convective interactions between oceanic lithosphere and asthenosphere influence of a transform fault abstract current estimates of the asthenospheric viscosity favor the occurrence of small scale convection ssc beneath oceanic lithosphere of sufficient age since transform faults tfs juxtapose lithospheres of distinct ages and thicknesses they are likely to influence their dynamics in this paper we describe 3d numerical models of oceanic spreading for two ridge segments offset by a tf in domains that are sufficiently large for ssc to fully develop a framework based on thermal convection with temperature dependent viscosity and both internal and basal heating is adopted and parameters such as surface velocity age offset at the tf activation energy mantle viscosity and internal heating rate are explored in our numerical experiments the onset time for ssc is identical on each side of the fracture zone fz it is not modified by the presence of a tf moreover scaling relationships for the onset time obtained in 2d experiments remain valid earlier studies focused on the influence of a tf based on 2d experiments corresponding to slices parallel to the ridge axis they noted the rapid development of a sinking thermal instability located at the step in the lithospheric relief our results indicate that the wake of the tf is characterized by the development of a lateral edge driven buoyant flow from the younger thinner side to the older thicker side of the fz however due to the global shearing context this instability does not sink before the onset of ssc characterized by small scale instabilities on each side of the fz as observed in earlier studies the essentially 3d velocity field combining this lateral flow and the prescribed surface spreading velocity results in an angle formed by the tf trail with the spreading direction we describe the parameters controlling this angle in our experiments and propose a scaling relationship in oceanic settings the specific geometry of the ridge axis segment length orientation of the tfs may affect this angle in addition a large scale mantle flow of other geodynamical origin could interact with the spreading flow and also modify the tf trail angle this departure at depth of the boundary between the thick and thin lithospheres from the surface fz may explain the presence of oceanic volcanoes on the older side of the fz usually considered as thicker

contrasted styles of rifting in the eastern gulf of aden a combined wide angle multichannel seismic and heat flow survey continental rifts and passive continental margins show fundamental along axis segmentation patterns that have been attributed to one or a number of different processes extensional fault geometry variable stretching along strike preexisting lithospheric compositional and structural heterogeneities oblique rifting and the presence or absence of eruptive volcanic centers the length and width scales of the rift stage fault bounded basin systems change during the late evolution of the new plate boundary and the role of magmatism may increase as rifting progresses to continental rupture along obliquely spreading ridges first order mid ocean ridge geometries originate during the synrift stage indicating an intimate relationship between magma production and transform fault spacing and location the gulf of aden rift is a young ocean basin in which the earliest synrift to breakup structures are well exposed onshore and covered by thin sediment layers offshore this obliquely spreading rift is considered magma poor and has several large offset transforms that originated during late stage rifting and control the first order axial segmentation of the spreading ridge widely spaced geophysical transects of passive margins that produce only isolated 2 d images of crust and uppermost mantle structure are inadequate for evaluation of competing rift evolution models using closely spaced new geophysical and geological observations from the gulf of aden we show that rift sectors between transforms have a large internal variability over short distances 10 km the ocean continent transition oct evolves from a narrow magmatic transition to wider zones where continental mantle is probably exhumed we suggest that this small scale variability may be explained 1 by the distribution of volcanism and 2 by the along strike differences in time averaged extension rate of the oblique rift system the volcanism may be associated with 1 the long offset alula fartak fracture zone which may enhance magma production on its younger side or 2 channeled flow from the afar plume material along the newly formed oct and the spreading ridge oblique extension and or hot spot interactions may thereby have a significant control on the styles of rifting and continental breakup and on the evolution of many magma poor margins

fault reactivation and rift localization northeastern gulf of aden margin 1 the gulf of aden and the sheba spreading ridge gulf of aden forms the southern boundary of the arabian plate its orientation 075 e and its kinematics about 030 e divergence are interpreted as the result of an oblique rifting in this contribution a field study in the northeastern gulf of aden allows us to confirm the oligo miocene synrift directions of extension and to precise the normal fault network geometry the synrift extensions are 020 e and 160 e possibly in this chronological order the normal faults strike 070 e 090 e and 110 e the results show that some characteristics are consistent with oblique rifting analogue models while some others are not especially fault reactivation of mesozoic structures is shown to have occurred significantly at the beginning and during rifting these data are therefore compared to analogue models of oblique reactivation and this comparison demonstrates that fault reactivation played a key role during the early stage of the gulf of aden rifting finally scenarios of the lithospheric evolution during the eastern gulf of aden opening preexisting weaknesses in the lithosphere or not are discussed to better constrain the deformation history of the northern margin especially we show that rift localization processes may imply stress rotations through time

rheological properties of water ice applications to satellites of the outer planets 1 abstract the icy moons of the outer solar system have not been quiescent bodies in part because many have a substantial water component and have experienced significant internal heating we can begin to understand the thermal evolution of the moons and the rate of viscous relaxation of surface topography because we now have good constraints on how ice in several of its polymorphic forms flows under deviatoric stress at planetary conditions details of laboratory derived flow laws for pure polycrystalline ice are reviewed in detail one of the more important questions at hand is the role of ice grain size grain size may be a dynamic quantity within the icy moons and it may or may not significantly affect rheology one recent beneficiary of revelations about grain size sensitive flow is the calculation of the rheological structure of europa s outer ice shell which may be no thicker than 20 km

evidence for ice flow prior to trough formation in the martian north polar layered deposits the relative importance of surface mass fluxes and ice flow in shaping the north polar layered deposits npld now or in the past remains a fundamental and open question motivated by observation of an apparent ice divide on gemina lingula also known as titania lobe we propose a two stage evolution leading to the present day topography on that lobe of the npld ice flow approximately balances surface mass fluxes in the first stage but in the second stage ice flow has minimal influence and topography is modified predominantly by the formation of troughs we focus here on evidence for the first stage by testing the fit of topography between troughs to an ice flow model we find that independent model fits on distinct flow paths closely match inter trough topography uniformly over a broad region on gemina lingula with mutually consistent and physically reasonable fitting parameters however our model requires ice to occupy and flow in spaces where troughs currently incise the ice we therefore infer that the troughs and the distribution of mass balance that caused them post date deposition of the inter trough material and its modification by flow because trough formation has apparently altered inter trough topography very little we infer that trough formation must have been rapid in comparison to the still unknown time scale of flow since troughs began to form we view the evidence for past flow as strong but we do not think that topographic evidence alone can be conclusive observations of englacial stratigraphy using orbital sounding radars will yield conclusive tests of our inferred mechanism for the formation of inter trough topography

deflection of dykes into sills at discontinuities and magma chamber formation abstract many rift zones contain composite volcanoes central volcanoes most of which are supplied with magma from shallow crustal magma chambers volcanotectonic studies of eroded rift zones such as in iceland as well as geophysical studies indicate that many shallow magma chambers evolve from sills here i provide field description of sills and extinct magma chambers in the quaternary and tertiary palaeorift zones of iceland focusing on the felsic pluton of slaufrudalur southeast iceland as a fine example of a magma chamber developed through the injection of sills in this paper i review analyse and develop further two related mechanisms for the deflection of dykes into sills and thus the potential initiation of a magma chamber namely a cook gordon debounding delamination and b stress barriers and propose a third mechanism c favourable material toughness ratios due to elastic mismatch difference in young s moduli or stiffnesses of layers in contact in the cook gordon mechanism a weak contact opens up as a result of dyke induced tensile stress this mechanism is likely to operate primarily at shallow depths as is supported by field observations and numerical models a stress barrier is a layer where the local stress is unfavourable to a particular type of rock fracture here a dyke field observations and numerical models show that on meeting a stress barrier a dyke either changes into a sill or becomes arrested the material toughness mechanism indicates that when the upper layer at a contact has the same or less stiffness than the lower layer hosting the dyke there is little tendency for the dyke to become deflected into the contact however when the upper layer is stiffer such as when a stiff basaltic lava flow is on the top of a soft pyroclastic layer the dyke tends to become deflected into the contact to form a sill field results suggest that all these mechanisms may cause dyke deflection into sills and may operate together particularly where the rock consists of alternating soft and stiff layers since the quaternary lava pile in iceland has many more soft hyaloclastite layers than the tertiary lava pile these mechanisms were probably very efficient in generating sills and by implication shallow magma chambers during the quaternary this may be one reason why in comparison with the tertiary lava pile the quaternary lava pile contains so many sills and extinct plutons and active shallow magma chambers

formation and mechanics of magma reservoirs in iceland summary provided magma flow in a partially molten layer follows darcy s law of flow in porous media it can be demonstrated that magma in the proposed magma layer beneath iceland flows towards regions of reduced crustal thickness in these regions magma accumulates to form partially molten magma reservoirs which are thus located in the magma layer these reservoirs either give rise to surface eruptions or feed totally molten shallow magma chambers located in the crust that may erupt it is argued that the melt fraction varies throughout a reservoir being highest near the top where the reservoir may be totally molten but that the average is 0 25 it is concluded that for individual eruptions the reservoirs behave on the whole as a poroelastic material it is suggested that formation of crustal magma chambers is facilitated by the occurrence of stress barriers that lead to the formation of thick sills while liquid such sills absorb the magma of all dykes that enter them and may evolve into magma chambers ideal sites for stress barriers and thus for magma chambers are formations where individual layers have different elastic properties i e where the crust behaves as a multilayer denoting the bulk volume of the reservoirs by vb the volume of the crustal chambers by v and the volume of lava and dyke material in individual eruptions by ve it is found that vb 5365ve and v 1850ve using the estimated volumes of exposed plutons as a basis it is found that provided the feeder reservoir does not participate in the eruption the volume of individual eruptions from elastic crustal magma chambers in iceland is less than 0 25 km3 usually less than 0 1 km3 and that typical central volcano eruptions should be about 0 02 km3 for eruptions of the order of 1 km3 or larger the reservoirs must supply magma during the eruption it is concluded that from the point of view of volume historical eruptions of typical size could have come from elastic crustal magma chambers

central gulf of aden conjugate margins yemen somalia tectono sedimentary and magmatism evolution in hybrid type margins abstract magma rich and magma poor passive margins are generally studied separately yet the spatial evolution from one type to another is not well understood central gulf of aden margins are at the transition between these two types of margins based on new seismic data published wells and gravity data we determine seismic stratigraphy and structural patterns we map the distinct crustal domains continental ocean continent transition oct oceanic domains and propose a tectonic evolution of the yemeni and somalian conjugate margins in an oblique rifting context the most striking results are the significant segmentation narrowness and asymmetry of conjugate margins as well as the gradual eastward decrease in the intensity of magmatic activity of these hybrid type margins west of bosaso hami fracture zone bhfz the hami ahl medo segment presents magmatic type margin features in the distal yemeni margin related to the strong influence from the afar plume seaward dipping reflectors develop since the late syn rift stage 21 18 ma near the xiis al mukalla fracture zone conversely east of bhfz the bosaso sayhut margins segment is characterized by magma poor margins for which the exhumation stage is characterized by the unroofing of mantle along multiple detachment faults in the oct and shortly followed by diachronous volcanism 18 ma the central gulf of aden is affected by a major uplift during the final syn rift stage and up to oct formation this event is associated with the formation of erosional surfaces in the east and with gravitational deformation taking place on decoupling surfaces at the base of the syn rift sequence lower ghaydah fm that lead to the sliding of syn rift units on top of the exhumed mantle rocks in the oct

structure and evolution of the eastern gulf of aden insights from magnetic and gravity data encens sheba md117 cruise summary magnetic and gravity data gathered during the encens sheba cruise 2000 june in the eastern gulf of aden provide insights on the structural evolution of segmentation from rifted margins to incipient seafloor spreading in this study we document the conjugate margins asymmetry confirm the location of the ocean continent transition oct previously proposed by seismic data and describe its deep structure and segmentation in the oct gravity models indicate highly thinned crust while magnetic data indicate presence of non oceanic high amplitude magnetic anomalies where syn rift sediments are not observed thus the oct could be made of ultra stretched continental crust intruded by magmatic bodies however locally in the north the nature of the oct could be either an area of ultra slow spreading oceanic crust or exhumed serpentinized mantle between the alula fartak and socotra fracture zones the non volcanic margins and the oct are segmented by two n027 e trending transfer fault zones these transfer zones define three n110 e trending segments that evolve through time the first evidence of oceanic spreading corresponds to the magnetic anomaly a5d and is thus dated back to 17 6 ma at least reconstruction of the spreading process suggests a complex non uniform opening by an arc like initiation of seafloor spreading in the oct the early segmentation appears to be directly related to the continental margin segmentation the spreading axis segmentation evolved from three segments 17 6 to 10 95 ma to two segments 10 95 ma to present at the onset of the spreading process the western segment propagated eastwards thus reducing the size of the central segment the presence of a propagator could explain the observed spreading asymmetry with the northern flank of the sheba ridge being wider than the southern one

source rock characteristics and hydrocarbon generation modelling of upper cretaceous mukalla formation in the jiza qamar basin eastern yemen abstract the upper cretaceous mukalla coals and other organic rich sediments which are widely exposed in the jiza qamar basin and believed to be a major source rocks were analysed using organic geochemistry and petrology the total organic carbon toc contents of the mukalla source rocks range from 0 72 to 79 90 with an average toc value of 21 50 the coals and coaly shale sediments are relatively higher in organic richness consistent with source rocks generative potential the samples analysed have vitrinite reflectance in the range of 0 84 1 10 ro and pyrolysis t max in the range of 432 454 c indicate that the mukalla source rocks contain mature to late mature organic matter good oil generating potential is anticipated from the coals and coaly shale sediments with high hydrogen indices 250 449 mg hc g toc this is supported by their significant amounts of oil liptinite macerals are present in these coals and coaly shale sediments and py gc s 2 pyrograms with n alkane alkene doublets extending beyond nc 30 the shales are dominated by type iii kerogen hi one dimensional basin modelling was performed to analysis the hydrocarbon generation and expulsion history of the mukalla source rocks in the jiza qamar basin based on the reconstruction of the burial thermal maturity histories in order to improve our understanding of the of hydrocarbon generation potential of the mukalla source rocks calibration of the model with measured vitrinite reflectance ro and borehole temperature data indicates that the present day heat flow in the jiza qamar basin varies from 45 0 mw m 2 to 70 0 mw m 2 and the paleo heat flow increased from 80 ma to 25 ma reached a peak heat flow values of approximately 70 0 mw m 2 at 25 ma and then decreased exponentially from 25 ma to present day the peak paleo heat flow is explained by the gulf of aden and red sea tertiary rifting during oligocene middle miocene which has a considerable influence on the thermal maturity of the mukalla source rocks the source rocks of the mukalla formation are presently in a stage of oil and condensate generation with maturity from 0 50 to 1 10 ro oil generation 0 5 ro in the mukalla source rocks began from about 61 ma to 54 ma and the peak hydrocarbon generation 1 0 ro occurred approximately from 25 ma to 20 ma the modelled hydrocarbon expulsion evolution suggested that the timing of hydrocarbon expulsion from the mukalla source rocks began from 15 ma to present day

crustal geometry of the northeastern gulf of aden passive margin localization of the deformation inferred from receiver function analysis summary here we use receiver function analysis to retrieve crustal thickness and crustal composition along the 35 my old passive margin of the eastern gulf of aden our aims are to use results from the 3 d seismic array to map crustal stretching across and along the aden margin in southern oman the array recorded local and teleseismic events between 2003 march and 2004 march seventy eight events were used in our joint inversions for vp vs ratio and depth the major results are 1 crustal thickness decreases from the uplifted rift flank of the margin towards the sheba mid ocean ridge we found a crustal thickness of about 35 km beneath the northern rift flank this value decreases sharply to 26 km beneath the post rift subsidence zone on the salalah coastal plain this 10 km of crustal thinning occurs across a horizontal distance of less than 30 km showing a localization of the crustal thinning below the first known rifted block of the margin 2 a second rift margin transect located about 50 km to the east shows no thinning from the coast to 50 km onshore the lack of crustal thickness variation indicates that the maximum crustal stretching could be restricted to offshore regions 3 the along strike variations in crustal structure demonstrate the scale and longevity of the regular along axis rift segmentation 4 extension is still observed north of the rifted domain 70 km onshore from the coast making the width of margin larger than first expected from geology 5 the crust has a felsic to normal composition with a probably strong effect of the sedimentary layer on the vp vs ratio comprised between 1 67 and 1 91

viscous flow rates of icy topography on the north polar layered deposits of mars we investigate the importance of viscous flow in shaping topography at the north polar layered deposits npld of mars by using finite element modeling to calculate the distribution of stresses and flow velocities present day impact craters on thenpldare too small and cold for viscous relaxation tohavebeen an important mechanism in controlling their current dimensions this effect may be ignored when analyzing crater size frequency distributions scarps at the npldmargins where avalanches of dust and carbon dioxide frost occur are sufficiently steep high andwarm to experience significant viscous flow we find flow velocities at the base of these steep scarps on the order of tens to hundreds of cm yr which are fast enough to significantly affect their slopeover kiloyear timescales alternatively the scarps could be close to steady state in which observed block falls provide a competing effect to viscous flow

styles of lithospheric extension controlled by underplated mafic bodies abstract the role of underplated mafic bodies upmb in the localization of deformation is examined using a two dimensional thermo mechanical finite element model rheological heterogeneity brought about by the upmb is linked with the following two main physical effects the upmb material is assumed to have 1 an anomalous high temperature and 2 a mafic crustal rock composition that is intrinsically weaker than the mantle the thermal effect will disappear rather quickly but the rock composition can have an effect at any stage of extension we show that the upmb has a strong influence on the style of lithospheric extension which depends on the thermal condition of the uppermost mantle at the time when the upmb is emplaced since the strength contrast between the weakened and non weakened regions is the most important factor the upmb works more efficiently on the localization of deformation for a colder uppermost mantle dependence of the localization of deformation on the temperature and thickness of the upmb i e more significant localization for the upmb with higher temperature and greater thickness also depends on the thermal condition of the uppermost mantle however the width of the upmb has a strong influence on the localization for any thermal condition of the uppermost mantle a greater amount of thinning is distributed into a narrower weakened region such a model behaviour implies that various styles of lithospheric extension including inward or outward migration of deformation and asymmetric extension can be simply obtained by considering the emplacement of the upmb which also plays an important role in controlling the onset of continental break up

tectono stratigraphic development of the qamar basin eastern yemen abstract the qamar basin is a polyphase rift basin located in eastern yemen adjacent to the indian ocean it probably originated as part of an extensive rift system which developed during the late jurassic to early cretaceous and led to the break up of gondwanaland rift reactivation during the mid cretaceous affected limited areas including the qamar basin a third episode of rifting occurred during the oligo miocene prior to sea floor spreading in the gulf of aden since spreading in the gulf began in the middle miocene the offshore qamar basin has been thermally subsiding in contrast the onshore part of the basin has been elevated by over 1 km the nature of the pre mid cretaceous basin fill is unknown during santonian to early campanian times thick fluvio deltaic clastics accumulated in a rapidly subsiding depocentre marked deepening during the early campanian led to deposition of deep water limestones during the late campanian prograding deep marine clastics infilled the basin minor transgression during the maastrichtian caused a resumption of marine carbonate deposition sedimentation was interrupted during the early palaeocene following which shallow marine carbonates were deposited over a large area of the arabian peninsula after a hiatus spanning the late eocene to late oligocene carbonate sedimentation resumed offshore in an active rift basin deep water carbonates and carbonate turbidites accumulated in lows whilst reefs developed on footwall highs from the late miocene onwards the reefs have been gradually drowned by rapidly accumulating clastics onshore the tertiary rift episode was associated with major uplift and only a thin veneer of recent wadi deposits overlies the rift topography the superposition of jurassic cretaceous and tertiary rifts which has not been described elsewhere in the region is tentatively related to the evolution of the indian ocean

internal structure of the martian south polar layered deposits 1 we investigate the three dimensional 3 d stratigraphic structure of the south polar layered deposits spld on mars prominent bench forming layers exposed on spld scarps were observed and mapped in three dimensions using high resolution topographic and imaging data sets using the 3 d location of exposures of one of these strata we can accurately describe the shape of that layer using simple mathematical functions analysis of these functions and the surface topography can be used to reliably predict where on other scarps this layer is exposed in general this bench forming layer and its surrounding strata is not flat and is well approximated as a parabolic dome near the center of the spld its curvature indicates that when deposited it was draped over a topographic dome similar in size to that of the present day the scarps in which this layer is exposed must have formed subsequently and have not been significantly modified by flow processes the basement topography exercises some control over the shape of the interior strata in extreme cases our successful layer fitting technique illustrates the regional uniformity in layer formation and the lack of major internal defects such as faulting within the spld we have mapped exposures of what appear to be this layer in scarps farther from the center of the deposits the position of these exposures can be used to modify the modeled parabolic shape at the periphery of the spld these peripheral elevations provide constraints on the role of flow in the overall shaping of the spld

on estimating length fluctuations of glaciers caused by changes in climatic forcing the reaction of alpine glaciers to shifts in the equilibrium line altitude ela is calculated by using a two dimensional numerical model to solve the full equations for the velocity and stress fields full system model in the absence of basal motion rates of advance and retreat of the snout of typically sized alpine glaciers are found to be insensitive to the details of the flow at the snout even when the glaciers are far from steady state a comparison of results obtained with a full system model and a shallow ice approximation sia model yields no significant differences in advance or retreat rates this assumption has been implicitly made in numerous previous climatic studies and is here shown to be well justified using a realistic mass balance altitude feedback only slight model dependent changes in steady state lengths are found the relative importance of mass balance and glacier dynamics for the transient response of alpine glaciers to changes in the ela is given a precise meaning by determining the model dependent additional shifts in ela needed for the sia and the full system models to produce identical changes in length for alpine glaciers these additional shifts in ela are on the order of 10 m which is within the error range of ela estimates it follows that at least in the absence of significant basal motion there is no need to include the effects of horizontal stresses when calculating the reaction of alpine glaciers to climatic changes attention should focus on accurate determination of the mass balance distribution and model tuning to give realistic ice thickness distributions

north polar stratigraphy and the paleo erg of mars an accurate self consistent way of coregistering the imaging and topographic data sets of the mars global surveyor mission was developed and used to begin a stratigraphic analysis of the northern polar region a distinct change in the layering style exists at a definite stratigraphic horizon near the base of the north polar layered deposits occurrences of the contact between two distinct layered units can be mapped hundreds of kilometers apart at nearly the same mars orbiter laser altimeter mola elevation the lower layered unit has a consistent association with sand dunes leading to the conclusion that it is an eroding sand rich deposit that predates most of the overlying north polar layered deposits which exhibits the expected features of a dust ice mixture these results suggest that an areally extensive erg was in existence before the present ice cap and that the present circumpolar erg is likely composed of material reworked from this older deposit the volume of this lower unit is estimated to be on the order of 10 5 km 3 the presence of this deposit implies that there existed a period in mars history when there was no icy polar cap a dramatic climatic change leading to the deposition of the upper layered icy unit in the present day polar layered deposits represents a major event in mars history however owing to uncertainties in the mechanics of layered deposits formation such an event cannot be dated at this time

geodynamics active passive margins passive margins were thought to be tectonically inactive documentation of a volcanic dyke intrusion along the eastern flank of the red sea rift proves this plate tectonic tenet wrong however with implications for hazard assessments in these regions

magmatism at continental passive margins inferred from ambient noise phase velocity in the gulf of aden non volcanic continental passive margins have traditionally been considered to be tectonically and magmatically inactive once continental breakup has occurred and seafloor spreading has commenced we use ambient noise tomography to constrain rayleigh wave phase velocity maps beneath the eastern gulf of aden eastern yemen and southern oman in the crust we image low velocities beneath the jiza qamar yemen and ashawq salalah oman basins likely caused by the presence of partial melt associated with magmatic plumbing systems beneath the rifted margin our results provide strong evidence that magma intrusion persists after breakup modifying the composition and thermal structure of the continental margin the coincidence between zones of crustal intrusion and steep gradients in lithospheric thinning as well as with transform faults suggests that magmatism post breakup may be driven by small scale convection and enhanced by edge driven flow at the juxtaposition of lithosphere of varying thickness and thermal age

crustal structure of the ocean continent transition at flemish cap seismic refraction results 1 we conducted a seismic refraction experiment across flemish cap and into the deep basin east of newfoundland canada and developed a velocity model for the crust and mantle from forward and inverse modeling of data from 25 ocean bottom seismometers and dense air gun shots the continental crust at flemish cap is 30 km thick and is divided into three layers with p wave velocities of 6 0 6 7 km s across the southeast flemish cap margin the continental crust thins over a 90 km wide zone to only 1 2 km the ocean continent boundary is near the base of flemish cap and is marked by a fault between thinned continental crust and 3 km thick crust with velocities of 4 7 7 0 km s interpreted as crust from magma starved oceanic accretion this thin crust continues seaward for 55 km and thins locally to 1 5 km below a sediment cover 1 9 3 1 km s oceanic layer 2 4 7 4 9 km s is 1 5 km thick while layer 3 6 9 km s seems to disappear in the thinnest segment of the oceanic crust at the seawardmost end of the line the crust thickens to 6 km mantle with velocities of 7 6 8 0 km s underlies both the thin continental and thin oceanic crust in an 80 km wide zone a gradual downward increase to normal mantle velocities is interpreted to reflect decreasing degree of serpentinization with depth normal mantle velocities of 8 0 km s are observed 6 km below basement there are major differences compared to the conjugate galicia bank margin which has a wide zone of extended continental crust more faulting and prominent detachment faults crust formed by seafloor spreading appears symmetric however with 30 km wide zones of oceanic crust accreted on both margins beginning about 4 5 m y before formation of magnetic anomaly m0 118 ma

the effects of martian orbital variations upon the sublimation and relaxation of north polar troughs and scarps abstract the north polar layered deposits pld of mars are climatologically significant because they represent the largest actively exchanging reservoir of martian water the kilometer scale topography of the north pld is dominated by troughs and scarps which exhibit highly correlated surface slopes and total depths the most widespread theories of pld evolution presume that the asymmetrical nature of north pld troughs characterized by equatorward facing slopes that are generally steeper than poleward facing slopes is the result of preferential h 2 o sublimation from equatorward facing trough walls however our orbitally modulated simulations indicate that there is no long term sublimation advantage of equatorward facing trough walls because of the effects of obliquity upon the slope dependence of sublimation rate in contrast we propose that viscous relaxation of subsurface water is consistent with the slope and depth distributions of north pld troughs and scarps the results of our finite element simulations suggest that a mere 2 k difference in the subsurface temperatures of opposing trough walls is sufficient to produce the observed slope disparity due to the slower rate of uplift of colder poleward facing trough walls both our sublimation and relaxation simulations indicate that present day north pld troughs have formed since 5 ma and are not sites of long term deposition additionally our results imply that glacial flow may govern the large scale evolution of the north pld especially at high obliquity

a flow model for the polar caps of mars a mechanical model with circular symmetry is examined to test the hypothesis that the martian ice caps are composed of flowing water ice together with some rock debris in contrast with most or all previous models no assumption of a steady state is made instead the accumulation and ablation is assumed to be insignificant and it is suggested that after a sufficient time the profile would have settled down to a particular collapsing form calculated by halfar 1983 higher modes of flow would have decayed relatively quickly to calculate the time constant it is necessary to consider carefully the distribution of temperature with depth the time constant is sensitive to the grain size which is assumed to be 1 10 mm and is a significant unknown as is also the effect of preferred crystal orientation apart from this the main uncertainty is the value of the upward heat flux with a heat flux of 30 mw m 2 the water ice hypothesis is consistent with an age of about 10 7 years for both the north and the south polar caps the north cap being the younger by a factor of about 7

mars landscape evolution influence of stratigraphy on geomorphology in the north polar region abstract lithology and physical properties of strata exposed at the earth s surface have direct influence on the erosion and geomorphic expression of landforms while this is well known on our planet examples on mars are just coming to light among the tens of thousands of airphoto quality images resolutions 1 5 12 m pixel acquired since 1997 by the mars global surveyor mgs mars orbiter camera moc specific examples occur among martian north polar layered materials which moc images reveal are divided into two distinct stratigraphic units a lower dark toned layered unit and a younger upper lighter toned layered unit the lower unit is less resistant to wind erosion than the upper unit the upper unit most likely consists of stratified dust and ice while the lower unit contains abundant poorly cemented sand sand is more easily mobilized by wind than dust the lower resistance to erosion of the lower unit results from the presence of sand where wind erosion in polar troughs has penetrated to the lower unit geomorphic change has proceeded more rapidly sand has been liberated from the lower unit and arcuate scarps have formed as the upper unit has been undermined wind erosion of the lower unit thus influences the geomorphology of the north polar region this result likely explains the genesis of the large polar trough chasma boreale and the relations between dunes and arcuate scarps that have puzzled investigators for nearly three decades the properties of the stratigraphic units suggest that the upper limit for the amount of water contained in the north polar layered materials may be 30 50 less than previously estimated

dynamics of an ice continent on titan 1 if the large high albedo surface feature on titan s leading hemisphere is an elevated continent composed mainly of water ice it will deform under its own weight we present a model for the axisymmetric spreading of this hypothesized continent based on the similarity solution of halfar 1983 and the approach of nye 2000 we find that the thickness of the model continent is dictated by the spreading time whereas topographic relief is controlled primarily by isostatic effects for a mantle density of 1000 kg m 3 the relief of an isostatically supported continent on titan is not likely to exceed 3 7 km

the effect of lower crustal flow on continental extension and passive margin formation the great variety of styles of continental extension may reflect different crustal thickness and thermal states of continental lithosphere during the initiation of rifting to investigate how these and other factors affect rifting and the development of passive continental margins we develop a simplified model of lithospheric extension we consider the evolution of extensional deformation for a three layer model lithosphere bounded laterally by much stronger lithosphere the cold part of the crust and mantle are treated as thin brittle plastic layers the lower crust is approximated as a thin viscous channel each brittle plastic layer can extend in only one location determined by the strength of the layer shear of the lower crust and buoyancy forces related to both crustal thickness variations and thermally induced density differences the lower crust flows in response to crustal thickness variations and is sheared when the loci of extension for the two brittle plastic layers are horizontally offset a situation we term shear decoupling as in previous studies we see three distinct patterns or modes of extensional deformation that occur under different sets of model parameters the core complex mode the wide rift mode and the narrow rift mode shear decoupling occurs only in cases with a crustal rheology at the weak end of the spectrum of laboratory estimated values we are aware of no observations that require that the upper crust and upper mantle strain at laterally displaced positions we show that for large magnitudes of extension there can be transitions between modes as inferred for some highly extended continental areas predicted patterns of crustal thickness and heat flow for some models are similar to observations at several rifted continental margins including very wide and asymmetric margins

fast computation of a viscoelastic deformable earth model for ice sheet simulations the model used by lingle and clark 1985 to approximate the deformation of the earth under a single ice stream is adapted to the purposes of continent scale ice sheet simulation the model combines a layered elastic spherical earth farrell 1972 with a viscous half space overlain by an elastic plate lithosphere cathles 1975 for the half space model we identify a new mathematical formulation essentially a time dependent partial differential equation which generalizes and improves upon the standard elastic plate lithosphere with relaxing asthenosphere model widely used in ice sheet simulation the new formulation allows a significantly faster numerical strategy a spectral collocation method based directly on the fast fourier transform we verify this method by comparing to an integral formula for a disk load we also demonstrate that the magnitudes of numerical errors made in approximating coupled ice flow earth deformation systems are significantly smaller than pairwise differences between several earth models our implementation of the lingle and clark 1985 model offers important features of spherical layered self gravitating viscoelastic earth models without the computational expense

from rifting to spreading in the eastern gulf of aden a geophysical survey of a young oceanic basin from margin to margin a geophysical survey in the eastern gulf of aden between the alula fartak 52 e and the socotra 55 e transform faults was carried out during the encens sheba cruise the conjugate margins of the gulf are steep narrow and asymmetric asymmetry of the rifting process is highlighted by the conjugate margins horst and graben in the north and deep basin in the south two transfer fault zones separate the margins into three segments whereas the present day sheba ridge is divided into two segments by a transform discontinuity therefore segmentation of the sheba ridge and that of the conjugate margins did coincide during the early stages of oceanic spreading extensive magma production is evidenced in the central part of the western segment anomaly 5d was identified in the northern and southern parts of the oceanic basin thus confirming that seafloor spreading in this part of gulf of aden started at least 17 6 ma ago

dynamics of volcanic systems in iceland example of tectonism and volcanism at juxtaposed hot spot and mid ocean ridge systems volcanic systems are swarms of tectonic fractures and basalt volcanoes formed as a result of plate pull as the plates are pulled apart associated with the mid ocean ridges and the magma dynamics of the iceland mantle plume most systems are 40 150 km long 5 20 km wide and develop a central volcano they supply magma to all eruptions in iceland data obtained in the last few years have greatly improved our knowledge of their volcanotectonic environment as a result the geometry of the plume is better constrained and the crust previously considered thin 10 km is now modeled as thick 20 40 km depending on the location of the volcanic systems their activity either decreases or increases faulting in the two main seismic zones from this we can infer that emplacement of the feeder dike to the largest historical eruption in iceland that of laki in 1783 increased shear stress in the south iceland seismic zone and almost certainly triggered the largest m 7 1 in 1784 historical earthquake in ic

magmatism of the eastern red sea margin in the northern part of yemen from oligocene to present the northern part of yemen during the last 30 ma has been the site of an intense magmatic activity which can be divided into three main episodes the first episode lasted from 30 to 18 ma and was characterized by two climaxes huge flood basalt eruptions between 30 to 26 ma were succeeded by massive ignimbrite sheet deposits and intrusion of silicic subvolcanic masses 22 to 20 ma ago contemporaneously along the present day western margin of the yemen plateau a basaltic dike swarm was emplaced additionally alkaline and peralkaline granites were intruded at the end of the first climax and throughout the second after a long quiescence magmatic activity occurred again in late miocene time about 10 ma with extrusion of lava flows in scattered areas e g al harf wadi as sirr maswar al hada jabal an nar the third magmatic episode has been concentrated in three well defined areas it began about 6 5 ma ago in the dhamar rada area and later in the san a amram and sirwah marib areas new chemical and isotopic data on the magmatic products of north yemen indicate that extensive crustal contamination together with simple crystal fractionation processes have been operating during the magmatic evolution the most primitive products of each of the three episodes share isotopic and petrological features suggesting a similar mantle source isotope geochemistry indicates an important asthenospheric component in the mantle source for the magmas of all three episodes finally a comparison between the coeval igneous rocks of the yemen and ethiopia plateaus is attempted

magmatic and tectonic extension at mid ocean ridges 1 controls on fault characteristics we use 2 d numerical models to explore the thermal and mechanical effects of magma intrusion on fault initiation and growth at slow and intermediate spreading ridges magma intrusion is simulated by widening a vertical column of model elements located within the lithosphere at a rate equal to a fraction m of the total spreading rate i e m 1 for fully magmatic spreading heat is added in proportion to the rate of intrusion to simulate the thermal effects of magma crystallization and the injection of hot magma into the crust we examine a range of intrusion rates and axial thermal structures by varying m spreading rate and the efficiency of crustal cooling by conduction and hydrothermal circulation fault development proceeds in a sequential manner with deformation focused on a single active normal fault whose location alternates between the two sides of the ridge axis fault spacing and heave are primarily sensitive to m and secondarily sensitive to axial lithosphere thickness and the rate that the lithosphere thickens with distance from the axis contrary to what is often cited in the literature but consistent with prior results of mechanical modeling we find that thicker axial lithosphere tends to reduce fault spacing and heave in addition fault spacing and heave are predicted to increase with decreasing rates of off axis lithospheric thickening the combination of low m particularly when m approaches 0 5 as well as a reduced rate of off axis lithospheric thickening produces long lived large offset faults similar to oceanic core complexes such long lived faults produce a highly asymmetric axial thermal structure with thinner lithosphere on the side with the active fault this across axis variation in thermal structure may tend to stabilize the active fault for longer periods of time and could concentrate hydrothermal circulation in the footwall of oceanic core complexes

multistage sedimentary and metamorphic origin of pyrite and gold in the giant sukhoi log deposit lena gold province russia gold mineralization at sukhoi log in eastern siberia is hosted in a deformed neoproterozoic organic bearing and pyritic black shale and siltstone sequence that is folded into a tight overturned anticline the deposit contains about 30 million ounces of gold at an average grade of 2 0 g t au and is one of the largest known undeveloped gold resources the high grade gold zone forms a gently dipping tabular body in the core of the anticline the best gold grades occur in narrow bedding parallel pyrite quartz veinlets that have been folded during the main deformation event lower grade gold is associated with disseminated pyrite developed in and around the high grade core of the deposit detailed paragenetic studies of the mineralization and host rocks have defined six stages of pyrite development in the carbonaceous sediments the two earliest forms of pyrite termed py1 and py2 are commonly developed in stratiform layers of micron sized crystals framboids and fine euhedra which are interpreted as synsedimentary to early diagenetic in origin coarser grained bedding parallel aggregates of inclusion rich pyrite termed py3 contain inclusions of arsenopyrite native gold and gold tellurides and are interpreted to form during late diagenesis and earliest deformation coarse euhedral pyrite py4 overgrows the earlier pyrite py1 py2 and py3 and the slaty cleavage developed in the host rocks indicating a syndeformation timing late stage inclusion free pyrite py5 overgrows and replaces earlier sulfides and is considered to be syn to late deformation laser ablation inductively coupled plasma mass spectrometry la icpms analyses of the various pyrite types indicate that the synsedimentary py1 contains the highest levels of invisible gold varying from 0 4 to 12 1 ppm with a mean of 3 22 ppm au and 1 900 ppm as py1 is also enriched in a suite of trace elements mo sb ni co se te ag cu pb zn mn ba cr u v which are similar to those concentrated by organic processes in euxinic sedimentary environments later generations of pyrite from py2 to py5 including pyrite in bedding parallel pyrite quartz veinlets contain progressively lower contents of invisible gold and most other trace elements however this metamorphic and postmetamorphic pyrite contains microinclusions of free gold arsenopyrite pyrrhotite sphalerite and chalcopyrite the paragenetic textural and chemical relationships at sukhoi log suggest that gold was clearly initially introduced prior to cleavage development accompanying sedimentation of the organic rich shales and fixed during diagenesis within the structure of diagenetic arsenian pyrite subsequently accompanying deformation gold was liberated from recrystallized diagenetic pyrite to become concentrated as free gold and gold tellurides within metamorphic pyrite and folded bedding parallel pyrite quartz veinlets two key processes are considered vital to the formation of the sukhoi log deposit original synsedimentary and early diagenetic concentration of gold dissolved within arsenian pyrite in organic rich black shales and metamorphic processes that liberated gold from the early forms of arsenian pyrite to be concentrated as free gold and gold tellurides within late diagenetic and metamorphic pyrite and associated pyrite quartz veinlets in the core of an overturned anticline these ore forming processes are unlikely to be unique to sukhoi log other black shale and turbidite hosted deposits that occur in rifted continental margin environments which have undergone collision and basin inversion may form by similar processes

evidence of partial melting beneath a continental margin case of dhofar in the northeast gulf of aden sultanate of oman summary gravity data and p wave teleseismic traveltime residuals from 29 temporary broad band stations spread over the northern margin of the gulf of aden dhofar region oman were used to image lithospheric structure we apply a linear relationship between density and velocity to provide consistent density and velocity models from mid crust down to about 250 km depth the accuracy of the resulting models is investigated through a series of synthetic tests the analysis of our resulting models shows 1 crustal heterogeneities that match the main geological features at the surface 2 the gravity edge effect and disparity in anomaly depth locations for layers at 20 and 50 km 3 two low velocity anomalies along the continuation of socotra hadbeen and alula fartak fracture zones between 60 and 200 km depth and 4 evidence for partial melting 3 6 per cent within these two negative anomalies we discuss the presence of partial melting in terms of interaction between the sheba ridge melts and its along axis segmentation

method for anisotropic crystal orientation fabrics detection using radio wave depolarization in radar sounding of mars polar layered deposits the polar layered deposits plds provide a wealth of information about the past climate evolution of mars surface mass fluxes and ice flow mainly governed topography and layering of the pld china s mars probe including an orbiter and a landing rover will be launched by 2020 a new type satellite borne mars penetrating radar instrument has been selected to be a part of the payloads on the orbiter its main scientific objectives are to map the distribution of water water ice and to detect the soil characteristics at global scale on the martian crust compared with mars advanced radar for subsurface and ionospheric sounding and shallow radar the biggest difference is that the antenna system of this mars penetrating radar consists of two dipole antennas mutually perpendicular this special configuration enables the investigation of the ice flow of pld by detecting and analyzing the features of anisotropic crystal orientation fabric cof thus relying on the fact that the radio waves are depolarized while passing through an anisotropic cof layer in this paper a method for anisotropic cof detection based on this radar system is proposed the radar echo formulation of anisotropic cof is derived and the ratio of the signals measured by the two perpendicular antennas is used to analyze the anisotropy of cof we demonstrate that the ratio is an ideal criterion for the detection and analysis of cof since it contains all parameters about the anisotropy feature of cof and it is independent of the attenuation in the propagation path and the reflection coefficient in order to verify the validity of the derived analytical expression of the ratio for the detection of cof finite different time domain simulations are carried out based on a simple model of the subsurface of pld which contains an anisotropic cof layer the advantages of this method the potential application scenarios and the effects of the martian environment are also discussed

forward modeling of ice topography on mars to infer basal shear stress conditions 1 understanding the history of ice caps on mars could reveal important information about martian geologic and climatic history to do this an ice reconstruction model is needed that operates over complex topography and can be constrained with a limited number of free parameters in this study we developed a threshold sliding model for ice cap morphology based on the classic model of nye later incorporated into the models of reeh and colleagues we have updated the nye reeh model with a new numerical algorithm although the model was originally developed to model perfectly plastic deformation it is applicable to any ice body that deforms when a threshold basal shear stress is exceeded the model requires three inputs a digital elevation model of bed topography a mask grid that defines the position of the ice terminus and a function defining the threshold basal shear stress to test the robustness of the model the morphology of the greenland ice sheet is reconstructed using an empirical equation between threshold basal shear stress and ice surface slope the model is then used to reconstruct the morphology of ice draping impact craters on the margins of the south polar layered deposits using an inferred constant basal shear stress of 0 6 bar for the majority of the examples this inferred basal shear stress value is almost 1 3 of the average basal shear stress calculated for the greenland ice sheet what causes this lower basal shear stress value on mars is unclear but could involve the strain weakening behavior of ice

mars evidence for geologically recent advance of the south polar cap an impact crater near the south pole characterized by pristine appearing secondary crater chains is partly covered with more than a kilometer of polar layered terrain deposits continuous with the main polar deposit the observed relationships strongly suggest that the crater formed near the edge of the cap in amazonian time was buried by polar cap advance and that some retreat occurred these data appear to indicate that the south polar cap has been active in the geologically recent past

age of the zambian copperbelt the sediment hosted stratiform cu co deposits and prospects of the central african copperbelt are characterized by two intimately associated mineralization styles disseminated sulfides and sulfide bearing quartz carbonate veins and veinlets it has been widely accepted that the disseminated mineralization was introduced during sediment diagenesis in a rift setting and possibly in multiple events spanning several hundred million years in contrast the veinlet hosted mineralization is commonly thought to have been derived either by remobilization of the disseminated sulfides during the lufilian collisional orogeny or introduced at broadly the same time s as the disseminated sulfides during diagenesis and subsequent orogeny the results of 15 re os molybdenite age determinations from cu co deposits and prospects across the zambian part of the central african copperbelt suggest that both the disseminated and veinlet mineralization styles were indeed generated together but in a 50 myr cambrian window 540 490 ma during the later stages of the lufilian collisional orogeny the molybdenite ages for the disseminated stratiform mineralization at two localities do not support the notion of syndiagenetic cu introduction the molybdenite ages also show that individual deposits formed over minimum time intervals of 10 24 myr

crustal structure of the northern part of the voring basin mid norway margin from wide angle seismic and gravity data abstract three regional ocean bottom seismograph obs profiles with a total of fifteen recovered obss were acquired in the northern part of the voring basin mid norway margin in 1996 the data have been modelled by inversion and forward modelling of the obs vertical components p waves the velocity is found to increase downwards within the sedimentary layers due to increasing depth of burial confining pressure within the two deepest sedimentary layers there seems to be an increase of 0 5 1 km s in velocity northwestwards this increase is most likely caused by high velocity sill intrusions in the sedimentary rocks emplaced during the rift episode leading to early eocene opening of the ne atlantic the presence of significant amounts of sills seems to terminate close to the nyk high along profile 5 dip profile and close to a dome northwest of the shelf edge along profile 6 dip profile the velocity in the top of this dome is anomalously high 3 4 km s at only 1 km depth beneath the seafloor suggesting that the high consists of volcanic rocks or heavily intruded sedimentary rocks the relatively low velocities derived from within the upper crystalline crust 5 9 6 2 km s confirms that the crust is of continental origin within the lower crust there seems to be a clear increase in velocity northwestwards from about 7 0 km s to about 7 4 km s suggesting that the amount of high velocity intrusions underplating in the lower crust decreases landwards the modelling of the strike profile profile 7 suggests that the transition from a lower crust dominated by magmatic intrusions to a more undisturbed continental lower crust is located approximately beneath this profile the model for profile 7 ties closely southwestwards to the models derived from obs data acquired in 1992 and northeastwards to lofoten obs data acquired in 1988 the models indicate that the thick magmatic intrusions in the sedimentary rocks and lower crust extend further landwards in the area of the 1992 survey and that the transition zone is related to the surt lineament the distribution of magmatic rocks thus seems to correlate strongly with pre breakup structures in addition to the p wave modelling gravity modelling has been performed along all profiles the gravity modelling confirmed all main aspects of the obs models and provided constraints on the crustal structures towards the ends of the profiles

tectono sedimentary evolution of the eastern gulf of aden conjugate passive margins narrowness and asymmetry in oblique rifting context abstract here we focus on the yet unexplored eastern gulf of aden on socotra island yemen southeastern oman and offshore conjugate passive margins between the socotra hadbeen shfz and the eastern gulf of aden fracture zones our interpretation leads to onshore offshore stratigraphic correlation between the passive margins we present a new map reflecting the boundaries between the crustal domains proximal necking hyper extended exhumed mantle proto oceanic and oceanic domains and structures using bathymetry magnetic surveys and seismic reflection data the most striking result is that the magma poor conjugate margins exhibit asymmetrical architecture since the thinning phase upper rupelian burdigalian their necking domains are sharp 40 10 km wide and their hyper extended domains are narrow and asymmetric 10 40 km wide on the socotra margin and 50 80 km wide on the omani margin we suggest that this asymmetry is related to the migration of the rift center producing significant lower crustal flow and sequential faulting in the hyper extended domain throughout the oligo miocene rifting far field forces dominate and the deformation is accommodated along ew to n110 e northward dipping low angle normal faults convection in the mantle near the shfz may be responsible of change in fault dip polarity in the omani hyper extended domain we show the existence of a northward dipping detachment fault formed at the beginning of the exhumation phase burdigalien it separates the northern upper plate oman from southern lower plate socotra island and may have generated rift induced decompression melting and volcanism affecting the upper plate we highlight multiple generations of detachment faults exhuming serpentinized subcontinental mantle in the ocean continent transition associated to significant decompression melting final detachment fault may have triggered the formation of a proto oceanic crust at 17 6 ma and induced late volcanism up to 10 ma finally the setting up of a steady state oceanic spreading center occurs at 17 ma

structure and evolution of the eastern gulf of aden conjugate margins from seismic reflection data the gulf of aden is a young and narrow oceanic basin formed in oligo miocene time between the rifted margins of the arabian and somalian plates its mean orientation n75 e strikes obliquely 50 to the n25 e opening direction the western conjugate margins are masked by oligo miocene lavas from the afar plume this paper concerns the eastern margins where the 19 35 ma breakup structures are well exposed onshore and within the sediment starved marine shelf those passive margins about 200 km distant are non volcanic offshore during the encens sheba cruise we gathered swath bathymetry single channel seismic reflection gravity and magnetism data in order to compare the structure of the two conjugate margins and to reconstruct the evolution of the thinned continental crust from rifting to the onset of oceanic spreading between the alula fartak and socotra major fracture zones two accommodation zones trending n25 e separate the margins into three n110 e trending segments the margins are asymmetric offshore the northern margin is narrower and steeper than the southern one including the onshore domain the southern rifted margin is about twice the breadth of the northern one we relate this asymmetry to inherited jurassic cretaceous rifts the rifting obliquity also influenced the syn rift structural pattern responsible for the normal faults trending from n70 e to n110 e the n110 e fault pattern could be explained by the decrease of the influence of rift obliquity towards the central rift and or by structural inheritance the transition between the thinned continental crust and the oceanic crust is characterized by a 40 km wide zone our data suggest that its basement is made up of thinned continental crust along the southern margin and of thinned continental crust or exhumed mantle more or less intruded by magmatic rocks along the northern margin abstract from author

a mechanism to thin the continental lithosphere at magma poor margins crustal thinning where continental plates break apart can be accomplished by a system of conjugate concave downward faults instead of multiple normal faults through exhumation of mid crustal and mantle material where continental plates break apart slip along multiple normal faults provides the required space for the earth s crust to thin and subside1 after initial rifting however the displacement on normal faults observed at the sea floor seems not to match the inferred extension2 here we show that crustal thinning can be accomplished in such extensional environments by a system of conjugate concave downward faults instead of multiple normal faults our model predicts that these concave faults accumulate large amounts of extension and form a very thin crust 10 km by exhumation of mid crustal and mantle material this transitional crust is capped by sub horizontal detachment surfaces over distances exceeding 100 km with little visible deformation our rift model is based on numerical experiments constrained by geological and geophysical observations from the alpine tethys and iberia newfoundland margins3 4 5 6 7 8 9 furthermore we suggest that the observed transition from broadly distributed and symmetric extension to localized and asymmetric rifting is directly controlled by the existence of a strong gabbroic lower crust the presence of such lower crustal gabbros is well constrained for the alpine tethys system4 9 initial decoupling of upper crustal deformation from lower crustal and mantle deformation by progressive weakening of the middle crust is an essential requirement to reproduce the observed rift evolution this is achieved in our models by the formation of weak ductile shear zones

timing and pt evolution of whiteschist metamorphism in the lufilian arc zambezi belt orogen zambia implications for the assembly of gondwana abstract one of the world s most extensive occurrences of whiteschists talc kyanite schists is located in south central africa with several exposures along a ca 700 km northwest southeast striking zone the metamorphic evolution and age relations of whiteschists and associated rocks from four localities three in the lufilian arc and one in the zambezi belt were investigated in the lufilian arc whiteschists associated garnet amphibolites and biotite kyanite garnet gneisses record peak metamorphic conditions of about documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes

anhydrite pseudomorphs and the origin of stratiform cu co ores in the katangan copperbelt democratic republic of congo the stratiform cu co ore mineralisation in the katangan copperbelt consists of dispersed sulphides and sulphides in nodules and lenses which are often pseudomorphs after evaporites two types of pseudomorphs can be distinguished in the nodules and lenses in type 1 examples dolomite precipitated first and was subsequently replaced by cu co sulphides and authigenic quartz whereas in type 2 examples authigenic quartz and cu co sulphides precipitated prior to dolomite and are coarse grained the sulphur isotopic composition of the copper cobalt sulphides in the type 1 pseudomorphs is between 10 3 and 3 1 relative to the vienna canyon diablo troilite indicating that the sulphide component was derived from bacterial sulphate reduction bsr the generation of text hco 3 during this process caused the precipitation and replacement of anhydrite by dolomite a second product of bsr is the generation of h2s resulting in the precipitation of cu co sulphides from the mineralising fluids initial sulphide precipitation occurred along the rim of the pseudomorphs and continued towards the core precipitation of authigenic quartz was most likely induced by a ph decrease during sulphide precipitation fluid inclusion data from quartz indicate the presence of a high salinity 8 18 eq wt nacl fluid possibly derived from evaporated seawater which migrated through the deep subsurface 87sr 86sr ratios of dolomite in type 1 nodules range between 0 71012 and 0 73576 significantly more radiogenic than the strontium isotopic composition of neoproterozoic marine carbonates 87sr 86sr 0 7056 0 7087 this suggests intense interaction with siliciclastic sedimentary rocks and or the granitic basement the low carbon isotopic composition of the dolomite in the pseudomorphs 7 02 and 9 93 relative to the vienna pee dee belemnite v pdb compared to the host rock dolomite 4 90 and 1 31 v pdb resulted from the oxidation of organic matter during bsr

crustal structure of the rifted volcanic margins and uplifted plateau of western yemen from receiver function analysis we analyse p wave receiver functions across the western gulf of aden and southern red sea continental margins in western yemen to constrain crustal thickness internal crustal structure and the bulk seismic velocity characteristics in order to address the role of magmatism faulting and mechanical crustal thinning during continental breakup we analyse teleseismic data from 21 stations forming the temporary young conjugate margins laboratory yocmal network together with gfz and yemeni permanent stations analysis of computed receiver functions shows that 1 the thickness of unextended crust on the yemen plateau is 35 km 2 this thins to 22 km in coastal areas and reaches less than 14 km on the red sea coast where presence of a high velocity lower crust is evident the average vp vs ratio for the western yemen plateau is 1 79 increasing to 1 92 near the red sea coast and decreasing to 1 68 for those stations located on or near the granitic rocks thinning of the crust and by inference extension occurs over a 130 km wide transition zone from the red sea and gulf of aden coasts to the edges of the yemen plateau thinning of continental crust is particularly localized in a 30 km wide zone near the coastline spatially co incident with addition of magmatic underplate to the lower crust above which on the surface we observe the presence of seaward dipping reflectors sdrs and thickened oligo miocene syn rift basaltic flows our results strongly suggest the presence of high velocity mafic intrusions in the lower crust which are likely either synrift magmatic intrusion into continental lower crust or alternatively depleted upper mantle underplated to the base of the crust during the eruption of the sdrs our results also point towards a regional breakup history in which the onset of rifting was synchronous along the western gulf of aden and southern red sea volcanic margins followed by a second phase of extension along the red sea margin

volcanism jump and propagation on the sheba ridge eastern gulf of aden segmentation evolution and implications for oceanic accretion processes the rifting between arabia and somalia which started around 35 ma was followed by oceanic accretion from at least 17 6 ma leading to the formation of the present day gulf of aden bathymetric gravity and magnetic data from the encens sheba cruise are used to constrain the structure and segmentation of the oceanic basin separating the conjugate continental margins in the eastern part of the gulf of aden between 51 degrees e and 55 5 degrees e data analysis reveals that the oceanic domain along this ridge section is divided into two distinct areas the eastern area is characterized by a shorter wavelength variation of the axial segmentation and an extremely thin oceanic crust in the western segment a thicker oceanic crust suggests a high melt supply this supply is probably due to an off axis melting anomaly located below the southern flank of the sheba ridge 75 km east of the major alula fartak transform fault this suggests that the axial morphology is produced by a combination of factors including spreading rate melt supply and the edge effect of the alula fartak transform fault as well as the proximity of the continental margin the oceanic domains have undergone two distinct phases of accretion since the onset of seafloor spreading with a shift around 11 ma at that time the ridge jumped southwards in response to the melting anomaly propagating ridges were triggered by the melting activity and propagated both eastward and westward the influence of the melting anomaly on the ridges decreased stopping their propagation since less than 9 ma from that time up to the present the n025 degrees e trending socotra transform fault developed in association with the formation of the n115 degrees e trending segment 2 in recent times a counter clockwise rotation of the stress field associated with kinematic changes could explain the structural morphology of the alula fartak and socotra hadbeen fracture zones

magmatism on rift flanks insights from ambient noise phase velocity in afar region during the breakup of continents in magmatic settings the extension of the rift valley is commonly assumed to initially occur by border faulting and progressively migrate in space and time toward the spreading axis magmatic processes near the rift flanks are commonly ignored we present phase velocity maps of the crust and uppermost mantle of the conjugate margins of the southern red sea afar and yemen using ambient noise tomography to constrain crustal modification during breakup our images show that the low seismic velocities characterize not only the upper crust beneath the axial volcanic systems but also both upper and lower crust beneath the rift flanks where ongoing volcanism and hydrothermal activity occur at the surface magmatic modification of the crust beneath rift flanks likely occurs for a protracted period of time during the breakup process and may persist through to early seafloor spreading

recent kinematics of the tectonic plates surrounding the red sea and gulf of aden this work was funded by the miur prin prot 20125jkany the geodas ship track data used in this study can be found at ngdc http www ngdc noaa gov mgg geodas trackline html

heat flow and hydrothermal circulation at the ocean continent transition of the eastern gulf of aden abstract in order to investigate the importance of fluid circulation associated with the formation of ocean continent transitions oct we examine 162 new heat flow hf measurements in the eastern gulf of aden obtained at close locations along eight seismic profiles and with multi beam bathymetry the average hf values in the oct and in the oceanic domain 18 m y are very close to the predictions of cooling models showing that the overall importance of fluids remains small at the present time compared to oceanic ridge flanks of the same age however local hf anomalies are observed although not systematically in the vicinity of the unsedimented basement and are interpreted by the thermal effect of meteoric fluids flowing laterally we propose a possible interpretation of hydrothermal paths based on the shape of hf anomalies and on the surface morphology fluids can circulate either along dip or along strike but are apparently focussed in narrow pipes in several locations in the oct there is no detectable hf anomaly while the seismic velocity structure suggests serpentinization and therefore past circulation we relate the existence of the present day fluid circulation in the eastern gulf of aden to the presence of unsedimented basement and to the local extensional stress in the vicinity of the socotra hadbeen fault zone at the scale of rifted margins fluid circulation is probably not as important as in the oceanic domain because it can be inhibited rapidly with high sedimentation rates serpentinization and stress release after the break up

layered azimuthal anisotropy of rayleigh wave phase velocities in the european alpine lithosphere inferred from ambient noise abstract passive seismic imaging of the earth is a rapidly developing field of study recent advances in noise cross correlation techniques allow imaging of isotropic surface wave and shear wave velocities in areas where earthquake numbers and distributions are insufficient to implement traditional earthquake based tomography furthermore advances in the theory underpinning surface wave inversion have led to depth dependent mapping of seismic anisotropy in the lithosphere and upper asthenosphere we show that by merging these two rapidly advancing fields we can invert noise based phase velocity measurements for azimuthally anisotropic phase speed thereby providing a highly resolved image of layered azimuthal anisotropy in continental crust we apply this new algorithm in the western alps an area of complex lithospheric structure alpine crustal thickening results from continental collision indentation of the ivrea mantle into the middle crust of the european plate rollback of the european lithospheric mantle and crustal slicing we find that anisotropy beneath the central alps is stratified in two layers one with an orogen parallel fast direction above 30 km depth and another with a strong orogen perpendicular fast direction at greater depth although our resolution is reduced outside the central alps we map orogen parallel anisotropy in the crust of the northern alpine foreland we interpret the results in the central alps as first order evidence for a model of azimuthal anisotropy in which 1 near vertical emplacement of crustal slices following detachment of the lithospheric mantle from the crust gives rise to orogen parallel fast directions of wave propagation in the crust and 2 dominantly horizontal tectonics in the thickened crustal root and uppermost mantle yield orogen perpendicular fast directions at greater depth

marine ice sheet dynamics part 1 the case of rapid sliding marine ice sheets are continental ice masses resting on bedrock below sea level their dynamics are similar to those of land based ice sheets except that they must couple with the surrounding floating ice shelves at the grounding line where the ice reaches a critical flotation thickness in order to predict the evolution of the grounding line as a free boundary two boundary conditions are required for the diffusion equation describing the evolution of the grounded ice thickness by analogy with stefan problems one of these conditions imposes a prescribed ice thickness at the grounding line and arises from the fact that the ice becomes afloat the other condition must be determined by coupling the ice sheet to the surrounding ice shelves here we employ matched asymptotic expansions to study the transition from ice sheet to ice shelf flow for the case of rapidly sliding ice sheets our principal results are that the ice flux at the grounding line in a two dimensional ice sheet is an increasing function of the depth of the sea floor there and that ice thicknesses at the grounding line must be small compared with ice thicknesses inland these results indicate that marine ice sheets have a discrete set of steady surface profiles if they have any at all and that the stability of these steady profiles depends on the slope of the sea floor at the grounding line

stress fields associated with oceanic transform faults abstract although their formation is commonly attributed to shear stresses oceanic transform faults contain many structural elements indicating extension across them examples include transform parallel tension fractures normal faults grabens dykes and small scale spreading centres at the junctions between transform faults and adjacent ridge segments there are commonly sets of oblique fractures a detailed field study of oblique fractures in the vicinity of a junction between a mid ocean ridge and an oceanic fracture zone in north iceland shows that they are mostly tension fractures and normal faults this paper presents the results of a boundary element study of the stress field associated with the junction between an oceanic transform fault and its ridge segments the results show that uniaxial tensile loading plate pull parallel with the transform fault tends to lock it but also gives rise to shear stress concentration in a zone that coincides with the fault this shear stress is primarily responsible for the strike slip faulting and associated earthquakes in the transform fault to generate the transform parallel fracture zone graben normal faults tensile fractures dykes and spreading centres however requires tensile loading parallel with the ridge axis the results suggest that biaxial ridge parallel and ridge perpendicular tensile loading generates a stress field that 1 unlocks and makes slip easier on the transform faults and 2 explains the oblique fractures at the ridge transform junctions

post rift volcanism and high heat flow at the ocean continent transition of the eastern gulf of aden a high heat flow 900 mw m 2 has been observed over a volcanic structure at the ocean continent transition in the eastern gulf of aden oman margin the anomaly is superposed to a progressive increase of heat flow across the margin and can be interpreted either by 1 heat refraction 2 fluid discharge or 3 cooling magma the two first explanations cannot be ruled out definitely by modelling analysis but require unlikely thermal conductivity or permeability values the third one implies that the latest activity of the volcano was about 100 000 years old and therefore continued c 18 ma after the break up of africa and arabia this potential mechanism is consistent with other lines of evidence of post rifting activity in the gulf of aden and could invalidate the conventional assumption that rifted margins become passive after the break up of continents

global upper mantle tomography with the automated multimode inversion of surface and s wave forms summary we apply the automated multimode inversion of surface and s wave forms to a large global data set verify the accuracy of the method and assumptions behind it and compute an s vvelocity model of the upper mantle crust 660 km the model is constrained with 51 000 seismograms recorded at 368 permanent and temporary broadband seismic stations structure of the mantle and crust is constrained by waveform information both from the fundamentalmode rayleigh waves periods from 20 to 400 s and from s and multiple s waves higher modes in order to enhance the validity of the path average approximation we implement the automated inversion of surface and s wave forms with a three dimensional 3 d reference model linear equations obtained from the processing of all the seismograms of the data set are inverted for seismic velocity variations also relative to a 3 d reference in this study composed of a 3 d model of the crust and a one dimensional 1 d global average depth profile in the mantle below waveform information is related to shear and compressional velocity structure within approximate waveform sensitivity areas we use two global triangular grids of knots with approximately equal interknot spacing within each a finely spaced grid for integration over sensitivity areas and a rougher spaced one for the model parametrization for the tomographic inversion we use lsqr with horizontal and vertical smoothing and norm damping we invert for isotropic variations in s and p wave velocities but also allow for s wave azimuthal anisotropy in order to minimize errors due to possible mapping of anisotropy into isotropic heterogeneity the lateral resolution of the resulting isotropic upper mantle images is a few hundred kilometres varying with data sampling we validate the imaging technique with a spectral element resolution test inverting a published global synthetic data set computed with the spectral element method using a laterally heterogeneous mantle model we are able to reconstruct the synthetic model accurately this test confirms both the accuracy of the implementation of the method and the validity of the jwkb and path average approximations as applied in it reviewing the tomographic model we observe that low s v velocity anomalies beneath mid ocean ridges and backarc basins extend down to 100 km depth only shallower than according to some previous tomographic models this presents a close match to published estimates of primary melt production depth ranges there in the seismic lithosphere beneath cratons unambiguous high velocity anomalies extend to 200 km pronounced low velocity zones beneath cratonic lithosphere are rare where present south america tanzania they are neighboured by volcanic areas near cratonic boundaries the images of these low velocity zones may indicate hot material possibly of mantle plume origin trapped or spreading beneath the thick cratonic lithosphere

influence of ice rheology and dust content on the dynamics of the north polar cap of mars abstract the evolution and dynamics of the north polar cap residual ice cap layered deposits complex of mars is simulated with a thermomechanical ice sheet model we consider a scenario with ice free initial conditions at 5 ma before present due to the large obliquities which prevailed prior to this time the north polar cap is then built up to its present shape driven by a parameterized climate forcing surface temperature surface mass balance based on the obliquity and eccentricity history the effects of different ice rheologies and different dust contents are investigated it is found that the build up scenarios require an accumulation rate of approximately 0 15 0 2 mm a 1 at present the topography evolution is essentially independent of the ice dynamics due to the slow ice flow owing to the uncertainties associated with the ice rheology and the dust content flow velocities can only be predicted within a range of two orders of magnitude likely present values are of the order of 0 1 1 mm a 1 and a strong variation over the climatic cycles is found for all cases computed basal temperatures are far below pressure melting

diagenetic origin of the stratiform cu co deposit at kamoto in the central african copperbelt sediment hosted stratiform ore deposits that underwent metamorphism and orogenesis are interpreted either to have a multiphase origin that might in part predate these events or to be exclusively related to fluid migration during the orogeny this controversy concerns the formation of many world class sediment hosted ore deposits such as in the central african copperbelt here we present re os dating results for disseminated and stratiform cu co sulfide pseudomorphs after anhydrite in nodules and layers from the copperbelt results demonstrate that at least some of the ores formed at around 800 ma during rifting and basin development and thus predate the pan african lufilian orogeny younger re os ages i e between 682 28 and 230 36 ma may be due to the replacement of the early cu co sulfides by younger sulfide phases alternatively these younger ages may result from the disturbance or resetting of the re os system during recrystallization metamorphism or the transition of chalcocite from its high to its low temperature polymorph

broad accommodation of rift related extension recorded by dyke intrusion in saudi arabia volcanic fields at the eastern margin of the red sea rift were regarded as seismically quiet until a swarm of 30 000 earthquakes struck in 2009 geological analyses reveal the intrusion of a volcanic dyke and indicate that rift related extension is spread over a broad region far from the rift axis