the galapagos rift at 86 w 5 variations in volcanism structure and hydrothermal activity along a 30 kilometer segment of the rift valley a 30 km segment of the galapagos rift near 86 w has been mapped in detail using the angus towed camera system the submersible alvin and multi narrowbeam sonar data recent volcanic activity and active hydrothermal circulation are evident along the entire length of the segment mapped there are however clear along strike variations in these processes which render previous two dimensional models obsolete although alternate explanations are possible eruptive sequences appear to begin with the outpouring of surface fed sheet flows and end with more channelized pillow flows in the western portion of the rift studied sheet flows dominate with the entire valley floor covered by recent flows associated with a broad shield volcano the eastern portion on the other hand is narrower consisting primarily of less voluminous pillow flows of apparently the same youthful age three possible models for the volcanic evolution of this rift segment are presented according to the first model the extrusive portion of the crust is formed by a distinct volcanic episode followed by a long period of volcanic quiescence the volcanic phase begins with voluminous sheet flows emerging from numerous eruptive fissures which in time evolve into a narrow pillow ridge farther along strike where the flows are smaller and the extrusive zone narrow the marginal portions undergo continued fissuring and subsequent uplift to form marginal highs and lows this deformational activity also affects the extrusive zone once volcanic activity ends converting the distinctly lobate topography of the active period into highly lineated fault controlled terrain according to the second model extension and voleanism can be viewed as a continuous process without major periods of volcanic quiescence the initial lava flows of a new eruptive sequence fill low areas frequently spilling over local sills and flooding much of the rift valley these sheet flows are then capped by pillow ridges but from an areal view sheet flows dominate since they can flow over much greater distances in the third model along strike differences in flow type arise from ephemeral differences in spreading rate along adjacent segments of the rift separated by transient transform faults or adjacent segments of en echelon spreading segments in this third model eruptions dominated by sheet flows are associated with a faster spreading element than eruptions dominated by pillow flows active hydrothermal vent fields were found throughout the study area as well as numerous yellow and ocher accumulations of what appear to be massive sulfide deposits these latter deposits are not associated with active venting apparently reflecting previous active hydrothermal phases when the exiting temperatures were sufficently high to permit surficial sulfide deposition the active vent fields exhibit along strike variations in their exiting temperatures either due to ephemeral differences in spreading rate or variations in the volcanic flow sequences

the juan de fuca ridge hot spot propagating rift system new tectonic geochemical and magnetic data underway geophysics deep tow camera work and preliminary geochemical and magnetic studies from a 1980 cruise to the juan de fuca ridge allow the following interpretations 1 the central third between 45 5 n and 46 5 n of the ridge is now actively spreading from a zone which is 10 to 20 km west of the axis bisecting the brunhes matuyama magnetic reversal boundary 2 the apparent terminus of surficial igneous activity at the northern tip of the active cobb propagator has been identified by dredging and camera work the tip appears curved or offset slightly to the west does not correspond to the northernmost tip of the central magnetic anomaly elvers et al 1974 and spreading has been initiated beneath sediments farther north and 3 iron and titanium in fresh glasses from the active volcanic zone exhibit a progressive enrichment northward then a pronounced reversal to more normal concentrations in the vicinity of the cobb offset

evolution of abyssal lavas along propagating segments of the galapagos spreading center abstract the unusual petrological diversity of abyssal lavas erupted along some segments of the galapagos spreading center is a direct consequence of the propagation elongation of these segments into older oceanic crust with increasing distance behind propagating rift tips relatively unfractionated morb erupted close to the tips are joined first by feti basalts bimodal assemblage and then by a wide range of basaltic and siliceous lavas further behind propagating rift tips this broad range diminishes again approaching the narrow compositional range of adjacent normal ridge segments these compositional variations reflect the evolution of the subaxial magmatic system beneath the newly forming spreading center as it propagates through a pre existing plate we envisage this evolution as proceeding from small isolated ephemeral magma chambers through increasing numbers of larger increasingly interconnected chambers to the steady state buffered system of a normal ridge throughout this evolution magma supply rates gradually increase and cooling rates of crustal magma bodies decrease high degrees of crystal fractionation are favored only when a delicate balance between cooling rate and resupply rate of primitive magma is achieved at other propagating and non propagating ridge transform intersections the degree to which the balance is achieved and the length of ridge over which it evolves control the distribution of fractionated lavas these effects may be evaluated provided a number of tectonic variables including transform length spreading and propagation rates are taken into account

volcanics and structure of the famous narrowgate rift evidence for cyclic evolution amar 1 a near bottom photographic survey on the mid atlantic ridge from the famous region south to transform b reveals structural and volcanologic evidence for an alternating widening and narrowing rift valley extension wedges appear to be propagating south from transform a and north from transform b these merge at mt mars where the rift valley is narrowest and shallowest narrowgate giving the valley a symmetrical hourglass shape three major volcanic en echelon lineaments trend n10 e 10 oblique to the strike of the inner walls faulting and fissuring are antisymmetrical across the rift valley in contrast to the major extension this contrasting antisymmetry reflects the surficial distribution of stress across crust of varying strengths whereas the large scale wedge phenomenon determines the periodicity of transition from a narrow valley to a wide valley using vector analysis the extension wedges are propagating at about 9 cm yr toward each other at this rate a narrow rift valley will be fully transformed into a wide rift in 0 64 m y

ridge crest volcanism developments in this area have preceeded very rapidly much of this progress has resulted from sophisticated multi channel seismic surveys of the ridge axes and especially from the detailed investigation or mapping of small areas using multibeam arrays towed cameras and submersibles the magma chamber the depth lateral dimensions of the magma chamber remain unconstrained multi channel seismic reflections from the mantle and from the top of the magma chamber under the crest of the east pacific rise at 9 n have been reported by stoffa et al 1980 and herron et al 1980 these have been re interpreted by hale et al 1982 as showing an assymetric chamber caused perhaps by assymetric spreading or by the intersections of the profile with an unmapped fracture zone herron 1982 using the same data has resolved the transition from the pillow basalt layer to the sheeted dyke complex at a depth of about 400 m all interpretations are consistent with the top of the magma chamber being at a depth of between 1 5 and 2 km below the sea floor in this area

reinterpretation of seismic reflection data over the east pacific rise multichannel seismic reflection data over the axial region of the east pacific rise are depth migrated using detailed velocity information from laboratory measurements of ophiolite samples the data are from the lamont doherty geological observatory s line 17 shot across the rise at 9 sup 0 n the migrated data are interpreted to show the structure of the seafloor and what we believe is the magma chamber roof the polarity and apparent root mean squared velocity of the magma chamber roof are asymmetric with respect to the topographic axis of the ridge the asymmetry is probably real and not an artifact of data collection modeling of the magnetic data gathered along the line shows that spreading has not been proceeding normally a reasonable explanation for the asymmetry is the possibility that the line intersects a transform fault or an abandoned ridge segment near the ridge axis the shape of the roof reflection is convex upward with an approximate slope of 10 sup 0 and a width of 4 km if extrapolated symmetrically to the other side of the ridge axis the magma chamber roof event is consistent with the funnel shaped chamber proposed by pallister and hopson 1981 for the samail ophiolite if the chambermore roof steepens rapidly beyond the extent of the reflection it would be consistent with the mush filled model of sleep 1975 1978 and dewey and kidd 1977 less

topographic forcing of supercritical convection in a porous medium such as the oceanic crust abstract we have performed laboratory experiments using a hele shaw cell to model a saturated porous layer with various sinusoidal upper boundaries our intent was to determine the range of conditions over which boundary topography can control the pattern of thermal convection within a porous layer and thereby take the first step toward understanding why heat flow seems correlated with hypsography in many areas of the ocean floor these experiments indicate that above the critical rayleigh number topography does not control the convection pattern except when the topographic wavelength is comparable to the depth of water penetration scaled to the depth of the layer the convective wavenumbers are restricted to values between 2 5 and 4 8 a range which brackets the natural wavenumber for convection in a porous slab with planar isothermal impermeable boundaries topographies within this range control the circulation pattern perfectly with downwelling under valleys and upwelling aligned with topographic highs other topographies do not force the pattern although in some cases the convection wavenumber may be a harmonic of the topographic wavenumber unforced circulation cells wander and vary in size because they are not locked to the topography for these experiments we employed eight different topographies with non dimensional wavenumbers between 1 43 and 8 17 and we studied the flow at rayleigh numbers between zero and five times the critical rayleigh number the amplitude of each topography tapered linearly over a factor of three to six from one end of the apparatus to the other and the mean topographic amplitude was 0 05 times the depth of the layer under these conditions amplitude has only a minor effect on the structural form and vigor of supercritical convection our results may apply to submarine geothermal systems sealed by a thin layer of impermeable sediment draped over the basement topography in this case the convection wavelength as measured perhaps by the spatial periodicity of conductive heat flow may be a good measure of the depth to which water penetrates the crust where the circulation correlates with the bottom topography it may be because the topographic wavelength is comparable to the depth to which water penetrates the porous crust

geophysical evidence for the absence of a crustal magma chamber under the northern juan de fuca ridge a contrast with rose results deep tow reflection data seismic refraction data to ocean bottom seismometers and gravity interpretations of the northern juan de fuca ridge all suggest a very narrow or absent crustal magma chamber under this portion of the rise axis the seismic features that have been suggested as indicating crustal magma chambers on other parts of the east pacific rise such as travel time delays and reflections from the top of a shallow magma reservoir are not found on the northern part of this ridge these results indicate that at spreading rates of 3 cm yr crustal cooling probably by water penetration is sufficiently rapid to prevent the existence of a steady state crustal magma chamber and therefore crustal accretion may be episodic

rivera ocean seismic experiment rose overview the rivera ocean seismic experiment rose was designed as a combined sea and land seismic program to utilize both explosive and earthquakes to study a number of features of the structure and evolution of a mid ocean ridge a major oceanic fracture zone and the transition region between ocean and continent the primary region selected for the experiment included the rivera fracture zone the crest and eastern flank of the east pacific north of the rivera and adjacent areas of baja california and mainland mexico the experiment included 1 study of the east pacific rise south of the orozco fracture zone primarily using ocean bottom recording and explosive sources 2 a seismicity program at the orozco and 3 a land based program of recording natural events along the coastal region of mexico a considerable amount of useful data was obtained in each of the three subprograms in the marine parts of the experiment we were able to address a variety of problems including structure and evolution of young oceanic crust and mantle structure and dynamics of the east pacific rise seismicity of the orozco fracture zone and partitioning of energy transmission between the ocean volume and the crust lithosphere on land the fortuitousmore occurrence of the petatlan m7 6 earthquake of march 14 1979 permitted the acquisition of an excellent data set of foreshocks and aftershocks of this large event which provide new insight into the filling of a major seismic gap in the region this overview describes the scientific rationale and the design of the experiments along with some general results less

internal structure and early thermal evolution of mars from mars global surveyor topography and gravity topography and gravity measured by the mars global surveyor have enabled determination of the global crust and upper mantle structure of mars the planet displays two distinct crustal zones that do not correlate globally with the geologic dichotomy a region of crust that thins progressively from south to north and encompasses much of the southern highlands and tharsis province and a region of approximately uniform crustal thickness that includes the northern lowlands and arabia terra the strength of the lithosphere beneath the ancient southern highlands suggests that the northern hemisphere was a locus of high heat flow early in martian history the thickness of the elastic lithosphere increases with time of loading in the northern plains and tharsis the northern lowlands contain structures interpreted as large buried channels that are consistent with northward transport of water and sediment to the lowlands before the end of northern hemisphere resurfacing

east pacific rise at 21 n the volcanic tectonic and hydrothermal processes of the central axis abstract photographs obtained by the angus survey system at 21 n reveal many similarities to the geological processes delineated at other spreading centers and in particular those observed in the galapagos rift at 86 w the region of recent volcanism is restricted to a narrow zone zone 1 approximately 1 km wide this suggests that the width of the magma chamber feeding these flows is also narrow at the top variations in sediment cover are used to subdivide the flows within zone 1 into three sheet flow pillow flow pairs the youngest pair appears to have erupted form a linear fissure 8 km long running parallel to the valley axis this fissure is part of a larger en echelon pattern of eruptive fissures active hydrothermal vents associated with the youngest flows are situated directly above the apparent eruptive fissure the high 350 c temperature for some of the fluids exiting from the vents suggests a highly restricted circulation system involving at times little to no mixing with cold seawater the lava terrain in the remainder of zone 1 and bordering regions is characterized by extensive fracturing further suggesting a narrow upper dimension to the magma chamber and also that subsurface hydrothermal deposition of massive sulfides may lead to a reduction in the mixing processes

constraints on the structure of the east pacific rise from gravity gravity data across the east pacific rise at 9 n and 12 n the rose area are characterized by a high of 12 20 mgal with a width of about 20 km it is shown that this axial gravity high is not adequately explained by uncompensated constant density topography or an isostatically compensated thermally expanded ridge crest it can be adequately explained by airy compensation with a depth of compensation of at least 20 km or by elastic plate compensation with a flexural rigidity between 1018 and 1020 n m the significance of the depth and flexural rigidity parameters is that in both cases they result in a gravity field from the compensation whose amplitude is below the data resolution for wavelengths less than about 50 km that is the gravity data do not require compensation of the rise axis for wavelengths less than about 50 km this suggests that these wavelengths are either supported mechanically or by other forces dynamic the mechanical interpretation is not consistent with partial melt under the axis indicating that dynamic forces may be important in supporting the rise axis the same conclusion is arrived at from the thermal model the thermal model can be made to fit the gravity data if we allow departures from isostasy and the addition of a dike like body about 2 km wide in the crust having a positive density contrast of about 0 25 mg m 3 this geometry is consistent with a seismically determined low velocity zone in the crust in the rose area in this model the nonhydrostatic pressures must be supported dynamically a general conclusion of this study for the portions of the east pacific rise studied here is that the axial topography is not buoyantly supported by low density material in the crust therefore either the crust under the rise axis mechanically supports the axial topography or a mass excess at the thermally expanded rise axis is supported by other forces such as plate driving forces the latter option is favored on the basis of heat flux and seismic data

geologic investigations in the cayman trough evidence for thin oceanic crust along the mid cayman rise the mid cayman rise is a 110 km long accreting plate boundary 2cm yr full rate characterized by a deep rift valley eighteen transponder navigated dives were made by the submersible alvin along the rift valley walls in two areas on opposite sides of the rift valley the relief of the walls is produced by a large number of inward facing small throw fault escarpments with vertical displacements ranging from a few tens of meters to perhaps as much as 400 m from these escarpments 142 samples were collected and gabbroic rocks 116 samples characterized by a wide range of compositions and a complex metamorphic history predominate in number over other rock types basalt serpentinized ultramafic we interpret the assemblage of rocks exposed on the escarpments to indicate that a shallow level magma reservoir exists beneath the axis of accretion at the mid cayman rise and creates anomalously thin oceanic crust with a carapace of only several hundreds of meters of diabase and basalt this carapace is und

constraints on the structure of the east pacific rise from seismic refraction data university of washington seismic data taken during rose phase 1 have been used to study the structure of the east pacific rise in the vicinity of 12 n off axis but within 50 km of the axis we find that the oceanic crust can be modeled with a rapid change in velocity in the top 2 5 km a zone of low but uniform velocity gradient from 2 5 to about 5 5 km and a transitional zone about 1 km thick to mantle material there is also evidence for substantial anisotropy in mantle velocity gradients with a positive velocity gradient in the spreading direction and a negative velocity gradient parallel to the rise under the rise axis the data allow at most a very narrow zone of low velocity less than about 2 km wide in the crust

evidence for extensive denudation of the martian highlands high resolution topographic data from the mars orbiter laser altimeter reveal evidence for widespread denudation from margaritifer sinus to northern arabia terra an area of 1 107 km2 a major resurfacing event is indicated by 1 a heavily degraded landscape hosting numerous inliers 2 truncation or absence of valley networks and 3 a break in slope at the edge of early to middle noachian plateau materials geomorphic mapping was completed on the type locale of denudation 0 30 s 0 30 w superposition relations and crater counts for geomorphic units indicate that large scale resurfacing took place in the late noachian eroding the martian highlands and resulting in transportation and deposition of 4 5 106 km3 of sediment in the northern plains this is equivalent to a 120 m thick uniform layer of sediment on the surface of mars north of 30 n geomorphic mapping and crater counts limit the timing of denudation to the late noachian an interval of 350 500 m y using this limit we estimate a minimum rate of denudation of 2 0 m yr comparable to denudation of typical slopes in a temperate maritime climate on earth the morphology rate of denudation and extensive nature of upland degradation suggest that precipitation fed surface runoff is the most likely geomorphic agent capable of such a process indicative of a warm wet mars during the late noachian epoch

galapagos hot spot spreading center system 2 87sr 86 sr and large ion lithophile element variations 85 w 101 w thirty eight samples of basalts dredged from the galapagos spreading center gsc between 85 w and 101 5 w longitudes have been analyzed for k rb cs ba and sr contents as well as for 87sr 86sr ratios basalts between 85 w and 89 w segment of the gsc range from 0 7025 to 0 7028 in 87sr 86sr and those between 95 5 w and 101 5 w range from 0 7026 to 0 7027 these basalts are characterized by lree depleted patterns basalts from 89 w to 95 5 w segment having slightly lree depleted to lree enriched patterns la sm e f between 0 8 and 2 4 range from 0 7027 to 0 7031 in 87sr 86sr the longitudinal 87sr 86sr variation shows a maximum at about 92 w where the gsc is nearest to darwin island the alkali and alkaline earth elements as well as several element ratios also show maxima in their longitudinal profiles but these occur at about 91 w the apparent shift of 87sr 86sr maximum with respect to that of the trace elements may be related to differences in the degree of partial melting the highest 87sr 86sr from the gsc is only slightly lower than the highest 87sr 86sr for tholeiitic basalts from the galapagos archipelago several possible hypotheses have been examined to account for these geochemical and sr isotopic variations in the light of the presently available data the most promising explanation seems to be a binary mixing of hot spot or plume type material with the source of normal mid ocean ridge basalts and resulting rift propagation both end members seem to be heterogeneous and therefore it is at present difficult to put severe constraints on this binary mixing nevertheless the flow pattern of mantle material from the hot spot does not appear to be radial but is preferentially channeled along the 91 w and 92 5 w fracture zones toward the rift

galapagos hot spot spreading center system 1 spatial petrological and geochemical variations 83 w 101 w we report on the petrology and geochemistry of basalts dredged at 40 50 km intervals along the galapagos spreading center between 83 sup 0 w and 101 sup 0 w 40 stations emphasis is on spatial variations of whole rock major elements rare earths trace metals of the first transition series and the nature of phenocryst assemblages and their abundances these results provide new constraints on the nature and scale of mantle source heterogeneities melting conditions thermal field and dynamics of crustal formation of the region we suggest that ridge segments outside the high magnetic amplitude zone are at a steady state as a result of passive seafloor spreading basalts from these segments are apparently derived from an asthenosphere relatively uniformally depleted in incompatible elements which appears of worldwide extent we reject vogt and deboer s 1976 model invoking damming at fracture zones of subaxial asthenosphere flow of crystal slushes and increasing fractional crystallization down the flow line because this model would not explain the gradients in ree observed about the galapagos platform our preferred model combines the mantle plume binary mixing model of schilling 1973 with the concept of recurring rift propagation proposed by hey 1977a we further propose that pulsating mantle plume flux perhaps inmore the form of a chain of blobs may initiate the development of new rifts and their propagation the present position of the tips of such new propagating rifts locate the wave fronts of such pulsating mantle plume flow a two million year period is suggested for the last 4 m y from wilson and hey s 1979 information rigorous testing of our preferred model is possible less

minor and trace element geochemistry of volcanic rocks dredged from the galapagos spreading center role of crystal fractionation and mantle heterogeneity a wide range of rock types abyssal tholeiite fe ti rich basalt andesite and rhyodacite were dredged from near 95 w and 85 w on the galapagos spreading center computer modeling of major element compositions has shown that these rocks could be derived from common parental magmas by successive degrees of fractional crystallization however the p2o5 k2o ratio averages 0 83 at 95 w and 1 66 at 85 w and implies distinct mantle source compositions for the two areas these source regions also have different rare earth element ree abundance patterns with la sm ef 0 67 at 95 w and 0 46 at 85 w the sequence of fractionated lavas differs for the two areas and indicates earlier fractionation of apatite and titanomagnetite in the lavas from 95 w the mantle source regions for these two areas are interpreted to be depleted in incompatible and volatile elements although the source region beneath 95 w is less severely depleted in la and k incompatible trace element abundances in 26 samples are used to infer that the range of fe ti rich basalt from 85 w represents 19 to 35 residual liquid following crystal fractionation of a mineral assemblage of plagioclase clinopyroxene and lesser olivine the most highly differentiated samples have also had less than 1 titanomagnetite removed most samples from 85 w can be related to a common parental magma that contained approximately 9 wt feo 1 wt tio2 and had an mg number mg 100 mg mg fe2 of about 65 although the samples from 95 w cannot all be derived from a common parental magma the inferred parental magmas may have been derived by varying degrees of partial melting of a common source the fractionation sequence consists of two parts an initial iron enrichment trend followed by a silica enrichment trend we interpret the trace element data to indicate that the most iron rich lavas represent about 32 residual liquid derived by crystal fractionation of plagioclase clinopyroxene and lesser olivine from a parental magma with an mg number of about 66 the silica enrichment trend results from crystallization of titanomagnetite and some apatite fractionation of pigeonite which is a minor phase in the major element models cannot be distinguished from clinopyroxene fractionation by using trace elements

microearthquakes in the black smoker hydrothermal field east pacific rise at 21 n in july and august 1980 an array of five ocean bottom seismographs was deployed within 3 km of the 350 c hydrothermal vents at the rivera submersible experiment rise site at 21 n on the east pacific rise two of these instruments were placed within 600 m of the vents using a transponder navigation network the array detected four basic types of events the first type consisted of local very small microearthquakes locations obtained for 11 of these events place three within 1 km of the vents with the others elsewhere along the rise crest they appear to originate either from movement on the faults in the area or from the hydrothermal system beneath this area a study of the s p times of this type indicates a maximum hypocentral depth of 2 3 km implying a similar limit to the depth of hydrothermal circulation and brittle fracturing in the vicinity of the vents the second type of event found consisted of emergent earthquakes that have many of the characteristics of volcanic harmonic tremor the frequency of these events falls in the 1 5 hz range and are similar in appearance to those seen at mount st helens prior to and during its may 1980 eruption they may be either hydrothermal or volcanic in origin the third type of event produced a very monochromatic high frequency seismogram with the energy concentrated at 20 hz these events also appear to have a local origin the particle motion of these earthquakes shows almost pure stonely wave propagation and they appear to originate from a regularly oscillating dilatational source the nature of the source of event types two and three is clearly not the usual sort of motion on a fault and is still not understood the fourth type of earthquake recordings were regional events apparently from the nearby rivera and tamayo fracture zones the low frequency acoustic noise in the water was 16 64 times higher within 300 m of the vents than it was 2 km away this implies that the black smoker vents may be generating a large acoustical disturbance as the 350 c water pours into the ocean the observations are among the first of seismicity on moderate to fast spreading ridges and many of the events appear to be related to the hydrothermal activity found there

oceanic ridge crest processes oceanic ridge crest processes constitute a vigorous frontier of multidisciplinary research with global ramifications in the earth atmospheric and biologic sciences this deep ocean frontier is gradually opening in response to investigations which are elucidating the role of these processes in the lithosphere hydrosphere atmosphere and biosphere and the application of advanced instrumental and interpretive techniques which are increasing the quality and quantity of data these techniques include crustal drilling with borehole logging deep sea drilling project reports high resolution bathymetry sea beam etc side scan sonar seamarc i and ii gloria etc imagery with video and photography multichannel seismics determination of water column properties ctd with rosette sampler etc and submersible applications

sh wave scattering and propagation analyses at irregular sites by time domain bem abstract this article presents numerical computations of two dimensional transient responses of canyons and alluvium deposits with various configurations subject to vertically and obliquely incident sh waves the time domain boundary element method bem is used for the analysis elementwise double integrals over space and time domains are performed analytically for the first time the substructure formulation is used for the analysis of alluvium deposits the validity of the present solution is examined for a semi circular canyon and an alluvium whose frequency domain solutions are available from the past publications in the following analyses the transient responses are computed for specific ricker wavelets under different angles of incidences for various site configurations the results show that the proposed technique is reliable and therefore can be used to study the seismic response at irregular sites

lava flow layer east pacific rise a multichannel seismic reflection profile across the east pacific rise near 9 n reveals an approximately 400 m thick layer at the top of the oceanic crust with seismic velocity of about 2 8 km s at the rise crest this velocity increases to about 3 8 km s 210 km east of the rise crest these low velocities suggest that this layer is probably a porous lava flow layer layer 2a of seismic refraction studies generated by volcanic activity at the spreading center reflections from the base of the layer imply a relatively sharp transition between the lava flows and the underlying layer presumably a sheeted dike complex the underlying layer is a zone about 1 4 km thick with velocity about 5 7 km s whose base may coincide with the top of a low velocity magma chamber from which the crust develops

propagating rift explanation for the tectonic evolution of the northeast pacific the pseudomovie abstract we have developed a new computer graphic technique to use in reconstructing plate tectonic evolution and to make movies illustrating this evolution using this technique we have deciphered the evolutionary history of the juan de fuca area in the northeast pacific and show that a combination of seafloor spreading and rift propagation can explain this seemingly complex area as a simple consequence of the interaction of two rigid plates

rates of surface processes on slopes slope retreat and denudation results taken from 270 publications on rates are summarized and collated with those from 149 publications reviewed previously young 1969 1974 the data are classified by major climatic zone normal or steep relief and consolidated or unconsolidated rocks representative rates and their ranges are given for soil creep solifluction surface wash solution chemical denudation rock weathering slope retreat cliff free face retreat marine cliff retreat and denudation the last being compared with representative rates of uplift solifluction is of the order of 10 times faster than soil creep but both cause only very slow ground loss solution is an important cause of ground loss for siliceous rocks on which it may be half as rapid as on limestones total denudation brought about mainly by surface wash reaches a maximum in the semi arid and probably also the tropical savanna zones acceleration of natural erosion rates by human activities ranges from 2 3 times with moderately intense land use to about 10 times with intensive land use and considerably higher still where there is recognized accelerated soil erosion where there is active uplift typical rates are of the order of 10 times faster than denudation although in some high steep mountain ranges these may approach equality

upper crustal velocity structure in the rose area of the east pacific rise we describe a method to constrain the seismic velocity structure of the upper 500 800 m of young oceanic crust conventional interpretations of seismic refraction profiles provide little or no information about this shallowmost crust the method described here requires the assumption of a linear velocity gradient and utilizes measured values of range travel time and slowness of the observed refracted waves results are reported from several refraction profiles that sample crust between 0 and 4 m y in age on the flanks of the east pacific rise at 12 n these data give an average gradient in the uppermost 800 m of the crust of 3 5 s 1 and a velocity at the seafloor of 2 5 km s ray path and travel time modeling show that the refracted waves sampling the uppermost part of the crust are effectively masked by reflections from the seafloor

the global topography of mars and implications for surface evolution elevations measured by the mars orbiter laser altimeter have yielded a high accuracy global map of the topography of mars dominant features include the low northern hemisphere the tharsis province and the hellas impact basin the northern hemisphere depression is primarily a long wavelength effect that has been shaped by an internal mechanism the topography of tharsis consists of two broad rises material excavated from hellas contributes to the high elevation of the southern hemisphere and to the scarp along the hemispheric boundary the present topography has three major drainage centers with the northern lowlands being the largest the two polar cap volumes yield an upper limit of the present surface water inventory of 3 2 to 4 7 million cubic kilometers

magnetotelluric experiment over the rose area seafloor observations in natural electromagnetic fluctuations diffusing into the earth have been made in the vicinity of the pacific rise at 12 sup 0 n and 21 sup 0 n to gather information on the electrical conductivity structure of the oceanic basement near a spreading ridge at 21 sup 0 n the close proximity of land baja california introduces a noticeable distortion of the em fields and consequently the magnetotelluric sounding for this area must be interpreted with caution at both locations the conductance of the upper 200 km appears exceptionally high and the conductivity rises rapidly below reaching 1 s m sup 1 or more at 350 km the statistical significance of a prominent conductivity increase near a depth of 22 5 km detected in the inverted data from 12 sup 0 n and of a less developed one at 30 km in the case of 21 sup 0 n is discussed records of the magnetic variations of 21 sup 0 n taken above the spreading center compared to records from 40 and 120 km to the west reveal a conspicuous enhancement of the high frequency horizontal variations in a direction slightly west of magnetic north we interpret this distinctive feature as the signature of an electric current sheet concentrated in an extremely highmore conductance relatively shallow layer electrically bridging the two seafloor areas adjacent to the spreading center an approximate calculation suggests a conductance in excess of 3000 s per meter of ridge crest in the close vicinity of the accretion center equivalent to 1 to 8 km of molten basalt depending on temperature 1000 sup 0 1200 sup 0 c and depth 2 10 km this structure is consistent with seismic refraction information for the same area and with stratigraphic reconstruction of a fossil accretion center in samail ophiolite an interpretation in terms of a well developed magma chamber is irresistable although confirmation of the existence of this feature as well as improvement of its resolution by means of additional em data would be of major interest less

geophysical evidence for the locations shapes and sizes and internal structures of magma chambers beneath regions of quaternary volcanism this paper is a review of seismic gravity magnetic and electromagnetic techniques to detect and delineate magma chambers of a few cubic kilometres to several thousand cubic kilometres volume a dramatic decrease in density and seismic velocity and an increase in seismic attenuation and electrical conductivity occurs at the onset of partial melting in rocks the geophysical techniques are based on detecting these differences in physical properties between solid and partially molten rock although seismic refraction techniques with sophisticated instrumentation and analytical procedures are routinely used for detailed studies of crustal structure in volcanic regions their application for magma detection has been quite limited in one study in yellowstone national park u s a fan shooting and time term techniques have been used to detect an upper crustal magma chamber attenuation and velocity changes in seismic waves from explosions and earthquakes diffracted around magma chambers are observed near some volcanoes in kamchatka strong attenuation of shear waves from regional earthquakes interpreted as a diffraction effect has been used to model magma chambers in alaska kamchatka iceland and new zealand one of the most powerful techniques in modern seismology the seismic reflection technique with vibrators was used to confirm the existence of a strong reflector in the crust near socorro new mexico in the rio grande rift this reflector discovered earlier from data from local earthquakes is interpreted as a sill like magma body in the kilauea volcano hawaii mapping seismicity patterns in the upper crust has enabled the modelling of the complex magma conduits in the crust and upper mantle on the other hand in the usu volcano japan the magma conduits are delineated by zones of seismic quiescence three dimensional modelling of laterally varying structures using teleseismic residuals is proving to be a very promising technique for detecting and delineating magma chambers with minimum horizontal and vertical dimensions of about 6 km this technique has been used successfully to detect low velocity anomalies interpreted as magma bodies in the volume range 10 3 10 6 km 3 in several volcanic centres in the u s a and in mt etna sicily velocity models developed using teleseismic residuals of the cascades volcanoes of oregon and california and kilauea volcano hawaii do not show appreciable storage of magma in the crust however regional models imply that large volumes of parental magma may be present in the upper mantle of these regions in some volcanic centres teleseismic delays are accompanied by p wave attenuation and linear inversion of spectral data have enabled computation of three dimensional q models for these areas the use of gravity data for magma chamber studies is illustrated by a study in the geysers clear lake volcanic field in california where a strong gravity low has been modelled as a low density body in the upper crust this body is approximately in the same location as the low velocity body delineated with teleseismic delays and is interpreted as a magma body in yellowstone national park magnetic field data have been used to map the depth to the curie isotherm and the results show that high temperatures may be present at shallow depths beneath the yellowstone caldera the main application of electrical techniques in magma related studies has been to understand the deep structure of continental rifts electromagnetic studies in several rift zones of the world provide constraints on the thermal structure and magma storage beneath these regions