the apparent paradox of abundant foramol facies in low latitudes their environmental significance and effect on platform development recent studies of low latitude carbonates from se asia and australia have revealed deposits and platform types that do not fit easily into the perceived view of warm water tropical carbonates the sediments are classified as a foramol or foralgal facies and are dominated by non framework building light dependent biota such as perforate larger benthic foraminifera coralline algae and sometimes halimeda although ancient foramol deposits are often interpreted as an indication of non tropical conditions foramol facies have long been known from the tropics yet the environmental conditions in which they form are poorly known the aims of this paper are to evaluate how these deposits are best classified and more importantly whether there are any overriding controls that influence their common development in low latitude areas and if resultant platform development is affected the deposits are all best grouped within the photozoan association james n p 1997 the cool water depositional realm in james n p clarke j a d eds cool water carbonates sepm special publication 56 1 20 an assignment synonymous with warm water conditions the facies were also found to develop in conditions of limited light or oligophoty all the platforms described are characterised by having extensive areas of oligophotic facies generally in water depths extending down from 20 m but sometimes in shallower waters platforms types vary from land attached rimmed shelves homoclinal and distally steepened ramps to isolated banks and platforms but all have extensive moderate to deep photic zone development of oligophotic facies some workers might assign these platforms to the incipiently drowned platform type read j f 1985 carbonate platform facies models american association of petroleum geologists bulletin 69 1 21 however a morphological assignment is preferred here to signify that many of the deposits did not or may not form through drowning limited light in moderate to deeper depths of the photic zone often in regions with a reduced depth euphotic high light zone is seen as key to the extensive development of these deposits and the resultant platforms these conditions are often developed in humid low latitude areas of high terrestrial runoff and therefore regions of reduced water salinity or in areas of nutrient upwelling reduced water clarity due to nutrients plankton blooms suspended particulate matter upwelling and strong currents all promote the development of oligophotic conditions and facies on a regional scale time periods of relative rises in sealevel can also promote oligophotic facies development the findings of this study have broader implications for the development of low latitude platforms beyond australasia and during time periods when non framework building oligophotic biota are dominant sediment contributors the reconstruction of past climatic conditions based on study of foramol grain assemblages requires careful study to distinguish warm water deposits from those formed in cool waters

oceanic transverse ridges a flexural response to fracture zone normal extension one of the most puzzling characteristics of sea floor morphology is the occurrence of anomalously shallow fracture zone parallel oceanic transverse ridges a model is proposed for the formation of transverse ridges near lat 21 and 24 n on the mid atlantic ridge in which the differential responses of large offset and small offset fracture zones to recent changes in spreading direction result in the generation of normal faults that coincide with the off axis traces of fracture zones numerical models of the flexural response of the lithosphere to normal faulting suggest that modest amounts of extension 5 km along low angle faults 45 are responsible for the transverse ridges

stratigraphic numerical modelling of a carbonate platform on the romanche transverse ridge equatorial atlantic abstract the romanche transverse ridge equatorial atlantic is located in the northern flank of the fracture zone opposite the eastern ridge transform intersection rti it constitutes a major positive topographic anomaly that reaches a height of over 4 km above the level predicted by the thermal subsidence curve a series of e w aligned peaks located on the crest of the transverse ridge were at sea level during early and middle miocene times they are presently capped by a 300 m thick shallow water carbonate platform that grew on a subsiding oceanic crust basement surface flattened by erosion at sea level these structures are now about 1 km below sea level high resolution seismic reflection surveys and multibeam morphobathymetry as well as study of samples recovered from the carbonate platform allowed a reconstruction of the paleoenvironment and of the vertical movements of the peaks starting from the lower miocene ages derived from microfossils suggest that the base of the carbonate platform dates from 17 25 m y ago and the sinking of the platform started between 18 and 13 m y ago these data were included in a numerical simulation model that takes into account thermal and tectonic subsidence growth potential of the carbonates subaerial and submarine erosion rates and mio pliocene absolute sea level fluctuations the results outline the subsidence history of the romanche transverse ridge and suggest post miocene subsidence faster than that predicted by the thermal cooling model

temperate carbonate platform drowning linked to miocene oceanographic events maiella platform margin italy carbonate platform drownings are frequent often synchronous global occurrences yet explanations for these world wide events remain unsatisfactory in the central apennines lower and middle miocene carbonate rocks deposited on a temperate ramp in the maiella platform margin record two episodes of platform drowning followed by hemipelagic sedimentation dated as latest oligocene aquitanian 26 23 ma and as burdigalian langhian 20 16 ma a high resolution stratigraphy based on strontium isotopes allows us to correlate key phases of platform evolution with events recorded in deep water ocean sediments this paper suggests that high weathering rates and nutrient input in the mediterranean during the early and middle miocene possibly linked to the uplift of the tibetan region set the preconditions for platform drowning which were ultimately caused by rapid eustatic sea level rises

carbonate systems along nutrient and temperature gradients some sedimentological and geochemical constraints research over the past several decades has clearly demonstrated that changes in the ocean environment have had major impacts on carbonate systems changes in climate ocean circulation and seafloor spreading rates have influenced temperature and seawater chemistry including carbonate saturation state and nutrient availability and thereby have determined boundary conditions for the biota that form carbonate platforms in turn the biota determine accumulation rates and facies zonations thus controlling platform geometry and facies dynamics in the first section of this paper we examine how nutrient availability influences carbonate facies associations we first discuss the role of temperature and nutrient gradients in the modern ocean and their influence on biotic associations then we discuss how carbonate sedimentation can be characterized along nutrient gradients in the second section we review proxies currently used to reconstruct paleoproductivity in open ocean environments and discuss their applicability to neritic carbonate systems we highlight the variety of existing proxies and their limitations and suggest that multiple lines of evidence are needed for valid interpretations our short review discusses sedimentological biogenic and geochemical proxies that can be used to reconstruct past nutrient fluxes and to constrain paleoceanographic controls over the distribution of carbonate associations however it also reveals that more data and case studies are needed that integrate shallow and deep water carbonate sequences and elucidate the links between temperature vs nutrient supplies changes and facies in ancient carbonate sequences

flexural uplift of a lithospheric slab near the vema transform central atlantic timing and mechanisms abstract the vema transverse ridge vtr is a prominent long and narrow topographic anomaly that runs for over 300 km along a sea floor spreading flow line south of the vema transform at 11 n in the atlantic it rises abruptly about 140 km from the axis of the mid atlantic ridge mar in 10 myr old crust and runs continuously up to 25 myr old crust it reaches over 3 km above the predicted lithospheric thermal contraction level it is absent in crust younger than 10 myr thus the uplift of the vtr must have ended roughly 10 ma the vtr is interpreted as the exposed edge of a flexured and uplifted slab of oceanic lithosphere that was generated at an 80 km long mar segment based on satellite gravimetry imagery this mar segment was born roughly 50 ma and increased its length at an average rate of 1 6 mm yr multibeam data show that the mar parallel sea floor fabric south of the vtr shifts its orientation by 5 to 10 clockwise in 11 12 myr old crust indicating a change at that time of the orientation of the mar axis and of the position of the euler rotation pole this change caused extension normal to the transform followed between 12 and 10 ma by flexure of the edge of the lithospheric slab uplift of the vtr at a rate of 2 to 4 mm yr and exposure of a lithospheric section vema lithospheric section or vls at the northern edge of the slab parallel to the vema transform ages of pelagic carbonates encrusting ultramafic rocks sampled at the base of the vls at different distances from the mar axis suggest that the entire vtr rose vertically as a single block within the active transform offset a 50 km long portion of the crest of the vtr rose above sea level subsided was truncated at sea level and covered by a carbonate platform subaerial and submarine erosion has gradually removed material from the top of the vtr and has modified its slopes spreading half rate of the crust south of the transform decreased from 17 2 mm yr between 26 and 19 ma to 16 9 mm yr between 19 and 10 ma to 13 6 mm yr from 10 ma to present the slowing down of spreading occurred close in time to the change in ridge transform geometry suggesting that the two events are related a numerical model relates lithospheric flexure to extension normal to the transform suggesting that the extent of the uplift depends on the thickness of the brittle layer consistent with the observed greater uplift of the older lithosphere along the vtr

plate kinematics of the south atlantic chron c34 to present a high resolution seafloor spreading history of the south atlantic since chron c34 is constrained by a combination of seasat altimeter data and underway marine geophysical data a set of 45 finite rotation poles defines the relative position of africa and south america at roughly 2 m y intervals a set of 12 stage poles constrain the relative motion of these two plates at 5 to 10 m y intervals the position of the stage poles continuously migrates reflecting the continuously changing azimuths of the fracture zones major changes in spreading direction are observed in the late cretaceous and early cenozoic as the fracture zones sweep out broad s shaped curves similar to the pattern seen on the kane fracture zone in the central atlantic small offset fracture zones were found to be the most accurate recorders of changes in plate motion large offset fracture zones such as the agulhas falkland fracture zone were the least reliable recorders at 30 s spreading rates decrease throughout the late cretaceous from a high of 75 mm yr at the end of chron c34 to a low of 30 mm yr around chron c27 a period of slow spreading between chron c30 and chron c20 corresponds to a zone of fracture zone proliferation an increase in the amplitude of geoid anomalies over fracture zones greater relief on topographic profiles and locally evidence of intraplate crustal deformation spreading rates increase at chron c20 to about 50 mm yr and then gradually decrease during the late paleogene and neogene a comparison of synthetic fracture zones based on the south atlantic stage poles to the observed trends of fracture zones in the equatorial atlantic indicates that the vema and marathon fracture zones were generated by south atlantic spreading as opposed to central atlantic spreading at least as far back as 35 m y ago and possibly 50 m y ago

miocene c isotope signature from the central apennine successions italy monterey vs regional controlling factors the c isotope composition of a miocene carbonate succession from the latium abruzzi sector of the central apennines was studied to evaluate the influence of local vs global factors in changing d 13 c we propose that the oligo miocene igneous activity of the western mediterranean and the closure of the indo pacific connection influenced climate change and mediterranean sea water circulation the huge aquitanian burdigalian volcanic activity during the formation of the

vertical crustal movements at the vema fracture zone in the atlantic evidence from dredged limestones abstract a crustal block shallower by over 3 km than the level predicted by the thermal contraction model for the ocean crust is located on the southern side of the vema transform in the atlantic lithology magnetic and gravity data suggest the anomalous block is an uplifted piece of crust and upper mantle and not a constructional volcanic feature the crest and upper slopes of the anomalous block are capped by shallow water reef limestones determination of paleofacies and ages of samples of these limestones suggest that 1 a the crest of the anomalous crustal block reached sea level and was exposed subaerially in middle pliocene time about 3 m y ago and subsequently sank to its present depth at an average rate of about 0 3 mm year that is a rate one order of magnitude faster than that predicted by the thermal contraction model for crust of the same age 2 b the anomalous block reached at or above sea level either continuously or intermittently throughout the late miocene from roughly 10 m y ago 3 c paleocene sediments dated at about 55 58 m y have been recovered from the uplifted block where assuming a spreading rate of 1 1 cm year the crust should not be more than about 35 m y old vertical crustal movements at the vema transform provide an example of vertical tectonism in the large oceanic transform fracture zones

himalayan tectonics weathering processes and the strontium isotope record in marine limestones the time evolution of the isotopic composition of seawater strontium the ratio of strontium 87 to strontium 86 over the last 500 million years has the form of an asymmetric trough the values are highest in the cambrian and recent 0 7091 and lowest in the jurassic 0 7067 superimposed on this trend are a number of smaller oscillations consideration of the geochemical cycle of strontium and the dynamics of weathering shows that only himalayan style continental collisions can influence the isotope ratio on the scale observed the contemporary himalayan orogeny is by far the largest since the late precambrian pan african event that produced the high in the cambrian

holocene tectonic uplift of the st peter and st paul rocks equatorial atlantic consistent with emplacement by extrusion abstract st peter and st paul rocks spspr are emergent portions of a peridotitic mass within the st paul fracture zone of the equatorial mid atlantic ridge where local seismicity is dominated by strike slip mechanisms we used subtidal red algae in living position death assemblages in intertidal conglomerates and marine terraces as sea level markers the highest deposits correspond to a marine terrace with a shoreline angle at 8 1 m asl that formed ca 6400 6200 cal yr bp after correction for the sea level changes derived by glacioisostatic modelling ams and radiometric 14 c age measurements on the holocene sea level markers indicate emergence during the last 6600 a at a minimum average rate of 1 5 mm a we propose that uplift resulted from peridotite emplacement by extrusion rather than volcanic activity or fault slip with the implication that the ridge has been subject to compression during the holocene

the paradox of drowned carbonate platforms and the origin of cretaceous pacific guyots geochemical stratigraphic and palaeolatitudinal data from deep boreholes drilled through pacific guyots flat topped seamounts help to explain the drowning of these cretaceous shallow water carbonate platforms that once thrived through the accumulation of biogenic and inorganic calcium carbonate sediment in mid oceanic regions the platforms drowned sequentially over a 60 million year interval while they were being transported northward by pacific plate motion through a narrow equatorial zone 0 10 s such platforms were apparently resistant to the effects of cretaceous oceanic anoxic events although the mechanism responsible for drowning remains unknown the tropics have not always been the refuge for atolls that they are today

non tropical carbonate deposits on the modern new zealand shelf abstract extensive ca 50 000 km 2 shallow marine platforms 70 caco 3 despite proximity to a tectonically active plate margin in these regions the terrigenous sediment supply is presently low and growth of epibenthos is fostered by firm substrates rock gravels shells seaweeds and the generally energetic nature and high nutrient levels of open shelf waters rapid transition into adjacent terrigenous dominated facies is characteristic irrespective of water depth the carbonates are coarse grained and fragmental carbonate mud is rare calcite dominates over aragonite high mg calcite widespread off northern new zealand is rare in the south skeletal material is dominated by bryozoans and bivalve molluscs with significant local contributions from foraminifers barnacles calcareous red algae and echinoderms the name bryomol is suggested for this distinctive temperate region skeletal carbonate facies which can be usefully subdivided based on dominant zoarial growth forms of the bryozoan component known to be habitat related bioerosion is an important mechanism of skeletal fragmentation and degradation many grains especially aragontic bivalves are infested by endolithic borers and have low preservation potential ages of skeletal material in the surficial deposits range from more than 20 000 years b p to modern which is consistent with both low rates of carbonate production and sediment accumulation and the wide range in preservation state of grains some data suggest that the skeletal carbonates are dispersed and mixed mainly during infrequent movement of sand ribbons sand waves and sand sheets driven by storm assisted tidal flows tracts of modern palimpsest and relict carbonates can occur in juxtaposition the facies characteristics of the new zealand shelf carbonate deposits contrast significantly with those of the classical bahaman type carbonate model however they are similar to those reported from many other mid to high latitude carbonate shelves and afford good analogues for most onland occurrences of new zealand cenozoic limestones

dissolution methods for strontium isotope stratigraphy guidelines for the use of bulk carbonate and phosphorite rocks diverse carbonate dissolution methods have been applied to bulk carbonate rocks in order to target least altered components for strontium isotope stratigraphy sis this is especially important for precambrian and cambrian studies for which no suitable skeletal material is available one proven method for bulk limestones involves the removal of up to a third of the powdered sample using an acid pre leach before partial dissolution of the rest of the sample using a weak acid or acid buffer solution we applied a similar technique to dissolve a range of lithologies limestone dolostone and phosphorite and compared the strontium isotopic composition of various leaches to contemporaneous seawater our results vindicate this approach and allow us to conclude that some dolomitic and phosphatic rock components may retain a near primary seawater 87sr 86sr composition once contaminant strontium from secondary calcite and other phases has been removed commonly applied trace element e g mn sr mg ca and isotopic c o screening cut offs were also examined in the light of these results and proved unreliable particularly in the case of dolostones and partially dolomitized limestones we recommend that rigid sample selection and sequential leaching procedures be applied to all sis studies of bulk materials and propose a general protocol for strontium isotope studies on marine authigenic rocks which may also be applicable to rare earth element and other geochemical studies of marine authigenic minerals in the future

nutrients sea level and tectonics constrains for the facies architecture of a miocene carbonate ramp in central italy in the western part of the central apennines lower middle miocene carbonates were deposited on a tropical subtropical carbonate ramp they record two long term cycles the first of which is illustrated in this paper between 21 and 17 5 ma miocene carbonates paraconformably overlying the cretaceous limestones record a transgressive event during a time of global 2nd order sea level lowstand it is postulated that this deviation is related to an increase in tectonic subsidence between 17 5 ma and 16 15 ma with a progressive relative sea level rise the inner middle ramp facies belt stepped back whereas the bryozoan dominated outer ramp facies belt stepped back but simultaneously prograded this bloom of suspension feeding organisms is interpreted to reflect an increased nutrient availability hence a change from oligotrophic to eutrophic conditions strontium isotope dates constrain correlation of the second phase with a eutrophic event possibly linked to the influence of the neighbouring apenninic accretionary wedge and foredeep system

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

tectonic rotation and diverse remanence carriers revealed by the paleomagnetism of eo oligocene turbidites from two ultrahelvetic units western alps haute savoie france abstract our paleomagnetic study of two ultrahelvetic units from the western alps shows that 1 one unit has experienced an important vertical axis tectonic rotation and 2 in turbidite deposits the nature of the remanence carrier can differ between coarse grained and fine grained intervals samples were collected at close intervals from weakly deformed turbidites in two different areas derochoir and marais characteristic remanent magnetization chrm was measured with alternating field and thermal demagnetization whereas clues for identifying the main remanence carrier were provided by coercivity spectra analysis in the derochoir area the bulk of the section consists of fine grained rocks magnetic intensity values are small directions are scattered and no polarity reversals were detected remanence was probably acquired during late diagenesis or regional tectonic deformation in coarser lithologies depositional chrm has probably been preserved by an early phase of cementation in the marais turbidites remanent magnetization is mainly carried by magnetite although some samples show clustered directions several polarity inversions have been observed suggesting that remanence was acquired soon after sediment deposition magnetic directions values indicate a counterclockwise rotation of about 110 of the marais sediments with respect to the stable european craton this movement which partly affects the underlying subalpine chains is probably related to sinistral strike slip motion along a major regional accident during the late oligocene or miocene

seismic constraints on the water flux delivered to the deep earth by subduction for the first time large quantities of water are identified throughout the length of a subducting plate and an efficient pathway is defined for injecting water into the deep 600 km earth by the incorporation of water into the mantle of an oceanic plate as serpentine water is thought to facilitate plate tectonics but the quantities of water involved and thus its effect are poorly known taking advantage of an anomalous seismic arrival recorded in the tonga fiji subduction zone the presence volume water content and flux of serpentine and water associated with the subducting oceanic mantle at tonga is quantified at 2 10 8 tg ma h 2 o many times larger than previous inferences based on surface observations water fluxes of this magnitude into the deep earth will substantially increase the role of water in hypotheses of plate tectonic movements mantle plumes and flood basalt activity

lower cretaceous to eocene sedimentary transverse ridge at the romanche fracture zone and the opening of the equatorial atlantic abstract a transform parallel transverse ridge runs for several hundred kilometers along the northern side of the romanche fracture zone rfz being most prominent opposite to the eastern mid atlantic ridge transform intersection rti seismic reflection profiles and rock sampling indicate that the western part of the transverse ridge is made of uplifted slivers of oceanic lithosphere in contrast the eastern part east of the rti consists of a thick 4 km sedimentary succession affected by faulting and thrusting we call it the romanche sedimentary sequence rss if this sedimentary sequence were underlain by oceanic crust its predicted age should be not more than 60 ma however dolomitized maiolica type pelagic limestones containing calpionellids of early cretaceous age 140 ma were sampled from the rss in addition to paleocene eocene quartzs and stones siltstones claystones diatomites and limestones we present a description of the rock samples and a reconstruction of their environment of deposition and of their age the rss may represent material deposited during initial continent continent transform motion in a narrow deep e w elongated basin that communicated with the central atlantic the rss was later subjected to transpression and uplift due to transform related tectonics gabbros but not basalts were sampled from the igneous basement underlying the rss it is not clear whether the rss overlies oceanic crust or some sort of pre oceanic northern red sea type crust the presence of lower cretaceous pelagic deposits near the romanche eastern rti implies an age for the initial stage of the opening of the equatorial atlantic older than the generally assumed aptian albian

nonvolcanic tectonic islands in ancient and modern oceans 1 most oceanic islands are due to excess volcanism caused by thermal and or compositional mantle melting anomalies we call attention here to another class of oceanic islands due not to volcanism but to vertical motions of blocks of oceanic lithosphere related to transform tectonics sunken tectonic islands capped by carbonate platforms have been previously identified along the vema and romanche transforms in the equatorial atlantic we reprocessed seismic reflection lines did new facies analyses and 87sr 86sr dating of carbonate samples from the carbonate platforms a 50 km long narrow paleoisland flanking the vema transform underwent subsidence erosion and truncation at sea level it was then capped by a 500 m thick carbonate platform dated by 87sr 86sr at 11 10 ma three former islands on the crest of the romanche transverse ridge are now at 900 m bsl they show horizontal truncated surfaces of oceanic crust capped by 300 m thick carbonate platforms with 10 6 ma sr isotopic ages these sunken islands formed due to vertical tectonics related to transtension transpression along long offset slow slip transforms another tectonic sunken island is atlantis bank an uplifted gabbroic block along the atlantis ii transform sw indian ridge 700 m bsl a modern tectonic island is st peter and st paul rocks a rising slab of upper mantle located at the st paul transform equatorial atlantic cold tectonic islands contrast with hot volcanic islands related to mantle thermal and or compositional anomalies along accretionary boundaries and within oceanic plates or to supra subduction mantle melting that gives rise to islands arcs

mechanism for generating the anomalous uplift of oceanic core complexes atlantis bank southwest indian ridge atlantis bank is an anomalously uplifted oceanic core complex adjacent to the atlantis ii transform on the southwest indian ridge that rises 3 km above normal seafloor of the same age models of flexural uplift due to detachment faulting can account for 1 km of this uplift postdetachment normal faults have been observed during submersible dives and on swath bathymetry two transform parallel large offset hundreds of meters normal faults are identified on the eastern flank of atlantis bank with numerous smaller faults tens of meters on the western flank flexural uplift associated with this transform parallel normal faulting is consistent with gravity data and can account for the remaining anomalous uplift of atlantis bank extension normal to the atlantis ii transform may have occurred during a 12 m y period of transtension initiated by a 10 change in spreading direction ca 19 5 ma this extension may have produced the 120 km long transverse ridge of which atlantis bank is a part and is consistent with stress reorientation about a weak transform fault

cenozoic carbonates in southeast asia implications for equatorial carbonate development cenozoic equatorial carbonate production was at its most extensive and diverse in the seas of southeast asia yet the carbonates of this region remain poorly documented this paper presents the first published review of carbonate deposition during the cenozoic collating sedimentological data for carbonate successions throughout the region current models of warm water carbonate platform evolution sedimentation and facies distribution inspired by studies of the classic sub tropical areas provide inadequate analogues for the evaluation of modern or cenozoic equatorial carbonates in southeast asia the equatorial carbonates differ in being dominated by bioclasts coated grains and associations with evaporites or dolomites are rare the carbonate systems occur in a range of depositional settings which were often affected by coeval tectonism siliciclastic input or volcanism an understanding of the carbonate depositional environments spatial facies distributions and controls on deposition and diagenesis is essential in order to characterize equatorial carbonate development and to evaluate their considerable economic potential

nd and sr isotopic variations in acidic rocks formed under a peculiar tectonic environment in miocene southwest japan initial nd and sr isotopic ratios were obtained for middle miocene igneous rocks as well as for related rocks from the outer zone of southwest japan to investigate the petrogenesis of acidic magmas and their relation to a peculiar tectonic environment bearing on the back arc spreading of the japan sea on the nd sr diagram data points for the acidic rocks fall in the nd sr quadrant occupying different positions from those for sedimentary and old crustal rocks and seem to define several subparallel lines which extend towards the lower righthand sedimentary field the s type acidic rocks occupy an intermediate position between i type rocks and sedimentary ones a fact suggesting mixing of an igneous component and a sedimentary one the linear mixing trend observed on the nd sr diagram can be attained in the restricted case that the igneous component has similar sr nd concentration ratios to that of the sedimentary one which implies an intermediate to acidic composition for the igneous component inconsistency between the elemental and isotopic variations observed may be reconciled by considering that mixing probably in the relatively deep part of the crust might have occured prior to chemical differentiation processes the episodic igneous activity and the high heat energy required to melt such materials involving sedimentary rocks may be explained by a model in which a hot mantle region probably corresponding to the rising part of the mantle convection supplied the heating energy to the outer zone of southwest japan when passing beneath southwest japan in the course of movement of the hot rising part from the shikoku basin areas to the japan sea area