oceanic plateau formation by seafloor spreading implied by tamu massif magnetic anomalies tamu massif is an immense mesozoic submarine volcano the main edifice of the shatsky rise oceanic plateau it is located at a spreading ridge triple junction but considered to be a shield volcano formed by effusive volcanism from an emerging mantle plume however it is unclear how tamu massif eruptions interacted with the spreading ridges which are enormous linear volcanoes themselves here we create a magnetic anomaly map for tamu massif which can provide clues about crustal formation for tamu massif we find dominantly linear magnetic field anomalies caused by crustal blocks of opposite magnetic polarity this pattern suggests that tamu massif is not a shield volcano but was emplaced by voluminous focused ridge volcanism if the magma source at the shatsky rise was a plume it was closely connected to and controlled by seafloor spreading by implication even the largest oceanic plateau edifices can be formed by seafloor spreading we suggest that the widely accepted analogy between continental flood basalts and oceanic plateaus requires reconsideration the shatsky rise oceanic plateau formed by spreading ridge volcanism according to analyses of linear magnetic anomalies over the tamu massif submarine volcano

double flood basalts and plume head separation at the 660 kilometer discontinuity several of the world s flood basalt provinces display two distinct times of major eruptions separated by between 20 million and 90 million years these double flood basalts may occur because a starting mantle plume head can separate from its trailing conduit upon passing the interface between the upper mantle and the lower mantle this detached plume head eventually triggers the first flood basalt event the remaining conduit forms a new plume head which causes the second eruptive event the second plume head is predicted to arrive at the lithosphere at least 10 million years after the first plume head in general agreement with observations regarding double flood basalts

northwest pacific tectonic evolution in the middle mesozoic abstract two magnetic bights implying pacific izanagi farallon p i f ridge ridge ridge rrr triple junctions occur in the northwest pacific basin one is formed in anomalies m26 m21 southeast of shatsky plateau while the second is formed in anomalies m10 cl1 the latter a new anomaly between m1 and m0 northeast of shatsky plateau the offset between these two bights is in a n40 e direction parallel to the p f relative motion during m20 to m10n time half spreading rates and directions and the assumption of symmetric orthogonal spreading permit the solution of the p i f velocity triangle and reveal an isosceles triangle with the i f rate equal to the p f rate this is a necessary condition for a stable fault fault ridge ffr triple junction which would migrate in the n40 e direction of the offset between magnetic bights the growth of the p f transform fault explains the increasing offset across the ends of anomalies m20 m10n projected onto the trace of shatsky plateau subsequent evolution led to the formation of a system of long transform faults offset by short spreading centers which we call the shatsky fracture zones and which extend eastward across the pacific from the plateau the original trace of the transform fault fracture zone that formed at the triple junction underlies the main mass of the shatsky plateau and would suggest that the formation of the plateau is at least coincidental with the existence of the ffr triple junction at 21 ma

an immense shield volcano within the shatsky rise oceanic plateau northwest pacific ocean most oceanic plateaux are massive basaltic volcanoes however the structure of these volcanoes and how they erupt and evolve is unclear because they are remote and submerged beneath the oceans here we use multichannel seismic profiles and rock samples taken from integrated ocean drilling program core sites to analyse the structure of the tamu massif the oldest and largest edifice of the shatsky rise oceanic plateau in the north western pacific ocean we show that the tamu massif is a single immense volcano constructed from massive lava flows that emanated from the volcano centre to form a broad shield like shape the volcano has anomalously low slopes probably due to the high effusion rates of the erupting lavas we suggest that the tamu massif could be the largest single volcano on earth and that it is comparable in size to the largest volcano in the solar system olympus mons on mars our data document a class of oceanic volcanoes that is distinguished by its size and morphology from the thousands of seamounts found throughout the oceans

a new mesozoic isochron chart of the northwestern pacific ocean paleomagnetic and tectonic implications a new mesozoic isochron chart of the northwestern pacific ocean has been completed by utilizing all the currently available data from 357 cruises our chart provides the first whole view of the mesozoic magnetic anomaly lineations in the northwestern pacific ocean and yields essential information for understanding the problems of the mesozoic magnetostratigraphic time scale and the evolutional history of the pacific plate the mesozoic magnetostratigraphic time scale must be recalibrated using not only the hawaiian lineation set but also other mesozoic lineation sets the lack of lineation m0 in the lineation sets other than the hawaiian lineation set indicates that a reorganization of the ridges except the pacific farallon ridge occurred between chrons m1 and m0 nearly 120 ma the eruption rate of the shatsky hotspot seems to be comparable to that of the reunion hotspot at their initial stage the growth of the pacific plate began in a small triangular region at about 190 ma

preserved paleo oceanic plateaus in accretionary complexes implications for the contributions of the pacific superplume to global environmental change abstract we have reinvestigated the mid cretaceous plume pulse in relation to paleo oceanic plateaus from accretionary prisms in the circum pacific region and we have correlated the pacific superplume activity with catastrophic environmental changes since the neoproterozoic the paleo oceanic plateaus are dated at 75 150 ma they were generated in the pacific superplume region and are preserved in accretionary prisms the volcanic edifice composed of both modern and paleo oceanic plateaus is up to 10 7 10 6 km 2 in area and 19 1 10 7 km 3 in volume the degassing rate of co 2 0 82 1 1 10 18 mol m y suggests a significant impact on cretaceous global warming the synchronous occurrence of paleo oceanic plateaus in accretionary complexes indicates that pacific superplume pulse activities roughly coincided at the permo triassic boundary and the vendian cambrian boundary interval the co 2 expelled by the pacific superplume probably contributed to environmental catastrophes the initiation of the pacific superplume contributed to the snowball earth event near the vendian cambrian boundary this was one of the most dramatic events in earth s history the scale of the pacific superplume activity roughly corresponds to the scale of drastic environmental change

volcanic evolution of the submarine super volcano tamu massif of shatsky rise new insights from formation microscanner logging imagery tamu massif the southernmost plateau of shatsky rise is recently reported as the largest single volcano known on earth this work seeks to understand the type of volcanism necessary to form such an anomalously large single volcano by integrating core and high resolution wireline logging data in particular resistivity imagery obtained by the formation microscanner in integrated ocean drilling program hole u1347a located on the eastern flank of tamu massif was used to construct a logging based volcanostratigraphy this model revealed two different volcanic stages formed tamu massif i the core part of the massif s basaltic basement was formed by a construction phase of volcanism with cyclic eruption events from a steady state magma supply and ii the very topmost basaltic section was formed by a depositional phase of volcanism during which long traveling lava flows were deposited from a distant eruption center

effects of ridge geometry on mantle dynamics in an oceanic triple junction region implications for the azores plateau abstract plate boundary geometry can affect the nature of magmatism along a mid ocean ridge the azores plateau is located in a complex geological setting that includes a triple junction tj an oblique and recently formed ultra slow spreading ridge a zone of diffuse seafloor deformation a major fracture zone and a postulated hotspot the precise character of the hotspot is somewhat debated as some lines of evidence indicate it may not be a classic deep seated plume however seismic and gravity data suggest plateau crustal thicknesses of 8 km or more implying some mechanism for excess melting to assess the role of ridge geometry in creating the azores plateau this study uses a finite element numerical model to isolate the effects of selected aspects of plate boundary configuration on mantle flow and melt production in a tj kinematically similar to the azores tj the model focuses on the slowest spreading ridge in the tj analogous to the terceira rift the effect of the varying ridge obliquity observed along the terceira rift is also assessed using an independent 1 d melting model in general relatively little melt production is predicted along the terceira rift analogue except for regions closest to the tj where the proximity of a faster spreading ridge increases temperatures within the melting zone in the 1 d melting model with mantle temperatures of 1350 c melt thicknesses of 2 km are calculated for the least oblique segments while more oblique segments produce little to no melt the presence of a long discontinuity simulating the gloria fz has little effect on mantle dynamics for axial distances

the mesozoic tectonic history of the magellan microplate in the western central pacific magnetic lineation mapping in the western central pacific has revealed a pair of opposite sensed fanned lineation patterns that define the accretionary boundaries of the fossil magellan microplate this tectonic synthesis results from extensive magnetic mapping of two new lineation patterns over a large area and extended mapping of previously identified lineations the entire evolutionary history of the magellan microplate is well constrained to a 9 m y period in the early cretaceous by synchronous spreading patterns and associated geologic data during this period the microplate grew and evolved as a generally rectangular structure to a final size of 700 km 600 km with spreading centers on two opposing sides and transform faults on the other two sides the lifetime and size of the magellan microplate are somewhat longer and larger respectively than presently active microplates on the east pacific rise however these modern structures are still evolving and growing and the tectonic behavior of the modern and cretaceous systems appears to be similar study of both active and fossilized microplates should provide additional insights on their common tectonic histories in particular we show that the magellan trough spreading center behaved as an asymmetric accretionary plate boundary that can be described with two separate poles of motion very close to this spreading center during much of its history the magellan trough spreading center then failed as a result of a larger ridge reorganization at the triple junction of the pacific farallon and phoenix plates at ml0n time microplate activity ceased when the microplate became welded to the pacific plate at m9 time

relative paleointensity stack during the last 250 kyr in the northwest pacific 1 we have conducted a paleomagnetic study of 10 sediment cores recovered from the northwest pacific the purpose is to establish a regional paleointensity stack during the last 250 kyrs nopapis 250 for global coverage of paleointensity data as well as a regional reference for paleointensity assisted stratigraphy in the studied cores reductive dissolution of magnetite appears to have occurred mainly in glacial periods such horizons accompany large decreases in s ratio s 0 3t 1 irm 0 3t sirm 2 as low as 0 90 the sediments are also intercalated by frequent volcanic ash layers prerequisites for reliable relative paleointensity estimation are not satisfied at these horizons we rejected unsuitable data by quantitative criteria based on s ratio and saturation isothermal remanent magnetization sirm and constructed the nopapis 250 curve from the remaining data in the construction of the curve we chose irm as a normalizer because acquisitions of anhysteretic remanent magnetization arm are considered to be influenced by magnetic interactions among magnetic grains in the northwest pacific sediments relative intensity variation of the nopapis 250 gives a standard deviation that is 33 of the average if the normal distribution is assumed the nopapis 250 curve generally shows similar variation patterns compared with other high resolution records reported from other regions many paleointensity lows are recognized in common six out of the nine lows can be correlated with reported excursions in literature

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

structural variation of the oceanic moho in the pacific plate revealed by active source seismic data abstract the characteristics of the oceanic moho are known to depend on various factors such as seafloor spreading rate crustal age and accretionary processes at a ridge however the effect of local magmatic activities on the seismic signature of the moho is poorly understood here an active source reflection and refraction survey is used to investigate crustal structure and moho characteristics along a 1000 km long profile southeast of the shatsky rise in a pacific ocean basin formed from the late jurassic to early cretaceous and spanning the onset of shatsky rise volcanism although the seismic velocity structure estimated from the refraction data showed typical characteristics of the oceanic crust of the old pacific plate the appearance of the moho reflections was spatially variable we observed clear moho reflections such as those to be expected where the spreading rate is fast to intermediate only at the southwestern end of the profile whereas moho reflections were diffuse weak or absent along other parts of the profile the poor moho reflections can be explained by the presence of a thick crust mantle transition layer which is temporally coincident with the formation of the shatsky rise we inferred that the crust mantle transition layer was formed by changes in on axis accretion process or modification of the primary moho by off axis magmatism induced by magmatic activity of the shatsky rise

evolution of the eastern indian ocean since the late cretaceous constraints from geosat altimetry we propose a new model for the tectonic evolution of the eastern indian ocean from the late cretaceous to the present two types of data are used to improve previously published reconstructions first recent reinterpretations of seafloor magnetic anomalies between australia and antarctica and in the wharton basin provide new constraints on spreading rates and the timing of major reorganizations second vertical deflection profiles i e horizontal gravity anomaly derived from 22 repeat cycles of geosat altimeter data reveal the tectonic fabric associated with fracture zones these new geosat data provide tight constraints on paleospreading directions for example three prominent fracture zones can be traced from south of tasmania to the george v basin antarctica providing an important constraint on the relative motions of australia and antarctica through the late eocene in addition the geosat profiles are used to locate the conjugate continental margins and continent ocean boundaries of australia and antarctica as well as the conjugate rifted margins of kerguelen plateau and broken ridge based on a compilation of magnetic anomaly data from the crozet basin the central indian basin the wharton basin and the australian antarctic basin ten plate tectonic reconstructions are proposed reconstructions at chrons 5 11 ma 6 21 ma 13 36 ma and 18 43 ma confirm that the southeast indian ridge behaved as a single plate boundary since chron 18 the constraints from the geosat data provide an improvement in the fit of the kerguelen plateau and broken ridge at chron 20 46 ma to avoid overlaps between broken ridge and the kerguelen plateau prior to their breakup our model includes relative motions between the northern and southern provinces of the kerguelen plateau finally we examine the implications of our model for the relative motions of india australia and antarctica on the tectonic evolution of the kerguelen plateau and broken ridge and the adjacent labuan basin and diamantina zone as well as the emplacement of the ninetyeast ridge and the kerguelen plateau over a fixed hot spot

structures in the jackass lakes pluton host rock system central sierra nevada california and inferred mid cretaceous farallon north america plate kinematics

rapid formation of the shatsky rise oceanic plateau inferred from its magnetic anomaly shatsky rise in the northwest pacific ocean is probably the oldest extant oceanic plateau and as with most such features its origin is uncertain both oceanic plateaus and continental flood basalts are thought to be formed by rapid voluminous eruptions that occur when the head of a newly born mantle plume ascends to the base of the lithosphere1 3 high eruption rates have been estimated for flood basalts for example 1 5km3 yr 1 for the deccan traps2 from dating of lava flows but the inaccessibility of oceanic plateaus makes it necessary to extrapolate dating information from a small number of samples and sites4 here we estimate the eruption rate of shatsky rise by a method that is indirect but has the virtue of sampling the entire volume of the plateau above the surrounding sea floor the main southern part of the plateau has a positive magnetic anomaly corresponding to a reversed geomagnetic polarity at the time of eruption using age constraints to identify the longest period of reversed polarity during which the plateau could have formed we estimate that 2 106 km3 of material erupted at a minimum rate of 1 7km3 yr 1 this issomewhat less than the rate of 8 22 km3 yr 1 estimated for the ontong java plateau4 but still represents a massive eruption consistent with the plume head hypothesis

late jurassic early cretaceous evolution of the eastern indian ocean adjacent to northwest australia over 9700 km of new aeromagnetic data were acquired off the northwest coast of australia and combined with existing magnetic data to map magnetic isochrons in the eastern indian ocean the isochrons were used to constrain a tectonic model of the evolution of the seafloor in the argo cuvier and gascoyne abyssal plains a complete set of anomalies from m26 through m16 was found in the argo abyssal plain trending generally n70 e spreading commenced in the center of the basin at or prior to m26 and propagated outward until at least m24 time anomalies m10 mo recording the separation of australia and india were found in the cuvier and gascoyne abyssal plains with a trend of about n30 e a significant crustal age discontinuity occurs in the vicinity of the joey rise where the two lineation sets converge because there appears to be no overlap of isochron ages in the two groups it is not necessary to postulate that a triple junction existed off northwest australia as has been previously suggested at m4 m5 time a 10 clockwise change in spreading direction occurred on the cuvier gascoyne spreading system this event triggered ridge jumps that transferred two pieces of the indian plate to the australian plate overlapping spreading on the forming and dying ridges curved fracture zones and lineations as well as fanned lineation trends suggest that the ridge jumps occurred by ridge propagation and that the transferred lithospheric blocks behaved as microplates for a brief interval of approximately 1 2 m y

copper nickel rich amalgamated ferromanganese crust nodule deposits from shatsky rise nw pacific 1 a unique set of ferromanganese crusts and nodules collected from shatsky rise sr nw pacific were analyzed for mineralogical and chemical compositions and dated using be isotopes and cobalt chronometry the composition of these midlatitude deep water deposits is markedly different from northwest equatorial pacific pcz crusts where most studies have been conducted crusts and nodules on sr formed in close proximity and some nodule deposits were cemented and overgrown by crusts forming amalgamated deposits the deep water sr crusts are high in cu li and th and low in co te and tl concentrations compared to pcz crusts thorium concentrations ppm are especially striking with a high of 152 mean 56 compared to pcz crusts mean 11 the deep water sr crusts show a diagenetic chemical signal but not a diagenetic mineralogy which together constrain the redox conditions to early oxic diagenesis diagenetic input to crusts is rare but unequivocal in these deep water crusts copper ni and li are strongly enriched in sr deep water deposits but only in layers older than about 3 4 ma diagenetic reactions in the sediment and dissolution of biogenic calcite in the water column are the likely sources of these metals the highest concentrations of li are in crust layers that formed near the calcite compensation depth the onset of ni cu and li enrichment in the middle miocene and cessation at about 3 4 ma were accompanied by changes in the deep water environment especially composition and flow rates of water masses and location of the carbonate compensation depth

volcanism and vertical tectonics in the pacific basin related to global cretaceous transgressions abstract the dominance of volcanic processes and the importance of vertical tectonics in the geological evolution of the pacific basin has been recognised since the time of charles darwin data gathered on several legs of the deep sea drilling project dsdp and numerous marine expeditions in the past decade have confirmed menard s postulate that the pacific basin was the scene of volcanism on an enormous scale in mesozoic time widespread mid plate volcanism between 110 and 70 m y b p characterised the area bounded by the line islands the mid pacific mountains and the nauru basin marshall islands heating of the pacific lithospheric plate during this period of volcanism resulted in regional uplift and the bathymetric evolution of the area diverged significantly from a normal parsons sclater subsidence curve the farallon plate now almost entirely subducted was also the scene of mid plate volcanism that produced such features as the nicoya plateau now found as an allochthonous ophiolitic terrain landward of the middle america trench large benthonic reef associated foraminifera comprising a pseudorbitoid fauna hitherto considered to be largely restricted to central america have now been additionally recorded from dsdp sites 165 315 and 316 in the line islands site 462 in the nauru basin and in new guinea the distribution of this fauna of campanian maastrichtian age is interpreted as indicating stepping stone connections aseismic ridges plateaus and seamounts between the caribbean farallon and pacific plates 70 80 m y b p similarities between the geology of the nauru basin and the caribbean ocean crust reinforce the interpretation of the latter as a former part of the farallon plate estimates of the sea level and continental freeboard change caused by the thermally induced uplift of the pacific and farallon plates as well as substantial areas in the atlantic and indian ocean basins indicate that such shallowing of the sea floor could have been the major factor in causing global cretaceous transgressions

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

a hotspot model for iceland and the voring plateau the proximity of a hotspot to a spreading center may result in the channeling of atmosphere to the section of rise crest closest to the hotspot this produces more basalt and thicker crust at these locations thus forming a plateau over time the geometric constraints of such a model predict a unique orientation location and age progression for a plateau formed by this mechanism the hotspot will channel material to the closest part of the rise therefore the orientation of the plateau will differ from that of a hotspot track by the component of absolute motion perpendicular to the rise axis the plateau will be symmetric with respect to the location of the rise axis at the time of formation also the age progression of the plateau will be contemporaneous with the age of formation of the seafloor on either side because the plateau is seafloor just with thicker crust a set of reconstructions based upon magnetic isochrones and a fixed hotspot reference frame is presented for the norwegian greenland sea as a means of evaluating the model s predictions by locating the iceland hotspot reconstructing the relative positions of the greenland and european plates and then assuming material would be channeled from the hotspot to the closest section of the rise crest we can trace the tectonic evolution of the greenland faeroe and voring plateaus the model is able to locate the plateaus explain their orientations and predict an age progression that satisfies observed age determinations the analysis demonstrates that both plateaus could have been formed by the iceland hotspot with the greenland faeroe plateau being in effect a continuation of the voring plateau which was cut off due to transform motion between the northern and southern spreading centers

mid ocean ridge jumps associated with hotspot magmatism abstract hotspot ridge interaction produces a wide range of phenomena including excess crustal thickness geochemical anomalies off axis volcanic ridges and ridge relocations or jumps ridges are recorded to have jumped toward many hotspots including iceland discovery galapagos kerguelen and tristan de cuhna the causes of ridge jumps likely involve a number of interacting processes related to hotspots one such process is reheating of the lithosphere as magma penetrates it to feed near axis volcanism we study this effect by using the hybrid finite element code flac to simulate two dimensional 2 d cross section viscous mantle flow elasto plastic deformation of the lithosphere and heat transport in a ridge setting near an off axis hotspot heating due to magma transport through the lithosphere is implemented within a hotspot region of fixed width to determine the conditions necessary to initiate a ridge jump we vary four parameters hotspot magmatic heating rate spreading rate seafloor age at the location of the hotspot and ridge migration rate our results indicate that the hotspot magmatic heating rate required to initiate a ridge jump increases non linearly with increasing spreading rate and seafloor age models predict that magmatic heating itself is most likely to cause jumps at slow spreading rates such as at the mid atlantic ridge on iceland in contrast despite the higher magma flux at the galapagos hotspot magmatic heating alone is probably insufficient to induce a ridge jump at the present day due to the intermediate ridge spreading rate of the galapagos spreading center the time required to achieve a ridge jump for fixed or migrating ridges is found to be on the order of 105 106 years simulations that incorporate ridge migration predict that after a ridge jump occurs the hotspot and ridge migrate together for time periods that increase with magma flux model results also suggest a mechanism for ridge reorganizations not related to hotspots such as ridge jumps in back arc settings and ridge segment propagation along the mid atlantic ridge

composition and timing of carbonate vein precipitation within the igneous basement of the early cretaceous shatsky rise nw pacific numerous calcium carbonate veins were recovered from the igneous basement of the early cretaceous shatsky rise during integrated ocean drilling program iodp expedition 324 the chemical sr ca mg ca and isotopic 87sr 86sr 143nd 144nd 18o 13c compositions of these veins were determined to constrain the timing of vein formation a dominant control by seawater chemistry on calcite composition is evident for most vein samples with variable contributions fromthe basaltic basement slightly elevated precipitation temperatures as inferred from oxygen isotope ratios indicative of hydrothermal vein formation are only observed at site u1350 in the central part of shatsky rise the highest 87sr 86sr ratios least basement influence of vein samples at each drill site range from 0 70726 to 0 70755 and are believed to reflect the contemporaneous seawater composition during the time of calcite precipitation in principle age information can be deduced by correlating these ratios with the global seawater sr isotope evolution since the sr isotopic composition of seawater has fluctuated three times between the early and mid cretaceous no unambiguous precipitation ages can be constrained by this method and vein precipitation could have occurred at any time between 80 and 140 ma however based on combined chemical and isotopic data and correlations of vein compositionwith formation depth and inferred temperature we argue for a rather early precipitation of the veins shortly after basement formation at each respective drill site

bathymetry of shatsky rise northwest pacific ocean implications for ocean plateau development at a triple junction oceanic plateaus are large igneous edifices thought to have been created by nascent mantle plumes but owing to sparse data their origins remain uncertain understanding plateau evolution is important because they are significant ocean features and may provide clues about mantle plume dynamics we constructed a bathymetry map of shatsky rise a large pacific plateau combining multibeam and wide beam echosounder data from 87 cruises and u s navy multibeam contours the rise consists of three large isolated volcanic edifices massifs surrounded by nearly normal lithosphere a linear volcanic ridge and a group of about 80 scattered seamounts massif flank slopes are typically gentle 1 5 and often parallel magnetic lineations or fracture zones the slope angles imply effusive volcanism similar to flood basalts whereas the rise shape suggests formation near the pacific izanagi farallon triple junction with modification of volcano flanks by spreading ridge tectonics edifice sizes and inferred ages imply a trend of decreasing volume and age from southwest to northeast furthermore gaps between massifs suggest episodic volcanism existing data are broadly consistent with the plume head hypothesis wherein the largest edifice formed by massive plume head eruptions the ridge formed from the plume tail and the two massifs in between represent a transition seamounts east of the rise are morphologically distinct which suggests that they may have been formed by a different source or mechanism than shatsky rise proper

pre cretaceous tectonic evolution of the pacific plate and extension of the geomagnetic polarity reversal time scale with implications for the origin of the jurassic quiet zone abstract linear magnetic anomalies resulting from seafloor spreading were mapped in the vicinity of the magnetic bight in the western pacific ocean new aeromagnetic data allowed the magnetic bight to be more accurately mapped from m21 to m28 and enabled the identification of low amplitude magnetic lineations in the jurassic quiet zone these lineations were formed by magnetic field reversals prior to m29 a revised jurassic geomagnetic polarity reversal time scale was constructed with nineteen reversals older than m29 numbered m30 m38 these reversals extend the record of geomagnetic polarity back in time by approximately 8 ma and are important constraints on the origin of the quiet zone in particular they imply that the jurassic was not a period of constant normal polarity an explanation offered by some authors further they cast doubt on a model of systematically decreasing geomagnetic field strength with increasing age during this period the early history of the northern pacific plate and pacific farallon izanagi p f i triple junction was traced by mapped magnetic isochrons the pacific plate seems to have evolved from a small plate that formed near the phoenix farallon izanagi triple junction about 180 188 m y ago at approximately 17 n 160 e in present coordinates until m21 time the evolution of the northern pacific plate was relatively simple and the p f i triple junction migrated north northwest with respect to the pacific

extinction dissolution and possible ocean acidification prior to the cretaceous paleogene k pg boundary in the tropical pacific abstract biotic perturbations and changes in ocean circulation during the maastrichtian stage of the latest cretaceous raise questions about whether the biosphere was preconditioned for the end cretaceous mass extinction of calcareous plankton a brief acme of inoceramid clams at 71 ma on shatsky rise in the tropical north pacific was followed by their extinction during the mid maastrichtian event at 70 1 ma associated with an abrupt warming of deep waters this was later followed by an interval of intense dissolution beginning 67 8 ma at odp site 1209 2387 m the late maastrichtian dissolution interval was initially gradual and is characterized by a low planktic benthic p b ratio highly fragmented planktic foraminifera mostly an absence of larger taxa low abundances of smaller taxa extremely low planktic foraminiferal numbers and low planktic foraminiferal and nannofossil species richness a partial recovery in carbonate preservation and calcareous plankton simple diversity began 250 kyr prior to the k pg boundary associated with the incursion of a younger more enriched 13c deep water mass although total abundances of planktic foraminifera in the sediment remained a tiny fraction of their earlier maastrichtian values a second brief dissolution event occurred 200 kyr before the boundary evidenced by renewed increase in planktic fragmentation but without a decrease in p b ratio our data show that changing deep water masses coupled with reduced productivity and associated decrease in pelagic carbonate flux was responsible for the first 1 6 myr dissolution interval while deccan traps volcanism may have caused surface ocean acidification 200 kyr prior to the k pg mass extinction event

nucleation and evolution of ridge ridge ridge triple junctions thermomechanical model and geometrical theory abstract ridge ridge ridge triple junctions are among the most remarkable features of global plate tectonics but their nucleation and evolution remains incompletely understood here we employ 3d numerical models to study the processes of the nucleation and evolution of triple junctions induced by multi directional lithospheric extension the simulations show that two major classes of junctions develop i transient quadruple and triple plate rifting junctions formed by the initial plate breakup that are gradually converted into ii stable triple oceanic spreading junctions formed by the accretion of new lithosphere quadruple junctions break into two diverging triple oceanic spreading junctions connected by a linear spreading center lengthening with time this process gradually decreases the length of deforming boundaries between four diverging rigid plates and thus the integral mechanical resistance of these boundaries to the spreading the geometry of triple oceanic spreading junctions varies from asymmetrical t junctions to ideal 120 junctions the structure of these junctions includes two crucial tectonic elements oceanic ridges spreading centers and intra plate ranges triple junction traces dividing crust accreted from different spreading centers the geometrical steady state is achieved within several million years we propose a new geometrical theory of a migrating steady state triple junction which describes its structure as a function of the relative plate velocities in a moving triple junction reference frame the relative plate velocities define the orientations and growth rates of respective intra plate ranges orientations and lengthening rates of oceanic ridges are in turn given by the average relative velocity of two adjacent plates the steady state triple junction migration velocity satisfies the condition of minimal cumulative ridge lengthening rate weighted by the square root of the spreading velocity our analytical theory agrees well with the results of the numerical simulations and natural data

using calculated chemical potential relationships to account for replacement of kyanite by symplectite in high pressure granulites calculated mineral equilibria are used to account for the formation of sapphirine plagioclase spinel plagioclase and corundum plagioclase symplectites replacing kyanite in quartz plagioclase garnet kyanite granulite facies gneisses from the southern domain of the athabasca granulite terrane a segment of the snowbird tectonic zone in northern saskatchewan canada metamorphic conditions of 14 kbar and 800 c are established for the high pressure garnet kyanite assemblage using constraints from p t pseudosections and zr in rutile thermometry replacement of kyanite by symplectites reflects the reaction of kyanite with the matrix following near isothermal decompression to 10 kbar the chemical potential gradients developed between the kyanite and the matrix led to diffusion that attempted to flatten the gradients kyanite persisting as a stable phase while it is consumed by symplectite from its edge in this local equilibrium model the mineral and mineral compositional spatial relationships are shown to correspond to paths in na2o cao k2o feo mgo in the model chemical system na2o cao k2o feo mgo al2o3 sio2 nckfmas with sio2 and al2o3 taken to be completely immobile the values of na2o and cao are constrained by fixing p t conditions and choosing appropriate na2o and cao values that correspond to the observed plagioclase compositions feo mgo diagrams show the corresponding spatial relationships with kyanite and the symplectite phases these results demonstrate that the replacement of kyanite by sapphirine plagioclase and spinel plagioclase appears to be metastable with respect to replacement by corundum plagioclase replacement by corundum plagioclase does also occur apparently overprinting pre existing symplectite and also kyanite ignoring corundum the resulting diagrams account for the spatial relationships and compositions observed in the spinel plagioclase and sapphirine plagioclase symplectites they are predicted to occur over both a wide range of p t conditions 6 11 kbar 650 850 c and plagioclase compositions xan 0 5 0 9 the wide range of p t conditions that may result in identical spatial and compositional relationships suggests that such reaction textures may be of limited use in accurately quantifying the p t conditions of retrograde metamorphism

paleo elevation and subsidence of 145ma shatsky rise inferred from co2 and h2o in fresh volcanic glass abstract shatsky rise a large mesozoic oceanic plateau in the northwest pacific consists of three massifs tamu ori and shirshov that formed near a mid ocean ridge triple junction published depth estimates imply that shatsky rise has not subsided normally like typical oceanic lithosphere we estimated paleo eruption depths of shatsky rise massifs on the basis of dissolved co2 and h2o in volcanic glass and descriptions of cores recovered from five sites of integrated ocean drilling program expedition 324 initial maximum elevations of shatsky rise are estimated to be 2500 3500 m above the surrounding seafloor and the ensuing subsidence of shatsky rise is estimated to be 2600 3400 m we did not observe the anomalously low subsidence that has been reported for both shatsky rise and the ontong java plateau although we could not resolve whether shatsky rise originated from a hot mantle plume or non plume fusible mantle uplift and subsidence histories of shatsky rise for the both cases are constrained based on the subsidence trend from the center of tamu massif 2600 m toward the flank of ori massif 3400 m in the case of a hot mantle plume origin shatsky rise may have formed on young 5 ma pre existing oceanic crust with a total crustal thickness of 20 km for this scenario the center of shatsky rise is subsequently uplifted by later prolonged crustal growth forming the observed 30 km thickness crust for a non plume origin shatsky rise may have formed at the spreading ridge center as initially thick crust 30 km thickness with later reduced subsidence caused by the emplacement of a buoyant mass perhaps a refractory mantle residuum beneath the center of shatsky rise

mantle flow and melting beneath oceanic ridge ridge ridge triple junctions abstract plate boundary geometry likely has an important influence on crustal production at mid ocean ridges many studies have explored the effects of geometrical features such as transform offsets and oblique ridge segments on mantle flow and melting this study investigates how triple junction tj geometry may influence mantle dynamics an earlier study georgen j e lin j 2002 three dimensional passive flow and temperature structure beneath oceanic ridge ridge ridge triple junctions earth planet sci lett 204 115 132 suggested that the effects of a ridge ridge ridge configuration are most pronounced under the branch with the slowest spreading rate thus we create a three dimensional finite element variable viscosity model that focuses on the slowest diverging ridge of a triple junction with geometry similar to the rodrigues tj this spreading axis may be considered to be analogous to the southwest indian ridge within 100 km of the tj temperatures at depths within the partial melting zone and crustal thickness are predicted to increase by 40 c and 1 km respectively we also investigate the effects of differential motion of the tj with respect to the underlying mantle by imposing bottom model boundary conditions replicating a absolute plate motion and b a three dimensional solution for plate driven and density driven asthenospheric flow in the african region neither of these basal boundary conditions significantly affects the model solutions suggesting that the system is dominated by the divergence of the surface places finally we explore how varying spreading rate magnitudes affects tj geodynamics when ridge divergence rates are all relatively slow i e with plate kinematics similar to the azores tj significant along axis increases in mantle temperature and crustal thickness are calculated at depths within the partial melting zone temperatures are predicted to increase by 150 c similar to the excess temperatures associated with mantle plumes likewise crustal thickness is calculated to increase by approximately 6 km over the 200 km of ridge closest to the tj these results could imply that some component of the excess volcanism observed in geologic settings such as the terceira rift may be attributed to the effects of tj geometry although the important influence of features like nearby hotspots e g the azores hotspot cannot be evaluated without additional numerical modeling

buried paleo channels on the new jersey continental margin channel porosity structures from electromagnetic surveying abstract we report on a marine electromagnetic em survey across two portions of the new jersey continental margin that have been previously shown to contain buried paleo channels the em method used provides bulk porosity estimates to depths of around 20 m below the seafloor and is thus able to place porosity constraints on the nature of the channel infill and the contrast in physical properties across the channel boundaries our data show that a key condition for the channels to have an electrical signature is that they incise an underlying regional unconformity r thought to represent a subaerially eroded surface exposed during the late wisconsinan glaciation channels that cut r are seen through increases in apparent porosity another seismically imaged channel sequence which lies within the outer shelf sediment wedge sequence above r does not have an electrical signature indicating that the sediments above and below the channel boundaries have similar physical properties

seismic stratigraphy of the shatsky rise sediment cap northwest pacific and implications for pelagic sedimentation atop submarine plateaus abstract pelagic sediments accumulated atop shatsky rise since early cretaceous time provide a valuable record of pacific paleoceanography the accepted sedimentation model invokes relatively continuous pelagic sediment accumulation punctuated by four regional hiatuses caused by changes in oceanic chemistry that framework was proposed more than two decades in the past and based on low quality single channel seismic profiles it was developed at the center of tamu massif the main topographic high of shatsky rise and extended to other highs with uncertainty owing to sparse data the availability of high quality seismic data and additional coring by the ocean drilling program and integrated ocean drilling program allows a re examination seismic and core data indicate that sedimentation atop shatsky rise has been complex with regional and local variability four major hiatuses were previously defined and tentatively correlated to nearby topographic highs these breaks occurred approximately during the aptian barremian and cenomanian coniacian ages of the cretaceous and during the paleocene and oligocene epochs of the cenozoic they are of variable length and tend to be of shorter duration at the summit of tamu massif but of longer durations elsewhere all produced erosional unconformities that may be related to mass wasting of the sediment cap edges each hiatus is approximately correlated with global falls of sea level suggesting a connection perhaps to intensified bottom currents at those times local variability is observed in the form of discontinuous layering non uniform sedimentation thinning of the sediment caps at their edges and local erosion near topographic highs these variations are likely the result of shifting bottom currents possibly resulting from interactions of internal tides with topography shatsky rise sedimentation is inherently punctuated and non uniform and the sedimentary record includes significant lateral variability other large oceanic plateaus are likely to exhibit similar sedimentary cap complexities

tectonic evolution of the north pacific during the cretaceous quiet period the cretaceous quiet period ended with a significant reorganization of the plate tectonics of the north pacific the farallon plate broke into three primary pieces 1 the kula plate 2 a large fragment transferred to the pacific plate and 3 the continued farallon plate the time of the rearrangement is constrained to just before magnetic anomaly 34 by its presence contrasts in tectonic spreading fabric tsf the emperor trough and the chinook trough our model is based on 1 a complete reevaluation of primary bathymetric and magnetic data 2 the distribution of major troughs emperor chinook hokkaido and krusenstern 3 mapping and analysis of tsf orientation and amplitude on quiet zone crust and 4 available deep sea drilling project drill hole data east west trending magnetic reversal lineations and tsf south of the aleutian trench are bounded on the west by the buldir fracture zone at 175 e the major troughs mark the abandonment and initiation of spreading in the reorganization their absence elsewhere precludes other long distance boundary relocations emperor trough is the abandoned pacific farallon spreading center rift initiation and early slow spreading have created the high amplitude topography of the chinook trough we believe that the hokkaido trough is the initiation site of a failed oceanic rift the incorporation of large amounts of farallon plate into the pacific plate obviates the need for the very rapid pacific farallon spreading rates of previous models and significantly alters the interpretation of mid pacific geology

continuous growth of hydrogenetic ferromanganese crusts since 17 myr ago on takuyo daigo seamount nw pacific at water depths of 800 5500 m ferromanganese crusts cover all outcrops on takuyo daigo seamount traversed during remotely operated underwater vehicle rov dives except in places covered by foraminifera sand takuyo daigo is a cretaceous seamount located in the northwest pacific ocean geological and bathymetric mapping provide the framework for this study chemical and mineralogical analyses of the hydrogenetic ferromanganese crusts show temporal and spatial variations typical of those found in previous studies outcrops from 800 to 5500 m water depths are covered with ferromanganese crusts up to 105 mm thick beryllium isotope dating shows that the crusts have apparently been growing continuously at all water depths even through the modern oxygen minimum zone omz contrary to some earlier models for deposition growth rates vary from 2 3 to 3 5 mm myr with fe or mn fluxes of 0 07 0 11 g cm2 myr since the early middle miocene co mn ratios decrease with water depth while fe mn and other metallic elements increase or show no change based on the analysis of the uppermost crust surface this is probably because co is the most abundant redox sensitive element derived from seawater that occurs in crusts

tectonic evolution of the northern pacific plate and pacific farallon izanagi triple junction in the late jurassic and early cretaceous m21 m10 abstract magnetic lineations in the northwestern pacific near the shatsky plateau were interpreted to trace the tectonic history of the northern pacific plate from the time of anomalies m21 to m10 at m22 time the pacific farallon izanagi p f i triple junction was located southwest of the plateau and migrating north northwest with respect to the pacific plate shortly thereafter the stress field acting on the three plates changed significantly and the pacific lzanagi ridge reacted by pivoting 24 clockwise from m21 to m19 time several ridge propagation events occurred while the ridge adjusted to the new spreading regime to the east of the m22 location of the junction it appears that this reorganization triggered a series of at least three ridge jumps along the pacific farallon ridge the geometry of magnetic isochron m21 suggests that several triple junctions existed simultaneously during the reorganization bordering one or more microplates differences in the spreading rate of the japanese magnetic lineations north of the plateau imply that one microplate which we call the shatsky microplate existed until about m15 m13 time it was probably captured by either the farallon or the izanagi plate and subsequently destroyed by subduction after m15 the p f i triple junction was reformed on seafloor that is now beneath the shatsky plateau the junction drifted generally northeast either in a ridge ridge ridge or in a ridge fault fault configuration perhaps alternating between the two from m11 m10 time the stress field on the three plates changed again and the pacific farallon ridge southeast of the plateau reoriented 15 clockwise at this time the p f i junction assumed a ridge ridge ridge geometry at the northeast end of the plateau

is there continental crust underneath the northern natal valley and the mozambique coastal plains 1 to draw conclusions about the crustal nature and history of the natal valley and the mozambique ridge systematic potential field data were obtained during the aistek iii cruise with r v pelagia in 2009 this paper presents and interprets the results of that expedition the new magnetic data reveal a pattern of linear magnetic spreading anomalies nw se trending in the southwestern part of the mozambique ridge and e w trending on its central part the ariel graben which separates the mozambique ridge from the northern natal valley is represented by a pronounced negative anomaly in the magnetic as well as the free air gravity field the northern natal valley bears a complicated pattern of anomalies with mainly sw ne trends in the northern natal valley no indications for a continent ocean boundary between continental crust in the north and oceanic crust in the south exist either in the free air gravity or in the magnetic field the magnetic wavelengths of the mozambique coastal plains are similar to those of the northern natal valley and the mozambique ridge particularly in the gravity data the mozambique coastal plains the northern natal valley and the mozambique ridge appear as one continuous geological province we interpret the region from the mateke sabi monocline in the north to the southwestern tip of the mozambique ridge as mainly being floored by thickened oceanic crust

the age of earth s largest volcano tamu massif on shatsky rise northwest pacific ocean this study presents laser step heating 40ar 39ar age determinations of basaltic lava samples from tamu massif the oldest and largest edifice of the submarine shatsky rise in the northwest pacific and earth s proposed largest volcano the rocks were recovered during integrated ocean drilling program expedition 324 which cored 160 m into the igneous basement near the summit of tamu massif the analyzed lavas cover all three major stratigraphic groups penetrated at this site and confirm a late jurassic early cretaceous age for the onset of shatsky rise volcanism lavas analyzed from the lower and middle section of the hole yield plateau ages between 144 4 1 0 and 143 1 3 3 ma with overlapping analytical errors 2 whereas a sample from the uppermost lava group produced a significantly younger age of 133 9 2 3 ma suggesting a late or rejuvenated phase of volcanism the new geochronological data infer minimum average melt production rates of 0 63 0 84 km3 a over a time interval of 3 4 million years consistent with the presence of a mantle plume

magnetic lineations within shatsky rise northwest pacific ocean implications for hot spot triple junction interaction and oceanic plateau formation oceanic plateaus are major ocean features yet their origins and development are poorly understood many are huge piles of basalt and it is widely accepted that mantle plumes are their source perhaps from eruptions of the voluminous head of an emerging plume shatsky rise is a basaltic plateau that formed during a period of geomagnetic reversals unlike many mid cretaceous plateaus so magnetic data can help us understand its tectonic history in this study we analyzed magnetic anomaly data from 131 cruises over and around shatsky rise and constructed a magnetic lineation chart for tectonic interpretation a significant finding is that magnetic lineations are traceable through low parts of the rise between volcanic massifs indicating nearly normal lithosphere and between large volcanic edifices many lineations form bights near the rise axis and show former locations of the pacific izanagi farallon triple junction they indicate that the junction was in a ridge ridge ridge rrr configuration and closely followed the rise axis from chron m20 146 ma to chron m4 127 ma lineation and bight geometries indicate that the triple junction jumped northeastward at least nine times often apparently in response to volcanic activity from the mantle plume some jumps however appear related to ridge reorientations numerous features within the rise are parallel to lineations or lineation offsets implying that the plateau formed near the triple junction and was modified by ridge faulting most of our observations support the hypothesis that shatsky rise was formed by a mantle plume that captured a triple junction

geochemical evidence for a mid cretaceous superplume basalt lavas with a high nb y ratio for a given nb zr ratio occur in the polynesian superswell region of the south pacific which probably formed by upwelling of a deep mantle superplume the distinctive geochemical characteristics of the polynesian basalts may be attributed to melting of a mantle source that is more enriched in a basaltic ancient mid oceanic ridge basalt component basalts displaying such chemical signatures have been found on shatsky rise the ontong java plateau and greenstones from subduction zone complexes of sakhalin island the occurrence of polynesian type basalts together with an estimate of their ages suggests that the south pacific superplume was active as long ago as 90 150 ma the superplume activity preceded the onset of the superchron supporting an idea that the superplume acted as a trigger for such a global event

rapid cenozoic subsidence in the gulf of mexico resulting from hess rise conjugate subduction enigmatic surface deflections occurred in north america starting from the cretaceous including the continental scale drainage reorganization and the long wavelength subsidence in the western interior seaway these surface undulations cannot be simply explained by sea level change or flexure loading coinciding with the large scale surface deflection the gulf of mexico gom has an immense paleocene sediment deposition probably caused by tectonic subsidence increasing evidence indicates a distinct seismic anomaly localized in the mantle below the gom with geodynamic models we show that the hess rise conjugate coincides with the position of the seismic anomaly the basalt eclogite transition in the hess conjugate can lead to a localized dynamic subsidence in the gom which is superimposed on the broad surface deflection caused by the farallon slab the hess conjugate transformed to eclogite could tilt the surface southward in the u s and help frame the gom as a main depocenter in the cenozoic

magnetic anomaly lineations from late jurassic to early cretaceous in the west central pacific ocean summary late jurassic to early cretaceous mesozoic magnetic anomaly lineations the phoenix magellan mid pacific mountains hawaiian and japanese lineation sets with fracture zones in the west central pacific ocean were identified more comprehensively than in any previous studies we fixed 2100 positions of identified magnetic anomalies based on magnetic data of 283 cruise tracks two remarkable fracture zones the phoenix and central pacific fracture zones were mapped and newly named our newly identified lineations from mlon to mo around the mid pacific mountains which belong to the hawaiian lineation set illustrated that the sea floor south of the mid pacific mountains has the same age as that of the north 132 118 ma our analysis of skewness parameters revealed that the older part of the phoenix set m17 m29 has skewness different from that of the younger part ml m14 implying an effect of magnetic overprints by the cretaceous volcanism it was confirmed that the spreading rate of the mesozoic pacific spreading system was the fastest in the world in the mesozoic a drastic change in spreading rates occurred simultaneously at the period between chrons m21 and m20 149 5 148 5 ma in all the mesozoic pacific spreading systems the event appears to be synchronous with events in other oceans such as the mesozoic atlantic and indian oceans

cretaceous paleomagnetic apparent polar wander path for the pacific plate calculated from deep sea drilling project and ocean drilling program basalt cores abstract the apparent polar wander path apwp of the pacific plate still has many uncertainties owing to the fact that paleomagnetic data are difficult to obtain for oceanic plates after more than three decades of coring by the deep sea drilling project dsdp and ocean drilling program odp there are now a large number of reliably dated basalt cores recovered from the pacific plate and this provides an opportunity to determine paleomagnetic poles based on igneous rock samples considered by many scientists to be the most reliable data type cretaceous pacific plate basalt core data were compiled corrected using a standard technique divided into groups based on age and combined to calculate five mean paleomagnetic poles with ages of 80 92 112 121 and 123 ma the latter two being for two different coeval regions in all pole analyses the lack of azimuthal orientation for cored samples leads to large uncertainties in pole locations along a nearly east west direction this difficulty was mitigated by using declination data from magnetic anomaly inversions of dated pacific seamounts for azimuth constraint the two nearly same age poles were calculated because paleocolatitudes from ontong java plateau ojp are discordant compared to those from other pacific locations i interpret the discordant ojp results to indicate that the plateau is on crust that had an early history as an independent plate the other poles 80 92 112 and 123 ma fall on a northeast trending line that suggests slow apparent polar wander during the early and mid cretaceous followed by rapid polar wander between 92 and 80 ma comparison of the 123 ma pole with previously published paleomagnetic data of jurassic age implies southward apparent polar wander followed by a turnaround because the 123 ma pole is the farthest from the geographic pole it implies the turnaround happened near that time and that the pacific plate has moved 40 northward since then the 80 ma pole stands 17 from the geographic pole indicating that 60 of the northward drift occurred prior to that time and 40 afterwards