the structure and fault activity of the makran accretionary prism the makran subduction zone has the highest incoming sediment thickness up to 7 5 km of any subduction zone these sediments have formed a wide accretionary prism 400 km seismicity in the makran is generally low however the margin experienced an mw 8 1 earthquake in 1945 which generated a significant regional tsunami seismic reflection data and swath bathymetry data from offshore pakistan are used to analyze the structure and fault activity of the outer accretionary prism the outer prism has a simple structure of seaward verging imbricate thrust faults many continuous for over 100 km along strike fault activity is analyzed using basin stratigraphy and fault geometry revealing a frontal continuously active zone a central intermittently active zone and a landward inactive zone over 75 of the faults in the seaward 70 km of the prism show evidence for recent activity the decollement occurs within the lower sediment section but steps onto the top basement surface in regions of elevated basement topography fault spacing 6 km and taper 4 5 are comparable to other margins such as s hikurangi cascadia and nankai suggesting that high sediment input is not leading to an unusual prism structure the decollement is unreflective which is unexpected considering other prism characteristics predicting a weak surface and may suggest a potentially stronger decollement than previously predicted this study provides a significant advance in our understanding of the structure of an end member convergent margin and demonstrates that systematic analyses of accretionary prism structure can help to elucidate subduction zone dynamics with ultimate relevance to seismogenic potential

water velocity estimation using inversion methods it is known that the propagation velocity of sound waves in water can vary over time for a 3d seismic survey if data are acquired in adjacent lines but at different dates this implies the same reflection point will be recorded at different times to take this effect into account in seismic processing it is necessary to measure the sound velocity in water i have developed a 3d tomographic method that directly estimates it it assumes a constant sound velocity for a group of shots belonging to a single sail line using a picked water bottom reflection and an initial depth and velocity model results good for use in subsequent processing can be obtained by estimating only two parameters the variation of the propagation velocity and a constant vertical shift of the reflector depth in relation to the initial model the method was tested with both synthetic and real data the real data results were validated using two methods first i analyzed the histogram of the residuals of the final updated model second i used a specially modified kirchhoff migration algorithm to migrate the sea bottom the main advantages of this method are that it takes into account the sea bottom dips to estimate the velocities and it can be applied to each sail line separately also the inversion is not ill conditioned provided that data with large enough offsets are used as a result the method is simple to apply

a new model for the emplacement of columbia river basalts as large inflated pahoehoe lava flow fields extensive flows of the columbia river basalt crb group in washington oregon and idaho are dominantly inflated compound pahoehoe sheet lavas early studies recognized that crb lavas are compound pahoehoe flows with textures suggesting low flow velocities but it was thought that the great thickness and extent of the major flows required very rapid emplacement as turbulent floods of lava over a period of days or weeks however small volume 1 km compound pahoehoe flows on kilauea hawai i demonstrate that such flows can thicken by at least an order of magnitude through gradual inflation and the same mechanism has been proposed for larger 10 20 km3 pahoehoe flows in iceland the vertical distribution of vesicles and other morphologic features within crb lava flows indicate that they grew similarly by inflation small pahoehoe lobes at the base and top of many crb pahoehoe lava flows indicate emplacement in a gradual piecemeal manner rather than as a single flood we propose that each thick crb sheet flow was active for months to years and that each group of flows produced by a single eruption a flow field was emplaced slowly over many years

structure of the makran subduction zone from wide angle and reflection seismic data abstract makran is one of the largest accretionary wedges on the globe formed by the convergence between the eurasian and the arabian plates it is characterised by an extremely high sediment input of 7 km and a shallow subduction angle we present seismic velocity models from four wide angle seismic lines that image the wedge sediments and the subducted oceanic crust three strike lines show complex but rather one dimensional structures where the observed seismic phases have been studied with the reflectivity method a 160 km long dip line was surveyed coincident with a mcs line collected by cambridge university in 1986 prestack depth migration of this data with the new wide angle velocities shows improved images compared to earlier results interpretation of the wide angle and the mcs data indicates that a decollement is developed along a bright reflector within the turbiditic sequence more than 3 km of sediment bypasses the first accretionary ridges underthrusting and is transported to greater depth this might help to explain the sparse earthquake activity associated with subduction here in the strike lines low velocity layers occur landward of the deformation front in zones where active thrusting or underplating takes place the oceanic crust is characterised by a strong velocity contrast between layer ii and layer iii

subaqueous explosive eruption and welding of pyroclastic deposits silicic tuffs infilling an ancient submarine caldera at mineral king in california show microscopic fabrics indicative of welding of glass shards and pumice at temperatures 500 degrees c the occurrence indicates that subaqueous explosive eruption and emplacement of pyroclastic materials can occur without substantial admixture of the ambient water which would cause chilling intracaldera progressive aggradation of pumice and ash from a thick fast moving pyroclastic flow occurred during a short lived explosive eruption of approximately 26 cubic kilometers of magma in water 150 meters deep the thickness high velocity and abundant fine material of the erupted gas solids mixture prevented substantial incorporation of ambient water into the flow stripping of pyroclasts from upper surfaces of subaqueous pyroclastic flows in general both above the vent and along any flow path may be the main process giving rise to buoyant convective subaqueous eruption columns and attendant fallout deposits

time lapse seismic monitoring some shortcomings in nonuniform processing conceptually repeat or time lapse seismic monitoring is straightforward seismic data sets acquired at different times are calibrated compared and the intersurvey differences analyzed and interpreted in terms of fluid changes or movement

arc continent collisions sediment recycling and the maintenance of the continental crust author posting geological society of london 2009 this is the author s version of the work it is posted here by permission of geological society of london for personal use not for redistribution the definitive version was published in geological society london special publications 318 2009 75 103 doi 10 1144 sp318 3

palaeoenvironmental signals in ancient urban setting the heavy rainfall record in sumhuram a pre islamic archaeological site of dhofar s oman a case study of an archaeological section in the pre islamic town of sumhuram third century bc fifth century ad dhofar s oman is presented sedimentological and palynological data have been integrated with the aim of finding signals of natural processes within an urban context two main types of deposits alternate in the section anthropogenic ad and waterlain wd the presence of laminated sediments within the wd deposits suggesting short term street flooding from heavy rainfall encouraged the development of a targeted palynological sampling strategy the results of the pollen analysis in these laminated sediments e g high pollen concentrations particularly of wet environment plants clumped pollen grains long distance pollen grains and overall good state of preservation suggest a stronger monsoonal influence than the present and high precipitation in s oman between 204 bc and ad 130

degassing of rhyolitic magma during ascent and emplacement the degassing history of a rhyolitic igneous system was documented from analyses of drill core samples through the extrusive and intrusive portions of obsidian dome and of surface samples of associated tephra the initial volatile composition of the inyo magma was estimated to be 4 0 wt h2o 500 ppm f 800 ppm cl and 80 ppm s retained volatile contents of glassy and crystalline samples reflect the effects of decompression and second boiling decompression is rapid and involves loss of water rich fluid until a close approach to lithostatic equilibrium is achieved second boiling is a slower process and produces a chlorine rich fluid some of which can be trapped during development of extremely fine crystallization textures nearly complete dewatering during decompression of surface extruded magma strongly undercools the system t 175 c suppressing crystallization and yielding glassy rhyolitic lava partial degassing of shallowly intruded magma permits pervasive crystallization even at high cooling rates the subvolcanic intrusive regime is the zone of maximum volatile release because second boiling is incomplete in extrusives and volatile bearing crystalline phases are stable in magma crystallized at greater depth

east africa continental margins new well data from somalia together with the history of sea floor spreading in the indian ocean derived from magnetic anomalies show that the east african margins from at 15 s into the gulf of aden comprise four distinct segments that formed successively by the southward drift of madagascar from somalia during the middle to late jurassic and early cretaceous by the northeastward drift of india along the owen transform during the late cretaceous and paleocene and by the opening of the gulf of aden during the neogene

accelerated mass flux to the arabian sea during the middle to late miocene possible links between orogenic uplift and the strengthening of the south asian monsoon are best examined in the arabian sea where the erosional flux is not dominated by the high himalaya as in the bengal fan but is controlled by tibet the karakoram and the hindu kush the erosional flux from these mountains into a basin where the monsoon is well dated allows a direct comparison of erosional tectonic and climatic processes new seismic data from the proximal indus fan permit the thickest part of this system to be dated for the first time through correlation to drilling sites after converting time sections to depth and then accounting for sediment compaction the data show a two to threefold increase in mass flux to the upper fan and continental shelf peaking during the middle miocene 16 11 ma and falling from the late miocene to the present increased sedimentation rates are accompanied by the growth of channel levee complexes high sediment flux does not correlate with monsoonal strengthening at 8 5 ma but follows uplift of the murray ridge uplift in tibet and the karakoram after 20 ma and the start of transpression along the karakoram fault at 18 ma 20 km of erosion

analytical numerical modeling of komatiite lava emplacement and thermal erosion at perseverance western australia abstract we have applied a thermal fluid dynamic geochemical model to investigate the emplacement and erosional potential of archean komatiite flows at perseverance western australia perseverance has been proposed as a site of large scale thermal erosion by large volume komatiite eruption s resulting in a 100 150 m deep lava channel containing one of the world s largest komatiite hosted fe ni cu pge sulfide deposits using constraints based on field theoretical and geochemical data we have modeled the emplacement of a range of flow thicknesses over felsic tuffaceous substrates with various degrees of consolidation and water contents thermo mechanical erosion becomes more effective for substrates that are increasingly unconsolidated and water rich for thermo mechanical erosion to be responsible for the formation of the 100 m deep concave perseverance embayment and the highly contaminated 10 20 perseverance komatiites the most likely scenarios require emplacement of thick e g 10 30 m turbulent channelized liquidus or superheated komatiite lavas over a welded or unconsolidated submarine tuff flow distances must have been long tens to hundreds of kilometers and flow volumes must have been very high hundreds to thousands of km 3 lava channels and tubes 110 km long have not been observed on earth but are consistent with those formed by low viscosity lavas on venus the moon mars and io flow volumes are consistent with those in continental flood basalt eruptions and oceanic plateau large igneous provinces and may represent the initial outpourings of komatiite lavas from archean mantle plume activity

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

core rotational dynamics and geological events a study of earth9s fluid core oscillations induced by lunar solar tidal forces together with tidal secular deceleration of earth9s axial rotation shows that the rotational eigenfrequency of the fluid core and some solar tidal waves were in resonance around 3 0 109 1 8 109 and 3 108 years ago the associated viscomagnetic frictional power at the core boundaries may be converted into heat and would destabilize the d thermal layer leading to the generation of deep mantle plumes and would also increase the temperature at the fluid core boundaries perturbing the core dynamo process such phenomena could account for large scale episodes of continental crust formation the generation of flood basalts and abrupt changes in geomagnetic reversal frequency

tectono stratigraphic development of the qamar basin eastern yemen abstract the qamar basin is a polyphase rift basin located in eastern yemen adjacent to the indian ocean it probably originated as part of an extensive rift system which developed during the late jurassic to early cretaceous and led to the break up of gondwanaland rift reactivation during the mid cretaceous affected limited areas including the qamar basin a third episode of rifting occurred during the oligo miocene prior to sea floor spreading in the gulf of aden since spreading in the gulf began in the middle miocene the offshore qamar basin has been thermally subsiding in contrast the onshore part of the basin has been elevated by over 1 km the nature of the pre mid cretaceous basin fill is unknown during santonian to early campanian times thick fluvio deltaic clastics accumulated in a rapidly subsiding depocentre marked deepening during the early campanian led to deposition of deep water limestones during the late campanian prograding deep marine clastics infilled the basin minor transgression during the maastrichtian caused a resumption of marine carbonate deposition sedimentation was interrupted during the early palaeocene following which shallow marine carbonates were deposited over a large area of the arabian peninsula after a hiatus spanning the late eocene to late oligocene carbonate sedimentation resumed offshore in an active rift basin deep water carbonates and carbonate turbidites accumulated in lows whilst reefs developed on footwall highs from the late miocene onwards the reefs have been gradually drowned by rapidly accumulating clastics onshore the tertiary rift episode was associated with major uplift and only a thin veneer of recent wadi deposits overlies the rift topography the superposition of jurassic cretaceous and tertiary rifts which has not been described elsewhere in the region is tentatively related to the evolution of the indian ocean

crustal structure of the murray ridge northwest indian ocean from wide angle seismic data the murray ridge dalrymple trough system forms the boundary between the indian and arabian plates in the northern arabian sea geodetic constraints from the surrounding continents suggest that this plate boundary is undergoing oblique extension at a rate of a few millimetres per year we present wide angle seismic data that constrains the composition of the ridge and of adjacent lithosphere beneath the indus fan we infer that murray ridge like the adjacent dalrymple trough is underlain by continental crust while a thin crustal section beneath the indus fan represents thinned continental crust or exhumed serpentinized mantle that forms part of a magma poor rifted margin changes in crustal structure across the murray ridge and dalrymple trough can explain short wavelength gravity anomalies but a long wavelength anomaly must be attributed to deeper density contrasts that may result from a large age contrast across the plate boundary the origin of this fragment of continental crust remains enigmatic but the presence of basement fabrics to the south that are roughly parallel to murray ridge suggests that it separated from the india seychelles madagascar block by extension during early breakup of gondwana

cretaceous sedimentary rocks on the masirah ophiolite sultanate of oman evidence for an unusual bathymetric history autochthonous sedimentary rocks on the masirah ophiolite preserve a record of vertical movements of their oceanic basement the masirah ophiolite was formed in the uppermost jurassic and subsequently underwent thermal subsidence for about 20 ma as documented by pelagic sediments submarine alkali basaltic volcanism in the lower barremian marks the onset of rapid basement uplift to or above sea level carbonate platforms grew in barremian mid aptian time on highs of oceanic basement the uplift is interpreted as the result of transpressive tectonism along a large oceanic fracture zone the platforms submerged in the mid aptian and reached the ccd in the albian ribbon cherts were deposited during the albian to santonian time the first influx of siliciclastic detritus shed from the arabian craton is recorded in coniacian sandstones ophiolitic conglomerates interfinger in the campanian to mid maastrichtian with continent derived sandstones reflecting regional compressive tectonism crystalline exotics deposited in the late maastrichtian are derived most likely from the precambrian basement of arabia whilst the origin of the sedimentary exotics dated triassic to cretaceous remains unclear the masirah ophiolite was overthrust at the cretaceous tertiary boundary by an upper ophiolite nappe and emplaced onto the margin of southeast oman

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

crustal structure in the western somali basin as part of integrated marine geophysical studies in the western somali basin we performed 118 sonobuoy experiments to define better the crustal structure of the margins and basin created by the separation of madagascar and africa after using t2 x2 conventional slope intercept methods and slant stacked t p techniques to analyse the data we combined our solutions with all previous velocity information for the area velocity functions were derived for the sediment coiumn and we detected a high velocity 4 58 0 29 km s 1 sediment layer overlying acoustic basement we confirmed that the crust is indeed seismically oceanic and that it may be considered either in terms of a layered model layers 2b 5 42 0 19 km s 1 2c 6 23 0 22 km s 1 3 7 03 0 25 km s 1 and mantle 7 85 0 32 km s 1 were identified or a more complex gradient model in which layer 2 is marked by a steeper velocity gradient than underlying layer 3 integrated igneous crustal thicknesses 1 62 0 22 s 5 22 0 64 km are significantly less than what is considered normal we present a revised seismic transect across the east african margin as well as total sediment thickness depth to basement and crustal thickness maps

dalrymple trough an active oblique slip ocean continent boundary in the northwest indian ocean the dalrymple trough marks part of the transform plate boundary between india and arabia in the northern arabian sea oblique extension is presently active across this portion of the boundary at a rate of a few millimetres per year and seismic reflection profiles across the trough confirm that it is an extensional structure we present new swath bathymetric and wide angle seismic data from the trough the bathymetric data show that the trough is bounded by a single steep 3 km high scarp to the southeast and a series of smaller en echelon scarps to the northwest wide angle seismic data show that a typical oceanic crustal velocity structure is present to the northwest with a crustal thickness of 6 km there is an abrupt change in crustal thickness and velocity structure at the northwestern edge of the trough and the trough itself is underlain by 12 km thick crust interpreted as thinned continental crust therefore we infer that dalrymple trough is an unusual obliquely extending plate boundary at which continental crust and oceanic crust are juxtaposed the extensional deformation is focused on a single major fault in the continental lithosphere but distributed over a region 60 km wide in the oceanic lithosphere

basanites related to late eocene extension from ne oman six small basanite intrusions were emplaced into the allochthonous batain nappes of ne oman along approximately north south to nnw sse lineaments they contain olivine clinopyroxene plagioclase analcite ulvospinel and apatite biotite calcite the basanites are primitive with mg numbers 100mg mg fe 2 from 63 18 to 69 37 and are nepheline normative compositional variation suggests fractionation of olivine and clinopyroxene thermobarometric studies on rare spinel lherzolite xenoliths suggests re equilibration at shallow depth 143 nd 144 nd and 87 sr 86 sr isotopic data and ree modelling indicate that the magmas were formed by 1 2 melting of an ocean island basalt oib like source containing about 3 4 garnet deviations from oib are explained by mobilization of incompatible elements from potassic layers in the lithosphere during ascent sedimentological studies and three new whole rock ar ar ages 39 77 0 26 37 63 0 46 and 36 51 0 21 ma lutetian to priabonian time suggest intrusion during maximum deepening of the abat trough movements were related to dextral strike slip movement along major tectonic structures during the initial stages of the opening of the gulf of aden the basanites were formed by decompressional melting of an asthenospheric mantle source facilitated by thinned lithosphere and depression of the solidus by h 2 o and co 2 rich fluids

extremely thin oceanic crust in the proto indian ocean evidence from the masirah ophiolite sultanate of oman the masirah ophiolite is a good example of thin oceanic crust below pillow lavas and a sheeted dike complex with a relatively normal thickness of 1 1 5 km the gabbroic lower crust barely exceeds 500 m in thickness in spite of this reduced thickness the oceanic crust preserves all members of a model ophiolite in a coherent lithostratigraphic sequence the crust was formed during the uppermost jurassic circa 150 ma when the indian madagascar plate separated from the african arabian plate and is therefore related to the opening of the coeval somali basin geological relationships indicate that this portion of oceanic crust was formed at a ridge transform intersect the peculiarly reduced thickness of the gabbro layer is interpreted as the result of a weak magma supply at the edge of a ridge segment rather than the consequence of a tectonic thinning the cooling effect due to the vicinity of two large continental lithospheric blocks indian madagascar and african arabian plates during this initial stage of the oceanization might have been an additional factor contributing to the reduction of the crustal thickness

the sawqirah contourite drift system in the arabian sea nw indian ocean a case study of interactions between margin reactivation and contouritic processes the relationships between oceanic circulation in the arabian sea and late cenozoic climate changes including variations in monsoon intensity at the million year time scale remain poorly investigated using multibeam and seismic data we provide the first description of a contourite drift in the arabian sea along the south eastern oman margin this contourite drift is referred as the sawqirah contourite drift system late miocene reactivation of the south eastern oman margin resulted in the formation of a complex anticline system which shaped the seafloor topography above which the sawqirah drift subsequently developed the drift resulted from the circulation of bottom currents within the north intermediate indian water major seismic unconformities identified within the sawqirah drift were tied to ocean drilling program odp drill holes and allowed defining distinct episodes of drift construction at least two of these unconformities record reorganizations of the oceanic circulation at 4 5 4 8 ma and 2 4 ma the 4 5 4 8 ma old unconformity is coeval with the onset of upwelling of deep and cold waters in the owen basin the 2 4 ma old unconformity records a major episode of indian monsoon intensification at the million year time scale over the arabian sea indicating strong coupling between oceanic and atmospheric circulation processes

extension across the indian arabian plate boundary the murray ridge summary seismic reflection profiles from the murray ridge in the gulf of oman northwest indian ocean show a significant component of extension across the predominantly strike slip indian arabian plate boundary the murray ridge lies along the northern section of the plate boundary where its trend becomes more easterly and thus allows a component of extension the dalrymple trough is a 25 km wide steep sided halfgraben bounded by large faults with components of both strike slip and normal motion the throw at the seabed of the main fault on the southeastern side of the halfgraben reaches 1800 m the northwest side of the trough is delineated by a series of smaller antithetic normal faults wide angle seismic gravity and magnetic models show that the murray ridge and dalrymple trough are underlain by a crystalline crust up to 17 km thick which may be continental in origin any crustal thinning due to extension is limited and no new crust has been formed we favour a plate model in which the indian arabian plate boundary was initially located further west than the owen fracture zone possibly along the oman continental margin and suggest that during the oligocene early miocene indian ocean plate reorganization the plate boundary moved to the site of the present owen fracture zone and that motion further west ceased at this time deformation began along the murray ridge with both the uplift of basement highs and subsidence in the troughs tilting the lowest sedimentary unit qalhat seamount was formed at this time subsequent sediments were deposited unconformably on the tilted lower unit and then faulted to produce the present basement topography the normal faulting was accompanied by hanging wall subsidence footwall uplift and erosion flat lying recent sediments show that the major vertical movements have ceased although continuing earthquakes show that some faulting is still active along the plate boundary

target oriented joint least squares migration inversion of time lapse seismic data sets two related formulations are proposed for target oriented joint least squares migration inversion of time lapse seismic data sets time lapse seismic images can be degraded when reservoir overburden is complex or when acquisition geometries significantly differ because the migration operator does not compensate for the resulting amplitude and phase distortions under these circumstances time lapse amplitudes are poor indicators of production related changes in reservoir properties to correct for such image degradation time lapse imaging is posed as joint inverse problems that utilize concatenations of target oriented approximations to the linear least squares imaging hessian in both formulations spatial and temporal constraints ensure inversion stability and geologically consistent time lapse images using two numerical time lapse data sets we confirmed that these formulations can attenuate illumination artifacts caused by complex overburden or geometry differences and that they yield better quality images than obtainable with migration

deformation at plate boundaries around the gulf of oman abstract seismic studies during charles darwin cruise 18 86 focused on three zones of deformation the makran accretionary prism the murray ridge owen fracture zone system and the continental margin of east oman a multichannel seismic reflection profile across the makran margin clearly imaged the imbricate thrust slices and their bounding faults a bottom simulating reflector bsr 500 800 m beneath the seabed marking the base of a gas hydrate stability zone may be traced across most of the profile the heat flow variation estimated from changes in the depth of this reflector shows evidence for localised fluid expulsion reflection profiles across the murray ridge show evidence for active extension along a series of major normal faults the rift structure is segmented with the fault polarity changing between segments a combined reflection and wide angle study of the east oman margin shows a deep offshore basin underlain by crust only 5 km thick and bounded by a basement ridge interpreted as a tilted fault block the wide angle data suggest a sleeply dipping moho beneath the margin the large lateral changes in crustal thickness may be explained by episodes of margin parallel shear

lithospheric strength variations as a control on new plate boundaries examples from the northern red sea region abstract the complex plate boundary between arabia and africa at the northern end of the red sea includes the gulf of suez rift and the gulf of aqaba dead sea transform geologic evidence indicates that during the earliest phase of rifting the red sea propagated nnw towards the mediterranean sea creating the gulf of suez subsequently the majority of the relative movement between the plates shifted eastward to the dead sea transform we propose that an increase in the strength of the lithosphere across the mediterranean continental margin acted as a barrier to the propagation of the rift a new plate boundary the dead sea transform formed along a zone of minimum strength we present an analysis of lithospheric strength variations across the mediterranean continental margin the main factors controlling these variations are the geotherm crustal thickness and composition and sediment thickness the analysis predicts a characteristic strength profile at continental margins which consists of a marked increase in strength seaward of the hinge zone and a strength minimum landward of the hinge zone this strength profile also favors the creation of thin continental slivers such as the levant west of the dead sea transform and the continental promontory containing socotra island at the mouth of the gulf of aden calculations of strength variations based on changes of crustal thickness geotherm and sediment thickness can be extended to other geologic settings as well they can explain the location of rerifting events at intracratonic basins of backarc basins and of major continental strike slip zones

investigating the strength and trend of seismic anisotropy in the western part of makran subduction zone and southeast of iran abstract the makran subduction zone located in southeastern iran and southern pakistan extends approximately 900 km along the eurasian arabian plate boundary where the arabian oceanic plate subducts beneath the eurasian plate using broadband seismic data recorded in the western part of makran in the southeast of iran seismic anisotropy was studied to better understand geodynamic processes of this subduction zone we analyzed shear wave splitting of core refracted shear wave phases sks skks and pks recorded by 17 broadband stations located in the western makran and southeast of iran from 542 earthquakes mw 6 0 that occurred between 2004 and 2017 at epicentral distances between 90 145 few stations close to the trench line exhibit a trench oblique trend in the west to a trench prependicular trend to the east the magnitude of splitting delay times suggests a mantle source of anisotropy our findings show that the majority of the fast axes of seismic anisotropy at farthest stations in the backarc region are oriented in the nw se direction with delay times created by crust and mantle varying between 0 8 and 1 3 s the orientations of these fast directions are almost parallel to surface structural features for stations near the minab strike slip fault system the transition from the makran subduction zone to the zagros collision zone the fast directions are sub parallel to the strike of this fault system indicating that simple shear is likely the main source of anisotropy

magmatic evolution and porphyry epithermal mineralization in the taftan volcanic complex southeastern iran abstract the taftan volcanic complex is located above the makran subduction zone in sistan and baluchestan province southeastern iran the earliest volcanic activity at taftan started in the late miocene 8 ma with eruption of andesitic to dacitic lava onto a cretaceous to eocene volcanic and sedimentary paleosurface 20 km to the northwest of the current volcanic edifice later plio pleistocene volcanism consisted of voluminous andesitic and dacitic lavas and pyroclastic flows 3 1 to 0 4 ma taftan and the neighbouring bazman volcano are the southeasternmost and youngest manifestations of arc related volcanism in iran which began with the eocene miocene urumieh dokhtar magmatic arc udma in northwest and central iran and extends into western pakistan chagai arc major porphyry cu mo au deposits are associated with miocene to pliocene intrusive rocks in the kerman section of the eocene neogene urumieh dokhtar magmatic arc in southeastern iran e g sar cheshmeh meiduk and at reko diq and saindak in the late cretaceous quaternary chagai arc in western pakistan in a gap of 300 km between these two belts several porphyry and epithermal prospects are exposed in the miocene pliocene sections of the taftan volcanic complex including the kharestan 6 10 0 80 ma and bidester porphyry epithermal cu au deposits 4 3 ma and the siah jangal epithermal au deposit late miocene in addition several argillic to advanced argillic and fumarolic alteration zones occur in plio pleistocene volcanic rocks around the current volcanic edifice these deposits have received limited exploration electron microprobe analyses of amphibole magnesio hastingsite phenocrysts and magnetite ilmenite mineral pairs from the taftan and bazman volcanic rocks indicate a change of crystallization temperature and magmatic oxidation state from 1000 c and fmq 1 in andesitic rocks to 900 c and fmq 2 in dacitic rocks magmatic water content was 4 wt as indicated by the ubiquitous presence of amphibole phenocrysts major and trace element compositions of the taftan and bazman volcanic rocks show calc alkaline to high k calc alkaline affinity with relative depletions of nb ta and ti and enrichments of large ion c elements lile th and u on normalized diagrams these trace element patterns including listric shaped normalized rare earth element profiles and relatively high sr y and la yb ratios are similar to those of fertile miocene igneous rocks from the kerman belt and chagai arc and suggest that the taftan suite in particular should be prospective for porphyry cu ore formation regionally there is no clear geochemical difference between the neogene kerman belt rocks which are thought to post date the onset of collision between the afro arabian and eurasian plates late oligocene to earliest miocene and the subduction related bazman taftan and chagai belt magmas the porphyry deposits formed in these distinct settings are also virtually indistinguishable this suggests that most of the processes affecting the geochemistry and metallogeny of the magmas in both settings take place in the lithosphere albeit that the ultimate source of the magmas is in the supra subduction zone asthenospheric mantle wedge in collisional environments subduction related material previously crystallized in the deep lithosphere is simply being remobilized

mantle xenolith bearing maastrichtian to tertiary alkaline magmatism in oman 1 mantle xenolith bearing alkali basalts basanites and tephrites with 1 5 2 k2o and 4 5 wt na2o occur as small 100 m plugs and dikes in the batain and haushi huqf areas and wnw of muscat their black color and the common presence of peridotite xenoliths allow a separation from older alkaline rocks and from the ophiolitic extrusive rocks two main dike directions approximately e w and n s oriented are observed intrusions seem to occur where these two fault systems intersect the basalts crosscut the upper maastrichtian siliciclastic rocks of the fayah formation and the nappe stack of the eastern ophiolite belt of the batain area they have not been observed intruding the tertiary shallow marine carbonates although k ar whole rock dating on these lavas yielded late eocene 37 1 to 44 1 ma ages the rocks are aphyric and fine grained with microgranular and less common microtrachytic texture they contain magmatic olivine fo80 82 nepheline ne82 86ks14 18 clinopyroxene di46en25wo28 with 2 5 wt al2o3 accessory phlogopite xmg0 6 with 5 5 wt tio2 in a microcrystalline or glassy matrix plagioclase is only observed in few samples locally occurring mm to cm sized immiscible leucocratic melt droplets consist of potassium feldspar an1 4ab37 39or60 70 plagioclase an20 22ab65or13 15 nepheline ne85ks15 phlogopite xmg0 8 with 6 7 wt tio2 and titanomagnetite with 8 wt tio2 more than 95 of the xenoliths are 5cm sized weakly to clearly foliated spinel peridotites of mantle origin sedimentary xenoliths hornfelses are rare and lower crustal xenoliths were not found the peridotite xenoliths consist of olivine fo90 92 enstatite en89fs09wo02 with 3 1 3 3 wt al2o3 diopside di45en26wo29 with 3 6 wt al2o3 and cr spinel with 60 mol spinel 20 hercynite and 15 16 magnesiochromite the age chemical composition and structural position suggest a relation with local tectonic movements associated with plate reorganization in the owens basin region prior to the red sea opening

the separation of madagascar and africa identification of a sequence of east west trending magnetic anomalies of mesozoic age in the western somali basin helps define the position of madagascar in the gondwana reconstruction the anomalies are symmetric about ancient ridge segments and are flanked to the north and south by the jurassic magnetic quiet zone the motion of madagascar relative to africa was from the north and began in the middle jurassic about the same time as the initial breakup of gondwanaland sea floor spreading ceased when madagascar assumed its present position in the early cretaceous

overburden complexity and repeatability of seismic data impacts of positioning errors at the oseberg field north sea repeatability of seismic data plays a crucial role in time lapse seismic analysis there are several factors that can decrease the repeatability such as positioning errors varying tide source variations velocity changes in the water layer marine data and undesired effects of various processing steps in this work the complexity of overburden structure as an inherent parameter that can affect the repeatability is studied a multi azimuth three dimensional vertical seismic profiling data set with 10 000 shots is used to study the relationship between overburden structure and repeatability of seismic data in most repeatability studies two data sets are compared but here a single data set has been used because a significant proportion of the 10 000 shots are so close to each other that a repeatability versus positioning error is possible we find that the repeatability decreases by a factor of approximately 2 under an overburden lens furthermore we find that the x and y components have approximately the same sensitivity to positioning errors as the z component for the same events in this three dimensional vertical seismic profiling experiment this indicates that in an area with complex overburden positioning errors between monitor and base seismic surveys are significantly more critical than outside such an area this study is based on a three dimensional three component vertical seismic profiling data set from a north sea reservoir and care should be taken when extrapolating these observations into a general four dimensional framework

the transition between the sheba ridge and owen basin rifting of old oceanic lithosphere summary magnetic quiet zones are present along the margins of the entire length of the gulf of aden to the owen fracture zone this includes the easternmost 300 km between the eastern edges of arabia and africa to the owen fracture zone where old oceanic lithosphere was rifted to form the sheba ridge within this easternmost region the boundary between the quiet zone and the old oceanic lithosphere is marked by ridge complexes the sharbithat ridge complex to the north and the error ridge complex to the south these ridge complexes which lack a magnetic signature occupy a structural position similar to the hinge zones at the continental margins to the west they appear to have formed early in the opening of the gulf of aden or perhaps to have been pre existing features the boundary between sheba ridge and the northern magnetic quiet zone is often marked by an abrupt end of the sheba ridge seafloor spreading magnetic anomaly pattern and a sharp basement deepening to the north the boundary between the east sheba ridge and northern magnetic quiet zone becomes less distinct near the owen fracture zone this is also accompanied by changes in the east sheba ridge specifically a decrease in magnetic anomaly amplitudes increase in ridge flank depths and a loss of the cooling curve ridge flank shape this may be the result of lower mantle temperatures in the vicinity of the owen fracture zone the sediments within the magnetic quiet zone can be divided into a lower disturbed unit and an upper unit consisting of flat lying reflectors the disturbance of the lower sediments may have resulted from a period of diffuse extension in the magnetic quiet zone prior to the establishment of the sheba ridge spreading centre the similarity of the easternmost quiet zone to quiet zones at rifted continental margins leads to the suggestion that these regions were formed by diffuse extension of old oceanic lithosphere using a two layer lithospheric attenuation model and assuming extension during a period of 15 myr followed by 10 myr of cooling the basement depths and heat flow measurements can be adequately matched the modelling implies 45 per cent crustal extension in the quiet zone the amount of extension calculated is compatible with documented motion between arabia and africa the old oceanic lithosphere thus must have been substantially thinned to a thickness similar to 10 14 myr old ocean before seafloor spreading was initiated at the sheba ridge

the owen ridge uplift in the arabian sea implications for the sedimentary record of indian monsoon in late miocene abstract the pelagic cover of the owen ridge in the arabian sea recorded the evolution of the indian monsoon since the middle miocene the uplift of the owen ridge resulted from tectonic processes along the previously unidentified miocene india arabia plate boundary based on seismic reflection data tied with deep sea drilling to track the miocene india arabia plate boundary we propose a new timing for the uplift of the owen ridge and highlight its impact on the record of climate changes in pelagic sediments the new dataset reveals a fracture zone east of the owen ridge corresponding to the fossil plate boundary and documents that the main uplift of the owen ridge occurred close to 8 5 ma and is coeval with a major uplift of the east oman margin late miocene deformation at the india arabia plate boundary is also coeval with the onset of intra plate deformation in the central indian ocean suggesting a kinematic change of india and surrounding plates in the late miocene the uplift of the owen ridge above the lysocline at 8 5 ma accounts for a better preservation of globigerina bulloides in the pelagic cover previously misinterpreted as the result of a monsoon intensification event

the deep structure of the east oman continental margin preliminary results and interpretation abstract we describe preliminary results of a coincident normal incidence and wide angle seismic experiment across the east oman continental margin just north of the masirah island ophiolite this margin is affected by tectonic deformation and may be the site of margin parallel shear in a zone parallel to the owen fracture zone the reflection profile is used to define upper crustal structure and shows a deep offshore basin dammed oceanwards by a ridge interpreted here as a rotated fault block wide angle data were collected using ten digital ocean bottom seismometers and 110 explosive shots preliminary raytracing of a crustal model based on the gravity model of whitmarsh 1979 shows rapid changes in crustal thickness across the margin a steep landward dipping reflector probably the moho lies beneath the continental slope the orientation of this reflector is exactly opposite to the direction of tectonic fabric predicted by a simple overthrust model of ophiolite emplacement from the ocean basin to the east

somali basin chain ridge and origin of the northern somali basin gravity and geoid low the northern somali basin located between chain ridge and the horn of africa north of 4 n is characterized by a distinct 5 m geoid low and by large negative gravity anomalies the boundaries of the basin are marked by steep gradients in both gravity and geoid basement in northern somali basin is 1 2 km deeper than on the carlsberg ridge flank to the southeast or on the sheba ridge flank to the north with a sharp discontinuity across the boundary of the basin the western somali basin to the south was created in the late jurassic and early cretaceous by the movement of madagascar away from africa reinterpretation of the magnetic anomalies in the western somali basin shows that they record both limbs of a mid ocean ridge that was active by m22 time kimmeridgian and died soon after mo aptian magnetic and gravity data allow the relict ridge crest to be traced from davie ridge near the african coast to the dhow vlcc ars fracture zone complex at 50 e davie ridge is a transform fault connecting the western somali basin spreading center with a similar age spreading center in the mozambique basin the dhow vlcc ars complex can be shown to continue north of the 4 n bathymetric high separating the northern and western somali basins and to intersect the african margin near 7 n the northern somali basin thus appears to be the third of a series of oceanic basins separated by long transform faults created during movement between east and west gondwanaland the original northern somali basin was split apart by the northward motion of india in the late cretaceous and chain ridge formed along the new boundary thermal and gravity modeling shows that the flexure resulting from differential subsidence across chain ridge combined with the difference in lithospheric thermal structure late jurassic vs early tertiary on either side of it explains well the amplitude and shape of the observed geoid step and gravity anomalies across chain ridge the geoid step up from the basin up to the african coast can be modeled as an edge effect between the juxtaposed jurassic oceanic and african continental crust thus the geoid and gravity low over the northern somali basin results from the superposition of a continental edge effect anomaly and the fracture zone edge effect anomaly

owen fracture zone the arabia india plate boundary unveiled we surveyed the owen fracture zone at the boundary between the arabia and india plates in the nw indian ocean using a high resolution multibeam echo sounder owen cruise 2009 for search of active faults bathymetric data reveal a previously unrecognized submarine fault scarp system running for over 800 km between the sheba ridge in the gulf of aden and the makran subduction zone the primary plate boundary structure is not the bathymetrically high owen ridge but is instead a series of clearly delineated strike slip fault segments separated by several releasing and restraining bends despite an abundant sedimentary supply by the indus river flowing from the himalaya fault scarps are not obscured by recent deposits and can be followed over hundreds of kilometres pointing to very active tectonics the total strike slip displacement of the fault system is 10 12 km indicating that it has been active for the past 3 to 6 ma if its current rate of motion of 3 1 mm yr 1 has remained stable we describe the geometry of this recent fault system including a major pull apart basin at the latitude 20 n and we show that it closely follows an arc of small circle centred on the arabia india pole of rotation as expected for a transform plate boundary

thermal structure and megathrust seismogenic potential of the makran subduction zone the makran subduction zone experienced a tsunamigenic mw 8 1 earthquake in 1945 and recent smaller earthquakes also suggest seismicity on the megathrust however its historical record is limited and hazard potential enigmatic we have developed a 2 d thermal model of the subduction zone the results are twofold 1 the thick sediment cover on the incoming plate leads to high 150 plate boundary temperatures at the deformation front making the megathrust potentially seismogenic to a shallow depth and 2 the shallow dip of the subducting plate leads to a wide potential seismogenic zone up to 350 km combining these results with along strike rupture scenarios indicates that mw8 7 9 2 earthquakes are possible in the seaward makran subduction zone these results have important earthquake and tsunami hazard implications particularly for the adjacent coastlines of pakistan iran oman and india as the makran has not been previously considered a likely candidate for a mw 9 earthquake

a komatiite analog to potential ultramafic materials on io ultramafic volcanism is one model which fits currently available data for some eruptions on jupiter s moon io assuming that such activity does occur it is possible to apply komatiitic analogs and terrestrial ultramafic flow models to io we have used such analogs and models along with galileo data to investigate the nature of ultramafic materials that are assumed to occur at high temperature hotspots on io the unusual komatiites of the commondale greenstone belt south africa are consistent with available galileo data on the temperatures and composition of potential ionian ultramafic materials these komatiites had high sio2 50 wt and mgo 31 wt contents which are inferred to result in high liquidus temperatures 1610 c low dynamic viscosities 0 2 pa s and low densities 2680 kg m and in the crystallization of orthopyroxene phenocrysts not found in other komatiites we used the commondale composition and the model of williams et al 1998 to investigate potential ultramafic flow behavior on io compared to their terrestrial counterparts ionian ultramafic lava flows would have less potential for turbulent flow lower maximum emplacement distances and lower thermal erosion rates because of the lower ionian gravity and their more silicic composition shallow 10 m thermal erosion channels seem less likely on io unless 1 lavas are superheated 2 lavas are more ultramafic 31 mgo 3 eruption rates eruption durations or flow volumes are high or 4 substrates are unconsolidated partly consolidated or volatile rich and thus more erodable if volatiles are present in the lava as vesicles then increasing lava vesicularity results in decreasing maximum emplacement distances decreasing thermal erosion rates and decreasing erosion channel depths for given eruption durations

late cretaceous early tertiary convergence between the indian and arabian plates recorded in ophiolites and related sediments abstract remnants of ocean floor forming the eastern ophiolite belt in oman and the western ophiolite belt in pakistan have a common plate tectonic history culminating in emplacement at the cretaceous tertiary boundary fragments of ocean floor in these two belts have ages between 150 and 65 ma and recorded tectonic events in the early indian ocean at 150 ma 130 120 ma 110 100 ma and 70 65 ma new radiometric and chronostratigraphic ages and paleomagnetic and sedimentary information are used to relocate these ophiolites in the frame of the evolving indian ocean which was characterised by the stepwise breakup of gondwana at 158 ma east and west gondwana 130 ma southern atlantic breakup of east gondwana 95 84 ma madagascar and india seychelles 65 ma india and seychelles and finally at 40 ma rifting between africa and arabia the 150 ma old oceanic rocks of masirah island in oman originally formed the extension of the basins now preserved along the eastern edge of the afro arabian plate masirah drifted together with india seychelles when a new ridge formed at approximately 130 ma separating microplates e g kabul block from the northern edge of greater india parts of this new ocean were later emplaced to form the western ophiolite belt in pakistan consumption of oceanic crust and emplacement of ophiolites during the closure of the neotethys occurred not only north of india seychelles but also along two or three subduction zones between the afro arabian plate and india seychelles documented by the formation of metamorphic soles beneath ophiolites relics of ocean floor formed approximately 65 70 ma ago now form the upper units in the western ophiolite belt of pakistan where they overlie an accretionary prism of pillow lavas of dominantly aptian albian age and sedimentary rocks the breakup of india and the seychelles at 65 ma was associated with the eruption of flood basalts deccan and seychelles and enhanced counter clockwise movement of india this breakup possibly caused the late jurassic early cretaceous fragments of ocean floor e g masirah to be thrust onto the eastern edge of oman the ophiolites which were emplaced onto the western edge of india drifted as part of this plate farther north where collision with eurasia and the accreted microplates occurred in the eocene 55 ma

sea bed diffractions and their impact on 4d seismic data a sea bed reflection is used to estimate changes in water layer velocities between timelapse seismic surveys such corrections are crucial in order to obtain high quality 4d seismic data this might be a challenge in areas where rough sea bed topography creates sea bed diffractions that interfere with the sea bed reflection as in several of the northern fields in the norwegian sea these diffractions and diffracted multiples are difficult to attenuate during data processing and become a source of noise in time lapse data in this work we study how cross correlation analysis of time lapse seismic data at a sea bed reflection may be perturbed by the presence of diffractions the seabed topography from the norne field is used in 2d finite difference modelling to explain some of the observed variation in a water layer time shift in field data we find that a rough sea bed and the diffracted energy it induces cause residual noise on the 4d data the variation to the water layer time shift increases with sea bed complexity and is amplified by interaction with other sources of non repeatability like water column variations mis positioning and strength of the ice scours creating the diffracted energy time shift variations with mis positioning and velocity changes between the surveys seem to best explain the observed variation in the time shift for time lapse seismic field data from norne

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

a proven method for acquiring highly repeatable towed streamer seismic data seismic reservoir monitoring has become an important tool in the management of many fields monitoring subtle changes in the seismic properties of a reservoir caused by production places strong demands on seismic repeatability a lack of repeatability limits how frequently reservoir changes can be monitored or the applicability of seismic monitoring at all in this paper we show that towing many streamers with narrow separation combined with cross line interpolation of data onto predefined sail lines can give highly repeatable marine seismic data results from two controlled zero time lag monitoring experiments in the north sea demonstrate high sensitivity to changing water level and variations in lateral positions after corrections by deterministic tidal time shifts and spatial interpolation of the irregularly sampled streamer data relative rms difference amplitude levels are as low as 12 for a deep structurally complex field and as low as 6 for a shallow structurally simple field reducing the deg

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

discrimination between phase and amplitude attributes in time lapse seismic streamer data time lapse seismic experiments aim to obtain information about production related effects in hydrocarbon reservoirs to increase the recovery percentage however nonrepeatability problems such as acquisition differences overburden effects and noise are often significantly stronger than the imprint of production changes in time lapse seismic data sets consequently it is very difficult to appraise the changes in petrophysical reservoir parameters over time we introduce a 4d monitoring approach based on the spectral ratio method this method produces two time lapse attributes the relative change in reflection coefficient and the traveltime shift at reflecting interfaces these attributes can be used for appraising production related changes in the subsurface the approach corrects for time invariant nonrepeatability effects in the overburden and source receiver coupling problems in time lapse surveys the validity of the method is limited to structurally simple overburden and reservoirs with weak lateral variations first we validate the methodology using a synthetic time lapse seismic experiment next we apply the method to a real time lapse data set from the troll west gas province in the north sea in the real example we could not detect movement in the fluid contact of 5 15 m the expected change in amplitude is less than 10 which is probably below the background noise level for this data set

tracking the paleogene india arabia plate boundary the location of the india arabia plate boundary prior to the formation of the sheba ridge in the gulf of aden is a matter of debate a seismic dataset crossing the owen fracture zone the owen basin and the oman margin was acquired to track the past locations of the india arabia plate boundary we highlight the composite age of the owen basin basement made of paleocene oceanic crust drilled on its eastern part and composed of pre maastrichtian continental and oceanic crust overlaid by ophiolites emplaced in early paleocene on its western side a major fossil transform fault system crossing the owen basin juxtaposed these two slivers of lithosphere of different ages and controlled the uplift of marginal ridges along the oman margin this transform system deactivated 40 myrs ago coeval with the onset of ultra slow spreading at the carlsberg ridge the transform boundary then jumped to the edge of the present day owen ridge during the late eocene oligocene period before seafloor spreading began at the sheba ridge this migration of the plate boundary involved the transfer of a part of the indian oceanic lithosphere formed at the carlsberg ridge to arabia this late eocene oligocene tectonic episode at the india arabia plate boundary is synchronous with a global plate reorganization event corresponding to geological events at the zagros and himalaya belts the owen ridge uplifted later in late miocene times and is unrelated to any major migration of the india arabia boundary

emplacement and erosion by archean komatiite lava flows at kambalda revisited we have developed a mathematical model to evaluate the flow and erosional potential of submarine channelized komatiite lavas at kambalda western australia field data from kambalda were used to constrain the choice of important input parameters and model results were compared with data from field studies and geochemical analyses our results suggest that thermal erosion is strongly dependent upon the nature and behavior of the substrate if the substrate is treated as an unconsolidated hydrous sediment that can be fluidized by vaporized seawater then our model predicts that an initially 10 m thick basal kambalda komatiite lava could have produced very high thermo mechanical erosion rates 23 10 m day crustal thicknesses of 5 20 cm at distances of 5 35 km from the source and a high degree of lava contamination 3 12 in contrast if the substrate is treated as a more consolidated anhydrous sediment that could not be fluidized then our model predicts that a kambalda komatiite flow would have had much lower thermal erosion rates 1 2 0 4 m day and degrees of contamination 1 3 and would have had crustal thicknesses of 5 20 cm at longer flow distances of 30 165 km field constraints are generally consistent with our predictions for a non fluidized substrate the reentrant embayments at kambalda are thought to form from either erosion of deep channels in a flat basaltic seafioor huppert et al 1984 or erosion of a thin 5 m sediment with minor undercutting of basalt in pre existing topography lesher et al 1984 our modeling indicates that the former was possible for long eruption durations months whereas the latter was possible for short eruption durations 2 weeks as the latter hypothesis is more consistent with the existing field evidence for thermal erosion at kambalda we believe it is the preferred interpretation

a summary of the geology and oil habitat of the eastern flank hydrocarbon province of south oman abstract the eastern flank hydrocarbon province borders the southeastern edge of the south oman salt basin in the southern part of oman an intensive exploration programme by pdo over the past seven years has led to the discovery of almost 2 10 9 m 3 of oil with current production of some 15000 m 3 d from six fields in stark contrast to other oil habitats of the arabian peninsula the main play involves rock units and geological events ranging in age from late precambrian to quaternary times while trap development and charging are closely related to syn tectonic salt dissolution the principal reservoirs and seals are poorly consolidated palaeozoic clastics which drape due to salt dissolution over residual cores principally composed of either lower palaeozoic sandstones haima group or late proterozoic carbonates huqf group or some combination of these oils have been geochemically correlated with algal source rocks of the huqf eastern flank structures are considered to have been charged from late mesozoic times onwards in conjunction with progressive subsurface salt removal and the onset of maturation of local source rocks the effectiveness of charging along the retreating salt edge is indicated by the small percentage of dry wells major oil zones are found in both anticlinal and truncation traps which are commonly developed around sandstone haima group cores oils show a wide range of characteristics but are typically heavy early expulsion crudes high sulphur with little associated gas bacterial transformation is not uncommon producing reservoirs mainly occur at relatively shallow depths 600 1200 m primary recovery factors of around 7 reflect the high density and viscosity of the eastern flank crudes and have initiated interest in enhanced oil recovery eor techniques two such pilot projects involving steam and polymer flooding are due to commence in 1984 and if successful will substantially increase the reserve base of these fields

organic geochemistry and basin modeling of late cretaceous harshiyat formation in the onshore and offshore basins in yemen implications for effective source rock potential and hydrocarbon generation abstract the purpose of this study is to present the source rock characteristics of late cretaceous harshiyat formation source rock in the onshore and offshore basins of yemen i e mukalla sayhut sayun masilah and jiza qamar source rock geochemical characterizations were completed and integrated to perform 1 d basin modeling to back predict source rock thermal maturation oil generation and expulsion the geochemical findings revealed that mudstone intervals within the harshiyat formation comprised favorable source rocks with total organic carbon toc content between 0 50 and 35 10 wt the organic matter intervals in the harshiyat mudstones are mainly types ii iii and iii kerogens with certain amount of type ii and i kerogens based on rock eval hi values ranging from 40 to 923 mg hc g toc mixed kerogen types were consistent with vitrinite and sapropelic organic matters as observed by visual kerogen examination and the distribution of biomarker fingerprints the existence of mixed types i ii and iii organic matter suggests that the harshiyat mudstones could contribute to oil and gas at sufficient thermal maturity levels the biomarker data of the organic matter in the harshiyat mudstones show that the source rock contains a mix marine phytoplanktonic bacteria terrestrial land plant enthronement and were deposited in suboxic anoxic conditions results of chemical and optical maturity indicators led to the conclusion of varying maturity stages from immature to late oil window levels for the mudstones of the harshiyat formation thermal maturation distributions inferred from the analyzed samples indicate that mudstone intervals in the offshore mukalla sayhut basin are extremely mature compatible with the late mature oil window and may have generated significant amounts of oil this interpretation was supported by strong geochemical similarity between the organic matter in the mudstones and those of the oils found in the offshore mukalla sayhut basin and demonstrated by the concentrations of n alkanes and isoprenoids as well as carbon isotopes in the offshore ras ghashwah 1x well burial and thermal models were constructed and used to address the effect of tectonic events in triggering oil generation and expulsion from mature harshiyat source rock in the offshore area kinetic database models for type ii and iii kerogen mixtures indicate that the initial conversion of kerogen to oil occurred since the late oligocene with transformation ratios tr that span between 10 and 50 furthermore the oil was expelled from organic matter intervals in the harshiyat source rock starting from late oligocene and continuing until late miocene with trs of 50 72 from that period up to the present the retained oil was cracked into wet gas in the gas window with an easy ro greater than 1 30 and with peak tr ratios greater than 72

a diffuse plate boundary model for indian ocean tectonics it is suggested that motion along the virtually aseismic owen fracture zone is negligible so that arabia and india are contained within a single indo arabian plate divided from the australian plate by a diffuse boundary the boundary is a zone of concentrated seismicity and deformation commonly characterized as intraplate the rotation vector of australia relative to indo arabia is consistent with the seismologically observed 2 cm yr of left lateral strike slip along the ninetyeast ridge north south compression in the central indian ocean and the north south extension near chagos

field test of a permanent in well fiber optic seismic system a deviated borehole was drilled and completed to test systems for deployment of permanent in well three component fiber optic seismic sensors two systems were developed and tested for deployment of these sensors the first system involved using a passive mandrel device to enable coupling of the sensors simply using the weight of the tubing the second system involved use of an active mandrel that incorporates a spring loaded pad device to isolate the sensor from the tubing vibration in addition reference wireline vertical seismic profile data were acquired both in the empty cased borehole and through the tubing at the same location as the fiber optic sensor analysis of the data showed that best results were obtained using the active design tubing related resonances were noted on data acquired using the passive design whereas the active design did a good job of discriminating among these undesirable signals