global marine gravity from retracked geosat and ers 1 altimetry ridge segmentation versus spreading rate 1 three approaches are used to reduce the error in the satellite derived marine gravity anomalies first we have retracked the raw waveforms from the ers 1 and geosat gm missions resulting in improvements in range precision of 40 and 27 respectively second we have used the recently published egm2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean third we have used a biharmonic spline interpolation method to construct residual vertical deflection grids comparisons between shipboard gravity and the global gravity grid show errors ranging from 2 0 mgal in the gulf of mexico to 4 0 mgal in areas with rugged seafloor topography the largest errors of up to 20 mgal occur on the crests of narrow large seamounts the global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate for rates less than about 60 mm a the typical ridge segment is 50 80 km long while it increases dramatically at higher rates 100 1000 km this transition spreading rate of 60 mm a also marks the transition from axial valley to axial high we speculate that a single mechanism controls both transitions candidates include both lithospheric and asthenospheric processes

relative ages of loess and till in two quaternary palaeosols in gorges valley mount kenya east africa two compound palaeosol profiles formed in loess and till were studied in the oldest early quaternary end moraine gorges age system on mount kenya although both tills appear to have similar weathering histories the overlying loessic sediments serving as secondary parent materials for the palaeosols are considered to have different ages on the basis of colour presence or absence of clay films and field texture especially percentage of silt palaeomagnetic determinations showed both tills to have reversed remanent magnetism suggesting that they were emplaced during the matuyama chron the overlying loessic sediments in profiles gor55 and gor58 show normal magnetism and grade upward into the lower zone of bioturbation in the a horizon complex however in gor58 the surface loessic sediments have normal remanent magnetism with considerable secular variations suggesting it was emplaced over a longer time frame during the brunhes chron i e 0 78 ma profile gor55 contains larger amounts of gibbsite indicating more aggressive leaching over time and an absence of secular magnetic variations suggests deposition over a shorter time interval the study of volcanic feldspars and ash using scanning electron microscopy in the two profiles shows that sand grain 63 250 m weathering is slightly different in the two tills relative weathering effects e g corrosion of grain surfaces and neoformation of clay minerals are greater in profile gor55 than in profile gor58 overall the data indicate that some sites in the lower tropical afroalpine timberline environment may have aeolian covers that are considerably younger than the deposits they overlie 1997 by john wiley sons ltd

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

ground motion and tectonics in the terceira island tectonomagmatic interactions in an oceanic rift terceira rift azores triple junction abstract the interpretation of high resolution topography bathymetry gps and insar data and detailed structural geology indicate that 1 terceira developed at the intersection of two major volcano tectonic lineaments wnw ese local tr s direction and nnw sse submarine chain of volcanoes here firstly recognised and coined terceira seamount chain 2 terceira is affected by four main fault systems the ca n165 normal faults dipping to east and west mostly across the middle of the island the n140 normal faults mostly making up the lajes graben the n110 faults with oblique striations normal dextral making up the main volcanic lineament and the more subtle n70 the transform direction related to the nubia eurasia plate boundary seismicity gps data and faults displacing the topography indicate that all systems are active 3 the whole island is subsiding at a rate of ca 5 mm yr as attested by both gps and insar data which is exceptionally high for the azores islands common explanations like thermal contraction or bending of the lithosphere or magmatic processes or collapse of the island under its own weight likely cannot justify the observed subsidence rate the estimated average of tr s subsidence rate is also not enough therefore we conclude that the measured 5 mm yr can be a peak 4 the fault geometry and kinematics are consistent with the current direction of maximum extension in the azores ca n65 and the rotation of nubia relative to eurasia 5 given that the ne shoulder of the lajes graben is moving upwards at 5 mm yr and sits directly on the tr s ne shoulder we conclude that the tr s shoulder is moving up most likely as a result of the elastic rebound associated with rifting the elastic rebound in both ne and sw tr s shoulders is most likely responsible for the observed ridge morphology all along the tr

thermal stresses and the bimodal distribution of elastic thickness estimates of the oceanic lithosphere observations of plate flexure beneath seamounts and at deep sea trenches suggest that the mechanical strength of the oceanic lithosphere increases with age the mechanical thickness h seems to follow the depth to a particular isotherm in the cooling plate model a detailed review of available estimates of elastic thickness te shows that the strength deduced from studies at seamounts is significantly different from the strength inferred at trenches and fracture zones whereas te beneath seamounts approximately follows the 400 c isotherm h at trenches follows the depth to the 700 800 c isotherms the cause of this bimodal distribution is not fully understood thermal bending stresses due to cooling of the lithosphere have been suggested to play an important role in the flexural deformation at transform faults and fracture zones and their distribution with depth is consistent with intraplate seismicity this paper evaluates the effects of thermal stresses on deformation beneath seamounts and at trenches and how their interaction with flexural stresses may modify plate strength the results indicate that thermal bending stresses will bias the moment curvature relationship such that a plate of a certain strength will appear relatively weaker beneath a seamount positive curvature but relatively stronger at a trench negative curvature this systematic bias appears to be in qualitative agreement with published te estimates for old lithosphere the theory predicts a diminishing amount of bias the general lack of sufficient te determinations from old ocean basins and the uncertainties associated with modeling of trenches prohibit a detailed analysis of this part of the te versus age relationship i conclude that the scatter in the elastic thickness estimates appears to be caused by a combination of lithospheric reheating during seamount emplacement bias introduced by thermal stresses and variable amount of yielding of the plate beneath seamounts in proportion to seamount size and lithospheric age at the time of loading

intraplate seismicity and stresses in young oceanic lithosphere we have conducted a systematic study of oceanic intraplate seismicity in the period 1964 1983 occurring in lithosphere younger than 35 million years focal mechanisms determined for 26 events using first motions p wave modeling and surface wave amplitude radiation patterns along with 13 previously determined mechanisms indicate a diversity of faulting styles and stress orientations in young oceanic lithosphere there is no evidence for a worldwide transition from extension to compression at a specific lithospheric age tensional axes of normal faulting events are oriented oblique to the spreading direction and do not show extension in the spreading direction congressional axes of thrust events show a weak preferred orientation in the spreading direction presumably due to ridge push stresses however stress axes of thrust events in very young lithosphere age 9 ma do not show a correlation with the spreading direction suggesting that the ridge push force is small near the ridge these observations indicate that tensional stress in the spreading direction is concentrated in a very narrow zone at the ridge axis in agreement with models which assume that the axial region has very little tensile strength normal faulting events occur at greater depths and temperatures than thrust events possibly because rocks under tensional stress have strength maxima at higher temperatures alternatively the depths of the normal faulting events may be a result of the higher thermal gradients at depth in young lithosphere if thermoelastic stresses are important in generating this seismicity differential thermal contraction along fracture zones does not seem to be a significant source of near ridge intraplate seismicity high seismicity rates in young lithosphere noted previously on a global scale seem characteristic of all three major ocean basins additionally most of the total oceanic intraplate moment release takes place in young lithosphere however most of the events and more than 96 of the total moment release of the near ridge intraplate seismicity are contained in five areas two zones in the indian ocean one near chagos bank and the other in the amsterdam st paul region indicate tension parallel to the trend of the plate boundary a region of high seismicity near the east pacific rise between 3 and 7 n shows both normal and thrust faulting while another region on the cocos plate west of the panama fracture zone is characterized by strike slip faulting thrust strike slip and normal faulting events are found in a small seismically active zone of the south central pacific the extremely uneven distribution of seismicity if not a result of the relatively short time period studied 20 years indicates local perturbations strongly affect near ridge seismicity

late neogene volcanics and interbedded palaeosols near mount kenya abstract two lava flows with interbedded palaeosols outcrop c 40 km sw of mount kenya near the amboni river north of mweiga kenya along the nyeri thompson falls road at 0 18 s 37 48 e these flows overlain by loess are principally trachyandesite and form the base of the mount kenya volcanic series which in the early literature is described as being of probable miocene pliocene age here we report 39 ar 40 ar dates c 5 2 5 5 ma and reversed magnetizations which establish a latest miocene to earliest pliocene age for these flows weathering characteristics of palaeosols interbedded with the lavas indicate generally dry climatic conditions during the late miocene punctuated with humid events during the pliocene and quaternary these late miocene quaternary palaeosols depict a relatively long and complex weathering history followed by loess deposition the palaeosols appear to have been episodically deflated initially in phase with the deposition of lavas when surfaces were devoid of vegetation and later during periods of climatic deterioration when wind systems intensified such weathering histories within palaeosol profiles are also documented on nearby mount kenya where well weathered lower palaeosol horizons developed on matuyama age tills are overlain by much younger less weathered horizons developed on brunhes age loess the geochronology of late miocene lavas reported here provides maximum ages for weathering histories of palaeosols formed in a xeric tropical highland climate

oblique reflections in the mars express marsis data set stable density structures in the martian ionosphere the mars advanced radar for subsurface and ionospheric sounding marsis onboard the european space agency s mars express mex spacecraft routinely detects evidence of localized plasma density structures in the martian dayside ionosphere such structures likely taking the form of spatially extended elevations in the plasma density at a given altitude give rise to oblique reflections in the active ionospheric sounder data these structures are likely related to the highly varied martian crustal magnetic field in this study we use the polar orbit of mex to investigate the repeatability of the ionospheric structures producing these anomalous reflections examining data taken in sequences of multiple orbits which pass over the same regions of the martian surface under similar solar illuminations within intervals lasting tens of days presenting three such examples or case studies we show for the first time that these oblique reflections are often incredibly stable indicating that the underlying ionospheric structures are reliably reformed in the same locations and with qualitatively similar parameters the visibility or lack thereof of a given oblique reflection on a single orbit can generally be attributed to variations in the crustal field within the ionosphere along the spacecraft trajectory we show that within these examples oblique reflections are generally detected whenever the spacecraft passes over regions of intense near radial crustal magnetic fields i e with a cusp like configuration the apparent stability of these structures is an important feature that must be accounted for in models of their origin

observations of the latitude dependence of the location of the martian magnetic pileup boundary 1 we report observations that show the dependence of the altitude of the magnetic pileup boundary mpb at mars on planetary latitude as seen by the mars global surveyor magnetometer electron reflectometer instrument the mpb is further away from mars on average at southern latitudes than at northern latitudes the data are consistent with a mpb distance mapped to the terminator plane that does not vary with latitude in the northern hemisphere but increases with increasing southern latitude in the southern hemisphere we also report increased variability in the mpb distance within the longitude range 90 270 e longitude in the southern hemisphere which is the region that contains the strongest crustal magnetic fields these trends are most obvious in a planet fixed coordinate system indicating a planet fixed driver of the mpb location the proposed mechanism is the local diversion of shocked solar wind flow by crustal magnetic fields

new constraints on relative motion between the pacific plate and baja california microplate mexico from gps measurements summary we present a new surface velocity field for baja california using gps data to test the rigidity of this microplate calculate its motion in a global reference frame determine its relative motion with respect to the north american and the pacific plates and compare those results to our estimate for pacific north america motion determination of pacific plate motion is improved by the inclusion of four sites from the south pacific sea level and climate monitoring project these analyses reveal that baja california moves as a quasi rigid block but at a slower rate in the same direction as the pacific plate relative to north america this is consistent with seismic activity along the western edge of baja california the baja california shear zone and may reflect resistance to motion of the eastern edge of the pacific plate caused by the big bend of the san andreas fault and the transverse ranges in southern california

bounds on geologically current rates of motion of groups of hot spots it is widely believed that groups of hot spots in different regions of the world are in relative motion at rates of 10 to 30 mm a 1 or more here we present a new method for analyzing geologically current motion between groups of hot spots beneath different plates in an inversion of 56 globally distributed equally weighted trends of hot spot tracks the dispersion is dominated by differences in trend between different plates rather than differences within plates nonetheless the rate of hot spot motion perpendicular to the direction of absolute plate motion vperp differs significantly from zero for only 3 of 10 plates and then by merely 0 3 to 1 4 mm a 1 the global mean upper bound on vperp is 3 2 2 7 mm a 1 therefore hot spots move slowly and can be used to define a global reference frame for plate motions

shapes of magnetically controlled electron density structures in the dayside martian ionosphere nonhorizontal localized electron density structures associated with regions of near radial crustal magnetic fields are routinely detected via radar oblique echoes on the dayside of mars with the ionospheric sounding mode of the mars advanced radar for subsurface and ionospheric sounding marsis radar onboard mars express previous studies mostly investigated these structures at a fixed plasma frequency and assumed that the larger apparent altitude of the structures compared to the normal surrounding ionosphere implied that they are bulges however the signal is subjected to dispersion when it propagates through the plasma so interpretations based on the apparent altitude should be treated with caution we go further by investigating the frequency dependence i e the altitude dependence of the shape of 48 density structure events using time series of marsis electron density profiles corrected for signal dispersion four possible simplest shapes are detected in these time series which can give oblique echoes bulges dips downhill slopes and uphill slopes the altitude differences between the density structures and their edges are in absolute value larger at low frequency high altitude than at high frequency low altitude going from a few tens of kilometers to a few kilometers as frequency increases bulges dominate in numbers in most of the frequency range finally the geographical extension of the density structures covers a wide range of crustal magnetic fields orientations with near vertical fields toward their center and near horizontal fields toward their edges as expected transport processes are suggested to be a key driver for these density structures

effects of crustal field rotation on the solar wind plasma interaction with mars the crustal remnant field on mars rotates with the planet at a period of 24 h 37 min constantly varying the magnetic field configuration interacting with the solar wind until now there has been no self consistent modeling investigation on how this varying magnetic field affects the solar wind plasma interaction here we include the rotation of this localized crustal field in a multispecies single fluid mhd model of mars and simulate an entire day of solar wind interaction under normal solar wind conditions the mhd model results are compared with mars global surveyor mgs magnetic field observations and show very close agreement especially for the field strength along almost all of the 12 orbits on the day simulated model results also show that the ion escape rates slowly vary with rotation generally anticorrelating with the strength of subsolar magnetic crustal sources with some time delay in addition it is found that in the intense crustal field regions the densities of heavy ion components enhance significantly along the mgs orbit implying strong influence of the crustal field on the ionospheric structures

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

stratigraphy of the gorges moraine system mount kenya palaeosol and palaeoclimate record moraines marking an early pleistocene glaciation on mt kenya are known from several valleys along the eastern and southeastern flanks of the mountain the most prominent group of end moraines delimiting the lowermost extent of the early pleistocene gorges glaciation 2900 m above sea level overlie either older till or weathered bedrock the latter composed of thin pedostratigraphic remnants covered with locally derived aeolian sediments truncated by the ingress of ice the loess tills and interbedded palaeosols of the gorges glaciation crop out adjacent to the nithi river which drains the gorges valley and abut the upper timberline of montane forest the lowermost palaeosols at these sites were formed either in till predating the gorges glaciation or in weathered bedrock of trachytic textured lavas similar to the lithology forming the base of the mt kenya volcanic series of miocene or pliocene age palaeomagnetism and weathering characteristics are here used to refine the age of sediments assigned to the gorges glaciation these normally magnetized deposits carry a persistent reversed overprint suggesting that they were deposited during one of the normal subchrons within the matuyama reversed chron they are underlain either by a reversely magnetized till or by weathered bedrock and lower palaeosol the latter exhibiting normal magnetization gauss with reversed overprint matuyama the sediments of the gorges glaciation are overlain at all three sites by normally magnetized loesses and palaeosols presumably of brunhes age but carrying a high percentage of well weathered recycled grains the normal magnetization and reversed overprint recorded in sediments of the gorges glaciation most probably span a considerable portion of the olduvai subchron 1 78 1 950 ma which persisted for sufficient time to accommodate an extensive montane glaciation followed by a prolonged period of weathering and soil formation the importance of these sediments in global palaeoclimate reconstruction and insolation changes is discussed the stratigraphic evidence contradicts the cosmogenic exposure ages presented by previous workers attempting to obtain true ages of deposits using exhumed coarse clastic sediment without analysis of complex pedostratigraphic complexes supplementary materials tables 1s 5s are available at www geolsoc org uk sup18592

the magnetic field draping direction at mars from april 1999 through august 2004 using more than five years of data from the magnetometer and electron reflectometer mag er on mars global surveyor mgs we derive the draping direction of the magnetic field above a given latitude band in the northern hemisphere the draping direction varies on timescales associated with the orbital period of mars and with the solar rotation period we find that there is a strongly preferred draping direction when mars is in one solar wind sector but the opposite direction is not preferred as strongly for the other solar wind sector this asymmetry occurs at or below the magnetic pileup boundary mpb is observed preferentially on field lines that connect to the collisional ionosphere and is independent of planetary longitude the observations could be explained by a hemispherical asymmetry in the access of field lines to the low altitude ionosphere or possibly from global modification of the low altitude solar wind interaction by crustal magnetic fields we show that the draping direction affects both the penetration of sheath plasma to 400 km altitudes on the martian dayside and the radial component of the magnetic field on the planetary night side

initial results from the maven mission to mars the mars atmosphere and volatile evolution maven mars orbiter has been gathering information on the mars upper atmosphere ionosphere and solar and solar wind interactions since its orbit insertion in september 2014 maven s science goals are to understand processes driving the escape of atmospheric gases to space at the present epoch and their variations with solar and local heliospheric conditions together with geographical and seasonal influences this introduction and the accompanying articles provide a selection of key results obtained up to the time of writing including measurements of the overall geometry and variability of the martian magnetosphere upper atmosphere and ionosphere and their responses to interplanetary coronal mass ejections and solar energetic particle influxes the ultimate goal is to use these results to determine the integrated loss to space through time and its role in overall mars atmosphere evolution

a proxy for determining solar wind dynamic pressure at mars using mars global surveyor data 1 we develop a method to infer the upstream solar wind pressure at mars using data from the mars global surveyor mgs magnetometer instrument the method assumes magnetic field pressure in the magnetic pileup region balances the incident solar wind dynamic pressure newtonian pressure balance conditions are imposed for positions off of the subflow point proof of concept exists from the mapping phase of the spacecraft mission when the orbital geometry at a constant altitude allows fits to magnetic field data as a function of solar zenith angle during the elliptical orbit phase of mgs when the spacecraft passes through a large range of altitudes such fits are not possible however the measured magnetic pressure in the magnetic pileup region is scaled by the same means as derived for the mapping orbits and reasonable upstream solar wind pressure is obtained we compare the distribution of solar wind pressure calculated using mgs data with the solar wind pressure measured upstream of the earth during the same time period and find good agreement further supporting this algorithm as a valid proxy for the incident solar wind pressure the response of the position of the magnetic pileup boundary to the inferred solar wind pressure also validates the method high solar wind ram pressure depresses the altitude of the magnetic pileup boundary

warping and cracking of the pacific plate by thermal contraction 1 lineaments in the gravity field and associated chains of volcanic ridges are widespread on the pacific plate but are not yet explained by plate tectonics recent studies have proposed that they are warps and cracks in the plate caused by uneven thermal contraction of the cooling lithosphere we show that the large thermoelastic stress produced by top down cooling is optimally released by lithospheric flexure between regularly spaced parallel cracks both the crack spacing and approximate gravity amplitude are predicted by elastic plate theory and variational principle cracks along the troughs of the gravity lineaments provide conduits for the generation of volcanic ridges in agreement with new observations from satellite derived gravity our model suggests that gravity lineaments are a natural consequence of lithospheric cooling so that convective rolls or mantle plumes are not required index terms 1208 geodesy and gravity crustal movements intraplate 8110 1236 geodesy and gravity rheology of the lithosphere and mantle 8160 1219 geodesy and gravity local gravity anomalies and crustal structure 3045 marine geology and geophysics seafloor morphology and bottom photography 8159 tectonophysics rheology crust and lithosphere keywords thermal contraction pacific plate gravity anomalies

strain rate dependency of oceanic intraplate earthquake b values at extremely low strain rates we discovered a clear positive dependence of oceanic intraplate earthquake oceq b values on the age of the oceanic lithosphere oceq b values in the youngest 20 ma oceanic lithosphere exceed 1 5 which is significantly higher than the average worldwide earthquake b value around 1 0 on the other hand the b value of intraplate earthquakes in the ninety east sumatra orogen where oceanic lithosphere has an anomalously higher strain rate compared with normal oceanic lithosphere is 0 93 which is significantly lower than the oceq b value about 1 9 with the same age 50 110 ma thus the variation in b values relates to the strain rate of the oceanic lithosphere and is not caused by a difference in thermal structure we revealed a negative strain rate dependency of the b value at extremely low strain rates 1 5 in oceanic lithosphere 20 ma old imply that future improvement in seismic observation will capture many smaller magnitude oceqs which will provide valuable information on the evolution of the oceanic lithosphere and the driving mechanism of plate tectonics

the plate tectonic approximation plate nonrigidity diffuse plate boundaries and global plate reconstructions the central assumption of plate tectonics that plate interiors are rigid remains a useful but uncertain approximation strain rates of stable plate interiors are bounded between 10 i12 10 i11 year i1 and 4 10 i10 year i1 the narrowness of all plate boundaries the other main assumption of plate tectonics as originally conceived is contradicted by many observations both in the continents and in the oceans some diffuse plate boundaries in both continents and oceans exceed dimensions of 1000 km on a side diffuse plate boundaries cover 15 of earth s surface the maximum speed of relative plate motion across any one diffuse plate boundary ranges from 2t o 15 mm year which is faster than some upper bounds on intraplate motion across stable plate interiors 2m m year i1 strain rates in diffuse plate boundaries can be as high as 10 i8 year i1 25 times higher than the upper bound on strain rates of stable plate interiors but 600 times lower than the lowest strain rates across typical narrow plate boundaries the poles of rotation of the plates flanking a diffuse oceanic plate boundary tend to be located in the diffuse boundary which is a consequence of the strong coupling across the boundary

active pacific north america plate boundary tectonics as evidenced by seismicity in the oceanic lithosphere offshore baja california mexico pacific ocean crust west of southwest north america was formed by cenozoic seafloor spreading between the large pacific plate and smaller microplates the eastern limit of this seafloor the continent ocean boundary is the fossil trench along which the microplates subducted and were mostly destroyed in miocene time the pacific north america plate boundary motion today is concentrated on continental fault systems well to the east and this region of oceanic crust is generally thought to be within the rigid pacific plate yet the 2012 december 14 m w 6 3 earthquake that occurred about 275 km west of ensenada baja california mexico is evidence for continued tectonism in this oceanic part of the pacific plate the preferred main shock centroid depth of 20 km was located close to the bottom of the seismogenic thickness of the young oceanic lithosphere the focal mechanism derived from both teleseismic p wave inversion and w phase analysis of the main shock waveforms and the 12 aftershocks of m 3 4 are consistent with normal faulting on northeast striking nodal planes which align with surface mapped extensional tectonic trends such as volcanic features in the region previous global positioning system gps measurements on offshore islands in the california continental borderland had detected some distributed pacific and north america relative plate motion strain that could extend into the epicentral region the release of this lithospheric strain along existing zones of weakness is a more likely cause of this seismicity than current thermal contraction of the oceanic lithosphere or volcanism the main shock caused weak to moderate ground shaking in the coastal zones of southern california usa and baja california mexico but the tsunami was negligible

pacific plate gravity lineaments diffuse extension or thermal contraction 1 we detect strain very close to zero on the pacific plate relative to the nazca plate for anomalies c8 c13 our observations are in the region of the satellite gravity lineaments 10 40 s in the central pacific strain was measured by comparing fracture zone spacing on the pacific and nazca plates in an approach similar to that of goodwillie and parsons 1992 but with an improved data set which allowed us to place tighter tolerances on estimated strain we used a standard finite rotation technique that rotates points of a given isochron and corresponding active transform fault from the pacific plate to the nazca plate our strain measurements discount the diffuse extension hypothesis for gravity lineament formation which requires at least 10 strain sandwell et al 1995 the best known hypothesis that small scale convection rolls in the upper mantle aligned in the direction of plate motion produce the lineaments was rejected because of observations of seamount chains in the gravity lows and observations of lineaments in seafloor topography younger than 2 3 ma we propose a third hypothesis that the gravity lineaments are signatures of the bending stresses resulting from vertical variations in cooling rate we interpret the lineament lows as areas of brittle failure and the highs as convex upward flexure due to greater contraction of the base of the plate association of the lows with volcanoes could result either from both being caused by cracks or by the load of volcanoes causing brittle failure we explored the feasibility of this idea using the equations for thermal bending moment and vertical lithosphere displacement as presented by turcotte 1974 and the equation for thermal stress over elastic thickness presented by parmentier and haxby 1986 results of this simple model show vertical displacement or convex upward flexure ranging from 80 m to 140 m with associated gravity anomalies of 6 10 mgal these amplitudes are within a factor of 2 of topographic and free air gravity anomaly amplitudes observed in the region of the gravity lineaments

horizontal thermal contraction of oceanic lithosphere the ultimate limit to the rigid plate approximation average strain rate inferred from seismic moment release newly created i e 0 1 ma old lithosphere is displaced by thermal contraction toward lithosphere in old ocean basins at a rate that is 1 35 of the half rate of seafloor spreading giving displacement rates of 0 1 to 1 1 km ma 1 or equivalently 0 1 to 1 1 mm a 1 predicted displacement rates parallel to midocean ridges depend strongly on lithospheric age and are proportional to the distance along which contractional strain rates are integrated displacement rates can be significant 1k m ma 1 for young lithosphere in particular the displacement of oceanic lithosphere adjacent to southern baja california relative to lithosphere near the pacific antarctic rise has an indicated north south displacement rate between 3 and 10 km ma 1 this suggests that global plate motion circuits based on the assumption of plate rigidity may be biased by a substantial velocity

magnetic lineations in the ancient crust of mars the mars global surveyor spacecraft in a highly elliptical polar orbit obtained vector magnetic field measurements above the surface of mars altitudes 100 kilometers crustal magnetization mainly confined to the most ancient heavily cratered martian highlands is frequently organized in east west trending linear features the longest extending over 2000 kilometers crustal remanent magnetization exceeds that of terrestrial crust by more than an order of magnitude groups of quasi parallel linear features of alternating magnetic polarity were found they are reminiscent of similar magnetic features associated with sea floor spreading and crustal genesis on earth but with a much larger spatial scale they may be a relic of an era of plate tectonics on mars

tests of the rigid plate hypothesis and bounds on intraplate deformation using geodetic data from very long baseline interferometry the geodetic results of ma et al 1994 from very long baseline interferometry are used to assess rates of intraplate deformation we analyze the resulting set of velocities of 18 plate interior sites nine on the north american plate four western european sites on the eurasian plate three on the pacific plate and two on the australian plate we formulate three alternative error budgets based partly on the work of ryan et al 1993a an optimistic error budget a conservative error budget and an upper bound error budget all three budgets assume the existence of a previously unmodeled error that is inversely proportional to the time span of observations at a site use of the conservative error budget indicates no significant motion between sites on any one of the four plates use of the upper bound error budget designed to be precisely large enough to be consistent with rigid plates places a usefully small upper bound on how fast any site might be moving relative to the rest of its plate speeds faster than 2 mm yr are excluded at the 95 confidence level for seven of the 18 plate interior sites adjustments of the site velocities for motion due to postglacial rebound as predicted from the model of tushingham and peltier 1991 give an insignificant improvement to the fit of the horizontal velocities to the rigid plate model the velocities of six sites near the western margin of the interior of the north american plate were also analyzed use of the conservative error budget indicates that a site near fairbanks alaska has an upper bound of 3 mm yr on its speed a site in nome alaska has an upper bound on its speed of 8 mm yr and a site in penticton british columbia has an upper bound on its speed of 4 mm yr of three sites on the colorado plateau two move marginally significantly and one moves insignificantly relative to stable north america upper limits on their speeds relative to stable north america range from 4 to 5 mm yr we conclude that the data do not yet demonstrate significant velocities between sites in the stable interior of any one plate but do provide useful and small upper bounds of 2 mm yr or less on the speeds of the best observed intraplate sites six sites near the western margin of the north american interior have upper bound speeds of 3 to 8 mm yr

tectonic implications of mars crustal magnetism mars currently has no global magnetic field of internal origin but must have had one in the past when the crust acquired intense magnetization presumably by cooling in the presence of an earth like magnetic field thermoremanent magnetization a new map of the magnetic field of mars compiled by using measurements acquired at an 400 km mapping altitude by the mars global surveyor spacecraft is presented here the increased spatial resolution and sensitivity of this map provide new insight into the origin and evolution of the mars crust variations in the crustal magnetic field appear in association with major faults some previously identified in imagery and topography cerberus rupes and valles marineris two parallel great faults are identified in terra meridiani by offset magnetic field contours they appear similar to transform faults that occur in oceanic crust on earth and support the notion that the mars crust formed during an early era of plate tectonics

thermal contraction and the state of stress in the oceanic lithosphere nonuniform contraction of the oceanic lithosphere as it cools and thickens following its formation at the axis of a spreading center results in a complex three dimensional state of deviatoric stress which can be separated into two parts a thermal bending stress due to changes in the vertical temperature distribution and a thermal contraction stress due to lateral variations in the vertically averaged temperature we examine thermal contraction stresses due to temperature changes in a thin semi infinite rectangular plate bounded by the spreading center axis two stress free boundaries representing transforms or fracture zones define the plate width or ridge segment length l with the bottom of the elastic plate defined by a prescribed temperature the plate thickens as the square root of age as expected for a thermal boundary layer due to vertical conductive cooling above this prescribed temperature elastic stresses are assumed to relax quickly stresses are obtained by properly accounting for the rate of accumulation of the vertically averaged stress as initially stress free material is added to bottom of the cooling thickening plate the state of thermal contraction stress calculated from this model is characterized by large tensile stresses at the boundaries of the plate and relatively low stresses in the plate interior at the ridge axis ridge parallel tensile stresses are about 300 mpa the same as if the plate were not allowed to contract in this direction along the transform boundary the maximum transform parallel tensile stress occurs at a distance l 2 from the ridge transform intersection where its magnitude is comparable to the stress at the ridge axis the tangential stresses at the plate boundary decrease rapidly with distance from the boundary at a distance of l 4 from the ridge axis the ridge axis parallel stresses are one tenth of their ridge axis magnitude the stress magnitudes are independent of both spreading rate and ridge segment length a large transform parallel tensile stress may control the length of transform offsets thermal bending moments are influenced by the large thermal contraction stresses near the ridge axis however a short distance from the ridge these moments attain their free horizontal contraction values which previous studies have shown to cause observable bending of the plate and a geoid anomaly at fracture zones flexure due to thermal bending moments will concentrate bending stresses at a distance from the fracture zone determined by the flexural length of the plate thus providing a natural length scale controlling the spacing of transforms

anisotropic horizontal thermal contraction of young oceanic lithosphere inferred from stress release due to oceanic intraplate earthquakes how freely the oceanic lithosphere contracts horizontally due to thermal contraction is important information because it reflects the boundary condition of the oceanic lithosphere which includes information regarding the magnitude of driving resisting forces of plate tectonics we investigated the horizontal thermal contraction of young oceanic lithosphere using an analysis of the intraplate stress release due to oceanic intraplate earthquakes oceqs and numerical simulations the stress release due to oceqs in young oceanic lithosphere 5 15 ma shows significant differences between the spreading directional component and the ridge parallel component the extensional stress release of the ridge parallel component is six times as large as that of the spreading directional component while the compressional stress release of the ridge parallel component is one seventh that of the spreading directional component we conducted a numerical simulation of the thermal stress evolution of the oceanic lithosphere to investigate how the difference in the horizontal contraction rates between the spreading direction and the ridge parallel direction can explain the observed anisotropic stress release the result indicates that young oceanic lithosphere 5 15 ma barely contracts in the ridge parallel direction only 0 30 of the spreading directional contraction rate while it contracts freely in the spreading direction due to the weakness of the oceanic ridge strength and the low viscosity asthenosphere from the results we constrained the magnitude of the basal traction working on the bottom of the oceanic lithosphere to be smaller than 0 44 mpa

a new gps velocity field for the pacific plate part 1 constraints on plate motion intraplate deformation and the viscosity of pacific basin asthenosphere

thermoelastic stress how important as a cause of earthquakes in young oceanic lithosphere we present a series of exploratory models which test the hypothesis that thermoelastic stress is a significant contributor to the state of stress in young oceanic lithosphere less than 35 m y in age in support of this hypothesis is the concentration of seismicity in lithosphere younger than 15 m y where cooling rates are relatively high most near ridge earthquakes have focal depths between the moho and the depth of the 800 c isotherm thrust and strike slip events dominate the shallowest seismicity while the focal mechanisms of the deeper events are generally characterized by normal faulting to assess the importance of thermal stress in the generation of near ridge earthquakes we model the lithosphere as an elastic half space in which the response to cooling is computed using the method of thermoelastic displacement potentials a key assumption in the models is that stress is relieved on time scales generally short compared with the age of the lithosphere a further assumption in some models is that material will not contribute thermal stress until it has cooled below an elastic blocking temperature normal faulting is predicted throughout the lithosphere in thermal stress models based on simple half space cooling models in which a cooled surface layer is incorporated to simulate the effect of hydrothermal circulation on shallow thermal structure predict stresses that can match the locations of both thrust and normal faulting earthquakes near mid ocean ridges the addition of a ridge push stress to the computed thermal stress enhances the likelihood of thrust faulting in the uppermost lithosphere at ages greater than about 15 m y but probably contributes little to the stress field at younger ages though these simple models are limited by several simplifying assumptions particularly in the immediate vicinity of the ridge axis they support the hypothesis that thermoelastic stress can play a major role in the tectonics of young oceanic lithosphere

a 50 degree spherical harmonic model of the magnetic field of mars this paper presents a 50 degree and order spherical harmonic model of the magnetic field of mars derived at 120 km altitude using the three orthogonal components of the vector magnetic field data acquired by the mars global surveyor within the 80 to 200 km altitude range the downward continued vector magnetic field components to the surface of mars delineate details of the martian magnetic field the strong magnetic anomalies in the southern hemisphere do not show compelling evidence for magnetic lineations similar to those associated with the seafloor spreading on earth the anomalies are more or less equidimensional the magnetic signatures surrounding the impact basins hellas argyre and isidis show that the prominent magnetic anomalies are older than the impact events an average magnetization of 2 a m is estimated for the upper crust surrounding hellas basin four isolated magnetic anomalies are modeled by vertical prisms with circular cross sections of 170 to 190 km radius and 20 to 30 km thickness the magnetization of the model prisms is 5 a m more than that of the surroundings

solar wind interaction with the martian upper atmosphere crustal field orientation solar cycle and seasonal variations a comprehensive study of the solar wind interaction with the martian upper atmosphere is presented three global models the 3 d mars multifluid block adaptive tree solar wind roe upwind scheme mhd code mf mhd the 3 d mars global ionosphere thermosphere model m gitm and the mars exosphere monte carlo model adaptive mesh particle simulator m amps were used in this study these models are one way coupled i e the mf mhd model uses the 3 d neutral inputs from m gitm and the 3 d hot oxygen corona distribution from m amps by adopting this one way coupling approach the martian upper atmosphere ion escape rates are investigated in detail with the combined variations of crustal field orientation solar cycle and martian seasonal conditions the calculated ion escape rates are compared with mars express observational data and show reasonable agreement the variations in solar cycles and seasons can affect the ion loss by a factor of 3 3 and 1 3 respectively the crustal magnetic field has a shielding effect to protect mars from solar wind interaction and this effect is the strongest for perihelion conditions with the crustal field facing the sun furthermore the fraction of cold escaping heavy ionospheric molecular ions 2 and or 2 total are inversely proportional to the fraction of the escaping ionospheric and corona atomic ion o total whereas 2 and 2 ion escape fractions show a positive linear correlation since both ion species are ionospheric ions that follow the same escaping path

effects of the crustal magnetic fields on the martian atmospheric ion escape rate eight years 2007 2015 of ion flux measurements from mars express are used to statistically investigate the influence of the martian magnetic crustal fields on the atmospheric ion escape rate we combine all aspera 3 ima measurements taken during nominal upstream solar wind and solar extreme ultraviolet euv conditions to compute global average ion distribution functions individually for the north south hemispheres and for varying solar zenith angles sza of the strongest crustal magnetic field escape rates are subsequently calculated from each of the average distribution functions the maximum escape rate 4 2 1 2 1024s 1 is found for sza 60 80 while the minimum escape rate 1 7 0 6 1024s 1 is found for sza 28 60 showing that the dayside orientation of the crustal fields significantly affects the global escape rate p 97 however averaged over time independent of sza we find no statistically significant difference in the escape rates from the two hemispheres escape from southern hemisphere 46 18 of global rate

control of mars global atmospheric loss by the continuous rotation of the crustal magnetic field a time dependent mhd study we present a time dependent mhd study of the controlling effects of the mars crustal field on atmospheric escape we calculate globally integrated planetary ion loss rates under quiet solar conditions considering the continuous rotation of crustal anomalies with the planet it is found that the rotating crustal field plays an important role in controlling atmospheric escape significant time variation of 20 and 50 is observed during the entire rotation period for o and for o2 and co2 respectively the control is exerted mainly through two processes first the crustal magnetic pressure over the subsolar regime controls solar wind penetration and mass loading and therefore the escaping planetary ion source there is a strong negative correlation between the magnetic pressure and ion loss with a time lag of 1 h for o and 2 5 h for o2 and co2 second the crustal magnetic pressure near the terminator region controls the cross section area between the induced magnetospheric boundary and 100 km altitude at the terminator the change in day night connection regulates the extent to which planetary ions created on the dayside can be ultimately carried away by the solar wind and escape mars there is a strong positive correlation between the cross section area and ion loss with no significant time lag as the planet rotates the dayside process and the terminator process work together to control the total amount of escaping planetary ions however their relative importance changes with the local time of the strong crustal field region

first measurements of the ionospheric plasma escape from mars before the phobos mission the magnetosphere of mars was relatively unexplored basically all data on the charged particle environment of mars had come from the soviet mars missions mars 2 3 4 and 5 here we report the results of an ion composition experiment on board the soviet phobos 2 spacecraft which allowed us to determine the loss of plasma from the ionosphere of mars because mars has a very small intrinsic magnetic field the ion outflow had been expected to be similar to that of venus surprisingly there were many similarities between the ionospheric outflow from mars and the earth the ion loss from mars results from both ion pick up that results from mass loading of the solar wind in the martian boundary layer and ionospheric o beams of energies up to several kev possibly from upward acceleration processes similar to those observed above the earth s auroral oval a preliminary estimate of the ionospheric outflow from mars indicates that the planet is losing oxygen at a rate of 3 x 1025 ions s 1 this corresponds to an evacuation of its present total atmospheric oxygen content contained in co2 and o2 in less than 100 million years

sand coatings in paleosols evidence of weathering across the plio pleistocene boundary to modern times on mt kenya abstract a comparison of younger post olduvai paleosol horizons with older buried horizons of plio pleistocene age shows that primary and secondary minerals and organic molecules in sand coatings in these paleosols remain intact and provide a record of past weathering events from ir spectra concentrations of organic and some inorganic constituents within these coatings reveal supporting evidence of variable weathering trends from late pliocene to holocene time with some variability hematite and goethite concentrations in sand coatings parallel similar variable concentrations of secondary fe extractable concentrations previously determined for pre and post olduvai paleosols in these mt kenya sections negative correlations between metahalloysite and gibbsite previously determined to result from aggressive leaching of si from metahalloysite in paleosols tend to follow similar distributions in the sand coatings reported here similarly additions of gibbsite in selected paleosol horizons are followed by reductions of lesser concentrations of metahalloysite suggesting that individual horizons within paleosols have independent leaching histories different from the entire paleo pedon as with these paleosols analyzed previously quartz exists in greater proportion within sands recovered from ah and ahb horizons and this mineral has previously been considered to be sourced from the older surrounding african crust rather than from local neogene volcanic sources the presence of vermiculite in sand coatings previously not detected in mt kenya paleosols may be related to the presence of cedar stands juniperus procera within the timberline zone of the mountain a forest species clay mineral correlate that may relate to past forest biome evolution specifically in the pre olduvai paleosols the occurrence of vermiculite in these older paleosols a clay species with a known affinity for conifers birkeland 1999 may indicate a higher timberline during pre olduvai time the outcome of this analysis shows that certain horizons act as a singular open weathering system responding to the throughput of either meteoric or groundwater in selective ways to produce a range of weathering products sourced from primary minerals and selective allochthonous mineral inputs from near and far more importantly sand coatings are shown to provide important paleoenvironmental data parallel to that recovered from macro paleosol systems moreover the wealth of paleoenvironmental data recovered from sand coatings rivals that in weathering rinds and the combined data are applicable to other paleoenvironments both on earth and mars

deformation of indian ocean lithosphere evidence for a highly nonlinear rheological law the width of diffuse oceanic plate boundaries is determined by the rheology of oceanic lithosphere here we apply thin viscous sheet models which have been successfully applied to deformation in several continental deforming zones to investigate the deformation of oceanic lithosphere in the diffuse oceanic plate boundaries between the india capricorn and australia plates we apply kinematic boundary conditions based on the current motion between these plates we neglect buoyancy forces due to plate thinning or thickening and assume that the thin viscous sheet has the same depth integrated nonlinear viscosity coefficient everywhere our initial models have only one adjustable parameter n the power law exponent with n 1 3 10 30 and 100 the predicted width of the deforming zone decreases with increasing n with n 30 explaining the observations this n value is higher than has been estimated for continental lithosphere and suggests that more of the strength of oceanic lithosphere lies in layers deforming by faulting or by dislocation glide than for continental lithosphere to obtain a stress field that better fits the distribution and type of earthquake focal mechanisms in the diffuse oceanic plate boundary we add a second adjustable parameter representing the effect of slab pull stretching the oceanic plate near the sumatra trench we show that an average velocity increment on this boundary segment of 5 mm a 1 relative to the average velocity of the india and australia plates fits the observed distribution of fault types better than velocities of 3 3 mm a 1 or 10 mm a 1

thermal stresses in the oceanic lithosphere evidence from geoid anomalies at fracture zones models for the thermal and mechanical evolution of the oceanic lithosphere predict the progressive development of large thermal stresses in the thickening plate however there has so far been little direct evidence for the magnitude and distribution of thermal stresses we present theoretical models which examine the effect of thermal stresses at fracture zones and show that an anomaly of the predicted form can be observed in geoid profiles which cross fracture zones specifically our models predict the development of thermal bending moments which depend on lithosphere thickness or age and therefore change across fracture zones including the effect of varying thermal bending moments thin plate theory predicts vertical nonisostatic displacements of the lithosphere by plate flexure the predicted amplitude of the resulting geoid anomaly is large enough to be observed in seasat altimeter profiles furthermore the general form of this anomaly differs sufficiently from other predicted components of the geoid anomaly at fracture zones to be discernable the anomaly due to thermal stresses has been clearly identified in geoid profiles across the clarion and the udintsev fracture zones the amplitude of this observed anomaly is well predicted if cooling lithosphere begins to accumulate elastic stresses at a temperature of 700 c consistent with the maximum depth of seismicity in the oceanic lithosphere the distribution of thermal stresses with depth is also consistent with focal mechanisms of intraplate earthquakes

magmatism in the evolution of the south china sea geophysical characterization abstract timing distribution and intensity of magmatism are critical to understanding deep lithospheric processes in the opening of the south china sea scs based primarily on multichannel seismic reflection data and aided with gravity and magnetic anomalies we identify various igneous features such as sills stocks volcanic edifices and volcanoes which are often characterized by high positive seismic amplitudes and or by chaotic or blank reflections beneath them because of their seismic shielding effect we estimate the timing of igneous activities and analyze their areal distribution in the scs region based on seismic stratigraphy we confirm that most igneous emplacements in the scs margins occurred after the cessation of seafloor spreading and are sparse and volumetrically small during the rifting and seafloor spreading phases supporting magma poor margins prior to the breakup of the continental lithosphere by contrast post spreading magmatic activities are widespread in the continental slope areas and in the central scs basin and are most likely triggered by extension in relation to cooling and subsidence of the oceanic and attenuated continental lithosphere which could accumulate a total thermal contractional displacement up to 24 km

magnetic field and plasma observations at mars initial results of the mars global surveyor mission the magnetometer and electron reflectometer investigation mag er on the mars global surveyor spacecraft has obtained magnetic field and plasma observations throughout the near mars environment from beyond the influence of mars to just above the surface at an altitude of 100 kilometers the solar wind interaction with mars is in many ways similar to that at venus and at an active comet that is primarily an ionospheric atmospheric interaction no significant planetary magnetic field of global scale has been detected to date 2 1021 gauss cubic centimeter but here the discovery of multiple magnetic anomalies of small spatial scale in the crust of mars is reported

thermal stress and the spacings of transform faults bathymetric charts are used with satellite altimeter profiles to locate major ridge transform intersections along five spreading ridges the ridges are the mid atlantic ridge the east pacific rise the chile rise the pacific antarctic rise and the southeast indian ridge analysis of these data show spacings between transform faults w increase linearly with spreading rate w 6 28 m y this linear correlation is explained by a thermoelastic model of a cooling strip of lithosphere spreading at a rate the traction free boundaries of the thin elastic strip simulate cracks in the lithosphere at transform faults a two dimensional thermoelastic solution for the in plane stress shows the largest stress component is tensional and parallel to the ridge stresses are zero at the ridge and increase as age to a maximum value at an age of w 4 all stress components are small for ages greater than w when the transform spacing is large compared with the spreading rate w 100 m y thermal stresses exceed the strength of the lithosphere for ages between 0 and 30 ma the observed maximum ratio of transform spacing to spreading rate w 10 m y results in low thermal stresses that only exceed the strength of the lithosphere for ages less than 1 ma thus transform faults relieve most of the thermal stress model predictions also agree with earthquake studies showing that normal faults in young lithosphere have tensional axes aligned with the ridge moreover oceanic intraplate earthquakes rarely occur in lithosphere older than 30 ma as predicted by the model these and other geophysical observations confirm turcotte s hypothesis that transform faults are thermal contraction cracks

accurate focal depth determination of oceanic earthquakes using water column reverberation and some implications for the shrinking plate hypothesis a b s t r a c t investigation of oceanic earthquakes is useful for constraining the lateral and depth variations of the stress and strain rate fields in oceanic lithosphere and the thickness of the seismogenic layer as a function of lithosphere age thereby providing us with critical insight into thermal and dynamic processes associated with the cooling and evolution of oceanic lithosphere with the goal of estimating hypocentral depths more accurately we observe clear water reverberations after the direct p wave on teleseismic records of oceanic earthquakes and develop a technique to estimate earthquake depths by using these reverberations the z h grid search method allows the simultaneous determination of the sea floor depth h and earthquake depth z with an uncertainty less than 1k m which compares favorably with alternative approaches we apply this method to two closely located earthquakes beneath the eastern pacific these earthquakes occurred in 25 ma old lithosphere and were previously estimated to have similar depths of 10 12 km we find that the two events actually occurred at dissimilar depths of 2 5 km and 16 8 km beneath the seafloor respectively within the oceanic crust and lithospheric mantle the shallow and deep events are determined to be a thrust and normal earthquake respectively indicating that the stress field within the oceanic lithosphere changes from horizontal deviatoric compression to horizontal deviatoric tension as depth increases which is consistent with the prediction of lithospheric cooling models furthermore we show that the p axis of the newly investigated thrust faulting earthquake is perpendicular to that of the previously studied thrust event consistent with the predictions of the shrinking plate hypothesis

the rigid plate and shrinking plate hypotheses implications for the azimuths of transform faults the rigid plate hypothesis implies that oceanic lithosphere does not contract horizontally as it cools hereinafter rigid plate an alternative hypothesis that vertically averaged tensional thermal stress in the competent lithosphere is fully relieved by horizontal thermal contraction hereinafter shrinking plate predicts subtly different azimuths for transform faults the size of the predicted difference is as large as 2 44 with a mean and median of 0 46 and 0 31 respectively and changes sign between right lateral rl slipping and left lateral ll slipping faults for the morvel transform fault data set all six plate pairs with both rl and ll slipping faults differ in the predicted sense with the observed difference averaging 1 4 0 9 95 confidence limits which is consistent with the predicted difference of 0 9 the sum squared normalized misfit r to global transform fault azimuths is minimized for 0 8 0 4 95 confidence limits where is the fractional multiple of the predicted difference in azimuth between the shrinking plate 1 and rigid plate 0 hypotheses thus observed transform azimuths differ significantly between rl slipping and ll slipping faults which is inconsistent with the rigid plate hypothesis but consistent with the shrinking plate hypothesis which indicates horizontal shrinking rates of 2 ma 1 for newly created lithosphere 1 ma 1 for 0 1 ma old lithosphere 0 2 ma 1 for 1 ma old lithosphere and 0 02 ma 1 for 10 ma old lithosphere which are orders of magnitude higher than the mean intraplate seismic strain rate of 10 6 ma 1 5 10 19 s 1

long term fault slip rates distributed deformation rates and forecast of seismicity in the western united states from joint fitting of community geologic geodetic and stress direction data sets 1 the long term average velocity field of the western united states is computed with a kinematic finite element code community data sets include fault traces geologic offset rates geodetic velocities principal stress directions and euler poles there is an irreducible minimum amount of distributed permanent deformation which accommodates one third of pacific north america relative motion in california much of this may be due to slip on faults not included in the model all data sets are fit at a common rms level of 1 8 datum standard deviations experiments with alternate weights fault sets and euler poles define a suite of acceptable community models in pseudoprospective tests fault offset rates are compared to 126 additional published rates not used in the computation 44 are consistent another 48 have discrepancies under 1 mm a 1 and 8 have larger discrepancies updated models are then computed novel predictions include dextral slip at 2 3 mm a 1 in the brothers fault zone two alternative solutions for the mendocino triple junction slower slip on some trains of the san andreas fault than in recent hazard models and clockwise rotation of some domains in the eastern california shear zone long term seismicity is computed by assigning each fault and finite element the seismicity parameters coupled thickness corner magnitude and spectral slope of the most comparable type of plate boundary this long term seismicity forecast is retrospectively compared to instrumental seismicity the western united states has been 37 below its long term average seismicity during 1977 2008 primarily because of temporary reduced activity in the cascadia subduction zone and san andreas fault system

variability of the altitude of the martian sheath received 31 march 2005 revised 18 august 2005 accepted 25 august 2005 published 24 september 2005 1 using electron energy spectra we identify time periods when the mars global surveyor mgs spacecraft is in or above the martian magnetic pileup boundary mpb we use more than five years of data to develop a statistical picture of the location of the mpb relative to the mgs mapping altitude near 400 km we show for the first time that the mpb location is sensitive to interplanetary magnetic field imf orientation and to martian season and confirm a dependence upon solar wind pressure we confirm that crustal magnetic sources raise the altitude of the mpb and demonstrate that sheath electrons populate magnetic cusp regions in the southern hemisphere during southern summer strong crustal fields near the subsolar point raise the altitude of the mpb over the entire dayside implying that martian crustal fields modify the solar wind interaction globally citation brain d a j s halekas r lillis d l mitchell r p lin and d h crider 2005 variability of the altitude of the martian sheath geophys res lett 32 l18203

magnetic reconnection in the ionosphere of mars the role of collisions this is an excel data file for an analysis of nasa maven date looking at magnetic reconnection in the ionosphere it will accompany a later paper in the j geophysical research on this topic

the angular velocities of the plates and the velocity of earth s centre from space geodesy summary using space geodetic observations from four techniques gps vlbi slr and doris we simultaneously estimate the angular velocities of 11 major plates and the velocity of earth s centre we call this set of relative plate angular velocities geodvel for geodesy velocity plate angular velocities depend on the estimate of the velocity of earth s centre and on the assignment of sites to plates most geodetic estimates of the angular velocities of the plates are determined assuming that earth s centre is fixed in an international terrestrial reference frame itrf and are therefore subject to errors in the estimate of the velocity of earth s centre in itrf2005 and itrf2000 earth s centre is the centre of mass of earth oceans and atmosphere cm the velocity of cm is estimated by slr observation of lageos s orbit herein we define earth s centre to be the centre of mass of solid earth ce we determine the velocity of ce by assuming that the portions of plate interiors not near the late pleistocene ice sheets move laterally as if they were part of a rigid spherical cap the geodvel estimate of the velocity of ce is likely nearer the true velocity of cm than are the itrf2005 and itrf2000 estimates because 1 no phenomena can sustain a significant velocity between cm and ce 2 the plates are indeed nearly rigid aside from vertical motion and 3 the velocity of cm differs between itrf2005 and itrf2000 by an unacceptably large speed of 1 8 mm yr 1 the velocity of earth s centre in geodvel lies between that of itrf2000 and that of itrf2005 with the distance from itrf2005 being about twice that from itrf2000 because the geodvel estimates of uncertainties in plate angular velocities account for uncertainty in the velocity of earth s centre they are more realistic than prior estimates of uncertainties geodvel differs significantly from all prior global sets of relative plate angular velocities determined from space geodesy for example the 95 per cent confidence limits for the angular velocities of geodvel exclude those of revel sella et al for 34 of the 36 plate pairs that can be formed between any two of the nine plates with the best constrained motion the median angular velocity vector difference between geodvel and revel is 0 028 myr 1 which is up to 3 1 mm yr 1 on earth s surface geodvel differs the least from the geodetic angular velocities that altamimi et al determine from itrf2005 geodvel s 95 per cent confidence limits exclude 11 of 36 angular velocities of altamimi et al and the median difference is 0 015 myr 1 geodvel differs significantly from nearly all relative plate angular velocities averaged over the past few million years including those of nuvel 1a the difference of geodvel from updated 3 2 myr angular velocities is statistically significant for all but two of 36 angular velocities with a median difference of 0 063 myr 1 across spreading centres eight have slowed down while only two have sped up we conclude that plate angular velocities over the past few decades differ significantly from the corresponding angular velocity averaged over the past 3 2 myr

implications of volcanism in coastal california for the neogene deformation history of western north america 1 the geologic record of coastal california includes evidence of numerous volcanic centers younger than 30 ma that do not appear to have erupted in an arc setting by correlating these volcanic centers with specific slab windows predicted from analysis of magnetic anomalies on the pacific plate we add new constraints to tectonic reconstructions since 30 ma our correlations such as erupting the morro rock islay hill complex south of the pioneer fracture zone and the iversen basalt south of the mendocino fracture zone require larger displacements within western north america than advocated by most previous authors specifically we infer at least 315 km of motion between the sierra nevada and rigid north america at an azimuth of about n60 w and at least 515 km between baja california and rigid north america in a similar direction a consequence of inferring a large displacement of baja california is that the pacific north american plate boundary must have developed most of its current form prior to 10 ma we interpret a slab window developing between cocos and monterey plates after 19 ma that reconstructs under nearly all of the southern california volcanic centers dated at 18 14 ma most of the sedimentary basins associated with volcanic rocks show brief periods of rapid subsidence synchronous with volcanism followed by slow subsidence of variable but often extended duration consistent with rapid extension of cold lithosphere over recently introduced hot asthenosphere

martian magnetic storms the response of mars to the major space weather events called interplanetary coronal mass ejections icmes is of interest for both general planetary solar wind interaction studies and related speculations on their evolutionary consequences especially with respect to atmosphere escape various particle and field signatures of icmes have been observed on phobos 2 mars global surveyor mgs mars express mex and now mars atmosphere and volatile evolution maven of these maven s combined instrumentation and orbit geometry is particularly well suited to characterize both the event drivers and their consequences however maven has detected only moderate disturbances at mars due in large part to the general weakness of the present solar cycle nevertheless the strongest event observed by maven in march 2015 provides an example illustrating how further insights can be gained from available models here we first look more closely at what previously run bats r us mhd simulations of the combined maven observations tell us about the march 2015 event consequences we then use analogous models to infer those same responses including magnetic field topology changes and ionospheric consequences to a hypothetical extreme icme at mars based on stereo a measurements in july 2012 the results suggest how greatly enhanced yet realistic solar wind pressure magnetic field and convection electric field combine to produce strong magnetospheric coupling with important consequences for upper atmosphere and ionosphere energization

the malpelo plate hypothesis and implications for nonclosure of the cocos nazca pacific plate motion circuit using global multiresolution topography we estimate new transform fault azimuths along the cocos nazca plate boundary and show that the direction of relative plate motion is 3 3 1 8 95 confidence limits clockwise of prior estimates the new direction of cocos nazca plate motion is moreover 4 9 2 7 95 confidence limits clockwise of the azimuth of the panama transform fault we infer that the plate east of the panama transform fault is not the nazca plate but instead is a microplate that we term the malpelo plate with the improved transform fault data the nonclosure of the nazca cocos pacific plate motion circuit is reduced from 15 0 mm a 1 3 8 mm a 1 to 11 6 mm a 1 3 8 mm a 1 95 confidence limits the nonclosure seems too large to be due entirely to horizontal thermal contraction of oceanic lithosphere and suggests that one or more additional plate boundaries remain to be discovered

mineralogy and chemistry of late pliocene early pleistocene paleosols on mount kenya weathering indices of relative age and paleoenvironmental reconstruction abstract iron and al extracts as weathering indices in paleosols have been used in many localities to determine relative age transformation of total chemical element concentrations to secondary forms translocation of organically complexed al and long standing inundation of soils with groundwater on mt kenya a succession of paleosols straddling the olduvai subchron are here analyzed to determine the degree to which fe al extracts assist paleoenvironmental reconstruction especially relative age determination genesis and translocation of alteration products and the effect of paleoclimatic deterioration with the advent of glaciation ca 2 0 ma and possibly before warmer humid climate from the onset of the plio pleistocene to the olduvai subchron thereafter reverting to a long episode of oscillating drier wetter ice age perturbations is written into the profile morphologies mineralogies and chemistries supporting earlier stratigraphic interpretations

source mechanisms of earthquakes near mid ocean ridges from body waveform inversion implications for the early evolution of oceanic lithosphere to investigate the early tectonic evolution of oceanic lithosphere we present a synthesis of the characteristics of near ridge earthquakes i e earthquakes which occur off the axis of major mid ocean spreading centers but in lithosphere less than 35 m y old near ridge seismicity for the past two decades has been rather evenly distributed along the major ridge systems with the exception of the central indian ocean and the eastern part of the cocos plate which have experienced a high rate of earthquake activity during that time for 32 near ridge earthquakes we determined the double couple orientation seismic moment centroid depth and source time function using a formal inversion technique based on matching synthetic and observed p and sh waveforms all types of faulting styles are observed seismogenic deformation in young oceanic lithosphere is concentrated in the first 15 m y and occurs almost entirely in the uppermost mantle from just below the moho to depths at which the temperature estimated from standard thermal models exceeds 800 c thrust faulting is not observed at depths greater than 10 km below the seafloor the deepest near ridge earthquakes are generally characterized by normal faulting focal mechanisms with the t axis oblique or perpendicular to the local spreading direction most of these earthquakes occur in the central indian ocean the transition from mechanisms characteristic of ridge axes to those characteristic of the near ridge environment occurs within the first few million years for lithosphere less than 35 m y old horizontal compressive stress associated with the cooling and subsidence of the lithosphere ridge push does not appear to be the dominant source of stress released in earthquakes this result may be used to place a lower bound of several hundred bars on the typical stress difference in young oceanic lithosphere this bound together with the observed depth of faulting implies that the strain rate associated with seismic deformation is locally at least 10 15 s 1 we propose that thermoelastic stresses associated with plate cooling play an important role in near ridge earthquakes this source of stress can account for the mechanisms and depths of many near ridge earthquakes and for the concentration of activity in very young lithosphere the comparative aseismicity of the oceanic crust may reflect the concentration of thermoelastic stress below the moho due to different material properties for the crust and upper mantle or the early release of thermoelastic stresses through hydrothermal circulation and cracking throughout the crust at or near the ridge axis the higher level of near ridge seismicity in the indian ocean may be the result of modification or enhancement of the more typical near ridge stress state by such local processes as the continental collision between india and asia or active secondary convection beneath the young lithosphere of the indian plate

pacific plate deformation from horizontal thermal contraction the central approximation of plate tectonics is that the plates are rigid which gives the theory its rigor and predictive power space geodetic measurements are consistent with the rigidity of stable plate interiors but some failures of plate circuit closure in particular of oceanic plates indicate that plates may be measurably non rigid we explore the hypothesis that horizontal thermal contraction causes deformation of oceanic plates here we show significant expected displacement fields due to thermal contraction for the pacific plate based on a previously proposed relationship between seafloor age and strain rate and on two end member assumptions on how strain compatibility is enforced the predicted maxi mum 2 2 mm yr southeastward motion of the northeastern part of the plate relative to the pacific antarctic rise may contribute to a large part of the non closure of the pacific north america plate motion circuit our predicted displacement rates cannot yet be confirmed by current space geodetic data and will require seafloor geodesy with 1 mm yr accuracy the spatial distribution of predicted moment rate agrees reasonably well with that of intraplate earthquake epicenters similar to what is observed for plate boundary zones our results suggest that plate scale horizontal thermal contraction is significant and that it may be partly released seismically

factors controlling the location of the bow shock at mars 1 during the first year of the mars global surveyor mgs mission 553 shock crossings have been identified from a total of 363 orbits the shape of the shock has been determined by examining the mgs spacecraft magnetometer electron reflectometer mag er data the location of the shock was found highly variable the present study shows that the high crustal magnetic sources found in the southern hemisphere do not seem responsible for the bow shock bs variability the present study shows that contrary to many expectations there is no obvious strong one to one correlation between the location of the highest crustal sources and the variability of the shock position on the other hand the shock appears farthest from mars in the hemisphere of locally upward interplanetary electric field consistent with the idea that mass loading play a role in controlling the bs location which confirms previous results