petrology and geochemistry of lavas from the sumisu and torishima backarc rifts thirteen dredge hauls from the active sumisu and torishima rift grabens west of the izu bonin arc at about 30 31 n 140 e recovered a suite of tholeiitic basalts to sodic rhyolites volcanism occurs along tensional faults within and bounding the rift grabens and along the transfer zones between adjoining rift segments the sumisu and torishima rift lavas differ significantly from the lavas of the adjacent arc volcanic centers in having loweral2o3 na2o ba zr v ti andba ce and higher abundances of the rare earth elements the rift lavas also have characteristics of backarc basin basalts in that they are enriched in al2o3 and depleted in total iron and tio2 relative to mid ocean ridge basalt characteristics which are consistent with a higher water content in the source thus the model of a progressive change in backarc basin basalt composition from arc like to mid ocean ridge like as a function of evolution of the basin as has been suggested from many backarc regions is not generally applicable the comparison of the sumisu and torishima rift lavas with mariana backarc basin lavas indicates that backarc basin basalts differ in composition from one basin to another the comparison of these backarc basin suites with mid ocean ridge suites from similar axial depths indicates that the overall control over the spectrum of backarc basin basalt compositions may be different extents of melting of the mantle the sumisu and torishima rift lavas formed by a slightly higher extent of melting than the mariana backarc basin basalts a phenomenon which is related to the depth of the ridge segments furthermore the data suggest a systematically higher extent of melting in the arc lavas than in the backarc lavas for both of these arc backarc systems consistent with a greater flux of water beneath the arc than beneath the backarc region

controls on back arc basin formation the relationship between subduction and back arc spreading has been well known since the early days of plate tectonics however the reasons why back arc basins are associated with some subduction systems but not all has remained elusive we examine the kinematic controls on subduction and back arc basins for both the present day and cenozoic to differentiate between the major competing hypotheses for back arc basin formation and to explain their temporal and spatial distribution our new data set of subduction and back arc basin parameters uses a new set of paleo oceanic age grids muller et al 2005 associated with a moving atlantic indian ocean hot spot reference frame o neill et al 2005 the plate model includes detailed reconstructed spreading histories of back arc basins based on marine geophysical and satellite gravity data our combined rotation and oceanic paleo age model provides the age distribution of subducting lithosphere through space and time convergence rates and the absolute motion of the downgoing and overriding plates we find that back arc basins develop when the age of subducting normal oceanic lithosphere is greater than 55 million years additionally we establish an age dip relationship showing that the intermediate dip angle of the subducting slab is always greater than 30 with back arc spreading our results suggest that back arc basin formation is always preceded by an absolute motion of the overriding plate away from the subduction hinge thereby creating accommodation space between the overriding and subducting plates once back arc extension is established it continues regardless of overriding plate motion indicating back arc spreading is not a simple consequence of overriding plate behaviour the landward migration of the overriding plate as a precursor to back arc extension may indicate that extension on the overriding plate is influenced by the oceanward lateral flow of the mantle however once back arc extension is established rollback of the subduction hinge appears to be the primary force responsible for the continued creation of accommodation space our analysis indicates the driving mechanism for back arc extension is a combination of surface kinematics properties of the downgoing slab the effect of lateral mantle flow on the slab and mantle wedge dynamics

backarc rifting constructional volcanism and nascent disorganised spreading in the southern havre trough backarc rifts sw pacific high resolution multibeam em300 and seabeam data of the southern havre trough sht combined with observations and sample collections from the submersible shinkai6500 and deep tow camera are used to develop a model for the evolution and magmatism of this backarc system the havre trough and the associated kermadec arc are the product of westward subduction at the pacific australian plate boundary detailed studies focus on newly discovered features including a seamount saito seamount and a deep graben ngatoroirangi rift 4000 m water depth floored with a constructional axial volcanic ridge 5 km in length and in excess of 200 m high both of which are characterised by pillow and lobate flows estimated at elongate volcanic ridges at 35 15 s and 34 30 s and backarc seamounts 35 30 s 178 30 e occur at the eastern margin of the sht similar seafloor morphology is observed in the central and western portions of the basin suggesting that recent volcanism may be broadly distributed across the backarc mass balance modelling indicates a maximum crustal thickness of 11 km to 4000 m spreading systems these discontinuous spreading systems are characterised by failed rifts rift segmentation and propagation successive episodes of magmatic intrusion into thinned faulted arc basement results in defocused asymmetrical accretion cross arc volcanic chains isolated volcanoes and underlying basement plateaus are interpreted to represent a cap of recent extrusives however they may also be composed entirely of newly accreted crust and the spatially extensive basement fabric of elongated volcanic ridges may be the surface expression of pervasive dike intrusion that has thoroughly penetrated and essentially replaced the original arc crust with newly accreted intrusives

east pacific rise 8 10 30 n evolution of ridge segments and discontinuities from seamarc ii and three dimensional magnetic studies magnetic bathymetric and seamarc ii side scan sonar data from the ridge flanks adjoining the overlapping spreading center osc at 9 03 n on the east pacific rise epr are used to establish the evolution of this ridge axis discontinuity and adjacent ridge segments within the past 2 4 m y a three dimensional inversion for magnetization shows the discontinuity evolved from one of small offset 2 km to an 8 km offset osc within the past approximately 1 0 m y the osc has left a v shaped discordant zone indicating an average southward migration of 42 km m y since the beginning of anomaly 2 time 1 8 m y the west flank discordant zone consists of discrete abandoned overlap basins bounded by highly magnetized ridge tips with anomalous lineations while the east flank discordant zone is a broad swath of high magnetization anomalous lineations and greater depths kinematic modelling indicates that southward migration has not been steady but has been accomplished by a series of episodic propagation events with rates ranging from 500 mm yr ridge tips have been abandoned at intervals of approximately 50 000 100 000 years both by linkage of neighboring ridge segments and by self decapitation magnetic modelling suggests rotations of overlap basins of 25 consistent with predictions of kinematic modelling orientation of magnetic isochrons indicate a 3 6 counterclockwise change in pacifie cocos plate motion within the past 1 m y assuming orthogonal spreading throughout this time although the discontinuity currently located at 9 03 n existed prior to this change in plate motion its growth into a right stepping 8 km offset osc may be causally related to this change the current configuration of intratransform spreading within the siqueiros has developed since onset of the brunhes anomaly 0 73 m y possibly in response to this recent counterclockwise change in plate motion in addition to the osc there is evidence for several small discontinuities offsetting the ridge during anomaly 2 time which have since migrated along the ridge and vanished an axial magnetization high is observed along the crest of the epr change in the magnitude of the axial magnetization high or shifts in its location relative to the bathymetric axis coincide with several devals within the area magnetization maxima are found at ridge transform intersections and the osc these highs may reflect presence of highly fractionated feti basalts erupted from magma chambers with restricted supply located adjacent to both large and short offset ridge axis discontinuities discrete magnetization highs are found off axis adjacent to clipperton and siqueiros fracture zones indicating that production of feti basalts at ridge transform intersections is episodic magnetizations are higher north of clipperton than south of it possibly reflecting a long lived starvation in magma supply north of clipperton and more abundant supply to the south most seamounts within the area have the same magnetization polarity as surrounding seafloor indicating that seamount formation occurs near the ridge axis the exceptions lie near magnetic reversal boundaries and are consistent with seamount formation as close as 10 km from the ridge axis few pronounced magnetization highs are associated with seamounts indicating that the magnetized source layer is no thicker beneath them than surrounding seafloor or that seamount basalts are substantially less magnetic than normal mid ocean ridge basalts

unzipping of the volcano arc japan abstract a working hypothesis for the recent evolution of the southern volcano arc japan is presented which calls upon a northward progressing sundering of the arc in response to a northward propagating back arc basin extensional regime this model appears to explain several localized and recent changes in the tectonic and magrnatic evolution of the volcano arc most important among these changes is the unusual composition of iwo jima volcanic rocks this contrasts with normal arc tholeiites typical of the rest of the izu volcano mariana and other primitive arcs in having alkaline tendencies high concentrations of light ree and other incompatible elements and relatively high silica contents in spite of such fractionated characteristics these lavas appear to be very early manifestations of a new volcanic and tectonic cycle in the southern volcano arc these alkaline characteristics and indications of strong regional uplift are consistent with the recent development of an early stage of inter arc basin rifting in the southern volcano arc new bathymetric data are presented in support of this model which indicate 1 1 structural elements of the mariana trough extend north to the southern volcano arc 2 2 both the mariana trough and frontal arc shoal rapidly northwards as the volcano arc is approached 3 3 rugged bathymetry associated with the rifted mariana trough is replaced just south of iwo jima by the development of a huge dome 50 75 km diameter centered around iwo jima such uplifted domes are the immediate precursors of rifts in other environments and it appears that a similar situation may now exist in the southern volcano arc the present distribution of unrifted volcano arc to the north and rifted mariana arc to the south is interpreted not as a stable tectonic configuration but as representing a tectonic snapshot of an arc in the process of being rifted to form a back arc basin

evolution of intra oceanic arc trench systems abstract intra oceanic island arc trench systems represent a locus of crustal thickening through the addition of mantle derived magmas which form the volcanoes and subjacent stratiform plutons the crust of immature island arcs and adjacent back arc basins is oceanic in chemistry in that it is dominated by mafic e g tholeiitic magmas a magma sequence is recognized in many arc systems which progress from boninite to arc tholeiite to calc alkaline and in some arcs to shoshonites the adjacent back arc basins are formed of morb like tholeiite arc systems may be dominated by extension rather than compression if the subducted and overriding plates are decoupled extension of the arc system may be associated with seaward retreat or roll back of the trench axis alternate cycles of extension and compression of the arc system can lead to both tectonic erosion of the fore arc and accretion of fragments from the incoming sea floor the izu bonin mariana tonga yap and palau trench arc systems are examples of different stages in a dynamic process which starts when subduction begins along old fracture zone traces crustal extension causes both fore arc subsidence and opening of back arc basins island arc volcanic chains and back arc basins are new crust formed largely of fractional melts of the mantle the evolution of an arc system may stop when blocking of the subduction zone or changes in relative plate motion causes overthrusting of back arc lithosphere on to former volcanic arcs many ophiolites preserve remnants of arc system crust and upper mantle

back arc extension in the okinawa trough the okinawa trough lying to the east of china is a back arc basin formed by extension within continental lithosphere behind the ryukyu trench arc system middle to late miocene uplift associated with normal faulting of the initially adjacent ryukyu nonvolcanic arc and the taiwan sinzi folded belt corresponds to the first rifting phase the timing of rifting is supported by the presence of marine sediments of corresponding age drilled in the northern okinawa trough the rifting occurred after a major early miocene change in the motion of the philippine plate with respect to eurasia and ceased during the pliocene a second rifting phase started about 2 m y ago at the plio pleistocene boundary and has continued until the present time it has proceeded to a more advanced stage in the middle and southern okinawa trough than it has farther north detailed bathymetric sea beam seismic reflection and magnetics data collected during the pop 1 cruise of the r v jean charcot reveal the principal features of the extensional processes the back arc spreading phase started very recently in the southern and middle okinawa trough as exemplified by several en echelon and in some cases overlapping active central graben oriented n70 e n80 e some of these depressions are intruded by volcanic ridges of fresh back arc basalt with associated large magnetic anomalies transform faults between these en echelon active rifts are not obvious we suggest that the major part of the southern okinawa trough is underlain by a thinned continental crust and that except for the system of en echelon rifts of the southern okinawa trough the back arc basin oceanic domain is limited to a width of a few tens of kilometers or less in the axial portion of the trough the system of axial back arc volcanic ridges that occur in the rifts ends at the latitude of okinawa island whereas active volcanoes in the ryukyu arc occur only north of okinawa island we refer to this transition between active arc and back arc volcanism as the volcanic arc rift migration phenomenon vamp globally back arc volcanism propagated from the southern okinawa trough to the okinawa vamp area rifting continues to occur in the northern okinawa trough but is not yet accompanied by associated volcanism the okinawa vamp area is characterized by a series of parallel basaltic ridges oriented n75 e with associated linear magnetic anomalies characteristic of dyke intrusions we suggest that the formation of the back arc oceanic domain took place along the axial back arc extensional zone trending n75 e and that this zone presently ends at the southern extremity of the active volcanic chain the initial phase of formation of back arc basin oceanic crust is non steady state and is characterized by the lack of a developed fracture zone pattern the termination of the vamp area in the direction of the volcanic zone of the arc is consistent with the suggestion of molnar and atwater that the volcanic arc is a fundamental line of weakness which determines where initial back arc spreading occurs apparently back arc extension initially occurred within the continental lithosphere located westward of the ryukyu arc along its whole length but the subsequent back arc volcanism was initiated in the southernmost portion of the region and then moved northward this migration was accompanied by the shutting down of volcanic activity along the abandoned portions of the arc it is this transfer of volcanism that we call the vamp process thus arc and back arc basin volcanism seem to be associated in such a manner that spreading tends to migrate simultaneously with a cessation in volcanic activity along the arc this interplay of arc and back arc activity is probably linked to changes in the parameters of plate convergence since the plate motion in the phillippine sea is oblique to the trench at least in the southern part of the okinawa trough we suggest that the oblique resisting force applied to the edge of the overriding plate engenders the development of en echelon extensional features within and behind the arc the motion of the ryukyu platelet with respect to eurasia is consequently an extensional strike slip motion trending roughly parallel to the okinawa trough

physical volcanology of the submarine mariana and volcano arcs narrow beam maps selected dredge samplings and surveys of the mariana and volcano arcs identify 42 submarine volcanos observed activity and sample characteristics indicate 22 of these to be active or dormant edifices in the volcano arc are larger than most of the mariana arc edifices more irregularly shaped with numerous subsidiary cones and regularly spaced at 50 70 km volcanos in the mariana arc tend to be simple cones sets of individual cones and volcanic ridges are elongate parallel to the trend of the arc or at 110 counterclockwise from that trend suggesting a strong fault control on the distribution of arc magmas volcanos in the mariana arc are generally developed west of the frontal arc ridge on rifted frontal arc crust or new back arc basin crust volcanos in the central mariana arc are usually subaerial large 500 km3 and spaced about 50 70 km apart those in the northern and southern marianas are largely submarine closer together and generally less than 500 km3 in volume there is a shoaling of the arc basement around iwo jima accompanied by the appearance of incompatible element enriched lavas with alkalic affinities the larger volcanic edifices must reflect either a higher magma supply rate or a greater age for the larger volcanos if the magma supply estimated at 10 20 km3 km of arc per million years at 18 n has been relatively constant along the mariana arc we can infer a possible evolutionary sequence for arc volcanos from small irregularly spaced edifices to large over 1000 km3 edifices spaced at 50 70 km the volcano distribution and basal depths are consistent with the hypothesis of back arc propagation into the volcano arc

shoshonitic magmas in nascent arcs new evidence from submarine volcanoes in the northern marianas volcanoes in the northern mariana arc between uracas lat 20 n and minami iwo jima 24 n are very active yet entirely submarine in contrast to the predominantly low k basaltic magmas of the central mariana arc the northern mariana arc is dominated by more siliceous melts in the south and by shoshonites in the north the northern arc melts have enrichments in ba 800 ppm rb 70 ppm sr 1000 ppm ce 50 ppm and ce yb n 24 which increase to the north as far as iwo jima lavas from volcanoes north of iwo jima lack these enrichments and are indistinguishable from those of the central maranas the shoshonites are unusual in occurring along the magmatic front of a primitive intra oceanic arc we hypothesize that they represent the reconstruction of a magmatic arc following melting of enriched mantle due to the propagation of the mariana trough spreading center northward through the volcano arc shoshonites thus may characterize the initial stages of arc construction after an episode of back arc rifting and need not be restricted to the mature stages of arc evolution this situation contrasts with subduction zone initiation where first melts may be boninites or low k tholeiites these differing initial melts converge toward tholeiitic and calc alkaline compositions as arcs evolve

subduction hinge migration along the izu bonin mariana arc abstract hinge migration or rollback is thought to have a significant influence on the tectonics of trench arc backarc systems where the backarc region is actively opening the hinge is usually either stationary or retreating in a mantle hotspot frame of reference the izu bonin mariana system represents a highly anomalous case in which the hinge is reportedly advancing behind the retreating eastern edge of the overriding philippine sea plate we have re evaluated the motion of the subduction hinge using the best available estimates of the motion of the philippine sea plate and estimated maximum rates of extension within the izu bonin arc if the plate motions have not changed appreciably the subduction hinge of the izu bonin trench has advanced over the last 5 ma the approximate average rate of hinge advance is 0 6 mm a 1 at the north end of the izu bonin arc increasing to 24 30 mm a 1 at 27 n the rate of advance of the mariana hinge at 18 n is 10 20 mm a 1 the dip of the subducted slab is strongly correlated with the motion of the philippine sea plate suggesting that the rapid retreat of the overriding plate affects mantle flow we have found a tentative hinge advance mechanism in a simple newtonian viscous corner flow model though the model has severe geometrical and rheological limitations it suggests that the retreat of the overriding plate gives rise to excess uplift pressures acting on the top surface of the subducted slab if the hinge is not assumed to be fixed the excess pressure will tend to rotate the slab arcward steepening the slab and advancing the subduction hinge thus the westward motion of the philippine sea plate may explain both the advance of the subduction hinge and the observed relationship between the motion of the philippine sea plate and the dip of the subducted slab along the izu bonin mariana trench

the west mariana ridge western pacific ocean geomorphology and processes from new multibeam data the western flank of the west mariana ridge and the distal parece vela basin have been mapped for the first time using a modern multibeam echosounder the new data reveal the ridge to be composed of 82 volcanic peaks of which 25 have summits with water depths of 1000 m and 11 have summits with depths of 500 m deep five of the volcanoes have summit craters and two are guyots the remaining 75 seamounts are peaked volcanoes one volcanic peak has a summit water depth of 16 m and two in the central region have summits as shoal as 7 m the erosional character of the volcanoes suggests a younging toward the south the western flank of the west mariana ridge is buried by a sediment apron that merges with the distal parece vela basin the sediment apron is incised by a series of canyon channel systems that head on the upper flank or summit of the west mariana ridge and trend downslope to the west several of the channels are sediment filled with an incised thalweg suggesting a multiphase evolution to the channel systems none of the channels have a pelagic drape whereas the seafloor adjacent to the channels does suggesting recent activity the lack of guyots the apparent younging of the volcanoes to the south and the youth of channel activity suggest the west mariana ridge may be either recently or is presently tectonically active with the evidence suggesting uplift

mode of seafloor spreading in the northern mariana trough abstract a geophysical study was conducted to clarify the mode of seafloor spreading in the northern mariana trough north of 20 n magnetic anomaly data and single channel seismic reflection profiles were taken along ship s tracks typically with a 2 4 mile spacing the transition from back arc rifting to true seafloor spreading occurs at a structural boundary which has ne sw strike at about 22 n south of this boundary clear magnetic lineations occur and axial rift valleys on the spreading ridges are visible on the seismic profiles our identification of the magnetic lineations shows that the accretion of oceanic crust started at about 3 5 ma with a spreading half rate of 2 cm yr the average spreading rate during the brunhes chron is of the order of 1 cm yr the rate increases northward and is accompanied by a decrease in the relative depths of the axial rift valleys a ridge axis discontinuity is found at about 21 n two spreading axes overlap each other about 50 km along strike and are offset about 30 km we interpret this discontinuity as an on going ridge jump from the western axis to the eastern one the southern end of the western axis is cut by a transform fault but the eastern axis is now propagating northwards arc volcanoes comprising the present volcanic front occur on the eastern flank of the spreading ridges suggesting basinward migration of the volcanic arc the spreading center of the mariana trough is located not at the center but on the eastern or the volcanic arc side of the trough in general offset is caused by the arcward ridge jump and the basinward migration of the volcanic arc asymmetric rifting before the beginning of the accretion of oceanic crust would be another reason for the offset a row of rift grabens along the eastern margin of the trough north of 22 n where the extension of island arc crust is concentrated continues to the spreading ridge south of 22 n the arcward offset of spreading ridges is also found in the lau basin suggesting that it is a common feature of the evolution of back arc basins

field experiments on the transport of rock fragments by animal trampling on scree slopes qualitative observations in an area with steep scree slopes in turkey suggested that trampling by goats and sheep may contribute significantly to the downslope movement of rock fragments in these environments therefore a series of field experiments was set up to determine the rate of rock fragment transport caused by trampling the experimental results allowed the calculation of a unit displacement distance from each trampling experiment which may then be used to extrapolate the measured transport rates over longer time periods and larger areas although such extrapolations should be interpreted with great caution the results demonstrate that trampling contributes significantly to rock fragment transport in such environments as calculated yearly displacement distances are equal to or higher than those reported in the literature for other geomorphic processes the observed travel distance distributions are not well described by a negative exponential distribution trampling causes a weak downslope sorting the coarsest clasts move farther but has a significant effect on vertical sorting and is one of the factors which can explain the layered structure of the scree slopes in the area

upper plate stress controls the distribution of mariana arc volcanoes we present a spatial analysis of volcano distribution and morphology in the young intraoceanic mariana arc both the quality of fit to idealised models and the divergence from those ideals indicate that mariana arc volcanoes are arranged into five great circle segments rather than a single small circle or multiple small circles the alignment of magmatic centres suggests that magma transport is controlled by the stress regime in the deep crust and or lithospheric mantle of the philippine sea plate into which the arc is emplaced and that arc normal tension is the dominant process operating in the deep lithosphere along the whole arc volcano morphologies indicate that the stress regime in the shallow crust varies between arc normal tension and compression which also implies that the stress field can vary with depth in the arc lithosphere we show that this horizontal and vertical stress partitioning can be related to the changing dip of the subducting plate and the breadth of the zone where it is coupled with the overriding plate the variation in stress regime is consistent with both the distribution of seismicity in the philippine sea plate and with the structural fabrics of the non volcanic part of the plate margin to the south our analysis suggests that the upper plate exerts the principal control on the distribution of volcanoes in the mariana arc where tension in the deeper parts of arc lithosphere is sufficiently concentrated then a distinct volcanic front is produced

3 d simulation of tectonic evolution of the mariana arc back arc system with a coupled model of plate subduction and back arc spreading abstract we developed a mechanical model for the nonlinear coupled system of plate subduction and back arc spreading on the basis of mathematical formulation for internal deformation due to a moment tensor in an elastic viscoelastic layered half space under gravity in our modeling the plate subduction and the back arc spreading are rationally represented by the increase of tangential displacement discontinuity at the plate interface and normal displacement discontinuity at the spreading center respectively through 3 d numerical simulation with this model we obtained a possible scenario for the tectonic evolution of back arc basins as follows at the first stage steady plate subduction gradually forms tensile stress fields in the back arc region when the accumulated tensile stress reaches a critical level back arc spreading starts at a structurally weak portion of the overriding plate the back arc spreading pushes out the frontal part of the overriding plate toward the plate boundary and leads to the increase of slip rates at the plate interface the local increase of slip rates at the plate interface produces additional tensile stress in the back arc region the incremental tensile stress is canceled out by further back arc spreading such a feedback mechanism is necessary to maintain steady back arc spreading the long duration of slip rate excess due to back arc spreading leads to the gradual protrusion of the plate boundary toward the descending plate trench retreat as the plate boundary moves away from the back arc spreading center the accumulation rate of tensile stress at the spreading center gradually decreases and the slip rate excess at the plate interface due to back arc spreading also decreases then the original back arc spreading center becomes less effective in releasing the tectonic tensile stress and new active back arc spreading will start somewhere closer to the plate boundary such a qualitative scenario accords with the evolution history of back arc basins in the mariana region we obtained the slip rate excess of 7 mm year at the plate interface and the 20 km trench retreat for the last 4 5 myr these simulation results are significantly smaller than observations in the mariana region the discrepancy between simulation results and observations can be ascribed to the effect of the negative buoyancy acting on cold descending slab which advances spontaneous plate subduction and directly increases the slip rate excess and the trench retreat in our model

evaluation of image analysis methods used for quantification of particle angularity angularity is an important parameter in the characterization of particle morphology that is used to interpret the transport history of particles in sedimentary deposits in the past visual classification using silhouette charts was widely used to determine particle angularity but this approach is subjective and time consuming with advances in modern image analysis techniques and low cost software packages it is possible to rapidly quantify particle angularity more objectively than using visual classification methods this study re examines the performance of three existing image analysis methods and one new image analysis procedure applied to six rock and sediment samples that were visually different in angularity to facilitate comparison between the angularity results measurements were reduced to rankings for each aggregate sample these results show that the four image analysis methods rank the angularity of the samples differently and that none rank the mean angularity index in the same order as the angularity ranking using visual classification therefore further research is needed to develop an image analysis method that can quantify the angularity of sedimentary particles more precisely

geophysical characteristics of the southern mariana trough 11 50 n 13 40 n despite slow opening rates generally inferred for the mariana trough the southernmost part of the basin has fast spreading geophysical and morphologic characteristics that are unlike the features of the basin to the north a side scan sonar and geophysical survey maps the eastern part of the basin and the seafloor spreading center between 11 50 n and 13 40 n and identifies the following characteristics the ridge across axis profile forms a triangular to rounded high with relief of 100 to 500 m and cross sectional area variations of 1 to 7 km2 the along axis mantle bouguer gravity gradient is 0 2 mgal km axial segmentation occurs as overlapping axes and small deviation in trend no transform fault offsets exist despite significant changes in the trend of the spreading center characteristics of the surrounding basin include shallower overall depth than in the north no well developed frontal arc high in the southernmost trough the close proximity of submarine arc type volcanoes to the spreading center and tectonic fabric that is at a high angle to the trend of the spreading center on the eastern flank but is concordant on the western flank these characteristics imply different tectonic and magmatic conditions in the southern trough from the rest of the basin we propose that these effects are related to 1 the geometry of trench rollback in the southern trough leading to trench parallel extension generating inward radiating extensional faults 2 decoupling of the trench parallel extensional strain by the spreading center so that it primarily affects the eastern flank of the basin and 3 augmentation of the spreading center s magmatic budget by arc magmatic sources contributing to its fast spreading character although these effects may be accentuated in the southern mariana trough by the geometry of trench rollback and position of the slab which here underlies the spreading center they reflect distinct volcanic and tectonic processes which are varyingly expressed in back arc systems but are not normally found at mid ocean ridges

early neoproterozoic multiple arc back arc system formation during subduction accretion processes between the yangtze and cathaysia blocks new constraints from the supra subduction zone ne jiangxi ophiolite south china the ne jiangxi ophiolite in the eastern jiangnan orogen is a tectonic melange that mainly consists of individual tectonic blocks comprising pyroxenite gabbro basalt diorite granite and chert in a matrix of serpentinite or tuffaceous greywacke a combined geochemical and geochronological study of the ne jiangxi ophiolite was undertaken to constrain the timing and tectonic setting of its formation the basalts were geochemically subdivided into three groups with different feot and tio2 contents group 1 basalts have the lowest feot 12 17 13 07 wt and tio2 1 48 1 62 wt contents and the lowest nb yb 0 80 0 88 and th nb 0 02 0 03 ratios furthermore they have normal mid ocean ridge basalt n morb like trace element patterns suggesting derivation from an n morb type mantle source without subduction input group 2 fe ti basalts have the highest feot 15 52 16 30 wt and tio2 3 06 3 23 wt contents nb yb and th nb ratios from 1 75 to 1 89 and from 0 11 to 0 15 respectively and trace element patterns similar to those of back arc basin basalts the geochemical characteristics suggest that group 2 basalts were derived from a slightly enriched morb type mantle source with a minor subduction contribution in contrast group 3 fe ti basalts have moderate feot 12 98 13 40 wt and tio2 2 37 2 71 wt contents and nb yb and th nb ratios from 1 28 to 1 45 and from 0 27 to 0 30 respectively these basalts further display markedly negative nb ta anomalies and show a geochemical affinity to island arc basalts iab indicating a slightly enriched morb type mantle source that was significantly influenced by subduction derived fluids and or melts sims zircon u pb dating on gabbros gave ages of 995 22 ma and 993 12 ma which are interpreted as the formation age of the ne jiangxi ophiolite positive zircon ehf t 8 8 to 13 8 values for the gabbros and whole rock end t 5 5 to 6 6 values for the basalts indicate that the ne jiangxi ophiolite originated from an isotopically homogeneous depleted mantle source the diversity of morb to iab like basalts and the presence of fe ti basalts favor a formation of the ne jiangxi ophiolite during the initial rifting phase of an intra oceanic back arc basin between an oceanic arc huaiyu terrane and the continental margin of the yangtze block jiuling terrane at ca 990 ma both the present and previous studies imply that multiple arc back arc systems formed during long lasting subduction accretion processes between the yangtze and cathaysia blocks during the early neoproterozoic

insights into the causes of arc rifting from 2 d dynamic models of subduction author s billen mi abstract 2017 american geophysical union all rights reserved back arc spreading centers initiate as fore arc or arc rifting events when extensional forces localize within lithosphere weakened by hydrous fluids or melting two models have been proposed for triggering fore arc arc rifting rollback of the subducting plate causing trench retreat or motion of the overriding plate away from the subduction zone this paper demonstrates that there is a third mechanism caused by an in situ instability that occurs when the thin high viscosity boundary which separates the weak fore arc from the hot buoyant mantle wedge is removed buoyant upwelling mantle causes arc rifting drives the overriding plate away from the subducting plate and there is sufficient heating of the subducting plate crust and overriding plate lithosphere to form adakite or boninite volcanism for spontaneous fore arc arc rifting to occur a broad region of weak material must be present and one of the plates must be free to respond to the upwelling forces

evolution and interaction of migrating cross arc volcanism and backarc rifting an example from the southern havre trough 35 20 37 s the active southern havre trough 35 20 37 s backarc basin is interpreted to form from the evolution and interaction of migrating cross arc magmatism and the progressive development of longitudinal rift grabens the proposed migration of the proto kermadec arc front from the colville arc at 5 ma to the active kermadec arc margin is recorded by the construction of arc edifices trailed across the intervening and contemporaneously rifting backarc complex migration trails are identified for at least four arc volcano sources the rumble v arc migration trail is the most prominent forming a continuous high standing magmatic arc ridge these postulated arc trails segment the backarc region and initially limit rift development to the intervening proto arc crust blocks early rifting between the arc volcanoes forms fully developed rift grabens which with progressive basin widening propagate longitudinally across the trails of migrating constructional arc magmatism a model of the balance between rates of constructional arc magma production mr and destructive back arc rifting extension ve is proposed when ve is high mr is insufficient to keep abreast of destructional rifting resulting in small isolated arc massifs quickly dismembered by rifting conversely when ve is low mr is sufficiently greater than rifting to produce a continuous high standing cross arc ridge which segments longitudinal rift development migrating arc magmatism will be best observed and preserved in rifting backarc basins when mr is 600 km3 m y 1 100 km 1 of plate boundary and ve is 25 mm a 1

a volcanic production rate for the mariana island arc abstract despite the importance of volcanic production rates along subducting plate boundaries few reliable values are available a well constrained rate has here been determined for the past 5 m y in the mariana island arc the calculation covers the period during the opening of the mariana trough an active back arc basin in which the volcanic centers are located and most volcanic products are contained about half the extruded volcanic rocks occur in central cones with the other half in a large volcanicclastic apron after correction of volumes for porosity and for changes in mineral densities the average extrusion rate of 12 4 km 3 km m y was calculated the local rate varies along the arc roughly as does the subduction rate before quantification of that correlation can be made however apparent temporal fluctuations of volcanism on a global scale which do not appear to be correlated with subduction rates must be examined and explained

the geometry of propagating rifts abstract the kinematics of two different processes are investigated both of which have been described as rift propagation courtillot uses this term to describe the change from distributed to localised extension which occurs during the early development of an ocean basin the term localisation is instead used here to describe this process to distinguish it from hey s type of propagation localisation generally leads to rotation of the direction of magnetisation to hey propagation means the extension of a rift into the undeformed plate beyond a transform fault detail surveys of the galapagos rift have shown that the propagating and failing rifts are not connected by a single transform fault but by a zone which is undergoing shear the principal deformation is simple shear and the kinematics of this deformation are investigated in some detail the strike of most of the lineations observed in the area can be produced by such deformation the mode of extension on the propagating rift appears to be localised for some periods but to be distributed for others neither simple kinematic arguments nor stretching of the lithosphere with conservation of crust can account for the observed variations in water depth

evolution of backarc rifting mariana trough 20 24 n the mariana trough is an actively opening backarc basin in the western pacific the trough formed by extension which longitudinally split an earlier arc massif creating a crescent shaped basin between the remnant west mariana ridge and eastern active volcanoes of the mariana arc opening increases southward from the volcano islands near 24 n where the two arcs join at 18 n in the central mariana trough the basin is widest and may be opening primarily by seafloor spreading we present a synthesis of closely spaced shipboard gravity magnetic and bathymetry measurements from the northern basin 20 24 n an area that undergoes a significant progressive southward increase in extension we have identified three stages in the evolution of rifting in this area 1 asymmetric rifting from 24 n to 22 15 n where faulting and magmatism have migrated laterally to remain near the active volcanic arc side of the basin 2 a localization of rifting from 22 15 n to 21 n where the primary zone of rifting separates from the active volcanic arc and 3 a further concentration of rifting from 21 n to 20 n leading to the formation of deep tectonic grabens near the center of the basin this last stage may be a precursor to or incipient seafloor spreading we describe a new type of magnetic lineation resolved in a three dimensional seafloor magnetization inversion of the trough between 20 and 24 n the detailed character of these lineations their association with tectonic structures and other geophysical observations indicate that they are not seafloor spreading lineations but rather result from magnetic intrusions and volcanism emplaced within preexisting less magnetic rifted arc crust the development of these well defined magnetization bands indicates that the zone of magmatism remains relatively narrow at any one time throughout the opening of the trough although tectonic deformation appears to be more widely distributed in addition the geometry of the magnetization bands with respect to the remnant arc border faults indicates that rifting propagated rapidly northward

observations of the terra nova bay antarctica polynya by modis ice surface temperature imagery from 2005 to 2010 abstract ice surface temperature ist imagery from the thermal infrared bands of the satellite borne moderate resolution imaging spectroradiometer modis has been used to analyze the activity of the terra nova bay polynya from 2005 to 2010 in clear sky conditions the shape of the open water area of the polynya i e the portion of the ocean free of sea ice is revealed by the skin sea ice temperatures close to the water freezing point provided by the modis derived ist product the swaths of the modis sensors onboard terra and aqua satellites over the terra nova bay consist of about 5500 scenes per year useful to estimate the cloud cover too the open water area was extracted from clear sky scenes and from scenes affected by fog but providing continuity of the shape of the polynya using about 40 of the available swaths the polynya was detected for more than 80 of the sea ice period with a revisit time below 24 h and for more than 50 with a revisit time below 3 h the open water area was extracted using thresholds of ist based on the ist values detected within the open water area and within the band of sea ice surrounding the polynya obtained scene by scene an ice free area larger than 100 km 2 was observed near the nansen ice shelf for more than 70 of time larger than 500 km 2 for 50 of time the mean annual open water area around 900 km 2 was close to the lower limit of the reported range except during 2006 around 600 km 2 due to the limited activity in the second half of the cold season the largest extents between 6000 and 8500 km 2 occurred in different periods of the antarctic winter and exceeded maxima estimated in previous studies by thermal infrared data polynya openings developed from a few hours to several days in the latter case sustained by subsequent katabatic events visible in the ist sequence peaks of growing rate above 300 km 2 h were observed during intense katabatic events with gusts of 50 m s the fluctuations of the open water area resulted well correlated with wind records available from local automatic weather stations within the main katabatic routes and followed katabatic events at intervals between 0 and 6 h the analysis of the ist data set derived from the thermal infrared bands of modis provides new polynya observations of high horizontal resolution 1 km and frequent revisit time during the winter season complementing previously available observations with passive microwave imagery

subduction zone infancy examples from the eocene izu bonin mariana and jurassic california arcs a new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the izu bonin mariana ibm arc system and then applied to late jurassic ophiolites of cailfornia the model accounts for several key observations about the earliest stages in the evolution of the ibm system 1 subduction nucleated along an active transform boundary which separated younger less dense lithosphere in the west from older more dense lithosphere to the east 2 initial arc magmatic activity occupied a much broader zone than existed later 3 initial magmatism extended up to the modern trench over a region now characterized by subnormal heat flow 4 early are magmatism was characterized by depleted tholeiitic and ultra depleted boninitic magmas indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth 5 early arc magmatism was strongly extensional with crust forming in a manner similar to slow spreading ridges and 6 crust production rates were 120 to 180 km 3 km ma several times greater than for mature arc systems these observations require that the earliest stages of subduction involve rapid retreat of the trench we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault this resulted in strong extension and thinning of younger more buoyant lithosphere to the west this extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere addition of water and asthenospheric upwelling led to catastrophic melting which continued until lithosphere subsidence was replaced by lithospheric subduction application of the subduction zone infancy model to the late jurassic ophiolites of california provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger sierran magmatic arc and the franciscan subduction complex provides a mechanism for the formation and subsidence of the great valley forearc basin and explains the limited duration of high t high p metamorphism experienced by franciscan melanges

lithospheric strength variations as a control on new plate boundaries examples from the northern red sea region abstract the complex plate boundary between arabia and africa at the northern end of the red sea includes the gulf of suez rift and the gulf of aqaba dead sea transform geologic evidence indicates that during the earliest phase of rifting the red sea propagated nnw towards the mediterranean sea creating the gulf of suez subsequently the majority of the relative movement between the plates shifted eastward to the dead sea transform we propose that an increase in the strength of the lithosphere across the mediterranean continental margin acted as a barrier to the propagation of the rift a new plate boundary the dead sea transform formed along a zone of minimum strength we present an analysis of lithospheric strength variations across the mediterranean continental margin the main factors controlling these variations are the geotherm crustal thickness and composition and sediment thickness the analysis predicts a characteristic strength profile at continental margins which consists of a marked increase in strength seaward of the hinge zone and a strength minimum landward of the hinge zone this strength profile also favors the creation of thin continental slivers such as the levant west of the dead sea transform and the continental promontory containing socotra island at the mouth of the gulf of aden calculations of strength variations based on changes of crustal thickness geotherm and sediment thickness can be extended to other geologic settings as well they can explain the location of rerifting events at intracratonic basins of backarc basins and of major continental strike slip zones

regional seismicity and on land deformation in the ryukyu arc implications for the kinematics of opening of the okinawa trough the stress field evolution and the kinematics of opening of the okinawa trough are investigated on the basis of earthquake focal mechanisms and structural data in the ryukyu arc and the okinawa trough focal mechanisms show that the crust underlying the arc and the trough undergoes extension along two suborthogonal directions a regional arc perpendicular extension and a local arc parallel extension both extensions are concurrent and related to the same regional stress field characterized by permutating horizontal 2 and 3 axes earthquake slip vectors reveal a southward motion of the ryukyu arc with respect to the south china block the current pole of opening of the okinawa trough is located around 16 n and 50 e fracture analysis in okinawa island allows identification of three episodes of extension a late miocene n40 w to n20 e extension episode i a late pliocene to early pleistocene n20 e extension episode ii and a latest pleistocene to present day n20 w extension episode iii episodes ii and iii are characterized by permutations between the two horizontal 2 and 3 axes by synthesizing regional deformation data and by comparing the geometry of the deformation with analogue models of oblique rifting we reconstruct the kinematics of opening of the okinawa trough since the late miocene the direction of divergence of the ryukyu arc has rotated clockwise from n150 e in the late miocene to nearly n s today

the lau havre taupo back arc basin a southward propagating multi stage evolution from rifting to spreading abstract the lau havre taupo lht system is an active 2000 km long back arc basin associated with pacific australian plate convergence in the sw pacific data from the lht back arc system include odp drillhole swath bathymetric and side scan imagery seismic reflection rock geochemistry magnetic anomaly heat flow shallow seismicity gravity and seismic refraction data the lht forms a longitudinal transect from southward propagating true oceanic spreading in the lau basin near tonga similarly southward propagating arc rifting in the havre trough and southward propagating continental rifting in the taupo volcanic zone tvz in northern new zealand four transitional tectonic settings are identified along the lht north to south between the continuum of spreading to rifting and comprise 1 oceanic spreading of the central lau spreading centre clsc propagating into oceanic crust formed by the eastern lau spreading centre elsc 2 oceanic spreading of the elsc propagating into rifted lau havre arc crust 3 rifting within havre arc crust propagating into rifting new zealand continental crust and 4 rifting of continental crust propagating into as yet non rifted but flexurally subsiding continental crust the enhanced spatial and temporal resolution of observed tectonic and magmatic processes along the strike of the lht is used to constrain a model for the evolution of back arc rifting to back arc spreading a sequence of five stages is identified and comprise 1 a combination of incipient simple and pure shear 2 early distributed and spatially heterogeneous half graben rifting with low angle synthetic and antithetic structures associated with rapid graben sediment infilling ephemeral early morb like rift magmatism and the initial trenchward migration of the arc front 3 a latter phase of half graben rifting mostly confined to a contiguous axial zone with continued ephemeral morb like rift magmatism and further trenchward migration of the arc front 4 initial oceanic spreading with evolving steady state morb magmatism along the axial rift graben system and 5 the reconfiguration of ridge geometry in response to changes in opening orientation leading to the development of ridge propagation into newly created oceanic crust rifting stages 2 and 3 can produce at least 120 km wide back arc crust by a balance of crustal extension with an estimated factor of 3 with rift and constructional arc magmatism in the lht system depending on latitude rates of widening during stages 2 and 3 vary between 15 and 45 mm yr 1 whilst stages 4 and 5 have full spreading rates of 90 mm yr 1

hydrothermal activity and volcano distribution along the mariana arc 1 submarine volcanic arcs extend for fully a third of the length of the mid ocean ridge system but systematic investigation of hydrothermal venting on those arcs is sparse here we report on the distribution of hydrothermal activity along the intraoceanic mariana arc we identify 76 volcanic edifices along 1370 km of arc grouped into 60 volcanic centers of which at least 26 20 submarine are hydrothermally or volcanically active the overall volcanic center density is 4 4 100 km of arc and that of active centers is 1 9 100 km an equal length of the tonga kermadec arc holds fewer centers 2 9 100 km but a similar density of active centers 1 8 100 km the lower percentage of active centers on the mariana arc results from its having both fewer caldera volcanoes since on both arcs calderas are twice as likely as cones to be active and a lower percentage of active calderas and cones active centers are found from 80 to 230 km above the subducting pacific slab and a quarter lie behind the arc front the frequency distribution of center spacing along the arc front peaks between 20 and 30 km and shows the asymmetric long tail shape typical for many other arcs especially those with volcano populations 20 there is no evidence for a regular spacing of volcanic centers using the new mariana data and recent data from the tonga kermadec arc we estimate that all intraoceanic arcs combined may contribute hydrothermal emissions equal to 10 of that from the global mid ocean ridge

control of seafloor aging on the migration of the izu bonin mariana trench article i nfo recent global kinematic studies reveal that most of the trenches roll back but a significant number of them advance toward the upper plate those advancing trenches are mostly located in the western pacific and correspond to the subduction of very old mesozoic oceanic lithosphere while retreating trenches are commonly explained by the slab pull action of the descending lithosphere the origin of advancing trenches is still debated since this relationship is dependent upon the adopted reference frame we select region where geological studies show the variability of trench migration style with time the izu bonin mariana ibm region represents a key example the detailed reconstruction of the trench migration of the ibm subducting system reveals that after a long episode of asymmetric rollback the ibm trench recently started advancing we propose that this change from retreating to advancing trench mode results from the subduction of progressively older and stiffer lithospheric material we test this hypothesis by means of two dimensional 2 d numerical models reproducing the effects of the lithospheric aging during subduction the result of our numerical tests shows that the entrance of old and stiff lithosphere forces the trench to advance because the increasing stiffness of the slab prevents the slab to unbend once it has subducted we adapt this physical result to the ibm evolution showing that the switch from trench retreat to trench advance can reconcile the shape of ibm slab as previously suggested finally we discuss the possibility that the subduction of the cretaceous lithosphere might have triggered the change of the motion of the pacific plate around 5 ma

propagation of rifting along the arabia somalia plate boundary the gulfs of aden and tadjoura the localization and propagation of rifting between arabia and somalia are investigated by assessing the deformation geometry and kinematics at different scales between the eastern gulf of aden and the gulf of tadjoura using bathymetric magnetic seismological and structural evidence large scale southwestward propagation of the aden ridge markedly oblique to the arabia somalia relative motion vector began about 30 myr ago between the error and sharbithat ridges it was an episodic process with stages of rapid propagation mostly at rates 10 cm yr interrupted by million year pauses on transverse discontinuities coinciding with rheological boundaries between different crustal provinces of the arabia somalia plate the longest pause was at the shukra el sheik discontinuity 45 e where the ridge tip stalled for 13 myr between 17 and 4 ma west of that discontinuity rifting and spreading took place at an azimuth n25 10 e and rate 1 2 0 3 cm yr different from those of the global arabia somalia motion vector n39 1 73 cm yr implying an additional component of movement n65 10 e 0 7 0 2 cm yr due to rotation of the danakil microplate at shukra el sheik the typical oceanic ridge gives way to a narrow wsw trending axial trough resembling a large fissure across a shallow shelf this trough is composed of about eight rift segments which result from normal faulting and fissuring along n110 n130 e trends all the segments step to the left southwestward mostly through oblique transfer zones with en echelon normal faults only two segments show clear significant overlap there is one clear transform the maskali fault between the obock and tadjoura segments the latter segment which encroaches onland is composed of two parallel subrifts iboli ambabbo that propagated northwestward and formed in succession the most recent southwestern subrift ambabbo represents the current tip of the aden ridge we propose a mechanical model in which the large scale propagation of the ridge followed a wsw trending zone of maximum tensile stress while the small scale propagation of its nw trending segments was dictated by the orientation of that stress oblique propagation was a consequence of passive lithospheric necking of the arabia somalia plate along its narrow section in map view between socotra and the kink of the red sea ethiopian rift above the afar plume individual ridge segments oriented roughly perpendicular to plate motion like lithospheric cracks were forced to jump southward because of confinement within the necking zone self sustaining plate scale necking may explain why the aden ridge did not connect with the red sea through bab el mandeb but continued straight into afar

the submarine flanks of anatahan volcano commonwealth of the northern mariana islands abstract the submarine flanks of anatahan volcano were surveyed comprehensively for the first time in 2003 and 2004 with multibeam and sidescan sonar systems a geologic map based on the new bathymetry and backscatter data shows that 67 of the volcano s submarine flanks are covered with volcaniclastic debris and 26 is lava flows cones and bedrock outcrops the island of anatahan is only 1 of the volume of the entire volcano which has a height from its submarine base of 3700 m and an average diameter of 35 km ne anatahan is a prominent satellite volcano located 10 km ne of the island but it is only 6 of anatahan s volume 40 km 3 vs 620 km 3 seventy eight submarine eruptive vents are mapped associated with lava flows and cones between depths of 350 and 2950 m and 80 of these vents are located in a cluster on the east flank of the volcano the distribution of cones and lava flows vs depth suggests a possible change in eruptive style from explosive to effusive between 1500 and 2000 m eruptive vents below 2000 m have produced mostly lava flows there is no evidence of major landslides on the submarine flanks of anatahan volcano in contrast to many basaltic islands and seamounts suggesting that mass wasting at felsic oceanic arc volcanoes may be characterized by sediment flows of unconsolidated volcaniclastic debris instead of mass movements of relatively large intact blocks

sea floor spreading in the lau back arc basin abstract we report the first 3 d magnetization inversion of bathymetry and magnetics data for the entire lau basin and confirm a history of southward propagating sea floor spreading during the past 4 myr magnetic lineations seafloor fabric and ridge segmentation patterns indicate that lithospheric accretion on discrete spreading centers in this back arc basin is fundamentally similar to that occurring on mid ocean ridges the brunhes chron spreading rates 65 100 mm yr are

sedimentary volcanic and tectonic processes of the central mariana arc mariana trough back arc basin formation and the west mariana ridge 1 we present new multichannel seismic profiles and bathymetric data from the central marianas that image the west mariana ridge wmr remnant arc both margins of the mariana trough back arc basin the modern arc and eocene frontal arc high these data reveal structure and stratigraphy related to three periods of arc volcanism and two periods of arc rifting we interpret the boundary between accreted back arc basin and rifted arc crust along the mariana trough and support these findings with drilling results and recent seismic refraction and gravity studies we show that with the exception of a few volcanoes behind the volcanic front that straddle the boundary between crustal types the modern mariana arc is built entirely on rifted arc crust between 14 and 19 n our data indicate that there is more accreted back arc seafloor to the west of the mariana trough spreading axis than to the east confirming previous evidence for an asymmetric basin the rifted margin of the wmr remnant arc forms a stepped pattern along the western boundary of the mariana trough between 15 30 and 19 n in this region linear volcanic cross chains behind the wmr are aligned with the trend of mariana trough spreading segments and the wmr ridges extend into the back arc basin along the same strike these ridges are magmatic accommodation zones which to the north along the izu bonin arc punctuate tectonic extension for the wmr we hypothesize that rift basins are more commonly the sites where spreading segment offsets nucleate whereas magmatic centers of spreading segments are sites where magmatism continues from arc volcanism through rifting to back arc spreading the mariana trough is opening nonrigidly and is characterized by two predominant abyssal hill trends nnw sse in the north and n s in the south between the only two basin crossing fracture zones at 15 5 and 17 5 n s axes propagated north at the expense of nnw axes

controls on back arc crustal accretion insights from the lau manus and mariana basins abstract together the lau manus and mariana basins encompass a broad range of conditions of back arc basin development marine surveys have determined the tectonic setting and reconnaissance scale geophysical and geochemical properties of the extension axes in these basins we review these data to examine crustal accretion characteristics in the back arc setting in each basin magmatism is enhanced in spreading centres near the arc volcanic front but decreases becoming deficient in axes further from the arc in the lau and manus basins the axes extend far behind the arc and develop typical mid ocean ridge mor like characteristics we propose that these variations are controlled by the subducting slab which shapes composition and flow within the mantle wedge water released from the slab lowers the mantle solidus and increases in concentration toward the volcanic arc spreading centres near the arc advect the hydrated mantle material enhancing melt production slab induced flow carries melt depleted mantle material back beneath the basin where it mixes with ambient mantle spreading centres further from the arc advect this mantle mixture and produce thinner than normal crust far from the arc spreading centres advect essentially mid ocean ridge basalt morb source mantle and their characteristics are mor like thus in addition to spreading rate effects on mantle melting that predominate at mors in back arc basins the subducting slab introduces additional systematic effects

episodicity in back arc tectonic regimes abstract the evolution of back arc basins is tied to the development of the dynamics of the subduction system they are a part of we present a study of back arc basins and model their development by implementing 3d time dependant computer models of subduction including an overriding plate we define three types of episodicity pseudo quasi and hyper episodicity and find evidence of these in nature observations of back arc basin ages histories of spreading quiescence and compression in the overriding plate give us an understanding of the time development of these subduction zones and back arc basins across the globe today a number of trenches are advancing the izu bonin trench the mariana trench the japan trench the java sunda trench and the central portion of the peru chile trench the andes subduction zone the izu bonin mariana and japan all have established back arc basins while the others have documented episodes of spreading quiescence compression or a combination of these the combination of advancing and retreating trench motion places these subduction zones in the category of hyper episodicity quasi episodicity in which the back arc shifts between phases of rifting spreading and quiescence is the dominant form of episodic back arc development in the present we find this type of episodicity in models for which the system is dynamically consistent that we have allowed the subducting plate s velocity to be determined by the sinking slabs buoyancy quasi and hyper episodicity are only found in subduction zones with relatively high subducting plate velocities between 6 and 9 cm year finally those subduction zones for which the subducting plate is moving slowly such as in the mediterranean or the scotia sea experience only pseudo episodicity where the spreading moves linearly towards the trench but often does so in discrete ridge jump events

3 d configuration of subducted lithosphere in the western pacific summary computer graphics techniques are used to portray the shape of the subducted lithosphere as seen from different views the configuration of the subducted lithosphere is represented by a surface that approximately fits the upper envelope of the slab like spatial distribution of earthquake hypocentres in the benioff wadati zone regions considered include tonga kermadec izu bonin mariana japankuril kamchatka and sunda banda in the western pacific these regions include all known zones of deep focus earthquakes except the tectonically complex regions of new hebrides solomon new britain molucca sea phillipines the mediterranean and south america shown at the same scale and with the effect of earth s sphericity included the views are very useful for illustrating variations in the configuration of the subducted lithosphere between different regions and within the same region the results illustrate the segmentation of the upper several hundred kilometres of subducted lithosphere into major units of relatively uniform configuration these segments are also clearly defined by major features of surface morphology of the trench and upper plate the distribution of subduction related volcanism and by the intersection with the subduction zone of major bathymetric features of the subducting plate in contrast the deeper portions of the slabs have a more complex and variable configuration discordances between the deep and shallow structures are often important on a regional scale and suggest that there are relative horizontal movements between the surface trace of the subduction zone and the material at depth into which the subducted lithosphere is sinking these relative movements appear to be intimately associated with the tectonic evolution of the region and indicate a regional aspect of subduction that is often overlooked in an attempt to explain the global variability of the shapes of subducted lithosphere although earthquake activity ceases at 680 km possibly because of a phase transition the geometry of the deep portion of subducted lithosphere suggests that subducted lithosphere does penetrate to greater depth

sedimentary processes in modern and ancient oceanic arc settings evidence from the jurassic talkeetna formation of alaska and the mariana and tonga arcs western pacific abstract sediment deposited around oceanic volcanic ares potentially provides the most complete record of the tectonic and geochemical evolution of active margins the use of such tectonic and geochemical records requires an accurate understanding of sedimentary dynamics in an arc setting processes of deposition and reworking that affect the degree to which sediments represent the contemporaneous volcanism at the time of their deposition we review evidence from the modern mariana and tonga arcs and the ancient arc crustal section in the lower jurassic talkeetna formation of south central alaska and introduce new data from the mariana arc to produce a conceptual model of volcaniclastic sedimentation processes in oceanic arc settings all three arcs are interpreted to have formed in tectonically erosive margin settings resulting in long term extension and subsidence debris aprons composed of turbidites and debris flow deposits occur in the immediate vicinity of arc volcanoes forming relatively continuous mass wasted volcaniclastic records in abundant accommodation space there is little erosion or reworking of old volcanic materials near the arc volcanic front tectonically generated topography in the forearc effectively blocks sediment flow from the volcanic front to the trench although some canyons deliver sediment to the trench slope most volcaniclastic sedimentation is limited to the area immediately around volcanic centers arc sedimentary sections in erosive plate margins can provide comprehensive records of volcanism and tectonism spanning 10 my the chemical evolution of a limited section of an oceanic arc may be best reconstructed from sediments of the debris aprons for intervals up to 20 my but no longer because subduction erosion causes migration of the forearc basin crust and its sedimentary cover toward the trench where there is little volcaniclastic sedimentation and where older sediments are dissected and reworked along the trench slope

geodynamic setting of izu bonin mariana boninites the izu bonin mariana ibm forearc is characterized by the occurrence of boninite like lavas the study of the cenozoic setting of the genesis of these boninitic lavas in light of modern geodynamic contexts in the tonga and fiji regions lead us to define three tectonic settings that favour the formation of boninites in back arc basins in addition to previous settings that involve the presence of a mantle plume 1 propagation at low angle between a spreading centre and the associated volcanic arc 2 intersection at a high angle of an active spreading centre and a transform fault at the termination of an active volcanic arc and 3 intersection at a right angle between an active spreading centre and a newly created subduction zone a geodynamic model of the philippine sea plate shows that boninites in the bonin islands are related to the second mechanism mentioned above whereas mariana forearc boninites are relevant to the third mechanism in the early eocene the transform plate boundary bounding the eastern margin of the philippine sea plate at the location of the present day mariana arc evolved into a subduction zone that trends perpendicular to the active spreading centre of the west philippine basin some where around 43 47 ma the presence of a mantle plume in the vicinity of the subduction zone bounding the northern ibm arc explains boninites that erupted in its northern part but only in early eocene time

a neogene back arc origin for the banda sea basins geochemical and geochronological constraints from the banda ridges east indonesia abstract dredgings conducted during the french indonesian cruises banda sea ii and iii collected volcanic rocks from several ridges of the banda sea area tukang besi ridge site 218 lucipara ridge sites 214 and 305 nieuwerkerk emperor of china sites 219 and 220 with the exception of one 46 ma old n morb type basalt thought to belong to an ophiolitic complex k ar and ar ar datings indicate that all the dredged volcanics are neogene they range in age from ca 10 ma tukang besi back arc basalts to 8 7 ma nieuwerkerk emperor of china calc alkaline andesites and to 7 3 ma lucipara oib type transitional basalts and cordierite bearing andesites radiogenic isotopic signatures of andesites are consistent with an afc assimilation coupled with fractional crystallization process involving assimilation of continental crust 8 3 ma old calc alkaline volcanic activity is also recorded on the wetar segment an inactive part of the east sunda arc and corresponding isotopic compositions are also consistent with an afc process involving continental crust these features suggest that lucipara nieuwerkerk emperor of china ridges and the wetar segment were representing a single volcanic arc 8 7 ma ago the corresponding calc alkaline activity was related to the subduction of the indian oceanic lithosphere beneath continental blocks of australian origin back arc opening processes occurred from 6 to 3 ma as a multi rift opening for the wetar basin and as a single rift opening for the damar basin while subduction related magmatism was still active in the wetar segment volcanic activity stopped at 7 ma in the nieuwerkerk emperor of china ridge on lucipara ridge 6 3 ma volcanic activity emplaced concomitantly transitional basalts and cordierite bearing andesites the mineralogical and chemical features of the latter are consistent with an afc process involving assimilation of continental crust by mantle derived basaltic magmas the end of magmatic activity on both volcanic segments at 3 ma is thought to result from the collision of timor with the wetar segment of the sunda arc

magnetic lineations in marginal basins of the western pacific this paper separates the small oceanic basins around the western perimeter of the pacific ocean into marginal basins that have formed through back arc extension and those that apparently have not and reviews our knowledge of magnetic lineation patterns observed in possible and probable back arc basins magnetic lineations in these basins resemble lineations commonly associated with the world s mid oceanic spreading systems indicating that similar processes of crustal accretion occur in both tectonic environments in some back arc basins of the southwestern pacific magnetic lineation and other evidence suggest that back arc basins can evolve through the interaction and growth of multi plate systems because of the small time and space characteristics of back arc basins compared with the world s major oceanic spreading systems tectonic conditions favourable for the generation of back arc basins are either relaxed rapidly or easily interrupted models proposed to account for back arc basin development include a local models where back arc extension is mechanically driven by the downgoing slab and b global plate kinematics models where conditions favourable for back arc extension are governed by the motion of the overriding plate relative to the trench axis

geological interpretation of volcanism and segmentation of the mariana back arc spreading center between 12 7 n and 18 3 n the relationships between tectonic processes magmatism and hydrothermal venting along 600 km of the slow spreading mariana back arc between 12 7 n and 18 3 n reveal a number of similarities and differences compared to slow spreading mid ocean ridges analysis of the volcanic geomorphology and structure highlights the complexity of the back arc spreading center here ridge segmentation is controlled by large scale basement structures that appear to predate back arc rifting these structures also control the orientation of the chains of cross arc volcanoes that characterize this region segment scale faulting is oriented perpendicular to the spreading direction allowing precise spreading directions to be determined four morphologically distinct segment types are identified dominantly magmatic segments type i magmatic segments currently undergoing tectonic extension type ii dominantly tectonic segments type iii and tectonic segments currently undergoing magmatic extension type iv variations in axial morphology including eruption styles neovolcanic eruption volumes and faulting reflect magma supply which is locally enhanced by cross arc volcanism associated with n s compression along the 16 5 n and 17 0 n segments in contrast cross arc seismicity is associated with n s extension and increased faulting along the 14 5 n segment with structures that are interpreted to be oceanic core complexes the first with high resolution bathymetry described in an active back arc basin hydrothermal venting associated with recent magmatism has been discovered along all segment types

a three plate kinematic model for lau basin opening 1 we present revised compilations of bathymetry magnetization acoustic imagery and seismicity data for the lau basin and surrounds we interpret these data to more precisely locate the plate boundaries in the region and to derive a three plate kinematic model for the opening of the lau basin during the brunhes chron our tectonic model includes the niuafo ou microplate which is separated from the australian plate along the peggy ridge lau extensional transform zone central lau spreading center and from the tongan plate along the newly discovered fonualei rift and spreading center our model shows that australia tonga plate motion between 15 5 s and 19 s is partitioned across the microplate and it resolves the former apparent conflict between geodetic versus seafloor spreading velocities regionally this implies that the angular rate of opening of the lau basin has been faster than that of the havre trough to the south and that the rapid current rates of pacific subduction at the tonga trench 240 mm yr at 16 s have been sustained for at least 0 78 m y

tectonics of the southern tip of the parece vela basin philippine sea plate we report new geophysical and petrological data collected at the southern tip of the parece vela basin in the philippine sea the parece vela basin which was formed as a backarc basin behind proto mariana arc trench system from late oligocene to middle miocene provides us a good opportunity to study the nature of successive backarc basin formations in the philippine sea and the relationship between are and backarc magmatisms regional bathymetric map derived from satellite altimetry shows that the southern tip of the basin now located just west of the yap arc trench system has unique morphological and tectonic features which include 1 the absence of spreading center or its trace 2 shallow average depth and 3 enigmatic curved structures our newly collected high resolution bathymetric data reveal that the spreading fabric similar to the central parece vela basin exists to the north of 9 degrees 20 n thus it appears that the present day yap arc and backarc region represent the western half of the seafloor that was produced by the early e w and the following ne sw spreading in the northern and central parece vela basin and that the eastern counterpart now lies west of the west mariana ridge unlike the northern parece vela basin there appears to be no evidence for a systematic propagation of spreading center in the southern part instead two rift segments one which extends from the central parece vela basin and the other which lies within the western remnant arc kyushu palau ridge overlap at the southern tip of the basin producing a complex seafloor that includes curvilinear deeps and deformed topographic highs c 2007 elsevier b v all rights reserved

traditional soil particle sphericity roundness and surface roughness by computational geometry definitions of soil particle sphericity roundness and roughness have existed since at least the 1930s in the 1950s charts of typical sphericity and roundness values were developed to alleviate tedious manual determination they allowed users to classify particles by visual comparison to typical particles possessing ranges of sphericity and roundness the original definitions and somewhat subjective chart methods are still widely used today this paper describes robust numerical methods based on computational geometry to determine precisely the traditional values from two dimensional images of particles statistical methods including locally weighted regression and k fold cross validation were used to discretise a mean particle surface and thereby quantify and remove roughness the paper details the algorithms for identifying particle corners and fitting circles to them for computation of roundness conclusions are also drawn regarding the most appropriate definition for particle sphericity from among f

tectonic volcanic and hydrothermal features of a nascent rift graben in the southern okinawa trough abstract high resolution geophysical mapping using an autonomous underwater vehicle auv and surface vessel revealed tectonic volcanic and hydrothermal features of a nascent rift graben in the southern okinawa trough ship based bathymetry identified ene wsw trending lineaments at a water depth of 1900 2000 m that form a rift graben the ishigaki rift which is 50 km long 7 km wide and has relief of 100 m shipboard em122 multibeam sonar detected acoustic bubble plumes and high backscatter intensity on the seafloor along the rift the auv high resolution bathymetry 2 m and sidescan imagery revealed the presence of a ropey texture with high acoustic reflectivity interpreted to be relatively young lava flows evidence of hydrothermal venting was observed at two previously unknown sites along the rift steep sided spires up to 23 m in height and a mound discharging smoke like acoustic plumes were identified the auv temperature sensor observed sharp temperature increases of up to 0 13 c around the plumes even at the vehicle s altitude of 100 m above the seafloor the presence of relatively young lava flows and indicators of high temperature hydrothermal venting and gas bubbles suggest the ishigaki rift is magmatically active compared with adjacent active back arc rifts the ishigaki rift has a shorter length narrower width and lower relief and occurs at a shallower water depth suggesting that the rift is relatively young and immature considering that the ishigaki rift is located near the arc volcanic front and that other mature rifts are typically located farther from the front it is suggested that a new axial rift system is evolving within the okinawa trough and that back arc rifting processes may be gradually migrating arcward in the trough during the quaternary

assessing automated image analysis of sand grain shape to identify sedimentary facies gran dolina archaeological site burgos spain abstract gran dolina is a cave sierra de atapuerca spain infilled by a 25 m thick sedimentary record divided into 12 lithostratigraphic units that have been separated into 19 sedimentary facies containing early and middle pleistocene hominin remains in this paper an automated image analysis method has been used to study the shape of the sedimentary particles since particle shape is interpreted as the result of sedimentary transport and sediment source this study can provide valuable data about the sedimentological mechanism of sequence formation the shape of the sand fraction in 73 samples from gran dolina site and sierra de atapuerca was analyzed using the malvern morphologi g3 an advanced particle characterization tool in this first complete test we used this method to the published sequence of gran dolina defined previously through field work observations and geochemical and textural analysis the results indicate that this image analysis method allows differentiation of the sedimentary facies providing objective tools to identify weathered layers and measure the textural maturity of the sediments channel facies have the highest values of circularity and convexity showing the highest textural maturity of particles on the other hand terra rossa and debris flow samples show similar values with the lowest particle maturity

on the mechanism of seismic decoupling and back arc spreading at subduction zones to address the problem of the great variability of the mechanical state of subduction zones we investigate the mechanics of back arc spreading and seismic decoupling back arc spreading is assumed to be due to rifting of the upper plate and hence occurs when trench normal tension reaches a critical value seismic decoupling is assumed to occur when the normal stress at the frictional interface is decreased by an amount sufficient to cross the friction stability transition two forces are important in this problem the first is a small component of the slab pull force which remains unbalanced by the subduction resistance and exerts a vertical suction force at the trench the second is a sea anchor force exerted on the slab that resists its lateral motion assumed to occur at the upper plate velocity both forces contribute to the coupling problem only the sea anchor force is responsible for back arc spreading the unbalanced slab pull force is determined from a force balance for subduction the sea anchor force is computed as the hydrodynamic resistance to the facewise translation of an elliptical disc through a viscous fluid the model predicts three regimes seismically coupled compressional arcs with advancing upper plates seismically decoupled extensional arcs with retreating upper plates and strongly extensional arcs which also have back arc spreading this model is applied in detail to the izu bonin marianas system it shows that back arc spreading occurs when the integrated tension in the upper plate exceeds a value of about 1 1013 n m 1 and requires a residual tension of about a third that to drive the back arc spreading once rifting is completed it shows why the plate interface near guam is seismically coupled while the plate boundary everywhere farther north is decoupled when applied globally it successfully predicts the state of seismic coupling and back arc spreading in more than 80 of the world s subduction zones of the remaining half can be seen to contain additional complications not included in the model about 10 of cases remain unexplained but some of these may have incorrectly determined seismic coupling coefficients

spreading process of the northern mariana trough rifting spreading transition at 22 n 1 we have conducted a geophysical survey of the northern mariana trough from 19 n to 24 n the trough evolves southward from incipient rifting to seafloor spreading within this region this study aims to clarify the location and time of the rifting to spreading transition which was controversial previously and processes of seafloor spreading after the transition the new data set includes swath bathymetry with side scan images and magnetic vector anomaly the mantle bouguer gravity anomaly mba was calculated using the free air gravity anomaly from satellite altimetry the rifting to spreading transition occurs at about 22 n which is proved by seafloor spreading fabric in the bathymetry clear magnetic lineations and the bull s eye pattern in mba four ridge segments separated by three nontransform discontinuities are recognized between 19 n and 22 n the northernmost segment has relatively abundant magma supply compared with the other segments which is estimated from a larger segment length shallower axial depths with no rift valley and lower mba the next segment to the south is on the other hand a magma starved segment with a prominent rift valley two anomalously deep grabens called the central grabens formed by amagmatic extension occur near the segment ends the succession of magma rich magma starved and normal segments with increasing distance from the volcanic arc is the same as the observation in the lau basin reported by martinez and taylor 2002 the magnetic anomaly revealed the detailed history of the spreading the seafloor spreading between 19 n and 20 n began prior to 5 ma and that between 20 n and 21 30 n began at about 4 ma spreading half rates in the western side of the spreading center were 2 to 3 cm year before 2 58 ma south of 21 30 n and during the matuyama chron north of 21 30 n but an average during the brunhes chron is 1 cm year or less orientations of the ridge axes which range from 20 to 0 at present have rotated about 20 clockwise since the start of the spreading these changes in rate and direction might be associated with changes in the motion of the philippine sea plate spreading has been asymmetric in the northern mariana trough the spreading rates of the western side of the spreading center have been significantly larger than the eastern counterpart in general the asymmetry may have been caused by an interaction of mantle upwelling systems under the volcanic front and the backarc spreading center and would be a characteristic of backarc spreading

early evolution of a young back arc basin in the havre trough back arc basins are found at convergent plate boundaries nevertheless they are zones of significant crustal extension that show volcanic and hydrothermal processes somewhat similar to those of mid ocean ridges accepted models imply the initial rifting and thinning of a pre existing volcanic arc until seafloor spreading gradually develops over timescales of a few million years the havre trough northeast of new zealand is a unique place on earth where the early stages of back arc basin formation are well displayed in the recent geological record here we present evidence that in this region rifting of the original volcanic arc occurred in a very narrow area about 10 15 km wide which could only accommodate minimal stretching for a very short time before mass balance required oceanic crustal accretion an initial burst of seafloor spreading started around 5 5 5 0 million years ago and concluded abruptly about 3 0 2 5 million years ago after which arc magmatism dominated the crustal accretion the sudden transition between these different tectonomagmatic regimes is linked to trench rollback promoted by gradual sinking of the subducting lithosphere which could have diverted the arc flux outside the region of seafloor spreading and induced the vertical realignment of surface volcanism with the source of arc melts at depth the havre tough back arc basin new zealand formed rapidly and in two phases initial limited seafloor spreading was followed by a transition to arc magmatism as shown by geophysical data and modelling

working with broken agents exploring computational 2d morphometrics for studying the post depositional history of potsherds abstract the study of size and shape of ceramic fragments is a common approach used for unraveling the depositional and post depositional history of archaeological assemblages similar to sediment particles the size and shape of a potsherd are altered under conditions of abrasion breakage or weathering occurring during or after deposition which can be the product of both human and non human agents thus the analysis of dimensional and morphological changes of fragments not only points towards specific post depositional processes but can also shed light on the nature of human interaction with broken pottery by coupling a traditional sedimentological framework for shape description and a computational approach to 2d morphometrics in this paper we present a quick reproducible and accurate method for studying the alteration of shape and size of potsherds three main shape descriptors i e sphericity roundness and convexity were tested on a combination of experimental and early neolithic potsherds in northwest romania from contexts where the conditions of fragment alteration were known or could be safely inferred in addition potsherds from an early neolithic pit discovered at c linesti oas d mbul sfintei m rii satu mare county romania were analysed for determining the pit infilling process and understanding the extent of the interaction between humans and broken pots results from morphometric tests show the reliability of the computational technique and shape descriptors for identifying different conditions of alteration of potsherds the analysis of the romanian assemblages provide insight into post depositional processes showing downslope movement as the main mechanism behind the infilling of the pit at c linesti oas d mbul sfintei m rii in addition the potsherds possessed a high level of erosion and fragmentation highlighting a long span of human interaction with broken materials at this site during early neolithic times

malaguana gadao ridge identification and implications of a magma chamber reflector in the southern mariana trough 1 six channel seismic reflection data reveal a magma chamber reflector beneath the malaguana gadao ridge the southernmost segment of the spreading center in the mariana trough for most of its length the spreading center in this active back arc basin is morphologically similar to slow spreading mid ocean ridges having a deep central graben flanked by a zone of abyssal hill fabric this southernmost segment however has a broad smooth cross section lacks a deep central graben and is thus similar in morphology to fast spreading ridges e g the east pacific rise we identify a magma chamber at 1 5 s two way travel time below the crest of the ridge observations from remotely operated vehicles along the ridge reveal not only fresh pillows lobate and sheet lava flows but also an abundance of volcaniclastic debris and intense hydrothermal activity these observations together with the fast spreading morphology of the ridge suggest that this segment has a considerably higher magma supply than is typical in the mariana trough we suggest that the volcanic arc or enhanced melting of a hydrated mantle is supplying volatile rich magma as evidenced by a highly negative coefficient of reflection 0 42 for this mcr and the presence of evolved highly vesicular lava and volcaniclastic materials the southeastern mariana back arc basin spreading ridge does not compare readily with mechanical models for global mid ocean ridge data sets because of marked asymmetry in both volcanism and deformation that may be the consequence of slab related geometry in this part of the convergent margin system

asymmetric accretion along the slow spreading mariana ridge 1 we report the results of several surface geophysical surveys of the central part of the mariana basin western pacific analysis of bathymetric and magnetic data along three distinct spreading segments indicates highly asymmetric spreading processes the half spreading rate being two to three times more important on the western side of the basin than on its eastern side such an asymmetry is as strong at the centers of segments as at their ends and does not systematically correlate with an asymmetric shape of the rift valley a half graben shape of the rift valley is observed only at segment ends at segments centers a symmetric graben shape characterizes the valley the asymmetric shape of the valley at segment ends probably reflects the localization of the tectonic strain due to changes in lithospheric rheology and thickness asymmetric spreading is likely accomplished through a different amount of deformation and magma emplacement on both sides of the spreading axis it may result from the combined effects of 1 asymmetric rheologic melting and stress conditions imposed by the influence of the pre existing volcanic arc and its melt sources on the eastern side of the basin 2 the northwestward movement of the philippine sea plate while the almost vertical pacific slab in the mariana subduction zone resists to this motion 3 a roll back component at the mariana subduction zone

fe ti basalts and propagating rift tectonics in the josephine ophiolite the jurassic josephine ophiolite of northwest california and southwest oregon is one of the largest and most complete ophiolites in north america and clearly formed in a suprasubduction zone setting the geochemistry of lavas and dikes is highly variable because of a range in magma types and different degrees of fractionation basalts rich in fe and ti occur in the uppermost part of the extrusive sequence and as late dikes cutting oceanic serpentinite they have morb mid oceanic ridge basalt affinities and appear to be unrelated to the rest of the ophiolite that has affinities dominantly transitional between iat and morb iat island arc tholeiite this late fe ti morb suite was erupted while the ophiolite was undergoing the last half of 50 of tilting of the entire crustal sequence and after widespread serpentinization of ultramafic cumulates and uppermantle peridotite related to freezing of the axial magma chamber fe ti basalts are characteristic of propagating spreading centers on mid ocean ridges and in at least one backarc basin geochemical structural stratigraphic and regional geologic constraints suggest formation of the josephine ophiolite by propagation of a spreading center into rifted island arc lithosphere that is preserved along the margin of the ophiolite another possible indication of propagating rift tectonics is the presence of off axis metalliferous sedimentary rocks that may have been deposited as hydrothermal plume fall out from a second propagating spreading center the lau basin provides modern analogues including spreading centers propagating into rifted arc lithosphere into older backarc ocean crust and across the arc into the forearc

curved grooves at the godzilla megamullion in the philippine sea and their tectonic significance deep sea core complexes are characterized by remarkably well developed grooves that formed below extensional detachment faults and reveal plate divergence direction during asymmetric plate spreading the godzilla megamullion in the parece vela basin in the philippine sea contains the longest known grooves on an oceanic core complex with individual groove lengths up to 60 km over much of the complex curved grooves reveal a 10 change in plate divergence direction during miocene back arc spreading behind the mariana island arc evaluation of groove and transform fault orientations indicates that the curved grooves do not reflect plate displacement about a fixed pole of rotation but rather reflect a change in the position of the rotation pole this in turn was associated with increasingly significant left lateral transform motion in the back arc parece vela basin

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

bathymetry and magnetic anomalies in the havre trough and southern lau basin from rifting to spreading in back arc basins abstract the havre trough and lau basin are active back arc basins related to pacific australian plate convergence these back arc basins provide insight into the evolution of rifting through various stages including a final stage of oceanic spreading we carried out a swath bathymetry and magnetic anomaly survey focused on the less studied havre trough and southern lau basin the western portion of the whole havre trough is deeper and more thickly sedimented whereas shallow bathymetry and volcanic edifices are dominant in the east we suggest that crustal rifting progresses asymmetrically with respect to the trough axis the active tectono magmatic zone migrates trenchward during ongoing widening rifting along normal faults trending in the longitudinal trough strike occurs in the southern havre trough volcanism is restricted to a line along the normal faults circular or elongated knolls in the northern havre trough indicate that en echelon type segmented rifting of short strike length and successive volcanic intrusion takes place volcanism in this area is dominated by diapir like magma intrusion into an extended and thinned arc crust the southernmost tip of the spreading center of lau basin is defined by the deep graben striking at n20 e located at 24 00 s 177 10 w

preferential rifting of continents a source of displaced terranes lithospheric rifting while prevalent in the continents rarely occurs in oceanic regions to explain this preferential rifting of continents we compare the total strength of different lithospheres by integrating the limits of lithospheric stress with depth comparisons of total strength indicate that continental lithosphere is weaker than oceanic lithosphere by about a factor of 3 also a thickened crust can halve the total strength of normal continental lithosphere because the weakest area acts as a stress guide any rifting close to an ocean continent boundary would prefer a continental pathway this results in the formation of small continental fragments or microplates that once accreted back to a continent during subduction are seen as displaced terranes in addition the large crustal thicknesses associated with suture zones would ironically make such areas likely locations for future rifting episodes this results in the tendency described as the wilson cycle of new oceans to open along the suture where a former ocean had closed

mantle wedge control on back arc crustal accretion at mid ocean ridges plate separation leads to upward advection and pressure release partial melting of fertile mantle material the melt is then extracted to the spreading centre and the residual depleted mantle flows horizontally away in back arc basins the subducting slab is an important control on the pattern of mantle advection and melt extraction as well as on compositional and fluid gradients modelling studies predict significant mantle wedge effects on back arc spreading processes here we show that various spreading centres in the lau back arc basin exhibit enhanced diminished or normal magma supply which correlates with distance from the arc volcanic front but not with spreading rate to explain this correlation we propose that depleted upper mantle material generated by melt extraction in the mantle wedge is overturned and re introduced beneath the back arc basin by subduction induced corner flow the spreading centres experience enhanced melt delivery near the volcanic front diminished melting within the overturned depleted mantle farther from the corner and normal melting conditions in undepleted mantle farther away our model explains fundamental differences in crustal accretion variables between back arc and mid ocean settings

dynamics of the ryukyu izu bonin marianas double subduction system trench motions represent the surface expression of the interaction between subducting plates and the underlying mantle but the inherent dynamics are not fully understood one interesting case is the migration of the izu bonin marianas trench ibm that accommodates the subduction of the pacific beneath the philippine sea plate psp which is in turn subducting beneath the eurasian plate along the ryukyu trench the history of the ibm trench is dominated by fast episodic retreat from 40 to 15 ma however around 10 5 ma the ibm trench reversed its motion from retreat to advance the switch in trench motion occurred soon after the breakoff of the psp slab along the ryukyu trench and the onset of new subduction and represents a fundamental change in the dynamics of the western pacific subduction zones here following the modelling study of ci kova and bina 2015 which suggested a link between ibm trench advance and ryukyu subduction we run 2 d numerical experiments to test the influence of a newly formed ryukyu slab on the established ibm subduction zone we run two dimensional numerical experiments to test the influence of this newly formed ryukyu slab on the ibm subduction zone the results from our geodynamic model compare favourably with the reconstructed trend indicating that the switch in trench motion along the ibm trench may indeed be related to the onset of a new subduction zone along the ryukyu trench our analysis substantiates the idea that advancing trench motions in the western pacific are due to the establishment of a double subduction system further analysis of such dynamics provides insights for the mechanisms controlling subducting plate and trench motions and mantle force transmission

three dimensional magnetic modeling of a propagating rift galapagos 95 30 w a combined deep tow and seabeam investigation at 95 30 w on the cocos nazca spreading center has revealed the crustal contact between the propagating rift and the dying rift systems the observed 27 km offset between the axes creates an ideal situation for the application of magnetic methods the normally magnetized crust of the propagating rift tip penetrates into older crust which was created when the earth s main field was reversed as the v shaped structure of the rift clearly violates the conventional two dimensional assumptions a full three dimensional analysis is required in addition the distorting influence of over 1000 m bathymetric relief must be removed from the observed field inversions have been performed on the gridded representations of the observed magnetic field and bathymetry working in the fourier domain the result is a gridded rock magnetization distribution the inversion of the surface data covers a large area 6000 km2 and demonstrates close agreement with magnetization amplitudes of rock samples at existing dredge sites in general the propagating rift process appears to be much more orderly than the dying rift process the magnetic polarity transition widths are narrower and the boundaries have fewer undulations than the dying rift which appears to be quite episodic in behavior the average propagation rate is 52 mm yr compared to the average spreading half rate of 29 mm yr the locations of the boundaries suggest that the acceleration to the normal spreading rate on the propagating rift requires about 250 000 years the inversion of the deep tow data near the seafloor provides a high resolution definition of the location of the tip of the propagating rift at 2 38 1 n 95 30 0 w

rifting history of the northern mariana trough seamarc ii and seismic reflection surveys seamarc ii sidescan sonar data and seismic reflection profiles of the northernmost portion of the mariana trough suggest a rifting rather than a seafloor spreading origin for lithosphere within this active backarc basin of the western pacific variations in morphology along the strike of the backarc basin suggest both a spatial and a temporal variation in the style of extension of the basin the northern mariana trough widens from 70 km near 23 n to 135 km near 21 30 n major active fault zones bound both sides of the backarc basin active rifting and volcanism are predominant in a roughly 20 km wide active rift zone along the eastern arc side of the basin from 24 n to 22 n the locus of active rifting has remained near the volcanic front of the arc during extension and is probably controlled by lithospheric weakness which is greatest at the volcanic front the zone of rifting intersects the active volcanic arc at nikko seamount the distinction between the mechanism for formation of the mariana trough proposed here and that of slow mid ocean ridge spreading is that no true plate boundary has developed in the northernmost mariana trough rather the backarc basin is composed of arc lithosphere intruded by arc and backarc basin magmas volcanism both in the rift zone and throughout the basin is fault controlled south of 22 n the zone of active rifting diverges westward from the arc as the transition to true seafloor spreading is approached the transition to true mid ocean ridge spreading occurs to the south of the survey area described here

silicic volcanism and back arc extension related to migration of the late cainozoic australian pacific plate boundary abstract the initial phases of large scale silicic and bimodal volcanism within an active miocene arc coromandel volcanic zone cvz are preserved on several small islands in northern new zealand major and trace element compositions classify the rocks as high k calc alkaline silicic lavas which have some of the low ion lithophile element and high field strength element characteristics of convergent margin lavas the silicic lavas 12 ma are spatially associated with a relatively primitive basalt of similar age 12 06 0 25 ma which is a sub alkaline tholeiite with an anomalously high tio2 content and as such it is not a typical arc basalt isotopically the samples indicate the presence of oceanic sediment in their petrogenesis an explanation for the geochemical and isotopic compositions of the northern cvz rocks involves migration of the australian pacific convergent margin orientation from a north south direction northland volcanic arc through a northeast southwest direction cvz and finally subparallel to the orientation of the presently active taupo volcanic zone the silicic eruptives thereby preserve a trail of volcanic edifices within a back arc basin the edifice trail exhibits a general northwest to southeast younging trend and lies perpendicular to the axis of the contemporaneous coromandel volcanic arc late cainozoic migration of the convergent margin and subduction system is proposed as the mechanism for extension across the back arc region of the cvz producing large scale back arc silicic volcanism and minor associated basalts perpendicular to the orientation of the main arc

on the evolution of marginal basins this paper presents a critique of the models proposed to explain the formation of back arc basins those marginal basins located behind active or inactive trench systems and whose origin is inferred to be subduction related several marginal basins are not generically back arc but rather are related to the initiation of subduction continental rifting or plate boundary readjustments models of back arc basin formation must be consistent with the observed characteristics of back arc basins which include the following 1 the crustal structure and magnetic lineation patterns of back arc and major ocean basins are similar implying that the processes of crustal accretion in both tectonic environments are also similar 2 arc trench systems without currently spreading back arc basins are in the majority 3 although petrographically similar to and within the compositional range of midocean ridge basalts morb back arc basin basalts babb show consistent geochemical differences from n type morb and are in many respects transitional toward island arc tholeiites iat 4 distinct babb and iat mantle sources may be in extremely close proximity during the initial rifting of an island arc 5 rifting of the volcanic arc may occur on either side of the line of active volcanoes 50 km and may vary from one side to the other along strike 6 a remnant arc is not always developed 7 back arc spreading may be initiated above subducted lithosphere but with time the center of spreading migrates away from the arc and no longer overlies a benioff zone and 8 there is no simple correlation between the timing of global plate reorganizations and the formation of back arc basins the three main classes of models proposed to explain back arc basin formation are mantle diapirism induced aesthenospheric convection and global plate kinematics because the driving mechanism for the first two classes of models is provided by the local subducting slab these models fail to explain the temporal and spatial distribution of back arc basins the third class of models proposes that back arc basins should form whenever global plate interactions require divergence between the overriding plate and the trench line the latter being observed to move only in a seaward direction with respect to a hot spot reference frame however the tectonic setting of several back arc basins suggests that they represent more than just a passive response to kinematic boundary conditions for example some dynamic back arc force seems required to move the new hebrides tonga and south sandwich forearc plates seaward neither the local nor the global models adequately specify the necessary and or sufficient conditions for back arc basin formation a better understanding of the processes associated with the initiation of rifting and of subduction is required arc rifting and back arc spreading are both secondary phenomena to subduction but they are also primary extensional phenomena fundamentally analogous to continental rifting and the initial spreading of major ocean basins

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