special collection on the june 1993 volcanic eruption on the coaxial segment juan de fuca ridge the mid ocean ridge system is the primary site of oceanic crustal accretion and a major source of chemical thermal flux from the earth s interior to the hydrosphere although geological mapping geophysical exploration and hydrographie chemical surveys have provided important insights into accretionary processes until recently there has been no method of monitoring the dynamics of these systems over long periods and large areas insights into spreading center dynamics have depended on serendipitous discoveries of evidence of activity based on water column anomalies baker et al 1987 bathymetric differences fox et al 1992 or direct field observations haymon et al 1993 in situ instrumentation has been used to monitor dynamics at a few selected sites for long periods but these instruments are generally unable to telemeter data in real time

the central anomaly magnetic high implications for ocean crust construction and evolution the central anomaly magnetic high camh is a long lineated short wavelength 10 km wide at the sea surface magnetic anomaly overlying many of the world s midocean spreading centers this anomaly is particularly well developed over the northern juan de fuca ridge including the northern symmetrical and endeavour segments the ubiquitous presence of this zone of high magnetic contrast directly in the zone of active oceanic plate formation is a key to our understanding of near axis crustal plate evolution recently obtained deep tow magnetometer data from segments of the juan de fuca spreading center have caused us to revise the present models of the source of the camh and to review the implications the new model would have on our understanding of the evolution of the magnetic structure of oceanic plates in this model we attribute the near axis magnetization contrast to tectonically controlled low temperature alteration of the upper vertical section of extrusive basement extensive and systematic fissuring of the upper extrusive rocks allows penetration of sea water and rapid alteration of the magnetic minerals in fissured areas approximately 3 km off axis the correlation between fissured crustal blocks and near bottom magnetic lows and unfissured crustal blocks with magnetic highs which is observed at the axis is also seen in a single off axis tow over an outer flanking ridge

single crystal 40ar 39ar dating of the olorgesailie formation southern kenya rift single crystal laser fusion sup 40 ar sup 39 ar analyses and several conventional bulk fusion sup 40 k sup 40 ar dates have been used to determine the age of volcaniclastic strata within the olorgesailie formation and of associated volcanic and sedimentary units of the southern kenya rift in the principal exposures along the southern edge of the legemunge plain the formation spans the interval from approximately 500 to 1 000 ka deposition continued to the east along the ol keju nyiro river where a tuff near the top of the formation has been dated at 215 ka in these exposures the formation is unconformably overlain by sediments dated at 49 ka a possible source for the olorgesailie tephra the ol doinyo nyokie volcanic complex contains as ash flow dated at approximately 1 ma extending the known age range of this complex to encompass that of virtually the entire olorgesailie formation in the legemunge plain these geologic examples illustrate the importance of the single crystal sup 40 ar sup 39 ar dating technique whereby contaminant altered or otherwise aberrant grains can be identified and eliminated from the determination of eruptive ages for reworked or altered pyroclastic deposits the authors have presented a computer modeling procedure based on an inverse isochron analysis thatmore promotes a more objective approach to trimming sup 40 ar sup 39 ar isotope data sets of this type less

seabeam depth changes associated with recent lava flows coaxial segment juan de fuca ridge evidence for multiple eruptions between 1981 1993 after a swarm of earthquakes was detected on the coaxial segment of the juan de fuca ridge in june july 1993 the area was resurveyed with seabeam multibeam sonar to search for depth changes associated with a submarine volcanic eruption quantitative comparison of the 1993 seabeam survey with surveys in 1981 82 and 1991 shows one area of seafloor depth change up to 29 m between 1991 93 exactly where a pristine lava flow was discovered in addition two other depth anomalies up to 37 m and 20 m are identified between 1981 91 evidence that other recent eruptions have occurred along this spreading ridge segment

magnetic properties of zero age oceanic crust a new submarine lava flow on the juan de fuca ridge a volcanic eruption that occurred in october 1993 on the juan de fuca ridge shows unusually high magnetization when compared with older samples from the off axis juan de fuca ridge and other pacific sites these data plus data from other young oceanic basalts indicate that oceanic crustal magnetization may decay rapidly by 10 in the first year the considerably slower decrease in magnetization for rocks from older off axis sites agrees with previous models of progressive low temperature oxidation of the mineral titanomagnetite the magnetization for newly erupted submarine basalts however is not consistent with the exponential decay predicted from older sites indicating that oxidation may not be the sole process responsible for the short term decay similarly weak field magnetic susceptibility of the younger rocks does not show an obvious corresponding decrease with increasing age as expected for the oxidation of titanomagnetite to titanomaghemite these preliminary results indicate that decay of magnetization of oceanic crustal rocks with time may have two distinct stages with only the later stage being due to the progressive oxidation of the magnetic minerals

detection of changes in ridge crest morphology using repeated multibeam sonar surveys a quantitative analysis method is developed to objectively identify significant bathymetric changes between multibeam sonar surveys of the same seafloor area the technique involves mathematical gridding of bathymetric surveys from different years co registration of grids based on prominent features covered in both surveys calculation of slope weighted depth difference grids and determination of the boundaries of statistically significant differences the resulting difference grids are used in interpreting geological processes and calculating the volume of new material added to the seafloor the method is applied to data from the southern juan de fuca ridge where a seafloor volcanic eruption of 0 05 km3 is inferred to have occurred between 1983 and 1987 camera tow data and submersible observations provide ground truth which confirms the validity of the quantitative analysis method and indicates empirically that the detection limit of the method allows new seafloor landforms measuring 1 105 m2 in area or about 200 300 m in diameter and 5 15 m in thickness to be detected through repeated sea beam bathymetric surveys

episodic dike swarms inferred from near bottom magnetic anomaly maps at the southern east pacific rise fissure eruption at 1728 0 s similar lows are observed at three other drained lava lake troughs including one which is at least 1800 years old residing 400 m away from the present day axis we attribute these lows to the presence of shallow dike swarms the degree to which other geologic features may contribute to the lows is constrained using geologic geophysical and geochemical observations and forward modeling compositional analyses of alvin samples at 1728 0 s do not support fe or ti variations as a primary source hypotheses requiring hydrothermal alteration and porosity variations are both inconsistent with geologic observations and near bottom gravity data analysis from similar areas previous mappings between paleointensity variations and the observed magnetic field over distances of several kilometers from the axis suggest that such variations do not create the field low the dominant source of the magnetization low is most likely the presence of a 100 200 m wide region of shallow dikes which are poorly magnetized relative to extrusives or a region heated above magnetic blocking or curie temperatures by intrusions during the most recent eruption though the latter interpretation cannot explain the low at the fossil trough in the first case this extrusive thinning implies a change in eruptive behavior over the last 750 1500 years given the local spreading rate for the latter case thermal models suggest the anomaly had to have been created by a dike swarms totaling at least 45 m width during the most recent eruption s corresponding to 300 years of plate spreading models indicate that the source of the low is centered slightly east of the axial trough this offset suggests that the axis has been progressively migrating westward over the past millennium consistent with other studies covering greater length and timescales westward migration provides an explanation for the preferential emplacement of recent lavas flows west of the axis evident in abe bathymetry and submersible observations index terms 3035 marine geology and geophysics midocean ridge processes 1517 geomagnetism and paleomagnetism magnetic anomaly modeling 8499 volcanology general or miscellaneous 3045 marine geology and geophysics seafloor morphology and bottom photography 3005 marine geology and geophysics geomagnetism 1550

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

alvin magnetic survey of zero age crust coaxial segment eruption juan de fuca ridge 1993 magnetic profiles obtained by deep submersible alvin across a newly erupted lava flow on the juan de fuca ridge jdfr show strong magnetic anomaly fields 20 000 nt and high crustal magnetization 50 a m contrast relative to the surrounding older lavas these results are consistent with previous ideas that extrusive crust is initially highly magnetized within the high amplitude magnetic anomaly over the center of the flow there is a clearly defined deep notch like magnetic low this notch may indicate thermal demagnetization associated with a subsurface feeder dike weakly magnetized intrusives or altered extrusive rock if the former mechanism is active the notch should disappear with time as the dike cools

magnetic anomalies at the puna ridge a submarine extension of kilauea volcano implications for lava deposition high resolution magnetic data collected along the axis and the south flank of the puna ridge the submarine extension of the east rift zone of kilauea volcano hawaii aid in the interpretation of magmatic processes crustal magnetization varies along the crest of the puna ridge three prominent magnetization highs are located between the depths of 1500 and 2500 m each of the magnetization highs is associated with distinct volcanic features a flow field on top of fissured terrain a lava plateau and a staircase of lava terraces the crustal magnetization highs can be explained by the presence of these volcanic features taking into account water depth and the size of the volcanic features and making simple assumptions about the rock magnetization no measurements of rock magnetization have been made on puna ridge samples we assume two values 50 and 10 a m based on values from mid ocean ridge basalts and the estimated ages of puna ridge rocks clague et al 1995 the two shallowest magnetization highs are adjacent to a landslide scarp lava flows overtop the scarp suggesting that a dike has been emplaced and eruptions have occurred since the landslide formed a possible scenario is that the collapse of the flank led to dike intrusion and eruption at this location because of a reduction in the horizontal stress within the edifice our observations suggest that flows are starting to fill in the gap caused by the landslide to rebuild this section of the north flank

quaternary mammals and the end villafranchian dispersal event a turning point in the history of eurasia the history of land mammals is characterized by dispersal events i e short periods of rapid intercontinental migrations and faunal replacements one of these the elephant equus event of lindsay et al may be dated somewhere between 3 and 2 5 ma a second event the wolf event may fall near 1 7 ma the end villafranchian event is discussed in detail it may be dated between 1 0 and 0 9 ma and is marked by a practically total rejuvenation of the fauna with the appearance of new types of adaptation unknown in the villafranchian this faunal revolution is easily recognized all over eurasia and is accompanied by wide ranging changes in climate and vegetation this whole sequence of phenomena may have been triggered by extensive tectonic movements in the mountain belts of central and southern asia

fission track dating of volcanically derived sedimentary rocks depositional ages of sedimentary rocks can be determined using fission track single grain ages on zircons from layers of volcanic ash or bentonite even when the layers have been contaminated by older grains this is done by compiling an age probability distribution or age spectrum for a sample from individual grain ages an age spectrum is a simple and unambiguous way of testing for contamination and extracting useful age information the youngest peak in the age spectrum approximates the time of deposition in most contaminated samples 30 or more grains should be counted to produce a reliable spectrum however useful reproducible ages can be obtained by counting less than 10 grains in samples where most of the older contaminating grains can be removed the few older grains that remain after removing the obviously abraded ones may then be eliminated by examining the age spectrum although ages determined in this way are probably not precise enough for use in defining stratigraphic boundaries they still provide a means of obtaining an isotopic age in sediments that cannot be dated by other radiometric methods 18 references 3 figures 3 tables

central anomaly magnetization high constraints on the volcanic construction and architecture of seismic layer 2a at a fast spreading mid ocean ridge the epr at 9 30 50 n abstract the central anomaly magnetization high camh is a zone of high crustal magnetization centered on the axis of the east pacific rise epr and many other segments of the global mid ocean ridge mor the camh is thought to reflect the presence of recently emplaced and highly magnetic lavas forward models show that the complicated character of the near bottom camh can be successfully reproduced by the convolution of a lava deposition distribution with a lava magnetization function that describes the variation in lava magnetization intensity with age this lava magnetization function is the product of geomagnetic paleofield intensity which has increased by a factor of 2 over the last 40 kyr and low temperature alteration which decreases the remanence of lava with exposure to seawater the success of the forward modeling justifies the inverse approach deconvolution of the magnetic data for lava distribution and integration of that distribution for magnetic layer thickness this approach is tested on two near bottom magnetic profiles al2767 and al2771 collected using alvin across the epr axis at 9 31 n and 9 50 n our analysis of these data produces an estimate of the relative thickness of the magnetic lava layer which is remarkably consistent with existing multichannel estimates of layer 2a thickness from lines cdp31 and cdp27 the similarity between magnetic layer and seismic layer 2a at the 9 10 n segment of the epr crest provides independent support to the notion that seismic layer 2a in young oceanic crust represents the highly magnetic lava layer and that the velocity gradient at the base of layer 2a is related to the increasing number of higher velocity dikes with depth in the lava dike transition zone the near bottom magnetic anomaly character of the camh is a powerful indicator of the emplacement history of upper crust at mors which allows prediction of the relative thickness and architecture of the extrusive lavas independent of other constraints

variations in oceanic crustal magnetization systematic changes in the last 160 million years the amplitudes of marine magnetic anomalies show a clear worldwide pattern of systematic variation when viewed as a function of age globally the amplitudes decrease with age over the first 20 to 30 million years a phenomenon that has been attributed to the low temperature oxidation of the magnetic mineral titanomagnetite in the upper extrusive rocks in oceanic crust older than 40 million years however the amplitudes of the magnetic anomalies increase with increasing age and remain at elevated levels for the entire period between 80 and 160 million years in order to examine the processes responsible for this elevated crustal magnetization in older oceanic crust we compiled all of the existing rock magnetic data from relevant deep sea drilling project and ocean drilling program sites that sampled extrusive rocks from normal ocean crust this compilation shows that the laboratory measurements of the magnetization of the upper basement rocks closely reflect that of the anomaly amplitudes both in the decrease due to oxidation in the first 20 to 30 million years and in the increase in magnetization in older 80 million years crust this positive correlation between anomaly amplitudes and upper crustal magnetization supports the argument that a major source of the marine magnetic anomalies is in the upper extrusive volcanic rocks further the curie temperature data show that oxidation of the magnetic minerals in oceanic basalts occurs largely within the first 30 million years and does not increase significantly beyond that point finally the strong positive correlation between the intensity of the magnetization of the drill core samples and the saturation magnetization argues that the higher magnetic anomaly amplitudes over older crust are related to a change in an intrinsic property of the upper crust specifically we propose that the observed elevated magnetization of older ocean crust is due primarily to an increased abundance of magnetic feti oxides in the older crustal rocks this large increase in abundance of the feti oxides may be related to a systematic increase in bulk feti content in older tholeiitic basalts or more likely is due to a difference in the partitioning of the iron and titanium content between the silicate and oxide phases

seafloor geomagnetic vector anomaly of the intersection of the mid atlantic ridge and the kane transform fault implications for magnetization of the oceanic crust we investigated the fine scale magnetic structure of the oceanic crust and upper mantle around the western intersection of the mid atlantic ridge and the kane transform fault using the geomagnetic vector field measured at the seafloor the strikes of magnetic boundaries were found to reflect of the local tectonics represented by fault strikes and dips the ridge transform intersection is characterized by magnetic boundary strikes rotated eastward the strike directions reflect the direction of recent magmatic activity these indicate that a few traces of faults extending obliquely from the ridge axis to the transform fault are associated with neovolcanism magnetic boundary strikes in the lower part of the transform valley wall and near a linear trace of the transform fault indicate deformation or tilting of the crust associated with transform faulting it is also indicated that the crustal block around the median ridge is tilted by uplift accompanied by injection or emplacement and serpentinization of peridotites the nodal basin consists of extrusive basalts in the e w direction and n s trending magnetic strikes submersible observation of in situ magnetization of oceanic crust shows that extrusive basalts are typically strongly magnetized the most recent neovolcanic ridge has a mean magnetization of 50 a m the mean magnetization of the ridge axis is 20 a m the magnetization of the 3 ma ridge flank in the inside corner high is about 10 a m the gabbroic layer in the transform valley has a magnetization less than 2 a m the serpentinized peridotites in the median ridge have magnetization less than i a m this magnetization records a reversal in polarity which is important because the serpentinized peridotites contribute to the marine geomagnetic anomaly pattern

ridge segmentation and the magnetic structure of the southern mid atlantic ridge 26 s and 31 35 s implications for magmatic processes at slow spreading centers along axis profiles of three dimensional magnetic inversions for the mid atlantic ridge mar 31 35 s show low magnetization near the middle of ridge segments and high magnetization at the segment tips for three adjacent spreading segments thus there is an inverse relation between axial magnetization and axial topography the ridge segment at 26 s on the mar has the same inverse relationship between magnetization and topography the common occurrence of this relationship suggests that it reflects a fundamental process of crustal accretion at the mar we analyze the rock magnetic properties from 42 locations within the four ridge segments in the south atlantic to constrain the inherent trade off between source intensity and source thickness in the magnetization model the natural remanent magnetization nrm intensities from the four ridge segments averaged together correlate with the magnetic inversion profiles this finding implies that changes in the magnetization of the extrusives may account for much of the observed magnetic anomaly amplitude variation a direct correlation of feo content and magnetization suggests that magnetic anomaly amplitudes may be an indicator of feti rich basalts at the slow spreading mar even though the iron content of the basalts from high magnetization areas is not as high as observed at pacific spreading centers despite the different magma plumbing systematics of the pacific spreading centers and the mar it appears that the segment scale magma system of the mar also results in segment scale crustal magnetization variations further evidence that the axial magnetic variations result from source intensity variations is that older isochrons have higher intensities near the ridge discontinuities similar to the behavior on axis between 0 and 5 ma the decay in magnetization is 50 independent of location within a spreading segment

variations in layer 2a thickness and the origin of the central anomaly magnetic high the seismically determined off axis thickening of the extrusive layer is apparently at odds with the magnetic anomaly high t ypic ally associated with the ridge crest the positive magnenzauon contrast at the ridge crest is most likely caused by rapid alteration of the extrusive source layer which occurs over spatial scales 2 3 km comparable to that of the proposed layer 2a thickening we present magnetic rema nence data from basalts dredged on and near the east pacific rise axis at 12on which are compatible with a rapid magnetization reduction 20 k y to decay to l e together with near bottom magnetic profiles from the ultra fast spreading east pacific rise at 19 5os these data suggest that previous estimates of the time constant of alteration inferred from slow spreading ridges 0 5 m y may be more than an order of magnitude too high seismic studies of the fast spreading east pacific rise epr indicate a rapid thickening of seismic layer 2a within a few kilometers of the ridge axis vera et al 1990 toomey et ai 1990 detrick et al 1993 these seismic data have been interpreted to reflect an approximate doubling of the extrusive layer thickness from 100 200 m to 300 500 m through addition of flows whi ch may extend 1 2 km off axis christeson et al 992 vera and diebold in press such thickening of the extrusive source layer had been argued on the basis of the widths of magnetic reversal transitions e g macdonald et al 1983 however this geometry for a uniformly magnetized source layer should produce a marked magnetic anomaly minimum over the ridge axis and is therefore difficult to reconcile with the pronounced anomaly high typically associated with the ridge crest klitgord 1976 sea surface magnetic profiles from the ultra fast spreading 77 mrn yr half rate epr near 19 5os figure 1 illustrate the short wavelength axial anomaly high within the 120 km wide central anomaly which has been termed the central anomaly magnetic high camh klitgord 1976 the camh typically has a wavelength of 8 10 km in sea surface profiles at in termediate spreading ridges 15 45 mm yr the camh occupies a larger proportion of the central anomaly and at slow spreading ridges it is not readily distinguishable in sea surface profiles from the central anomaly because the wavelengths become similar near bottom magnetic profiles figure 1 typically have higher amplitude and shorter wavelength 2 4 kin axial anomalies as expected from the closer proximity to the source klitg6rd et al 1975 miller 1977 perram et al 1990 and even allow distinction of the camh from the central anomaly at some slow spreading ridges macdonald 1977 the higher temporal resolution of faster spreading ridges therefore provides the best opportunity to evaluate the processes responsible for the camh thickness variations in the extraslye layer at fast spreading fi dges have a profound effect on the axial magnetic anomaly since the presence of an axial magma chamber and the low re rnanent intensity of intrusive rocks e g smith and banerice 1986 pariso et al 1991 minimize deeper source contributions in the near ridge environment based on seismic

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

reduction of potential field data to a horizontal plane most existing techniques for potential field data enhancement and interpretation require data on a horizontal plane hence when observations are made on an irregular surface reduction to a horizontal plane is necessary to effect this reduction an equivalent source distribution that models the observed field is computed on a mirror image of the observation surface this irregular mirror image surface is then replaced by a horizontal plane and the effect of the equivalent sources is computed on the required horizontal level this calculated field approximates the field reduced to a horizontal plane the good quality of this approximation is demonstrated by two dimensional synthetic data examples in which the maximum errors occur in areas of steep topographic gradients and increased magnetic field intensity the approach is also applied to a portion of a helicopter borne aeromagnetic survey from the gaspe region in quebec canada where the results are a horizontal shifting of anomaly maxima of up to 150

crustal magnetization in the south atlantic from inversion of magnetic anomalies a method for inverting magnetic anomalies in the presence of topography parker and huestis 1974 is used to analyze and describe crustal magnetization along 12 profiles in the south atlantic the profiles are approximately 2000 km long and are almost perpendicular to the trend of the magnetic anomalies a bandpass filter is used to remove long wavelength anomalies unassociated with seafloor spreading processes a filter that passes only wavelengths between 200 and 3 km produces a magnetization that approximates the positive and negative aspects of the geomagnetic reversal sequence the positive aspects originating from normally magnetized portions of the crustal source and negative aspects originating from reversely magnetized portions of the crustal source a 6 km thick crust is assumed in order to produce magnetizations consistent with dredged oceanic rocks a long wavelength pattern is evident in the magnetization solutions the highest magnetizations are associated with the ridge crest 1 35 1 74 a m within the western section of the study area a sharp decrease in magnetization occurs within 100 km of the ridge crest 4 5 ma a minimum in magnetization is reached at 400 km from the axis or at 18 ma 0 51 0 57 a m the magnetization then gradually increases out to 1100 km from the ridge axis or 50 ma 0 91 0 93 a m and dramatically decreases to 1900 km or 86 ma 0 34 0 43 a m within the eastern section of the study area a sharp decrease in magnetization occurs within the first 100 km of the ridge crest 5 ma a minimum in the magnetization occurs at approximately 400 km from the ridge axis or at 20 ma 0 54 0 56 a m the magnetization then gradually increases out to 1200 km from the ridge axis or to 60 ma 1 15 1 24 a m an overall pronounced decrease occurs to 1900 km from the ridge axis or to 95 ma 0 36 0 40 a m in order to explain this behavior the field intensity may have changed with a time scale of several millions of years alternatively the higher magnetizations at 50 60 ma could be explained by a uniform intensity of magnetization in a thicker crust

the yanbian terrane southern sichuan province sw china a neoproterozoic arc assemblage in the western margin of the yangtze block abstract the magmatic and tectonic history of the yangtze block and its possible affinity with other neoproterozoic arc terranes are important elements in the reconstruction of neoproterozoic plate tectonics the yanbian terrane in the western margin of the yangtze block is a typical arc assemblage composed of a flysch type sedimentary sequence intruded by gabbroic and granodioritic plutons the sedimentary sequence consists chiefly of tuffaceous material with interlayered chert sandstone and pillow basalts laser ablation icp ms u pb dating of detrital zircons from the sandstones yield ages as young as 840 ma the gaojiacun and lengshuiqing mafic intrusions are dated at 812 3 ma and 806 4 ma respectively using the shrimp zircon u pb technique geochemical data show that both the gaojiacun and lengshuiqing intrusions have arc signatures with nd t values of 1 5 to 6 0 initial 87 sr 86 sr ratios of 0 705 0 706 and pronounced negative nb ta and zr hf anomalies their geochemical variations are best explained by fractional crystallization without major crustal contamination the yanbian terrane represents a typical arc assemblage formed on the western edge of the yangtze block during neoproterozoic time the sedimentary sequence was deposited in an oceanic setting probably in a back arc basin environment the depleted subduction modified lithospheric mantle wedge above the subduction zone was the source of melts from which the mafic plutons were crystallized this scenario suggests subduction of oceanic lithosphere eastward present day orientation underneath the yangtze block

fine scale magnetic anomaly field over the southern juan de fuca ridge axial magnetization low and implications for crustal structure sea surface deep tow and submersible magnetic surveys show that the spreading axis of the cleft segment of the southern juan de fuca ridge sjdf is overlain by a distinctive magnetic anomaly low in contrast to the more typical magnetic anomaly high found over the adjacent vance segment and most mid ocean ridge spreading centers forward iterative thickness modeling based on the near bottom magnetic data shows that this axial magnetic low can be produced by a thin magnetic source layer beneath the cleft axial valley thick crust is required to produce the magnetic anomaly high over the vance axial valley this magnetic modeling demonstrates that variations in the base of the magnetic source layer which is assumed to be equivalent to the base of the extrusive lavas can produce a significant proportion of the observed short wavelength magnetic anomaly signal these magnetic layer thickness models also correlate quite well with nearby seismic results which show that seismic layer 2a thins beneath the axial valley of the cleft and is thicker beneath the vance axial valley recent seismic results over other fast and medium spreading rifts also show rapid changes in the thickness of the uppermost crust near the axis of spreading which could produce significant magnetic contrasts the results of this study and the correlation with independent seismic measurements suggest that while magnetization may vary due to geomagnetic behavior or because of petrology or alteration first order change in upper crustal thickness is one of the most important variables to consider in the production of fine scale magnetic anomalies in young ocean crust

high resolution bathymetric surveys using scanning sonars lava flow morphology hydrothermal vents and geologic structure at recent eruption sites on the juan de fuca ridge the coaxial and cleft segments of the juan de fuca ridge have isolated chronic high temperature and focused hydrothermal vent sites both segments also have experienced recent volcanic eruptions which produced extensive ephemeral low temperature and diffuse hydrothermal venting to study the geologic setting of these sites high resolution bathymetric surveys at eight locations on the coaxial and cleft segments were collected between 1993 and 1999 two 675 khz scanning sonar systems were used mesotech on the submersible alvin and imagenex on the remotely operated vehicle jason the bathymetry from these surveys can be gridded at a scale of 2 4 m and contoured at 1 m and thus can resolve many fine scale features on the seafloor that are indistinguishable in multibeam bathymetry collected at the sea surface bathymetric data at this resolution are particularly useful for identifying geologic features related to diking faulting and lava flow emplacement for example the high resolution bathymetric maps show that submarine fissure eruptions that form pillow lavas last long enough to become localized and to produce point source constructs along their length and their extrusion rate is low enough that no significant drainback occurs in contrast lobate sheet flows are formed by short lived high effusion rate eruptions in which no localization of output occurs along the eruptive fissure and inflation is quickly followed by drainback resulting in extensive collapse features however if the process of submarine lava flow inflation occurs at a slower rate and over a longer period of time it can create lava rises up to 25 m high with distinctive structure and morphology the scanning sonar data also show that fissures and grabens have formed or reactivated where dikes approach the surface adjacent to recent eruptive sites the fine scale bathymetry establishes that all the hydrothermal vent sites studied at the coaxial and cleft segments are located along prominent volcanic or tectonic extensional structures which provide the physical pathway for fluids from the subsurface to the seafloor furthermore the fine scale morphology of recent lava flows can be used as a qualitative indication of eruption duration

mid pleistocene change in large mammal faunas of east africa abstract single crystal 40ar 39ar age estimates of 392 000 4000 to 330 000 6000 yr from lainyamok a middle pleistocene fossil locality in the southern kenya rift document the oldest evidence from sub saharan africa of a diverse large mammal fauna consisting entirely of extant species the inferred age of this fauna implies an upper limit for extinction of species that characterize well calibrated mid pleistocene fossil assemblages in east africa for its age and species richness the lainyamok fauna is surprising for its lack of extinct forms e g the bovine pelorovis well documented in later faunal assemblages of east and south africa definitive presence of the south african blesbok damaliscus dorcas is also unexpected especially as this alcelaphine bovid is the dominant large mammal in the lainyamok fauna these age estimates and the faunal composition at lainyamok indicate that geographic ranges and taxonomic associations of extant largebodied mammals were susceptible to wide fluctuations in sub saharan africa over the past 330 000 yr this inference is consistent with the hypothesis of nonanalogue or ephemeral biotas believed to characterize late quaternary ecosystems of northern continents

thickness of a submarine lava flow determined from near bottom magnetic field mapping by autonomous underwater vehicle magnetic field surveys obtained near the seafloor can map the boundaries of recent volcanic eruptions and can provide thickness estimates of these lava flow units independent of bathymetry differencing methods magnetic thickness estimation requires knowledge of the intensity of magnetization of the new lava and surrounding terrain but this can be satisfactorily obtained by representative sampling of the various volcanic units while bathymetry differencing requires pre existing data to assess the thickness of new lava eruptions magnetic surveys can be obtained after an eruption has occurred in this study near bottom magnetic surveys were obtained using an autonomous underwater vehicle auv which operates without a tether or human intervention auv technology offers rapid deployment and an efficient surveying approach for remotely mapping recent lava eruption sites on the seafloor

detection and response to volcanic hydrothermal events on the mid ocean ridge the mid ocean ridge is by far the most continuous volcanic system on the planet but since it lies beneath more than 2 km of seawater a thorough study of its thermal and chemical interaction with the ocean remains very difficult until 1990 the end of the period covered by the last iugg report literally all of our understanding of the dynamics of magmatic accretion on the mid ocean ridge was based on inferences from 1 geochemical geophysical and morphologic studies of long sections of the ridge using a variety of instruments mounted on ships hulls or towed on long cables near the seafloor and 2 submersible based mapping and sampling of small sections of the ridge particularly hydrothermally active ones these types of studies have continued to flourish parmentier 1995 humprhis 1995 but the past 4 years have also seen the first near real time observations of accretion events on two pacific spreading centers and the development of technology and techniques that can monitor the occurrence and extent of such events over a selected portion of the ridge system these monitoring capabilities allow interdisciplinary studies of seafloor hydrothermal systems so that the thermal and chemical effects of hydrothermal venting on the oceans can be assessed

pleistocene sea level fluctuations and human evolution on the southern coastal plain of south africa abstract humans evolved in africa but where and how remain unclear here it is proposed that the southern coastal plain scp of south africa may have served as a geographical point of origin through periodic expansion and contraction isolation in response to glacial interglacial changes in sea level and climate during pleistocene interglacial highstands when sea level was above 75 m human populations were isolated for periods of 360 3400 25 yr generations on the scp by the rugged mountains of the cape fold belt climate and vegetation barriers the scp expands five fold as sea level falls from 75 to 120 m during glacial maxima to form a continuous unobstructed coastal plain accessible to the interior an expanded and wet glacial scp may have served as a refuge to humans and large migratory herds and resulted in the mixing of previously isolated groups the expansive glacial scp habitat abruptly contracts by as much as one third in 300 yr during the rapid rise in sea level associated with glacial terminations rapid flooding may have increased population density and competition on the scp to select for humans who expanded their diet to include marine resources or hunted large animals successful adaptations developed on an isolated scp are predicted to widely disperse during glacial terminations when the scp rapidly contracts or during the initial opening of the scp in the transition to glacial maxima the hypothesis that periodic expansion and contraction of the scp as well as the coastal plain of north africa contributed to the stepwise origin of our species over the last 800 thousand years kyr is evaluated by comparing the archeological dna and sea level records these records generally support the hypothesis but more complete and well dated records are required to resolve the extent to which sea level fluctuations influenced the complex history of human evolution

variations in oceanic crustal structure and implications for the fine scale magnetic anomaly signal marine magnetic anomalies are traditionally modelled using a constant thickness source layer usually defined as the extrusive lavas associated with seismic layer 2a recent seismic studies reveal that the thickness of layer 2a can vary by more than a factor of two an observation which has important implications for the magnetic anomaly source layer we model deep tow magnetometer data from the endeavour segment of the northern juan de fuca ridge in combination with nearby seismic data and find a positive correlation between the thickness of the seismically defined layer 2a and the magnetic anomaly source layer this suggests that short wavelength magnetic anomalies observed within individual magnetic polarity units can arise from variations in the thickness of the extrusives rather than geomagnetic field variations or secondary magnetization