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

self similarity of the largest scale segmentation of the faults implications for earthquake behavior article i nfo article history earthquakes are sensitive to the along strike segmentation of the faults they break especially in their initiation propagation and arrest we examine that segmentation and search whether it shows any specific properties we focus on the largest scale fault segmentation which controls the largest earthquakes it is well established that major segments within faults markedly shape their surface cumulative slip length profiles segments appear as large slip bumps separated by narrow pronounced slip troughs inter segments we use that property to examine the distribution location number length of the major segments in 927 active normal faults in afar east africa of various lengths 0 3 65 km cumulative slips 1 1300 m slip rates 0 5 5 mm yr and ages 10 4 10 6 yr this is the largest fault population ever analyzed to identify the major bumps in the slip profiles and determine their number location and length we analyze the profiles using both the classical fourier transform and a space frequency representation of the profiles the s transform which is well adapted for characterizing local spectral properties our work reveals the fol lowing results irrespective of their length 70 of the slip profiles have a triangular envelope shape in conflict with the elastic crack concept irrespective of their length the majority of the faults at least 50 70 have a limited number of major segments between 2 and 5 and more commonly equal to 3 5 the largest scale segmentation of the faults is thus self similar and likely to be controlled by the fault mechanics the slip deficits at the major inter segment slip troughs tend to smooth as the faults accumulate more slip resulting in increased connection of the major segments the faults having accumulated more slip therefore generally appear as un segmented 10 30 our observations therefore show that whatever the fault on which they initiate large earthquakes face the same number of major segments to potentially break the number of segments that they eventually break seems to depend on the slip history structural maturity of the fault

mesozoic volcanism in the middle east geochemical isotopic and petrogenetic evolution of extension related alkali basalts from central lebanon mesozoic picritic and alkali basalts from central lebanon represent a significant part of an extension related upper jurassic to upper cretaceous discontinuous volcanic belt which occurs throughout the middle east volcanism was associated with an episode of intraplate extension that followed a period of continental break up where mesozoic micro continental blocks separated from gondwana as the neotethys ocean opened in jurassic times this volcanic episode produced mafic lava flows ranging in thickness from 5 to 20 m along with some minor pyroclastic flows these flows are stratigraphically intercalated with thick carbonate platform deposits the basalts are made up of about 15 20 olivine fo 78 91 30 35 clinopyroxene salite 40 50 plagioclase an 56 71 and opaque fe ti oxides 5 geochemically the rocks exhibit a relatively wide range of sio 2 40 4 to 50 5 wt and mgo 5 1 to 15 5 wt contents are relatively enriched in tio 2 1 7 to 3 7 wt and vary in composition from alkali picrite and basanite to alkali basalt the mg numbers range from 0 56 to 0 70 with an average of 0 63 the rocks are enriched in incompatible trace elements such as zr 86 247 ppm nb 16 66 ppm and y 19 30 ppm such compositions are typical of those of himu oib and plume related magmas the ree patterns are fractionated la yb n 11 lree enriched and are generally parallel to subparallel elemental ratios such as k p 1 1 4 7 la ta 11 13 la nb 0 57 0 70 nb y 0 68 1 55 and th nb 0 20 0 36 suggest that crustal contamination was minor or absent this may be related to a rapid ascent of the magma in agreement with the nature mafic oceanic like and the small thickness about 12 km of the mesozoic crust of the eastern mediterranean region the 143 nd 144 nd isotopic compositions of the lavas range from 0 512826 to 0 512886 and 87 sr 86 sr from 0 702971 to 0 703669 suggesting a himu like mantle source trace element compositions indicate a melt segregation depth of 100 110 km well within the garnet lherzolite stability field the geochemical characteristics of the rocks are typical of within plate alkali basalts and suggest that the magmas were derived from a fertile possibly plume related enriched mantle source petrogenetic modelling indicates that the magmas were produced by very small degrees of batch partial melting f 1 5 of a primitive garnet bearing mantle source garnet lherzolite

permian early mesozoic tectonism and continental margin formation in israel and its implications for the history of the eastern mediterranean summary the early mesozoic tectonic reorganization of the tethys region also affected the levant area here two processes were superimposed a the pattern of long wavelength vertical motions of the arabo nubian platform changed in the late permian when israel and nearby areas began to subside b rifting occurred in this subsiding area probably in several phases late anisian and ladinian carnian norian and liassic differential movements reached 2 3 km and magmatism occurred in the liassic and perhaps also in the triassic the tectonism was strongly felt up to 50 km landward of the present coast the crust was thinned and modified under the present continental margin and in the main rifts passive margin conditions were established in late liassic times over the previously faulted area later in the jurassic a carbonate shelf was constructed along this subsiding margin its basinward edge was 1 1 5 km high which shows that the adjacent se mediterranean was already a deep sea hence this basin and its passive margin are considered to have been shaped by the early mesozoic rifting which is recognized in israel the rifting process probably signifies oblique separation of the tauride block from the levant part of gondwana

petroleum prospects of lebanon some remarks from sedimentological and diagenetic studies of jurassic carbonates abstract only seven exploration wells have been drilled in lebanon to date failing to encounter oil or gas in commercial quantities and to penetrate into unexposed pre jurassic reservoirs this paper shows that the sedimentological and diagenetic characteristics of the major lebanese jurassic carbonate rock unit the kesrouane formation thickness 1000 m negate jurassic hydrocarbon prospectivity in northern mount lebanon the jurassic platform in this region was subjected to a first emergence and invasion by meteoric waters in the late jurassic causing a pervasive cementation a second phase of uplift and meteoric flushing occurred in the cenozoic thus only the areas located far from the jurassic and post cretaceous phreatic realms e g offshore tripoli nw lebanon may have been less affected by meteoric diagenesis and still provide promising prospects a 1993 seismic survey offshore tripoli confirmed the presence of late triassic evaporites in northern lebanon this also adds confidence to the results of the nahr ibrahim electrical survey which earlier suggested the presence of these evaporites in the subsurface of central lebanon the pre jurassic sequence hence is sealed and protected from the meteoric invasion observed in the overlying jurassic rocks accordingly the qartaba horst structure central northern lebanon would be equally if not more attractive for a pre jurassic play

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

the paradox of drowned reefs and carbonate platforms shallow water carbonate platforms and reefs are drowned when tectonic subsidence or rising sea level outpaces carbonate accumulation and benthonic carbonate production ceases drowned platforms are common in the geologic record but they present a paradox if one considers rates of processes involved during the early holocene when sea level rose at rates of 6 000 to 10 000 m yr mm 1 000 yr most reefs and platforms were outpaced by the rising sea during the late holocene with sea level rising 500 to 3 000 m yr in the atlantic caribbean area reefs and platforms started to recover built to sea level and prograded seaward 1 000 m yr is thus a conservative estimate of the average growth potential of modern reefs and platforms independently accumulation rates of prograding platforms in the geologic record suggest growth potential in excess of several hundred microns per year the growth potential of 1 000 m yr exceeds any relative rise of sea level caused by long term processes in the geologic record newly formed ocean crust subsides at a maximum of 250 m yr basin subsidence averages 10 to 100 m yr and sea level rises due to increased sea floor spreading amount to less than 10 m yr rapid pulses of relative rise of sea level or reduction of benthic growth by deterioration of the environment remain the only plausible explanations of drowning the geologic record shows examples of both of these processes global mass extinctions of reefs and platforms occurred in the middle cretaceous eustatic rise due to submarine volcanism or desiccation of a small ocean basin and the late devonian global crisis of ocean environment extraterrestrial cause drowning controlled by regional tectonics prevailed in the jurassic and early cretaceous of the tethyan realm and the drowning of mesozoic platforms in the western north atlantic seems to have been dictated by plate tectonic drift to higher latitudes

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

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

numerical modelling of quaternary deformation and post rifting displacement in the asal ghoubbet rift djibouti africa over the last three decades a host of information on rifting process relating to the geological and thermal structure long time scale deformation quaternary and holocene and rifting cycle displacement across the asal ghoubbet rift has been made available these data are interpreted with a two dimensional thermo mechanical model that incorporates rheological layering of the lithosphere dyke inflation and faulting active fault locations and geometry are mainly controlled by both thermal structure and magma intrusion into the crust the distributed slip throughout the inner rift is related to the closeness of magma chamber leading to additional stress into the upper thinned crust assuming a constant arabia somalia motion of 11 mm year the variation of subsidence rate between the last 100 and 9 ka is associated with a decrease of the average injection rate from 10 to 5 mm year these values about equal to the regional opening rate suggest that both volcanism and tectonic play an equivalent role in the rifting process our modelled sequence of events gives one possible explanation for both vertical and horizontal displacements observed since the 1978 seismovolcanic crisis although part of the post rifting deformation could be due to viscous relaxation the high opening rate in the first years after the event and the abrupt velocity change in 1984 1986 argue for a large dyke inflation of 12 cm year ending in 1985 the asymmetric and constant pattern of the gps velocity since 1991 suggests that present post rifting deformation is mainly controlled by fault creep and regional stretching this study demonstrates the internal consistency of the data set highlights the role of magmatism in the mechanics of crustal stretching and reveals a complex post rifting process including magma injection fault creep and regional stretching d 2005 elsevier b v all rights reserved

bookshelf faulting and horizontal block rotations between overlapping rifts in southern afar lateral slip on initially rift parallel normal faults may be a particularly efficient mechanism to accomodate strain between overlapping oceanic rifts it occurs in southern afar where clockwise block rotations result from distributed dextral shear between the overlapping ghoubbet asal manda inakir and manda hararo abhe bad rifts faulting observed during the 1969 serdo earthquakes and on spot images is consistent with the shear being taken up by left lateral slip on steep nw se striking faults which formed as normal faults before extensional strain became localized in the two rifts this bookshelf faulting accounts quantitatively for the 14 5 7 5 rotation documented by paleomagnetism in the 1 8 0 4 ma old afar stratoid basalts given the 17 5 5 mm yr rate of separation between arabia and somalia

tectono stratigraphic evolution of the western margin of the levant basin offshore cyprus abstract stratigraphic interpretation of twenty four 2d seismic reflection profiles coupled with available well data from the ocean drilling program odp offshore cyprus enabled identification of the main unconformities and the seismic packages in the western part of the levant basin and their correlation with major geodynamic events the basic concepts of seismic stratigraphy were applied to improve our understanding of the tectonostratigraphy and the sedimentary architecture of eratosthenes isolated carbonate platform this platform is composed of dominant retrograding and aggrading mesozoic carbonate build ups and its evolution partly resembles that of the egyptian margin deep water mixed carbonates and siliciclastics probably prevailed in the basin during the early cretaceous during the late cretaceous these platforms were drowned following the convergence of the two plates both the eratosthenes seamount esm area and the levant basin progressively became part of a foreland basin along the cyprus arc different units in the eratosthenes carbonate platforms ecps and the transition to the deep levant basin are detailed here for the first time we present a 3d conceptual model for the evolution of the western margin of the levant basin we discuss the effect of the miocene collision between the cyprus arc and eratosthenes to understand the presence of upper miocene mass transport complexes mtcs in the deep basin and the tilting of the top of the adjacent ecps we briefly examine the deformation mechanisms of the messinian salt near the paleo shelves of the ecps

fault propagation and climatic control of sedimentation on the ghoubbet rift floor insights from the tadjouraden cruise in the western gulf of aden summary a detailed geophysical survey of the ghoubbet al kharab djibouti clarifies the small scale morphology of the last submerged rift segment of the propagating aden ridge before it enters the afar depression the bathymetry reveals a system of antithetic normal faults striking n130 e roughly aligned with those active along the asal rift the 3 5 khz sub bottom profiler shows how the faults cut distinct layers within the recent up to 60 m thick sediment cover on the floor of the basin a large volcanic structure in the centre of the basin the ghoubbet volcano separates two sedimentary flats the organization of volcanism and the planform of faulting with en echelon subrifts along the entire asal ghoubbet rift appear to confirm the westward propagation of this segment of the plate boundary faults throughout the rift have been active continuously for the last 8400 yr but certain sediment layers show different offsets the varying offsets of these layers dated from cores previously retrieved in the southern basin imply holocene vertical slip rates of 0 3 1 4 mm yr 1 and indicate a major decrease in sedimentation rate after about 6000 yr bp and a redistribution of sediments in the deepest troughs during the period that preceded that change

constrains on the origin and history of the eastern mediterranean basin abstract the basin formed along the northern passive margin of gondwanaland as a relict of the mesozoic neotethys in early mesozoic time though direct information from the basin itself is meagre the geology of its passive margins and the regional framework of the tethys evolution constrain the formation and history of the basin the eastern mediterranean and its passive margins are interpreted to have formed as a result of several faulting and continental breakup phases from early permian to middle jurassic times before pangaea s complete disintegration this led to the detachment of the eratosthenes and tauride blocks from the margins of the levant and herodotus basins respectively the latter merged in the northeast with the cyprus basin that extended along arabia s northern margin the main rifting probably occurred in the triassic leading to seafloor spreading in the herodotus and cyprus basins while along the levant margin important rifting also occurred in the jurassic during the initial rifting stages extension perpendicular to the levant margin was important but subsequently the tauride block drifted mainly northward the existence of the passive margins and slopes of the eastern mediterranean is documented at least from the end of the middle jurassic until then the basin was probably narrow 200 300 km but during the main phase of the africa laurasia separation it grew to twice its present width or more this scenario depends critically on the interpretation that the tauride block was originally located next to northeastern africa on the western side of the eratosthenes block plate convergence and consumption of the northern portion of the basin have taken place since mid cretaceous times but the active convergent boundary on its northern side formed only in the neogene by modification of earlier structures some intra plate deformation also took place especially in the neogene but it hardly changed the original configuration of the remaining portion of the basin

tectono stratigraphic evolution of the northern levant basin offshore lebanon abstract seismic interpretation constrained by a detailed assessment of the levant paleogeography allowed subdividing the sedimentary infill of the northern levant basin offshore lebanon in eight major seismic packages fifteen seismic facies have been identified with distinctive characteristics the levant basin architecture is pre determined by a late paleozoic early mesozoic rift that led to the formation of a passive margin dominant aggrading carbonate platforms are observed along the levant margin and deepwater mixed settings i e carbonates and siliciclastics are suggested to prevail in the basin the collision of afro arabia with eurasia led to the development of a flexural basin in the northernmost offshore lebanon since the late cretaceous a southward migration of this flexural depocenter in the miocene is hindered by the change in the stress field along the latakia ridge and by the westward escape of the anatolian plate in late miocene and pliocene times interplay between major geodynamic events as well as sea level fluctuations in the mesozoic and cenozoic induced important marginal uplifts and emersion sediments sourced from the erosion of nubian siliciclastic material and from the exposed granitic red sea rift shoulders and arabian shield were driven into the levant basin the sediment sources diversity the mechanisms of sediment transport through varied pathways i e the levant margin canyons the latakia region and the nile delta deep sea cone are expected to strongly impact the reservoir characteristics and prospectivity of the northern levant basin

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

some outstanding issues in the geology of lebanon and their importance in the tectonic evolution of the levantine region abstract the concept of pervasive and disruptive late neogene quaternary strike slip tectonics in lebanon has made establishing a satisfactory pre neogene palaeogeographic and palaeotectonic history for the area difficult research has allowed clarification of three fundamental issues that permit the setting of lebanon against its adjacent regions to be more firmly assigned the first issue is the identification of the significance of the 100 km long nne ssw trending monocline of the western lebanon flexure this feature is interpreted as a modified and rejuvenated early mesozoic hinge line that is presumed to be the northern continuation of the carmel sinai hinge line to the south the second issue is the correlation of the mount lebanon bekaa and anti lebanon complex with the adjacent intracontinental palmyride fold belt of syria through recognition in each of common divisions in structural style between a higher northern zone dominated by broadly spaced box folds and a lower southern zone with more numerous closely spaced and more internally complex folds the third issue involves the identification of major two stage late cretaceous to palaeogene syrian arc tectonism within lebanon resolution of these issues suggests a tectonic model for the region whereby the levantine region formed in two main phases the first of these was the multiphase late palaeozoic early mesozoic rifting of the area which imposed a fundamental extensional fabric over much of the region the second tectonic phase was the subsequent inversion of these extensional structures in the two stage syrian arc deformation of senonian and late palaeogene age which gave rise to the main lebanon structures it is this syrian arc tectonism rather than the late cenozoic strike slip that was probably the main structuring agent which affected the lebanon area the transpression from the braided dead sea fault motion although locally complex is an essentially neotectonic overprint on an already existing fabric offset of syrian arc structures in lebanon suggests that the total sinistral motion of the yammouneh fault segment of the dead sea fault is around 47 km

reflux stratabound dolostone and hydrothermal volcanism associated dolostone a two stage dolomitization model jurassic lebanon the kesrouane formation which is characterized by pervasive dolomitization has a stratigraphic thickness that exceeds 1000 m it is part of a broad carbonate platform deposited in the levant region and represents 60 of the lebanese jurassic rocks two genetically distinct dolostones are recognized within this unit 1 fine to medium crystalline non planar grey dolostone and 2 coarse crystalline planar beige dolostone the former is stratabound and of early jurassic age 87 sr 86 sr 0ae707455 this dolostone locally exhibits pseudomorphs of evaporite nodules pointing towards seepage reflux dolomitization by hypersaline to marine related fluids exposures of the coarse crystalline dolostone are associated with regional pre cretaceous faults along which late jurassic volcanics also occur sedimentological and diagenetic considerations coupled with microthermometry support a hydrothermal origin for this dolostone with th values of primary inclusions between 50 and 80 c the related dolomitizing fluids are mesosaline 3ae5 12ae0 eq wt nacl and are believed to result from the mixing of evaporative brines and sea water dolomitization is thus believed to have occurred in two stages whereby fluids invaded the host rocks first by seepage reflux explaining the resulting early jurassic stratabound dolostone and later through fracture flow along the faults associated with the late jurassic volcanism explaining the coarse crystalline hydrothemal dolostone

contemporary holocene and quaternary deformation of the asal rift djibouti implications for the mechanics of slow spreading ridges because the frequency and character of rifting events along mid ocean ridges are largely unknown how the repetition of such events gives rise to rift structures is unexplored the asal rift in the afar depression of djibouti africa provides the world s best subaerial analogue for young slow spreading mid ocean ridges seismic geodetic and field observations of a seismovolcanic event in 1978 at asal yield estimates of the fault and dike locations geometry displacement and volume of basalt extruded in a rifting event a 6 9 kyr old lake shore highstand at asal has been warped downward by 70 m providing a holocene measure of the vertical deformation across the rift the rift topography furnishes an older datum which we infer to be 34 6 kyr old using the holocene deformation rate we find that faults throughout the rift valley are active holocene slip rates diminish beyond 4 km from the rift axis late quaternary rates decrease beyond 6 7 km the holocene slip rates are used to estimate repeat times by taking the displacement on the faults which slipped in 1978 as characteristic we find tectonic events on individual faults recur every 200 300 years half the rift faults slipped together in the 1978 event if this is typical then groups of faults are activated every 100 150 years we suggest that half the events take place in the rift axis accompanied by volcanic extrusion the remainder occur peripheral to the neovolcanic zone and involve fault slip only both events having a repeat time of 200 300 years given the 10 km width of the rift and its 16 mm yr 1 spreading rate the mean age of the material in the rift should be 350 kyr an order of magnitude older than the inferred age of the formation of the rift topography the subsidence rate of the rift axis during the past 35 kyr is 8 9 mm yr 1 with the rate of infilling by volcanic extrusion 1 mm yr 1 the resulting net subsidence rate about equal to the half spreading rate of the rift could not be sustained for 300 kyr without significant infilling by lavas thus both observations suggest that the long term vertical deformation in the rift has not been steady state instead we suggest that there is a rifting filling cycle at asal with the most recent filling episode ending 35 kyr

propagation of an oblique spreading centre the western gulf of aden abstract the east west trending gulf of aden rift between arabia and somalia is spreading along a n35 e direction it propagates westward inside the african continent toward the afar triple junction oblique to the spreading direction these combined propagation and oblique rifting generate a special faulting pattern associated with an irregular distribution of volcanism we analyse this pattern using a structural map derived from the analysis of multibeam bathymetric data and backscattering images from the 1995 tadjouraden cruise in the western gulf of aden the rift valley is bounded by faults trending from n110 e to n130 e in the eastern part of the surveyed area where the valley trends n90 e and by large faults in the central western part where the valley is oriented n70 e the direction of extension deduced from the fault histogram is n37 e 10 over the whole area whatever the rift trend therefore this bend of the plate boundary does not result from a change in the stretching direction this bending increases the rifting obliquity westward and therefore modifies the faulting pattern faults display two distinct patterns either en echelon on the walls indicating a right lateral component of shear or purely normal and parallel to the rift trend inside the valley volcanoes are mostly located in the eastern part of the rift where they are aligned into swarms trending roughly perpendicular to the extension direction deduced from fault directions backscattering images show that recent lava flows fill the axial valley of the eastern part whereas in the central and western parts volcanoes are scattered in the middle part of the valley this distribution is interpreted as a westward propagation of the volcanism and as the direction of deformation migration the thickness of the upper brittle layer of the lithosphere is estimated from the width if the inner graben is inside the rift it indicates a deepening of the brittle ductile transition from east 6 km to west 16 km these values are typical of oceanic and continental systems respectively and consistent with the depth of earthquakes in the western part this points out the existence of a transitional area between 44 e and 45 20 e characterised by the lack of recent volcanism the nature of this mechanical transition is discussed in terms of propagation of an spreading centre into a continental or transitional afar type lithosphere

palaeoenvironments and palaeohydrology of a tropical closed lake lake asal djibouti since 10 000 yr b p in eastern africa republic of djibouti the hypersaline lake asal occupies an exceptional geological topographical and hydrological situation at 155 m below sea level and at 12 km from the sea it lies in the youngest and the deepest basin in a chain of closed depressions of central afar decreasing in elevation from west to east and represents the hydrogeological low of the whole system presently infiltration of marine water represents the major water input to the lake due to high evaporation conditions halite precipitates halocene water level fluctuations 315 m sedimentary conditions from fresh oligosaline waters to heavy brines and palaeohydrology are reconstructed field observations are supported by 85 14c dates on inorganic carbonates mollusc shells stromatolites and hydrothermal travertines stable isotope analyses on carbonates and sulphates mineralogy geochemistry and studies of biological remains discussion of the water balance for the major stages is based on reasonable assumptions about holocene conditions of precipitation and evaporation after an arid terminal pleistocene phase marked by coarse deposits due to exceptional floods the holocene cycle begins with complex strontium rich carbonate phases aragonite high and low mg calcites and dolomite these carbonates could reflect the first underground marine supply and waters concentrated through evaporation a high lake level ca 160 m a s l from ca 8600 to 6000 yr b p corresponds to the deposition of high mg calcite precipitated in a deep fresh water lake lake asal was draining the awash river system ethiopian plateau and escarpment through groundwater circulations the terminal basin of the system was probably the present day marine gulf of ghoubbat al kharab hydrologically separated from the open ocean by a sill at 4 m below sea level possibly due to fracture opening a dramatic regression in the lake level from 160 to 150 m occurred between ca 6000 and 5000 yr b p the inputs of continental water were consequently overwhelmed by marine infiltrations the carbonate phase exhibits clear resemblance with that of the transgressive episode from 5000 yr b p lake asal experienced minor water level fluctuations aragonite rich carbonates precipitated followed by calcium sulphates and halite stable isotopes of gypsum reflect its marine origin and various diagenetic processes the holocene evolution of asal which clearly differs from that of the neighbouring closed basins is due to both its specific geological and hydrological setting and to the regional climatic changes

depositional sequences in shallow carbonate dominated sedimentary systems concepts for a high resolution analysis abstract oxfordian and berriasian sections representing shallow water carbonate dominated sedimentary systems have been studied in the swiss and french jura in spain and in normandy they all display a hierarchical stacking of depositional sequences facies evolution and stacking pattern allow to define elementary small scale medium scale and large scale sequences some depositional sequences display well marked sequence boundaries others are limited by transgressive or maximum flooding surfaces the hierarchical organisation of such sequence stratigraphic elements implies that sea level fluctuations were an important factor in their formation and that these fluctuations had different frequencies the superposition of high frequency sea level changes on a long term sea level trend leads to repetition of diagnostic surfaces defining sequence boundary and maximum flooding zones wherein the corresponding high frequency surfaces are well developed chronostratigraphic tie points permit us to estimate the duration of large scale sequences this time control and the observed hierarchical stacking suggest that the high frequency sea level changes were controlled by climatic cycles in the milankovitch frequency band the variability of stacking pattern and facies evolution between sections illustrates the complexity of the studied environments furthermore because of the minimal accommodation space available in these shallow water settings much of the geologic time is not recorded nevertheless detailed analysis of the depositional sequences allows the interpretation of the evolution of the sedimentary system with a high time resolution thus there is a potential to monitor sedimentological ecological and diagenetic processes on a time scale of 20 to 100 ka

subsidence history crustal structure and evolution of the somaliland yemen conjugate margin 1 we have used biostratigraphic data from deep exploration wells to determine the tectonic subsidence history of the somaliland northwestern somalia yemen conjugate margin a poorly known margin in the central part of the gulf of aden bathymetry and magnetic anomaly data suggest the gulf of aden is a young feature that formed following the rifting apart and breakup of the african and arabian plates 32 ma our tectonic subsidence data suggest however that the present day gulf of aden developed on an earlier mesozoic rift system the oldest episode of rifting initiated at 156 ma and lasted for 10 ma and had a nw se trend we interpret the rift as a late stage event associated with the breakup of gondwana and the separation of africa and madagascar at 80 ma there is evidence of an intermediate rift event which correlates with a rapid increase in spreading rate on the ridges separating the african and indian and african and antarctica plates and a contemporaneous slowing down of africa s plate motion the combined effect of all three rifting events has been to thin the crust and upper mantle by up to a factor of 2 the amount of thinning deduced from the wells is in accord with the crustal structure inferred from available seismic refraction data and process oriented gravity and flexure modeling the margin is asymmetric with a steeper gradient in the moho on the yemen side than the somaliland side the main discrepancy is on the yemen side where the gravity derived moho is 10 km deeper than the well derived moho we attribute the discrepancy to the addition of material at the base of the crust since rifting possibly magma sourced from the afar plume

volcanic rifting at martian grabens 1 a large fraction of surface extension on mars occurred at segmented grabens having width length ratios akin to oceanic rifts on earth association with volcanic landforms such as pit craters clearly suggests interconnection between tectonic and magmatic processes a martian rift evolution model is proposed on the basis of new geomorphological and structural interpretations of imagery high resolution digital elevation models dems scaled experimental modeling and three dimensional boundary element modeling of magmatic and tectonic processes and a comparison with terrestrial rifts the dems were obtained from mars observer laser altimeter viking orbiter stereo images or a combination of both comparison of terrestrial rifts included afar iceland and the east pacific rise the ambient extensional stress field induced by regional body forces is combined at depth with decompression melting and mantle plume thermal anomaly resulting in emplacement of elongated magma reservoirs along the grabens injection of dikes above the reservoirs and flood basalt eruption result in a reservoir underpressurization of up to hundreds of mpa and induces surface collapse each collapse event is associated with an eruption of volcanic volumes akin to those of individual flow eruptions in large terrestrial igneous provinces the geometry and mechanisms of graben formation and surface collapse are described and used to infer reservoir depth and width we conclude from this study that giant dike swarms akin to typical giant terrestrial dike swarms are unlikely to underlie the volcanic martian grabens on the basis one graben one dike rather every volcanic graben segment appears to own a local dike swarm perhaps analogous to the dike swarms in the icelandic fissure zones index terms 5475 planetology solid surface planets tectonics 8149 5480 planetology solid surface planets volcanism 8450 6225 planetology solar system objects mars 8121 tectonophysics dynamics convection currents and mantle plumes 8010 structural geology fractures and faults keywords rifting graben pit crater magma chamber dyke

the gulf of aden structure and evolution of a young ocean basin and continental margin new marine geophysical data are used to describe the structure and history of the gulf of aden magnetic anomaly data shows seafloor spreading magnetic anomalies of sheba ridge from the axial anomaly to anomaly 5 10 m y b p between the owen fracture zone and 45 sup 0 e and to anomaly 2 3 m y b p or anomaly 3 4 m y b p west of 45 sup 0 e the data does not support the two episodes of seafloor spreading recently proposed landward of the seafloor spreading magnetic anomalies is a magnetic quiet zone of uncorrelatable anomalies the magnetic quiet zone boundary is also a structural boundary effectively marking the edge of sheba ridge with deeper basement lacking a significant topographic gradient found on the landward side a magnetic quiet zone is found not only where sheba ridge splits continental lithosphere but also on east sheba ridge where the ridge splits the old oceanic lithosphre of the owen and somali basins there the position occupied by the continental margin within the gulf is marked by nonmagnetic ridge complexes that stretch from the continents to the owen fracture zone the magnetic quiet zone boundary is not an isochron in either the gulf of aden ormore the red sea suggesting that significant horizontal motions can occur prior to the initiation of seafloor spreading the offset on the dead sea rift is used to estimate that from 80 to 160 km of opening amounting to between 65 and 200 extension of the initial rift valley occurred in the gulf of aden and red sea prior to the establishment of a mid ocean ridge it is suggested that the development of a new ocean basin occurs in two stages the first involves diffuse extension over an area perhaps 10 km wide in a rift valley environment without an organized spreading center this is followed by concentration of the extension at a single axis and the beginning of true seafloor spreading less

strain accumulation across the asal ghoubbet rift djibouti east africa the asal ghoubbet rift in the republic of djibouti is a subaerial continuation of the gulf of aden ridge system many geological and tectonic arguments indicate that it represents one of the most typical spreading segments of the world rift system geodetic measurements regularly conducted from 1979 to 1983 across this area show an accumulation of extension in the inner floor of the rift valley with a rate of about 60 mm yr perpendicular to the normal faults that bound the rift periodic leveling surveys across major faults in the rift show vertical creep movements of about 5 to 10 mm yr although the observed deformations could be contamined by postseismic relaxation after a rifting episode in november 1978 the regularity and the magnitude of these movements during the last two years argue for a steady state mechanism of deformation of the rift valley the deformation is uniformly distributed in time and in space inside the inner floor of the rift the necking of the lithosphere in the central part of the rift and the high temperature gradient imply a ductile rheology in the inner floor which is stretched in a steady state process

emplacement of giant mud volcanoes in the south caspian basin 3d seismic reflection imaging of their root zones exceptional quality 3d data for the largest mud volcano yet described provide the first detailed imaging of the plumbing architecture that connects a major volcanic edifice to its source layer at depth the volcano is in the south caspian basin and consists of an extruded submarine mud bicone 10 km wide and 1 4 km thick overlying an oval caldera 1 2 1 6 km in width and 0 5 km in depth the caldera narrows downwards into a zone of collapsed country rock forming a downward tapering cone 1 km in height the vertex of which is located close to the top of the mud source layer the imaged structural elements lead to an evolutionary model a narrow steep fluidization pipe fed the oldest pioneer cone we propose that numerous additional fluidization pipes injected the country rock forming a densely intruded cylindrical zone similar to gryphon swarms observed at outcrop onshore wall rock erosion and compaction of the intruded zone led to collapse of the downward tapering cone that linked upwards into ring faults that define the caldera margins later mud flowage focused on the conical sheared margins volumetric contraction of the extruded volcanic cone led to an unusual concentric system of minor outward facing normal faults this model has many similarities to syntheses of igneous maar diatreme caldera systems for which it may be analogous

crustal configuration in the northern levant basin based on seismic interpretation and numerical modeling abstract the interpretation of five 2d pstm seismic reflection sections 14 s twt covering the northern levant basin revealed a total of 10 horizons among which one is interpreted as an interface that may represent the moho the interpretation of seismic packages and their bounding surfaces as well as the seismic facies analysis were constrained by published 2d seismic interpretations of the northern lebanese offshore a total of nine seismic packages are identified in the basin with ages varying from the mid jurassic to the quaternary the filling of the basin is made up of thick cenozoic and mesozoic strata deposited above rifted triassic early jurassic interval the sediments are deposited in deep water mixed settings resulting from high stand systems various types of carbonate platforms and low stand systems siliciclastic and carbonate deep water turbidite complexes carbonate and siliciclastic systems are sealed by 1 1 5 km of evaporites and underlie plio quaternary hemipelagic and pelagic sediments intercalated by turbiditic sheets the time horizons were converted into depth using two methods the first one is based on stacking velocities and the second one on velocities resulting from refraction data 2d crustal modeling was achieved by integrating free air gravity anomaly geoid heights and topography data on the five interpreted pstm seismic lines the models representing five sections across the northern levant basin show a progressively attenuated crystalline crust in an ew direction away from the basin s eastern margin the crystalline crust is best interpreted as a strongly thinned continental crust under the levant basin represented by two distinct components an upper and a lower continental crust the moho appears to be situated between 20 and 23 km in the central and southern lebanese offshore estimated surface heatflow in the basin is around 40 mw m2 which is lower than reported values for the onshore and the margin these differences in heatflow values between the offshore the margin and the onshore have an important impact on hydrocarbon maturation and assessment of potential petroleum systems

field geometry petrography and geochemistry of a dolomitization front late jurassic central lebanon this contribution describes the field geometry petrography and geochemistry of a well exposed dolomitization front in upper jurassic carbonates and attempts to highlight the sedimentological structural and relative sea level controls on multiphase dolomitization and related diagenetic events the data presented reflect the superposition of various diagenetic phases which have resulted in a single dolostone body whose dimensions are well defined in the field local microbial intraclastic dolomites of late tithonian age accumulated in a hypersaline lagoon during relative sea level fall these pre date beige hydrothermal dolostones 51 to 55 mol caco3 18o 9 3 to 4 0 v pdb 13c 1 5 to 2 1 v pdb 87sr 86sr 0 70742 matrix porosity 6 klinkenberg permeability 0 5 md whose dolomitizing fluid circulated along faults and invaded the nearby facies first the burrows were dolomitized then the bulk rocks resulting in the investigated tongue shaped dolomite body upon late jurassic early cretaceous uplift near surface water percolated through and altered the underlying beige dolostones this event was followed by a ferroan dolomite cement phase which occurred during further burial this contribution featuring a well defined geometric pattern of a dolomitization front with a large petrographic and geochemical data set may also serve as a case study illustrating the complexity of superimposed diagenetic processes which have to be taken into account in modelling exercises of multiphase hydrothermal dolomitization

tectonics and magmatism associated with mesozoic passive continental margin development in the middle east early triassic cretaceous rifting and magmatism occurred along the western edge of the arabian peninsula from egypt in the south to syria in the north associated with the development of the neotethyan passive continental margin which is preserved in the present day eastern mediterranean a change in the direction and rate of extension during the early cretaceous had a profound effect upon the developing continental margin and controlled the pattern of rifting magmatism and sedimentation extensive late jurassic early cretaceous volcanism occurred throughout the region and is clearly rift related major and trace element including rare earth element and sr nd isotopic compositions suggest derivation of the most primitive mafic magmas from an enriched mantle source with minimal evidence of crustal contamination the magmas appear to have ascended rapidly to the surface through large scale faults isopach maps have been constructed for a series of time intervals during the mesozoic using well data from egypt israel iraq jordan lebanon and syria these reveal the distribution and orientation of the main depocentres the main tectonic feature from the late triassic until the late jurassic was a basin which extended northeastwards from northern israel to the palmyrides in central syria by the early cretaceous however deposition switched to a n s oriented basin in central and northern israel and lebanon the distribution of late jurassic early cretaceous magmatism coincides with this new basin and indicates the location of the major extensional fault zones

tectonics of the westernmost gulf of aden and the gulf of tadjoura from submersible observations the gulf of tadjoura separating the danakil and somalia plates1 represents the western extension of the sheba ridge between arabia and somalia in the gulf of aden fig 1 it has been assumed that a simple plate tectonic scheme could be applied to the area with ne sw transform faults offsetting roughly east west zones of accretion represented by the major bathymetrie troughs1 8 fig 1c more recently this boundary has been interpreted as a westward propagating rift with its present day tip on land in the region of the asal lake9 10 here we present data from dives with the submersible cyana which show that there have been two episodes of extensional faulting an older inactive phase parallel to the general ene wsw direction of the gulf and a younger fault set along discontinuous zones striking 140 there is no surface evidence for any transform fault zone in the area covered and the incipient spreading segments whose geometry implies extension on 050 do not define a plate boundary the faulting observed is at odds with that predicted by orthodox plate tectonics

reconciliation of late quaternary sea levels derived from coral terraces at huon peninsula with deep sea oxygen isotope records a major discrepancy between the late quaternary sea level changes derived from raised coral reef terraces at the huon peninsula in papua new guinea and from oxygen isotopes in deep sea cores is resolved the two methods agree closely from 120 ka to 80 ka and from 20 ka to 0 ka ka 1000 yr before present but between 70 and 30 ka the isotopic sea levels are 20 40 m lower than the huon peninsula sea levels derived in earlier studies new high precision u series age measurements and revised stratigraphic data for huon peninsula terraces aged between 30 and 70 ka now give similar sea levels to those based on deep sea oxygen isotope data planktonic and benthic 18o data using the sea level and deep sea isotopic data oxygen isotope ratios are calculated for the northern continental ice sheets through the last glacial cycle and are consistent with results from greenland ice cores the record of ice volume changes through the last glacial cycle now appears to be reasonably complete