venusian channels and valleys distribution and volcanological implications abstract nearly complete image coverage of venus by magellan enabled us to map various venusian channel and valley landforms and to examine their associations with other geological units global mapping reveals a nonrandom distribution the highest total concentration is in the equatorial regions characterized by highlands rift and fracture zones and associated volcanic features many channels associated with flow deposits are similar to typical terrestrial lava drainage channels they are observed to be associated with a wide range of volcanic edifices such as coronae shield volcanoes and rift and fracture zones one type of channel similar morphologically to lunar sinuous rilles is classified as a venusian sinuous rille based on the close associations of many venusian sinuous rilles with coronae we hypothesize that mantle plume or blob volcanism has caused high effusion and sustained lava eruptions essential for sinuous rille formation mantle derived high temperature low viscosity lava eruptions are responsible for the efficient erosional processes particularly for thermal erosion that seems to be at least partially required for some sinuous rille formation many valley networks are observed in highlands and in association with coronae fracture systems and source zones for low viscosity lavas both key to network formation were probably concentrated at highlands and coronae canali type channels which are morphologically unlike other known volcanic channels are limited to certain plains regions a wide range of low viscosity lava types is possible for the channel forming lava depending on the formation mechanism their lengths exceed the lengths of other common volcanic channel types on venus implying a large volume of lava and long duration of the eruption the close association of canali with plains regions implies that canali formation is probably related to the emplacement of plains a hypothesized global resurfacing event late in venusian history may be responsible for canali formation

effects of birefringence within ice sheets on obliquely propagating radio waves in this paper effects of birefringence on radio waves obliquely propagating though polar ice sheets are examined to facilitate interpretations of bistatic and side looking radar data a formalism applicable for arbitrary radar configurations is developed to predict the returned power from within and beneath the ice sheets that have arbitrary alignments of ice crystals ice fabrics we applied this formalism to a range of ice fabrics found in ice cores and assessed the effects of birefringence in terms of ray path configurations ice fabrics and radar frequency predicted frequency dependence of the bed return power replicates prominent features observed at greenland ngrip ice core site results show that birefringence in ice of 1 km or more thickness with strong weak fabric can reduce the power returned from the bed 2 db or more at frequencies higher than 200 mhz 20 mhz as compared to isotropic ice this suggests that quantitative interpretation of the power returned from the bed requires careful assessment of birefringence almost everywhere over the ice sheets application of this formalism also suggests a radar frequency range usable for attenuation measurements possible effects of fabric on synthetic aperture radar processing and a feasibility of remote sensing of ice fabric

the paleomagnetism of single silicate crystals recording geomagnetic field strength during mixed polarity intervals superchrons and inner core growth 1 the basic features of the geomagnetic reversal chronology of the last 160 million years are well established the relationship between this history and other features of the field however has been elusive the determination of past field strength paleointensity is especially challenging commonly accepted results have come from analyses of bulk samples of lava historic lavas have been shown to faithfully record the past field strength when analyzed using the thellier double heating method data from older lavas however tend to show effects of in situ and laboratory induced alteration here we review an alternative approach single plagioclase crystals can contain minute magnetic inclusions 50 350 nm in size that are potential high fidelity field recorders thellier experiments using plagioclase feldspars from an historic lava on hawaii provide a benchmark for the method rock magnetic data from older lavas indicate that the feldspars are less susceptible to experimental alteration than bulk samples this resistance is likely related to the lack of clays in addition magnetic minerals are sheltered by the encasing silicate matrix from natural alteration that can otherwise transform the well defined thermoremanent magnetization into an irresolute chemical remanent magnetization if there is a relationship between geomagnetic reversal frequency and paleointensity it should be best expressed during superchrons intervals with few or no reversals thellier data sets based on single plagioclase crystals from lavas erupted during the cretaceous normal polarity superchron 83 120 million years ago suggest a strong 12 1022 am2 stable field consistent with an inverse relationship between reversal frequency and paleointensity superchrons may represent times when the pattern of core mantle boundary heat flux allows the geodynamo to operate at peak efficiency as suggested in some numerical models thellier data from single plagioclase crystals formed during times of moderate 10 reversals million years reversal occurrence suggest a weaker and more variable field these paleointensity data together with a consideration of paleomagnetic directions suggest that geomagnetic reversals field morphology secular variation and intensity are related the linkages over tens of millions of years imply a lower mantle control on the geodynamo on even longer timescales the magnetization held by plagioclase and other silicate crystals can be used to investigate the proterozoic and archean geomagnetic field during the onset of growth of the solid inner core data from plagioclase crystals separated from mafic dikes together with directional data from whole rocks indicate a dipole dominated field similar to that of the modern 2 5 2 7 billion years ago older archean rocks are of great interest for paleomagnetic and paleointensity investigations because they may record a time when the compositionally driven convection of the modern dynamo may not have been operating and a solid inner core did not play its current role in controlling the geometry of outer core flow most rocks of this age have been affected by low grade metamorphism investigations using single silicate grains provide arguably our best hope of seeing through secondary geologic events and reading the early history of the geodynamo absolute paleointensity measurements of the oldest rocks on the planet will require the further development of methods to investigate silicate crystals with exsolved magnetic minerals that address the uncertainties posed by thermocrystallization remanent magnetization anisotropy and slow cooling fortunately prior work in rock magnetism together with advances in analytical equipment and techniques provides a solid foundation from which these frontier issues can be approached

east pacific rise 8 10 30 n evolution of ridge segments and discontinuities from seamarc ii and three dimensional magnetic studies magnetic bathymetric and seamarc ii side scan sonar data from the ridge flanks adjoining the overlapping spreading center osc at 9 03 n on the east pacific rise epr are used to establish the evolution of this ridge axis discontinuity and adjacent ridge segments within the past 2 4 m y a three dimensional inversion for magnetization shows the discontinuity evolved from one of small offset 2 km to an 8 km offset osc within the past approximately 1 0 m y the osc has left a v shaped discordant zone indicating an average southward migration of 42 km m y since the beginning of anomaly 2 time 1 8 m y the west flank discordant zone consists of discrete abandoned overlap basins bounded by highly magnetized ridge tips with anomalous lineations while the east flank discordant zone is a broad swath of high magnetization anomalous lineations and greater depths kinematic modelling indicates that southward migration has not been steady but has been accomplished by a series of episodic propagation events with rates ranging from 500 mm yr ridge tips have been abandoned at intervals of approximately 50 000 100 000 years both by linkage of neighboring ridge segments and by self decapitation magnetic modelling suggests rotations of overlap basins of 25 consistent with predictions of kinematic modelling orientation of magnetic isochrons indicate a 3 6 counterclockwise change in pacifie cocos plate motion within the past 1 m y assuming orthogonal spreading throughout this time although the discontinuity currently located at 9 03 n existed prior to this change in plate motion its growth into a right stepping 8 km offset osc may be causally related to this change the current configuration of intratransform spreading within the siqueiros has developed since onset of the brunhes anomaly 0 73 m y possibly in response to this recent counterclockwise change in plate motion in addition to the osc there is evidence for several small discontinuities offsetting the ridge during anomaly 2 time which have since migrated along the ridge and vanished an axial magnetization high is observed along the crest of the epr change in the magnitude of the axial magnetization high or shifts in its location relative to the bathymetric axis coincide with several devals within the area magnetization maxima are found at ridge transform intersections and the osc these highs may reflect presence of highly fractionated feti basalts erupted from magma chambers with restricted supply located adjacent to both large and short offset ridge axis discontinuities discrete magnetization highs are found off axis adjacent to clipperton and siqueiros fracture zones indicating that production of feti basalts at ridge transform intersections is episodic magnetizations are higher north of clipperton than south of it possibly reflecting a long lived starvation in magma supply north of clipperton and more abundant supply to the south most seamounts within the area have the same magnetization polarity as surrounding seafloor indicating that seamount formation occurs near the ridge axis the exceptions lie near magnetic reversal boundaries and are consistent with seamount formation as close as 10 km from the ridge axis few pronounced magnetization highs are associated with seamounts indicating that the magnetized source layer is no thicker beneath them than surrounding seafloor or that seamount basalts are substantially less magnetic than normal mid ocean ridge basalts

high resolution 40ar 39ar dating of the oldest oceanic basement basalts in the western pacific basin 1 we report new 40ar 39ar ages for the oldest pacific oceanic floor at ocean drilling program site 801c in the pigafetta basin and site 1149d close to the izu bonin subduction zone in the nadezhda basin these ages were determined by applying high resolution incremental heating experiments including 15 30 heating steps to better resolve the primary argon signal from interfering alteration signatures in these low potassium ocean crust basalts combined with previous results from pringle 1992 for site 801b and 801c we arrive at a multistage history for the formation of the pigafetta ocean crust the oldest part of the pacific plate was formed at the spreading ridges at 167 4 1 4 3 4 ma n 2 2 internal absolute error offering an important calibration point on the geological reversal timescale grts since it represents the old end of the mesozoic magnetic anomalies this mid ocean ridge basalt sequence however is overlain by more tholeiites and alkali basalts that were formed 7 3 1 5 myr later around 160 1 0 6 ma n 7 2 internal error the older age group is confirmed independently by radiolarian ages ranging from late bajocian to middle bathonian 167 173 ma bartolini and larson 2001 and by profound differences in the structural characteristics of this basement section pockalny and larson 2003 thin layers comprising hydrothermal deposits separate these sequences which in addition to the difference in isotopic age show distinct major and trace element compositions this indicates that key volcanic and hydrothermal activity took place 400 600 km away from the spreading ridges on the basis of a jurassic 66 km myr half spreading rate in the pacific it remains unclear if these processes were active continuously after the initial formation of the pacific oceanic crust but all our observations seem to point to an episodic history site 1149d gives another important calibration point on the grts of 127 0 1 5 3 6 ma n 1 2 internal absolute error for anomaly m12 that is slightly younger when compared to current timescale compilations 134 2 2 1 ma gradstein et al 1995 this might suggest that the dated basalt from site 1149d does not represent the age of the ocean crust formed at its ridge axis it may also be part of the early cretaceous intraplate events that have produced dolerite sills in the pacific crust at sites 800 and 802 around 114 126 ma

atmospheric effects on ejecta emplacement laboratory experiments allow the investigation of complex interactions between impacts and an atmosphere although small in scale they can provide essential first order constraints on the processes affecting late stage ballistic ejecta and styles of ejecta emplacement around much larger craters on planetary surfaces the laboratory experiments involved impacting different fine grained particulate targets under varying atmospheric pressure and density different gas compositions during crater formation ballistic ejecta form the classic cone shaped profile observed under vacuum conditions as atmospheric density increases for a given pressure however the ejecta curtain bulges at the base and pinches above this systematic change in the ejecta curtain reflects the combined effects of deceleration of ejecta smaller than a critical size and entrainment of these ejecta within atmospheric vortices created as the outward moving wall of ejecta displaces the atmosphere additionally a systematic change in emplacement style occurs as a function of atmospheric pressure largely independent of density contiguous ejecta rampart superposing ballistically emplaced deposits 0 06 to 0 3 bar ejecta flow lobes 0 3 to 0 7 bar and radial patterns 0 8 bar underlying processes controlling such systematic changes in emplacement style were revealed by observing the evolution of the ejecta curtain by changing target materials including layered targets and low density particulates by varying atmospheric density by changing impact angle and by comparing the ejecta run out distances with first order models of turbidity flows three distinct ejecta emplacement processes can be characterized ejecta ramparts result from coarser clasts sorted and driven outward by vortical winds behind the outward moving ejecta curtain this style of wind modified emplacement represents minimal ejecta entrainment and is enhanced by a bimodal size distribution in the ejecta such eddy supported flows are observed to increase in run out distance scaled to crater size with increasing atmospheric pressure by analogy with turbidity flows this scaled distance should increase as r1 2 for a given atmospheric pressure and degree of entrainment ejecta flows with much greater run out distances develop as the turbulent power in atmospheric response winds increase such flows overrun and scour the inner ejecta facies thereby producing distinct inner and outer facies the degree of ejecta entrainment depends on the dimensionless ratio of drag to gravity forces acting on individual ejecta and the intensity of the winds created by the outward moving curtain entrainment increases with increasing atmospheric density and ejection velocity crater size but decreases with ejecta density and size the intensity of curtain generated winds increases with ejection velocity crater size the dimensionless drag ratio characterizing the laboratory experiments can be applied to mars since the reduced atmospheric density is offset by the increased ejection velocities for kilometer scale events for a given crater size ejection velocity and atmospheric conditions a wide range of nonballistic ejecta emplacement styles could occur simply by varying ejecta sizes even without the presence of water alternatively the onset crater diameter for nonballistic emplacement styles can reflect the range of ejecta sizes possible from the diverse martian geologic history massive basalts to fine grained aeolian deposits scaling considerations further predict that ejecta run out distances scaled to crater size on mars should increase as r1 2 hence long run out flows dependent on crater diameter need not reflect depth to a buried reservoir of water on venus however the dense atmosphere maximizes entrainment and results in ejecta flow densities approaching a constant fraction of the atmospheric density under such conditions ejecta run out distances should decrease as r 1 2

downhole magnetic measurements of odp hole 801c implications for pacific oceanic crust and magnetic field behavior in the middle jurassic 1 downhole horizontal and vertical magnetic field measurements within the 474 m thick jurassic crustal section drilled at odp hole 801c in the western pacific show anomalies that are in phase indicating that this site formed in the southern hemisphere and moved across the paleoequator to its present location at 18 6 n the inclination computed from the horizontal and vertical anomaly logging data varies significantly downhole and can be explained by progressive rotation of the lava sequence as it is buried during accretion this is compatible with the observed dips in the formation micro scanner fms data which show up to 42 of rotation toward the fossil spreading axis we restore the magnetic inclination to prerotated values and obtain an estimated pretilt inclination of 39 9 6 6 corresponding to a paleolatitude of 22 7 5 s the magnetic logging data document six polarity units within the drilled volcanic extrusive crust the upper 132 m thick logged section of lavas contains four polarity units which apparently formed 7 myr after the formation of the underlying basement the lower 212 m logged section of the hole is reversely magnetized with an intervening normal polarity zone more than one reversal downhole would indicate a rapid reversal rate given the time it takes to form the extrusive section of oceanic crust 45 000 years at a fast spreading rate of 66 km myr previous results from a contemporaneous section in spain suggest rapid reversals throughout the late bajocian early bathonian stage 170 165 ma nevertheless the complicated formation history prevents quantification of the geomagnetic reversal rate for the 801c crustal section

modeling magnetostratigraphy in a borehole the magnetic field along a hole bored through a sequence of dipping layers with varying magnetization and planar interfaces is calculated the emphasis is on recovering remanent magnetization polarity transitions for magnetostratigraphic applications dating correlations although intensity of remanent magnetization can in itself be a useful rock property results are presented for various cases of geologic interest in the form of axial vertical profiles and section maps of the holes at varying distances from polarity reversal interfaces the vertical profiles demonstrate a resolving power of about six times the hole radius meaningful magnetostratigraphies can be expected for rocks with a magnetization larger than 10 3am 1 for instruments with a sensitivity of 0 1 nt in a number of natural occurrences it may not be necessary to resort to progressive demagnetization to recover the polarity sequence depending on which magnetic field and magnetization component one looks at the section maps display c

impact crater outflows on venus morphology and emplacement mechanisms many of the 932 impact craters discovered by the magellan spacecraft at venus are associated with lobate flows that originate at or near the crater rim they extend for several to several hundred kilometers from the crater and they commonly have a strong radar backscatter a morphologic study of all identifiable crater outflows on venus has revealed that many individual flows each consist of two areas defined by distinct morphologic features these two areas appear to represent two stages of deposition for each flow the part of the flow that is generally deposited closest to the crater tends to be on the downrange side of the crater flows in the downrange direction and it is interpreted to be a late stage ejecta in many cases this proximal part of the flow is too thin to completely bury the large blocks in subjacent ejecta deposits dendritic channels present in many proximal flows appear to have drained liquid from the proximal part in the downhill direction and they debouch to feed the outer part of the flows this distal part flows downhill fills small grabens and is ponded by ridges behavior that mimics that of volcanic lava flows the meandering and dendritic channels and the relation of the distal flows to topography strongly suggest that the distal portion is the result of coalescence and slow drainage of impact melt from the proximal portion impact melt forms a lining to the transient crater and mixes turbulently with solid clasts and part of this mixture may be ejected to form the proximal part of the flow during the excavation stage of crater development a statistical study of the venusian craters has revealed that in general large craters produced by impacts with relatively low incidence angles to the surface are more likely to produce flows than small craters produced by higher angle impacts the greater flow production and downrange focusing of the proximal flows with decreasing incidence angle indicate a strong control of the flows by the impactor flight direction and a high downrange velocity imparted to the proximal flow material in lower angle impacts on the moon small flows interpreted to be composed of impact melt are observed atop the ejecta of large fresh craters on earth melt rich suevite deposits form the uppermost layer of ejecta of some fresh craters these features albeit much smaller may be analogous to the flows on venus numerical models have predicted that larger volumes of impact melt would be produced on venus than on the cooler terrestrial bodies due to high atmospheric and target temperatures perhaps 3 times the volume produced on the moon for a given crater diameter

a multichannel seismic investigation of upper crustal structure at 9 n on the east pacific rise implications for crustal accretion reprocessed multichannel seismic profiles from the 9 n segment of the east pacific rise reveal prominent shallow subbasement events these events are identified as wide angle reflections from the base of seismic layer 2a based upon modeling of expanding spread profile data and velocity functions the layer 2a reflections typically increase from 0 15 s after the seafloor reflection at the rise axis to 0 3 0 45 s within 1 2 km of the axis corresponding to an increase in layer thickness of 200 600 m no further systematic increase in layer thickness is observed although lateral variability of the order of a few hundred meters in thickness is observed at greater offsets from the rise axis however the intermittent character of the imaged layer 2a reflection is attributed to focusing and defocusing of energy by the seafloor bathymetry rather than necessarily to intrinsic lateral variability at the base of the layer the base of layer 2a is interpreted as corresponding to the transition between the extrusive section pillow basalts and sheet flows and a sheeted dike complex the rapid thickening of the layer near the rise axis is attributed to successive lava flows burying the initially shallow top of the sheeted dike complex as the layer passes through the neovolcanic zone lateral variability of layer 2a can significantly affect the imaging of the underlying axial magma chambers as average velocities within layer 2a are approximately half that of layer 2b for an along axis profile apparent along axis variability in the depth of the axial magma chamber is traced to variability in the thickness of layer 2a caused by wandering of the profile relative to axis within the resolution of the data the time delay of the magma chamber reflection relative to the base of layer 2a is constant

origin of the pacific jurassic quiet zone understanding the marine magnetic anomaly record is critical for constructing realistic geodynamo models of global geomagnetic field polarity reversal mechanisms and long term geomagnetic field behavior one of the least understood portions of the marine magnetic anomaly record is also the oldest part of the record the jurassic quiet zone jqz where anomalies become weak and difficult to correlate the reason for the existence of the jqz is unclear it has been suggested that the jqz is a true polarity superchron similar to the cretaceous normal superchron continental magnetostratigraphic studies have suggested that the jqz is a period of rapid polarity reversal of low field intensity or both we show results of a deep tow survey of pacific jurassic crust that confirms the existence of magnetic anomalies within the jqz we tie ocean drilling program hole 801c 167 4 ma into the record and show that seafloor spreading magnetic anomalies are present around the hole and extend to 170 ma crust we find a rise in reversal rate with increasing age with reversal rates over 10 rev m y at 160 ma and at 167 ma anomaly amplitudes decrease in the record from 155 ma until 162 ma where low amplitude anomalies are difficult to correlate prior to 167 ma anomalies regain amplitude and remain strong until the end of our record at 170 ma the jqz thus appears to be a combination of low amplitude magnetic anomalies combined with rapid field fluctuations which could be due to either intensity or polarity changes

ice volume estimation inferred from ice thickness and surface measurements for continental glacier wind river range wyoming usa the wind river range in wyoming usa contains the largest concentration of glacial mass in the rocky mountains of the contiguous usa despite this distinction only a few field or remotely sensed studies providing glacier volume changes have been published the current study focuses on continental glacier located on the northern end of the range and uses two field datasets high accuracy gps surface elevation points and ice penetrating radar transects of the glacier bed to create a three dimensional model of glacier volume current surface elevations are compared with historical elevation data to calculate surface elevation change over time an average thinning rate of 13 8 7 8 m 0 30 0 17 m a 1 between 1966 and 2012 was found surface elevation change rates varied across the glacier ranging from 0 30 to 0 98 m a 1 taking into account variable melt rates across the glacier along with a glacial volume of 72 1 106 10 8 106 m3 we estimate that continental glacier will be reduced in volume by 43 over the next 100 years and will disappear completely over the next 300 400 years if current climatic conditions persist

crystal orientation fabrics within the antarctic ice sheet revealed by a multipolarization plane and dual frequency radar survey 1 to investigate the viscosity structure of ice sheets induced by crystal orientation fabric cof we carried out a multipolarization plane and dual frequency radar survey in east antarctica radar surveys were done along a 670 km long flow line from dome fuji toward the coast and two transverse lines of 300 km and 20 km length respectively the radar echoes were highly dependent on the polarization plane for ice depths between about 40 and 60 of the ice thickness in the lower reaches of the convergent ice flow sector approaching the outlet glacier when the polarization was perpendicular to the ice flow echoes were about 10 db stronger than when the polarization was parallel to the ice flow this feature was not clear in the upper part of this convergent flow sector farther inland where ice flow is divergent or parallel the radar echo varied by several decibels because of changes of the radar polarization and had maxima in two orientations dual frequency data showed that the cause of the reflections was changes in cof multipolarization data identified anisotropic reflectivities and birefringence as causes of the anisotropic radar echoes in the lower and upper reaches respectively with the aid of ice core based studies on cof we show that ice is composed of stacked layers of single pole and vertical girdle fabrics in the lower reaches in contrast we argue that changes of single pole clustering cause isotropic reflectivities in the upper reaches we also discuss on the development of cof along ice flow and its implication to ice sheet dynamics

an improved coherent radar depth sounder the university of kansas developed a coherent radar depth sounder during the 1980s this system was originally developed for glacial ice thickness measurements in the antarctic during the field tests in the antarctic and greenland we found the system performance to be less than optimum the field tests in greenland were performed in 1993 as a part of the nasa program for arctic climate assessment parca we redesigned and rebuilt this system to improve the performance the radar uses pulse compression and coherent signal processing to obtain high sensitivity and fine along track resolution it operates at a center frequency of 150 mhz with a radio frequency bandwidth of about 17 mhz which gives a range resolution of about 5 m in ice we have been operating it from a nasa p 3 aircraft for collecting ice thickness data in conjunction with laser surface elevation measurements over the greenland ice sheet during the last 4 years we have demonstrated that this radar can measure the thickness of more than 3 km of cold ice and can obtain ice thickness information over outlet glaciers and ice margins in this paper we provide a brief survey of radar sounding of glacial ice followed by a description of the system and subsystem design and performance we also show sample results from the field experiments over the greenland ice sheet and its outlet glacicrs

intraplate volcanism of the western pacific new insights from geological and geophysical observations in the pigafetta basin understanding intraplate volcanism is a key to deciphering the earth s magmatic history one of the largest intraplate volcanic events occurred during the mid cretaceous roughly 75 to 125 ma in the western pacific to investigate the origin and effects of this volcanism on various earth systems we present the first comprehensive study of volcanism in the pigafetta basin using seismic surveys magnetic and gravity modeling and ocean drilling program drill core and well log data from site 801 our results show that intraplate volcanism in the pigafetta basin coincides with the rest of the western pacific seamount provinces supporting the previously suggested plumelets scenario for the origin of intraplate volcanism during the mid cretaceous volcanic events we also discover that the late stage volcanism does not overprint the remanant magnetization acquired by the jurassic ocean crust in the pigafetta basin and hence marine magnetic anomalies recorded in the jurassic basement are preserved also the formerly identified rough smooth boundary rsb is indistinguishable from any other rough smooth topographic boundaries throughout the survey area suggesting that the rsb is unlikely to be a cretaceous sill jurassic basement boundary lastly the apparent ages and spatial distribution of volcanic features suggests a dynamic history of hydrothermal circulation in the pigafetta basin indicating that hydrothermal circulation was ongoing well past 100 ma this article is protected by copyright all rights reserved

paleomagnetic inclinations in dsdp hole 417d reconsidered secular variation or variable tilting 1 in deep sea drilling program dsdp hole 417d in the western central north atlantic dips of pillow margins measured in the upper and lower part of basement correlate with tilts predicted from paleomagnetic inclinations in the corresponding parts of the hole a 40 westward dip of the seafloor around 417d matches the tilt predicted from inclinations and pillow margin dips in the uppermost lavas the correlation suggests that most of the large long wavelength inclination variation in hole 417d is caused by variable tilting about an axis more or less parallel to the paleo rift rather than by secular variation the variable tilt in hole 417d is interpreted as the sum of rotation due to lava loading along the mesozoic paleo rift and a later block rotation of 40 in the rift valley walls this interpretation allows a reconstruction of the paleo rift environment in which the 417d upper crust formed

seismic constraints on shallow crustal emplacement processes at the fast spreading east pacific rise we present the results of nine on bottom seismic refraction experiments carried out over young east pacific rise crust the experiments are unusual in that both the source and receiver are located within a few meters of the seafloor allowing high resolution determinations of shallow crustal structure three experiments were located within the axial summit caldera asc over zero age crust the seismic structure at these three locations is fundamentally the same with a thin 60 m surficial low velocity 2 5 km s layer a 100 to 150 m thick transition zone with velocities increasing by 2 5 km s and a layer with velocities of 5 km s at a depth beneath the seafloor of 130 190 m the surficial low velocity layer and transition zone are defined as seismic layer 2a and the 5 km s layer is defined as the top of layer 2b both the surficial low velocity layer and the transition zone double in thickness within 1 km of the rise axis we model layer 2a as the extrusive sequence and transition zone and the 2a 2b boundary as the top of the sheeted dikes the primary implication of this interpretation is that the depth to the top of the sheeted dikes deepens from 150 m to 300 m within 1 km of the asc the thickening of the extrusive layer is interpreted to be due to lava that either overflows the asc walls is emplaced through eruptions outside of the asc or travels laterally from the asc through conduits the most probable cause for the thickening of the transition zone is sill emplacement outside of the asc either from magma that does not reach the surface in an off axis eruption or magma that is transported laterally during the drainage process creating the asc we suggest that the mechanism controlling the magnitude and rate of the dike subsidence is the mechanism that determines the thickness of the extrusive section and the total thickness of layer 2a

deep tow magnetic anomaly study of the pacific jurassic quiet zone and implications for the geomagnetic polarity reversal timescale and geomagnetic field behavior 1 the jurassic quiet zone jqz is a region of low amplitude magnetic anomalies whose distinctive character may be related to geomagnetic field behavior we collected deep tow magnetic profiles in pigafetta basin western pacific where previous deep tow data partially covered the jqz sequence our goals were to extend the survey through the jqz examine anomaly correlations and refine a preliminary geomagnetic polarity timescale gpts model we collected a series of closely spaced profiles over anomaly m34 and ocean drilling program hole 801c to examine anomaly correlation in detail one profile in between previous profiles and two long profiles extending the survey deeper into the jqz anomaly features can be readily correlated except in a region of low amplitude short wavelength anomalies in the middle of the survey area low amplitude zone or laz the small multiprofile surveys demonstrate anomaly linearity implying that surrounding anomalies are also linear and likely result from crustal recording of geomagnetic field changes we constructed a gpts model assuming that most anomalies result from polarity reversals the polarity timescale is similar to the polarity sequences from previous studies but its global significance is uncertain because of problems correlating anomalies in the laz and the ambiguous nature of the small jqz anomalies overall anomaly amplitude decreases with age into the laz and then increases again implying low geomagnetic field strength perhaps related to a rapidly reversing field other factors that may contribute to the laz are interference of anomalies over narrow crustal polarity zones and poorly understood local tectonic complexities

implications for crustal accretion at fast spreading ridges from observations in jurassic oceanic crust in the western pacific 1 downhole logging data and basement stratigraphy interpretations are used to determine the spreading environment and crustal accretion history of the ocean basement cored at odp hole 801c located in the jurassic magnetic quiet zone of the western pacific high resolution microresistivity data obtained with the formation microscanner are used to measure the dip of the extrusive layers and indicate a 10 30 increase in dip down the drill hole with lava flow contacts dipping back toward the original ridge axis this structural pattern and the high proportion of massive flows relative to pillow units are consistent with prevailing crustal accretion models proposed for faster spreading ridges e g 60 km m y full rate a detailed analysis of the age data basement lithology related geochemistry and structural attitudes suggest the shallowest 100 m of the drilled section e g sequences i iii were emplaced just off the ridge sequence iii or significantly farther off axis up to 5 15 m y later sequences i and ii the remainder of the drilled section e g sequences iv viii has geochemical lithological and physical trends that are assumed to be representative of crust created at faster spreading ridges the pattern of dipping lava flow contacts from this deeper section of the drill hole suggests lava flows emanating from the ridge axis are limited to 1 2 km off axis our results suggest ocean crust drilled at hole 801c was created at faster spreading rates however caution should be used when incorporating sequences i iii into geochemical reference sections for faster spreading ridges

modeling of fluidized ejecta emplacement over digital topography on venus the flow computer model of mcewen and malin 1989 modified for application to the study of venus fluidized ejecta blankets febs demonstrates that relatively low viscosities yield strengths and initial velocities are required to duplicate the observed flow paths of the outflow materials the model calculates the velocities and simulated flow paths of gravity flows over magellan topography the model is formulated to determine flow movements from initial conditions gravitational acceleration and resistance to motion as described by coulomb viscous and turbulent resistance forces successful duplication of observed feb flow paths has been achieved for the feb craters addams isabella and cochran when used as a simple energy line model the model requires low coefficients of friction to extend febs to near their observed termini in the synthetic aperture radar sar imagery although the resulting straight flow lines do not follow the observed flow paths well for bingham flow the model requires low values of viscosity and yield strength which are more similar to pyroclastic or debris flows than basaltic lavas flows of 100 m depth require 1 to 2 orders of magnitude higher values of both viscosity and yield strength than 10 m deep flows the complicated nature of the flow lines for the low velocity model suggests that febs were probably emplaced under variably laminar and turbulent flow conditions where underlying topography influenced both the direction and energy of flow materials

alteration and self reversal in oceanic basalts 1 we report on magnetic properties and compositions of titanomaghemites from moderately to highly oxidized basalts recovered at nine drilling sites in the pacific ocean these new data together with similar data sets allow us to test compositional fields of titanomaghemite that could carry self reversed components of natural remanent magnetization above room temperature these compositions have been proposed on the basis of theoretical considerations of cation distributions during ionic reordering associated with low temperature oxidation maghemitization the data indicate that very high oxidation states z 0 9 and relatively high ti contents x 0 6 are needed to produce natural self reversed components this compositional range is much more limited than predicted the severe restrictions on compositions and oxidation state strongly suggest that self reversed magnetizations should not be a general feature of the magnetization carried by oceanic basalt instead the formation of self reversed components may signify unusual conditions such as extreme fluid flow and iron removal which could result in the required oxidation states while maintaining stability of the cation deficient titanomaghemite lattice structure

magnetization of 0 29 ma ocean crust on the mid atlantic ridge 25 30 to 27 10 n a sea surface magnetic survey over the west flank of the mid atlantic ridge from 0 to 29 ma crust encompasses several spreading segments and documents the evolution of crustal magnetization in slowly accreted crust we find that magnetization decays rapidly within the first few million years although the filtering effect of water depth on the sea surface data and the slow spreading rate 13 km m y preclude us from resolving this decay rate a distinctly asymmetric along axis pattern of crustal magnetization is rapidly attenuated off axis suggesting that magnetization dominated by extrusive lavas on axis is reduced off axis to a background value off axis we find a statistically significant correlation between crustal magnetization and apparent crustal thickness with thin crust tending to be more positively magnetized than thicker crust indicative of induced magnetization in thin inside corner ic crust in general we find that off axis segment ends show an induced magnetization component regardless of polarity and that ic segment ends tend to have slightly more induced component compared with outside corner oc segment ends possibly due to serpentinized uppermost mantle at ic ends we find that remanent magnetization is also reduced at segment ends but there is no correlation with inside or outside corner crust even though they have very different crustal thicknesses this indicates that remanent magnetization off axis is independent of crustal thickness bulk composition and the presence or absence of extrusives remanence reduction at segment ends is thought to be primarily due to alteration of lower crust in oc crust and a combination of crustal thinning and alteration in ic crust from all these observations we infer that the remanent magnetization of extrusive crust is strongly attenuated off axis and that magnetization of the lower crust may be the dominant source for off axis magnetic anomalies

geomagnetic polarity reversal model of deep tow profiles from the pacific jurassic quiet zone the jurassic magnetic quiet zone jqz contains magnetic lineations but their low amplitudes make correlation and interpretation difficult part of the problem is the separation of source and sensor for old deep ocean crust we increased anomaly amplitudes by collecting magnetic data along two deep tow profiles over western pacific jqz lineations a magnetic polarity reversal timescale was constructed by matching deep tow anomalies with a simple rectangular block magnetization model for oceanic crust the polarity sequence covers 11 m y 156 167 5 ma and contains 88 pre m29 polarity chrons extending to chron m41 a limitation of this model is its poor representation of the oldest anomalies upward continued to sea level on deep tow profiles these anomalies have both long and short wavelength components but only the latter are easily modeled on a datum close to the source an alternative polarity model was constructed to match the anomalies upward continued to sea level this model retains only 44 of the deep tow model polarity chrons because of short wavelength attenuation by upward continuation because of the inferred periods and magnetization contrasts we think many of the short wavelength anomalies represent paleofield intensity fluctuations in contrast polarity reversals have been documented by prior magnetostratigraphic work for the younger part of the timescale covered by our model thus our data may show a transition from a geomagnetic field behavior dominated by intensity fluctuations to one dominated by reversals

giant debris avalanches from the colima volcanic complex mexico implications for long runout landslides 100 km and hazard assessment at least two giant volcanic debris avalanche deposits are associated with the colima volcanic complex located in the western part of the trans mexican volcanic belt one avalanche originated from volcan de colima 3820 m probably 4300 yr ago a much larger avalanche originated from nevado de colima 4240 m 18500 yr ago it traveled more than 120 km from its source and covered an area of at least 2200 km 2 almost twice the area of any previously described avalanche deposit this older debris avalanche is the second largest by volume known has the longest travel distance yet reported and has one of the lowest height length ratios 0 04 the avalanche material was probably hot during emplacement and after initial slope failure may have behaved similarly to a pyroclastic flow such large long runout landslides from volcanic or nonvolcanic constructs present a serious geologic hazard that must be considered in risk assessment

towards the in situ measurement of the remanent magnetization of oceanic basalts summary recent developments in electromagnetic double dipole devices make these systems convenient for both magnetic susceptibility and resistivity measurements the coupling of such an electromagnetic logging tool with a magnetic probe supplying the three components of the total magnetic field in the well allows in siru determination of magnetic parameters this paper shows how to i interpret the raw data in terms of magnetic susceptibility and resistivity of the surrounding medium ii determine the horizontal and vertical components of the total magnetization and iii separate the remanent magnetization which seems to be possible at many points of the logging

jurassic magnetostratigraphy 3 bathonian bajocian of carcabuey sierra harana and campillo de arenas subbetic cordillera southern spain abstract four sections in majocian bathonian middle jurassic pelagic limestone with standard ammonite zonation have yielded magnetic polarity sequences magnetic directions in these red to white limestones were obtained by thermal demagnetization and were stable from about 300 c to in excess of 450 c the polarity patterns indicate that the majority of the bajocian and bathonian is characterized by quite frequent reversals of the magnetic field lengthy periods of constant polarity particularly constant normal polarity were not observed the average frequency of reversals is about 6 per ammonite zone which roughly may be interpreted as a frequency of a reversal every 260 000 years a rate comparable to that of the miocene pliocene paleolatitudes of these sites 25 28 are about 10 south of their present positions variable clockwise block rotations within the subbectic region have rotated these sites relative to stable iberia

a u pb and 40ar 39ar time scale for the jurassic published time scales provide discrepant age estimates for jurassic stage boundaries and carry large uncer tainties the u pb or 40 ar 39 ar dating of volcaniclastic rocks with precisely known stratigraphic age is the preferred method to improve the calibration a radiometric age database consisting of fifty u pb and 40 ar 39 ar ages was com piled to construct a revised jurassic time scale accepted ages have a precision of 5 ma 2 or better and are con fined to no more than two adjacent stages the majority of these calibration points result from integrated bio and geochronologic dating in the western north american cordillera and have not been previously used in time scales di rect dates are available only for the triassic jurassic boundary and the initial boundary of the crassicosta chron and the callovian stage the chronogram method was used to estimate all early and early middle jurassic zone boundaries attempted here for the first time late middle jurassic substage boundaries and late jurassic stage boundaries signif icant improvement is achieved for the pliensbachian and toarcian where six consecutive zone boundaries are deter mined the derived zonal durations are disparate varying between 0 4 and 1 6 ma the latest jurassic isotopic database remains too sparse therefore chronogram estimates are improved using interpolation based on magnetochronology the initial boundaries of jurassic stages are proposed as follows berriasian jurassic cretaceous 1418 18

nature of the jurassic magnetic quiet zone the nature of the jurassic quiet zone jqz a region of low amplitude oceanic magnetic anomalies has been a long standing debate with implications for the history and behavior of the earth s geomagnetic field and plate tectonics to understand the origin of the jqz we studied high resolution sea surface magnetic anomalies from the hawaiian magnetic lineations and correlated them with the japanese magnetic lineations the comparison shows the following i excellent correlation of anomaly shapes from m29 to m42 ii remarkable similarity of anomaly amplitude envelope which decreases back in time from m19 to m38 with a minimum at m41 then increases back in time from m42 and iii refined locations of pre m25 lineations in the hawaiian lineation set based on these correlations our study presents evidence of regionally and possibly globally coherent pre m29 magnetic anomalies in the jqz and a robust extension of hawaiian isochrons back to m42 in the pacific crust

nature of radio echo layering in the antarctic ice sheet detected by a two frequency experiment a two frequency radio echo sounding experiment was carried out at dome fuji the second highest dome in east antarctica and along a 1150 km long traverse line from the dome to the coast the goal was to determine the dominant causes of the radio echo internal reflections and to investigate their possible changes with depth ranges and regions from the two frequency 60 mhz and 179 mhz radio echo responses at various sites we distinguished four zones each of the zones is characterized by a dominant cause of radio echo internal reflection as follows in the pd zone changes in dielectric permittivity are mainly due to density fluctuations in the pcof zone changes in dielectric permittivity are mainly due to changes in crystal orientation fabrics and in the ca zone changes in electrical conductivity are mainly due to changes in acidity induced by past volcanic eruptions in each of these three zones the changes occur commonly along isochrones in addition a basal echo free zone the fourth zone was found to appear always below the pcof zone these four zones and their distribution suggested variations of the physical conditions within the ice sheet

rapid development of anisotropic ice crystal alignment fabrics inferred from englacial radar polarimetry central west antarctica 1 anisotropy in englacial radar power was measured using 60 mhz and 179 mhz copolarized pulse modulated radar at 19 sites in central west antarctica the study region is a 100 300 km2area near the west antarctic ice sheet divide that separates ice flow toward the ross and amundsen embayments the frequency dependence of the returned power indicates that most of the radar data are affected by vertical variations in the crystal orientation fabric cof though the 60 mhz data are more affected by acidity contrasts in the top 1000 m significant polarimetric variations occur at most sites likely due to effects of the anisotropic cof patterns more anisotropic variations occur at sites with significant horizontal strain whereas more isotropic variations occur at sites where vertical compression dominates azimuthal shifts with depth of the principal axes of cof were found in shallow ice near the current flow divide and at greater depths over locations of rough bed the former indicates that the divide has differentially migrated resulting in a rotation of the principal cof axes nevertheless the regionally consistent radar signatures suggest that the first order ice properties in this area have remained constant and that no major changes in the strain configuration or ice topography have occurred for the past five to eight thousand years we conclude that shallow polarimetric features can be related to the current strain configurations and that englacial polarimetric features can help constrain current ice rheology and evolution of the ice topography

paleomagnetic constraints on deformation of superfast spread oceanic crust exposed at pito deep rift 1 the uppermost oceanic crust produced at the superfast spreading 142 km ma 1 full spreading rate southern east pacific rise epr during the gauss chron is exposed in a tectonic window along the northeastern wall of the pito deep rift paleomagnetic analysis of fully oriented dike 62 and gabbro 5 samples from two adjacent study areas yield bootstrapped mean remanence directions of 38 9 8 1 16 7 15 6 n 23 area a and 30 4 8 0 25 1 12 9 n 44 area b both are significantly distinct from the geocentric axial dipole expected direction at 23 s regional tectonics and outcrop scale structural data combined with bootstrapped remanence directions constrain models that involve a sequence of three rotations that result in dikes restored to subvertical orientations related to 1 inward tilting of crustal blocks during spreading area a 11 area b 22 2 clockwise vertical axis rotation of the easter microplate a 46 b 44 and 3 block tilting at pito deep rift a 21 b 10 these data support a structural model for accretion at the southern epr in which outcrop scale faulting and block rotation accommodates spreading related subaxial subsidence that is generally less than that observed in crust generated at a fast spreading rate exposed at hess deep rift these data also support previous estimates for the clockwise rotation of crust adjacent to the easter microplate dike sample natural remanent magnetization nrm has an arithmetic mean of 5 96 a m 3 76 which suggests that they significantly contribute to observed magnetic anomalies from fast to superfast spread crust

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

an unrecognized major collision of the okhotomorsk block with east asia during the late cretaceous constraints on the plate reorganization of the northwest pacific abstract interactions at plate boundaries induce stresses that constitute critical controls on the structural evolution of intraplate regions however the traditional tectonic model for the east asian margin during the mesozoic invoking successive episodes of paleo pacific oceanic subduction does not provide an adequate context for important late cretaceous dynamics across east asia including continental scale orogenic processes significant sinistral strike slip faulting and several others the integration of numerous documented field relations requires a new tectonic model as proposed here the okhotomorsk continental block currently residing below the okhotsk sea in northeast asia was located in the interior of the izanagi plate before the late cretaceous it moved northwestward with the izanagi plate and collided with the south china block at about 100 ma the indentation of the okhotomorsk block within east asia resulted in the formation of a sinistral strike slip fault system in south china formation of a dextral strike slip fault system in north china and regional northwest southeast shortening and orogenic uplift in east asia northeast striking mountain belts over 500 km wide extended from southeast china to southwest japan and south korea the peak metamorphism at about 89 ma of the sanbagawa high pressure metamorphic belt in southwest japan was probably related to the continental subduction of the okhotomorsk block beneath the east asian margin subsequently the north northwestward change of motion direction of the izanagi plate led to the northward movement of the okhotomorsk block along the east asian margin forming a significant sinistral continental transform boundary similar to the san andreas fault system in california sanbagawa metamorphic rocks in southwest japan were rapidly exhumed through the several kilometer wide ductile shear zone at the lower crust and upper mantle level accretionary complexes successively accumulated along the east asian margin during the jurassic early cretaceous were subdivided into narrow and subparallel belts by the upper crustal strike slip fault system the departure of the okhotomorsk block from the northeast striking asian margin resulted in the occurrence of an extensional setting and formation of a wide magmatic belt to the west of the margin in the campanian the block collided with the siberian margin in northeast asia at about 77 ma a new oceanic subduction occurred to the south of the okhotomorsk block ending its long distance northward motion based on the new tectonic model the abundant late archean to early proterozoic detrital zircons in the cretaceous sandstones in kamchatka southwest japan and taiwan are interpreted to have been sourced from the okhotomorsk block basement which possibly formed during the late archean and early proterozoic the new model suggests a rapidly northward moving okhotomorsk block at an average speed of 22 5 cm yr during 89 77 ma it is hypothesized that the okhotomorsk east asia collision during 100 89 ma slowed down the northwestward motion of the izanagi plate while slab pull forces produced from the subducting izanagi plate beneath the siberian margin redirected the plate from northwestward to north northwestward motion at about 90 89 ma

impact cratering on venus physical and mechanical models results are presented on laboratory theoretical and numerical simulations of various phenomena related to impact cratering processes specific to the venusian environment such processes are influenced mainly by two factors the high atmospheric density and the high surface temperature model results are presented for the influence of the atmosphere on high velocity meteoroids the influence of high velocity meteoroids on the atmosphere the influence of atmospheric shock waves on the venusian surface and the influence of high crustal temperature on impact melting and melt cooling new modeling techniques based on the two dimensional free lagrangian method have been employed and show that an icy body of 1 km radius may reach the bottom of the venusian atmosphere in a partially dispersed state and form an impact crater approximately 20 km in diameter a significant amount of energy is transferred to the venusian atmosphere during meteoroid transit and the gas dynamic flow will be similar to a strong explosion modeling shows that a significant wake of low density will form behind the meteoroid and have an important influence on crater formation and ejecta distribution according to our estimates the boundary of the flow of dense gas may correspond to the region of hummocky ejecta deposition observed for venusian impact craters the collapse of the gas rock debris column on the boundary between the low and high density regions may result in a turbiditelike flow outside the hummocky ejecta area we carried out preliminary laboratory simulations of the interaction of meteoroid shock waves with the surface considerable surface disruption is possible even under the trajectory of an oblique impact the possibility of back venting caused by shock wave pressuring and depressuring in ground pores may provide an explanation for the origin of dark margins the differences between venus and earth for impact melting and melt cooling were examined melt generation on venus should be 20 to 40 greater than on earth for the same crater diameter melt volumes should stay molten about an order of magnitude longer on venus leading to substantial melt flows and smooth crater floors

borehole magnetic surveys in weakly magnetic sediments chicxulub impact crater versus strongly magnetic volcanics bathurst mining camp borehole navigation surveys performed using a triaxial fluxgate magnetometer record the change in orientation of the magnetic vector versus depth variations in the orientation of the magnetic vect

dipole strength and variation of the time averaged reversing and nonreversing geodynamo based on thellier analyses of single plagioclase crystals 1 single plagioclase crystals separated from lavas can contain minute magnetic inclusions that faithfully record the geomagnetic field while the silicate host limits natural and experimentally induced alteration here we use paleointensity analyses of plagioclase crystals to examine three characteristic geodynamo regimes spanning the last 200 myr an interval of moderate reversal frequency during the paleogene the nonreversing field of the cretaceous normal polarity superchron and a period of very high reversal occurrence during the late jurassic an inverse relationship between reversal rate and intensity is supported furthermore the strength of the time averaged reversing field is more variable than that of the nonreversing field these observations are consistent with an active lower mantle shaping core mantle boundary heat flux and efficiency of the geodynamo on timescales of tens of millions of years

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

venus topography and kilometer scale slopes during the first 8 months of the magellan mission the radar altimeter has made some three million measurements of the surface of venus covering the latitude range from 85 n to 80 s methods involving range correlation doppler filtering multi burst summation and range migration are used to focus the observations and to achieve high surface resolution results are presented as maps of the global distribution of topography meter scale roughness and power reflection coefficient the results are similar to those reported in previous experiments surface heights exhibit a unimodal distribution with more than 80 of the surface lying within 1 km of the 6051 84 km mean radius but the higher resolution of the magellan altimeter has disclosed several surprisingly steep features e g the southwest face of the maxwell montes the southern face of the danu montes and the chasmata to the east of thetis regio where average kilometer scale slopes of greater than 30 are not uncommon this conclusion is corroborated by close inspection of synthetic aperture radar imagery

geology and distribution of impact craters on venus what are they telling us magellan has revealed an ensemble of impact craters on venus that is unique in many important ways we have compiled a data base describing the 842 craters on 89 of venus surface mapped through orbit 2578 the craters range in diameter from 1 5 to 280 km we have studied the distribution size density morphology geology and associated surface properties of these craters both in the aggregate and for some craters in greater detail we find that 1 the spatial distribution of craters is highly uniform 2 the size density distribution of large craters diameters 35 km is similar to the young crater populations on other terrestrial planets but at a much lower density that indicates an average age of only about 0 5 ga based on the estimated population of venus crossing asteroids 3 unlike the case on other planets the density of small craters diameters 35 km declines rapidly with decreasing diameters because of atmospheric filtering 4 the spectrum of crater modification differs greatly from that on other planets 62 of all craters are pristine only 4 are embayed by lavas and the remainder are affected by tectonism but none are severely and progressively depleted as extrapolated from the size density distribution of larger craters 5 large craters have a progression of morphologies generally similar to those on other planets but small craters are typically irregular or multiple rather than bowl shaped 6 diffuse radar bright or dark features surround some craters and 367 similar diffuse splotches with no central crater are observed and 7 other crater features unique to venus include radar bright or dark parabolic arcs opening westward and extensive outflows originating in crater ejecta the first three of these observations are entirely unexpected we interpret them as indicating that the planet s cratering record was erased by a global resurfacing event or events the latest ending about 0 5 ga after which volcanic activity declined but did not cease entirely since the last resurfacing event a maximum of 10 of the planet has been resurfaced and only about 4 of the craters have been obliterated convective thermal evolution models support this interpretation arkani hamed and toksoz 1984 observations 3 7 confirm quantitatively the expectation that the dense atmosphere of venus has strongly affected the production of craters large impactors have been relatively unaffected intermediate sized ones have been fragmented and have produced overlapping or multiple craters a narrow size range has produced shock induced splotches but no craters and the smallest bodies have had no observable effect on the surface the number of craters eliminated by the atmospheric filter is enormous about 98 of the craters between 2 and 35 km in diameter that magellan might have observed on a hypothetical airless venus unique crater related features such as parabolas and outflow deposits demonstrate the roles of venus high atmospheric density and temperature in modifying the crater formation process finally heavily fractured craters and lava embayed craters are found to have higher than average densities along the major fracture belts and rifted uplands connecting aphrodite terra and atla beta themis and phoebe regiones these craters thus provide physical evidence for recent volcanic and tectonic activity at a low level

internal and basal ice changes near the grounding line derived from radio echo sounding evidence of changing basal and internal ice properties near the grounding line was derived from airborne radio echo sounder observations of the ice sheet around the sor rondane mountains antarctica from the trailing figure of the bottom echo signal the roughness of the ice bottom near the grounding line was inferred results show that the specular components of scattering begin to appear on the ice shell side of the grounding line furthermore double trip echoes were observed with at strong scattering in the shelf area and their boundary of occurrence was very close to the grounding line this is evidence of interaction between ice and sea water at the bottom of the ice shelf we also examined the occurrence of internal layered echoes in most of the area around the mountains internal echoes were observed continuously but they were not found at or close to the ice shelf the boundary between the appearance and disappearance of internal layer echoes is distinct and occurs 20 30 km inland from the grounding line these results suggest that some major change may occur in the internal ice on the inland side of the grounding line

paleointensity results from the jurassic new constraints from submarine basaltic glasses of odp site 801c 1 tholeiite of the oldest oceanic crust was drilled during odp legs 129 and 185 at hole 801c in the western pacific fresh appearing submarine basaltic glass sbg was recovered from the tholeiites 167 ma which has been shown to be nearly ideal for determining absolute paleointensity paleointensities of the younger off axis alkalic basalts 160 ma overlying the tholeiites had been studied earlier here we report results from the older tholeiitic on axis sequence we subjected a total of 73 specimens from 17 cooling units to absolute paleointensity experiments of these 30 specimens and six cooling unit averages met our strictest reliability criteria yielding an average of 11 9 3 9 t the bulk of evidence suggests a paleolatitude of the site of 14 s with an uncertainty of 10 this translates the intensity to a value for the virtual axial dipole moment of 28 zam2 slightly lower than values determined from the plagioclase crystals in the three cooling units of the younger alkalic basalts overlying the tholeiites tarduno cottrell 2005 our value is low when compared to the long term median value of the field of 42 zam2 our results and those of the published literature therefore support the contention of a low magnetic field strength in the jurassic average of 28 14 zam2 n 138 individual estimates as initially suggested by prevot et al 1990 our interpretation of the body of available data argue for low field strengths for the entire jurassic extending into the early cretaceous

radio echo layering in polar ice sheets and past volcanic activity a recent study1 has shown how acidity profiles along dated greenland ice cores can reveal large volcanic eruptions in the northern hemisphere during the past 10 000 yr it has been suggested that these layers of acidic ice in the polar ice sheets may also be detected by airborne radio echo sounding res techniques as stratification echoes2 4 explosive volcanic eruptions eject large amounts of so2 into the stratosphere5 where it forms an h2so4 aerosol6 studies of antarctic snow7 and greenland ice cores2 show that this material can be deposited in layers of large areal extent on polar ice sheets calculations based on observations of these slightly acidic layers show that they should give rise to radar reflections of similar magnitude to those observed for stratification echoes new res data from the antarctic enables the present comparison to be made between observed layer power reflection coefficients prcs and calculated values for reflections from acidic ice layers and from layers of ice of changed density a gap in layering has been identified which seems to coincide with a similar gap reported in greenland at crete8 profiles of layer prc against age show a common pattern for many sites on the antarctic ice sheet this prc age profile may provide a record of explosive volcanic activity for the southern hemisphere over the past 150 000 yr

pacific microplate and the pangea supercontinent in the early to middle jurassic new biostratigraphic data based on radiolarians recovered from deep within the oceanic crustal section of ocean drilling program hole 801c in the western pacific along with existing radiometric information date this oceanic crust as late bajocian early bathonian 170 165 ma the overlying basal sediments at hole 801c are essentially identical in age middle bathonian 164 162 ma to the basal sediments at deep sea drilling program hole 534a in the central atlantic we estimate the time of formation of the pacific plate as 175 170 ma and the time of initial separation of the pangea supercontinent in the central atlantic as 190 180 ma we also identify a time of extensive subduction zone magmatism 175 159 ma at the eastern and western edges of pangea we suggest that the initial plate separation of pangea increased subduction rates at its outer margins and altered the plate boundaries in the pacific superocean leading to formation of the pacific plate

tectonic rotations in extensional regimes and their paleomagnetic consequences for oceanic basalts paleomagnetic samples of many deep sea drilling project dsdp basalts have shallower inclinations than would be inferred from the paleolatitude at which the basalts were extruded at mid ocean ridges rather than arising from peculiarities of the geomagnetic field the anomalous magnetic directions may result from tectonic rotations along the listric normal faults which often characterize extensional tectonic regimes several lines of evidence support this conclusion observations of the basin and range province an extensional tectonic regime occurring in continental crust observations of the troodos complex cyprus an oceanic extensional tectonic regime now exposed on land and analysis of possible fault patterns associated with the magma chambers postulated to occur at some mid oceanic ridges the tectonic rotations resulting from movement along the listric normal faults occur about a horizontal axis generally perpendicular to the extension direction and typically amount to 30 to 50 although rotations as large as 70 to 90 are not uncommon the paleomagnetic consequences of these rotations can be calculated for dsdp basalts if the original strike and latitude of a ridge are known at four dsdp sites for which adequate data are available namely sites 319 332 410 and 417 rotations ranging from 50 to 70 are required to explain the paleomagnetic results if the original strike and location of the ridge are not known as is the case for most of the older parts of the pacific ocean the paleomagnetic data and the inferred rotation determine a relationship between possible ridge strikes and locations this information combined with a similar but independent relationship derived from the marine magnetic anomalies puts severe constraints on the original strike and location of the ridge

direct measurement of magnetic reversal polarity boundaries in a cross section of oceanic crust magnetic field measurements made by submersible define the cross sectional geometry of a magnetic polarity rever sal boundary and the vertical variation of crustal magnetization in upper oceanic crust measured polarity boundaries show a systematic pattern of shallow dip towards the spreading axis within the upper extrusive lavas and steeper dip in the lower extrusive lavas this geometry is a consequence of the em placement of extrusive lava at a midocean ridge reversal boundary geometry and magnetization estimates are used to calculate the magnetic contribution of the extrusive lava se quence to the overlying marine magnetic anomaly signal from the forward modeling the highly magnetized extrusive lavas contribute the majority 50 75 of the observed sea surface magnetic anomaly consistent with the extrusive crust forming the primary source layer for young marine magnetic anomalies