rise and fall of the southern santa cruz mountains california from fission tracks geomorphology and geodesy the santa cruz mountains are closely associated with a left bend along the right lateral san andreas fault the loma prieta area on the northeast side of the san andreas consists of fault bounded blocks that rise along active deeply rooted reverse and oblique slip faults six samples from a transect across this area yield concordant apatite fission track ages averaging 4 6 0 5 ma these ages date the time of cooling below 110 c and suggest that about 3 km of unroofing has occurred over the last 4 6 m y allowing for current elevations of about 1 km this suggests an average uplift rate of the order of 0 8 mm yr over the last 4 6 m y to further define the extent and distribution of this young uplift we used morphometric analyses of the youthful topography of the area steep drainage slopes and high local relief indicate that the area northeast of the san andreas forms a well defined zone of high uplift in contrast the region on the southwest side of the san andreas is characterized by broad upwarping and folding more subdued topography old fission track ages and mean quaternary uplift rates of 0 1 0 4 mm yr geodetic data 1906 san francisco earthquake and subsequent strain transients 20 years of interseismic deformation 1989 loma prieta earthquake and 2 years of post loma prieta earthquake strain show that the southern santa cruz mountains repeatedly rise and subside through a complex sequence of bay area deformation events an additional deformation element that involves reverse slip averaging 2 3 mm yr along the foothills thrust system must occur to explain the longer term uplift pattern in the loma prieta area since the late pliocene the asymmetry in deformation on opposite sides of the san andreas probably reflects the contrasting rock types on opposite sides of the fault the influence of preexisting structures and the interaction with neighboring faults of the san andreas system

seismotectonics of the san andreas fault system between point arena and cape mendocino in northern california implications for the development and evolution of a young transform the northernmost and relatively youthful segment of the san andreas fault system is situated within a 100 km wide zone of northwest trending strike slip faults that includes from west to east the san andreas maacama and bartlett springs faults although the san andreas fault is the principal strike slip fault in this system it has been virtually aseismic since the 1906 earthquake moderate levels of seismicity locate to the east along the maacama fault and to a lesser extent the bartlett springs fault at focal depths typical of other strike slip faults within the san andreas fault system in central california north of the san andreas fault system within the cape mendocino area earthquakes occur at depths of up to 40 km and primarily reflect internal deformation of the subducting gorda slab and slip along the mendocino fracture zone seismicity along the maacama and bartlett springs faults is dominated by right lateral to oblique reverse slip along fault planes that dip 50 75 to the northeast the northern extent of seismicity along these faults terminates near the surface projection of the southern edge of the gorda slab the onset of seismicity along these faults may be related to the abrupt change in the elastic thickness of the north american plate as it enters the asthenospheric window the maacama and bartlett springs faults are strike parallel with active reverse faults within the forearc region of the cascadia subduction zone this preexisting structural fabric of northwest trending reverse faults in the forearc area appears to have strongly influenced the initial slip and complexity of these faults continuation of the moderately dipping maacama fault to the southeast along the steeply dipping healdsburg and rodgers creek fault zones and the near vertical hayward and calaveras fault zones in the san francisco bay area suggests that these faults evolve toward a more vertical dip to minimize the shear stresses that tend to resist plate motion

a geometric technique for relocating hotspots and refining absolute plate motions an age independent geometric relationship is presented that links hotspots to the seamounts which they produce and so permits the use of undated seamounts to refine the motion of tectonic plates this technique has the potential to rigorously assess hotspot fixity and to locate extinct hotspots the present application of this method points to a recent change in pacific plate motion and suggests a relocation of the louisville hotspot to the hollister ridge south of the eltanin fracture zone

the ionospheric response to interplanetary magnetic field variations evidence for rapid global change and the role of preconditioning in the magnetosphere we have found observational evidence for a rapid communication of interplanetary magnetic field imf changes to the global ionosphere and evidence for the state of the magnetosphere in the previous hour conditioning this response these conclusions are drawn from a case study of sunward flow bursts on the nightside polar cap boundary observed by geomagnetically conjugate hf radars the flow burst excitation consists of two factors 1 at the time of the flow burst the magnetosphere still held a memory of the stable and northward imf period that had persisted up until 1 hour before the flow burst internal condition during the northward imf period a theta aurora associated with a sunward flow channel was formed in the polar cap after that the imf turned southward and the transpolar arc decayed antisunward however by the time of the flow burst i e 1 hour after the imf southward turning the sun aligned arc had not yet completely vanished and in the poleward expanded portion of the northern plasma sheet there was still a remnant of the sunward flow channel susceptive to an external forcing 2 one hour after the southward turning of the imf a sharp imf transition from southward to northward bz impinged on the dayside magnetopause external condition on arriving at the dayside cusp ionosphere the bz transition signal pervaded the entire polar cap ionosphere instantaneously 1 min and reached the nightside plasma sheet there the remnant of the sunward flow channel was reactivated by the bz transition and a sunward flow burst was observed first in the northern ionosphere and then in the southern ionosphere with a 7 min time delay thus the sunward flow burst represents a rapid global response of the ionosphere starting 2 3 min after the imf change at the subsolar magnetopause

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

morphology and distribution of fossil soils in the permo pennsylvanian wichita and bowie groups north central texas usa implications for western equatorial pangean palaeoclimate during icehouse greenhouse transition analysis of stacked permo pennsylvanian palaeosols from north central texas documents the influence of palaeolandscape position on pedogenesis in aggradational depositional settings palaeosols of the eastern shelf of the midland basin exhibit stratigraphic trends in the distribution of soil horizons structure rooting density clay mineralogy and colour that record long term changes in soil forming conditions driven by both local processes and regional climate palaeosols similar to modern histosols ultisols vertisols inceptisols and entisols all bearing morphological mineralogical and chemical characteristics consistent with a tropical humid climate represent the late pennsylvanian suite of palaeosol orders palaeosols similar to modern alfisols vertisols inceptisols aridisols and entisols preserve characteristics indicative of a drier and seasonal tropical climate throughout the lower permian strata the changes in palaeosol morphology are interpreted as being a result of an overall climatic trend from relatively humid and tropical moist conditions characterized by high rainfall in the late pennsylvanian to progressively drier semi arid to arid tropical climate characterized by seasonal rainfall in early permian time based on known late palaeozoic palaeogeography and current hypotheses for atmospheric circulation over western equatorial pangea the pennsylvanian palaeosols in this study may be recording a climate that is the result of an orographic control over regionalscale atmospheric circulation the trend towards a drier climate interpreted from the permian palaeosols may be recording the breakdown of this preexisting orographic effect and the onset of a monsoonal atmospheric circulation system over this region

northern east pacific rise evolution from 25 m y b p to the present the northeast pacific topography and magnetic lineations 25 m y b p to the present record the traces of three major spreading reorganizations only one spreading center is observed today but there is evidence for several ephemeral episodes of twin spreading accompanying the evolution from an extensive pacific guadalupe plate boundary to a much shortened pacific cocos and pacific rivera plate boundary the 25 m y b p plate reorganization culminated with the formation of the guadalupe plate bound by the murray fracture zone to the north and the cocos nazca spreading ridge to the south between 25 and 12 5 m y b p spreading continued while the plate retained the same general outline the 12 5 11 m y b p reorganization resulted in the creation of a much shortened pacific cocos plate boundary located in its early stages over the mathematician seamounts and a much reduced cocos plate the last reorganizaton 6 5 3 5 m y b p resulted in the abandonment of the mathematician spreading ridge as a pacific cocos plate boundary in favor of the east pacific rise

implication for mantle flow from the hawaiian emperor chain abstract assuming that rayleigh benard flow occurs within the upper mantle ascending plumes must undergo horizontal displacement during ascent that can be traced at the surface it is shown that the hawaiian emperor bend cannot be formed if the convection rate is of the same order of magnitude as the ascent rate of the plumes the plume patterns obtained by push pull or lithospheric sliding are similar to that of the hawaiian emperor chain hence the present results support the sliding model

plate reorganization a cause of rapid late neogene subsidence and sedimentation around the north atlantic subsidence analysis of wells in the central north sea and labrador grand banks and oft the west greenland scotian shelf and united states atlantic margin shows distinct quantitative stratigraphic correlation patterns of circum north atlantic sites a significant departure from the overall decrease in subsidence for the pliocene occurs in many wells when the rate is found to have increased one or more orders of magnitude from oligocence miocene rates wells were selected along transects from shore to basin to find if relative basin position is influenced by differential basin subsidence although stratigraphic resolution is not detailed more basinward sites experienced up to four times larger subsidence rates in the late neogene than in the oligocene miocene with a peak in the pliocene wells at the basin edge experienced much less subsidence or showed uplift the observations are consistent with a rapid change of intraplate stress at the cause for this observed transition in neogene subsidence major reorganizations of spreading direction and rate occurred during the pliocene along the entire atlantic spreading system possibly in conjunction with more global changes in plate motions we propose that the associated changes in intraplate stress caused the excess margin subsidence relative uplift along basin edges is consistent with this mechanism of relative movement and may explain apparent eustatic changes in sea level

coherent mesoscale convection patterns during northward interplanetary magnetic field all sky imaging photometer asip and coincident de 2 satellite plasma drift and particle data have been combined to study polar ionospheric convection in the presence of subvisual intensity soft particle excited f region 6300 a sun aligned polar cap arcs coincident de b drift meter data identify these arcs electrodynamically as lines of negative electric field divergence based on conductivities derived from the de b low altitude plasma instrument measured particle fluxes and the measured electric field gradients the divergence of horizontal current across these particle impact excited arcs is in good quantitative agreement with upward birkeland currents carried by the measured particle fluxes although velocity structure can be found without arcs given the asip identified condition of stable weak hundreds of rayleighs 6300 a sun aligned arcs in the polar cap electric field negative divergence is consistently found these arcs of the order of 100 km in width are found by asips in the polar cap about half the time under bz 0 interplanetary magnetic field conditions they are regularly seen by asip data to extend 1000 to over 2000 km in the sunward direction and to persist in time often for over an hour we are thus led to conclude that velocity gradients of this noon midnight elongated scale are typical of bz 0 conditions we further conclude that combined polar asip images and electrostatic potentials calculated along transpolar satellite tracks offer a valuable diagnostic for polar ionospheric convection studies under bz 0 conditions the asip time continuous two dimensionality and the satellite equipotential scaling allow individual snapshots of these polar convection boundaries application here demonstrates highly anisotropic temporally stable convection with greater order than has previously been suspected

recent movements of the juan de fuca plate system analysis of the magnetic anomalies of the juan de fuca plate system allows instantaneous poles of rotation relative to the pacific plate to be calculated from 7 ma to the present by combining these with global solutions for pacific america and absolute relative to hot spot motions a plate motion sequence can be constructed this sequence shows that both absolute motions and motions relative to america are characterized by slower velocities where younger and more buoyant material enters the convergence zone pivoting subduction the resistance provided by the youngest portion of the juan de fuca plate apparently resulted in its detachment at 4 ma as the independent explorer plate in relation to the hot spot framework this plate almost immediately began to rotate clockwise around a pole close to itself such that its translational movement into the mantle virtually ceased after 4 ma the remainder of the juan de fuca plate adjusted its motion in response to the fact that the youngest material entering the subduction zone was now to the south differences in seismicity and recent uplift between northern and southern vancouver island may reflect a distinction in tectonic style between the normal subduction of the juan de fuca plate to the south and a complex underplating occurring as the explorer plate is overridden by the continent the history of the explorer plate may exemplify the conditions under which the self driving forces of small subducting plates are overcome by the influence of larger adjacent plates the recent rapid migration of the absolute pole of rotation of the juan de fuca plate toward the plate suggests that it too may be nearing this condition

relationship between tectonics volcanism sedimentation and basin development late cenozoic central part of northern honshu japan abstract a pronounced interplay between tectonics volcanism and sedimentation can be established in the volcanic arc region of the central part of northern honshu during the early miocene to quaternary interval four stages are recognized with the type of volcanism and sedimentation varying according to the tectonic stress regime 1 1 rifting stage 22 15 ma ns trending half grabens were formed under an extensional stress regime related to the rifting of the sea of japan basins were rapidly filled by intermediate to felsic volcanic products which are often associated with subaerial fluvial and lacustrine sediments 2 2 back arc basin opening stage 15 13 ma with more advanced stagess of rifting subsidence from crustal stretching resulted in marked marine incursion and an overall transgressive succession developed during this stage the back arc side of northern honshu subsided rapidly to middle bathyal environments at the same time associated with the opening of the sea of japan large volumes of regionally extensive mafic lavas and associated hyaloclastites were erupted within submarine grabens in contrast in the present backbone ranges felsic volcanism occurred within bathyal to shallow marine environments igneous rock types in this stage display a bimodal mafic felsic chemistry 3 3 transitional stage 13 2 4 ma a thermal subsidence substage 13 8 ma an intermediate stress regime developed which included phases of both weak extension and compression consequently grabens along the sea of japan ceased to form later lithospheric cooling eventually led to thermal subsidence of the back arc region and hemipelagic sediments mantled the previously active mafic volcanic apron volcanism at this time was mainly restricted to a narrow zone along the present backbone ranges where felsic to intermediate volcanic products accumulated within shallow marine environments b incipient compressional substage 8 2 4 ma hemipelagic sedimentation continued on the back arc side of northern honshu along the present backbone ranges the intermediate stress regime continued with formation of many large caldera structures associated with subaerial felsic volcanism 4 4 shortening deformation stage 2 4 ma present the change of the absolute direction of motion of the pacific plate at the early to middle pliocene changed the regional stress regime of the volcanic arc from intermediate to compression movement of back arc thrusts along the sea of japan coast began 2 3 ma and led to uplift of the volcanic arc continued compression in the volcanic arc produced narrow uplifted zones with associated basins bounded by reverse faults these basins were filled rapidly by terrestrial sediments containing upward coarsening sequences under this highly compressional tectonic stress regime volcanism became intermediate mafic in composition with large stratovolcanoes becoming prominent

reconciling late neogene pacific absolute and relative plate motion changes new models of pacific absolute plate motion relative to hot spots and models of relative plate motion involving the pacific plate all agree there was a significant change in the late neogene chron 3a 5 89 ma reflecting a more northerly absolute motion than previously determined as pacific absolute plate motion became slightly more northerly left stepping transform segments came under compression some left stepping segments became microplates with clockwise rotation others show clear evidence of compressional deformation conversely right stepping transforms came under tension and many developed intratransform spreading centers or show similar evidence for transform magmatism several large left stepping transform offsets represent portions of the nazca plate protruding into the pacific and as such act as obstacles to the more northerly pacific absolute plate motion we suggest these obstructions act to enhance the generally tensile equatorial pacific stress regime caused by distant slab pull as a consequence the greater french polynesia region has experienced diffuse volcanism that increased considerably following the chron 3a plate motion change we propose that since the hawaii emperor bend time the distant pacific slab pull and the friction between the large buoyant ontong java plateau and the northern margin of the australia plate have produced episodes of increased tensile stresses in the equatorial pacific we believe these stress excursions are responsible for much of the intraplate volcanism observed in a wide triangular region from samoa to the east pacific and pacific antarctic rises

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

the development of two oceanic microplates easter and juan fernandez microplates east pacific rise in this paper we synthesize and discuss the results of recent detailed geophysical surveys and reconstructions of two active mid ocean ridge microplates on the east pacific rise we quantify their development in terms of the principal dimensions of the microplates and their boundaries and the temporal changes in these dimensions the results are discussed in the framework of the roller bearing microplate model of schouten and others which postulates that microplates are driven by shear couples imposed on them by the motions of their bounding major plates we test the specific predictions of this model for the geometry of the pseudofaults traces of the propagating tips of the microplates spreading boundaries and the histories of rotation of the microplates both microplates are currently rotating at the rates predicted by the model on the assumption of complete coupling no slip or deformation between the microplates and the major plates at instantaneous points of contact near the propagating tips of the spreading boundaries juan fernandez microplate was also rotating at the predicted rate just before 2 ma when it was driven by the nazca pacific couple rather than the nazca antarctic couple that currently applies however earlier rotations were slower than predicted implying some degree of decoupling or deformation at earlier times the microplates appear to have grown from moderate sized c 100 km probably left stepping offsets of the east pacific rise since about 5 ma the nature of these offsets is currently uncertain but they may have been propagating rifts which changed from shearing by bookshelf faulting to rigid block microplate rotation on reaching a critical size once rigid block rotation began the microplates grew by seafloor spreading accompanied by propagation of their bounding spreading centres the effects of the neighbouring easter mantle plume and pacific nazca antarctic triple junction can be discerned in the bathymetry and ridge propagation rates but do not appear to have had a major influence on the microplates development

response of the magnetosphere ionosphere system to a sudden southward turning of interplanetary magnetic field 1 a sudden southward turning of interplanetary magnetic field imf is simulated by the university of michigan s bats r us model the main goal of this study is to determine the time delay as well as physical processes between when an imf discontinuity reaches the bow shock when reconnection is initiated at the magnetopause and when the ionosphere starts to react while observations or empirical models might give an estimate of the time delay for the propagation of the discontinuity from the bow shock to the magnetopause the global mhd simulation provides a more comprehensive insight of responses of the magnetosphere ionosphere system an idealized north to south imf transition is modeled using a solar wind velocity of 400 km s after the southward imf encounters the bow shock it takes about 6 min for the north to south imf transition front to arrive at the subsolar geomagnetic field the ionospheric response to this sudden southward imf turning is delayed by another 4 min during which the magnetosphere undergoes a conversion from cusp reconnection to subsolar reconnection and the alfven wave propagation to the ionosphere takes place thereafter changes in the ionosphere and ground magnetic perturbations associated with the southward imf are observed these responses appear to be globally onset as described in many other studies the time it takes from the encounter of the imf transition with the bow shock to when the ionospheric reaction takes place varies with the solar wind speed ranging from nearly 15 min for a solar wind speed of 300 km s to just over 6 min for solar wind speeds of 600 km s

northern east pacific rise magnetic anomaly and bathymetric framework the oceanic crust in the eastern pacific between 7on and 30on and east of 127ow contains a fairly complete history of the spreading centers associated with the east pacific rise since 25 m y b p late oligocene in this paper we have summarized the seafloor spreading magnetic anomaly data and the bathymetric data that reflect the record of this tectonic history the well defined magnetic lineations north of the clarion fracture zone in the mouth of the gulf of california and on the east flank of the east pacific rise epr are carefully examined and used to provide a guide for interpreting the spreading pattern between the clarion and clipperton fracture zones southward of the rivera fracture zone over the mathematician ridge and over the entire epr east of the mathematician ridge between the rivera and siqueiros fracture zones the bathymetric data provide a trace of the fracture zone pattern in each of the above mentioned areas the fracture zone bathymetry and the seafloor spreading magnetic lineations on the epr south of the rivera fracture zone have a distinctive fanning pattern caused by close poles of rotation and plate boundary reorganizations all these data provide a good record of the plate reorganizations in the middle miocene at magnetic anomaly 5a time 12 5 to 11 m y b p in the late miocene at magnetic anomaly 3 4 time 6 5 m y b p and in the pliocene at magnetic anomaly 2 3 time 3 5 m y b p several abandoned spreading centers including the mathematician ridge were left behind as a result of these reorganizations the mathematician ridge is shown to be a set of ridges and trough whose origin is related to the tectonic activity associated with each of the above mentioned reorganizations since anomaly 5a

a tectonic model for ridge transform ridge plate boundaries implications for the structure of oceanic lithosphere abstract the first order geologic and morphologic relationships at along and proximal to ridge transform ridge plate boundaries are used to construct an empirical and speculative tectonic model the distinctive but variable morphotectonic fabric and crustal structure of the transform domain are the product of a tectonic continuum that ranges from very slow rates of strain associated with very thick edges of lithosphere slowly slipping ridge transfrom ridge plate boundaries to extremely high rates of strain associated with very thin edges of lithosphere fast slipping ridge transform ridge plate boundaries the geometry of a ridge transform intersection necessitates the juxtaposition of a relatively cold thick edge of lithosphere against the truncated end of an accreting plate boundary the cold face of lithosphere cools the adjacent wedge of asthenosphere rising beneath the axis of accretion and restricts the amount of partial melting thus attenuating the volume of basaltic melt segregated from the asthenosphere per unit time the shallow level manifestation of this cold edge effect is a thinner oceanic crust at depth the evolving lithosphere thickens rapidly as the cold edge is approached creating changes in the properties of the young lithosphere and forming a weld of upper mantle material against the cold edge of truncating lithosphere the ridge transform weld of upper mantle material creates a shear couple in the lithosphere underlying the intersection resulting in the progressive reorientation of the maximum tensile stress from normal to the ridge axis at some distance from the ridge transform intersection to an oblique angle near the boundary the brittle carapace of oceanic crust that overlies the mantle weld will deform accordingly with the development of oblique dip slip faults the model predicts that the geologic expressions of this cold boundary effect will become more dramatic with increasing thickness of the truncating edge at very slow rates of accretion 100 km juxtapose thick 30 50 km edges of lithosphere against the accreting plate boundary all but nullifying the processes that lead to the emplacement of normal oceanic lithosphere in this environment a relatively strong mantle weld will form at the ridge transform intersection and the thick edges of opposing lithosphere along the length of the transform will tend to confine the strike slip tectonism to a relatively narrow and temporally stable principal transform displacement zone in contrast at very fast rates of accretion large offset transforms 100 km place relatively thin edges of lithosphere

late cenozoic tectonic evolution of southwestern california we synthesize available geologic and geophysical data from southern california and adjoining areas in order to develop a reconstruction of the tectonic evolution of the southern coast ranges western transverse ranges and borderland regions of california since 30 ma premises include specified relative plate motions for times prior to 10 5 ma and after 3 ma a mid tertiary bight in the continental margin of southern california midcrustal detachment faults under much of california and adjoining regions rotation of the western transverse ranges about an eastern pivot and specified fault displacement histories prior to 18 ma about 90 of the tangential component of pacific north america relative motion was accommodated on an offshore dextral fault system near the toe of the continental slope from 18 ma to 5 5 ma dextral slip was accommodated predominantly on the offshore system but also on a second inboard system that included the san andreas fault fault bounded crustal blocks between the offshore and inboard fault systems underwent translation rotation and extension from 18 ma to 11 5 ma but not from 11 5 ma to 5 5 ma since 5 5 ma dextral slip has been accommodated predominantly on the inboard fault system dextral slip ceased on the offshore fault system north of the transverse ranges but continues on faults west of or within baja california the san gregorio san simeon hosgri fault system has accommodated less than 5 km of the late cenozoic dextral slip the distribution of dextral displacement in the plate boundary zone has been controlled by abrupt jumps of the position of a southern triple junction and to a lesser degree changes in magnitude of pacific north america relative motion based on our reconstuction the tangential component of pacific north america relative motion was 90mm yr for the period 10 5 ma to 5 5 ma and 60mm yr for the period 5 5 ma to 3 ma

stable isotope constraints on the elevation history of the sierra nevada mountains california research on the uplift history of the sierra nevada mountain range has yielded seemingly conflicting results some studies argue for substantial uplift within the past 3 5 m y others suggest that high elevations may have existed since the cretaceous the rain shadow across the sierra nevada is associated with a strong isotopic gradient with lower 18o values in precipitation on the leeward side of the range reconstruction of the 18o value of meteoric water as a monitor of paleoelevation has focused mainly on the leeward side of the sierras but interpretation of the results of these studies may be complicated by shifts in global climate and regional moisture sources we address these concerns by analyzing the 18o value of tooth enamel bioapatite from contemporaneous mammalian fossils on either side of the present sierra range by sampling across the range 18o differences induced by a rain shadow can be isolated from other complicating factors our results indicate that the sierra rain shadow has existed since at least 16 ma which is an important constraint on models for the tectonic evolution of the western united states unfortunately temporal resolution for localities is too coarse to differentiate between glacial and interglacial localities during the past 2 m y so we cannot evaluate if there was a latest cenozoic pulse of uplift or elevation loss

magnitude and timing of new hebrides arc rotation paleomagnetic evidence from nendo solomon islands paleomagnetic results from nendo record a total rotation of the northwestern corner of the new hebrides arc of 52 12 clockwise reversed magnetic polarity and a nannofossil assemblage constrain the age of the nendo pole to 4 29 3 58 ma paleomagnetic data from malekula farther south in the same arc imply a rotation of only about 28 since the late miocene probably because these poles have been affected by rotation of malekula in response to collision of the arc with the d entrecasteaux zone aseismic ridge a new late miocene reconstruction of the new hebrides and vitiaz arcs based on the paleomagnetic evidence requires a rotation of 39 around the point of intersection of the two arcs the same rotation restores both the northwestern and southeastern segments of the new hebrides arc although rotation of the southeastern segment commenced earlier a series of sequential movements of segments of the arc occurring at different times but acting to keep the arc roughly collinear may be driven by a mechanism which minimizes deformation of the subducted plate propagation of spreading into the northwestern corner of the north fiji basin after 4 3 ma is implied by the paleomagnetic results and is consistent with spreading anomaly patterns in the basin when the new hebrides and vitiaz arcs are restored to their position relative to the lau ridge at 10 ma the fiji platform can be accommodated after clockwise rotation of 140 to a position in line with the new hebrides arc the large rotation for fiji is consistent with paleomagnetic evidence indicating at least 115 of rotation

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

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

new insight into the dynamic development of the southern alps new zealand from detailed thermochronological investigation of the mataketake range pegmatites

the relationship between late cenozoic tectonic events and stress field and basin development in northeast japan the regional tectonic stress field basin development and crustal deformation of the ne japan arc in the interval between 32 ma to the quaternary can be synthesized based on dike vein and fault orientation data as well as on the compilation of the regional geology an extensional stress field became prevalent from 32 ma and major normal faulting started at 25 20 ma which resulted from back arc rifting normal faulting trending nearly parallel to the arc propagated from the present japan sea coast to the forearc side following the trenchward migration of the main volcanic field from 20 to 15 ma normal faults with an oblique trend to the arc developed due to the counterclockwise rotation of ne japan the rapid clockwise rotation of sw japan since 16 ma produced a nw se directed transtensional stress regime in the ne japan arc due to crustal stretching associated with this pull apart movement the back arc side of the ne japan arc subsided rapidly to middle bathyal environments after the termination of the opening of the japan sea at about 14 ma a neutral stress regime prevailed which included phases of both weak extension and compression lithospheric cooling eventually led to thermal subsidence of the back arc region and igneous underplating caused uplift of the axial zone of the volcanic arc the increase in velocity of the westward motion of the pacific plate at around 4 ma produced strong compression across the arc reactivating most of the miocene normal faults and uplifting the volcanic arc the greatest crustal shortening occurred in areas that were stretched the most in the miocene within the volcanic arc which implies tectonic inversion

ground based observations of ion neutral coupling at thule and qânâq greenland during december 1988 24 hours of darkness and clear sky conditions permitted continuous observations of the o i 6300 a airglow by a fabry perot interferometer located at thule air base greenland thus a continuous record of the f region neutral winds was obtained for that month during this same time period a digital ionosonde located at q n q greenland 110 km north of thule air base was in operation measuring electron density profiles and f region ion drifts this combination of ground based observations allowed the investigation of ion neutral coupling at a temporal resolution of about 15 min interplanetary magnetic field imf data from the imp 8 satellite were also available from december 16 to 24 and indicated intervals of bz northward imf conditions during this period here we investigate the observed response of the neutral wind to convection changes in the ion drift inside the polar cap for southward and northward imf bz conditions in particular we establish a control day illustrating the typical antisunward neutral wind and ion drift patterns observed for southward bz over thule and q n q and we compare it with observations made when the imf bz is directed northward the observations during periods of northward bz display sunward directed ion drifts over the polar cap accompanied by decreasing antisunward directed neutral winds we investigate these times of northward bz further and demonstrate that the ion drag term alone cannot describe the observed response in the neutral wind during northward imf

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

the determination of time stationary two dimensional convection patterns with single station radars at the present time most ground based radar estimations of ionospheric convection use observations from single station facilities this approach requires certain assumptions as to the spatial and or temporal uniformity of the convection in this paper the authors present a critical examination of the accuracy of these vector velocity determinations using realistic modeled flow patterns that are time stationary but not spatially uniform they find that under certain circumstances the actual and inferred flow fields show considerable discrepancy sometimes not even agreeing in the sense of flow direction specifically they show that the natural curvature present in ionospheric convection on varying spatial scales can introduce significant error in the velocity estimate particularly when the radius of curvature of the flow structure is less than or equal to the radar range to the scattering volume the presence of flow curvature cannot be detected by radars which determine velocities from measurements in two viewing directions and it might not be detected by radars using azimuth scanning techniques thus they argue that every effort should be made to measure the ionospheric convection by bidirectional or multidirectional observations of a common ionospheric volume and that a synthesis of coherent and incoherent radar observations from different sites ismore preferable to multidirectional single station observations using either radar alone these conclusions are applicable to any doppler measurement technique and are equally valid for high latitude wind patterns using fabry perot interferometer techniques less

fault displacement gradient folds and the structure at lost hills california u s a abstract geometric models of fault propagation folds require that the underlying fault tip propagate upward across layering however related folds can be generated if the tip remains stationary or even moves back along the ramp as a pre existing fault is reactivated fault propagation folds detachment folds as well as those produced when there is no propagation are generated by a displacement gradient along the underlying fault and as a general class can be called fault displacement gradient folds new geometric equations are derived which relate fold shape to displacement gradient and fault propagation folds are a special case of these more general equations growth sequences associated with these structures have been used to distinguish fault propagation folds from fault bend folds when the underlying structure is obscured a displacement gradient fold in which the fault tip retreats back down the ramp generates a growth sequence that in some cases could be confused with that of a fault bend fold the structure at lost hills in the san joaquin valley of california u s a has been interpreted as a fault bend fold but it may be a displacement gradient fold which formed as a pre existing fault was reactivated during a pliocene change in plate motion

pliocene change in pacific plate motion the absolute motion of the pacific plate has been nearly constant during the past 15 myr but epp1 recognized that some change took place between 15 myr and the present and tentatively chose 1 myr for the time of the change cox and engebretson2 found a change in pacific absolute motion based on hotspot tracks and a change in pacific antarctic relative motion the pliocene deformation in california and the juan de fuca rotations led them to postulate a 5 myr age for the change the manner and timing of the change are explored more thoroughly here and we put the time for the change in motion of the pacific plate at between 5 and 3 2 myr basing the younger limit on the correlation of computed hotspot tracks with pacific and nazca hotspot traces the change may explain several observations on the neighbouring continental and oceanic plate boundaries including direct adjustments at the pacific spreading ridges a likely mechanism for the change is a large clockwise torque caused by acceleration of the sinking slab beneath the kurile trench where a fossil spreading ridge is inferred to have been subducted

superposed epoch analysis of the ionospheric convection evolution during substorms imf by dependence we present superposed epoch analyses of the average ionospheric convection response in the northern and southern hemispheres to magnetospheric substorms occurring under different orientations of the interplanetary magnetic field imf observations of the ionospheric convection were provided by the super dual auroral radar network superdarn and substorms were identified using the far ultraviolet fuv instrument on board the imager for magnetopause to aurora global exploration image spacecraft we find that during the substorm growth phase the expected imf by dependent dawn dusk asymmetry is observed over the entire convection pattern but that during the expansion phase this asymmetry is retained only in the polar cap and dayside auroral zone in the nightside auroral zone the convection is reordered according to the local substorm electrodynamics with any remaining dusk dawn asymmetry being more closely related to the magnetic local time of substorm onset itself only weakly governed by imf by owing to the preponderance of substorms occurring just prior to magnetic midnight the substorm asymmetry tends to be an azimuthal extension of the dusk convection cell across the midnight sector a manifestation of the so called harang discontinuity this results in the northern southern hemisphere nightside auroral convection during substorms generally resembling the expected pattern for negative positive imf by when the preexisting convection pattern in the northern southern hemisphere is driven by positive negative imf by the nightside auroral convection changes markedly over the course of the substorm to establish this same harang configuration

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

change in motion of pacific plate at 5 myr bp alignments of volcanoes such as the hawaiian seamount chain record the passage of oceanic plates over mantle plumes or hot spots and changes in the orientation of a seamount chain may indicate changes in the direction of motion of the plate within the hotspot frame of reference1 this hypothesis can be tested for the pacific plate by comparing data from the hawaiian volcanoes with tectonic movements inferred from the geology of adjacent plates we have now found that the motion of the pacific plate changed to a more northerly direction 5 myr ago probably as a result of the detachment of a piece of pacific sea floor which had been subducted beneath the fiji plateau during the tertiary

episodic cenozoic tectonism and the development of the nw european passive continental margin the north atlantic margins are archetypally passive yet they have experienced post rift vertical movements of up to kilometre scale the cenozoic history of such movements along the nw european margin from ireland to mid norway is examined by integrating published analyses of uplift and subsidence with higher resolution tectono stratigraphic indicators of relative movements including results from the stratagem project three episodes of epeirogenic movement are identified in the early mid and late cenozoic distinct from at least one phase of compressive tectonism two forms of epeirogenic movement are recognised referred to as tilting coeval subsidence and uplift rotations 1 over distances of 100s of kilometres and sagging strongly differential subsidence rotations up to 4 over distances 100 km each epeirogenic episode involved relatively rapid 10 ma km scale tectonic movements that drove major changes in patterns of sedimentation to find expression in regional unconformity bounded stratigraphic units early cenozoic tilting late paleocene to early eocene c 60 50 ma caused the basinward progradation of shelf slope wedges from elongate uplifts along the inner continental margin and from offshore highs mid cenozoic sagging late eocene to early oligocene c 35 25 ma ended wedge progradation and caused the onset of contourite deposition in deep water basins late cenozoic tilting early pliocene to present 4 0 5 ma again caused the basinward progradation of shelf slope wedges from uplifts along the inner margin including broad dome like features and from offshore highs the early mid and late cenozoic epeirogenic episodes coincided with atlantic plate reorganisations but the observed km scale tectonic movements are too large to be accounted for as flexural deflections due to intra plate stress variations mantle lithosphere interactions are implied but the succession of epeirogenic episodes of differing form are difficult to reconcile with the various syn to post rift mechanisms of permanent and or transient movements proposed in the hypothetical context of a plume beneath iceland the epeirogenic movements can be explained as dynamic topographic responses to changing forms of small scale convective flow in the upper mantle tilting as coeval upwelling and downwelling above an edge driven convection cell sagging as a loss of dynamic support above a former upwelling the inferred cenozoic succession of epeirogenic tilting sagging and tilting is proposed to record the episodic evolution of upper mantle convection during ocean opening a process that may also be the underlying cause of plate reorganisations the postulated episodes of flow reorganisation in the ne atlantic region have testable implications for epeirogenic movements along the adjacent oceanic spreading ridge and conjugate continental margin as well as on other atlantic type passive margins

the response of high latitude convection to a sudden southward imf turning we describe the response of the high latitude convection to a sudden southward turning of the imf in this event bz recorded at the wind satellite switched from 6 nt to 6 nt in 2 minutes and effects in the ionosphere were observed by the superdarn hf radars in the vicinity of noon mlt the convection switched suddenly 2 min from sunward to antisunward with the velocity change reaching magnitudes as great as 1500 m s in about 6 minutes surprisingly the responses at the other mlts distributed from noon to midnight began at virtually the same time meridional flows near 22 mlt were observed to reverse direction in about 4 minutes with the velocity change reaching magnitudes as great as 750 m s in about 8 minutes this example shows that the convection can respond to a sudden imf change practically simultaneously 2 min over the entire high latitude zone and reconfigure from a pattern characteristic of bz to one characteristic of bz in a very short 2 4 min time

empirical high latitude electric field models electric field measurements from the dynamics explorer 2 satellite have been analyzed to extend the empirical models previously developed from dawn dusk ogo 6 measurements j p heppner 1977 the analysis embraces large quantities of data from polar crossings entering and exiting the high latitudes in all magnetic local time zones paralleling the previous analysis the modeling is based on the distinctly different polar cap and dayside convective patterns that occur as a function of the sign of the y component of the interplanetary magnetic field the objective which is to represent the typical distributions of convective electric fields with a minimum number of characteristic patterns is met by deriving one pattern model bc for the northern hemisphere with a y interplanetary magnetic field imf and southern hemisphere with a y imf and two patterns models a and de for the northern hemisphere with a y imf and southern hemisphere with a y imf the most significant large scale revisions of the ogo 6 models are 1 on the dayside where the latitudinal overlap of morning and evening convection cells reverses with the sign of the imf y component 2 on the nightside where a westward flow region poleward from the harang discontinuity appears under model bc conditions and 3 magnetic local time shifts in the positions of the convection cell foci the modeling above was followed by a detailed examination of cases where the imf z component was clearly positive northward neglecting the seasonally dependent cases where irregularities obscure pattern recognition the observations range from reasonable agreement with the new bc and de models to cases where different characteristics appeared primarily at dayside high latitudes the analysis of these differences for strong bz conditions using passes through different mlt zones led to finding that the deviations from both the bc and de models could be modeled by extending the evening cell foci toward and beyond noon with a deformation that roughly resembles a rotational twist of the extended evening cell the morning cell accommodation to the model bc deformation produces a narrow tongue of eastward convection near 75 invariant latitude on the dayside the model de deformation compresses the morning cell toward the night hours such that the sunward convection in the morning cell is diverted to polar latitudes in the 0400 to 0700 mlt sector in both cases the deformations of the two cell patterns lead to sunward convection in dayside polar regions while maintaining the integrity of the nightside convection pattern thus the nightside dilemmas that plague three and four cell models designed to explain sunward convection in polar regions under bz conditions do not appear

response of intracontinental deformation in the central andes to late cenozoic reorganization of south american plate motions new stratigraphic chronologic and fault kinematic data from the quebrada del toro in northwestern argentina provide a detailed view of late cenozoic kinematic reorganization in the southern central andes the quebrada del toro is a thrust bounded basin filled with 2 km of synorogenic clastic deposits ranging in age from early miocene to holocene as observed elsewhere in northwestern argentina faults in the quebrada del toro record an early phase of horizontal nw se contraction beginning between early and late miocene and ending after 0 98 ma and a later phase of horizontal ne sw contraction beginning between late miocene and 4 17 ma and still active these results require both kinematic regimes to have been active between 4 17 and 0 98 ma compatible with timing and kinematic data from the adjacent central andes and best explained by a temporal pattern of serial recurrence of the kinematic regimes a hypothesis that intraplate deformation in the central andes is driven by the absolute motion of the south american plate with respect to the hotspot reference frame is tested by comparing quebrada del toro kinematics to plate reconstructions and space based geodetic results nazca south america relative motion and absolute nazca plate motion show consistent azimuths throughout the past 20 5 myr in contrast absolute south american plate motion azimuths mimic the changing contraction directions determined from intraplate andean faults although the timing of the south american plate motion change is poorly resolved by current data it is compatible with the quebrada del toro data absolute upper plate motion might control intraplate kinematics along many of the earth s convergent margins