paleoenvironnements et dynamique de faunes de trilobites dans l ordovicien llanvirn superieur caradoc basal du massif armoricain france abstract during the ordovician the armorican massif is generally considered as a platform domain a shallow marine environment characterized by endemic invertebrate faunas however approximately from the late llanvirn to the end of the llandeilo different trilobite associations develop their geographic distribution seems to be mainly related to bathymetric variations the interpretations proposed in this paper are supported by lithological and sedimentological data and comparisons with living marine crustaceans the possible mode of life of some trilobites has also been taken into consideration thus at the eastern end of the armorican median synclinorium and south of rennes martigne ferchaud synclinorium the presence of parabarrandia a mesopelagic trilobite from open shelf indicates amongst other things a marked increase of depth from west to east and from north to south in the southeastern part of the armorican massif ancenis region the development of a cyclopygid biofacies indicates the beginning of a slope opening onto a deep marine environment south armorican ocean in some cases uralichas ectillaenus panderia the exceptionally large dimensions of the dorsal exoskeletons would result from the low water temperature at high latitudes which agrees well with the existence of a gondwanan glaciation at the end of the ordovician

cadomian terranes wrench faulting and thrusting in the central europe variscides geophysical and geological evidence sixty five per cent of the paleozoic basement of western and central europe is hidden by a sedimen tary cover and or sea this work aims to remove that blanket to detect new structures which could used to build a more comprehensive model of the variscan or ogeny it is based on the interpretation of various forms of data a published gravity maps corrected for the ef fects of the crust mantle boundary topography and light sedimentary basins b aeromagnetic maps c measurements of densities and d induced and rema nent magnetizations on rocks from paleozoic outcrops of the upper rhenish area from the northern b ohem ian massif to the eastern paris basin the saxothurin gian is characterized by a 500 km long belt of gravity highs the most important being the kraichgau high most of the corresponding heavy bodies are buried un der a post early visdan cover they are interpreted as relics of late proterozoic terranes overlain by an early to middle paleozoic sequence equivalent to the bo hemian t errane in the bohemian massif the most probable continuation of these dense bohemian ter ranes toward the west is the southern channel north ern brittany cadomian terrane the gravity lows are correlated with variscan granites and pre and early variscan metagranites gravity and magnetic maps demonstrate large scale displacement in devonian early carboniferous times along the parallel and equidistant nw se striking vistula elbe bavarian bray and south armorican dextral wrench faults in the vosges schwarzwald and central massif the faults continue with the east west striking lalaye lubine ba den baden and marche faults and with south vergent thrusts the bavarian faults shift the kraichgau terrane by 150 km relative to the bohemian terrane whereas the offset of the north ern brittany cadomian relative to the northern vosges kraichgau terranes is estimated at 400km along the bray fault sinistral wrench faults are the ne sw striking sillon houiller rheingraben rodl vitis and diendorf faults the southern vosges schwarzwald devonian dinanti an basin is interpreted as a pull apart basin at the south easterly extremity of the bray fault the bohemian and kraichgau body form allochthonous terranes which were thrust over the saxothuringian crust thrusting to the north west was accompanied by back thrusting and led to the forma tion of pop up structures contemporaneous dextral and sinistral wrench faulting resulted in transpressive strain during collision the zonal structure of the varis cides in the sense of kossmat 1927 is relevant only to the rhenohercynian foreland belt kossmat 1927 al ready spoke of a moldanubian region because it dis plays no real zonal structure the saxothuringian zone was formed by terrane accretion their apparent zonal structure is not a pre collisional feature but only the result of accretion and collision

trench migration and upper plate strain over a convecting mantle trench motion and upper plate deformation ultimately respond to mantle flow herein i build upon the mantle flow model results of conrad and behn 2010 and compute the drag forces underneath all plates and show that they control the dynamics of plates and plate boundaries the small misfit angle between between the traction azimuths of mantle traction and absolute plate motion corroborates the idea that convective mantle drag is a prominent driver of plate tectonics less intuitive is the fact that the interplay between the drag forces from the upper and lower plates that amounts to 5 to 8 5 1012nm 1 per unit trench length dictates both trench migration rates and upper plate deformation at odds with the classic view that assigns the prime role to the idiosyncrasies of subduction zones slab age interplate friction water content etc i find that the intrinsic properties of subduction zones in fact only modulate this behavior more specifically the mean value of the integrated trenchward mantle drag force from the lower and upper plates from 2 to 6 5 1012nm 1 controls upper plate deformation conversely it is the difference between the lower and upper plates mantle drag forces from 3 to 10 1012nm 1 that controls trench migration rates in addition i find that a minimum trenchward force of 2 5 1012nm 1 must be supplied by mantle drag before trenches can actually advance and before upper plates undergo compression this force results from the default tendency of slabs to rollback when solely excited by their own buoyancy and is thus the effective tensional force that slab pull exerts on the plate interface

global marine gravity from retracked geosat and ers 1 altimetry ridge segmentation versus spreading rate 1 three approaches are used to reduce the error in the satellite derived marine gravity anomalies first we have retracked the raw waveforms from the ers 1 and geosat gm missions resulting in improvements in range precision of 40 and 27 respectively second we have used the recently published egm2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean third we have used a biharmonic spline interpolation method to construct residual vertical deflection grids comparisons between shipboard gravity and the global gravity grid show errors ranging from 2 0 mgal in the gulf of mexico to 4 0 mgal in areas with rugged seafloor topography the largest errors of up to 20 mgal occur on the crests of narrow large seamounts the global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate for rates less than about 60 mm a the typical ridge segment is 50 80 km long while it increases dramatically at higher rates 100 1000 km this transition spreading rate of 60 mm a also marks the transition from axial valley to axial high we speculate that a single mechanism controls both transitions candidates include both lithospheric and asthenospheric processes

a recipe for microcontinent formation accreted slivers of continental margins are common in the geologic record but the processes that lead to their formation are poorly understood we observe an association of plume related microcontinent isolation and subsequent long term asymmetries in oceanic crustal accretion based on four recent examples the seychelles in the indian ocean jan mayen in the norwegian greenland sea and the east tasman plateau and the gilbert seamount complex in the tasman sea these microcontinents formed by rerifting of a young continental margin 25 m y old in the vicinity of a mantle plume stem followed by asymmetric seafloor spreading two dimensional numerical stochastic basin modeling suggests that a yield strength minimum along the landward edge of a rifted margin thermally enhanced by heating from a mantle plume may cause a spreading ridge to jump onto this zone of weakness this action isolates a passive margin segment the association of large igneous provinces and microcontinents should be useful for identifying similar events in the geological record

the relationship between rifting and magmatism in the northeastern arabian sea the rifting of the seychelles microcontinent from india involved two phases of extensional activity the initial separation of the laxmi ridge from india was accompanied by extensive magmatism but the later separation of the seychelles from the laxmi ridge was only weakly magmatic the causal mechanisms linking continental flood basalts lithospheric extension and mantle plumes as well as the relative timing of extension and volcanism are controversial1 2 3 4 the eruption of the deccan flood basalts was approximately contemporaneous with the separation of the seychelles microcontinent from india however between these continental blocks lies the enigmatic laxmi ridge and the sequence of extensional events that formed these various tectonic elements is poorly understood here we present wide angle seismic data along a profile across laxmi ridge that permit delineation of offshore igneous bodies associated with the deccan magmatism these bodies are similar to those associated with flood basalt volcanism and rifting in the atlantic region and elsewhere1 from the geometry of these bodies we infer that there were two periods of extension the first phase which involved extension between laxmi ridge and the indian subcontinent was accompanied by significant deccan related magmatism full development of a continental margin was achieved during the second phase of weakly magmatic extension between laxmi ridge and the seychelles we suggest that between these rifting events the region passed beyond the reach of lateral flow from the source region of the deccan flood basalts

shear sense a new approach that resolves conflicts between criteria in metamorphic rocks investigation of microstructural relationships in major movement zones in metamorphic rocks where the sense of displacement is known from regional geological relationships indicates numerous problems with current concepts of shear sense criteria and their application the direction of apparent shearing commonly conflicts from one criterion to another e g from the symmetry of quartz c axis orientation diagrams to the asymmetry of extensional crenulation cleavages this implies that interpretations of shear sense along foliations from some mesoscale and microscale criteria have been erroneous a new approach to interpreting shear sense involving the use of strain fields resolves conflicts in mesoscopic and microscopic criteria and provides a method for determining coherent shear sense histories extending back before the last shearing event for any foliated metamorphic rock it also provides a powerful tool for determining the structural metamorphic path that a rock has followed within an orogen for determination of the shear sense on the last foliation developed in a rock this approach uses geometries developed around competent heterogeneities such as quartz pebbles pegmatite pods veins porphyroclasts porphyroblasts and breccia clasts a shear sense history is derived by applying this approach to earlier foliations preserved within the heterogeneities and their strain shadows

paleozoic laurentia gondwana interaction and the origin of the appalachian andean mountain system laurentia the rift bounded precambrian nucleus of north america may have broken out from a neoproterozoic supercontinent between east and west gondwana several lines of evidence suggest that the appalachian margin of laurentia subsequently collided with the proto andean margin of the amalgamated gondwana supercontinent in different relative positions during early and mid paleozoic time in route to final docking against northwest africa to complete the assembly of pangea hence the appalachian and andean orogens may have originated as a single mountain system the overall hypothesis retains the same paleomagnetic and paleobiogeographic controls as previous global reconstructions for the paleozoic era laurentia gondwana collisions may help to explain contemporaneous unconformities in the paleozoic sedimentary cover of the laurentian gondwanan and baltic cratons

evidence for seafloor spreading in the laxmi basin northeastern arabian sea abstract marine magnetic anomaly data from the laxmi basin for the first time reveal the presence of fairly correlatable nnw trending magnetic lineations these magnetic lineations are symmetric about a central negative magnetic anomaly and the axis of symmetry coincides with a characteristic short wavelength free air gravity low the anomalies are interpreted as representing a two limbed seafloor spreading sequence which can be equated to the a28 a33 interval of the geomagnetic polarity reversal timescale these results suggest that the laxmi basin is underlain by an oceanic crust evidence of seafloor spreading in this basin possibly implies a pre a27 spreading episode in the evolutionary history of the arabian sea

paleomagnetic evidence for tectonic rotations in the variscan mountain belt a critical review of devonian and carboniferous paleomagnetic results from the hercynian basement of western and central europe reveals large rotations their geographic distribution suggests a tectonic origin for large scale curved structures such as the ibero armorican arc or the variscan v together with a coherence of the hercynian domain with time geodynamic models which can have produced the observed rotation pattern are discussed it appears that lateral compression due to the accommodation of continental collision as in the india asia convergence is a plausible mechanism although variscan and himalayan mountain belts have different characteristics

evidence of lower mantle slab penetration phases in plate motions it is well accepted that subduction of the cold lithosphere is a crucial component of the earth s plate tectonic style of mantle convection but whether and how subducting plates penetrate into the lower mantle is the subject of continuing debate which has substantial implications for the chemical and thermal evolution of the mantle here we identify lower mantle slab penetration events by comparing cenozoic plate motions at the earth s main subduction zones with motions predicted by fully dynamic models of the upper mantle phase of subduction driven solely by downgoing plate density whereas subduction of older intrinsically denser lithosphere occurs at rates consistent with the model younger lithosphere of ages less than about 60 myr often subducts up to two times faster while trench motions are very low we conclude that the most likely explanation is that older lithosphere subducting under significant trench retreat tends to lie down flat above the transition to the high viscosity lower mantle whereas younger lithosphere which is less able to drive trench retreat and deforms more readily buckles and thickens slab thickening enhances buoyancy volume times density and thereby stokes sinking velocity thus facilitating fast lower mantle penetration such an interpretation is consistent with seismic images of the distribution of subducted material in upper and lower mantle thus we identify a direct expression of time dependent flow between the upper and lower mantle

strike slip duplexes abstract strike slip fault systems often contain zones of steep imbricate faults geometrically similar to imbricate fans and duplexes in dip slip thrust and normal fault systems they are evident in map view rather than in vertical sections examples of duplexes are cited from both active and ancient systems and from theoretical and physical models duplexes may form at bends on strike slip faults by a process kinematically analogous to the sequential imbrication of ramps on dip slip faults however some may form and many may initiate as non sequential riedel fractures at fault offsets or on straight fault segments this process is more marked than in dip slip systems where primary anisotropy such as bedding exerts more control on fault geometry strike slip duplexes may be shunted along the fault system parallel to the regional slip vector however duplexes or individual horses will usually also move up or down perpendicular to the slip vector because of the unconstrained upper surface to the fault system these factors mean that no section through a strike slip system should be expected to area balance the faults of strike slip duplexes and imbricate fans may root in kinematically necessary low dip faults or may converge downwards and appear in vertical sections as flower structures

a precambrian microcontinent in the indian ocean ridges of thick raised crust on the indian ocean floor were thought to be mostly volcanic seamounts formed above the reunion mantle plume dating of zircon minerals in mauritian lavas however indicates that fragments of an ancient microcontinent may be preserved beneath the seamounts contributing to the thickened crust

the ancenis terrane an exotic duplex in the hercynian belt of armorica western france abstract the ancenis terrane bordering the loire france consists of precambrian brioverian basement and three paleozoic sedimentary sequences marine ordovician to devonian shelf sediments lower carboniferous synorogenic intermontane sediments the culm and coal bearing namurian deposits the terrane is completely bound by faults and its geology is in marked contrast with that of its neighbours new data and observations along the terrane s southern boundary show that the ordovician devonian sequence occupies a transpressional shear zone and can be described as a tectonic melange the culm outcrop previously interpreted as a sinistral pull apart structure is reinterpreted as a lower carboniferous dextral pull apart and it is shown that carboniferous transpression and thrusting remodelled the pull apart basin into its present mica fish shape with its remarkable concentric structure and within a plate tectonic setting of mega dextral displacements along the northern boundary of gondwana the ancenis terrane can be interpreted as an exotic duplex

the saint georges sur loire olistostrome a key zone to understand the gondwana armorica boundary in the variscan belt southern brittany france in the southern part of the french armorican massif the ligerian domain is located along the boundary between gondwana and armorica lithological geochemical and structural data on the saint georges sur loire unit which is the northern part of the ligerian domain allow us to distinguish two sub units a southern sub unit formed by various blocks chert limestone sandstone rhyolite mafic rocks of silurian to middle devonian age included as olistoliths in a middle late devonian terrigeneous matrix overthrusts a sandstone pelite northern sub unit both units experienced two deformation events the first one is a top to the nw thrusting and the second one is a left lateral wrenching the saint georges sur loire unit is an accretionary prism formed during the late devonian closure of the layon rift coeval with the main phase of the variscan orogeny the layon rift which according to the mafic olistoliths was partly floored by oceanic crust appears as a buffer structural zone that accounts for the lack in central brittany of any tectonic or sedimentary echo of the closure of the medio european ocean the tectonic evolution of the saint georges sur loire unit supports a polyorogenic model for this part of the variscan belt

analysis of variscan dynamics early bending of the cantabria asturias arc northern spain calcite twinning analysis in the cantabria asturias arc caa of northern spain provides a basis for evaluating conditions of variscan stress and constrains the arc s structural evolution twinning typically occurs during earliest layer parallel shortening offering the ability to define early conditions of regional stress results from the somiedo correcilla region are of two kinds early maximum compressive stress oriented layer parallel and at high angles to bedding strike d1c1 and later twin producing compression oriented sub parallel to strike d2c1 when all d1 compressions are rotated into a uniform east west reference orientation a quite linear north south trending fold thrust belt results showing a slight deflection of the southern zone to the south southeast north south directed d2c1 compression was recorded prior to bending of the belt calcite twinning data elucidate earliest structural conditions that could not be obtained by other means whereas the kinematics of arc tightening during d2 is constrained by paleomagnetism a large and perhaps protracted d2c1 is suggested by our results as manifested by approximately 50 arc tightening prior to acquisition of paleomagnetic remagnetizations throughout the caa early east west compression d1c1 likely resulted from the ebro aquitaine massif docking to laurussia whereas the north directed collision of africa d2c1 produced clockwise bending in the northern zone radial folding in the hinge and rotation of thrusts in the southern zone fl 2000 elsevier science b v all rights reserved

the formation of an orocline by multiphase deformation a paleomagnetic investigation of the cantabria asturias arc northern spain the variscan orogeny in the cantabria asturias arc caa of northern spain represents a multiphase deformation history associated with pangea s amalgamation to determine the timing and kinematic history of deformation a paleomagnetic investigation was carried out on devonian limestones in three structural domains of the caa s hinge zone two characteristic remagnetizations in the caa are distinguished an early permian b component that postdates initial westphalian stephanian aged folding f1 and f2 and a late carboniferous c component that postdates f1 deformation both the b and c magnetizations experienced a later f3 folding phase the kinematics and geometry of post magnetization deformation are determined by bringing measured magnetic directions into agreement with reference directions these structural corrections also allow separation of deformation events in time we conclude that generally east west trending but variably plunging fold axes characterize f3 folding in the hinge zone of the caa this post magnetization deformation involved significant amounts of tilting and sub vertical axis rotation which together produced clockwise rotation in the north of the arc and counterclockwise rotation in the south of the arc to form the horseshoe shaped orocline that is observed today f3 fold axes change from near vertical to moderately inclined between and within structural domains due to the structural fabric imposed by f1 and f2 based on calculated f3 fold properties we can reconstruct pre existing f1 and f2 structural geometries of the hinge zones this analysis shows that f1 and f2 structures are the result of late carboniferous deformation as part of the same east west but temporally discrete compression regimes on the other hand f3 is controlled by permian north south

deformation partitioning and porphyroblast rotation in meta morphic rocks a radical reinterpretation abstract most porphyroblasts never rotate during ductile deformation provided they do not intern ally deform during subsequent events with the exception of relatively uncommon but spectacular examples of spiralling garnets instead the sur rounding foliation rotates and reactivates due to partitioning of the deformation around the porphyroblast consequently porphyroblasts commonly preserve the orientation of early foliations and stretching lineations within strain shadows or inclusion trails even where these structures have been rotated or obliterated in the matrix due to subsequent deformation these relationships can be readily used to help develop an understanding of the processes of foliation development and they demonstrate the prominent role of reactivation of old foliations during subse quent deformation they can also be used to determine the deformation history as porphyro blasts only rotate when the deformation cannot partition and involves progressive shearing with no combined bulk shortening component

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

evidence for pre orogenic early devonian rifting in the variscan belt stratigraphy and structure of the palaeozoic cover of the mauges unit upper allochthon armorican massif france the palaeozoic sediments from the mauges unit armorican massif france are the best preserved pre orogenic sequences belonging to the upper allochthon two coherent sequences are identified the southern unit ch teaupanne unit represents the cover of the proterozoic basement and consists of ordovician sediments unconformably overlain by emsian carbonates followed by emsian to earliest eifelian immature sandstones the northern unit tombeau leclerc unit consists of an hirnantian to emsian condensed sequence in reverse position that has been thrust over the southern unit the devonian unconformity is interpreted as evidence for an early devonian extension recorded by normal faults affecting both the early devonian limestones and the underlying ordovician series this crustal extension recorded here for the first time is possibly related to the opening of a back arc basin saint georges sur loire associated with the subduction of an ocean located further south galicia brittany massif central ocean

foraminiferal assemblage and reef check census in coral reef health monitoring of east brazilian margin abstract human activity is changing environmental conditions on a global scale among the ecosystems that are affected by human activities coral reefs are among the most prominent in brazil the coral reefs of the corumbau marine extractive reserve cmer and abrolhos national marine park anmp in bahia state have some of the highest coral cover in the south atlantic ocean hard coral cover algal cover and foraminiferal population distribution patterns were used to assess the coral reef benthic environments and define a background that can be used in worldwide comparisons in future studies to compare these two monitoring approaches in different coral reef environments relative frequency data for occurrence of hard coral and algal cover using point intercept transects as proposed by the reef check protocol and foraminiferal samples were collected from corumbau nearshore and abrolhos offshore in april 2005 the foraminiferal assemblage was evaluated using the foram index fi foraminifera in reef assessment and monitoring which provides a numeric diagnosis of suitability of benthic habitat to support calcifying organisms that host algal symbionts originally developed for caribbean reef areas coral cover in the surveyed areas both in corumbau and in abrolhos ranged from 13 to 37 while high foraminiferal diversities h were found in all stations dominance of symbiont bearing taxa of amphistegina lessonii and archaias angulatus only occurred at two shallow stations mato verde and siriba both in abrolhos where fi 4 00 stations located in corumbau and abrolhos had fi values

evidence for a variscan suture zone in the vendee france a petrological study of blueschist facies rocks from bois de cene in the bois de cene area blueschist facies rocks characterized by glaucophane and or chloritoid provide evidence for a suture zone in the variscan this terrain is considered to be the eastern equivalent of the ile de groix high pressure metamorphic terrain petrological study of the two characteristic types of rocks found in the area shows that the primary high pressure paragenesis was modified during a retrogression which followed substantial decompression probably at constant or decreasing temperature the simplest interpretation is that this retrogression followed tectonic emplacement within a nappe pile

the rapid drift of the indian tectonic plate the breakup of the supercontinent gondwanaland into africa antarctica australia and india about 140 million years ago and consequently the opening of the indian ocean is thought to have been caused by heating of the lithosphere from below by a large plume whose relicts are now the marion kerguelen and reunion plumes plate reconstructions based on palaeomagnetic data suggest that the indian plate attained a very high speed 18 20 cm yr 1 during the late cretaceous period subsequent to its breakup from gondwanaland and then slowed to approximately 5 cm yr 1 after the continental collision with asia approximately 50 myr ago the australian and african plates moved comparatively less distance and at much lower speeds of 2 4 cm yr 1 refs 3 5 antarctica remained almost stationary this mobility makes india unique among the fragments of gondwanaland here we propose that when the fragments of gondwanaland were separated by the plume the penetration of their lithospheric roots into the asthenosphere were important in determining their speed we estimated the thickness of the lithospheric plates of the different fragments of gondwanaland around the indian ocean by using the shear wave receiver function technique we found that the fragment of gondwanaland with clearly the thinnest lithosphere is india the lithospheric roots in south africa australia and antarctica are between 180 and 300 km deep whereas the indian lithosphere extends only about 100 km deep we infer that the plume that partitioned gondwanaland may have also melted the lower half of the indian lithosphere thus permitting faster motion due to ridge push or slab pull

geochemical and isotopic characteristics of blueschist facies rocks from the ile de groix armorican massif northwest france abstract nine samples of blueschist facies metabasic rocks and four samples of associated metasedimentary lithologies were analysed for ree contents and nd isotopic composition sampling includes the main rock types exposed on groix concentrating on those metabasic rocks with least dispersed rb sr systems the ree abundance patterns appear not to have suffered significant alteration since emplacement of the protolith magma despite subsequent blueschist facies metamorphism the data when integrated into a wider geochemical data base allow the identification and characterization of both tholeiitic and alkaline magma types in the original igneous suite which was differentiated from depleted mantle sources the basalts probably originated in an oceanic context some sediments are predominantly volcaniclastic in which a continental crustal component may be recognized other sediments are closer in composition to continental crust with variable additions of tholeiitic or alkali basalt debris the volcanic sedimentary assemblage on groix is suggestive of ocean island magmatic activity associated with a sedimentary sequence derived from a nearby continent

nature of the crust in the laxmi basin 14 20 n western continental margin of india 1 the nature of the crust in the laxmi basin western margin of india is an uncertain issue more importantly this has implications on paleogeographic reconstructions of the western indian ocean we have analyzed three geophysical data sets and modeled gravity and magnetic anomalies for determining nature of the crust basement of the laxmi basin includes numerous highs which make the basement uneven and shallower compared to the western basin the laxmi basin is characterized by a broad gravity high and a narrower prominent gravity low within it while within the basin the broad anomaly gradually increases toward north the panikkar ridge is associated with the gravity low which is comparable at least in sign to known negative gravity anomaly of the laxmi ridge intrusive structures mapped in the laxmi basin coincide with significant magnetic anomalies which were earlier interpreted as seafloor spreading anomalies model studies reveal that the laxmi basin consists of 14 km thick stretched continental crust in which magmatic bodies have been emplaced whereas the panikkar ridge remains less altered stretched continental crust the crust of the laxmi basin is mostly thinner than crust under the laxmi ridge and continental margin in addition to the rift drift related stretching of the continental margin the laxmi basin possibly has undergone extra stretching in e w direction during the pre tertiary period at 68 ma deccan volcanism on western india may have disrupted the initial conditions that were leading to onset of spreading in the basin subsequently the reunion hot spot had emplaced the volcanic material within the stretched thinned continental crust we interpret the laxmi basin as a failed rift undergone stretching following intraplate kinematics prior to deccan volcanism

crustal control of ridge segmentation inferred from observations of the reykjanes ridge large amplitude variations in topography and inferred crustal thickness along the axes of mid ocean ridges often referred to as segmentation1 are mainly observed at slow spreading ridges2 4 this observation has led to the suggestion that mantle processes give rise to segmentation only when spreading rates are low5 6 here we make the alternative proposal that the development of segmentation is controlled by the temperature of the crust segmentation cannot develop when the lower crust is hot enough to undergo rapid ductile flow thermal models predict that thick crust at a slow spreading ridge may be as hot as normal thickness crust along fast spreading ridges we accordingly test our hypothesis at a slow spreading ridge characterized by thick crust the reykjanes ridge topography and gravity data along the reykjanes ridge axis indeed show an absence of segmentation suggesting that the thermal state of the crust rather than any mantle process controls the development of this structure

the breakup of east gondwana assimilating constraints from cretaceous ocean basins around india into a best fit tectonic model published models for the cretaceous sea oor spreading history of east gondwana result in unlikely tectonic scenarios for at least one of the plate boundaries involved and or violate particular constraints from at least one of the associated ocean basins we link east gondwana spreading corridors by integrating magnetic and gravity anomaly data from the enderby basin off east antarctica within a regional plate kinematic framework to identify a conjugate series of east west trending magnetic anomalies m4 to m0 126 7 120 4 ma the mid ocean ridge that separated greater india from australia antarctica propagated from north to south starting at 136ma northwest of australia and reached the southern tip of india at 126 ma sea oor spreading in the enderby basin was abandoned at 115 ma when a ridge jump transferred the elan bank and south kerguelen plateau to the antarctic plate our revised plate kinematic model helps resolve the problem of successive two way strike slip motion between madagascar and india seen in many previously published reconstructions and also suggests that sea oor spreading between them progressed from south to north from 94 to 84 ma this timing is essential for tectonic ow lines to match the curved fracture zones of the wharton and enderby basins as greater india gradually began to unzip from madagascar from 100 ma in our model the 85 east ridge and kerguelen fracture zone formed as conjugate anks of a leaky transform fault following the 100ma spreading reorganization our model also identi es the afanasy nikitin seamounts as products of the conrad rise hotspot

evidence for a lost ocean in variscan terranes of the western massif central france plate tectonics is now commonly invoked to explain the variscan orogeny1 3 but only the presence of ophiolites can provide real evidence for an early variscan ocean none has been described so far in south western europe with the possible exception of a few scattered lenses4 within the low grade moeche unit in cabo ortegal spain and the chamrousse ophiolite5 6 french alps which displays no clear relationships to documented variscan terranes to fill this gap eclogites with mid ocean ridge basalt morb compositions have been tentatively ascribed to an oceanic origin7 and various serpentinites and amphibolites have been inferred to derive from ophiolites1 3 but many geologists are still reluctant to admit that the variscan orogeny resulted from the closure of a true ocean here we describe the remains of an ophiolite found in high grade crystalline rocks in the european variscan fold belt in limousin western french massif central this ophiolite comprises diopside bearing harzburgites depleted harzburgites dunites wehrlites and morb type gabbros and has structural petrological and geochemical characteristics typical of oceanic lithosphere thereby providing evidence for a lower palaeozoic ocean

mantle conveyor beneath the tethyan collisional belt abstract collisional belts are generated by the arrival of continental lithosphere into a subduction zone the tethyan suture from the bitlis to the himalayas is a prime example where the arabian and indian plates collided with eurasia during the cenozoic while the kinematics of this process are well established its dynamics are more uncertain india and arabia intriguingly keep advancing in spite of large collisional resisting forces and in the absence of a substantial upper mantle slab driving force at present day we perform global mantle circulation computations to test the role of deep mantle flow as a driving force for the kinematics of the tethyan collisional belt evaluating different boundary conditions and mantle density distributions as inferred from seismic tomography or slab models our results show that mantle drag exerted on the base of the lithosphere by a large scale convective conveyor belt with an active upwelling component is likely the main cause for the ongoing indentation of the indian and arabian plates into eurasia

paleogene opening of drake passage the timing of events leading to the earliest connection between the pacific and atlantic oceans at drake passage is controversial but important because gateway opening probably had a profound effect on global circulation and climate a rigorous new analysis of marine geophysical data demonstrates a major change in the motion of the south american and antarctic plates at about 50 ma from n s to wnw ese accompanied by an eightfold increase in separation rate this would have led to crustal extension and thinning and perhaps the opening of small oceanic basins with the probable formation of a shallow b1000 m gateway during the middle eocene no change in south american antarctic motion is observed near the eocene oligocene boundary but a deep water connection developed between 34 and 30 ma when continued extension led to the initiation of seafloor spreading at the west scotia ridge these timings correlate with events seen in the oxygen isotope record from benthic foraminera and support the view that drake passage opening was the trigger for abrupt eocene oligocene climate deterioration and the growth of extensive antarctic ice sheets d 2005 elsevier b v all rights reserved

subduction history of the tethyan region derived from seismic tomography and tectonic reconstructions 1 in the mantle underneath the tethyan suture zone large volumes of positive velocity anomalies have been imaged by seismic tomography and interpreted as the present day signature of subducted tethyan lithosphere we investigate the mesozoic cenozoic subduction history of the region by integrating independent information from mantle tomography and tectonic reconstructions three different subduction scenarios for the tethyan oceanic lithosphere representative for the available tectonic reconstructions are used to predict the present thermally anomalous volumes associated with the lithospheric surface subducted since the late mesozoic next these predicted thermal volumes and their expected positions are compared to the relevant anomalous volumes derived from seismic tomographic images in this analysis we include among others the possible effects of ridge subduction and slab detachment after the cenozoic continental collisions absolute plate motion and slab thickening in the mantle our preferred subduction model comprises the opening of large back arc oceanic basins within the eurasian margin the model points to slab thickening by a factor of 3 in the mantle in which case the estimated volumes allow for active oceanic spreading 1 2 5 cm yr in the tethyan lithosphere during convergence our results further indicate the occurrence of early oligocene slab detachment underneath the northern zagros suture zone followed by both westward and eastward propagation of the slab tear and diachronous eocene to miocene slab detachment below the eastern to western himalayas free sinking rates of the detached material of 2 cm yr in the lower mantle provide the best fit between the tomographic mantle structure and our tethyan subduction model

ridge push mantle plumes and the speed of the indian plate summary the buoyancy of lithospheric slabs in subduction zones is widely thought to dominate the torques driving plate tectonics in late cretaceous and early paleogene times the indian plate moved more rapidly over the mantle than freely subducting slabs sink within it this signal event has been attributed to arrival of the deccan rmantle plume beneath the plate but itisunknowninwhichproportionstheplumeactedtoalterthebalanceofexistingplatedriving torques and to introduce torques of its own our plate kinematic analysis of the mascarene basin yields a detailed indian plate motion history for the period 89 60ma plate speed initially increases steadily until a pronounced acceleration in the period 68 64ma after which it abruptly returns to values much like those beforehand this pattern is unlike that suggested to result from the direct introduction of driving forces by the arrival of a thermal plume at the base of the plate a simple analysis of the gravitational force related to the indian plate s thickening away from its boundary with the african plate suggests instead that the sudden acceleration and deceleration may be related to uplift of part of that boundary during a period when it was located over the plume head in this instance torques related to plate accretion and subduction may have contributed in similar proportions to drive plate motion

the mortagne granite pluton france emplaced by pull apart along a shear zone structural and gravimetric arguments and regional implication the emplacement mechanism and kinematics and the 3 d geometry of the variscan granite pluton of mortagne are interpreted using systematic measurements of the granite structure allied with micro structural observations from a detailed gravimetric study it is proposed that magma infilled a pull apart structure which formed along a jog of a northwest southeast trending sinistral zone of shear a second episode of shearing under a dextral regime and independent of emplacement was responsible for perigranitic tectonites these successive transcurrent opposite movements already documented for variscan times in western france are discussed concerning the age of the mortagne pluton 313 15 m y

a new model for the hercynian orogen of gondwanan france and iberia abstract the hercynian orogen of gondwanan france and iberia is a collage of disparate terranes separated by faults plate reconstructions indicate that the orogen developed during a massive dextral transpression as laurentia slid along the boundary of gondwana and as laurentia baltica and east avalonia rotated clockwise to form pangea it is proposed therefore that the collage is the result of this transpression and that it is an amalgamation of displaced tectonostratigraphic terranes which represent the dismembering of the gondwanan shelf with its stable platform ordovician devonian sedimentary sequence and the neighbouring rheic ocean specifically it is proposed that 1 the slice of the gondwanan shelf that now forms north brittany was displaced dextrally from nw africa around iberia by more than 2000 km 2 the ophiolites oceanic material and volcanic arcs of south armorica and the massif central represent slices of the rheic ocean displaced dextrally along the gondwana margin and shuffled in amongst slices of gondwanan shelf 3 the convergent component of the transpression is represented by thrusts throughout the region the leon terrane of france and ophiolite bearing klippe in nw iberia the ibero armorican arc was formed by wrapping the mobile dextral shear belts about a rigid iberian basement block the dextral porto tomar shear zone represents the more important shearing in iberia and the iberian sinistral shears are interpreted as bookshelf type structures subsidiary to the dextral shearing

cretaceous to present kinematics of the indian african and seychelles plates an iterative inverse model of seafloor spreading data from the mascarene and madagascar basins and the flanks of the carlsberg ridge describes a continuous history of indian african plate divergence since 84 ma visual fit modelling of conjugate magnetic anomaly data from near the seychelles platform and laxmi ridge documents rapid rotation of a seychelles plate about a nearby euler pole in palaeocene times as the euler pole migrated during this rotation the amirante trench on the western side of the plate accommodated first convergence and later divergence with the african plate the unusual present day morphology of the amirante trench and neighbouring amirante banks can be related to crustal thickening by thrusting and folding during the convergent phase and the subsequent development of a spreading centre with a median valley during the divergent phase the model fits fracture zone trends in the north arabian and east somali basins suggesting that they formed in india africa plate divergence seafloor fabric in and between the basins shows that they initially hosted a segmented spreading ridge that accommodated slow plate divergence until 71 69 ma and that upon arrival of the deccan re union plume and an increase to faster plate divergence rates in the period 69 65 ma segments of the ridge lengthened and propagated ridge propagation into the indian continental margin led first to the formation of the laxmi basin which accompanied extensive volcanism onshore at the deccan traps and offshore at the saurashtra high and somnath ridge a second propagation episode initiated the ancestral carlsberg ridge at which seychelles india and india africa plate motions were accommodated with the completion of this propagation the plate boundaries in the mascarene basin were abandoned seafloor spreading between this time and the present has been accommodated solely at the carlsberg ridge

statistical trends in the intraplate stress field the world stress map wsm database contains thousands of intraplate stress indicators with the potential to provide important constraint for arguments about the relationship between tectonic stresses and both the kinematics and dynamics of plate motion previous studies which relied almost exclusively on visual inspection of the data established the existence of broad regions of uniform maximum horizontal compressive stress orientation shmax and stress regimes on a regional scale in the present study we present a statistical analysis of the wsm stress indicators with the aim of quantifying trends in both the shmax orientations and stress regimes the analysis was carried out within 5 5 bins which provide a resolution of several hundred kilometers only the 4537 high quality wsm indicators with rating of a to c were used in the analysis we present results for two types of analysis on the information contained within the bins first we evaluate the spatial distribution of the average stress regime normal strike slip or thrust second we apply the rayleigh test a standard statistical method in the analysis of directional data to the distribution of shmax orientations to test the null hypothesis that the orientations are random an important aspect of our study is the quantification of the conclusions drawn from visual inspection of the world stress map our results indicate that broad regions of uniform shmax orientations exist in most continental regions at high confidence levels 90 and 95 and are less robust in the slowest moving continental plates we also quantify the predominance of strike slip and compressional stress regimes in continental regions importantly our analysis provides information about trends in the shmax orientations in regions where large amounts of scatter in the directional data prevented conclusions being drawn from visual inspection of the data for example in western north america and continental australia furthermore we find a strong correlation between average shmax orientations and both the ridge push torque and the absolute plate velocity azimuths our observation that a greater number of shmax orientations correlate with the ridge push torque directions is further evidence that the intraplate stress field is strongly influenced by the ridge push force

paleopositions of the western europe hercynides during the late carboniferous deduced from paleomagnetic data consequences for stable europe abstract paleomagnetic investigations have been carried out in different areas of the variscan belt armorican massif central massif ardenne vosges and the black forest in order to unravel the paleogeographic evolution of the hercynides during the carboniferous subsequent to important southerly or northerly drifts the central massif and vosges underwent a large rotation of a still undetermined direction in latest visean namurian times during the namurian westphalian the whole belt was oriented n s all investigated areas yield wsw ene directions for that period these directions question the validity of carboniferous mean poles available for stable europe a european plate rotated counterclockwise by about 40 relative to its permian position would be more consistent with these results during the latest westphalian stephanian period i e within the 310 290 ma timespan the western europe hercynides rotated clockwise by 40 50 perhaps together with the whole of europe late variscan strike slip faulting and overthrusting probably result from that rotation

age of seychelles india break up many continental flood basalt provinces are spatially and temporally linked with continental break up establishing the relative timing of the two events is a key step in determining their causal relationship here we investigate the example of the deccan traps and the separation of india and the seychelles whilst there has been a growing consensus as to the age of the main phase of the deccan emplacement 65 5 1 ma chron 29r the age of the rifting has remained unclear we resolve this issue through detailed seafloor magnetic anomaly modeling supported by wide angle and reflection seismic results of the north seychelles and conjugate laxmi ridge gop rift margins and geochemistry and 40ar 39ar geochronology of rocks from the north seychelles margin we show that syn rift volcanics offshore the seychelles islands in the form of seaward dipping reflectors were most likely erupted during chron 28n and the first organized seafloor spreading at the carlsberg ridge also initiated during this chron at 63 4 ma the severing of the seychelles occurred by a south eastward ridge propagation that was completed by the start of chron 27n 62 ma a brief pre 28r phase of seafloor spreading occurred in the gop rift possibly as early as 31r 32n 71 ma initial extension at the margin therefore preceded or was contemporaneous with the deccan emplacement and separation of the seychelles was achieved less than 3 5 ma afterwards this is the shortest time interval between flood basalt emplacement and break up yet reported for any continental flood basalt rifted margin pair a contributing factor to the apparently short interval in the deccan case may be that rifting occurred by a ridge jump into already thinned continental lithosphere however we conclude that external plate boundary forces rather than the impact of a mantle plume were largely responsible for the rifting of the seychelles from india

quantifying the net slab pull force as a driving mechanism for plate tectonics 1 it has remained unclear how much of the negative buoyancy force of the slab fb is used to pull the trailing plate at the surface into the mantle here i present three dimensional laboratory experiments to quantify the net slab pull force fnsp with respect to fb during subduction results show that fnsp increases with increasing slab length and dip up to 8 12 of fb making fnsp up to twice as large as the ridge push force the remainder of fb is primarily used to drive rollback induced mantle flow 70 to bend the subducting plate at the trench 15 30 and to overcome shear resistance between slab and mantle 0 8

ile de groix retrogression and structural developments in an extensional regime abstract the paleozoic metamorphic rocks of ile de groix france record a retrogressive metamorphism from eclogite facies through blueschist to greenschist facies produced during progressive uplift and exhumation and accompanied throughout by high extensional strain extension was accompanied by formation of a generally flat lying foliation the result of either pure shear or conjugate simple shearing with top to the northwest and southeast shear senses equally important the foliation is either wrapped around pods of relatively competent and or higher grade rock elongate in the direction of stretching or folded on axes parallel to the stretching direction these structures represent the effects of the intermediate strain axis which transiently and locally fluctuated between being one of extension or shortening large scale open folds of foliation may represent a wrapping of foliation about two giant pods which define the entire island we suggest the so called lawsonite pseudomorphs might possibly be altered calcium bearing plagioclase which indicates a simpler path of retrogression than would lawsonite the plate tectonic setting of groix might be solved by applying the concept of suspect tectonostratigraphic terranes to gondwanan france and iberia and invoking large scale displacements

granite intrusion along strike slip zones in experiment and nature granite intrusion in the upper crust along a crustal scale shear zone has been modelled by injecting a newtonian fluid low viscosity silicone putty into a sandpack containing a ductile layer of silicone putty which acted as a weak level along which the injected material could spread the strike slip regime was obtained using two mobile rigid basal plates sliding horizontally boundary conditions were chosen in order to analyse the influence of thicknesses of the brittle cover and of the volume of injected material on the pattern of the intrusion experiments showed that 1 the presence of a soft layer between two competent units allowed the formation of laccolithic intrusions by lateral expansion in the emplacement site 2 intrusions are elongate and their long axis tends to track the principal stretching direction associated with the strike slip regime 3 this direction controls the emplacement beginning with the first stages of injection 4 the lateral expansion of the intrusion is locally controlled by arrays of riedel faults formed in the overburden 5 the control of faults on the intrusion pattern increases with increasing brittle ductile ratio i e with increasing depth and or decreasing thickness of the weak layer 6 intrusions are sigmoidal or lozenge shaped in horizontal view 7 the greater the brittle ductile ratio the less sigmoidal are the intrusions 8 feeding pipes shifted with respect to the strike slip zone result in asymmetric intrusions with the development of a sheared tail trailing behind the intrusion examples of leucogranites of the south armorican shear zone south brittany france emphasize that our experiments can explain the geometry of many syntectonic granites emplaced along strike slip zones they further shed some light on mechanisms of pluton intrusion in the upper crust

acceleration and deceleration of india asia convergence since the cretaceous roles of mantle plumes and continental collision 1 a strong 50 35 ma decrease in india asia convergence is generally ascribed to continent continent collision however a convergence rate increase of similar magnitude occurred between 65 50 ma an earlier increase occurred at 90 ma both episodes of accelerated convergence followed upon arrival of a mantle plume below and emplacement of a large igneous province lip on the indian plate we here first confirm these convergence rate trends reassessing the indo atlantic plate circuits then using two different numerical models we assess whether plume head arrival and its lateral asthenospheric flow may explain the plate velocity increases and whether decreased plume flux and increasing continent plume distance may explain deceleration even without continental collision the results show that plume head arrival can indeed lead to absolute indian plate motion accelerations on the order of several cm yr followed by decelerations on timescales similar to the reconstructed fluctuations the 90 ma increase could potentially be explained as response to the morondova mantle plume alone the 65 50 ma convergence rate increase however is larger than can be explained by plume head spreading alone we concur with previous hypotheses that plume induced weakening of the indian continental lithosphere asthenosphere coupling and an increased slab pull and ridge push efficiency are the most likely explanations for the large convergence rate increase the post 50 ma decrease is best explained by orogeny related increased trench resistivity decreased slab pull due to continental subduction and possibly restrengthening of lithosphere asthenosphere coupling upon plume demise

global plate motion frames toward a unified model 1 plate tectonics constitutes our primary framework for understanding how the earth works over geological timescales high resolution mapping of relative plate motions based on marine geophysical data has followed the discovery of geomagnetic reversals mid ocean ridges transform faults and seafloor spreading cementing the plate tectonic paradigm however so called absolute plate motions describing how the fragments of the outer shell of the earth have moved relative to a reference system such as the earth s mantle are still poorly understood accurate absolute plate motion models are essential surface boundary conditions for mantle convection models as well as for understanding past ocean circulation and climate as continent ocean distributions change with time a fundamental problem with deciphering absolute plate motions is that the earth s rotation axis and the averaged magnetic dipole axis are not necessarily fixed to the mantle reference system absolute plate motion models based on volcanic hot spot tracks are largely confined to the last 130 ma and ideally would require knowledge about the motions within the convecting mantle in contrast models based on paleomagnetic data reflect plate motion relative to the magnetic dipole axis for most of earth s history but cannot provide paleolongitudes because of the axial symmetry of the earth s magnetic dipole field we analyze four different reference frames paleomagnetic african fixed hot spot african moving hot spot and global moving hot spot discuss their uncertainties and develop a unifying approach for connecting a hot spot track system and a paleomagnetic absolute plate reference system into a hybrid model for the time period from the assembly of pangea 320 ma to the present for the last 100 ma we use a moving hot spot reference frame that takes mantle convection into account and we connect this to a pre 100 ma global paleomagnetic frame adjusted 5 in longitude to smooth the reference frame transition using plate driving force arguments and the mapping of reconstructed large igneous provinces to core mantle boundary topography we argue that continental paleolongitudes can be constrained with reasonable confidence

indian and african plate motions driven by the push force of the reunion plume head mantle plumes are thought to play an important part in the earth s tectonics yet it has been difficult to isolate the effect that plumes have on plate motions here we analyse the plate motions involved in two apparently disparate events the unusually rapid motion of india between 67 and 52 million years ago and a contemporaneous transitory slowing of africa s motion and show that the events are coupled with the common element being the position of the indian and african plates relative to the location of the reunion plume head the synchroneity of these events suggests that they were both driven by the force of the reunion plume head the recognition of this plume force has substantial tectonic implications the speed up and slowdown of india the possible cessation of convergence between africa and eurasia in the palaeocene epoch and the enigmatic bends of the fracture zones on the southwest indian ridge can all be attributed to the reunion plume an analysis of the plate motions involved in two apparently disparate tectonic events the unusually rapid motion of india between 67 million and 52 million years ago and a transitory slowing of africa s motion at around the same time has uncovered a possible common cause for the two the reunion volcanic hotspot beneath the indian ocean has been active for about 67 million years and the close coupling of the motion of india and africa strongly suggests that it is the waxing and waning of the reunion plume head that is driving these motions

tectonic evolution of the antarctic phoenix plate system since 15 ma abstract joint inversion of magnetic isochron and fracture zone data from the extinct antarctic phoenix spreading system in sw drake passage yields seven new finite reconstruction poles the inversion results are very well constrained for such a short length of plate boundary although this is partly because the finite poles are located close to the reconstructed region the optimum use of fracture zone identifications from satellite derived free air gravity data is also important as the stability of stage poles throughout the short intervals in the model affirms the model results describe a well organised spreading system since magnetic anomaly chron c5ad 15 ma in which the phoenix plate rotated about stage poles nearby to the southwest stage pole locations are broadly consistent with a hypothesis of pivoting subduction as the driving force of phoenix plate movement and there is some evidence in the progression of stage poles for late stage movement of the subduction pivot in response to the changing azimuth of the subduction zone at which the phoenix plate was being consumed the model kinematics alone provide no unequivocal support for previous interpretations of disruption of the subducted part of the phoenix plate the very latest stages of spreading saw falling spreading rates between magnetic anomaly chrons c4 8 1 ma and c2a 3 3 ma when the antarctic phoenix ridge became extinct this is consistent with an increase in shear stress across the plate bounding shackleton fracture zone due to a plate reorganisation in the neighbouring scotia sea following the cessation of spreading on the west scotia ridge

petrogenesis of hercynian leucogranites from the southern armorican massif contribution of ree and isotopic sr nd pb and o geochemical data to the study of source rock characteristics and ages abstract ree geochemistry and isotopic pb sr nd o compositions have been studied in up to nine separate leucogranite massifs from three structurally adjacent zones in south brittany ages decrease from 340 ma in the north to 300 ma in the south whereas initial 87 sr 86 sr ratios increase from 0 706 to 0 710 0 717 ree abundances are relatively low la n 20 80 chondrites with a strong enrichment of lree relative to hree la n lu n 13 4 26 6 pb isotope ratios are homogeneous for a given massif but heterogeneous among massifs and plot very close to a normal terrestrial lead growth curve at 300 ma ago 18 o 16 o ratios are high and relatively uniform at 10 7 to 12 8 mean 11 9 u pb studies on zircon indicate the presence of an inherited radiogenic pb component and that the sediment dominated source region had been metamorphosed during the siluro devonian ligerian negativee nd t values imply that the source region had been enriched in lree prior to the melting episode based on the combined data large scale partial melting of crustal material dominantly of sedimentary origin is proposed the model age of the source rocks is not very old 600 700 to 1200 ma the pb and sr isotopic variations observed among different massifs is probably the result of age differences in the source materials of the different leucogranites

crenulated seafloor evidence for spreading rate dependent structure of mantle upwelling and melting beneath a mid oceanic spreading center abstract the availability of dense geosat gm satellite altimetry data south of 30 s 1 provides the first opportunity to compare at 25 km resolution seafloor generated at ridges spreading with a wide range of spreading rates the axis of the slow spreading southwest indian ridge as well as the seafloor that it has generated has a distinctly crenulated appearance this crenulated seafloor shows regularly spaced gravity lineations in the spreading direction crenulations that record stationary centers of mantle upwelling over the life of the southwest indian ridge a similar pattern of crenulation is seen along sections of the slow spreading northern mid atlantic ridge but along the faster intermediate spreading southern mid atlantic ridge while regularly spaced highs are apparent along sections of the spreading axis stationary crenulation does not persist off the axis this pattern implies that the faster spreading southern mid atlantic ridge has variable centers of time dependent upwelling that migrate along the ridge crenulations are not visible near the axis of the fast spreading pacific antarctic ridge crenulation amplitudes decrease strongly with increasing spreading rate the absence of crenulation at high spreading rate could be due to a 3d 2d transition in flow structure or the effect of mechanisms to redistribute crust along axis while crenulations are not ubiquitous even at slowly spreading ridges where they exist they provide a time record to observe the characteristics of mid ocean ridge upwelling melting and melt migration not only beneath active spreading centers but through a much longer spreading history