crustal structure of the northern nova scotia rifted continental margin eastern canada 1 the nova scotia continental margin off eastern canada marks a transition from a volcanic to a nonvolcanic style of rifting the northern nonvolcanic segment of the margin was studied by a 490 km long refraction seismic line with dense air gun shots coincident with previous deep reflection profiles a p wave velocity model was developed from forward and inverse modeling of the wide angle data from 19 ocean bottom seismometers and coincident normal incidence reflection profiles the continental crust has a maximum thickness of 36 km and is divided into three layers with velocities of 5 7 6 9 km s crustal thinning down to 3 km occurs in a 180 km wide zone and the sediment cover in this area is up to 15 km thick farther seaward a 150 km wide transition zone is observed with a 5 km thick lower layer 7 2 7 6 km s interpreted as partially serpentinized mantle at the landward end this layer is overlain by highly altered continental crust 5 4 km s extending up to the seaward limit of the jurassic salt province farther seaward the upper layer is interpreted as exhumed and highly serpentinized mantle 5 1 km s separated from the lower layer by subhorizontal reflectivity which probably represents a serpentinization front oceanic crustal thickness is 4 km with layer 2 velocities of 4 6 5 0 km s layer 3 velocities of 6 4 6 55 km s are lower than typical lower oceanic crust velocities but consistent with a low magma supply and increased tectonism as observed on the reflection profile this reduced magma production might be related to the proximity of the newfoundland transform margin

crustal structure variations along the nw african continental margin a comparison of new and existing models from wide angle and reflection seismic data abstract deep seismic data represent a key to understand the geometry and mechanism of continental rifting the passive continental margin of nw africa is one of the oldest on earth formed during the upper triassic lower liassic rifting of the central atlantic ocean over 200 ma we present new and existing wide angle and reflection seismic data from four study regions along the margin located in the south offshore dakhla on the central continental margin offshore safi in the northern moroccan salt basin and in the gulf of cadiz the thickness of unthinned continental crust decreases from 36 km in the north to about 27 km in the south crustal thinning takes place over a region of 150 km in the north and only 70 km in the south the north moroccan basin is underlain by highly thinned continental crust of only 6 8 km thickness the ocean continent transition zone shows a variable width between 40 and 70 km and is characterized by seismic velocities in between those of typical oceanic and thinned continental crust the neighbouring oceanic crust is characterized by a thickness of 7 8 km along the complete margin relatively high velocities of up to 7 5 km s have been imaged between magnetic anomalies s1 and m25 and are probably related to changes in the spreading velocities at the time of the kimmeridgian tithonian plate reorganization volcanic activity seems to be mostly confined to the region next to the canary islands and is thus not related to the initial opening of the ocean which was associated to only weak volcanism comparison with the conjugate margin off nova scotia shows comparable continental crustal structures but 2 3 km thinner oceanic crust on the american side than on the african margin

inverse square root dependence of mid ocean ridge flank roughness on spreading rate the topographic roughness of mid ocean ridge flanks is known to increase with decreasing spreading rate1 4 but the exact form of this relationship has not been established as the topographic features that make up ridge flank roughness abyssal hills are created near the axes of mid ocean ridges by faulting and volcan ism5 7 the relationship between roughness and spreading rate may shed light on the process of crustal accretion at spreading centres here i present measurements of ridge flank roughness on profiles crossing the world mid ocean ridge system which show that roughness is proportional to the inverse square root of the spreading rate this result is consistent with some very simple inferences of how the topographic roughness of mid ocean ridge flanks scales with the lithospheric thickness near ridge axes as determined by thermal models

the role of partial melting in the 15 ma geochemical evolution of gran canaria a blob model for the canary hotspot the subaerial portion of gran canada canary islands was built by three cycles of volcanism a miocene cycle 8 5 15 ma a pliocene cycle 1 8 60 ma and a quaternary cycle 1 8 0 ma only the pliocene cycle is completely exposed on gran canaria the early stages of the miocene cycle are submarine and the quaternary cycle is still in its initial stages during the miocene sio2 saturation of the mafic volcanics decreased systematically from tholeiite to nephelinite for the pliocene cycle sio2 saturation increased and then decreased with decreasing age from nephelinite to tholeiite to nephelinite sio2 saturation increased from nephelinite to basanite and alkali basalt during the quaternary in each of these cycles increasing melt production rates sio2 saturation and concentrations of compatible elements and decreasing concentrations of some incompatible elements are consistent with increasing degrees of partial melting in the sequence melilite nephelinite to tholeiite the mafic volcanics from all three cycles were derived from co2 rich garnet lherzolite sources phlogopite ilmenite sulfide and a phase with high partition coefficients for the light rare earth elements lree u th pb nb and zr possibly zircon were residual during melting to form the miocene nephelinites through tholeiites phlogopite ilmenite and sulfide were residual in the source of the pliocene quaternary nephelinites through alkali basalts highly incompatible element ratios e g nb u pb ce k u nb pb ba rb zr hf la nb ba th rb nb k nb zr nb th nb th la and ba la exhibit extreme variations in many cases larger than those reported for all other ocean island basalts but these ratios correlate well with degree of melting survival of residual phases at higher degrees of melting during the miocene cycle and differences between major and trace element concentrations and melt production rates between the miocene and pliocene tholeiites suggest that the miocene source was more fertile than the pliocene quaternary source s we propose a blob model to explain the multi cycle evolution of canary volcanoes and the temporal variations in chemistry and melt production within cycles each cycle of volcanism represents decompression melting of a discrete blob of plume material small degree nephelinitic and basanitic melts are derived from the cooler margins of the blobs whereas the larger degree tholeiitic and alkali basaltic melts are derived from the hotter centers of the blobs the symmetrical sequence of mafic volcanism for a cycle from highly undersaturated to saturated to highly undersaturated compositions reflects melting of the blob during its ascent beneath an island in the sequence upper margin corelower margin volcanic hiatuses between cycles and within cycles represent periods when residual blob or cooler entrained shallow mantle material fill the melting zone beneath an island

intraseasonal teleconnection between north american and western north pacific monsoons with 20 day time scale abstract based on a recently released high resolution reanalysis dataset for the north american region the intraseasonal variability isv with a time scale of about 20 days of the north american monsoon nam is examined the rainfall signals associated with this phenomenon first emerge near the gulf of mexico and eastern pacific at about 20 n they subsequently migrate to the southwestern united states along the slope of the sierra madre occidental the rainfall quickly dissipates upon arrival at the desert region of arizona and new mexico aznm the enhanced rainfall over aznm is accompanied by strong southeasterly low level flow along the gulf of california this pattern bears strong resemblance to the circulation related to gulf surge events as documented by many studies the southeasterly flow is associated with an anomalous low vortex over the subtropical eastern pacific ocean off california and a midlatitude anticyclone over the central united states in the lower troposphere this flow pat

a transition phase teleconnection of the pacific quasi decadal oscillation the atmospheric circulation patterns associated with the pacific quasi decadal oscillation qdo are investigated using available observational data from 1948 to 2007 previous studies indicate that the pacific qdo is characterized by a distinct lifecycle in the form of sea surface temperature sst patterns in the warm and cool phases of the pacific qdo the sst patterns resemble those associated with the el nino southern oscillation enso during the warm cool and cool warm transitions of the pacific qdo recurrent sst patterns are also clearly visible the rotated empirical orthogonal function analysis on the 10 15 year filtered data shows that the evolutions of sst and atmospheric circulation are well coupled while the warm cool phase pacific qdo generates an enso like circulation pattern the transition phases form a distinct short wave train emanating from southeast asia towards north america this short wave train is particularly robust in the streamfunction of water vapor flux diagnostic analyses of the heat budget the stationary wave flux and a barotropic model indicate that this short wave train is thermodynamically maintained and is likely forced by diabatic heating near southeast asia additional modulations of forcing sources in the western and eastern tropical pacific on this short wave teleconnection are also discussed

the gibraltar subduction a decade of new geophysical data the gibraltar arc spans a complex portion of the africa eurasia plate boundary marked by slow oblique convergence and intermediate and deep focus seismicity the seemingly contradictory observations of a young extensional marine basin surrounded by an arcuate fold and thrust belt have led to competing geodynamic models delamination and subduction geophysical data acquired in the past decade provide a test for these models and support a narrow east dipping subduction zone seismic refraction studies indicate oceanic crust below the western gulf of cadiz tomography of the upper mantle reveals a steep east dipping high p wave velocity body beneath gibraltar the anisotropic mantle fabric from sks splitting shows arc parallel fast directions consistent with toroidal flow around a narrow westward retreating subducting slab the accompanying wsw advance of the rif betic mountain belt has constructed a thick pile of deformed sediments an accretionary wedge characterized by west vergent thrust anticlines bathymetric swath mapping images an asymmetric embayment at the deformation front where a 2 km high basement ridge has collided subduction has slowed significantly since 5 ma but deformation of recent sediments and abundant mud volcanoes suggest ongoing activity in the accretionary wedge three possible origins for this deformation are discussed gravitational spreading overall nw se convergence between africa and iberia and finally a wsw tectonic push from slow but ongoing roll back subduction in the absence of arc volcanism and shallow dipping thrust type earthquakes evidence in favor of present day subduction can only be indirect and remains the object of debate continued activity of the subduction offers a possible explanation for great m 8 5 earthquakes known to affect the area like the famous 1755 great lisbon earthquake recent gps studies show sw motion of stations in n morocco at velocities of 3 6 mm yr indicating the presence of an independent block a rif betic alboran microplate situated between iberia and africa

mechanisms of interannual variability of the southwest united states summer rainfall maximum abstract the mid summer rainfall singularity of the southwest united states principally arizona exhibits marked variations on interannual and decadal time scales examination of the synoptic mechanisms involved in these variations is undertaken here in particular associations between the rainfall the dominant latitude of the summertime mid tropospheric subtropical ridge str over the southwest united states and the sea surface temperatures ssts of the equatorial and north pacific region are documented the analysis utilizes a composite approach for sets of extreme years chosen on the basis of the rainfall and circulation anomalies it is found that northward southward displaced seasonal str is associated with wetter drier summers in arizona further these extremes have tended to follow winters characterized by positive negative phases of the pacific north america pna teleconnection pattern the latter association arises at least in part from the memory imparted to the atmosphere by th

geophysical evidence for a transform margin offshore western algeria a witness of a subduction transform edge propagator for the first time a deep seismic data set acquired in the frame of the algerian french spiral program provides new insights regarding the origin of the westernmost algerian margin and basin we performed a tomographic inversion of traveltimes along a 100 km long wide angle seismic profile shot over 40 ocean bottom seismometers offshore mostaganem northwestern algeria the resulting velocity model and multichannel seismic reflection profiles show a thin 3 4 km thick oceanic crust the narrow ocean continent transition less than 10 km wide is bounded by vertical faults and surmounted by a narrow almost continuous basin filled with miocene to quaternary sediments this fault system as well as the faults organized in a negative flower structure on the continent side marks a major strike slip fault system the extremely sharp variation of the moho depth up to 45 3 degrees beneath the continental border underscores the absence of continental extension in this area all these features support the hypothesis that this part of the margin from oran to tenes trending n65 n70 degrees e is a fossil subduction transform edge propagator fault vestige of the propagation of the edge of the gibraltar subduction zone during the westward migration of the alboran domain

lower crustal intrusion on the north atlantic continental margin near the faroe islands in the north atlantic the majority of basalt is intruded into the continent ocean transition rather than extruded onto the surface this melt is intruded into the lower crust as sills which cross cut the continental fabric rather than as an underplate of 100 melt as has often been assumed when continents break apart the rifting is sometimes accompanied by the production of large volumes of molten rock1 2 3 the total melt volume however is uncertain because only part of it has erupted at the surface furthermore the cause of the magmatism is still disputed specifically whether or not it is due to increased mantle temperatures we recorded deep penetration normal incidence and wide angle seismic profiles across the faroe and hatton bank volcanic margins in the northeast atlantic here we show that near the faroe islands for every 1 km along strike 360 400 km3 of basalt is extruded while 540 600 km3 is intruded into the continent ocean transition we find that lower crustal intrusions are focused mainly into a narrow zone 50 km wide on the transition although extruded basalts flow more than 100 km from the rift seismic profiles show that the melt is intruded into the lower crust as sills which cross cut the continental fabric rather than as an underplate of 100 per cent melt as has often been assumed evidence from the measured seismic velocities and from igneous thicknesses are consistent with the dominant control on melt production being increased mantle temperatures with no requirement for either significant active small scale mantle convection under the rift or the presence of fertile mantle at the time of continental break up as has previously been suggested for the north atlantic ocean4 5 6

crustal structure of the central nova scotia margin off eastern canada summary the central nova scotia margin off eastern canada is located at a transition from a volcanic margin in the south to a non volcanic margin in the north in order to study this transition a wide angle refraction seismic line with dense airgun shots was acquired across the central nova scotia margin the 500 km long transect is coincident with previous deep reflection profiles across the lahave platform and extending into the sohm abyssal plain a p wave velocity model was developed from forward and inverse modelling of the wide angle data from 21 ocean bottom seismometers and coincident normal incidence reflection profiles the velocity model shows that the continental crust is divided into three layers with velocities of 5 5 6 9 km s 1 the maximum thickness is 36 km a minor amount 5 km of thinning occurs beneath the outer shelf while the major thinning to a thickness of 8 km occurs over the slope region the seaward limit of the continental crust consists of 5 km thick highly faulted basement there is no evidence for magmatic underplating beneath the continental crust on the contrary a 4 km thick layer of partially serpentinized mantle 7 6 7 95 km s 1 begins beneath the highly faulted continental crust and extends 200 km seawards forming the lower part of the oceancontinent transition zone the upper part of the transition zone consists of the highly faulted continental crust and 4 to 5 km thick initial oceanic crust the continent ocean boundary is moved 50 km farther seawards compared to an earlier interpretation based only on reflection seismic data the oceanic crust in the transition zone consists of layer 2 and a high velocity lower crustal layer layer 2 is 1 3 km thick with velocities of 5 6 6 0 km s 1 the high velocity lower crustal layer is 1 2 km thick with velocities of 7 25 7 4 km s 1 suggesting a composite layer of serpentinized peridotite and gabbroic layer 3 oceanic crust with normal thickness of 5 7 km and more typical layer 3 with velocities of 6 95 7 3 km s 1 is observed at the seaward end of the profile

impact of the quasi biweekly oscillation over the western north pacific on east asian subtropical monsoon during early summer 1 the impact of quasi biweekly oscillation qbwo over the western north pacific on east asian summer monsoon easm is investigated the life cycle of qbwo is divided into eight phases defined by the two leading principal components pc1 and pc2 of an empirical orthogonal function analysis subtropical rainfall shows significant changes with a northwestward propagation of convection from equatorial regions to the south china sea scs the most significant variations occur in qbwo phases 3 and 4 enhanced convection over scs and phases 7 and 8 reduced convection over scs the east asia mei yu significantly decreases in qbwo phases 3 and 4 but increases in phases 7 and 8 the qbwo influences easm through modulating the subtropical monsoon flow and extratropical circulation the response of lower tropospheric atmosphere to qbwo shows a northwestward propagation and a downstream wave train that extends northward into the western north pacific modulating the scs monsoon trough and the easm flow associated with moisture transportation the mid tropospheric extratropical circulation and the western pacific subtropical high also show obvious changes accompanying qbwo evolution resulting in circulation patterns associated with cold air activity moreover changes with qbwo are found in the upper tropospheric east asian westerly jet stream and the south asian high and these changes contribute to upper level divergence over subtropical east asia

large igneous province on the us atlantic margin and implications for magmatism during continental breakup rifted continental margins commonly include sections of igneous rock more than twice as thick as normal oceanic crust explanations for this voluminous magmatic accretion during rifting include plume models1 3 which require a deep seated thermal or chemical anomaly in upwelling mantle and non plume models4 7 which call on broad shallow thermal anomalies and or rapid upwelling of mantle through the melting zone new seismic models from two transects across the continent ocean transition on the us atlantic margin8 10 confirm the presence of a 20 25 km thick igneous section here we argue that the similarity of the crustal structure on these and two previous transects spanning 1 000 km of the margin and the association of thick igneous crust with the east coast magnetic anomaly11 imply that the thick igneous section extends along the entire margin and may have a volume of as much as 3 2 106 km3 the distribution of volcanic and plutonic rocks details of the seismic structure and lack of independent evidence for a hotspot are difficult to reconcile with plume models and suggest that non plume processes created the thick igneous crust

principal component analysis of the summertime winds over the gulf of california a gulf surge index a principal component analysis of the summertime near surface quick scatterometer quikscat winds is used to identify the leading mode of synoptic scale variability of the low level flow along the gulf of california during the north american monsoon season a gulf surge mode emerges from this analysis as the leading eof with the corresponding principal component time series interpretable as an objective index for gulf surge occurrence this index is used as a reference time series for regression analysis and compositing meteorological fields of interest to explore the relationship between gulf surges and precipitation over the core and marginal regions of the monsoon as well as the manifestation of these transient events in the large scale circulation it is found that although seemingly mesoscale features confined over the gulf of california gulf surges are intimately linked to patterns of large scale variability of the eastern pacific itcz and greatly contribute to the definition of the northward extent of the monsoonal rains

the southernmost margin of the tethys realm during the mesozoic and cenozoic initial geometry and timing of the inversion processes 1 along the northern border of africa pangea breakup has been diachronic during the jurassic the alpine tethys propagated northeastward from the atlantic to the alps during the permian the neo tethys propagated westward from oman to northwestern arabia then a secondary and late branch of neo tethys gave birth to the east mediterranean basin finally the two oceans connected at end of jurassic times achieving the development of africa northern plate boundary by the late cretaceous convergence between africa and eurasia led to the progressive closure of the tethys realm the continental collision is not completely achieved and the different segments of the confrontation zone maghreb central and east mediterranean zagros and oman expose different stages of the process however we emphasize the existence of synchronous geodynamic events from one end of the system to the other although they do not have the same meaning two of them are particularly important the campanian santonian c s event corresponds to 1 obduction and exhumation of high pressure low temperature metamorphic rocks around the arabian promontory 2 inversion along the margins of the east mediterranean basins and 3 lithosphere buckling in the atlas system maghreb and adjacent sahara platform the middle late eocene mle event corresponds to 1 the onset of collision at the northern corner of arabia 2 the onset of slab retreat in the mediterranean and 3 inversion along the margin of the east mediterranean as well as in the atlas the c s event coincides with a change in plate kinematics resulting in an abrupt increase of convergence velocity the mle event coincides with a period of strong coupling between the africa and eurasia plates and an abrupt decrease of convergence velocity in the middle of the system the central mediterranean seems to escape to the effects of convergence and is the site of quite permanent extensional movements since the triassic

crustal structure of the sw moroccan margin from wide angle and reflection seismic data the dakhla experiment part a wide angle seismic models abstract a total 1500 km of seismic reflection and wide angle profiles were acquired off the southern moroccan margin during the dakhla cruise a joint project of ifremer the universities of brest el jadida and lisbon and total the shots along two profiles parallel to the margin and two profiles perpendicular to the margin were also recorded by ocean bottom seismometers obs the profiles perpendicular to the margin were additionally extended on land using 14 stations on the northern profile and 11 stations on the southern profile modelling of the reflection and wide angle seismic data reveals a 10 km deep sedimentary basin including two high velocity carbonate layers lateral crustal thinning is observed from a 27 km thick crystalline continental crust to a 7 km thick oceanic crust occurring over less than 100 km the crystalline continental crust can be divided into two distinct layers of 12 and 15 km thickness the oceanic crust east of the magnetic anomaly m25 displays higher velocities in layer 3 than west of the magnetic anomaly the change in seismic velocity suggests a possible link to changes in accretionary processes of the oceanic crust some regions show seismic velocities between 6 8 and 7 4 km s which could be explained by slightly elevated mantle temperatures during accretion of the crust

the role of premagmatic rifting in shaping a volcanic continental margin an example from the eastern north american margin both magmatic and tectonic processes contribute to the formation of volcanic continental margins such margins are thought to undergo extension across a narrow zone of lithospheric thinning 100 k

spreading rate dependence of melt extraction at mid ocean ridges from mantle seismic refraction data a variety of observations indicate that mid ocean ridges produce less crust at spreading rates below 20 mm yr 1 refs 1 3 reflecting changes in fundamental ridge processes with decreasing spreading rate the nature of these changes however remains uncertain with end member explanations being decreasing shallow melting or incomplete melt extraction each due to the influence of a thicker thermal lid here we present results of a seismic refraction experiment designed to study mid ocean ridge processes by imaging residual mantle structure our results reveal an abrupt lateral change in bulk mantle seismic properties associated with a change from slow to ultraslow palaeo spreading rate changes in mantle velocity gradient basement topography and crustal thickness all correlate with this spreading rate change these observations can be explained by variations in melt extraction at the ridge with a gabbroic phase preferentially retained in the mantle at slower spreading rates the estimated volume of retained melt balances the approximately 1 5 km difference in crustal thickness suggesting that changes in spreading rate affect melt extraction processes rather than total melting

hotspot volcanism close to a passive continental margin the canary islands the canarian archipelago is a group of volcanic islands on a slow moving oceanic plate close to a continental margin the origins of the archipelago are controversial a hotspot or mantle plume a zone of lithospheric deformation a region of compressional block faulting or a rupture propagating westwards from the active atlas mountains fold belt have been proposed by different authors however comparison of the canarian archipelago with the prototypical hotspot related island group the hawaiian archipelago reveals that the differences between the two are not as great as had previously been supposed on the basis of older data quaternary igneous activity in the canaries is concentrated at the western end of the archipelago close to the present day location of the inferred hotspot this is the same relationship as seen in the hawaiian and cape verde islands the latter archipelago associated with a well defined but slow moving mantle plume shows anomalies in a plot of island age against distance which are comparable to those seen in the canary islands these anomalies cannot therefore be used to argue against a hotspot origin for the canaries individual islands in both archipelagoes are characterized by initial rapid growth the shield building stages of activity followed by a period of quiescence and deep erosion erosion gap which in turn is followed by a post erosional stage of activity the absence of post shield stage subsidence in the canaries is in marked contrast with the major subsidence experienced by the hawaiian islands but is comparable with the lack of subsidence evident in other island groups at slow moving hotspots such as the cape verdes comparison of the structure and structural evolution of the canary islands with other oceanic islands such as hawaii and reunion reveals many similarities these include the development of triple mercedes star rift zones and the occurrence of giant lateral collapses on the flanks of these rift zones the apparent absence of these features in the post erosional islands may in part be a result of their greater age and deeper erosion which has removed much of the evidence for their early volcanic architecture we conclude that the many similarities between the canary islands and island groups whose hotspot origins are undisputed show that the canaries have been produced in the same way

quasi periodic oscillations in a symmetric general circulation model abstract long lime integrations with a symmetric version of the clas climate model with hydrology have shown that the hadley circulation has well defined strong and weak episodes this oscillation of the hadley circulation seems to occur in two dominant range of periodicities one with periods between 10 and 15 days and another with periods between 20 and 40 days this quasi periodic behavior of the hadley circulation is seen only when the moist convective heating is determined by dynamics if the latent beating in the atmosphere is prescribed and kept artificially fixed this oscillation of the hadley circulation disappears thus this oscillation of the hadley circulation appears as a result of interactions between moist convective and dynamical processes a wave phenomenon is seen in the lower atmosphere that propagates toward the position of maximum radiative beating this wave perturbation has a length scale of about 15 20 degrees latitude in the north south direction this phenomenon has large ampli

simulation of the intraseasonal variability over the eastern pacific itcz in climate models during boreal summer convective activity over the eastern pacific epac inter tropical convergence zone itcz exhibits vigorous intraseasonal variability isv previous observational studies identified two dominant isv modes over the epac i e a 40 day mode and a quasi biweekly mode qbm the 40 day isv mode is generally considered a local expression of the madden julian oscillation however in addition to the eastward propagation northward propagation of the 40 day mode is also evident the qbm mode bears a smaller spatial scale than the 40 day mode and is largely characterized by northward propagation while the isv over the epac exerts significant influences on regional climate weather systems investigation of contemporary model capabilities in representing these isv modes over the epac is limited in this study the model fidelity in representing these two dominant isv modes over the epac is assessed by analyzing six atmospheric and three coupled general circulation models gcms including one super parameterized gcm spcam and one recently developed high resolution gcm gfdl hiram with horizontal resolution of about 50 km while it remains challenging for gcms to faithfully represent these two isv modes including their amplitude evolution patterns and periodicities encouraging simulations are also noted in general spcam and hiram exhibit relatively superior skill in representing the two isv modes over the epac while the advantage of spcam is achieved through explicit representation of the cumulus process by the embedded 2 d cloud resolving models the improved representation in hiram could be ascribed to the employment of a strongly entraining plume cumulus scheme which inhibits the deep convection and thus effectively enhances the stratiform rainfall the sensitivity tests based on hiram also suggest that fine horizontal resolution could also be conducive to realistically capture the isv over the epac particularly for the qbm mode further analysis illustrates that the observed 40 day isv mode over the epac is closely linked to the eastward propagating isv signals from the indian ocean western pacific which is in agreement with the general impression that the 40 day isv mode over the epac could be a local expression of the global madden julian oscillation mjo in contrast the convective signals associated with the 40 day mode over the epac in most of the gcm simulations tend to originate between 150 e and 150 w suggesting the 40 day isv mode over the epac might be sustained without the forcing by the eastward propagating mjo further investigation is warranted towards improved understanding of the origin of the isv over the epac

deep structure of the continental margin and basin off greater kabylia algeria new insights from wide angle seismic data modeling and multichannel seismic interpretation abstract during the algerian french spiral survey aimed at investigating the deep structure of the algerian margin and basin two coincident wide angle and reflection seismic profiles were acquired in central algeria offshore greater kabylia together with gravimetric bathymetric and magnetic data this 260 km long offshore onshore profile spans the balearic basin the central algerian margin and the greater kabylia block up to the southward limit of the internal zones onshore results are obtained from modeling and interpretation of the combined data sets the algerian basin offshore greater kabylia is floored by a thin oceanic crust 4 km with p wave velocities ranging between 5 2 and 6 8 km s in the northern hannibal high region the atypical 3 layer crustal structure is interpreted as volcanic products stacked over a thin crust similar to that bordering the margin and related to miocene post accretion volcanism these results support a two step back arc opening of the west algerian basin comprising oceanic crust accretion during the first southward stage and a magmatic and probably tectonic reworking of this young oceanic basement during the second westward opening phase the structure of the central algerian margin is that of a narrow 70 km magma poor rifted margin with a wider zone of distal thinned continental crust than on the other margin segments there is no evidence for mantle exhumation in the sharp ocean continent transition but transcurrent movements during the second opening phase may have changed its initial geometry the plio quaternary inversion of the margin related to ongoing convergence between africa and eurasia is expressed by a blind thrust system under the margin rising toward the surface at the slope toe and by an isostatic disequilibrium resulting from opposite flexures of two plates decoupled at the continental slope this disequilibrium is likely responsible for the peculiar asymmetrical shape of the crustal neck that may thus be a characteristic feature of inverted rifted margins

the relationship between tropical easterly waves and surges over the gulf of california during the north american monsoon abstract coastally trapped disturbances which occur over the gulf of california and are commonly referred to as gulf surges are an important mechanism in the transport of low level moisture into the southwestern united states to determine whether or not gulf surges develop in association with the passage of tropical easterly waves as hypothesized in previous studies standard surface observations and european centre for medium range weather forecasts reanalysis data during the months of july and august over a 14 yr period are examined results indicate that tropical easterly wave troughs often cross western mexico 1 3 days prior to surge onset at yuma arizona indicating a strong correlation between these two events this analysis documents yet another mechanism by which the tropics influences convection in the midlatitudes

tropical convection and precipitation regimes in the western united states abstract the authors have documented the relationship between tropical convection and precipitation regimes in the western united states circulation patterns associated with precipitation regimes are described and physical mechanisms are proposed contributions from intraseasonal and interannual bands are examined when enhanced convection is located in the western pacific dry conditions in the southwest sw and wet conditions in the pacific northwest pnw are observed fluctuations in both intraseasonal and interannual bands contribute to the rainfall variability enhanced convection in the western pacific is accompanied by suppressed convection in the central pacific the associated rossby wave vorticity source s anomalies keep the pacific jet west of 150 w a westward shift of the storm track to the north pacific also contributes to dry conditions in the sw and wetness in the pnw when enhanced tropical convection is located near 150 e substantial contributions to outgoing longwave radiation ano

incipient subduction at the contact with stretched continental crust the puysegur trench a seismic benioff zone and plate kinematics show puysegur trench south of new zealand transitioning to subduction because the local structure and its influence on subduction initiation is poorly understood we conducted a seismic survey with ocean bottom seismometers and multichannel seismic profiles our early results show that the overriding pacific plate beneath the solander basin is composed of block faulted and thinned continental crust and the inner trench wall of northern puysegur ridge is composed of folded and faulted sediment the megathrust interface has been imaged and shows 500 m of downgoing undisturbed sediments combining plate kinematic history with seismic velocity inferred density we show that the density difference across the plate boundary changed as oblique strike slip plate motion juxtaposed dense oceanic crust with thinned continental crust the density difference rapidly increased 18 to 15 ma coincident with subduction initiation suggesting that compositional differences have a large influence on subduction initiation

teleconnections associated with northern hemisphere summer monsoon intraseasonal oscillation the boreal summer intraseasonal oscillation bsiso has strong convective activity centers in indian i western north pacific wnp and north american na summer monsoon sm regions the present study attempts to reveal bsiso teleconnection patterns associated with these dominant intraseasonal variability centers during the active phase of ism a zonally elongated band of enhanced convection extends from india via the bay of bengal and philippine sea to tropical central pacific with suppressed convection over the eastern pacific near mexico the corresponding extratropical circulation anomalies occur along the waveguides generated by the north african asian jet and north atlantic north european jet when the tropical convection strengthens over the wnpsm sector a distinct great circle like rossby wave train emanates from the wnp to the western coast of united states us with an eastward shift of enhanced meridional circulation in the active phase of nasm large anticyclonic anomalies anchor over the western coast of us and eastern canada and the global teleconnection pattern is similar to that during a break phase of the ism examination of the evolution of the bsiso teleconnection reveals quasi stationary patterns with preferred centers of teleconnection located at europe russia central asia east asia western us and eastern us and canada respectively most centers are embedded in the waveguide along the westerly jet stream but the centers at europe and russia occur to the north of the jet induced waveguide eastward propagation of the iso teleconnection is evident over the pacific north america sector the rainfall anomalies over the elongated band near the monsoon domain over the indo western pacific sector have an opposite tendency with that over the central and southern china mexico and southern us providing a source of intraseasonal predictability to extratropical regions the bsiso teleconnection along and to the north of the subtropical jet provides a good indication of the surface sir temperature anomalies in the nh extratropics

salt tectonics along the atlantic margin of morocco abstract the moroccan salt basin appears to be very unique in the sense that it is not sedimentary loading but tectonic inversion which appears to drive the latest stages of salt related deformation in the central part of the basin the gravity potential to maintain salt tectonics is provided by the differential uplift of the atlas mountains located adjacent and striking almost perpendicular to the margin along the offshore part of the moroccan salt basin there are many play types most of them related to the triassic syn rift salt toe thrust anticlines at the basinwards edge of the salt basin form very large structures traps associated with salt tongues and diapirs define a more classical salt flank play numerous salt tongues sheets and canopy complexes provide for a gulf of mexico style subsalt play despite the numerous untested play types there have been only four deep water exploration wells drilled in the entire moroccan salt basin none of them having subsalt penetrations

crustal structure of the sw moroccan margin from wide angle and reflection seismic data the dakhla experiment part b the tectonic heritage abstract the dakhla 2002 experiment on the northwest african margin obtained deep penetration multichannel reflection mcs and wide angle seismic obs data these results were combined with total magnetic intensity data kinematic reconstructions and geological field studies to decipher the early tectonic and sedimentary history of the south moroccan margin adjacent to the reguibat shield the structural study of this margin shows that the crust thins from about 28 km beneath the un thinned continental domain to less than 10 km over a lateral distance of about 120 km the dakhla margin differs from the structural scheme of the adjacent african margin segments there the moho rise generally observed just above the deepening of the top of the basement is achieved with an offset of around 50 km with respect to the latter in early jurassic time the break up of continental crust in what is now the central atlantic led to the formation of two basins one on either side of the reguibat which correspond to the older parts of segments of the late palaeozoic hercynian orogen the dakhla segment appears as a precambrian cratonic zone squeezed between two orogenic segments which have remained unaffected by break up processes and the lower crust of this particular domain has behaved differently from all other neighbouring appalachian north america and mauritanide domains this observation points to the important role of tectonic inheritance in the structural development of passive continental margins

seaward dipping reflectors offshore the southeastern united states seismic evidence for extensive volcanism accompanying sequential formation of the carolina trough and blake plateau basin deep penetration multichannel seismic reflection profiles off the southeastern united states reveal widespread seaward dipping reflectors sdrs similar features have been imaged and sampled on other north atlantic rifted margins where voluminous volcanism has accompanied continental breakup beneath the carolina trough are two sets of sdrs one along a basement hinge zone and another seaward of the east coast magnetic anomaly axis the hinge sdrs lie beneath and apparently developed prior to a flood basalt that erupted at 184 3 ma and is marked by a prominent reflector j beneath the northern blake plateau basin only the hinge sdrs are observed but they developed after j we suggest that the inferred north to south age difference of sdr emplacement implies a heretofore unrecognized time transgressive breakup of northwest africa and north america during the early middle jurassic

origin of the intraseasonal variability over the north pacific in boreal summer abstractthe spatial structure and temporal evolution of the intraseasonal oscillation iso in boreal summer over the midlatitude north pacific ocean are investigated through the diagnosis of ncep reanalysis data it is found that the midlatitude iso has an equivalent barotropic structure with maximum amplitude at 250 hpa initiated near 120 w the iso perturbation propagates westward at a phase speed of about 2 4 m s 1 and reaches a maximum amplitude at 150 w a diagnosis of barotropic energy conversion shows that the iso gains energy from the summer mean flow in the iso activity region a center followed column averaged vorticity budget analysis shows that the nonlinear eddy meridional vorticity transport plays a major role in the growth of the iso perturbation there is a two way interaction between iso flows and synoptic eddies while a cyclonic anticyclonic iso flow causes synoptic scale eddies to tilt toward the northwest southeast northeast southwest direction the tilted synoptic eddies then

the formation mechanism of the bonin high in august the bonin high is a subtropical anticyclone that is predominant near japan in the summer this anticyclone is associated with an equivalent barotropic structure often extending throughout the entire troposphere although the equivalent barotropic structure of the bonin high has been known for years among synopticians because of its importance to the summer climate in east asia there are few dynamical explanations for such a structure the present paper attempts to provide a formation mechanism for the deep ridge near japan we propose a new hypothesis that this equivalent barotropic ridge near japan is formed as a result of the propagation of stationary rossby waves along the asian jet in the upper troposphere the silk road pattern first the monthly mean climatology is examined in order to demonstrate this hypothesis it is shown that the enhanced asian jet in august is favourable for the propagation of stationary rossby waves and that the regions of descent over the eastern mediterranean sea and the aral sea act as two major wave sources second a primitive equation model is used to simulate the climatology of august the model successfully simulates the bonin high with an equivalent barotropic structure the upper tropospheric ridge is found to be enhanced by a height anomaly of more than 80 m at 200 hpa when a wave packet arrives sensitivity experiments are conducted to show that the removal of the diabatic cooling over the asian jet suppresses the silk road pattern and formation of an equivalent barotropic ridge near japan while the removal of the diabatic heating in the western pacific does not copyright 2003 royal meteorological society

characteristics and mechanisms of the subseasonal eastward extension of the south asian high abstractthis study investigates the features of eastward extension of the south asian high sah and its connection with diabatic heating and rainfall over eastern asia on subseasonal time scales the causes of sah s eastward extension are examined by potential vorticity pv diagnosis with emphasis on the joint role of diabatic heating feedback and midlatitude wave train the sah s eastward extension features eastward propagation of a wave train across eurasia among the wave train the migration of weak high from the western flank of the tibetan plateau tp to the east of tp contributes to the sah s eastward extension at the early stage when the sah approaches its easternmost position a strong negative pv positive geopotential height center prevails to the east of the tp at 200 hpa the associated anomalies in diabatic heating and rainfall include the anomalous heating and above normal rainfall over the south china sea scs and subtropical western pacific occurring 12 days before the sah s eastern

opening of the central atlantic ocean implications for geometric rifting and asymmetric initial seafloor spreading after continental breakup study of the deep structure of conjugate passive continental margins combined with detailed plate kinematic reconstructions can provide constraints on the mechanisms of rifting and formation of initial oceanic crust in this study the central atlantic conjugate margins are compared based on compilation of wide angle seismic profiles from nw africa nova scotian and u s passive margins the patterns of volcanism crustal thickness geometry and seismic velocities in the transition zone suggest symmetric rifting followed by asymmetric oceanic crustal accretion conjugate profiles in the southern central atlantic image differences in the continental crustal thickness while profiles on the eastern u s margin are characterized by thick layers of magmatic underplating no such underplate was imaged along the african continental margin in the north two wide angle seismic profiles acquired in exactly conjugate positions show that the crustal geometry of the unthinned continental crust and the necking zone are nearly symmetric a region including seismic velocities too high to be explained by either continental or oceanic crust is imaged along the canadian side corresponding on the african side to an oceanic crust with slightly elevated velocities these might result from asymmetric spreading creating seafloor by faulting the existing lithosphere on the canadian side and the emplacement of magmatic oceanic crust including pockets of serpentinite on the moroccan margin after isochron m25 a large scale plate reorganization might then have led to an increase in spreading velocity and the production of thin magmatic crust on both sides

imaging a lithospheric detachment at the continent ocean crustal transition off morocco abstract the sismar seismic survey on the moroccan atlantic margin recorded deep penetration images of the continent ocean boundary this paper focuses on the 3d observation of a landward dipping reflector overlaid by a layered unit the deep part of the reflector plunges towards the continent beneath thinned continental crust whereas its upper part finishes at the top of the basement in a transition zone where likely volcanics dipping reflectors are visible and where a magnetic anomaly named s1 marks the continent ocean boundary location as the landward dipping reflector crosses the moho it is interpreted as a lithospheric detachment with associated volcanic material originating from the asthenospheric mantle observations are used i to build a 2d model to explain the mechanisms of the lithospheric breaking off between moroccan margin and its conjugate canadian margin the landward dipping reflector allowing mantle exhumation ii to propose an ene wnw trend for the initial rifting extension in this part of atlantic during late triassic direction compatible with that of the southern grand banks transform margin

deep crustal structure of the north west african margin from combined wide angle and reflection seismic data mirror seismic survey the structure of the moroccan and nova scotia conjugate rifted margins is of key importance for understanding the mesozoic break up and evolution of the northern central atlantic ocean basin seven combined multichannel reflection mcs and wide angle seismic obs data profiles were acquired along the atlantic moroccan margin between the latitudes of 31 5 and 33 n during the mirror seismic survey in 2011 in order to image the transition from continental to oceanic crust to study the variation in crustal structure and to characterize the crust under the west african coast magnetic anomaly wacma the data were modeled using a forward modeling approach the final models image crustal thinning from 36 km thickness below the continent to approximately 8 km in the oceanic domain a 100 km wide zone characterized by rough basement topography and high seismic velocities up to 7 4 km s in the lower crust is observed westward of the west african coast magnetic anomaly no basin underlain by continental crust has been imaged in this region as has been identified north of our study area comparison to the conjugate nova scotian margin shows a similar continental crustal thickness and layer geometry and the existence of exhumed and serpentinized upper mantle material on the canadian side only the oceanic crustal thickness is lower on the canadian margin

crustal structure gravity anomalies and flexure of the lithosphere in the vicinity of the canary islands summary simple elastic plate models have been used to calculate the flexure of the lithosphere caused by volcanic loading at the canary islands and sediment loading at the moroccan continental margin by comparing the calculated flexure to observations based on seismic refraction and free air gravity anomaly data constraints have been placed on the long term 106 years elastic thickness of the lithosphere te the best fit between the calculated and observed flexure in the vicinity of the canary islands is for te 20 km this value of te also explains seismic reflection data in regions that flank the island provided that the lithosphere underlying the moroccan margin is sufficiently weak te 5 km for sediment loading to contribute little to the island flexure a backstripping study in which gravity data are used to constrain the value of te supports the suggestion that the lithosphere underlying the moroccan margin like its conjugate at the baltimore canyon trough is weak although sediment loading at the moroccan margin appears therefore to have exerted little influence on the structure of the canary islands there is evidence that island flexure may have influenced the stratigraphic development of the moroccan margin especially in the region of the slope anticline a te of 20 km is about 15 km lower than would be expected on the basis of the thermal age of the oceanic lithosphere that underlies the canary islands similar low values have been reported from oceanic islands in the pacific superswell region of french polynesia where they have been attributed to re heating of the lithosphere by one or more hotspots a general e w age progression of the volcanic rocks suggests that the canary islands were also generated by a hotspot they lack however the topographic swell and gravity geoid high which usually accompanies these features one possibility is that the low te values are the result of a pre existing weakness in the oceanic crust a te of 20 km is large enough however for a significant part of the mantle to still be involved in the support of the island loads a more likely explanation is thermal weakening by a hotspot which has been localized enough to reduce te but not to produce a swell or gravity geoid high irrespective of its cause the low te suggests that oceanic lithosphere does not necessarily progressively increase its strength with age so that even 140 ma old lithosphere is vulnerable and can in some regions be significantly weakened by later thermal and mechanical processes

initial rifting and breakup between nova scotia and morocco insight from new magnetic models1 2 1this article is one of a series of papers published in this cjes special issue on the theme of mesozoic cenozoic geology of the scotian basin 2earth sciences sector contribution 20120024 new models of magnetic data are presented that provide insight into the early stages of rifting and breakup between morocco and nova scotia the margins which began forming during the late triassic rifting and middle jurassic separation of the north american and african plates display considerable variability in deeper crustal structure faulting style and basin geometry along their length the central and northeastern portions of the nova scotia margin show characteristics of a magma poor margin with a high degree of crustal thinning and a complex ocean continent transition zone with little direct evidence of volcanism in contrast the margin to the southwest of nova scotia has clearly recognized characteristics of a volcanic style rifted margin including seaward dipping reflector sdr sequences that are interpreted as rift related volcanic flows overlying basement these sdrs are coincident with a strong linear magnetic anomaly the east coast magnetic anomaly ecma which shares many characteristics with the west african coast magnetic anomaly wacma both magnetic anomalies change character and diminish in amplitude northward along the margins the new models show the expected decrease in magnetic source material towards the northeastern end of the margin and suggest that modest amounts of igneous material emplaced at or near the edge of the thinned continental crust will satisfy the observed anomalies in this magma poor section of the margin these new interpretations and supporting evidence indicate volcanism was a factor along much of the margin during early rifting between nova scotia and morocco resume de nouveaux modeles de donnees magnetiques sont presentes ils donnent des apercus des premieres etapes de distension et de rupture entre le maroc et la nouvelle ecosse les bordures ont commence ase former durant la distension

hotspot tracks and the early rifting of the atlantic abstract many hotspot tracks appear to become the locus of later rifting as though the heat of the hotspot weakens the lithosphere and tens of millions of years later the continents are split along these weakened lines examples are the west coast of greenland east coast of labrador madeira hotspot the south coast of mexico north coast of honduras guyana hotspot and the south coast of west africa north coast of brazil st helena hotspot a modern day analog of a possible future rift is the snake river plain where the north american continent is being pre weakened by the yellowstone hotspot track this conclusion is based on reconstructions of the motions of the continents over hotspots for the past 200 million years the relative motions of the plates are determined from magnetic anomaly isochrons in the oceans and the motion of one plate is chosen ad hoc to best fit the motions of the plates over the hotspots however once the motion of this one plate is chosen the motions of all the other plates are prescribed by the relative motion constraints in addition to the correlation between the predicted tracks and sites of later continental breakup exposed continental shields correlate with the tracks their exposure may be the result of hotspot induced uplift which has led to erosion of their former platform sediment cover

lower crustal composition and crustal lineaments on the voring margin ne atlantic a review abstract a prominent lower crustal high velocity layer observed in the voring basin ne atlantic has been investigated with regard to composition and spatial link to crustal scale lineaments in the southernmost part of the basin the lower crustal layer is interpreted as eclogites based on anomalously high p wave velocities 8 4 km s in the remainder of the basin the layer is preferably interpreted as mafic intrusions emplaced during the last phase of rifting but it cannot be excluded that the body consists of older caledonian mafic rocks direct link with high grade caledonian rocks to the east and westwards to east greenland cannot be made since these rocks are mainly of felsic composition the crustal scale lineaments inferred from wide angle seismic data appear to represent the oceanward extension of post caledonian detachments the spatial variation in the thickness of the layer is within the late rifting magmatic hypothesis partly interpreted in terms of pre magmatic variations in thickness of the lithosphere in addition lateral variations in extension mantle upwelling and decompressional melting during the last phase of rifting may also have contributed

an alternative early opening scenario for the central atlantic ocean abstract the opening of the central atlantic ocean basin that separated north america from northwest africa is well documented and assumed to have started during the late jurassic however the early evolution and the initial breakup history of pangaea are still debated most of the existing models are based on one or multiple ridge jumps at the middle jurassic leaving the oldest crust on the american side between the east coast magnetic anomaly ecma and the blake spur magnetic anomaly bsma according to these hypotheses the bsma represents the limit of the initial basin and the footprint subsequent to the ridge jump consequently the evolution of the northwest african margin is widely different from the northeast american margin however this setting is in contradiction with the existing observations in this paper we propose an alternative scenario for the continental breakup and the mesozoic spreading history of the central atlantic ocean the new model is based on an analysis of geophysical data including new seismic lines an interpretation of the newly compiled magnetic data and satellite derived gravimetry and recently published results which demonstrate that the opening of the central atlantic ocean started already during the late sinemurian 190 ma based on a new identification of the african conjugate to the ecma and on the extent of salt provinces off morocco and nova scotia the identification of an african conjugate magnetic anomaly to bsma the african blake spur magnetic anomaly absma together with the significant change in basement topography are in good agreement with that initial reconstruction the early opening history for the central atlantic ocean is described in four distinct phases during the first 20 myr after the initial breakup 190 170 ma from late sinemurian to early bajocian oceanic accretion was extremely slow 0 8 cm y at the time of blake spur 170 ma early bajocian a drastic change occurred both in the relative plate motion direction from nnw sse to nw se and in the spreading rate an increase to 1 7 cm y after a small increase between chron m25 154 ma kimmeridgian and chron m22 150 ma tithonian the spreading rate slowed down to about 1 3 cm y and remained fairly constant until chron m0 125 ma barremian aptian boundary in addition kinematic reconstructions illustrate a significant spreading asymmetry during the early history of the central atlantic ocean the accretion rates were higher on the american side and led to the formation of more oceanic crust on this plate we infer that this asymmetry could be related to the fact that the thermal anomaly responsible for the significant magmatism of the central atlantic magmatic province camp was preferentially located below the african plate

the low level circulation of the north american monsoon as revealed by quikscat five years 1999 2003 of near surface quikscat ocean winds over the gulf of california and northeast pacific ocean are used to characterize the changes in the low level circulation associated with the north american monsoon our analysis shows that the onset of the summer season is accompanied by a seasonal reversal of the flow along the gulf of california with the establishment of a time mean southerly wind throughout the gulf this reversal not evident in the global reanalysis products occurs in late spring and precedes the onset of the monsoonal rains in the core of the monsoon the time mean flow is found to be modulated by transient events namely gulf surges detected in the near surface wind field as periods of enhanced southerly flow which typically originate at the southern end of the gulf and propagate northward the histogram of the summertime along shore winds identifies these surges as a distinct population of events readily distinguishable from the background flow

surges over the gulf of california during the mexican monsoon abstract northward surges of relatively cool moist maritime air from the tropical pacific into the southwestern united states occur via the gulf of california every summer during the mexican monsoon season these surges advect large amounts of moisture northward and promote increased convective activity in arizona mesoscale model output from 32 successive 24 h simulations are used to examine both the large scale and mesoscale features associated with surge events model data are compared to high resolution aircraft measurements on two days showing clearly that the model is capable of reproducing many of the detailed structures found in the observations results from a large scale analysis indicate that strong surges are produced in the model when a midlatitude trough is in the proper phase relationship with a tropical easterly wave weak surges occur when this relationship is missing

circumglobal teleconnection in the northern hemisphere summer analysis of the 56 yr ncep ncar reanalysis data reveals a recurrent circumglobal teleconnection cgt pattern in the summertime midlatitude circulation of the northern hemisphere this pattern represents the second leading empirical orthogonal function of interannual variability of the uppertropospheric circulation the cgt having a zonal wavenumber 5 structure is primarily positioned within a waveguide that is associated with the westerly jet stream the spatial phases of cgt tend to lock to preferred longitudes the geographically phase locked patterns bear close similarity during june august and september but the pattern in july shows shorter wavelengths in the north pacific north america sector the cgt is accompanied by significant rainfall and surface air temperature anomalies in the continental regions of western europe european russia india east asia and north america this implies that the cgt may be a source of climate variability and predictability in the above mentioned midlatitude regions the cgt has significant correlations with the indian summer monsoon ism and el nino southern oscillation enso however in normal ism years the cgt enso correlation disappears on the other hand in the absence of el nino or la nina the cgt ism correlation remains significant it is suggested that the ism acts as a conductor connecting the cgt and enso when the interaction between the ism and enso is active enso may influence northern china via the ism and the cgt additionally the variability of the cgt has no significant association with the arctic oscillation and the variability of the western north pacific summer monsoon the circulation of the wave train shows a barotropic structure everywhere except the cell located to the northwest of india where a baroclinic circulation structure dominates two possible scenarios are proposed the abnormal ism may excite an anomalous west central asian high and downstream rossby wave train extending to the north pacific and north america on the other hand a wave train that is excited in the jet exit region of the north atlantic may affect the westcentral asian high and thus the intensity of the ism it is hypothesized that the interaction between the global wave train and the ism heat source may be instrumental in maintaining the boreal summer cgt

quasi biweekly oscillation over the tibetan plateau and its link with the asian summer monsoon abstract intraseasonal variation isv is especially prominent and unique in the asian summer monsoon region in this work the dominant isv mode over the tibetan plateau tp in the summer monsoon season june august together with its structure and evolution is identified using station observations global precipitation climatology project precipitation data and era interim during 1979 2011 results indicate that quasi biweekly oscillation qbwo is the dominant mode of isv over the tp and is significant in terms of the circulation precipitation and diabatic heating fields in particular the qbwo is closely related to the onset and active break phases of the tp summer monsoon in most cases the qbwo originates from the equatorial western pacific and first propagates northwestward to the bay of bengal and northern india then northward to the southeastern tp and finally eastward to the east asian area showing a clockwise propagation pathway two main mechanisms are responsible for the northward

downstream development of the summertime tropical cyclone submonthly wave pattern in the extratropical north pacific abstract the impact of tropical perturbation on the extratropical wave activity in the north pacific in the submonthly time scale is demonstrated here previous studies identified a tropical cyclone tc submonthly wave pattern which propagated north northwestward in the philippine sea and recurved in the oceanic region between japan and taiwan this study found that after the arrival of the tc submonthly wave pattern at the recurving region the eastward propagating wave activity in the extratropical north pacific was significantly enhanced it is suggested that the tc submonthly wave pattern which is originated in the tropical western north pacific enhances the eastward energy propagation of rossby wave like perturbation in the extratropical north pacific and may have an impact on the long range weather predictability in the eastern north pacific and western north america

plate tectonic reconstructions and paleogeographic maps of the central and north atlantic oceans1 2 1this article is one of a series of papers published in this cjes special issue on the theme of mesozoic cenozoic geology of the scotian basin 2earth sciences sector contribution 20120172 we have established a new plate kinematic model of the central and north atlantic oceans between north america africa meseta iberia flemish cap and galicia bank from late triassic to late cretaceous to better understand the nature and timing of rifting of nova scotia and morocco conjugate continental margins since late triassic the maps of salt distributions at the sinemurian pliensbachian limit 190 ma after salt deposition and in middle bajocian 170 ma show that an area of the nova scotia margin is devoid of allochthonous salt and that an area of similar size located oceanward of the west african coast magnetic anomaly shows salt deposits suggesting that a portion of the nova scotia margin with its overlying salt deposits could have been transferred onto the moroccan side right after the formation of the conjugate east coast magnetic anomaly and west african coast magnetic anomaly seven paleogeographic maps from late triassic to late cretaceous are presented with structural elements and ma

tectonic history of the western tethys since the late triassic the tectonic history of the western tethys since the late triassic is illustrated through a set of computer generated plate reconstructions which are based on a rigorous plate motions model of this region the model is constrained by the atlantic plate kinematics and on land geologic evidence and defines 13 tectonic phases spanning the time interval from the late ladinian 230 ma to the present the kinematics associated with the late triassic western tethyan rifts produced the detachment of a large composite fragment from the northern margin of gondwana it can be considered as the eastern propagation of the central pangea breakup during the early jurassic these rift zones became inactive while new zones of extension formed along the southern margin of eurasia the eastern margin of iberia and within the rifted northern gondwana fragment itself plate motions associated with the first two extensional centers can still be considered as an eastern branch of the central atlantic plate kinematics conversely the kinematic parameters of the latter rift result from the composition of the euler rotation describing the central pangea breakup and the euler pole of closure of the paleo tethys ocean the late triassic early jurassic rifting phases determined the formation of a number of independent microplates at the interface between africa and eurasia starting from the early cretaceous convergence between africa and eurasia triggered further deformation within the dispersed continental fragments and the formation of backarc basins at the active margins ultimately leading to an increase in the number of tectonic elements that were moving independently in the western tethyan region during the late cretaceous and the cenozoic the proposed tectonic evolution of the western tethys area is compatible with both global scale plate kinematics and geological constraints from on land data observed across the present day mosaic of displaced terranes surrounding the mediterranean region

the crustal structure of the central mozambique continental margin wide angle seismic gravity and magnetic study in the mozambique channel eastern africa abstract the continental margin of mozambique formed during the initial dispersal of gondwana about 180 ma due to the lack of deep seismic and dense potential field data many details of the timing and geometry of the early breakup in this region remained unknown to date to close this gap a research project mobamasis mozambique basin marine seismic survey with the french research vessel r v marion dufresne ii was conducted in 2007 this paper presents the results of p wave magnetic and 2d gravity modelling along two parallel seismic refraction profiles between 37 and 41 e crossing the mozambique rifted margin the crust shows the characteristics of normal to slightly thickened oceanic crust a lower crustal high velocity body with p wave velocities of 7 0 7 5 km s is observed along both profiles its origin is discussed in the context of upper mantle convection and thermal properties the existing magnetic anomaly identifications have been extended to older ages we postulate that the oldest oceanic crust near the central mozambique continental margin has been formed around m41n 166 ma closer to the coast a pronounced negative magnetic anomaly exists that we interpret to coincide with the continent ocean transition this implies that the position of the continent ocean transition is located significantly closer to the shoreline than proposed before

the transition from a volcanic to a nonvolcanic rifted margin off eastern canada three multichannel seismic reflection profiles were collected on the rifted continental margin southeast of nova scotia eastern canada the profiles cross the east coast magnetic anomaly ecma which parallels much of the margin of eastern north america south of the grand banks and which is usually associated with the transition from continental to oceanic crust studies to the south of the work reported here suggest that the ecma may be related to the emplacement of large thicknesses of late rift stage or early drift stage igneous material which is characterized by seaward dipping reflections in basement and a high velocity lower crustal layer the seismic data show that seaward dipping reflections sdr continue northward into the study area and support the correlation between the sdr unit and the presence of a well developed ecma magnetic modeling confirms this association although it does not rule out an additional contribution to the magnetic anomaly from an edge effect or suture just north of the study area the ecma diminishes and is no longer well developed the sdr unit also terminates and it is not observed over most of the nova scotian margin if our understanding of the origin of these features is correct then their disappearance marks a transition from a volcanic margin in the south to a nonvolcanic margin in the north the association of the transition with significant changes in the prerift fabric of the adjacent continental crust in the trend of synrift extensional structures and in the width of the zone of thinned continental crust below the margins must be clues to the deeper processes controlling the amount of volcanism produced we suggest that these clues are consistent with small scale convection as a mechanism for delivering large melt volumes to crustal depths during rifting

pacific east asian teleconnection part ii how the philippine sea anomalous anticyclone is established during el nino development the anomalous philippine sea anticyclone psac conveys impacts of el nino to east asian climate during the mature and decay of an el nino from the winter to ensuing summer it is shown that the anomalous psac forms in fall about one season prior to the peak el nino its strength increases with the el nino intensity and its sign reverses during a la nina the psac formation concurs with abnormal deepening of the east asian trough and with increasing number of northward recurvature of tropical storms in the western pacific the psac establishment is abrupt coupling with a swing from a wet to dry phase of an intraseasonal oscillation iso and often concurrent with early retreat of the east asian summer monsoon the iso becomes inactive after psac establishment the development of the psac is attributed to combined effects of the remote el nino forcing tropical extratropical interaction and monsoon ocean interaction the developing el nino induces off equatorial as cending rossby wave responses and land surface cooling in northeast asia both deepen the east asian trough in fall and induces vigorous tropical extratropical exchange of air mass and heat which enhances the cold air outbreak and initiation of the psac through exciting descending rossby waves the el nino induced indonesian subsidence generates low level anticyclonic vorticity over south asia which is advected by mean monsoon westerly instigating the anomalous psac the iso interacting with the underlying ocean plays a critical role in the abrupt establishment of psac the wind evaporation entrainment feedback tends to amplify suppress iso before after winter northeasterly monsoon commences suggesting the roles of atmosphere ocean inter action and the seasonal march of background winds in changing the philippine sea iso intensity and maintaining psac

warm season subseasonal variability and climate extremes in the northern hemisphere the role of stationary rossby waves abstractthis study examines the nature of boreal summer subseasonal atmospheric variability based on the new nasa modern era retrospective analysis for research and applications merra for the period 1979 2010 an analysis of the june july and august subseasonal 250 hpa meridional wind anomalies shows distinct rossby wave like structures that appear to be guided by the mean jets on monthly subseasonal time scales the leading waves the first 10 rotated empirical orthogonal functions reofs of the 250 hpa wind explain about 50 of the northern hemisphere wind variability and account for more than 30 60 of the precipitation surface temperature variability over a number of regions of the northern middle and high latitudes including the u s northern great plains parts of canada europe and russia the first reof in particular consists of a rossby wave that extends across northern eurasia where it is a dominant contributor to monthly surface temperature and precipitation variability and

structural development of the jurassic magnetic quiet zone off morocco and identification of middle jurassic magnetic lineations 1 geophysical investigations carried out during two meteor cruises have revealed weak linear magnetic anomalies in those parts of the jurassic magnetic quiet zone jmqz off morocco that are not affected by cenozoic igneous activity the linear magnetic anomalies are not correlated with variations in relief or structure of oceanic crust using the reversal sequence m25 m41 the anomalies of the jmqz can be modeled with a half spreading rate of 2 2 cm yr extrapolation of the weak lineations back to the slope anomaly s1 results in a breakup age of 170 ma according to this interpretation the crust of the jmqz off morocco has formed during the time 170 155 ma bajocian to oxfordian however much lower spreading rates around 1 cm yr cannot be excluded s1 coincides with a poorly developed seaward dipping reflector sequence sdrs which is most likely its source the sdrs corroborates the earlier claim that s1 marks the ocean continent transition seaward of s1 is a 70 km wide strip of horizontal reflectors that become landward dipping to the west in the uppermost part of the basement it parallels s1 over a distance of 200 km and may indicate excessive magma supply within the first 2 m y of seafloor spreading similar landward dipping reflectors are observed also in the conjugate sohm abyssal plain off nova scotia the average magnetization of the landward dipping reflectors is much lower than that of the sdrs a lower crustal body with a seismic velocity of 7 3 km s which formed during the neogene beneath the eastern part of the essaouira rise has only a weak magnetization 0 5 a m

effects of canary hotspot volcanism on structure of oceanic crust off morocco analysis of over 6400 km of multichannel seismics mcs and 50 sonobuoy reflection and refraction experiments reduced both in the domain of x t and tau p shows that a region within the jurassic quiet zone off morocco underwent dramatic changes as a result of the passage of the lithosphere over the canary hotspot commencing approximately 60 ma a seismic unit ucf interpreted as volcanic in origin is observed within the sediments in a region characterized by a broad bathymetric swell it shows diffractions from its upper surface and an internally chaotic seismic facies and pinches out between sedimentary units of continuous subparallel facies a velocity inversion is noted between the ucf 4 7km s and the underlying sediment 3 1 km s the ucf is time transgressive it lies near the cretaceous s tertiary boundary in the northern portion of the study area and is younger to the south kinematic studies of the movement of the canary hotspot relative to africa show that the hotspot first appeared off nw africa about 60 ma and was located beneath oceanic crust in the region where the ucf is observed depth to basement measurements in areas not effected by the hotspot show a consistent linear trend of increased depth with age in areas effected by the hotspot the thermal rejuvenation is evident as basement depths shoal with increased proximity to the present hotspot the reheating of the crust resets the thermal age of the lithosphere with many of the properties of crust of a younger age subsidence curves of the reheated crust off morocco show good correlation to subsidence curves of other reheated crust on a global basis a zone characterized by high crustal velocities 7 1 7 4 km s and greater crustal thicknesses by 1 2 km is observed in an area that corresponds to the bathymetric swell the region of the ucf and the reelevated basement the high velocities and increased crustal thickness are interpreted to be the result of underplating and assimilation of existing oceanic crust caused by the canary thermal anomaly the presence of high crustal velocities coupled with a thickened crustal section has been noted on various passive margins of the world they have generally been attributed to the thermal processes associated with continental rifting off morocco we believe that similar thermally induced phenomena have occurred but that here the heat anomaly was the midplate volcanism associated with the canary hotspot

northward propagation initiation and termination of boreal summer intraseasonal oscillations in a zonally symmetric model abstracta simple multilayer zonally symmetric model using a multicloud convective parameterization and coupled to a dynamical bulk atmospheric boundary layer is used here to simulate boreal summe

emag2 a 2 arc min resolution earth magnetic anomaly grid compiled from satellite airborne and marine magnetic measurements 1 a global earth magnetic anomaly grid emag2 has been compiled from satellite ship and airborne magnetic measurements emag2 is a significant update of our previous candidate grid for the world digital magnetic anomaly map the resolution has been improved from 3 arc min to 2 arc min and the altitude has been reduced from 5 km to 4 km above the geoid additional grid and track line data have been included both over land and the oceans wherever available the original shipborne and airborne data were used instead of precompiled oceanic magnetic grids interpolation between sparse track lines in the oceans was improved by directional gridding and extrapolation based on an oceanic crustal age model the longest wavelengths 330 km were replaced with the latest champ satellite magnetic field model mf6 emag2 is available at http geomag org models emag2 and for permanent archive at http earthref org cgi bin er cgi s erda cgi n 970

subsalt exploration potential of the moroccan salt basin the moroccan salt basin is one of the remaining underexplored west african salt basins the dimensions of this salt basin are about 100 km in average width and almost 1000 km in length along the atlantic margin of morocco figure 1 as opposed to the aptian salt basins of the south atlantic in northwestern africa the salt was deposited during the last stage of rifting prior to continental breakup in the upper triassic lower jurassic hafid 2000 tari et al 2003

deep structure of the ocean continent transition in the southern iberia abyssal plain from seismic refraction profiles the iam 9 transect at 40 20 n we present a crust and mantle velocity structure for the west iberia passive continental margin derived from a 320 km long wide angle seismic profile acquired in the southern iberia abyssal plain we observe a 170 km wide ocean continent transition zone which includes a pair of overlapping peridotite ridges and is bounded by oceanic crust and landward by fault bounded blocks of continental crust the profile lies 40 km south of the transect sampled by ocean drilling program odp legs 149 and 173 the transition zone structure can be divided into an upper layer 2 4 km thick with velocities of between 4 5 and 7 0 km s 1 and generally a high velocity gradient 1 s 1 and a lower layer up to 4 km thick with a velocity of 7 6 km s 1 and a low velocity gradient a weak moho reflection in this zone was seen only on wide angle profiles at an offset of 30 km the upper layer has a distinctly lower velocity than thinned continental crust adjacent to the continental slope conversely the lower layer has too high a velocity to be magmatically intruded or underplated lower continental crust on the coincident seismic reflection profile fault bounded crustal blocks identified in unequivocal extended continental crust are not observed in the transition zone the upper layer has velocity bounds and gradient similar to oceanic layer 2 observed west of the peridotite ridges but no oceanic layer 3 velocity structure is present while magnetic anomalies have been identified within the transition zone they have not been modeled successfully as seafloor spreading magnetic anomalies nor do they generally form long linear margin parallel features finally odp boreholes 40 km north of our profile and within the interpreted transition zone have recovered up to 140 m thick sections of serpentinite and serpentinized peridotites with little evidence of mafic igneous material we conclude that the transition zone cannot be dominantly composed of either extended continental crust or oceanic crust although current melting models predict a considerably thicker crust of decompression melt products we interpret this region as exposed upper mantle peridotite with little or no synrift extrusive material and limited amounts of synrift material intruded within the serpentinized peridotite

seismic evidence for the presence of jurassic oceanic crust in the central gulf of cadiz sw iberian margin abstract we investigate the crustal structure of the sw iberian margin along a 340 km long refraction and wide angle reflection seismic profile crossing from the central gulf of cadiz to the variscan continental margin in the algarve southern portugal the seismic velocity and crustal geometry model obtained by joint refraction and reflection travel time inversion reveal three distinct crustal domains the 28 30 km thick variscan crust in the north a 60 km wide transition zone offshore where the crust abruptly thins 20 km and finally a 7 km thick and 150 km wide crustal section that appears to be oceanic in nature the oceanic crust is overlain by a 1 3 km thick section of mesozoic to eocene sediments with an additional 3 4 km of low velocity unconsolidated sediments on top belonging to the miocene age gulf of cadiz imbricated wedge the sharp transition between continental and oceanic crust is best explained by an initial rifting setting as a transform margin during the early jurassic that followed the continental break up in the central atlantic the narrow oceanic basin would have formed during an oblique rifting and seafloor spreading episode between iberia and africa that started shortly thereafter bajocian and lasted up to the initiation of oceanic spreading in the north atlantic at the tithonian late jurassic earliest cretaceous the velocity model displays four wide prominent south dipping low velocity anomalies which seem to be related with the presence of crustal scale faults previously identified in the area some of which could well be extensional faults generated during this rifting episode we propose that this oceanic plate segment is the last remnant of an oceanic corridor that once connected the alpine tethys with the atlantic ocean so it is in turn one of the oldest oceanic crustal fragments currently preserved on earth the presence of oceanic crust in the central gulf of cadiz is consistent with geodynamic models suggesting the existence of a narrow westward retreating oceanic slab beneath the gibraltar arc alboran basin system

the crustal structure of the nw moroccan continental margin from wide angle and reflection seismic data summary the atlantic margin off morocco with its neighbouring jurassic oceanic crust is one of the oldest on earth it is conjugate to the nova scotia margin of north america the sismar marine seismic survey acquired deep reflection seismic data as well as wide angle seismic profiles in order to image the deep structure of the margin characterize the nature of the crust in the transitional domain and define the geometry of the synrift basins we present results from the combined interpretation of the reflection seismic wide angle seismic and gravity data along a 440 km long profile perpendicular to the margin at 33 34 n extending from nearly normal oceanic crust in the vicinity of coral patch seamount to the coast at el jadida and approximately 130 km inland the shallow structure is well imaged by the reflection seismic data and shows a thick sedimentary cover that is locally perturbed by salt tectonics and reverse faulting the sedimentary basin thickens from 1 5 km on the normal oceanic crust to a maximum thickness of 6 km at the base of the continental slope multichannel seismic mcs data image basement structures including a few tilted fault blocks and a transition zone to a thin crust a strong discontinuous reflection at 12 s two way travel time twt is interpreted as the moho discontinuity as a result of the good data quality the deep crustal structure depth and velocity field is well constrained through the wide angle seismic modelling the crust thins from 35 km underneath the continent to approximately 7 km at the western end of the profile the transitional region has a width of 150 km crustal velocities are lowest at the continental slope probably as a result of faulting and fracturing of the upper crust uppermantle velocities could be well defined from the ocean bottom seismometer obs and land station data throughout the model

lithosphere tearing at step faults response to edges of subduction zones abstract slab edges are a relatively common feature in plate tectonics two prominent examples are the northern end of the tonga subduction zone and the southern end of the new hebrides subduction zone near such horizontal terminations of subduction trenches ongoing tearing of oceanic lithosphere is a geometric consequence we refer to such kinks in the plate boundary as a subduction transform edge propagator or step other steps are the north and south ends of the lesser antilles trench the north end of the south sandwich trench the south end of the vrancea trench and both ends of the calabria trench volcanism near steps is distinct from typical arc volcanism in some cases slab edges appear to coincide with mantle plumes using 3d mechanical models we establish that step faults are stable plate tectonic features in most circumstances in the probably rare cases that the resistance to fault propagation is high slab break off will occur relative motion along the transform segment of the plate boundary often is non uniform and the step is not a transform plate boundary in the rigid plate tectonics sense of the phrase step propagation may result in substantial deformation rotation topography and sedimentary basins with a very specific time space evolution surface velocities are substantially affected by nearby steps

the crustal structure of the canary basin accretion processes at slow spreading centers multichannel seismic reflection and gravity data define the structure of mesozoic ocean crust of the canary basin formed at slow spreading rates single and multichannel seismics show a transition from smooth to rough basement topography from jurassic to cretaceous crust and a coeval change in crustal structure internal reflectivity of the rough basement area comprises upper upper middle or whole crust cutting discrete dipping reflections lower crustal reflectivity is almost absent and reflections from the crust mantle transition are short and discontinuous or absent for several kilometers in contrast crust in the smooth basement area is characterized by sparse lower crustal events and common reflections from the crust mantle boundary the crustal structure of fracture zones in the rough basement area is associated with depressions in the basement top and in most cases with thin crust in the smooth basement area fracture zones exhibit neither a clear topographic expression nor crustal thinning we interpret these characteristics as indicative of an increase in extensional tectonic activity and decrease in magmatic activity at the spreading ridge associated with a general decrease of spreading rate from jurassic to cretaceous times in addition the crust imaged across the path of the cape verde hot spot in the canary basin exhibits a widespread lower crustal reflectivity very smooth topography and apparently thick crust our data document significant changes in the structure of crust formed at slow spreading rates which we attribute to thermal changes in the lithosphere due either to variations in spreading rate or to the presence of a hot spot beneath the mesozoic mid atlantic ridge

airborne doppler observations of a landfalling cold front upstream of steep coastal orography abstract this study uses airborne doppler radar observations to describe the mesoscale structure and evolution of a cold frontal system as it made landfall on the mountainous coast of oregon and northern california on 1 december 1995 during the coastal observations and simulations with topography experiment this section of coastline constitutes a steep approximately two dimensional north south oriented orographic barrier the front exhibited a northeast southwest orientation and thus intersected the axis of high terrain at an acute angle the along barrier pressure gradient and low level winds increased with time along the coastal zone and reached a maximum as the front made landfall stably stratified prefrontal flow was strongly blocked by the orography resulting in a confluent transition from pervasive southwesterly winds offshore to a narrow zone of accelerated south southwesterly flow near the coast where wind speeds approached 30 m s 1 at a height of 750 m above mean sea level postfrontal flow

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

breakup of pangaea and plate kinematics of the central atlantic and atlas regions summary a new central pangaea fit type a is proposed for the late ladinian 230 ma together with a plate motions model for the subsequent phases of rifting continental breakup and initial spreading in the central atlantic this model is based on 1 a reinterpretation of the process of formation of the east coast magnetic anomaly along the eastern margin of north america and the corresponding magnetic anomalies at the conjugate margins of northwest africa and the moroccan meseta 2 an analysis of major rifting events in the central atlantic atlas and central mediterranean and 3 a crustal balancing of the stretched margins of north america moroccan meseta and northwest africa the process of fragmentation of central pangaea can be described by three major phases spanning the time interval from the late ladinian 230 ma to the tithonian 147 7 ma during the first phase from the late ladinian 230 ma to the latest rhaetian 200 ma rifting proceeded along the eastern margin of north america the northwest african margin and the high saharan and tunisian atlas determining the formation of a separate moroccan microplate at the interface between gondwana and laurasia during the second phase from the latest rhaetian 200 ma to the late pliensbachian 185 ma oceanic crust started forming between the east coast and blake spur magnetic anomalies whereas the morrocan meseta simply continued to rift away from north america during this time interval the atlas rift reached its maximum extent finally the third phase encompassing the time interval from the late pliensbachian 185 ma to chron m21 147 7 ma was triggered by the northward jump of the main plate boundary connecting the central atlantic with the tethys area therefore as soon as rifting in the atlas zone ceased plate motion started along complex fault systems between morocco and iberia whereas a rift drift transition occurred in the northern segment of the central atlantic between morocco and the conjugate margin of nova scotia the inversion of the atlas rift and the subsequent formation of the atlas mountain belt occurred during the oligocene early miocene time interval in the central atlantic this event was associated with higher spreading rates of the ridge segments north of the atlantis fz an estimate of 170 km of dextral offset of morocco relative to northwest africa in the central atlantic is required by an analysis of marine magnetic anomalies five plate tectonic reconstructions and a computer animation are proposed to illustrate the late triassic and jurassic process of breakup of pangaea in the central atlantic and atlas regions

formation and evolution of the nova scotian rifted margin evidence from deep seismic reflection data a deep marine seismic reflection profile was obtained across the mesozoic rifted continental margin off nova scotia eastern canada this profile crosses the seotian basin one of the deepest basins on the margin of eastern north america and it complements other deep crustal seismic data on this margin the seismic data have been interpreted in conjunction with gravity anomaly and subsidence data they show significant thinning of the continental crust over a zone about 200 km wide the mode of extensional deformation is probably a combination of pure and simple shear there is evidence for simple shear in the crust the continent ocean boundary lies near the seaward edge of synrift salt below the continental rise a 100 km wide zone of very thin approximately 9 km or less continental or transitional crust extends seaward from the outer shelf to this boundary reflectivity of the oceanic crust adjacent to the margin shows evidence of progressive igneous construction perhaps modified by extensional faulting this margin is nonvolcanic and the transition to the volcanic margin off the eastern united states occurs about 500 km southwest of the seismic line the width of the zone of crustal extension is much greater on this nonvolcanic margin segment than it is on the volcanic margin to the south it seems likely that the prerift fabric of the continental lithosphere controls this width a narrow rift may be prone to vigorous asthenospheric convection and therefore to more voluminous volcanism however significantly narrower zones of crustal extension occur on other nonvolcanic margins so factors in addition to rift width such as the rate of rifting may also be important

the role of inherited lithospheric heterogeneities in defining the crustal architecture of rifted margins and the magmatic budget during continental breakup during the final stage of continental rifting stretching localizes in the future distal domain where lithospheric necking occurs resulting in continental breakup in magma poor margins the lithospheric necking is accompanied by crustal hyperextension serpentinization and exhumation of mantle lithosphere in the continent ocean transition domain cot in magma rich margins the necking is accomplished by the emplacement of large amounts of volcanics in the cot in the form of seaward dipping wedges of flood basalts sdrs this study examines the factors controlling the final crustal architecture observed in rifted margins and the magmatic budget during continental breakup using observations from the labrador sea the latter shows magma rich breakup with sdrs documented in the north and magma poor breakup with a wide domain of exhumed serpentinized mantle recorded in the south the pre rift strength of the lithosphere defined by the inherited thermal structure composition and thickness of the lithospheric layers controls the structural evolution during rifting while variations in the magmatic budget associated with breakup are controlled primarily by the interaction between the pre rift inheritance the timing and the degree of mantle melting in relation to lithospheric thinning and mantle hydration

meteorological tower measurements of a surface cold front abstract measurements from the boulder atmospheric observatory meteorological research tower are used to describe the structure and physical processes of a strong surface cold front analysis reveals that the horizontal gradients in temperature and wind velocity at the front are concentrated within a 200 m distance and measured vertical velocities exceed 5 m s 1 at the leading edge of the front calculations with the miller frontogenesis equation show the magnitude and relative importance of cross frontal confluence versus differential vertical motion tilting in forcing frontogenesis

evidence for active subduction beneath gibraltar we report on a marine seismic survey that images an active accretionary wedge west of gibraltar ramp thrusts offset the seafloor and sole out to an east dipping decollement indicating ongoing westward vergent tectonic shortening new traveltime tomographic re sults image a slab of oceanic lithosphere descending from the atlantic domain of the gulf of cadiz passing through intermediate depth 60 120 km seismicity beneath the west ernmost alboran sea and merging with a region of deep focus earthquakes 600 660 km below granada spain together these new data provide compelling evidence for an active east dipping subduction zone

the effect of the mjo on the north american monsoon abstract the effect of the madden julian oscillation mjo in the eastern pacific on the north american monsoon is documented using ncep ncar reanalysis and daily mean precipitation data from 1958 to 2003 it is found that positive zonal wind anomalies in the eastern tropical pacific lead to above normal precipitation in northwest mexico and arizona from several days to over a week later this connection between the tropical pacific and monsoon precipitation appears to be limited to regions influenced by moisture surges from the gulf of california as a similar connection does not exist for new mexico precipitation the evidence suggests that the mjo might affect monsoon precipitation by modulating the strength of low level easterly waves off the coast of mexico which in turn triggers the development of a gulf surge

crustal thickness and earthquake distribution south of the logachev seamount knipovich ridge 1 during rv polarstern cruise ark xxiv 3 2009 a geophysical study along the ultraslow spreading knipovich ridge was conducted the survey located in the rift valley south of the logachev seamount 76 36 n provides a crustal thickness of 4 5 km in the amagmatic parts and 5 7 km underneath the seamount itself the velocity depth function indicates the presence of a thick oceanic layer 2 4 5 km but no indication for a thick oceanic layer 3 the only exception is the area underneath the logachev seamount where velocities higher than 6 km s are detected this indicates a stronger and focussed melt supply underneath the seamount local seismicity was analysed for two days in total 191 quakes were identified in the vicinity of the rift valley with magnitudes up to ml 2 6 at least 48 of them are located in the upper mantle up to 18 km below sea level supporting models predicting a cold mantle or conductive cooling underneath ultraslow ridges to explain the reduced melt supply

ambient seismic noise tomography of sw iberia integrating seafloor and land based data we used ambient seismic noise recorded by 24 broadband ocean bottom seismometers obs deployed in the gulf of cadiz during the ec funded nearest project and seven broadband land stations located in the south of portugal to image the sedimentary and crustal structure beneath the eastern atlantic and sw iberia we computed ambient noise cross correlations to obtain empirical green s functions egfs between all station pairs using land seismometers and both obs sensors seismometers and hydrophones despite the great difference in the recording conditions and local crustal structure between the obss and land stations we could compute egfs by applying a linear cross correlation with running absolute mean average time normalization followed by a time frequency phase weighted stack dispersion analysis was then applied to the egfs between 4 and 20s period the obtained dispersion curves allowed mapping the lateral variation of rayleigh wave group velocities as a function of period finally dispersion curves extracted from each cell of the 2d group velocity maps were inverted as a function of depth to obtain the 3d distribution of the shear wave velocities the 3 d shear wave velocity model computed from joint inversion of obss and land stations data allowed to estimate the thickness of sediments and crust and the moho depth despite the gap that exists between the obss and land station locations our model displays a good correlation with the known geological structure the derived sedimentary layer and crustal thicknesses and the obtained moho depth are locally in agreement with the models proposed by other studies using near vertical refraction and wide angle seismic profiling we conclude that ambient noise tomography could be a valuable tool to image oceanic domains and also that it is possible to integrate seafloor and land based stations to derive a structure model in the transition domain between continent and ocean