ocean crust formation processes at very slow spreading centers a model for the mohns ridge near 72 n based on magnetic gravity and seismic data the mohns ridge in the norwegian greenland sea is one of the slowest spreading centers of the mid ocean ridge system 8 mm yr half rate sea beam data acquired with r v jean charcot near 72 n show that its rift valley floor is characterized by en echelon volcanic ridges oriented obliquely relatively to the average strike of the ridge axis these ridges are regularly spaced along the axis about every 40 km and are separated by nontransform discontinuities sharp positive magnetic anomalies centered over the topographic highs suggest that they are eruptive centers considered as the surficial expression of active spreading cells over the rift valley bouguer anomalies obtained by subtracting the predicted effects due to seafloor topography from the measured free air gravity field are consistent with a low density body within the lower crust having its upper surface lying at about 2 km below the sea surface this body if it exists probably corresponds to the zone of low viscosity that can be inferred from the model of chen and morgan 1990b which predicts the existence of a decoupling region between the upper crust and the asthenophere below its width varies rapidly along strike from less than about 5 km to more than 15 km in plan view it has a pinch and swell form which defines a series of spreading cells the center of one cell being where the bouguer anomaly is widest short wavelength less than 10 to 20 km along strike variations such as bouguer anomaly lows centered on the topographic highs reflect local effects associated with the presence of the eruptive centers seismic tomography data from a 20 10 km active oblique volcanic ridge near 72 22 n tend to indicate that the links between the main low velocity body at depth and the magma injections centers which lie within the rift valley inner floor are probably complex

evaluation of recent tectonomagmatic discrimination diagrams and their application to the origin of basic magmas in southern mexico and central america discrimination diagrams to decipher tectonic settings have been in use for nearly 40 years although old diagrams have been extensively used the recent ones based on discriminant functions of ratio variables with or without log transformation proposed during 2004 2010 for the discrimination of four tectonic settings of island arc continental rift ocean island and mid ocean ridge were newly evaluated to show their high success rates of 57 3 100 and 58 5 100 for major element and immobile element based diagrams respectively for the continental arc of the andes evaluated for its similarity to island arc these four sets of diagrams showed success rates of 62 1 83 8 these four sets of five diagrams per set were therefore used to infer tectonic setting of the mexican volcanic belt mvb los tuxtlas volcanic field ltvf and central american volcanic arc cava using this approach the mvb especially its western central and eastern parts and the ltvf of southern mexico show a dominantly continental rift setting and the cava shows an arc setting the west central part of the mvb is consistent with dual tectonics of arc and rift these results confirm the application of an unusual mantle upwelling rift model for the mexican on land volcanism whereas the conventional plate tectonic subduction model seems to be applicable for the cava from guatemala to north western costa rica

plate tectonic development of the south west indian ocean a revised reconstruction of east antarctica and africa a revised sequence of reconstructions for east antarctica relative to africa is generated from published seafloor spreading and fracture zone anomalies these date from the cretaceous tertiary boundary to the late jurassic and lead to a revised predrift reconstruction for east and west gondwanaland euler pole 1 67 s 35 99 e rotation angle 53 43 this reconstruction derived wholly from marine data places east antarctica adjacent to the lebombo mountains and south of and subparallel to the sabi acid volcanic suite supporting the postulate that these rocks represent mesozoic plate boundaries this refit implies that part of the coastal plain of mozambique is underlain by oceanic or very highly extended continental crust the reconstruction is supported by the alignment of 1 the cape fold belt with its antarctic equivalents and 2 the western limit of pan african reactivation of the mozambique mobile belt with the western limit of the sverdrupfjella metamorphic suite antarctica initially moved along a transform fault lying immediately east of and subparallel to the lebombo mountains the eastern face of the tugela cone the southeastern face of the falkland plateau in its reconstructed position and the eastern flank of the agulhas plateau in its reconstructed position at m10 time a triple junction formed near the tip of the falkland plateau implying that the oldest lineated magnetic anomalies in the eastern weddell sea associated with this triple junction cannot predate m10 at m2 time the mozambique ridge rifted from the astrid ridge in a ridge jump episode such an event may also explain the presence of magnetic anomalies in the southern mozambique basin gunnerus ridge conrad rise and del cano rise are all tectonically associated with the madagascar ridge

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

gondwana breakup messages from the north natal valley the natal valley offshore mozambique is a key area for understanding the evolution of east gondwana within the scope of the integrated multidisciplinary pamela project we present new wide angle seismic data and interpretations which considerably alter geoscience paradigms these data reveal the presence of a 30 km thick crust that we argue to be of continental nature this falsifies all the most recent palaeo reconstructions of the gondwana this 30 km thick continental crust 1 000 m below sea level implies a complex history with probable intrusions of mantle derived melts in the lower crust connected to several occurrences of magmatism which seems to evidence the crucial role of the lower continental crust in passive margin genesis

the nature and occurrence of discrete zoning in plagioclase from recently erupted andesitic volcanic rocks montserrat abstract recently erupted rocks from montserrat lesser antilles were found to be coarsely porphyritic including plagioclase magnesio hornblende orthopyroxene titano magnetite and rare quartz phenocrysts zonations within plagioclase show three distinct periods within the evolution of the andesitic magmas initial steady state oscillatory overgrowths an49 54 represent an early period of growth within a confined andesitic magma chamber interaction with an intruding more mafic hotter magma resulted in thick zones of dissolution features that cut through these early growth layers the dissolution zones and crystal fragments are overgrown by thin calcic an86 plagioclase rims a plagioclase growth rate of 10 7cm s suggests that the calcic plagioclase rims were formed in approximately 10 h consistent with the idea that mineral growth may increase magma viscosity inducing conduit plugging and shallow pressurization resulting in cycles of dome inflation eruption and explosive activity thermodynamic modeling of heating and mixing of the andesitic magma with a basaltic liquid produces plagioclase of composition an79 not the observed composition of the late plagioclase an86 we conclude that the final plagioclase grew under non equilibrium conditions as a result of the mixing event

stress transfer relations among the earthquakes that occurred in kerman province southern iran since 1981 summary we explore the possible stress triggering relationship of the m 6 4 earthquakes that occurred in kerman province southern iran since 1981 we calculated stress changes due to both coseismic sudden movement in the upper crust and the time dependent viscous relaxation of the lower crust and or upper mantle following the event four events of m 6 4 occurred between 1981 and 2005 on and close to the gowk fault show a clear coulomb stress load to failure relationship the 2003 m 6 5 bam earthquake however which occurred approximately 95 km sw of the closest gowk event shows a very weak stress relation to preceding earthquakes the coseismic static stress change at the hypocentre of the bam earthquake is quite small 0 006 bars the time dependent post seismic stress change could be 26 times larger or 7 times lower than that of coseismic static stress alone depending on the choice of viscoelastic crustal model and the effective coefficient of friction given the uncertainties in the viscoelastic earth models and the effective coefficient of friction we cannot confidently conclude that the 2003 bam event was brought closer to failure through coseismic or post seismic stress loading interestingly the southern gowk segment with a similar strike to that of the bam fault experienced a stress load of up to 8 3 bars between 1981 and 2003 and is yet to have a damaging earthquake

some insight on why bam iran was destroyed by an earthquake of relatively moderate size 1 the bam iran earthquake of 2003 resulted in one of the worst human disaster in recent years yet the magnitude of the event mw 6 6 was relatively moderate we show that the remarkable recording of the ground motion produced in the city itself contains some clues which help explain this disaster we identify three factors whose unfortunate combination led to the strong ground shaking which destroyed the city 1 the rayleigh like speed of the rupture 2 the high slip velocity which exceeded 2m s over a large part of the fault 3 the strong directivity which focused the elastic energy released directly toward the city

an early cretaceous extinct spreading center in the northern natal valley abstract we have identified an extinct e w spreading center in the northern natal valley on the basis of magnetic anomalies which was active from chron m11 133 ma to 125 3 ma just before chron m2 124 ma in the early cretaceous seafloor spreading in the northern natal valley accounts for approximately 170 km of north south motion between the mozambique ridge and africa this extension resolves the predicted overlap of the continental central and southern mozambique ridge and antarctica in the chron m2 to m11 reconstructions from mesozoic finite rotation parameters for africa and antarctica in addition the magnetic data reveal that the mozambique ridge was an independent microplate from at least 133 to 125 ma the northern natal valley extinct spreading center connects to the spreading center separating the mozambique basin and the riiser larsen sea to the east it follows that the northern mozambique ridge was either formed after the emplacement of the surrounding oceanic crust or it is the product of a very robust spreading center to the west the extinct spreading center connects to the spreading center separating the southern natal valley and georgia basin via a transform fault prior to chron m11 there is still a problem with the overlap of mozambique ridge if it is assumed to be fixed with respect to either the african or antarctic plates some of the overlap can be accounted for by jurassic deformation of the mozambique ridge mozambique basin and dronning maud land it appears though that the mozambique ridge was an independent microplate from the breakup of gondwana 160 ma until it became part of the african plate 125 ma

anomalous seafloor mounds in the northern natal valley southwest indian ocean implications for the east african rift system abstract the natal valley southwest indian ocean has a complicated and protracted opening history as has the surrounding southwest indian ocean recently collected multibeam swath bathymetry and 3 5 khz seismic data from the natal valley reveal anomalous seafloor mounds in the northern natal valley the significance of these domes as recorders of the geological history of the natal valley and se african margin has been overlooked with little attempt made to identify their origin evolution or tectonic significance this paper aims to describe these features from a morphological perspective and to use their occurrence as a means to better understand the geological and oceanographic evolution of this basin the seafloor mounds are distinct in both shallow seismic and morphological character from the surrounding seafloor of the natal valley between 25 km and 31 km long and 16 km and 18 km wide these features rise some 400 m above the sedimentary deposits that have filled in the natal valley such macro scale features have not previously been described from the natal valley or from other passive margins globally they are not the result of bottom water circulation salt tectonics rather igneous activity is favoured as the origin for these anomalous seafloor features we propose a hypothesis that the anomalous seafloor mounds observed in the natal valley are related to igneous activity associated with the ears the complicated opening history and antecedent geology coupled with the southward propagation of the east african rift system creates a unique setting where continental rift associated features have been developed in a marine setting

near field probabilistic seismic hazard analysis of metropolitan tehran using region specific directivity models ground motions are affected by directivity effects at near fault regions which result in low frequency cycle pulses at the beginning of the velocity time history the directivity features of near fault ground motions can lead to significant increase in the risk of earthquake induced damage on engineering structures the ordinary probabilistic seismic hazard analysis psha does not take into account such effects recent studies have thus proposed new frameworks to incorporate directivity effects in psha the objective of this study is to develop the seismic hazard mapping of tehran city according to near fault psha procedure for different return periods to this end the directivity models required in the modified psha were developed based on a database of the simulated ground motions the simulated database was used in this study because there are no recorded near fault data in the region to derive purely empirically based pulse prediction models the results show that the directivity effects can significantly affect the estimate of regional seismic hazard

gravity anomalies flexure and crustal structure at the mozambique rifted margin abstract the free air gravity edge effect anomaly associated with passive continental margins is one of the most distinctive features of the marine gravity field early studies attributed the edge effect to the juxtaposition of thick continental with thin oceanic crust while thinning of the crust due to rifting at the time of continental break up is a major contributor it is now recognised that geological processes such as sedimentation and magmatic underplating also contribute the different amounts of sediment and underplate at rifted margins help in fact to explain the diversity of the edge effect although they obscure the mechanics styles and geometry of rifting in this paper combined 2 dimensional flexural backstripping and gravity modelling techniques are used to quantify the contribution of rifting sediment loading and magmatic underplating to the edge effect anomaly at the mozambique margin this margin formed as the result of a two stage break up the first of which resulted in the separation of africa and madagascar in the early jurassic by comparing the observed edge effect to calculations based on simple elastic plate models we have been able to constrain the elastic thickness te and hence flexural rigidity of the lithosphere that underlies the sediments which accumulated at the mozambique margin following break up the model that best explains the shape i e amplitude and wavelength of the edge effect high is one in which the earliest jurassic early cretaceous sediments were deposited on relatively weak lithosphere te 17 5 km while the later miocene recent sediments accumulated on relatively rigid lithosphere te 40 km these results suggest that sedimentation at the mozambique margin has involved lithosphere which responds to loads in a similar manner as would oceanic lithosphere therefore the thinned transitional crust that underlies much of the coastal plain of the mozambique margin is probably of oceanic rather than continental origin

reassembling gondwana a new high quality constraint from vibroseis exploration of the sub ice shelf geology of the east antarctic continental margin the breakup of gondwana is manifested by coeval early jurassic karoo magmatism in south africa and east antarctica in south africa the large volumes of volcanic rocks of the adjoining lebombo and mwenetzi save monoclines represent a volcanic rift margin and in east antarctica a corresponding feature the explora wedge is buried below sediments and floating ice shelves on the continental margin of dronning maud land we use the seismic vibrator source to explore the sub ice geology in antarctica and the new seismic reflection and available regional aeromagnetic data enable us to outline a dogleg landward extent of the explora wedge in dronning maud land the congruent inboard wedge geometries on the two continents define a high quality constraint which facilitate for the first time a geologically consistent and tight reconstruction of africa relative to east antarctica within gondwana the uncertainties in correlations of major geological features mobile belts from one continent to the other may now be of the order of ten s of kilometers rather than hundreds of kilometers

pre eruption history of phyric basalts from dsdp legs 45 and 46 evidence from morphology and zoning patterns in plagioclase phyric basalts recovered from dsdp legs 45 and 46 contain abundant plagioclase phenocrysts which occur as either discrete single grains megacrysts or aggregates glomerocrysts and which are too abundant and too anorthitic to have crystallized from a liquid with the observed bulk rock composition almost all the plagioclase crystals are complexly zoned in most cases two abrupt and relatively large compositional changes associated with continuous internal morphologic boundaries divide the plagioclase crystals into three parts core mantle and rim the cores exhibit two major types of morphology tabular with a euhedral to slightly rounded outline or a skeletal inner core wrapped by a slightly rounded homogeneous outer core the mantle region is characterized by a zoning pattern composed of one to several spikes plateaus superimposed on a gently zoned base line with one large plateau always at the outside of the mantle and by in most cases a rounded internal morphology the inner rim is typically oscillatory zoned the width of the outer rim can be correlated with the position of the individual crystal in the basalt pillow the presence of a skeletal inner core and the concentration of glass inclusions in low an zones in the mantle region suggest that the liquid in which these parts of the crystals were growing was undercooled some amount the resorption features at the outer margins of low an zones indicate superheating of the liquid with respect to the crystal

3 d velocity structure of the 2003 bam earthquake area se iran existence of a low poisson s ratio layer and its relation to heavy damage abstract to understand the generation mechanism of the bam earthquake mw 6 6 we studied three dimensional v p v s and poisson s ratio structures in the bam area by using the seismic tomography method we inverted accurate arrival times of 19490 p waves and 19015 s waves from 2396 aftershocks recorded by a temporal high sensitivity seismic network the 3 d velocity structure of the seismogenic region was well resolved to a depth of 14 km with significant velocity variations of up to 5 the general pattern of aftershock distribution was relocated by using the 3 d structure to delineate a source fault for a length of approximately 20 km along a line 4 5 km west of the known geological bam fault this source fault dips steeply westward and strikes a nearly north south line the main shallow cluster of aftershocks south of the city of bam is distributed just under the minor surface ruptures in the desert the 3 d velocity structure shows a thick layer of high v s and low minimum 0 20 at a depth range of 2 6 km the deeper layer with a thickness of about 2 km appears to have a low v s and high maximum 0 28 from 6 km depth beneath bam to a depth of 9 km south of the city the inferred increase of poisson s ratio from 2 to 10 km in depth may be associated with a change from rigid and sio 2 rich rock to more mafic rock including the probable existence of fluids the main seismic gap of aftershock distribution at the depth range of 2 to 7 km coincides well with the large slip zone in the shallow thick layer of high v s and low the large slip propagating mainly in the shallow rigid layer may be one of the main reasons why the bam area suffered heavy damage

surface displacements and source parameters of the 2003 bam iran earthquake from envisat advanced synthetic aperture radar imagery 1 the mw 6 6 26 december 2003 bam iran earthquake was one of the first earthquakes for which envisat advanced synthetic aperture radar asar data were available using interferograms and azimuth offsets from ascending and descending tracks we construct a three dimensional displacement field of the deformation due to the earthquake elastic dislocation modeling shows that the observed deformation pattern cannot be explained by slip on a single planar fault which significantly underestimates eastward and upward motions se of bam we find that the deformation pattern observed can be best explained by slip on two subparallel faults eighty five percent of moment release occurred on a previously unknown strike slip fault running into the center of bam with peak slip of over 2 m occurring at a depth of 5 km the remainder occurred as a combination of strike slip and thrusting motion on a southward extension of the previously mapped bam fault 5 km to the east

geophysical and geochemical constraints on crustal accretion at the very slow spreading mohns ridge the composition of upper mantle and lower crustal material at very slow spreading centers cannot be reliably determined by seismic studies alone since the range of p wave velocities for serpentinized peridotites and gabbros overlap additional information provided by the major and rare earth element ree content of the basalts is useful to constrain interpretations of seismic data refraction seismic data from the very slow spreading 16 mm a full rate mohns ridge in the norwegian greenland sea yields a highly variable thin crust of 4 0 0 5 km thickness analysis of s waves suggests that layer 3 is composed primarily of gabbro containing at most a small percentage 20 of mantle material the na8 content of mohns ridge basalts suggests a magmatic crustal thickness of 4 5 km inversion of the ree concentrations yields a melt thickness of 5 km this agreement between seismic and geochemical data suggests that neither large quantities of mantle material are found in the lower crust nor is a large volume of basaltic magma frozen in the upper mantle

validation of teleseismic inversion of the 2004 mw 6 3 les saintes lesser antilles earthquake by 3d finite difference forward modeling we study the 21 november 2004 mw 6 3 les saintes earthquake that occurred south of the island of guadeloupe at a shallow depth damaging some buildings on the island the objective of this work is to assess the potential of a teleseismic source study to reproduce local ground motion the velocity model of this area is not currently well known and the near field seismograms are affected by paths and site effects we first analyze this earthquake as a point source and then as an extended fault using teleseismic broadband waveform inversion approach from the p and sh waveforms at stations well distributed in azimuth we obtain two stable and reliable solutions of the spatiotemporal history of the earthquake rupture on each nodal plane the models show a rupture duration of 6 sec and a maximum slip of about 1 m but have different locations of the main asperity we then carry out the forward modeling of the seismic wave propagation at the regional scale coupled with the obtained kinematic source models taking into account the topography the bathymetry and the sea layer we investigate if one of the fault models could produce synthetics that match the local observed ground motions the simulation results provide a better understanding of the uncertainty of source path and site effects even if our knowledge of the underground geological structure is limited to the 1d stratified model it is inferred that the 1d model does not seem to be appropriate in the northern direction middle of guadeloupe island due to the unknown basin structure while it is relatively sufficient in the eastern and the northwestern directions from the source by careful comparison of the synthetic and observed displacement seismograms in terms of main phase forms and arrival times at low frequency especially at the closest station gbga the fault model with northeast faulting is estimated to be the more relevant to the measured local ground motions

reconstruction of the east africa and antarctica continental margins the early jurassic separation of antarctica from africa plays an important role in our understanding of the dispersal of gondwana and pangea previous reconstruction models contain overlaps and gaps in the restored margins that reflect difficulties in accurately delineating the continent ocean boundary cob and determining the amount and distribution of extended continental crust this study focuses on the evolution of the african margin adjacent to the mozambique basin and the conjugate antarctic margin near the riiser larsen sea satellite derived gravity data have been used to trace the orientations and landward limits of fracture zones a 3 d gravity inversion has produced a crustal thickness model that reliably quantifies the extent and amount of stretched crust crustal thicknesses together with fracture zone terminations reveal cobs that are significantly closer to the african and antarctic coasts than previously recognized correlation of fracture zone azimuths and identified cobs suggests antarctica began drifting away from africa at approximately 171 ma in a roughly sse direction an areal balancing method has been used to restore the crust to a uniform prerift thickness so as to perform a nonrigid reconstruction for both nonvolcanic and volcanic margins both margins reveal a trend of increasing extension from east to west our results suggest africa underwent extension of 60 120 km while antarctic crust was stretched by 105 180 km various models tested to determine the direction of extension during rifting suggest that antarctica moved away from africa in a wnw ese direction during the period between 184 and 171 ma prior to the onset of seafloor spreading

magma chamber processes in central volcanic systems of iceland constraints from layered gabbro of the austurhorn intrusive complex new field work and petrological investigations of the largest gabbro outcrop in iceland the hvalnesfjall gabbro of the 6 7 ma austurhorn intrusive complex have established a stratigraphic sequence exceeding 800 m composed of at least 8 macrorhythmic units the bases of the macrorhythmic units are composed of 2 10 m thick melanocratic layers rich in clinopyroxene and sometimes olivine relative to the thicker overlying leucocratic oxide gabbros while the overall compositional variation is limited mg clinopyroxene 72 84 an plagioclase 56 85 the melanocratic bases display spikes in mg and cr2o3 of clinopyroxene and magnetite indicative of magma replenishment some macrorhythmic units show mineral trends indicative of up section fractional crystallisation over up to 100 m whereas others show little variation two populations of plagioclase crystals large an rich and small less an rich indicate that the recharge magma carried plagioclase xenocrysts high an type the lack of evolved gabbros suggests formation in a dynamic magma chamber with frequent recharge tapping and fractionation modelling of these compositional trends shows that the parent magma was similar to known transitional olivine basalts from iceland that had undergone about 20 crystallisation of olivine plagioclase and clinopyroxene and that the macrorhythmic units formed from thin magma layers not exceeding 200 300 m such a mushy magma chamber is akin to volcanic plumbing systems in settings of high magma supply rate including the mid ocean ridges and present day magma chambers over the iceland mantle plume the austurhorn central volcano likely formed in an off rift flank zone proximal to the iceland mantle plume during a major rift relocation

terminology for layered intrusions the terminology of igneous cumulates was originally formulated on the concept that the rocks were concentrations of minerals accumulated by crystal settling it is now well established for several major intrusions that plagioclase that had been typed as cumulus should in fact have floated in its parental magmatic liquid nevertheless the terminology has been valuable in the classification of the rocks of these and other intrusions and it has been useful in certain aspects of their genetic interpretation the name cumulate is therefore redefined so that crystal settling is a possible but not essential process in the origins of the rocks to which it is applied the overall nomenclature of layered intrusions is then reviewed and definitions of names are refined and systematized to clarify their applications careful distinction is made between descriptive and genetic names a cumulate is defined as an igneous rock characterized by a framework of touching mineral crystals and grains that evidently were concentrated through fractional crystallization of their parental magmatic liquids a layer is a sheetlike unit of cumulate that is a distinctive entity in its compositional and or textural features a lamina is a thin 2 5 cm sharply defined layer igneous lamination is a general name for planar structure that is pervasive through cumulates on the scale of their grain size layering is the overall structure and fabric manifest through combinations of layers laminae and lamination the difference between layers and layering is essentially the difference between individuals and a population and the terms that describe one are not necessarily appropriate to the description of the other but the fundamental variants of all of layering lamination grading in layers and layer contacts can essentially be described by one or more of the four modifiers grain size modal textural and cryptic where the last refers to cumulus miner al compositions a system for subdividing layered intrusions is proposed in which distinction is made between general divisions ranked as series zones and subzones and prominent units that are called groups members rhythmic units and cyclic units applications to the skaergaard bushveld muskox stillwater and other intrusions are briefly described

fault mechanics and post seismic deformation at bam se iran the extent to which aseismic deformation relaxes co seismic stress changes on a fault zone is fundamental to assessing the future seismic hazard following any earthquake and in understanding the mechanical behaviour of faults here we use models of stress driven afterslip and viscoelastic relaxation in conjunction with post seismic insar measurements to show that there has been minimal release of co seismic stress changes through post seismic deformation following the 2003 mw 6 6 bam earthquake our analysis indicates the faults at bam remain predominantly locked suggesting that the co plus interseismically accumulated elastic strain stored downdip of the 2003 rupture patch may be released in a future mw 6 earthquake our observations and models also provide an opportunity to probe the growth of topography at bam we find that for our modelled afterslip distribution to be consistent with forming the sharp step in the local topography over repeated earthquake cycles and also to be consistent with the geodetic observations requires either 1 far field tectonic loading equivalent to a 2 10 mpa deviatoric stress acting across the fault system which suggests it supports stresses 60 100 times less than classical views of static fault strength or 2 that the fault surface has some form of mechanical anisotropy potentially related to corrugations on the fault plane that controls the sense of slip

advanced insights into magmatism and volcanism of the mozambique ridge and mozambique basin in the view of new potential field data summary a new plate tectonic model is presented which describes the emplacement of the mozambique ridge off southeast africa as the result of long lasting volcanic activity 140 122 ma during the initial opening between africa and antarctica thus an oceanic origin for the mozambique ridge is proposed this model is based on a new and systematic high resolution magnetic anomaly data set acquired across the mozambique ridge and throughout the mozambique basin data from the mozambique basin allow the identification of mesozoic magnetic anomalies from m0r to m26 124 61 155 3 ma with previously unmatched accuracy small scale fracture zones are recognized by offsets of magnetic anomalies in the westernmost part of the basin additionally a bend in the major fracture zones f and e between m17r and m18n 142 84 144 04 ma and a recognized sinusoidal change in spreading direction with an amplitude of about 15 indicate that the basin experienced several small scale changes in spreading direction through time a maximum change in spreading direction to almost 0 at around m11n 135 69 ma is followed by a short lived increase in spreading half rate from 23 5 km ma 1 to about 27 5 km ma 1 in the time frame from m10r to m9n 134 30 132 83 ma we propose that this is related to the initial opening of the south atlantic ocean represented by the onset of seafloor spreading between the falkland plateau and africa in the conjugate georgia and natal basins across the mozambique ridge high amplitude magnetic anomalies at major structural boundaries suggest that the different plateaus of the ridge were formed at different times a simple 2 d gravity and magnetic model for the ridge supports the hypothesis of multiple volcanic episodes which formed the ridge though long lasting volcanic activity between about 140 and 122 ma together with new and verified rotation parameters from the mozambique basin and its conjugate the riiser larsen sea antarctica a series of plate tectonic reconstructions are presented which demonstrate when and how the different parts of the ridge evolved through time

mesozoic and early cenozoic sediment influx and morphology of the mozambique basin abstract the mozambique basin is one of the oldest extensional sedimentary basins developed along the eastern african margin the basin hosts a continuous record of sediments since the jurassic separation of antarctica from africa the objectives of this study were to extend the regional stratigraphic framework north of the zambezi delta into the deep abyssal plains and review the early evolution of the mozambique basin using nine multi channel seismic reflection profiles we identify six major stratigraphic units that were deposited in jurassic early cretaceous late cretaceous paleogene neogene and quaternary times mesozoic sedimentation rates of 5 10 cm kyr and 1 3 cm kyr during the paleogene are calculated in the deeper basin the presence of shales in neighbouring wells on the shelf implies an euxinic environment in the rapidly subsiding basin until early cretaceous times the mesozoic sediments have a high seismic velocity that exceeds 4 5 km s except in a distinct early cretaceous low velocity 3 7 km s zone that may indicate the presence of undercompacted overpressured shales in spite of the fact that the zambezi catchment was much smaller in pre miocene times the high late cretaceous sedimentation rates can be attributed to rapid denudation of the african continent after a major tectonic uplift episode at approximately 90 ma increased sediment influx into the basin from the zambezi in late cretaceous times resulted in the formation of an elongated submarine fan lobe into the mozambique channel north of beira high strong north south bottom currents commenced within the channel in late cretaceous times forcing the aggradation of sediments on the southern flank of the lobe in addition we observe several current controlled sediment deposits in the deeper basin that are influenced by north south bottom currents low paleogene sedimentation rates are attributed to a sediment starved basin during a relative quiet tectonic phase onshore

basin analysis and hydrocarbon generation of the south mozambique graben using extensional models of heat flow abstract the northern south mozambique graben was assessed for hydrocarbon potential by reconstructing its structural burial and thermal histories using a seismic line and an array of basin modelling techniques the structural and depositional history was determined by simultaneously palinspastically restoring and backstripping a depth converted cross section the thermal history was predicted from extensional models which relate palaeoheat flow to basement subsidence maturation potential cross sections were derived by applying a generation model to each of the pseudo wells assuming each layer to be a potential source rock considering the range of uncertainty when using these models this analysis suggested that regardless of the extension model type used and within the maximum and minimum limits of the palaeoheat flow the eastern side of the northern south mozambique graben is more hydrocarbon prone than the west in addition the timing of hydrocarbon generation regardless of the palaeoheat flow possibilities modelled is favourably after source and seal rock deposition and after the development of trapping structures it is apparent from this study that a stratigraphic well with down hole thermal indicator measurements and bottom hole temperature information would greatly enhance the understanding of the hydrocarbon potential of this basin

strong motion and teleseismic waveform inversions for the source process of the 2003 bam iran earthquake the bam earthquake m w 6 5 occurred on 26 december 2003 in southeastern iran causing a major disaster in the city of bam a remarkable vertical peak ground acceleration value of 988 cm s2 and two horizontal pulses were recorded inside the damaged city previous analyses showed that the earthquake was caused by a subsurface rupture on an unknown strike slip fault in this study we attempt to determine the precise fault location and source process of the 2003 bam earthquake by performing a multiple time window linear waveform inversion of teleseismic and strong motion data both individually and jointly we examined the general features of the fault location and the source process by analyzing the teleseismic displacement waveforms and determined the precise features and fault geometry by inverting the three components of strong motion velocity records the final estimate of the source process of the 2003 bam earthquake was determined by joint inversion of the datasets our results show that a single fault model characterized by the appropriate location of the hypocenter rake angle variations and the rayleigh like speed of the rupture front can satisfactorily explain the three components of the strong motion records at bam station

near fault strong motion complexity of the 2000 tottori earthquake japan from a broadband source asperity model abstract the october 6 2000 tottori earthquake that occurred in central japan was an intermediate size strike slip event that produced a very large number of near field strong motion recordings the large amount of recorded data provides a unique opportunity for investigating a source asperity model of the tottori earthquake that combined with a hybrid strong motion simulation technique is able to reproduce the observed broadband frequency near fault ground motion we investigated the optimum source asperity parameters of the tottori earthquake by applying a genetic algorithm ga inversion scheme to optimise the fitting between simulated and observed response spectra and peak ground acceleration pga values we constrained the initial model of our inversion by using the heterogeneous slip distribution obtained from a kinematic inversion of the source of previous studies we used all the observed near fault ground motions 100 m from the borehole strong motion network of japan kik net which are little affected by surficial geology site effects the calculation of broadband frequency strong ground motion 0 1 10 hz is achieved by applying a hybrid technique that combines a deterministic simulation of the wave propagation for the low frequencies and a semi stochastic modelling approach for the high frequencies for the simulation of the high frequencies we introduce a frequency dependent radiation pattern model that efficiently removes the dependence of the pattern coefficient on the azimuth and take off angle as the frequency increases the good agreement between the observed and simulated broadband ground motions shows that our inversion procedure is successful in estimating the optimum asperity parameters of the tottori earthquake and provides a good test for the strong ground motion simulation technique the ratio of background stress drop to average asperity stress drop from our inversion is nearly 50 in agreement with the theoretical asperity model of das and kostrov das s kostrov b v 1986 fracture of a single asperity on a finite fault a model for weak earthquakes earthquake source mechanics agu pp 91 96 and an empirical ratio of asperities to rupture area seismol res lett 70 1999 59 80 the simulated radiation pattern is very complex for epicentral distances within half the fault length but it approaches the radiation of a double couple point source for larger distances the rupture velocity and rise time have a significant influence on the peak ground velocity pgv distribution around the fault an increase in rupture velocity produces a similar effect on the ground motion as a reduction in rise time

the 2003 december 26 bam earthquake iran mw 6 6 aftershock sequence summary from 2003 december 29 to 2004 january 30 a dense seismological network of 23 stations was installed in the epicentral area of the 2003 december 26 bam earthquake to study the aftershock seismicity we select the 331 earthquakes recorded at a minimum of 10 stations with rms less than 0 1 s and uncertainties less than 1 km to infer the precise geometry of the seismicity in the fault region we also process the data with the double difference technique to confirm the results the aftershock cluster is 25 km long trends north south and is located 5 km west of the bam baravat escarpment exactly beneath the observed surface breaks at depth aftershocks are concentrated between 6 and 20 km beneath the upper layer of relatively low velocity that experienced the maximum slip and they dip slightly westward the southernmost part of the aftershock cluster is narrow and defines the rupture zone that is likely the bam baravat fault at depth however it is unlikely that it is connected at surface to the bam baravat escarpment but more likely to the co seismic ruptures south of bam on the contrary the distribution of the northernmost aftershocks spread into a more complex pattern which is consistent with a northward propagation of the rupture along the fault plane the focal mechanisms are consistent with right lateral strike slip faulting on n s trending faults parallel to the bam baravat escarpment

mesozoic seafloor spreading in the southern weddell sea a re evaluation of weddell sea magnetic anomaly data in the light of satellite gravity maps provides important constraints on plate kinematics prior to chron c34 83 ma although m series anomalies are difficult to identify the evidence favours younger ages than previously suggested based on the assumption that these anomalies formed at the south america antarctica plate boundary geosat derived free air gravity maps show a distinct anomaly anomaly t associated with the southern termination of a herring bone pattern of flow lines the associated magnetic anomaly has been identified previously as anomaly m20 on the basis of a model of very slow spreading but is interpreted here as an edge effect associated with an abrupt decline in spreading rate which occurred between m4 126 ma and m0 118 ma and which induced the fine segmentation pattern to the north although anomalies are present to the south of anomaly t they are of reduced amplitude and are difficult to correlate over distances of more than c 100 km nevertheless they appear to strike approximately w e sub parallel to the younger anomalies to the north and show a resemblance to anomalies m4 m12 modelled at a rate of 15 km ma 1 flow line modelling based on published rotations for the southwest indian ridge and southern mid atlantic ridge gives a reasonable fit to the observed trends of weddell sea gravity ridges two alternative models for the m series reconstruction of africa and antarctica are employed corresponding to the two m2 fits suggested by bergh although it is not possible to reject either of the models the tighter fit of the two gives slightly better agreement with weddell sea trends in both models a drop in rates occurs at m0 time anomalies farther south correspond with previously identified features such as the orion explora and andenes anomalies and the polarstern bank all characterized by magnetic anomaly highs attributed to excess volcanism during early break up

the 2003 bam se iran earthquake precise source parameters from satellite radar interferometry summary differential radar interferometry provided high quality near field deformation data for the 2003 bam earthquake and therefore strong constraints on its source parameters the ruptured fault segments could be clearly detected by using a sobel edge filter on the phase unwrapped deformation field the estimated total rupture length is about 24 km more than 80 per cent of the seismic moment was released from its southern segment of about 13 km where the slip reached a maximum of up to 270 cm resulting in a stress drop of at least 6 mpa in addition optical remote sensing data show that the bam fault is not a single fault but consists of a 4 5 km wide fault system with the known main branch running between the city of bam and baravat the fault ruptured by the bam earthquake appears to continue the nw branch of this fault system from bam city southwards based on these results we suggest that the bam earthquake ruptured a hidden or new fault and that in this process an unusually strong asperity was involved

mesozoic magnetic anomalies in the southern natal valley the recognition of mesozoic magnetic anomalies m0 m12 within the submarine natal valley south west indian ocean provides a further constraint on the relative motion between south america falkland plateau and africa during the early opening phase of west gondwanaland fig 1 colinear with m12 magnetic anomaly g ref 1 is delineated north of the south tugela ridge2 an acoustic basement high which is suspected to have been emplaced within continental crust immediately north of the continental oceanic crust boundary cob during the initial rifting phase starting from a revised west gondwana reconstruction3 and using palaeo poles of rotation1 3 the falkland plateau can be rotated into the natal valley superposing the corresponding georgia basin and natal valley magnetic anomalies this achieves an optimum continuity of inter continental structural lineaments3 by overlapping of an anomaly intermediate to m10 and m11 m10n4 this suggests that the natal valley cob is located between anomalies m10 and m11 implying that in this zone of south west gondwana seafloor spreading commenced at 125 myr

magma ascent and crustal accretion at ultraslow spreading ridges constraints from plagioclase ultraphyric basalts from the arctic mid ocean ridge plagioclase ultraphyric basalts pubs with up to 54 plagioclase phenocrysts were dredged in the rift valley and adjacent flanks of the ultraslow spreading mohns and knipovich ridges the pubs show large variations in crystal morphologies and zoning the large variations suggest that single basalt samples contain a mixture of plagioclase crystals that aggregated at different levels in the magma conduits resorbed crystals and repeated reverse zones suggest that the magma reservoirs were replenished and heated several times thin concentric zones with melt inclusions and sharp reductions in the anorthite content of 3 7 are common between the reverse zones these zones and skeletal crystals with distinctly lower anorthite contents than massive crystals are interpreted to be the result of rapid crystalliztion during strong undercooling the changes between short periods of cooling and longer periods with reheating are explained by multiple advances of crystal rich magma into cool regions followed by longer periods of gradual magma inflow and temperature increase the porphyritic basalts are characterizd by more depleted and more fractionated compositions than the aphyric basalts with lower la sm n k 2 oa nd mg numbers this relationship and the observation that pubs are sampled only close to segment centres along these ridges suggests that the pubs formed by higher degrees of melting and evolved in more long lived magma reservoirs we propose that the zoning patterns of plagioclase crystals and crystal morphologies of these pubs reflect the development and flow of magma through a stacked sill complex like conduit system whereas the aphyric equivalents represent later flow of magma through the conduit the formation of voluminous higher degree melts may trigger the development of the magma conduits and explain the generally depleted compositions of pub magmas

effect of latent heat of freezing on crustal generation at low spreading rates lithospheric structure changes at low spreading rates 40 mm a 1 such that steady state molten rock cannot exist above the normal moho depth of 6 km dredge statistics of recovered lithologies indicate that the percentage of basalt is significantly decreased compared to faster ridges and that the mode of crust generation is variable one important effect on lithospheric structure is the latent heat released when basaltic magma freezes within the mantle instead of being transported all the way to the surface using thermal models we show that freezing of melt at mantle depths buffers temperature due to the latent heat of crystallization two quasi stable seafloor spreading patterns are imposed on the model 1 upwelling of magma along a narrow axial zone and 2 pure shear extension over a broad zone the variability in crustal structure predicted by these models explains variability observed in dredge lithology statistics at eight slow to ultraslow spreading ridge segments for example the gakkel eastern volcanic zone and sparsely magmatic zone are both spreading at 12 mm a 1 but the former is composed of 7 peridotite whereas the latter is 46 peridotite this difference can be explained by a change from a narrow axis regime with 2 km thick crust to a wide axis regime with a crust mantle mix overall dredge statistics and thermal models suggest that some but not all latent heat of ascending magmas is released at mantle depths and that various ridge axial morphologies can occur

new continental margin magnetic anomalies of east antarctica over the past decade australian norwegian and russian marine surveys have collected integrated seismic gravity and magnetic data in the southern indian ocean the more than 350 000 line km of new airborne and marine magnetic observations for the east antarctic continental margin have been compiled into an improved definition of crustal magnetic anomaly patterns this compilation provides important new constraints on the breakup processes and igneous activity related to the formation of the passive margin of east antarctica the eastern sector of the map from bruce rise in the west to the d urville sea in the east is largely dominated by seafloor spreading magnetic anomalies the adelie rift block of highly stretched and extensively faulted continental crust is associated with a smooth anomaly fabric abrupt magnetic anomaly changes along the oceanic continent transition in the cooperation sea including the enderby basin anomaly extend for more than 1680 km from the kerguelen plateau towards the cosmonaut sea three sectors of the east antarctic continental margin exhibit pronounced disparities in the anomaly patterns that strongly suggest different modes of seafloor formation strong positive seafloor magnetic anomalies mark the southern margin of the kerguelen plateau the maud rise and adjacent areas in the riiser larsen sea the new compilation suggests that at least 300 km of the enderby basin and shackleton basin may be part of the cretaceous kerguelen volcanic province and possibly maps an abandoned fossil spreading center in the central enderby basin the majority of the published age models for the enderby basin and australian sector of the east antarctic margin are not in agreement with the structural grain of magnetic anomalies in the newly compiled map

petrological variability of recent magmatism at axial seamount summit juan de fuca ridge 1 a combined study of mapping observational age constraint and geochemical data at the summit of axial seamount juan de fuca ridge has revealed its recent petrological history multiple basalt types erupted at the summit in a time sequence at least three different magma batches have been present beneath the axial summit caldera during the last millennium each with a range in differentiation the first prior to 1100 ce was compositionally diverse dominantly aphyric t morb the second from 1220 to 1300 ce was dominantly plagioclase phyric more mafic n morb erupted mostly in the central portion of the caldera since 1400 ce lavas have been more differentiated and nearly aphyric t morb mostly erupted in the caldera s rift zones parental magmas vary subtly due to small coupled differences in the degree of melting and sources but all share a uniform differentiation trend indicating pooling at similar depths thus melts percolate through melt rich lenses that remain partially isolated in space and or time centennial magmatic timescales at axial seamount are similar to those for fast spreading ridge segments the fluctuation between aphyric and plagioclase phyric lava likely reflects different pathways or velocities of melt migration

the initial gondwana break up a synthesis based on new potential field data of the africa antarctica corridor abstract the initial opening of the africa antarctica corridor in the heart of gondwana is still enigmatic due to missing information on the origin of major crustal features and the exact timing of the onset of the first oceanic crust in the jurassic therefore in 2014 new ship borne magnetic data were systematically acquired in the northern mozambique basin and across beira high which we merged with all accessible magnetic data in the mozambique basin herein distinct magnetic lineations are observed which allow a refined identification of a whole set of jurassic magnetic spreading anomalies constraining the timing of the onset of oceanization beginning at m38n 2n 164 1 ma in combination with high resolution potential field data from the conjugate antarctic margin well expressed fracture zones can be traced throughout the africa antarctica corridor and allow the precise rotation of antarctica back to africa the initial fit depicts striking continuations of onshore tectonic features across the plate boundaries taking onshore aeromagnetic data of both margins into account within a tight gondwana fit the beira high can be restored along the major sinistral namama orvin shear zone of the east african antarctic orogen the beira high represents a continental block which was detached from antarctica by 157 ma at the latest simultaneously the antarctic plate cleared the area of the mcp however the crustal nature of the southern mcp remains ambiguous the northern natal valley and the mozambique ridge consist of thick oceanic crust being emplaced between m26r m18n 157 1 144 ma and m18n m6n 144 131 7 ma respectively about the half of this crust was won from the antarctic plate by a series of southwards directed ridge jumps to the northern boundary of the explora wedge a refined kinematic break up model constrained by the most extensive magnetic dataset is presented describing consistently the initial opening of the africa antarctica corridor and the somali basin

magma mixing recharge and eruption histories recorded in plagioclase phenocrysts from el chichon volcano mexico operative throughout the 200 ky history of the el chichon magma consistent core to rim decreases of sr sr ratios and coincident system increases in sr concentrations in plagioclase phenocrysts of varying size 1 cm to 2 mm are reported from samples of the 1982 and pre 1982 200 ka eruptions of el chichon volcano maximum sr sr ratios of 0 7054 significantly higher than the wholerock isotopic ratios 0 7040 0 7045 are found in the cores

simulation of selected strong motion records of the 2003 mw 6 6 bam earthquake se iran the modal summation ray tracing methods in the wkbj approximation

the zambezi delta mozambique channel east africa high resolution dating combining bio orbital and seismic stratigraphies to determine climate palaeoprecipitation and tectonic controls on a passive margin abstract the zambezi delta draining the southern african plateau and the southern part of the east african rift is one of a the largest delta of africa with a long lasting history starting during early cretaceous with more than 12 km of sediments deposited the zambezi delta is therefore a unique archive of the past topographic evolution of southern and eastern africa and their related deformations but also of the climate changes global and regional consequences of local topographic growths understanding this archive supposes to get a high resolution dating of the sediments our two objectives are here 1 to construct an age model of the zambezi cenozoic delta using a combination of biostratigraphy orbital stratigraphy and sequence stratigraphy and 2 to determine the palaeoprecipitation variations of the zambezi catchment from the oligocene to present day in a known tectonic framework the neogene sequences were dated at high resolution assuming that the third order sequences are of eustatic origin and record long term eccentricity cycles the sequences were correlated in ages on the calculated earth orbital solutions of laskar for the time intervals provided by the biostratigraphy nannofossils planktonic foraminifers the palaeoprecipitation record was based on the definition of a humidity index based on pollen analysis and associated botanical associations the late oligocene was a quite wet period getting dryer in the uppermost chattian the base tortonian 11 ma was a humid period the messinian was a dry period with a slight increase of the humidity during the zanclean and a sharp increase around the zanclean piacenzian boundary the zambezi delta has recorded the uplifts of the southern african plateau around 85 ma and around 25 ma and those of the southward migration of the east african rift since 5 5 ma

cretaceous reconstructions of east antarctica africa and madagascar abstract some cretaceous reconstructions of east antarctica africa and madagascar overlap the mozambique ridge onto the margin of dronning maud land and others place madagascar west of the gunnerus ridge into the riiser larsen sea our m10ny 131 9 ma m0 120 4 ma 96 ma and 34y 83 ma marine free air gravity and topography reconstructions illustrate a solution to the overlap problem and also demonstrate that madagascar separated from antarctica east of the gunnerus ridge our reconstructions chronicle the growth of the conjugate mozambique and riiser larsen basins created at the southwest indian spreading ridge well defined spreading corridors identified by conjugate fracture zones and mesozoic magnetic anomaly identifications extend from one basin into the other we find that the eastern scarp of the mozambique ridge aligns with the astrid ridge and there is no room for madagascar west of the gunnerus ridge the overlap of the mozambique ridge is solved by taking into account the motion of an independent mozambique ridge microplate that existed from chron m11 to circa m2

source fault structure of the 2003 bam earthquake southeastern iran inferred from the aftershock distribution and its relation to the heavily damaged area existence of the arg e bam fault proposed 1 we investigate the hypocenter distribution of aftershocks of the december 26 2003 bam earthquake mw 6 5 by using a temporal seismic network the hypocenters distribute linearly over about 20 km in parallel with a line 3 5 km west of the geological bam fault and extend from the south of bam city to the heavily damaged area in the eastern part of the city including the historical mud brick citadel arg e bam based on the hypocenter distribution we propose a schematic 3 d structural model of a new fault that we have named the arg e bam fault we suggest that the bam earthquake occurred not in the bam fault but in the arg e bam fault bam city is located just above northern part of the arg e bam fault and also on the rupture propagation direction from the asperity this may be one of the main reasons why eastern part of bam city suffered the heaviest damage

evolution of the glorieuses seamount in the sw indian ocean and surrounding deep somali basin since the cretaceous abstract little is known about the geological history of the glorieuses seamount including basic information about its age and origin related to the regional evolution of the southern tip of the somali basin this study focused on describing and reconstructing the long term stratigraphic evolution of the glorieuses seamount sw indian ocean to identify the mechanisms that have occurred through time to finally shape the emerged modern islands distinct terrace levels currently submerged along the flanks of the seamount and surrounding seamounts have already been interpreted as resulting from successive carbonate development and back stepping episodes over the last 62 myr new isotopic and biostratigraphic dating on the flanks of the seamount coupled with sequence stratigraphic interpretation of seismic profiles acquired in the adjacent basin provide new constraints for the late cretaceous and cenozoic vertical evolution of the seamount topped by carbonate platforms and sedimentation in the surrounding deep basin even if starved steep slopes prevent a straightforward source to sink continuity between the platform and the basin domains our findings propose a consistent chronostratigraphic framework for the identified seismic markers and sequences in the deep basin and discuss a long term geological model that includes the main driving factors behind deposition volcanic events subsidence vs uplift phases climate and hydro dynamism changes and their quantitative impact on the evolution of the isolated carbonate sedimentary system our results show that i the glorieuses volcanic seamount emerged from two successive late cretaceous magmatic pulses firstly during the turonian then during the maastrichtian ii at least two potential uplift phases are recognized during the tertiary paleogene and or the eocene and tortonian iii basinal sedimentation recorded an abrupt change probably related to major regional hydro dynamical changes in late eocene times in the western indian ocean iv the export of sediments from the platform towards the basin numerous gravity flow processes is strongly enhanced after the mid miocene and is probably linked to the onset of the asian monsoon winds and bipolar circulation finally the glorieuses seamount although located in the vicinity of the comoros islands appears to have a much longer history and is geologically more comparable to the nearby seychelles this long term study has enabled us to associate the glorieuses seamount with the sse nnw madagascar seychelles alignment rather than with the comoro hot spot evolution

numerical simulations of crystallization processes of plagioclase in complex melts the origin of major and oscillatory zoning in plagioclase numerical simulations of the growth of a large crystal face of plagioclase in response to an instantaneous undercooling below the equilibrium temperature are presented for model granodiorite and basalt melts with varying water contents the simulations suggest that the anorthite content of plagioclase decreases uniformly from the composition in equilibrium with the bulk melt as undercooling is increased and that the water content in the melt has little influence on this result comparison of the simulations with sharp compositional changes in natural profiles suggests that undercoolings of tens of degrees c can be rapidly imposed on plutonic phenocrysts large changes of undercooling most likely result from chilling of the magma and local convection around growing crystals the observation in experiments that growth rate does not increase rapidly with increasing water content in the starting melting composition can be attributed to the concentration of water at the crystal face during growth the action of water to reduce liquidus temperature and undercooling has a greater effect on growth rate than its action to increase transport rates even at large undercooling there is no significant increase in temperature at the interface caused by the release of heat of crystallization

spatial relation between main earthquake slip and its aftershock distribution 1 we examine where aftershocks occur relative to the spatial distribution of the main shock slip using data from several recent large earthquakes no universal relation between high and low moment regions and high or low aftershock occurrence or vice versa is found we generally find that few and usually the smaller aftershocks occur in the high slip regions of the fault a notable exception to this being the great 1996 biak indonesia subduction zone earthquake in all cases aftershocks occur on favorably oriented planes of weakness in regions of increased postseismic stress generally they are clustered at both ends of faults but examples where aftershocks occur only at one end or where there is no clustering at the ends are found aftershock clusters are also found at the edge of unbroken barriers and regions of rapid transition from high to low slip within the main fault area we identify examples of geometrical and inhomogeneous barriers and sharp and dull stress concentrations rupture in the main shock is generally found to nucleate in the region of low slip or at the edge of high slip regions the 1996 biak earthquake again being the only exception nucleating in a very high slip region off fault aftershocks are found for all earthquakes in this study and they sometimes rupture the nodal plane conjugate to the main shock fault plane

preliminary analysis of the knipovich ridge segmentation influence of focused magmatism and ridge obliquity on an ultraslow spreading system abstract bathymetry gravity and deep tow sonar image data are used to define the segmentation of a 400 km long portion of the ultraslow spreading knipovich ridge in the norwegian greenland sea northeast atlantic ocean discrete volcanic centers marked by large volcanic constructions and accompanying short wavelength mantle bouguer anomaly mba lows generally resemble those of the gakkel ridge and the easternmost southwest indian ridge these magmatically robust segment centers are regularly spaced about 85 100 km apart along the ridge and are characterized by accumulated hummocky terrain high relief off axis seamount chains and significant mba lows we suggest that these eruptive centers correspond to areas of enhanced magma flux and that their spacing reflects the geometry of underlying mantle upwelling cells the large scale thermal structure of the mantle primarily controls discrete and focused magmatism and the relatively wide spacing of these segments may reflect cool mantle beneath the ridge segment centers along the southern knipovich ridge are characterized by lower relief and smaller mba anomalies than along the northern section of the ridge this suggests that ridge obliquity is a secondary control on ridge construction on the knipovich ridge as the obliquity changes from 35 to 49 from north to south respectively while spreading rate and axial depth remain approximately constant the increased obliquity may contribute to decreased effective spreading rates lower upwelling magma velocity and melt formation and limited horizontal dike propagation near the surface we also identify small magmatically weaker segments with low relief little or no mba anomaly and no off axis expression we suggest that these segments are either fed by lateral melt migration from adjacent magmatically stronger segments or represent smaller discrete mantle upwelling centers with short lived melt supply

cretaceous palaeopositions of the falkland plateau relative to southern africa using mesozoic seafloor spreading anomalies summary mesozoic seafloor spreading anomalies mo m10 have been identified in the natal valley between the south eastern african margin and the mozambique ridge these identifications confirm that seafloor spreading north and south of the falkland agulhas fracture zone fafz began simultaneously the natal valley anomalies are offset 1300km by the fafz from their equivalents in the southern cape basin their positions confirm that the tugela ridge marks the continent ocean boundary cob in the natal valley we have computed successive falkland plateau palaeopositions using natal valley and georgia basin anomalies these show that the offset in spreading ridges at the fafz remained 1300km long from m10 to mo time by anomaly 34 time the offset was 1270 km therefore no major ridge jumps had occurred by then dating mo as 108 myr bp and anomaly 34 as 80 myr bp the average halfspreading rate immediately south of the fafz for the cretaceous normal polarity epoch is 4 2 cm yr using this we date a the change in early pole of rotation at 105 myr b a reconstruction which juxtaposes salt boundaries in the brazil and angolan basins at 103 7 myr c final separation of the falkland plateau from southern africa at 98 3 myr d the formation of the oceanic northern part of the agulhas plateau at 97 3 90 7 myr comparison of magnetic data with implied cob positions in the southernmost cape and argentine basins and the georgia basin suggests continental separation began 122 1 27 myr bp with undeformed magnetic anomalies dating from 122 myr m10 these dates are consistent with micropalaeontological and sedimentological data around southern africa

crustal evolution and the temporality of anorthosites abstract types of rock such as komatiite which formed entirely or dominantly during restricted time periods in solar system history are important indicators of how planetary bodies evolved in deep time in this paper we characterize three different types of temporally restricted anorthosites and discuss their significance to the broad scale evolution of planetary processes primordial anorthosites which constitute the bulk of the lunar crust were sampled during apollo and luna missions and have subsequently been identified in our meteorite collections they are characterized by very calcic plagioclase an93 98 and all have magmatic crystallization ages 4 3 ga suggesting that the earliest planetary crust forming processes on the moon and possibly elsewhere involved substantial if not total melting the magma ocean hypothesis which has endured for nearly 50 years argues that the lunar anorthositic rocks represent global flotation cumulates of plagioclase which crystallized after extensive precipitation and sinking of olivine and pyroxene another possibility revitalized by critical filtering of age data involves serial magmatism whereby the lunar crust was constructed by younger smaller episodic magmatic events on earth two distinct types of temporally restricted anorthosites offer opportunities to understand the evolution of terrestrial geodynamics tectonics and magmatic processes archean megacrystic anorthosites formed only between 3 73 and 2 49 ga as small bodies

is there continental crust underneath the northern natal valley and the mozambique coastal plains 1 to draw conclusions about the crustal nature and history of the natal valley and the mozambique ridge systematic potential field data were obtained during the aistek iii cruise with r v pelagia in 2009 this paper presents and interprets the results of that expedition the new magnetic data reveal a pattern of linear magnetic spreading anomalies nw se trending in the southwestern part of the mozambique ridge and e w trending on its central part the ariel graben which separates the mozambique ridge from the northern natal valley is represented by a pronounced negative anomaly in the magnetic as well as the free air gravity field the northern natal valley bears a complicated pattern of anomalies with mainly sw ne trends in the northern natal valley no indications for a continent ocean boundary between continental crust in the north and oceanic crust in the south exist either in the free air gravity or in the magnetic field the magnetic wavelengths of the mozambique coastal plains are similar to those of the northern natal valley and the mozambique ridge particularly in the gravity data the mozambique coastal plains the northern natal valley and the mozambique ridge appear as one continuous geological province we interpret the region from the mateke sabi monocline in the north to the southwestern tip of the mozambique ridge as mainly being floored by thickened oceanic crust

relationship between plagioclase crystallization and cooling rate in basaltic melts rock textures commonly preserve a record of the near surface crystallization history of volcanic rocks under conditions of simple cooling without convection or mixing textures will reflect sample cooling rate the temperature at which crystallization was initiated and the distribution of mineral phase precipitation across the crystallization interval compilation of plagioclase size and number density data on natural dike sill and lava lake and experimental samples suggests that 1 growth and nucleation rates of plagioclase in natural basaltic samples are a predictable function of cooling rate and 2 the observed crystallization rate dependence on cooling rate is similar to that observed in experiments initiated at subliquidus temperatures comparison of natural and experimental samples thus suggests that most basalts crystallize under conditions of heterogeneous nucleation with the number density of preexisting nucleii partially controlling textural responses to cooling rate changes time scales of crystallization and cooling in magmatic systems are intimately linked through a balance between heat removal from the system and heat evolved through crystallization evaluation of textural data in the context of recent numerical models of crystallization in simple one and two component systems provides new insight into regularities in the crystallization behavior of basaltic magmas for example the rate of change in crystal size and number density as dictated by mass balance has been used as a measure of the relative importance of time scales of crystallization and cooling in numerical models of crystallizing systems in natural samples plagioclase size scales with the length scale of cooling such that a logarithmic plot of grain size as a function of normalized distance across the dike has a slope that appears approximately independent of dike width solidification time comparison with available textural data for other phenocryst phases suggests that the same may be true for pyroxene and magnetite crystallization with each phase having a characteristic slope probably controlled by the thermodynamic properties of the crystallizing phase measured crystal size distributions are unimodal and show maximum frequencies in the smaller size classes distributions broaden and the grain size at peak frequency increases with increasing crystallization times decreasing cooling rates in contrast partially crystallized makaopuhi lava lake samples have crystal size distributions that decrease exponentially with increasing crystal size measured size distributions in dikes can be explained by late stage modification of makaopuhi type distributions through loss of small crystals possibly the consequence of growth without nucleation finally this compilation of the textural response of basaltic magmas to changes in cooling rate suggests that empirical calibrations of crystallization rate dependence on cooling rate from natural samples provide a reasonable model for plagioclase crystallization in near surface basaltic systems predicted growth rates will be slow and relatively constant 10 10 10 11 cm s for crystallization times expected in most shallow volcanic systems 1000 years

petrology geochemistry and original tectonic setting of basalts from the mozambique basin and ridge dsdp sites 248 249 and 250 and from the southwest indian ridge dsdp site 251 abstract basalts from dsdp sites 248 249 250 and 251 in the southwestern indian ocean formed in a complex tectonic region affected by the separation of africa and south america the different ages and variable geochemical features of these dsdp basalts probably reflect this tectonic complexity for example site 251 on the flanks of the southwest indian ridge is represented by normal morb which probably originated at the southwest indian ridge site 250 in the mozambique basin includes an older incompatible element enriched unit which may represent basalt associated with the prince edward fracture zone the upper unit is normal morb basalts at site 248 also in the mozambique basin are geochemically very unlike morb and have strong continental affinities they are also comparable in age to some of the continental karroo basalts they appear to be related to a subcontinental mantle source or to contamination by continental basement associated with the tectonic elevation of the mozambique ridge basalts from site 249 on the mozambique ridge are relatively weathered but appear to be normal morb their age location and composition are consistent with their origin at an early cretaceous rift which has been postulated to have separated the falkland plateau from the mozambique ridge

sedimentology and depositional architecture of a submarine delta fan complex in the durban basin south africa abstract the seismic stratigraphy evolution and depositional framework of a sheared passive margin the durban basin of south east africa are described based on single channel 2d seismic reflection data six seismic units a f are revealed separated by major sequence boundaries these are compared to well logs associated with the seismic data set internal seismic reflector geometries and sedimentology suggest a range of depositional regimes from syn rift to upper slope and outer shelf nearshore and continental facies are not preserved with episodic shelf and slope sedimentation related to periods of tectonic induced base level fall the sedimentary architecture shows a change from a structurally defined shelf shearing phase to shallow ramp and then terminal passive margin sedimentary shelf settings sedimentation occurred predominantly during normal regressive conditions with the basin dominated by the progradation of a constructional submarine delta tugela cone during sea level lowstands lst the earlier phases of sedimentation are tectonic controlled however later stages appear to be linked to global eustatic changes

geology basin analysis and hydrocarbon potential of mozambique and the mozambique channel abstract the phanerozoic structural and stratigraphic history of mozambique can be separated into a karoo and a post karoo phase outcrops of karoo rocks limited to the zambezi valley and small outliers in the mozambique belt to the north show an eastward facies change marked by an increased argillaceous content and the appearance of carbonates the wide coastal plain and immediate offshore of southern mozambique is marked by extensional horsts and grabens with a mesozoic fill covered by cenozoic sediments little is known of lower stratigraphic horizons in this coastal province there is a facies transition from continental clastics in the northwest and west to more lacustrine and marine deposits to the southeast and east upwards the succession passes into the cenozoic without a noticeable break in the offshore in the north seismic lines help define two basins the zambezi delta basin and the mozambique channel basin separated by the beira high in the south several offshore grabens have been defined one of which the south mozambique graben has been examined here by an integrated basin analysis technique the technique which uses available depositional structural and thermal data provides information on maturation and the timing of hydrocarbon maturation the analysis showed variation in the extension rate with time and two phases of rifting the greatest rate of extension accumulation and subsidence between 96 and 76 ma depends upon interpretational assumptions during this time interval fault blocks were systematically tilted domino fashion this event broadly coincides with the rifting between madagascar and antarctica a period of plate reorganization the earlier rifting phase was probably related to the strike slip separation of madagascar from africa modelling heat flow suggests that the later event is the more important from the maturation hydrocarbon standpoint the offshore and potentially the deeper onshore post karoo basins have traps reservoirs and seals with the presumed source rock within lower cretaceous or jurassic strata there is at the present time no means to evaluate the potential of the karoo rocks

the petrogenesis of plagioclase phyric basalts at mid ocean ridges 1 plagioclase ultraphyric basalts pubs have been sampled along most mid ocean ridges with ultraslow to intermediate spreading rates over the past 40 years the prevalent models for their origin assume positive buoyancy of plagioclase in basaltic liquids resulting in selective concentration of plagioclase phenocrysts by floatation however when the global population of pub lavas is examined this hypothesis becomes less compelling pub host lavas demonstrate a large range of compositions and densities similar to aphyric glasses from the same ridge segments most importantly the majority of pub host liquids are less dense than their phenocryst cargo meaning that plagioclase floatation within a magma chamber cannot be the driving force for phenocryst enrichment furthermore pub lavas have never been sampled on axis at fast spreading centers or from locations with noted contemporaneous axial magma chambers where pubs should be abundant if plagioclase is buoyant in mid ocean ridge basalt morb instead we argue that the high modal abundance of plagioclase results from interaction between magma and preexisting zones of crystal cumulates within the lower crust possibly followed by loss of olivine during magma ascent pubs erupt when the magma maintains an ascent velocity greater than the settling rate of the plagioclase phenocrysts which precludes long crustal residence times for these magmas in addition to being a proxy for lower spreading rates our findings also suggest that pub eruption can also be used as a proxy for the absence of a magma chamber or transport through a conduit system that bypassed an axial chamber

the african plate a history of oceanic crust accretion and subduction since the jurassic we present a model for the jurassic to present evolution of plate boundaries and oceanic crust of the african plate based on updated interpretation of magnetic gravity and other geological and geophysical data sets location of continent ocean boundaries and age and geometry of old oceanic crust jurassic and cretaceous are updated in the light of new data and models of passive margin formation a new set of oceanic palaeo age grid models constitutes the basis for estimating the dynamics of oceanic crust through time and can be used as input for quantifying the plate boundary forces that contributed to the african plate palaeo stresses and may have influenced the evolution of intracontinental sedimentary basins as a case study we compute a simple model of palaeo stress for the late cretaceous time in order to assess how ridge push slab pull and horizontal mantle drag might have influenced the continental african plate we note that the changes in length of various plate boundaries especially trenches do not correlate well with absolute plate motion but variations in the mean oceanic crust age seem to be reflected in acceleration or deceleration of the mean absolute plate velocity

role of continental break up mantle plume development and fault reactivation in the evolution of the gamtoos basin south africa abstract in response to the break up of gondwana the gamtoos basin rifted in the oxfordian kimmeridgian with reactivation of cape fold belt thrusts as extensional structures rifting was rapid with an initial divergent syn rift sequence passing into a parallel passive infill sequence as rifting waned dextral movement on the agulhas falkland fracture zone during the valanginian resulted in reactivation of oxfordian kimmeridgian structures with extensional compressional and strike slip senses of movement post rift thermal subsidence phase of basin evolution began during latest valanginian times with passive infill and onlap of the topography created during the valanginian subsequently the basin began to be uplifted due to the westward migration of the falkland plateau and natal valley spreading centre past the basin resulting in a non depositional hiatus and local channelling before erosion during the cenomanian following renewed subsidence the basin experienced uplift during the palaeocene and eocene the cause of which remains problematic but is contemporaneous with igneous intrusion in southern africa

magnetic lineations within shatsky rise northwest pacific ocean implications for hot spot triple junction interaction and oceanic plateau formation oceanic plateaus are major ocean features yet their origins and development are poorly understood many are huge piles of basalt and it is widely accepted that mantle plumes are their source perhaps from eruptions of the voluminous head of an emerging plume shatsky rise is a basaltic plateau that formed during a period of geomagnetic reversals unlike many mid cretaceous plateaus so magnetic data can help us understand its tectonic history in this study we analyzed magnetic anomaly data from 131 cruises over and around shatsky rise and constructed a magnetic lineation chart for tectonic interpretation a significant finding is that magnetic lineations are traceable through low parts of the rise between volcanic massifs indicating nearly normal lithosphere and between large volcanic edifices many lineations form bights near the rise axis and show former locations of the pacific izanagi farallon triple junction they indicate that the junction was in a ridge ridge ridge rrr configuration and closely followed the rise axis from chron m20 146 ma to chron m4 127 ma lineation and bight geometries indicate that the triple junction jumped northeastward at least nine times often apparently in response to volcanic activity from the mantle plume some jumps however appear related to ridge reorientations numerous features within the rise are parallel to lineations or lineation offsets implying that the plateau formed near the triple junction and was modified by ridge faulting most of our observations support the hypothesis that shatsky rise was formed by a mantle plume that captured a triple junction

near fault strong motion complexity of the 2003 bam earthquake iran and low frequency ground motion simulation summary the 2003 december 26 bam earthquake was an intermediate size strike slip event that occurred beneath the bam city causing more than 26000 fatalities the record of the only near fault station bam station shows a clear long period pulse in almost fault normal direction suggesting a near fault forward directivity effect contributing to the heavy damage observed in bam city in this study the long period 0 1 1 5hz strong ground motions were simulated to answer some ambiguities in source parameters for fully description of the observed motion in different components at bam station to this end the hisada s kinematic model accounting for both heterogeneous source characteristics and underground geology is applied to simulate the long period ground motion different source scenarios suggested by previous studies are examined and the optimum scenario that could provide the best fitting between the simulated and observed motions for different components was introduced the slip model and asperities as well as velocity structure parameters are optimized in order to have the best possible fit between the simulated and observed velocity time histories and spectra the good agreements between the simulated and observed low frequency motions indicate a successful identification of source and slip parameters of the bam earthquake and provide a good test for the simulation technique furthermore the uniqueness of the identified source parameters and the slip model is further checked against the observed near and far field recordings of the bam earthquake a good agreement between the simulated and the observed waveforms provides a base for model verification and results confirmation

a revised fit of south america and south central africa abstract a new set of parameters for the total plate tectonic reconstruction of south america and south central africa is presented euler pole 46 75 n 32 65 w rotation angle 56 40 this fit is constrained by at least three pre drift tectonic features crossing from one continent to the other 1 the geophysically defined eastern and western boundaries of the submarine jurassic outeniqua basin south africa and the falkland plateau basin 2 the late precambrian transcurrent fault and mylonite belts of pernambuco brazil and foumban west africa and 3 the triassic northern tectonic front of the cape fold belt and the major morphological feature on the falkland plateau with which it is closely lined up isotopic ages of falkland plateau gneisses correspond to cape pluton and cape fold belt ages suggesting their palaeoposition was within the realm of the cape fold belt in addition the bathymetrically and geophysically defined northeastern apex of the falkland plateau fits into the re entrant angle defined on the south african margin by the steep southeast facing sheared agulhas margin and the southern face of the tugela cone simultaneously known precambrian outcrops in northeastern brazil and in the gulf of benin area of west africa are juxtaposed rather than overlapped reconstructions producing a closer fit of these cratonic areas are considered untenable

the jurassic history of the africa antarctica corridor new constraints from magnetic data on the conjugate continental margins abstract finding the best fit for east and west gondwana requires a detailed knowledge of the initial jurassic movements between africa and antarctica this study presents results of systematic and densely spaced aeromagnetic measurements which have been conducted in 2009 2010 across the astrid ridge antarctica and in the western riiser larsen sea to provide constraints for the early seafloor spreading history between both continents the data reveal different magnetic signatures of the northern and southern parts of the astrid ridge which are separated by the astrid fracture zone the southern part is weakly magnetised corresponding to the low amplitude anomaly field of the southwestern riiser larsen sea in contrast the northern astrid ridge bears strong anomalies of positive value furthermore several sets of trends are visible in the data in the mozambique channel we extended the existing magnetic spreading anomaly identifications combined with the spreading anomalies in the conjugate riiser larsen sea they were used to establish a new model of the relative movements of africa and antarctica after the breakup of gondwana in jurassic times a detailed model for the emplacement of the mozambique ridge is now incorporated the model postulates a tight fit between africa and antarctica and two stages of breakup the first of which lasting until 159 ma m33n during this stage antarctica rotated anticlockwise with respect to africa the grunehogna craton cleared the coastal plains of mozambique to a position east of the mozambique fracture zone the southern astrid ridge is interpreted to consist of oceanic crust also formed during this first stage prior to the riiser larsen sea during the second stage antarctica moved southward with respect to africa forming the mozambique basin and the conjugate riiser larsen sea the mozambique ridge and the northern natal valley were formed at different spreading centres being active subsequently

crustal structure of a super slow spreading centre a seismic refraction study of mohns ridge 72 n summary a series of eight high resolution seismic refraction profiles from the ultra slow spreading 16 mm yr 1 full spreading rate mohns ridge in the norwegian greenland sea has been treated with modern inversion methods the profiles were shot parallel to the ridge at an off axis distance of 0 135 km corresponding to crustal ages of 0 22 ma the resulting models are constrained by synthetic seismograms and gravity modelling the crustal thickness in all profiles is well below the global average for typical oceanic crust and shows a high variability with a mean thickness of 4 0 0 5 km this is mainly due to a very thin and variable lower crustal layer layer 3 generally the crust is thicker beneath basement highs and thinner beneath basins implying local isostatic compensation the top of the basement layer 2a consists of a zone with low p wave velocities 2 5 3 0 km s 1 the mean thickness of this layer decreases with distance from the ridge beneath it lies a layer with slightly higher velocities layer 2b its thickness shows less variability along a given profile and an overall increase with age the combined average thickness of the upper two layers remains nearly constant indicating that the boundary between layer 2a and 2b may represent an alteration front upper mantle velocities are generally slow around 7 5 km s 1 for the profile directly within the rift valley a model without a third layer incorporating a constant gradient up to upper mantle velocities and a model with a moho depth inferred from neighbouring profiles and upper mantle velocity as slow as 7 2 km s 1 fit the seismic and gravity data equally well the crustal structure is not mature below the ridge these observations support previous models suggesting the presence of low densities and velocities at about 2 km below the rift axis poisson s ratios determined from converted s wave modelling are incompatible with a layer 3 consisting of purely serpentinized peridotite however a volume fraction of 10 40 per cent serpentinite cannot be ruled out

magmatic segmentation of the northern knipovich ridge evidence for high pressure fractionation at an ultraslow spreading ridge 1 the ultraslow spreading knipovich ridge shows axial topographic highs that are associated with off axis linear arrays of seamounts that parallel the spreading direction these linear arrays suggest that the segmentation has been stable for at least 7 8 m y axial topographic highs are marked with distinct gravity anomalies suggesting that they represent thicker crust and hence volcanic segment centers dredging on the northern knipovich ridge between 76 30 and 77 50 n shows that the axial and off axial seamounts and the deeper axial rift valley between the segment centers are composed of volcanic rocks the basalts are subdivided into high intermediate and low feo groups low feo basalts with generally more enriched radiogenic isotope ratios and ree patterns were only dredged midway between segment centers whereas dredges close to the segment centers comprised high and intermediate feo basalts the variations in isotopes and ree patterns suggest that the mantle below the northern knipovich ridge is heterogeneous and the along axis variations can be explained by higher degrees of melting of this heterogeneous source close to segment centers the high and intermediate feo basalts define different mgo feo liquid lines of descents that may reflect differences in the crystallization pressures or the water contents of the magmas similar water contents of the high and intermediate feo basalts suggest that the differences in feo primarily are a pressure effect and that the intermediate feo group fractionated at higher pressure than the high feo group the high feo basalts show both low and high pressure phenocryst assemblages suggesting crystallization at high pressures 6 7 kbar followed by shallow low pressure fractionation on the basis of the new data we propose that magmas at this ultraslow spreading ridge fractionate at mantle depth except at segment centers where magma productivity occasionally is high enough to sustain shallow level magma reservoirs

isostatic response of the lithosphere beneath the mozambique ridge sw indian ocean and geodynamic implications summary the sw indian ocean is characterized by the presence of several aseismic features the mozambique ridge an elongated feature lying roughly parallel to the se coast of africa is by far the least known of those structures mainly due to the scarcity of marine data to date the crustal nature and the origin of the ridge are still controversial points since knowledge of the origin of the mozambique ridge is important for a better understanding of the evolution of the sw indian ocean the isostatic response of the lithosphere beneath the ridge is analysed in order to characterize its effective elastic thickness and the emplacement process of the feature two different approaches are applied the direct computation of the geoid anomaly over the ridge by means of a 2 5 dimensional method and the computation of the admittances between the bathymetry and both the geoid and gravity anomalies both approaches point to a local isostatic response of the lithosphere the crustal thickness beneath the mozambique ridge ranges from 17 to 30 km and the average density from 2 78 to 2 7 103 kg m 3 varying with locality in good agreement with the few existing refraction profiles based on our results on the geochemical similarity between the basalts cored at the dsdp site 249 and the mor basalts erlank reid 1974 thompson et al 1982 and on the present knowledge of the sw indian ocean kinematics martin hartnady 1986 we propose an on ridge origin for the mozambique ridge the ridge would have been formed by the anomalous activity of a spreading axis linking the northern mozambique and the transkei basins accretion centres between m10 and m2 times at the m2 epoch a ridge jump event would have caused the spreading to cease the astrid ridge a poorly known aseismic structure lying close to the coast of antarctica near 15 e may be the antarctic counterpart of the mozambique ridge formed by the same accretionary phenomenon the two ridges breaking apart at m2 time due to the ridge jump event however any admittance computation at such high latitudes is impossible at present due to the very poor quality of both bathymetry and altimetry and so this hypothesis cannot be verified

three dimensional simulation of the near fault ground motion for the 1995 hyogo ken nanbu kobe japan earthquake abstract the 17 january 1995 hyogo ken nanbu earthquake is a typical example showing that the ground motions along basin edge faults can be very destructive in this study we simulate the near fault ground motion from this earthquake based on a kinematic fault model and a simplified 3d velocity structure of the kobe area the kinematic earthquake rupture and the wave propagation are modeled using a 3d finite difference method fdm our simulation identifies the basin edge effect as an important factor that influenced the ground motion amplification pattern in the kobe area we found that the coupling of the source directivity and basin edge effects causes impulsive ground motions with extremely high amplitude at periods greater than 1 sec and in a narrow zone offset less than 1 km from the basin edge the combination of these effects acted to create a fairly continuous band of amplification that extends about 30 km in an elongated zone parallel to the basin edge boundary in some areas localized site effects might have been as important as the abovementioned effects but they cannot explain the continuity of the extended east west zone of damage

slip distribution aftershocks and co seismic fault of some large persian earthquakes this investigation concentrates on slip distribution aftershocks mainshock location and related co seismic faults for seven continental large earthquakes in the persian plateau most of the earthquakes m s 5 7 7 7 1981 2005 with both measurable surface slip along a mature rupture and a precisely aftershock surveying with a local seismological network in iran are evaluated in most of the case studies we observe an acceptable correlation between maximum surface slip along the fault trace and the asperity an area which is recognized as an obvious gap or an area with lack of aftershocks on a fault plane on the other hand we conclude that the aftershocks which are associated with immeasurable or smaller surface slips mainly concentrate at the outside of the asperities also the mainshocks in most of the earthquakes are in neighboring with the big asperities and segment the co seismic faults we could see a gradual increase in the co seismic slip when the magnitude of the earthquake increases but examples with defuse aftershocks and with no specific pattern are also found

oblique rifting in a slow spreading ridge in oceanic rifts the spreading direction is usually nearly perpendicular to the ridge axis but at the reykjanes1 and mohns2 4 ridges this direction is highly oblique 30 to 40 to the axis these ridges possess axial valleys containing oblique raised features highs corresponding to second or third order ridge axis discontinuities5 at the reykjanes ridge searle and laughton1 observed small highs which they interpreted as volcanic domes here we present a fault map of the mohns ridge obtained from image processing of seabeam data we argue that the oblique highs in this ridge are of tectonic origin the axial valley is interpreted as the surface trace of a deformable band of oceanic lithosphere inherited from an earlier stage of perpendicular spreading we describe laboratory experiments on brittle ductile models which support this interpretation in which stretching at 30 to a deformable band reproduces the en echelon horst and graben pattern of the mohns ridge

low degree melting under the southwest indian ridge the roles of mantle temperature conductive cooling and wet melting abstract both low mantle temperatures and conductive cooling have been suggested as the cause of the atypically thin oceanic crust and the incompatible element enrichment characteristic of very slow spreading ridges here we present a model of melting under the southwest indian ridge swir which takes into account mantle temperature conductive cooling source composition and wet melting the model parameters are constrained by oceanic crustal thickness lava chemistry and isotopic composition and water content the results suggest that conductive cooling to a depth of around 20 km expected in areas with a full spreading rate of 15 mm yr is necessary to generate the swir lava chemistry but not that from faster spreading rate ridges at 23 n on the mid atlantic ridge or 45 n on the juan de fuca ridge the mantle potential temperatures of 1280 c estimated for the swir lavas are close to the global average of the upper mantle mantle water contents of 150 300 ppm can explain the observed melt water contents and allow sufficient melting at depth to explain the observed heavy rare earth element depletions in the melts

source process and slip model of 2005 dahuiyeh zarand earthquake iran using inversion of near field strong motion data summary on 2005 february 22 the dahuiyeh zarand earthquake mw 6 5 struck one of the most seismically active regions in south central iran east of zarand city in kerman province causing more than 500 fatalities the causative fault of the 2005 dahuiyeh zarand earthquake a reverse fault striking nearly ew and dipping to the north was located within a mountainous region and therefore more difficult to identify compared to the range bounded faults its identification after the 2005 event is very important for both the estimation of seismic hazard as well as for the damage and fatality functions we have inverted six three component near field strong motion waveforms to obtain the complete earthquake rupture history and slip distribution accelerograms are bandpass filtered with 0 2 1 0 hz and a length of 15 17 s of the waveforms is inverted the lack of absolute timing has been successfully overcome by estimating from the velocity model of the region the propagation of p and s waves from the epicentre to the stations the final fault slip model and the estimated source parameters are able to explain the observed waveforms the rupture is found to be bilateral with a maximum slip of 2 4 m concentrated on two asperities in the west and east sides of the nucleation point at depths of 6 12 km the western asperity is located to the east of zarand city and beneath the dahuiyeh village which might explain why the dahuiyeh village was totally destroyed by this earthquake

ambient noise tomography of the east african rift in mozambique seismic ambient noise tomography is applied to central and southern mozambique located in the tip of the east african rift ear the deployment of mozart seismic network with a total of 30 broad band stations continuously recording for 26 months allowed us to carry out the first tomographic study of the crust under this region which until now remained largely unexplored at this scale from cross correlations extracted from coherent noise we obtained rayleigh wave group velocity dispersion curves for the period range 5 40 s these dispersion relations were inverted to produce group velocity maps and 1 d shear wave velocity profiles at selected points high group velocities are observed at all periods on the eastern edge of the kaapvaal and zimbabwe cratons in agreement with the findings of previous studies further east a pronounced slow anomaly is observed in central and southern mozambique where the rifting between southern africa and antarctica created a passive margin in the mesozoic and further rifting is currently happening as a result of the southward propagation of the ear in this study we also addressed the question concerning the nature of the crust continental versus oceanic in the mozambique coastal plains mcp still in debate our data do not support previous suggestions that the mcp are floored by oceanic crust since a shallow moho could not be detected and we discuss an alternative explanation for its ocean like magnetic signature our velocity maps suggest that the crystalline basement of the zimbabwe craton may extend further east well into mozambique underneath the sediment cover contrary to what is usually assumed while further south the kaapval craton passes into slow rifted crust at the lebombo monocline as expected the sharp passage from fast crust to slow crust on the northern part of the study area coincides with the seismically active nne ssw urema rift while further south the mazenga graben adopts an n s direction parallel to the eastern limit of the kaapvaal craton we conclude that these two extensional structures herald the southward continuation of the ear and infer a structural control of the transition between the two types of crust on the ongoing deformation

magma emplacement at anomalous spreading ridge constraints due to plagioclase crystals from basalts of marsili seamount southern tyrrhenian back arc abstract plagioclase crystals from four basalt samples dredged from different portions of the marsili seamount southern tyrrhenian italy provide a detailed record of the changes occurring in the surrounding magma during its ascent in the crust the textural features and chemical zoning i e an content and fe mg sr and ba concentrations of the plagioclases from each sample show that aggregation of various generations of crystals occurred in the host melt prior to eruption plagioclases resulting from crystallisation in small sill intrusions where local flotation of plagioclase may have occurred are ubiquitous instead large up to 1 mm homogeneous an rich plagioclases are rare and have only been found in two of the studied basalts recovered from the northern and southern portions of the volcano respectively these crystals record crystallisation events occurring long before eruption and probably derive from deep seated crystalline material at the highest an values the sr and ba concentrations of the studied plagioclases also indicate the existence of two distinct mafic magmas within the lower crustal plumbing system of the marsili volcano one with lower sr and ba contents than the other three of the four studied basalts came from magma with low sr and ba whereas the fourth which erupted from the southern portion of the volcano sampled magma richer in sr and ba the ascent of magma caused resorption of the previously formed plagioclase crystals and regrowth of normally or inversely zoned portions depending on the water saturated or water undersaturated conditions of the surrounding magma respectively small shallow magmatic reservoirs may be located under the southern part of the marsili volcano as testified by the resorbed sodic cores an52 57 found in the two basalt samples from this portion

plagioclase ultraphyric basalts of the galapagos archipelago abstract basalts containing as much as 50 plagioclase phenocrysts that attain lengths of several centimeters are common on the northern islands of the galapagos archipelago mineralogic and geochemical evidence requires that the large anorthitic phenocrysts nucleated and grew from a less evolved magma than that in which they were erupted we suggest that plagioclase ultraphyric basalts pubs form when a buoyant phenocryst laden magma enters a chamber containing a denser more evolved magma as the new magma mixes with the old anorthitic plagioclase phenocrysts may accumulate near the roof in a hybrid liquid thinner crust under the northern islands increases the probability that viscous plagioclase rich magmas can ascend to shallow magma chambers and may account for the fact that pubs are more common on the northern islands than in the rest of the archipelago

the 2003 bam iran earthquake rupture of a blind strike slip fault a magnitude 6 5 earthquake devastated the town of bam in southeast iran on 26 december 2003 surface displacements and decorrelation effects mapped using envisat radar data reveal that over 2 m of slip occurred at depth on a fault that had not previously been identified it is common for earthquakes to occur on blind faults which despite their name usually produce long term surface effects by which their existence may be recognised however in this case there is a complete absence of morphological features associated with the seismogenic fault that destroyed bam

thermal and kinematic control on ocean ridge magma fractionation contrasts between atlantic and pacific spreading axes differences in the lithology and chemistry of mid ocean ridge basalts generated at pacific and atlantic spreading axes reflect a simple correlation to the spreading rate on the basis of experimental data and thermal models these differences are attributed to variation in the nature of magma supply and fractionation processes as a function of spreading rate rather than to processess in the upper mantle thermal and kinematic models for ocean ridges predict stable steady state magma reservoirs at faster spreading axes 5 cm yr and more restricted transient systems at slower spreading

trace element partitioning between high an plagioclase and basaltic to basaltic andesite melt at 1 atmosphere pressure we determined plagioclase melt partition coefficients for 18 elements by performing controlled cooling rate 1 atmosphere experiments using both natural and synthetic basaltic 51 wt sio2 and basaltic andesite 56 wt sio2 powders in a vertical quench furnace the experiments produced an69 to an87 composition plagioclase three starting powders were gorda ridge basalt synthetic diopside 40 albite 28 anorthite 32 mixture and arenal volcano costa rica basaltic andesite the gorda and synthetic powders were doped at both low concentrations 20 200 ppm and high concentration 200 5000 ppm whereas the arenal powder was doped only at high concentrations resulting in two doped gorda powders low np and high sdp two doped diopside albite anorthite powders low dad high sdd and one doped arenal powder high ar99 2 trace elements concentrations in both glass and plagioclase were measured by secondary ion mass spectrometry sims and or by laser ablation inductively coupled plasma mass spectrometry la icp ms results for the partitioning of trace elements between plagioclase and melt at different doping levels demonstrate both adherence to henry s law and good agreement between the different analytical techniques in general plagioclase melt partition coefficients determined in the an69 73 range are similar to other published values however some of those occurring for plagioclase compositions an75 are distinctly lower than those predicted by current regression formulations we applied a two lattice melt model to these data to account for differences in melt composition and temperature and found that there were no aberrations associated with partition coefficients a new set of regression formulations is determined involving the newly determined dataset for plagioclase with an contents between 75 and 87

geophysical evidence for the crustal variation and distribution of magmatism along the central coast of mozambique abstract for our understanding of the timing and geometry of the initial gondwana break up still a consistent image of the crustal composition of the conjugated margins of central mozambique and antarctica and the location of their continent ocean boundaries is missing in this regard a main objective is the explanation for the source of the different magnetic signature of the conjugate margins based on a revised investigation of wide angle seismic data along two profiles across the mozambican margin by means of an amplitude modelling this study presents the crustal composition across and along the continental margin of central mozambique supported by 2d magnetic modelling the results are compared to the conjugate margin in antarctica and allow new conclusions about their joined tectonic evolution an observed crustal diversity between the north eastern and south western parts of the central mozambican margin testifies to the complex break up history of this area conspicuous is the equal spatial extent of the hvlcb along the margin of 190 215 km the onset of oceanic crust at the central mozambican margin is refined to chron m38n 2n 164 1 ma magnetic modelling supports the presence of reversed polarized sdrs in the continent ocean transition that were mainly emplaced between 168 5 and 166 8 ma m42 m40 inferred sdrs in the riiser larsen sea might be emplaced sometime between 166 8 and 164 1 ma m39 m38 but got overprinted by normal polarized intrusions of a late stage of rift volcanism causing the opposite magnetic signature of the conjugate margins the distribution of the magmatic material along the central coast of mozambique clearly indicates the eastern extension of the north eastern branch of the karoo triple rift along the entire margin the main magmatic phase affecting this area lasted for at least 12 myr between 169 and 157 ma followed by the cease of the magmatism perhaps due to the relative southwards motion of the magmatic centre