earliest sea floor spreading magnetic anomalies in the north arabian sea and the ocean continent transition summary magnetic and gravity data collected during a gloria survey of the indus fan provide new information on the earliest sea floor spreading history of the arabian sea a negative gravity anomaly correlates with the buried laxmi ridge this ridge is interpreted here to be a sliver of continental crust adjacent to the oceancontinent transition which bounds thinned probably intruded transitional crust to the ne the oldest sea floor spreading anomaly is anomaly 28 65 66 ma breakup occurring at the time of the deccan traps volcanic event the earliest oceanic crust formed from two phases of rift propagation which accommodates the angular disparity between the e w trending anomalies in the western arabian sea and the ne sw trending western part of the laxmi ridge flow line projection shows that the laxmi ridge forms the conjugate structure to the northern mascarene plateau margin

fluid inclusion characteristics and geological significance of the dajinshan w sn polymetallic deposit in yunfu guangdong province abstract the dajinshan tungsten tin polymetallic deposit is a quartz vein type ore deposit located in western guangdong province the ore bodies show a fairly simple shape and mainly occur as tungsten tin polymetallic bearing sulfide quartz veins including quartz vein quartz greisens and sulfide quartz veins and their distribution is spatially related to dajinshan granitoids the formation of the deposit experienced three stages a wolframite molybdenite quartz stage a wolframite cassiterite sulfide quartz stage and a fluorite calcite carbonate stage based on detailed petrographic observations we conducted microthermometric and raman microspectroscopic studies of fluid inclusions formed at different ore forming stages in the dajinshan tungsten tin polymetallic deposit identifying four dominant types of fluid inclusions aqueous two phase inclusions co2 bearing inclusions solid or daughter mineral bearing inclusions and gas rich inclusions the gas compositions of ore forming fluids in the dajinshan tungsten tin polymetallic deposit are mostly co2 ch4 and h2o the hydrogen oxygen and sulfur isotopic data imply that the ore forming fluids in the dajinshan tungsten tin polymetallic deposit were mainly derived from magmatic fluids mixed with meteoric water in the ore formation process these results indicate that the fluid mixing and boiling led to the decomposition of the metal complex in ore forming fluids and ore deposition

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

heat flow and age of the gulf of oman we present the results of twenty heat flow stations in the gulf of oman which are used to infer the first reliable age estimates for the basin a mean surface heat flux of 42 6 3 6 mw m 2 exhibits no significant regional variation after correction for thick and rapidly deposited sediments this yields an age of 70 to 100 ma according to oceanic thermal models this age is also consistent with the sediment corrected basement depths of 5 5 6 0 km and with formation during the cretaceous quiet zone the latter can explain the absence of magnetic sea floor spreading lineations heat flow measurements are also used to confirm the presence of gas hyrdate layers the measured thermal gradient yields a depth for the solid to free gas phase transition which is the same as that deduced from bright spots seen on seismic reflection profiles

fluid evolution of triassic and jurassic w mineralization in the xitian ore field south china constraints from scheelite geochemistry and microthermometry abstract rare earth element ree enriched scheelite is widely distributed in many ore deposits and has been utilized in a broad range of geochronological and geochemical studies in an effort to decipher skarn and quartz vein hosted w mineralization process scheelite from triassic and jurassic granites skarn ores and quartz vein hosted ores in the xitian ore field south china were analyzed for major and minor element content and fluid inclusion microthermometry all scheelite grains show homogeneous backscatter electron and cathodoluminescence response and uniform w and ca distribution the chondrite normalized ree patterns of scheelite from triassic granites to skarn ores show similar pr sm enrichments and negative eu anomalies but different hree contents in contrast the chondrite normalized ree patterns of scheelite from jurassic granites to quartz vein hosted ores changes from flat with slight lree enrichment to concave with remarkable lree depletion the homogenization temperatures and salinities of fluid inclusions from the scheelite in triassic granites and skarn ores are similar both reduce significantly from the jurassic granites to quartz vein hosted ores combined with pressure ph and mineral assemblage analysis we propose that the ree signatures in the ore forming fluids of skarn type w deposits are mainly controlled by crystallization of ree enriched skarn minerals e g garnet diopside epidote the similarity in lree patterns and y ho ratios between the triassic granite and skarn ore scheelite indicates that the skarn ore forming fluid was derived from the triassic granite while the distinct ree patterns and variable y ho ratios in scheelite from jurassic granites and quartz vein hosted ores imply possible fluid mixing for jurassic w mineralization in light of these results ree rich scheelite could serve to discriminate fluid source and document mineralization processes in a complex ore forming system

geodynamic evolution of the offshore indus basin pakistan the western indian plate passive continental margin a complex meso cenozoic history characterizes the offshore indus basin pakistan including rifting drifting collision transpression and recent inverted extension that is poorly understood we use reflection seismic data to document the geometry of the structures that accommodated the deformation and offshore well data to correlate the sedimentary sequences which were affected by the different tectonic episodes synthetic seismograms have been prepared for calibrating seismic data with well information for identification of key reflections representative of major lithological horizons seismic stratigraphy helped in dividing the sedimentary successions into rifted margin and transpressive margin sequences additionally the basement represented by chaotic and semitransparent reflections and moho discontinuity by moderate amplitude bright reflections are also identified on seismic records syn depositional normal faulting in extensional regime cretaceous oligocene are identified on seismic profiles that converted into strike slip faults in miocene and listric faults in recent sediments forming inverted extensional regime we infer that the basin inversion from extension to inverted extension and transpression occurred in recent times number of depositional features including shelves deltaic deposits canyons and channel levee complexes have been identified that developed in different tectonic episodes five tectonic subsidence curves using back stripping procedure have been calculated in order to constrain the geodynamic evolution of the meso cenozoic sedimentary succession of the offshore indus pakistan the analysis of tectonic subsidence curves covering a time span of 140 ma allowed us to quantify the total subsidence and to document the activities of syn rift and post rift faulting systems different stages in terms of duration and magnitude of subsidence uplift trends have been identified in the evolution of the basin two uplift events are prominent in eocene similar to 40 ma and miocene time similar to 12 ma a rapid resumption of subsidence with non conformity in early palaeocene is seen along the margin

gravity anomalies and associated tectonic features over the indian peninsular shield and adjoining ocean basins abstract a combined gravity map over the indian peninsular shield ips and adjoining oceans brings out well the inter relationships between the older tectonic features of the continent and the adjoining younger oceanic features the nw se ne sw and n s precambrian trends of the ips are reflected in the structural trends of the arabian sea and the bay of bengal suggesting their probable reactivation the simple bouguer anomaly map shows consistent increase in gravity value from the continent to the deep ocean basins which is attributed to isostatic compensation due to variations in the crustal thickness a crustal density model computed along a profile across this region suggests a thick crust of 35 40 km under the continent which reduces to 22 20 24 km under the bay of bengal with thick sediments of 8 10 km underlain by crustal layers of density 2720 and 2900 2840 kg m3 large crustal thickness and trends of the gravity anomalies may suggest a transitional crust in the bay of bengal up to 150 200 km from the east coast the crustal thickness under the laxmi ridge and east of it in the arabian sea is 20 and 14 km respectively with 5 6 km thick tertiary and mesozoic sediments separated by a thin layer of deccan trap crustal layers of densities 2750 and 2950 kg m3 underlie sediments the crustal density model in this part of the arabian sea east of laxmi ridge and the structural trends similar to the indian peninsular shield suggest a continent ocean transitional crust cotc the cotc may represent down dropped and submerged parts of the indian crust evolved at the time of break up along the west coast of india and passage of reunion hotspot over india during late cretaceous the crustal model under this part also shows an underplated lower crust and a low density upper mantle extending over the continent across the west coast of india which appears to be related to the deccan volcanism the crustal thickness under the western arabian sea west of the laxmi ridge reduces to 8 9 km with crustal layers of densities 2650 and 2870 kg m3 representing an oceanic crust

growth and demise of a paleogene isolated carbonate platform of the offshore indus basin pakistan effects of regional and local controlling factors based on high resolution seismic and well datasets this paper examines the evolution and drowning history of a paleocene eocene carbonate platform in the offshore indus basin of pakistan this study uses the internal seismic architecture well log data as well as the microfauna to reconstruct factors that governed the carbonate platform growth and demise carbonates dominated by larger benthic foraminifera assemblages permit constraining the ages of the major evolutionary steps and show that the depositional environment was tropical within oligotrophic conditions with the aid of seismic stratigraphy the carbonate platform edifice is resolved into seven seismic units which in turn are grouped into three packages that reflect its evolution from platform initiation aggradation with escarpment formation and platform drowning the carbonate factory initiated as mounds and patches on a cretaceous paleocene volcanic complex further the growth history of the platform includes distinct phases of intraplatform progradation aggradation backstepping and partial drownings the youngest succession as late stage buildup records a shift from benthic to pelagic deposition and marks the final drowning in the early eocene the depositional trend of the platform controlled by the continuing thermal subsidence associated with the cooling of volcanic margin lithosphere was the major contributor of the accommodation space which supported the vertical accumulation of shallow water carbonate succession other factors such as eustatic changes and changes in the carbonate producers as a response to the paleogene climatic perturbations played secondary roles in the development and drowning of these buildups

metallogenesis at the carris w mo sn deposit geres portugal constraints from fluid inclusions mineral geochemistry re os and he ar isotopes abstract the carris orebody consists of two partially exploited w mo sn quartz veins formed during successive shear stages and multipulse fluid fillings they cut the variscan post d3 geres i type granite the most important ore minerals are wolframite scheelite molybdenite and cassiterite there are two generations of wolframite the earlier generation of wolframite is rare and has the highest wo4mn content 91 mol and the most common wolframite contains 26 57 mol wo4mn re os dating of molybdenite from the ore quartz veins and surrounding granite yields ages of 279 1 2 ma and 280 3 1 2 ma respectively which are in very good agreement with the previous id tims u pb zircon age for the carris granite 280 5 ma 3he 4he ratio of pyrite ranging between 0 73 and 2 71 ra 1 ra 1 39 10 6 and high 3he 36ar 0 8 5 10 3 indicate a mixture of a crustal radiogenic helium fluid with a mantle derived fluid the fluid inclusion studies on quartz intergrown with wolframite and scheelite beryl and fluorite reveal that two distinct fluid types were involved in the genesis of this deposit the first was a low to medium salinity aqueous carbonic fluid co2 between 4 and 14 mol with less than 1 95 mol n2 which was only found in quartz associated with wolframite the other was a low salinity aqueous fluid found in all the four minerals the homogenization temperatures indicate minimum entrapment temperatures of 226 310 c average 280 c for the h2o co2 n2 nacl fluid and average temperatures of 266 c for scheelite and 242 c 190 c and 160 c for the last generations of beryl fluorite and quartz respectively it was estimated that wolframite was deposited 7 km depth assuming a lithostatic pressure probably due to strong pressure fluctuation caused by seismic events triggered by brittle tectonics during the exhumation event precipitation of scheelite and sulphides took place later at the same depth but under a hydrostatic or suprahydrostatic pressure regime and probably caused by mixing between the magmatic hydrothermal fluid and meteoric waters that deeply penetrated the basement during post variscan decompression

structural framework of the gulf of elat aqaba northern red sea the gulf of elat aqaba occupies the southern part of the dead sea rift the rift is considered to be a plate boundary of the transform type partially leaky which connects seafloor spreading in the red sea with the sagros taurus zone of continental collision the deep water in the gulf of elat up to 1850 m provides a rare opportunity to examine the process of continental rifting by marine geophysical techniques the bathymetry alone provides much information about fault patterns in this area the fragmentation of the once continuous arabian african platform is a complicated process it shapes the structure of the gulf which has developed through continuing tectonism primarily consisting of faulting recent geophysical and geological studies of the gulf of elat including bathymetry bottom photographs continuous seismic profiles seismic refraction gravity magnetics heat flow and coring provide new information about the shallow and deep crustal structure of this important segment of the world rift system the shallow structure of the gulf is dominated by three elongated en echelon basins which strike n20 25 e undulations in the floors of the basins produce several distinct deeps these basins are interpreted as pullaparts the new data from the gulf suggest that classical models for the formation of these structures should be modified only one of the longitudinal faults of each depression is a strike slip fault while the other is predominantly a normal fault the situation with the other two sides of the basin which are supposed to be composed of normal faults may also be more complex than previously thought at least in one case one such boundary is composed of a strike slip fault while the other has no significant fault crustal models of the gulf of elat based on gravity data indicate that the basins are rather shallow and do not extend into the lower crust the fill of the basins extends to about 5 km below the seafloor in the northern and southern basins and less in the central basin most significant magnetic anomalies in the gulf of elat extend from land into the sea none of them however extends from one coast to another across the gulf this supports the geological evidence for a shear between the arabia and sinai plates the magnetic field over the southern third of the gulf is rather smooth and markedly different from that of the other parts the nature of the magnetics the crustal structure of the western margin and the heat flow values indicate a thinner crust and different tectonic processes in the southern part of the gulf overall it seems that within the southern gulf of elat a transition occurs between crustal spreading that takes place in the red sea to a rifting without spreading that takes place along the dead sea transform spreading activity propagates from south to north the most active place is the central basin which is being propagated northward into the shallow northern basin

origin of the northern indus fan and murray ridge northern arabian sea interpretation from seismic and magnetic imaging abstract the nature and origin of the sediments and crust of the murray ridge system and northern indus fan are discussed the uppermost unit consists of middle miocene to recent channel levee complexes typical of submarine fans this unit is underlain by a second unit composed of hemipelagic to pelagic sediments deposited during the drift phase after the break up of india seychelles africa a predrift sequence of assumed mesozoic age occurring only as observed above basement ridges is composed of highly consolidated rocks different types of the acoustic basement were detected which reflection seismic pattern magnetic anomalies and gravity field modeling indicate to be of continental character the continental crust is extremely thinned in the northern indus fan lacking a typical block faulted structure the indian continent ocean transition is marked on single mcs profiles by sequences of seaward dipping reflectors sdr in the northwestern arabian sea the indian plate margin is characterized by several phases of volcanism and deformation revealed from interpretation of multichannel seismic profiles and magnetic anomalies from this study thinned continental crust spreads between the northern murray ridge system and india underneath the northern indus fan

carbonate hosted talc deposits in the contact aureole of an igneous intrusion hwanggangri mineralized zone south korea geochemistry phase relationships and stable isotope studies many talc deposits occur in the hwanggangri mineralized zone hmz in dolomitic marbles of the cambro ordovician samtaesan formation within 1 km of the contact with the cretaceous muamsa granite talc commonly forms fine grained fibrous aggregates or pseudomorphs after tremolite abundant tremolite is included as impurities in the talc ore talc generally was derived from tremolite in calc silicate rock within the dolomitic marble calc silicate rock consisting mainly of tremolite and diopside was generated from silicic metasomatism during the prograde stage which promoted decarbonation reactions until dolomite was exhausted locally hydrothermal alteration of calc silicate rock to talc is marked by the addition of mg and si and the leaching of ca cr co and ni were relatively immobile during the retrograde stage contact metamorphism related to the granite intrusion generated the successive appearance of tremolite diopside and forsterite or wollastonite bearing assemblages in the marble depending on the bulk rock composition the xco2 content of the metamorphic fluids rose initially above xco2 0 6 and decreased steadily toward a water rich composition with increasing temperature above 600 c in the calcitic marble while buffered reaction of the dolomitic marble occurred at higher xco2 conditions above 600 c talc mineralization developed under metastable conditions with infiltration of large amounts of igneous fluids along a fault shattered zone during the retrograde stage and is characterized by the loss of ca2 with the addition of mg2 oxygen and carbon isotopic variations of carbonate and calc silicate minerals are in agreement with theoretical relationships determined for decarbonation products of contact metamorphism talc formation temperatures obtained from oxygen isotope fractionation t xco2 relationships and activity diagrams range from 380 to 400 c

resolution of the scripps noaa marine gravity field from satellite altimetry the july 1995 declassification of the entire geosat gm satellite altimeter data set enabled a joint scripps noaa effort to compute a new version 7 2 marine gravity field on a 2 minute grid this gravity field covers the world oceans between 72 n and 72 s and is derived from a combination of ers 1 and geosat gm and erm data an earlier noaa geosat only gravity field solution was confined to the southern latitudes because the 1992 declassification was limited to gm data south of 30 s a simple coherence analysis between accurately navigated ship gravity profiles and comparable gravity profiles obtained from the gravity grids reveals that the scripps noaa gravity field is coherent with ship gravity down to 23 30 km this slight increase in resolution over the previous noaa geosat only gravity field short wavelength resolution of 26 30 km implies that the increased spatial coverage provided by the ers i altimeter when combined with geosat improves the solution coherence analyses between satellite gravity and ship topography and ship gravity and ship topography show that even shorter wavelength gravity anomalies 13 km are present in sea surface measurements made by ship even so the scripps noaa marine gravity field does an excellent job of resolving most of the short wavelength gravity anomalies covering the world oceans

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

ion microprobe 232th 208pb ages from high common pb monazite morefield mine amelia county virginia implications for alleghanian tectonics monazite ce th po4 from a pegmatite in the morefield mine of the eastern piedmont of central virginia has unusually high and variable amounts of common pb leading to problematic interpretations of its u th pb ages and how the monazite relates to nearby granite intrusions and faults to address these issues we analyze a single large monazite grain from the pegmatite using electron microprobe analysis empa n 64 laser ablation inductively coupled plasma mass spectrometry la icp ms n 58 and secondary ion mass spectrometry sims n 59 the monazite study grain exhibits compositional variations in proximity to microcracks consistent regions of secondary alteration and recrystallization although the compositions of these regions fit the ideal stoichiometry of monazite they have lower si th u and y and higher p rare earth element ree and ca concentrations compared to visibly unaltered portions of the grain la icp ms and sims analyses demonstrate that common pb as proxied by 204pb is enriched in proximity to microcrack regions and correlates with 137ba sims 232th 208pb analysis from grain regions with lowest contents of common pb 208pb comprises 99 of pb isotopes yields two sets of ages 263 5 3 0 ma 1 mswd 1 7 n 11 and 234 1 3 3 ma 1 mswd 0 4 n 13 regionally the ages are similar to the youngest appalachian pegmatite bodies emplaced during the terminal alleghanian laurentia africa collision however the monazite ages are younger than locally surrounding intrusions the closest intrusive in distance 30 km and age is the petersburg granite 296 33 0 11 ma zircon 238u 206pb the emplacement of which coincided with activity along the hylas fault because the fault experienced a brittle ductile transition in the late permian 262 ma coeval with the older 232th 208pb monazite age group we postulate that the hylas fault may have operated as a fluid migration system between the petersburg granite and morefield mine pegmatite the younger monazite age coincides with triassic normal and or sinistral faulting linked to the development and deformation of local rift basins

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

evidence for motion between nubia and somalia along the southwest indian ridge the east african rift marks the northern boundary of the nubian west african and somalian east african plates and has formed by horizontal stretching due to the separation of these plates1 south of 20 s any expression of deformation or seismicity due to the relative motion of these two distinct plates vanishes although the boundary must continue until it intersects another plate boundary the nearest such boundary is that of the antarctic plate marked by the southwest indian ridge but previous analyses of plate motion data have indicated no significant difference between nubia antarctica and somalia antarctica motion2 3 here we show using a large compilation of plate motion data that nubia antarctica motion does differ from somalia antarctica motion and we determine a relative angular velocity of the two plates that has compact confidence limits our analysis places the pole of rotation near to the southern limit of african seismicity implying that the southern part of the nubian somalian plate boundary is a diffuse zone of convergence up to 2 mm yr 1 whereas up to 6 mm yr 1 of separation is accommodated across the east african rift about half the separation rate of the slowest mid ocean ridge

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

origin and evolution of ore forming fluids of the larong w mo deposit eastern tibet constraints from fluid inclusions h o isotopes and scheelite geochemistry abstract the larong porphyry w mo deposit formed in the main collisional orogenic stage of lhasa and qiangtang terranes is the first discovered giant w mo deposit in the leiwuqi zogang metallogenic belt sanjiang region the w mo mineralization occurs as veins and stock works mainly hosted in the granodiorite porphyry monzogranite porphyry biotite granite porphyry and surrounding schist four types of fluid inclusions fis including pc type pure vapor co2 c type co2 h2o w type aqueous two phase and l type pure liquid h2o fis have been identified from stage i quartz scheelite alkaline feldspar veins stage ii including three sub stages quartz molybdenite pyrite chalcopyrite veins and stage iii quartz calcite pyrite veins the total homogenization temperatures of fis vary widely 187 437 c with multiple peaks indicating multistage fluid activities the ore forming fluids of the main mineralization stages are characterized by co2 enrichment high medium temperature 270 440 c and medium low salinity 2 4 14 4 wt nacl equiv the homogenization temperature of w type inclusions is obviously higher than that of c type inclusions and the salinity is relatively lower implying multiple immiscibility of fluids with continuous escape of co2 h o isotopic compositions suggest that the primary ore forming fluids dominantly originated from magma water mo concentrations and eu values in scheelite indicate a decreasing trend of redox state of ore fluids from stage i to stage ii to stage iii the relatively higher temperature co2 rich and oxidized fluids gradually evolved into relatively lower temperature co2 poor and reduced fluids here we propose that the decrease of temperature and oxygen fugacity and increase of s2 activity and ph values of the ore fluids caused by continuous fluid rock interaction and loss of co2 could account for the successive mineralization of w and mo at the larong deposit

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

wolframite solubility and precipitation in hydrothermal fluids insight from thermodynamic modeling abstract wolframite a solid solution series between hubnerite mnwo4 and ferberite fewo4 is the main tungsten bearing ore mineral in vein type tungsten deposits much progress has been made on characterizing the mineralizing fluids precipitating wolframite but it remains poorly understood what are the mechanisms depositing wolframite effectively and the variables controlling mn fe ratios in wolframite this study examines chemical controls on wolframite solubility in hydrothermal fluids and mn fe ratios in wolframite using thermodynamic models for fluids in the systems of w mn cl na o h and w fe cl na o h the fluid buffered models are established based on the characteristics of vein type tungsten deposits the modeling results indicate that solubilities of hubnerite and ferberite in hydrothermal fluids at typical w mineralizing conditions 300 400 c 500 1500 bars 5 15 wt nacl equivalent ph 4 6 reach up to hundreds of ppm the predominant tungsten manganese and iron species in fluids are h w o 4 m n c l 2 0 and f e c l 4 2 respectively an increase in ph and a decrease in fluid temperature and salinity are efficient mechanisms precipitating hubnerite and ferberite from acidic hydrothermal fluids hubnerite and ferberite are also significantly dissolved in alkaline hydrothermal fluids but their solubilities in alkaline fluids are insensitive to fluid temperature and salinity this may affect w mineralization at stages with different ph levels e g the greisen stage and the alkaline stage the mn fe ratio of mineralizing fluids is the principal variable controlling mn fe ratios in wolframite more supply of fe than mn from the fertile magma tends to produce fe rich mineralizing fluids this may be one of reasons why ferberite is more common than hubnerite in tungsten deposits and why hubnerite is more likely to precipitate at an early stage

the seismic structure of thinned continental crust in the northern bay of biscay summary the stretching and thinning of the continental crust which occurs during the formation of passive continental margins may cause important changes in the velocity structure of such crust further crust attenuated to a few kilometres thickness can be found underlying oceanic water depths this paper poses the question of whether thinned continental crust can be distinguished seismically from normal oceanic crust of about the same thickness a single seismic refraction line shot over thinned continental crust as part of the north biscay margin transect in 1979 was studied in detail tau p inversion suggested that there are differences between oceanic and continental crust in the lower crustal structure this was confirmed when synthetic seismograms were calculated the thinned continental crust p 7 0 exhibits a two gradient structure in the non sedimentary crust with velocities between 5 9 and 7 4 km s l an upper 0 8 s l layer overlies a 0 4 s l layer no layer comparable to oceanic layer 3 was detected the uppermost mantle also contains a low velocity zone

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

composition of arsenopyrite from topaz greisen veins in southeastern missouri arsenopyrite occurs in greisen sulfide veins hosted by unmetamorphosed precambrian granite and rhyolite in the silver mine district of southeastern missouri greisenization and sulfide mineralization appear to have been a continuous depositional sequence which recorded falling temperature in a near surface vein environment textural criteria imply that equilibrium existed between arsenopyrite and pyrite and that this pair crystallized in an intermediate paragenetic position between the greisen and hydrothermal stages thirty eight electron microprobe spot analyses of 15 arsenopyrite crystals from the einstein and gabriel veins failed to disclose chemical zoning of as s the compositional range of the analyzed arsenopyrites is 32 9 to 31 0 atomic as a range of arsenopyrite crystallization temperature from 485 c 15 to 455 c 15 is indicated for the gabriel vein in contrast arsenopyrites from the einstein vein record a lower and broader crystallization range of 440 c 15 to 368 c 15

evaluation of the volatilization and infiltration effects on the stable isotopic and mineralogical variations in the carbonate rocks adjacent to the cretaceous muamsa granite south korea the muamsa granite in the hwanggangri mineralized zone south korea intruded the cambro ordovician samtaesan formation which dominantly consists of calcitic marble and dolomitic marble with minor intercalations of pelitic rock and amphibolite intrusion of the granite caused an isotopic depletion of 13c and 18o in the carbonate rocks calcitic carbonates show systematic depletion from unrecrystallized limestone through calcitic marble to skarn it was initiated with volatilization accompanying calcsilicate forming reactions and then with increased permeability h2o rich siliceous fluids of igneous origin actively infiltrated the system promoting fluid rock interaction for the skarn fluid infiltration yielded fluid rock ratios as high as 4 2 in open system and caused increased 18o depletion rates compared to the volatilization stage more than 70 of the isotopic depletion in the calcitic marble is attributed to the infiltration process with such strong infiltration mineral assemblages such as phlogopite in the absence of k feldspar phlogopite diopside and wollastonite are commonly observed in the marble the dolomitic marble was comparatively less depleted in heavy isotopes compared to the calcitic marble and formed under a wide range of xco2 and low fluid rock ratios 1 0 active decarbonation in dolomitic marble was restricted by silica poor dolomite protolith which caused limited fluid infiltration and resulted in high xco2 mineral assemblages represented by dolomite calcite diopside forsterite and more 13c depleted nature compared to dolomite calcite tremolite forsterite assemblage

an infrared microthermometric study of fluid inclusions in coexisting quartz and wolframite from late mesozoic tungsten deposits in the gannan metallogenic belt south china abstract numerous wolframite quartz vein type tungsten deposits are present in the gannan metallogenic belt south china including the piaotang and dangping deposits in the chong you yu ore cluster the dajishan deposit in the jiulianshan ore cluster and the pangushan deposit in the yushan ore cluster the ores generally consist of quartz and wolframite with variable amounts of base metal sulfides wolframite and quartz are intergrown but the genetic relationships between them are still ambiguous quartz contains two phase aqueous inclusions type i and two or three phase co 2 bearing inclusions type ii but wolframite only contains two phase aqueous inclusions type iw the homogenization temperatures for primary inclusions in wolframite are generally 40 to 100 c higher than those in quartz in a single deposit moreover there are variable amounts of co 2 in inclusions in quartz but no co 2 rich inclusions are identified in coexisting wolframite type i fluid inclusions in quartz from the piaotang deposit show that fluid mixing is predominant in the formation of quartz veins whereas fluid inclusions from the dangping deposit suggest that simple cooling is the primary factor quartz from the dajishan and pangushan deposits contains both type i and coexisting type ii inclusions indicating a process of fluid immiscibility the homogenization temperatures of two phase aqueous inclusions type iw in wolframite from all of these deposits show a clearly decreasing trend with constant salinities implying a simple cooling process hence simple cooling fluid mixing and fluid immiscibility take place in quartz but only simple cooling happens in wolframite our results show that the gangue and ore minerals from the late mesozoic tungsten deposits in the gannan metallogenic belt have different homogenization temperatures volatile compositions and fluid processes the gangue minerals do not provide reasonable estimates of the depositional conditions of the ore minerals even though they are intimately intergrown

constraints on plate motions in southern pakistan and the northern arabian sea from the focal mechanisms of small earthquakes the focal mechanism and depth were determined for nine small earthquakes m0 1025 dyn cm m 5 5 that occurred in southern pakistan and the northern arabian sea from an analysis of the vertical component of rayleigh waves in combination with limited first motion data focal parameters were determined from the rayleigh waves by using an event pair method of analysis for earthquakes that are located very close to each other 50 km the event pair method is able to remove a significant proportion of propagation effects at all periods in the range of interest 20 50 s for events separated by more than 100 km the propagation effects are reduced for only the longer periods 40 50 s the earthquakes that were studied provide evidence for a model of plate interactions in the vicinity of the southern pakistan triple junction the owen fracture zone is a transform fault that accommodates right lateral motion between the indian and arabian plates the plate boundary in the vicinity of the murray ridge is also partially made up of transform segments that strike subparallel to the owen fracture zone spreading centers may also exist in the vicinity of the murray ridge but were not documented by seismic or other evidence the slip azimuths for earthquakes along this boundary are significantly more northerly than those predicted by various regional and worldwide models of plate motion the arabian plate is being subducted beneath the eurasian plate along the southern coast of pakistan slip vectors for earthquakes along this boundary trend northnortheasterly in general agreement with predicted directions left lateral motion is documented along the boundary between the indian and eurasian plates in southern pakistan the predicted direction of relative motion between these plates is not significantly different from that observed two of the earthquakes studied appear to be intraplate in nature the depth and focal mechanism of one intraplate event which may have occurred within the accretionary prism along the makran coast however are based on limited data the other intraplate earthquake occurred within the sedimentary deposits along the western edge of the indian subcontinent near the indian eurasian plate boundary appendix is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b83 009 2 50 payment must accompany order

distal relationship of the taihexian pb zn au deposit to the dengfuxian magmatic hydrothermal system south china constraints from mineralogy fluid inclusion h o pb and in situ s isotopes abstract the taihexian pb zn au vein style deposit is located in the well preserved dengfuxian magmatic hydrothermal w sn ore field south china the deposit can be divided into three mineralization stages i pyrite arsenopyrite chalcopyrite gold tourmaline quartz stage ii pyrite arsenopyrite chalcopyrite sphalerite galena quartz stage and iii pyrite galena sphalerite quartz calcite fluorite stage liquid rich nacl aqueous inclusions vapor rich nacl aqueous inclusions co2 rich inclusions and three phase co2 h2o inclusions occur in quartz and calcite of the ore veins at taihexian the ore veins of the three stages formed at around 310 c 210 c and 170 c respectively precipitated from h2o nacl co2 ch4 fluids with salinities averaging around 5 wt nacl eq the ore forming mechanisms include fluid immiscibility a potential trigger for metal precipitation from hydrothermal fluids during stage i fluid mixing during stage ii and cooling during stage iii oxygen 18ofluid 1 5 6 9 and hydrogen dfluid 61 to 53 isotopes suggest that the ore forming fluids originated from a mixture of magmatic water with ground water and or meteoric water sulfur isotopes 34s 7 8 3 1 suggest a magmatic origin for sulfur in stages i and ii and then possibly mixing with sulfur from sedimentary rocks in stage iii lead isotopes 206pb 204pb 18 05 18 65 207pb 204pb 15 53 15 75 208pb 204pb 37 91 38 88 indicate that ore metals originated mainly from the upper crust with minor mantle contributions comparing arsenopyrite sulfur isotope and fluid inclusion geothermometers it is estimated that the ore formation pressure was about 58 120 mpa corresponding to a depth of approximately 2 2 4 5 km taihexian appears to form part of a regional magmatic hydrothermal system centered on the dengfuxian pluton

successive shifts of the india africa transform plate boundary during the late cretaceous paleogene interval implications for ophiolite emplacement along transforms abstract the arabian sea in the nw indian ocean is a place where two major transform boundaries are currently active the owen fracture zone between india and arabia and the owen transform between india and somalia these transform systems result from the fragmentation of the india africa transform boundary which initiated about 90 myrs ago when the india seychelles block separated from madagascar to move towards eurasia therefore the geological record of the arabian sea makes it possible to investigate the sensitivity of a transform system to several major geodynamic changes here we focus on the evolution of the india africa transform system during the 47 90 ma interval we identify the late cretaceous 90 65 ma transform plate boundary along chain ridge in the north somali basin from 65 to 42 47 ma the india africa transform is identified at the chain fracture zone which crossed both the owen basin and the north east oman margin finally the transform system jumped to its present day location in the vicinity of the owen ridge these shifts of the india africa boundary with time provide a consistent paleogeographic framework for the emplacement of the masirah ophiolitic belt which constitutes a case of ophiolite emplaced along a transform boundary the successive locations of the india africa boundary further highlight the origin of the owen basin lithosphere incoming into the makran subduction zone

anomalous subsidence on the rifted volcanic margin of pakistan no influence from deccan plume abstract the role of hotter than ambient plume mantle in the formation of a rifted volcanic margin in the northern arabian sea is investigated using subsidence analysis of a drill site located on the seismically defined somnath volcanic ridge the ridge has experienced 4 km of subsidence since 65 ma and lies within oceanic lithosphere we estimate crustal thickness to be 9 5 11 5 km curiously 600 c especially considering the rapid northward drift of india relative to the deccan reunion hotspot we suggest that this reflects very slow lithospheric growth possibly caused by vigorous asthenospheric convection lasting 28 m y and induced by the steep continent ocean boundary post rift slow subsidence is also recognized on volcanic margins in the ne atlantic and se newfoundland and cannot be used as a unique indicator of plume mantle involvement in continental break up

along axis variations in seafloor spreading in the mark area recent investigations with the manned submersible alvin and the angus deep towed camera sled greatly extended the known range of variations in the style of seafloor spreading along the axis of the mid atlantic ridge five transects of the spreading centre at intervals of10 20 km south of the kane fracture zone at 24 n latitude demonstrate dramatic changes in the style and magnitude of tectonic extension development of the neovolcanic zone expression of hydrothermal venting types oflithologic exposures and morphology of the median valley

multiple fluids involved in granite related w sn deposits from the world class jiangxi province china abstract this paper aims at providing new insights into w sn ore forming processes within one of the largest granitic provinces in the world nanling range south china which was emplaced during the jurassic cretaceous period the origin composition and pressure temperature conditions of fluids involved in the w sn ore forming processes have been investigated by microthermometry raman spectroscopy la icpms hydrogen isotope analyses of fluid inclusions and oxygen isotope analyses of minerals from the maoping and piaotang w sn deposits for each deposit pre quartz syn wolframite and cassiterite and post ore quartz topaz and fluorite minerals were studied in both deposits the vast majority of fluid inclusions are aqueous with salinities between 0 0 and 12 6 wt equiv nacl and homogenization temperatures between 136 and 349 c a minor proportion 5 of inclusions observed in the ore stage quartz from maoping have aquo carbonic compositions for both deposits four compositional groups are defined early quartz fluid inclusions are characterized by salinities between 0 4 and 9 0 wt equiv nacl trapping temperatures between 150 and 350 c and pressures between 20 and 150 mpa la icpms analyses of these fluid inclusions reveal a wide range of na k and li concentrations as well as relatively low metal contents w at piaotang fluid inclusions in wolframite as well as in post ore quartz and fluorite have salinities of 5 6 to 12 6 wt eq nacl trapping temperatures between 150 and 400 c and pressures of 20 to 150 mpa la icpms analyses of these fluid inclusions reveal similar compositions to early quartz fluid inclusions values of 18o in wolframite and quartz crystals range from 0 8 to 5 2 vsmow while the d values of fluid inclusions range from 66 to 62 vsmow collectively the data suggest the involvement of four aqueous fluid end members mixed episodically in the mineralization process a a low salinity low temperature metal poor low 18o low d fluid derived from meteoric water b a high salinity high temperature metal poor high 18o low d fluid derived from a differentiated peraluminous granitic magma c a high salinity high temperature metal rich lower d magmatic fluid derived from a more differentiated peraluminous granitic magma and d a high salinity high temperature metal rich high 18o low d magmatic fluid this study shows that multiple fluids with distinct magmatic and meteoric origins were involved in the formation of these w sn deposits and that the dilution of metal bearing magmatic fluids by meteoric fluids was probably the main driver for ore deposition the common fluid history of the two deposits studied as well as similarities with other deposits in the jiangxi province points toward common ore forming processes at the regional scale

fluid inclusion and isotopic c h o s and pb constraints on the origin of late mesozoic vein type w mineralization in northern guangdong south china abstract northern guangdong of south china occurs many large tungsten deposits including shirenzhang meiziwo and yaoling quartz vein type wolframite deposits in this study we carried out a detailed study on fluid inclusions and c h o s pb isotopic analyses of mineral separates for these three deposits in order to resolve the origin and evolution of ore forming fluids and the ore deposition mechanism the ore veins in these deposits present a similar mineral paragenesis that could be divided into three stages i pre ore stage ii syn ore stage and iii post ore stage the pre ore stage is the magmatic hydrothermal transition stage which is marked by fluid exsolution from the highly fractionated granites accompanied with potassic alteration and greisenization the syn ore stage can be further divided into two stages silicate oxide stage stage ii 1 that is characterized by significant tin tungsten mineral deposition and sulfide stage stage ii 2 that is characterized by abundant sulfide minerals deposition the post ore stage is dominated by quartz and fluorite the three deposits show similar temperature and salinity variations for syn ore and post ore stages in the shirenzhang deposit the fluid temperature and salinity range from 239 to 301 c and 1 4 to 8 7 wt nacl equiv in stage ii 1 from 206 to 256 c and 1 4 to 7 0 wt nacl equiv in stage ii 2 and from 186 to 236 c and 1 4 to 8 6 wt nacl equiv in stage iii in the meiziwo deposit the fluid temperature and salinity range from 242 to 310 c and 1 1 to 8 6 wt nacl equiv in stage ii 1 from 196 to 252 c and 1 4 to 8 1 wt nacl equiv in stage ii 2 and from 188 to 234 c and 1 2 to 7 3 wt nacl equiv in stage iii in the yaoling deposit the fluid temperature and salinity range from 230 to 304 c and 1 2 to 9 0 wt nacl equiv in stage ii 1 from 203 to 258 c and 1 4 to 6 5 wt nacl equiv in stage ii 2 and from 184 to 231 c and 1 6 to 8 3 wt nacl equiv in stage iii the ore forming fluids in the three deposits belong to a medium temperature low salinity h2o nacl system with trace amounts of volatile components including co2 ch4 and n2 the c h o isotope data 13cco2 18 9 to 6 7 18owater 1 1 to 6 3 dh2o 78 to 55 indicate that the ore forming fluids were mainly magmatic water which might be modified by fluid rock interaction and mixing with meteoric water sulfur 34s 7 0 to 0 3 and pb isotope data 206pb 204pb 18 531 18 693 207pb 204pb 15 728 15 753 208pb 204pb 38 902 39 081 suggest that the ore metal and sulfur are of magmatic origin the involvement of both organic matter in metasedimentary rocks during fluid rock interaction and oxidized meteoric water may have been responsible for the negative 34s and 13c values fluid boiling fluid rock interaction and minor input of meteoric water might have been effective factors for the precipitation of wolframite cassiterite and scheelite the sulfide precipitation may have resulted from cooling and dilution of magmatic fluids by mixing with meteoric water

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

thin crust beneath ocean drilling program borehole 735b at the southwest indian ridge abstract a wide angle seismic experiment at the atlantis ii fracture zone southwest indian ridge together with geochemical analyses of dredged basalt glass samples from a site conjugate to ocean drilling program hole 735b has allowed determination of the thickness and the most likely lithological composition of the crust beneath hole 735b the measured na8 composition of 3 3 0 1 corresponds to a melt thickness of 3 1 km a result consistent with rare earth element inversions which indicate a melt thickness of between 1 5 and 4 5 km the seismic crustal thickness to the north and south of the atlantis platform on which hole 735b is located is 4 1 km and probably consists largely of magmatic material since the seismic and inferred melt thicknesses agree within experimental uncertainty beneath hole 735b itself the moho is at a depth of 5 1 km beneath the seafloor the seismic model suggests that on average about 1 km of upper crust has been unroofed on the atlantis platform however allowing for the inferred local unroofing of 2 km of upper crust at 735b the base of the magmatic crust beneath this location is probably about 2 km beneath the seafloor and is underlain by a 2 3 km thick layer of serpentinised mantle peridotite the p wave velocity of 6 9 km s for the serpentinised peridotite layer corresponds to a 35 10 vol serpentine content the moho beneath hole 735b probably represents a serpentinisation front

seafloor topography and tectonic elements of the western indian ocean the break up of gondwanaland and dispersal of several of its component continental fragments which eventually formed the margins of the indian ocean have produced an ocean basin of enormous variety both in relief and in origin of seafloor features the western half of the indian ocean alone contains every type of tectonic plate boundary both active and fossil and along with some of the deepest fracture zones the most complex mid ocean ridge configurations and some of the thickest sedimentary sequences in the world s ocean basins this ocean is one of the most diverse on the face of the globe we explore the evolution of the morphology of the indian ocean floor and discuss the effect of its variations maxima and minima on the interconnectivity of the ocean s water masses

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

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

densities and porosities in the oceanic crust and their variations with depth and age we have used drilling and downhole logging results from deep sea drilling project holes 395a and 418a measured properties of core samples from the seafloor and from ophiolites and seismic velocity structure models of the oceanic crust to constrain empirical statistical relationships between in situ densities porosities and seismic velocities and to estimate densities and porosities in the oceanic crust with the following principal results our review of drilling results and studies of ophiolites suggests that the velocity structure of the oceanic crust is more closely related to changes in porosity and alteration or metamorphic grade than to its igneous structure the transition from layer 2a 3 7 0 4 km s 1 to 2b 5 2 0 4 km s 1 occurs within the extrusive pile the top of layer 2c 6 2 0 2 km s 1 is associated with the top of the sheeted dike complex and with the first appearance of greenschist facies alteration products ophiolite studies have shown that the top of layer 3 occurs within the sheeted dikes where the metamorphic grade of alteration products increases from greenschist to amphibolite facies a comparison of reprocessed and corrected downhole logs from holes 395a and 418a indicates that in situ densities and fractional porosities differ by 0 2 mg m 3 and 0 07 respectively these differences are consistent with prevailing views of the progressive alteration of the upper oceanic crust statistical relationships between in situ density porosity and slowness are highly linear and indicate that the density slowness relation estimated from laboratory samples pb 3 81 0 02 6 0 0 1 s applies to in situ properties as well we obtained a similar empirical relationship for porosities from the hole 418a log data f 0 35 0 03 2 37 0 15 s r2 0 84 s e 0 015 applying these empirical models to seismic velocity structure models yields the following crustal densities and porosities layer 2 2 62 2 69 mg m 3 0 10 0 12 and layer 3 2 92 2 97 mg m 3 the estimated average density of the oceanic crust is 2 86 0 03 mg m 3 densities and porosities estimated from a seismic crustal structure model for old atlantic crust agree very well with the logging data from hole 418a suggesting that in situ densities and porosities can be estimated from seismic velocities high density and porosity gradients are characteristic of the upper oceanic crust increasing densities reflect both a progressive decrease of porosity and increasing grain density with depth based on the seismic properties of layer 2a we estimate that the layer thins at an approximate rate 20 to 45 m m y 1 while the velocity increases by 0 040 km s 1 m y 1 the corresponding rates of change of density and porosity are 0 016 0 009 mg m 3 m y 1 and 0 005 0 003 m y 1 respectively

structure and early evolution of the arabian sea and east somali basin summary the laxmi ridge is a large scale basement high buried beneath the sediments of the indus fan the location of the ocean continent transition oct on this margin has previously been proposed at either the southern edge of the laxmi ridge or beyond it towards the india pakistan shelf the former explains the margin parallel laxmi basin as thinned continental crust the latter as a failed rift of earlier seafloor spreading to examine the structure of this margin a reassessment of marine magnetic data has detailed seafloor spreading magnetic anomalies prior to anomaly 24 in both the arabian and east somali basins the previously identified anomaly 28 is not interpreted as a seafloor spreading anomaly but as a magnetized basement feature adjacent to and merging with the ridge the laxmi spur new gravity models across the laxmi ridge and adjacent margin using ship and satellite data corroborate the existence of underplated crust beneath the laxmi ridge and basin and the location of the oct at the southern edge of the ridge the results are not compatible with the existence of a pre anomaly 28 phase of seafloor spreading although large scale intrusions may be the origin of some of the basement features in the laxmi basin the models also identify the laxmi spur as a low density feature with a natural remanent magnetization nrm compatible with serpentinization the laxmi ridge is mapped to the southeast where it appears to terminate at a point coinciding with the appearance of e w magnetic lineations and gravity anomalies at 15 5 n thereafter it becomes indistinct this is interpreted as necessary in the reconstruction to the mascarene plateau to avoid continental overlap

infrared microthermometric and stable isotopic study of fluid inclusions in wolframite at the xihuashan tungsten deposit jiangxi province china the xihuashan tungsten deposit jiangxi province china is a world class vein type ore deposit hosted in cambrian strata and mesozoic granitic intrusions there are two major sets of subparallel ore bearing quartz veins the ore mineral assemblage includes wolframite and molybdenite with minor amounts of arsenopyrite chalcopyrite and pyrite there are only two phase aqueous rich inclusions in wolframite but at least three major types of inclusions in quartz two or three phase co2 rich inclusions two phase pure co2 inclusions and two phase aqueous inclusions indicating boiling fluid inclusions in wolframite have relatively higher homogenization temperatures and salinities 239 380 c 3 8 13 7 wt nacl equiv compared with those in quartz 177 329 c 0 9 8 1 wt nacl equiv these distinct differences suggest that those conventional microthermometric data from quartz are not adequate to explain the ore formation process enthalpy salinity plot shows a linear relationship implying mixing of different sources of fluids although boiling occurred during vein type mineralization it seems negligible for wolframite deposition mixing is the dominant mechanism of wolframite precipitation in xihuashan 34s values of the sulfides range from 1 6 to 0 1 indicative of a magmatic source of sulfur 18o values of wolframite are relatively homogeneous ranging from 4 8 to 6 3 oxygen isotope modeling of boiling and mixing processes also indicates that mixing of two different fluids was an important mechanism in the precipitation of wolframite

ore forming mechanism of quartz vein type w sn deposits of the xitian district in se china implications from the trace element analysis of wolframite and investigation of fluid inclusions abstract the xitian w sn district is located in eastern hunan province of se china and comprises the heshuxia goudalan and longshang w sn deposits our study on the occurrence mineral assemblage and geochemistry of the orebodies shows that quartz vein orebodies are dominant mode of mineralization and can be divided into three stages stage i quartz molybdenite wolframite i stage ii quartz wolframite ii polymetallic sulfides and stage iii quartz pyrite fluorite the elements nb ta sc sn zn cr v mo cu and zr are abundant in both wolframite i and wolframite ii wolframite i contains higher concentrations of nb 1408 9245 ppm ta 241 2407 ppm and sc 123 521 ppm than wolframite ii nb 144 1080 ppm ta 3 76 315 ppm and sc 1 86 11 44 ppm these results indicate that wolframite i was formed closer to the host rock and at a greater ore forming depth than wolframite ii the total ree concentrations of wolframite i and ii range from 34 96 to 133 36 ppm and 21 30 to 43 73 ppm respectively both wolframite i and wolframite ii have low lree concentrations 0 00 0 05 and 0 08 8 18 ppm respectively and high hree concentrations 34 96 133 31 and 21 23 41 87 ppm respectively the enrichment of hrees in wolframite may be due to the fact that the sizes of the hrees3 0 94 1 02 a in combination with nb5 or ta5 0 72 a are closer to that of w6 0 68 a in combination with ca2 1 08 a or mg2 0 80 a than that of the lrees3 1 03 1 13 a making the coupled substitution easier a study of fluid inclusions in the coexisting gangue minerals shows that the homogenization temperatures of the fluid inclusions decrease from stage i to stage iii stage i 187 382 c average 275 c stage ii 122 278 c average 175 c stage iii 92 172 c average 139 c whereas the salinities of the fluid inclusions increase from stages i to ii and decrease from stages ii to iii stage i 1 8 18 2 wt nacleqv average 10 3 wt nacleqv stage ii 7 3 24 4 wt nacleqv average 14 9 wt nacleqv stage iii 0 2 5 3 wt nacleqv average 1 5 wt nacleqv raman spectroscopy reveals that the fluid inclusions mainly contain h2o co2 is minor and h2s ch4 and n2 are rare we conclude that the formation of wolframite was driven by post magmatic thermodynamic processes the ore forming fluid flowed through a multi fractured low pressure zone in the fault system in the district and a temperature pressure decrease led to fluid immiscibility characterized by co2 escaping in a low ph and high eh environment in stage i crust mantle mixed fluid and meteoric water mixing in stage ii and natural cooling of the fluid system in stage iii

the polymetallic magmatic hydrothermal xiangdong and dalong systems in the w sn cu pb zn ag dengfuxian orefield se china constraints from geology fluid inclusions h o s pb isotopes and sphalerite rb sr geochronology vein type deposits including the xiangdong w sn and dalong pb zn deposits occur in or near the dengfuxian composite granite pluton comprising predominantly triassic and minor jurassic intrusions liquid rich nacl aqueous inclusions vapor rich nacl aqueous inclusions liquid rich cacl2 nacl aqueous inclusions two phase ch4 rich inclusions three phase co2 h2o inclusions and three phase calcite bearing inclusions occur in the quartz veins at xiangdong whereas only liquid rich nacl aqueous inclusions occur at dalong the xiangdong veins formed at temperatures near 241 c from nacl cacl2 h2o ch4 co2 fluids averaging 11 4 wt nacl eq the dalong deposit formed at temperatures near 186 c from nacl h2o ch4 fluids averaging 6 2 wt nacl eq the ore forming mechanisms at xiangdong include fluid immiscibility during stage i fluid mixing during stage ii and mixing with meteoric water accompanied by cooling during stage iii the ore forming mechanisms at dalong include cooling and mixing with meteoric water oxygen and hydrogen isotopes suggest that the ore forming fluids from both deposits originated as mixtures of magmatic water with various amounts of meteoric water sulfur and strontium isotopes suggest an igneous origin for both deposits and possibly mixing with s and sr from sedimentary rock for dalong lead isotopes indicate that ore metals originated mainly from the upper crust with minor mantle contributions sphalerite from dalong gives a rb sr isochron age of 151 6 7 1 ma consistent with the mineralization age of xiangdong both the w sn and pb zn ore forming events are closely related to late jurassic magmatism which occurred in an environment of lithospheric extension and thinning

re os sm nd u pb and stepwise lead leaching isotope systematics in shear zone hosted gold mineralization genetic tracing and age constraints of crustal hydrothermal activity abstract a combined re os sm nd u pb and stepwise pb leaching pbsl isotope study of hydrothermal mo w bearing minerals and base metal sulfides from two adjacent shear zone hosted gold deposits ran kimberley in the harare shamva greenstone belt zimbabwe constrain the timing of the mineralizing events to two periods during an initial late archean event 2 60 ga a first molybdenite scheelite bearing paragenesis was deposited in both shear zone systems followed by a local reactivation of the shear systems during an early proterozoic 1 96 ga tectono thermal overprint during which base metal sulfides and most of the gold was re deposited while pbsl has revealed an open system behavior of the u pb systematics in molybdenite and wolframite from the ran mine initial archean re os ages are still preserved implying that this system in these minerals was more resistant to the overprint a similar retentivity could be shown for the sm nd system in scheelite and powellite associated with the above ore minerals re os isotopic data from the proterozoic mineralization in the kimberley mine point to a recent gain of re most pronouncedly affecting fe rich sulfides such as pyrrhotite a significant re loss in powellitic scheelite an alteration phase of molybdenite bearing scheelite coupled with a marked loss of u in w mo ore minerals complements the observation of a major re uptake in fe sulfides during oxidizing conditions in a weathering environment pyrrhotite under these conditions behaves as an efficient re sink lead isotope signatures from pbsl residues of molybdenite powellite and quartz indicate a continental crustal source and or contamination for the mineralizing fluid by interaction of the fluids with older sedimentary material as represented by the direct host country rocks our investigation reveals the potential of the re os isotopic system applied to crustal hydrothermal ore minerals for genetic tracing and dating purposes the simplified chemical separation of re and os from geological material used in this study together with improvements of chemical yields will enable high precision data to be collected rapidly on crustal material with low os concentrations in the future

centroid depths of mid ocean ridge earthquakes dependence on spreading rate from inversions of teleseismic p and sh waveforms we have determined the source parameters of 50 large earthquakes that occurred during 1962 1983 on slowly spreading mid ocean ridges all events are characterized by predominantly normal faulting on planes that dip at approximately 45 and strike parallel to the local trend of the ridge axis centroid depths range from 1 to 6 km beneath the seafloor the p waves from these earthquakes show strong water column reverberations suggesting that fault rupture extended to the seafloor under the assumption that the centroid depth marks the mean depth of fault slip earthquake faulting extended to a depth of 2 10 km for these earthquakes the water depths inferred from the predominant periods of the water column reverberations constrain the epicenters of these earthquakes to lie within the relatively deep inner floor of the median valley the maximum centroid depths of ridge crest earthquakes decrease with increasing spreading rate and the maximum seismic moment may also decrease with increasing spreading rate these results indicate a general decrease with spreading rate in maximum thickness of the mechanically strong layer beneath the ridge axis region the concentration of large earthquakes within the deepest parts of the median valley and the depth extent of seismic faulting support lithospheric necking models for the origin of the median valley

origin and evolution of w mo producing fluids in a granitic hydrothermal system geochemical studies of quartz vein deposits around the susan granite hwanggangri district republic of korea the hwanggangri mineralized district is located approximately 130 km southeast of seoul within the paleozoic ogcheon belt of south korea the jungbo suri and deogman mines in the district occur around the late cretaceous whole rock k ar age 89 0 or 2 0 ma susan granite and are located on w mo bearing quartz veins these veins fill fractures in the granite and its host rocks which comprise early paleozoic limestone rich sedimentary rocks the k ar date of vein muscovite 88 58 or 1 95 ma is very similar to that of the susan granite 89 04 or 1 95 ma indicating that w mo mineralization was associated with intrusion of the granite molybdenite and wolframite rich deposits occur within or near 285 degrees c from h 2 o co 2 fluids with salinities of 0 4 to 9 7 wt percent nacl equiv probably in response to ph increase due to boiling including co 2 unmixing of the hydrothermal fluid at pressures between 150 and 220 bars following w mo deposition the high temperature fluids gave way to progressively cooler more dilute fluids associated with scheelite base metal sulfide deposition 200 degrees 320 degrees c 0 7 7 2 wt nacl equiv and carbonate fluorite deposition 105 degrees 225 degrees c 0 0 3 2 wt nacl equiv these trends are interpreted as the result of progressive mixing of magmatic fluids with cooler more dilute meteoric waters there is a systematic decrease in the measured and calculated isotopic compositions of hydrothermal waters with increasing paragenetic time and decreasing temperature stage i delta 18 o water 8 0 to 1 5 per mil and delta d water 56 to 74 per mil stage ii delta 18 o water 1 9 to 5 6 per mil and delta d water 88 to 95 per mil stage iii delta 18 o water 0 9 to 3 0 per mil and delta d water 93 to 97 per mil the trend reflects the progressive increase of meteoric water involvement in the mineralizing system during the cooling history of the susan granite stock molybdenum and tungsten were partitioned preferentially into a coexisting magmatic hydrothermal fluid delta 18 o 8ppm delta d 56 to 72ppm sulfur isotope analyses of hydrothermal sulfide minerals suggest an igneous source of sulfur with a delta 34 s sub sigma s value near 4 per mil following deposition of molybdenite and wolframite from magmatic fluids wolframite was dissolved or replaced by scheelite in the outer and or shallower portions of the hydrothermal system this remobilization of tungsten and later redeposition as scheelite at temperatures of 200 degrees 280 degrees c from fluids with delta 18 o 1 2ppm accompanied mixing with low temperature meteoric fluids increasing involvement of progressively larger volumes of meteoric water resulted in successive deposition of base metal sulfides fluorite and carbonates

neotectonics of the owen fracture zone nw indian ocean structural evolution of an oceanic strike slip plate boundary the owen fracture zone is a 800 km long fault system that accommodates the dextral strike slip motion between india and arabia plates because of slow pelagic sedimentation rates that preserve the seafloor expression of the fault since the early pliocene the fault is clearly observed on bathymetric data it is made up of a series of fault segments separated by releasing and restraining bends including a major pull apart basin at latitude 20 n some distal turbiditic channels from the indus deep sea fan overlap the fault system and are disturbed by its activity thus providing landmarks to date successive stages of fault activity and structural evolution of the owen fracture zone from pliocene to present we determine the durability of relay structures and the timing of their evolution along the principal displacement zone from their inception to their extinction we observe subsidence migration in the 20 n basin and alternate activation of fault splays in the vicinity of the qalhat seamount the present day owen fracture zone is the latest stage of structural evolution of the 20 myr old strike slip fault system buried under indus turbiditic deposits whose activity started at the eastern foot of the owen ridge when the gulf of aden opened the evolution of the owen fracture zone since 3 6 myr reflects a steady state plate motion between arabia and india such as inferred by kinematics for the last 20 myr period the structural evolution of the owen fracture zone since 20 myr including fault segments propagation and migration pull apart basin opening and extinction seems to be characterized by a progressive reorganisation of the fault system and does not require any major kinematics change

from rifted margins to foreland basins investigating provenance and sediment dispersal across desert arabia oman u a e arid arabia where chemical weathering and anthropic modifications are negligible is a huge natural laboratory for the study of sediment provenance mafic and ultramafic detritus from ophiolite belts is found along the oman mountains and on masirah island obduction orogen provenance dominant cellular serpentinite grains from residual mantle harzburgites of the sama il ophiolite contrast with gabbroic diabase and basaltic grains from igneous crustal rocks of the masirah ophiolite detritus from autochthonous basement and cover rocks is found along the arabian sea margin rift shoulder provenance transition from carbonate dominated lithic sands undissected stage to hornblende rich plagioclase arkoses dissected stage is best documented at the northern edge of the gulf of aden along the older strike slip owen basin margin only locally are feldspars hornblende and garnet derived from small outcrops of pan african basement modern eolian sands from yemen to the arabian persian gulf display marked enrichment in quartz low plagioclase feldspar ratios and depleted dense mineral assemblages with rounded epidotes hornblende garnet and ultrastables pointing to extensive recycling also fostered by quaternary changes the rub al khali and wahibah sand seas have distinct quartz feldspar ratios reflecting largely separate transport pathways dune fields fed chiefly from rift highlands in the west and south mix with orogenic detritus in the northeast carbonate dominated sands in coastal uae are mostly derived from the zagros mountains blown by northerly shamal winds across the gulf during pleistocene lowstands to reach the backbulge depozone and the foreland beyond contributions from the oman mountains reach only 5 10 of bulk sediment along their southern apron and mostly indirectly through deflation of plio pleistocene fans the sama il ophiolite supplies enstatite and diopside to the eastern uae and wahibah sands the masirah ophiolite supplies amphiboles and clinopyroxenes locally along the batain coast the composition of desert sediments faithfully mirrors tectonic history of source terranes deformation style during orogeny and unroofing level during extension bulk petrography coupled with dense mineral analysis thus represents a powerful tool to investigate not only provenance and sediment transport but also crustal dynamics and geologic evolution at continental scale

unfolding impacts of freaky tectonics on sedimentary sequences along passive margins pioneer findings from western indian continental margin offshore indus basin abstract the western passive continental margin of the indian plate wcmi serves as an elegant laboratory for understanding the syn sedimentary rift drift tectonics preserving a variety of episodic structural deformations and complex sedimentary infill here we present a multidisciplinary study on offshore indus basin oib that is less investigated compared to adjacent marginal basins along wcmi reflection seismic gravity borehole and biostratigraphic data have been used to reconstruct the tectono sedimentary history of the meso cenozoic succession in oib seismic based dynamic stratigraphy helped in mapping seven different depositional sequences grounded on morpho tectonic considerations the seismic expression of these sequences provided an idea of the depositional styles with different tectonic episodes since rifting in jurassic to basin inversion in miocene continued to present time the oib initially filled from the north with the sediments rapidly prograding southwards in general the basin deepens in the central and northwestern part where the formations achieve maximum depths the analysis of 3d structural models revealed that the region experienced poly phase deformation from the late cretaceous to recent times that include steeply dipping normal faults horst and graben half graben half graben with growth faulting flower structures anticline syncline and uplift with an angular unconformity the most remarkable tectonic activity happened at the early miocene and resulted in the reactivation of extensional faults into strike slip faults giving rise to a spectacular structural inversion due to transform movement between indian and arabian plates that continues to present day the most predominant faults are oriented in s20 50 e with azimuth as 120 160 and the maximum dip angles are 80 in the southwest flank of the basin the main fault trend is s20 e n20 w and dip angles ranging between 40 and 80 and above a thick crust and the ocean continent transition oct is imaged with massive felsic mafic and ultramafic intrusions by the help of forward gravity modeling an initial onset of syn rift tectonic subsidence between 90 and 68 ma with a post rift general increase between 68 and 54 ma is comprehended while two uplift events at 40 ma and 18 ma are documented through backstripping analysis it is inferred that the region is rapidly under complex structural inversion with growing folding faulting and flowering structures this study could help in explaining the unanswered reasons of structural complexities depositional styles and basin inversion along worldwide passive margins

the s reflector west of galicia spain evidence from prestack depth migration for detachment faulting during continental breakup the mechanism of continental extension and breakup and the role of detachment faults in these processes are currently the subject of intense debate one possible detachment fault is the s reflector imaged as an undulating in time locally discontinuous reflection on existing seismic profiles west of iberia here we present new images in depth of the four margin normal profiles across the west galicia rifted margin where the s reflector was originally defined and is best imaged it is shown that faults bounding wedge shaped units of late synrift sediments which hence were active during rifting immediately prior to breakup appear to detach at shallow levels onto the s reflector s itself appears as a continuous locally domal feature and does not generally appear offset the waveform of s is compatible with a reflection from a single interface such as a sharp tectonic boundary the depth sections show that s was active at 1 3 km below the seafloor during final rifting s is interpreted as a brittle detachment fault which controlled the final breakup of the continent west of galicia furthermore the data provide constraints on the sense of shear of s analogy with detachment terranes the present synrift and structural dips of s and the identification of a breakaway to s imply that s accommodated top to the west shear toward the east of the profiles s becomes more complex possibly because of different phases of detachment faulting and the development of both incisement and excisement structures from the geometry of wedge shaped sedimentary units deposited during faulting above s it also appears that s was active at an angle of 20 or less and hence may be considered a genuine low angle normal fault

paleogeographic maps of the continents bordering the indian ocean since the early jurassic paleomagnetic data from the african plate from the early jurassic onward are critically re evaluated and updated resulting in a set of 30 paleopoles from which an apparent polar wander path is derived using 20 ma intervals this path is then used in conjunction with kinematic models of the indian and atlantic oceans to place other continents bordering the indian ocean eurasia india islands and plateaus madagascar seychelles mascarene and blocks involved in the collision of india with eurasia south tibet indochina in an absolute paleogeographic frame fixed with respect to africa paleogeographic maps are presented for the early jurassic pre breakup the jurassic cretaceous boundary anomaly ml5 the early cretaceous m2 the late cretaceous a 34 the cretaceous tertiary boundary a 29 the early eocene a 24 the middle eocene a 21 and the oligocene miocene boundary a 6 observed and predicted paleomagnetic directions for landmasses other than africa i e for western europe china korea india and australia which are not used in deriving the maps are found to be in very satisfactory agreement the quality of reconstructions as measured by rms discrepancies between observed and predicted paleolatitudes is of the order of 5 the main features of the paleogeographic maps are discussed with the aim of relating oceanic and on land geodynamic events the constraints provided by tibetan data on the india asia collision and the 20 ma reorganization of geology and plate boundaries in a band extending from the south china sea through tibet to the red sea system are discussed in more detail we emphasize the episodic nature of the breakup of gondwanaland and formation of the indian ocean and subsequent interaction with eurasia in which six major discrete events are identified separated by 30 ma phases of rather uniform plate motion

present day motion along the owen fracture zone and dalrymple trough in the arabian sea all plate motion data available for describing the present day motion between the arabian and indian plates are analyzed to understand the present day tectonic role of the presumed plate boundary the owen fracture zone and dalrymple trough in the northern arabian sea all prior plate motion models are shown to be inconsistent with some of the data along the presumed boundary in particular published global plate motion models predict some convergence across the owen fracture zone and dalrymple trough convergence along the owen fracture zone is contradicted by earthquake focal mechanisms which show right lateral strike slip convergence along the dalrymple trough is contradicted by observed normal faulting in the trough and by normal and strike slip focal mechanisms for earthquakes occurring along or near the trough the sense and rate of motion is further constrained by spreading rates along the sheba ridge northwest of the owen fracture zone and along the carlsberg ridge southeast of the owen fracture zone to estimate the present day motion we determine 43 3 m y average spreading rates from sheba ridge carlsberg ridge and central indian ridge magnetic profiles and have combined them with eight transform azimuths and 15 earthquake slip vectors along these ridges and the arabia india boundary we determine an arabia india euler vector that predicts 2 mm yr of right lateral strike slip motion along the owen fracture zone with a 95 confidence interval of 0 7 mm yr which excludes the faster rates of slips predicted by several prior models the direction of motion is 35 50 counterclockwise from that predicted by published global plate motion models if we omit the data along the arabia india boundary the motion differs insignificantly from zero suggesting that magnetic profiles from the carlsberg and sheba ridges give spreading rates too imprecise to detect the slow arabia india motion if the owen fracture zone and dalrymple trough are regarded as a plate boundary it is the slowest slipping plate boundary known on earth with motion about 100 times slower than the fastest slipping faults along the east pacific rise

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

the metamorphosed molybdenum vein type deposit of the alpeinerscharte tyrol austria and its relation to variscan granitoids the molybdenite deposit of the alpeinerscharte austria is situated in variscan greenschist to amphibolite facies metamorphosed granodiorites and granites of the western tauern window these granitoids represent strongly fractionated calc alkaline i type magmas with minor s type components and reveal post orogenic affinities molybdenum contents average 4 3 ppm are slightly above the general background of average granites molybdenite mineralization is restricted to narrow quartz veins and quartz vein selvages which are presently composed of biotite and almandine grossular garnet these selvages show geochemical features typical of intermediate argillic alteration in a hydrothermal system postdating granite intrusion instability of plagioclase causes removal of na ba sr pb and eu while k and ca remain nearly constant rare earth elements apart from eu and metals are extremely enriched application of fe mg exchange garnet biotite and oxygen isotope quartz garnet quartz plagioclase geothermometers to vein selvage mineral assemblages reveals temperatures of the late alpine 35 55 ma metamorphic overprint 540 c 7 10 kbar leucocratic rocks composed of mainly orthoclase and plagioclase are occasionally spatially related to molybdenite bearing veins they are interpreted as episyenites formed by hydrothermal alteration of the host granite this episyenitic alteration is characterized by a mass loss of ca 30 relative enrichment of plagioclase components extreme depletion of si and minor depletion of fe zn cu and mo

development of asymmetric basins along continental transform faults abstract many basins associated with wrench or transform faults are asymmetrical both longitudinally and laterally the sense of basin asymmetry changes along the strike of transform faults geophysical data from the dead sea rift and other transforms indicate that wrench induced asymmetric basins are bounded on only one side by a transform fault this implies that during the evolution of such basins maximum compression was parallel to the transform the direction of extension can change during the geologic evolution of a basin a possible explanation of this observation takes the strength of the fault into into consideration in the case of a weak fault in a strong crust the horizontal principal stresses would probably rotate to orientations parallel and perpendicular to the fault thus minimizing the shear stress on the main fault basins formed by regional extension such as in the east african rift system sometimes show similar features recent studies have shown that these basins often undergo oblique extension this suggests that the asymmetry in these basins may result from simultaneous strike slip motion and fault normal extension similar to the way asymmetry is produced in large transform basins periods of oblique extension can alternate with periods of orthogonal extension the asymmetry formed during periods of oblique extension is often preserved in the stratigraphic record

evolution of the gorda escarpment san andreas fault and mendocino triple junction from multichannel seismic data collected across the northern vizcaino block offshore northern california the gorda escarpment is a north facing scarp immediately south of the mendocino transform fault the gorda juan de fuca pacific plate boundary between 126 w and the mendocino triple junction it elevates the seafloor at the northern edge of the vizcaino block part of the pacific plate 1 5 km above the seafloor of the gorda juan de fuca plate to the north stratigraphy interpreted from multichannel seismic data across and close to the gorda escarpment suggests that the escarpment is a relatively recent pop up feature caused by north south compression across the plate boundary close to 126 w the vizcaino block acoustic basement shallows and is overlain by sediments that thin north toward the gorda escarpment these sediments are tilted south and truncated at the seafloor by contrast in a localized region at the eastern end of the gorda escarpment close to the mendocino triple junction the top of acoustic basement dips north and is overlain by a 2 km thick wedge of pre 11 ma sedimentary rocks that thickens north toward the gorda escarpment this wedge of sediments is restricted to the northeast corner of the vizcaino block unless the wedge of sediments was a preexisting feature on the vizcaino block before it was transferred from the north american to the pacific plate the strong spatial correlation between the sedimentary wedge and the triple junction suggests the entire vizcaino block with the san andreas at its eastern boundary has been part of the pacific plate since significantly before 11 ma

early spreading and continental to oceanic basement transition beneath the indus deep sea fan northeastern arabian sea abstract a seismic survey to study the indus deep sea fan penetrated into basement this enabled the mapping of basement structures and a description of the geodynamical evolution of a part of the northeastern arabian sea the main tectonic structure of the basement is an e w graben gop rift with a prominent central e w horst the palitana horst this rift is cut obliquely by several ne sw faults large e w magnetic anomalies are not the continuation of the magnetic anomaly sequence of the oceanic arabian basin however these anomalies are linked to inverse magnetisation in the crust suggesting that the rift is either underlain by stretched continental crust intruded by linear volcanic bodies related to the deccan traps or by oceanic crust older than chron 28 64 ma as proposed in the laxmi basin southeastwards accompanying these structures ne sw faults with a strike slip pattern may have played the role of transform direction during the separation of the laxmi ridge from the indian continent we propose that rifting and oceanic spreading have occurred around a triple junction connecting the oceanic laxmi basin the narmada son lineament and the gop rift this phase preceded oceanic spreading in the arabian basin this interpretation is in agreement with the previously proposed initial separation of india and the seychelles before chron 28 the laxmi ridge could be a part of the seychelles microplate subsequently left attached to india

how continents break up insights from papua new guinea the woodlark basin in the western pacific forms a continuous system of active continental rifting evolving to well developed seafloor spreading thin sediment cover in the basin and a dominantly nonvolcanic rift phase permit basement fabric and structures to be imaged by swath mapping and seismic reflection data in the continental and oceanic parts of the basin magnetic isochrons indicate a single euler pole of opening for most of the basin history and allow us to infer the opening kinematics along the rifted margins in agreement with rigid plate tectonic models continental rifting initiated geologically synchronously at 6 ma along the length of the protomargins within a deforming plate boundary zone strain localization and seafloor spreading however developed in a time transgressive fashion from east to west within this zone of deformation spreading centers formed within the rheologically weaker protocontinental margins surrounded by stronger oceanic lithosphere in the solomon and coral seas the transition to spreading occurred after a rather uniform degree of continental extension 200 40 km both early and late stage rifting involved high and low angle normal faults we identify distinct styles in the transition from rifting to spreading which we refer to as nucleation propagation and stalling these breakup styles impart varyingly concordant to discordant relationships between the adjacent oceanic and continental rift structures continental transform margins which are or were juxtaposed against the ends of spreading centers show no evidence for thermal uplift or igneous underplating the initial spreading segments achieved much of their length at nucleation within rift basins separated along strike by accommodation zones with subsequent lengthening by spreading propagation into rifting continental crust this early propagation and the subsequent development of transform faults between initially nontransform spreading segment offsets produced rift and spreading segmentation boundaries that are not simply correlated the spreading centers nucleated approximately orthogonal in strike to the opening direction but as the protomargins were oblique to this direction nucleation jumps occurred in order to maintain the new spreading centers within the protomargins thus stepwise spreading nucleation in order to remain within a rheologically weak zone rather than rupturing of the lithosphere by stress concentration at the tip of a propagating ridge axis is the dominant form of the rifting to spreading transition in the woodlark basin