imaging subduction from the trench to 300 km depth beneath the central north island new zealand with vp and vp vs summary recent dense deployments of portable digital seismographs have provided excellent control on earthquakes beneath the central north island of new zealand here we use a subset of the best recorded earthquakes in an inversion for the 3 d vp and vp vs structure the data set includes 39 123 p observations and 18 331 s observations from 1239 earthquakes and nine explosions the subducted plate is imaged as a high vp low vp vs feature vp within the mantle of the subducted slab is almost always 8 5 km s 1 which requires the ca 120 myr slab to be unusually cold the low vp vs within the subducted plate closely parallels the lower plane of the dipping seismic zone it most likely indicates fluid resulting from dehydration of serpentine in the slab mantle and the earthquakes themselves are likely to be promoted by dehydration embrittlement we identify a region with vp 8 0 km s 1 directly above the dipping seismic zone can be interpreted as sinking entrained with the motion of the subducted slab and forming a viscous blanket that insulates the slab from the high temperature mantle wedge material in the overlying low vp region can be interpreted as rising within a return flow within the wedge the volcanic front appears to be controlled by where this dipping low vp region meets the base of the crust the thickness of the backarc crust also shows significant variation along strike in the central taupo volcanic zone tvz the crust is ca 35 km thick while southwest of mt ruapehu the crust thickens by ca 10 km there is no significant low vp zone in the mantle wedge in this southwestern region suggesting that this thicker crust has choked off mantle return flow the seismic tomography results when combined with constraints on mantle flow from previous shear wave splitting results provide a plausible model for both the distribution of volcanism in the central north island and the exceptional magmatic productivity of the central tvz

the calcarenite di gravina formation in matera southern italy new insights for coarse grained large scale cross bedded bodies encased in offshore deposits excellent outcrops of the upper pliocene lower pleistocene calcarenite di gravina around matera southern italy provide continuous exposure of coarse grained clastic basin margin shoreline to offshore facies among these facies the most conspicuous and volumetrically important are the transition slope deposits that form large scale high angle cross bedded lithosomes these are laterally extensive parallel with the paleoshoreline and show seaward progradation we interpret them to represent avalanches of sediment swept out onto a depositional slope below wave base from the shoreface zone by storm waves and wind driven currents three types of building blocks are recognized based on bedding patterns and facies architecture embryonic parasequences mature parasequences and simple sequences parasequences formed during stillstands of sea level and simple sequences during high frequency cycles of relative change of sea level these building blocks are stacked in a backstepping configuration and onlap onto cretaceous limestone substrate backstepping is believed to be due to a tectonically forced transgression that is punctuated by high frequency cycles of sea level modern analogs for these building blocks are the holocene prograding prisms detected in high resolution seismic lines of the mediterranean shelves the reflection patterns of these seismic units resemble the bedding architecture of the matera simple sequences and parasequences and show similarities of shape size position and orientation of coastal setting and direction of progradation comparison of the holocene prisms and the pliocene pleistocene accretional units in matera indicates that they may represent the same begin page 662 genetic process progradation of clastic prisms below the wave base level the matera accretional units also show similarities with other examples of laterally extensive large scale cross bedded sand bodies encased in offshore deposits such as some sandbanks differentiating between these laterally extensive and seaward prograding cross bedded sand bodies and some coarse grained gilbert type deltas is difficult however if interpretations are based only on two dimensional 2 d outcrops in dip section without high resolution data it is also difficult to distinguish between transition slope and prograding shoreface lithosomes this difficulty may be acute where only seismic or well log data are available however certain other architectural characteristics such as stacking and preservation of facies belts and position of bounding surfaces as well as differences in associated sedimentary structures and fossil content may be used for interpretation the matera example provides a mechanism for emplacement in offshore settings of elongate and strandline parallel sand and gravel deposits that prograde seaward and that preserve a coarsening upward internal succession this article offers an interpretation for other ancient examples of large scale cross bedded lithosomes encased in offshore deposits

the peopling of eastern beringia and its archaeological complexities debates over meaningful archaeological units typologies or technocomplexes have a lengthy history in archaeology and the issue is particularly convoluted in eastern beringia categorizing the early prehistoric tool industries of the pleistocene holocene transition is pertinent to understanding the colonization of eastern beringia and ultimately the americas yet alaska archaeologists continue to disagree on a unified culture history the primary point of contention surrounds the presence or absence of microblade technology in central alaska and the meaning of the nenana and denali complexes while some interpret the former as a unique manifestation representing a separate migratory population others disagree and the denali complex has become a catchall category for a variety of artifact types leading to questions over its conceptual validity this assessment tests specific questions pertinent to the relationship between prehistoric tool use and ecotones in an attempt to explain the presence or absence of particular artifact types and land use strategies through time it also reflects on issues caused by repeated occupations palimpsest assemblages and other taphomonic processes that influence archaeological consensus results indicate separate phases in the initial colonization of alaska and form a testable hypothesis based on functional land use properties associated with the denali complex era in a way that distinguishes it from other early technologies in the region the approach contributes to a longstanding discussion over subjectivity among archaeological categories and explains assemblage variability in eastern beringia with widespread implications for interpreting the initial peopling of the americas

fore arc deformation and underplating at the northern hikurangi margin new zealand geophysical investigations of the northern hikurangi subduction zone northeast of new zealand image fore arc and surrounding upper lithospheric structures a seismic velocity vp field is determined from seismic wide angle data and our structural interpretation is supported by multichannel seismic reflection stratigraphy and gravity and magnetic modeling we found that the subducting hikurangi plateau carries about 2 km of sediments above a 2 km mixed layer of volcaniclastics limestone and chert the upper plateau crust is characterized by vp 4 9 6 7 km s overlying the lower crust with vp 7 1 km s gravity modeling yields a plateau thickness around 10 km the reactivated raukumara fore arc basin is 10 km deep deposited on 5 10 km thick australian crust the fore arc mantle of vp 8 km s appears unaffected by subduction hydration processes the east cape ridge fore arc high is underlain by a 3 5 km deep strongly magnetic 3 3 a m high velocity zone interpreted as part of the onshore matakaoa volcanic allochthon and or uplifted raukumara basin basement of probable oceanic crustal origin beneath the trench slope we interpret low seismic velocity high attenuation low density fore arc material as accreted and recycled suggesting that underplating and uplift destabilizes east cape ridge triggering two sided mass wasting mass balance calculations indicate that the proposed accreted and recycled material represents 25 100 of all incoming sediment and any remainder could be accounted for through erosion of older accreted material into surrounding basins we suggest that continental mass flux into the mantle at subduction zones may be significantly overestimated because crustal underplating beneath fore arc highs have not properly been accounted for

geometry of the hikurangi subduction thrust and upper plate north island new zealand 1 we use 2800 line km of seismic reflection data to map the offshore character and geometry of the hikurangi subduction thrust and outer forearc wedge to depths of 15 km for 200 km along strike south of hawke bay the subduction thrust is relatively smooth dips less than 8 and the wedge is characterized by accretion of young sediment and topographic slopes of less than 3 in hawke bay and north for 200 km a kink in the subduction thrust is apparent with a downdip increase in dip to angles greater than 8 at depths of 10 15 km there is a corresponding steepening of the topographic slope to greater than 3 outboard of the kink and the wedge is characterized by lithified sedimentary rock and slope failure the kink in the subduction thrust is a locus of inherent weakness in the subducting slab we suggest its occurrence relates to a northward increase in subduction rate that controls initial slab dehydration and fluid release rates and hence intraslab deformation patterns the subduction thrust geometry in combination with a northward increase in subducting plate roughness and decrease in the amount of sediment accreted causes the observed spatial change in character of the subduction thrust and forearc wedge

depositional facies and stratal geometry of an upper carboniferous prograding and aggrading high relief carbonate platform cantabrian mountains n spain seismic scale continuous exposures of an upper carboniferous bashkirian moscovian carbonate platform n spain provide detailed information about the lithofacies and stratal geometries quantified with differential global positioning system measurements of microbial boundstone dominated steep prograding and aggrading platform margins progradational and aggradational platform to slope transects are characterized by distinct lithological features and stratal patterns that can be applied to the understanding of geometrically comparable high relief depositional systems the bashkirian is characterized by rapid progradation at rates of 415 970 m my 1 characteristic outerplatform facies are high energy grainstones with coated intraclasts ooids and pisoids moderate energy algal skeletal grainstones to packstones and lower energy algal packstone and boundstone units the moscovian aggradational phase is characterized by aggradation rates of 108 m my 1 coated grain shoals are less common whereas crinoidal bars nucleated in well circulated settings below wave base boundstones form a belt 30 300 m wide at the platform break and interfinger inwards with massive algal skeletal wackestones mud rich banks the progradational phase has divergent outer platform strata with basinward dips of 12 to 2 steep clinoforms with dips of 20 28 are 650 750 m in relief and possibly sigmoidal to concave in the lower part the basinward dipping outer platform strata might be depositional for less than 6 consistent with lithofacies deepening seaward the basinward dip is attributed to the downward shift of upperslope boundstone forced by late highstand and relative sea level fall and to compaction induced differential subsidence during progradation the aggradational phase is characterized by horizontally layered platform strata clinoforms steepen to 30 45 reaching heights of 850 m and are planar to concave the evolution from progradation to aggradation at the bashkirian moscovian boundary is attributed to increased foreland basin subsidence and decreased boundstone accumulation rates progradation was primarily controlled by boundstone growth rather than by highstand shedding from the platform top within the major phases aggradational progradational increments are produced by third to fourth order relative sea level

quantitative compositional analysis of a triassic carbonate platform southern alps italy abstract we have estimated abundance and distribution of automicrite marine cements and skeletal grains in the triassic sella massif an isolated platform flanked by steep 25 35 clinoforms 108 samples were taken at constant intervals from measured sections of the major zones of the platform edifice the platform top margin upper slope and lower slope in a first step carried out in the field and on hand specimen purely detrital deposits were separated from automicrite facies i e beds with automicrite cement filled primary vugs and admixtures of skeletal carbonate and lithoclasts in the second step samples with automicrite facies were thin sectioned and point counted the categories used for point counting were a automicrite b vugs and cement c microspar or neomorphic spar d skeletal grains and e internal sediments at the platform top 46 of samples are pure detrital deposits 27 consist of automicrite facies and 27 are too strongly altered by dolomitization to allow classification at the margin upper slope 68 of samples consist of automicrite facies 22 are pure detrital sediments and 10 are strongly altered at the lower slope 63 are detrital deposits 10 automicrite facies and 27 are extensively dolomitized the most important contributors to the automicrite facies are automicrite 41 on the platform top 29 on the margin upper slope 28 on the lower slope and early marine cement 35 on the platform top 48 on the margin upper slope 27 on the lower slope the amount of skeletal grains is less than 10 the automicrite facies stabilized the platform margin and upper slope automicrite abundant early marine cements and micro organisms such as tubiphytes formed a rigid framework thus substituting for the lack of a metazoan reef on the upper slopes the framework of automicrite facies stabilized the slope but intermittently the automicrite layers are frequently dissected by sediment filled fractures or are broken into clasts we assume that they slid on the layers of loose detritus bigger slides turned into rubbly debris flows that formed metre thick breccias at the lower slope and the proximal basin floor the planar shape and steep angle of the clinoforms indicate that the large scale geometry of the slope was not controlled by the automicrite but rather by non cohesive layers of sand and rubble piled up to the angle of repose the production mode of the sella is comparable of that of a mud mound factory this factory was highly productive in 1 ma the platform aggraded over 300 m and prograded over 2000 m in all directions

progradation geometries of carbonate platforms examples from the triassic of the dolomites northern italy various types of progradation of triassic carbonate platforms are described from the dolomites of the southern alps the internal and external geometric relationships are exposed in spectacular natural sections and moreover their scale 500 1000 m of thickness is such that they can be compared with features found in seismic profiles the different types of progradation are controlled by a number of factors which normally interact with each other these factors include rate of basinal sedimentation rate of subsidence width of the platform depth of the surrounding basin and eustatic variations of sea level progradation is not a continuous process but episodic moments of massive debris input during which the platform advances alternate with long periods of negligible progradation during which basinal sediments accrete and onlap the toe of slope upper boundary relationships of the prograding platforms include offlap toplap and erosional truncation lower boundary relationships are horizontal climbing and descending progradations a variety of phenomena and circumstances have caused the cessation of progradation of the triassic platforms they include volcanism collapse of margins drowning rapid relative rise of sea level subaerial exposure relative fall of sea level and probably a natural decay of the system in the triassic of the dolomites two main progradation models can be put forward in the ladinian model progradation took place simultaneously with aggradation relative rise of sea level whereas the characteristic feature of the carnian model is toplap relative stillstand of sea level

steep microbial boundstone dominated plaform margins abstract seaward progradation of several kilometers has been documented mostly for leeward margin low angle carbonate slope systems with a dominant platform top sediment source however steep and high relief margins fronting deep basins can also prograde and as such are somewhat perplexing characteristics of two prograding carboniferous examples provide a model which may apply elsewhere 1 outcrops in asturias northern spain serve as important analogs for 2 hydrocarbon reservoirs in steep sided isolated platforms of the north caspian basin kazakhstan such as tengiz seismic and well data from tengiz corroborate outcrop patterns for slope development showing progradation of up to 5 and more than 10 km respectively despite the high relief up to 600 m and steep 20 35 nature of these margins the two examples share a highly productive microbial boundstone factory extending from the platform break down the slope to nearly 300 m or more depth and a lower slope dominated by mega breccias and grain flow deposits derived from the margin and slope itself the broad depth range of microbial boundstone increases the potential for production during both lowstands and high stands of sea level and thereby facilitates progradation independent from platform top derived sediment rapid in situ lithification of the boundstone provides stability to the steep slopes but also leads to readjustment through shearing and avalanching what controls the microbial cement boundstone formation remains a debate but its presence is a key factor controlling the progradational geometry of these and possibly other margins this new model of slope shedding has implications for slope readjustment processes and resulting architecture sequence stratigraphic interpretation reservoir characterization and reservoir modeling especially the isotropic character of microbial boundstone will reduce the potential for coherent seismic reflections to develop and possibly invoke under certain stress regimes shattering and fracturing thereby generating significant non matrix permeability key considerations are the contrasts with the bahamian high stand shedding depositional model slope progradation rates that range from 450 to 1500 m my and net growth rates of in situ boundstone of 1000 m my comparable to or higher than accretion rates for metazoan skeletal reef growth

facies analysis and palaeoenvironmental interpretation of the late oligocene attard member lower coralline limestone formation malta the oligocene represents a key interval during which coralline algae became dominant on carbonate ramps and luxuriant coral reefs emerged on a global scale so far few studies have considered the impact that these early reefs had on ramp development consequently this study aimed at presenting a high resolution analysis of the attard member of the lower coralline limestone formation late oligocene malta in order to decipher the internal and external factors controlling the architecture of a typical late oligocene platform excellent exposures of the lower coralline limestone formation occurring along continuous outcrops adjacent to the victoria lines fault reveal in detail the three dimensional distribution of the reef associated facies a total of four sedimentary facies have been recognized and are grouped into two depositional environments that correspond to the inner and middle carbonate ramp the inner ramp was characterized by a very high energy shallow water setting influenced by tide and wave processes this setting passed downslope into an inner ramp depositional environment which was colonized by seagrass and interfingered with adjacent areas containing scattered corals the middle ramp lithofacies were deposited in the oligophotic zone the sediments being generated from combined in situ production and sediments swept from the shallower inner ramp by currents compositional characteristics and facies distributions of the attard ramp are more similar to the miocene ramps than to those of the eocene an important factor controlling this similarity may be the expansion of the seagrass colonization within the euphotic zone this expansion may have commenced in the late oligocene and was associated with a concomitant reduction in the aerial extent of the larger benthonic foraminifera facies stacking pattern analysis shows that the depositional units parasequences at the study section are arranged into transgressive regressive facies cycles this cyclicity is superimposed on the overall regressive phase recorded by the attard succession furthermore a minor highstand correlated with the ru4 ch1 sequence and subsequent minor lowstand ch2 sequence have been recognized the biota assemblages of the attard member suggest that carbonate sedimentation took place in subtropical waters and oligotrophic to slightly mesotrophic conditions the apparent low capacity of corals to form wave resistant reef structures is considered to have been a significant factor affecting substrate stability at this time the resulting lack of resistant mid ramp reef frameworks left this zone exposed to wave and storm activity thereby encouraging the widespread development of coralline algal associations dominated by rhodoliths

glacial versus interglacial sedimentation rates and turbidite frequency in the bahamas the southern tongue of the ocean is a 1300 m deep flat floored basin in the bahamas that receives large amounts of sediment from the carbonate platforms surrounding it on three sides we have examined five 8 13 m long piston cores and determined bulk sedimentation rates turbidite frequency and turbidite accumulation rates for the past two glacial and interglacial periods the mean of bulk sedimentation rates is four to six times higher in interglacial periods average accumulation rates of recognizable turbidites are higher by a factor of 21 to 45 and interglacial turbidite frequency is higher by a factor of 6 to 14 sediment composition indicates that increased interglacial rates are due to higher accumulation of platform derived material additional data from other bahamian basins as well as published material from the caribbean strongly suggest that highstand shedding is a general trend in pure carbonate depositional systems carbonate platforms without a siliciclastic component export more material during highstands of sea level when the platform tops are flooded and produce sediment the response of carbonate platforms to quaternary sea level cycles is opposed to that of siliciclastic ocean margins where sediment is stored on the inner shelf during highstands and passed on to continental rises and abyssal plains during lowstands of sea level

seismic reflection imaging of ultradeep roots beneath the eastern aleutian island arc seismic reflection data show that the eastern aleutian arc is characterized by reflectors that extend continuously from the lower arc crust to 50 km depth which is considerably deeper than the crustal thickness of 27 35 km previously inferred from coincident wide angle seismic surveys because the upper mantle is commonly homogeneous and therefore nonreflective relative to the overlying crust we interpret these reflectors to be gabbro garnet gabbro and pyroxenite intrusions within two 50 km wide roots that represent a 25 km thick heterogeneous transition from mafic lower crustal rocks to ultramafic mantle rocks we suggest that the reflectivity is linked to repeated differentiation and intrusion of mantle derived melts into the subarc lithosphere and that the depth of these roots shows that fractionation of arc crust can extend well below the seismically determined moho because these deep roots are not evident beneath the central aleutian arc either the roots form sporadically perhaps as a consequence of an elevated magmatic supply or such roots ultimately founder into the underlying mantle due to their relatively high mass density

crustal seismic reflection measurements across the northern extension of the taupo volcanic zone north island new zealand abstract a crustal seismic reflection profile across the offshore taupo volcanic zone otvz has been reprocessed to improve the lower crust upper mantle image the otvz is underlain by several strong sub horizontal crustal reflections that may correspond to shear zones or mafic sills a reflector interpreted to correspond to moho lies at a depth of about 8 8 5 s twt seconds two way traveltime about 20 km under the western margin of the raukumara peninsula and rises to a depth of about 6 s twt about 16 km under the active eastern part of the otvz and can be traced at about this depth across most of the otvz underlying this inferred moho are two diffuse zones of discontinuous reflectivity one underlying the active eastern otvz and a less well defined one underlying the landward extension of the havre trough they extend from about 15 to 35 km depth within the mantle wedge and are inferred to be imaging a zone of depleted upper mantle associated with crustal extension a localised zone of deep 30 50 km reflectivity and seismicity occurs under the eastern otvz margin and may relate to fluid flow from the hikurangi plateau being subducted further east

three dimensional velocity structure of the northern hikurangi margin raukumara new zealand implications for the growth of continental crust by subduction erosion and tectonic underplating traveltimes between shots from nine marine seismic reflection lines and nine onshore recorders were used to construct a 3 d p wave velocity model of the northern hikurangi subduction margin new zealand from north to south between raukumara basin and raukumara peninsula the moho of the overriding plate increases in depth from 17 to similar to 35 km low seismic p wave velocities of 3 5 5 0 km s are localized within a similar to 10 km thick prism in the lower crust of the overriding plate immediately updip of the intersection between the subduction thrust and moho and beneath the topographic crest of east cape ridge and the raukumara range southward this region of low seismic velocities and surface uplift increases in distance from the trench as the thickness of the crust in the overriding plate increases we interpret this low velocity volume to be underplated sedimentary rocks and crustal materials that were tectonically eroded by subduction beneath the trench slope the buoyancy and low strength of these subducted materials are proposed to assist the escape from a subduction channel near the base of the crust and drive local rock uplift in the middle crust our observations of very low velocity suggest high fluid filled porosities of 12 18 and the implied buoyancy anomaly may enhance underplating at greater depths the process is driven by the contrast between upper crustal quartz feldspar mineralogy and the denser diabase or olivine rich lithologies of the lower crust and mantle we estimate a rate of lower crustal underplating at the northern hikurangi margin of 20 7 km 3 ma 1 km 1 since 22 ma we suggest that underplating provides an efficient means of accreting subducted sediment and tectonically eroded material to the lower crust and that the flux of forearc crustal rocks into the mantle at subduction zones may be systematically overestimated

imaging the hikurangi subduction zone new zealand using teleseismic receiver functions crustal fluids above the forearc mantle wedge summary we image the hikurangi subduction zone using receiver functions derived from teleseismic earthquakes migrated receiver functions show a northwest dipping low shear wave feature down to 60 km depth which we associate with the crust of the subducted pacific plate receiver functions rf at several stations also show a pair of negative and positive polarity phases with associated conversion depths of 20 26 km where the subducted pacific plate is at a depth of 40 50 km beneath the overlying australian plate rf inversion solutions model these phases with a thin low s wave velocity zone less than 4 km thick and an s wave velocity contrast of more than 0 5 km s 1 with the overlying crust we interpret this phase pair as representing fluids near the base of the lower crust of the australian plate directly overlying the forearc mantle wedge

controls of facies and sediment composition on the diagenetic pathway of shallow water heterozoan carbonates the oligocene of the maltese islands relatively few studies have so far addressed diagenetic processes in heterozoan carbonates and the role that sediment composition and depositional facies exert over diagenetic pathways this paper presents a study of oligocene shallow water heterozoan carbonates from the maltese islands we investigate stratigraphic distribution abundance and timing of diagenetic features and their relationship to sediment composition and depositional facies the studied carbonate rocks comprise rud to packstones of the heterozoan association predominantly containing coralline red algae bryozoans echinoids and benthic foraminifers xrd analyses show that all high mg calcite has been transformed to low mg calcite and that no aragonite is preserved diagenetic processes include dissolution of aragonitic biota neomorphism of high mg calcitic biota to low mg calcite and cementation by fibrous bladed epitaxial and blocky cements stable isotopes on bulk rock integrated with petrographic data suggest that the study interval was not exposed to significant meteoric diagenesis we interpret early cementation to have taken place in the marine and marine burial environment the distribution and abundance of early diagenetic features determining the diagenetic pathway can be related to the primary sediment composition and depositional texture sorting and micrite content are important controls over the abundance of diagenetic features

transgressive regressive cycles and jurassic palaeogeography of northeast iberia abstract a correlation and sequence stratigraphy study of jurassic successions has been carried out in the main sedimentary basins of northeast iberia i e asturias basque cantabrian and iberian basins based on the identification of transgressive regressive cycles the development and palaeogeographic evolution of the epicontinental carbonate platforms of northeast iberia were largely controlled by major tectonic activity at three main intervals at the beginning of the jurassic in the lower middle jurassic transition and during the uppermost jurassic respectively during early jurassic times northeast iberia was the site of a single and large carbonate ramp opened to the north this carbonate ramp suffered a progressive drowning evolving from an inner to hemipelagic ramp systems with the local development of suboxic environments in the deepest areas located to the north during the middle jurassic different open carbonate platforms were formed including the development of swells in intermediate areas during the upper jurassic the outer ramp areas were progressively moving to the east iberian basin and the ammonite faunas showing a markedly tethyan affinity thereafter three first order t r cycles bounded by major discontinuities associated with significant time gaps are identified they extend respectively from the latest rhaetian to the early aalenian from the middle aalenian to the early oxfordian and from the early oxfordian to the late berriasian major transgressive peaks occurred at the middle toarcian bifrons zone at the late bajocian upper niortense and garantiana zones and at the mid kimmeridgian divisum zone each of the first order cycles includes four second order t r cycles cycles 1 1 1 4 are identified in the northern basins cycles 2 1 2 4 display some differences in age of transgressive peaks from one basin to another cycles 3 1 3 4 are mainly identified in the iberian basin the correlation with other separated boreal and thethysian basins demonstrates that the number and age of t r cycles varies from one basin to another and are mainly controlled by the local or regional tectonic development the transgressive peaks may reflect episodes of eustatic rise during the jurassic however their different age from one basin to another is explained by different subsidence evolution

a comparison of shear strength in calcareous and siliciclastic marine sediments abstract more than 2000 measurements of shear strength versus depth in fine grained marine sediments were collected from the literature and our own experiments in order to study the relationship between carbonate content and strength behavior of marine sediments depositional environments include slope rise and abyssal plain with most data originating from the slopes the depth range is restricted to 6 m subbottom depth in order to minimize diagenetic effects contour plots of shear strength versus depth for calcareous and siliciclastic sediments clearly reveal different trends whereas shear strength in siliciclastic sediments shows low variability and increases only gradually with depth the shear strength in the calcareous population is highly variable and increases rapidly with depth the difference in strength behavior of the two sediment groups has implications for the stability of submarine slopes previous research has shown that carbonate sediments build up steeper and more variable slopes than siliciclastic sediments this difference in slope angle can be explained at least in part by the higher internal strength of the carbonate sediments

a three dimensional image of shallow subduction crustal structure of the raukumara peninsula new zealand summary earthquake arrival time data from a 36 station deployment of portable seismographs on the raukumara peninsula have been used to determine the 3 d vp and vp vs structure of this region of shallow subduction a series of inversions have been performed starting with an inversion for 1 d structure then 2 d and finally 3 d this procedure ensures a smooth regional model in places of low resolution the subducted plate is imaged as a northwest dipping feature with vp consistently greater than 8 5 km s 1 in the uppermost mantle of the plate structure in the overlying plate changes significantly along strike in the northeast there is an extensive low velocity zone in the lower crust underlying the most rapidly rising part of the raukumara range it is bounded on its arcward side by an upwarp of high velocity a viable explanation for the low velocity zone is that it represents an accumulation of underplated subducted sediment while serpentinization of the uppermost mantle may be responsible for the adjacent high velocity region the low velocity zone decreases and the adjacent high velocity region is less extensive in the southwest this change is interpreted to be related to a change in the thickness of the crust of the overlying plate in the northeast the crust is thinner and subducted sediment ponds against relatively strong uppermost mantle while in the southwest the crust is thicker and the relatively weak lower crust allows sediment subduction to greater depths a narrow zone of high vp vs parallels the shallow part of the plate interface this suggests elevated fluid pressures with the distribution of earthquakes about this zone further suggesting that these pressures may be close to lithostatic the plate interface at 20 km depth beneath the raukumara peninsula may thus be a closed system for fluid flow similar to that

terrestrial and gps measurements of deformation across the taupo back arc and hikurangi forearc regions in new zealand geodetic measurements of deformation across the taupo back arc and hikurangi forearc regions of new zealand are derived from global positioning system gps measurements made in 1990 and 1991 and triangulation observations made in the 1920s 1950s and 1970s the gps horizontal coordinate differences have precisions of 5 6 mm and the triangulation observations have precisions of 0 7 1 2 arc sec these different kinds of observation allow simultaneous estimation of strain parameters and single epoch coordinates for a total of 184 stations at 68 of which gps observations were made under the assumption that no shift of the scale or orientation of the gps reference frame is embodied in the data from 13 gps stations within the taupo volcanic zone tvz common to the 1990 and 1991 surveys the rotational dilatational and shear components of the deformation rate tensor within the tvz can be estimated for that 1 year interval the principal extension rate from 1990 to 1991 is 0 21 0 09 10 6 yr 68 confidence at an azimuth of 124 13 corresponding to 8 4 mm yr extension over 40 km neither the orthogonal principal extension rate 0 02 0 03 10 6 yr nor the rotation rate 0 06 0 04 rad yr is significant the dilatation rate of 0 23 0 09 10 6 yr is therefore produced by uniaxial extension the data from 75 stations distributed from behind the back arc region and well into the forearc region which were surveyed in at least two of the triangulation or gps epochs between the 1920s and 1991 allow the spatial variation of the shear components of the deformation rate tensor to be estimated the maximum engineering shear rates lie in the range 0 1 0 2 10 6 yr within the tvz and maintain these values with similar orientations in the forearc region to the east the western and southern margins of active extension are reasonably well determined by changes in orientation and magnitude of the shear component these results provide confirmation of the previously less well determined deformation field and in comparison with subduction models indicate that the surface deformation is reflecting variation from north to south of coupling of the subduction plate interface and that both trench suction and gravitational collapse probably contribute to the extension in the forearc region

crustal heterogeneity and subduction processes 3 d vp vp vs and q in the southern north island new zealand summary along the southern hikurangi subduction zone in the southern north island new zealand the subduction zone changes from volcanic with moderately weak interseismic coupling to non volcanic with strong coupling in wellington to investigate how material heterogeneity relates to subduction processes we compute 3 d seismic properties using 342 earthquakes and 2 shots recorded by a 165 station seismic array the time base for some telemetered temporary stations was not stable which required revising the inversion code to allow an additional origin time related to the telemetered stations progressive inversions from 2 d to fine 3 d were carried out for vp vp vs and qp combined interpretation of these three seismic properties highlights features of the subduction zone the subducted slab is imaged as a high velocity high q feature in the overlying crust from southeast to northwest there is low velocity in the accretionary wedge high velocity in the axial ranges and low velocity in the wanganui basin high vp vs and low qp characterize the crust east of the north island dextral fault belt high vp vs is imaged above the subducted slab to 40 km depth in the northern half of the study area low qp is also prominent above the slab particularly trenchwards of the volcanic region in marked contrast to the high vp vs the low qp above the slab extends as a continuous feature up to the low qp in the shallow crust underlying the volcanic region the low qp and high vp vs are correlated with the region of weaker coupling and are consistent with significant subducted sediments with associated fluid release vp vs is particularly sensitive to overpressured fluid in the compressional forearc whereas qp also images the hydrostatically pressured fluid transport in the more permeable extensional volcanic region the rheology of the lower crust of the overlying plate is a factor in large scale subduction coupling as evidenced by the terrane boundaries and haast schist in the section near wellington the changing rheology of the lower crust of the overlying plate in the southern north island also provides a possible explanation for the southward cessation of volcanism

compositional variations during phases of progradation and retrogradation of a triassic carbonate platform picco di vallandro durrenstein dolomites italy in this paper data are presented on the composition of sediments deposited at the toe of slope during progradation or retreat of triassic carbonate platforms in the dolomites italy for this purpose a succession was studied from the toe of slope of a triassic carnian carbonate platform picco di vallendro durrenstein dolomites italy the microfacies analysis of selected calciturbidites sequences revealed a reduced input of oncolites and ooids during progradation and an increase in clasts the main input however was derived from the reefs on the platform retrogradation of the platform showed an increase of filaments and radiolarians open ocean biota as well as carbonate mud and a reduced input of grains that originated within the reefs on the platform both during progradation and retrogradation parts of the platform were flooded and could produce excess sediment that could be exported to the surrounding basins however the absence of platform interior biota documents that progradation occurred from sediments of the reefal belt probably during relative sea level lowstands carbonate composition varies systematically with toe of slope progradation retrogradation and thus argues for carbonate production as the main driver of the geometries observed at the toe of slope

plate interface properties in the northeast hikurangi subduction zone new zealand from converted seismic waves arrival time inversion of local earthquakes in the northeast hikurangi subduction zone has indicated high vp vs at the plate interface but the thickness of the anomalous zone is poorly constrained by the p and s travel times we investigate this plate interface zone further by modeling s to p converted phases using synthetic seismograms for upgoing rays of earthquakes in the subducting plate mantle the modeling indicates a low velocity layer at the plate interface which is generally 1 2 km thick and has vp of 5 0 5 35 km s and vp vs of 2 this is consistent with a subducting sediment channel with near lithostatic fluid the low rigidity indicates a weak zone which will have a strong influence on the distribution of deformation the modeling approach shows promise in defining rheological parameters at the plate interface and mapping their variation across the seismogenic zone of the subduction thrust

clinoform composition and margin geometries of a lower cretaceous carbonate platform vercors se france the concept of sequence stratigraphy uses the stratal geometries at the margins of carbonate platforms to define depositional sequences and systems tracts the aim of this study was to research if prograding purely aggrading and retrograding phases of a cretaceous carbonate platform showed differences in the composition and facies type of the slope sediments the vercors plateau in se france provides excellent outcrops to study this relationship continuous exposure of the platform to basin transition allowed direct examination of the margin geometries five successions were measured and sampled in great detail samples were thin sectioned and point counted using point count groups characterizing palaeoenvironments along the platform to basin transect the composition logs as well as the numerical analysis of the point count data both show a clear relationship between grain composition and stratal geometry the prograding and purely aggrading intervals are similar in composition as they are both relatively coarse grained and enriched in platform biota and limeclasts retreating units are relatively fine grained and rich in basinal grains small benthic foraminifers sponge spicules non carbonate grains and embedding material these observations suggest that the retreating intervals represent incipient drownings of the platform however rather than distinctly separated groups the different phases of platform development form a continuous range of variation between high platform input and high basinal input compared to the prograding units the purely aggrading intervals are relatively rich in peloids which may be suggestive of relatively low energy conditions on the platform during aggradation the compositional analysis also revealed significant variation in the frequency of ooids but these variations showed no relationship with the progradation aggradation or retreat of the platform

reef geometries erosion surfaces and high frequency sea level changes upper miocene reef complex mallorca spain the upper miocene reef complex of mallorca is a 20 km prograding unit which crops out in sea cliffs along the southern side of the island these vertical and exceptionally clean outcrops permit i identification of different facies lagoon reef front reef slope and open platform and their geometries and boundaries at different scales ranging from metre to kilometre and ii construction of a 6 km long high resolution cross section in the direction of reef progradation this cross section shows vertical shifts of the reefal facies and erosion surfaces linked to a general progradational pattern that defines the accretional units four hierarchical orders of magnitude 1 m to 4 m of accretional units are identified by consideration of the vertical facies shifts and by which erosion surfaces are truncated by other erosion surfaces all these orders show similar patterns horizontal beds of lagoonal facies in the upper part landward reefal and slope facies with sigmoidal bedding in the central part and open platform facies with subhorizontal bedding in the lower part basinwards the boundaries are erosion surfaces horizontal over the lagoon facies dipping basinwards over the reef front facies and connecting basinwards with their correlative conformities over the reef slope and open platform facies the four orders of accretional units are interpreted in terms of four 1 m to 4 m hierarchies of sea level cycles because i there is a close relation between the coral growth and the sea surface ii there are vertical shifts in the reefal facies and their relation to the erosion surfaces and iii there was very little tectonic subsidence in the study area during the late miocene additionally all these units can be described in terms of their position relative to the sea level cycle i the reefs prograde on the open platform sediments during low stands of sea level ii aggradation of the lagoon reef and open platform facies dominates during sea level rises and the lagoonal beds onlap landwards upon the previous erosion surface iii reefal progradation occurs during high stands of sea level and iv the 2 m sea level fall produces an off lapping reef and there is progradation with downward shifts of the reefal facies and erosion landward on the emerged older reefal units a erosion surfaces the 3 m and 4 m sea level falls produce only erosion b and c erosion surfaces although precise age data do not exist at present some speculations upon the frequency of these miocene relative sea level cycles can be made by comparisons with pleistocene cyclicity there is a good correlation between the miocene 2 m cycles and the 100 ka pleistocene cycles consequently the 1 m cycles can be assigned to a fourth order in relation to previously proposed global cycles and the 2 m to fifth order cycles all these accretional units could be defined as sequences according to the definition as commonly used in sequence stratigraphy however they represent higher than third order sea level cycles but are not parasequences the term subsequence therefore is suggested to define a part of a sequence bounded by erosion surfaces mostly subaerial and their correlative conformities basinwards a hierarchy of subsequences can be established

shear wave velocity variation across the taupo volcanic zone new zealand from receiver function inversion summary the taupo volcanic zone tvz new zealand is a region characterized by very high magma eruption rates and extremely high heat flow which is manifest in high temperature geothermal waters the shear wave velocity structure across the region is inferred using non linear inversion of receiver functions which were derived from teleseismic earthquake data results from the non linear inversion and from forward synthetic modelling indicate low s velocities at 6 16 km depth near the rotorua and reporoa calderas we infer these low velocity layers to represent the presence of high level bodies of partial melt associated with the volcanism receiver functions at other stations are complicated by reverberations associated with near surface sedimentary layers the receiver function data also indicate that the moho lies between 25 and 30 km deeper than the 15 2 km depth previously inferred for the crust mantle boundary beneath the tvz

three dimensional attenuation structure of the hikurangi subduction zone in the central north island new zealand summary we use waveform data from 2708 earthquakes recorded by dense seismograph deployments in the central north island of new zealand to image the 3 d attenuation structure of the hikurangi subduction zone down to ca 300 km depth attenuation images are obtained by determining the quality factor of p waves qp using a t inversion with a previously determined 3 d seismic velocity model we have included limited frequency dependence for qp with qp being frequency independent above 10 hz and having a frequency dependence of f 10 0 5 below 10 hz the qp images provide further constraint on the large scale features associated with subduction and magmatism beneath the central north island and serve to refine interpretations of crust and upper mantle structure from vp and vp vs the subducted plate is the most prominent feature in the qp images its high qp 900 1200 is consistent with the ca 120 myr old slab being relatively cold qp is better at resolving the slab than vp the coincidence of a strong gradient in qp with the upper plane of the dipping seismic zone indicates that even below 75 km depth the upper envelope of seismicity provides a good estimate of the location of the plate interface the mantle wedge is generally imaged as a relatively low qp 400 feature below 50 km depth however there are significant changes evident in the wedge along the strike of the subduction zone the most pronounced low qp in the mantle wedge occurs from ca 50 to 85 km depth beneath the productive rhyolite dominated central segment of the taupo volcanic zone suggesting a close link between volcanism and low qp in the shallow mantle wedge there is a strong correlation between low qp l owvp and high vp vs in this part of the mantle wedge suggesting that high temperature is the controlling influence on qp there within 30 km of the surface of the slab there is a large change in qp but only a modest change in vp our results are consistent with a fluid rich viscous blanket being entrained with the motion of the subducted slab down to ca 140 km depth where hydrous melting of mantle peridotite initiates trenchward of the volcanic front a strong horizontal gradient to higher qp is imaged in the mantle wedge we interpret this as a stagnant mantle nose not involved in the corner flow of the adjacent mantle the distribution of qp within both the slab and the overlying plate provides support for dehydration embrittlement being important in promoting seismicity in the mantle of the slab where qp is high at shallow depth in the forearc it is usually underlain by low qp near the plate interface this suggests that competent terranes in the forearc may act as aquicludes increasing fluid content in the rocks below and hence might control the distribution of coupling at the plate interface

double seismic zone and dehydration embrittlement of the subducting slab 1 dehydration embrittlement of metamorphosed oceanic crust and mantle in the subducting slab may be responsible for the occurrence of intermediate depth earthquakes we explore the possibility that this hypothesis can explain the morphology of the double seismic zones observed in northeast japan southwest japan northeast taiwan northern chile cape mendocino and eastern aleutians we calculate transient temperature structures of slabs based on geologically estimated subduction histories of these regions we then determine dehydration loci of metamorphosed oceanic crust and serpentinized mantle using experimentally derived phase diagrams the depth range of the dehydration loci of metamorphosed oceanic crust and serpentine is dependent on slab age the dehydration loci of serpentine produce a double layered structure because the upper dehydration loci of serpentine are mostly located in the wedge mantle above the slab we regard the upper plane seismicity representing dehydration embrittlement in the oceanic crust and we fix the slab geometry so that the upper plane seismicity is just below the upper surface of the slab we find that the lower plane seismicity is located at the lower dehydration loci of serpentine which indicates that the morphology of the double seismic zones is consistent with the dehydration embrittlement

subduction of a wedge shaped philippine sea plate beneath kanto central japan estimated from converted waves and small repeating earthquakes 1 we estimate the configuration of the philippine sea plate phs subducted beneath kanto to understand the seismotectonics in the tokyo metropolitan area in addition to small repeating earthquakes that delineate both the upper and lower boundaries of the phs previously unrecognized converted waves at its upper boundary from lower boundary repeater sources provide a new constraint on the position of the boundary the results from 794 s to p and 212 p to s converted waves together with 200 repeater locations show that 1 the phs has a wedge like shape decreasing in thickness from 50 km beneath tokyo to zero at its northeastern limit and 2 the phs is relatively flat in the eastern part of kanto and bends upward near its northeastern limit from the fact that the phs in the izu bonin forearc also has a similar wedge like shape we infer that its shape was formed before its subduction on the other hand the relatively flat phs at its northeastern limit and its upward bend are probably a consequence of ongoing deformation from an interaction with the underlying thick pacific plate pac the history of the phs as a forearc before subduction and its contact with the pac after subduction suggests that the temperature of this part of the phs is colder than other regions which probably causes anomalously deep seismicity on the phs in kanto colder phs temperatures imply that the source regions of large earthquakes may extend deeper than expected an important fact when assessing the locations of disastrous earthquakes for tokyo

antigorite high pressure stability in the system mgosio2h2o msh abstract the breakdown reactions of antigorite 1 forming talc forsterite water at low pressures and 2 forming forsterite clinoenstatite water at high pressures were determined in reversed equilibrium experiments results on reaction 1 were found to be in good agreement with former experimental determinations by both johannes johannes w 1975 zur synthese und thermischen stabilitat von antigorit fortschr mineral beih 53 36 and evans et al evans b w johannes w oterdoom h trommsdorff v 1976 stability of crysotile and antigorite in the serpentinite multisystem schweiz mineral petrogr mitt 56 79 93 from our experiments the invariant point i1 interconnecting the two reactions can be located at about 15 kbar 650 c this is consistent with the thermodynamic calculations using the dataset of berman berman r g 1988 internally consistent thermodynamic data for minerals in the system j petrol 29 445 522 however it is in contrast to recent experimental studies of ulmer and trommsdorff ulmer p trommsdorff v 1995a serpentine stability to mantle depths and subduction related magmatism science 268 858 861 who determined i1 at 21 kbar 730 c our pt conditions for i1 could be confirmed by equilibrium experiments on reaction 10 talc forsterite clinoenstatite water which is generated at i1 as well up to about 25 kbar the breakdown reaction 2 is nearly pressure independent towards still higher pressures the dp dt slope of reaction 2 bends and becomes negative schreinemakers analysis as well as thermodynamic calculations of the upper pressure stability of antigorite show that the possible antigorite breakdown reaction 3 antigorite clinoenstatite brucite water and reaction 4 brucite clinoenstatite forsterite water could originate at a new invariant point i2 provided that the reactions 2 and 11 brucite antigorite forsterite water intersect bracketing equilibrium 4 and combining these results with those on reaction 2 i2 was located at only about 51 kbar 490 c compared to 77 kbar 680 c according to berman s data however when taking into account the dense hydrous magnesium silicate dhms phase a mg7si2o8 oh 6 the phase relations of antigorite are changed resulting i in the metastability of i2 and reaction 3 and ii in a new invariant point i7 at about 44 kbar and 580 c generating the new antigorite breakdown reaction 16 antigorite phase a clinoenstatite water on the basis of these new data on the stability of antigorite earlier conclusions about dehydration depths in subducted serpentine bearing oceanic lithosphere have to be reconsidered the maximum pressure stability of antigorite according to reaction 16 extends between 44 and 55 kbar that is between about 130 and 160 km depths as opposed to about 75 kbar 220 km following ulmer and trommsdorff see above because many different thermal regimes are possible in subduction zones no specific dehydration depth can be expected but rather more continuous dehydration fronts in space and time

three dimensional attenuation model of the shallow hikurangi subduction zone in the raukumara peninsula new zealand 1 the material properties of the shallow subduction zone are imaged with data from a passive seismic deployment in the raukumara peninsula attenuation images are obtained using t values measured from 2848 spectra of p wave arrivals the t values are inverted for frequency independent three dimensional 3 d qp using a 3 d velocity model linked nodes in a preliminary 2 d inversion are used to obtain the most reliable initial model for 3 d inversions the results show the benefits of obtaining 3 d q in addition to 3 d vp and vp vs q is related to localized permeability and heterogeneity and thus 3 d q is an important tool for interpreting active tectonic regions qp in the shallow overlying plate generally corresponds to the mapped geology two high qp features 600 represent subducted seamounts or cretaceous volcanics within the overlying plate the gradient above the subducting plate is the strongest feature in the qp model with the slab having high qp 600 900 below 20 km depth and above the plate interface there is a zone of 50 decrease in qp and high vp vs about 10 km thick which represents actively subducting sediment and has distributed microearthquake activity associated with active underplating

carbonate shedding and sedimentary cyclicities of a distally steepened carbonate ramp miocene great bahama bank depositional geometries as imaged in seis mic lines logging data and quantitative petrographic data were used to analyse the slope and toe of slope deposits of the miocene distally steepened carbonate ramp of great bahama bank the shedding pattern along the slope of this ramp is more complex than it is along the slope of the pliocene pleistocene flat topped carbonate platform compositional changes and compositional trends of periplatform sediments correlate with the positions of geophysically defined sequence boundaries two types of depositional sequences occur the first sequence is characterised by laterally traceable sequence internal reflections whereas the second type contains major intrasequen tial incisions erosional incisions and sedimentary infills of these canyons by turbidites formed during sea level lowstands as is indicated by the composition of the turbidites having a mixture of shallow water and pelagic particles highstand turbidites are charac terised by more extensive laterally traceable geome tries and by the occurrence of abundant shallow water particles in contrast to highstand turbidites shed from the pliocene pleistocene flat topped platform shal low water components in highstand turbidites of the miocene ramp are of skeletal origin the stacking pat tern of the periplatform deposits is controlled by sea level fluctuations four orders of sea level changes are distinguished third order cycles are delimited by sequence boundaries and fourth 100 000 a order cycles govern the bundling of turbidites into packages fifth 40 000 a and sixth 23 000 a order cycles are recorded in the background sediments and resolvable with spectral analysis

calciturbidite composition related to exposure and flooding of a carbonate platform triassic eastern alps the compositional variation of calciturbidites pedata potschen formation deposited adjacent to the dachstein formation carbonate platform in the triassic hallstatt basin was analysed using detailed field measurements and point counting of thin sections the 35 m long section is located in the northern calcareous alps of austria six point count groups were distinguished separating basinal from platform derived input summary statistics cluster and correspondence analysis of the point count data reveals a close relationship between the biota present on the dachstein carbonate platform and the calciturbidite composition the variations in turbidite composition are attributed to fluctuations in sea level and resulting flooding and exposure of the platform which alternately created and destroyed shallow water habitats on the platform top

a steep fronted carboniferous carbonate platform clinoformal geometry and lithofacies picos de europa nw spain a steep margined carbonate platform is developed in the carboniferous synorogenic foreland basin of northern spain dips of 60 90 produced during late carboniferous thrusting enable cross sections of a 4 km wide portion of the marginal area of this platform las llacerias outcrop to be studied in aerial photographs at a seismic scale three stratal domains are observed 1 a horizontal bedded platform 2 a clinoformal bedded margin with a relief of up to 500 m and 3 a low angle toe of slope where slope beds interfinger with basin sediments the slope shows well bedded sigmoidal clinoforms with depositional dips ranging from 15 to 32 based on lithology and stratal patterns four facies groups have been recognized 1 a flat topped platform in which thick algal boundstone skeletal packstone grainstone and peloidal micrite wackestone with a poorly rhythmic character prevail 2 the platform margin and upper slope characterized by microbial boundstone spanning a bathymetric range of 150 m measured from the break of slope 3 a slope predominantly composed of margin derived rudstones and breccias and 4 a toe of slope to basin zone where a cyclic alternation of spiculitic siltstones packstone to grainstone calciturbidites and rudstone breccia is visible five successive stages of platform development are deduced 1 bashkirian flooding of the pre existing serpukhovian platform giving rise to the nucleation of a low angle ramp to the south east of the study area with microbial mud mound accumulations and breccias and calciturbidites on the margins 2 early moscovian an influx of siliciclastic sediment buried part of the platform and reduced the area of carbonate sedimentation 3 moscovian aggradation and progradation of the carbonate system produced an extensive steep margined and flat topped shallow water platform shelf system 4 latest moscovian earliest kasimovian drowning of the platform and 5 kasimovian covering of the platform by marly calcareous ramp sediments

pennsylvanian sea level cycles nutrient availability and brachiopod paleoecology abstract as with modern organisms the spatial and temporal distribution of fossil communities was controlled by both the physical setting in which the organisms lived and by the organisms physiology and interactions by studying the sedimentological and geochemical context of fossil communities it is possible to assess the relative importance of the physical setting and the organisms physiology comparison of pennsylvanian brachiopod associations with changing sedimentological context water depth facies and nutrient availability indicates that body size is a function of water depth and nutrient availability for most spire bearing athyridids and spiriferids brachiopods but rarely for productid brachiopods spire bearing brachiopods dominate the associations in high nutrient settings and productid brachiopods dominate the associations in low nutrient settings this difference suggests that physiological differences between brachiopod orders such as lophophore filtering efficiency play an important role in controlling their distribution

fore arc mantle wedge seismicity under northeast new zealand abstract relocated upper mantle seismicity using a 3d velocity model delineates three tightly clustered about 12 km diameter zones of seismicity that occur in the mantle wedge under the fore arc region of the northern hikurangi margin north island new zealand these clusters extend from the subducting plate at about 55 km depth up to about 35 km and coincide with strong seismic reflectivity recorded active source seismic reflection measurements suggesting a fluids association they have maximum magnitudes of about mw 4 5 the clusters correspond closely with a change from low to high vp and qp and with average vp vs that is interpreted to be caused by the transition from serpentinised to normal peridotite the location of this transition appears to correspond to the locus of dehydration of peridotite suggesting that the seismicity is caused by dehydration embrittlement the release of these fluids where the serpentinised mantle wedge reaches the locus of dehydration may provide a mechanism for transferring additional fluids or fluid fluxed partial melt into the adjacent back arc basin and facilitate the generation of voluminous magmas that are erupted there the occurrence of serpentinite in the fore arc mantle under northeastern north island may influence the shallow extent of interplate coupling under the region the location and spacing of the clusters is consistent with thermally driven diapiric or fluid upwellings but they also show a spatial relationship with proposed lithospheric fractures and associated volcanic seamounts on the subducting hikurangi plateau

distribution and geometry of facies and early diagenesis the key to accommodation space variation and sequence stratigraphy upper cretaceous congost carbonate platform spanish pyrenees abstract through only moderate structural deformation the upper cretaceous carbonates of the tremp region in the spanish pyrenees can be studied in the form of a platform to basin cross section comparable in scale to that of a seismic section this study examines the upper turonian to mid coniacian congost platform and its associated basinal succession the sedimentology facies geometries and cycle stacking patterns along with evidence of exposure of the platform are interpreted in terms of fluctuations in relative sea level and variations in accommodation space differences in the facies and early diagenesis of carbonates from different parts of the congost platform suggest two separate phases of platform development an earlier more land ward platform was dominated by clinoforms prograding on to a sloping basin floor with a thin lagoonal sand bank facies upon the platform itself supplying abundant excess sediment to the clinoforms a later more basin ward platform has a more complex internal geometry of depositional units and marginal rudist coral buildups the interpretations are used to propose a new sequence stratigraphic architecture for the congost platform involving two sequences the second produced through dynamic forced regression sequence boundaries are characterised by subaerial exposure on the platform top that can be correlated with hardgrounds and or glauconitic horizons within the more basinal locations overlain by deeper water facies the presence of a submarine onlap surface and down slope slide deposits immediately above the sequence boundary suggests that the major transgressive event that followed sequence boundary development was brought about by local extensional tectonic activity the small scale cyclicity within the sequences and the major falls in sea level that produced the sequence boundaries are interpreted to have resulted largely from eustatic processes probably with tectonic enhancement

modelling the interplate domain in thermo mechanical simulations of subduction critical effects of resolution and rheology and consequences on wet mantle melting abstract the present study aims at better deciphering the different mechanisms involved in the functioning of the subduction interplate a 2d thermo mechanical model is used to simulate a subduction channel made of oceanic crust free to evolve convergence at constant rate is imposed under a 100 km thick upper plate pseudo brittle and non newtonian behaviours are modelled the influence of the subduction channel strength parameterized by the difference in activation energy between crust and mantle e a is investigated to examine in detail the variations in depth of the subduction plane down dip extent z coup first simulations show that numerical resolution may be responsible for an artificial and significant shallowing of z coup if the weak crustal layer is not correctly resolved second if the age of the subducting plate is 100 myr subduction occurs for any e a the stiffer the crust is that is the lower e a is the shallower z coup is 60 km depth if e a 20 kj mol and the hotter the fore arc base is conversely imposing a very weak subduction channel e a 135 j mol leads there to an extreme mantle wedge cooling and inhibits mantle melting in wet conditions partial kinematic coupling at the fore arc base occurs if e a 145 kj mol if the incoming plate is 20 myr old subduction can occur under the conditions that the crust is either stiff and denser than the mantle or weak and buoyant in the latter condition cold crust plumes rise from the subduction channel and ascend through the upper lithosphere triggering 1 partial kinematic coupling under the fore arc 2 fore arc lithosphere cooling and 3 partial or complete hindrance of wet mantle melting z coup then ranges from 50 to more than 250 km depth and is time dependent if crust plumes form finally subduction plane dynamics is intimately linked to the regime of subduction induced corner flow two different intervals of e a are underlined 80 120 kj mol to reproduce the range of slab surface temperature inferred from geothermometry and 10 40 kj mol to reproduce the shallow hot mantle wedge core inferred from conditions of last equilibration of near primary arc magmas and seismic tomographies therefore an extra process controlling mantle wedge dynamics is needed to satisfy simultaneously the aforementioned observations a mantle viscosity reduction by a factor 4 20 caused by metasomatism in the mantle wedge is proposed from these results i conclude that the subduction channel down dip extent z coup should depend on the subduction setting to be consistent with the observed variability of sub arc depths of the subducting plate surface

characteristic seismic activity in the subducting plate boundary zone off kamaishi northeastern japan revealed by precise hypocenter distribution analysis using ocean bottom seismometers 1 high seismic activity prevails along the plate boundary to the east of northeastern japan to understand how this seismic activity is related to subduction process hypocenter locations are re determined using data obtained over 6 years by fiber cabled permanent ocean bottom seismometers off kamaishi the double difference method is adopted to obtain the relative location in more detail as a result of this analysis a number of seismic clusters including small repeating earthquakes are identified along the plate boundary this observation supports the hypothesis that seismic coupling is very low in this region and there only small asperities are distributed along the plate boundary off kamaishi a number of events with down dip compressional focal mechanisms are identified corresponding to the upper plane seismicity of the double seismic zone in this area although most of these events are located in the crust of the subducting pacific plate some are located in the mantle of the slab

carbonates and relative changes in sea level abstract in the geologic record some of the most accurate gauges of changes in sea level are the sediment type geometry and diagenesis of carbonate shelves and platforms this is because carbonates frequently occur at or very near sea level and are usually less compacted than siliciclastics world wide changes in relative sea level the sum of eustatic sea level changes sedimentation and crustal movements have occurred repeatedly and cyclicly through geologic time producing characteristic responses in carbonates 1 1 relative rises in sea level usually caused by the cumulative effect of tectonic subsidence and eustatic rise may result in the following 1 1 a drowned carbonate reefs or platforms here carbonate growth potential is exceeded by relative sea level rise and is characterized by shallow water sediments overlain by hardgrounds and or deep water sediments some of which may be condensed sequences 1 2 b platforms where only the fast growing rim and patches of the interior are able to match sea level rise while the remainder of the platform is drowned temporarily 1 3 c platforms which keep up and maintain a flat top at sea level and contain shallow water sediments whose thickness at the least matches the height of the sea level rise if terrigenous supply is limited prograding sheets of shelf carbonate occur frequently capped by supratidal evaporites with prograding shelf margin carbonate clinoforms and turbidites if terrigenous supply is high the shelf carbonates encroach on deltaics 2 2 relative drops in sea level caused by crustal uplift or by subsidence being outpaced by a eustatic drop in sea level cause karst and soil development over shelves and platforms deposition of deep water evaporites in adjacent semi enclosed basins and in open marine basins the deposition of deltaic and aeolian clastics that bypassed the shelf falls are accompanied by platform wide fresh water diagenesis during relative sea level rises marine diagenesis is common in the subtidal portions of the shoaling upward carbonates and fresh water diagenesis and dolomitization and sulphate deposition is common in their intertidal and supratidal portions the stratigraphic significance of these reponses to relative sea level change is that many are tied to eustatic events and so are predictable within a basin of deposition

compositional changes in sigmoidal carbonate clinoforms late tithonian eastern sardinia italy insights from quantitative microfacies analyses quantitative analysis of sediment composition was performed on a kilometre wide section of upper tithonian low relief up to 70 m gently inclined 3 to 15 sigmoidal carbonate clinoforms eastern sardinia to identify changes in sediment composition along the slope and across the studied succession these changes may reflect modifications of the carbonate factory and of processes responsible for sediment transport point count analysis of carbonate microfacies q mode r mode cluster analysis and spearman s rank provided a composition based classification of microfacies and highlighted relationships among sediment components the studied clinoforms are mainly composed of non skeletal grains 70 such as peloids and lithoclasts together with micrite and cements and only a limited contribution from coated grains 2 among skeletal grains 28 the greatest contribution derives from a coral stromatoporoid encruster reef that provided 15 of the components crinoids brachiopods and other along slope thriving biota provided nearly 5 of the allochems whilst fragments of molluscs gastropods bivalves and diceratids from the backreef sourced another 2 the contribution of platform interior biota is negligible 1 the association of composition based facies varies along the slope the upper slope beds consist of coral stromatoporoid grainstone to rudstone the middle slope deposits are dominated by encruster lithoclast grainstone and packstone at the lower slope peloidal lithoclastic packstone as well as brachiopod crinoidal wackestone prevail also the association of skeletal grains changes along the slope the encruster frame builder association typifies the upper slope whilst encrusters characterize the middle slope sediments in the lower slope encrusters are equally represented as the brachiopod crinoid association along slope compositional changes evidence a scarce downslope transport of frame builders and a progressive enrichment in along slope thriving biota quantitative analysis of microfacies allowed the sigmoidal clinoforms to be grouped into six sets each set gathers sigmoids with a similar sediment composition coated grains are dominant in the first set whilst they are lacking in the overlying sets reflecting a change in the carbonate factory other major compositional changes among the sets concern the relative amounts of peloids micrite frame builders corals and stromatoporoids and encrusters the contribution of peloids varies inversely to that of cements and micrite as evidenced in the third and fifth sets which respectively record the highest occurrence of peloids or cement and micrite variations in the amount of frame builders and encrusters are instead non linear high percentages of both frame builders and encrusters as recorded in the second and fifth sets are related to low amounts of peloids and lithoclasts that probably reflect episodes of reduced background sedimentation this study demonstrates that quantitative analysis of carbonate microfacies represents a powerful tool that can improve the reconstruction of the stacking pattern in carbonate slope successions both in outcrop and in subsurface settings

peloidal fabrics in upper jurassic reefal limestones weald basin southern england abstract coraliferous limestones from the corallian group upper jurassic of subsurface of the weald basin southern england record the localized development of superimposed patch reefs and banks the reefal limestones exhibit a variety of peloidal fabrics in particular well sorted peloids 10 60 m in diameter with a well developed euhedral rim of fibrous sparry calcite around a micritic anhedral nucleus they occur in confined intraskeletal pores corals molluscs and borings growth cavities between framebuilders and as stromatolitic crusts the peloids in interskeletal cavities and as crusts resemble microbial precipitates and merge into spongiostromate fabrics the distribution of these peloidal fabrics seems to be facies related and is best developed in zones which had relatively low framework growth rates low sedimentation rates and moderate energy levels the peloids are identical in form and occurrence to those in some ancient and quaternary reefs the association of the peloids with stromatolitic coatings and spongiostromate fabrics leads us to interpret the peloids as microbial precipitates either by direct precipitation or by induction the abundance of microbial carbonate in these reefal limestones places them in an intermediate category between frame built reefs and microbial mud mounds

an inverted continental moho and serpentinization of the forearc mantle volatiles that are transported by subducting lithospheric plates to depths greater than 100 km are thought to induce partial melting in the overlying mantle wedge resulting in arc magmatism and the addition of significant quantities of material to the overlying lithosphere asthenospheric flow and upwelling within the wedge produce increased lithospheric temperatures in this back arc region but the forearc mantle in the corner of the wedge is thought to be significantly cooler here we explore the structure of the mantle wedge in the southern cascadia subduction zone using scattered teleseismic waves recorded on a dense portable array of broadband seismometers we find very low shear wave velocities in the cold forearc mantle indicated by the exceptional occurrence of an inverted continental moho which reverts to normal polarity seaward of the cascade arc this observation provides compelling evidence for a highly hydrated and serpentinized forearc region consistent with thermal and petrological models of the forearc mantle wedge this serpentinized material is thought to have low strength and may therefore control the down dip rupture limit of great thrust earthquakes as well as the nature of large scale flow in the mantle wedge

temporal stability of deformation rates comparison of geological and geodetic observations hikurangi subduction margin new zealand abstract we examine how well contemporary horizontal velocity fields from global positioning system gps geodetic measurements describe long term deformation rates million year rates of extension shortening strike slip and vertical axis rotations are compared with modelling of gps velocities from the last 10 15 years in the hikurangi subduction margin new zealand within measurement uncertainties of mainly 50 the rates of deformation and vertical axis rotations for each time interval are usually indistinguishable the comparable deformation rates are consistent with the notion of near constant rates since 1 5 ma during this time oblique relative plate motion was partitioned with the majority 85 of margin normal motion accommodated on the plate interface and most 60 margin parallel motion by strike slip faulting and vertical axis rotations in the upper plate prior to 1 5 ma gps velocities cannot be reliably used to describe deformation and rotations of the hikurangi margin because at this time the taupo rift commenced opening the boundary between raukumara and wairoa rotational domains was abandoned and the rates of upper plate shortening increased the broad compatibility of gps and geological deformation and rotation rates since 1 5 ma suggests that the build up of contemporary elastic strains in the north island measured by gps is largely uniform for much of the duration of each seismic cycle the good agreement between the gps and geological deformation rates in the north island is consistent with the view that contemporary elastic strains will mainly be converted to permanent dip slip on the subduction thrust and to displacement on upper plate faults during future large magnitude earthquakes

lithospheric structure of an active backarc basin the taupo volcanic zone new zealand seismic data from both explosive and earthquake sources have been used to model the crustal and upper mantle velocity structure beneath the taupo volcanic zone tvz an active backarc basin in central north island new zealand volcanic sediments with p wave velocities of 2 0 3 5 km s 1 reach a maximum thickness of 3 km beneath the central tvz underlying these sediments to 16 km depth is material with velocities of 5 0 6 5 km s 1 interpreted as quartzo feldspathic crust east and west of the tvz crust with similar velocities is found to depths of 30 and 25 km respectively beneath the tvz material with p wave velocities of 6 9 7 3 km s 1 is found from 16 to 30 km depth and is interpreted as heavily intruded or underplated lower crust the base of the crust at 30 km depth under the tvz is marked by a strong seismic reflector interpreted as the moho modelling of arrivals from deep 40 km earthquakes near the top of the underlying subducting pacific plate reveals a region with low mantle velocities of 7 4 7 8 km s 1 beneath the crust of the tvz this region of low mantle velocities is best explained by the presence of partially hydrated upper mantle resulting from dehydration of hydrous minerals e g serpentinite carried down by the underlying subducting plate within the lower crust beneath the tvz a region of high 0 34 poisson s ratio is observed indicating the presence of at least 1 per cent partial melt this melt probably fractionates and assimilates crustal material before some of it migrates into the upper crust where it provides a source for the voluminous rhyolitic magmas of the tvz

pliocene pleistocene platform facies transition recorded in calciturbidites exuma sound bahamas abstract the composition of pliocene pleistocene calciturbidites of odp hole 632a exuma sound bahamas has been determined by point count analysis of thin sections in order to correlate basin platform events which influenced sedimentation strontium isotope chronostratigraphy has been used to correlate basin turbidite compositional variations to the evolution of the platform as documented by magnetostratigraphy magnetostratigraphy of great bahama bank cores shows that between 3 4 and 2 5 ma late pliocene ooids and peloids became the dominant sediments across the platform after this period the platform has been subjected to a series of subaerial exposure events this led to slow accumulation on the platform and starvation of the basin around 1 ma the platform was reflooded the transgression being followed by an increased production of ooids there is a change in calciturbidite composition in the earliest pleistocene coinciding with a regional unconformity pleistocene turbidites containing abundant skeletal material and ooids overlie pliocene early pleistocene turbidites rich in mud and skeletal material the duration of the hiatus represented by the unconformity based on 87 sr 86 sr dating is 1 0 0 7 ma prior to this hiatus sedimentation rates in the basin are an order of magnitude lower than present day accumulation rates we conclude that reflooding of the platform around 0 8 ma led to shedding of both non skeletal and skeletal sediments the change of great bahama bank from a pliocene reef rimmed skeletal platform to the present day flat topped bank dominated by non skeletal sediments can be correlated with the observed change in turbidite composition