tectonic interactions between india and arabia since the jurassic reconstructed from marine geophysics ophiolite geology and seismic tomography gondwana breakup since the jurassic and the northward motion of india toward eurasia were associated with formation of ocean basins and ophiolite obduction between and onto the indian and arabian margins here we reconcile marine geophysical data from preserved oceanic basins with the age and location of ophiolites in nw india and se arabia and seismic tomography of the mantle below the nw indian ocean the north somali and proto owen basins formed due to 160 133 ma n s extension between india and somalia subsequent convergence destroyed part of this crust simultaneous with the uplift of the masirah ophiolites most of the preserved crust in the owen basin may have formed between 84 and 74 ma whereas the mascarene and the amirante basins accommodated motion between india and madagascar east africa between 85 and circa 60 ma and 75 and circa 66 ma respectively between circa 84 and 45 ma oblique arabia india convergence culminated in ophiolite obduction onto se arabia and nw india and formed the carlsberg slab in the lower mantle below the nw indian ocean the nne ssw oriented slab may explain the anomalous bathymetry in the nw indian ocean and may be considered a paleolongitudinal constraint for absolute plate motion nw india asia collision occurred at circa 20 ma deforming the sulaiman ranges or at 30 ma if the hindu kush slab north of the afghan block reflects intra asian subduction our study highlights that the nw india ophiolites have no relationship with india asia motion or collision but result from relative india africa arabia motions instead key points we present a new tectonic model for the evolution of nw indian ocean subducted slab under the carlsberg ridge resulted from arabia india convergence

initial chronology of a recently discovered hydrothermal field at 14 45 n mid atlantic ridge two expeditions of the sevmorgeologija association 1991 1994 led to the discovery of two new hydrothermal sites on the mid atlantic ridge mar south of the 15 20 north fracture zone one around 14 45 n and the other around 14 42 n the northern one between 14 45 and 14 45 3 n has been studied in detail about 12 mounds have been mapped and 3 of them have been sampled using a large hydraulic grab sampler the largest one is about 200 m long and 200 m wide when progressively moving up on the slope of an uplifted block of the rift valley floor the sulphide samples have revealed ages ranging from about 10 ka to 60 ka the ages were obtained using the 230th 234u dating method used for chronological studies of diverse hydrothermal fields the general picture of this lateral location of the samples of different ages provides evidence of a shift in the focus of hydrothermal activity with time moreover there were rejuvenation stages of hydrothermal activity including black and white smokers

true triaxial stresses and the brittle fracture of rock this paper reviews the efforts made in the last 100 years to characterize the effect of the intermediate principal stress 2 on brittle fracture of rocks and on their strength criteria the most common theories of failure in geomechanics such as those of coulomb and mohr disregard 2 and are typically based on triaxial testing of cylindrical rock samples subjected to equal minimum and intermediate principal stresses 3 2 however as early as 1915 boker conducted conventional triaxial extension tests 1 2 on the same carrara marble tested earlier in conventional triaxial compression by von karman that showed a different strength behavior efforts to incorporate the effect of 2 on rock strength continued in the second half of the last century through the work of nadai drucker and prager murrell handin wiebols and cook and others in 1971 mogi designed a high capacity true triaxial testing machine and was the first to obtain complete true triaxial strength criteria for several rocks based on experimental data following his pioneering work several other laboratories developed equipment and conducted true triaxial tests revealing the extent of 2 effect on rock strength e g takahashi and koide michelis smart wawersik testing equipment emulating mogi s but considerably more compact was developed at the university of wisconsin and used for true triaxial testing of some very strong crystalline rocks test results revealed three distinct compressive failure mechanisms depending on loading mode and rock type shear faulting resulting from extensile microcrack localization multiple splitting along the 1 axis and nondilatant shear failure the true triaxial strength criterion for the ktb amphibolite derived from such tests was used in conjunction with logged breakout dimensions to estimate the maximum horizontal in situ stress in the ktb ultra deep scientific hole

isostatic response of the alula fartak and owen transforms in the eastern gulf of aden and the adjoining arabian sea summary the alula fartak and owen transforms are the active parts of major fracture zones with a distinct topographic expression in the eastern gulf of aden and the arabian sea respectively while the alula fartak transform offsets the sheba ridge by about 180 km and is associated with a broad steep sided valley with a relief of nearly 3 5 km the owen transform offsets the carlsberg ridge by nearly 300 km and is associated with a broad step like valley surrounded by deeper water depths the gravity and topography data along several profiles selected across these two transforms have been analysed using cross spectral analysis in order to investigate their isostatic compensation the observed admittance estimates have been compared with three theoretical isostatic compensation models two local compensation models airy i and ii and one regional compensation plate model comparing the longer wavelength admittance estimates suggests that the regional compensation model gives the best fit for both the alula fartak transform and the owen transform with effective elastic thickness te of 5 km and slightly less than 10 km respectively for the alula fartak transform the airy ii model might also be acceptable though with large scatter in the observed values it suggests a mean value of 9 km for the mantle layer with a 6 km thick crust for the owen transform on the other hand the two local compensation models failed the difference in te estimate between the two transforms could be ascribed to differences in thermal structure arising from their varied tectonic history a comparison with the isostatic response estimates of transform fracture zones along the slow spreading mid atlantic ridge suggests that the regional compensation model is generally applicable for transform fracture zone topography along such mid ocean ridges

actinide series disequilibrium as a tool to establish the chronology of deep sea hydrothermal activity this paper describes the different radiochronological methods used to date geologically recent i e 400 000 years deep sea hydrothermal deposits as well as the basic conditions necessary to obtain reliable dates the limitations of the different techniques also are described using measurements of 210pbpb 228th228ra 230th234u 231pa235u and 228ra226ra the authors have undertaken an exhaustive chronological study of the hydrothermal deposits along the east pacific rise the mid atlantic ridge and in some back arc basins the objectives of this study were to obtain regional chronologies and to establish a general synthesis on the evolution of the hydrothermal processes at the scale of the mid oceanic ridge system some results obtained by other authors are included in this synthesis the dependence of the general trends of temporal development of the hydrothermal chimneys edifices and fields on their tectonic settings is discussed this study demonstrates that hydrothermal activity does not represent a regular input of matter to the ocean and that its pulsed character must be taken into account in all modeling attempts chemical biological and tectonic affected by hydrothermal processes in areas of rapid spreading like the east pacific rise recent and fossil deposits are spatially separated by contrast at the slow spreading mid atlantic ridge fossil and present activity are found in the same location

calyptogena cemented rocks and concretions from the eastern part of nankai accretionary prism age and geochemistry of uranium calyptogena valves included in a carbonate rich cement and fragments of a carbonate rich chimney have been examined for their stable isotopic c and o composition 14c activity and uranium series disequilibrium the fossil shells were formed essentially with seawater carbon and a negligible contribution of cold seepage organic carbon as shown by their 13c values this allows the14c method to be used to determine their age a fairly good concordance between the14c and230th234u ages of the youngest shells gives confidence in the dating of the older samples using the latter technique thus thecalyptogena are dated at ca 150 000 and 20 000 yrs b p they have been preserved from dissolution by rapid cementation by a supersaturated carbonate solution the cement is especially rich in uranium as high as 75 ppm whose source is seawater the enrichment is due to local reducing conditions brought about by the bacterial decomposition of the soft tissues of the bivalves shortly after death thecalyptogena that probably developed between these two events the events of ca 20 000 and 150 000 yrs have not been preserved from dissolution because as is presently the case the cold seepages were undersaturated with calcium carbonate the two events probably represent periods of intense fluid venting connected with tectonic activity

geochemistry of metalliferous hydrothermal deposits in the aeolian arc tyrrhenian sea abstract in shallow water areas of the submerged volcanic complex around the island of panarea aeolian archipelago hydrothermal precipitation of both low temperature fe oxyhydroxide rich red muds and crusts and high temperature sediment hosted massive sulfides was discovered during an integrated high resolution survey iron rich crusts were also found on the bathymetric high of secca del capo north of salina island the exhalative iron rich sediments occur in small closed depressions or in proximity to faults and scarps at water depths ranging from 55 to 285 m the principal chemical characteristics of these deposits are high but variable fe content ranging from 12 2 to 45 and low contents of the transition elements mn cu zn ni and co the low contents of cu ni and co suggest a hydrothermal origin the fe oxyhydroxide deposits are enriched in light rare earth element ree 35 110 chondrite compared to heavy ree 10 25 chondrite their ree patterns are similar to those of associated calc alkaline volcanics negative slope of light ree and a horizontal distribution of the heavy ones this contrasts with the pattern for iron and manganese oxides of hydrogenous origin which have tilted trends of heavy ree paralleling that of seawater the mineralogy of the polymetallic sulfide deposits consists of galena sphalerite pyrite and barite in the form of silt sand grains and decimeter sized fragments disseminated in clay 30 cm below the seabed at a waterdepth of 80 m the chemistry of the aeolian iron rich low temperature deposits and of the high temperature ba rich pb and zn sulfides suggests that they are genetically analogous to respectively the kuroko type iron formation tetsusekiei and the black ore exposed in the miocene hokuroku district of japan

on isostasy at atlantic type continental margins cross spectral techniques have been used to analyze the relationship between free air gravity anomalies and topography of approximately 100 profiles over the continental margins of eastern north america southwest africa and the coral sea lord howe rise the resulting admittance functions for these margins have been interpreted in terms of the flexure model of isostasy the relatively young coral sea lord howe rise margin is associated with the lowest value of the effective elastic thickness te 5 km while the relatively old eastern north american margin is associated with the highest value 10 te 20 km thus te appears to increase with the age of the margin these results which are consistent with flexure studies from the major ocean basins have been interpreted in terms of a simple model in which the flexural strength of the lithosphere increases with age the model suggests that a low rigidity plate or airy type model is most applicable early in rifting history while a high rigidity plate is more applicable later in margin evolution thus sediments at a margin initially load a relatively thin and hot plate while sediments later in margin evolution load a relatively thick and cold plate the model explains a number of features of the tectonic stratigraphic evolution of margins these include a change in flexural style from a narrow and deep basin to a broad and shallow basin coastal plain onlap and uplift in the continental interior furthermore the flexure model of isostasy contributes in a major way to observed free air gravity anomalies at continental margins in particular the model explains the existance of airy isostatic gravity anomalies at margins the amplitude and wavelength of these anomalies are a function of the sediment supply the type of mechanisms controlling sediment deposition up building and or out building and the thermal structure history and age of the margin

hydrothermal manganese plumes in the mid atlantic ridge rift valley the concentration of dissolved manganese in sea water can be used as a chemical tracer of hydrothermal activity on the underlying sea floor here we present vertical profiles of seawater manganese concentrations above the rift valley of the mid atlantic ridge mar the values obtained indicate the presence of plumes that are presently active generated by hydrothermal vents in the rift valley these plumes are confined to the rift valley and do not spill over into the adjacent deep ocean basins differences between them suggest that there are at least five hydrothermal sources along this 1 700 km long section of mid ocean ridge

the early cenozoic tectonic history of the southeast pacific abstract new marine geophysical data collected in the southeast pacific enable us to define better the early cenozoic tectonic history of the area and to interpret previously unexplained bathymetric features in terms of plate tectonic processes we have identified a major re organization in plate boundaries at anomaly 21 time when a large fragment was broken off the pacific plate and joined to the antarctic plate this interpretation is based on three pieces of evidence 1 a major ese trending feature the humboldt fracture zone marks the southern boundary of the farallon antarctic anomalies identified by weissel et al 1 2 a north south striking trough the hudson trough which represents the scar left by the pacific antarctic ridge when it rifted across old pacific crust 3 a sequence of north south striking magnetic lineations south of the humboldt fracture zone and east of the hudson trough is correlated to anomalies 27 to 22 and identified with pacific aluk spreading

lamprophyric volcanism of cenozoic age on the sierra leone rise implications for regional tectonics and the stratigraphic time scale abstract igneous and sedimentary rocks have been recovered by dredging the upper slopes of krause seamount a feature lying on the sierra leone rise near 9 n 20 w in the eastern equatorial atlantic the igneous samples consist of lamprophyres and an ankaramite which are rarely encountered in an oceanic setting associated with these rocks are breccias made up of lamprophyric fragments set in a lithified calcareous matrix containing a rich planktonic foraminiferal assemblage of early eocene ypresian subzone p8a age whole rock k ar dating of the igneous samples proved unsuccessful owing to advanced alteration and consequent argon loss however within the breccias are numerous fresh crystal clasts of ti rich amphibole pyroxene and biotite which have compositions closely similar to phenocrysts observed in the lamprophyric rocks suggesting that they are derived directly from them k ar dating of separates of the three minerals yielded ages in the range 53 3 55 4 ma the concordant results provide a maximum age for the ypresian subzone p8a and are consistent with its position within recently established cenozoic time scales they indicate that krause seamount is appreciably younger than the adjacent gambia and sierra leone basins in common with other occurrences of lamprophyric rocks in the oceans those exposed on krause seamount are located in an area of old oceanic basement close to a large fracture zone which has been reactivated by late stage volcanic activity these relatively rare alkaline rocks may be generated by remelting of a volatile rich metasomatized layer which is present in the upper mantle beneath mesozoic ocean floor

on the initiation of subduction zones analysis of the relation between intraplate stress fields and lithospheric rheology leads to greater insight into the role that initiation of subduction plays in the tectonic evolution of the lithosphere numerical model studies show that if after a short evolution of a passive margin time span a few tens of million years subduction has not yet started continued aging of the passive margin alone does not result in conditions more favorable for transformation into an active margin

active and relict sea floor hydrothermal mineralization at the tag hydrothermal field mid atlantic ridge the tag hydrothermal field is a site of major active and inactive volcanic hosted hydrothermal mineralization in the rift valley of the slow spreading mid atlantic ridge at 26 degree n the axial high is the principal locus of present magmatic intrusions the tag field contains three main areas of present and past hydrothermal activity 1 an actively venting high temperature sulfide mound 2 two former high temperature vent areas 3 a zone of low temperature venting and precipitation of fe and mn oxide deposits the volcanic centers occur at the intersections between ridge axis parallel normal faults and projected axis transverse transfer faults the intersections of these active fault systems may act as conduits both for magmatic intrusions from sources beneath the axial high that build the volcanic centers and for hydrothermal upwelling that taps the heat sources radiometric dating of sulfide samples and manganese crusts in the hydrothermal zones and dating of sediments intercalated with pillow lava flows in the volcanic center adjacent to the active sulfide mound indicate multiple episodes of hydrothermal activity throughout the field driven by heat supplied by episodic intrusions over a period of at least 140 times 10 sup 3 yr the sulfide deposits are built by juxtaposition and superposition during relatively long residence times near episodic axial heat sources counterbalanced by mass wasting in the tectonically active rift valley of the slow spreading oceanic ridge hydrothermal reworking of a relict hydrothermal zone by high temperature hydrothermal episodes has recrystallized sulfides and concentrated the first visible primary gold reported in a deposit at an oceanic ridge

the geological setting of the ultramafic hosted logatchev hydrothermal field 14 45 n mid atlantic ridge and its influence on massive sulfide formation the logatchev hydrothermal field 14 degrees 45 n on the mar is one of a few submarine hydrothermal systems associated with ultramafic rocks it is situated on the eastern inner flank of the rift valley wall 7 km away from the spreading axis and its formation has previously been linked to detachment faulting and core complex formation geological mapping during various rov dives geological sampling and shallow drilling reveal a structural control of hydrothermal activity as well as its location in a debris flow consisting of heterogeneous ultramafic and mafic intrusive rocks the mixed mafic ultramafic host rock lithology is in agreement with published vent fluid and gas chemical data showing characteristics for interaction with mafic as well as with ultramafic rocks massive sulfide formation is more focused than previously thought and likely limited to a thin veneer at the seafloor the logatchev hydrothermal field shows a number of peculiarities that are unusual for most other hydrothermal systems one of these are so called smoking craters seafloor depressions that are several meters wide characterized by an elevated crater rim made up partly of sulfide talus but also of abundant wall rock material at these smoking craters hydrothermal venting occurs directly from holes within the craters and from small cm to dm high cu rich chimneys occurring at the crater rim based on geological mapping and sampling we suggest that these smoking craters are the product of processes related to the regional and local geological setting in an ultramafic hosted off axis location with abundant landslides as well as off axis gabbroic intrusions providing the heat for the hydrothermal convection cell c 2009 elsevier b v all rights reserved

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

chronology of selected hydrothermal mn oxide deposits from the transatlantic geotraverse tag area mid atlantic ridge 26 n abstract radiometric ages on discrete layers in some hydrothermally deposited mn oxide birnessite samples from the tag hydrothermal field on the mid atlantic ridge range from 16 000 to 4 000 years with marked increased activity noted about 15 000 and 7 000 years ago these observations along with previously published data and inferences on different kinds of hydrothermal activity in the tag field suggest episodic multistage hydrothermal events ranging from high temperature black smokers precipitating sulfides to low temperature events precipitating fe and mn oxides over this entire time range black smokers are presently active the location of the various deposits on the eastern wall of the median valley at this latitude suggests that the high temperature events occur at greater depths

off ridge alkaline magmatism and seamount volcanoes in the masirah island ophiolite oman abstract the masirah ophiolite offers an unique opportunity to study well preserved small seamount structures obducted seamounts have not been described up to now and from the present day ocean floor they are almost exclusively known from bathymetric studies the thin oceanic crust of the masirah ophiolite was formed at a ridge transform intersect in upper jurassic time it was overprinted and reworked by a major intra oceanic tectono magmatic event at mid cretaceous time that has been well dated owing to the presence of interstratified sedimentary rocks late hauterivian to early barremian c 130 125 ma this mid cretaceous magmatism produced alkaline volcanic rocks ranging in chemistry from alkalibasalts to rhyolites volcanism occurred in a nw se extensional regime small elongate submarine volcano structures seamounts developed within widespread alkalibasaltic pillow lava and pillow breccia deposits which are interfingered with deep marine pelagic sediments the volcanoes reached a maximum of a few kilometres in diameter and a few hundred metres in height the seamounts are built up of basic to acid subvolcanic stock or sheet like intrusions several generations of dikes vent agglomerates and pyro to epiclastic deposits the latter range from coarse breccias to finely stratified lapilli and record explosive volcanism in a deep marine environment in the magma chambers under the volcanoes local differentiations to trachytic and rhyolitic members took place the alkaline rocks show a pronounced ocean island basalt oib character indicating the considerable contribution of a mantle plume source hotspot as cause of the volcanism we propose a combination of original transform setting followed by drift past the marion hotspot during the major plate tectonic reorganization between greater india madagascar and africa starting in mid cretaceous time

radiochronological investigation of hydrothermal deposits from the meso zone central indian ridge abstract hydrothermal activity in the indian ocean has hitherto only been poorly documented first indications of hydrothermal mineralization were observed in 1987 during the gemino cruise over the 4th ridge segment of the central indian ridge 270 km north of the rodriguez triple junction subsequent cruises in 1993 hydrotrunc and in 1995 hydrock were carried out in order to obtain detailed information about the geological setting and the extent and mineral distribution of the field but primarily to sample the first massive sulfides of the indian ocean floor this hydrothermal field which was called the meso zone was studied by detailed mapping and by ocean bottom photography hydrothermal precipitates were recovered with a tv grab sampler at four sampling locations two in the northern part of the mineralized field called the talus tips site and two in the central part called the sonne field the sulfide samples were age dated using the 230 th 234 u method they show different periods of activity at each site 52 5 22 7 3 and 16 2 ka for the main events at the talus tips site and 18 2 and 12 5 1 5 ka for the sonne field we conclude from these data that the two sites were not active at the same time if it is assumed that there is a complete cessation of activity between 52 and 23 ka for more recent periods from 23 to 10 ka it may be postulated that the deep seated hydrothermal circulation is continuous but that the discharge of hot fluid occurs alternately at the sonne field or at the talus tips site comparing our results with those obtained previously at the different hydrothermal sites we favor the conclusion of episodic activity moreover in the sonne field jasper formation resulting from a low temperature hydrothermal event dated at around 11 ka is presently the youngest hydrothermal formation in the entire meso zone

northern east pacific rise evolution from 25 m y b p to the present the northeast pacific topography and magnetic lineations 25 m y b p to the present record the traces of three major spreading reorganizations only one spreading center is observed today but there is evidence for several ephemeral episodes of twin spreading accompanying the evolution from an extensive pacific guadalupe plate boundary to a much shortened pacific cocos and pacific rivera plate boundary the 25 m y b p plate reorganization culminated with the formation of the guadalupe plate bound by the murray fracture zone to the north and the cocos nazca spreading ridge to the south between 25 and 12 5 m y b p spreading continued while the plate retained the same general outline the 12 5 11 m y b p reorganization resulted in the creation of a much shortened pacific cocos plate boundary located in its early stages over the mathematician seamounts and a much reduced cocos plate the last reorganizaton 6 5 3 5 m y b p resulted in the abandonment of the mathematician spreading ridge as a pacific cocos plate boundary in favor of the east pacific rise

geochemical time series of hydrothermal fluids from the slow spreading mid atlantic ridge implications of medium term stability abstract time series investigations of hydrothermal fluids from different geological settings at the slow spreading mid atlantic ridge mar indicate a surprising medium term stability of their chemical composition over up to 20 years this does not only apply to tectonically controlled systems with exposure of mantle rocks where magmatic activity is limited such as the logatchev 1 hydrothermal field at 14 45 n we also found the 407 c hot and extremely metal rich fluids at the 5 s site turtle pits 2986 m water depth to discharge at constant composition indicating supercritical phase separation for at least 4 years which contrasts with rather dynamic volcanically driven hydrothermal systems at the fast spreading east pacific rise epr possibly fluid pathways and pressure temperature conditions are more stable in tectonically controlled settings at mar than at the epr where frequent volcanic activity affects permeability within the hydrothermal systems despite some spatial variability hydrothermal fluids discharging at the shallower ca 810 870 m menez gwen hydrothermal field at 37 50 n seem to represent a stable discharge of phase separated vapor type fluids over 16 years this demonstrates that medium term stability of hydrothermal fluid composition seems to be a general feature at slow spreading ridges as a consequence hydrothermal vent organisms at the mar may live in a more stable chemical environment compared to vent systems at fast spreading ridges considering the fact that slow spreading ridges account for more than half of the submarine ridge system such a constant emanation of hydrothermal fluids makes hydrothermal vent systems on slow spreading ridges a medium term stable source of fe and other micronutrients into the ocean

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

global positioning system measurements of deformations associated with the 1987 superstition hills earthquake evidence for conjugate faulting large station displacements observed from imperial valley global positioning system gps campaigns are attributed to the november 24 1987 superstition hills earthquake sequence thirty sites from a 42 station gps network established in 1986 have been reoccupied during 1988 and or 1990 displacements at three sites within 3 km of the surface rupture approach 0 5 m eight additional stations within 20 km of the seismic zone are displaced at least 10 cm this is the first occurrence of a large earthquake ms 6 6 within a preexisting gps network best fitting uniform slip models of rectangular dislocations in an elastic half space indicate 130 8 cm right lateral displacement along the northwest trending superstition hills fault and 30 10 cm left lateral displacement along the conjugate northeast trending elmore ranch fault the geodetic moments are 9 4 1025 dyn cm and 2 3 1025 dyn cm for the superstition hills and elmore ranch faults respectively consistent with teleseismic source parameters the data also suggest that postseismic slip along the superstition hills fault is concentrated at shallow depths distributed slip solutions using singular value decomposition indicate near uniform displacement along the elmore ranch fault and concentrated slip to the northwest and southeast along the superstition hills fault a significant component of nonseismic secular displacement is observed across the imperial valley which is attributed to interseismic plate boundary deformation

u th systematics and 230th ages of carbonate chimneys at the lost city hydrothermal field the lost city hydrothermal field lchf is a serpentinite hosted vent field located 15 km west of the spreading axis of the mid atlantic ridge in this study uranium thorium u th geochronological techniques have been used to examine the u th systematics of hydrothermal fluids and the 230th ages of hydrothermally precipitated carbonate chimneys at the lchf fluid sample analyses indicate that endmember fluids likely contain only 0 0073 ng g u or less compared to 3 28 0 03 ng g of u in ambient seawater for fluid samples containing only 2 21 ambient seawater 1 1 11 mmol kg mg th concentration is 0 11 0 13 pg g and surrounding seawater concentrations average 0 133 0 016 pg g the 230th 232th atomic ratios of the vent fluids range from 1 10 10 6 to 11 5 10 6 are less than those of seawater and indicate that the vent fluids may contribute a minor amount of non radiogenic 230th to the lchf carbonate chimney deposits chimney 238u concentrations range from 1 to 10 g g and the average chimney corrected initial 234u is 147 2 0 8 which is not significantly different from the ambient seawater value of 146 5 0 6 carbonate 232th concentrations range broadly from 0 0038 0 0003 to 125 16 ng g and 230th 232th atomic ratios vary from near seawater values of 43 8 10 6 up to 530 25 10 3 chimney ages corrected for initial 230th range from 17 6 yrs to 120 13 kyrs the youngest chimneys are at the intersection of two active steeply dipping normal faults that cut the atlantis massif the oldest chimneys are located in the southwest portion of the field vent deposits on a steep fault bounded wall on the east side of the field are all 4 kyrs old indicating that mass wasting in this region is relatively recent comparison of results to prior age dating investigations of submarine hydrothermal systems shows that the lchf is the most long lived hydrothermal system known to date it is likely that seismic activity and active faulting within the atlantis massif and the atlantis fracture zone coupled with volumetric expansion of the underlying serpentinized host rocks play major roles in sustaining hydrothermal activity at this site the longevity of venting at the lchf may have implications for ecological succession of microorganisms within serpentinite hosted vent environments

the breakup of east gondwana assimilating constraints from cretaceous ocean basins around india into a best fit tectonic model published models for the cretaceous sea oor spreading history of east gondwana result in unlikely tectonic scenarios for at least one of the plate boundaries involved and or violate particular constraints from at least one of the associated ocean basins we link east gondwana spreading corridors by integrating magnetic and gravity anomaly data from the enderby basin off east antarctica within a regional plate kinematic framework to identify a conjugate series of east west trending magnetic anomalies m4 to m0 126 7 120 4 ma the mid ocean ridge that separated greater india from australia antarctica propagated from north to south starting at 136ma northwest of australia and reached the southern tip of india at 126 ma sea oor spreading in the enderby basin was abandoned at 115 ma when a ridge jump transferred the elan bank and south kerguelen plateau to the antarctic plate our revised plate kinematic model helps resolve the problem of successive two way strike slip motion between madagascar and india seen in many previously published reconstructions and also suggests that sea oor spreading between them progressed from south to north from 94 to 84 ma this timing is essential for tectonic ow lines to match the curved fracture zones of the wharton and enderby basins as greater india gradually began to unzip from madagascar from 100 ma in our model the 85 east ridge and kerguelen fracture zone formed as conjugate anks of a leaky transform fault following the 100ma spreading reorganization our model also identi es the afanasy nikitin seamounts as products of the conrad rise hotspot

how accurately can we date the duration of magmatic hydrothermal events in porphyry systems an invited paper determining the absolute duration of magmatic hydrothermal events leading to the formation of porphyry systems i e including porphyry copper skarn and epithermal deposits is one of the key questions in ore geology this is so because the duration of magmatic hydrothermal events in porphyry systems is instrumental to the development of genetic models necessary to explore a category of mineral deposits that provide most of the copper and significant amounts of base and precious metals to our economy the problem of determining the absolute duration of magmatic hydrothermal events in porphyry systems has been addressed through thermal modeling of cooling intrusions and time needed to precipitate specified metal amounts from active hydrothermal systems with known metal concentrations and fluid fluxes both these methods have shown that the likely duration of hydrothermal systems is on the order of a few tens of kilo annum ka isotopic dating in contrast is the only possible way to determine the life span of magmatic hydrothermal events in fossil porphyry systems analytical and methodological developments during the last decade in the fields of the most robust isotopic systems commonly used for absolute dating u pb 40ar 39ar re os allow us to date minerals with internal precisions 0 2 2 for a 10 ma old mineral this corresponds to a 20 ka uncertainty which is marginally sufficient to discriminate the duration of hydrothermal systems at the tens of kilo annum scale however many geochronological studies on fossil porphyry systems have shown that these are most often formed through repeated cycles of several intrusion events which extend the overall life of the porphyry systems to a few 0 x and up to 2 ma in some cases internal precisions of the above mentioned dating methods allow us in theory to comfortably discriminate events at the 0 x scale and the combination of u pb 40ar 39ar and reos geochronology is a tool widely used by ore geologists to bracket the duration of cyclic magmatic hydrothermal events in porphyry systems in this review we discuss some fundamental problems that are systematically overlooked in most geochronological studies trying to bracket the life span of porphyry systems we show that if these problems are not adequately taken into account and tackled the result will be that fundamentally wrong life spans of porphyry systems will be estimated we also provide basic guidelines to follow when trying to resolve the duration of magmatic hydrothermal events in porphyry systems with the highest accuracy and precision currently achievable

estimation of maximum horizontal principal stress magnitude from stress induced well bore breakouts in the cajon pass scientific research borehole in this paper we utilize the mechanical properties of core samples detailed observations of stress induced well bore breakouts and estimates of the magnitude of shmin from hydraulic fracturing experiments to construct a vertical profile of the maximum horizontal principal stress shmax to 3 5 km depth in the cajon pass borehole as in essentially all other boreholes the hydraulic fracturing stress measurements in the cajon pass borehole yielded appreciably more and more reliable data on the magnitude of the least principal horizontal stress shmin than the maximum principal horizontal stress shmax to utilize the breakout observations to constrain the magnitude of shmax a brittle failure criterion was used that is based on the effective strain energy concept this failure criterion also allows us to account for the polyaxial stress around the borehole and to incorporate the role of pore fluid pressure on rock strength in various ways a comparison of the shmax profile estimated from the breakouts with estimates of shmax obtained from the hydraulic fracturing tests indicates that they compare fairly well for the case when the effect of pore pressure on hydraulic fracture initiation is negligible the overall stress state to a depth of 3 5 km in the well corresponds to a normal strike slip faulting regime the shmax profile indicates several marked decreases in stress magnitude associated with major fault zones an observation consistent with the shmin profile obtained from hydraulic fracturing tests

northern east pacific rise magnetic anomaly and bathymetric framework the oceanic crust in the eastern pacific between 7on and 30on and east of 127ow contains a fairly complete history of the spreading centers associated with the east pacific rise since 25 m y b p late oligocene in this paper we have summarized the seafloor spreading magnetic anomaly data and the bathymetric data that reflect the record of this tectonic history the well defined magnetic lineations north of the clarion fracture zone in the mouth of the gulf of california and on the east flank of the east pacific rise epr are carefully examined and used to provide a guide for interpreting the spreading pattern between the clarion and clipperton fracture zones southward of the rivera fracture zone over the mathematician ridge and over the entire epr east of the mathematician ridge between the rivera and siqueiros fracture zones the bathymetric data provide a trace of the fracture zone pattern in each of the above mentioned areas the fracture zone bathymetry and the seafloor spreading magnetic lineations on the epr south of the rivera fracture zone have a distinctive fanning pattern caused by close poles of rotation and plate boundary reorganizations all these data provide a good record of the plate reorganizations in the middle miocene at magnetic anomaly 5a time 12 5 to 11 m y b p in the late miocene at magnetic anomaly 3 4 time 6 5 m y b p and in the pliocene at magnetic anomaly 2 3 time 3 5 m y b p several abandoned spreading centers including the mathematician ridge were left behind as a result of these reorganizations the mathematician ridge is shown to be a set of ridges and trough whose origin is related to the tectonic activity associated with each of the above mentioned reorganizations since anomaly 5a

crustal structure of the gulf of aden southern margin evidence from receiver functions on socotra island yemen breakup of continents in magma poor setting occurs primarily by faulting and plate thinning spatial and temporal variations in these processes can be influenced by the pre rift basement structure as well as by early syn rift segmentation of the rift in order to better understand crustal deformation and influence of pre rift architecture on breakup we use receiver functions from teleseismic recordings from socotra which is part of the subaerial oligo miocene age southern margin of the gulf of aden we determine variations in crustal thickness and elastic properties from which we interpret the degree of extension related thinning and crustal composition our computed receiver functions show an average crustal thickness of 28 km for central socotra which decreases westward along the margin to an average of 21 km in addition the crust thins with proximity to the continent ocean transition to 16 km in the northwest assuming an initial pre rift crustal thickness of 35 km undeformed arabian plate we estimate a stretching factor in the range of 2 1 2 4 beneath socotra our results show considerable differences between the crustal structure of socotra s eastern and western sides on either side of the hadibo transfer zone the east displays a clear intracrustal conversion phase and thick crust when compared with the western part the majority of measurements across socotra show vp vs ratios of between 1 70 and 1 77 and are broadly consistent with the vp vs values expected from the granitic and carbonate rock type exposed at the surface our results strongly suggest that intrusion of mafic rock is absent or minimal providing evidence that mechanical thinning accommodated the majority of crustal extension from our observations we interpret that the western part of socotra corresponds to the necking zone of a classic magma poor continental margin while the eastern part corresponds to the proximal domain

uranium lead isotopic ages of the samail ophiolite oman with applications to tethyan ocean ridge tectonics plagiogranites are a minor but widespread component of the samail ophiolite plutonic member they crystallized from the most fractionated melts generated by magmatic crystallization and differentiation of a steady state magma chamber beneath the tethyan spreading ocean ridge and their ages are thought to mark the time of ocean crust formation isotopic u pb ages of zircons from 13 plagiogranites collected along a 270 km segment of the samail ophiolite subparallel to the regional trend of the sheeted dike complex the former spreading ridge axis direction define a narrow time interval of 93 5 97 9 m y with a pronounced clustering about 95 m y the zircon ages of the plagiogranites agree remarkably well with the early cenomanian to early turonian biostratigraphic ages of sediments that are intercalated within the ophiolite pillow lavas and that lie just above them tippit et al 1981 the agreement of radiometric and biostratigraphic ages provides strong support for the conclusion that the plagiogranite u pb ages closely date the time span of ocean crust formation no step changes in age patterns are observed along the ridge axis sheeted dike direction suggesting that there are no major internal offsets of the ophiolite by transform or other faults along most of the traverse one possible exception occurs at the southeastern end of the sampled interval ibra area where a 3 m y discontinuity might be caused by an unmapped fault assuming that the regional trend of the sheeted dikes n10 25 w marks the direction of the former spreading ridge axis the present array of sample localities spans a distance of 130 to 195 km normal to that axis i e in the spreading direction the data as a whole do not define a clear cut age trend normal to the spreading axis but by eliminating samples that may be aberrant due to faulting the data array suggests a pattern of increasing ages from east to west this would indicate that the samail ophiolite is derived from the western southwestern side of the tethyan spreading system a conclusion reached independently from geological evidence also the gentle age gradient in the spreading direction i e between 130 and 195 km of ocean crust were formed within an age span of no more than 2 m y suggests a very fast spreading rate this conclusion also is supported by independent geologic evidence

structure of the sheeted dike complex of the samail ophiolite near ibra oman the sheeted dike complex is a regionally mappable geologic unit with a consistent position in the samail ophiolite stratigraphy discontinuous exposures of the complex in the ibra area of the southeast oman mountains provide structural data that allow reconstruction of a paleo spreading ridge axis of 347 for the region and suggests a maximum spreading width of 275 km for the ophiolite contact relations within the well exposed al ahmadi section indicate that the complex was formed by multiple parallel intrusion over a narrow zone at the paleo ridge axis split dikes are abundant facing directions indicate a spreading ridge southwest of the present outcrop area in conflict with other spreading axis direction indicators

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

rifting of old oceanic lithosphere geophysical data from five regions in the pacific and indian oceans reveal that long distance 400 km spreading center jumps have occurred in the past the present day seafloor morphology is used to develop a scenario for a spreading center jump the major events are 1 thinning and weakening of the lithosphere at the future rifting site 2 rifting of the weakened lithosphere during rifting the crack is filled from above by normal faulting and wedge subsidence viscous upwelling fills the crack from below 3 spreading at the rift site results in a ridge bounded by two troughs spreading ceases at the dying spreading center resulting in a deep central graben surrounded by flexural ridges periods of slow spreading at both spreading centers produce rough topography 4 ageing and cooling that produce a general deepening of the abandoned spreading ridge and also reduce the thermal contrast across the fossil rifting site the new spreading center develops into a normal spreading rift the major topographic expressions apparent in the seafloor today are the deep trough of the abandoned spreading center and the proximal and distal troughs which formed when the emerging spreading center bisected the fossil rifting site the proximal trough nearer the new spreading ridge and the distal trough farther from the new ridge are first order topographic features 100 1000 km long and 300 km wide resembling fracture zones with which they are often confused they share with fracture zones the characteristic of bringing together fragments of lithosphere of different ages but unlike fracture zones they are generally parallel to magnetic lineations

hydrothermal sulfide accumulation along the endeavour segment juan de fuca ridge highlights high resolution auv bathymetry is used to identify hydrothermal sulfides on the seafloor 1 2 mt of massive sulfide is identified along 24 km of mid ocean ridge length massive sulfide accumulated at a rate of 400 t yr efficiency of sulfide deposition is 5 of total mobilized metals and reduced sulfur current global seafloor sulfide estimates likely underestimate amount present by about a factor of four abstract hydrothermal sulfide deposits that form on the seafloor are often located by the detection of hydrothermal plumes in the water column followed by exploration with deep towed cameras side scan sonar imaging and finally by visual surveys using remotely operated vehicle or occupied submersible hydrothermal plume detection however is ineffective for finding hydrothermally inactive sulfide deposits which may represent a significant amount of the total sulfide accumulation on the seafloor even in hydrothermally active settings here we present results from recent high resolution autonomous underwater vehicle based mapping of the hydrothermally active endeavour segment of the juan de fuca ridge in the northeast pacific ocean analysis of the ridge bathymetry resulted in the location of 581 individual sulfide deposits along 24 km of ridge length hydrothermal deposits were distinguished from volcanic and tectonic features based on the characteristics of their surface morphology such as shape and slope angles volume calculations for each deposit results in a total volume of 372 500 m3 of hydrothermal sulfide sulfate silica material for an equivalent mass of 1 2 mt of hydrothermal material on the seafloor within the ridge s axial valley assuming a density of 3 1 g cm3 much of this total volume is from previously undocumented inactive deposits outside the main active vent fields based on minimum ages of sulfide deposition the deposits accumulated at a maximum rate of 400 t yr with a depositional efficiency proportion of hydrothermal material that accumulates on the seafloor to the total amount hydrothermally mobilized and transported to the seafloor of 5 the calculated sulfide tonnage represents a four fold increase over previous sulfide estimates for the endeavour segment that were based largely on accumulations from within the active fields these results suggest that recent global seafloor sulfide resource estimates which were based mostly on the sizes and distribution of hydrothermally active deposits may be similarly underestimating the amount of sulfide along the global submarine neovolcanic zones

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

the effect of the intermediate principal stress on fault formation and fault angle in siltstone abstract we conducted true triaxial compression tests on specimens prepared from two siltstone core sections one above and one below the chelungpu fault taiwan for different constant 2 and 3 magnitudes the maximum principal stress 1 was raised until a post failure stage was reached and a through going fault had developed despite differences between the properties of the two cores in all tests peak 1 increased as 2 was set at higher levels than 3 in contrast to mohr coulomb condition predictions the fault normal vector was aligned with the 3 direction and made an angle with 1 direction the angle which corresponds to fault dip in case of normal faulting increased monotonically with 2 for fixed 3 a variation that is also inconsistent with mohr coulomb theory the results of shear band localization theory are used with fault angles observed for axisymmetric compression and deviatoric pure shear to infer properties of the inelastic constitutive behavior these properties are significantly different for the two cores using them to predict for other deviatoric stress states yields good agreement with the observations for core ii and acceptable agreement for core i the results are used to predict the angle variation for constant mean normal stress decreases as the deviatoric stress state varies from axisymmetric extension to axisymmetric compression and at fixed deviatoric stress state decreases monotonically with increasing mean normal stress

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

new age data for mid atlantic ridge hydrothermal sites tag and snakepit chronology revisited the chronologies of tag and snakepit hydrothermal fields have been established using 210pb pb 230th 234u and 14c dating at the tag field a mn oxide record indicative of low temperature events began at least 125 000 years and possibly 140 000 years ago with maximum intensities at 15 000 7000 and 4000 years before present high temperature events giving rise to sulfide deposits began about 100 000 years ago and have been intermittent to the present day a presently active site has experienced intermittent pulses of activity every 4000 to 6000 years over the past 20 000 years decrease in activity is often marked by low temperature aragonite precipitation in chimney conduits at 4000 7000 and 9000 years ago after a period of quiescence lasting about 4000 years this site was reactivated about 50 years ago the snakepit field is much younger and no sulfides older than 4000 years have been recovered relict sulfide deposits are dated between 2000 and 4000 years old indicating this site was active during a quiescent period at tag reactivation of snakepit took place about 80 years ago and is presently concurrent with that of tag comparison with hydrothermal sites on the east pacific rise suggests that on slow spreading ridges the major fracture systems focussing the hydrothermal discharge can be reactivated at intervals and new deposits precipitated on top of older ones while on faster spreading ridges each pulse of activity is separated in space and time resulting in discrete deposits

effects of finite rifting times on the development of sedimentary basins abstract most thermo mechanical models for the development of sedimentary basins have assumed that the rifting responsible for the formation of the basin occurred instantaneously and have examined the post rift development of the basin this assumption greatly simplifies the mathematical treatment but is not in accord with what is found in nature where 10 to 50 m y rifting events commonly accompany the formation of sedimentary basins and continental margins the effects of a finite rifting time on the development of sedimentary basins are examined using an analytic technique which allows an arbitrary rifting history in both time and space and which considers the effects of both vertical and horizontal heat transfer this technique allows the thermal structure of the lithosphere to be calculated throughout the rifting event and thus permits the subsidence history and surface heat flow of the developing basin to be traced the effect of a finite duration extension event is that heat is lost during rifting increasing the syn rift subsidence at the expense of the post rift lateral heat flow which was not included in previous studies of the effect of finite rifting times has a significant effect on the subsidence history distribution of sediments and thermal history in particular the post rift subsidence is decreased by more than 25 for a 20 m y rifting event and by more than 10 15 for a rifting event as short as 10 m y this will significantly decrease the subsidence rates in the post rift stage and implies that inferences concerning the structure development and thermal history of the basin derived from using curves to interpret backstripped subsidence can be greatly in error variations in syn rift sediment accumulation and lithospheric thermal structure at the end of rifting resulting from different rifting histories can interact with other factors such as the flexural response of the lithosphere to sediment loading to affect the final width of the basin the total amount of sediments that accumulate and the basin stratigraphy

the sinai triple junction revisited abstract this paper is a summary of a more detailed analysis of the kinematics of the sinai triple junction courtillot et al 1987 accurate kinematic data are lacking along the red sea and they can be supplemented by bathymetric topographic and geological data pertaining to the three arms of the entirely continental sinai triple junction motions across the northern red sea and along the gulf of elat are an order of magnitude larger than across the gulf of suez the direction of motion there remains a major uncertainty a possible kinematic model is highlighted in which right lateral strike slip motion and small pull apart basins occur along the gulf of suez in agreement with recent field observations in egypt early miocene is marked by major geodynamical changes all along the northern boundaries of the african and indian plates we suggest that rifting in the arabian sea gulf of aden red sea and gulf of suez was initiated at the end of the first phase of continental extrusion of indochina when the tibetan plateau began to rise and spreading in the south china sea came to a halt

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

true triaxial strength and failure characteristics of cubic coal and sandstone under different loading paths abstract in deep underground coal mining engineering activities are performed within anisotropic in situ stress fields due to engineering disturbances and tectonic stress many such activities involve the development of excavations in soft rock and anisotropic coal accordingly studying the mechanical properties of soft rocks is important for the stability of deep underground excavations in this study the deformation strength and failure characteristics of soft sandstone and raw coal under two different true triaxial loading paths were investigated using a self developed true triaxial test apparatus the results indicated that the inelastic strain in the pre peak stage of sandstone and coal gradually increased with increasing intermediate principal stress also the strength drop in the post peak stage increased the crack initiation stress crack damage stress and peak strength of sandstone and coal first increased and then decreased with increasing intermediate principal stress for a given 3 moreover with increasing intermediate principal stress the failure mode of sandstone and coal changed from shear to tensile shear and from brittle to semi brittle the linear mogi criteria as found to characterize the true triaxial strength of coal well while the modified lade criteria was more applicable to soft sandstone owning to the symmetry assumption the linear mogi criteria predicted low strength when the intermediate principal stress coefficient exceeded to 0 5 in addition the peak strength curve of rock on the plane and the influence of weak structures on the failure mode of anisotropic coal was discussed weak structures have an important influence on the failure mode of coal which depends on the strength difference between the structural plane and the coal rock mass the strength envelope on the plane had a significant stress lode angle effect which gradually decreased as the mean stress increased

oceanic transform earthquakes with unusual mechanisms or locations relation to fault geometry and state of stress in the adjacent lithosphere on oceanic transforms most earthquakes are expected to occur on the principal transform displacement zone ptdz and to have strike slip mechanisms consistent with transform parallel motion we conducted a search for transform earthquakes departing from this pattern on the basis of source mechanisms and locations taken from the harvard centroid moment tensor catalogue and the bulletin of the international seismological centre respectively events with unusual mechanisms occur on several transforms we have determined the source mechanisms and centroid depths of 10 such earthquakes on the st paul s marathon owen heezen tharp menard and rivera transforms from inversions of long period body waveforms relative locations of earthquakes along these transforms have been determined with a multiple event relocation technique much of the anomalous earthquake activity on oceanic transforms is associated with complexities in the geometry of the ptdz or the presence of large structural features that may influence slip on the fault reverse faulting earthquakes occur at a compressional bend in the owen transform in the area of mount error and at the st paul s transform near st peter s and st paul s rocks a normal faulting earthquake on the heezen transform is located at the edge of a pull apart basin marking an extensional offset of the fault normal faulting earthquakes along the tharp menard and rivera transforms may also be related to extensional offsets some events with unusual mechanisms occur outside of the transform fault zone however and do not appear to be related to fault zone geometry for instance earthquakes with mechanisms indicating reverse faulting on ridge parallel fault planes are located near the ridge transform intersections of the st paul s and the marathon transforms possible additional contributors to the occurrence of anomalous earthquakes include recent changes in plate motion differential lithospheric cooling and the development of a zone of weakness along the fault zone but we do not find strong evidence to confirm the influence of these processes

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

black smokers massive sulphides and vent biota at the mid atlantic ridge the discovery of black smokers massive sulphides and vent biota in the rift valley of the mid atlantic ridge demonstrates that this assemblage of hydrothermal phenomena is not limited to intermediate to fast spreading oceanic ridges hydrothermal exchange processes may thus be important at the ridges which extend through the atlantic ocean and western indian ocean comprising more than half of the 55 000 km global length of seafloor spreading centres

the application of a matsuoka nakai lade duncan failure criterion to two porous sandstones abstract we summarize two suites of recent true triaxial tests in two porous sandstones coconino and bentheim one suite of tests conducted under constant 2 and 3 which revealed the effect of 2 and 3 on rock failure the other suite of tests performed under constant lode angle determined the failure dependencies on mean stress and lode angle building and modifying failure descriptions by matsuoka nakai mn and lade duncan ld an explicit failure criterion modified mnld that generalizes a class of failure forms is used to fit the novel loading path data of both sandstones the determined failure descriptions fit both rocks well over a wide range of failure conditions then the generated failure descriptions were further used to predict failure conditions under the constant 2 and 3 loading path the predictions by the modified mnld are generally consistent with these data the predictions also successfully replicated the variation of 1 peak with 2 under constant 3 suggesting the potential of the modified mnld to clarify the effect of 2 on rock failure

origin of seaward dipping reflectors in oceanic crust off the norwegian margin by subaerial sea floor spreading a remarkable layered acoustic stratification is observed in the upper oceanic crust adjacent to the norwegian continental margin it comprises a seaward dipping complex of reflectors in the form of a wedge we suggest that it is a layered igneous sequence that results when crustal accretion occurs at a subaerial spreading axis and that this phenomenon may commonly occur during the earliest phase of ocean basin genesis

sulfide geochronology along the endeavour segment of the juan de fuca ridge 1 forty nine hydrothermal sulfide sulfate rock samples from the endeavour segment of the juan de fuca ridge northeastern pacific ocean were dated by measuring the decay of 226ra half life of 1600 years in hydrothermal barite to provide a history of hydrothermal venting at the site over the past 6000 years this dating method is effective for samples ranging in age from 200 to 20 000 years old and effectively bridges an age gap between shorter and longer lived u series dating techniques for hydrothermal deposits results show that hydrothermal venting at the active high rise sasquatch and main endeavour fields began at least 850 1450 and 2300 years ago respectively barite ages of other inactive deposits on the axial valley floor are between 1200 and 2200 years old indicating past widespread hydrothermal venting outside of the currently active vent fields samples from the half graben on the eastern slope of the axial valley range in age from 1700 to 2925 years and a single sample from outside the axial valley near the westernmost valley fault scarp is 5850 205 years old the spatial relationship between hydrothermal venting and normal faulting suggests a temporal relationship with progressive younging of sulfide deposits from the edges of the axial valley toward the center of the rift these relationships are consistent with the inward migration of normal faulting toward the center of the valley over time and a minimum age of onset of hydrothermal activity in this region of 5850 years

heat flow and hydrothermal circulation at the ocean continent transition of the eastern gulf of aden abstract in order to investigate the importance of fluid circulation associated with the formation of ocean continent transitions oct we examine 162 new heat flow hf measurements in the eastern gulf of aden obtained at close locations along eight seismic profiles and with multi beam bathymetry the average hf values in the oct and in the oceanic domain 18 m y are very close to the predictions of cooling models showing that the overall importance of fluids remains small at the present time compared to oceanic ridge flanks of the same age however local hf anomalies are observed although not systematically in the vicinity of the unsedimented basement and are interpreted by the thermal effect of meteoric fluids flowing laterally we propose a possible interpretation of hydrothermal paths based on the shape of hf anomalies and on the surface morphology fluids can circulate either along dip or along strike but are apparently focussed in narrow pipes in several locations in the oct there is no detectable hf anomaly while the seismic velocity structure suggests serpentinization and therefore past circulation we relate the existence of the present day fluid circulation in the eastern gulf of aden to the presence of unsedimented basement and to the local extensional stress in the vicinity of the socotra hadbeen fault zone at the scale of rifted margins fluid circulation is probably not as important as in the oceanic domain because it can be inhibited rapidly with high sedimentation rates serpentinization and stress release after the break up

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

limiting envelopes of a dry porous limestone under true triaxial stress states abstract the present study investigates the influences of the confining pressure and the intermediate principal stress on the mechanical behaviour of a porous limestone true triaxial tests are carried out on a broad range of confining stresses covering both brittle and ductile regimes the experiments included the special cases of triaxial compression extension and isotropic compression as well as tests at constant levels of the intermediate principal stress the resulting stress strain curves and yield stresses are then discussed three dimensional yield envelopes are subsequently built in meridian and octahedral planes the shape of the octahedral sections changes with the mean stress from triangular to hexagonal and then quasi circular at even higher mean stresses the evolution continues towards a triangle with apexes oriented in the direction of triaxial extension stress states various existing yield criteria are fitted and compared to the experimental data finally a three dimensional yield surface is proposed combining the previous observations

the magnetic quiet zone in the eastern gulf of aden implications for the early development of the continental margin summary the magnetic quiet zone in the eastern gulf of aden is located between the oceanic crust of sheba ridge and the continental crust of arabia and somalia and is separated from both by important structural boundaries the seaward boundary is marked by the end of the seafloor spreading magnetic anomaly sequence and by a basement depth discontinuity the landward boundary is marked by escarpments made up of a series of normal faults these escarpments extend from 2 3 km below sea level to 1500 m above sea level and are equivalent of the hinge zone found at mature continental margins the magnetic field in the quiet zone is flat in some areas and in others is characterized by anomalies of up to several hundred gammas which are correlatable over distances of up to about 20 km and which appear related to basement topography the basement lacks the topographic slope characteristic of mid ocean ridge flanks and is characterized by moderately rough relief the crustal structure appears quite heterogeneous and where the crustal thicknesses have been determined they are slightly greater than those of oceanic crust new heat flow measurements show high values 95 7 123 3 mw m 2 in the quiet zone with values decreasing from sheba ridge toward the coast the unusual structure of the quiet zone and the observations that more opening appears to have occurred between arabia and somalia than can be accounted for by the oceanic crust of sheba ridge leads to the suggestion that the magnetic quiet zone was generated by diffuse extension of continental crust through a combination of rotational listric faulting and dyke injection this possibility is investigated using both a stretching or lithospheric attenuation model and a model in which a portion of the extension occurs through dyke injection it is found that these models can adequately match the observed heat flow and basement depths although very large amounts of extension 4 6 are required in the deep seaward portion of the quiet zone this results in more extension than is compatible with the documented motion between arabia and africa however formation of the magnetic quiet zone occurred over a period of 10 15 myr rather than instantaneously as assumed in the simple models when the effects of a finite length rifting episode are considered less extension is required and the observed geophysical data are consistent with a diffuse extension origin for the magnetic quiet zone

geodynamics of the middle east domain since the oligocene research summary several analogue models were merged to link the post oligocene closure of the neotethys seaway with the break up of the gulf of aden and the red sea the seaway closure since the early neogene which was a result of northwards subduction of neotethyan lithosphere under south central eurasia generated two styles of kinematic convergence its western branch forms the tethyan subduction under south central eurasia along the hellenic deformation front generating the break up of the aegean sea and the westward migration of anatolia the eastern branch comprises the oblique convergence of arabia with eastern anatolia and iran along the bitlis zagros suture which gradually shifted from subduction to collision because the velocity of plate collision is slower than that of subduction the subducting section of the underthrust slab moves faster than the colliding segment consequently the underthrust arabian slab rotated counterclockwise and developed detachment zones such extension triggered the resumption of the propagation of the carlsberg ridge into the gulf of aden after being stalled for nearly 30 ma it is presumed further that the break up of the red sea in the mid miocene was superimposed on the regional uplift and exhumation of the arabo nubian massif which was caused by a mantle plume in the oligocene a plume that subsequently affected the ethiopian and the east african plateaux in the neogene

thinned and subsided continental crust on the rifted margin of eastern canada crustal structure thermal evolution and subsidence history summary seismic refraction measurements were carried out using ocean bottom seismometers over foundered continental crust in orphan basin and flemish pass on the continental margin north east of newfoundland tau p travel time inversion synthetic seismogram analysis and conventional layered model calculations were applied to the data the results show that these outer regions of the margin are underlain by thinned continental crust with a total depth to the m discontinuity of about 22 km there are two main crustal layers with p velocities of about 6 1 and 7 0 km s 1 which appear to be homogenous and separated by sharp interfaces these are overlain by a layer in which p velocities are about 5 5 km s 1 this is interpreted to be precambrian or palaeozoic basement mesozoic and cenozoic sediments cover the basement rocks and are over 4 km thick the results imply that crustal thinning to about 50 per cent of the original crustal thickness occurred the gravity anomaly data show that the thin crust has a maximum horizontal extent of about 450 km from the ocean continent boundary near orphan knoll landward to the outer continental shelf the subsidence history and thermal evolution of the region was computed assuming that the observed thinning is produced by horizontal extension of the lithosphere it is suggested that extension can only satisfy the observed crustal structure and elevation of the margin during the rift phase if more extension took place in the lower lithosphere than in the upper lithosphere the computed subsidence is compared to the observed subsidence and the total amounts of subsidence are similar however the shape of the observed subsidence curves measured in deep exploratory wells differs significantly from the predicted subsidence assuming cooling began when final continental breakup occurred in the late cretaceous the temperature distribution within the lithosphere due to extension may be related to the flexural rigidity of the plate as a function of both time and position across the margin therefore it is suggested that the response of the lithosphere to sediment loading and the large amplitude of the gravity shelf edge anomalies are directly related to the thermal history of the region

evolution of the south asian monsoon wind system since the late middle miocene abstract the evolution and timing of the inception of the south asian monsoon sam has been one of the most debated climatic mysteries amongst climate workers the himalaya tibetan plateau htp complex has been implicated as the main driver for sam circulation however there are intense debates as to whether there was a critical elevation of the htp complex that drove the sam and at what time such a critical elevation was attained also the role of the htp complex in driving the sam has been questioned in recent climate model studies the model simulation suggests a major intensification of the sam as early as 30 ma whereas marine records from the arabian sea indicate a major strengthening of the south asian summer or southwest monsoon winds between 10 and 8 ma the continental vegetation on the other hand captured a major transition from c 3 to c 4 type during 8 7 ma but whether this change was local or global and whether this transition was driven by monsoonal precipitation is not unanimously accepted our new record from ocean drilling program holes located off the oman margin and on the owen ridge western arabian sea shows appearance of monsoon wind proxy planktic foraminifer globigerina bulloides a significant increase in total organic carbon and a negative shift in stable carbon isotope record of benthic foraminifera at 12 9 ma these proxies indicate that present day south asian monsoon wind system began to develop during the late middle miocene 12 9 ma and summer monsoon was in its full strength in the late miocene 7 ma from 11 to 7 ma the summer monsoon was weaker when winter monsoon was stronger

hydrothermal activity on a 105 year scale at a slow spreading ridge tag hydrothermal field mid atlantic ridge 26 n new radiometric dates are presented for the hydrothermal deposits in the alvin and mir relict hydrothermal zones located on the lower east wall of the rift valley within the tag hydrothermal field and placed in context of the evolution of the tag field as a whole over the past 105 years the field includes an active high temperature sulfide mound situated near the base of the east wall and a low temperature zone at middepth on the east wall all within a 5 by 5 kilometer area the oldest hydrothermal deposits sampled in the mir relict zone the low temperature zone and an inactive sulfide mound in the alvin relict zone are low temperature manganese crusts dated at 140 000 125 000 and 74 000 years respectively high temperature activity began about 100 000 years ago in the mir zone a 50 000 year high temperature event took place at the active sulfide mound in the mir zone and at the inactive sulfide mound in the alvin zone this event is followed by sporadic activity at the mir zone and the active sulfide mound until 10 000 and 5000 years ago respectively a reactivation of the system affected the mir zone from about 2000 to 600 years ago the presently active sulfide mound was reactivated only 60 years ago after a quiescence of about 4000 years the radiochronologic record reveals that the hydrothermal activity has been episodic with a major high temperature event throughout the tag field at 50 000 years and other more localized events

sulfide geochronology along the northern equatorial mid atlantic ridge abstract hydrothermal processes and seafloor massive sulfide sms deposits have different characteristics at fast and slow spreading mid ocean ridges one such parameter is the age of a sms deposit which differs by 1 2 orders of magnitude between the fast spreading east pacific rise epr and the slow spreading mid atlantic ridge mar the large collection of sms samples dated from the 18 hydrothermal fields of the northern equatorial part of the mid atlantic ridge 194 samples demonstrates a relatively old average age of hydrothermal fields here 66 ka with the oldest one estimated as ca 223 ka peterburgskoye field based on geochronological data it was confirmed that hydrothermal discharge has an episodic character active and inactive periods of the sms formation alternate the distribution of events at all hydrothermal fields demonstrates that maximum activity occurred at 38 35 30 20 and 8 2 ka and increased with time based on statistical analyses dating variations can be explained as a superposition of several periods of activity with the duration of 15 10 and 5 ka relationship between the age and distance from the axial rift zone as well as between the age and aerial distribution is different for sms deposits hosted by basalts and by gabbro peridotites depending on their geological setting on the particular mar segment this difference can be explained by a variety of hydrothermal processes determined by tectonic or magmatic segment evolution and symmetrical or asymmetrical mode of accretion escartin et al 2008

plate tectonic models for indian ocean intraplate deformation abstract the equatorial region of the conventionally defined indo australian plate has long been recognized as containing a type example of intense intraplate deformation we trace the development of tectonic models for the area to illustrate techniques for the analysis of such deformation the identification of anomalous seismicity near the ninetyeast and chagos laccadive ridges demonstrated the existence of the deformation focal mechanisms from recent and historic earthquakes showed strike slip motion occurring along the ninetyeast ridge seismic moment data allowed the rate to be estimated similar studies showed north south tension in the chagos bank region and north south compression in the region between the ninetyeast and chagos ridges global plate motion studies indicated non closure of the indian ocean triple junction suggesting the conventional plate geometry was inadequate for a rigid plate description of the area gravity and marine geophysical data indicated intense north south compressional deformation south of the bay of bengal these observations are reconciled by a plate motion model in which australia and india lie on distinct plates divided by a boundary that intersects the central indian ridge near the equator in this model arabia usually considered a separate plate has negligible motion relative to india the resulting euler vector for australia relative to indo arabia lies just east of the central indian ridge and predicts approximately 0 5 1 5 cm yr compression in the central indian basin and 1 5 2 cm yr strike slip motion along the northern ninetyeast ridge consistent with the seismological and geophysical data in contrast to conventional oceanic plate boundaries the boundary deformation is distributed over a wide zone this diffuse nature may reflect either the boundary s recent inception or slow rate of motion analysis of seismicity and deformation in the boundary zone should offer insights into the mechanics of its development and its implications for the evolution of plate boundaries

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

redox changes in a seafloor hydrothermal system recorded in hematite chalcopyrite chimneys abstract hydrothermal chimneys composed of hematite and chalcopyrite grow at the irina ii vent site logatchev hydrothermal field mid atlantic ridge hematite and chalcopyrite form layers that repeatedly alternate within the chimney wall the repeating alternation of hematite and chalcopyrite implies for repeating changes in the physico chemical parameters of the vent fluid the modeled stability diagrams of hematite and chalcopyrite at the physico chemical conditions of the irina ii hydrothermal site showed that the precipitation of these two fe minerals is primarily governed by the activity of h2s and ph fe isotope studies suggest that the successive deposition of chalcopyrite 56 54fe 0 20 and hematite 56 54fe 0 60 layers in the irina ii chimneys is a result of redox changes in the hydrothermal fluid the replacement of hematite by either magnetite or chalcopyrite at some places also suggests for a redox inversion in the hydrothermal system from oxic to reduced chondrite normalized rare earth element distribution patterns of the hematite layers show a strong negative eu anomaly which is interpreted to be a result of crystal chemical control on the rare earth element fractionation during hematite precipitation sr isotope systematics indicate significant and variable incorporation of seawater from 40 to 80 in the hydrothermal system nd pb isotope studies show that nd and pb in chalcopyrite and hematite layers are derived mainly from the underlying mafic and ultramafic rocks the chimneys are young 5 13 1 28 kyrs b p and their wall thickened inward the duration of redox cycles oxic and reducing within the scale of a single chimney is about several hundred years the hematite layers are enriched in w in sn and u indicating that primary hematite hydrothermal deposits at the seafloor may be a potential source for these strategic trace elements high concentrations of these elements in the studied hematite layers can be explained by their easy ion substitution for fe3 in the hematite crystal lattice whereas the most plausible source of the elevated w in and sn contents are the hydrothermal fluids the seawater is the possible major source of u

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

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