triple junction and ridge hotspots earthquakes faults and volcanism in afar the azores and iceland the afar depression of the horn of africa the azores archipelago and iceland are zones of plate divergence and mantle plume discharge strike slip focal mechanisms dominate in major earthquakes among nodal planes a particular fault plane can be identified by precise epicentral location from the strike of the aftershock zone or by relation to field evidence of active faulting these fault planes are orthogonal to the directions expected in previous models for transform faults along a continuous divergent plate boundary recent volcanic features are consistently at small angles with the fault planes rotation of blocks within a deforming zone is implied by the direction of fault planes the sense of motion along them and their location on a series of parallel features transverse to the deformed zone the drift of oceanic or continental large lithospheric plates deforms a zone of lithosphere deriving from both the plume discharge and ridge accretion while the zone is created

the red beds series in the erta ale segment north afar evidence for a 6 ma old post rift basin prior to continental rupturing abstract the erta ale rift segment north afar is regarded as the most mature rift part within the entire afar rift system very little is known about its deformation history because of limited exposures of geological records in its inner floor except volcanics of the erta ale chain and the poorly known red beds series along the flanks of the depression an integrated study combining sedimentological geochemical 40ar 39ar radiometric and tectonic approaches has been devoted to red beds series flanking the depression to the sw our new results allow to argue that 1 the 300 m thick red beds exposed section comprises alluvial deposits that enclose 2 basaltic lavas and related sill intrusions that both yield 6 ma 40ar 39ar ages and display similar geochemical affinities 3 the red beds series locally overlap unconformably basement bounding terrains and 4 are involved in a limited number of low displacement normal faults that recorded a modest amount of extension

dike fault interaction during the 2004 dallol intrusion at the northern edge of the erta ale ridge afar ethiopia received 17 july 2012 revised 29 august 2012 accepted 4 september 2012 published 12 october 2012 1 during continental rifting the interaction between faulting and magmatic intrusions is not well understood using insar and seismicity we show that a 0 06 km 3 dike was intruded along the dallol segment ethiopia and was accompanied by a mw 5 5 earthquake and associated fault slip along the western flank of the rift the intrusion was fed by a previously unidentified magma chamber under dallol the total seismic moment release was 2 3 10 17 nm 10 of the geodetic moment this is a higher proportion than during the 2005 2009 dabbahu rifting episode which ranged between 1 4 of the geodetic moment a larger component of faulting occurs at dallol than at dabbahu segment a feature we interpret to be related to the proximity 10 km of the dallol segment to the rift margin where welldeveloped faults facilitate slip citation nobile a c pagli d keir t j wright a ayele j ruch and v acocella 2012 dike fault interaction during the 2004 dallol intrusion at the northern edge of the erta ale ridge afar ethiopia geophys res lett 39 l19305 doi 10 1029 2012gl053152

differentiating flow melt or fossil seismic anisotropy beneath ethiopia ethiopia is a region where continental rifting gives way to oceanic spreading yet the role that pre existing lithospheric structure melt mantle flow or active upwellings may play in this process is debated measurements of seismic anisotropy are often used to attempt to understand the contribution that these mechanisms may play in this study we use new data in afar ethiopia along with legacy data across ethiopia djibouti and yemen to obtain estimates of mantle anisotropy using sks wave splitting we show that two layers of anisotropy exist and we directly invert for these we show that fossil anisotropy with fast directions oriented northeast southwest may be preserved in the lithosphere away from the rift beneath the main ethiopian rift and parts of afar anisotropy due to shear segregated melt along sharp changes in lithospheric thickness dominates the shear wave splitting signal in the mantle beneath afar away from regions with significant lithospheric topography melt pockets associated with the crustal and uppermost mantle magma storage dominate the signal in localized regions in general little anisotropy is seen in the uppermost mantle beneath afar suggesting melt retains no preferential alignment these results show the important role melt plays in weakening the lithosphere and imply that as rifting evolves passive upwelling sustains extension a dominant northeast southwest anisotropic fast direction is observed in a deeper layer across all of ethiopia this suggests that a conduit like plume is lacking beneath afar today rather a broad flow from the southwest dominates flow in the upper mantle

propagating rifts and continental breakup a simple quantitative model is proposed whereby continental breakup starts with a phase of continental rifting and continues with the propagation of rifts toward locked zones these zones are deformed in the rifting process while oceanic crust is generated in the wake of the rift tip the deformed zones will cause overlaps when a continental reassembly is attempted the model accounts for the shape and distribution of magnetic quiet zones at ocean continent boundaries and for the detailed geometry of some v shaped zones of oceanic magnetic anomalies the former provide an estimate of the scale at which the hypothesis of rigid plates fails and the latter are used to measure the velocity of rift propagation illustrations are provided for present rifting in the red sea and gulf of aden and for early rifting of the south atlantic and breakup of pangea

recent seismicity of the east african rift system and its implications abstract the seismicity of the east african rift system and southern red sea is studied here location of earthquake epicenters in east africa shows that there is a seismicity gap in space and time between the main ethiopian rift system and the eastern rift however distribution of earthquake epicenters together with the energy mapping suggest a continuity of seismic activity or stress field from the main ethiopian rift system to the western rift system via the southernmost rifts of ethiopia in general except for some earthquakes which occurred at different complex tectonics regions mechanisms of earthquakes studied here show dominantly normal faulting suggesting that the rift system is an extensional zone on the continent the presence of greater focal depth earthquakes to the southern part of the rift system may indicate that softer materials at a shallower depth are present in afar and neighboring regions than in the remaining part of the east african rift system this interpretation is supported by other geophysical studies low electrical resistivity and gravity data performed in afar it is also supported by low and high stress drops found for the northern part afar depression and southern part of the east african rift system respectively

insar observations of post rifting deformation around the dabbahu rift segment afar ethiopia summary increased displacement rates have been observed following manylarge earthquakes and mag matic events although an order of magnitude smaller than the displacements associated with the main event the post seismic or post rifting deformation may continue for years to decades after the initial earthquake or dyke intrusion due to the rare occurrence of subaerial rifting events thereareveryfewobservationstoconstrainmodelsofpost riftingdeformation in2005 september a 60 km long dyke was intruded along the dabbahu segment of the nubia arabia plate boundary afar ethiopia marking the beginning of an ongoing rifting episode con tinued activity has been monitored using satellite radar interferometry and data from global positioning system instruments deployed around the rift in response to the initial intrusion using multiple satellite passes we are able to separate the rift perpendicular and vertical displacement fields around the dabbahu segment rift perpendicular and vertical rates of up to 180 and 240 mm yr 1 respectively here we show that models of viscoelastic relaxation alone are insufficient to reproduce the observed deformation field and that a large portion of the observed signal is related to the movement of magma within the rift segment our models suggest upper mantle viscosities of 10 18 19 pas overlain by an elastic crust of between 15 and 30km to fit the observations inflation and deflation of magma chambers in the centre of the rift and to the south east of the rift axis is required at rates of 0 13 and 0 08km 3 yr 1

seismological evidence of active faulting in the tendaho rift afar triangle ethiopia a temporary network has been used to study the seismic activity in the central northern part of the tendaho rift afar triangle ethiopia small size earthquakes m 2 5 characterize the seismicity during the time interval 1993 1994 when the seismic array was operating shocks are mainly located in the central part of the tendaho rift and their epicentral distribution highlights a remarkable coincidence with nw se trending tectonically active structures focal depths are confined in the range of 3 8 km the few acceptable fault plane solutions suggest the occurrence of both strike slip and dip slip rupture mechanisms which are often characterized by a nodal plane oriented approximately nw se a detailed study of earthquake waveforms using polarization analysis showed the occurrence of splitting phenomena for the s waves despite the differences in focal mechanisms the similarity of the faster s wave directions nw se to nnw sse suggests that the direction of geological structures and or the systems of fractures adjacent to the faults could bias shear wave propagation the splitting time between fast and slow shear waves shows no systematic relationship with either hypocentral distance or focal depth suggesting that the anisotropic volume is constrained to depths shallower than 3 km

kinematics of the asal rift djibouti determined from the deformation of fieale volcano because of its subaerial exposure the asal rift segment provides an exceptional opportunity to quantify the deformation field of an active rift and assess the contribution of tectonics and volcanism to rifting processes the present topography of the asal rift results from the tectonic dismemberment during the last 100 000 years of a large central volcanic edifice that formed astride the rift zone 300 000 to 100 000 years ago three dimensional deformation of this volcano has been quantified from the combined analysis of the topography and geology the analysis indicates that spreading at 17 to 29 millimeters per year in a n40 5 e direction accounts for most of the separation between arabia and somalia the small topographic subsidence relative to extension suggests that tectonic thinning of the crust has been balanced by injection and underplating of magmatic material of near crustal density the methodology developed in this study could also be applied to quantify deformation in relatively inaccessible areas where the main available information is topography or bathymetry

geographical distribution of surface wave velocities and 3 d upper mantle structure in africa summary long period records of surface waves from wwssn gdsn ida and geoscope stations are analysed to retrieve purely continental dispersion of the african continent in the period range of 20 200 s for rayleigh waves and 20 125 s for love waves the phase and group velocities are obtained by two station and single station methods with earthquakes within or near the continent a regionalization without any a priori constraints is performed over the whole african continent to retrieve local phase and group velocities phase and group velocity geographical distributions show in the northeast of africa near the red sea a region with lower velocities extending to the west and high velocities in the northwestern and southern parts of africa at long periods there is a general agreement with global models but the resolution is improved in this regional study in the resulting 3 d structure cratonic areas show a fast lithosphere from the nw to the se whereas the high sv velocities of the nw area extend deeper than 200 km lower velocities appear below this depth in the centre in the red sea region low velocities are locally deep rooted with a slow lithospheric layer 150 km thick extending to the west in this region polarization anisotropy is evidenced by love rayleigh incompatibility short period lateral variations are used to model the crustal thickness

seafloor spreading event in western gulf of aden during the november 2010 march 2011 period captured by regional seismic networks evidence for diking events and interactions with a nascent transform zone in november 2010 intense seismic activity including 29 events with a magnitude above 5 0 started in the western part of the gulf of aden where the structure of the oceanic spreading ridge is characterized by a series of n115 trending slow spreading segments set within an ew trending rift using signals recorded by permanent and temporary networks in djibouti and yemen we located 1122 earthquakes with a magnitude ranging from 2 1 to 5 6 from 01 november 2010 to 31 march 2011 by looking in detail at the space time distribution of the overall seismicity and both the frequency and the moment tensor of large earthquakes we reexamine the chronology of this episode in addition we also interpret the origin of the activity using high resolution bathymetric data as well as from observations of sea floor cable damage caused by high temperatures and lava flows the analysis allows us to identify distinct active areas firstly we interpret that this episode is mainly related to a diking event along a specific ridge segment located at e044 in light of previous diking episodes in nearby subaerial rift segments for which field constraints and both seismic and geodetic data exist we interpret the space time evolution of the seismicity of the first few days migration of earthquakes suggests initial magma ascent below the segment center this is followed by a southeastward dike propagation below the rift immediately followed by a northwestward dike propagation below the rift ending below the northern ridge wall the cumulative seismic moment associated with this sequence reaches 9 1 1017 nm and taking into account a very low seismic versus geodetic moment we estimate an horizontal opening of 0 58 to 2 9 m the seismic activity that followed occurred through several bursts of earthquakes aligned along the segment axis which are interpreted as short dike intrusions implying fast replenishment of the crustal magma reservoir feeding the dikes over the whole period the opening is estimated to 1 76 8 8 m across the segment a striking feature of this episode is that the seismicity remained confined within one individual segment whereas the adjacent en echelon segments were totally quiescent suggesting that the magma supply system of one segment is disconnected from those of the neighboring segments second we identify activity induced by the first intrusion with epicenters aligned along a n035 e trending 30 km long at the northwestern end of the active opening segment this group encompasses more than 7 earthquakes with magnitude larger than 5 0 and with strike slip focal mechanisms consistent with the faults identified in the bathymetry and the structural pattern of the area we propose that a transform fault is currently in formation which indicates an early stage of the ridge segmentation at the locus of the trend change of the spreading ridge which also corresponds to the boundary between a clear oceanic lithosphere and the zone of transform between continental and oceanic crust

the kerguelen isles southern indian ocean new results on deep structure from refraction profiles abstract additional seismic refraction profiles shot across the kerguelen archipelago terres australes et antarctiques francaises known as taaf located in the southern indian ocean substantiate the oceanic type crust of the archipelago and the kerguelen plateau as well the crust mantle boundary is better modelled by a transition than by a discrete interface seismic velocities of 6 60 6 80 km s are in the range of those within oceanic layer 3 velocities of 7 2 7 5 km s observed at long ranges may be interpreted according to at least two distinct hypotheses 1 1 notwithstanding their rather low values they represent upper mantle material and might be assigned to peridotites serpentinised by hydrothermal circulations as reported for various oceanic and continental features 2 2 these velocities are assigned to the lower crust intruded by melt produced by adiabatic decompression of upper mantle material during rifting underplating a small increase in mantle temperature caused by the presence of heat sources i e the southeast indian ridge and the kerguelen mantle plume nearby may have produced huge quantities of melt facilited by the slow movement of the antarctic plate then building up the aseismic kerguelen plateau

surface morphology of active normal faults in hard rock implications for the mechanics of the asal rift djibouti abstract tectonic stretching models have been previously proposed to explain the process of continental break up through the example of the asal rift djibouti one of the few places where the early stages of seafloor spreading can be observed in these models deformation is distributed starting at the base of a shallow seismogenic zone in which sub vertical normal faults are responsible for subsidence whereas cracks accommodate extension alternative models suggest that extension results from localised magma intrusion with normal faults accommodating extension and subsidence only above the maximum reach of the magma column in these magmatic rifting models or so called magmatic intrusion models normal faults have dips of 45 55 and root into dikes vertical profiles of normal fault scarps from levelling campaign in the asal rift where normal faults seem sub vertical at surface level have been analysed to discuss the creation and evolution of normal faults in massive fractured rocks basalt lava flows using mechanical and kinematics concepts we show that the studied normal fault planes actually have an average dip ranging between 45 and 65 and are characterised by an irregular stepped form we suggest that these normal fault scarps correspond to sub vertical en echelon structures and that at greater depth these scarps combine and give birth to dipping normal faults the results of our analysis are compatible with the magmatic intrusion models instead of tectonic stretching models the geometry of faulting between the fieale volcano and lake asal in the asal rift can be simply related to the depth of diking which in turn can be related to magma supply this new view supports the magmatic intrusion model of early stages of continental breaking

strike slip faults in a rift area a transect in the afar triangle east africa abstract the afar triangle a diffuse triple junction where the red sea ethiopian and gulf of aden rifts converge is examined along an e w cross section in order to recognize traces of strike slip faulting summarily known from earlier studies both field evidences from slickensides and airphotograph or satellite image data indicate that strike slip faults although less numerous than normal ones occur throughout this area these faults mainly strike parallel or at small angles relative to rifting axes rather than transversal to them as would be expected if they were transforms strike slip subparallel to rifts is explained through lateral displacement between the major lithospheric plates around the junction or subordinately by a domino fault mechanism in zones of diffuse transform deformation faults at small angles with the rift axes often constitute conjugate systems suggesting along axis compression which is considered to be frequently induced by the lateral intraplate shift mentioned above in other cases this compression may develop in intervals between mantle plumes wedging up along a rift or at the head of a propagating rift the main lateral displacements among the boundary lithospheric plates during the last million of years are supposed to have been sinistral this does not challenge the notion of mainly divergent plate movements but adds to this divergence an anticlockwise shift of the plates around the junction

the lithosphere asthenosphere system in the calabrian arc and surrounding seas southern italy a fairly detailed structural model of the lithosphere asthenosphere system thickness s and p wave velocities of the crust and of the uppermost mantle layers has been defined in the calabrian arc region southern tyrrhenian sea calabria and the northwestern part of the ionian sea in southern italy using seismic data from literature as a priori constraints of the nonlinear inversion of surface wave data

extension and stress during continental breakup seismic anisotropy of the crust in northern afar studies that attempt to simulate continental rifting and subsequent breakup require detailed knowledge of crustal stresses however observational constraints from continental rifts are lacking in addition a knowledge of the stress field around active volcanoes can be used to detect sub surface changes to the volcanic system here we use shear wave splitting to measure the seismic anisotropy of the crust in northern afar a region of active magma rich continental breakup we combine shear wave splitting tomography with modelling of gravitational and magmatic induced stresses to propose a model for crustal stress and strain across the rift results show that at the ethiopian plateau seismic anisotropy is consistently oriented n s seismic anisotropy within the rift is generally oriented nnw sse with the exception of regions north and south of the danakil depression where seismic anisotropy is rift perpendicular these results suggest that the crust at the rift axis is characterized by rift aligned structures and melt inclusions consistent with a focusing of tectonic extension at the rift axis in contrast we show that at regions within the rift where extension rate is minimal the seismic anisotropy is best explained by the gravitationally induced stress field originating from variations in crustal thickness seismic anisotropy away from the rift is controlled by a combination of inherited crustal structures and gravitationally induced extension whereas at the dabbahu region we show that the stress field changes orientation in response to magmatic intrusions our proposed model provides a benchmark of crustal stress in northern afar which will aid the monitoring of volcanic hazard in addition we show that gravitational forces play a key role in measurements of seismic anisotropy and must be considered in future studies we demonstrate that during the final stages of continental rifting the stress field at the rift axis is primarily controlled by tectonic extension but that gravitational forces and magmatic intrusions can play a key role in the orientation of the stress field

seismotectonics in the republic of djibouti linking the afar depression and the gulf of aden abstract the arta seismological array around the gulf of tadjurah monitored earthquake activity for over 15 years epicentral locations and focal mechanisms are compared with tectonic evidence from surface breaks traces of active faults and the spatial distribution of swarms of aftershocks strike slip motion on near vertical faults predominates in the gulf left lateral strike slip on n150 e oriented faults is suggested at variance with the emphasis in previous models on either 1 a continuous plate boundary with spreading centres connected by ene trending right lateral transform faults 2 a propagating rift oriented approximately east west or 3 a few broad zones of extension between n150 e normal faults left lateral strike slip motion occurring on a series of parallel faults results in clockwise rotation of the blocks between the faults the type of deformation has similarities to that further west but is not obviously controlled by a distinct pair of overlapping rifts as proposed inland by tapponnier et al 1990 it is suggested that the stresses induced by the northeasterly drift of arabia alone may deform the southern part of the more easterly oriented zone of lithosphere derived of plume and ridge which separates arabia from africa

orientation of absolute african plate motion revealed by tomomorphometric analysis of the ethiopian dome the ethiopian dome comprising the ethiopian and yemeni plateaus formed as a result of the uplift of red sea gulf of aden and ethiopian rift shoulders the uplift was essentially due to a mantle plume activity a digital elevation model of this region before rifting constructed by using the rotation of its constituent plates and pulling down the rift shoulders reveals that important high areas existed before rifting these highs are interpreted to be the result of mantle plume activity a fast fourier transform was used to smooth the digital elevation model prerift dome before a tomomorphometric analysis was applied to quantify the orientation of each digital elevation model horizontal slice this procedure allowed us to determine a bearing of n30 e for absolute african plate motion since the paleogene

post rifting relaxation in the afar region ethiopia 1 crustal accretion at divergent plate boundaries typically occurs via the periodic intrusion of dikes but their emplacement and the associated deformation are rarely observed the few existing observations at subaerial rifts show that these diking events are followed by a decadal scale period with extension rates faster than the secular divergent plate motion this transient accelerated deformation has been explained by continued subsurface magma injection or by relaxation in the viscoelastic mantle of the stress changes imparted by dike opening for the first time gps measurements were collected within a few months of a rifting event at a major plate boundary the september 2005 60 km long dike intrusion in the dabbahu segment afar ethiopia extension rates for the first 3 years greatly exceed the plate motion nubia arabia secular divergence rate even at sites located more than 60 km from the rift axis here we show that these observations are consistent with stress relaxation in a viscoelastic upper mantle with a viscosity of about 5 1018 pa s overlain by a 12 14 km thick elastic crust the alternative model of continued diking requires continuous opening well below the moho and is therefore unlikely instead magma injection in afar since june 2006 has taken the form of smaller discrete diking events tapping into a mid crustal melt reservoir under the segment center

khmer sandstone quarries of kulen mountain and koh ker a petrographic and geochemical study abstract sandstone from several ancient quarries and natural outcrops located at the eastern foothill of kulen mountain and in koh ker northern cambodia has been characterized by means of petrographic analysis scanning electron microscopy and geochemical analysis the samples have been collected during a series of field surveys organized jointly by the metropolitan museum of art and the authority for the protection and management of angkor and the region of siem reap apsara the data show how the sandstone quarried from these sites is characterized by an overall petrographic and geochemical homogeneity and is consistent with the upper terms of the lower middle jurassic terrain rouge formation this result suggests that petrography and geochemistry of sandstone belonging to the terrain rouge formation might not be unambiguously characteristic of single quarry sites but rather representative of vast portions of the sedimentary sequence

a new kinematic approach for the danakil block using a digital elevation model representation abstract data from the literature are integrated in a regional digital elevation model dem in order to analyse the motion of the danakil block with regard to the arabian and somalian plates the application of the poles and angles of rotation taken from the literature induces a superposition of the danakil block on the arabian plate and the formation of a gap in the afar region the determination of new poles of rotation using a best fitting procedure allows one to avoid these drawbacks in all cases calculations are carried out keeping the nubian plate stationary the present approach shows that the danakil block is an independent entity and is not related to the nubian and arabian plates between the oligocene and the miocene it has been submitted to a n20 e sinistral strike slip motion anterior to the rotation itself that was started in the middle miocene this rotation is directed by a mechanical couple due to the combination of the red sea and north afar extensions

seismic surveys in the northern red sea asymmetric crustal structure abstract wide angle reflection and refraction seismic warrs surveys at key sections of the northern red sea and adjacent areas enabled a better definition of the crust and upper mantle structure a fundamental asymmetry of the region was revealed by the results of profiles which cover both flanks of the red sea main trough the coastal plains and the bordering precambrian shields this asymmetry is clearly expressed by the presence of different crustal types on both flanks the western flank shows oceanised crust within only 20 km off the african coast while the whole eastern flank is underlain by a continental type of crust likewise the shape of the continent ocean transitions differs significantly on both flanks the arabian margin extends almost into the centre of the red sea and shows considerable stretching and attenuation of the continental crust while the african margin on the other hand even thickens towards the escarpment and appears very abrupt the influence of the red sea opening as reflected by the decrease of seismic velocities and the increase of poisson s ratio in the upper mantle due to partial melting is strictly limited to the area seaward of the egyptian escarpment the asymmetry in the northern red sea is regarded to be the expression of strike slip processes in the initial stages of red sea evolution

strike slip faults parallel to crustal spreading axes data from iceland and the afar depression slickenside studies in regions of crustal spreading such as iceland and the afar depression east africa reveal that a significant number of faults parallel and close to rift axes are strike slip rather than normal therefore the pattern of brittle deformation in these regions does not conform to the classic two dimensional schemes of oceanic tectonics and pre oceanic rifting dip slip and strike slip faulting presumably alternated along or in the vicinity of spreading axes indicate a varying stress field and a combination of transverse and longitudinal movements in iceland strike slip faults parallel to rifts are observed both west and east of the rift system as well as in a median area between overlapping rifts the mechanisms proposed for their origin include accommodation of oblique convergence or divergence of crustal sections due to variations of spreading directions along axis and the interaction of overlapping rifts in the afar depression this kind of fault is recorded west of the rift of asal and can be imputed to reflect an interaction among rifts in the vicinity of the afar triple junction rift parallel strike slip faults cannot however be assumed to be a feature of all crustal spreading axes due to the peculiarity of the examined regions both of them are hot spot areas and the afar depression lies at a triple junction

consideration on the construction process and the sandstone quarries during the angkor period based on the magnetic susceptibility abstract detailed magnetic susceptibility measurement was conducted on the sandstones used for the angkor monuments constructed in the period spanning the preah ko and angkor wat styles and the construction process of the buildings and quarries of the sandstones was considered combined with the previous study on the sandstones of the bayon style uchida e cunin o shimoda i suda c nakagawa t 2003 the construction process of the angkor monuments elucidated by the magnetic susceptibility of sandstone archaeometry 45 221 232 the magnetic susceptibility measurement revealed that there were 7 sandstone quarries corresponding to stages i ii iii iva v vii and viiib during the angkor period the sandstones used for the monuments belonging to stage i show average magnetic susceptibility values ranging from 1 1 to 2 3 10 3 si units in the bakheng style period stage ii the average magnetic susceptibility of the supplied sandstones decreased gradually from around 10 to 1 10 3 si units over time the magnetic susceptibility range of the sandstones used for the monuments of stage iii was from 2 3 to 3 0 10 3 si units in the early angkor wat style period ivb the average magnetic susceptibility of the supplied sandstones increased over time reflecting the supply of the sandstones from two different quarries that is the quarry corresponding to the khleang and baphuon style monuments stage iva with low average magnetic susceptibilities of 1 1 to 2 4 10 3 si units and the quarry corresponding to the main angkor wat style monuments stage v with high average magnetic susceptibilities ranging from 2 8 to more than 4 3 10 3 si units the sandstones of stage vii show low average magnetic susceptibility around 1 10 3 si unit the sandstones of stage vid is a mixture of sandstones of stages v and those of stage vii the sandstones with high magnetic susceptibilities are found in the monuments belonging to stage viiib reflecting the supply from the new sandstone quarry

nature and provenance of the sandstone used for bayon style sculptures produced during the reign of jayavarman vii under jayavarman vii 1182 83 ca 1218 ce the khmer empire reached its apex leaving a heritage of major construction works and unique artistic production the stone materials of several sculptures produced under his reign were characterized and compared to possible geological sources in northern and eastern cambodia the data suggest that a specific type of sandstone rich in volcanic detritus was deliberately selected and quarried from a triassic sedimentary sequence exposed far from angkor the main political and economic center at that time

length and timescales of rift faulting and magma intrusion the afar rifting cycle from 2005 to present although fault and magmatic processes have achieved plate spreading at mid ocean ridges throughout earth s history discrete rifting episodes have rarely been observed this paper synthesizes ongoing seismic structural space based geodetic and petrologic studies from the subaerial red sea rift in ethiopia where a major rifting episode commenced in september 2005 our aims are to determine the length and timescales of magmatism and faulting the partitioning of strain between faulting and magmatism and their implications for the maintenance of along axis segmentation most of the magma for the initial and subsequent 12 intrusions was sourced from the center of the dabbahu manda hararo rift segment strain is accommodated primarily by axial dike intrusions fed from mid segment magma chamber s these findings show that episodic approximate century interval rapid opening of discrete rift segments is the primary mechanism of plate boundary deformation the scale 65 km 8 km and intensity of crustal

current plate boundary deformation of the afar rift from a 3 d velocity field inversion of insar and gps extension faulting and magmatism are the main controls on the magnitude and localization of strain at mid ocean ridges however the temporal and spatial patterns of such processes are not clear since the strain distribution has not been resolved in the past at sufficient spatial resolution and over extended areas interferometric synthetic aperture radar insar and gps data with unprecedented resolution are now available to us from the afar rift of ethiopia here we use a velocity field method to combine insar and gps to form the first high resolution continuous three dimensional velocity field of afar we study an area that is 500 km wide and 700 km long covering three branches of the afar continental rift and their triple junctions our velocity field shows that plate spreading is currently achieved in afar in contrasting modes a transient postdiking deformation is focused at the dabbahu rift segment while in central afar spreading is distributed over several overlapping segments and southern afar exhibits an interdiking deformation pattern focused at the asal ghoubbet segment we find that current spreading rates at dabbahu following the 2005 2010 intrusions are up to 110 mm yr 6 times larger than the long term plate divergence a segment centered uplift of up to 80 mm yr also occurs indicating that magma flow is still a primary mechanism of deformation during postdiking on the other hand no vertical displacements are currently observed in central and southern afar suggesting lack of significant magmatic activity at shallow levels

lithospheric structure of a nascent spreading ridge inferred from gravity data the western gulf of aden the aden spreading ridge somalia arabia plate boundary does not connect directly to the red sea spreading ridge it propagates toward the east african rift through the afar depression where the presence of a hot spot has been postulated from seismological and geochemical evidence the spreading direction n37 e is highly oblique to the overall trend n90 e of the ridge we present and interpret new geophysical data gathered during the tadjouraden cruise r v l atalante 1995 in the gulf of aden west of 46 e these data allow us to study the propagation of the ridge toward the afar and to discuss the processes of the seafloor spreading initiation we determine the lithospheric structure of the ridge using gravity data gathered during the cruise with the constraint of available refraction data a striking bouguer anomaly gradient together with the identification of magnetic anomalies defines the geographical extent of oceanic crust the inversion of the bouguer anomaly is performed in terms of variations of crustal thickness only and then discussed with respect to the expected thermal structure of the mantle lithosphere which should depend not only on the seafloor spreading but also on the hot spot beneath east africa our results allow us to define three distinct lithospheric domains in the western gulf of aden east of 44 45 e the lithosphere displays an oceanic character thermal subsidence recorded for the last 10 ma and constant crustal thickness between 43 30 e and 44 10 e the lithosphere is of continental type but locally thinned beneath the axial valley the central domain defined between 44 10 e and 44 45 e is characterized by a transitional lithosphere which can be seen as a stretched continental crust where thick blocks are mixed with thinned crust it displays en echelon basins that are better interpreted as extension cells rather than accretion cells

the red sea gulf of aden re assessment of hydrocarbon potential abstract the results and conclusions of the undp world bank red sea gulf of aden regional hydrocarbons study project june 1989 january 1992 are summarized these derive from detailed examination of pooled public sector data provided to the project by the participating bordering states in the region these included samples and logs from about 56 wells drilled in the two basins out of a total of 71 and about 30 000 line km of reflection seismic data selected from 110 000 km available well logs were digitized and correlated and basinwide biostratigraphic lithostratigraphic and palaeoenvironmental studies source rock organic richness quality and maturity studies and sedimentological analyses studies were performed by contracted specialist staff in the uk and france these were integrated with basinwide seismic structural and isopach mapping for selected regional reflectors intervals by national task forces in cairo under project supervision academically acquired crustal seismic and other geophysical data and oil industry velocity and other borehole data were also integrated for the red sea modelling utilizing recently acquired geophysical data indicates crustal asymmetry for the red sea rift with oceanic crust of pull apart basin type flooring some portions of offshore egypt and sudan on the west and stretched continental crust underlying the eastern arabian flank these interpretations furthermore suggest that sinistral strike slip processes controlled the initial break up of the arabian plate shaping the african flank as a sharp plate boundary similar processes appear to have initially shaped the somalia flank of the gulf of aden rift only later did true seafloor spreading propagate in the eastern gulf of aden 10 12 ma about a regional lithostratigraphic nomenclature scheme for the miocene recent syn and post rift red sea successions keyed to detailed biostratigraphical distribution charts and microfaunal assemblage characteristics was erected tied to the well documented and understood petroliferous gulf of suez basin scheme this allows facies variations within the basin to be compared and allows consistent interpretation and correlations across the basin to which local informal nomenclature can be related pre rift pre oligocene marine sediments are present in most of the gulf of aden jurassic eocene but only incompletely on the basin margins in the red sea jurassic in the south upper cretaceous early palaeogene in the north significant pre rift oil gas prone upper jurassic cretaceous and palaeogene source rocks occur in gulf of aden sub basins initiated by late jurassic early cretaceous tensional tectonics with maturity levels ranging from early late to post mature whereas early synrift source rocks are generally immature in the red sea good quality pre rift source rocks outcrop in the north and south but are probably overmature for oil in the basin except in the extreme south if present good syn pre and post salt and post rift source levels occur throughout the basin but in the central southern sectors only the post salt remains within the oil window because of high geothermal gradients both basins are little explored attractive pre rift jurassic eocene exploration plays in rotated fault and horst blocks in predominantly carbonate but subordinate sandstone reservoirs are present in the gulf of aden confirmed by an important undeveloped eocene light oil discovery in offshore yemen with subordinate synrift potential where maturity is established relative continental shelf narrowness is economically limiting but wider sectors occur in the red sea the principal exploration plays are in miocene synrift pre and post salt successions rotated fault blocks occur in the pre salt but maturity for oil in the deeper levels is restricted to the northern sector these levels in the central and southern sectors are overmature except in the southernmost portion but excellent potential exists in the synrift post salt and early post rift sequences with a number of attractive play types reservoirs for synrift plays are mainly sandstones and potential is confirmed by several undeveloped gas and condensate discoveries and numerous oil seepages throughout the basin relatively wide continental shelves occur in the south and along much of the eastern flank

the afar depression transition between continental rifting and sea floor spreading abstract detailed geophysical and geological studies were undertaken in the 1970 s in the afar depression ethiopia in order to study the processes of continental break up occurring in this unique setting the afar is located between the ethiopian and somali plateaus and the red sea we have re evaluated the available seismic refraction and gravity data using modern interpretation techniques and incorporated new information derived from magnetic gravity seismicity and seismic refraction data the results of the re evaluation indicate that the afar depression is a transition zone between the continental rifts of kenya and the present ongoing sea floor spreading of the red sea and the gulf of aden in the depression itself the crust is greatly attenuated and is underlain by a low velocity 7 4 7 5 km s high temperature upper mantle material however it is still a continental crust which thins to the northeast and east towards the red sea and the gulf of aden it is not a newly formed oceanic crust as has been suggested in the past the trends of the areas of crustal thinning are indicated by the gravity anomalies the trend of the ethiopian rift reaches south afar where the trend of crustal thinning changes and becomes parallel to the red sea trend it is offset en echelon from the spreading area of the red sea which is marked by an alignment of gravity maxima the gulf of aden trend continues west towards the ethiopian rift the gravity trends coincide with areas of maximum crustal thinning and may thus suggest a possible triple junction in development

transform margin of the northern levant eastern mediterranean from formation to reactivation 1 the levant continental margin eastern mediterranean is usually defined as a passive margin that developed between the late paleozoic early mid mesozoic previous studies suggest that the margin is composed of two segments with different structural characteristics the southern segment south of the carmel structure was formed by rifting whereas the origin of the northern segment is not known the present study focuses on the northern segment which is a key for understanding the tectonic history of the eastern mediterranean investigation is carried out offshore on two scales 1 deep crustal observations analysis of seismic refraction and magnetic data and their comparison with other continental margins worldwide and 2 shallow crustal scale observations of recent tectonic deformations based on multichannel and single channel seismic reflection profiles integration of the deep and shallow observations with reevaluation of geological data onland show strong structural resemblance between the northern levant segment and other transform margins a modified scenario is suggested for the formation and development of the levant basin and its margins combining all previous studies into one simple picture where a signature of older structures on more recent features is evident located in the midst of the progressive afro eurasian collision the northern levant presents a unique lifecycle for transform margins worldwide from formation to reactivation

identification of a magma chamber in the ghoubbet asal rift djibouti from a magnetotelluric experiment abstract a very broad band 10 3 to 10 4 hz magnetotelluric investigation of the axial zone of the ghoubbet asal rift djibouti has revealed a shallow 2 4 km magma chamber which can be mapped in some detail the suggested roof of the chamber is shallowest very close to the ardoukoba volcano which was built during the november 1978 rifting episode

seismic structure of the uppermost mantle beneath the kenya rift a major goal of the kenya rift international seismic project wrisp 1990 experiment was the determination of deep li ospheric structure in the re action wide angle reflection part of the krisp effort the experiment was designed to obtain arrivals to distances in excess of 400 km phases from interfaces within the mantle vyere recorded from many shotpoints and by design the best data were obtained along the axial profile reflected arrikals from two thin 10 km high velocity layers were observed along this profile and a refracted arrival was observed from the upper high velocity layer these mantle phases were observed on record sections from four axial profile shotpoints so overlapping and reversed coverage was obtained both high velocity layers are deepest beneath lake turkana and become more shallow southward as the apex of the kenya dome is approached the first layer has a velocity of 8 05 8 15 km s is at a depth of about 45 km beneath lake turkana and is observed at depths of about 40 km to the south before it disappears near the base of the crust the deeper layer has velocities ranging from 7 7 to 7 8 km s in the south to about 8 3 km s in the north has a similar dip as the upper one and is found at depths of 60 65 km mantle arrivals outside the rift valley appear to correlate with this layer the large amoungs of extrusive volcanics associated with the rift suggest comp itional anomalies as an explanation for the observed velocity structure however the effects of the large heat anomaly associated with the rift indicate that composition alone cannot explain the high velocity layers observed these layers require some anisotropy probably due to the preferred orientation of olivine crystals the seismic model is consistent with hot mantle material rising beneaith the kenya dome in the southern kenya rift and north dipping shearing along the rift axis near the base of the lithosphere beneath the northern kenya rift this implies lithosphere thickening towards the north and is consistent with a thermal thinning of the lithosphere from below in the south changing to thinning of the lith phere due to stretching in the north

uppermost mantle pn velocity model for the afar region ethiopia an insight into rifting processes the afar depression ethiopia offers unique opportunities to study the transition from continental rifting to oceanic spreading because the process is occurring onland using traveltime tomography and data from a temporary seismic deployment we describe the first regional study of uppermost mantle p wave velocities vpn we find two separate low vpn zones as low as 7 2 km s 1 beneath regions of localized thinned crust in northern afar indicating the existence of high temperatures and potentially partial melt the zones are beneath and off axis from contemporary crustal magma intrusions in active magmatic segments the dabbahu manda hararo and erta ale segments this suggests that these intrusions can be fed by off axis delivery of melt in the uppermost mantle and that discrete areas of mantle upwelling and partial melting thought to characterize segmentation of the uppermost mantle at seafloor spreading centres are initiated during the final stages of break up

comparison of dike intrusions in an incipient seafloor spreading segment in afar ethiopia seismicity perspectives 1 oceanic crust is accreted through the emplacement of dikes at spreading ridges but the role of dike intrusion in plate boundary deformation during continental rupture remains poorly understood between 2005 and 2009 the 70 km long dabbahu manda hararo rift segment in ethiopia has experienced 14 large volume dike intrusions 9 of which were recorded on temporary seismic arrays a detailed comparison of the seismic characteristics of the seismically monitored dikes is presented with implications for dike intrusion processes and magmatic plumbing systems all of the migrating swarms of earthquakes started from a 80 of energy is released during the propagation phase with minimal seismic energy release after the dike propagation ceases we interpret that faulting and graben formation above the dikes occurs hours after the passage of the dike tip coincident with the onset of low frequency earthquakes dike lengths show no systematic reduction in length with time suggesting that topographic loading and stress barriers influence dike length as well as changes in tectonic stress the propagation velocities of all the dikes follow a decaying exponential northward propagating dikes had faster average velocities than those that propagated southward suggesting preconditioning by the 2005 megadike or ongoing heating from a subcrustal magma source north of the midsegment

nature of the crust under afar new igneous not thinned continental abstract thinned continental crust is considered absent from beneath afar except for isolated remnants such as comprise the danakil block the ethiopian plateau sialic crust thins abruptly across the plateau afar margin to abut new igneous crust under afar generated during the early development of the red sea basin analyses of stretching and sea floor spreading amounts elsewhere in the red sea and gulf of aden basins are employed to support this concept the dual layering of the afar crust and the similarity of p wave velocities in these layers to velocities in sialic crust lead to the proposal that new continental crust can be generated at magmatic rift zones

the red sea 50 years of geological and geophysical research abstract the red sea displays lithosphere that is in transition from rifting to drifting and therefore provides enormous opportunity to understand incipient seafloor spreading development despite more than 50 years of extensive geological and geophysical research there remains significant conjecture concerning the timing of sea floor spreading initiation the extent of spreading along the axis of the red sea and the geodynamic processes responsible for the onset of crustal extension red sea tectonic models based on geological data are dominated by single stage rift models involving protracted stretching of continental crust followed by sea floor spreading at ca 5 ma and include both asymmetric and symmetrical extension models geophysical data suggest that an oceanic crust occurs beneath the red sea shelves although exactly how this crust has evolved remains undecided equally arguments have been made in favor of the sedimentary shelves forming on extended continental crust and that oceanic crust is restricted to the present day red sea axial valley however our synthesis shows that none of the models proposed so far is applicable to the entire red sea basin the distribution and timing of the red sea crustal extension and sea floor spreading may have been influenced by subduction related far field forces due to complex plate interactions along the northern edge of the arabian plate during early miocene our reassessment suggests that following the arrival of the afar plume the southern red sea underwent rapid extension culminating in the transition from rifting to seafloor spreading which stalled during the early miocene in the central red sea due to either slowing of the convergence between arabia and eurasia or the onset of the arabian plate passive margin collision with eurasia off axis magmatism is focused on the arabian margin and afar depression and has been episodically active since the miocene we consider a formation of the red sea as a hybrid process involving both active and passive rifting

thermal models of dyke intrusion during development of continent ocean transition a consensus has emerged in recent years from a variety of geoscientific disciplines that extension during continental rifting is achieved only partly by plate stretching dyke intrusion also plays an important role magma intrusion can accommodate extension at lower yield stresses than are required to extend thick strong unmodified continental lithosphere mechanically thereby aiding the breakup process dyke intrusion is also expected to heat and thereby weaken the plate but the spatial extent of heating and the effect of different rates of magmatic extension on the timescales over which heating occurs are poorly understood to address this issue a numerical solution to the heat flow equation is developed here to quantify the thermal effects of dyke intrusion on the continental crust during rifting the thermal models are benchmarked against a priori constraints on crustal structure and dyke intrusion episodes in ethiopia finite difference models demonstrate that magmatic extension rate exerts a first order control on the crustal thermal structure once dyke intrusion supersedes faulting and stretching as the principal extensional mechanism the crust will heat and weaken rapidly less than 1 ma in the main ethiopian rift mer the majority of present day extension is focused on 20 km wide quaternary recent axial magmatic segments that are mostly seismogenic to mid crustal depths and show p wave seismic velocities characteristic of heavily intruded continental crust when reviewed in light of our models these observations require that no more than half of the mer s extension since 2 ma has been achieved by dyke intrusion magmatic heating and weakening of the crust would have rendered it aseismic if dyke intrusion accounted for the entire 6 mm yr extension rate in the older faster extending 16 mm yr red sea rift rsr in afar dyke intrusion is expected to have had a more dramatic impact on crustal rheology accordingly effective elastic plate thickness and moho depth in the danakil region of northernmost afar are markedly reduced and seismicity is shallower than in the mer thermally driven variations in crustal rheology over time in response to dyke intrusion thus play an important role in the development of continent ocean transition

block rotation and continental extension in afar a comparison to oceanic microplate systems the reorganization of oceanic spreading centers separating major plates often appears to occur by a process in which discrete microplates form and evolve by rift propagation to see whether such microplate behavior has implications for continental rifting we investigate the application of a microplate model to the afar region at the nubia somalia arabia triple junction studies of marine magnetic anomalies volcanic ages bathymetry and seismicity suggest that the westward propagating gulf of aden spreading center has propagated into eastern afar within the past 2 m y causing rifting and extension within the continent we derive constraints on the extension history from the geometry and timing of rift formation and from paleomagnetic data indicating that pliocene to pleistocene age rocks have undergone a clockwise rotation of 11 we suggest that the history of rifting the rotation and several other features of the regional geology can be described by combining features of an oceanic microplate model and the concept of rift localization previously proposed for afar in this scenario motion occurring on several rifts within an extensional zone preceding the propagating spreading center is gradually transferred to a single rift while motion is transferred the overlap region between the growing and dying rifts acts as one or more microplates or blocks that rotate relative to the surrounding major plates the rifting history and rotations in eastern afar are thus related to the rift propagation and localization that occurs as the plate boundary evolves provided the constraints we use are appropriate our model better describes the regional kinematics than alternative block models including one based on bookshelf faulting if the tectonics of afar are typical for continental breakup they have interesting implications for the geometry of passive margins in particular asymmetric rifted margins can be produced if the final location of the rift axis is not at the center of the zone of initially disrupted lithosphere additionally if the rate of rift propagation and the rate and location of rift localization are not uniform then along axis structural variations will result

the afar depression interpretation of the 1960 2000 earthquakes summary we studied the seismic activity of the afar depression ad and adjacent regions during the period 1960 2000 we define seven distinct seismogenic regions using geological tectonic and seismological data based on the frequency magnitude relationships we obtain b values of about 1 for the different regions the pattern of the distribution of the location of epicentres fits with the known active fault zone in the ad and the axial volcanic ridges the bab el mandab area and the danakil aysha a blocks are less active for 125 intermediate to strong earthquakes the seismic moment and source parameters were calculated the results of the fault plane solutions for the afar depression indicate mainly strike slip and normal sense of movement originating from fault planes striking nw se these results indicate a clockwise block rotation described previously as a bookshelf model in central ad there are a few rightlateral faults east of massawa with e w striking fault planes at the southern red sea north of the danakil block the mixed focal mechanisms with axial plane striking nw se comprise several reverse faulting strike slip motion and normal faulting right lateral movement was also calculated for a cluster of seismic events between the manda hararo and alyata volcanic ridges along nw se striking faults along the n s striking faults in the escarpment at the western afar margins there are two distinct clusters of epicentres the strong earthquakes at the southern cluster exhibit normal or strike slip motions the intermediate to small earthquakes in the northern cluster exhibit reverse and strike slip motions mainly normal faults were calculated along ne sw striking faults of the ethiopian east african rift estimates of the seismic efficiency suggest that the maximal values are about 50 per cent or less implying that most of the motion is taken aseismically

implications of longeval lava lakes for geomorphological and plutonic processes at erta ale volcano afar erta ale volcano sited within the afar triangle of ethiopia is one of the least visited of perennially active subaerial volcanoes satellite images recorded over the past thirty years illustrate the sustained activity of the volcano and complement the few brief periods of field observation we consider that erta ale s lava lakes have probably persisted for 90 years placing them among the longest lived yet recorded despite sustained surface heat fluxes of 100 400 mw the net surface growth rate integrated over the interval 1968 1995 was only 10 kg s 1 we speculate that this minimal lava discharge cannot be used to infer a low magma supply rate to the volcano since heat losses from die lava lakes inhibit eruption by increasing magma density this implies growth of the underlying crust by formation of dikes and sills containing cumulates accommodated and possibly promoted by regional extension and intrusion loading open vent degassing and rift kinematics are clearly reflected in erta ale s morphology which is characterised by very gentle 3 slopes and a summit caldera

the nature of the crust beneath the afar triple junction evidence from receiver functions the afar depression is an ideal locale to study the role of extension and magmatism as rifting progresses to seafloor spreading here we present receiver function results from new and legacy experiments crustal thickness ranges from 45 km beneath the highlands to 16 km beneath an incipient oceanic spreading center in northern afar the crust beneath afar has a thickness of 20 26 km outside the currently active rift segments and thins northward it is bounded by thick crust beneath the highlands of the western plateau 40 km and southeastern plateau 35 km the western plateau shows vp vs ranging between 1 7 1 9 suggesting a mafic altered crust likely associated with cenozoic flood basalts or current magmatism the southeastern plateau shows vp vs more typical of silicic continental crust 1 78 for crustal thicknesses 2 0 can only be explained by significant amounts of magmatic intrusions in the lower crust this suggests that melt emplacement plays an important role in late stage rifting and melt in the lower crust likely feeds magmatic activity the crust between the location of the miocene red sea rift axis and the current rift axis is thinner 2 0 than beneath the eastern part of afar 26 km vp vs 1 9 this suggests that the eastern region contains less partial melt has undergone less stretching extension and has preserved a more continental crustal signature than west of the current rift axis the red sea rift axis appears to have migrated eastward through time to accommodate the migration of the afar triple junction

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

tetrahedrite mineral chemistry and metal zoning a thermodynamic assessment from the rajpura dariba polymetallic deposit india the polymetallic ore deposit at rajpura dariba comprising a metamorphosed synsedimentary banded ore facies and a younger unmetamorphosed pegmatoidal vein ore facies of minor intensity displays a conspicuous base metal zoning in the banded ore from the footwall to the hanging wall in the classic sequence of cu pb zn zn fe that notationally registers the fluid flow direction tetrahedrite is of widespread occurrence samples of the banded ore collected systematically from footwall to hanging wall at various mine levels register extensive substitution of ag for cu zn for fe and as for sb in the mineral in contrast tetrahedrite from the vein ores exhibits much restricted compositional variability in particular its silver content is too low to allow any correlation of variation in the ag and sb contents in general the footwall samples have high amounts of as and low amounts of ag hanging wall samples have low amounts of as and high amounts of ag whereas samples from the interveining region record intermediate values of ag and as these features confirm the existence and preservation of a footwall to hanging wall tetrahedrite compositional zoning which indicates that the fluid flow direction is consistent with that shown by the base metal zonation trend anomalous samples whose spatial positions do not match those prescribed by their ag and as contents are interpreted to be the result of metamorphic reequilibration and or localized interaction with younger vein ore fluid which partially modified the pristine chemical signature the tetrahedrite zoning patterns compare favorably with those derived from curves of constant ag cu sub 1 and as sb sub 1 exchange potentials mu sub ag cu sub 1 and mu sub as sb sub 1 respectively passing through the appropriate compositional coordinates in terms of as as sb x 3 and ag ag cu x 4 most of the footwall samples plot on higher mu sub as sb sub 1 and lower mu sub ag cu sub 1 curves of constant exchange potentials from footwall proximal to hanging wall distal through the intermediate zone samples there is an initial decreasing trend in mu sub as sb sub 1 and an increasing trend in mu sub ag cu sub 1 values the anomalous samples constitute exceptions to this pattern the study indicates that in rajpura dariba a relict tetrahedrite compositional zoning trend persists parallels the base metal zonation trend and is only partially modified due to the later metamorphism and or vein forming episodes

crustal structure of the main ethiopian rift from gravity data 3 dimensional modeling abstract a three dimensional interpretation of the newly compiled bouguer anomaly map of the main ethiopian rift is presented a high resolution 3 d model constrained with the seismic results reveals a possible crustal thickness and density distribution beneath the graben the bouguer anomalies along the axial portion of the rift floor as deduced from the results of the regional and residual separation are mainly caused by the deep seated structures the inferred zone of intrusion which is the main subject of the present study coincides with the maximum gravity anomaly of the rift floor the intrusion is displaced at several sectors along the east west direction and the two major displacements coincide with the locations of the major rift offsets on the surface because of the asthenospheric uplift the crust under the main ethiopian rift is slightly thinned the zone of crustal thinning 31 km coincides with the location of the intrusion beneath the rift floor and the maximum of which is attained in the northern and central sectors of the graben the trend of the crustal thinning zone which is from south to north is the same as the one obtained in the afar depression the southeastern and western plateaus on the other hand show by far the largest crustal thickness in the region 38 51 km in contrast to the afar depression where the crust is partly oceanized the thickness and density of the crust suggest that the main ethiopian rift is underlain by a purely continental crust the deep and relatively large nature of the intrusion leads to the conclusion that a large scale asthenospheric upwelling might be responsible for the thinning of the crust and subsequent rifting of the graben

saudi arabian seismic refraction profile a traveltime interpretation of crustal and upper mantle structure abstract the crustal and upper mantle compressional wave velocity structure across the southwestern arabian shield has been investigated by a 1000 km long seismic refraction profile the profile begins in mesozoic cover rocks near riyadh on the arabian platform trends southwesterly across three major precambrian tectonic provinces traverses cenozoic rocks of the coastal plain near jizan and terminates at the outer edge of the farasan bank in the southern red sea more than 500 surveyed recording sites were occupied and six shot points were used including one in the red sea two dimensional ray tracing techniques used to analyze amplitude normalized record sections indicate that the arabian shield is composed to first order of two layers each about 20 km thick with average velocities of about 6 3 km s and 7 0 km s respectively west of the shield red sea margin the crust thins to a total thickness of less than 20 km beyond which the red sea shelf and coastal plain are interpreted to be underlain by oceanic crust a major crustal inhomogeneity at the northeast end of the profile probably represents the suture zone between two crustal blocks of different composition elsewhere along the profile several high velocity anomalies in the upper crust correlate with mapped gneiss domes the most prominent of which is the khamis mushayt gneiss based on their velocities these domes may constitute areas where lower crustal rocks have been raised some 20 km two intracrustal reflectors in the center of the shield at 13 km depth probably represent the tops of mafic intrusives the mohorovicic discontinuity beneath the shield varies from a depth of 43 km and mantle velocity of 8 2 km s in the northeast to a depth of 38 km and mantle velocity of 8 0 km s depth in the southwest near the shield red sea transition two velocity discontinuities occur in the upper mantle at 59 and 70 km depth the crustal and upper mantle velocity structure of the arabian shield is interpreted as revealing a complex crust derived from the suturing of island arcs in the precarnbrian the shield is currently flanked by the active spreading boundary in the red sea

the protracted development of the continent ocean transition in afar continental breakup and the transition to seafloor spreading is characterized by extensional faulting thinning of the lithosphere and at magmatic margins voluminous intrusive and extrusive magmatism1 2 3 4 it is difficult to discriminate between different mechanisms of extension and magmatism at ancient continental margins because the continent ocean transition is buried beneath thick layers of volcanic and sedimentary rocks5 6 and the tectonic activity that characterized breakup has ceased instead the timing of these mechanisms is inferred from theoretical models or from the geological record preserved at the fully developed ancient rifted margins1 5 7 8 ongoing rifting in ethiopia offers a unique opportunity to address these problems because it exposes subaerially the transition between continental rifting towards the south and seafloor spreading further northward here we synthesize constraints on the spatial and temporal evolution of magmatism and extension in ethiopia we show that although intrusion of magma maintains crustal thickness during the early stages of the continent ocean transition subsidence of the margin below sea level and eruption of voluminous basalt flows is initiated by late stage thinning of the heavily intruded weakened plate just before the onset of seafloor spreading we thus conclude that faulting stretching and magma intrusion are each important but at different times during breakup

quarries and transportation routes of angkor monument sandstone blocks sandstone blocks are the main construction materials used in the angkor monuments in cambodia however a thorough study of the quarries has not yet been carried out we conducted a field investigation of sandstone quarries from the angkor period at the southeastern foot of mt kulen which is approximately 35 km northeast of the angkor monuments as a result we discovered more than 50 sandstone quarries on the basis of the measurements of magnetic susceptibilities and thicknesses step heights we found that they were quarried at different times these four quarrying areas were identified as the quarries d to g inferred by uchida et al 2007 in addition we investigated a canal that was identified on satellite images connecting quarry sites at the foot of mt kulen to the angkor monuments the field investigation suggests a high probability that the canal was used for the transportation of sandstone blocks from mt kulen

redistribution of the lithosphere deformation by the emplacement of underplated mafic bodies implications for microcontinent formation abstract sedimentary basin migration and microcontinent formation could be the result of deformation redistribution in which the emplacement of an underplated mafic body upmb may play an important role in this study a 2d finite element model is used to examine the redistribution of the deformation that can be triggered by a upmb it is shown that three modes of deformation redistribution can exist 1 a shift completed mode where the deformation is completely redistributed into the newly weakened region 2 a transition mode where the deformation is redistributed but extension is accommodated by thinning in both regions 3 a shift failed mode where the deformation is not redistributed whether or not the deformation is redistributed depends on the configuration of the upmb and on the initial rheological heterogeneity in the initially deformed region however it becomes difficult for any upmb to initiate the redistribution of the deformation once the stretching factor in the first deformed region exceeds a critical value the microcontinent formations observed in the indian ocean in the norwegian greenland sea in the red sea afar region and in the western mediterranean are particularly discussed in relation to the transition mode providing some important geodynamic implications

constraining the african pole of rotation abstract in the absence of well defined transform faults in the east african rift system for constraining the plate kinematic reconstruction the pole of relative motion for the african nubian and somalian plates has been determined from residual motion if africa and somalia are to continue to drift apart along the east african rift system which would then evolve into a series of ridges offset by transform faults then incipient transform faults that may reflect the direction of relative motion should already be in place along the east african rift system the incipient transforms along the east african rift system are characterized by shear zones such as the zambezi shear zone in the south and the aswa and hamer shear zones in the north some of these shear zones have been associated with recent strike slip faulting in the nw se direction during periods of earthquakes provided that these consistently nw se oriented strike slip movements in the shear zones give the direction of relative motion of the adjacent plates then they can be used to constrain the position of the africa somalia euler pole due to the fact that identifying transform faults in the east african rift system is difficult and because the genesis of transform faults characterizing a plate boundary at an inception stage is not well known the discussion here is limited to the northern segment of the east african rift system where shear zones are better characterized by the existing geophysical data the characterizing features vary with latitude indicating the complexity of the problem of the genesis of transform faults i believe however that the relatively well defined intra continental transform fault in the northern east african rift system which is characterized by strike slip faulting and earthquakes constrains the pole of relative motion for the african and somalian plates to a position near 1 5 s and 29 0 e

evidence for anisotropy in north east africa from geographical and azimuthal distribution of rayleigh wave velocities and average upper mantle structure long period surface waves from wwssn gdsn sro and geoscope stations are analysed to obtain the dispersion of surface waves in the north east of africa lateral variations of rayleigh wave velocities are retrieved simultaneously with the average azimuthal anisotropy as a first result the low velocity anomaly under the red sea is apparent at all periods a second result is a nearly north south direction of maximum group and phase velocities this azimuthal anisotropy direction is in agreement with the direction of absolute plate motion in this region anisotropy is also evidenced by simultaneous inversion of love and rayleigh wave dispersion along the path tam agd in the resulting models isotropic and anisotropic very low mantle sv wave velocities extend up to the moho for sh waves we find a strong discrepancy with sv waves sh wave velocities being higher by 5 down to 200 km under the crust this is characteristic of lithospheric polarization anisotropy the s wave anisotropy polarization and azimuthal found in this region including the red sea is compatible with the absolute motion of the african plate it can be interpreted considering the low lithospheric s velocities as a consequence of plastic flow in the weak lithosphere of an anomalous mantle this anisotropy evidenced here on s waves may be due as pn and lithospheric s wave anisotropy found in other continental areas to dynamical processes related to plate motion

the development of late stage continental breakup seismic reflection and borehole evidence from the danakil depression ethiopia during continental breakup the locus of strain shifts from a broad region of border faulting and ductile plate stretching to a narrow zone of magma intrusion in a young ocean basin recent studies of volcanic rifts and margins worldwide suggest this shift occurs subaerially before the onset of seafloor spreading we test this hypothesis using recently acquired seismic reflection and borehole data from the danakil depression ethiopia a unique region of transition between continental rifting and seafloor spreading our data located near dallol 30 km northwest of the erta ale volcanic segment reveal a remarkably thick 1 km sequence of young 100 ka evaporites in a basin bound by a major 400 m throw east dipping normal fault to generate such a large amount of subsidence in such a relatively short time we propose that upper crustal extension in danakil is currently dominated by faulting not magmatic intrusion given the region s markedly thinned crust 15 km thick relative to elsewhere in afar where magma assisted rifting dominates and maintains crustal thickness at 25 km mechanical extension in danakil is likely coupled with ductile extension of the lower crust and mantle lithosphere despite proximity to the voluminous lavas of the active erta ale volcanic segment evidence for igneous material in the upper 2 km of the 6 to 10 km wide basin is limited late stage stretching was likely aided by thermal strain induced lithospheric weakening following protracted magma assisted rifting basin formation immediately prior to the onset of seafloor spreading may also explain the accumulation of thick marine seepage fed evaporite sequences akin to those observed for example along the south atlantic rifted margins

the gravity field of the red sea and east africa abstract reevaluation of all gravity data from the red sea the gulf of aden and east africa permitted the compilation of a new bouguer anomaly map the intensity of the gravity field and its regional pattern correlate closely with the topographic features of the region the maximum bouguer values 100 mgal are located over the median troughs of the red sea and gulf of aden dense juvenile oceanic crust in these rifts and intruding magmas in stretched continental areas produce excess mass responsible for the anomaly highs in the red sea the orientation of the gravity highs is nw se in the south turning to ne sw in the north almost parallel to the aqaba dead sea strike this pattern reveals that the present basin axis is not identical with that which formed the tertiary coastal margins and the pre red sea zones of crustal weakness in the gulf of aden new oceanic crust along the tadjura trench and its eastward extension is also expressed in the bouguer anomaly map by gravity highs and a sharp bending of the isolines a maximum of approx 150 mgal is located over the central section of the sheba ridge bouguer gravity values over the east african and yemen plateaus are of the order of 180 to 240 mgal indicating significant crustal thickening on the somali plateau the marda fault also has a strong gravity signature that can be traced towards somalia by constraining crustal thickness and structure with seismic data and density values from the velocity distribution by means of the nafe drake and birch relationships we computed density models for the crust and upper mantle the crustal thickness is of the order of 40 km beneath the plateaus and only 5 to 6 km at the oceanized parts in the central and southern portions of the red sea median trough the flanks of the southern red sea and the corresponding arabian side are underlain by 12 to 16 km thick stretched continental type crust oceanization offshore sudan and egypt is asymmetrical the continental crust terminates abruptly at about 20 km off the coastline followed by an oceanic crust of early miocene age that was produced in pull apart basins by contrast the eastern side of the red sea trough offshore saudi arabia is floored by stretched continental crust that extends far into the sea seafloor spreading and the generation of oceanic crust in organized spreading centres are limited to the median troughs off sudan and the northern part of ethiopia and commenced approx 5 m y bp they are absent in the northern red sea where crustal fracturing occurs only in pull apart basins of dead sea aqaba orientation distributed in en echelon pattern

shear controlled evolution of the red sea pull apart model abstract results of seismic and other geophysical investigations suggest that strike slip processes controlled the break up of the arabian plate from africa and initiated the red sea rift early oceanisation was facilitated by nucleation of pull apart basins and massive intrusives the evolution of the red sea has gone through different stages it was a zone of structural weakness already during the pan african orogeny approximately 600 ma a major reactivation however that gradually led to the present day configuration was initiated during the late oligocene with intense magmatic activity and the development of a continental rift wrench faulting played a key role in the early evolution of the red sea as it shaped most of its western flank as a sharp plate boundary and resulted in the generation and rapid oceanisation of linearly arranged pull apart basins spatial distribution of these basins reflects the geometry of the strike slip zone which was controlled by pre existing fault systems like the najd shear system the central african fault zone or the onib hamisana and baraka suture zones strike slip motion along the latter zones of weakness influenced mainly the egyptian and sudanese coastal areas arabia was therefore separated from africa by oceanisation in those regions where pull apart basins developed they were still connected in the in between segments by stretched continental crust with arabia as the moving and africa as the stable plate the eastern red sea flank was formed by pure shear through stretching thinning and diffuse extension as a consequence the eastern and western flanks of the red sea are asymmetrical the acceleration of the movement of arabia in early middle miocene could no longer be accommodated by the opening in the gulf of suez and consequently the dead sea strike slip fault developed approximately 14 ma ago since plate motion was still oblique to the major structural trends the pull apart evolution on the western flank continued and the oceanisation was accentuated while in the north the eastern flank developed en echelon fractures of aqaba orientation seafloor spreading commenced 5 ma ago at parts of the central and southern red sea this final stage is also responsible for the bathymetry with maximum depth values of 3000 m in the seafloor spread areas and a much shallower basin in the north the metalliferous red sea deeps are either associated with this infant mid ocean ridge or with the en echelon fractures in the north

seismic refraction studies of the afro arabian rift system a brief review abstract the crustal and uppermost mantle structure of major units of the afro arabian rift system has been consecutively investigated by seismic refraction surveys in the jordan dead sea rift the red sea the afar depression and the east african rift of kenya with the exception of the jordan dead sea transform the entire afro arabian rift system is underlain by anomalous mantle with pn velocities less than 8 km s while under the rift flanks the velocity is clearly equal to or above 8 0 km s various styles of rifting have been found oceanic crust floors the axial trough of the southern red sea rift thinned continental crust underlies the margins of the red sea as the afar depression and the northern kenya rift on the other hand 30 35 km thick continental crust is found both under the jordan dead sea rift where strike slip rifting is active and thinning towards the mediterranean occurs and under the central kenya rift where updoming is apparently the controlling feature while the transition from thinned continental to 5 6 km thick oceanic crust in the centre of the red sea appears to be more gradual the transition from rift related structure to undisturbed continental crust of 40 5 km thickness is mostly rather abrupt the seismic data indicate various stages of rifting evidenced by different styles of crustal strucre and they imply the presence of heated uppermost mantle under most parts of the rift system possibly related to plume activity local volcanism may disrupt and or underplate the crust in places altering in particular the structure of the lower crust progressive thickening of the rifted crust away from the oceanized centres in the southern red sea and gulf of aden towards north and south may be viewed as an evolutionary sequence which however may be difficult to explain when viewing the afro arabian rift system as an active rift controlled by plume activity

crustal structure and magmato tectonic processes in an active rift asal ghoubbet afar east africa 2 insights from the 23 year recording of seismicity since the last rifting event 1 the asal ghoubbet ag rift has sustained a major volcano tectonic rifting episode in 1978 and has been subsequently monitored with continuous geodetic and seismological surveys it is therefore an ideal place to study the transient magmato tectonic processes that operate after a rifting episode we examine the space time evolution of 2500 md 2 8 earthquakes recorded in the rift from 1979 to 2001 we focus on the relationships between this seismic activity and both the three dimensional structure of the rift and its postrifting behavior depicted from geodesy the results highlight the major role of the central magmatic system fieale shark bay on the structure seismic activity and overall behavior of the rift from 1978 to 1986 the rift opens at a fast rate yet mainly aseismically the opening is magmatically driven and accommodated since 1986 when the opening rate abruptly decreased the seismicity is concentrated in the central part of the rift and reveals pulses of activity of the central volcanic system these pulses result from that magmatic zone undergoing alternating stretching and inflating episodes thus while the plate driven induced stresses have been rebuilding in the rift since 1987 the rift opening is still essentially accommodated in the axial magmatic zone the ag rift has thus sustained a postrifting unsteady opening over more than 23 years following its stretching episode that transient opening has essentially occurred aseismically and most tectonic faults remain relaxed and locked