new evidence for crustal accretion in the outer mariana fore arc cretaceous radiolarian cherts and mid ocean ridge basalt like lavas new age determinations on radiolarian cherts foraminifers and volcanic rocks document the presence of allochthonous fragments of cretaceous oceanic plate suggesting accreted terrane in the outer mariana fore arc more than 50 km from the trench three dredges from 3km 2 area along a steep scarp recovered a diverse assemblage of rocks representing an ophiolite suite chert mafic and intermediate lavas and intrusive rocks trace element patterns of the lavas suggest at least three tectonic associations island arc ocean island and oceanic plate the cherts contain two deep water assemblages of radiolaria of middle to late valan ginian 131 138 ma and albian 97 112 ma age foraminifers recovered with the chert are aptian to albian in age the lavas record a wide range of k ar ages 85 ma for a metabasalt with trace element signatures of mid ocean ridge basalt 71 ma for a highly metamorphosed alkalic basalt and 39 ma for a fresh glassy boninite these ages imply multiple volcanic events and at least two tectonic settings for magma genesis the cherts and metabasalts are too old to have formed in situ or to be part of trapped west philippine basin crust the mix of old oceanic plate with younger are rocks requires complex tectonic relations we suggest that one or more fragments of cretaceous oceanic plate chert mid ocean ridge basalt and alkalic lavas were accreted to the mariana fore arc and have been extensively faulted and probably intruded by arc lava island arc tholeiite and boninite

evidence of late holocene shoreline progradation in the coast of kerala south india obtained from osl dating of palaeo beach ridges abstract the luminescence chronology of beach ridges in the central part of kerala along the west coast of india has been made for the first time in order to reconstruct past sea level highstands and coastline progradation transitional boundaries between shoreface to foreshore and foreshore to dune aeolian environments were used as marker horizons for determining the palaeo shoreline as well as sea level indicators the study presents the results of optically stimulated luminescence osl dating of sand samples collected from wave and wind built horizons derived from quartz using single aliquot regenerative dose sar protocol the osl ages from most of the sites are consistent with each other indicating a stage of coastline progradation between 3 and 5 ka and widespread beach ridge formation all along the west coast of kerala during this period the osl ages of transition levels together with the elevation measured using differential global positioning system dgps indicates that the sea level was between 3 5 and 4 5 m above present msl during the late holocene period it may also imply the emergence of the coast due to neo tectonic activity as the sea level curve for the east coast from reported ages indicates a mean elevation between 1 5 and 2 5 m the ages are discussed in the light of reported events such as sea level changes coastal geomorphology hydrological variations and neo tectonics

cenozoic plate interaction of the australia and philippine sea plates hit and run tectonics abstract recent studies in northwest new guinea have shown the presence of at least two marginal basins of different age both of which formed in back arc settings the older basin opened between the middle jurassic and early cretaceous a remnant of which is now preserved as the new guinea ophiolite its obduction started at 40 ma and it was finally emplaced on the australian margin at 30 ma the younger basin was active during the oligocene to middle miocene and was obducted in the early pliocene studies of the western edge of the philippine sea also reveal an important deformation of the philippine arc in the oligocene which hitherto has remained unexplained using information from these systems paleomagnetic results kinematic reconstructions and geochemistry of the supra subduction ophiolite we present a plate model to explain the region s eo oligocene development we suggest that an extensive portion of oceanic crust extended the australian plate a considerable distance north of the australian craton as australia began its steady 7 8 cm year northward drift in the early eocene this lithosphere was subducted thus the portion of the philippine sea plate carrying the taiwan philippine arc to its present site may have actually been in contact with the ophiolite now in new guinea and obduction led to deformation of the philippine sea plate itself neogene plate kinematics transported the deformed belt in contact with the sunda block in the late miocene and pliocene this interpretation has implications for the origin for the philippine sea plate and the potential incorporation of continental fragments against its boundaries

gravity signatures of terrane accretion abstract in modern collisional environments accreted terranes are bracketed by forearc gravity lows a gravitational feature which results from the abandonment of the original trench and the initiation of a new trench seaward of the accreted terrane the size and shape of the gravity low depends on the type of accreted feature and the strength of the formerly subducting plate along the central american trench the accretion of gorgona island caused a seaward trench jump of 48 to 66 km the relict trench axes show up as gravity lows behind the trench with minimum values of 78 mgal n of gorgona and 49 mgal s of gorgona respectively these forearc gravity lows have little or no topographic expression the active trench immediately seaward of these forearc gravity lows has minimum gravity values of 59 mgal n of gorgona and 58 mgal s of gorgona respectively in the north the active trench has a less pronounced gravity low than the sediment covered forearc in the mariana arc two cretaceous seamounts have been accreted to the eocene arc the northern seamount is most likely a large block the southern seamount may be a thrust slice these more recent accretion events have produced modest forearc topographic and gravity lows in comparison with the topographic and gravity lows within the active trench however the minimum values of the mariana forearc gravity lows are modest only by comparison to the mariana trench 216 mgal their absolute values are more negative than at gorgona island 145 to 146 mgal we speculate that the forearc gravity lows and seaward trench jumps near gorgona island were produced by the accretion of a hotspot island from a strong plate the mariana gravity lows and seaward trench jumps or thrust slices were the result of breaking a relatively weak plate close to the seamount edifice these gravity lows resulting from accretion events should be preserved in older accreted terranes

three dimensional crustal structure of the mariana island arc from seismic tomography 1 a three dimensional 3 d seismic refraction survey was acquired over the mariana volcanic arc at 14 5 18 5n and 145 147e first arrival traveltimes from this survey and from a separate 2 d survey acquired approximately perpendicular to the arc have been simultaneously inverted for a 3 d p wave velocity model using seismic tomography subject to smoothness constraints the active arc which initiated only 3 4 ma ago has an average crustal thickness of 18 km approximately 40 km to the east the inactive remnant of the rifted eocene arc has an average crustal thickness of 21 km due primarily to a thicker lower crustal layer with velocities of 6 5 7 0 km s 1 crustal production clearly varies both temporally and spatially with some crustal layers including the igneous forearc crust varying in thickness by a factor of up to 2 along strike average p wave velocities within the upper crust of the modern arc are 240 360 m s 1 lower than in the eocene arc but are 280 m s 1 higher within the lower crust middle crust with velocities of 6 0 6 5 km s 1 is best developed beneath the eocene arc these results suggest an evolution of arc structure with increasing age we infer closure of fractures and porosity in the upper crust through hydrothermal circulation and a reduction in the mafic character of the middle to lower crust as a result of intracrustal differentiation although tonalitic rocks may predominate in the transition from upper to middle crust the bulk of the crust is essentially basaltic

geochemical and sedimentological characteristics of surface sediments from ashtamudi estuary southern india implications for provenance and modern sedimentary dynamics geochemical and end member mixing analyses emma of the grain size distribution were conducted on recent surface sediments from ashtamudi estuary southern india to investigate the hydrodynamic factors that influence the depositional processes in the region the complex interplay between the natural fluvial and marine and anthropogenic influences on the ashtamudi estuary have been delineated based on the inter relationships between geochemical elements and the end members em derived from the grain size parameters si al and fe are the major elements in the sediment of ashtamudi estuary constituting 26 47 9 21 and 8 14 respectively four grain size end members were identified with dominant modes of 7 em1 6 em2 4 5 em3 and 3 3 em4 corresponding to fine silt medium silt coarse silt and fine sand respectively the high contributions of al fe cr and ni combined with the em1 and em2 in the upstream of the estuary indicated that the fine grained sediments were derived from the fluvial inputs into the basin the marine tidal influenced lower estuary is characterised by high concentrations of si ti ca sr and coarse fraction end member em3 the elemental concentration of cu zn and co along with em4 shows higher values along the shore region suggesting human interventions in the basin this integrated geochemical analysis and emma provide advancement in the knowledge of the transportation mechanisms and regional sediment dynamics from the estuarine system in southern india

cyanobacteria and testate amoeba as potential proxies for holocene hydrological changes and climate variability evidence from tropical coastal lowlands of sw india cyanobacteria and thecamoebians are the two major constituents of non pollen palynomorphs npp found in the palynological preparations the former is well represented in the sediment archives while the latter has restricted occurrence in the holocene sequence apart from the conventional organic matter of pollen and spores well preserved morphotypes of gleotrichia and rivularia are the best forms of cyanobacteria found in almost all the studied profiles and their abundance towards early to middle holocene coincides with the heavy rainfall and wet climate conditions during the holocene climate optimum hco contrary to cyanobacterial diversity thecamoebians are restricted to late holocene in most of the studied profiles and their occurrence is correlated with the prevailing dry and stressful environmental conditions the major elements of thecamoebians identified belong to arcellaceans centropyxids and cucurbitella types abundance of centropyxids and arcellaceans in varapuzha west kadungallur koonamavu pachcha and ernakulam indicates low humidity and low precipitation however the relative abundance of centropyxids in mid holocene at ernakulam and koonamavu reflects the early colonization of this type competing with the cyanobacteria whereas arcellaceans are essentially confined to late holocene dominance of cyanobacteria and its proliferation is directly related to freshwater influx of the rainfall as well as the local hydrological regimes associated with the active aquifers controlled by the geomorphological features of the region in which the wetland system sustained considering the signatures of cyanobacteria and thecamoebians observed in the sediment archives their occurrence in the wetlands during the holocene corresponds to the hydrodynamic regimes brought in by the freshwater influx as a result of monsoon variability the well preserved cyanobacterial signatures in sediment archives of kerala coast and their scarcity or even absence elsewhere along the west coast of india indicates that they are exclusively adapted to the wetland habitats where the hydrodynamic regimes are regularly controlled by fresh water influx in the form of extended rainy days of the monsoon system

mesozoic tectonic evolution of the proto south china sea a perspective from radiolarian paleobiogeography abstract the proto south china sea pscs represents the lithosphere that formerly occupied the region southeast of south china where the present day south china sea is located the subduction and demise of the pscs have attracted wide attention whereas the pre subduction history particular the mesozoic of the region remains relatively neglected in this paper the radiolarian paleobiogeography of the region is analyzed to identify the paleobiogeographic affinity and paleo connectivity of the pscs with the pacific and tethys oceans during the mesozoic then the continent ocean configuration and tectonic evolution of the pscs and adjacent areas are discussed through the integration of paleobiogeographical results and associated paleomagnetic stratigraphic and petrologic data the main conclusions are as follows 1 during the triassic the pscs was situated in the transition zone between the pacific and tethys oceans and its paleobiogeography was influenced by both oceans the pscs was characterized by complex tectonic overprinting by the pacific and tethyan tectonic regimes 2 in the early middle jurassic the pscs was well connected to the pacific ocean but poorly connected to the tethys ocean consistent with the paleobiogeography of the pscs being mainly constrained by the pacific ocean the pscs was part of the paleo pacific slab 3 during the late jurassic cretaceous the pscs was in a semi closed environment the late jurassic early late cretaceous pscs was part of a west facing andean type subduction zone and the late late cretaceous pscs was situated in an extensional tectonic setting

holocene environmental changes as recorded in sediments of a paleodelta southwest coast of india abstract sedimentary record of a 40 m deep borehole drilled in a paleodelta region of southwest coast of india has been studied for sedimentary proxies texture clay minerals geotechnical parameters and pollens the sedimentary record spanning from 12 ka to recent in age exhibits two different sedimentary environments of deposition sediments record high values of moisture content organic carbon and plasticity index sediment texture and geotechnical properties indicate a distinct change in depositional environment from marine to fluvial during the sea level fall i e after 7 ka it also suggests that major rise of sea level from 11 to 7 ka and regression from 7 ka to 5 ka contributed to the changes in the environment of deposition the downcore increase of illite and decrease of kaolinite at 12 m depth 6 ka and an upward increase of smectite and kaolinite and decrease of illite concentration support the major fall in sea level in the region that accounted for the change of depositional environment the pollen records reveal the abundant occurrence of semi evergreen type of mangroves during early to mid holocene thus the multi proxy record provides an evidence of change in the environment of deposition from marine to fluvial which might have been influenced by neotectonics sea level variations and monsoonal intensity in the region

influence of the mendocino triple junction on the tectonics of coastal california abstract the migration of the mendocino triple junction through central and northern california over the past 25 30 million years has led to a profound change in plate interactions along coastal california the tectonic consequences of the abrupt change from subduction plate interactions north of the triple junction to the development of the san andreas transform system south of the triple junction can be seen in the geologic record and geophysical observations the primary driver of this tectonism is a coupling among the subducting juan de fuca gorda north american and pacific plates that migrates with the triple junction this coupling leads to ephemeral thickening of the overlying north american crust associated uplift and subsequent subsidence and a distinctive sequence of fault development and volcanism

on ferroan a type granitoids their compositional variability and modes of origin we recognize eight types of ferroan granitoid that can be distinguished on the basis of major element chemistry these include alkalic granitoids that may be metaluminous or peralkaline alkali calcic granitoids that may be metaluminous peraluminous or peralkaline calc alkalic granitoids that may be metaluminous or peraluminous and rare calcic ferroan granitoids these granitoids may form by two distinct end member processes extreme differentiation of basaltic melts results in ferroan granitoids that are either peralkaline alkalic and alkali calcic or metaluminous alkalic alkali calcic and calc alkalic with alkalinity increasing with increasing pressure of differentiation partial melting of tonalitic to granodioritic crust produces alkali calcic to calc alkalic granitoids that are metaluminous at low pressures and peraluminous at high pressures it is likely that a combination of these two processes plays some role in the formation of most ferroan granitoids most granitoids that are referred to as ferroan however the term has become more confusing than edifying because it has been applied to a broad spectrum of granitoid compositions with varying petrogenesis for this reason we recommend that the term be discontinued and ferroan used in its stead

cretaceous fore arc basalts from the tonga arc geochemistry and implications for the tectonic history of the sw pacific the tonga fore arc preserves a complex history of subduction initiation back arc basin formation and arc volcanism which has extended from the cretaceous to the present in this paper we discuss the geochemistry of a cretaceous basalt dolerite gabbro suite recovered in two dredges from the tonga fore arc at 19 s the geochemistry of the tonga fore arc suite is unlike that of the uniformly depleted morb basalts of the subducting pacific plate and therefore is unlikely to be accreted from pacific cretaceous crust the 102 ma age obtained for one tongan fore arc dolerite is contemporaneous with a major phase of cretaceous subduction related volcanism recorded both in detrital zircon age populations and associated volcanics from new caledonia and new zealand we believe that the tonga fore arc basalts are a remnant of a hypothesized once extensive cretaceous back arc basin called the east new caledonia basin which we propose to have existed from 102 to 50 ma the allochthonous poya terrane of new caledonia is geochemically very similar to the tonga fore arc basalts and represents a younger 84 55 ma remnant of the same basin subduction related cretaceous volcanics from the sw pacific representing both arc and back arc settings all appear to have similar zr nb values suggesting a common mantle component in their petrogenesis the tonga fore arc basalts are also similar to fore arc basalts recovered from the izu bonin mariana fore arc but unlike these basalts they are not associated with subduction initiation

a trapped philippine sea plate origin for morb from the inner slope of the izu bonin trench basement outcrops sampled by submersible and dredge from the inner slope of the izu bonin trench at 32 n and 6200 6700 m water depth have a distinct mid ocean ridge basalt morb chemistry unlike any other rocks previously sampled in the izu bonin arc they are low k tholeiites with moderate tio2 0 7 1 8 wt extremely low ba 1 5 7 parts per million low ba la 1 2 3 and are depleted in light rare earth elements these samples could represent either an accreted piece of subducting pacific plate or a trapped remnant of philippine sea plate on which the izu bonin arc was built although their major and trace element chemistry do not help to distinguish their source the sr nd and pb isotopes clearly support a philippine sea plate origin the isotopic signature of the inner trench slope samples matches that of philippine sea plate lavas with 87 sr 86 sr 0 70321 0 70373 143 nd 144 nd 0 513057 0 513077 and 206 pb 204 pb 18 2 18 5 the samples have elevated 207 pb 204 pb 15 3 15 5 and 208 pb 204 pb 38 0 38 2 values compared to the northern hemisphere reference line nhrl and their isotopic signature is distinct from the mesozoic pacific morb being subducted these are the first samples of trapped philippine sea oceanic crust discovered in the izu bonin mariana arc they require that models for the formation of intra oceanic arc crust account for pre existing oceanic crust and that estimates of arc magma production rates are lowered accordingly fl 1999 elsevier science b v all rights reserved

late quaternary evolution of ashtamudi sasthamkotta lake systems of kerala south west india abstract sedimentological and palynological analysis of two borehole samples collected from the confluence of kallada river with the ashtamudi lake in the south west coast of india have been addressed while ascertaining the late quaternary evolution of the two borehole samples the one collected from pangod 9 03 n 76 42 e reveals a fining upward sequence with medium to coarse sand 3 05 m at base organic carbon rich silt and fine sand dominated sediments 2 m at the middle and yellowish brown muddy sediments 3 2 m at the top the middle layer embeds a suite of partly carbonized sub fossil logs of wet evergreen to semi evergreen vegetation the palynoflora of this layer reveals that the depositional site is within the tidal limit and deposition occurred under high precipitation and atmospheric humidity the similarity in 14 c dates of a wood at 5 m bgl 7490 90 yrs bp and the embedding sediments 7480 80 yrs bp indicate quick burial of the riparian vegetation the west kallada borehole 9 30 n 76 37 e reveals middle to late holocene sequence of clayey silt 6250 110 yrs bp 3880 80 yrs bp and sand resting unconformably over greyish white clayey sand with pebbles and granules derived from laterite provenance palynological analysis shows that the holocene sedimentation took place under marine nearly marine environment and later changed to brackish water and finally to freshwater environment marine transgression 6000 yrs bp coupled with heavy rainfall in the hinterlands was responsible for faster sedimentation in the region the heavy mineral contents especially opaques garnet and sillimanite in the sediment samples of the study area as well as the bathymetric configuration of the ashtamudi sasthamkotta and chelupola lakes reiterate the fact that these lakes have been evolved from an embayment consequent to incomplete partial silting up during early to middle holocene higher sea levels and also under high rainfall of the holocene climatic optimum of around 10 000 7000 yrs bp

stratigraphical and palynological appraisal of the late quaternary mangrove deposits of the west coast of india the organic deposits derived from the mangrove swamps form reliable stratigraphic markers within the late quaternary sequence of kerala konkan basin three generations of such deposits have been identified the older one is dated to around 43 000 40 000 14c yr b p with a few dates beyond the range of radiocarbon the younger ones date from the middle holocene to latest pleistocene 10 760 4540 14c yr b p and the late holocene 4000 14c yr b p pollen analyses confirm that the deposits are mostly derived from the mangrove vegetation peat accumulation during the period 40 000 28 000 14c yr b p can be correlated with the excess rainfall 40 100 greater than modern values of the asian summer monsoon the low occurrence of mangrove between 22 000 and 18 000 14c yr b p can be attributed to the prevailing aridity and or reduced precipitation associated worldwide with last glacial maximum because exposure surfaces and ferruginous layers are commonly found in intervals representing this period the high rainfall of 11 000 4000 14c yr b p is found to be the most significant as the mangrove reached an optimum growth around 11 000 14c yr b p but with periods of punctuated weaker monsoons from the present and previous studies it has been observed that after about 5000 or 4000 14c yr b p the monsoons became gradually reduced leading to drying up of many of the marginal marine mangrove ecosystems a case study of hadi profile provided an insight to the relevance of magnetic susceptibility to record the ecological shift in late holocene

basalts erupted along the tongan fore arc during subduction initiation evidence from geochronology of dredged rocks from the tonga fore arc and trench a wide variety of different rock types were dredged from the tonga fore arc and trench between 8000 and 3000 m water depths by the 1996 boomerang voyage 40ar 39ar whole rock and u pb zircon dating suggest that these fore arc rocks were erupted episodically from the cretaceous to the pliocene 102 to 2 ma the geochemistry suggests that mor type basalts and dolerites were erupted in the cretaceous that island arc tholeiites were erupted in the eocene and that back arc basin and island arc tholeiite and boninite were erupted episodically after this time the ages generally become younger northward suggesting that fore arc crust was created in the south at around 48 52 ma and was extended northward between 35 and 28 ma between 9 and 15 ma and continuing to the present day the episodic formation of the fore arc crust suggested by this data is very different to existing models for fore arc formation based on the bonin marianas arc the bonin marianas based models postulate that the basaltic fore arc rocks were created between 52 and 49 ma at the beginning of subduction above a rapidly foundering west dipping slab instead a model where the 52 ma basalts that are presently in a fore arc position were created in the arc back arc transition behind the 57 35 ma loyalty three kings arc and placed into a fore arc setting after arc reversal following the start of collision with new caledonia is proposed for the oldest rocks in tonga this is followed by growth of the fore arc northward with continued eruption of back arc and boninitic magmas after that time

holocene climatic vicissitudes and sea level changes in the south western coast of india appraisal of stable isotopes and palynology abstract the southern kerala coast in sw india has experienced dramatic changes in climate and sea level during holocene these changes are apparently well preserved in the sedimentary archives of the estuarine basins that are entrenched on the uplifted neogene deposits here we report the sedimentary processes palaeoclimate and sea level records decoded from two borehole cores puthenthuruthu and munrothuruthu borehole cores retrieved respectively from the marine and fluvial ends of the second largest estuarine basin in sw india the ashtamudi estuary using sedimentology palynology and stable isotopes as the major proxies the heavy rainfall event at the latter half of early holocene 6117 101 bp is reflected well in the isotopic signatures of the puthenthuruthu borehole core the higher level of lighter isotopes of oxygen and carbon at 10 15 m below ground level is a clear indication of freshwater influx from the hinterland due to intensified monsoon activity this in turn was responsible for the development of bay head delta in the fluvial end of the estuary during early middle holocene period palynological contents below the level indicate a facies change as freshwater and terrestrial elements dominate over marine contents further the occurrence of desmids shows the marine facies is being gradually replaced by freshwater to continental facies this was followed by the deposition of littoral sands as flood tide islands within the estuarine basin the palynological investigations of sediments with radiocarbon age of 4350 90 bp in the munrothuruthu borehole core show a dry phase with high saline depositional environment in the beginning which is followed by influx of freshwater as indicated by the higher content of lighter isotopes of carbon and oxygen in the calcareous nodules algal pisolites embedded in the sediments the occurrence of pteridophytic spores and a few pollen of cullenia in the palynological preparations in the upper half of the silt and clay dominated intervening layers reiterate this view

evolution of the coastal wetland systems of sw india during the holocene evidence from marine and terrestrial archives of kollam coast kerala abstract kerala coast in southwest india is endowed with an interlacing network of wetlands evolved during the late quaternary the wetlands in the southern part of kerala especially around kollam district show antecedent characteristics and are incised on the neogene sediments these wetlands include 20 0 35 0 m thick deposits of holocene sediments containing good archives of landform evolution and climate changes holocene sedimentation was initiated at about 8920 110 bp and is represented mainly by sub fossil wood peat or carbonaceous clays subsequent deposition of brackish water to marine sediments occurred in the middle holocene the proposed evolutionary model reveals that many wetland bodies including sasthamkotta lake chelupola lake chittumalachira lake and kotta lake were evolved from the proto estuarine basins the upper arms of the pre holocene ashtamudi and paravur estuaries were cut off due to progradation of alluvial sediments during the holocene climatic optimum

signatures of late quaternary land sea interactions and landform dynamics along southern kerala coast sw india abstract the kerala coast displays a spectrum of varied geomorphic features that are evolved as a function of late quaternary land sea interaction the southern kerala coast south of achankovil shear zone asz has been characterised by cliffed coast with entrenched estuarine basins pocket beaches promontories older and younger strand plains etc that are evolved as a result of the transgressive regressive phases to which the coast has been subjected during the late quaternary period in spite of having varied geomorphic features lacuna in comprehending the evolutionary phases of the coastal landform features is the primary impediment that bolstered us to unravel the late quaternary land sea interactions and landform changes of the coast between thottapally and vizhinjam moreover the last two centuries are known to have been witnessing many natural as well as human induced environmental perturbations in the area the present study attempts to reconstruct t he late quaternary landform dynamics based on the satellite data while the last century changes were extracted from the digital shoreline analysis system dsas the evidences based on geomorphic variations suggest that the present study area hosts two distinct paleo coastlines 1 coincides with the eastern boundary of the older strand plains accreted during late pleistocene and 2 marks the boundary between older and younger strand plains accreted during early middle holocene a sector wise analysis reveals that the northern half of the study area is characterised by many coast perpendicular estuarine basins with well developed bay head deltas in its fluvial end and flood tide islands near the estuarine mouth on the contrary the southern half is characterised by pocket beaches and coast parallel backwater bodies developed during the regressive phases of the sea a high resolution study of the shoreline changes during the period 1920 2018 reveals that the younger strandlines are vulnerable to severe coastal erosion and shoreline retreat at many places compared to coastal accretion the study stresses the imminent need for continuous monitoring and implementation of site specific mitigation measures for the conservation and management of this coast known for its outstanding natural beauty and strategic beach placer deposits

slow rates of subduction erosion estimated from subsidence and tilting of the tonga forearc the convergent margins of the western pacific are marked by widespread tectonic erosion of the overriding plate by the subducting oceanic lithosphere at ocean drilling program site 841 on the mid trench slope of the tonga forearc rates of tectonic trenchward tilting and subsidence of the basement appear to be linked supporting models of basal erosion focused near the trench rates of subsidence significantly exceed those predicted from plate cooling models and require crustal thinning to account for the difference in this extensional environment estimated extension at site 841 is 280 tilting and subsidence rates peaked ca 32 34 ma just after spreading in the south fiji basin at 35 ma however the trenchward direction of tilt and higher extension on the trench slope than on the tonga platform suggest a link to tectonic erosion near the trench rather than extension focused in the arc edifice since this time ca 135 km has been eroded from the edge of the plate of which 80 km is related to the ongoing collision of the louisville ridge a probable paleo collision event at 16 ma caused the forearc at site 841 to be uplifted and tilted arcward forming a major unconformity due to mass wasting steady state erosion averages 1 5 km m y there is no compelling evidence to suggest loss of several hundred kilometers of arc crust since the eocene and thus no reason to suspect that subduction initiation and related magmatism may have predated the ca 45 ma boninitic magmatic activity

residual liquid origin for a monzonitic intrusion in a mid proterozoic anorthosite complex the sybille intrusion laramie anorthosite complex wyoming the sybille intrusion 100 km 2 is one of three large monzonitic intrusions in the 1 43 ga laramie anorthosite complex of southeastern wyoming the petrographic geochemical isotopic and geophysical characteristics of sybille monzonitic rocks are consistent with an origin by extensive crystallization of liquids residual to nearby anorthositic cumulates ferrodiorites and contamination by archean wall rocks the exposed part of the intrusion is composed mainly of coarse grained monzosyenites with abundant alkali feldspar phenocrysts the monzosyenites preserve mineralogical evidence for high crystallization temperatures 1000 c mid crustal emplacement pressures 3 kbar relatively reduced crystallization conditions 2 log units below the fayalite magnetite quartz fmq oxygen buffer and they crystallized in the presence of a co2 rich fluid phase fuhrman et al 1988 frost and touret 1989 the eastern monzosyenites those adjacent to contemporaneous anorthosite are distinguished by an anhydrous mineral assemblage fo16 fo8 olivine high ca pyroxene lacking modal quartz silica contents of 60 wt and smaller eu anomalies eu eu 1 2 to 1 3 abundant xenoliths of archean wall rocks and anorthosite from the adjacent intrusions in all monzosyenites attest to a stoping emplacement mechanism near the roof of the chamber we propose that the monzosyenites represent a relatively thin 0 5 1 0 km thick roof to a magma chamber dominated by dense ferrodioritic cumulates at depth extensive open system fractionation of a ferrodioritic parent magma residual after crystallization of anorthosite produced fe enriched monzodioritic and or monzonitic magma in the upper part of the chamber and complementary fe and ti rich cumulates in the lower levels we have corroborated the production of monzonitic liquids from crystallization of ferrodiorite through a series of reconnaissance equilibrium crystallization experiments the presence of dense ferrodioritic cumulates at depth is consistent with the prominent positive gravity anomaly associated with the sybille intrusion hodge et al 1973 in the upper parts of the chamber the fractionated monzodioritic and or monzonitic magmas eventually became saturated in alkali feldspar owing to density contrasts the alkali feldspar phenocrysts floated to the roof of the chamber thus producing the exposed porphyritic monzosyenites in addition the roof of the chamber was the site of significant melting of archean gneiss and locally metapelite the sr and nd isotopic compositions of the monzosyenites with sr isotopic ratios becoming increasingly radiogenic from east i sr 0 7059 and initial nd 2 5 to west i sr 0 7092 and initial nd 2 6 are consistent with a 5 to 15 addition of archean orthogneiss to a ferrodioritic parent magma that had isotopic characteristics similar to adjacent anorthositic rocks the stratigraphic and compositional similarity of the sybille monzosyenites to mangerites in the bjerkreim sokndal intrusion of the rogaland anorthosite complex southern norway indicates that similar open system magmatic processes are capable of having produced high temperature k rich monzonitic rocks in other proterozoic anorthosite complexes

late proterozoic evolution of the northern part of the hamisana zone northeast sudan constraints on pan african accretionary tectonics new structural data from the red sea hills northeast sudan add constraints to accretionary tectonic processes that acted in the arabian nubian shield during the late proterozoic geometric constraints provided by field data are used together with landsat thematic mapper imagery to interpret the regional distribution and structural relations of spectrally and lithologically distinctive rock assemblages in contrast to interpretations reported elsewhere these studies indicate that the hamisana zone a prominent structural feature in the northern red sea hills post dated terrane accretion it is characterized by east west crustal shortening and by steep folds and thrust faults calc alkaline magmatism was contemporaneous with and outlasted main phase deformation thus deformation occurred in an intra arc setting thrust faults were reactivated after east west contraction compromising their usefulness in delineating 9sutures9 or as piercing points across older faults pan african accretionary processes may have been analogous to phanerozoic ophiolite and island arc accretion in the western north american cordillera where penetrative deformation occurred in response to periodic intra plate shortening events rather than an ultimate collision of unrelated crustal fragments such an analogy implies a mechanism for crustal thickening by structural as well as igneous processes during evolution of an accretionary belt in a long lived convergent margin setting consistent with the long standing observation that pan african magmatism became more evolved with time this view emphasizes that an interplay of deformation and magmatism resulted in protracted accretion and crustal thickening in contrast to models where crustal shortening results directly from collision of a volcanic arc microcontinent or other buoyant features

evolution of the mariana convergent plate margin system the mariana convergent plate margin system of the western pacific provides opportunities for studying the tectonic and geochemical processes of intraoceanic plate subduction without the added complexities of continental geology the system s relative geologic simplicity and the well exposed sections of lithosphere in each of its tectonic provinces permit in situ examination of processes critical to understanding subduction tectonics its general history provides analogs to ancient convergent margin terranes exposed on land and helps to explain the chemical mass balance in convergent plate margins the mariana convergent margin s long history of sequential formation of volcanic arcs and extensional back arc basins has created a series of volcanic arcs at the eastern edge of the philippine sea plate the trenchward edge of the overriding plate has a relatively sparse sediment cover rocks outcropping on the trench s inner slope are typical of the early formed suprasubduction zone s lithosphere and have been subjected to various processes related to its tectonic history pervasive forearc faulting has exposed crust and upper mantle lithosphere many large serpentinized peridotite seamounts are within 100 km of the trench axis from these we can learn the history of regional metamorphism and observe and sample active venting of slab fluids ocean drilling recovered suprasubduction zone lava sequences erupted since the eocene that suggest that the forearc region remains volcanologically dynamic seismic studies and seafloor mapping show evidence of deformation throughout forearc evolution large portions of uplifted southern forearc are exposed at the larger islands active volcanoes at the base of the eastern boundary fault of the mariana trough vary in size and composition along strike and record regional differences in source composition their locations along strike of the arc are controlled in part by cross arc structures that also facilitate formation of submarine volcano chains extending from the base of the fault westward into the back arc basin the western boundary is the west mariana ridge the western portion of the volcanic arc active prior to formation of the mariana trough the trough evolved in a two stage extension process of rifting and subsequent seafloor spreading the back arc basin varies along strike from rifted arc lithosphere with scattered volcanoes but no real spreading center in the north to a complex mid ocean type spreading center south of 20 n the change from initial rifting to true seafloor spreading is also evident across the mariana trough from rifted topography near the west mariana ridge to spreading ridges in the central to eastern basin south of 20 n this morphologic change indicates an early stage of extension with basin and range type topography predominant and volcanism restricted to fissure eruptions along fault block boundaries the spreading ridges and abyssal hill morphology evolved later as new lithosphere was generated at elongate volcanic ridges located in the center of rift valleys the center of extension intersects the active volcanic front differently at either end of the mariana trough in the north extension is by rifting of arc lithosphere where it intersects the arc in the south a major strike slip fault extends from the trench axis across the forearc through the volcanic arc and into the back arc basin arc magmas apparently leak along this fault zone into the forearc and the back arc spreading center the complexity of interrelated tectonism and magmatism in this convergent margin is daunting but studies of arc systems such as this provide the best hope of interpreting many of the exposed terranes accreted to continents comparison of subaerial terranes with recent studies of intraoceanic convergent margins will add to our understanding of plate interactions and of the evolution of the volcanic arcs and extensional back arc basins generated within such environments

tectonic setting and paleoenvironment of terranes in the southern new england orogen eastern australia as constrained by radiolarian biostratigraphy abstract radiolarians are abundant in the gamilaroi djungati and anaiwan terranes of the new england orogen in eastern australia these microfossils present the first age constraints on the timing of development of hitherto undated lithologies within the various juxtaposed terranes of the orogen they give a biostratigraphic framework for interpreting the history of marine sedimentation in these terranes and have significant implications for published tectonic models radiolarians also show that the stratigraphically lowermost rocks of the gamilaroi terrane are most probably of devonian age better stratigraphic resolution permits interpretation of these rocks as part of an intra oceanic island arc succession which accreted to the eastern margin of australia gondwana around the end of the devonian this intra oceanic arc is distinct from a younger carboniferous superposed continental arc sequence which constitutes a successor basin that developed over the gamilaroi terrane subsequent to its accretion to the margin of australia radiolarians provide the basis for dating siliceous lithologies which dominate the djungati terrane and permit a more detailed analysis of the history of this terrane during the middle silurian through late devonian the djungati terrane was part of an oceanic basin which was isolated from any source of terrigenous sedimentation the djungati terrane was subsequently influenced by volcanic island arc activity and was tectonically disrupted during the latest devonian to early carboniferous radiolarian age data show that age correlations inferred on the basis of similarities in detrital sandstone petrography between lithostratigraphic units in the gamilaroi and djungati terranes are not always appropriate djungati terrane has been widely interpreted as a subduction complex related to the gamilaroi terrane radiolarian data now elucidate much of the structural complexity of this terrane significant differences between the djungati terrane and the style of well documented subduction complexes include the close spacing of zones of radiolarian chert the dominance of this lithology over others and the lateral extent of many of the chert horizons the anaiwan terrane has features which are characteristic of many well documented subduction complexes radiolarians are abundant in this terrane and can be used to show that it developed along the eastern margin of australia in response to early carboniferous subduction of late devonian to early carboniferous oceanic crust packages of chert accreted into the subduction complex are progressively younger towards the northeast indicating that subduction was directed to the southwest

holocene and late pleistocene relative sea level fluctuations along the east coast of india abstract hermatypic coral colonies and intertidal fossil bearing grainstone packstone and sandstone beds of holocene and of a late pleistocene highstand are exposed above the present high tide level htl at a number of locations along the east coast of india from cape comorin to rameswaram being a passive margin boundary free from indications of late quaternary seismogenic movements this sector provides a benchmark for defining minor relative sea level perturbations during the late pleistocene and holocene highstands along a tropical coast lying between latitudes 5 n and 10 n a series of marine terraces carved on and locally blanketted by late pleistocene biotic and terrigenous accumulations occur at different elevations up to 4 4 m above ltl at manappad point possibly signifying discrete stillstand episodes followed by abrupt intervals of rising falling sea level sea level indicators of the holocene highstand occur in this sector as well as along the fringes of the godavari delta further north the holocene highstand reached nearly 3 m above ltl at 7 3 ka remained stable for approximately 1 7 kyr and was followed by a relative sea level fall between 5 2 and 4 2 ka there was a second pulse of relative sea level rise of a few metres leading to a fresh spurt in coral growth along the northern coast of mandapam and rameswaram this was nearly contemporaneous with fresh melting of ice sheets of antarctica the little ice age lia witnessed a minor 1 m relative sea level fall along this coast resulting in rapid diagenetic hardening and infiltration of goethite into the emerged foreshore sand at karikovil and its neighbourhood this was followed by a rise of the sea level during the last few centuries

multiple ophiolite generation preserved in the northern philippines and the growth of an island arc complex abstract although all oceanic arcs grow through the addition of subduction generated magmas the geology of the northern philippines demonstrates that a major contribution to arc crustal growth can come from repeated episodic intra arc back arc and or fore arc oceanic crust generation with subsequent preservation of the basic ultrabasic units in the arc complex at least five episodes of oceanic crust generation are represented in the northern philippines by preserved ophiolitic sequences and recent intra arc seafloor spreading each episode is distinct in age as confirmed by modern dating techniques with the ages ranging from pre jurassic to quaternary although the philippines is widely regarded as an amalgamation of allochthonous terranes a review of the available data shows that there is currently no compelling evidence that these ophiolites are of exotic origin and that they have been tectonically accreted to the philippine arc complex rather the evidence suggests that most and possibly all of the ophiolites were generated as back arc fore arc or intra arc crust within the philippine arc complex hence there is a close spatial association of several ophiolitic terranes of diverse ages spanning 150 myr that formed as part of the arc complex such an association may have arisen from episodic generation of oceanic crust during periods of local extension in a suprasubduction zone setting which has experienced changing and possibly overlapping subduction from the east and west sides in the current reference frame disruption of the ophiolitic basement terranes has been and continues to be effected primarily by wrench faulting this style of arc growth has implications for the paleotectonic interpretation of ancient ophiolite arc terranes in continents and the petrologic evolution of island arcs

cyanobacteria as potential biomarkers of hydrological changes in the late quaternary sediments of south kerala sedimentary basin india abstract cyanobacteria tolerate extreme hydrological regimes and their mucilaginous filaments account for binding and preservation of delicate structures as the hydrological set up is controlled by rainfall they serve as biomarkers to distinguish palaeoenvironmental scenario and ecological shifts ranging from late pleistocene to early holocene 40 ka 2 5 ka in the studied boreholes of the south kerala basin ayiramthengu panavally and west kallada the accrued cyanobacteria microflora is unique as it has different morphotypes of rivularia sp and gloeotrichia sp at varying levels in the sediment profiles probably reflecting the signatures of hydrological modifications associated with the development of wetland system the response of cyanobacteria is similar in panavally and ayiramthengu from early to late holocene with short signals of pauses probably relating to aberrations in precipitation however the late pleistocene and late holocene periods show a poor record of cyanobacteria reflecting decline in rainfall and prevalence of long spell of dry phase from early holocene to middle holocene the turnover of cyanobacteria has been prolific which may be due to the holocene optimum rainfall relatively larger size and better carbonised and smaller size and lighter coloured morphotypes are correlated with such hydrological modifications as a result of high and low rainfall respectively during the early holocene and late holocene as the cyanobacteria have the potential for adaptation and stabilization of numerous specialized eco and morphotypes in time and space they provided additional palaeodata as good biomarkers of climatic change

lateral growth of the continental crust through deep level subduction accretion a re evaluation of central greek rhodope abstract the rhodope massif of northern greece and southern bulgaria has been variably interpreted as a fragment of pre alpine possibly hercynian or precambrian age continental crust that was actively involved in european margin convergent tectonics during the mesozoic and tertiary alternatively we propose that for central greek rhodope the lithological associations and tectonothermal history are consistent with progressive south directed mesozoic crustal growth through deep level subduction accretion a process which involved the near continuous transferral of material of oceanic and continental margin affinity from footwall to hanging wall within a north directed palaeotethyan subduction system central greek rhodope comprises a sequence of schists gneisses and migmatites of wide compositional variation that have undergone intense tectonic intermixing extensive occurrences of marble are associated with amphibolites variably retrogressed eclogites meta gabbros and ultramafic rocks quartzo feldspathic gneisses are abundant throughout the area and are associated with psammitic meta greywacke sequences pelites and quartzites the presence of intercalated marble slivers and thin manganiferous and iron rich siliceous bands is significant the various components exposed in the subduction accretion complex were first underthrust northwards to deep crustal levels subjected to high pressure eclogite facies metamorphism and underplated to the base of the overriding eurasian plate this latter accretion event was characterised by intense southwest directed ductile thrusting and pervasive amphibolite facies metamorphic overprinting it was accompanied by migmatisation and intracrustal melt generation and followed by subduction related magmatism deep level subduction accretion leads to southward growth of the continental margin which in turn leads to subduction retreat allowing the encroachment of the magmatic arc from the north subsequent extensional tectonic exhumation brought the complex up to its present position within the southern margin of eurasia this new interpretation may have implications for other high grade terrains within convergent and collisional systems that have been assumed to represent ancient or pre existing continental crust if so then the importance of deep level subduction accretion as a mechanism of continental crustal growth has hitherto been underestimated