magma systems related to the red sea opening abstract four distinct magma environments can be recognized in association with the red sea basin 1 1 mid ocean ridge basalts within the present actively spreading axial trough 2 2 basaltic dikes and layered gabbros within the extended precambrian crust on the arabian margin 3 3 granites and rhyolites developed in the extended precambrian crust and 4 4 plateau basalts asymmetrically distributed on the arabian flanks of the red sea these magmas developed as the result of the initial break up of africa and the establishment of an oceanic spreading center within the red sea original asymmetric continental extension occurred on east dipping detachment faults in the oligocene and caused morb like magmas to invade an extending precambrian crust contemporaneous small scale mantle convection initiated limited partial melting under the arabian plate to form the undersaturated alkali olivine plateau basalts slow extension through the miocene allowed an evaporite basin to develop in the red sea while small volumes of rift and plateau basalts formed on the arabian plate a small change in plate motion in the pliocene initiated the seafloor spreading in the axial trough where morb magmas formed oceanic crust rise of the asthenosphere produced small scale thermal perturbations under the arabian plate controlled by an east dipping master detachment fault small scale mantle convection extended partial melting 2 5 eastward under the arabian plate producing more undersaturated diverse alkali olivine plateau basalts distinct chemical and isotopic signatures of these four magma types lead to obvious actualistic correlations with the geodynamic setting

nature of the early proterozoic outokumpu assemblage eastern finland abstract the 1970 ma old outokumpu assemblage consists of serpentinites gabbro and mafic metavolcanites with pelagic and chemogenic sediments lying in the early proterozoic 2200 1950 ma karelides of eastern finland its formation occurred after the development of the jatulian 2200 2000 ma quartzite dominated shelf sediments and prior to the deposition of the kalevian flysch the outokumpu assemblage has been compared to an ophiolite and though the lithologies are all present sheeted dykes excepted the internal stratigraphy suggests otherwise the mafic metavolcanites have a more depleted chemistry than n morb and were erupted into shallow water along with the deposition of pelagic shales carbonates and chemogenic sediments including metalliferous exhalites ultramafic sills intruded these rocks at a later stage prior to rapid subsidence and flysch deposition the recently discovered jormua ophiolite complex may constitute part of the depositional basement to the outokumpu assemblage and the kalevian flysch which formed over stretched crust in an ensialic basic

three dimensional dynamical model of continental rift propagation and margin plateau formation we investigate the dynamics of rift propagation through continental lithosphere and the resulting differential extension along passive margins using a full three dimensional finite element implementation of the yield stress envelope model in its initial configuration the three dimensional model represents a 1000 km by 1000 km area of continental lithosphere an initial weak trend representing a suture zone or other preexisting weakness within the continental lithosphere extends 500 km from one model edge to the center of the model and terminates the model region beyond the weak trend is uniform model conditions are varied by changing the thickness and composition of the crust the geotherm and extension rate the strength of the initial weakness and the dimensions of the preweakened and uniform regions for each model breakup nucleates at a point on the weak trend in response to an applied constant extension rate the break propagates rapidly along the weak trend and then slows as it crosses the uniform region this produces a locked zone in which breakup is delayed resulting in an increase in the width of the model margin analogous to offsets associated with margin plateaus the total extension required to tear across the 1000 km length of the model is 210 km for 40 km thick crust 400 km for 45 km thick crust and 710 km for 50 km thick crust smaller increases in total extension occur for elevated geotherms and reduced extension rates this indicates that prior extension estimates based on subsidence which exceed 600 km for margin plateaus can be explained by rift propagation in three dimensions differential extension along the model margin is linearly related to the length of the locked zone the slope of the line which we call is a function of the rheological properties of the locked zone and reflects the extent to which continental lithosphere within the locked zone resists rift propagation model calculations for crustal thickness from 35 km to 50 km and compositions from quartz diorite to granite predict a range in from 0 2 to 1 3 we compare values computed from our model to those estimated from the subsidence of bonavista platform and rockall plateau conjugate margins in the north atlantic basin and georges bank and blake plateau and their conjugate margins in the central atlantic basin these results indicate that the differential extension associated with these locked zones which ranges from 170 km to 550 km can be predicted in terms of their initial crustal thickness and their length along the margins

a seismic refraction survey of the imperial valley region california the u s geological survey conducted an extensive seismic refraction survey in the imperial valley region of california in 1979 the imperial valley is located in the salton trough an active rift between the pacific and north american plates forty shots fired at seven shot points were recorded by 100 portable seismic instruments at typical spacing of 0 5 1 km more than 1300 recording locations were occupied and more than 3000 usable seismograms were obtained we analyzed five profiles using a standard ray tracing program constructed a contour map of reduced travel times from our most widely recorded shot point and modeled an existing gravity profile across the salton trough results are itemized 1 all models have in common a sedimentary layer vp 1 8 5 0 km s a transition zone vp 5 0 5 65 km s a basement vp 5 65 km s in the imperial valley 5 9 km s on the bordering mesas and subbasement vp 7 2 km s 2 the sedimentary layer ranges in thickness along the axis of the salton trough from 3 7 km salton sea to 4 8 km u s mexican border on the bordering mesas it is quite variable in thickness 3 the transition zone is about 1 km thick in most places in the imperial valley there are no marked velocity discontinuities in this zone between the sedimentary layer and basement on the bordering mesas however there is a discontinuity at the top of this zone 4 there are apparently two types of basement on the bordering mesas basement is crystalline igneous and metamorphic rocks in the imperial valley basement is mostly lower greenshist facies sedimentary rocks based primarily on the smooth transition in character from sediment to basement arrivals the low value of basement velocity and the fact that deep 4 km wells in the valley penetrate only the upper part of the known cenozoic stratigraphic column for the salton trough 5 the subbasement or intermediate crustal layer ranges in depth along the axis of the salton trough from 16 km salton sea to 10 km u s mexican border gravity modeling requires that this layer deepen and or pinch out beneath the bordering mesas and mountain ranges based on its high velocity and the presence of intrusive basaltic rocks in the sedimentary section in the imperial valley the subbasement is thought to be a mafic intrusive complex similar to oceanic middle crust 6 several structures are seen that affect basement transition zone and deeper parts of the sedimentary layer they include a scarp along the imperial fault as much as 1 km down to the northeast and a scarp passing roughly along the topographic boundary between the imperial valley and the bordering mesa to the west as much as 3 km down to the east we interpret the latter scarp to be the suture or rift boundary between the older crystalline basement on the mesa and the younger metasedimentary basement in the imperial valley 7 on a contour map of reduced travel time from our most widely recorded shot point subtle patches of early arrivals among otherwise late arrivals in the central imperial valley correlate well with known geothermal resource areas having reservoir temperatures of more than 150 c apparently the salton trough is a location where new crust is being generated as the rift opens mafic intrusive rocks fill it from below as sedimentary rocks fill it from above rifting and intrusion produce high heat flow that metamorphoses the sedimentary rocks to shallow depth metasedimentary basement in the imperial valley and thus consolidates the new crust

radiolarian stratigraphy across the oligocene miocene transition abstract recently the international union of geological sciences commission on stratigraphy working group on the paleogene neogene boundary proposed that the oligocene miocene boundary be placed at the base of chron c6cn2n at 23 8 ma on the cande and kent 1992 magnetic time scale where it is approximated by planktic foraminifera at the first occurrence of globorotalia kugleri and by calcareous nannofossils at the last occurrence of sphenolithus ciperoensis and the first and last occurrences of sphenolithus delphix and s capricornutus herein we show that in terms of radiolarians the base of chron c6cn2n can be correlated with the upper part of the lychnocanoma elongata zone between the last occurrence of artophormis gracilis 23 94 ma and the first occurrence of cyrtocapsella tetrapera 23 69 ma since the proposed stratotype at lemme carrosio italy does not contain radiolarians at the boundary we re examined 13 dsdp sites and established the stratigraphic sequence of 29 first and last radiolarian occurrences and one evolutionary transition across the boundary nine of these sites contain both calcareous and siliceous microfossils and thus allow for an integrated biostratigraphy paleomagnetic stratigraphy is not available for any of the dsdp cores examined however use of hodell and woodruff s 1994 strontium isotope curve from dsdp site 289 has permitted calibration of several low latitude microfossil datum levels against the geomagnetic polarity scale two new species lychnocanoma apodora and eucyrtidium plesiodiaphanes are described

the composition and emplacement of basaltic magmas produced during the development of continental margin basins the gulf of california mexico the gulf of california is a region of active ensialic basin development and transform faulting legs 64 and 65 of the deep sea drilling project have demonstrated the diversity of basement stratigraphy drilling at sites 474 482 483 and 485 at the mouth of the gulf recovered massive basaltic flows and occasional pillow lavas and sills whereas drilling in the central part of the gulf in the guaymas basin recovered a sequence of sediments and massive doleritic sills very high sedimentation rates over 1000 m ma are characteristic of the guaymas basin and prevent extrusion of the basaltic magma onto the seafloor thus resulting in at least partial intercalation of oceanic layers 1 and 2 such a process of ocean crust formation could be locally important during the early stages of ocean and marginal basin development when sedimentation rates may be very high over 200 samples of basalts and dolerites from the gulf of california have been analysed by x ray fluorescence and neutron activation techniques basalts from the mouth of the gulf are tholeiitic and contain very low abundances of the more hygromagmatophile more hyg elements k rb ba th ta nb and light ree relative to the hyg elements y zr ti and heavy ree thus resembling depleted n type morb from elsewhere along the e pacific rise epr dolerites from the guaymas basin also have a tholeiitic chemistry and some epr basalt characteristics e g la n sm n

abyssal teleconnections interactive dynamics of the deep ocean circulation abstract a teleconnective linkage between two abyssal water masses consists of one or more zones of interaction arranged sequentially in space and time the properties spatial extent and advective nature of one of these water masses are directly influenced by the presence of the other via the zones of interaction the sequence of components in the linkage may in some instances be configured linearly and respond in an irreversible fashion in other instances the linkage is not unidirectional but may include one or more feedback loops representing the dependence of each water mass upon the properties and extent of the other three types of water mass interaction are represented in the modern oceans these are designated as 1 re inforcing 2 lateral blocking re direction and 3 wedging the criteria for discriminating between these types are geometrical yet the physical processes and rates of interaction may be distinguishable between and perhaps characteristics of these different configurations regions of water mass interaction are marked by sharp gradients of water column properties each of which is spatially restricted to a narrow interval of the overall mixing zone sediment characteristics exhibit a similar pattern there is a broad transition zone within which each geological gradient is restricted to a relatively narrow interval one must distinguish between sediment indicators which reflect water properties and those which reflect water column advection and avoid inferring one type of waters characteristic from geological indices which are inappropriate for identifying such characteristics it is not possible to specify a priori all of the components of a teleconnection linkage we can identify critical zones of interaction and search for evidence of threshold events which contributed to the initiation or the breaking of a particular linkage in order to establish the spatial and temporal sequence of components in a complex linkage we must identify the frequency response and phase response spectra in high resolution time series of appropriate paleocenographic indices at key locations

latest oligocene through early middle miocene diatom biostratigraphy of the eastern tropical pacific abstract study of dsdp sites 71 77 and 495 has allowed the development of a refined diatom biostratigraphy for the latest oligocene through early middle miocene of the eastern tropical pacific which is well correlated to the low latitude zonations for planktonic foraminifers coccoliths and radiolarians six zones and 7 subzones are proposed and correlation with high latitude diatoms zonations for the north pacific the norwegian sea and the southern ocean is suggested by the discovery of selected diatoms in these tropical sediments which were previously thought to be restricted to high latitudes six new species and one new variety of diatoms which are stratigraphically useful are proposed actinocyclus hajosiae n sp a radionovae n sp coscinodiscus blysmos n sp c praenodulifer n sp craspedodiscus rydei n sp thalassiosira bukryi n sp and coscinodiscus lewisianus var robustus n var

active deformation and shallow structure of the wagner consag and delfin basins northern gulf of california mexico oblique rifting began synchronously along the length of the gulf of california at 6 ma yet there is no evidence for the existence of oceanic crust or a spreading transform fault system in the northern gulf instead multichannel seismic data show a broad shallow depression 70 200 km marked by active distributed deformation and six 10 km wide segmented basins lacking well defined transform faults we present detailed images of faulting and magmatism based on the high resolution and quality of these data the northern gulf crust contains a dense up to 18 faults in 5 km complex network of mainly oblique normal faults with small offsets dips of 60 80 and strikes of n n30 e faults with seafloor offsets of tens of meters bound the lower and two upper delfin basins these subparallel basins developed along splays from a transtensional zone at the nw end of the ballenas transform fault twelve volcanic knolls were identified and are associated with the strands or horsetails from this zone a structural connection between the two upper delfin basins is evident in the switching of the center of extension along axis sonobuoy refraction data suggest that the basement consists of mixed igneous sedimentary material atypical of mid ocean ridges on the basis of the near surface manifestations of active faulting and magmatism seafloor spreading will likely first occur in the lower delfin basin we suggest the transition to seafloor spreading is delayed by the lack of strain partitioned and focused deformation as a consequence of shear in a broad zone beneath a thick sediment cover

miocene biochronology and paleoceanography of the north pacific abstract biostratigraphic correlation based on microfossil datum levels directly or indirectly tied to the paleomagnetic time scale provides a high resolution time control for the miocene in the equatorial and middle latitude north pacific faunal changes and abundance fluctuations of planktic foraminiferal species combined with the oxygen pacific faunal changes and abundance fluctuations of planktic foraminiferal species combined with the oxygen isotope record of foraminifers reveal the paleoclimatic and paleoceanographic history the planktic foraminiferal assemblage change in the early miocene extinction of oligocene fauna and rise of a highly diverse neogene fauna appears to be related to increased water mass stratification in the world oceans presumably resulting from the establishment of circum antarctic circulation an increase in the siliceous productivity in the eastern equatorial pacific region between 20 and 18 ma suggests that the vertical and horizontal circulation was intensified at that time climates cooled rapidly during the middle miocene between 14 and 13 ma suggesting the growth of a major east antarctic ice sheet paleoclimatic conditions remained generally cool although oscillating during the late miocene in the late early to middle miocene faunal provincialism developed between low and middle latitudes and by late miocene time a distinct provincialism similar to the present was established

crustal refraction surveys across the trans hudson orogen williston basin of south central canada eight reversed seismic refraction profiles have been recorded across the phanerozoic williston sedimentary basin that overlies the margins of the superior and wyoming archean cratons and the proterozoic trans hudson orogen these data have been modeled using two dimensional ray tracing techniques to match the times and amplitudes of primary and coherent secondary arrivals within the southern extension of the superior craton margin thompson belt a high velocity 6 4 km s lid overlies a narrow region of high electrical conductivity and a zone of westerly dipping reflections is nearly coincident with a weak velocity discontinuity that seems to delineate the western edge thompson fault of the craton margin complex zones of low velocity material and the north american central plains electrical conductivity anomaly are concluded to lie within the southern extension of the reindeer south indian lakes terrain a component of the trans hudson orogen a high velocity 7 0 km s lower crustal layer which represents a zone of complexity separating normal crust from the upper mantle extends throughout the region the crust is relatively thick with depths to moho varying smoothly between 41 and 48 km there is no evidence in the relatively low resolution seismic refraction data for abrupt changes in the level of the moho but there may be a minor 3 4 km thinning of the crust near the center of the williston basin

geology and seismic structure of the northern section of the oman ophiolite in the north oman mountains a continuous ophiolite succession is exposed from tectonized harzburgites and dunites at the base through layered gabbros and peridotites high level gabbros and plagiogranite to a dike swarm and pillowed volcanics overlain by pelagic shales the upper part of this sequence possesses a static metamorphic overprint which passes downward from greenschist facies in the lowermost volcanics and most of the dike swarm to amphibolite facies in the lowermost dike swarm and in the high level intrusives these pervasively altered rocks are underlain by layered gabbros and peridotites where hydration is restricted to fractures compressional and shear wave velocities have been measured to confining pressures of 6 kbar for 139 water saturated cores from this sequence the samples were field oriented and collected from known stratigraphic levels which allows the construction of velocity depth profiles and an examination of anisotropy within the ophiolite compressional wave velocities measured for the basalt section are quite variable ranging from 4 5 to 6 0 km s at in situ pressures within the sheeted dike section compressional and shear wave velocities vary from 5 4 to 6 3 km s and from 2 9 to 3 6 km s respectively from the basalt sediment contact through the dike section velocities increase rapidly with depth this gradient is related to increasing metamorphic grade and a decrease in grain boundary porosity the top of seismic layer 3 is located near the lower boundary of the sheeted dike swarm and marks the transition from greenschist facies to amphibolite facies metamorphics the dikes decrease in abundance downward into the high level intrusives coarse grained gabbroic and granitoid rocks which often contain abundant hornblende with isotropic textures and lacking compositional layering mean compressional and shear wave velocities increase uniformly from 6 7 and 3 6 km s respectively in the highlevel intrusives to 7 5 and 3 9 km s respectively at the base of the layered sequence measured velocities of the ultramafic tectonites are highly variable because of the anisotropy and serpentinization petrofabrics of relic olivine in the wadi ragmi region show strong preferred fabrics for the tectonites and calculated compressional wave velocities for the serpentine free rocks range from 7 8 to 8 5 km s with the fast direction parallel to the direction of spreading inferred from dike orientations in excellent agreement with observed upper oceanic mantle seismic anisotropy the crust mantle boundary which is defined by the abrupt increase in seismic velocities coincides with the petrological contact between layered gabbros and peridotites and tectonized harzburgite and dunites

late pleistocene palaeocenography of the tasman sea implications for the dynamics of the warm pool in the western pacific quantitative analyses of planktonic foraminifera from 47 core top samples spatially distributed through the tasman sea and cores rc12 113 e36 23 dsdp sites 588 592 593 and odp site 828a from a n s transect supplemented with stable isotope analyses indicate that three planktonic foraminiferal assemblages defined by factor analysis on core top material correlated with three hydrographic modes defined on the basis of sea surface temperatures temperature profiles seasonal changes in temperature and mixed layer depth similar assemblages were found in down core material along the tasman sea transect that combined with semi quantitative analyses of foraminiferal assemblages provinces and the coiling ratio of globorotalia truncatulinoides allows reconstruction of the position of the tasman front or the subtropical divergence during the late brunhes chron the results indicate that the tasman front migrated from its present position at 30 s to 26 s during the last glacial maximum the northward migration of the tasman front may indicate a reduction in volume of the equatorial warm pool and or an increase in the strength of cool subtropical water masses the late pleistocene distribution of g truncatulinoides left coiling from appears to be related to the formation of subantarctic mode water thermostads this relationship may explain the evolutionary origin of this species in the southwest pacific at 2 4 ma simultaneously with the onset of late pliocene glaciations the past position of the subtropical convergence is reconstructed by means of coilingv directions in neogloboquadrina pachyderma the convergence apparently migrated slightly north of 43 s core e36 23 during glacial stages 6 10 and 12

constraints on the structure of the east pacific rise from seismic refraction data university of washington seismic data taken during rose phase 1 have been used to study the structure of the east pacific rise in the vicinity of 12 n off axis but within 50 km of the axis we find that the oceanic crust can be modeled with a rapid change in velocity in the top 2 5 km a zone of low but uniform velocity gradient from 2 5 to about 5 5 km and a transitional zone about 1 km thick to mantle material there is also evidence for substantial anisotropy in mantle velocity gradients with a positive velocity gradient in the spreading direction and a negative velocity gradient parallel to the rise under the rise axis the data allow at most a very narrow zone of low velocity less than about 2 km wide in the crust

reconstruction of oceanic circulation patterns in the tropical pacific across the early middle miocene boundary as inferred from radiolarian assemblages abstract although there have been attempts to infer neogene oceanic circulation patterns in the tropical pacific on the basis of multiple marine proxies oceanic circulation patterns across the early middle miocene boundary are still poorly understood despite paleoclimate having significantly changed during this interval in this study we reconstruct the changes in tropical oceanic circulation patterns in the pacific across the early middle miocene boundary based on radiolarian assemblages obtained at integrated ocean drilling program site u1335 in the eastern tropical pacific the radiolarian upwelling taxa increased during four intervals 18 4 18 1 ma 16 9 15 7 ma 15 2 14 6 ma 14 2 13 4 ma radiolarian temperature index rti was calculated using the relative abundances of the warm water cluster b1 and cool water taxa cluster b2 in order to evaluate relative temperature fluctuations of the surface water the sea surface temperature index was relatively high from 16 8 to 16 0 ma and gradually decreased from 16 0 to 14 6 ma subsequently the temperature index decreased stepwise at 13 9 and 13 4 ma and became relatively low from 13 4 to 12 7 ma comparison of the rti record in the eastern tropical pacific to the climatic index based on the benthic foraminiferal 18o record shows that variability in the two records follows a roughly similar trend from the early to the middle miocene however the sea surface temperature record does not indicate a sharp increase in warmth in the eastern tropical pacific during the mid miocene climatic optimum 1 mmco 1 as suggested by the benthic isotopic record this is likely because of increased upwelling of cool nutrient rich water at this time beginning in the latest early miocene 17 ma radiolarian assemblages were dominated by different taxa in the eastern and western tropical pacific this pattern is interpreted as indicating a shallower thermocline in the east and a deeper thermocline in the west based on the relative abundance of the upwelling taxa there is an overall increasing trend in this difference since the latest early miocene we tie these events to the effective closure of the indo pacific seaway and the development of a substantial western pacific warm pool along with the development of a strong equatorial undercurrent

subsidence history of australia s southern margin constraints on basin models the subsidence history of australia s southern margin is shown to consist of two distinct tectonic phases and has important implications for models of sedimentary basin formation the first phase is characterized by rapid tectonic subsidence about 50 m m y which lasted for about 25 30 m y during the early cretaceous the period of rapid subsidence represents the rifting phase of margin development this early stage is correlated with associated rifting phenomena faulting and extension of the brittle upper crust formation of grabens and half grabens and the rapid accumulation of volcanogenic detritus otway group the amount of extension 20 50 is estimated from a comparison of observed and modeled tectonic subsidence at several exploratory wells along the margin the model includes uniform extension and thinning of the continental lithosphere a prolonged extensional event followed by simple cooling horizontal and vertical conduction of heat and radioactive heat production by the crust the rift phase may have begun earlier but the most rapid and widespread subsidence occurred during the early cretaceous the end of rapid and widespread subsidence occurred during the early cretaceous the end of rapid subsidence coincides closely with the age of the oldest sea floor spreading magnetic anomaly about 90 ma adjacent to the continent ocean boundary cob the second stage of margin development is the post rift or drift phase a period of much slower rates of subsidence and relative tectonic quiescence subsidence during the post rift stage is modeled as cooling and contraction of extended continental lithosphere estimates of extension and thinning from subsidence modeling agree with seismic refraction measurements of the thickness of continental crust near the cob estimates of extension from the geometry of basement blocks near the cob do not agree with the analysis from subsidence modeling upper crustal extension seems to record only a small proportion of total lithospheric extension alternative or supplementary mechanisms of extension are explored with emphasis on implications from a model that includes localized shear along a lithospheric detachment system

radiolarian faunal turnover across the oligocene miocene boundary in the equatorial pacific ocean abstract the global warming trend of the latest oligocene was interrupted by several cooling events associated with antarctic glaciations these cooling events affected surface water productivity and plankton assemblages well preserved radiolarians were obtained from upper oligocene to lower miocene sediments at ocean drilling program odp leg 199 sites 1218 and 1219 in the equatorial pacific and 110 radiolarian species were identified four episodes of significant radiolarian faunal changes were identified middle late oligocene 27 5 to 27 3 ma latest oligocene 24 4 ma earliest miocene 23 3 ma and middle early miocene 21 6 ma these four episodes approximately coincide with increases and decreases of biogenic silica accumulation rates and increases in 18o values coded as oi and mi events these data indicate that antarctic glaciations were associated with change of siliceous sedimentation patterns and faunal changes in the equatorial pacific radiolarian fauna was divided into three assemblages based on variations in radiolarian productivity species richness and the composition of dominant species a late oligocene assemblage 27 6 to 24 4 ma a transitional assemblage 24 4 to 23 3 ma and an early miocene assemblage 23 3 to 21 2 ma the late oligocene assemblage is characterized by relatively high productivity low species richness and four dominant species of tholospyris anthophora stichocorys subligata lophocyrtis nomas and lithelius spp the transitional assemblage represents relatively low values of productivity and species richness and consists of three dominant species of t anthophora s subligata and l nomas the characteristics of the early miocene assemblage are relatively low productivity but high species richness the two dominant species present in this assemblage are t anthophora and cyrtocapsella tetrapera the most significant faunal turnover of radiolarians is marked at the boundary between the transitional early miocene assemblages we also reviewed changes in other microfossil assemblages in the low latitudes during the late oligocene through early miocene the microfossil assemblages of major groups show sequential changes near the oligocene miocene o m boundary 23 8 ma many extinction events and some first occurrences of calcareous nannofossils and many occurrences of radiolarians are found from about 24 8 to 23 3 ma and first occurrences of planktic foraminifers and diatoms followed from 23 2 through 22 ma hence the o m boundary is identified as a significant level for microfossil evolutions

structure and variability of oceanic crust on the flanks of the east pacific rise between 11 and 13 n we use over 500 seismograms collected by five ocean bottom seismometers to examine the structure and variability of 0 5 m y old crust in the rivera ocean seismic experiment rose area of the east pacific rise between 11 and 13 n there are no significant differences among the first arrival travel times for 10 independent refraction line segments along the isochron amplitude patterns recorded on the individual lines however suggest that structural boundaries within the upper crust vary by hundreds of meters in depth over lateral distances of tens of kilometers within this 200 km segment of lithosphere on the basis of the modeling of amplitude patterns at ranges of 5 10 km and of comparisons with the upper crust sampled at deep sea drilling project hole 504b and in ophiolite complexes layer 2 can be divided into three units 2a corresponds to extrusive volcanics with a steep velocity gradient 2b to extrusives with a near zero velocity gradient and 2c to a complex region of transition from extrusives to sheeted dikes the total thickness of layer 2 varies by about 0 7 km over the length of the line another amplitude peak which varies in range between 14 and 21 km may be related to the downward transition from sheeted dikes to isotropic gabbros within layer 3 the variability of these two amplitude patterns provides direct evidence that the along strike structure of oceanic crust in the rose area is definably heterogeneous and that the process of crustal accretion is likewise variable on the scale resolved by amplitude and travel time information

paleozoic echinoderm hangovers waking up in the triassic echinoderms are among the marine invertebrates that underwent the most severe losses at the end permian extinction the prevailing paradigm claims an extreme bottleneck with only very few if not single holdovers hangovers herein sparking the post paleozoic radiation here we identify previously overlooked triassic echinoids ophiuroids and asteroids as unambiguous members of paleozoic stem groups these echinoderm hangovers occurred almost worldwide and had spread into a wide range of paleoenvironments by the late triassic our discovery challenges fundamentals of echinoderm evolution with respect to end permian survival and sheds new light on the early evolution of the modern clades in particular on triassic ghost lineages i e inferred but undocumented fossil record of the crown group look alikes of the paleozoic hangovers

focal depths and mechanisms of large earthquakes on the arctic mid ocean ridge system as part of a global study of the source characteristics and tectonic implications of large earthquakes on mid ocean ridges we report on the focal depths and mechanisms of the six largest earthquakes that have occurred in the last 20 years on the arctic mid ocean ridge system for each earthquake we invert the long period p and sh waveforms to estimate the parameters of the best fitting point source including seismic moment centroid depth double couple source orientation and source time function three of the earthquakes occurred on the oceanic spreading center in the eurasian basin along ridge segments spreading at half rates of 4 6 mm yr these events have mechanisms very similar to those of ridge crest earthquakes on the mid atlantic ridge almost pure normal faulting on planes that dip at approximately 45 and strike parallel to the rift axis moments of 4 5 1024 dyn cm centroid depths of 1 2 km beneath the seafloor and water depths inferred from the predominant period of water column reverberations appropriate to epicentral locations within the median valley the remaining three earthquakes also characterized by normal faulting are associated with the continuation of the divergent plate boundary 2 3 mm yr half rate onto the continental shelf of the laptev sea where the crust becomes transitional in nature one of the largest known spreading center earthquakes august 25 1964 m0 1 1026 dyn cm occurred where the oceanic ridge intersects the outer edge of the continental slope waveform inversion for this event can resolve unilateral rupture from north to south landward along a fault at least 30 km in length the preferred centroid depth is 5 km beneath the seafloor in crustal material with an unusually low shear velocity but a centroid depth as great as 15 km cannot be ruled out two earthquakes beneath the continental shelf have significantly greater centroid depths 10 20 km than mid ocean ridge earthquakes indicating a thicker brittle regime and a cooler thermal structure than are typical of oceanic spreading centers the tectonic environment of these events is more representative of rifted continental lithosphere than of a mid ocean ridge

heat flow hydrothermal circulation and basalt intrusions in the guaymas basin gulf of california abstract the guaymas basin is part of an en echelon series of short seafloor spreading segments in the gulf of california the basin is separated from adjacent deeps by long northwest southeast striking transform faults and divided into subparallel northern and southern troughs which trend northeast to southwest rapid sedimentation 1 to 5 m ka 1 results in a striking morphological contrast between seafloor spreading in the guaymas basin and that in more mature mid ocean settings basalt does not extrude onto the seafloor in the basin as flows and pillows but instead intrudes into a thick sediment layer the presence of relatively unconsolidated sediments lining the floor of the basin make this setting ideal for the study of axial heat flow and the thermal budget of oceanic crustal formation thermal data delineate several modes of heat transfer in both troughs of the basin with negligible heat flow in some areas and values greater than 3 w m 2 in others broad heat flow highs several square kilometers in area result from a combination of deep conductive heat flow slow intergranular fluid circulation and vigorous hydrothermal venting areas of low heat flow between heat flow highs and near normal faults which bound the basin may be indicative of hydrothermal recharge the geometry of faults shallow sills and broad northeast southwest structural trends in the basin largely seems to govern the distribution of heat flow the highest values are clustered within a few isolated zones in each trough with a total area of about 30 km 2 variations in heat flow of two orders of magnitude occur over distances as small as a few hundred meters fourteen hydrothermal vents in the southern trough of the guaymas basin have been sampled directly and found to have temperatures in excess of 250 c locally conductive heat flow at the seafloor is significantly raised in the vicinity of these vents but the general pattern appears to require more deeply seated conductive heat sources the mean heat flow in the southern trough is 453 342 s d mw m 2 a simple analysis of the heat budget in the southern trough suggests that the total intruded section between the top of the magma chamber and the seafloor may be unusually thin perhaps only several hundred meters in thickness

nature of the crust under afar new igneous not thinned continental abstract thinned continental crust is considered absent from beneath afar except for isolated remnants such as comprise the danakil block the ethiopian plateau sialic crust thins abruptly across the plateau afar margin to abut new igneous crust under afar generated during the early development of the red sea basin analyses of stretching and sea floor spreading amounts elsewhere in the red sea and gulf of aden basins are employed to support this concept the dual layering of the afar crust and the similarity of p wave velocities in these layers to velocities in sialic crust lead to the proposal that new continental crust can be generated at magmatic rift zones

crustal underplating and evolution in the red sea rift uplifted gabbro gneiss crustal complexes on zabargad and brothers islands the nature of the crust of the red sea outside the oceanic axial trough segments can be inferred from uplifted crustal sections exposed on the islands of zabargad and the brothers in the central and northern red sea zabargad exposes in addition to mantle derived peridotites a crustal section consisting of metagabbros and metapyroxenites associated with silicic gneisses and cut by doleritic dikes seismic velocities of samples from this section are compatible with crustal velocities obtained by seismic refraction experiments in the central and northern red sea the zabargad peridotitesilicic gneiss gabbroic association provides a sample of upper mantle lower crust in a continent before the inception of a major rifting event the petrology and mineral chemistry of the zabargad metagabbros suggest that they were originally part of a basic layered complex which crystallized at relatively high pressure 900 1000 mpa or 9 10 kbar i e over 30 km deep the gneisses have tonalitic trondhjemitic affinity and were originally equilibrated in the garnet granulite facies implying also high pressure and derivation from the lower continental crust this suggests that after the subcrustal continental lithosphere along the proto red sea rift had been consumed possibly by convective thinning but before any significant thinning of the crust basaltic melts intruded the base of the continental crust underplating it with gabbroic complexes thus crustal rifting was probably preceded by a thermal anomaly in the mantle which supports an active rifting hypothesis the gabbro pyroxenite complex and the gneisses underwent subsequently retrometamorphic events under decreasing pt conditions from granulite to amphibolite to greenschist facies implying decompression and uplift probably related to the gradual thinning of the continental crust during rifting during this stage additional gabbroic intrusions and basaltic dikes exemplified by the gabbros and dolerites from the brothers islands were emplaced at decreasing depths beneath the thinned continental crust the northern red sea crust is presently in this stage it is neither oceanic nor continental it lacks a continuous thick basaltic upper crustal layer and is made prevalently of gabbroic and doleritic intrusives with relicts of sialic continental crust subsequently basaltic injections became progressively focused in a narrow axial zone and seafloor spreading initiated at discrete equidistant hot points as observed presently in the central red sea which by axial propagation evolved into nearly continuous zones of oceanic crust accretion as observed presently in the southern red sea processes similar to those proposed here for the evolution of the red sea rift can be documented in passive margins of the atlantic and in some alpine ophiolite complexes

oceanic ridge crest processes oceanic ridge crest processes constitute a vigorous frontier of multidisciplinary research with global ramifications in the earth atmospheric and biologic sciences this deep ocean frontier is gradually opening in response to investigations which are elucidating the role of these processes in the lithosphere hydrosphere atmosphere and biosphere and the application of advanced instrumental and interpretive techniques which are increasing the quality and quantity of data these techniques include crustal drilling with borehole logging deep sea drilling project reports high resolution bathymetry sea beam etc side scan sonar seamarc i and ii gloria etc imagery with video and photography multichannel seismics determination of water column properties ctd with rosette sampler etc and submersible applications

geologic and seismic velocity structure of the crust mantle transition in the bay of islands ophiolite complex geological investigations of the bay of islands ophiolite complex show that while a typical layered ophiolite suite is present the thickness of major lithologic units is extremely variable from place to place the composition and internal structure of map scale kilometers across lithologic units as well as the contacts that bound them are laterally variable inferred velocity depth functions of the crust mantle moho transition reconstructed for this terrane as oceanic lithosphere suggest an extremely complex internal seismic structure the crust mantle transition from mafic to ultramafic lithologies varies across the ophiolite from a sharp geologic and seismic velocity discontinuity to a complexly interlayered transition zone as much as 3 km thick the mafic ultramafic transition is characterized by laterally discontinuous lithologic units on the order of hundreds of meters to a few kilometers thick and up to several kilometers long these units megalenses may be composed of lithologies with higher or lower seismic velocities than those of the surrounding units solid state deformation has produced seismically anisotropic materials in the upper mantle and lower crustal units substantial relief and slopes of up to at least 12 over lateral distances of about 10 km occur on the top of the crust mantle transition zone as well as other geologic unit contacts although these types of geological features are striking to the field geologist working in ophiolites and are significant to the understanding of crustal accretion processes many of them occur on such a small scale that they might go undetected in different types of seismic experiments in contemporary oceanic lithosphere

the gneiss of zabargad island deep crust of a rift abstract the gneisses of the zabargad island are in contact with lherzolites that were emplaced by mantle diapirism during the early rifting of the red sea the gneisses and the peridotites at their contact develop similar structures kinematics and common granulite metamorphism away from the contact the shear strain and associated recrystallizations decrease rapidly thus the zabargad gneisses are metamorphosed and deformed by the mantle diapiric intrusion this is confirmed by preliminary isotopic dates the gneisses have a tripartite polygenic composition of felsic granulites metagabbros and amphibolites the metagabbros and amphibolites are metamorphosed only by the mantle diapir intrusion which also induces a partial anatexis of the gneisses the felsic granulites record a proceeding high p granulite facies metamorphism they are assumed to represent screens of ancient continental crust

ocean crustal dynamics the study of oceanic crust continues to be important because of the presence of economic resources in oceanic areas and because many fundamental problems of geologic evolution are best solved from studies of the ocean although modeling and syntheses of existing data remain important key breakthroughs in the future will come from the application of new technology such as multichannel towed seismic arrays deep towed side scan sonars improved thermal probes deep drilling and satellite altimeters

paleodepth distribution of neocene deep sea hiatuses the depth of formation of miocene to middle pliocene deep sea hiatuses nh1 to nh8 has been determined using 152 deep sea drilling project sites in the atlantic pacific and indian oceans megahiatuses representing maxima in deep sea erosion occur at three water depths below 3800 m at intermediate depths between about 2000 and 3000 m and in shallower waters above 1500 m paleodepth both the paleodepths and distribution of these hiatus maxima suggest that flow of antarctic bottom water antarctic intermediate water and north atlantic deep water masses largely caused this erosion correlation of the hiatuses with oxygen isotope carbonate and sea level records indicates that prior to 11 ma brief hiatuses including those resulting from nondeposition in high productivity equatorial regions largely correlate with global cooling episodes high carbonate content and lowstands of sea level during the last 11 m y hiatuses also seem to correlate with cooling episodes but carbonate dissolution is characteristic and sea levels may have been rising or at a lowstand during the nh4 nh7 and nh8 intervals it is possible that a rise in sea level corresponds with polar cooling but there is uncertainty in correlation brief hiatuses during rising sea levels can be explained by basin shelf fractionation of carbonates hiatuses during sea level lowstands and cooling episodes may result from intensified bottom water circulation and increased corrosiveness of bottom water due to higher levels of co2 and increased productivity during increased upwelling supplement appendix 1 is availablewith entire article on microfiche orderfrom american geophysical union 2000florida avenue n w washington dc20009 document p87001 2 50 paymentmust accompany order

the morton black hypothesis for the thinning of continental crust revisited in western afar abstract geological observations along the western margin of afar show that the morton black model in which thinning of upper continental crust at a nascent continental margin is accomplished through block faulting and tilting is not wholly applicable rather dikes are concentrated into swarms suggestive of important direct dilatation faults are concentrated into sets within which no block tilting steeper than 45g is observed therefore attenuation of the afar margins requires a significant role for processes other than block tilting and in some margin sectors the transition from continental to neo oceanic crust may be relatively abrupt

paleo oceanography global ocean evolution paleoceanography one of the youngest branches of earth sciences is concerned with the development of oceanic circulation through geologic time this is a multifaceted and integrated endeavor since none of the subdisciplines is easily separated from the others the field is holistic in character and includes the history and effects of bottom intermediate and surface circulation patterns planktonic and benthonic biogeographic development and rates of organic evolution the history of biogenic productivity and dissolution and their effect on sediment distribution and the consequences of sea level change paleoceanography was largely born of the deep sea drilling project dsdp and continues to be nourished by it because problems are of global scope the appetite for new sediment core material is large and is being supplied by numerous expeditions of dsdp this article is a brief overview of a number of advances that have occurred in paleoceanography and related fields during the period from 1979 to 1982 because of page limitations all publications cannot be cited which is not meant to indicate any lack of importance

latest cretaceous early neogene oxygen and carbon isotopic record at dsdp sites in the indian ocean abstract stable isotopic data of calcareous nannofossil monogeneric and monospecific planktic and benthic foraminifera from five indian ocean dsdp sites 212 217 220 237 and 253 leads to the following paleoclimatic and paleoceanographic conclusions the latest cretaceous oxygen isotopic record implies a cooling 3 4 c during the maastrichtian at the cretaceous tertiary boundary only a minor warming about 2 c has been recorded the parallel 13 c decrease of more than 1 indicates a significant decrease in productivity during the latest paleocene a positive 13 c excursion was detected in sites 217 and 237 this transient enrichment in 13 c may be due to productivity changes on continents and or a change in the storage rate of organic matter in marginal basins or shelf areas the most striking feature in the oxygen isotopic record is noted at the early middle eocene transition the shift towards more positive values which were probably enhanced to a certain extent by a preceding diagenetic alteration delineates a dramatic climatic deterioration at high and mid latitudes during the earlier tertiary near the eocene oligocene boundary a cooling is evident within the latest eocene interval during the earliest oligocene time a hiatus at sites 217 and 253 partially obscures the climatic record several climatic fluctuations have been noted during the oligocene a cooling at the base of zone np 23 a warming at the top of zone np 23 through np 24 and a cooling during zone np 25 the miocene oxygen isotopic record is dominated by changes in surface and bottom water environments during zone nn5 the decreasing and then increasing 18 o values together with the subsequent steepening of the vertical 18 o gradient point towards major climatic instabilities these events coincide with the mid miocene build up of antarctic ice sheets during the latest miocene to the earliest pliocene the 18 o record of planktic foraminifera indicates a significant warming of the indian ocean at mid latitudes the 13 c record during the oligocene and miocene reveals several cycles 13 c enrichments np 24 nn2 nn5 nn9 and base nn 11 which are most likely related to changes in storage rates of organic matter and biological productivity due to climatic changes and transgression regression cycles in addition changes in the circulation patterns may also have influenced the carbon isotopic record

chromite pods in a lherzolite massif collo algeria evidence of oceanic type mantle rocks along the west mediterranean alpine belt abstract the high temperature lherzolite massifs of ronda southern spain and beni bousera northern morocco are ascribed by various authors to sub continental mantle diapirs emplaced within a granulite metamorphic aureole in continental rifts along transcurrent lithospheric faults the collo plagioclase lherzolite massif with associated kinzigite and related anatectic rocks belongs to the same type of geodynamic context about 17 chromitite bodies are found in the collo lherzolite massif their structural textural and geochemical characteristics are those of chromitite pods that are typical of ophiolite complexes i e of an oceanic lithosphere the collo massif is an atypical high temperature lherzolite massif displaying both continental and oceanic characteristics it could correspond to a mantle intrusion in a transition zone from continental to oceanic rift type

novel type of crust produced during continental rifting a normal oceanic crust cannot be generated after continental crust splitting of rifts filled by a thick pile of young sediments the magma released by the rising mantle is trapped mainly in these sediments as basalts sills and plugs the sediments are themselves metamorphosed by the heat flux from the underlying mantle a metasedimentary crust is formed in direct contact with an abnormal mantle vp 7 5 km s l produced by partial serpentinization

zabargad and the isotopic evolution of the sub red sea mantle and crust abstract zabargad island in the red sea exposes ultramafic bodies in contact with partially retrograded granulite facies lower crustal rocks the contact could represent the sub red sea moho prior to the opening of the red sea the ultramafic rocks are largely protogranular spinel lherzolites that grade locally to porphyroclastic to mylonitic amphibole peridotites some of the peridotites are infiltrated by high mg gabbros troctolites the peridotites and the lower crustal rocks are cut by diabase dikes the zabargad peridotites are isotopically heterogeneous on a scale of meters or less sr and nd isotopic ratios of clinopyroxenes cpx from spinel lherzolites lie in the depleted portion of the 143 nd 144 nd 87 sr 86 sr mantle array whereas cpx and amphiboles from amphibole peridotites fall off the array towards the low 143 nd 144 nd and sol 87 sr 86 sr field that may characterize portions of the sub continental mantle cpx from troctolites plot in the middle of the 143 nd 144 nd 87 sr 86 sr mantle array and have higher 87 sr 86 sr ratios and more depleted lree patterns than the intruded spinel lherzolites and amphibole peridotites all whole rock lherzolites and peridotites and most diabase dikes from zabargad as well as a variety of rocks from other parts of the red sea area morb glasses a diabase dike from the brothers island basanites and clinopyroxenes from associated nodules from the ataq diatreme in yemen define a linear array on a sm nd isochron diagram with a best fit age of ca 675 ma and an initial nd of 6 7 this array is probably a mixing line between depleted morb type mantle and a low 143 nd 144 nd 87 sr 86 sr metasomatic component the age defined by the linear array may date either an episode of pan african mantle homogenization or in our preferred model the pan african segregation of a previously homogeneous mantle into two or more reservoirs of identical 143 nd 144 nd but different sm nd ratios the subsequent partial remixing of these isotopic reservoirs during metasomatism associated with the opening of the red sea did not perturbed the slope of the array the lree enriched reservoir may have been the pan african lower crust since there is field and textural evidence that the metasomatic fluids were derived from or equilibrated with the adjacent crustal complex alternatively the lree enriched component may have been a mantle reservoir similar to that which generated basalts and nodules from the nearby ataq diatreme the troctolites are isotopically distinct from the host peridotites and retain igneous textures suggesting they intruded the peridotites during the opening of the red sea their isotopic patterns are similar to those of recent volcanics from afar portions of the ataq diatreme and some zabargad dikes indicating derivation from a similar mantle source their very lree depleted patterns suggest they are either re melted fractions of previously very depleted spinel lherzolites or early formed mineral aggregates that became enmeshed in the peridotites as fractionating melt ascended to higher levels our results indicate that at least two major processes metasomatism and intrusion have modified the isotopic pat terns of the spinel lherzolites of zabargad had the resultant chemically and isotopically heterogeneous bodies stayed in the mantle instead of being uplifted to their present positions they would have proven to be enigmatic sources for future basalts and xenoliths