propagation of rifting along the arabia somalia plate boundary into afar it is generally accepted that the aden ridge has propagated westward from 58 e to the western tip of the gulf of aden tadjoura at the edge of afar here we use new tectonic and geochronological data to examine the geometry and kinematics of deformation related to the penetration of that ridge on dry land in the republic of djibouti we show that it veers northward forming a narrow zone of dense faulting along the northeastern edge of the afar depression the zone includes two volcanic rifts asal ghoubbet and manda inakir connected to one another and to the submarine part of the ridge by transfer zones both rifts are composite divided into two or three disconnected parallel nw se striking subrifts all of which appear to have propagated northwestward in asal ghoubbet as in manda inakir the subrifts appear to have formed in succession through north directed jumps from subrifts more farther south at present the northernmost subrifts manda and dirko koma of the manda inakir rift form the current tip of the northward propagating arabia somalia plate boundary in afar we account for most observations by a mechanical model similar to that previously inferred for the gulf of aden in which propagation is governed by the intensity and direction of the minimum horizontal principal stress 3 we interpret the northward propagation on land almost orthogonal to that in the gulf to be related to necking of the central afar lithosphere where it is thinnest such necking may be a consequence of differential magmatic thickening greater in the center of the afar depression where the ethiopian hot spot enhanced profuse basaltic effusion and underplating than along the edges of the depression the model explains why the aden ridge foregoes its wsw propagation direction constant from 58 e to asal ghoubbet at a smaller scale individual rifts and subrifts keep opening perpendicular to the arabia somalia or danakil somalia motion vector and propagate northwestward concurrently such lithospheric cracks are forced to jump northward such that the plate boundary remains inside the regional n s necking zone changes of obliquity between the directions of overall and local propagation may account for different segmentation patterns a small angle promoting long en echelon subrifts and a high angle smaller nested subrifts within subrifts the propagation mechanism is thus similar whether in oceanic or continental lithosphere the principal change being the overall propagation path here governed by thickness changes rather than by the geometry in map view as previously inferred for the rest of the aden ridge finally because the same mechanism has led rifting along the red sea to propagate southward and jump to the western edge of afar the arabia somalia and arabia nubia plate boundaries tips have missed each other and keep overlapping further leading to strain transfer by large scale bookshelf faulting

a fractal model for crustal deformation abstract we hypothesize that crustal deformation occurs on a scale invariant matrix of faults for simplicity we consider a two dimensional pattern of hexagons on which strike slip faulting occurs the behavior of the system is controlled by a single parameter the fractal dimension deformation occurs on all scales of faults the fractal dimension determines the fraction of the total displacement that occurs on the first order or primary faults the value of the fractal dimension can be obtained from the frequency magnitude relation for earthquakes our results are applied to the san andreas fault system in central california earthquake studies give d 1 90 we associate the main strand of the san andreas fault with the primary faults of our fractal system we predict that the relative velocity across the main strand is 2 93 cm yr the remainder of the relative velocity of 5 5 cm yr between the pacific and north american plates occurs on higher order faults the predicted value is in reasonably good agreement with the value 3 39 0 29 cm yr obtained from geological studies

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

geology geochronology and rift basin development in the central sector of the main ethiopia rift based on stratigraphic relationships and k ar dating of volcanic rocks from both of the escarpments flanking plateaus and from the rift floor of the central sector of the main ethiopian rift six major volcanic episodes are recognized in the rift9s development over a time span from the late oligocene to the quaternary using the k ar data correlation of volcanic units from the six periods of activity throughout the study area forms the basis for establishing six time stratigraphic chronozones for the central sector that are related to volcanism in the ethiopian cenozoic volcanic province the oldest basalt and rhyolite flows exposed along the rift margins of the central sector are time correlative to or older than those in river canyons for example blue nile on the adjacent northwest plateau a thinned mesozoic stratigraphic sequence along the guraghe western rift margin suggests that doming may have preceded volcanism and rifting of the cenozoic by late miocene time at least by 8 3 ma and 9 7 ma the eastern and western faulted margins respectively of the rift had formed at guraghe and at agere selam as indicated by containment of flows of that age within the rift wall during eruption a paroxysm of calc alkaline ignimbrite activity produced voluminous flows nearly fully contained within the rift during the pliocene epoch the munesa crystal tuff 3 5 ma a prominent marker tuff exposed on both rift margins is present at depth in a geothermal well beneath the rift floor and indicates a minimum of 2 km of downthrow in the central sector since its eruption structural and stratigraphic relationships in the central sector indicate a two stage rift development this is characterized by an early phase late oligocene or early miocene of a series of alternating opposed half grabens along the rift with alternating polarity such as that in the present gregory and western rifts of east africa and symmetrical rifts that evolved from these grabens in late miocene or early pliocene time thus evolution from alternating half graben to a full symmetrical graben with a medially located neovolcanic zone that is bifurcated to marginal grabens in the northern part of the study area may be a fundamental part of the rifting process the study indicates that there are major petrologic and tectonic differences between the main ethiopian rift and the gregory kenyan rift

large scale flexuring and antithetic extensional faulting along a nascent plate boundary in the se afar rift newstructuraldatafromtheseafarmarginindjiboutileadusto discriminate two recent 1 ma extensional fault networks between the afar accreted crust and the south danakil block to the east emphasis is placed on the n s oriented makarassou fault system which dissects the prominent westward riftward facing topographic flexure between the two structural domains the most striking characteristic of the makarassou fault belt is a closely spaced network of continentward dipping extensional faults bounding domino blocks of 3 1 ma tilted lavas of the stratoid series the makarassou fault flexure system is interpreted as an extensional faulted monocline that developed in response to differential subsidence and crustal downwarping of the afar magmatically accreted transitional crust large scale flexuring and associated structures along the easternflank of the asal manda inakir incipient plate boundary are regarded as indicative of a nascent volcanic rifted margin in se afar

shear wave processing of sonic log waveforms in a limestone reservoir sonic full waveforms were recorded in a shale limestone interval with an experimental 12 channel acoustic logging device well defined shear pseudo rayleigh waves were observed throughout most of the 142 m interval and were used to distinguish lithologic boundaries and zones of fracturing as interpreted by logs the high signal to noise ratio of the waveforms permitted meaningful shear velocities to be obtained by intervals in this well a correlation of changes in the vp vs ratio with depth aided in identifying some compositional changes in the well but such a correlation could not be used to distinguish fracturing in the limestone shear wave amplitudes however were effective for identifying lithologic changes as well as fractures from the sonic log shear amplitudes were lower in the shale than in the limestone and shear wave attenuation increased in fractured zones there was no strong correlation between the degree of fracturing and the attenuation of stoneley waves in general shear wave pr

continental crust composition constrained by measurements of crustal poisson s ratio deciphering the geological evolution of the earth s continental crust requires knowledge of its bulk composition and global variability the main uncertainties are associated with the composition of the lower crust seismic measurements probe the elastic properties of the crust at depth from which composition can be inferred of particular note is poisson s ratio this elastic parameter can be determined uniquely from the ratio of p to s wave seismic velocity and provides a better diagnostic of crustal composition than either p or s wave velocity alone1 previous attempts to measure have been limited by difficulties in obtaining coincident p and s wave data sampling the entire crust2 here we report 76 new estimates of crustal spanning all of the continents except antarctica we find that on average increases with the age of the crust our results strongly support the presence of a mafic lower crust beneath cratons and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions followed by magmatic underplating or a model in which crust formation processes have changed since the precambrian era

a recipe for microcontinent formation accreted slivers of continental margins are common in the geologic record but the processes that lead to their formation are poorly understood we observe an association of plume related microcontinent isolation and subsequent long term asymmetries in oceanic crustal accretion based on four recent examples the seychelles in the indian ocean jan mayen in the norwegian greenland sea and the east tasman plateau and the gilbert seamount complex in the tasman sea these microcontinents formed by rerifting of a young continental margin 25 m y old in the vicinity of a mantle plume stem followed by asymmetric seafloor spreading two dimensional numerical stochastic basin modeling suggests that a yield strength minimum along the landward edge of a rifted margin thermally enhanced by heating from a mantle plume may cause a spreading ridge to jump onto this zone of weakness this action isolates a passive margin segment the association of large igneous provinces and microcontinents should be useful for identifying similar events in the geological record

extent of power law scaling for natural fractures in rock new data sets from natural faults and extension fractures exhibit simple power law scaling across 3 4 4 9 orders of magnitude regardless of rock type or movement mode the data show no evidence of natural gaps or scaling changes each data set consists of independent measurements made at different observational scales a power law regression to the subset of smaller fractures in each case provides an extrapolation that accurately predicts associated larger fractures consequently data representing a limited range of fracture sizes may be used to characterize a much broader spectrum of fracture sizes

confirmation of arabia plate slow motion by new gps data in yemen 1 during the last 10 years a network of about 30 gps sites was measured in djibouti east africa additional points were also measured in yemen oman ethiopia iran and on la reunion island merged with data from the available international gps service permanent stations scattered on the different plates in the area eurasia anatolia africa arabia somalia this unique data set provides new insight on the current deformation in the africa somalia arabia triple junction area and on the arabian plate motion here we show that coherent motions of points in yemen bahrain oman and iran allow us to estimate a geodetically constrained angular velocity for the arabian plate 52 59 n 15 74 w 0 461 myr in itrf2000 this result differs significantly from earlier determinations and is based upon our vectors in yemen they provide new additional data and better geometry for angular velocity determination combined with the african and somalian motions this new angular velocity results in predicted spreading rates in the red sea and the gulf of aden which are 15 20 lower than those measured from oceanic magnetic anomalies and thus averaged over the last 3 myr with respect to eurasia the geodetic motion of arabia is also about 30 slower than predicted by nuvel 1a on the basis of the kinematic results presented here and on other evidence for a similar slower geodetic rate of the indian plate we suggest that the whole collision zone between africa arabia india on one hand and eurasia on the other hand has slowed down in the last 3 myr

seismic study of the crust of the northern red sea and gulf of suez we report the results of fifteen expanding spread profiles esp s and a seismic wide angle reflection refraction une performed during march april 1986 in the gulf of suez and the egyptian part of the northern red sea area north of 25 n four 16 4 air guns were used as a sound source on board r v le suroit and a 96 channel 2 4 km long streamer was towed by a supply vessel the whity tide most of the profiles show good crustal reflection and refraction arrivals and often good moho arrivals obtained for a distance of 80 to 100 km we present the results of x t and p analysis obtained by a velocity inversion performed in the p plane and by ray tracing modeling of both the p and the x t sections the velocity models are computed for planar and linear gradient velocity layers the northernmost part of the red sea appears to be characterized by a continental type crust extremely thinned 4 lying at a mean depth of 7 8 km whereas the moho is at a mean 14 15 km depth the southern part shows a seismic velocity structure of an oceanic type except in the 40 km closest to the coastline in both parts seismic waves progressively get more attenuated with distance from the shore to the axial zone which is presently tectonically active the difference between the northern continental and southern oceanic zone is related to the termination of the levant fault the northern continental area appears to represent the termination of the levant fault as a zone of distributed deformation

semi empirical treatment of wave transformation outside and inside the breaker line a semi empirical method is developed which predicts the wave height shoaling up to the break point on laboratory and natural beaches linear shoaling is enhanced with empirical curves for the wave shape obtained from field measurements of wave height through the break point and the surf zone made on an ocean beach in southern australia from 1987 to 1989 wave height shoaling breaking and attenuation are unified in a mixed lagrangian eulerian numerical model of random wave height transformation which with the semi analytical enhancements provides improved predictions of the height transformation of individual waves through the surf zone this is demonstrated for a wide variety of cases from the field and laboratory

thermophysical properties of the earth s crust in situ measurements from continental and ocean drilling knowledge of the variation of thermal conductivity and heat production with depth is critical to understanding the thermal state of the crust continental and oceanic drill holes provide unique opportunities for the calibration of empirical and theoretical models relating crustal thermal properties to physical and chemical properties these calibrations are based on more than 30 000 well log measurements of acoustic velocity both compressional and shear density porosity radioelement concentrations k u th and major element chemistry al si ca fe s mg na in formations ranging in age from 5 to 620 ma and in composition from granitic to gabbroic the mineralogical data available from induced gamma ray spectroscopy logs when combined with known mineral thermal conductivities in a model for the thermal conductivity of a crystalline aggregate enable the in situ application of a laboratory estimation technique and yield continuous conductivity profiles however mineralogy based conductivity models cannot account for the anisotropic conductivity observed in foliated metamorphic rocks a phonon conduction model which utilizes acoustic velocity and bulk density measurements is accurate within 15 in both isotropic and anisotropic formations of the upper continental crust and in oceanic crustal layers 2c and 3 in oceanic crustal layers 2a and 2b however the effects of fracturing on compressional and shear velocities lead to inaccurate results the heat flow heat production relationship which provides one method of determining subsurface heat production has well documented limitations allis 1979 and rybach and buntebarth 1982 1984 on the basis of laboratory measurements have proposed quantitative relationships between heat production and rock compositional and physical properties the well log k u and th data indicate that heat production can be predicted from compositional data within a factor of 2 or 3 predictions of heat production from acoustic velocity and bulk density measurements are only accurate to within an order of magnitude three factors account for these results 1 the natural variability of velocity and density for a given rock composition 2 the tendency of u and th to associate with secondary rather than primary minerals and 3 u and th redistribution through fluid flow and metamorphism thus seismic velocity based heat production profiles can at best be taken as first order estimates which may misrepresent the complexity of the continental crust however the correlation of formations of known heat production with subsurface structures when combined with a suite of existing techniques for estimating heat production may lead to more accurate results

acoustic modes of propagation in the borehole and their relationship to rock properties acoustic waveform measurements in boreholes have important applications in fracture hydrology and radioactive waste disposal but ambiguities in existing interpretation techniques remain a problem we have addressed the problem by using residue theory to predict the relative excitation of various modes contained in experimental waveforms a plane geometry model involving a layer of fluid between two elastic half spaces is shown to provide velocity dispersion curves for propagating modes that are very similar to those for the fluid filled borehole we use the plane geometry model to illustrate the effects of the confined borehole fluid on surface and body waves traveling along the borehole in the elastic solid we also computed excitation functions for some of the lowest order symmetric modes calculated the time domain response of the trapped modes following the shear head waves and compared them to waveforms recorded in boreholes through several homogeneous formations the insight into the mode composit

scaling properties of normal faults in the central afar ethiopia and djibouti implications for strain partitioning during the final stages of continental breakup abstract the central afar depression offers an opportunity for quantitative analysis of the final stages of continental breakup at a divergent triple junction here quaternary faulting dissects plio pleistocene flood basalts providing a regionally pervasive datum to quantify the regional finite strain and fault scaling laws to this end we mapped and measured more than 8500 normal faults using remote sensing products fault length generally follows a power law distribution but exponential distributions are found in areas of magmatic extension fault throw and fault length scale following a power law relationship where t 0 031 l 0 96 similar to previous studies in afar and elsewhere in the world combining the power law distribution with this throw scaling enables a comprehensive assessment of quaternary finite strain throughout the region strain is greatest near manda inakir lake asal and the dobe hanle accommodation zone the makarassou fault zone shows low strain suggesting it does not kinematically link the manda inakir and asal rift segments the presence of moderate to high strain in the amagmatic portions of the central afar rift suggests that those areas accommodate a significant proportion of plate motion this may provide a future path for propagation between the gulf of aden rift and the red sea rift

magma influence on propagation of normal faults evidence from cumulative slip profiles along dabbahu manda hararo rift segment afar ethiopia abstract measuring displacement length profiles along normal faults provides crucial information on fault growth processes here based on satellite imagery and topography we analyze 357 normal faults distributed along the active rift of dabbahu manda hararo dmh afar which offers a unique opportunity to investigate the influence of magmatism on fault growth processes our measurements reveal a large variety of slip profiles that are not consistent with elastic deformation their analysis contributes towards a better understanding of the lateral propagation of faults especially when nucleation points and existence of barriers are included using the fault growth model of manighetti et al 2001 we determine the preferred direction of lateral propagation for each fault our results suggest that lateral propagation of faults is easier away from areas where magma has been stored for long time at crustal depth and has thus modified the thermo mechanical properties of the host rock however these areas correspond also to areas where the initiation of fault growth appears as easiest along the rift in combining these results with the analysis of rift width and the position of magma reservoirs along dmh rift we show that fault growth keeps track of the magma presence and or movement in the crust

stratigraphy of the archean western superior province from p and s wave receiver functions further evidence for tectonic accretion the archean western superior province in canada represents the nucleus of the north american continent whose origin has been speculated to be the result of widespread crustal accretion some 2 7 ga ago in this paper crustal and upper mantle seismic discontinuities beneath the western superior province of the canadian shield are imaged with teleseismic p to s and s to p converted phases using the receiver function method three crustal discontinuities are observed the moho ranging in depth between 38 and 47 km and dipping to the south and two intra crustal discontinuities having depths of approximately 15 and 30 km the crustal discontinuities undulate laterally and often lose continuity possibly indicating an imbricated structure and or regions of velocity gradients in the shallow lithosphere a positive discontinuity is imaged at approximately 65 km depth and is consistent with earlier refraction and wide angle reflection results additionally two zones of negative receiver function amplitudes at 55 km depth are observed and are coincident with a region of anomalous tomographic low p and s wave velocities as well as a zone of high electrical conductivity the images for the crust and shallow upper mantle when integrated with previous geophysical studies are consistent with ideas of continental root formation due to imbrication of archean subducted material and accretion of island arcs observed in surface geology

strain transfer between disconnected propagating rifts in afar we showed before that both the aden and red sea plate boundaries are currently rifting and propagating along two distinct paths into afar through the opening of a series of disconnected propagating rifts here we use new geochronological tectonic and paleomagnetic data that we acquired mostly in the southeastern part of afar to examine the geometry kinematics and time space evolution of faulting related to strain transfer processes it appears that transfer of strain is accommodated by a bookshelf faulting mechanism wherever rifts or plate boundaries happen to overlap without being connected this mechanism implies the rotation about a vertical axis of small rigid blocks along rift parallel faults that are shown to slip with a left lateral component which is as important as their normal component of slip rates of 2 3 mm yr by contrast where rifts do not overlap either a classic transform fault maskali or an oblique transfer zone mak arrasou kinematically connects them the length of the aden red sea overlap has increased in the last 0 9 myr as the aden plate boundary propagated northward into afar as a consequence the first order blocks that we identify within the overlap did not all rotate during the same time span nor by the same amounts similarly the major faults that bound them did not necessarily initiate and grow as their neighboring faults did despite these variations in strain distribution and kinematics the overlap kept accommodating a constant amount of strain 7 to 15 of the extension amount imposed by plate driving forces which remained distributed on a limited number seven or eight of major faults each one having slipped at constant rates 3 and 2 mm yr for vertical and lateral rates respectively the fault propagation rates and the block rotation rates that we either measure or deduce are so fast 30 130 mm yr and 15 38 myr respectively that they imply that strain transfer processes are transient as has been shown to be the case for the processes of tearing rift propagation and strain jumps in afar

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

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

slip accumulation and lateral propagation of active normal faults in afar we investigate fault growth in afar where normal fault systems are known to be currently growing fast and most are propagating to the northwest using digital elevation models we have examined the cumulative slip distribution along 255 faults with lengths ranging from 0 3 to 60 km faults exhibiting the elliptical or bell shaped slip profiles predicted by simple linear elastic fracture mechanics or elastic plastic theories are rare most slip profiles are roughly linear for more than half of their length with overall slopes always 0 035 for the dominant population of nw striking faults and fault systems longer than 2 km the slip profiles are asymmetric with slip being maximum near the eastern ends of the profiles where it drops abruptly to zero whereas slip decreases roughly linearly and tapers in the direction of overall aden rift propagation at a more detailed level most faults appear to be composed of distinct shorter subfaults or segments whose slip profiles while different from one to the next combine to produce the roughly linear overall slip decrease along the entire fault on a larger scale faults cluster into kinematically coupled systems along which the slip on any scale individual fault or fault system complements that of its neighbors so that the total slip of the whole system is roughly linearly related to its length with an average slope again 0 035 we discuss the origin of these quasilinear asymmetric profiles in terms of initiation points where slip starts and barriers where fault propagation is arrested in the absence of a barrier slip apparently extends with a roughly linear profile tapered in the direction of fault propagation

amount of extension on small faults an example from the viking graben extension estimates based on palinspastic restoration of only the largest faults in a deformed terrane may be significantly deficient due to neglect of small faults i e unsampled faults most of which have smaller displacements than do sampled faults small faults individually account for little extension but there may be large numbers of them the cumulative extension on small faults can be calculated from the fractal size distributions of fault displacements independent extension estimates support the inference from fault population statistics that small faults account for significant amounts of extension for example restorations of mesozoic faults interpreted from seismic reflection profiles across the viking graben in the north sea yield extension estimates that are only 40 75 of independent estimates the fractal size distribution of fault displacements in the viking graben is consistent with the hypothesis that the remaining 25 60 of extension resulted from displacements on small faults

a further note on earthquake size distributions abstract at the scale range of 1 to 10 km faults are not continuous surfaces but are tabular bodies composed of a mesh of subparallel unconnected strands or subfaults the cumulative length distribution of these subfaults is observed to be a power law with an approximate exponent of 2 and an upper fractal limit at w the seismogenic width if it is assumed that small earthquakes with lengths l w represent the rupture of these subfaults this offers a physical explanation for small earthquakes having their observed power law distribution with exponent 2 3 this further implies that any earthquake will be composed of a population of subevents involving the rupture of this population of subfaults precisely what was proposed by frankel 1991 to explain the 2 high frequency falloff of earthquake displacement spectra

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

upper mantle seismic wave velocity effects of realistic partial melt geometries we investigate seismic wave velocity reduction resulting from the presence of partial melt in the upper mantle the amount of shear and bulk modulus reduction produced by the presence of a connected network of realistically shaped and naturally organized melt inclusions is found using finite element calculations the geometries of the inclusions are taken directly from laboratory experiments of mantle melting with finite element meshes constructed to conform to these shapes the shear and bulk moduli of the composite material are found for both the unrelaxed isolated inclusions and relaxed pressure equalized inclusions cases by assigning appropriate material properties to the fluid modulus reduction from deformation simulations of a solid containing realistically shaped and ellipse shaped melt inclusions quantify the effect of melt pocket cuspateness and melt pocket organization on seismic velocity reduction the three dimensional response is estimated from two dimensional distributions of the melt phase by determining the mode ii and mode iii components of elastic modulus reduction separately and summing their effects in general cuspate and naturally organized melt inclusions cause greater velocity reduction it is shown that v and vs reduction per percent partial melt are at least 3 6 and 7 9 respectively even higher values for velocity reduction are possible above 1 melt fraction if melt exists only in tubules below 1 melt fraction the lower more conservative values of velocity reduction are 070 greater for vp and 84 greater for vs than the analytically determined values for ellipsoidal inclusions somewhat greater effects are possible if nonrandom organization of melt occurs on scales greater than our model

the manda inakir rift republic of djibouti a comparison with the asal rift and its geodynamic interpretation abstract asal ghoubbet and manda inakir two axial zones and the mak arrassou n s strike slip zone are located between the afar depression where significant crustal thinning is shown by geophysical data and the relatively thicker continental crust of the danakil horst these three active structures were formed in response to the counterclockwise motion of the danakil horst the two modes of response rift and strike slip faults are related to the orientation of the horst boundaries the boundaries perpendicular to the opening direction lead to extensional zones and rifts asal and manda lnakir the parallel boundaries give strike slip faults mak arrassou the two narrow subsided axial structures rift in rift located on the crustal discontinuity seem to represent a stage where after crustal thinning between 4 and 1 ma when the stratoid series was erupted the breaking point was reached these two axes though not genuine oceanic crust the older continental series are found under the axial series represent a proto oceanic stage and foreshadow the future ridges they also indicate the southwest border of the arabian plate including the danakil horst

secular and tidal strain across the main ethiopian rift using a combination of laser ranging and gps data acquired between 1969 and 1997 we derive a separation velocity for the somali and nubian plates in ethiopia 4 5 1 mm yr at n108 10e this vector is orthogonal to the nne trending neotectonic axis wonji fault belt of the ethiopian rift axis current rifting is concentrated within a 33 km wide zone that includes a 7 km wide belt of late quaternary faulting where maximum surface strain rates are comparable to those at active plate boundaries 0 1 strain yr the strain field suggests that thin 5 km elastic crust separates thick continental lithosphere a geometry quite different from oceanic rifting and a mechanical configuration that favors the amplification of regional strain semidiurnal strain tides however as measured by kinematic gps methods are not amplified along or across the rift indicating that the rift zone s low rigidity applies only at periods of years

effects of water and melt on seismic velocities and their application to characterization of seismic reflectors the effects of a silicate melt and water on seismic velocities are compared at relatively small fluid fraction less than 20 vol on the basis of a theoretical model of composite materials to show that the two fluids are clearly characterized by the velocity ratio vp vs for a silicate melt vp vs increases with increasing fluid fraction when a significant reflection is expected vp vs becomes much larger than 2 for water vp vs decreases as the fluid fraction increases to 10 vol then it increases but it remains similar to a solid state value about 1 8 unless the amount of water exceeds 15 vol this will be a good measure to distinguish between two candidates for seismic reflectors partially molten rocks and rocks containing free water if a reflector can be treated as a thin low velocity layer the velocity ratio in it can be estimated from the frequency dependence of reflection coefficient

propagation of rifting along the arabia somalia plate boundary the gulfs of aden and tadjoura the localization and propagation of rifting between arabia and somalia are investigated by assessing the deformation geometry and kinematics at different scales between the eastern gulf of aden and the gulf of tadjoura using bathymetric magnetic seismological and structural evidence large scale southwestward propagation of the aden ridge markedly oblique to the arabia somalia relative motion vector began about 30 myr ago between the error and sharbithat ridges it was an episodic process with stages of rapid propagation mostly at rates 10 cm yr interrupted by million year pauses on transverse discontinuities coinciding with rheological boundaries between different crustal provinces of the arabia somalia plate the longest pause was at the shukra el sheik discontinuity 45 e where the ridge tip stalled for 13 myr between 17 and 4 ma west of that discontinuity rifting and spreading took place at an azimuth n25 10 e and rate 1 2 0 3 cm yr different from those of the global arabia somalia motion vector n39 1 73 cm yr implying an additional component of movement n65 10 e 0 7 0 2 cm yr due to rotation of the danakil microplate at shukra el sheik the typical oceanic ridge gives way to a narrow wsw trending axial trough resembling a large fissure across a shallow shelf this trough is composed of about eight rift segments which result from normal faulting and fissuring along n110 n130 e trends all the segments step to the left southwestward mostly through oblique transfer zones with en echelon normal faults only two segments show clear significant overlap there is one clear transform the maskali fault between the obock and tadjoura segments the latter segment which encroaches onland is composed of two parallel subrifts iboli ambabbo that propagated northwestward and formed in succession the most recent southwestern subrift ambabbo represents the current tip of the aden ridge we propose a mechanical model in which the large scale propagation of the ridge followed a wsw trending zone of maximum tensile stress while the small scale propagation of its nw trending segments was dictated by the orientation of that stress oblique propagation was a consequence of passive lithospheric necking of the arabia somalia plate along its narrow section in map view between socotra and the kink of the red sea ethiopian rift above the afar plume individual ridge segments oriented roughly perpendicular to plate motion like lithospheric cracks were forced to jump southward because of confinement within the necking zone self sustaining plate scale necking may explain why the aden ridge did not connect with the red sea through bab el mandeb but continued straight into afar

normal faulting during the august 1989 earthquakes in central afar sequential triggering and propagation of rupture along the dobi graben in august 1989 an earthquake sequence including ten events with 6 3 m 5 5 in the first two days produced widespread ground deformation in the dobi graben of central afar numerous surface breaks with complex geometry including fresh scarplets with vertical throws up to 30 cm high and open fissures up to 30 cm wide were observed coseismic slip incremented the deformation normal faulting block tilting and counterclockwise rotation of basaltic slices accumulated in the last 2 m y in the transfer zone between the dobi and hanle grabens by combining maps of surface ruptures relative event relocations with the local djibouti network published focal mechanisms and source sizes we tentatively relate most of the main shocks of the sequence to slip on individual faults the largest shocks at 11h16 on 20 august 1989 ms 6 2 and at 1h09 on 21 august 1989 ms 6 3 ruptured southern segments of the southwestern bounding fault of the graben a dozen other faults also slipped along the edges of and inside the graben on average triggered seismic fault ing propagated about 35 km northwestward along the graben in about 20 hr slip on the main faults was coupled with slip on secondary antithetic faults branching from them at depth although the dobi earthquakes ruptured part of the fault array between the asal rift 1978 sequence and the serdo region 1969 sequence an approximately 50 km long gap subsists along the der ela half graben we infer the patterns of sur face faulting in the dobi sequence which coinvolved bookshelf faulting about both horizontal and vertical axes to typify the complexity of coseismic stress release in central afar and in other active zones of distributed extension e g iceland abruzzi basin and range online material additional photos and descriptions of surface effects of the dobi earthquake sequence

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

modern sedimentation on the shoreface and inner continental shelf at wrightsville beach north carolina u s a abstract the geologic framework and surficial morphology of the shoreface and inner continental shelf off the wrightsville beach north carolina barrier island were mapped using high resolution sidescan sonar bathymetric and seismic reflection surveying techniques a suite of over 200 diver vibracores and extensive seafloor observations by divers the inner shelf is a sediment starved active surface of marine erosion modern sediments where present form a patchy veneer over tertiary and quaternary units the lithology of the underlying units exerts a primary control on the distribution texture and composition of surficial sediments as well as inner shelf bathymetry the shoreface is dominated by a linear cross shore morphology of rippled scour depressions rsds extending from just seaward of the surf zone onto the inner shelf on the upper shoreface the rsds are incised up to 1 m below surrounding areas of fine sand and have an asymmetric cross section that is steeper sided to the north on the inner shelf the rsds have a similar but more subdued cross sectional profile the depressions are floored primarily by shell hash and quartz gravel vibracore data show a thick up to 1 5 m sequence of rsd sediments that unconformably overlies ancient coastal lithosomes in this sediment starved inner shelf setting rippled scour depressions probably form initially on preexisting coarse sediment substrates such as modern lag deposits of paleofluvial channel lithosomes or ancient tidal inlet thalwegs interannual observations of seafloor morphologic change and the longer term record contained in vibracores suggest that the present seafloor morphology is either relatively stable or represents a recurring preferential morphologic state to which the seafloor returns after storm induced perturbations the apparent stability is interpreted to be the result of interactions at several scales that contribute to a repeating self reinforcing pattern of forcing and sedimentary response which ultimately causes the rsds to be maintained as sediment starved bedforms responding to both along shore and across shore flows sediment accumulation from over 30 years of extensive beach nourishment at wrightsville beach appears to have exceeded the local shoreface accommodation space resulting in the leaking of beach and shoreface sediment to the inner shelf a macroscopically identifiable beach nourishment sediment on the shoreface and inner shelf was used to identify the decadal scale pattern of sediment dispersal the nourishment sediment is present in a seaward thinning wedge that extends from the beach over a kilometer onto the inner shelf to waters depths of 14 m this wedge is best developed offshore of the shoreline segment that has received the greatest volume of beach nourishment

three dimensional seismic imaging of a protoridge axis in the main ethiopian rift models of continental breakup remain uncertain because of a lack of knowledge of strain accommodation immediately before breakup our new three dimensional seismic velocity model from the main ethiopian rift clearly images mid crustal intrusions in this active transitional rift setting supporting breakup models based on dike intrusion and magma supply the most striking features of our velocity model are anomalously fast elongate bodies velocity v p 6 5 6 8 km s extending along the rift axis interpreted as cooled mafic intrusions these 20 km wide and 50 km long bodies are separated and laterally offset from one another in a right stepping en echelon pattern approximately mimicking surface segmentation of quaternary volcanic centers our crustal velocity model combined with results from geologic studies indicates that below a depth of 7 km extension is controlled by magmatic intrusion in a ductile middle to lower crust whereas normal faulting and dike intrusion in a narrow zone in the center of the rift valley control extension in the brittle upper crust this zone is inferred to be the protoridge axis for future seafloor spreading

use of a high precision gravity survey to understand the formation of oceanic crust and the role of melt at the southern red sea rift in afar ethiopia the red sea arm of the triple junction in northeastern ethiopia provides an opportunity to investigate rift forming processes at divergent boundaries in an attempt to study the subsurface especially the distribution and role of melt in the rifting process we carried out a high precision gravity survey with a mean square error of 0 011 mgal assisted by differential global positioning system measurements the profile is 162 km long and strikes ene wsw across the southern part of the red sea rift at a latitude of approximately 11 75 n modelling of the bouguer anomaly constrained by a priori information showed detailed in rift variations in the crustal structure and the distribution of melt beneath the rift axis our interpretation suggested that the process of continental break up is governed by crustal stretching and rifting accompanied by the emplacement of melt into the lower crust above a lower density upper mantle in addition we interpreted the thickness of the crust beneath this part of the rift axis to be 25 km the subsurface distribution of density beneath the profile shows that the south central part of the red sea rift has modified thinned crust intruded by high density material which resembles the crust formed during seafloor spreading

high resolution estimates of nubia somalia plate motion since 20 ma from reconstructions of the southwest indian ridge red sea and gulf of aden

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

precambrian crustal evolution seismic constraints from the canadian shield whether or not plate tectonic processes operated on a younger hotter earth remains ambiguous seismic data from new networks in the hudson bay region of the canadian shield where the precambrian geological record spans more than 2 billion years offer fresh scope to address this problem using receiver function analyses we show that the crust of the rae domain which exhibits ages of paleo to neoarchean 3 9 2 7 ga is likely felsic to intermediate in composition average vp vs 1 73 and seismically transparent with a sharp moho there is little evidence for modern style plate tectonics and based on the simplicity and spatial extent of the felsic crust models favouring vertical tectonic processes such as crustal delamination or plume activity appear better suited to the results data from the hearne domain which exhibits widespread 2 7 ga granite and greenstone geology show a more complex crust with higher vp vs ratios consistent with a greater mafic component the trans hudson orogen tho proposed to be a himalayan scale mountain belt during the paleoproterozoic is thought to have formed during the 1 8 ga collision of the superior and churchill plates results from the quebec baffin island segment of the tho appear to map out the first order shape of the underthrusting superior plate with elevated vp vs ratios likely representing the rifted margin of the superior craton consistently thicker crust is observed beneath central and southern baffin island 43 km coincident with widespread high grade metamorphic surface geology these features can be explained by crustal thickening due to stacking of accreted terranes during continent continent collision analogous to the present day tibetan plateau followed by erosion when reviewed in light of age and compositional constraints from the geological record our seismic observations point towards secular crustal evolution from non plate tectonic during the paleo to mesoarchean evolving towards fully developed modern style plate tectonics during the paleoproterozoic

sorted bed forms as self organized patterns 2 complex forcing scenarios 1 we employ a numerical model to study the development of sorted bed forms under a variety of hydrodynamic and sedimentary conditions results indicate that increased variability in wave height decreases the growth rate of the features and can potentially give rise to complicated a priori unpredictable behavior this happens because the system responds to a change in wave characteristics by attempting to self organize into a patterned seabed of different geometry and spacing the new wavelength might not have enough time to emerge before a new change in wave characteristics occurs leading to less regular seabed configurations the new seabed configuration is also highly dependent on the preexisting morphology which further limits the possibility of predicting future behavior for the same reasons variability in the mean current magnitude and direction slows down the growth of features and causes patterns to develop that differ from classical sorted bed forms spatial variability in grain size distribution and different types of net sediment aggradation degradation can also result in the development of sorted bed forms characterized by a less regular shape numerical simulations qualitatively agree with observed geometry spacing and height of sorted bed forms also in agreement with observations is that at shallower depths sorted bed forms are more likely to be affected by changes in the forcing conditions which might also explain why in shallow waters sorted bed forms are described as ephemeral features finally simulations indicate that the different sorted bed form shapes and patterns observed in the field might not necessarily be related to diverse physical mechanisms instead variations in sorted bed form characteristics may result from variations in local hydrodynamic and or sedimentary conditions

p wave attenuation measurements from laboratory resonance and sonic waveform data we computed compressional wave velocity vp and attenuation 1 qp from sonic log waveforms recorded in a cored 30 m thick dolostone reservoir using cores from the same reservoir laboratory measurements of vp and qp were also obtained we used a resonant bar technique to measure extensional and shear wave velocities and attenuations in the laboratory so that the same frequency range as used in sonic logging 5 25 khz was studied having the same frequency range avoids frequency dependent differences between the laboratory and in situ measurements compressional wave attenuations at 0 mpa confining pressure calculated on 30 samples gave average qp values of 17 experimental and geometrical errors were estimated to be about 5 percent measurements at elevated effective pressures up to 30 mpa on selected dolostone samples in a homogeneous interval showed mean qp and qs to be approximately equal and consistently greater than 125 at effective stress of 20 mpa and at room temperature the mean qp over

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

a mantle magma reservoir beneath an incipient mid ocean ridge in afar ethiopia the buoyancy of magma should cause it to rise into the crust preventing it from ponding in the uppermost mantle magnetotelluric data from the dabbahu rift segment ethiopia identify a magma reservoir that extends well into the mantle beneath the rift and is so large that it should persist for thousands of years

ray path interpretation of the crustal structure beneath saudi arabia abstract interpretation of a long range seismic refraction line in saudi arabia has shown that beneath the arabian shield velocity generally increases with depth from about 6 km s 1 at the surface to about 7 km s 1 at the top of the crust mantle transition zone the base of this transition zone moho occurs at 37 44 km in depth intracrustal discontinuities can also be recognized the most important being in the 10 20 km depth range and separating the upper from the lower crust laterally the variations in the intracrustal discontinuities and the total crustal thickness can be correlated with previously defined tectonic regions beneath the red sea shelf and coastal plain the crust including 4 km of sediments is only 15 17 5 km thick with the aid of both seismic and gravity data an abrupt steeply dipping transition from the crust of the red sea shelf and coastal plain to that of the arabian shield has been derived with a jump of more than 20 km in moho depth this appears to be the major discontinuity between the red sea depression and the arabian continental shield

formation of ripple bands in a wave convergence zone abstract sidescan sonographs of the whangarei harbor ebb tidal delta in northeastern new zealand revealed distinct bands of ripples of short wavelength lying in bands on otherwise undistinguished flat sand the ripple bands occurred in a confined and patchworked region across the delta extending some 4 km offshore and 1 0 km wide combined surface gravity wave refraction studies numerical hydrodynamic modeling and field measurements indicated that both the ripples and the bands were wave induced the ripples were found to have symmetrical profiles and wavelengths increasing shorewards both indicative of a wave generation mechanism the bands were aligned with the dominant surface wave orientation and typical bed orbital velocities exceeded the sediment threshold speed while tidal currents were perpendicular to the crests and velocities were below the sediment threshold the wave refraction study showed that the ebb tidal delta bathymetry concentrates surface gravity wave energy along a distended zone across the delta and this distinctive band of wave height reinforcement corresponded with the location of the bands of ripples numerical hydrodynamic modeling demonstrated that the shoaling waves induce a net shoreward circulation capable of transporting winnowed fines away from the ripple bands the regional wave height reinforcement and the associated shoreward current can explain the zonal nature of the bands the measured coarseness of the ripple sediments compared with neighboring sites and the presence of the bands in topographic lows

shear wave attenuation and dispersion in melt bearing olivine polycrystals 2 microstructural interpretation and seismological implications 1 the torsional forced oscillation tests of melt bearing olivine aggregates reported by jackson et al 2004 consistently show a peak in attenuation that is absent from melt free aggregates tested under similar conditions and grain sizes characterization by sem shows that the melt resides in triple junction tubules and larger pockets as previously described tem imaging and eds analysis reveals that olivine olivine grain boundaries are characterized by a region 1 nm wide which is structurally and chemically distinct from olivine grain interiors from the possible mechanisms that can produce an anelastic attenuation peak melt squirt can be eliminated for our samples and experimental conditions we attribute the observed attenuation peak to elastically accommodated grain boundary sliding requiring that the grain boundaries are weak relative to olivine grain interiors but have a significantly higher viscosity than bulk melt while the nanometer scale grain boundary structure in the melt bearing aggregates is essentially the same as for melt free aggregates studied previously elastically accommodated sliding in the latter is apparently inhibited by tight three grain edge intersections the exponentially increasing high temperature background attenuation in both types of aggregate is attributed to diffusionally accommodated grain boundary sliding extrapolation to mantle grain sizes shows that the broad peak may be responsible for nearly frequency independent attenuation in partially molten regions of the upper mantle

faulting and block rotation in the afar triangle east africa the danakil crank arm model several domains of contrasted extensional deformation have been identified in the southern afar triangle east africa from fault patterns analyzed with panchromatic stereoscopic spot systeme probatoire d9observation de la terre images stretching directions and statistical orientation and offset variations of faults fit with the danakii crank arm model of sichler a 10 sinistral rotation of the danakil block explains the fault geometry and dextral block rotation in the southern part of the afar triangle as well as the oblique extension in the tadjoura gulf analogue modeling supports this interpretation

wave mechanisms responsible for grain sorting and non uniform ripple distribution across two moderate energy sandy continental shelves abstract mechanisms responsible for observed ripple geometry and grain size trends at two open coast sites are considered the field sites are two moderate energy sandy shelves on the inner continental shelf out to 50 m depth at east gippsland in southeastern australia and at pakiri in northeast new zealand both are characterised by bands of medium sediments inshore and offshore separated at 20 45 m depth by a zone with significantly coarser grain size 0 9 mm cf 0 3 mm video observations of the seabed at 126 sites on the east gippsland shelf revealed that the band was also characterised by larger wavelength ripples 1 0 m cf 0 3 m the size of the band stretching over an average of 20 km from 20 45 m depth and its presence at both field sites suggests that the observed characteristics are a long term response to physical forcing possibly in near equilibrium with modern processes numerical modelling demonstrated that grain sorting on the shelf is initiated by the presence of maxima in predicted ripple activity and seabed roughness around 35 m depth the local grain size evidently increases due to winnowing of the fines positive feedback through dependence of ripple size on grain size causes further lengthening of ripple wavelength until the pattern has evolved to become highly pronounced on the shelf and self sustaining but limited by reducing sediment mobility as grain size increases

new age constraints on the timing of volcanism and tectonism in the northern main ethiopian rift southern afar transition zone ethiopia abstract forty new k ar and 40 ar 39 ar isotopic ages from the northern main ethiopian rift mer southern afar transition zone provide insights into the volcano tectonic evolution of this portion of the east african rift system the earliest evidence of volcanic activity in this region is manifest as 24 23 ma pre rift flood basalts transition zone flood basalt activity renewed at approximately 10 ma and preceded the initiation of modern rift margin development bimodal basalt rhyolite volcanism in the southern afar rift floor began at approximately 7 ma and continued into recent times in contrast post subsidence volcanic activity in the northern mer is dominated by mio pliocene silicic products from centers now covered by quaternary volcanic and sedimentary lithologies unlike other parts of the mer mio pliocene silicic volcanism in the mer afar transition zone is closely associated with fissural basaltic products the presence of pliocene age ignimbrites on the plateaus bounding the northern mer whose sources are found in the present rift indicates that subsidence of this region was gradual and that it attained its present physiography with steep escarpments only in the plio pleistocene large 7 5 ma silicic centers along the southern afar and northeastern mer margins apparently formed along an e w oriented regional structural feature parallel to the already established southern escarpment of the afar the addis ababa rift embayment and the growth of 4 5 3 ma silicic centers in the addis ababa area are attributed to the formation of a major cross rift structure and its intersection with the same regional e w structural trend this study illustrates the episodic nature of rift development and volcanic activity in the mer afar transition zone and the link between this activity and regional structural and tectonic features

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

correction to transient rift opening in response to multiple dike injections in the manda hararo rift afar ethiopia imaged by time dependent elastic inversion of interferometric synthetic aperture radar data 1 interferometric synthetic aperture radar insar data spanning the time intervals separating thirteen dike intrusions in the manda hararo dabbahu rift afar ethiopia from 2005 to 2009 show that transient deformation occurs in the inter diking period this deformation can be explained by the presence of seven inflating or deflating pressure sources by combining the data acquired on four different insar tracks through time dependent elastic models we are able to track these deformation modes with a time resolution smaller than 1 month sustained deflation of a deep magma reservoir at dabbahu in the 6 months following the main rifting event of 2005 and slow decelerating post eruptive re inflation of two shallow magma reservoirs below dabbahu and gabho volcanoes are monitored a deflation signal of deep origin on the neighboring rift system is also detected possibly caused by outflow of material from a preexisting reservoir into the deep plate boundary in contrast rapidly evolving deformation is observed at the center of the manda hararo rift segment transient deformation events are monitored in the weeks months following the diking events with pulses of localized rift opening after the dike intrusions followed by an exponential like decay of opening rate this signal may be associated with the replenishment of the central magma reservoir involved in feeding the 2005 2009 dikes alternatively the predominantly horizontal mode of deformation suggests an interaction between the response of the lithosphere to tectonic strain accumulation and the process of hydraulic connectivity within the central magma plumbing system

size distributions for large and small earthquakes small and large earthquakes obey different scaling laws and inhabit distinct populations both characterized by power laws with different exponents when considering earthquakes on a single fault or fault segment the one large earthquake that occurs during a seismic cycle is discontinuous with the population of small earthquakes here it is shown that these two observations together with the scaling laws are self consistent when proper account is taken of the size distribution of faults

p and s wave delays beneath intraplate volcanic ridges and gravity lineations near the east pacific rise the gravity lineations intraplate melting petrology and seismic expedition glimpse explored the origin of two intraplate non hot spot ridge systems associated with gravity lineations in the south central pacific using the 11 month ocean bottom seismometer obs deployment of the glimpse experiment we determined the average p and s station delays for the region across the array station delays varied by 1 22 s for p waves and 2 13 s for s waves the presence of ultraslow shear velocity

contrasting structural styles of the eastern and western margins of the southern red sea the 1988 sonne experiment abstract seismic investigations in the sudanese and yemeni sections of the red sea showed contrasting structural styles across the two surveyed areas of the opposing flanks off sudan seaward of an about 50 km broad zone of stretched continental crust different domains of oceanic crust were identified the oldest of these domains was found under an up to 6 km thick cover of sediments which include a 1 to 2 km thick pre evaporite sequence these sediments are interpreted to be part of a pull apart basin generated during the initial stages of red sea evolution towards the red sea axial trough a structural boundary separates this from younger oceanic crust which is covered only by post evaporite sediments this sequence thins progressively from about 800 m at the boundary to zero at the axial trough off yemen in contrast oceanic crust is confined to the axial trough zone which terminates southward of 16 n further south to 14 n no oceanic crust has yet been formed stretched continental crust was found across the entire flank of the southern red sea and at the tihama coastal plain further inland beginning at the escarpment the crust thickens in several major steps to a thickness of 35 km in central yemen

crustal thinning in relationship to the evolution of the afro arabian rift system a review of seismic refraction data abstract crustal and upper mantle structure of major units of the afro arabian rift system has been progressively investigated by seismic refraction surveys in the jordan dead sea rift the red sea the afar depression and in the east african rift of kenya the two extreme structures are oceanic crust flooring the axial trough of the southern red sea rift and 30 35 km thick continental crust underneath the jordan dead sea rift and its flanks underlain by a mantle with pn velocities of 8 0 8 2 km s the remainder of the units have a crustal structure of transitional to uncertain nature in the southeastern red sea the change from 15 18 km thick transitional crust to the 40 km thick continental crust of the arabian shield is probably very abrupt and is the major lateral change in structure in this region in the northern afar depression the crust is only 15 20 km thick in the southern afar depression anomalous mantle with 7 2 7 3 km s velocities occurs at depths greater than 25 km also here an abrupt change to a 40 km thick shield crust with a velocity of 8 0 km s is evident from seismic and gravity data crustal thickness of the east african rift system amounts to 30 36 km a recent trial experiment in the eastern rift of kenya indicates a 35 36 km thick crust underlain by a low velocity upper mantle with 7 5 km s at 23 24 km depth a thin high velocity lens of 7 1 km s velocity may be present within the 22 km thick lower crustal layer averaging 6 6 km s velocity investigations of local earthquake body waves in the western rift provide information that the crust here is of continental type with a thickness of about 32 km overlying mantle material of 8 0 km s p velocity and 4 6 km s s velocity in conclusion a model of preferential rifting of continents is applied to the afro arabian rift system where rifting seems to progress into areas with thick continental crust as seen for example in the levant and the east african rift systems instead of using paths in thin crust

fault strain and seismic coupling on mid ocean ridges the contribution of extensional faulting to seafloor spreading along the east pacific rise epr axis near 3 s and between 13 n and 15 n is calculated using data on the displacement and length distributions of faults obtained from side scan sonar and bathymetric data it is found that faulting may account for of the order of 5 10 of the total spreading rate which is comparable to a previous estimate from the epr near 19 s given the paucity of normal faulting earthquakes on the epr axis a maximum estimate of the seismic moment release shows that seismicity can account for only 1 of the strain due to faulting this result leads us to conclude that most of the slip on active faults must be occurring by stable sliding laboratory observations of the stability of frictional sliding show that increasing normal stress promotes unstable sliding while increasing temperature promotes stable sliding by applying a simple frictional model to mid ocean ridge faults it is shown that at fast spreading ridges 90 mm yr the seismic portion of a fault ws is a small proportion of the total downdip width of the fault w the ratio ws w interpreted as the seismic coupling coefficient x and in this case x 0 in contrast at slow spreading rates 40 mm yr ws w and therefore x 1 which is consistent with the occurrence of large magnitude earthquakes mb 5 0 to 6 0 occurring for example along the mid atlantic ridge axis

seismicity during lateral dike propagation insights from new data in the recent manda hararo dabbahu rifting episode afar ethiopia seismicity released during lateral dike intrusions in the manda hararo dabbahu rift afar ethiopia provides indirect insight into the distribution and evolution of tensile stress along this magma assisted divergent plate boundary in this paper 5 dike intrusions among the 14 that form the 2005 present rifting episode are analyzed with local and regional seismic data during dike intrusions seismicity migrates over distances of 10 15 km at velocities of 0 5 3 0 km h away from a single reservoir in the center of the rift segment confirming the analogy with a slow spreading mid ocean ridge segment comparison with geodetic data shows that the reservoir is located 7 km down rift from the topographic summit of the axial depression dikes emplaced toward the north are observed to migrate faster and to be more voluminous than those migrating southward suggesting an asymmetry of tension in the brittle elastic lithosphere seismicity during dike injections is concentrated near the propagating crack front in contrast faults and fissures in the subsurface appear to slip or open aseismically coeval with the intrusions the seismic energy released during dike intrusions in the manda hararo rift appears to be primarily modulated by the local magnitude of differential tensile stress and marginally by the rate of stress change induced by the intrusion the low level of seismic energy accompanying dike intrusions despite their significant volumes is likely an indicator of an overall low level of tension in the lithosphere of this nascent plate boundary

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