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

implications of estimated magmatic additions and recycling losses at the subduction zones of accretionary non collisional and collisional suturing orogens

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

attempts to model the lower cretaceous pelagic sedimentary cover of shatsky rise nw pacific ocean from downhole geophysical records abstract ocean drilling program odp leg 198 to shatsky rise cored lower cretaceous sediments at three sites sites 1207 1213 and 1214 unfortunately core recovery was poor at all sites most likely due to the contrast in physical properties between poorly cemented very soft carbonate ooze chalk and well cemented very hard biosiliceous chert downhole geophysical logs from two sites 1207 and 1213 provide continuous in situ records of physical and chemical properties that can be used to elucidate the sedimentary history of shatsky rise in the early cretaceous the downhole logs show that at both sites two stratigraphic intervals dated as late barremian aptian and early albian are characterised by low porosity high resistivity sediments the timing of these intervals is synchronous at both sites and it has been previously proposed that the resistivity highs are the result of increased biosiliceous sedimentation and subsequent silica cementation in order to test this hypothesis we use matrix algebra to invert the logging data from site 1207 to produce two compositional models the first reconstructs lithology the second reconstructs mineralogy the results of the lithology model suggest that the abundance of chert is highest during the resistivity highs the results of the mineralogical model are more complex because of the probable presence of additional minerals such as barite

estimating phase velocity and attenuation of guided waves in acoustic logging data in acoustic logging the guided waves which propagate along the borehole are affected by the properties of the formation for acoustic logging tools with many receivers array processing methods can be used to calculate the phase velocity and attenuation of these waves this information is crucial in estimating formation properties like permeability transverse isotropy and s wave velocity

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

microplate tectonics along a superfast seafloor spreading system near easter island there is a growing easter microplate between the pacific and nazca plates the eastern and western boundaries of the microplate are actively spreading and propagating north and south respectively the northern and southern boundaries are broad transform zones most of the interior of the microplate contains the sort of oblique sheared structures characteristic of zones of transferred lithosphere between propagating and failing spreading centres

the late cretaceous to recent tectonic history of the pacific ocean basin abstract a vast ocean basin has spanned the region between the americas asia and australasia for well over 100 myr represented today by the pacific ocean its evolution includes a number of plate fragmentation and plate capture events such as the formation of the vancouver nazca and cocos plates from the break up of the farallon plate and the incorporation of the bellingshausen kula and aluk phoenix plates which have been studied individually but never been synthesised into one coherent model of ocean basin evolution previous regional tectonic models of the pacific typically restrict their scope to either the north or south pacific and global kinematic models fail to incorporate some of the complexities in the pacific plate evolution e g the independent motion of the bellingshausen and aluk plates thereby limiting their usefulness for understanding tectonic events and processes occurring in the pacific ocean perimeter we derive relative plate motions with 95 uncertainties for the pacific farallon vancouver kula pacific bellingshausen pacific and early pacific west antarctic spreading systems based on recent data including marine gravity anomalies well constrained fracture zone traces and a large compilation of magnetic anomaly identifications we find our well constrained relative plate motions result in a good match to the fracture zone traces and magnetic anomaly identifications in both the north and south pacific in conjunction with recently published and well constrained relative plate motions for other pacific spreading systems e g aluk west antarctic pacific cocos recent pacific west antarctic spreading we explore variations in the age of the oceanic crust seafloor spreading rates and crustal accretion and find considerable refinements have been made in the central and southern pacific asymmetries in crustal accretion within the overall pacific basin where both flanks of the spreading system are preserved have typically deviated less than 5 from symmetry and large variations in crustal accretion along the southern east pacific rise i e pacific nazca farallon spreading appear to be unique to this spreading corridor through a relative plate motion circuit we explore the implied convergence history along the north and south america s where we find that the inclusion of small tectonic plate fragments such as the aluk plate are critical for reconciling the history of convergence with onshore geological evidence

anomalous late jurassic motion of the pacific plate with implications for true polar wander abstract true polar wander or tpw is the rotation of the entire mantle crust system about an equatorial axis that results in a coherent velocity contribution for all lithospheric plates one of the most recent candidate tpw events consists of a 30 rotation during late jurassic time 160 145 ma however existing paleomagnetic documentation of this event derives exclusively from continents which compose less than 50 of the earth s surface area and may not reflect motion of the entire mantle crust system additional paleopositional information from the pacific basin would significantly enhance coverage of the earth s surface and allow more rigorous testing for the occurrence of tpw we perform paleomagnetic analyses on core samples from ocean drilling program odp site 801b which were taken from the oldest available pacific crust to determine its paleolatitude during the late jurassic and early cretaceous 167 133 ma we find that the pacific plate underwent a steady southward drift of 0 49 0 74 my 1 except for an interval between kimmeridgian and tithonian time 157 147 ma during which it underwent northward motion at 1 45 0 76 my 1 1 this trajectory indicates that the plates of the pacific basin participated in the same large amplitude 30 rotation as continental lithosphere in the 160 145 ma interval such coherent motion of a large majority of the earth s surface strongly supports the occurrence of tpw suggesting that a combination of subducting slabs and rising mantle plumes was sufficient to significantly perturb the earth s inertia tensor in the late jurassic

source parameters of regional small to moderate earthquakes in the yunnan sichuan region of china abstract we apply a grid search waveform inversion technique to determine moment tensors for 73 small to moderate earthquakes that occurred in the yunnan sichuan region of china in 2003 and 2004 and were recorded by a passcal deployment together with 6 small to moderate events recorded by the yunnan province digital seismic network ydsn that occurred in 2000 to 2001 the advantage of studying these earthquakes down to an m w 3 03 is that the large number of solutions present a picture of the interaction of the four tectonic blocks in the region we find distinct patterns in the orientation of the maximum compressive stress axes derived from the moment tensor solutions they are compatible with the clockwise rotation of the eastern himalayan syntaxis northeastward compressive motion and the south southeast movement of the east tibetan plateau crustal material and the counterclockwise rotation at the east of the red river fault we also find a subtle variation in source depth for crustal sources across the region

extraction of reflected events from sonic log waveforms using the karhunen loeve transform abstractsonic reflection logging a recently developed borehole geophysical scheme is in principle capable of providing a clear view of outside the well bore in this type of acoustic well logging a key technical obstacle is that the reflected wave signal is almost entirely obscured by the directly arriving p s and stoneley wave modes effective extraction of these reflection signals from the full acoustic waveforms is therefore a critical data processing step we have examined the use of the karhunen loeve kl transform combined with a band limiting filter as a technique for the extraction of reflections of interest from a mixture with directly arriving wave modes of much higher amplitude under the assumption that large energy squared amplitude differences exist between each wave component the direct stoneley wave s wave and the p wave are eliminated sequentially by subtracting the most significant principal components after which the remaining signal is seen to be dominated by reflected

the early cenozoic tectonic history of the southeast pacific abstract new marine geophysical data collected in the southeast pacific enable us to define better the early cenozoic tectonic history of the area and to interpret previously unexplained bathymetric features in terms of plate tectonic processes we have identified a major re organization in plate boundaries at anomaly 21 time when a large fragment was broken off the pacific plate and joined to the antarctic plate this interpretation is based on three pieces of evidence 1 a major ese trending feature the humboldt fracture zone marks the southern boundary of the farallon antarctic anomalies identified by weissel et al 1 2 a north south striking trough the hudson trough which represents the scar left by the pacific antarctic ridge when it rifted across old pacific crust 3 a sequence of north south striking magnetic lineations south of the humboldt fracture zone and east of the hudson trough is correlated to anomalies 27 to 22 and identified with pacific aluk spreading

effects of ridge crest trench interaction on antarctic phoenix spreading forces on a young subducting plate precise measurements of spreading rates on marine magnetic profiles collected to the west of the antarctic peninsula have enabled some consideration of the forces governing plate motion since antarctic phoenix motion has been controlled by the local rather than global force balance over the last 35 m y the total effective driving force per unit length of trench is calculated to have ranged between 2 6 and 3 6 1012 n m which is much less than is commonly thought necessary to support subduction conventional calculations may overestimate slab pull for old slabs because they neglect the effect of extensional disruption in limiting the contribution to the balance of forces at the trench the low estimate of driving forces obtained here implies that resistive forces are also smaller than is generally assumed driving forces show a strong correlation with observed spreading rates which indicates that resistive forces were largely velocity dependent fluid migration up the subduction zone may elevate temperatures in and around the shear zone reducing resistive forces below the levels required by purely conductive models changes in convergence rate may affect the depths of both the brittle ductile deformation boundary and the basalt eclogite phase change causing a negative feedback which would appear as a velocity dependent resistive force the different driving forces acting on the ne and sw parts of the phoenix plate as a consequence of older oceanic lithosphere at the trench in the ne caused antarctic phoenix spreading to take place about a near pole to the sw since 21 m y ago and ultimately resulted in disruption of the phoenix plate about 9 m y ago spreading rates decreased abruptly about 6 m y ago probably because of e w compression across the long transform faults bounding the phoenix plate however spreading on the last three segments of the antarctic phoenix ridge continued at least until 4 m y ago either spreading stopped progressively from sw to ne or the final stage took place about a very near pole to the sw a magnetic quiet zone extends up to 95 km from the margin between the tula fracture zone and the north anvers fracture zone and is thought to indicate that the ridge crest became buried by terrigenous sediment prior to collision the absence of a magnetic quiet zone associated with the most recent ridge crest trench collisions suggests a change in sedimentary regime during the late miocene anomalously fast apparent spreading rates between 23 and 21 m y ago are thought to indicate an error in this part of the magnetic reversal time scale

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

seamount statistics in the pacific ocean we apply the wide beam sampling technique of jordan et al 1983 to approximately 157 000 km of wide beam profiles to obtain seamount population statistics for eight regions in the eastern and southern pacific ocean population statistics derived from wide beam echograms are compared with seamount counts from sea beam swaths and with counts from bathymetric maps we find that the average number of seamounts with summit heights h h is well approximated by the exponential frequency size distribution h oe h the exponential model for seamount sizes characterized by the single scale parameter 1 is found to be superior to a power law self similar model which has no intrinsic scale in describing the average distribution of pacific seamounts and it appears to be valid over a size spectrum spanning 5 orders of magnitude in abundance large scale regional variations in seamount populations are documented we observe significant differences in seamount densities across the murray fracture zone in the north pacific and the eltanin fracture zone system in the south pacific the eltanin discontinuity is equally evident on both sides of the pacific antarctic ridge in the south pacific regions symmetrically disposed about the ridge axis have very similar seamount densities despite the large difference between pacific plate and antarctic plate absolute velocities evidently any differences in the shear flows at the base of the pacific and antarctic plates do not affect seamount emplacement systematic variations in o and are observed as a function of lithospheric age with the number of large seamounts increasing more rapidly than small seamounts these observations have been used to develop a simple model for seamount production under the assumptions that 1 an exponential size frequency distribution is maintained 2 production is steady state and 3 most small seamounts are formed on or near the ridge axis the limited data available from this study appear to be consistent with the model but they are insufficient to provide a rigorous test of the assumptions or determine accurately the model parameters however the data from the south pacific indicate that the off axis production of large seamounts probably accounts for the majority of seamounts with summit heights greater than 1000 m

formation of curved seafloor fabric by changes in rift propagation velocity and spreading rate application to the 95 5 w galapagos propagator the geometry of oceanic spreading centers can change by rift propagation in which a new ridge segment propagates and preempts a preexisting one as the locus of relative motion between the two plates shifts the dying and growing ridge segments are joined by a boundary usually a transform which migrates with time causing seafloor to be transferred from one plate to the other as a result originally ridge parallel isochrons in a zone between the growing and failed rifts can be reoriented to trends oblique to both the ridge and spreading direction an attractive feature of this model is that the reorientation occurs by strictly rigid plate tectonics since relative motion occurs only between points on different plates recent surveys of the galapagos propagating rift system at 95 w show seafloor fabric curving away from the growing rift and toward the failed rift these observations have been interpreted as requiring a zone of distributed simple shear between the two rifts such behavior would represent a local deviation from rigid plate tectonics since relative motion would be distributed over a finite region not part of either of the two plates here we explore possible rigid plate models for the formation of curved seafloor lineaments by rift propagation we find that such fabric can be generated by changes either in the velocity rate or direction of rift propagation or in the spreading rate during rift propagation the curvature of various tectonic elements is diagnostic of the different possibilities the geometry observed at the galapagos can result from either rift propagation acceleration or a spreading rate decrease during the last few hundred thousand years the reverse curvature could result from either deceleration of rift propagation or an increase in spreading rate propagator acceleration and spreading rate slowing which produce similar structural trends could in principle be distinguished using spreading rate data at galapagos however since either would have occurred within the present period of normal polarity both are consistent with the magnetic anomaly data we thus conclude that the data interpreted as requiring a shear zone are equally consistent with two distinct models based on rigid plate tectonics it may however be possible to discriminate between rigid and shear models using fabric observations at other propagators the shear model requires that the sense of curvature at galapagos be a ubiquitous feature of active propagators in contrast the rigid plate tectonic model allows for fabric with a variety of orientations including reverse curvature formed by either propagation deceleration or increasing spreading rate and linear fabric formed by constant propagation and spreading velocities

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

new seismic data support cenozoic rifting in george vi sound antarctic peninsula summary new multifold seismic reflection and wide angle reflection refraction data across george vi sound antarctic peninsula show the presence of graben and horst structures indicating an extensional origin the data suggest that rocks of an accretionary complex and fore arc basin underlie the sound and are in faulted contact along its eastern boundary with volcanic and plutonic rocks of the associated mesozoic arc of western palmer land a cover of possible syn and postglacial cenozoic deposits drapes the structures the combination of new seismic synthetic aperture radar and previously acquired data suggests subduction related rifting in the sound was segmented with opening in the south predominately by normal extension whilst in the north dextral transtension predominated

microplate tectonics new insights from micro blocks in the global oceans continental margins and deep mantle abstract any plate has a growth process from small to large the micro blocks or micro plates are sometimes the precursors of large plates the origin growth aborting extinction and residual process of micro blocks are of great significance for the study of plate tectonics and pre plate tectonics the micro block can be divided into continental oceanic and mantle micro blocks according to their compositions in this paper the micro blocks in the global oceans have been summarized according to the following five environments mid ocean ridge system subduction system transform fault system deep sea intraplate system and extension rift system we first propose a genetic classification of micro blocks comprising detachment derived rifting derived transform derived propagation derived ridge jumping derived subduction derived accretion derived collision derived and delamination derived micro blocks the different types of micro block boundaries such as active or fossil detachment fault subduction zone mid ocean ridge transform fault fracture zone transfer fault accommodation zone lithosphere scale strike slip fault pseudofault intra oceanic convergent zone overlapping spreading centre non transform offset rheological crustal or mantle discontinuity are systematically discussed for different micro blocks thus the number of triple junctions will be more than the 16 in the traditional plate tectonic theory a stability analysis of these triple junctions is the key to understanding the causes of micro blocks however the micro block has no ultimate cause so it is unnecessary to pursue one ultimate cause or initiation of plate tectonics these micro blocks within oceanic basins along oceanic margins or within the deep mantle can not only be used to develop deep ocean fine structural analysis and plate tectonic reconstruction but also to explain the causes of micro blocks in some orogens this will enrich the research of more detailed pre orogenic or syn orogenic evolution of orogenic belts and even extend to the study of early precambrian pre plate tectonic mechanisms the micro block can be a transition among microplate plate and terrane under a plate tectonic regime it can also be formed in inter sphere tectonic processes it helps better our understanding of regimes of cratonic basin formation and intra continental deformation which are the difficulties faced by the plate tectonics theory we speculate that the micro block tectonics theory is a unified tectonic theory of cross layer cross phase cross space time scale and cross planet

ultrahigh resolution marine magnetic anomaly profiles a record of continuous paleointensity variations a distinctive pattern of small scale marine magnetic anomalies 25 100 nt amplitude 8 25 km wavelength tiny wiggles is superimposed on the more generally recognized seafloor spreading pattern between anomalies 24 and 27 in the indian ocean by normalizing and stacking multiple profiles it is demonstrated that this pattern of tiny wiggles is a high resolution recording of paleodipole field behavior between chrons c24 and c27 the pattern of tiny wiggles between anomalies 26 and 27 is compared to an ultrafast spreading 82 mm yr half rate profile from the southeast pacific where a similar signal is observed confirming the paleodipole field origin of the anomalies two basic models are considered in which the tiny wiggles are attributed either to short polarity intervals or to paleointensity fluctuations we conclude that tiny wiggles are most likely caused by paleointensity fluctuations of the dipole field and are a ubiquitous background signal to most fast spreading magnetic profiles the implications of this study are that 1 tiny wiggles may provide information on the temporal evolution of the geomagnetic dynamo 2 the small scale anomalies observed in the jurassic quiet zones may be due to paleointensity fluctuations 3 tiny wiggles are potential time markers in large regions of uniform crustal polarity such as the cretaceous quiet zones and 4 much of the variance in anomaly profiles normally attributed to crustal eraplacement processes particularly at fast and ultrafast spreading rates is actually due to intensity variations in the paleomagnetic field

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

magnetic anomaly map of ori massif and its implications for oceanic plateau formation abstract many oceanic plateaus have been emplaced at or adjacent to mid ocean ridges to explain plateau volume and thickened crust compared to normal oceanic crust hotspot ridge interaction is commonly assumed but the manner of interaction remains unclear the shatsky rise oceanic plateau is a large volcanic mountain that formed at a triple junction during late jurassic and early cretaceous time recent drilling and seismic investigations suggest that the intermediate edifice in the rise ori massif is a central volcano paradoxically magnetic lineations were traced across parts of ori massif implying formation at a spreading ridge in this study we examined magnetic anomalies over and around ori massif to obtain insights about the formation of this volcanic edifice magnetic data from 21 cruises were corrected combined and gridded to construct a magnetic anomaly map forward and inverse magnetic modeling was done to investigate the magnetic structure of ori massif the results imply that this large volcanic edifice is predominantly characterized by linear magnetic anomalies resulting from alternating normal and reversed polarity magnetization blocks analogous to magnetic anomalies recorded by spreading ridges this magnetic structure is not expected for a central volcano that was built by long runout lava flows implying that ori massif eruptions must have been constrained near the ridge axis magnetic bights on the north and south boundaries of ori massif imply that it was bracketed by triple junctions indicating complex ridge tectonics during the formation of shatsky rise the surprising finding that ori massif is traversed by coherent linear magnetic anomalies indicates that oceanic plateaus can record seafloor spreading magnetic anomalies despite large crustal thickness other oceanic plateaus also record linear magnetic anomalies implying a link between divergent plate boundaries and oceanic plateau volcanism

palaeo digital elevation models for use as boundary conditions in coupled ocean atmosphere gcm experiments a maastrichtian late cretaceous example abstract palaeogeography specifically palaeotopography and palaeobathymetry provides an essential boundary condition for computer based atmosphere and ocean modelling it also provides the geographic context for understanding surface processes palaeodrainage palaeoweathering and biotic interactions palaeoecology palaeobiogeography with increased model resolution coupled ocean atmosphere general circulation models gcms and the addition of vegetation soil weathering ice and chemical modules there is now a need for more robust detailed palaeotopographies and palaeobathymetries that are fully integrated with the processes being modelled especially the hydrological system here we present a new geographic information system gis based hydrologically correct palaeo digital elevation model dem for the maastrichtian late cretaceous we describe the methods and concepts used to construct the map and draw attention to the limits imposed by scale and uncertainty and how these factors must be considered as part of the error analysis of derived model results the underlying palaeogeography is one of a series of 27 global maps that represent the stages of the cretaceous and sub epochs of the cenozoic each map is generated at a scale of 1 30 million in arcview gis and arcinfo using data from the lead author s own databases of lithologic tectonic and fossil information the lithologic databases of the paleogeographic atlas project the university of chicago a survey of published literature and dsdp odp data interpretations of elevation are derived following the methods outlined in ziegler et al ziegler a m rowley d b lottes a l sahagian d l hulver m l gierlowski t c 1985 paleogeographic interpretation with an example from the mid cretaceous annual review of earth and planetary sciences 13 385 425 an understanding of the tectonic regime and evolution of each feature and the age depth relationship for the ocean the global palaeo dem was derived using the elevation contours from the paleogeography and the suite of hydrological tools available in arcinfo grid the palaeo dem has been constrained by defining areas of internal palaeodrainage palaeoriver mouths and known palaeoriver courses the maastrichtian has been completed first to provide the boundary conditions for a series of coupled atmosphere ocean experiments when integrated with the results of the coupled ocean atmosphere model the result is a powerful tool for understanding surface processes and an important step towards the development of a continuous series representing a fully evolving palaeolandscape

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

the effect of velocity structure errors on double difference earthquake location 1 we show that relative earthquake location using double difference methods requires an accurate knowledge of the velocity structure throughout the study region to prevent artifacts in the relative position of hypocenters the velocity structure determines the ray paths between hypocenters and receivers these ray paths and the corresponding ray take off angles at the hypocenters determine the partial derivatives of travel time with respect to the hypocentral coordinates which form the inversion kernel that maps double differences into hypocentral perturbations thus the large scale velocity structure enters into the core of the double difference technique by employing a 1d layered model with sharp interfaces to perform double difference inversion of synthetic data generated using a simple 1d gradient model we show that inappropriate choice of the velocity model combined with unbalanced source receiver distributions can lead to significant distortion and bias in the relative hypocenter positions of closely spaced events

the cenozoic subduction history of the pacific margin of the antarctic peninsula ridge crest trench interactions new magnetic anomaly identifications w of the shackleton fracture zone show 5 spreading sections separated by fracture zones in the 2 most southerly the ridge crest collided with a trench at the margin of the antarctic peninsula only 6 5 and 4 ma ago the latest of a series of collisions starting at the base of the peninsula so ma ago following each collision spreading and subduction both stopped opposite the south shetland trench and actively extending bransfield strait spreading also stopped 4 ma ago but before the spreading centre reached the trench the tendency of the subducting plate to continue sinking probably initiated the opening of the bransfield strait the parallelism between fracture zone orientation on the descending plate and convergence direction previously thought responsible for the tectonic segmentation of the peninsula was effective for only 10 30 ma before collision before that either the fracture zones did not extend to the trench or the subducting ocean floor formed at the farallon phoenix nazca aluk boundary with fracture zones oblique to the subduction direction the aleutian arc kula ridge model in which arc magmatism virtually ceases when ocean floor younger than 25 30 ma occurs beneath the arc fits the distribution of antarctic peninsula radiometric dates explaining a 50 60 ma gap between collisions and youngest ages in the s and possibly the migration of the youngest activity towards the trench thus gaps in the geologic record of arc magmatism need not imply cessation of subduction the progressive steepening of the peninsular margin towards the 4 ma collision site suggests tectonic erosion of the fore arc as the ridge crest approached

late cretaceous and paleogene tectonic evolution of the north pacific ocean the late cretaceous history of the northern pacific ocean has not been adequately deciphered largely because a major plate reorganization occurred during the cretaceous magnetic quiet interval using primary data to reconstruct plate motions from fracture zone trends and late cretaceous seafloor spreading magnetic anomalies allows formulation of a reasonable sequence of events that accounts for all the geologic features of that region especially the emperor and chinook troughs the primary event in our reconstruction is the subduction of the old northwest pacific triple junction new relative plate motions imposed by formation of convergent boundaries along both the northern pacific and farallon plates caused the farallon plate to crack this subdivision occurred 82 m y ago and resulted in the formation of the kula and chinook plates the chinook plate was bounded on the north by the chinook kula ridge the western arm of the great magnetic bight on the west by the southern emperor trough a slowly spreading rift valley on the south by the mendocino transform and on the east by the chinook farallon ridge the northern emperor trough formed the initial western boundary of the kula plate from 82 m y ago to 50 m y ago this configuration was stable the chinook plate expanded the chinook kula ridge migrated north and the kula plate was subducted with the subduction of this ridge the chinook plate adhered to the pacific plate and the pacific farallon ridge became the north south trending feature reflected by anomalies 22 and younger this reconstruction obviates the awkward late cretaceous shifting of triple junctions found in other proposed histories and provides reasonable explanations for the several structural features of the region to achieve closure of vector triangles representing the relative late cretaceous plate motions some extension must occur presumably across the eastern mendocino fracture zone between the northern and southern portions of the farallon plate these two plates apparently were independent units between 105 and 50 m y ago the initiation and cessation of rapid north farallon north american convergence 82 and 50 m y ago may have determined the nature and duration of the laramide orogeny

epicentre accuracy based on seismic network criteria summary we establish reliable and conservative estimates for epicentre location accuracy using data that are readily available in published seismic bulletins a large variety of seismic studies rely on catalogues of event locations making proper assessment of location uncertainty critical event location and uncertainty parameters in most global regional and national earthquake catalogues are obtained from traditional linearized inversion methods using a 1 d earth model to predict traveltimes reported catalogue uncertainties are based on the assumption that error processes are gaussian zero mean and uncorrelated unfortunately these assumptions are commonly violated leading to the underestimation of true location uncertainty especially at high confidence levels we find that catalogue location accuracy is most reliably estimated by station geometry we make use of two explosions with exactly known epicentres to develop local network location 0 2 5 accuracy criteria using monte carlo simulations of network geometry we find that local network locations are accurate to within 5 km with a 95 per cent confidence level when the network meets the following criteria 1 there are 10 or more stations all within 250 km 2 an azimuthal gap of less than 110 3 a secondary azimuthal gap of less than 160 and 4 at least one station within 30 km to derive location accuracy criteria for near regional 2 5 10 regional 2 5 20 and teleseismic 28 91 networks we use a large data set of exceptionally well located earthquakes and nuclear explosions beyond local distances we find that the secondary azimuthal gap is sufficient to constrain epicentre accuracy and location error increases when the secondary azimuthal gap exceeds 120 when station coverage meets the criterion of a secondary azimuth gap of less than 120 near regional networks provide 20 km accuracy at the 90 per cent confidence level while regional and teleseismic networks provide 25 km accuracy at the 90 per cent confidence level

late cretaceous to early tertiary subduction history of the antarctic peninsula quantitative estimates of the rates and azimuths of phoenix plate convergence with the antarctic peninsula have been derived from plate rotation calculations for two periods in the late cretaceous and early tertiary 83 0 67 7 and 61 1 53 4 ma published marine magnetic anomaly identifications and flow lines interpreted from gravity anomaly maps were used in simple spherical geometry calculations to derive phoenix pacific stage rotations these were combined with published pacific antarctic rotation data to determine contemporaneous phoenix antarctic stage rotations the results indicate a significant change in azimuths of relative motion between the late cretaceous and early tertiary late cretaceous and perhaps earlier oblique subduction probably caused migration of fore arc slivers along the margin resulting in variations in width of the accretionary prism comparison between synthetic magnetic profiles and a 900 km long magnetic profile across ocean floor produced at the antarctic phoenix ridge during the early tertiary establishes the time of a major decrease in spreading rate and hence also in convergence rate as chron c23r 52 3 ma the associated change in subduction dynamics may have caused the dextral transtensional deformation observed in the george vi sound region and initiated uplift of blueschist facies rocks now exposed on smith island the calculated convergence history does not provide a simple explanation for the occurrence of high mg andesite lavas on alexander island which were erupted more than 20 ma before the antarctic phoenix ridge arrived at the margin the existence of these lavas implies either earlier subduction of another ridge or slab break up beneath alexander island

the morphology of propagating spreading centers new and old recent seabeam investigations over the east pacific rise in the northeast pacific have shown the existence of a propagating spreading center at 18 n the propagating and dying spreading centers are joined by a zone of orthogonal lineations which appear to be the traces of a continuously migrating transform fault similar orthogonal lineations occur at 16 n 107 30 w on the western side of the trace of a 5 m y old propagating spreading center the amplitude and the complexity of the topographic signature over the earlier propagating spreading center are higher and greater than over the presently active center suggesting that the relief of the earlier center may owe its origin to differential subsidence of thermally contrasting segments of lithosphere analogous to fracture zone topography of similar origin traces of former propagating spreading centers are thus a combination of two sets of landforms those resulting from propagation and those resulting from the juxtaposition of two blocks of lithosphere of different thickness

40ar 39ar dating constraints on the high angle normal faulting along the southern segment of the tan lu fault system an implication for the onset of eastern china rift systems abstract high angle normal faulting in eastern china was an important tectonic process responsible for the rifting of the eastern asian continental margin along the southern segment of the tan lu fault system part of the eastern china rift system 55 70 east dipping normal faults are the oldest structures within this rift system chlorite pseudotachylite and fault breccia are found in fault zones which are characterized by microstructures and syn deformation chlorite minerals aligned parallel to a down dip stretching lineation 40ar 39ar dating of syn deformation chlorite and k feldspar from the fault gouge zone yields cooling ages of 75 70 ma interpreted as the timing of slip along the normal faults this age is older than that of opening of the japanese sea and back arc extension in the west pacific but similar to the onset of the indo asian soft collision

segmentation of mid ocean ridges studies of mid ocean ridges in the pacific and atlantic oceans show that the volcanism that forms the oceanic crust along the spreading plate boundaries is concentrated at regular intervals related to spreading rate this observation and a new calculation for a rayleigh taylor type of gravitational instability of a partially molten mantle region growing under spreading centres yield reasonable estimates of upper mantle viscosities

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

fluctuations in biosiliceous production and the generation of early cretaceous oceanic anoxic events in the pacific ocean shatsky rise ocean drilling program leg 198 integration of geophysical logs with sedimentological biostratigraphic and physical properties data from poorly recovered chert rich sequences at ocean drilling program leg 198 sites 1207 and 1213 allows us to propose an aptian albian early cretaceous depositional history for shatsky rise north pacific ocean the logs indicate that the lower aptian and lower albian deposits are relatively lithified and unporous compared to contiguous sediments these characteristics are best explained by increased siliceous cementation that resulted from an elevated flux of radiolarians to the seafloor the intervals of increased biosiliceous production may record a biotic response to enhanced nutrient delivery coeval with but longer in duration than oceanic anoxic events oaes 1a early aptian and 1b early albian on shatsky rise oae 1a is represented by organic carbon rich sediments but oae 1b is not thus black shale deposition in the pacific ocean was only possible when a carbon burial threshold was crossed we speculate that this threshold was related to the balance between productivity and carbon oxidation and that ultimately the supply of biolimiting nutrients through hydrothermal alteration of basalt may have been the key to the development of black shale in the pacific basin

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

southern california hypocenter relocation with waveform cross correlation part 1 results using the double difference method we present the results of relocating 327 000 southern california earthquakes that occurred between 1984 and 2002 we apply time domain waveform cross correlation for p and s waves between each event and 100 neighboring events identified from the catalog based on a 3d velocity model to simplify the computation we first divide southern california into five polygons such that there are 100 000 events or less in each region the polygon boundaries are chosen to lie in regions of sparse seismicity we calculate and save differential times from the peaks in the cross correlation functions and use a spline interpolation method to achieve a nominal timing precision of 0 001 sec these differential times together with existing p and s phase picks are input to the double difference program of waldhauser and ellsworth 2000 2002 to calculate refined hypocenters we divide the southern california region into grid cells and successively relocate hypocenters within each grid cell the overall resulting pattern of seismicity is more focused than the previously determined pattern from 1d or 3d models the new improved locations are more clustered in many cases by a factor of two or three and often show clear linear alignments in particular the depth distribution is improved and less affected by layer boundaries in velocity models or other similar artifacts

parametric estimation of phase and group slownesses from sonic logging waveforms sonic logging waveforms consist of a mixture of nondispersive waves such as the p and s headwaves and dispersive waves such as the stoneley and pseudo rayleigh waves in monopole logging and the flexural wave in dipole logging conventionally slowness dispersion curves of various waves are estimated at each frequency independent of data at other frequencies this approach does not account for the fact that slowness dispersion functions in sonic logging are continuous and in most cases smooth functions of frequency we describe a parametric slowness estimation method that uses this property by locally approximating the wavenumber of each wave as a linear function of frequency this provides a parametric model for the phase and group slownesses of the waves propagating across the receiver array the estimation of phase and group slownesses is then carried out by minimizing the squared difference between the predicted and observed waveforms the minimization problem is nonlinear and is solved by an it

identifying active structures using double difference earthquake relocations in southwest british columbia and the san juan islands washington this paper applies double difference earthquake relocation techniques to investigate sources of seismicity in southwest british columbia canada and the san juan islands washington the study area is a complex region of deformation and has the potential for large earthquakes in the north americancrust double difference earthquake relocation techniques are applied to identify otherwise hidden active struc tures that may pose a hazard to nearby population and infrastructure we present evi dence for previously unrecognized active structures using precise relative earthquake relocations obtained using both catalog and waveform cross correlation data results have significantly reduced errors over routine catalog locations and show lineations in areas of clustered seismicity in southwest british columbia these lineations or streaks appear to be hidden structures that do not disrupt near surface sediments however in thesanjuanislandstheidentifiedlineationcouldberelatedtorecentlymappedsurface expressions offaults identifiedfromseismic reflection andmultibeam bathymetric sur veys we use avariety of velocity models for therelocations and find that inappropriate models lead to artifacts at layer boundaries and increased vertical errors

crustal structure interpreted from magnetic anomalies this review discusses publications during the last quadrennium 1987 1990 that used aeromagnetic data marine magnetic data satellite magnetic data and rock magnetic and petrologic data to provide information on the sources of magnetic anomalies the publications reviewed reflect increased integration of rock magnetic property and petrologic studies with magnetic anomaly interpretation studies particularly in deep crustal magnetization exploration for hydrocarbons and inversion of marine magnetic anomalies interpretations of aeromagnetic data featuring image display techniques and using the horizontal gradient method for locating magnetization boundaries became standard

ridge spotting a new test for pacific absolute plate motion models relative plate motions provide high resolution descriptions of motions of plates relative to other plates yet geodynamically motions of plates relative to the mantle are required since such motions can be attributed to forces e g slab pull and ridge push acting upon the plates various reference frames have been proposed such as the hot spot reference frame to link plate motions to a mantle framework unfortunately both accuracy and precision of absolute plate motion models lag behind those of relative plate motion models consequently it is paramount to use relative plate motions in improving our understanding of absolute plate motions a new technique called ridge spotting combines absolute and relative plate motions and examines the viability of proposed absolute plate motion models we test the method on six published pacific absolute plate motions models including fixed and moving hot spot models as well as a geodynamically derived model ridge spotting reconstructs the pacific farallon and pacific antarctica ridge systems over the last 80 myr all six absolute plate motion models predict large amounts of northward migration and monotonic clockwise rotation for the pacific farallon ridge a geodynamic implication of our ridge migration predictions is that the suggestion that the pacific farallon ridge may have been pinned by a large mantle upwelling is not supported unexpected or erratic ridge behaviors may be tied to limitations in the models themselves or for indo atlantic models discrepancies in the plate circuits used to project models into the pacific realm ridge spotting is promising and will be extended to include more plates and other ocean basins

geography and history of the louisville hotspot chain in the southwest pacific the louisville ridge is a 4300 km long late cretaceous and cenozoic hotspot chain the south pacific equivalent of the hawaiian emperor chain its northwestern end is being consumed by the tonga kermadec subduction zone but seabeam and magnetic mapping shows that the chain still includes more than 60 major volcanoes of both normal and reversed polarity distributed along a 75 km wide band that can be fitted to small circles about three successive poles of pacific plate hotspot rotation this band obliquely crosses fracture zones of the eltanin system but there is little interaction and no evidence of any genetic connection between the two structures forty of the louisville volcanoes grew above sea level and are preserved as high latitude i e coral free guyots they are spaced less than 100 km apart along most of the chain but there are none within 750 km of the inferred present location of the hotspot beneath a swell at the southeast end of the chain the rate of volcano building by the louisville hotspot declined sharply about 20 m y ago after being remarkably constant at 3 4 103 km3 m y for the previous 50 m y and none of the louisville volcanoes built during the past 10 12 m y the time of most profuse hawaiian volcanism has reached sea level however a seamount from which pleistocene lavas were dredged rises to within 540 m of the sea surface from the crest of the hotspot swell at 50 5 s 139 2 w guyot heights demonstrate that a hotspot swell several hundred meters high has persisted throughout the known life of the chain and sometimes had an isostatic depression on its loaded crest the depths of guyot shelf breaks increase systematically to the northwest from 530 to 2100 m proportional to the square root of volcano age with some scatter and tilting caused by plate boundary tectonism and local isostatic subsidence and uplift

a statistical correlation between ridge crest offsets and spreading rate ridge crest offsets follow the statistical distribution n a exp b ln l lo where n is the cumulative number of ridge crest offsets with lengths greater than l lo is the cutoff length and a and b are constants we measured a and b for the slow spreading mid atlantic ridge the intermediate spreading juan de fuca ridge and the fast spreading east pacific rise the predicted mean distances between offsets greater than 4 km long are 30 32 and 74 km on the slow intermediate and fast spreading ridge crests these distances are in agreement with observations of other workers and calculations of modal ridge lengths

intra oceanic subduction shaped the assembly of cordilleran north america the western quarter of north america consists of accreted terranes crustal blocks added over the past 200 million years but the reason for this is unclear the widely accepted explanation posits that the oceanic farallon plate acted as a conveyor belt sweeping terranes into the continental margin while subducting under it here we show that this hypothesis which fails to explain many terrane complexities is also inconsistent with new tomographic images of lower mantle slabs and with their locations relative to plate reconstructions we offer a reinterpretation of north american palaeogeography and test it quantitatively collision events are clearly recorded by slab geometry and can be time calibrated and reconciled with plate reconstructions and surface geology the seas west of cretaceous north america must have resembled today s western pacific strung with island arcs all proto pacific plates initially subducted into almost stationary intra oceanic trenches and accumulated below as massive vertical slab walls above the slabs long lived volcanic archipelagos and subduction complexes grew crustal accretion occurred when north america overrode the archipelagos causing major episodes of cordilleran mountain building a new explanation for the origin of the accreted terranes that form the mountainous cordillera of western north america is proposed and tested stationary intra oceanic subduction deposited massive slab walls in the mantle and grew volcanic archipelagos at the surface which were overridden by and accreted to north america during cretaceous times the generally accepted account of how the geology of western north america developed involves the oceanic farallon plate acting as a conveyor belt sweeping crustal terranes onto the continental margin as it moves beneath the north american plate this model fails to explain many aspects of this accreted terrane and here karin sigloch and mitchell mihalynuk present an alternative model based on new tomographic images of lower mantle slabs and modelling of deep mantle structure in the region they propose that during the cretaceous period there were a series of island arcs west of north america beneath which proto pacific plates subducted into almost stationary trenches long lived volcanic archipelagos and subduction complexes grew above the slabs and then accreted onto north america as it overrode the archipelagos causing episodes of cordilleran mountain building

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

the tethyan pacific during late jurassic and cretaceous times abstract during mesozoic times the pacific ocean occupied almost an entire hemisphere plus a great wedge of ocean that lay between eastern gondwana and laurasia and extended westward from the pacific into the gradually widening tethys sea in the mediterranea atlantic and gulf of mexico as the atlantic widened the pacific narrowed apace and the pacific plate expanded at the expense of bordering plates during the late jurassic and cretaceous connections between the atlantic and pacific via passageways between small continental plates in central america gradually widened permitting a globe circling mainly west flowing current system in low latitudes except for tectonic slices preserved along the adjacent american and asian continents all pre jurassic pacific oceanic sediments have been subducted middle and late jurassic oceanic crust is inferred from magnetic data and one drill hole reached middle jurassic crust pacific cretaceous pelagic sedimentary rocks are typically nannofossil limestone above the calcite compensation depth and brown zeolitic clay and radiolarian mudstone below chert is common as nodules in both limey and clayey sediments and siliceous sediments appear to have been deposited over a wide band of latitudes in the cretaceous lower cretaceous black shale is present on several plateaus at paleodepths of 500 1500 m but contemporaneous sediments in adjacent basins are oxidized cretaceous vulcanism was on a stupendous scale in the western equatorial pacific a region of about 4x106 km2 centered on the ontong java plateau is covered almost entirely by mid plate tholeiitic flood basaltic flows and sills probably emplaced mainly during the earliest aptian emplacement of these huge masses in a relatively short time may have had important consequences not only for global sea level but also for the chemistry of the oceans and atmosphere a striking link to tethyan facies in the other hemisphere is the presence of shallow water platform and reefal sediments on the summits of many pacific cretaceous seamounts several dozen early cretaceous seamounts built to heights above sea level and doubtless many more existed on crust now subducted along the margins of the pacific as the seamounts subsided shallow water rudist coral platforms gradually covered them eventually forming atolls the platforms are the pacific expression of the urgonian platforms of the tethys many perhaps most of the pacific early cretaceous platforms drowned in the latest albian and are guyots today new volcanic chains rose later in the cretaceous but there is a gap in rudist reef development from the cenomanian through the santonian campanian and maestrichtian rudist reefs flourished in equatorial latitudes but hermatypic reefs appear to have been absent during the paleocene

estimating slowness dispersion from arrays of sonic logging waveforms acoustic wave propagation in a fluid filled borehole is affected by the type of rock which surrounds the hole more specifically the slowness dispersion of the various body wave and borehole modes depends to some extent on the properties of the rock we have developed a technique for estimating the dispersion relations from data acquired by full waveform digital sonic array well logging tools the technique is an extension of earlier work and is based on a variation of the well known prony method of exponential modeling to estimate the spatial wavenumbers at each temporal frequency this variation known as the forward backward method of linear prediction models the spatial propagation by purely real valued wavenumbers the prony exponential model is derived from the physics of borehole acoustics under the assumption that the formation does not vary in the axial or azimuthal dimensions across the aperture of the receiver array but can vary arbitrarily in the radial dimension the exponential model fits

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

monitoring the earthquake source process in north america with the implementation of the usgs national earthquake information center prompt assessment of global earthquakes for response system pager rapid determination of earthquake moment magnitude is essential especially for earthquakes that are felt within the contiguous united states we report an implementation of moment tensor processing for application to broad seismically active areas of north america this effort focuses on the selection of regional crustal velocity models codification of data quality tests and the development of procedures for rapid computation of the seismic moment tensor we systematically apply these techniques to earthquakes with reported magnitude greater than 3 5 in continental north america that are not associated with a tectonic plate boundary using the 0 02 0 10 hz passband we can usually determine with few exceptions moment tensor solutions for earthquakes with m w as small as 3 7 the threshold is significantly influenced by the density of stations the location of the earthquake relative to the seismic stations and of course the signal to noise ratio with the existing permanent broadband stations in north america operated for rapid earthquake response the seismic moment tensor of most earthquakes that are m w 4 or larger can be routinely computed as expected the nonuniform spatial pattern of these solutions reflects the seismicity pattern however the orientation of the direction of maximum compressive stress and the predominant style of faulting is spatially coherent across large regions of the continent

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

apparent polar wander of the mean lithosphere reference frame summary apparent polar wander in the mean lithosphere no net rotation no net torque uniform drag reference frame is compared with apparent polar wander in the hotspot reference frame over the past 100 myr palaeomagnetic poles and plate rotations previously used to determine an apparent polar wander path for the hotspot reference frame are here used to determine an apparent polar wander path in the mean lithosphere reference frame we find that the two paths are similar especially for late cretaceous time when a 10 20 shift of the pole occurred to first order the hotspots and lithosphere as a whole moved in unison relative to the palaeomagnetic axis during late cretaceous time a non dipole field explanation for the apparent shift can probably be excluded however either motion of the time averaged geomagnetic axis relative to the spin axis or polar wandering could have caused this shift the latter being the more likely explanation

structural evolution of preexisting oceanic crust through intraplate igneous activities in the marcus wake seamount chain multichannel seismic reflection studies and seismic refraction surveys with ocean bottom seismographs in the marcus wake seamount chain in the northwestern pacific ocean reveal p wave velocity structures of hot spot origin seamounts and adjacent oceanic crust inside the seamounts are central high velocity 6 5 km s structures extending nearly to the top that may indicate intrusive cores thick sediment layers up to 4 km with p wave velocities of 4 5 km s have accumulated on seafloor that predates seamount formation downward crustal thickening of up to 2 km was documented beneath a large seamount cluster but thickening was not confirmed below a small seamount cluster volume ratios of an intrusive core to a seamount body are 15 20 indicating that most of the supplied magma was consumed in forming the thick sedimentary and volcaniclastic layer constituting the seamount flanks underplating and downward crustal thickening may tend to occur when second or later intrusive cores are formed in a seamount p wave velocities in the lowest crust and in the uppermost mantle below the seamount chain are 0 1 0 2 km s higher and 0 3 0 5 km s lower respectively than velocities below oceanic crust we explain this difference as a result of sill like intrusion of magma into the lower crust and uppermost mantle reflected waves observed at offsets 200 km are from mantle reflectors at depths of 30 45 km and 55 70 km the shallower reflectors may indicate structures formed by intraplate igneous activities and the deeper reflectors may correspond to the lithosphere asthenosphere boundary

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

processing array acoustic logging data to image near borehole geologic structures imaging near borehole structures using acoustic logging data depends on the data processing method used we demonstrate that extracting the up and downgoing reflections from the data and using those separately for the imaging can significantly improve the image quality a parametric wave separation is first applied to array data to separate direct and reflection waves in particular we use a transmitter array gathered from successive source positions to extract the upgoing reflection reflection data can also be enhanced by stacking data along the reflection moveout in array using approximate structural dip information implementation of this method to acoustic logging data processing improves imaging quality making it possible to image near borehole geologic structures using conventional array acoustic logging a potentially important application of drilling steering is using logging while drilling acoustic measurements

geometry and earthquake potential of the shoreline fault central california abstract the shoreline fault is a vertical strike slip fault running along the coastline near san luis obispo california much is unknown about the shoreline fault including its slip rate and the details of its geometry here i study the geometry of the shoreline fault at seismogenic depth as well as the adjacent section of the offshore hosgri fault using seismicity relocations and earthquake focal mechanisms the optimal anisotropic dynamic clustering oadc algorithm ouillon et al 2008 is used to objectively identify the simplest planar fault geometry that fits all of the earthquakes to within their location uncertainty the oadc results show that the shoreline fault is a single continuous structure that connects to the hosgri fault discontinuities smaller than about 1 km may be undetected but would be too small to be barriers to earthquake rupture the hosgri fault dips steeply to the east while the shoreline fault is essentially vertical so the hosgri fault dips towards and under the shoreline fault as the two faults approach their intersection the focal mechanisms generally agree with pure right lateral strike slip on the oadc planes but suggest a non planar hosgri fault or another structure underlying the northern shoreline fault the shoreline fault most likely transfers strike slip motion between the hosgri fault and other faults of the pacific north america plate boundary system to the east a hypothetical earthquake rupturing the entire known length of the shoreline fault would have a moment magnitude of 6 4 6 8 a hypothetical earthquake rupturing the shoreline fault and the section of the hosgri fault north of the hosgri shoreline junction would have a moment magnitude of 7 2 7 5

convective partial melting 1 a model for the formation of thick basaltic sequences during the initiation of spreading stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure temperature environment where partial melting occurs the amount of melt liberated is however insufficient to account for the thickness of igneous crust emplaced near the continent ocean boundary at some passive margins these igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide seismic studies also suggest that the transition from continental to oceanic crust on these volcanic margins is relatively abrupt their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading to account for these observations we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle deep hot mantle material is transported upward by the convection and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling numerical calculations show that convective partial melting can provide substantial additions to melting due to passive upwelling alone using this model we propose an evolutionary scheme for volcanic passive margins emphasize the contrast with nonvolcanic margins and describe conditions which might lead to the development of both types

tectonic fabric map of the ocean basins from satellite altimetry data abstract satellite altimetry data provide a new source of information on the bathymetry of the ocean floor the tectonic fabric of the oceans i e the arrangement of fracture zones ridges volcanic plateaus and trenches is revealed by changes in the horizontal gravity gradient as recorded by satellite altimetry measurements seasat and geosat altimetry data have been analyzed and a global map of the horizontal gravity gradient has been produced that can be used to identify a variety of marine tectonic features the uniformity of the satellite coverage provides greater resolution and continuity than maps based solely on ship track data this map is also the first global map to incorporate the results of the geosat mission and as a result new tectonic features are revealed at high southerly latitudes this map permits the extension of many tectonic features well beyond what was previously known for instance various fracture zones such as the ascension tasman and udintsev fracture zones can be extended much closer to adjacent coninental margins the tectonic fabric map also reveals many features that have not been previously mapped these features include extinct ridges minor fracture zone lineations and seamounts in several areas especially across aseismic plateaus or along the margins of the continents the map displays broad gravity anomalies whose origin may be related to basement structures

inversion of p and sv waves from multicomponent offset vertical seismic profiles downgoing waves in multicomponent vsp experiments are used to obtain seismic p and s wave velocities as a function of depth and angle of incidence if p and sv waveforms do not overlap in time at the depth of interest local velocities of the medium are obtained by separate analysis of these events the apparent velocity of the event p or sv is computed from the moveout across several neighboring depth locations the angle of incidence of the same event is computed from the particle motion hodogram within an appropriately chosen time window then the local medium velocity p wave or s wave depending on the chosen event is given by the apparent velocity multiplied by the cosine of the angle of incidence layer interfaces with reasonably sharp velocity contrasts are efficient p wave to sv wave converters even at moderate angles of incidence in offset vsp experiments converted sv waves are generated with varying strengths at practically all depths consequently the converted sv waveforms partially o

characteristics of waveforms recorded by azimuthally spaced hydrophones of sonic logging tool for incident plane waves abstractthe borehole acoustic reflection survey bars is a method for imaging near borehole structures using waveforms acquired by a sonic logging tool signals from a monopole or dipole source are reflected from geologic interfaces and recorded by arrays of the hydrophones of the same tool due to the nearly 1d configuration of the source and hydrophones bars provides only 2d images in which the dimensions are the axis of the well and the distance from the center of the well we have developed methods for identifying the azimuthal direction of the reflector which is not given by the bars image by using the differences of the signals recorded by hydrophones at varying azimuths we have conducted a theoretical analysis for incident plane waves as follows analytic solutions in the frequency domain are numerically computed at discrete frequencies and converted to time domain synthetics using the fast fourier transform for a newly derived model that includes the effect of the tool in the fluid filled bo

equator crossing of shatsky rise new insights on shatsky rise tectonic motion from the downhole magnetic architecture of the uppermost lava sequences at tamu massif 1 shatsky rise is a large igneous province lip currently located in the northwestern pacific new downhole magnetic logging data from integrated ocean drilling program iodp hole u1347a at tamu massif of shatsky rise captured the magnetic architecture in the uppermost lava sequence providing a rare opportunity to investigate a time series of the intra plate volcanism in conjunction with the pacific plate construction history centered at the triple junction logging data results indicate that tamu massif was formed during normal polarity periods south of the paleoequator and crossed the equator at some point in the m19 m17 period combining these new observations with previous interpretations of the massif s tectonic history a time series of the latitudinal tectonic motion of a lip and the underlying pacific plate during the plateau formation is postulated

late cretaceous genesis of the kula plate abstract we propose that the pacific kula ridge began spreading approximately 85 m y b p during late cretaceous time extrapolation of the great magnetic bight backwards in time results in an implausible ridge configuration this implies that plate velocity vectors for the pacific kula and farallon plates were not constant during this interval evidence for splitting of the kula plate from the pacific plate along the chinook trough is the relationship between the north striking amlia and adak fracture zones the chinook and emperor troughs and the magnetic lineations south of the aleutian trench if this hypothesis is correct it will require that mesozoic reconstructions of the pacific basin and their relation to cenozoic reconstructions be re examined a previously unrecognized mesozoic plate may be required we propose calling this the izanagi plate

fault structure and mechanics of the hayward fault california from double difference earthquake locations 1 the relationship between small magnitude seismicity and large scale crustal faulting along the hayward fault california is investigated using a double difference dd earthquake location algorithm we used the dd method to determine high resolution hypocenter locations of the seismicity that occurred between 1967 and 1998 the dd technique incorporates catalog travel time data and relative p and s wave arrival time measurements from waveform cross correlation to solve for the hypocentral separation between events the relocated seismicity reveals a narrow near vertical fault zone at most locations this zone follows the hayward fault along its northern half and then diverges from it to the east near san leandro forming the mission trend the relocated seismicity is consistent with the idea that slip from the calaveras fault is transferred over the mission trend onto the northern hayward fault the mission trend is not clearly associated with any mapped active fault as it continues to the south and joins the calaveras fault at calaveras reservoir in some locations discrete structures adjacent to the main trace are seen features that were previously hidden in the uncertainty of the network locations the fine structure of the seismicity suggests that the fault surface on the northern hayward fault is curved or that the events occur on several substructures near san leandro where the more westerly striking trend of the mission seismicity intersects with the surface trace of the aseismic southern hayward fault the seismicity remains diffuse after relocation with strong variation in focal mechanisms between adjacent events indicating a highly fractured zone of deformation the seismicity is highly organized in space especially on the northern hayward fault where it forms horizontal slip parallel streaks of hypocenters of only a few tens of meters width bounded by areas almost absent of seismic activity during the interval from 1984 to 1998 when digital waveforms are available we find that fewer than 6 5 of the earthquakes can be classified as repeating earthquakes events that rupture the same fault patch more than one time these most commonly are located in the shallow creeping part of the fault or within the streaks at greater depth the slow repeat rate of 2 3 times within the 15 year observation period for events with magnitudes around m 1 5 is indicative of a low slip rate or a high stress drop the absence of microearthquakes over large contiguous areas of the northern hayward fault plane in the depth interval from 5 to 10 km and the concentrations of seismicity at these depths suggest that the aseismic regions are either locked or retarded and are storing strain energy for release in future large magnitude earthquakes

relocations of five yingjiang sequences china myanmar border details of seismogenic faults abstract five moderate sized earthquakes struck the yingjiang area china myanmar border where there is a lack of historical earthquakes and provided us an opportunity to study the seismogenic fault we relocate sequences via the hypodd method using the cross correlated arrival times the phase picking errors by the human analysts had only 0 01 s difference before and after 2009 however the p phase picking precision increased from the tenths to the hundredths place after 2009 and the relocation errors of hypodd simultaneously decreased by a factor of 2 the rupture dimensions are about 2 4 km for the five sequences the hypodd relocation errors are only 0 36 or 0 63 km for the two test clusters which are much smaller than the dimensions of the rupture areas the synthetic tests show no location bias from the distribution of the yunnan regional seismic network the relocations identify four linear features with the northwest north northwest or northeast direction for the 21 march 2008 3 september 2008 10 march 2011 and 30 may 2014 sequences orthogonal to the known major fault traces the relocations also suggest that the 10 march 2011 sequence is on a double layered fault dipping roughly 71 to the southwest focal mechanisms reveal a mixture of strike slip normal and reverse faulting

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

reference events for regional seismic phases at ims stations in china seismic event location within the context of monitoring the compre hensive nuclear test ban treaty entails a priori knowledge of the travel time of seismic phases for a given source to stations of the international monitoring system ims such travel time information or ground truth gt is provided empirically by seismic reference events events that have well determined hypocenter locations epicenters typically known to 5 km with high confidence and origin times in this study we present new reference events for the calibration of six seismic stations of the ims in china a region with high seismic activity we use the annual bulletin of chinese earthquakes which lists about 1000 earthquakes in and near china each year with consistent phase picks at regional stations to determine precise relative earthquake locations from double difference cluster analysis the resulting high resolution image of active faulting at seismogenic depths in areas of dense seismicity is correlated with the tectonic structure derived from mapped fault information at the surface to validate the absolute locations we generated 59 reference events with m 3 5 distributed in six clusters in central and eastern china and recorded by at least one of the six ims stations the scatter in relative travel time residuals is reduced from 1 28 sec before to 0 61 sec after relocation consistent with the relocated po sitions of the events the degree of correlation between seismicity structure and well characterized fault data indicates that in four clusters the locations of the new ref erence events are accurate to within 5 km gt5 and in two clusters within 10 km gt10

imaging of near borehole structure using full waveform sonic data the full waveforms recorded by an array of receivers in a modern borehole sonic tool contain secondary arrivals that are reflected from near borehole structural features these arrivals are used to form an image of the near borehole structural features in a manner similar to seismic migration possible uses of this technique include horizontal well logging structural dip and contour determination fault salt dome pinnacle reef and fracture zone imaging and eor steam flood monitoring since both the source and the receivers pass through structures that cross the borehole the downdip structure and the updip structure can be imaged separately the technique involves a backprojection of the recorded data into a matrix of accumulation bins representing distances radially out from the borehole and along the borehole axis separate matrices are formed for the updip and for the downdip raypaths the basic technique is illustrated with synthetic data generated to approximate the case of a sonic tool logging

the seismic moho structure of shatsky rise oceanic plateau northwest pacific ocean abstract oceanic plateaus are large igneous provinces formed by extraordinary eruptions that create thick oceanic crust whose structure is poorly known owing to the lack of deep penetration seismic data multichannel seismic mcs reflection and wide angle refraction data allow us to show moho structure beneath a large part of the shatsky rise oceanic plateau in the northwest pacific ocean moho reflectors in the two data sets can be connected to trace the interface from the adjacent abyssal plain across much of the interior the reflectors display varied character in continuity shape and amplitude similar to characteristics reported in other locations beneath normal crust the moho is observed at 13 km depth 7 km below the seafloor in mcs data and disappears at 20 km depth 17 km below the seafloor beneath the high plateau moho at the distal flanks dips downward towards the center with slopes of 0 5 1 increasing to 3 5 at the middle flanks seismic moho topography is consistent with airy isostasy confirming this widely applied assumption data from this study show that crustal thickness between the massifs in the interior of the plateau is nearly twice normal crustal thickness despite the fact that this crust records apparently normal seafloor spreading magnetic lineations the moho model allows improved estimates of plateau area 5 33 10 5 km 2 and volume 6 90 10 6 km 3 the latter assuming that the entire crust was formed by shatsky rise volcanism because the massifs formed at spreading ridges this study is unique in showing moho depth and structure over an extraordinarily large area beneath an oceanic plateau giving insight to plateau structure and formation

origin of the louisville ridge and its relationship to the eltanin fracture zone system we have combined shipboard and seasat altimeter derived data in an intergrated geological and geophysical study of the louisville ridge a 3500 km long seamount chain extending from the tonga trench to the eltanin fracture zone a break in the smooth trend of the ridge at latitude 37 5 s has been recognized in both bathymetric and altimetric data the 40ar 39ar dating of rocks dredged either side of the break suggest that it is analogous to the bend in the hawaiian emperor seamount chain although the general trend of the ridge can be fit by small circles about pacific absolute motion poles determined from other seamount chains the new bathymetric and age data allow us to refine pacific absolute motion poles the continuity in smooth trend of the ridge and the eltanin fracture zone suggests some relationship between them however a major offset developed on this transform between 60 and 80 ma prior to the oldest dated rocks from the ridge although magmatism was more or less continuous on the ridge during 28 60 ma it probably occurred on crust with little or no offset thus magmatism appears to have been little influenced by the developing fracture zone by 28 ma the distance between the magmatic source and the fracture zone had decreased sufficiently for a portion of the ridge to have been emplaced on crust with an offset after about 12 ma however volcanic activity on the louisville ridge apparently waned despite a possible influence on the magmatism of the fracture zone

tectonics and evolution of the juan fernandez microplate at the pacific nazca antarctic triple junction in this study magnetic and bathymetric data from an r v endeavor survey are combined with earthquake mechanism studies to produce a tectonic model for the juan fernandez microplate at the pacific nazca antarctic triple junction magnetic lineations along the east ridge of the microplate fan to the south indicating that the nazca juan fernandez pole is near the northern end of this feature relative motions of the juan fernandez pacific nazca antarctic four plate system are determined using relative plate motion inversions the resulting tectonic model predicts that the northern boundary of the juan fernandez microplate is a zone of compression and that the west ridge and southwestern boundary are spreading obliquely the southeastern boundary is predicted to be obliquely divergent but present data fail to constrain its geometry we also present a schematic evolutionary model which relates the formation of the juan fernandez microplate to differential spreading rates at the triple junction and discuss related complexities the future evolution of the fourplate system depends on whether one or both ridges remain active

the magallanes and deseado fault zones major segments of the south american scotia transform plate boundary in southernmost south america tierra del fuego the 600 km long magallanes fault zone and the newly discovered 100 km long deseado fault zone represent large but little understood segments of the south american scotia sam sco transform on the south american continent 50 56 s latitude kinematic analyses of fault populations near mount hope on tierra del fuego indicate that these fault zones have accommodated sinistral strike slip motion with a probable component of normal slip on vertical subvertical faults during cenozoic time regional geological data and overprinting relationships between the strike slip oblique slip faults and mid cretaceous to oligocene contractional structures suggest that sinistral motion in the mount hope segments of the two fault zones occurred significantly since 60 ma and dominantly since 30 ma with the most recent episode of activity occurring during the quaternary results from mapping show 20 25 km of cumulative sinistral separation of a thrust contact across a well exposed segment of the magallanes fault zone north and east of mount hope although the exact amount of displacement accommodated by this segment of the magallanes fault zone remains uncertain the 20 25 km of cumulative sinistral separation combined with an estimate of 3 km of vertical separation is consistent with small magnitudes of displacement of the order of tens of kilometers this result apparently contrasts with predictions based on plate reconstructions that large amounts of strike slip displacement 200 km have been accommodated internally within southernmost south america since early cenozoic time a low magnitude of displacement along the mount hope segment of the magallanes fault zone suggests that 1 strike slip motion has been partitioned across the continent since about 30 ma and or 2 the mount hope segment of the magallanes fault zone represents a young part of the modern plate boundary that migrated to its present location during the last few million years

tectonic evolution of the antarctic phoenix plate system since 15 ma abstract joint inversion of magnetic isochron and fracture zone data from the extinct antarctic phoenix spreading system in sw drake passage yields seven new finite reconstruction poles the inversion results are very well constrained for such a short length of plate boundary although this is partly because the finite poles are located close to the reconstructed region the optimum use of fracture zone identifications from satellite derived free air gravity data is also important as the stability of stage poles throughout the short intervals in the model affirms the model results describe a well organised spreading system since magnetic anomaly chron c5ad 15 ma in which the phoenix plate rotated about stage poles nearby to the southwest stage pole locations are broadly consistent with a hypothesis of pivoting subduction as the driving force of phoenix plate movement and there is some evidence in the progression of stage poles for late stage movement of the subduction pivot in response to the changing azimuth of the subduction zone at which the phoenix plate was being consumed the model kinematics alone provide no unequivocal support for previous interpretations of disruption of the subducted part of the phoenix plate the very latest stages of spreading saw falling spreading rates between magnetic anomaly chrons c4 8 1 ma and c2a 3 3 ma when the antarctic phoenix ridge became extinct this is consistent with an increase in shear stress across the plate bounding shackleton fracture zone due to a plate reorganisation in the neighbouring scotia sea following the cessation of spreading on the west scotia ridge

tests of fixity of the indo atlantic hot spots relative to pacific hot spots rates of inter hot spot motion have been debated for decades herein we present updated predictions for the tracks of the tristan da cunha reunion and iceland hot spots assuming them to be fixed relative to pacific hot spots uncertainties in pacific hot spot rotations which include uncertainties in the current locations of hot spots of 100 200 km are combined with uncertainties in relative plate motions accumulated through the plate circuit to obtain the final uncertainty in the predicted positions including uncertainties of 150 200 km in the current locations of the indo atlantic hot spots improvements to reconstruction methods to relative plate reconstructions to age dates along the tracks and to the geomagnetic reversal timescale lead to significant changes from prior results when compared with the observed tracks the predicted tracks indicate nominal rates of motion of only 2 6 mm a 1 of these indo atlantic hot spots relative to pacific hot spots over the past 48 ma within the uncertainties the rates range from no motion to rates as high as 8 13 mm a 1 for reconstructions prior to 48 ma b p however the apparent rates of inter hot spot motion are much larger 46 55 20 mm a 1 if the motion occurred entirely between 68 ma b p and 48 ma b p either hot spots moved rapidly before 48 ma b p and slowed drastically at 48 ma b p or global plate circuits through antarctica become less reliable as one goes increasingly further into the past most paleomagnetic data favor the latter explanation

waveform cross correlation based differential travel time measurements at the northern california seismic network we processed the complete digital seismogram database for northern california to measure accurate differential travel times for correlated earthquakes observed at common stations correlated earthquakes are earthquakes that occur within a few kilometers of one another and have similar focal mechanisms thus generating similar waveforms allowing measurements to be made via cross corre lation analysis the waveform database was obtained from the northern california earthquake data center and includes about 15 million seismograms from 225 000 local earthquakes between 1984 and 2003 a total of 26 billion cross correlation measurements were performed on a 32 node 64 processor linux cluster using improved analysis tools all event pairs with separation distances of 5 km or less were processed at all stations that recorded the pair we computed a total of about 1 7 billion p wave differential times from pairs of waveforms that had cross correlation coefficients cc of 0 6 or larger the p wave differential times are often on the order of a factor of ten to a hundred times more accurate than those obtained from routinely picked phase onsets 1 2 billion s wave differential times were mea sured with cc 0 6 a phase not routinely picked at the northern california seismic network because of the noise level of remaining p coda we found that approxi mately 95 of the seismicity includes events that have cross correlation coefficients of cc 0 7 with at least one other event recorded at four or more stations at some stations more than 40 of the recorded events are similar at the cc 0 9 level indicating the potential existence of large numbers of repeating earthquakes large numbers of correlated events occur in different tectonic regions including the san andreas fault long valley caldera geysers geothermal field and mendocino triple junction future research using these data may substantially improve earthquake lo cations and add insight into the velocity structure in the crust

post cratonization deformation processes and tectonic evolution of the north china craton abstract the timing of events during the tectonic evolution of the north china craton ncc and the east china region is debated extension compression transpression and shearing occurred throughout the ncc since 320 ma these main structural stages and the temporal evolution of the structural framework and kinematics are summarized we present here data from multiple structural and magmatic stages related to temporally varying kinematics and deformation mechanisms during the evolution of the post cratonization of the ncc the timing of each stage is constrained using isotopic dating along with the timing of magmatic intrusion dyke intrusion and basin formation associated with tectonic transformation of the ncc age data indicate distinct structural and magmatic stages at 1600 1500 1300 1100 445 315 270 200 170 155 130 110 75 65 and 25 20 ma relating to basin formation and continental uplift or collapse these stages define the tectonic evolution and structural transformation of the ncc and associated de rooting reactivation reworking and orogenic growth the tectonic setting and movement direction of adjacent plates changed multiple times during the various stages the stages are as follows separation of the ncc from the gondwana supercontinent and northwards drift formation of the central asian orogenic belt caob and northwards subduction of the yangtze plate resulting in closure of the qinling ocean the rapid initial formation of the western pacific plate and subsequent subduction and final closure of the mongol okhotsk ocean combined with closure of the bangong nujiang suture zone and southwards movement of the siberian continent as well as thinning of the north china cratonic lithosphere moreover temporal and spatial changes in deformation magmatism and basin formation as well as kinematic and mechanical transformations occurred in response to the changing tectonic setting and movement direction of surrounding plates these various factors resulted in a complex structural evolution of the ncc since 1 8 ga

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