uncertainties in the relative positions of the australia antarctica lord howe and pacific plates since the late cretaceous we determined parameters that describe finite rotations and their uncertainty regions for relative plate motion at the spreading centers between the pacific and antarctica plates between australia and antarctica and between the lord howe rise and australia we combined these to yield a range of possible finite rotations describing the relative positions of the pacific australia antarctica and lord howe plates since the late cretaceous if the pacific australia plate boundary has had its present trend since anomaly 18 time reconstructions show 330 110 km of motion of the pacific plate relative to the lord howe rise since anomaly 5 time 9 8 m y 420 110 km since anomaly 6 time 19 5 m y 770 330 km since anomaly 13 time 35 6 m y and 820 260 km since anomaly 18 time 43 0 m y we examined two cases for times prior to anomaly 18 assuming a late cretaceous age of australia antarctica separation if a plate boundary existed between the lord howe rise and pacific plates since the late cretaceous with no plate boundary in antarctica reconstructions with the lord howe rise fixed predict 610 200 km of westward motion of the pacific plate between the times of anomalies 31 and 22 followed by 260 100 km of northward motion between the times of anomalies 22 and 18 if the lord howe rise was fixed to the pacific plate until the eocene but a plate boundary existed between east and west antarctica reconstructions show very little motion across this boundary between the times of anomalies 31 and 22 followed by convergence between the times of anomalies 22 and 18 this second case also brings 70 80 m y paleomagnetic poles from the pacific and east antarctica plates into better agreement than the first case but large uncertainties in the reconstructions do not allow the first case to be conclusively eliminated

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

mesozoic aseismic ridges on the farallon plate and southward migration of shallow subduction during the laramide orogeny we propose that an aseismic ridge of late jurassic and early cretaceous age on the farallon plate was subducted beneath the unites states cordillera during the laramide orogeny the relative buoyancy of the aseismic ridge caused shallow subduction which resulted in a magmatic lull 70 40 ma in the existing near trench magmatic arc and laramide uplift and faulting of crystalline basement 1000 1500 km inland from the trench the timing and latitudinal limits of the subduction of the aseismic ridge are similar to the timing and latitudinal limits of both the laramide orogeny and the magmatic lull in the western cordillera because the north northeast trending aseismic ridge was moving northeast relative to the north american plate the point of collision of the aseismic ridge with north america and therefore the locus of shallow subduction migrated southward with time causing the well documented southward migration of the magmatic lull

paleomagnetic test of the emperor fracture zone hypothesis late cretaceous paleomagnetic data from the pacific plate were used to test farrar and dixon s 1981 hypothesis that 1700 km of strike slip accumulated during the early tertiary along the emperor fracture zone system an 8000 km long nw se trending feature which consists mainly of the emperor trough the gardner seamounts and the line islands the data strongly disagree with this hypothesis instead the data are consistent with no motion having occurred between seafloor east of the emperor fracture zone and seafloor west of the emperor fracture zone

upper cretaceous paleomagnetic data from shikotan island kuril arc implications for plate kinematics abstract maastrichtian tuffaceous sandstones and siltstones were sampled from four sites in the northern part of shikotan island in the lesser kuril islands a characteristic component chrm isolated from most samples passes the reversal and fold tests and is most probably a primary remanence the chrm mean direction d 334 i 56 k 36 95 3 2 is 14 3 5 steeper than the pacific reference direction and 13 4 more gentle than the eurasian reference direction we assume that the lesser kuril islands together with their southwestern extension into the nemuro peninsula hokkaido were originally an island arc situated at about 36 n in the central west pacific soon afterwards probably by the end of the late cretaceous the nemuro shikotan island arc became inactive and started moving with the pacific plate in the miocene about 15 m y ago the nemuro shikotan island arc collided with the eurasian plate overrode the subduction zone and occupied its present day position on the pacific side of the kuril island arc although this scenario fits the available paleomagnetic data and kinematics of the pacific plate itself it disagrees somewhat with the kinematic models for the north pacific as a whole in particular by the end of the late cretaceous the northern margin of the pacific plate together with the extinct nemuro shikotan island arc should have been in an area close to the transform or ridge type kula pacific boundary the available paleomagnetic data from northeast asia however indicate that active islands arcs existed in mid northern latitudes of the modern pacific ocean in the cretaceous

sediment delivery and depositional processes along the eastern aleutian trench the aleutian trench contains vast volumes of clastic sediment sourced mainly from the glacial erosion of alaska and controlled by allocyclic influences gloria geologic long range inclined asdic and multi channel seismic surveys conducted by the us geological survey revealed details of both the geometry and distribution of the trench sediments and their associated supply systems milankovitch induced climatic cycles cause the episodic release of sediment transported as both unconfined and confined turbid flows the latter routed along submarine channels that cross the abyssal plain of the gulf of alaska and discharge into the trench small volumes of sediment enter the trench directly from glacier snouts at the transform trough end of the trench and from the chugach mountains in the north a major channel system extends along the trench axis which acts as a conduit for turbid sediment flows the channel also reworks and redistributes material supplied from the accretionary prism and the oceanic plate subduction of seamount chains coupled with the westward increasing obliquity of plate collision and accretionary prism sourced slump which may be seismically triggered segment the trench environment dividing the axial channel system into discrete flow patterns this effect of compartmentalization creates petrographically distinct segments of fill partly owing to the primary source landscape of accreted terranes formed of ancient island arcs this serves to emphasize the cyclic nature of earth processes

mechanisms for ballistic block ejection during the 2016 2017 shallow submarine eruption of bogoslof volcano alaska ejection of ballistic blocks was a characteristic feature of the 2016 2017 bogoslof eruption high resolution satellite images acquired throughout the duration of the 9 month long eruptive period permitted the recognition and mapping of ballistic blocks on the surface of bogoslof island many of the satellite images recorded the accumulation of ballistic material over several individual eruptive events but a few images recorded the effects of a single event the nonuniform spatial distribution of blocks suggests that some of the eruption columns were inclined ballistic trajectories were estimated using the eject model and indicate that accumulation of blocks on bogoslof island required launch angles of 45 80 and initial velocities of 50 100 ms 1 to reproduce observed travel distances the amount of ballistic fallout observed in satellite data indicates that there must have been a shallow submarine source of rock within the conduit upper edifice system dense accidental cryptodome trachyandesite and juvenile basalt to trachybasalt scoria make up the bulk of the surface ejecta abundant accidental fragments and inclined eruption columns point to periodic vent wall collapse and jetting around edges of temporarily blocked vents as the likely cause of ballistic ejection

tectonic and geologic implications of the zodiac fan aleutian abyssal plain northeast pacific the zodiac fan a large body of upper eocene through at least lower oligocene turbidities underlies the aleutian abyssal plain and lies just south of the aleutian trench and the alaskan peninsula in the northeastern north pacific the fan deposits cover an area of more than 1 10 6 km 2 and contain at least 280 000 km 3 of terrigenous sediment the most striking feature of the fan is its well developed channel distributary system which persists nearly to the plain limits levee overbank deposits associated with the channels are the dominant sedimentary style a process believed to predominate on fans receiving primarily fine grained detritus four major channels have been identified on the fan with the following relative age relation older than sagittarius aquarius taurus and sagittarius seamap recently obtained seismic information implies that seamap channel may be older than aquarius channel if seamap is the youngest channel a depositional interval of 8 m y 40 32 m y b p is indicated for the fan as a whole if taurus is the youngest as we believe the interval of deposition is greater than 8 m y but is probably less than 16 m y 40 24 m y b p nanno flora pollen and spores obtained from deep sea drilling site 183 near the northern margin of the fan indicate that the fan formed in a nontropical northern environment and that the source terrane had a climate similar to or slightly cooler than that recorded from coeval onshore deposits in the eastern gulf of alaska on the basis of the interval of deposition the volume of the fan and sediment yield from climatically similar modern drainages a minimum drainage of 500 000 km 2 is believed necessary to have supplied the sediment to form the fan assuming no sediment was deposited before it reached the fan tertiary plate motion models requiring large amounts of relative convergence along the aleutian trench are judged unworkable as such reconstructions require the zodiac fan to have formed 1 500 to 3 000 km from the nearest continental landmass and separated from it by topographic barriers requiring that the drainage be inflated in size manyfold to overcome anticipated sediment losses during such a lengthy transport as the minimum drainage 500 000 km 2 is already equal to one half of the state of alaska any major expansion is judged unreasonable this requires that relative convergence at the aleutian trench be limited to less than 500 km from 40 m y b p to present a possible method by which this limitation can be satisfied is to allow a significant portion of southern alaska to move in concert with the pacific plate since the upper eocene

geology and eruptive history of bogoslof volcano bogoslof volcano is a shallow submarine subaerial volcano in the southern bering sea about 100 km west of the community of dutch harbor alaska the subaerial parts of the volcano consist of two small islands bogoslof island and fire island that together have a total area of about 1 6 km2 bogoslof was first depicted on a russian map in 1772 and since then has been observed and visited occasionally the volcano has had at least nine periods of eruptive activity since 1796 and all of its historical eruptions have been similar in style historical bogoslof eruptions involved the effusion of basalt trachybasalt basaltic trachyandesite and trachyandesite lava domes with above sea level relief of 100 200 m many of the eruptions are accompanied by the formation of tuff rings and ejection of ballistic particles historical observations suggest that eruption clouds are relatively ash poor minor ash fallout has typically occurred within about 100 km of the volcano many of the historical eruptions began at vents that were below sea level and thus seawater has played an important role in the style of eruptive activity exhibited by the volcano at times eruptive activity has been characterized by surtseyan style eruptions and magma interaction with wet vent fill deposits at other times the eruptive style has been more magmatically driven and has resulted in the formation of pyroclastic flows and small ash clouds preliminary studies of the deposits produced during the 2016 2017 eruption indicate vertical sequences of coarse grained horizontally bedded pyroclastic flow and fall deposits with numerous blocks bombs and lapilli of dense juvenile and accidental lithic material these deposits were emplaced by near vent pyroclastic flows surges and explosions some of which originated from shallow highly crystalline cryptodomes

stress modeling of tectonic blocks at cape kamchatka russia using principal stress proxies from high resolution sar new evidence for the komandorskiy block abstract a new data set of 851 lineaments mapped from european remote sensing satellites 1 and 2 full resolution synthetic aperture radar sar data are interpreted as geological proxies for stresses resulting from plate and block collision near the cape kamchatka region of the kamchatka peninsula using a geographical information systems based analysis numerical manifold method nmm analysis is used to model the stress field within the cape kamchatka region resulting from the collision of lithospheric plates and blocks results of our nmm model using different plate motion and plate configuration in the region are compared with orientation data for the mapped set of lineaments these data suggest that the lineaments observed in sar cannot be fully explained by a simple two plate model in this in the cape kamchatka region as an alternative we propose that the data can be explained by the existence of the previously proposed komandorskiy block recent global positioning satellite measurements in the aleutian islands support our lineament derived model and show that the near islands komandorskiy island block of the extreme western aleutians is moving independently of the north american plate

age dependence of skewness of magnetic anomalies above seafloor formed at the pacific kula spreading center the skewness of magnetic anomalies corresponding to chrons 25 to 32r 56 72 ma was investigated on 22 north pacific profiles recording pacific kula spreading from 172 estimates of skewness we find that it varies with age with irregular scatter superimposed on a steady increase with age of 1 20 0 27 m y skewness differs significantly between anomalies differing in age by as little as a million years an analysis of the errors in the skewness estimates indicates significant errors that are systematic for an anomaly of a given age but varying irregularly with age the significant increase with age of phase shift may reflect an increase with age of anomalous skewness or the northward motion of the pacific plate or both if it is due entirely to pacific plate motion and an age independent value of anomalous skewness of 14 found in prior studies is assumed the rate of northward motion is 0 70 0 26 m y four times faster than estimated from seamount paleomagnetic poles and paleolatitudes determined from sediment cores

seismic interpretation of pelagic sedimentation regimes in the 18 53 ma eastern equatorial pacific basin scale sedimentation and infilling of abyssal valleys understanding how pelagic sediment has been eroded transported and deposited is critical to evaluating pelagic sediment records for paleoceanography we use digital seismic reflection data from an integrated ocean drilling program site survey amat03 to investigate pelagic sedimentation across the eastern central equatorial pacific which represents the first comprehensive record published covering the 18 53 ma eastern equatorial pacific our goals are to quantify 1 basin hill scale primary deposition regimes and 2 the extent to which seafloor topography has been subdued by abyssal valley filling sediments the eastern pacific seafloor consists of a series of abyssal hills and basins with minor late stage faulting in the basement ocean crust rarely outcrops at the seafloor away from the rise crest both hills and basins are sediment covered the carbonate compensation depth is identified at 4440 m by the appearance of acoustically transparent clay intervals in the seismic data overall we recognized three different sedimentation regimes depositional high sedimentation rate transitional and minimal sedimentation low sedimentation rate regimes in all areas the sedimented seafloor mimics the underlying basement topography although the degree to which topography becomes subdued varies depositional regimes result in symmetric sedimentation within basins and subdued topography whereas minimal sedimentation regimes have more asymmetric distribution of sediments within topographic lows and higher seafloor relief regardless of sedimentation regime enhanced sediment deposition occurs within basins however we observe that basin infill is rarely more than twice as thick as sediment cover over abyssal hills if this variation is due to sediment focusing the focusing factor in the basins as measured by 230th is no more than a factor of 1 3 of the total vertical particulate rain

relative motions between oceanic plates of the pacific basin appendix tables are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b84 012 2 50 payment must accompany order relative motion poles describing the displacement histories between the pacific plate and once adjacent oceanic plates farallon kula izanagi i izanagi ii and phoenix were derived for the late mesozoic and cenozoic eras because fracture zone and magnetic anomaly data are generally available from the pacific plate but not from adjacent plates a new method of analysis for onesided data was required this analysis produced stage poles and rates of relative plate motion and estimates of their confidence regions the following are the main conclusions drawn from our analysis 1 for time intervals of the order of 107 years termed stages relative motion poles for plate pairs remained nearly fixed between stages shifts in poles were commonly both large and abrupt within stages rates of plate motion were commonly observed to change markedly indicating that plates changed speed more frequently than they changed direction 2 the relative motions of all of the plates analyzed changed at about chron m11 135 ma chron 34 85 ma and chron 25 56 ma 3 during the early cretaceous there were five oceanic plates in the pacific basin rather than the four recognized by previous workers 4 to determine the number of farallon plates that existed to the east of the pacific plate during the time interval from chron 34 85 ma to chron 25 56 ma fracture zones and magnetic anomalies that record pacific farallon spreading from the northern central and southern pacific plate were analyzed separately and collectively the analysis shows that a single pacific farallon relative motion pole and a single rate are consistent with all of the data 5 spreading rates along the pacific kula ridge decreased markedly between chrons 32b and 25 72 56 ma probably in response to the arrival of buoyant kula lithosphere at a subduction zone northwest of the bering sea 6 soon after chron 25 56 ma a major reorganization is recorded along the pacific farallon boundary the kula farallon boundary is also thought to have changed during this reorganization 7 kula pacific anomalies north of anomaly 25 are modeled by assuming continued kula pacific spreading after chron 25 56 ma 8 probably by chron 18 43 ma pacific kula spreading had ceased and the kula farallon ridge system had evolved into alignment with the pacific farallon spreading direction however the timing of these events is uncertain because pacific kula data younger than chron 25 56 ma are sparse

evolution and strength of pacific fracture zones previous studies have shown that pacific fracture zones are strong in some locations sustaining the stresses associated with differential subsidence across a locked fault while at other locations they show signs of weakness acting as preferential conduits for volcanism or supporting anomalously low shear stresses we find that half of all geosat crossings of central pacific fracture zones are inconsistent with a flexurally maintained scarp we test two hypotheses for the origin of such anomalous crossings 1 anomalous structures formed at the transform fault during times of changes in plate motions and preserved at strong fracture zones and 2 anomalous structures representing a posttransform response of the fracture zone to subsequent tectonic activity we find that three quarters of the anomalous crossings occur over the parts of fracture zones that formed during or immediately subsequent to times of changes in spreading directions with the exception of several locations overprinted by hot spot volcanism these same crossings show no obvious correlation with regional tectonic processes these observations suggest that most anomalous fracture zone topography is inherited from the transform fault and is not a product of subsequent activity consistent with the hypothesis that fracture zones generally remain strong throughout their lifetimes

sedimentological constraints on the evolution of the cordilleran arc new insights from the sonsela member upper triassic chinle formation petrified forest national park arizona usa the sonsela member of the upper triassic chinle formation in petrified forest national park arizona forms a distinctive part of an extensive fluviolacustrine complex that blanketed the colorado plateau region and surrounding areas upper triassic sediments were deposited within a continental backarc setting synchronous with the development of the cordilleran magmatic arc although the chinle formation was derived from a broad variety of source terranes petrographic and geochemical data clearly fingerprint the cordilleran arc as a region important to provenance the sonsela member exhibits a stratigraphically abrupt change in depositional style and sedimentary architecture that likely reflects the dynamics of the evolving backarc fluvial system and syndepositional arc magmatism the stratigraphic boundary between the two disparate depositional styles is marked by a laterally persistent red silcrete horizon that is also associated with a biostratigraphically significant faunal turnover the lower sonsela member a coarse grained assemblage dominated by persistent sheet sandstone and conglomerate bodies reflects a coarse grained progradation of sediment that suggests low rates of dynamic subsidence and high rates of sediment input the upper sonsela member characterized by an overall finer grained assemblage and ribbon sand bodies with an increased proportion of encasing mudstone reflects an increasing rate of basin subsidence that likely persisted throughout the deposition of the overlying petrified forest member these packages reflect a sharp change from lower to higher sinuosity river systems that closely coincides with the increased rate of subsidence across the regional stratigraphic marker previous studies have suggested the importance of the silcrete horizon as a key biostratigraphic marker associated with a significant faunal turnover this study supports the hypothesis that this horizon may represent a regional unconformity as opposed to local channeling perhaps the product of an abrupt change in basin geometry followed by an increase in basin subsidence because the sonsela member is the stratigraphically lowest unit of the chinle formation dominated by volcanic clastic detritus on the colorado plateau architectural analysis of the sonsela member serves as the most reliable indicator of short term fluctuations in arc dynamics of the poorly preserved and disjointed mesozoic portion of the cordilleran magmatic arc

lithospheric structure of the yukon northern canadian cordillera obtained from magnetotelluric data 1 two goals of lithoprobe s geoscientific studies in the phanerozoic accretionary cordillera of western north america were to define the subsurface geometries of the terranes and to infer the physical conditions of the crust these questions were addressed in canada s southern cordillera a decade ago and have more recently been addressed in the northern cordillera of which one component of the new studies is magnetotelluric mt profiling from ancestral north american rocks to the coast we present a resistivity cross section and its interpretation of the northern cordillera derived from modeling data from 42 mt sites along a 470 km long ne sw profile beneath the coast belt southwestern end of the profile a deep crustal low resistivity layer dips inland we interpret the crustal part of this conductor as being due to metasedimentary rocks emplaced and metamorphosed during paleocene kula plate subduction a strong lateral transition in lithospheric mantle resistivity exists below the intermontane belt that is spatially coincident with changes in chemical and isotopic characteristics of tertiary to recent alkaline lavas suggesting that isotopically enriched lithosphere related to the coast belt basalts extends partly beneath the intermontane belt the unusually high lower crustal resistivity in the intermontane and omineca belts similar in value to the resistivity found in the unextended part of central british columbia excludes the presence of fluids or conducting metasediments finally our resistivity model displays strong lateral variation of the middle and lower crust between different terranes within the same belt as a result of the complex structural evolution of the lithosphere index terms 0925 exploration geophysics magnetic and electrical methods 1515 geomagnetism and paleomagnetism geomagnetic induction 8110 tectonophysics continental tectonics general 0905 8120 tectonophysics dynamics of lithosphere and mantle general keywords magnetotelluric accreted terranes northern canadian cordillera

an imbrication model for the rajang accretionary complex in sarawak borneo abstract the structural framework of the rajang accretionary complex is interpreted on the basis of field surveys conducted around the lupar fault zone in southwestern sarawak bedding dips in the rajang accretionary complex are generally southward while the complex as a whole becomes younger northward the complex is interpreted as a series of thrust slices formed by accretion at a subduction trench each slice forms a unit 10 to 15 km in width thrust slices of this order of this order of magnitude are consistent with those formed by accretion processes at modern subduction trenches it is suggested that accretion of late jurassic to cretaceous oceanic crust occurred initially from the pacific eastern side of borneo in the late cretaceous forming part of an arc system extending all along the eastern margin of asia from japan to kalimantan subsequently in the early tertiary due to bending of the southern end of the arc system in borneo the direction of subduction changed so that accretion occurred from the north

eocene underplating along the kodiak shelf alaska implications and regional correlations structural geology and geophysical data from the kodiak shelf suggest that the mesozoic rocks exposed on the shelf are structurally underlain at about 12 km depth by several kms of eocene age strata kinematic data from the late cretaceous to paleocene ghost rocks formation indicate that this formation and probably all of the kodiak islands were uplifted vertically to nearly their present elevations landward tilting and imbrication are not indicated the age of uplift is indicated by a regional angular unconformity of early eocene to early oligocene age that separates deep sea rocks from shallow water to non marine rocks the uplift of the accretionary prism is believed to have been caused by underplating of an eocene sedimentary sequence because 1 a band of seismic reflections that occur 12 to 20 km beneath the shelf is interpreted as the top of the underplated material and 2 an obductively offscraped sequence of eocene deep sea rocks crops out on the seaward side of the kodiak shelf suggesting that a thick trench fill sequence may have been present prior to uplift of the prism the underplated material is interpreted to be part of either a previously unrecognized turbidite fan of early eocene age or a proximal equivalent of the zodiac fan of late eocene to early oligocene age other possible on land remnants of the underplated material may be present in prince william sound the montague belt the gulf of alaska lower sections of the yakutat block and in the coast ranges of oregon and washington the large volume of underplated material beneath the kodiak shelf suggests that underplating may be the dominant process in the growth of convergent margins

a geologic test of the kula pacific ridge capture mechanism for the formation of the west philippine basin in spite of nearly 30 yr of investigation the origin of the west philippine basin remains elusive this basin occupies nearly half of the philippine sea plate which remains one of the most poorly understood of the major lithospheric plates on earth establishing the origin of the west philippine basin is fundamental to deciphering the evolution of the western pacific basin including the india asia collision and the opening of numerous marginal basins in the region in this paper we combine new on land kinematic data from southwest japan with existing marine magnetic data from the ancient kula pacific ridge to test an early tectonic model that suggests that the west philippine basin arose by trapping of a part of the kula pacific ridge during the middle eocene the results suggest that this mechanism is viable but that the ridge capture event may have occurred during the middle eocene 10 m y earlier than originally proposed

plate motions in the north pacific the 43 ma nonevent the hawaiian emperor seamount chain in the north pacific ocean is commonly considered to have been produced by motion of the pacific plate over a hotspot if the hotspot is assumed to have remained fixed in a mantle frame of reference the 60 change in trend between the hawaiian and emperor portions of the chain results from a change in motion direction of the pacific plate relative to the mantle at 43 ma during middle eocene time such a large and abrupt direction change should result in significant plate reorganizations and tectonic events in the pacific and surrounding plates but no evidence for these events can be found in addition in this paper it is shown that there were no changes in relative plate motions between pacific area plates that can be associated with the hawaiian emperor bend this is done by calculating motion of the pacific farallon and kula plates relative to asia and north america using the plate circuit farallon pacific antarctica africa north america asia with no major regional tectonic event at the time of the bend it is suggested that there was no plate reorganization at 43 ma and that the emperor portion of the seamount chain was formed by a nonstationary hotspot

temperature control of continental lithosphere elastic thickness te vs vs article i nfo the elastic thickness of the continental lithosphere is closely related to its total strength and therefore to its susceptibility to tectonic deformation and earthquakes recently it has been questioned whether the lithosphere thickness and strength are dependent on crustal and upper mantle temperatures and compositions as predicted by laboratory data we test this dependence regionally by comparison in northwestern north america of the effective elastic thickness te from topography gravity coherence with upper mantle temperatures mapped by shear wave tomography velocities vs and other temperature indicators the te values are strongly bimodal as found globally less than 20 km for the hot cordillera backarc and over 60 km for the cold stable craton these te correspond to low vs beneath the cordillera and high vs beneath the craton model temperature depth profiles are used to estimate model te for comparison with those observed only limited areas of intermediate thermal regimes i e thermotectonic ages of 300 ma have intermediate te that suggest a weak lower crust over a stronger upper mantle there are large uncertainties in model te associated with composition water content strain rate and decoupling stress threshold however with reasonable parameters model yield stress envelopes correspond to observed te for thermal regimes with 800 900 c at the cordillera moho and 400 500 c for the shield in agreement with temperatures from vs and other estimators our study supports the conclusion that lithosphere elastic thickness and strength are controlled primarily by temperature and that laboratory based rheology generally provides a good estimate of the deformation behaviour of the crust and upper mantle on geological time scales

fast instantaneous oceanic plate velocities recorded by the cretaceous laytonville limestone paleomagnetic analysis and kinematic implications new paleomagnetic results from 101 89 ma laytonville limestone of the franciscan central belt melange of northern california confirm a southern hemisphere origin as first proposed by alvarez et al 1980 discovery and study of new laytonville limestone outcrops extend the paleomagnetic record and provide a definitive determination of age stratigraphic polarity and hemisphere of deposition paleontological determinations show that five sections are presently overturned and four are presently right side up as expected from a random distribution of nine blocks in a melange the exact accretion time is unknown but geologically reasonable estimates range between 70 and 50 ma leading to averaged estimates of poleward velocity between 30 and 14 cm yr the averaged values are obtained by dividing the total paleolatitude shift by the difference between magnetization and estimated accretion ages they reflect net poleward transport over a long time interval and are minimum estimates because they record only the poleward component of motion an instantaneous poleward velocity is recorded by the systematic inclination decrease of 160 paleomagnetic samples with respect to decreasing age over the 12 m y section conservative estimates place this minimum velocity at 15 cm yr the consistency between the values predicted by the average velocity calculations and those measured within the outcrops serves as an important check on the primary nature of the magnetization this minimum velocity is much higher than any present day plate velocity the subduction of old oceanic crust at the boundary of a relatively small plate is hypothesised as a driving mechanism to explain the measured velocities coupled with the minimum instantaneous poleward velocities this hypothesis suggests that the velocities of subducting plates may vary up to a factor of 5 similar plate geometries and resultant velocities have probably existed at other times in the past but their discovery may have been hindered by low preservation potential

opening of the sea of japan back arc basin by asthenospheric injection abstract the position of a volcanic front on an arc is governed by dehydration reactions of amphibole chlorite in down dragged hydrated peridotite at the base of the mantle wedge as these reactions are essentially pressure dependent ca 3 5 gpa the paleo position of a volcanic front offers an estimate for the depth of the subducted oceanic lithosphere beneath it this paper evaluates volcanism in the northeast japan arc since the oligocene and discusses the mass transportation in the mantle wedge during the episode of back arc extension in the sea of japan with reference to the migration of the volcanic front the volcanic front during the oligocene ca 30 ma was located along the western coast of the present northeast japan are and migrated about 200 km trenchwards at 23 ma before the opening of the sea of japan back arc basin it follows that the angle of subduction became steeper during the period of 30 23 ma triggered by the injection of the asthenosphere into the mantle wedge the back arc extension of the sea of japan between 21 and 14 ma is a manifestation of the dynamic process of the lithosphere due to the asthenospheric injection the temperature of the injecting asthenospheric mantle was higher than that of the surrounding upper mantle or lithosphere this caused the characteristic volcanism at 23 22 ma 1 high magnesian andesite volcanism took place in the near trench region 2 across arc compositional variation was not observed 3 the volume of volcanic materials increases towards the back arc side of volcanic arc the volcanic front has migrated over a short distance inwards less than 100 km since 15 ma possible as a result of cooling in the mantle wedge because both amphibole and chlorite break down at slightly higher pressures at lower temperatures the upper mantle materials which injected into the mantle wedge and caused back arc spreading are identical to those of the mid oceanic ridge basalt source in sr nd isotopic compositions because volcanic rocks in the back arc region of the northeast japan arc show secular variation in isotopic compositions from enriched to depleted isotopic characteristics during the last 30 ma

the late cretaceous paleomagnetic field in north america a south american perspective determining the late cretaceous paleomagnetic pole for north america has been difficult because of the lack of suitable rocks of that age in cratonic areas to provide the necessary data as an alternative different studies have appealed to paleomagnetic data from rocks in western north america using paleopoles from stable areas in neighboring continents it is suggested that the available late cretaceous paleomagnetic record in western north america should be analyzed in terms of rigid body deformations rather than be used to represent the cratonic reference field

dynamic topography and vertical motion of the u s rocky mountain region prior to and during the laramide orogeny dynamic topography of earth s surface occurs in response to hydrodynamic stresses due to mantle flow beneath a flexible lithosphere here we compare the predicted dynamic topography from an inverse convection model that includes flat slab subduction with the known geologic history of the western united states from late cretaceous through paleocene time to evaluate the validity of the model results downwarping behind the evolving cordilleran volcanic arc took place after its inception in late triassic time culminating in the formation of the western interior seaway in late cretaceous time subsidence behind arcs is a consistent prediction of most models of dynamic topography a more detailed convection model is evaluated here by comparing it to the geologic history of the central rocky mountain region from 100 to 50 ma the match of predicted and observed tectonic subsidence is good up until the time that local deformation by the laramide orogeny begins this is most likely due to local flexural effects of mountain building overwhelming regional dynamic effects maps for successive time intervals show that the model matches the geologic history quite well through time and space in particular the subduction of the previously postulated oceanic plateau the conjugate shatsky rise has a significant impact on surface movements 1 from 95 to 88 ma the locations of most significant regional unconformities match the eastward migration of a zone of high topography 2 from 90 to 85 ma the zone of maximum subsidence coincides with the motion of the leading edge of the conjugate shatsky rise 3 from 78 to 60 ma the site of initiation of laramide deformation migrates coincidently with the position of the center of the conjugate shatsky rise 4 from 70 to 60 ma the time transgressive deposition of thin fluvial conglomerate units in the southern part of the study area is generally coincident with surface uplift caused by the trailing part of the conjugate shatsky rise these results suggest that the inverse model approximates the vertical motion history quite well synchroneity of the position of the center of the passing conjugate shatsky rise the landward limit of the evolving cordilleran volcanic arc and the initiation of laramide deformation suggests that the farallon plate became coupled with the overlying north american plate as the subducted oceanic plateau passed beneath progressively enhanced mechanical coupling between the plates was likely the impetus for laramide shortening this comparison of model results with surface geologic history provides a means to validate but not verify predictions of dynamic mantle flow models

late cenozoic tectonics of the southern chile trench marine geophysical data from off shore of southern chile are used to define the interaction between the chile ridge and chile trench during late cenozoic time we identify three distinct ridge trench collision events between 14 and 10 ma a 700 km long nearly continuous section of the chile ridge was sub ducted between 55 s and 48 s shorter sections of the ridge offset by large transform faults were subducted at 6 and 3 ma between 48 s and 47 s at the present day triple junction the subduction of the ridge has a strong influence on the chile trench in this region the landward trench slope has undergone a recent episode of subduction driven tectonic erosion the trench slope is narrower and steeper than along other sectors of the margin and the trench axis has migrated shoreward evidence exists for late neogene and quaternary uplift and plutonism on the adjacent continental margin south of 48 s where collision took place between 10 and 14 ma the effects of collision are much less pronounced in particular the landward trench slope does not appear to have been subjected to extensive tectonic erosion we conclude that the configuration of the spreading centers and transform faults on the chile rise is the chief factor controlling ridge trench tectonic interaction tectonic erosion of the landward trench slope and tectonic activity in the adjacent continental margin are much greater when short sections of ridge offset by large transform faults are subducted a major sub marine channel leads along the trench axis south ward from the triple junction this channel cuts across the outer trench rise and carries sediment westward to the mornington abyssal plain the paleogene tectonic history of the southern chile trench includes a southward migrating ridge trench collision involving the farallon aluk spreading center in eocene time

global plate motions relative to the hot spots 64 to 56 ma we examined the early tertiary global plate velocities with respect to the hot spots and compared these velocities both to those at present and to early tertiary velocities inferred by assuming that no net torque is exerted on the lithosphere as a whole in our reconstruction the velocities of the pacific and african plates were inferred directly from their hot spot tracks and the velocities of other plates with respect to the hot spots were calculated from the motion of the pacific or african plate and by relative motions estimated from seafloor spreading data the relative motion between assumed separate north pacific and south pacific chatham rise plates was estimated by assuming the hot spots in the atlantic ocean basin have been fixed with respect to those in the pacific ocean basin thereby avoiding the use of a poorly defined relative motion circuit we found that the characteristics of early tertiary plate motions with respect to the hot spots resemble those at present the root mean square velocity of every major continental plate with respect to the hot spots during the early tertiary exceeds its present velocity but with the exception of the indian plate is less than the root mean square velocity of every early tertiary oceanic plate equatorial lithosphere moved faster than polar lithosphere during the early tertiary but the difference is less than at present over the interval 64 to 56 ma the no net torque absolute velocities differ from the assumed fixed hot spot velocities by only 0 04 ma which is insignificant thus two dissimilar approaches fixed hot spots and no net torque of the lithosphere when applied to identical early tertiary relative plate velocities yield similar absolute plate velocities we attribute the differences between these two reference frames found in prior investigations to relative plate velocities different than we assume here

cenozoic convergence along the california coast a qualitative test of the hot spot approximation the displacement history of the san andreas fault is characterized by three phases a post mid cretaceous to early eocene period of active right slip an eocene to miocene period of inactivity and a post miocene period of accelerating right slip it has been suggested that the proto san andreas offsets resulted from oblique convergence in the early tertiary this concept requires a mid tertiary period of normal convergence correlative with the period of inactivity and thus provides a basis for testing reconstructed plate motions assuming that hot spots constitute a fixed frame of reference and that the motion of north america has been constant farallon north america convergence vectors have been computed for three cenozoic phases of plate motion ending at 55 42 and 30 m y b p although there is some overlap between the early tertiary period of oblique convergence and the mid tertiary phase of inactivity on the san andreas the calculated convergence vectors are generally consistent with the displacement history of the fault moderate oblique convergence is indicated for the early cenozoic with some reduction in the oblique component between 55 and 42 m y b p and a period of nearly normal convergence is indicated for the period between 42 and 30 m y b p thus the two hypotheses that the proto san andreas was driven by oblique convergence and that hot spots approximate a fixed frame of reference are not mutually exclusive although this type of study cannot prove either hypothesis the results lend some support to each

relating rapid plate motion variations to plate boundary forces in global coupled models of the mantle lithosphere system effects of topography and friction abstract recent high resolution models of past plate motions and their comparison with plate motion models inferred from space geodetic techniques reveal a number of short term variations in global plate velocities over the past 10 myrs such variations serve as powerful probe into the nature and magnitude of plate boundary forces because they are unlikely to originate from changes in mantle buoyancy forces which evolve on longer time scales here we explore the constraints of the velocity record using a novel coupled modeling approach of global neo tectonic simulations combined with realistic plate driving forces obtained from mantle circulation models mcms to arrive at simple global budgets of mantle lithosphere and plate boundary forces we focus on three plate boundary systems along the nazca south america plate margin the aleutian trench and the india australia plate boundary to show that gravitational spreading from high topography in the andes and tibet contributes substantially to the global plate tectonic force balance and that this contribution is sufficient to explain some 35 of recent velocity changes over the earth s surface including among others the observed 30 convergence reduction between the nazca south america plates our models make a number of specific predictions such as significant lateral variations in plate coupling forces along a given margin revealed by trench parallel gravity and bathymetry anomalies and the occurrence of large earthquakes as well as differences by as much as a factor of five from margin to margin they also support the notion of a relatively young plate boundary separating the india and australia plates which has been previously suggested based on independent observations importantly we find that the modeled nazca south america convergence reduction explains recent spreading rate variations in the south atlantic and south pacific which points to the importance of far field effects on the adjacent continents in explaining the spreading record of oceanic basins our numerical results demonstrate a that detailed budgets of forces acting upon plates can be obtained and b support the notion of strong forcing along weak plate boundaries

comparison of seafloor tectonic fabric at intermediate fast and super fast spreading ridges influence of spreading rate plate motions and ridge segmentation on fault patterns we have conducted a comparative study of the tectonic morphology of young seafloor using seamarc ii side scan sonar surveys of the intermediate spreading ecuador rift the fast spreading east pacific rise epr 8 30 10 n and the super fast spreading epr 18 19 s we find that characteristics of fault populations are not only a function of spreading rate but also vary along axis within individual ridge segments i e with proximity to large and short offset discontinuities we also find that fault azimuths can be used to examine plate kinematics on a finer scale than can be obtained using magnetic data alone most of the variation in fault populations with spreading rate can be explained by an inverse relationship between spreading rate and thickness of the brittle layer for example regions of super fast spreading are characterized by the largest numbers of short faults the smallest average fault spacing and throw and the highest fault density in addition clusters of short closely spaced antithetic faults subsidiary to long master inward dipping faults are common within the super fast spreading area presumably the result of a thinner weaker brittle layer faults facing away from the ridge axis occur in increasing numbers with increasing spreading rate such that few outward facing faults are found at slow to intermediate rates and approximately equal numbers of inward and outward facing faults are observed at the fastest rates rapid thickening of the brittle layer with distance from the ridge may account for the predominance of inward facing faults at slower spreading rates outward facing faults at all spreading rates have shorter mean lengths and lower vertical offsets these differences may reflect the shorter time outward facing faults are active owing to increasing strength of the lithosphere with distance from the ridge fault lengths and spacings in all areas approximate exponential distributions the extensional strain represented by fault populations is calculated from the displacement and length distributions of faults and strain estimates of 4 are obtained for each area assuming that fault spacing reflects fracture depth extent where faults initiate we infer a brittle layer thickness of 1 km when faulting begins fault populations are examined for ridge segment scale variations in amagmatic extension we see evidence for greater amagmatic extension associated with long term reduced magma supply along the eastern third of the ecuador rift evidence for local increased brittle extension is also found within 15 km of transform faults discordant zones left by overlapping spreading centers oscs are characterized by low fault abundances at oscs discrete events of ridge tip propagation may accommodate extension taken up elsewhere along the ridge by normal faulting fault azimuths do appear to be useful indicators of plate motion within the epr 8 30 10 n area fault trends record a recent change in pacific cocos plate motion 3 6 at 1 m y consistent with magnetic anomaly and fault lineation data from elsewhere along the northern epr within the ecuador rift fault azimuths scatter within 3 of predicted trends and are consistent with constant spreading about one pole for the past 1 5 m y

palaeomagnetism of palaeogene volcanic series of the kamchatskymys peninsula east kamchatka the motion of an active island arc abstract the paleocene tarkhov and eocene baklan formation are the lower and upper members respectively of a continuousisland arc sequence at the kamchatsky mys peninsula on the pacific side of the kamchatka peninsula we studied paleocene tuffs at five localities and eocene tuffs and basalts at nine localities from beds of various attitude thermal demagnetization revealed consistent directions of the characteristic component chrm of natural remanent magnetization in both formations normal and reversed chrm directions in the paleocene rocks are roughly antiparallel thus constituting a positive reversal test at the same time the fold test mcfadden and jones 1981 is not straightforward as locality means display a banana shaped distribution most probably because of local rotations inclination only data however show considerable decrease in dispersion after tilt correction thus constituting a positive fold test and implying a prefolding and most prprobably primary age of the chrm in these rocks the formation mean inclination of 57 5 3 8 calculated with the aid of inclination only statistics corresponds to a palaeolatitude of 38 1 4 1 n chrm directions isolated from the eocene baklan formation are well grouped and pass both the fold and reversal tests the formation mean inclination of 65 0 4 9 corresponds to a palaeolatitude of 47 0 6 4 n geological data point to uninterrupted accumulation of the studied island arc complex and hence to continuous activity of the subduction zone and related island arc the difference between the paleocene and eocene palaeolatitudes of 8 9 6 3 is statistically significant thus implying an absolute motion of the active subduction zone and related island arc during the paleocene to eocene no other active subduction zone existed between the studied area and eurasia we think that the studied island arc belonged to the pacific plate and was moving northwestward while the oceanic part of the eurasian or north american plate was subducted southward underneath this arc this island arc became inactive in the late eocene and was transported with the pacific plate which started subducting under the eurasia margin since that time

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

high resolution inversion for south atlantic plate kinematics using joint altimeter and magnetic anomaly data we present an inversion for plate kinematics that solves for finite rotation parameters using fracture zone fz and magnetic anomaly location data jointly we define misfit functions that incorporate properties unique to each data type in particular the fz misfit function does not depend upon alignment of conjugate fz traces in the same way as magnetic lineations under the finite rotations this property is useful for fz locations in which the signals on conjugate sides of the ridge may include systematic differences or where data from one side of the ridge are sparse or missing formal error bounds estimated for the pole parameters show that the magnetic and fz data are complementary in their information content error bounds computed for the joint inversion are substantially smaller than for either the fz or magnetics data used separately indicating that simultaneous use of the data in an inversion is crucial we apply this method to seasat altimeter data and magnetic anomaly picks in the south atlantic we solve for finite poles corresponding to magnetic anomalies 5 6 8 13 21 22 25 30 33 33r and 34 these define a smooth path over the past 84 m y indicating that the plate motions have not been as erratic as found previously

rayleigh wave phase velocities in the pacific with implications for azimuthal anisotropy and lateral heterogeneities summary the lateral distribution of fundamental mode rayleigh wave phase velocities in the pacific has been calculated in order to determine the variation in velocity as a function of the age of the oceanic plate the importance of azimuthal anisotropy and the presence of secondary lateral heterogeneities the data set used in this study consists of phase velocity measurements in the period range 20 125 s for 178 paths traversing the pacific both the pure path and spherical harmonic inversion techniques are used in this investigation with an emphasis placed on determining the effectiveness and resolving power of these methods in calculating the velocity distribution in an oceanic regime the first order velocity age relation which dominantly characterizes surface wave dispersion in an oceanic environment can be adequately modelled by either the pure path or spherical harmonic techniques azimuthal anisotropy is the dominant second order effect and is best developed in regions less than 80 myr in age in this age zone the faster direction of wave propagation is either in the direction of fossil seafloor spreading or in the direction of present day absolute plate motion in the western pacific there is a correlation with paleo relative motion fossil seafloor spreading for periods less than 50 s lateral velocity heterogeneities which are not related to the age of the oceanic plate are found by pure path inversions and are correlated with regions associated with shallow residual depth anomalies this effect slow velocities with respect to the velocity age model monotonically decreases as a function of period and is indicative of a shallow origin age independent anomalies were also modelled by the application of the pure path and spherical harmonic methods in a sequential inversion

large scale fractures related to inception of the yellowstone hotspot during middle miocene time western north america was subject to flood basalt volcanism dike swarm injection and broad scale fracturing and folding of the crust we propose a simple model to account for these events and for a regional pattern of geologic and geophysical features aeromagnetic maps reveal some of the most important elements of this pattern which are several narrow arcuate anomalies here referred to as the northern nevada rifts these rifts extend hundreds of kilometers across nevada and are likely caused by highly magnetic middle miocene mafic dikes with the aid of filtering techniques the anomalies can be traced into oregon together with other geologic features such as fold axes dike swarms and faults they produce a spoke like pattern fanning over 2208 of arc that converges toward a point near the oregon idaho border lat 448n a possible cause for this pattern is a point source of stress at the base of the crust related to the formation of the yellowstone hotspot the spoke like pattern however does not persist at large distances from the emerging hotspot several hundred kilometers to the south the northern nevada rifts deviate significantly 308 from a radial trend we show that a simple model imposing a point source of stress at the base of the crust and a regional stress field aligned with the presumed middle miocene stress direction fits the observed fracture pattern it thus accounts for both the radial pattern present near the nascent hotspot and the far field pattern due to regional stresses

the regional tectonic fabric of the east pacific rise from 12 50 n to 15 10 n seamarc ii backscatter data for the east pacific rise are used to create structural maps of ridge parallel fabric on the crest and flanks of the rise from 12 50 n to 15 10 n the structural data are statistically analyzed to describe the spacing density azimuth facing direction and length of faults results of the statistical studies are compared with predictions for the width of the zone of active fault formation models for the generation of abyssal hills plate kinematic predictions models of along strike segmentation and observations of the asymmetric subsidence of oceanic crust on the cocos and pacific plates comparisons of the number of faults with distance from the rise crest and examination of the stratigraphic relationship between seamounts and the ridge parallel tectonic fabric illustrate that the zone where new faults are created is located within a few kilometers of the ridge crest fault density data reveal that fault distributions do not resemble periodic processes examination of fault density and spacing data along strike indicates that there is a high probability that the mean number of faults per crustal block differs significantly from north to south changes in along strike statistics correlate well with the occurrence of three overlapping spreading centers located within the survey area additionally fault spacing s determined from seamarc i and seamarc ii backscatter imagery demonstrate that quantitative analyses of seafloor fabric are dependent upon the instrument used to collect the data analysis of inward and outward facing faults indicates that especially in the northern portion of the survey area half graben models are better predictors of abyssal hill morphology than full graben models within the survey area inward facing faults are more abundant and affect more of the oceanic crust than outward facing faults although the cocos and pacific plates subside at different rates this asymmetry is not reflected in the tectonic component of morphology fault azimuth is observed to vary as a function of crustal age although the overall trend of the change in azimuthal values agrees with the trend predicted by relative poles of opening for the pacific and cocos plates the variability in azimuthal data suggests that other processes contribute to the orientation of lineations formed near the axis of the east pacific rise finally the statistical analyses demonstrate that all fault parameters are better correlated about the ridge axis than along strike of the axis the differences between the northern and southern portions of the survey area are reflected in the seamarc ii bathymetric data which depict a continuous narrow ridge crest in the southern region and an irregularly shaped ridge crest in the northern region that shoals and deepens every 30 to 40 km this along strike variability observed over distances of less than 100 km suggests that large scale plate stresses are not the only processes responsible for generating the tectonic fabric observed on the flanks of the east pacific rise

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

new cambrian and ordovician paleomagnetic poles for the north china block and their paleogeographic implications multicomponent magnetization has been revealed in several cambrian and ordovician sedimentary units from localities in hebei and shandong provinces of the north china block ncb a well defined component with lower unblocking temperature is characterized by northerly declination and moderate to steep inclination with both polarities and postdates the late cretaceous folding the pole position derived from the overprinted directions is close to both the cretaceous pole for the ncb as well as the present north pole the overprint is not a simple thermal remanent magnetization but is probably a chemical remanent magnetization imposed at moderately elevated temperature during the major episode of regional deformation in the late mesozoic the higher unblocking temperature characteristic components pass reversal baked contact and fold tests and are internally consistent with rocks of different lithologies from widely distributed localities moreover the data exhibit an overall similarity in the magnetic polarity pattern in the stratigraphic sequence at sampled localities hence the high unblocking temperature characteristic component was probably acquired at or near the time of deposition the cambrian and ordovician paleopoles are somewhat different from the other coeval poles for the ncb which are based on much less data and do not pass as many reliability tests but all imply that the ncb was in the equatorial zone in the early paleozoic

guadalupe pluton mariposa formation age relationships in the southern sierran foothills onset of mesozoic subduction in northern california we report a new 153 2 ma sims u pb date for zircons from the hypabyssal guadalupe pluton which crosscuts and contact metamorphoses upper crustal mariposa slates in the southern sierra a 950 m thick section of dark metashales lies below sandstones from which clastic zircons were analyzed at 152 2 ma assuming a compacted depositional rate of 120 m myr accumulation of mariposa volcanogenic sediments which overlie previously stranded middle jurassic and older ophiolite chert argillite belts in the sierran foothills began no later than 160 ma correlative oxfordian kimmeridgian strata of the galice formation occupy a similar position in the klamath mountains we speculate that the late jurassic was a time of transition from 1 a mid paleozoic middle jurassic interval of mainly but not exclusively strike slip and episodic docking of oceanic terranes 2 to transpressive plate underflow producing calcalkaline igneous arc rocks outboard blueschists at 170 150 ma whose erosion promoted accumulation of the mariposa galice overlap strata 3 continued transpressive underflow attending 200 km left lateral displacement of the klamath salient relative to the sierran arc at 150 140 ma and development of the apparent polar wander path cusps for north and south america and 4 then nearly orthogonal mid and late cretaceous convergence commencing at 125 120 ma during reversal in tangential motion of the pacific plate after 120 ma nearly head on subduction involving minor dextral transpression gave rise to voluminous continent building juvenile and recycled magmas of the sierran arc providing the erosional debris to the great valley fore arc and franciscan trench

plate interior polyphase fault systems and sedimentary basin evolution a case study of the east gobi basin and east gobi fault zone southeastern mongolia abstract structural interpretation of 2 d seismic reflection data and subsurface outcrop correlations reveal six distinct phases of deformation recorded in the paleozoic basement rocks and mesozoic cenozoic basin fill of the east gobi basin egb southeastern mongolia these phases include arc accretion and arc continent collision in late paleozoic time late triassic sinistral shear zone development early jurassic fold and thrust belt style shortening middle jurassic lower cretaceous extension and rift basin development middle cretaceous shortening with basin inversion and regional unconformity development and late cretaceous oligocene thermal subsidence with renewed paleogene left lateral strike slip faulting across the fault zone the five post amalgamation deformation phases are localized along the east gobi fault zone suggesting that preexisting structures and boundary conditions exert fundamental controls on the long term evolution of intracontinental basins such as the egb subsurface geophysical data and outcrop correlations demonstrate that the main subbasins of the egb contain major differences in basement metamorphic and structural fabrics basin fill patterns and distinct mesozoic cenozoic fault generations potential causes related to far field boundary conditions and other driving factors are suggested for each of the major deformation phases

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

plates and their motions the last reviews which covered research in plate motions were those of herron and pitman 1975 on marine magnetic anomalies and dewey 1975 on plate tectonics both reporting on research for the period 1971 1974 in the last 12 years there has been considerable refinement of relative motion models as well as further development of global models also major effort has gone into defining reference frames for the study of plate motions and discriminating between them more recently the role of errors in individual plate rotations and the associated uncertainties in reconstructions have received attention in this report i concentrate on work published in the last 4 years 1983 1986 citing a minimum of publications from previous years however some papers in press for early 1987 have been included this report first reviews work on relative plate motions for the present and past and reconstructions and then models for these motions global ones for driving forces and local ones determining subduction history and back arc spreading the problem of reference frames both hotspot and paleomagnetic is discussed next i end with a review of work on the errors in plate motions

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

paleogeographic evolution of the united states pacific northwest during paleogene time paleogeographic reconstructions for oregon and washington during paleogene time illustrate a major transition from a dominantly compressional prior to middle eocene time to an extensional tectonic regime this transition resulted in the development of three phases of paleogene basin evolution in the united states pacific northwest during the initial phase basins formed along the continental margin during collision of oceanic islands sediments in these basins were derived from nearby orogenic highlands the second phase of basin development began in middle eocene time and consisted of rapid subsidence of individual basins that formed within a broad forearc region nonmarine basins that formed during this phase were caused by extension possibly associated with transcurrent faulting rapid sedimentation in both marine and nonmarine basins during this time consisted dominantly of sandstone derived from cretaceous plutonic sources far to the east the final stage of basin development was the modification

western united states extension how the west was widened abstract cenozoic extension in the western united states presents a complex interrelation of extension volcanism and plate boundary tectonics that defeats simple notions of active or passive rifting forces driving extension can originate at plate boundaries through basal traction basal normal forces or from buoyancy forces internal to the crust and lithospheric mantle the latter two are most responsible for driving extension where it is observed in the basin and range the complex evolution of the northern basin and range probably represents removal or alteration of mantle lithosphere interacting with buoyancy stored in the crust in contrast crustal buoyancy forces combined with a divergent plate boundary between about 28 and 16 ma to drive extension in the southern basin and range the central basin and range most likely extended as a result of boundary forces external to itself but arising from buoyancy forces elsewhere in the western united states

the strength of faults in the crust in the western united states abstract the strength of secondary faults within plate boundary zones and of master faults like the san andreas has been controversial for decades we use a global finite element code with a variable resolution grid and global plate driving forces to determine whether the effective friction on the san andreas fault is high 0 6 1 intermediate 0 3 0 5 or low 0 2 whether a single value of can be used for all mapped faults within california and whether weakening of the ductile lower crust associated with faulting is important we compare our model results with existing data on fault slip rates gps velocities stress field and earthquake depth distribution the comparison indicates that all faults are weak 0 2 and that additional weakening of major faults is important all viable solutions also indicate that weakening of the lower crust below major faults is necessary the strongest faults in the region have in the range 0 2 0 05 the san andreas fault is a very weak fault among weak faults with

the scientific work 1957 1987 of allan cox during the summer of 1951 allan cox then a 25 year old chemistry major at the university of california berkeley became curious about geology and worked as a geology field assistant to clyde wahrhaftig thus did allan cox begin a momentous career in geomagnetism paleomagnetism and plate tectonics which left its indelible mark in the sciences themselves in the workings of earth sciences through his leadership roles in the american geophysical union the geological society of america and the national academy of sciences and most importantly in the lives and works of many junior scientists in the united states and throughout the world twenty three years ago i was one of those junior scientists whose life was touched and transformed by this unique individual over the course of ensuing years it is quite possible that but for a chance meeting with allan at the 1965 iaga assembly in pittsburgh i would have not seen for a long time the important ways by which basic rock magnetism can and does affect progress in geomagnetism and paleomagnetism

southern hemisphere origin of the cretaceous laytonville limestone of california new paleomagnetic paleontologic and stratigraphic data from outcrops of the laytonville limestone 101 to 88 million years old support a southern hemisphere origin a paleomagnetic megaconglomerate test is statistically significant and suggests magnetization at 14 5 south predating late cretaceous to eocene 70 to 50 million years ago accretion rapid kula plate movement or the existence and demise of a now vanished oceanic plate or both are required to accommodate the greater than 50 of poleward displacement implied by the paleomagnetic data this rapid motion brings into question the validity of a speed limit for absolute plate velocity based on present day plate motions

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

a revised identification of the oldest sea floor spreading anomalies between australia and antarctica we propose that magnetic anomalies south of australia and along the conjugate margin of antarctica that were originally identified as anomalies 19 to 22 may be anomalies 20 to 34 the original anomaly identification has two troublesome aspects 1 it does not account for an extra anomaly between anomalies 20 and 21 and 2 it provides no explanation for the rough topography comprising the diamantina zone with our revised identification there is no extra anomaly and the diamantina zone is attributed to a period of very slow spreading 4 5mm yr half rate between 90 and 43 m y the ages bounding the interval of slow spreading 90 and 43 m y correspond to times of global plate reorganizations our revised identification opens up the possibility that part of the magnetic quiet zone south of australia formed during the cretaceous long normal polarity interval breakup of australia and antarctica probably occurred sometime between 110 and 90 m y b p the breakup unconformity identified by falvey in the otway basin may correspond to a eustastic sea level change

global tectonics since the breakup of pangea 180 million years ago evolution maps and lithospheric budget abstract pangea the earth s youngest single supercontinent broke up 180 million years ago tectonic plates were subsequently formed by dispersal of the continental fragments and accretion of new oceanic lithosphere the configurations of all the major lithospheric plates at 0 20 65 95 140 180 ma bp are reconstructed on six globes of the earth each with a radius of 10 cm it appears that plate boundaries maintain a remarkably close fit on model globes of constant radii if the reconstructions include the recovery of subducted spreading patterns this is illustrated with maps in equatorial orthographic oblique orthographic and transverse hammer aitoff projections the snug fit of the tectonic plates at every tested time since the breakup of pangea 180 ma bp is consistent with the theory of plate tectonics on a non expanding earth and contradicts rapidly expanding earth models the areas of oceanic lithosphere produced and consumed during the past 180 ma bp are estimated from surface measurements of the globes reconstructed on the basis of particular assumptions these measurements suggest a consistent increase in the production rate of oceanic lithosphere during the past 140 ma it was decided to revise the assumptions and see if alternative reconstructions of the ancient spreading patterns on the floors of the tethys and eo pacific oceans could avoid implying an increase of lithospheric production rates with time this appeared to be possible the revised maps suggest that ophiolites older than 180 ma bp may have been obducted in cenozoic collision zones of the himalayas andes rockies and the western part of the banda arc timor new guinea estimates of the ocean floor production and consumption budget appear to be quite similar for both map series and only the possible ranges are summarized here time averaged for the past 180 ma world wide production and consumption of oceanic lithosphere appears to have varied between 2 6 and 3 5 km2 a 1 at most the mean of world wide spreading velocities ranges between 2 4 and 3 5 cm a 1 and world wide means of subduction velocities lie between 4 2 and 6 3 cm a 1 more accurate estimates are possible on the basis of particular assumptions and have been specified for the various time intervals distinguished i e the neogene 0 20 ma bp palaeogene 20 65 ma bp upper cretaceous 65 95 ma bp lower cretaceous 95 140 ma bp and middle and upper jurassic 140 180 ma bp

tectonic history of the juan de fuca ridge over the last 40 million years interpretation of a new compilation of magnetic anomaly data for north of the mendocino fracture zone and younger than chron 15 39 ma indicates a transition from a pattern of stable transform faults to a pattern of persistent ridge propagation reorganizations of the propagation pattern occurred at 30 ma in response to a counterclockwise change in direction of relative motion and at 19 ma in response to a clock wise change in addition there are significant changes in the gradient of the spreading rate apparently without major changes in motion direction at 27 and 24 ma the change at 30 ma is apparently the result of the separation of the juan de fuca plate from the rest of the farallon plate along the pioneer fracture zone the pattern of absolute motion after 30 ma does not generally support menard s pivoting subduction model and suggests that juan de fuca plate motion is being significantly affected by forces transmitted across the mendocino transform fault

a revised kinematic model for the relative motion between pacific oceanic plates and north america since the late cretaceous 1 the convergence between the pacific farallon and kula oceanic plates and the north american plate has provided a driving force for tectonic processes that have shaped the western margin of north america yet reconstructions of plate motion in the pacific basin have traditionally relied on the assumption of fixed hot spots in the last decade substantial cretaceous to paleogene motion between the pacific and atlantic hot spots has been recognized providing motivation to reevaluate the history of relative motion between pacific oceanic plates and north america by means of plate circuit reconstructions this paper presents new kinematic models based on two alternative plate circuits linking the pacific plate to north america since late cretaceous time when compared to models relying on fixity of pacific and indo atlantic hot spots our reconstructions suggest that the pacific kula and pacific farallon ridges were 600 1000 km closer to the western margin of north america and hence that the farallon and kula plates were smaller in late cretaceous to middle eocene time these findings cast significant doubt on the viability of a configuration of the farallon kula ridge that juxtaposes the kula plate with the southern part of the paleo north american margin during this time interval the results of our new reconstructions also suggest more oblique convergence between the farallon or kula plate and north america from the late cretaceous to the middle eocene related to the combination of dominantly northwest motion of the pacific plate and steady southwest motion of north america this provides an efficient mechanism for the coast parallel translation of some accreted terranes of north america it may also explain the structure of geological units in western north america such as the central belt of the franciscan complex which show a dominant coast parallel fabric formed during an interval coeval with the episode of oblique convergence

paleomagnetism of the resurrection peninsula alaska implications for the tectonics of southern alaska and the kula farallon ridge the rocks of the resurrection peninsula compose an ophiolite 57 1 ma within the vast accretionary prism of the chugach prince william terrane paleomagnetic data from pillow basalt and sheeted dikes of the ophiolite yield a mean paleolatitude of 54 7 which implies 13 9 of poleward displacement the characteristic component of the remanence resides in a ti poor titanomagnetite with a blocking temperature close td 580 c a positive fold test and the petrology of the magnetic minerals lead us to conclude that the characteristic remanent magnetization chrm in the sheeted dikes originated as a primary thermoremanent magnetization trm we hypothesize that the chrm of the pillow basalt originated as a chemical remanent magnetization crm acquired soon after formation of the rocks during hydrothermal metamorphism structural data indicate polyphase deformation with folding about vertical and horizontal axes the paleomagnetic data pass a fold test only when we incorporate this interpretation in our structural correction geologic relationships lead us to interpret the resurrection peninsula ophiolite as a fragment of the extinct kula farallon ridge that accreted to the chugach prince william terrane soon after formation the paleomagnetic paleolatitude indicates that the ridge intersected the north american continental margin in the vicinity of northern washington in late paleocene time the paleostrike of the sheeted dikes n26 e 12 shows that the ridge trended nne according to current plate tectonic models if the chugach prince william terrane moved north with the full coastwise displacement of the kula plate the resurrection peninsula ophiolite would have arrived at its present position at 45 ma this age is perhaps slightly later than that of the terranes that lay immediately inboard peninsular and wrangellia terranes indicating some relative displacement between those terranes and the chugach prince william terrane such displacement in turn agrees with studies which imply a coast plutonicmetamorphic complex provenance for the chugach prince william terrane

testing the fixed hotspot hypothesis using ar 40 ar 39 age progressions along seamount trails abstract hotspots and their associated intra plate volcanism producing seamount trails have become an accepted fact in geology from a conceptual theory the azimuths and age progressions of these seamount trails provide the only means to determine absolute plate motions with respect to an independent reference frame of fixed hotspots however the presumed fixity of hotspots is in disagreement with recent paleomagnetic studies and global circuit plate reconstructions for the hawaiian emperor seamount trail in this study we provide independent evidence suggesting that hotspots are not fixed relative to each other we use a straightforward test that compares the observed 40ar 39ar age progressions along pacific seamount trails 0 140 myr with the pacific plate velocities as predicted by their poles of plate rotation i e euler poles in most of these comparisons the age progressions were found incompatible with published euler poles or with a new set of euler poles as derived in this study using discrete seamount locations digitized from the bathymetry maps of smith and sandwell eos 77 1996 315 science 277 1997 1956 1921 we conclude that the relative motion between hotspots may be required to reconcile the observed age progressions with the predicted plate velocities from their modeled euler poles on average the pacific hotspots may show motion at 10 60 mm yr over the last 100 myr partly attributed to individual hotspot motion whereas systematic motion of these hotspots due to true polar wander may account for the remainder

propagation as a mechanism of reorientation of the juan de fuca ridge we present a revised model of the tectonic evolution of the juan de fuca ridge by propagating rifting the new model has three different relative rotation poles covering the time intervals 17 0 8 5 ma 8 5 5 0 ma and 5 0 ma to the present the rotation pole shifts at 8 5 and 5 0 ma imply clockwise shifts in the direction of relative motion of 10 to 15 at each of these shifts the pattern of propagation reorganizes and the new ridges formed by propagation are at an orientation closer to orthogonal to the new direction of motion than the orientation of the preexisting ridges the model containing a total of seven propagation sequences shows excellent agreement with the isochrons inferred from the magnetic anomaly data except in areas complicated by the separate explorer and gorda plates the agreement between model and data near the explorer plate breaks down abruptly at an age of about 5 ma indicating that the probable cause of the rotation pole shift at that time was the separation of the explorer plate from the juan de fuca plate

laboratory observation of acoustic fluidization in granular fault gouge and implications for dynamic weakening of earthquake faults 1 several lines of evidence including remote triggering of earthquakes and modulation of seismic tremor by earth tides suggest that faults weaken when subject to shaking and dynamic stresses associated with the passage of seismic waves however the origin of such dynamic weakening is poorly understood here we explore the role of acoustic resonance for dynamic fault weakening using laboratory measurements experiments were conducted using a split hopkinson pressure bar assembly with dynamic stressing via impact loading samples were composed of crushed rock particles from mine tailings with a particle size distribution similar to that found in a natural fault gouge we used pulse shaper techniques and carefully evaluated dynamic stresses recorded at the front and rear of the sample to ensure that dynamic force balance was satisfied our experiments document acoustic induced fluidization and dramatic dynamic weakening frictional strength and elastic modulus of a simulated fault gouge are reduced by a factor of 5 10 via acoustic fluidization we find a threshold acoustic pressure for fluidization that varies systematically with gouge zone properties our observations could help explain dynamic fault weakening and triggering of earthquake fault slip by dynamic stressing

paleomagnetic data from alaska reliability interpretation and terrane trajectories virtually the entire body of paleomagnetic data collected from southern alaska depicts a clear decrease in paleolatitude with increasing age strongly suggesting that southern alaska represents a displaced terrane in this paper paleomagnetic studies from southern alaska have been classified with respect to a quality index that is based on four criteria these criteria are the presence of both polarities of magnetic remanence stepwise thermal or alternating field demagnetization of specimens principal component analysis of demagnetization data and a successful fold test of the 51 studies compiled only four from southern alaska and one from western canada are demonstrated to satisfy all criteria and fall therefore in the category of group 3 highly reliable two studies from southern alaska satisfy three of the four criteria lacking both polarities of characteristic remanence and are judged to be of group 2 reliable two of these paleomagnetic studies constrain the accretion time of the southern alaska terrane to the relatively stationary region of central alaska north of the denali fault four paleomagnetic studies from the southern alaska terrane show a distinct paleolatitude anomaly when compared with their expected paleolatitudes from the north american apparent polar wander path using the model of engebretson et al 1984 a series of models are presented to best fit these highly reliable and reliable paleomagnetic studies the model preferred in this article assumes an accretion time with north america of 50 ma and documents pre 50 ma displacement of the southern alaska terrane on the kula plate if the ghost rocks paleomagnetic magnetizations plumley et al 1983 are assumed to be of earliest tertiary age this model fits all of the low paleolatitudes observed in southern alaska models incorporating coastwise translation of the southern alaska terrane along the western boundary of the north america plate and a 50 ma suturing age of this terrane to north america do not satisfy the paleomagnetic constraints four of the reliable studies can be modeled assuming strike slip motion of the southern alaska terrane along the western boundary of the north america plate if a 40 ma suturing age is assumed in this model however the paleomagnetic data must be presumed to represent the highest permissible paleolatitude and the ghost rocks formation paleolatitudes are problematically lower than is possible to model with coastwise translation

the mathematician paleoplate the pacific seafloor is littered with small fragments of lithosphere captured from adjacent plates by past plate boundary reorganizations one of the clearest examples of such a reorganization is documented in the mathematician seamounts region where a distinctive geomorphology and well developed magnetic anomalies are present this reorganization involved a short lived microplate between the failing mathematician ridge and a new propagating spreading center the east pacific rise it produced a transfer of a fragment of lithosphere from the farallon to the pacific plate and also created a number of landforms and magnetic patterns within and on the margins of the captured fragment these make up the mathematician paleoplate in many cases two sides of a microplate are active spreading ridges a microplate evolves into a paleoplate when dual spreading ceases and full spreading resumes at the prevailing spreading ridge we define a paleoplate as the area of the seafloor from the axis of a failed rift to the boundary of resumed full spreading it includes a fragment of captured lithosphere and the lithosphere slowly accreted to it during the period of dual spreading prior to complete abandonment of the failed rift the mathematician paleoplate has the following boundaries and components from west to east the axis of the mathematician failed rift the fragment of captured farallon plate a complex rift initiation site at the moctezuma trough a zone of slow spreading and an as yet ill defined eastern boundary where dual spreading stopped and full spreading resumed the northern boundary of the paleoplate is the rivera fracture zone its southeastern boundary a now inactive transform fault the west o gorman fracture zone in this case as well as in other more poorly documented ones relict landforms and magnetic patterns are carried on the aging lithosphere away from the spreading ridge recording a former geometry

south pacific hotspot swells dynamically supported by mantle flows the dynamics of mantle plumes and the origin of their associated swells remain some of the most controversial topics in geodynamics here we construct a numerical model of the mantle flow beneath the french polynesia region our study is based on a new regional seismic tomography model which high resolution allows obtaining information at the scale of plumes we find excellent correlations between the observed and the modeled dynamic swells between the modeled flow pattern and the active volcanism and between the buoyancy fluxes obtained from our numerical model and the ones deduced from the swells morphology these outstanding fits reveal for the first time that a direct link exists between the surface observations and mantle flows