late ordovician glacial record of the anti atlas morocco abstract late ordovician glaciogenic deposits are exposed intermittently along an 800 km long outcrop belt in the anti atlas mountains of southern morocco these deposits are of economic significance as potential oil bearing sandstones in the tindouf and boudenib basins and thus are here re examined as analogues to subsurface hydrocarbon reservoirs glaciogenic deposits of the upper second bani formation rest unconformably upon underlying shallow marine clastic deposits the unconformity is characterised by a series of palaeovalleys some 0 5 1 0 km wide and up to 100 m deep which may have been cut under elevated hydrostatic pressures as tunnel valleys beneath a late ordovician ice sheet the valleys and intervalley areas are filled with glaciogene sediments categorized into five facies associations namely 1 a tabular sandstone association shallow marine shoreface deposits 2 a massive sandstone and conglomerate ice contact debrites 3 meandriform sandstone deposits ice proximal sandur 4 stratified diamictites ice rafted debris and 5 sigmoidally bedded sandstones intertidal sandstones deformation in these sediments is ubiquitous and includes soft sediment striated pavements metre scale duplex systems thrust and fold belts of deformation affecting some tens of metres of sediment and pervasive lineations these features are interpreted to record the complex nature of deformation processes operating beneath a late ordovician ice sheet including sliding at the ice bed interface folding and deformation within the sediment column and a series of complex ramps detachments and shear zones within an unconsolidated pile of sediment beneath the ice sheet

the fossil accretionary wedge of the bay of biscay critical wedge analysis on depth migrated seismic sections and geodynamical implications abstractthe accretionary wedge of the bay of biscay is an east west compressive belt buried under recent sediments of the abyssal plain at the north iberian margin this structure formed through the partial closure of the previously extended biscay basin during the cenozoic north south collision between europe and iberia the same collision that produced the cantabrian pyrenean range on land three north south seismic sections have been prestack depth migrated showing a narrow tapered wedge 7 8 whose internal structure corresponds to a set of south dipping thrusts converging toward a basal decollement there are differences along strike within the wedge thrust spacing the dip of the basal thrust and the thickness of the sediments at the trough augment toward the east increasing its overall size the two dimensional velocity models obtained through migration analysis reflect values between 2000 km s at the sea floor 4500 m and 5000 km s at 12 km depth the syntectonic package thickness varies fr

new constraint on the division of cambrian time a u pb zircon age of 530 7 0 9 ma for a volcanic ash in the lower placentian series of the avalonian terrane in southern new brunswick provides a precise point for calibration of early cambrian biologic evolution the ash is from the middle rusophycus avalonensis zone in saint john new brunswick this date is on subtrilobitic lower cambrian strata that may be approximately correlative with the sub tommotian manykaian stage of siberia and the lowest meishucunian stage of the south china platform this age when combined with published geochronologic data from the cambrian suggests that the manykaian stage may have lasted approximately 13 m y and that the tommotian may have been considerably shorter

compression of oceanic lithosphere an analysis of intraplate deformation in the central indian basin intraplate compressional deformation in the central indian basin is expressed as linear e w trending topographic undulations and geoid anomlies with characteristic spacings of approximately 200 km to elucidate the nature of this deformation we develop models of an oceanic lithosphere in a state of horizontal compression the lithosphere is treated as a viscous or plastic layer of uniform strength which overlies a viscous half space in which strength decreases exponentially with depth we compare models in which deformation with a dominant wavelength develops in response to flexural buckling and the hydrodynamic growth of instabilities for the viscous flexural buckling model deformation is constrained to occur in the antisymmetric mode by flexural folding which is characterized by uniform bending of the layer in the hydrodynamic flow model a range of deformational styles are possible in a strong viscous lithosphere deformation of the layer occurs by flexural folding at a wavelength which agrees with that predicted from flexural buckling theory in a lithosphere of intermediate strength the layer deforms by folding characterized by thickening which localizes beneath topographic highs in a relatively weak lithosphere the layer incurs an even greater amount of localized thickening and deforms in the symmetric or pinch and swell mode by inverse boudinage if the layer has a plastic rheology deformation occurs by inverse boudinage independent of the yield stress results from models in which the layer folds either flexurally or with periodic thickening are consistent with the observed depth distribution of seismicity in the central indian basin and with experimental rock rheological data because of the trade off between buoyancy forces and layer strength in determining the dominant wavelength a lithosphere which was overlain by weak sediments when intraplate deformation initiated can be characterized by a compressive layer strength almost a factor of 3 less than a lithosphere which was initially sediment free for a range of plausible models which consider both a sediment covered and sediment free lithosphere the observed wavelength requires deviatoric stresses in the indian plate of several hundreds of mpa

weak zone formation for initiating subduction from thermo mechanical feedback of low temperature plasticity abstract we have addressed the problem of subduction initiation with a solid mechanical and fluid dynamical approach using a finite element method the setup has been extended by a rate sensitive coupling at the bottom of a semi brittle lithosphere and a fully coupled thermo mechanical model the central element of our model is a broad asymmetric sedimentary loading function at the passive continental margin which grows with time to 15 km two fundamentally different modes of shear zone formation have been found depending on the rheology of the creep layer mode 1 for cases of low or absent yield stress in the creep layer only the semi brittle top develops a weak zone while the rate sensitive layer acts as a decoupling shear zone mode 2 for cases with a yield strength in the creep layer strain rates above 10 15 s 1 after yielding the entire mechanical lithosphere fails on a major shear zone mode 1 fails to model subduction initiation while mode 2 creates a weak major shear zone that severs through the entire lithosphere

convection experiments and the driving mechanism the problem of mantle convection is discussed in light of laboratory and numerical experiments since wegener s time our knowledge of earth behaviour has increased so that we are in a better position to explain continental drift the explanation of this phenomenon must inevitably include mantle convection the problem has three aspects the physical laws the boundary conditions and the material we find that a likely model is high rayleigh number convection in a medium of temperature pressure and stress dependent rheology in this model we expect a strong and heavy upper thermal boundary layer and a fluid and light lower boundary layer the upper one consists of the lithospheric plates which highly organize the large scale circulation while the lower one at the core mantle boundary becomes unstable in the form of diapirs plumes or blobs in a fashion rather independent of the large scale circulation this model has the potential of reconciling the conflicting views of horizontal and vertical tectonics

palaeogeographical controls on the cambrian trilobite immigration and evolutionary patterns reported in the western gondwana margin abstract southward drifting of the western gondwanan margin during the cambrian has been demonstrated by means of both palaeomagnetic methods and lithological indicators of climate such as carbonates and evaporites recent improvements in biostratigraphical correlations permit an enhanced understanding of the climatic and palaeobiogeographical constraints that controlled the distribution of cambrian benthic communities palaeogeographical and biogeographical reconstructions based on trilobites are reported in this paper in order to test interaction between migration speciation and extinction rates the variability of the documented biogeographical patterns is directly related to species diversity in which wider distribution coincides with transgressive trends and subsequent connection of neighbouring platforms early cambrian trilobite faunas show a high degree of both substrate control and endemicity although transgressions led to parallel shifts in faunal compositions by contrast mid cambrian trilobite faunas are relatively uniform across western gondwana and latest mid and late cambrian associations document influence of an increased similarity with asian trilobite faunas

duration of the early cambrian u pb ages of volcanic ashes from avalon and gondwana volcanic zircons from three ashes give a u pb date of 511 1 ma on trilobite bearing upper lower cambrian upper branchian series strata of southern new brunswick that correlate into the siberian middle botomian toyonian stage interval this very young age on the late but not latest early cambrian of avalon is consistent both with a 519 1 ma age on the older caerfai bay shales of south wales that are tentatively correlated into strata with the oldest avalonian trilobites lower branchian and with a 517 1 5 ma age on the antatlasia gutta pluviae zone trilobites of morocco determination of a 522 2 ma zircon age on the moroccan subtrilobitic lower cambrian lie de vin formation is consistent with an earlier reported 521 7 ma age from the lie de vin but suggests that a 526 4 ma age on australian trilobite bearing lower cambrian rocks may be too old a 33 ma duration of the avalonian early cambrian and an 8 ma length of the avalonian trilobite bearing lower cambrian support proposals that

time specific black mudstones and global hyperwarming on the cambrian ordovician slope and shelf of the laurentia palaeocontinent abstract the early paleozoic featured nine intervals of strong expansion of an upper slope dysoxic anoxic d a water mass with eustatic rise or epeirogenic transgression strong expansion of this d a water mass led to deposition of time specific macroscale alternations of dark grey black mudstone within oxic green to red mudstone on the middle lower slope this d a facies even onlapped warm carbonate and cool water siliciclastic shelves as in the mesozoic d a muds were deposited in shallow water perhaps tens of metres deep with sea level rise these nine d a macroscale alternations correspond to intervals of global hyperwarming times of very intense greenhouse conditions that resulted from a feedback initiated by higher insolation and heat storage as shallow seas onlap tropical palaeocontinents warm epeiric seas heated the ocean and thermal expansion accelerated eustatic rise ever more extensive epeiric seas heightened oceanic and global temperature as heat storage capacity increased deep ocean circulation intensity fell below that of a greenhouse interval and lead to d a deposition low on the slope and on the platforms to provide the signature of global hyperwarming global hyperwarming differs from a hothouse interval as it does not require co 2 input from large igneous provinces to produce high temperatures and never shows deep sea anoxia late ordovician and late devonian black mudstones that cover much of laurentia record epeirogenic transgressions that led to global hyperwarming and suggest that cold water upwelling or plant terrestrialisation had nothing to do with epeiric sea anoxia global hyperwarming reduced oxygen solubility in these seas and erosion of orogens produced muddy water that limited light penetration and promoted shallow water anoxia the global hyperwarming hypothesis means that relative eustatic and epeirogenic sea levels complement the effect of global pco 2 on climate and sea level must also be regarded as a primary driver of phanerozoic climate

fast ductile failure of passive margins from sediment loading sediment loading at continental margins has been interpreted as a very inefficient mechanism for initiating subduction we have re investigated this problem using a time dependent non linear ductile failure analysis in which the effects of power law plastic strain hardening are assessed the results follow classical flexural rigidity solutions until wholesale lithosphere failure occurs sediment loading causes two shear zones to develop from the bottom of the lithosphere upwards for strong strain hardening shear zones nucleate earlier but the plate may never break for small hardening the instability grows much faster and the plate can break under the sediment load with a limiting yield stress lower than 0 4 gpa when the ductile faults growing from the top and the bottom of the plate merge in the center the flexurally supported gravitational energy is released the lithosphere becomes unstable and shears off super exponentially within a few million years

interlaminated ice proximal glacimarine sediments in muir inlet alaska muir inlet in glacier bay alaska is a glacial fjord receiving a tremendous volume of sediment annually the rate of sediment accumulation is greatest proximal to muir glacier about 9 m yr 1 and decreases away from the glacier the primary sediment sources are meltwater streams discharging at subglacial and ice marginal positions to form overflows interflows and underflows continuous turbidity currents overflows and interflows interact with diurnal tidal currents and their volume and sediment concentration varies diurnally and annually with meltwater discharge these effects produce cyclic deposits of a thin fine grained sand or silt lamina that grades normally to a thicker poorly to very poorly sorted mud lamina this lamina couplet is termed a cyclopel underflows are suggested to occur in this glacimarine environment because of conditions unique to subglacial fluvial systems underflow deposits occur only in proximal positions 0 5 km from glacier face and are coarse grained reverse to normal graded and exhibit an increase in sorting and sand content up layer ice rafted debris identified as particles 177 m is ubiquitous though low 5 by weight and occurs as isolated particles frozen pellets or as lenses that in cores may have a lamina appearance proximally ice rafted debris is difficult to identify because proximal sediment is often as coarse grained deposited sediment may be reworked by tidal currents and sediment gravity flows depositional processes operating in muir inlet produce interlaminated sand silt clay that characterizes sediment proximal to a glacier and fines seaward to mud sediment is classified into one of three sediment types 1 1 type i sediment is very fine grained mean 8 65 7 17 o low in sand 0 1 11 2 and very poorly to poorly sorted it is the dominant sediment type in muir inlet and is transported by plumes and deposited by suspension settling 2 2 type ii sediment is fine to coarse grained mean 6 70 3 12 o low to high in sand 5 1 86 6 and very poorly to moderately sorted it represents reworked sediment proximal plume deposits or coarse grained laminae of cyclopels 3 3 type iii sediment is coarse grained mean 3 89 2 38 o high in sand 58 0 100 0 and poorly to well sorted it is deposited by sediment gravity flows or underflows

first ediacaran fauna found in western africa and evidence for an early cambrian glaciation ediacara type body fossils are reported here for the first time from the west african craton in algeria this assemblage comprises medusoids comparable to medusinites sp and nimbia a trace fossil and a new genus of body fossil with uncertain affinity the fossils are in sandstones of the neoproterozoic cover separated by a major unconformity from the overlying tillite considered until now to be late neoproterozoic most ediacaran fauna radiated after the varanger ice age and tillites associated in ahaggar with molasse of the pan african belt are overlain by carbonates that were deposited during deglaciation and that locally contain early cambrian shelly fossils therefore we interpret the glaciogenic diamictite as early cambrian in age

the role of elasticity in slab bending previous studies showed that plate rheology exerts a dominant control on the shape and velocity of subducting plates here we perform a systematic investigation of the role of elasticity in slab bending using fully dynamic 2 d models where an elastic viscoelastic or viscoelastoplastic plate subducts freely into a purely viscous mantle we derive a scaling relationship between the bending radius of viscoelastic slabs and the deborah number de which is the ratio of maxwell time over deformation time we show that de controls the ratio of elastically stored energy over viscously dissipated energy and find that at de 10 2 substantially less energy is required to bend a viscoelastic slab to the same shape as a purely viscous slab with the same intrinsic viscosity elastically stored energy at higher de favors retreating modes of subduction via unbending while trench advance only occurs for some cases with de 1 where most zones have low de 0 1 slabs with de 10 2 either have very low viscosities or they may be yielding in which case our de estimates may be underestimated by up to an order of magnitude potentially pointing towards a significant role of elasticity in 60 of the subduction zones in support of such a role of elasticity in subduction we find that increasing de correlates with increasing proportion of larger seismic events in both instrumental and historic catalogues

reconstructing the avalonia palaeocontinent in the cambrian a 519 ma caliche in south wales and transcontinental middle terreneuvian sandstones an early cambrian caliche on the st non9s formation emended is the base of the caerfai bay formation unit term changed at caerfai bay south wales subaerial exposure and the caliche mean the two formations were not genetically related units the st non9s is an older sand sheet likely tidalitic not delta related referred to avalonian depositional sequence ads 2 and the caerfai bay is a shallow mud basin unit refered to ads 4a the similar random formation upper ads 2 in north american avalonia has a basal age of c 528 ma and is unconformably overlain by red mudstones or sandstones in fault bounded basins on the avalonian inner platform coeval british sandstones lower hartshill wrekin st non9s brand hills are unconformably overlain by latest terreneuvian ads 3 or epoch 2 ads 4a units dates of 519 ma on caerfai bay ashes give an upper bracket on the late appearance of avalonian trilobites and suggest an ads 2 4a hiatus of several million years post st non9s and post random basin reorganization led to abundant caerfai bay formation volcanic ashes and sparse brigus formation ashes in newfoundland the broad extent of erosional sequence boundaries that bracket lithologically similar to identical units emphasize that east and west avalonia formed one palaeocontinent the inner platform in southern britain was larger than the midlands craton a tectonically defined later palaeozoic area unrelated to terminal ediacaran early palaeozoic depositional belts the cool water successions of early palaeozoic avalonia were distant from coeval west gondwanan carbonate platforms

folding of oceanic lithosphere folding of the lithosphere just south of the bay of bengal appears as 1 undulations in acoustic basement topography and 2 as linear geoid undulations in the seasat altimeter data from the seasat data we find that the east west trending folds have wavelengths ranging from 130 to 250 km and clustering about 190 km the horizontal gravity disturbances due to the folds range in amplitude from 15 to 50 mgal elastic models of oceanic lithosphere have in the past been used to demonstrate the implausibility of lithosphere buckling or folding in response to compression these elastic models typically predict that compressive stresses of about 5 gpa are required to buckle oceanic lithosphere with an age comparable to that of the northeastern indian ocean 40 70 ma these stresses exceed the strength of lithospheric rock we use an elastic plastic model to show that oceanic lithosphere of this age should have a net compressive strength equal to about 12 of the elastic buckling stress we further demonstrate that loads approaching the net compressive strength cause the lithosphere to fold with a wavelength about 200 km i e the wavelength observed from seasat our results reinforce earlier speculation that this folding may be related to the himalayan orogeny

the elastic properties of the lithosphere beneath scotian basin abstract to assess the possibility that the north atlantic ocean may subduct at scotian basin east of canada we investigate the present compensation state of this deep basin a fourier domain analysis of the bathymetry depth to basement and observed gravity anomalies over the oceanic area east of nova scotia indicates that the basin is not isostatically compensated moreover the analysis emphasizes that in addition to the sediments density perturbations exist beneath the basin the load produced by the sediments and these density perturbations must have been supported by the lithosphere we simulate the flexure of the lithosphere under this load by that of a thin elastic plate overlying an inviscid interior it is shown that a plate with a uniform rigidity does not adequately represent the lithosphere beneath the basin as well as the oceanic lithosphere far from the basin rather the rigidity of the lithosphere directly beneath the basin is about one to two orders of magnitude smaller than elsewhere we relate this weakening to the thermal blanketing effects of the thick sediments and the fact that the lithosphere has a temperature dependent rheology we suggest that this weak zone would have a controlling effect on the reactivation of normal faults at the hinge zone of the basin that were formed during the break up of africa and north america and were locked in the early stages after the break up the weak zone would facilitate reactivation of the faults if tensional stresses were produced by possible reorientation of the spreading direction of the north atlantic ocean in the future the reactivation of the faults would create a free boundary condition at the hinge zone allowing further bending of the lithosphere beneath the basin and juxtaposition of this lithosphere to the mantle beneath the continent this may provide a favorable situation for initiation of slow subduction due to subsequent compressional forces

cephalopod ancestry and ecology of the hyolith allatheca degeeri s l in the cambrian evolutionary radiation abstract pyritized elongate conical conchs of allatheca degeeri s l are common in dysoxic dark gray mudstone intervals in the early cambrian upper terreneuvian series 2 boundary interval cuslett formation at keels eastern newfoundland wave oriented horizontal specimens are most abundant in this cool water high latitude off shore shelf facies of the early palaeozoic avalon microcontinent based on conch morphology shell microstructure and the operculum the species is an orthothecid hyolith comparison with the sizes of the early shells of planktic gastropods indicates a non planktic life mode of a degeeri s l hatchlings although buoyancy calculations show that small juveniles with septate conchs to ca 17 mm long could have been nektic planktic if smaller a degeeri s l individuals had a non benthic mode of life they and pseudoconodonts were the oldest skeletalized pelagic nektic animals in the cambrian evolutionary radiation most a degeeri s l conchs at keels are horizontally embedded and show a bimodal wave determined orientation but about 10 of the large conchs are vertically embedded with their aperture down as larger shells were not neutrally buoyant the vertical orientations of about 10 of the conchs suggests an infaunal likely detritivore life mode suggestive of a scaphopod available morphologic and taphonomic evidence suggests that the vertically embedded conchs are in situ remains of dead benthic animals that colonized the bottom in better oxygenated intervals based on the current knowledge of early palaeozoic hyolith and cephalopod larval and adult morphologies existing hypotheses of a planktic origin of cephalopods from hyolith ancestors are evaluated and no evidence for such an evolutionary relationship is concluded to exist

seismotectonics of the nubia plate compressive margin in the south tyrrhenian region italy clues for subduction inception 1 seismological and structural data are used to constrain the active tectonics of the nubia plate compressive margin in southern italy in this region compressional displacements have resumed at the rear of the maghrebian orogenic wedge since about 700 500 ka and are presently accommodated along a seismic belt in the south tyrrhenian area where a passive margin has developed during late neogene quaternary times earthquake data from the south tyrrhenian belt are analyzed with bayloc a probabilistic nonlinear location method results show that the seismic belt is segmented and involves a series of ne sw and nw se elongated clusters the ne sw clusters are interpreted as high angle reverse fault zones verging toward the southeast whereas the perpendicular clusters are interpreted as possible strike slip fault zones transferring the contractional displacements to advanced segments of the belt in the southeast brittle deformations observed in the quaternary volcanic island of ustica are consistent with the geometry and kinematics of the seismic belt deduced from the seismological data the south tyrrhenian active belt may constitute an early stage of subduction of the tyrrhenian oceanic crust beneath sicily a similar scenario has been hypothesized also for the westward prolongation of the south tyrrhenian belt off the algerian coast by integrating instrumental and historical seismic data with tectonic evidences we infer that the south tyrrhenian belt is probably capable of producing earthquakes with a maximum size close to magnitude 7 the analyzed data suggest a multifault system behavior for the studied compressional belt this inference is important to estimate and mitigate the seismic and tsunamic hazards in such a densely populated region

trench parallel shortening in the forearc caused by subduction along a seaward concave plate boundary insights from analogue modelling experiments abstract three dimensional thermo mechanical analogue experiments are employed to test the hypothesis that oceanic subduction along a seaward concave plate boundary can generate trench parallel shortening in the forearc near the axis of curvature the model deformation is analyzed with a particle imaging velocimetry piv system that allows for comparison of forearc deformation along the oblique limbs of the curved plate boundary and near the axis of curvature moreover piv allows for separation of the trench parallel and trench perpendicular components of strain regardless of trench orientation the resulting deformation maps show a remarkable symmetry and indicate drag of the forearc above the interplate coupling area towards the axis of curvature trench perpendicular profiles show that along the oblique limbs of the plate boundary the forearc is submitted to trench normal shortening and trench parallel shearing but not trench parallel shortening or extension this contrasts with the situation near the axis of symmetry where the forearc is submitted to trench parallel and trench perpendicular normal shortening but is not sheared the experimental results confirm that trench normal thrusts observed in the fore arc of the central andes can be a mechanical consequence of subduction along a seaward concave plate boundary if the degree of interplate coupling is large

tectonics of the tonga kermadec trench and ozbourn seamount junction area abstract seismic profiling together with seismic refraction data and dredging enabled us to study the upper crustal structure of the tonga kermadec trench and the ozbourn seamount junction area horst graben structures have been revealed both on the insular and oceanic slopes of the trench two systems of faults are characteristic one transverse and the other parallel to a general trend of the tonga kermadec trench grabens of the trench insular slope are partly compensated by sediments unlike those of the oceanic slope which are not ponded by sediments on both slopes the faults are normal this testifies to the extensional conditions within the upper crustal part of this region the tonga kermadec uplift insular slope of the trench has undergone intensive with amplitide to 5 7 km vertical movements uplifting and then subsidence in eocene oligocene pliocene and quaternary times

slab breakoff insights from 3d thermo mechanical analogue modelling experiments abstract the detachment or breakoff of subducted lithosphere is investigated using scaled three dimensional thermo mechanical analogue experiments in which forces are measured and deformation is monitored using high speed particle imaging velocimetry piv the experiments demonstrate that the convergence rate in a subduction zone determine if and when slab detachment occurs slow subduction experiments with scaled convergence rates 1 cm yr 1 have lower peclet numbers and are characterized by lower tensile strength subducted lithosphere causing detachment to occur when the downward pull force exerted by a relatively short subducted slab is relatively low therefore when continental collision is preceded by slow oceanic subduction the hot and weak subducted lithosphere need not be very long or extremely negatively buoyant to cause detachment under such conditions detachment may occur sooner after the onset of continental subduction than previously predicted in contrast if collision is preceded by rapid subduction 10 cm yr 1 breakoff will be delayed and occur only when the convergence rate has slowed sufficiently to thermally weaken the slab and cause its eventual failure the analogue experiments further confirm that slab detachment occurs diachronously as it propagates along the plate boundary stereoscopic piv reveals a characteristic strain pattern that accompanies the detachment horizontal contraction and subsidence with scaled values up to 1200 m in the trench and forearc area precedes the passage of the detachment and is followed by horizontal extension and uplift up to 900 m high frequency monitoring captures rapid propagation of the detachment along the plate boundary at scaled rates of up to 100 cm yr 1 however this rate is not constant and interaction between the slab and lower mantle or opening of a backarc basin in the upper plate can reduce or stop slab breakoff propagation altogether

mixed siliciclastic carbonate deposition in an early cambrian oxygen stratified basin chapel island formation southeastern newfoundland abstract the upper portion of member 3 and member 4 of the uppermost precambrian vendian lower cambrian chapel island formation represents a large scale shoaling up deposit dominated by shelf siliciclastic mudstone medium scale higher order shoaling up cycles at the top of member 3 and in member 4 are capped by peritidal limestones that contain a wide variety of stromatolitic buildups and algal mud mounds sedimentation took place in an oxygen stratified basin with accumulation of gray dysaerobic mudstone offshore and green and red mudstone in shallower more oxygenated waters biofacies models for dysaerobic basins can be applied to this sequence and help in understanding the distribution of trace fossils skeletal fossils shells and pyritic steinkerns and diagenetic features these da a are important for the understanding of the stratigraphic distribution of shelly fossils within this lowest cambrian unit a mixed carbonate siliciclastic facies model is presented in which carbonate sediments formed in intertidal areas along the shoreline of a siliciclastic muddy shelf the carbonate shoreline prograded during periods of increasing rate of relative sea level fall or decreasing rate of relative sea level rise restriction of carbonate sediments to the peritidal environment may have been due to a combination of higher rates of carbonate sediment production and accumulation in the peritidal zone seaward dilution by siliciclastic mud and a paleogeographic position near the latitudinal climatic extreme for carbonate production

horizontal lithosphere compression and subduction constraints provided by physical modeling physical modeling of the subduction is performed with a two layered mantle model elastico plastic lithosphere and low viscosity asthenosphere and is governed by the criteria of similarity compression of the lithosphere in the area of a passive continental margin has been shown to produce a buckling instability in the oceanic plate with wavelengths of 200 km on the average later a localization of deformation occurs in sagging at some distance from the margin where a strongly dislocated linear ridge is formed due to the thrusting the plate then experiences a failure along the inclined zone and subduction starts the inner trench slope which forms therefore has a scraped structure and include a block of crushed and dislocated oceanic crust and sediments in the lower section if there is an old inclined fault striking across the compression of an oceanic lithosphere it is on that fault that a subduction zone is initiated the inner trench slope has then a different structure and forms due to normal faulting in the frontal part of the overriding plate the formation of a subduction zone requires a compression that is smaller than that in the preceding case by a factor of 2 or 3 the subducting plate experiences an elastico plastic bending and under specific conditions thrusting along the zone dipping from under the overriding plate oceanward and crossing the entire lithosphere the best agreement between generalized relief in subduction zone in the model and nature is achieved when a shear yield limit s 1 3 108 pa modulus of elasticity e about a few times 1011 pa and a thickness h 60 km are adopted for the real lithosphere

large scale deformation associated with ridge subduction summary continuum models are used to investigate the large scale deformation associated with the subduction of aseismic ridges formulated in the horizontal plane using thin viscous sheet theory these models measure the horizontal transmission of stress through the arc lithosphere accompanying ridge subduction modelling was used to compare the tonga arc and louisville ridge collision with the new hebrides arc and d entrecasteaux ridge collision which have disparate arc ridge intersection speeds but otherwise similar characteristics models of both systems indicate that diffuse deformation low values of the effective stress strain exponent n are required to explain the observed deformation deformation is somewhat insensitive to the vertically integrated strength of the arc inversely proportional to the argand number ar but indicates that the arc lithosphere is not extremely weak ar 100 low values of both ar and n suggest that the thermal structure is typical of cold or normal arcs and that deformation is dominated by flow in the lower crust and mantle in addition low values of n approaching newtonian flow may indicate that specific deformation mechanisms dictate deformation of the arc lithosphere possible mechanisms include low stress grain size dependent creep pyroxenite controlled rheology and mechanisms associated with water weakening changes in the boundary conditions greatly affect deformation within island arcs high rates of arc ridge intersection speed tonga louisville system yield arc parallel tension and crustal thickening in the wake of ridge subduction in contrast low rates of arc ridge intersection speed new hebrides d entrecasteaux system yield compressional deformation directly arcward of the collision zone and transverse strike slip faulting adjacent to the region of compressional deformation localized regions of extensional deformation along the frontal part of the arc adjacent to the collision zone may contribute to the formation of re entrants

paleomagnetic constraints on the tectonic evolution of the costa rican subduction zone new results from sedimentary successions of iodp drill sites from the cocos ridge the near flat subduction of the cocos ridge cr along the middle american trench mat plays a pivotal role in governing the geodynamic evolution of the central american convergent margin elucidating the onset of its subduction is essential to understand the tectonic evolution and seismogenesis of the costa rican convergent margin a typical erosive convergent margin and modern example of a flat slab subduction initial subduction of the cr has been previously investigated by examining upper plate deformation that was inferred to have resulted from the initial cr subduction however little attention has been paid to the extensive sedimentary archives on the cr that could hold important clues to the initial cr subduction drilling on the cr during iodp expedition 344 discovered a pronounced sedimentary hiatus at site u1381 here we present paleomagnetic and rock magnetic results of the cenozoic sedimentary sequences at this site that bracket the hiatus between ca 9 61 and 1 52 ma we also examine the areal extent timing and geologic significance of the hiatus by analyzing sedimentary records from five other odp iodp sites on cr and cocos plate the analyses show that the hiatus appears to be regional and the presence absence of the sedimentary hiatus at different locations on cr implies a link to the onset of cr shallow subduction as a result of either bottom current erosion or cr buckling upon its initial collision at the mat records directly from cr thus provide a new window to unraveling the geodynamic evolution of the central american margin

density and visco elasticity of natrosol 250 hh solutions determining their suitability for experimental tectonics abstract analogue models often require that materials with specific physical properties be engineered to satisfy scaling conditions to achieve this goal we investigate the rheology of aqueous solutions of natrosol 250 hh a rheology modifier employed in various industries to thicken viscous solutions we report the rheological properties as functions of the concentration and temperature and discuss the advantages and limitations of these materials in view of their use in analogue modelling experiments the solutions are linear visco elastic for low stresses or strain rates becoming shear thinning for larger stresses for the typically slow analogue experiments of tectonics the solutions can be considered linear visco elastic with a maxwell relaxation time much smaller than the characteristic observation time this simplification is even more appropriate when the solutions are employed at temperatures higher than 20 c since the solutions then display a behaviour that is more viscous less elastic at the same shear rate while the newtonian viscosity reduces and the shear rate limit between newtonian and shear thinning behaviours increases the newtonian viscosity is shown to increase non linearly with concentration and decrease non linearly with temperature with concentrations between 0 and 3 and temperature between 20 and 40 c the viscosity varied between 10 1 and 4000 pa s while the density remained close to the density of water natrosol 250 hh thus offers the possibility to control the viscosity of a solution without significantly affecting the density thereby facilitating the design and setup of analogue experiments

the role of lithospheric heterogeneities in the location of the cenozoic intraplate deformation of iberia from finite element modeling received 31 may 2011 revised 28 november 2011 accepted 12 december 2011 published 9 february 2012 1 the cenozoic evolution of the central system basement uplift and of the continental duero and madrid basins on the iberian peninsula occurred in a compressive context triggered by the alpine orogeny although several models of deformation have been suggested the factors that led to the development of these have not been analyzed to date we present here finite element modeling of the intraplate deformation of the iberian lithosphere during the cenozoic this modeling has taken into account the role played by 1 the tectonic shortening originating from the pyrenean and betic collisions 2 the weight of tertiary continental sediments and 3 thermomechanical heterogeneities present in the lithosphere the numerical model represents a lithospheric section with a creep type mechanical behavior perpendicular to the central system and the two sedimentary basins both initial conditions and the results obtained have been specified based on the available geophysical and geological data this study has enabled us to simulate the present lithospheric structure of the central iberian peninsula a the formation of the central system relief and its horizontal shortening b the basement folding in the duero and madrid basins c the development of the root of the mountain range and d the depth of the lithosphere asthenosphere border in the interior of iberia the deformations occurred through a large scale folding and thickening mechanism by the shortening of a lithosphere that was initially thinner and with lateral contrasts in temperatures and composition

first middle ordovician biota from southern new brunswick stratigraphic and tectonic implications for the evolution of the avalon continent a limestone boulder in the triassic lepreau formation near saint john new brunswick has yielded the first diverse marine fauna from the sub caradoc ordovician of the western avalon continent this fauna includes the first arenig conodonts recovered from avalon and represents an unexposed interval in southern new brunswick association of the conodonts drepanoistodus and baltoniodus and the trilobites neseuretus nileus and stapeleyella emphasizes the faunal dissimilarity of avalon and laurentia through the late middle ordovician extension of the ranges of neseuretus cf neseuretus parvifrons and stapeleyella from britain into new brunswick further emphasizes that eastern and western avalon were confluent parts of a unified insular avalon continent that originated in the latest precambrian this fauna correlates with the lower amorphognathus lenodus variabilis zone kundan stage of baltica and the terminal arenig upper middle ordovician lower darriwilian stage of avalonian britain available evidence suggests that an arenig cover sequence with local shallow water hematitic iron ore quartz arenite and rare limestone extended across the avalonian marginal and inner platforms from eastern newfoundland to the boston massachusetts region this western avalonian arenig shows the greatest similarity with the arenig of the welsh borderlands phosphatic fossils from the boulder have a thermal alteration index much lower than that of nearby lower paleozoic outcrops and suggest derivation of the boulder from a weakly heated avalonian succession brought into the bay of fundy region by post ordovician transcurrent faulting

at what stress level is the central indian ocean lithosphere buckling internal contrasts in strength are responsible for lithospheric buckling these are quantified by comparing the indian ocean data to two dimensional visco elasto plastic numerical models where the material properties depend on temperature and pressure the central indian basin is known for its intraplate seismicity and long wavelength undulations of the sea floor and associated gravity signatures to simulate the amplitudes of undulations that reach 1 km within about 11 ma of compression and 60 km of shortening the required mean yield strength of the lithosphere is 400 mpa if either a hydrostatic fluid pressure is considered in the crust or a mechanical decoupling at the depth of the moho small crustal wavelengths are superimposed on the long wavelength deformation in agreement with observations it is then possible to match the alternative indication that buckling commenced only 4 ma ago with a total amount of shortening of 30 km and with a required yield strength of 200 mpa about 10 of homogeneous thickening accompanies buckling taking into account variable thermal conductivity demonstrates that a change in the geotherm is sufficient to increase buckling amplitudes by 10 within 5 ma fl 2000 elsevier science b v all rights reserved

tectonic history of northern new caledonia basin from deep offshore seismic reflection relation to late eocene obduction in new caledonia southwest pacific new high quality multichannel seismic reflection data from the western new caledonia offshore domain allow for the first time the direct continuous connection of seismic reflectors between the deep sea drilling project 208 drill hole on the lord howe rise and the new caledonia basin a novel seismic interpretation is hence proposed for the northern new caledonia basin stratigraphy which places the eocene oligocene unconformity deeper than previously thought and revisits the actual thickness of the pre oligocene sequences a causal link is proposed between the obduction of the south loyalty basin over new caledonia nc and the tectonic history of the northern new caledonia basin here it is suggested that as the south loyalty basin was being obducted during early oligocene times the nc basin subsided under the effect of the overloading and underthrusted to accommodate the compressional deformation which resulted in 1 the uplift of the northern fairway ridge and 2 the sinking of the western flank of new caledonia this event also had repercussions farther west with the incipient subsidence of the lord howe rise

the early palaeozoic of the north atlantic region as a test case for the use of fossils in continental reconstruction abstract fossils have an important contribution to make in the reconstruction of early palaeozoic palaeogeography the north atlantic regions include well known early palaeozoic fossil faunas which should relate to the dispositions of continental plates during the period however different versions of ordovician palaeogeography based on fossils are described in the literature and in this paper we evaluate the influences which produce these discrepancies the most reliable results are obtained by combining analysis of faunal similarity especially shelf faunas with mapping the distribution of those biofacies which relate to former deep water sites planktic organisms may not be reliable palaeocontinent discriminants cladistic analyses of different groups of organisms living at the same early ordovician time point to the usefulness of trilobite data for palaeogeographic reconstruction such data indicate a three fold division of the north atlantic region into baltica laurentia and gondwana from the cambrian to at least the mid ordovician paris and robardet 1990 denied the existence of a tornquist s sea separating baltica from gondwana in the early ordovician it is considered that their different reconstruction was because the planktic organisms on which they based their maps were not affected by this important oceanic separation to the same extent as shelly benthos on the other hand there is no evidence of deeper water biofacies along the supposed line of rheic ocean separation of avalonia from gondwana until the late ordovician however there is some evidence of deeper biofacies along the nantes sardic line new term extending from southern armorica to sardinia

morphostructure of an incipient subduction zone along a transform plate boundary puysegur ridge and trench multibeam bathymetric and geophysical data reveal a major strike slip fault that extends along the summit of the puysegur ridge east of the puysegur trench the northward structural development of this ridge trench system illustrates the evolution of an incipient subduction zone along a transform plate boundary that has been subjected to increasing transverse shortening during the past 10 m y at the southern end of the trench where subduction has not yet started the puysegur ridge has a narrow 50 km steep sided cross section and the axial strike slip fault separates a shallow 125 625 m flat topped eastern crest from a deeper 400 1600 m western crest these characteristics indicate differential uplift during the initial stage of shortening on the lower plate an incipient 5 2 km deep trench developed in conjunction with normal and reverse faults suggesting strong interplate coupling across the trench northward the ridge broadens linearly to 80 km wide its western flank has locally collapsed and the ridge summit has subsided possibly by 1 5 km suggesting that the interplate coupling decreases and that a benioff zone is being formed concomitant to the northward ridge evolution the trench deepens to 6 2 km and normal fault throws increase along its outer wall indicating greater flexure of the downgoing plate

upper lower cambrian depositional sequence in avalonian new brunswick the hanford brook formation emended is a thin up to 42 m upper lower cambrian depositional sequence that is unconformably bounded by the lower lower cambrian random formation and the middle middle cambrian fossil brook member of the chamberlain s brook formation these stratigraphic relationships of the trilobite bearing hanford brook formation indicate deposition on the avalonian marginal platform in the saint john new brunswick region and provide more evidence for a uniform latest precambrian cambrian epeirogenic history and cover sequence in avalon the hanford brook formation is a deepening shoaling sequence with i lower transgressive sandstone deposited in episodically high energy environments st martins member new ii highstand regressive dysaerobic mudstone fine grained sandstone with volcanic ashes somerset street member new and iii upper regressive planar and hummocky cross stratified sandstone long island member new trilobites are common in the distal somerse

iceberg drop dump and grounding structures from pleistocene glacio lacustrine sediments scotland abstract dropstone dump and grounding structures resulting from the melt out of debris contained in floating and grounded icebergs are described from a pleistocene ice marginal glacio lacustrine sequence in scotland dropstones display bending penetration rucking and complete rupture of stratum occurring beneath them and onlap above them with the degree of deformation varying as a function of the size shape and axial disposition of the clast and of the sediment type into which they fell dump structures are conical mounds of gravel or diamict formed by the break up and overturning of dirt laden icebergs and the consequent release of large quantities of debris to the lake floor grounding structures are caused by the grounding of icebergs the down warping of underlying lake floor se iment and the subsequent in situ melt out of contained debris to form isolated troughs of diamict

thrust initiation and propagation during shortening of a 2 layer model lithosphere analogue modelling was used to investigate thrust initiation and propagation in the lithosphere the model comprised a top layer of light sand model brittle crust overlying a layer of heavy silicone putty model ductile lower crust both resting upon higher density and lower viscosity natural honey model ductile mantle the experiments show the following 1 thrust initiation may occur in two main modes fold first or fault first they seem to be the result of presence or absence of an initial perturbation close to the piston respectively the fault first mode eventually goes into a fold first mode after a certain amount of deformation 2 in both modes thrusts localized around the inflection points of both fore and back limbs of buckles as a result of tectonic horizontal gravitational vertical and elastic forces 3 thrust propagation in the fault first mode occurred by thrust wedge thickening and downward flexure due to isostatic compensation which led to forward formation of bulges and subsequent amplification by buckling continued shortening increased the stresses that reached the brittle yield of sand and new thrusts originated 4 bulging ahead of thrusts tended to develop into asymmetric buckles with the shorter limb back limb steeper towards the piston 5 as a whole thrust initiation and propagation seem to depend mostly on how stresses evolve and propagate through the model and how material strength varies with progressive deformation the main stresses at play are born at the moving piston model horizontal tectonic force from thickening and isostatic compensation in the model gravitational forces and from elastic resistance to buckling

acritarchs from the maclean brook formation southeastern cape breton island nova scotia canada new data on middle cambrian lower furongian acritarch zonation abstract we present here the results of a pilot study on cambrian acritarchs from cape breton island nova scotia based on a material from the maclean brook formation in the mira river valley an assemblage from the base of the formation contains 8 species including eliasum llaniscum cristallinium dubium and symplassosphaeridium cambriense and correlates with a position close to the paradoxides davidis p forchhammeri zone boundary an assemblage from the upper part of the formation contains 16 species of which 3 are new including cristallinium aciculatum petaloferidium lacrimiferum n sp pirea orbicularis stelliferidium pingiculum s magnum n sp s albanii n sp and timofeevia microretis correlated to the olenus zone the upper assemblage is particularly noteworthy as it provides the first direct evidence for the olenus zone in the maclean brook formation furthermore it contains stratigraphically significant taxa known from sections on baltica and northern gondwana but which have not been reported previously on avalonia the distinction between stelliferidium and timofeevia is discussed in part based on new data on timofeevia lancarae from its type area in northern spain it is suggested that many reports of furongian timofeevia phosphoritica and t lancarae are better referred to stelliferidium

structure and development of the sagami trough and the boso triple junction abstract the sagami trough is located at the particular plate margin where the izu forearc is subducted underneath the honshu forearc at its southeastern end the world s only known ttt type triple junction boso triple junction has developed several different kinds of basins occur in different segments along the sagami trough and at the triple junction the bathymetric geologic and geophysical data obtained during the kaiko project and from additional studies are summarized together with our onland studies we suggest that the right lateral oblique plate motion formed an eduction margin in the sagami basin while a normal subduction margin and an oblique subduction margin have been formed in the middle sagami trough basin these tectonic phenomena resulted from the long lasting compressional covergence between the philippine sea plate and eurasian plate since the early or middle miocene the north basin on the northeasternmost margin of the philippine sea plate near the boso triple junction has developed as a stretched basin due to the westward motion of the philippine sea plate with respect to the eurasian plate

precambrian cambrian boundary global stratotype ratified and a new perspective of cambrian time the precambrian cambrian boundary global stratotype is the base of the phycodes pedum zone at fortune head southeastern newfoundland the initial two stage radiation of the phanerozoic fauna the appearance of phanerozoic aspect trace producers and subsequent origin of diverse skeletalized metazoans was earliest cambrian fossil assemblages with deep complex burrows or originally mineralized skeletal fossils with exception of cloudina are early cambrian or younger correlation into avalon indicates that strata once referred to the proterozoic e g manykaian or nemakit daldynian and rovno stages are lowest cambrian and that the siberian tommotian stage is middle lower cambrian the thick sequence and complex epeirogenic history of the placentian series as compared with the trilobite bearing branchian series suggest that most early cambrian time is represented by the subtrilobitic lower cambrian

the transylvania basin transfer zone between coeval extending and contracting regions inferences on the relative importance of slab pull and rift push in arc back arc regions 1 structural analysis and paleostress fault slip inversion of the transylvanian basin and its border regions with the surrounding east and south carpathians and the pannonian basin e g the apuseni mountains reveal at least two distinct tectonic phases for the neogene evolution for the eastern part of the pannonian basin system our main focus is on the role of the transylvanian basin and its border regions in the neogene as a transition zone between the extending pannonian basin and the converging east and south carpathians the results provide constraints on the mechanisms responsible for as well the formation of the transylvanian basin itself as on the relation between the basin formational processes operating in the pannonian basin and the contractional history of the east and south carpathian mountain belt in the middle miocene 17 14 12 myr the transylvanian basin underwent e w extension coeval with the first and major phase of overall e w extension affecting the pannonian basin and with submarine accretionary wedge tectonics in the east carpathian arc in the late miocene to early pliocene 12 8 6 myr the transylvanian basin experienced a strong pulse of e w oriented shortening coeval with the climax of compression in the east carpathians and a second rift phase affecting the pannonian basin continued contractional deformation affected the transylvanian basin during the pliocene and pleistocene with compression directions shifting to ese wnw directions

tectonics and structure of the queen charlotte fault zone haida gwaii and large thrust earthquakes abstract this article provides a summary of the structure and tectonic history of the queen charlotte transform fault zone off western canada as background to understanding the 2012 m w 7 8 thrust earthquake off haida gwaii there was margin subduction prior to the eocene the fault zone then became the mainly transcurrent pacific north america boundary there was mid tertiary oblique extension then 15 20 oblique convergence from 6 ma to the present that resulted in underthrusting and subduction initiation the total underthrusting has been too small for benioff wadati seismicity or arc volcanics but is indicated by 1 a trench the queen charlotte trough into which the oceanic plate bows downward and an offshore flexural bulge the oshawa rise 2 the queen charlotte terrace an accretionary sedimentary prism 3 seismic receiver function delineation of the underthrusting pacific plate 4 heat flow decreasing landward as predicted for underthrusting 5 low gravity offshore and high onshore consistent with underthrusting and 6 late tertiary uplift and erosion of the west coast of the islands oblique convergence is partitioned into nearly margin normal underthrusting i e m w 7 8 event relative to the terrace which is moving along the margin and margin parallel on the queen charlotte strike slip fault just off the coast that produced the 1949 m s 8 1 earthquake landward on the mainland global positioning system data suggest slow coast parallel shear with no historical seismicity the convergence rate decreases to the north of haida gwaii off dixon entrance but large thrust earthquakes are possible to the south underthrusting of the winona basin margin also could generate large earthquakes

water and geodynamics hydrogen is the most abundant element fig 1 in the galaxy and our solar system lodders 2003 therefore it is not astonishing that hydrogen is a key player in the geodynamic evolution of planets its fate in the early earth after condensation of the solar nebula the accretion of our planet and hydrogen reprocessing through early asteroidal and cometary bombardment dauphas et al 2000 and segregation of a proto earth into iron core and silicate mantle is described elsewhere in this volume marty and yokochi 2006 figure 1 abundance of elements in the solar system in numbers of atoms per 106 atoms of silicon lodders 2003 this chapter concerns itself with the geodynamics of the modern earth where nowadays hydrogen occupies only a very small mass fraction less than 0 1 wt although its abundance is thus drastically diminished it is still believed to have a profound influence on the geodynamics of the present planet the reason why hydrogen can have a profound influence on geodynamics even when it is diluted to ppm levels relies on its fundamental influence on both rock mechanics kohlstedt 2006 and the melting relationships kohn 2006 however both the micromechanical mechanism and the large scale implications are not yet fully understood in this review geodynamic processes which may be triggered by a rock mechanical hydrogen threshold value are reviewed in addition to resulting styles of planetary convection controlled by melt and rock mechanics this review will conclude with a more speculative discussion of the possible implications for planetary evolution from an early earth to the present day and highlight areas of future work following current practice in the literature the term water will be used interchangeably for hydrogen recent breakthroughs in computational geodynamics are throwing a new light on the possible evolution of planetary tectonics simulations

stratigraphy of the southern norfolk ridge and the reinga basin a record of initiation of tonga kermadec northland subduction in the southwest pacific abstract seismic stratigraphic interpretations of a large new 2d seismic reflection dataset from the reinga basin region northwest of new zealand constrain the history of cretaceous fragmentation of gondwana and cenozoic initiation of tonga kermadec northland subduction the southern norfolk ridge system lay in a proximal location to plate boundaries that were active during both cretaceous and cenozoic events and persistent marine conditions led to a relatively complete record of sedimentation cretaceous extension was followed by regional subsidence and transgression two cenozoic contractional events are separated by substantial 1 km subsidence late eocene contraction led to reverse faulting and folding between new caledonia and the southern norfolk ridge system and topographic highs created in the north western sector were locally eroded by wave abrasion reinga basin was located at the southern tip of this event which had only limited impact in adjacent northern new zealand regional oligocene and miocene subsidence was contemporaneous with late oligocene and early miocene emplacement of nappes in northern new zealand and the onset of arc volcanism early miocene contraction in the reinga basin led to formation of the wanganella ridge these events can be related to calculated plate motions and are shown to be consistent with models of induced subduction nucleation that require c 150 km of convergence the first contraction is associated with regional inception of tonga kermadec subduction whereas oligocene to miocene events and the onset of hikurangi northland subduction were in response to a local change in plate boundary displacement rate and consequent linkage between subduction north of new zealand with alpine fault formation farther south

field relations geochemistry and petrogenesis of the cretaceous basaltic jutal dykes kohistan northern pakistan a 75 ma cretaceous suite of low medium k basalt to basaltic andesite dykes intrudes the kohistan terrane between jutal and chalt gilgit area northern pakistan orientational data suggest they intruded in response to nw se to n s tension the dykes cut the albian aptian chalt volcanic group and the 102 ma matum das trondhjemite pluton they cut the fold structures and penetrative fabrics associated with the collision between kohistan and eurasia and thus constrain the age of formation of the northern suture the dykes define two groups on the basis of trace element geochemistry and geochemical trends a depleted d type group has low concentrations of incompatible elements 2 8 x primordial mantle and ce yb n ratios

early cambrian continental reconstructions climatically related sedimentary facies and faunal distributions have been combined with palaeomagnetic data to make provisional reconstructions of the early cambrian world laurentia baltica and siberia appear to have formed a continental group which rifted apart from each other prior to 600 ma the consolidation of much of gondwana probably occurred in the pan african orogeny well before the cambrian though the assembly of several east asian terranes is still speculative the archaeocyathan reefs of siberia southern europe and morocco with their bigotinid trilobite fauna suggest that these areas were adjacent to each other and at low latitudes avalonia had close faunal links with western gondwana but lacked bigotinids and archaeocyathans and may have been situated off west africa and florida during the early cambrian there was a northward migration of laurentia towards the equator as its separation from baltica and siberia increased and a southward movement of gondwana

temporal relationship between cratonization and glaciation the vendian early cambrian glaciation in western gondwana abstract cratonization has long been said to favour glaciation cratons would be ideal cradles for the development of continental ice caps in western africa the widespread continental glaciation developed near the proterozoic paleozoic boundary can no longer be considered as vendian according to the recent discovery of an early cambrian microfauna in eastern senegal it extends until atdabanian time 530 520 ma the age of its beginning lacks in precision it can be vendian or early cambrian in the hoggar fold belt it constitutes the lower part of molassic intermontane sequences where it is about 30 40 ma younger than the pan african cratonization dated at 600 ma this extensive cratonization closed the old long lived pan african brasiliano cycle 1000 600 ma and is responsible for the aggregation of west gondwana in the south american paraguay belt glacial sequences also postdate brasiliano cratonization dated at 590 ma they are probably vendian overlain by an ediacara like fauna and slightly older than those of western africa the glaciation was restricted to the shoulders of rifted basins initiating the young short lived pan african brasiliano cycle 600 or slightly younger to 520 ma world distribution of continents indicate that the widespread late vendian early cambrian ice cap glaciation of west africa developed at middle latitudes 50 30 whereas the probably vendian glaciation of the paraguay belt restricted to local highlands occupied high to middle latitudes 60 in western gondwana glaciation and cratonization occurring near the proterozoic cambrian boundary appear as distinct and temporary unrelated phenomena

subduction of the lithosphere and back arc dynamics insights from physical modeling physical modeling of the subduction of an elastico plastic lithosphere underlained by a low viscosity asthenosphere and driven by both horizontal compressional force and gravitational sinking of the subducting plate has shown the subduction regime to be strongly affected by 1 a where a and 1 are the asthenosphere and lithosphere densities at 0 a steady state process develops which requires a horizontal lithosphere compression of h 0 3 0 4 s 108 pa where s is the lithosphere yield limit the central part of the leading wedge of the overriding plate is nonisostatically elevated in this case especially in the region of the frontal arc which corresponds to a free air gravity anomaly maximum of 200 400 mgal a decrease in 0 compression of the overriding plate is considerably less from the onset of subduction and then decreases during subduction at some point compression changes into tension tensional nonhydrostatic stresses grow during the process to a value near h 0 1 0 2 s 0 4 108 pa both subsidence of the overriding wedge below the isostatic equilibrium level and an increase in the benioif dip angle accompany this process the gravity anomaly maximum is displaced into the volcanic arc in this subduction regime where it is normally less than 150 mgal if some weaknesses are present in the overriding plate tensional stresses result in a failure of the lithosphere at these locations and the broken lithosphere segment the arc plate starts to move counter to the subduction plate the arc plate slides along the underthrusting plate s surface without loss of contact and causes back arc opening hydrostatic suction prevents the plates from being separated this effect is caused by the peculiarity of a mechanical system whereby a low viscosity asthenosphere and a plastic lithosphere which effective yield limit is considerably lower than the hydrostatic stresses in the plate if the overriding lithosphere is not weakened enough to fail under the existing tension then the hanging slab breaks down under its own weight when it reaches a specific length both interacting plates undergo isostatic recovery in this case and the whole cycle is repeated again the hydrostatic suction effect also provides back arc tension under other conditions such as when asthenospheric flow forces the subducted plate to move away from the island arc

structure and tectonics of 85 e ridge from analysis of geopotential data abstract the bay of bengal bob sedimentary basin is one of the largest offshore sedimentary basins of the world having the thickest accumulation of sediments which mask the underlying crust and it is the geopotential data that offers the unique opportunity of seeing below the thick sediments satellite derived free air gravity fag data marine magnetic data bathymetry available seismic profiles and isopach maps over the bay of bengal are utilized in the present paper to throw light on the nature and tectonics of the bob with particular emphasis on the 85 e ridge combined analysis of gravity and magnetic data along six ew profiles of length varying from 325 to 440 km is undertaken incorporating seafloor spreading anomalies in the model of the several theories put forth for the evolution of the 85 e ridge the present study supports the sagging of the crust and rules out the hotspot trace and magmatic under plating theories the crustal model derived from the magnetic profile analysis suggests that the 85 e ridge does not extend to the north of 15 n latitude and this is supported by the euler solutions of the fag and the long wavelength filtered fag however the intermediate wavelength filtered fag suggests that the 85 e ridge extends up to the ocean continent boundary thereby suggesting that the 85 e ridge thins out dramatically to the north of 15 n latitude analysis of gravity data along 17 n latitude profile predicts that in the northern segment the 85 e ridge is a geo morphological feature within the sediments above the basement whereas to the south of 15 n latitude it is a feature within the oceanic crust below the basement further the reverse magnetization associated with the seafloor spreading anomalies lying on the 85 e ridge belongs either to the albian period or the anomaly a34 a33 the 85 e ridge would be younger than these seafloor spreading anomalies and would have formed due to horizontal compressional forces of the lithosphere preceding development of the subduction zone at the andaman trench

revised wonoka isotopic anomaly in south australia and late ediacaran mass extinction the global late ediacaran shuram wonoka carbon isotope anomaly has been regarded as the largest and longest known isotopic anomaly in the ocean assuming that all ediacaran carbonate is marine disregarding carbonate in south australia shown here to be palaeosol or palaeokarst the synchronous marine organic carbonate excursion is only 8 for 13 c organic and 6 for 13 c carbonate and lasted less than a million years this revised magnitude and duration is comparable with perturbations across the permian triassic boundary and correlative with a global late ediacaran acritarch mass extinction like permian triassic isotopic excursions the revised organic carbonate wonoka excursion may also have been a greenhouse palaeoclimatic warm spike which terminated valley incision and glacioeustatic drawdown during the mid ediacaran fauquier glaciation and preceded chill of the late ediacaran billy springs glaciation supplementary material measured sections and tables of mineral and grain size proportions major element and stable isotope analyses are available at www geolsoc org uk sup18756

a missing link in the peri gondwanan terrane collage the precambrian basement of the moroccan meseta and its lower paleozoic cover this article provides stratigraphic and geochronological data from a central part of gondwana s northern margin the moroccan meseta domain this region located to the north of the anti atlas area with extensive outcrops of precambrian and lower paleozoic rocks has hitherto not received much attention with regard to its precambrian geology detrital and volcanic zircon ages have been used to constrain sedimentary depositional ages and crustal affinities of sedimentary source rocks in stratigraphic key sections based on this a four step paleotectonic evolution of the meseta domain from the ediacaran until the early ordovician is proposed this evolution documents the transition from a terrestrial volcanic setting during the ediacaran to a short lived carbonate platform setting during the early cambrian the latter then evolved into a rifted margin with deposition of thick siliciclastic successions in graben structures during the middle to late cambrian the detritus in these basins was of local origin

models of lithospheric scale deformation during plate collision effects of indentor shape and lithospheric thickness abstract analogue models designed to test the effects of lithosphere thickness and indentor shape during continental collision indicate that strength and density differences between lithosphere types control structural vergence during collision models presented here include layers of putty and sand to form four three and two layer lithosphere types craton arc and oceanic equivalent respectively which collide during unidirectional shortening in most cases the stronger thicker four layer lithosphere does not deform but simply acts to bulldoze weaker material aside deformation concentrated in the thinnest layers consistently verges away from the indenting continent irrespective of indentor shape lateral strike slip component deformation occurs in the early stages of most models even with simple unidirectional shortening and is enhanced by highly oblique indentor shapes the effects of extrusion during collision tested by repeating models with and without a free edge which allows extrusion consistently indicate that with the ability to extrude structures are smaller and more numerous without extrusion fewer larger structures tend to develop two geological examples from the precambrian part of the tasman orogenic belt and the modern day collision at the leading edge of australia illustrate aspects of the models

structure and origin of the 85 e ridge the submerged 85 e ridge in the bay of bengal trends approximately n s between 19 n and 6 n latitudes off the southeast coast of sri lanka it takes an arcuate shape and seems to terminate with the northward extension of the afanasy nikitin seamounts situated around 2 s latitude the ridge is characterized by positive magnetic 100 400 nt and negative free air gravity 60 mgal anomalies with variable widths of 100 180 km magnetic model studies revealed that the rocks of the 85 e ridge are magnetized with reversed polarity the well defined geophysical anomalies and lack of magnetic polarity reversals together with the deep burial nature of the ridge may not favor a hotspot origin two alternative processes for the ridge emplacement have been suggested ridge emplacement may be 1 due to shearing of the lithosphere caused by stretching and compressional forces associated at the time of major plate reorganization immediately after the evolution of the early cretaceous crust in the bay of bengal more precisely at m0 isochron or during the middle albian reversals within the cretaceous long normal polarity k t superchron epoch when the earth s magnetic polarity changed from normal to reversed polarity and or 2 due to sagging followed by deformation produced by the buckling instability of the oceanic plate caused by horizontal compressional forces on the passive continental margin however more marine geophysical data are required to support the postulated coincidence of the ridge with a reversed polarity magnetic anomaly and their associated model further the rajmahal traps normally polarized and the 85 e ridge reversely polarized appear to be associated with two different episodes of eruption that might have been triggered by the kerguelen mantle plume the 85 e ridge seems to extend into the onshore west bengal basin as a subsurface ridge and merges with the reported nne ssw trending zone of strong geophysical anomalies east of rajmahal traps up to 25 n latitude

early paleozoic avalon gondwana unity an obituary response to palaeontological evidence bearing on global ordovician silurian continental reconstructions by r a fortey and l r m cocks

an experimental study of subduction and slab migration results of fluid dynamical experiments on the behavior of subducted slabs are presented that address two important characteristics of subduction slab penetration through the transition zone and horizontal slab migration cold negatively buoyant molded slabs of concentrated sucrose solution with viscosities of 3 5 106 p were introduced into a more dilute two layered sucrose solution representing the upper and lower mantle the transition zone was modeled by a step increase in both density and viscosity ratios of slab thickness to upper layer depth the relative viscosities in each layer and the effective rayleigh number were close to mantle values the initial configuration consisted of two plates separated by a trench gap with one plate attached to a dipping slab simulating a developing subduction zone with an overriding plate two different boundary conditions were used at the trailing ridge end of the subducting plate 1 fixed end corresponding to zero ridge motion and 2 free end corresponding to zero ridge resistance based on results from 15 experiments with various combinations of upper layer lower layer and slab densities u l s and viscosities we find that slab penetration depth is sensitive to both the normalized slab density anomaly r s l s u and the dip angle three penetration regimes are recognized r 0 5 in which the slab sinks into the lower layer without distortion 0 2 r 0 5 which permits limited penetration the amount dependent on dip angle in this regime the slab develops a thick root just beneath the transition zone and r 0 2 in which the slab is deflected by the transition and little or no penetration occurs retrograde slab motion and trench migration occurred in nearly every case with the overriding plate moving parallel to but faster than the trench causing the gap between subducting and overriding plates to close with time prograde trench motion was never observed and the trench remained stationary only in the extreme case free ridge condition with equal upper and lower layer densities these results suggest that the transient extension found in many back arc basins may be a direct consequence of the tendency for retrograde subduction and the amount of extension may be governed in part by the degree of slab penetration through the transition zone

late ordovician glaciation initiated by early land plant evolution and punctuated by greenhouse mass extinctions abstractlate ordovician hirnantian glaciation indicated by periglacial paleosols tillites and glacial pavements in saharan africa has been attributed to advances in weathering and carbon sequestration due to evolution of early land plants in the same way that devonian permian glaciation has been attributed to evolution of forests and quaternary glaciations to evolution of grasslands two problems for carbon cycle explanations of this glaciation are an hirnantian co2 greenhouse estimated at 4035 ppmv from mass balance models and possibly related hirnantian mass extinctions this estimate of high co2 is a 10 m yr average here evaluated against high resolution sequences of paleosols that give estimates not only of co2 but also of paleoclimate and vegetation through the late ordovician katian paleosols of the eastern united states show increases in depth and in degree of carbon consumption and storage comparable with those preceding glaciation of the devonian permian and quaternary early thalloid l

first evidence for cambrian glaciation provided by sections in avalonian new brunswick and ireland additional data for avalon gondwana separation by the earliest palaeozoic abstract the first evidence for cambrian glaciation is provided by two successions on the avalon microcontinent the middle lowest cambrian middle terreneuvian series and fortunian stage stage 2 boundary interval has an incised sequence boundary overlain by a fluvial lowstand facies and higher olive green marine mudstone on hanford brook southern new brunswick this succession in the lower mystery lake member of the chapel island formation may be related to melting of an ice sheet in avalon the evidence for this interpretation is a muddy diamictite with outsized up to 10 cm in diameter proterozoic marble and basalt clasts that penetrated overlying laminae in the marine mudstone that eustatic rise was associated with the mudstone deposition is suggested by an approximately coeval rise that deposited sediments with watsonella crosbyi zone fossils 650 km away in avalonian eastern newfoundland a sea level rise within the watsonella crosbyi chron at ca 535 ma may correspond to a unnamed negative 13 c excursion younger than the basal cambrian excursion bace and the zhuce excursion in stage 2 of the upper terreneuvian series cambrian dropstones are now also recognized on the northern gander margin of avalon in continental slope rise sedimentary rocks in southeast ireland although their age early middle cambrian is poorly constrained dropstones in the booley bay formation provide additional evidence for cambrian glaciation on the avalon microcontinent besides providing the first evidence of cambrian glaciation these dropstone deposits emphasize that avalon was not part of or even latitudinally close to the terminal ediacaran cambrian tropical carbonate platform successions of west gondwana

cambrian evolutionary radiation context correlation and chronostratigraphy overcoming deficiencies of the first appearance datum fad concept abstract use of the first appearance datum fad of a fossil to define a global chronostratigraphic unit s base can lead to intractable correlation and stability problems fads are diachronous they reflect species evolutionary history dispersal biofacies preservation collection and taxonomy the cambrian evolutionary radiation is characterised by diachronous fads biofacies controls and provincialism of taxa and ecological communities that confound a stable lower cambrian chronostratigraphy cambrian series and stage definitions require greater attention to assemblage zone successions and non biostratigraphic particularly carbon isotope correlation techniques such as those that define the ediacaran system base a redefined basal cambrian trichophycus pedum assemblage zone lies above the highest ediacaran type biotas vendobionts putative metazoans and calcareous problematica such as cloudina and the basal asteridium tornatum comasphaeridium velvetum zone acritarchs this definition and the likely close correspondence of evolutionary origin and local fad of t pedum preserves the fortune head newfoundland gssp of the cambrian base and allows the presence of sub cambrian branched ichnofossils the sub tommotian equivalent base of stage 2 a suggested laolinian stage should be defined by the i l4 zhuce 13 c positive peak bracketed by the lower ranges of watsonella crosbyi and aldanella attleborensis molluscs and the skiagia ornata fimbrioglomerella membranacea zone acritarchs the w crosbyi and a attleborensis fads cannot define a stage 2 base as they are diachronous even in the newfoundland type w crosbyi zone the series 2 base cannot be based on a species fad owing to the provincialism of skeletalised metazoans in the terreneuvian series 2 boundary interval and global heterochrony of the oldest trilobites a series 2 and stage 3 a suggested lenaldanian series and zhurinskyan stage new gssp base is proposed at the siberian lower atdabanian 13 c iv peak which correlates into south china avalonia and morocco and assigns the oldest trilobites to the terminal terreneuvian series

role of lateral mantle flow in the evolution of subduction systems insights from laboratory experiments summary we present 3 d laboratory experiments constructed to investigate the pattern of mantle flow around a subducting slab under different boundary conditions in particular we present a set of experiments characterized by different conditions imposed at the trailing edge of the subducting plate that is plate fixed in the far field plate detached in the far field imposed plate motion experiments have been performed using a silicone slab floating inside a honey tank to simulate a thin viscous lithosphere subducting in a viscous mantle for each set we show differences between models that do or do not include the possibility of out of plane lateral flow in the mantle by varying the lateral boundary conditions our results illustrate how a subducting slab vertically confined over a 660 km equivalent depth can be influenced in its geometry and in its kinematics by the presence or absence of possible lateral pathways on the basis of these results we show implications for natural subduction systems and we highlight the importance of suitable simulations of lateral viscosity variations to obtain a realistic simulation of the history of subduction

regional stress field of the indian plate we have calculated the regional stress field in the indian plate implementing dependence of slab pull and ridge push on the age of the oceanic lithosphere in the finite element procedure the high level of the calculated stress field order of a few kbar and the dominance of compression in the plate are consequences of the unique dynamic situation of the present day indian plate the calculated stress field explains the concentration of intraplate deformation in the region of the ninetyeast ridge and yields insight into the variations in stress directions in the australian continent

early subduction related orogeny in the northern andes turonian to eocene magmatic and provenance record in the santa marta massif and rancheria basin northern colombia terra nova 23 26 34 2011 abstract the timing of orogeny in the northern andes and the mechanism driving it are still debated we have studied the age composition and provenance of granitoids and sandstones of the santa marta massif and rancheria basin northern colombia to relate deep seated and surface tectonic processes attending the late cretaceous palaeogene history of the northern andes our results indicate the development of five tectonic episodes 1 collision of northwestern south america with a 92 80 ma caribbean arc 70 ma 2 late collisional to early subduction metamorphism and magmatism 65 ma 3 distal accumulation of a thick siliciclastic sequence 60 58 ma 4 renewed arc magmatism 58 50 ma and 5 magmatic quiescence and block uplift post 50 ma the first episodes are related to the onset of subduction and the last episode is related to shallow subduction and oblique convergence similar events in colombia and ecuador reveal that the late cretaceous eocene orogeny of the northern andes was influenced by the collision and subduction of the caribbean oceanic plate

lower cambrian apatitic stromatolites and phospharenites related to the glacio eustatic cratonic rebound sahara algeria abstract at the end of the early cambrian glacial event occurring on the west african craton the tillite surface was marked especially in the northeastern part of the taoudenni basin by noticeable topographic relief after deglaciation the eustatic rise in sea level led to deposition over all the west african craton of a thin but widespread carbonate unit containing lower cambrian shelly fossils at the ice cap periphery isostatic rebound of the craton resulted in uplift of the topographic highs and weathering of the carbonate during fluctuations of relative sea level before the return to marine transgressive conditions a succession of events took place first in the submerged areas stromatolitic phosphorite in flat domal to columnar buildups accreted followed by growth of dolomitic ministromatolites laterally phospharenitic beds oncolites and phosphatic grains accumulated under water in the depressions the stromatolites and oncolites are made of carbonate fluorapatite characteristic of coastal environments glauconite is closely related to apatite and suggests conditions of a sea level rise in respect to phosphatization precipitation of dolomite calcite and evaporite postdates phosphatization and glauconitization indicating a regression and a chemical concentration due to a warm and perhaps arid climate apatitic stromatolites and oncolites resulted from in situ precipitation within bacterial mats this phosphogenesis corresponds to the major early paleozoic phosphogenetic event

plate tectonic models for indian ocean intraplate deformation abstract the equatorial region of the conventionally defined indo australian plate has long been recognized as containing a type example of intense intraplate deformation we trace the development of tectonic models for the area to illustrate techniques for the analysis of such deformation the identification of anomalous seismicity near the ninetyeast and chagos laccadive ridges demonstrated the existence of the deformation focal mechanisms from recent and historic earthquakes showed strike slip motion occurring along the ninetyeast ridge seismic moment data allowed the rate to be estimated similar studies showed north south tension in the chagos bank region and north south compression in the region between the ninetyeast and chagos ridges global plate motion studies indicated non closure of the indian ocean triple junction suggesting the conventional plate geometry was inadequate for a rigid plate description of the area gravity and marine geophysical data indicated intense north south compressional deformation south of the bay of bengal these observations are reconciled by a plate motion model in which australia and india lie on distinct plates divided by a boundary that intersects the central indian ridge near the equator in this model arabia usually considered a separate plate has negligible motion relative to india the resulting euler vector for australia relative to indo arabia lies just east of the central indian ridge and predicts approximately 0 5 1 5 cm yr compression in the central indian basin and 1 5 2 cm yr strike slip motion along the northern ninetyeast ridge consistent with the seismological and geophysical data in contrast to conventional oceanic plate boundaries the boundary deformation is distributed over a wide zone this diffuse nature may reflect either the boundary s recent inception or slow rate of motion analysis of seismicity and deformation in the boundary zone should offer insights into the mechanics of its development and its implications for the evolution of plate boundaries

obduction why how and where clues from analog models obduction is an odd geodynamic process characterized by the emplacement of dense oceanic ophiolites atop light continental plates in convergent settings we herein present analog models specifically designed to explore the conditions i e sharp increase of plate velocities herein coined as acceleration slab interaction with the 660 km discontinuity ridge subduction under which obduction may develop as a result of subduction initiation the experimental setup comprises an upper mantle modeled as a low viscosity transparent newtonian glucose syrup filling a rigid plexiglas tank and high viscosity silicone plates convergence is simulated by pushing a piston with plate tectonics like velocities 1 10 cm yr onto a model comprising a continental margin a weakness zone with variable resistance and dip w an oceanic plate with or without a spreading ridge a preexisting subduction zone s dipping away from the piston and an upper active continental margin below which the oceanic plate is being subducted at the start of the model as for the neotethyan natural example several configurations were tested over thirty five parametric models with special emphasis on comparing different types of weakness zone and the degree of mechanical coupling across them measurements of displacements and internal deformation allow for a precise and reproducible tracking of deformation models consistently demonstrate that once conditions to initiate subduction are reached obduction may develop further depending on the effective strength of w results 1 constrainthe range of physical conditions required for obduction to develop nucleate and 2 underline the key role of such perturbations for triggering obduction particularly plate acceleration they provide an explanation to the short lived peri arabic obduction which took place along thousands of km almost synchronously within 50 10 myr from turkey to oman while the subduction zone beneath eurasia became temporarily jammed they also demonstrate that the emplacement of dense oceanic material on continental lithosphere is not a mysterious process requiring extraordinary boundary conditions but results from large scale normal oceanic then continental subduction processes

periodic instabilities during compression of the lithosphere 2 analogue experiments we have modeled the behavior of the continental and oceanic lithospheres under compression using materials with analogous properties in laboratory experiments to study the development of lithospheric buckling periodic instabilities which are a major deformation process during the compression of the lithosphere have already been described by several authors using an analytical perturbation method at small strains analogue experiments corroborate most of the results obtained by the perturbation method 1 the deformation modes geometrical relationships of interfaces and related wavelengths are mainly dependent on the spatial distribution of the brittle layer s and 2 the amplitude of buckling is an exponential function of the horizontal strain some departure from the perturbation method occurs when there are two instabilities growing concurrently the breakdown of the exponential growth occurs for strains of about 5 and is concomitant with the appearance of thrust faults in experiments including one brittle layer which model the compression of the oceanic lithosphere faults are regularly located at the inflection points of the folds in experiments including two brittle layers which model continental lithosphere faults form more complicated patterns with an asymmetrical deep thrust overlain by a fan shaped symmetrical thrust system in the upper brittle layer such fault geometries give some new highlights on typical compressive geological structures such as those encountered in central asia

from subduction to collision control of deep processes on the evolution of convergent plate boundary 1 using laboratory experiments we investigate the dynamics of the collisional process that follows the closure of an oceanic basin the evolution of these experiments systematically shows four successive episodes of deformation which correspond to 1 the initiation of oceanic subduction 2 a mature period of oceanic subduction 3 an episode of continental subduction during which the trench absorbs all the convergence and the superficial tectonic regime does not change significantly within the continental plates and 4 continental collision that starts when the trench locks and convergence is absorbed by a system of thrust faults and folds we observe that the amount of continental material that subducts before the onset of collision depends on the slab pull exerted by the subducted oceanic lithosphere the slab pull force in turn depends on the amount of subducted oceanic material on the thickness of the convective mantle and also on the rheology of the slab our experiments indeed suggest that parts of the oceanic slab may separate from the superficial slab to sink rapidly in the mantle decreasing the slab pull level and triggering the rapid onset of collision we observe two possible modes of slab deformation slab break off and development of viscous instabilities we define two dimensionless rheological numbers to characterize the possible occurrence of these modes of deformation in all cases oceanic closure is followed by episode of continental subduction during which the continental lithosphere may reach depths varying between 50 and 450 km prior to the onset of continental collision

seafloor spreading magnetic anomalies south off sri lanka abstract results obtained from compilation and reinterpretation of about 21 200 line km of bathymetry magnetic and satellite gravity data between 10 s to 10 n latitudes and 75 to 90 e longitudes south off sri lanka are presented here magnetic data and the synthetic seafloor spreading model reveal the presence of mesozoic anomaly sequence m11 through m0 south of sri lanka the oldest magnetic anomaly m11 134 ma occurs between 110 and 140 km away from the sri lankan coast the seafloor created during the early cretaceous is estimated to have evolved with variable half spreading rates ranging from 5 5 to 1 53 cm yr the trends of the fracture zones inferred from the offsets in the magnetic anomalies have been constrained using the satellite gravity mosaic the cretaceous magnetic quiet zone cmqz 121 84 ma crust between the isochrons m0 and a34 has an unequal width and widens from about 170 km in the west 80 e longitude to about 500 km towards east 85 e longitude plate reconstruction models for 160 fit 134 m11 121 m0 and 84 ma a34 are generated under the constraints of newly identified magnetic anomaly isochrons and fracture zones the mismatch in the a34 reconstruction is attributed to the emplacement of the ob lena and marion dufresne seamounts at the spreading ridge axis and or the frequent ridge jumps in the middle cretaceous during the major plate reorganization

physical modelling of lithosphere subduction in collision zones abstract physical mechanical modeling of continental collision is performed with a 4 layered mantle model compositionally distinct layers of hydrocarbons are used to model a lithosphere which consists of two crustal layers the upper one quasi brittle the lower one plastic and a plastic mantle layer the lower crustal layer the crustal asthenosphere has a low yield limit a low viscosity liquid water represents the asthenosphere under the lithosphere lateral compression of the model plate by a piston is the driving mechanism of collision similarity criteria are satisfied in the modeling the collisional process was investigated with various boundary conditions and values of the model parameters the yield limit and the thickness of the crustal asthenosphere were varied as well as the action of the piston on the plate the force was applied either throughout the plate section or on its mantle layer only in the latter case this layer slipped relative to the upper crust hence different results were obtained a more realistic pattern seems to occur when the yield limit of the crustal asthenosphere is not too small and free sliding of the crustal layers over the mantle lithosphere is not possible the upper crustal layer then experiences failure in the collision zone and thrusting develops there intensive thickening of the lower crust was observed the mantle layer being subducted like oceanic lithosphere in a subduction zone

strain partitioning and interplate friction in oblique subduction zones constraints provided by experimental modeling physical modeling of oblique subduction is performed to study the mechanism of strain partitioning the model is two layer and includes the elasto plastic lithosphere the overriding and subducting plates and the low viscosity liquid asthenosphere the subduction is driven by a push force from a piston and a pull force when the density contrast ap between the subducting plate and the asthenosphere is positive we vary both ap and the interplate friction frictional stresses slip partitioning is obtained only in the models with high interplate friction and only when the overrid ing plate contains a weak zone this zone in the models corresponds either to locally thinned litho sphere or to cut fault the horizontal trench normal component of the interplate friction force ffh can be comparable with the absolute value of the horizontal component of the nonhydrostatic interplate pressure force fph in the subduction zone ffh is always negative compression while fph can be either negative compressional subduction regime or positive extensional regime high friction which promotes partitioning can completely cancel the extensional suction force fph back arc tension and strike slip faulting appear thus as conflicting processes although they can coexist in the same subduction zone depending on the relative values of relevant forces it ap pears that high friction can exist only in compressional subduction zones where partitioning should develop more easily this conclusion is supported by the comparison of two oblique subduction zones having similar geometry the compressional southern kurile zone strong partitioning and extensional southern ryukyu zone no lithospheric scale partitioning other factors controlling the strain partitioning are the length of the oblique subductlon zone the boundary conditions at the transverse limits of the forearc sliver and of course the obliquity of subduction