the tectonothermal evolution of the cadomian belt of northern brittany france a neoproterozoic volcanic arc abstract the neoproterozoic cadomian belt northern brittany france results from the accretion of crustal blocks that record distinctive p t deformation paths in addition abundant gabbro dioritic to granitic intrusions during and after deformation influence the thermal pattern and are used as time markers as a whole the deformation history of the cadomian belt is characterised by a migration in time of the active zones which become younger towards the southeast relics of eocadomian rocks reveal that arc development took place at about 660 640 ma abundant calc alkaline magmatism intruding strongly deformed palaeoproterozoic orthogneisses characterise the northernmost unit of the cadomian belt i e the tregor unit which cooled before 600 ma the lanvollon basin a short lived intra arc rift containing abundant mafic and felsic volcanics followed by greywackes opened at about 600 580 ma the subsequent tectonothermal history of the cadomian belt is divided into two major episodes the first episode is related to the shortening of the intra arc rift and its southwestward thrusting over the guingamp unit this led to crustal thickening inducing partial melting in the guingamp unit exhumation of the latter rocks is accommodated by late extensional shearing along the boundary between the saint brieuc and guingamp units cooling of the saint brieuc unit is dated at about 570 560 ma the second episode is dominated by sinistral wrenching mainly localised in the eastern part of the cadomian belt at the boundary between the domnonean i e the arc domain and mancellian possibly a fore arc basin domains several shear zones developed during this episode some of them being associated with minor granitic intrusions dated around 550 ma the sinistral shear located southeast of the arc domain is reworking or crosscutting earlier high temperature fabrics within the saint brieuc unit in addition the vertical component of this shearing episode contributes significantly to the exhumation of the saint malo migmatites near the proterozoic cambrian boundary cordierite biotite granitoids intruded at shallow depths the fougeres unit their emplacement outlasting deformation of the sedimentary sequences minor stocks of muscovite leucogranites of cambrian age are possible equivalents of the abundant volcanism in an adjacent nw se trending rift this marks a major change in the kinematic framework with predominance of extension finally minor reactivation of the cadomian structures occurred during the carboniferous associated with the variscan orogeny

mesozoic plate motion history below the northeast pacific ocean from seismic images of the subducted farallon slab the high resolution seismic imaging of subducted oceanic slabs1 2 has become a powerful tool for reconstructing palaeogeography3 the images can now be interpreted quantitatively by comparison with models of the general circulation of the earth s mantle4 here we use a three dimensional spherical computer model of mantle convection5 6 to show that seismic images of the subducted farallon plate provide strong evidence for a mesozoic period of low angle subduction under north america such a period of low angle subduction has been invoked independently to explain rocky mountain uplift far inland from the plate boundary during the laramide orogeny7 the computer simulations also allow us to locate the largely unknown kula farallon spreading plate boundary the location of which is important for inferring the trajectories of suspect terrain across the pacific basin8

the surface tectonics of mantle lithosphere delamination following ocean lithosphere subduction insights from physical scaled analogue experiments many postulated lithospheric removal events occur in regions with an earlier history of subduction but the relationship between the two processes has not been explored in this work we use physical scaled analogue experiments to investigate the evolution from ocean lithosphere subduction to collision and possible delamination of the mantle lithosphere from the crust we test how varying the magnitude of plate convergence alters the behavior of the subduction delamination model our experiments show that a retreating ocean proplate can evolve to continental mantle lithosphere delamination negative surface topography is supported at the delamination hinge and this migrates back with the peeling lithosphere with high plate convergence delamination is suppressed rather the crust and mantle lithosphere split at the collision zone in a form of flake tectonics as oncoming procrust is accreted on top of the retroplate and the promantle lithosphere subducts below localized high topography develops at this zone of crustal accretion and thickening the results suggest that delamination may be a continental continuation of plate retreat and that lithospheric removal is triggered by the transition from one process to another

cosmic markers 40ar 39ar dating and paleomagnetism of the kt sections in the anjar area of the deccan large igneous province abstract bhandari et al bhandari et al geophys res lett 22 1995 433 436 bhandari et al geol soc am spec paper 307 1996 417 424 reported the discovery of iridium bearing sediments sandwiched between basalt flows in the anjar area kutch province india they concluded that the signature of the k t impact had been recorded and that onset of volcanism in the deccan traps preceded the k t boundary excluding the possibility of a causal connection this paper reports complementary analyses of anjar outcrops by a joint indo french team where we focused on cosmic markers iridium and spinels in the intertrappean sediments and 40ar 39ar dating and paleomagnetism of the lava flows anomalous ir concentrations up to 0 4 ng g are confirmed with up to three thin and patchy enriched layers which cannot be traced throughout the exposed sections despite careful search no ni rich spinels were found eight basalt samples provided 40ar 39ar results four on plagioclase bulk samples four on whole rocks spectra for whole rocks all indicate some amount of disturbance and ages based on plagioclase bulk samples seem to be consistently more reliable hofmann et al earth planet sci lett 180 2000 13 28 the three flows underlying the ir bearing sediments are dated at 66 5 ma and two overlying flows at 65 ma magnetic analyses both thermal and by alternating fields uncovered clear reversed primary components in the upper flows and more disturbed normal components in the lower flows with evidence for an additional reversed component there are reports bajpai geol soc india mem 37 1996 313 319 bajpai j geol soc london 157 2000 257 260 that the intertrappean sediments contain uppermost maastrichtian dinosaur and ostracod remains above the uppermost ir bearing level and may not be mechanically disturbed we propose the following scenario to interpret these multiple field and analytical observations deccan trap volcanism started within uppermost maastrichtian normal chron c30n at 66 5 67 ma in the anjar area volcanism then stopped at least locally and lacustrine sediments were deposited over a period that could be in the order of 1 2 ma the k t bolide impact was recorded as a deposit of ir and possibly though not necessarily spinels volcanism resumed shortly after the k t boundary within reversed chron c29r as witnessed by the three reversely magnetised overlying basalt flows dated 65 ma this was responsible for erosion and destruction of part of the uppermost sediments including spinels if there were any and heterogeneous and non uniform redeposition of ir at a number of underlying sedimentary levels this was also responsible for the partial remagnetisation of the underlying flows these findings generally confirm and complement those of bhandari et al bhandari et al geophys res lett 22 1995 433 436 bhandari et al geol soc am spec paper 307 1996 417 424 and are compatible with the occurrence of the k t impact at the paleontological k t boundary and of deccan trap volcanism straddling the boundary and starting before the impact anjar provides evidence for minor volcanism somewhat earlier than suggested by some authors though still within normal chron c30n there is no indication contradicting the view that the bulk of deccan trap volcanism occurred over only three chrons c30n c29r c29n courtillot evolutionary catastrophes the science of mass extinctions cambridge university press 1999 courtillot et al earth planet sci lett 80 1986 361 374 vandamme et al rev geophys 29 1991 159 190

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

emplacement of lobate sheet flows with hyaloclastites onto soft sediment the erquy neoproterozoic lava pile armorican massif france abstract the neoproterozoic volcano sedimentary succession of the heussaye headland erquy belongs to the cadomian block located in the north east of the armorican massif france the volcanism dated at approximately 608 7 ma zircon u pb data occurred in a back arc basin this site exhibits a noteworthy verticalized pile of moderately metamorphosed greenschist facies basaltic andesite flows of arc back arc tholeiite affinity alternating with metasedimentary layers sandstones and siltstones a large range of volcanic facies characteristic of an underwater soft substrate environment can be continuously studied for over than 200 m parallel to the flow edges they include pillow lavas lava lobes fluidal peperites and sediment matrix hyaloclastites the presence of parallel lava lobes in a sector where one lava flow is capped by a variably thick hyaloclastite carapace together with the occurrence of asymmetric turbiditic folds in interbedded metasedimentary layers suggests the existence of an easterly facing gentle paleoslope the sudden thinning of this hyaloclastite carapace in two spots located approximately 60 m from each other is attributed to the synvolcanic activity of two conjugate normal paleofaults this hypothesis is supported by the observation of a hyaloclastite accumulation in one of the two spots close to a strike slip fault which may have been extensional at the time of the volcanism water entered the lava flow along the faults resulting in an interstitial network of hyaloclastites by quench fragmentation in the upper part of the hanging wall block all these observations are consistent with a model of lava emplacement along a subaqueous slope resulting from synmagmatic extensional faulting in a marginal basin context

brioverian volcanism and cadomian tectonics baie de st brieuc brittany stages in the evolution of a late precambrian ensialic basin

late cretaceous india madagascar fit and timing of break up related magmatism a u pb zircon age of 91 2 0 2 myr from western india st mary islands confidently links india with the late cretaceous magmatic province in madagascar 84 92 ma and the u pb age is within analytical error of the u pb age of the analalava gabbro pluton 91 6 0 3 myr in northeastern madagascar palaeomagnetic data from india and madagascar allow us to postulate a new india madagascar fit euler latitude 14 24 longitude 38 8 and rotation angle 69 2 this fit is applicable to the late cretaceous directly prior to and during the early phase of madagascar india separation in our late cretaceous reconstruction south west india runs roughly subparallel with the first known break up related magnetic anomaly a34 it maintains a close connection between mada gascar and india but places india slightly rotated compared to the eastern margin of madagascar and more northerly compared with some reconstructions st mary magmatism is linked to the initial break up between india and madagascar and magmatism probably resulted from rift related extensional processes initially induced by the marion hotspot underlying southern madagascar during the late cretaceous

the karnataka late cretaceous dykes as products of the marion hot spot at the madagascar india breakup event evidence from 40ar 39ar geochronology and geochemistry two late cretaceous mafic dykes with an ene strike that is orthogonal to the west coast of india and located nearly 200 km inland around huliyardurga karnataka state yield 40ar 39ar plateau ages of 90 0 1 0 and 87 5 0 9 ma these fe ti enriched tholeiites are essentially co eval with at least four other igneous suites widely scattered in southern india namely the south and north kerala dykes the agali anaikatti dykes of central kerala tamil nadu and lavas of st mary islands off the west coast of india the karnataka cretaceous dykes are also co eval and compositionally very similar to the fe ti enriched tholeiitic lavas and dykes around mananjary a major phase of late cretaceous magmatism along the eastern rifted margin of madagascar which are believed to be products of the marion hot spot that extruded at 88 ma synchronous with the india madagascar break up event the age and compositional similarities between these late cretaceous magmatic rocks from india and madagascar constitute a clear evidence for extension of the thermal manifestations deep into the indian peninsula

geochemistry and geotectonic interpretation of the penthievre crystalline massif northern brittany france abstract in the armorican massif of brittany and normandy northern france and the channel islands u k an episode of crust formation spanning the period from 700 ma to the early palaeozoic can be recognized and is attributed to the cadomian orogenic cycle during the early part of this orogenic cycle the components of the penthievre crystalline massif were emplaced to produce the basement upon which local brioverian volcanism and sedimentation took place three separate episodes of crustal accretion have been recognised in the penthievre crystalline massif the first is represented by the yffiniac group which is dominantly of basic to intermediate composition with a minor acidic component this suite which was metamorphosed at amphibolite facies shows geochemical characteristics of island arc tholeiitic to continental calc alkaline igneous activity ree profiles are flat to lree enriched cen ybn in the range 1 18 4 89 incompatible element spidergrams show relative enrichment of for example th u lree and lil elements and depletion of ta nb and ti ta la and nb la ratios are low the second suite is represented by the port morvan group a linear belt of leucocratic orthogneisses containing net veined rafts of deformed yffiniac group rocks the port morvan group gneisses are trondhjemitic to quartz dioritic in composition with na2o k2o up to 32 9 the third assemblage comprises the jospinet group and the fort de la latte complex these are part of a compositionally expanded mantle derived plutonic suite ranging from ultrabasic to aplitic in composition flat chondritic ree profiles characterise gabbros whereas lree enriched profiles characterise calc alkaline and alkali calcic granitoid plutons there is evidence in penthievre for protracted destructive plate margin activity and associated plutonism of andean type the penthievre crystalline massif probably represents the roots of juxtaposed island arc and continental margin terranes produced in three crust forming episodes during the cadomian orogenic cycle brioverian sedimentation and volcanism took place in ensialic basins on rifted crust the closure of these brioverian basins was accompanied by the emplacement of calc alkaline plutons into the crystalline basement

behaviour of rb sr whole rock and u pb zircon systems during partial melting as shown in migmatitic gneisses from the st malo massif ne brittany france the age of regional metamorphism and partial melting in the late proterozoic succession of the st malo area has been established at around 540 ma whole rock homogenization of the sr isotope systems has been achieved only in the anatectic granites whereas the associated metatexitic and diatexitic gneisses remain highly heterogeneous at lower grade than the sillimanite isograd metasedimentary gneisses contain detrital zircons which record the regional metamorphic event as a u pb lower intercept age 536 14 ma in the sillimanite zone u pb zircon lower intercept ages are around 500 10 ma and suggest a recent loss of radiogenic pb from the observed overgrowths zircons from the anatectic granites do not yield u pb intercept ages due to the presence of inherited radiogenic pb but the finer fractions define a discordant array whose upper intercept on concordia yields 555 13 ma an age significantly older than the partial melting event indicated by the lower grade zircon ages the lower intercept defined by the fine grained anatectic zircons yields a date in the future negative ordinate value in their regional context the st malo migmatites can be considered as resulting from late stage crustal melting in the cadomian orogenic belt following genesis of calc alkaline magma at an active margin around 600 ma ago an age of 540 ma for partial melting in the st malo migmatites places the end of the cadomian orogeny near the precambrian cambrian boundary

surface expression of eastern mediterranean slab dynamics neogene topographic and structural evolution of the southwest margin of the central anatolian plateau turkey 1 the southwest margin of the central anatolian plateau has experienced multiple phases of topographic growth including the formation of localized highs prior to the late miocene that were later affected by wholesale uplift of the plateau margin our new biostratigraphic data limit the age of uplifted marine sediments at the southwest plateau margin at 1 5 km elevation to 7 17 ma and regional lithostratigraphic correlations imply that the age is 6 7 ma single grain ca tims u pb zircon analyses from a reworked ash within the marine sediments yield dates as young as 10 6 ma indicating a maximum age that is consistent with the biostratigraphy our structural measurements within the uplifted region and fault inversion modeling agree with previous findings in surrounding regions with early contraction followed by strike slip and extensional deformation during uplift focal mechanisms from shallow earthquakes show that the extensional phase has continued to the present broad similarities in the change in the tectonic stress regime after 8 ma and the onset of surface uplift after 7 ma imply that deep seated process es caused post 7 ma uplift the geometry of lithospheric slabs beneath the plateau margin pliocene to recent alkaline volcanism and the uplift pattern with accompanying normal faulting point toward slab tearing and localized heating at the base of the lithosphere as a probable mechanism for post 7 ma uplift of the southwest margin considering previous work in the region there appears to be an important link between slab dynamics and surface uplift throughout the anatolian plateau s southern margin

palaeomagnetism and 36ar40ar vs 39ar40ar isotope correlation ages of dyke swarms in central kerala india tectonic implications palaeomagnetic results and whole rock 36ar40ar vs 39ar40ar isotope correlation ages are presented for dyke swarms occurring within the granulite terrain in central kerala west coast of india palaeomagnetic data show that the nnw sse leucogabbro dykes sampled are normally magnetised d 307 and i 57 whereas most nw se dolerite dykes are reversely magnetised d 163 and i 61 one dolerite dyke shows normal polarity the present 40ar39ar isotope ages for dolerite dykes and previously reported k ar ages for leucogabbros suggest 81 2 ma for acquisition of the leucogabbro directions and 69 1 ma for acquisition of the reverse magnetic directions within the dolerites based on palaeomagnetic and isotopic age data the dolerite dykes appears to represent feeder systems for the deccan volcanic sequences we suggest that the deccan eruptions were widespread and probably extended southward at least into central kerala the entire magmatic episode may have occurred between ca 70 and 65 ma during the 31r period probably corresponding to the cretaceous tertiary boundary the leucogabbros may correspond to the 34n superchron and represent a plume related magmatic episode precursory to the main volcanic event these data suggest that an early intraplate plume tectonic setting since ca 80 ma preceded lithospheric thinning asthenospheric upwelling and consequent massive flood volcanism during the period 70 65 ma this culminated in ocean floor spreading between the seychelles mascarene and continental india

a review of indian phanerozoic palaeomagnetism implications for the india asia collision abstract an overview is presented of the indian apparent polar wander path apwp for the phanerozoic and in particular for post late palaeozoic times this apwp is compiled on basis of data available at october 1981 from peninsular and extrapeninsular indo pakistan and from dsdp cores from the indian plate one of the more important and newly recognized features of this apwp is a large scale triassic jurassic loop this loop indicates a changeover from a late palaeozoic early mesozoic northwards and counter clockwise rotational movement with greater india reaching moderately low southern latitudes into a southwards and clockwise rotational movement during the early to middle jurassic recognizable likewise in apwp s from other gondwana continents this loop reflects the opening of the neotethys studies of extrapeninsular regions up to and north of the indus tsangpo suture zone have shown wide spread presence of magnetic overprints which delineate two regionally confined age groups younger overprints 20 40 m y predominate in the more external thrust zones older overprints 50 60 m y in contrast are found in the more internal zones both north and south of the indus tsangpo suture zone the latter are interpreted to reflect a late phase of relaxation in the early tertiary collision of greater india with south central asia or off shore island arcs which occurred at equatorial to low northern palaeolatitudes 0 10 n subsequent northwards movement over 2500 3000 km or more and impingement of greater india into southern asia resulted into large scale underthrusting of greater india along the main central thrust beneath southern tibet and to clockwise rotation of thrust units in the western himalaya a discrepancy between indian palaeomagnetic data and results available todate from southern tibet is discussed

crustal growth and late precambrian early palaeozoic plate tectonic evolution of england and wales comparison of the basement rocks of britain reveals major contrasts to the n and s of the iapetus suture solway shannon line north of the suture polymetamorphic high grade basement schists and gneisses and igneous rocks date from about 2900 ma to c 400 ma the latter age corresponding to closure of the iapetus ocean by contrast s of the suture in england and wales the basement is dominantly low grade metamorphic rock together with intrusive and extrusive igneous and sedimentary rocks there is little geological or geochemical evidence for the presence of basement older than c 900 ma below this area and the predominant expression of crustal growth in england and wales is indicated by radiometric ages between c 700 ma and 450 ma reflecting late precambrian palaeozoic magmatism and metamorphism palaeomagnetic data from scotland demonstrate that the crust in northern britain developed as an integral part of the laurentian shield during mid proterozoic times 1600 1000 ma the palaeomagnetic record from the pre ordovician rocks of england and wales shows that this area was unrelated to the laurentian shield prior to closure of the iapetus ocean the apparent polar wander a p w path is that of a distinct microplate involved in rapid growth of iapetus during early cambrian times and emplaced in its present configuration adjacent to northern britain as a result of subduction of the ocean basin between cambrian and silurian times we conclude that whereas the basement to the north of the iapetus suture formed an essentially continental unit from 2900 to 400 ma the basement of england and wales is a result of crustal growth from about 900 ma to 400 ma and formed by accretion of island arcs associated accretionary prisms and fore arc basin sediments within the iapetus ocean

geodetic constraints on present day motion of the arabian plate implications for red sea and gulf of aden rifting 1 five years of continuously recording gps observations in the kingdom of saudi arabia together with new continuous and survey mode gps observations broadly distributed across the arabian peninsula provide the basis for substantially improved estimates of present day motion and internal deformation of the arabian plate we derive the following relative geodetic euler vectors latitude n longitude e rate myr counterclockwise for arabia nubia 31 7 0 2 24 6 0 3 0 37 0 01 arabia somalia 22 0 0 5 26 2 0 5 0 40 0 01 arabia india 18 0 3 8 87 6 3 3 0 07 0 01 arabia sinai 35 7 0 8 17 1 5 0 0 15 0 04 and arabia eurasia 27 5 0 1 17 6 0 3 0 404 0 004 we use these euler vectors to estimate present day stability of the arabian plate the rate and direction of extension across the red sea and gulf of aden and slip rates along the southern dead sea fault south of the lebanon restraining bend 4 5 4 7 0 2 mm yr left lateral 0 8 1 1 0 3 mm yr extension and the owens fracture zone 3 2 2 5 0 5 mm yr right lateral increasing from north to south 1 2 mm yr extension on a broad scale the arabian plate has no resolvable internal deformation weighted root mean square of residual motions for arabia equals 0 6 mm yr although there is marginally significant evidence for n s shortening in the palmyride mountains syria at 1 5 mm yr we show that present day arabia plate motion with respect to eurasia is consistent within uncertainties i e 10 with plate tectonic estimates since the early miocene when arabia separated from nubia we estimate the time of red sea and gulf of aden rifting from present day arabia motion plate tectonic evidence for a 70 increase in arabia nubia relative motion at 13 ma and the width of the red sea and gulf of aden and find that rifting initiated roughly simultaneously 2 2 myr along the strike of the red sea from the gulf of suez to the afar triple junction as well as along the west gulf of aden at 24 2 2 ma based on the present kinematics we hypothesize that the negative buoyancy of the subducted ocean lithosphere beneath the makran and the zagros fold thrust belt is the principle driver of arabia eurasia convergence and that resisting forces associated with arabia eurasia continental collision have had little impact on plate motion

petrology geochemistry and magnetic properties of sadara sill evidence of rift related magmatism from kutch basin northwest india abstract mafic volcanic rocks of the mesozoic kutch basin represent the earliest phase of deccan volcanic activity an olivine clinopyroxene plagioclase phyric undersaturated basalt occurs as a sill near sadara in the pachham upland northern kutch the sadara sill is deformed and emplaced along faults the sill is alkaline in character and is transitional between basalt and basanite compared to primitive mantle the sadara sill is enriched in sr ba pb and lree but depleted in nb cr y cs and lu fractional crystallization of olivine and clinopyroxene from an alkaline mafic melt generated by low degree partial melting of mantle peridotite can explain the observed chemical variation in the sill irm and l f test experiments and mineral analyses show titano magnetite as the major remanence carrying magnetic mineral af and thermal demagnetizations of the sadara sill yielded a mean chrm direction as d 315 6 i 43 0 95 9 78 k 25 38 and the corresponding vgp at 25 s 114 6 e dp dm 6 58 11 6 the sadara sill pole is significantly different from those of the deccan 65 ma and the rajmahal traps 118 ma but is close to the cretaceous poles of 85 91 ma rock units from southern india this suggests a pre deccan age for the sill

characterization of the causative fault system for the 2001 bhuj earthquake of mw 7 7 abstract precise hypocenters erh m w 2 0 5 3 delineate a east west trending blind thrust dipping 45 towards south named as north wagad fault nwf about 25 km north of kachchh main land fault kmf as the causative fault for the 2001 bhuj earthquake of m w 7 7 the aftershock zone involves a crustal volume of 60 40 35 km 3 lying between kmf and nwf the waveform inversion of 45 aftershocks of m 3 0 5 3 suggest that the e w trending south dipping reverse faulting mainly characterizes the aftershock zone however some reverse faulting along nw se as well as ne sw planes and some strike slip faulting along nw se trending vertical plane are also noticed the estimated p axes point on an average towards n s while t axes orient in e w agreeing well with the prevailing regional plate tectonic stress directions the estimated velocity model from one dimensional inversion of 8000 p and 5000 s travel times from 600 aftershocks deciphers a detailed crustal structure of the region upper 0 6 km on an average suggests a low velocity zone characterizing the jurassic and younger sediments the depth range 6 42 km is characterized on an average by larger values of v p 6 31 to 6 98 km s with an average of 6 71 km s v s 3 64 to 4 05 km s with an average of 3 85 km s and v p v s 1 69 to 1 81 with an average of 1 75 the high crustal velocities can be attributed to the existence of a high velocity mafic intrusive rift pillow structure beneath the region probably emplaced during the rifting time this brittle competent high velocity zone beneath the epicentral area in response to the compression due to the northward movement of the indian plate could induce sufficient local stress perturbation for generating large intraplate earthquakes of m w 7 5 in the lower crust further within the overall higher velocity depth range a layer characterized by large values of v p 6 98 km s v s 3 854 km s and v p v s 1 81 is found in the depth range of 20 5 30 km which is inferred to be fractured and saturated high crack density and probably fluid filled presence of numerous fractures in this layer is confirmed by a high b value 0 75 1 1 in this layer this layer might have facilitated the nucleation process of the 2001 bhuj earthquake the major rupture and 47 of the aftershocks have occurred within this layer

a single cause for uplift of the central and eastern anatolian plateau regional observations suggest that the central anatolian plateau central turkey has risen by 1 km since the tortonian 8 ma while significant crustal shortening did not occur this uplift was preceded by the onset of widespread volcanism 14 9 ma the lithospheric context of these events is however unknown for the eastern anatolian plateau similar events have been attributed to the late stage evolution of the northern neotethys slab resulting in delamination and slab breakoff recent tomographic results indicate that this slab extended beneath both below the eastern and central anatolian plateau just prior to delamination we propose a new lithospheric scenario for the regional evolution in the aegean anatolian near east region that combines a recent compilation of surface geology data with the structure of the upper mantle following the cretaceous eocene closure of the northern neotethys africa eurasia convergence was accommodated by horizontal subduction at a trench that was located south of anatolia like before the closure the northern neotethys slab continued to sink into the deeper mantle beneath the izmir ankara erzincan suture in the early miocene 20 15 ma the northern neotethys slab started to retreat southward to the trench resulting in delamination of the lithospheric mantle the last part of this scenario is testable whether delamination can explain the uplift of both the central and eastern anatolian plateau in the east uplift due to collision of arabia is included we use a coupled thermal flexural model of the lithosphere the model results show that delamination can explain the average present day long wavelength topography of the central anatolian plateau for the eastern anatolian plateau delamination explains half the present day elevation we find that a single delamination event also accounts for the present day surface heat flow and curie point depth in both plateaus we therefore propose to refer to central and east anatolia since the middle miocene as the anatolian plateau

the geochemical characteristics of granitoids in contrasting arcs and comments on magma sources geochemical variations in modern subduction related igneous suites with respect to arc maturity in time and space are illustrated using data for both volcanic suites basalt andesite and dacite and intrusive granitoid suites diorite tonalite granodiorite and granite from circum pacific arcs using trace element data we suggest that two groups of processes control the natural variation in the parental magmas of both suites a subduction zone enrichment of lithospheric mantle locally coupled with crustal assimilation allied with fractional crystallization afc in zones of thickened crust all of which yield magmas with enhanced concentrations of the large ion lithophile lil elements k rb th u lrees etc b with increasing distance from the active trench contributions from within plate sub continental lithosphere producing mantle derived magmas with enhanced levels of high field strength hfs elements among which nb ta hf and y are particularly distinctive thus even for the evolved granitoids of intrusive arc series ratios of hfs lil elements not significantly affected by crystal fractionation such as ta nb k rb la may throw some light on the origin of mafic intermediate precursor magmas in terms of these elements we suggest the following groupings for interpreting the tectonic associations of granitoid suites 1 primitive calcic arc granitoids with low lil and hfs element abundances 2 normal calc alkaline continental arc granitoids with enhanced lil element abundances and low hfs lil ratios 3 mature alkali calcic arc granitoids with high levels of lil and hfs elements and higher hfs lil ratios 4 back arc anorogenic alkaline granitoids with the highest levels of hfs elements

the cadomian active margin north armorican massif france a segment of the north atlantic panafrican belt abstract within the european variscan belt the armorican massif northwestern france contains an isolated fragment of the north atlantic panafrican belt the cadomian block mostly unaffected by variscan deformation this block is composed of neoproterozoic rocks ranging in age from 620 to 540 ma brioverian locally overlying a composite basement of icartian 2000 ma or pentevrian 750 ma age the cadomian tectonics the last magmatic manifestations of which are dated at around 540 ma were sealed by a cover of early cambrian rocks in normandy and early ordovician rocks in brittany five units are distinguished in this cadomian block tregor saint brieuc guingamp saint malo and fougeres although the units are bound by major faults the overall coherence of their different protolithic characteristics suggests that they can be fitted into a simple geodynamic model the tregor unit represented by the north tregor batholith of crustal affinity and its icartian host rock probably reflects an early cadomian evolution the saint brieuc unit essentially made up of magmatic rocks of juvenile affinity includes two assemblages juxtaposed by the cadomian tectonics 1 a volcanosedimentary succession formed in a typical island arc setting reflecting subduction around 610 ma and 2 a volcanic plutonic complex emplaced around 590 ma in an extensional active continental margin of pentevrian age the guingamp unit predominantly migmatitic includes a tectonic slice of deep origin the extrusion of which accompanied the exhumation of the migmatites it may represent the trace of a possible suture the saint malo unit is composed of brioverian detrital rocks the significance of which is uncertain passive continental margin or accretionary prism it is structured around a late migmatitic dome 540 ma the fougeres unit corresponds to the outer domain of the cadomian orogeny it was only weakly deformed by cadomian tectonics and is intruded by the large mancellian batholith of crustal origin dated at 540 ma together the structural units are interpreted in the framework of an evolving active margin juxtaposing continental margin marginal basin and volcanic arc before being shortened during the cadomian convergent tectonics

geology of the saucer shaped sill near mahad western deccan traps india and its significance to the flood basalt model an 22 m thick saucer shaped sill occurs near mahad and is exposed as a curvilinear miniature ridge within the deccan traps the sill has variable dips 42 55 it has a 7 1 km long axis and 5 3 km short axis aspect ratio of 1 4 and is larger than the mv sill of the golden valley sill complex south africa and the panton sill australia the sill has distinct glassy upper and lower chilled margins with a coarse grained highly jointed core the samples from the margin are invariably fractured and iron stained because of deuteric alteration the rock from the sill is plagioclase phyric basalt at least three thick sill like apophyses emanate from the base of the main sill the apophyses change direction because of bending and thinning from a horizontal concordant sheet at the top to a discordant inclined form that bends again to pass into a lower horizontal concordant sheet we interpret such features as nascent saucer shaped sills that did not inflate to form nested sills geochemically the sill consists of poorly differentiated tholeiitic basalt that has a restricted geochemical range critical trace element ratios and primitive mantle normalised trace and ree patterns indicate that the sills have geochemical affinities to the poladpur chemical type and that the pahoehoe flow they intrude belongs to the bushe formation calculated magmatic overpressures during sill emplacement range from 8 4 to 11 3 mpa for young s modulus e 5 gpa and 16 7 to 22 5 mpa for e 10 gpa and depth to magma chamber ranges from 8 5 to 11 5 km e 5 gpa and 17 1 to 22 9 km e 10 gpa consistent with petrological and gravity modelling the volume of the mahad sill is approximately 276 km3 and is constant irrespective of the variations in the values of host rock young s modulus in 1980 cox j petrol 21 629 650 1980 proposed a conceptual model of the crust mantle section beneath the karoo cfb which is considered as the fundamental model for flood basalt volcanism our paper confirms the presence of a sill plus the inferred substructure beneath mahad that are compatible with predictions of that model in lips saucer shaped sills are formed in areas experiencing extensional tectonics where processes such as the cook gordon delamination and dundurs elastic extensional mismatch between layered sedimentary rocks or lava flows are responsible for the deflection of dykes into sills a similar process is envisaged for the formation of the mahad sill