an early proterozoic foreland basin upper pethei group northwest canada abstract the upper pethei group is a 170 235 m thick largely limestone succession located in the east arm of great slave lake northwest canada strata accumulated in a paleoproterozoic foreland basin during the late stages of marine deposition rocks include nine predominantly stromatolitic facies that are grouped into five facies assemblages 1 marginal peritidal complex 2 back barrier lagoon 3 shelf margin 4 subtidal shelf and 5 slope and basin carbonate sedimentation on the platform involved precipitation of synsedimentary cements within and on stromatolite laminae water column precipitation of carbonate mud and generation and transport of nonskeletal grains stromaclasts ooids oncoids peloids and intraclasts the upper pethei platform is interpreted to have evolved through three basic platform geometries peritidal rimmed shelf subtidal rimmed shelf and subtidal open shelf in response to changes in the rate of creation of accommodation space the peritidal complex and rim behaved in a manner consistent with models of phanerozoic reefal rims and alternatively kept up gave up or caught up to sea level in response to fluctuations in the rate of change of relative sea level and lateral variations in sediment production and accumulation three scales of cycle are recognized 1 overall formation scale upward deepening of the platform 2 laterally traceable decameter scale shallowing upward cycles separated by marine flooding surfaces and 3 laterally discontinuous meter scale shal owing upward peritidal cycles the first two cycle types are interpreted to have been allogenic controlled by changes in eustasy and subsidence rates meter scale cycles are interpreted as mainly autogenic having formed in an aggradational tidal island model the upper petrel platform is similar to other paleoproterozoic foreland basin carbonate platforms important differences however include 1 laterally discontinuous peritidal cycles 2 lack of widespread sand shoals inboard of the shelf rim 3 a thick predominantly carbonate slope and basin succession 4 moderate to low platformal micrite content 5 extensive lagoonal ooid sands and 6 low contents of terrigenous clastics

hotspots and sunspots surface tracers of deep mantle convection in the earth and sun abstract the pattern of new appearances of hotspots on the earth is investigated using available age data the worldwide total of cenozoic and mesozoic hotspots predicted by extrapolation from the observed number of continental flood basalts is 40 in agreement with crough s counts there are found to be no true antipodal pairs in one interpretation of the data new appearances of hotspots occur first at high latitudes in both hemispheres and then migrate toward the equator shortly before the migration cycle is finished the next cycle begins a relative deficiency of hotspots however is observed in very low equatorial and very high polar regions in addition hotspots occupy two large hemispherical groups in longitude that slowly drift in the same direction as the axial body rotation an observed magnetic superchron seems to end when a new hotspot migration cycle in latitude begins but consecutive superchrons show opposite polarity although the hotspot data are rather sparse their suggested patterns resemble the well known patterns of complexes of active regions on the sun as exemplified by proxy data such as sunspots both the earth and the sun possess large convecting rotating and magnetized mantles in which the characteristic surface tracers are believed to reflect the patterns of convective phenomena very deep in the mantle even though the tracers themselves hotspots and sunspots are certainly not analogs of each other the present study supports existing theoretical ideas that the large scale patterns of deep mantle convection in the earth and sun may fundamentally resemble each other despite the enormous difference in their molecular viscosities magnetic polarity reversal histories also show close similarities in the two bodies suggesting the operation of basically similar convective dynamos however the sun has displayed no known analog of the earth s magnetically disturbed intervals

volcanic development of small oceanic central volcanoes on the flanks of the east pacific rise inferred from narrow beam echo sounder surveys abstract narrow beam bathymetric surveys of about 30 volcanoes on young crust of the cocos plate were collected in order to help determine the origin and evolution of oceanic central volcanoes the volcanoes are located on ocean crust 1 5 7 5 m y in age and range in size from to 600 km 3 these volcanoes differ greatly in shape development of linear features small scale morphologic characteristics and crater development we conclude that these large differences in morphologic characteristics are due to the complex interaction of many factors in their development such as conduit geometry magma ascent rates eruption rate history tectonic activity crater formation and filling and others elongation directions of individual volcanoes and clear linear bathymetric features near these volcanoes are among the lines of evidence that strongly suggest these volcanoes originate and evolve on and near oceanic fracture zones their origin and morphologic evolution is thus closely tied to the tectonic processes and thermal regimes at fractures we speculate that these volcanoes on fast spreading lithosphere are generated by episodic pulses of melt production beneath ridge crests in the vicinity of fracture zones petrologic evidence shows that these volcanoes cannot be fed from sub axial magma chambers beneath the east pacific rise instead they tap the same mantle source as the ridge crest at mantle depth

fine grained agglutinated elongate columnar stromatolites tieling formation ca 1420 ma north china the mesoproterozoic tieling formation near jixian northern china contains thick beds of vertically branched laterally elongate columnar stromatolites carbonate mud is the primary component of both the stromatolites and their intervening matrix its abundance is attributed to water column whiting precipitation stimulated by cyanobacterial photosynthesis neomorphic microspar gives the stromatolites a streaky microfabric and small mud flakes are common in the matrix the columns consist of low relief mainly non enveloping laminae that show erosive truncation and well defined repetitive lamination in plan view the columns form disjunct elongate ridges 10 cm wide separated by narrow matrix filled runnels the stromatolite surfaces were initially cohesive rather than rigid and prone to scour and are interpreted as current aligned microbial mats that trapped carbonate mud the pervasive ridge runnel system suggests scale dependent biophysical feedback between i carbonate mud supply ii current duration strength and direction and iii growth and trapping by prolific mat growth together these factors determined the size morphology and arrangement of the stromatolite columns and their laminae as well as their branching patterns alignment and ridge runnel spacing ridge runnel surfaces resemble ripple mark patterns but whether currents were parallel and or normal to stromatolite alignment remains unclear the formation and preservation of tieling columns required plentiful supply of carbonate mud mat building microbes well adapted to cope with this abundant sediment and absence of both significant early lithification and bioturbation these factors were time limited and tieling stromatolites closely resemble coeval examples in the belt purcell supergroup of laurentia the dynamic interactions between mat growth currents and sediment supply that determined the shape of tieling columns contributed to the morphotypical diversity that characterizes mid late proterozoic branched stromatolites this article is protected by copyright all rights reserved

an active thermoelastic rift mechanism finite element solutions for stresses and displacements during rift development are obtained for a variable thickness elastic lithosphere plate subjected to thermoelastic loading by conductive heating from an axial line source in the asthenosphere the asthenosphere is modelled as a fluid foundation the prerift lithosphere is modelled as an infinite continuous plate a semi infinite plate is used to characterize the rifted lithosphere horizontal restraints are removed at the rift axis to model the transition from a continuous to separated lithosphere plate during rifting lithosphere strength envelope curves derived from olivine flow laws are used to determine the elastic lithosphere depth in the solutions conductive heating of a variable thickness continuous lithosphere plate by a 2000 w m line source at 60 km depth produces high stresses oriented orthogonal to the forming rift axis and small vertical displacements the maximum uplift obtained in the continuous lithosphere plate models subjected to thermoelastic loading was 0 12 km at a distance of 120 km from the forming rift axis this small uplift is accompanied by shallow tensile stresses of 80 mpa at a distance of 5 km from forming rift axis which decrease to 6 mpa at a distance of 210 km shallow horizontal thermoelastic tensile stresses are over 5 times greater than the horizontal stresses produced by thermal buoyancy loads near the forming rift axis the transition from a continuous to rifted lithosphere is accompanied by an increase in lithosphere uplift at the rift axis from 0 11 to 0 42 km and a reduction in shallow horizontal stress magnitude by a factor of 30 thermoelastic loading may be a source of shallow tensile stress in the lithosphere and a contributing factor for the initiation of rifting thermal loading may also produce postrifting topography changes thermoelastic loading of the lithosphere is proposed as a possible driving mechanism for rift formation which produces separation and the transition from a continuous to rifted lithosphere plate

carbonate ramp depositional systems abstract the classification tectonic settings stratigraphy and early diagenesis of carbonate ramp systems are reviewed carbonate ramps are common in all geological periods but were dominant at times when reef constructing organisms were absent or inhibited ramps can be subdivided into inner mid and outer ramp environments the mid ramp zone extends from fair weather wave base to normal storm wave base although the water depths which these boundaries represent vary an additional outer slope environment occurs on distally steepened ramps as with siliciclastic shelves a range of wave storm and tide dominated ramps can be recognised and this forms the most convenient basis for ramp classification the carbonate productivity profile of ramps differs from that of rimmed shelves with the inner ramp showing lower production rates than comparable shallow water facies on rimmed shelves the zone of greatest organic carbonate sediment production appears to have shifted from the mid ramp to the inner ramp since the late jurassic carbonate ramps occur in most types of sedimentary basin but are best developed where subsidence is flexural and gradients are slight over large areas as in foreland and cratonic interior basins and along passive margins the featureless depositional profiles of many ramps means that sequence geometries are best observed on regional seismic lines some show low angle sigmoidal or shingled clinoforms suggesting that ramps may seldom be homoclinal but possess subtle slope geometries which reflect depositional environments because of their low angle slopes ramps respond differently to rimmed shelves during relative sea level changes although results seem to be strongly dependent on the rate of relative sea level change during a minor fall shallow ramp facies belts will simply shift basinwards in a forced regression in contrast the whole surface of a steep sloped flat topped rimmed shelf may be exposed so that sediment production ceases or is drastically reduced during a major fall shallow ramp bounded basins may empty completely conversely ramps also flood gradually whereas rimmed shelves do so more rapidly homoclinal ramps develop no resedimented lowstand deposits rimmed shelves and distally steepened ramps in contrast may develop lowstand talus or turbiditic wedges distally steepened ramps may behave more like homoclinal ramps during minor base level falls and like rimmed shelves during major base level falls many ramps consist of layered successions of several ramp sequences stacked one upon the other ramp stacks of this sort may show gross vertical accretion but individual ramp sequences seldom appear to develop in a keep up style apart from minor organic buildups as with many rimmed shelves steepening of the outer ramp margin due to tectonism slope inheritance or differential sedimentation may promote the development of a distally steepened ramp or rimmed shelf a wide variety of organisms have constructed buildups in mid and outer ramp environments isolated buildups may seed early in ramp development accrete to wave base or sea level and continue growth by stacking through successive ramp sequences so that depositional and diagenetic features within them are in concert with those of the shallow ramp the location of isolated buildups on ramps is governed by tectonism halokinesis antecedent topography or by the subtle slope geometry of the previous ramp sequence diagenesis on ramps shows some major variations compared with diagenesis on steep sloped flat topped carbonate platforms ramp bounded basins may form prolific petroleum sourcing and reservoiring systems and offer a range of subtle stratigraphic play types and lateral facies variations which determine reservoir quality and distribution isolated buildups in the mid and outer ramp environments represent one of the commonest petroleum reservoirs in ramp systems and tend to have their foundations in transgressive systems tracts grainstone and packstone reservoirs are widespread and range from shoreline carbonate sandbodies to major detached shoal complexes or shoals over offshore highs grainstone sandbodies occur in both highstand systems tracts and in prograding lowstand wedges

antipodal hotspot pairs on the earth the results of statistical analyses performed on three published hotspot distributions suggest that significantly more hotspots occur as nearly antipodal pairs than is anticipated from a random distribution or from their association with geoid highs and divergent plate margins the observed number of antipodal hotspot pairs depends on the maximum allowable deviation from exact antipodality at a maximum deviation of 700 km 26 to 37 of hotspots form antipodal pairs in the published lists examined here significantly more than would be expected from the general hotspot distribution two possible mechanisms that might create such a distribution include 1 symmetry in the generation of mantle plumes and 2 melting related to antipodal focusing of seismic energy from large body impacts

phanerozoic hot spot traces and paleogeographic reconstructions of the siberian continent based on interaction with the african large low shear velocity province abstract we review intraplate magmatism in siberia and its folded surroundings from 480 ma to the present we describe several large igneous provinces lips and the intervals in which they were continuously formed within the limits of the siberian continent the altay sayan early paleozoic magmatic area 598 446 ma the altay sayan lip 408 393 ma the viluy lip 380 350 ma the barguzin vitim lip 310 275 ma the late paleozoic rift system of central asia 318 242 ma the siberian traps and west siberian rift system 250 249 ma the east mongolian and west trans baikalian lip 228 195 ma and a number of various aged late mesozoic and cenozoic rift zones and magmatic areas from 160 ma to the present day following lawver and muller 1994 kharin 2000 lawver et al 2002 and chernysheva et al 2005 we accept the position of the icelandic hot spot under the siberian trap area at the permo triassic boundary that enables us to estimate the geographic coordinates of the siberian trap location at 250 ma they are the same as iceland today presently the icelandic hot spot is situated above the african large low shear velocity province llsvp that indicates a hot mantle plume we suggest a set of paleogeographic reconstructions of the siberian continent for which we evaluate the paleolatitude based on the paleomagnetic data and estimate the paleolongitude position by placing siberia above the african llsvp furthermore we estimate the geographic coordinates for other ancient hot spots in the framework of the african llsvp that we consider to be responsible for the intraplate magmatism during different time periods in the phanerozoic eon available rare element and isotopic characteristics of the intraplate magmatic rocks of siberia enable us to determine three primary sources moderately depleted mantle prema enriched mantle em i and em ii of the mantle origin magma we propose that the model explains the interaction of the hot mantle plume including hot spots plume tails with the siberian intraplate magmatism areas throughout the phanerozoic eon

early proterozoic melt generation processes beneath the intra cratonic cuddapah basin southern india early proterozoic tholeiitic lavas and sills were emplaced during the initial phase of extension of the intra cratonic cuddapah basin southern india ar 40 ar 39 laser fusion determinations on phlogopite mica from the tadpatri fm mafic ultramafic sill complex constrain the age of the initial phase of extension and volcanism in the basin at 1 9 ga despite their early proterozoic age the igneous rocks are unmetamorphosed undeformed and remarkably fresh they exhibit a wide range in mgo contents 4 28 wt and have undergone varying degrees of accumulation or crystal fractionation variable la nb ratios 1 2 3 7 and epsilon nd values 1 to 10 suggest that some but not all of the mafic rocks have been affected by crustal contamination this appears to have taken place in magma chambers at similar to9 kbar i e the base of the continental crust forward modelling of major and trace elements fe and nd and inverse modelling of rare earth elements suggest that the primary cuddapah melts were generated by similar to10 15 partial melting of a lherzolite mantle source this corresponds to a mantle potential temperature of similar to1500degreesc the thickness of the mechanical boundary layer predicted by the geochemical modelling is 70 km with a minimum initial lithospheric thickness of 120 kin this corresponds to a stretching factor of 1 6 1 8 richter s 1988 secular cooling model for the earth predicts that at 1 9 ga the ambient mantle had a potential temperature of similar to1500degreesc i e similar to200degreesc hotter than phanerozoic mantle if the cooling model is correct then proterozoic lithospheric stretching and mantle melting beneath the intra cratonic cuddapah basin could have been caused by passive rather than active rifting

flattening of the sea floor depth age curve as a response to asthenospheric flow the flattening of sea floor depths from the age dependence predicted by considering the cooling plate as a growing thermal boundary layer is a fundamental constraint on the evolution of oceanic lithosphere1 previous explanations for the flattening have included reheating from convective instabilities that form beneath lithosphere older than 80 myr ref 2 thermal rejuvenation of lithosphere that passes over stationary hotspots3 5 or a whole mantle flow forced by two converging plates6 we suggest here that the flattening of old ocean floors can perhaps best be explained as a dynamic phenomenon reflecting flow in asthenosphere underlying the oceanic lithosphere applying the model to the pacific plate we show that a solution for flow in an asthenosphere low viscosity channel which is consumed by plate accretion and the subduction of lithosphere at trenches and replenished by near ridge upwelling and near ridge hotspots generates the observed 1 km of sea floor flattening for asthenospheric viscosities of 2 1018 pa s and an absolute pacific plate motion of 100 mm yr 1 our model can also explain the asymmetric subsidence of the south american and african plates away from the mid atlantic ridge

the modern geoid and ancient plate boundaries abstract the non hydrostatic geoid is dominated by three large anomalies an area of high gravity potential in the equatorial pacific another stretching from greenland through africa to the southwest indian ocean and a semi continuous low region passing from hudson s bay through siberia to india and on to antarctica none of these three high amplitude greater than 60 m and long wavelength anomalies corresponds to present day plate boundaries however if the modern geoid is plotted over the positions of continents and plate boundaries at 125 ma b p reconstructed relative to hotspots a strong correlation emerges the modern geoidal low corresponds in position to the areas of subduction surrounding the pacific 125 ma ago the geoidal high now centered on africa is entirely contained within ancient pangaea and the equatorial pacific high overlies the location of the spreading centers preserved in the magnetic anomalies of the central pacific the most plausible cause of the large geoidal undulations is lower mantle convection only weakly coupled to plate motions the correspondence between modern geoid and ancient plate boundaries implies either that the coupling was much more intimate in the past or that there is a lag of at least 100 ma in response of the lower mantle to upper mantle conditions

mantle convection with internal heating and pressure dependent thermal expansivity recent laboratory work 1 suggests that the thermal expansivity of the mantle decreases strongly with pressure determines the buoyancy of plumes and the rate at which plumes lose their thermal signature as they rise and so may be expected to have a strong influence on the temperature and velocity structure of the mantle numerical simulations were conducted on convection in an internally heated compressible mantle including constant and pressure dependent and thermal conductivityk they show that a pressure dependent allows the existence of thermal plumes rising from the core mantle boundary cmb however the plumes are weak and dissipate higher in the mantle

fabrics and kinematic indicators associated with the local structures along finlay ingenika fault mcconnell creek area north central british columbia structures associated with dextral transcurrent displacement on the finlay ingenika fault are characterized predominantly by subvertical to vertical faults intersections of the fault planes with planar fabrics are all subvertical this geometry together with the common occurrence of subhorizontal slickenlines and stretching lineations in some strongly deformed fault zones indicates their strike slip nature principal directions of the strain ellipsoids determined from the deformed volcanic fragments along one of the faults are in accordance with those of the magnetic susceptibility ellipsoids from the adjacent plutonic rocks indicating that the plutonic rocks experienced the same deformation as the late triassic takla group volcanics regional cleavage within the fault bounded blocks indicates that the blocks were nonuniformly rotated clockwise about subvertical axes in response to progressive dextral transcurrent displacement on the finlay ingenika fault the mode of deformation observed in the a

paleoproterozoic stark formation athapuscow basin northwest canada record of cratonic scale salinity crisis abstract the paleoproterozoic stark formation a thick breccia unit formed during the transition from marine to non marine foreland basin fill in the athapuscow basin northwest territories canada four types of breccia occur in the stark formation 1 bedded breccias at the base of the unit overlying the basinal facies of the underlying pethei group are interpreted as debris flows 2 massive chaotic breccias occur in the lower part of the stark formation throughout the basin and these are interpreted to be evaporite collapse breccias 3 siltstone and mudstone breccia occurs in the upper part of the stark formation and is also interpreted as evaporite collapse breccia though much less evaporite was involved in its formation than the underlying chaotic breccia and 4 laminar accretionary breccia consisting of laminar carbonate clasts commonly with flat bottoms and convex up upper surface geometries these clasts occur in lenticular pods and are interpreted to form in caves following dissolution of evaporites or carbonates the widespread presence and abundance of evaporite pseudomorphs especially halite in all breccia types the stratigraphic restriction of the stark formation and irregular filling above this unit indicates that all but the bedded breccia formed by the dissolution and collapse of evaporites chaotic breccia directly above underlying pethei group shallow water and deep water carbonates indicates the evaporates formed across the platform and adjacent deep basin foundered megaclasts up to 1 5 km long and 40 m thick in basinal settings suggest cumulative evaporite thicknesses on the order of a few tens to a few hundreds of meters carbonate clasts in all the breccias contain abundant wave ripples planar laminations stromatolites and ooids these structures along with a paucity of subaerial exposure features indicates nearly all carbonate deposition in the stark was subaqueous thus by inference the evaporites in the stark formation were also partially subaqueous displacive halite pseudomorphs are common throughout all facies in the stark formation silicified and dolomite filled halite pseudomorphs are common along the contact between the evaporite collapse breccia and underlying carbonate platform pethei group and surrounding large clasts in basinal settings silicified hopper crystal casts within deep water rhythmites of the uppermost pethei group indicate precipitation of halite over deep waters gypsum pseudomorphs are exceedingly rare and anhydrite pseudomorphs were not observed the dominance of halite features in this basin and two penecontemporaneous basins kilihigok basin and wopmay orogen surrounding the slave craton suggests that relative to the present day seawater at 1 9 to 1 8 ga was oversaturated with respect to calcium carbonate had higher hco3 concentration and possibly was depleted in so42 the dearth of extensive thick bedded marine gypsum in the archean and early paleoproterozoic can be explained by low atmospheric oxygen content prior to 2 ga in conjunction with increased carbonate saturation before 1 6 ga

distribution of mantle up welling determined from plate motions a case for large scale benard convection the number and geometric distribution of putative mantle up welling centers and the associated convection cell boundaries are determined from the lithospheric plate motions as given by the 14 euler poles of the observational nuvel model for an assumed distribution of up welling centers called cell cores the corresponding cell boundaries are constructed by a voronoi division of the spherical surface the resulting polygons are called benard cells by assuming the flow kinematics within a cell the viscous coupling between the flow and the plates is estimated and the euler poles for the plates are computed under the assumption of zero net torque the positions of the cell cores are optimized for the hs2 nuvel1 euler poles by a method of successive approximation subplex convergence to one of many local minima occurred typically after 20 000 iterations cell cores associated with the fourteen hs2 nuvel1 euler poles converge to a relatively small number of locations 8 to 10 depending on interpretation irrespective of the number of convection cells submitted for optimized distribution from 6 to 50 these locations are correlated with low seismic propagation velocities in tomography uniformly occur within hotspot provinces and may specifically be associated with the hawaiian iceland reunion kerguelen indian ocean easter island melanesia society islands south pacific azores cape verde canary islands tristan da cunha south atlantic balleny islands and possibly yellowstone hotspots it is shown that arbitrary euler poles cannot occur in association with mantle benard convection irrespective of the number and the distribution of convection cells nevertheless eight of the observational euler poles including the five that are accurately determined in hs2 nuvel1 australia cocos juan de fuca pacific and philippine are benard valid i e can be explained by our benard model five of the remaining six observational poles must be relocated within their error ellipses to become benard valid the eurasia pole alone appears to be in error by 115 and may actually lie near 40 n 154 e the collective results strongly suggest benard like mantle convection cells and that basal shear tractions are the primary factor in determining the directions of the plate motions as given by the euler poles the magnitudes of the computed euler vectors show however that basal shear cannot be the exclusive driving force of plate tectonics and suggest force contributions of comparable magnitude for perhaps half of the plates from the lithosphere itself specifically subducting slab pull and continental collision drag which are provisionally evaluated the relationship of the putative mantle benard polygons to dynamic chaos and turbulent flow is discussed

dolomitization on an evaporitic paleoproterozoic ramp widespread synsedimentary dolomite in the denault formation labrador trough canada abstract the denault formation 2 1 1 9 ga crops out in the labrador trough northeastern quebec and western labrador rocks surrounding the town of schefferville quebec contain textural characteristics consistent with deposition on the middle and outer portions of a storm influenced shallow ramp mid ramp facies consist of intraclastic grainstones with hummocky cross stratification hcs swaley cross stratification scs current ripples and graded event beds further outboard grainstones grade into deeper water laminites that are composed of even mm scale couplets of flat lying organic and dolomudstone laminae scours within the laminites suggest periodic storm activity laminites gradually grade into outer ramp deep water shales an isolated eastern stromatolitic buildup is separated from these ramp facies by 50 km present day this succession can be interpreted as the remnant of a near continuous margin or may simply represent an isolated accumulation that developed on a pre existing topographic high the presence of gypsum pseudomorphs in all lithofacies indicates that the denault margin was restricted and evaporitic four paragenetic stages are recognized in the diagenetic evolution of the denault formation 1 carbonate deposition contemporaneous marine cementation authigenic gypsum growth and precipitation of authigenic chert 2 synsedimentary mimetic dolomite precipitation 3 pore rimming and pore occluding shallow burial dolomite cement and 4 fabric destructive sutured anhedral burial dolomite gypsum crystals occur in all lithofacies form the nuclei of interstitial dolomite rhombs average 10 m in length and often display swallowtail twinning paleoproterozoic ocean water had very low concentrations of dissolved sulfate and evaporation in restricted settings would have been required to form gypsum formation of microcrystalline gypsum across this restricted ramp facilitated dolomite precipitation by increasing pore water mg ca ratios and lowering its dissolved sulfate concentrations such an interpretation may explain why there is an abundance of synsedimentary dolostone in the precambrian and the relative paucity of phanerozoic analogs

heterogeneity in southern central indian ridge morb implications for ridge hot spot interaction between the rodrigues triple junction and the marie celeste fracture zone basalts from the central indian ridge cir exhibit an enrichment in incompatible elements that increases in intensity northward in addition h2o tio2 al 8 and dy yb ratios increase while na 8 remains unchanged and fe 8 decreases evolution of the enriched magma appears to be affected by elevated water contents which lower the mantle solidus thereby increasing the initial depth of melting as well as delaying plagioclase crystallization however the enrichment affecting the northern samples is not a just function of hydrous mantle melting and crystallization instead of trending toward a small melt fraction from the mantle as predicted by hydrous melting models the cir samples lie on a mixing line between n morb and a source component that closely resembles present day reunion hot spot lavas thus it appears that while hydrous melting and crystallization affect the cir the enriched and wet mantle originates from the reunion hot spot where it migrates eastward toward the cir against the direction of motion of the lithosphere

synsedimentary cements as paleoproterozoic platform building blocks pethei group northwestern canada peritidal and subtidal carbonates of the pethei group are a complex arrangement of fibrous and micritic synsedimentary precipitates carbonate grains and detrital micrite synsedimentary precipitates present largely as stromatolitic laminae constitute the bulk of the pethei platform providing about 75 of the total carbonate they occur as stromatolitic precipitates precipitates closely associated with stromatolitic laminae which constitute about 60 of the rock and encrusting precipitates precipitates outside stromatolites and between grains i e cement which constitute about 15 of the rock the most abundant precipitates are fibrous in the form of stromatolitic microdigitate stalks tufa and masses of radial fans as well as encrusting pore filling fans isopachous rinds and laminar crusts micrite precipitates occur as stromatolitic clotted micrite and encrusting isopachous rinds the ratio of fibrous to micritic precipitates is 2 1 stromatolitic precipitates are arranged in three distinct microfabrics 1 ubiquitous spar micrite couplets of fibrous and clotted micrite laminae 2 deep water vertical fibrous masses that precipitated over micritic precipitate cores and 3 supratidal microdigitate crusts of branching fibrous stalks micritic cements are interpreted to be of biogenic origin having precipitated somehow in association with microorganisms fibrous precipitates are interpreted to be largely abiotic since stromatolitic and encrusting precipitates are petrographicaily similar they probably share similar origins there is little evidence of sediment trapping and binding as a mechanism for stromatolite formation the abundance of synsedimentary precipitates varies with platform geometry subtidal ramps subtidal rimmed shelves subtidal open shelves peritidal rimmed shelves while the basic building blocks of the pethei platform synsedimentary precipitates carbonate grains and detrital micrite are the same as those in phanerozoic platforms their distribution and relative proportions are different these differences reflect a proterozoic carbonate factory which extended across the entire platform and which produced carbonate largely by in situ precipitation

steady plumes produced by downwellings in earth like vigor spherical whole mantle convection models if mantle thermal upwellings plumes are the cause of volcanic hot spots then observations suggest that plumes are relatively fixed with nonuniform distribution and limited lifetimes to date fixity of upwellings has only been shown in models of convection at either low vigor or with layering though studies where the lower mantle has high viscosity do frequently show upwellings with much lower drift velocities than the surface velocity since more vigorous convection traditionally shows more time dependence fixity of upwellings has not been expected for nonlayered convection at earth like vigor rather we might expect slow but increasing drift velocities i have undertaken numerical models of whole mantle convection in three dimensional spherical geometry at approaching earth like vigor surprisingly these simulations show prominent steady virtually fixed plumes arising from self organization controlled by the smaller but numerous cold downwellings if downwelling subduction dominates earth s interior dynamics then this work suggests that plume fixity need not require layering the regular spacing and permanence of model upwellings contrast with observations of hot spots though in these models which do not simulate plates the fixity and regular spacing result from the freedom of downwellings to occur everywhere except where plumes reach the surface i suggest that on earth this feedback is weakened by the presence of plates but that whole mantle thermal convection can still produce relatively fixed plumes though we might expect them to be weaker more mobile and transient

patterns of hot spot volcanism the aim of this paper is to identify the characteristic features in terms of geophysically measured parameters of the hot spot sites over the earth s surface we use a pattern recognition approach which identifies associations as well as single parameters and our statistical algorithm allows us to work at a given significance level avoiding any overfit we use the following parameters absolute plate velocity of the site minimum distance between neighboring hot spots minimum distance from a ridge minimum distance from a trench geoid anomaly of harmonic degrees 2 10 and 11 36 area of the host plate lithospheric thickness and stress state the hot spots generally defined as volcanoes not obviously related to plate boundaries are found to occur in sites characterized by either one of two main significant patterns 1 positive values of geoid anomalies with harmonic degree between 2 and 10 which is characteristic of most of the hot spots with a track of extinct volcanoes 2 clustered volcanoes distance between volcanoes less than 900 km in slow moving plates velocity 2 5 cm yr a third pattern representative of isolated volcanoes distance between volcanoes greater than 900 km anomalously close to mid ocean ridges distance from the ridge less than 600 km can also be tentatively identified this result implies the existence of two main types of hot spot volcanism the first one of sublithospheric origin with dynamics which is not influenced by surface tectonics and the second one mainly due to a favorably soft lithosphere a third type would probably consist of anomalous and magmatically very productive parts of mid ocean ridges

models for the hotspot distribution published hotspot catalogues all show a hemispheric concentration beyond what can be expected by chance cumulative distributions about the center of concentration are described by a power law with a fractal dimension closer to 1 than 2 random sets of the corresponding sizes do not show this effect a simple shift of the random sets away from a point would produce distributions similar to those of hotspot sets the possible relation of the hotspots to the locations of ridges and subduction zones is tested using large sets of randomly generated points to estimate areas within given distances of the plate boundaries the probability of finding the observed number of hotspots within 10 of the ridges is about what is expected

marine carbon reservoir corg ccarb coupling and the evolution of the proterozoic carbon cycle the post 2 0 ga proterozoic c isotope record reveals two distinct yet interrelated trends a stepwise increase in average 13c from 0 calculated with respect to the peedee belemnite isotope standard prior to ca 1 3 ga to 5 in the neoproterozoic and a concomitant increase in the magnitude of isotopic excursions steady state and non steady state models suggest that these fundamental changes are best explained by a combination of evolving burial fluxes and a secular decrease in the size of the marine dissolved inorganic carbon dic reservoir the dic reservoir size affects the sensitivity of the isotopic system to biogeochemical perturbation major rearrangements of carbon cycling during the proterozoic in part related to the evolving marine carbon reservoir permit elevated 13c values to be sustained for geologically long time spans recognition of this dependence on dic reservoir size provides for the first time a direct link between changing carbonate precipitation styles and the marine c isotope record and may help constrain estimates of proterozoic p co2

block rotations along the strike slip finlay ingenika fault north central british columbia implications for paleomagnetic and tectonic studies structure developed in association with oblique motions on the dextral strike slip finlay ingenika fault fif in the mcconnell creek area north central british columbia is dominated by subvertical to vertical strike slip faults some of the faults cut the area into discrete fault bounded blocks several kilometres wide stress tensors for 24 sites were inverted from the regional cleavage which predates the block bounding faults the site mean stress tensors indicate that the fault bounded blocks were rotated clockwise about a subvertical axis during progressive oblique motions on the fif rotation varies systematically being maximal 58 7 3 3 close to the fif and minimal 0 0 1 6 about 20 km away paleomagnetic samples were collected from 13 sites in the widespread early jurassic to cretaceous plutonic rocks interpretable magnetic components of presumed late cretaceous age were obtained from six sites the observed paleopole is significantly different from the late cretaceous reference pole for cratonal north america cna after corrections for the local block rotations the precision is much improved k increased from 48 to 383 and the paleopole moves closer to the reference pole but is not coincident with it rotation about the eulerian pole for the best fitting small circle to the tintina trench and northern rocky mountain trench fault zone however brings the observed pole into coincidence with that for cna and requires 670 km of dextral displacement on the fault zone it is evident therefore that local structures associated with large dextral strike slip faults could account for at least part of the paleomagnetic disparity between some western parts of the canadian cordillera and cna

on the applicability of thermal conduction models to mid plate volcanism comments on a paper by gass et al the proposition that mid plate hot spot magmatism is strongly discouraged by a combination of thick lithosphere and rapid plate motion is examined empirically on a global basis and found to be inadequate although hot spots perhaps tend to avoid cratonic areas the nonrandom worldwide distribution of hot spots appears primarily to reflect the state of the sublithospheric mantle plate thickness or speed is of lesser importance the high level of african mid plate volcanism since 25 m y b p appears to be part of a global pattern controlled by events in the mantle rather than a result of the africa plate coming to rest over sublithospheric thermal sources about 40 m y b p

pb sr and nd isotope systematics of uranium mineralised stromatolitic dolomites from the proterozoic cuddapah supergroup south india constraints on age and provenance abstract the pb sr and nd isotopic compositions were determined on uranium mineralised and barren stromatolitic dolomite samples from the vempalle and tadpatri formations of the cuddapah supergroup in southern india subsamples from individual handspecimens were analyzed to minimize the potential of heterogeneous initial pb compositions all of the u mineralised samples gave pb pb ages close to 1800 ma but with high mswd values presumably due to local redistribution of u and pb in the samples the best pb pb age of 1756 29 ma is interpreted as the time of u mineralisation and as a minimum age for carbonate sedimentation and dolomitisation within the cuddapah supergroup the sr isotopic compositions of the samples vary with the mineralised samples being more radiogenic even the lowest 87 sr 86 sr of the barren sample exceeds the seawater 87 sr 86 sr about 1800 ma ago on the other hand the nd systematics of the samples does not appear to have been disturbed by processes subsequent to carbonate sedimentation the calculated initial nd isotopic compositions suggest a dominant continental crustal source that was at least 600 ma old at the time of deposition of the carbonate sediment

plate kinematic constraints from mantle benard convection general kinematic implications for plate tectonics are determined for rayleigh benard convection of the mantle the continuum of all possible configurations of benard polygons is probed by large random samples of global configurations 450 000 to 54 000 000 for each of which the euler poles are determined on the basis of viscous coupling across the asthenosphere two computationally related methods lead first to euler pole restrictions for fourteen plates and second to restrictions on the benard cell configuration result no 1 euler poles occur in global preference patterns which are determined exclusively by the shape of the plate the observational hs2 nuvel1 model poles occur near regions preferred by benard convection eurasia excluded the agreement is best for the most accurate observational poles result no 2 seven specific mantle benard cells are indicated by present day plate motions the upwelling centers correlate with hotspot domains the major global subduction zones correlate with benard model downwelling this result is independent of the euler pole accuracy used in its determination and is consistent with the distribution of low seismic p wave propagation velocities determined by tomography and with shear wave splitting analysis within the asthenosphere conclusions the results suggest that the bulk mantle is divided into less than ten benard convection cells globally cf fohlmeister and renka 2002 each of which extends from the asthenosphere to the core mantle boundary turbulent flow and other perturbations of the benard kinematics appear to be limited these primally poloidal flow kinematics provide basal shear forces as a major component in driving plate tectonics and are specifically configured for the directions of plate motions the benard model is incomplete without a dynamic contribution from the lithosphere which represents a separate convection layer of the distinct polar kinematics of rigid plates the complete hybrid mechanism for driving plate tectonics includes lithospheric buoyancy dynamics specifically from the subducting pacific plate slabs to compensate for plate slowing due to the back flow sector of the hawaiian convection cell and collision drag dynamics principally for smaller plates or continental margins

geodynamical implications from the correlation of surface geology and seismic tomographic structure abstract we have investigated the relationship between the lateral seismic structure in the mantle and the geological features associated with major mantle upwellings the oceanic ridge system and hotspots using the technique of correlation analysis we have identified that these features extend quite deeply into the mantle to a depth of around 1300 km this finding is surprising for the ridges because it indicates that the style of upwelling is rather active and is somewhat two dimensional in character the confidence level of the correlation between ridges and slow seismic regions is maintained at a level of greater than 90 to a depth of 1300 km this is followed by a sharp decrease to 50 at 1700 km a similar correlation pattern is found for the hotspots in both cases the correlation maxima lie close to a depth of 1000 km where remnants of old slabs have recently been revealed by correlation analysis these findings suggest that dynamical processes at a depth of around 1000 km can be linked with surface tectonic activity this correlation disappears at a depth of around 1500 1700 km certain features of the surface tectonic pattern can again be recognized around a depth of 2000 km and in the deepest mantle where major hotspot plumes and the remnant old lithosphere are the dominant features there may be some dynamical relationship between the ridge system and the two megaplumes we have proposed a physical mechanism based on viscous heating within megaplumes to explain the two peaks in the correlation between the hotspots and the seismic heterogeneities

tomography the geoid and plate motions the dynamics of the earth s mantle and its relationship to mantle structure as revealed by seismic tomography the geoid and plate motions are discussed in a critical review of u s research from the period 1987 1990 sections are devoted to plates and mantle convection seismic anomalies flow and the geoid subducted slabs and the 670 km discontinuity the physical properties of the mantle plumes and computational and experimental modeling efforts also included is a comprehensive bibliography for the period 248 refs

uplift and heat flow following the injection of magmas into the lithosphere summary the thermal effect of a rapid injection of hot magmas into the lower part of the lithosphere is modelled as an increase in heat production through the invaded region the change in surface heat flow and the uplift resulting from the thermal expansion are determined in three dimensional axially symmetric geometry they are expressed as the space time convolutions of a green s function with the anomalous heat production the anomalies with shorter wavelength compared to the lithospheric thickness are attenuated this filtering affects the surface uplift more than the heat flow anomaly the attenuation effect is larger when only the lower part of the lithosphere is invaded the uplift time constant is of the same order as the heat conduction time if the lower lithosphere is invaded by magmas at a moderate rate i e the rate of injection does not exceed the equivalent of 0 1 per cent of the lithospheric volume in 106yr fifty per cent of the total uplift takes place in about 80x 106yr for a lithosphere lookm thick the uplift is slightly faster when the whole lithosphere is invaded the heat flow anomaly is delayed when the lower part of the lithosphere is invaded the spatial extent and the timing of the uplift and heat flow anomalies are critical in determining the mechanism s feasibility magma injections explain rapid uplifts 100 m lo6 yr only if the magma is supplied at a very high rate i e at least 10 per cent of the lithosphere volume per 106yr it is a feasible mechanism for uplifts that occur over longer periods of time 30 x 106yr such as those that seem to have occurred when the african plate came to rest with respect to the mantle

periodic hotspot distribution and small scale convection in the upper mantle hotspots are commonly attributed to mantle plumes and to a first order the hotspot distribution is correlated with long wavelength geoid anomalies and with higher temperatures in the lower mantle suggesting that the plumes are generated in the lower mantle i show that the second order distribution is periodic with a typical spacing of 800 1000 km such a spacing would be too narrow for plumes to ascend directly from the lower mantle but is close to the thickness of the upper mantle the alignment of convection cells in the upper mantle seems to control the periodicity to reconcile the first a and second order distributions assuming limited mass fluxes across the 670 km boundary a lateral deflection of plumes as they breach the transition zone from below is required

u pb geochronology and sr nd isotope compositions of groundmass perovskite from the newly discovered jurassic chidliak kimberlite field baffin island canada abstract we report the u pb age and sr nd isotope composition for perovskite isolated from forty six kimberlite samples located in the newly discovered chidliak field on baffin island canada the minimum duration of kimberlite magmatism in this field was 17 9 m y from 157 0 to 139 1 ma and represents a new jurassic kimberlite field in ne canada the most prolific period of kimberlite magmatism occurred between 152 and 142 ma 80 of dated kimberlites kimberlitic perovskite from these intrusions display a range in 87 sr 86 sr 0 7043 to 0 7030 and e nd t values 3 9 to 0 4 overlapping the isotopic field previously defined for southern african group i kimberlites the ages and isotopic compositions obtained for chidliak magmatism are identical to a number of jurassic kimberlite fields in eastern north america and sw greenland some of this jurassic kimberlite magmatism has a link to one or more mantle plume hotspot tracks but the chidliak kimberlites have an origin in the deep subcontinental lithospheric mantle and are part of a jurassic magmatic province that erupted along both margins of davis strait linked to upwelling asthenosphere continental rifting and mesozoic cenozoic development of oceanic crust in the labrador sea basin in contrast the location of other eastern north american jurassic kimberlites when plotted on a jurassic continental reconstruction aligns closely to the northernmost projection of contours 2 4 of the african large low shearwave velocity province consistent with a link to mantle plumes derived from the african mantle plume generating zone

capture of the canary mantle plume material by the gibraltar arc mantle wedge during slab rollback summary recent evidence suggests that a portion of the canary plume travelled northeastwards below the lithosphere of the atlas mountains in north africa towards the alboran domain and was captured 10ma ago by the gibraltar subduction system in the western mediterranean the capture would have been associated with the mantle return flow induced by the westwardretreating slab that would have dragged and trapped a portion of the plume material in the mantle wedge of the gibraltar subduction zone such material eventually contaminated the subduction related volcanism in the alboran region in this work we use scaled analogue models of slab plume interaction to investigate the plausibility of the plume capture an upper mantle scaled model combines a narrow 400 km edge fixed subduction plate with a laterally offset compositional plume the subduction dominated by slab rollback and toroidal mantle flow is seen to increasingly impact on the plume dynamics as the area of influence of the toroidal flow cells at the surface is up to 500 1350 km 2 while the plume head initially spreads axisymmetrically it starts being distorted parallel to the plate in the direction of the trench as the slab trench approaches the plume edge at a separation distance of about 500 km before getting dragged towards mantle wedge when applied to the canary plume gibraltar subductionsystem ourmodelsupportstheobservationallybasedconceptualmodelthatmantle plume material may have been dragged towards the mantle wedge by slab rollback induced toroidal mantle flow using a scaling argument for the spreading of a gravity current within a channel we also show that more than 1500 km of plume propagation in the sublithospheric atlas corridor is dynamically plausible

stratigraphic and structural development of the precambrian cuddapah basin s e india abstract the present work recognizes a subdivision of the cuddapah basin into 3 separate main units of deposition some of the earlier correlations proved to be untenable for practical reasons king s widely used stratigraphy is retained where possible in a revised form we recognise the cuddapah group 2000 1490 1450 ma nallamalai group 1490 1450 1090 870 ma and the kurnool group 1090 870 500 ma on the basis of surface geology mapping and photo interpretation we discern 5 tectonic zones the boundaries of these zones coincide with deep crustal faults encountered in a deep seismic sounding survey three phases of tectonic activity are 1 1 crustal movements resulting in broad folds affecting the cuddaph rocks exposed in the western area followed by 2 2 folding of the nallamalai rocks east of the rudravaram line and 3 3 later folding and metamorphism in the eastern tectonic zones structural sections allow an interpretation of an eastward continuation of the cuddapah basin rocks into the highly metamorphosed series underlying the eastern ghats complex a colour geological map and sections are included

mapping the lower mantle determination of lateral heterogeneity in p velocity up to degree and order 6 the data from the international seismological centre bulletins for the years 1964 1979 are used to derive a three dimensional model of lateral variations of the p velocity in the lower mantle unlike previous studies in which such perturbations were represented by a blocklike parameterization we seek the solution in the form r k 0k l 0l m 0l k r kalm cos m kblm sin m plm cos some 500 000 travel time residuals for teleseismic distances from 5000 earthquakes are used in an iterative procedure to derive the coefficients a and b the maximum k and l used are 4 and 6 respectively in each iteration in addition to solving for the three dimensional structure we also relocate all the earthquakes perturb the average travel time curve and determine station corrections for over 1000 stations particular attention is given to the problem of weighting the individual observations in order to avoid as much as possible the bias due to the uneven distribution of sources and receivers the resulting model shows a high level of perturbations just below the 670 km discontinuity and just above the core mantle boundary where the maximum perturbations reach 1 1 5 of the average velocity even for this highly smoothed model at a depth of 2000 km the rms perturbations are some 3 to 4 times lower than at either of the two boundaries the model predicts well the large scale pattern of observed travel time residuals for various source regions except for the distinct effects of the subduction zones the most striking large scale three dimensional feature of the model is a ring of high velocities circumscribing the pacific basin within a depth range from 1000 km to the core mantle boundary if the proportionality between perturbations in the velocity and temperature is assumed the derived pattern of anomalies could be consistent with a two boundary layer model of convection in the lower mantle some caution however should be exercised in interpreting the results near the 670 km discontinuity where the resolving power of our data set is limited by the lack of adequate control of the upper mantle structure at the same time a significant global increase in the level of perturbations in the lowermost mantle is established beyond question the results of dziewonski et al 1977 on the correlation between the equivalent geoid and the observed geoid for the angular order numbers 2 and 3 is confirmed but there is no significant correlation for degrees from 4 to 6 these calculations are made for the rigid earth richards and hager this issue show that introduction of finite viscosity can change the sign of a geoid anomaly in addition recent studies of lateral variations in the shear velocity reveal the presence of large scale anomalies in the upper mantle thus while the inferences made here with respect to the origin of the gravest orders of the geopotential field are not conclusive they point the way in which results from seismology can be used to address some of the basic questions of geodynamics

lifestyles of the palaeoproterozoic stromatolite builders in the vempalle sea cuddapah basin india abstract the distribution and changing pattern of stromatolites in the vempalle formation of the cuddapah basin india have been investigated with emphasis on external morphologies internal fabrics growth patterns and sediment associations the stromatolitic limestone occurs in a low angle ramp type carbonate platform with complete exposures from shallow shelf to basinal facies provides record of changes in reef building capacity of stromatolites with change in the depositional milieu changing pattern and style of the stromatolites clearly reflect depth partitioned growth of the microbial community small and scattered stromatolites commonly with wavy parallel laminations or stratifications occurred in inner ramp settings where they are build up to the sea level tepee structure and desiccation cracks in associated sediments and salt pseudomorphs point to shallow water to occasional exposure condition large bioherms with columnar and conical stromatolites developed in the headland mid ramp which were in continuous interaction with strong waves of the open sea growth of the bioherms changed the landform with time and generated a high sloping edge with a sheer drop that extended out into the sea forming distally steepened ramp outer ramp lithofacies characterized by thick layers of shale succession with thin beds of laterally persistent stromatolitic beds with low synoptic relief build ups these biotic components along with the absence of wave related structures place the outer ramp below the base of wave action zone ooid banks developed in the mid ramp area are in the high surfing zone high flux of nutrients and high light show positive correlation thus the high growth pattern is best observed within the photic zone where wave action is maximum the ability to change from low synoptic relief algal laminites to high synoptic relief columnar stromatolites point to their adaptive power the key factor that controlled the shape of these palaeoproterozoic stromatolites is the dynamic interactions between mat growth currents and sediment supply presence of concentric and radial fibrous ooids indicates change in climate and salinity of the sea

proterozoic evolution of eastern dharwar and bastar cratons india an overview of the intracratonic basins craton margins and mobile belts abstract based on critical analysis of stratigraphic succesions in the proterozoic intracratonic basins geochronological data and possible connections or disconnections with adjacent fold belts and craton margin mobile belts an overview of the proterozoic evolution of the eastern dharwar and bastar cratons in india is presented the chattisgarh and cuddapah basins in these cratons with archean nuclei host sedimentary successions ranging in age from the paleoproterozoic to the ediacaran period and representing several silicilastic and carbonate cycles the proterozoic successions of the pranhita godavari valley pgv basin along the dharwar bastar join record multiple rifting since late neoarchean suturing of the two cratons we postulate a first order stratigraphic correlation across the cratons and their join and discuss how major sea level fluctuations coupled with regional tectonic events shaped the development of unconformity bound successions adjoining fold belts and the mobile belts with supposed global tectonic connections the oldest paleoproterozoic intracratonic sedimentation is preserved in the cuddapah and pgv basins while sedimentation in the bastar craton is largely mesoproterozoic the fluviodeltaic to shallow marine neoproterozoic sedimentation followed emplacement of end mesoproterozoic kimberlites and lamproites indicating thermal rejuvenation of the indian continental crust the allochthonus nallamalai fold belt in eastern cuddapah basin and the early mesoproterozoic fold belt in pgv basin point to craton margin crustal convergences which punctuated the paleoproterozoic sedimentation widespread development of carbonate bearing sequences in the mesoproterozoic to neoproterozoic successions point to peneplanation of the cratons and stable platformal sedimentation the neoproterozoic sedimentation was terminated following the docking of the eastern ghats belt to the eastern margin of the dharwar bastar ensemble and possible final tectonic hiccups in the central indian tectonic zone bordering the bastar craton the available data and their analysis rule out any foreland basin type connection between the eastern ghats belt and cratonic sedimentation in bastar and eastern dharwar

antipodal hotspots and bipolar catastrophes were oceanic large body impacts the cause abstract one aspect of the hotspot distribution that has received little attention is its antipodal character of 45 primary hotspots found in most hotspot compilations 22 49 form antipodal pairs within observed hotspot drift limits 20 mm yr in addition the available ages or possible age ranges for both hotspots of an antipodal pair tend to be similar 10 myr difference or overlap monte carlo simulations indicate that the antipodal primary hotspots locations and ages are not due to chance at the 99 confidence level p

characteristics of palaeoproterozoic stromatolites in china in china palaeoproterozoic stromatolites are best represented by taxa from the hutuo group of the wutai mountains and the changchengian group of the yanshan range which all belong to palaeoproterozoic rocks with a time range from 2500 to 1600 ma from the present data palaeoproterozoic stromatolite assemblages are mainly characterized by the following features 1 1 the palaeoproterozoic stromatolite succession contains many special elements such as forms of pilbaria externia kussoidella confunda discorsia vertexa tibia dongyeella jiananella shugongsiella yaochicunia zhongtiaoshania which so far have not been found in meso and neoproterozoic rocks 2 2 the assemblages of the palaeoproterozoic stromatolites cannot be compared wich those of the meso and neoproterozoic stromatolites in representative elements 3 3 the microstructures of the palaeoproterozoic stromatolites are dominated by banded and streaky types but grumelous type which is found frequently in meso to neoproterozoic rocks has been found only occasionally 4 4 the stromatolite building microorganisms of the palaeoproterozoic stromatolites are dominated by quite thin filaments 5 m in width and solitary coccoids and display more primitive characteristics in their morphology and structure and are of a lower grade in evolution failure to distinguish between the palaeoproterozoic stromatolite succession and the meso to neoproterozoic one using tge above characteristics can result in incorrect correlations in the study of proterozoic stratigraphy

lithospheric age dependence of off ridge volcano production in the north pacific data for numbers of seamounts on north pacific ocean crust of different age indicate that the production rate of new off ridge seamounts volcanoes is proportional to the inverse of the square root of the age of the lithosphere this observation is consistent with several hypotheses which have been offered to explain the origin of off ridge oceanic volcanism and in combination with petrologic and paleomagnetic evidence for small oceanic volcanoes leads to a new self consistent model for off ridge volcanism in this model the production rate of off ridge volcanoes is controlled primarily by the availability of fracture zone conduit systems in the thickening lithosphere in contrast mantle plume or hotspot volcanoes may punch through the oceanic lithosphere decreasing production rate of off ridge volcanoes on old oceanic lithosphere may be associated with decreasing extent of partial melting of chemically and isotopically heterogeneous mantle material

coldspots and hotspots global tectonics and mantle dynamics of venus magellan radar images and altimetry data and pioneer venus gravity data of major highlands and lowlands on venus are examined with the objective of relating these tectonic and volcanic landforms to convection within venus mantle two roughly circular lowlands atalanta and lavinia planitiae are bowl shaped depressions which contain compressional tectonic features and lack hotspot related features such as coronae and shield volcanoes they are identified as surface expressions of young mantle coldspots or regions of mantle downwelling highlands can be divided into two distinct groups volcanic rises and plateau shaped highlands volcanic rises are domical circular to elongate regions characterized by volcanic construction extensional tectonism large positive gravity and geoid anomalies centered on the highlands and large apparent depths of compensation volcanic rises include beta bell atla and western eistla regiones and are identified as surface expressions of large mantle hotspots or plumes plateau shaped highlands are steep sided elevated regions that range from circular alpha regio to elongate or irregular ovda regio tellus regio in shape their surfaces are dominated by complex ridged terrain rather than the shield volcanoes and flows that dominate volcanic rises most plateau shaped highlands contain compressional structures some of which commonly lie along their margins and have small gravity and geoid anomalies and small apparent depths of compensation compared to volcanic rises plateau shaped highlands which include those mentioned above and western ishtar terra thetis regio and phoebe regio are identified as surface expressions of mantle coldspots or regions of downwelling the highland coldspot features are elevated as a consequence of crustal thickening crustal thickening is absent or minor at lowland coldspots the importance of mantle downwelling in the tectonic deformation of venus suggests that its mantle like the mantle of earth is strongly heated from within the major venusian hotspots attest to the existence of large mantle plumes carrying heat upward from the planet s core

calcitic aragonitic and mixed calcitic aragonitic ooids from the mid proterozoic belt supergroup montana a great variety of ooid types occurs within the siyeh and snowslip formations of the mid proterozoic belt supergroup montana cortical layers are inferred to have been composed either of calcite in a radial concentric or radial with dark rays fabric or aragonite in a radial or concentric fabric the calcitic cortical layers record their original fabrics but the originally aragonitic cortical layers have been replaced by calcite in a range of textures and by quartz and dolomite some formerly aragonitic cortical layers are replaced by calcite spar which contains relics of the original cortical structure others consist of calcite spar without inclusions or columnar calcite which grew radially from the nucleus commonly a calcitic ooid some ooids were wholly composed of calcite others were of aragonite but two phase ooids were common mostly consisting of an inner calcitic part and an outer aragonitic part probable microdolomite inclusions suggest a high mg content of the calcitic cortical layers the depositional environment of these oolites was probably analogous to baffin bay texas where a similar range of ooid types is forming today

modulation of mantle plumes and heat flow at the core mantle boundary by plate scale flow results from laboratory experiments we report results from analog laboratory experiments in which a large scale flow is imposed upon natural convection from a hot boundary layer at the base of a large tank of corn syrup the experiments show that the subdivision of the convective flow into four regions provides a reasonable conceptual framework for interpreting the effects of large scale flow on plumes region i includes the area of the hot thermal boundary layer tbl that is thinned by the large scale flow thereby suppressing plumes region ii encompasses the critically unstable boundary layer where plumes form region iii is the area above the boundary layer that is devoid of plumes region iv comprises the area of hot upwelling and plume conduits quantitative analysis of our experiments results in a scaling law for heat flux from the hot boundary and for the spatial extent of plume suppression when applied to the earth s core mantle boundary cmb our results suggest that large scale mantle flow due to sinking lithospheric plates can locally thin the tbl and suppress plume formation over large fractions of the cmb approximately 30 of heat flow from the core may be due to increased heat flux from plate scale flow furthermore cmb heat flux is non uniformly distributed along the cmb with large areas where heat flux is increased on average by a factor of 2 as a consequence the convective flow pattern in the outer core may be affected by cmb heat flux heterogeneity and sensitive to changes in plate scale mantle flow becauseofplumesuppressionanddfocusingtofhotmantlefromthecmbintozonesofupwellingflow plumeconduits hotspots

at least three scales of convection in a mantle with strongly temperature dependent viscosity abstract we studied the convective patterns developing at high rayleigh numbers and intermediate viscosity ratios in a fluid with a strongly temperature dependent viscosity within this sluggish lid regime three different scales of convection develop the largest convective scale is cellular with cold downwelling sheets of viscous fluid encasing hotter less viscous parts of the tank within each of those cells develop several typically 3 7 hot 3d upwelling plumes upon impinging under the cold thermal boundary layer each plume in turn generates locally a small ring of cold material which does not reach the bottom of the tank applying those results to the earth s mantle we suggest that the large scale features of mantle convection and the co existence of several scales of convection i e slabs and plumes are produced by thermal convection in a mantle with high rayleigh number and a strongly temperature dependent viscosity material this generates a convective pattern in two large scale cells the pacific and the indo atlantic boxes our experiments further suggest that what has been named the two hot superplumes i e the two seismically slow regions encased within the subduction rings are in fact each constituted of several hot instabilities moreover plumes impacts under the lithosphere should be surrounded by cold rings of small extent the asthenosphere appears therefore as the graveyard of both small scale convection and hot plumes generated in the system