geological hazards vulnerability and risk assessment using gis model for glenwood springs colorado glenwood springs colorado lies at the junction of the roaring fork and colorado rivers surrounded by the steep peaks of the colorado rocky mountains large parts of the region have had intensive sheet erosion debris flows and hyperconcentrated floods triggered by landslides and slumps the latter come from unstable slopes in the many tributary channels on the mountainsides causing concentration of debris in channels and a large accumulation of sediment in colluvial wedges and debris fans that line the river valleys many of the landslide and debris flow deposits exist in a state resembling suspended animation ready to be destabilized by intense precipitation and or seismic activity during this century urban development in the roaring fork river valley has increased rapidly the city of glenwood springs continues to expand over unstable debris fans without any construction of hazard mitigation structures since 1900 glenwood springs has had at least 21 damaging debris flows and floods on july 24 1977 a heavy thunderstorm spread a debris flow over more than 80 ha of the city this paper presents a method that uses geographic information systems gis to assess geological hazards vulnerability and risk in the glenwood springs area the hazards evaluated include subsidence rockfall debris flows and floods and in this paper we focus on debris flows and subsidence information on topography hydrology precipitation geomorphic processes bedrock and surficial geology structural geology soils vegetation and land use was processed for hazard assessment using a series of algorithms arc info and grass gis softwares were used to produce maps and tables in a format accessible to urban planners after geological hazards were defined for the study area we estimated the vulnerability ve of various elements for an event of intensity i risk is assessed as a function of hazard and vulnerability we categorized the study area in 14 classes for planning procedures 7 classes defined as areas suitable for human settlement and 7 classes defined as unsuitable for building and most effectively reserved for parks and forests

the southeast indian ridge between 88 e and 118 e variations in crustal accretion at constant spreading rate the temperature of the mantle and the rate of melt production are parameters which play important roles in controlling the style of crustal accretion along mid ocean ridges to investigate the variability in crustal accretion that develops in response to variations in mantle temperature we have conducted a geophysical investigation of the southeast indian ridge seir between the amsterdam hotspot and the australian antarctic discordance 88 e 118 e the spreading center deepens by 2100 m from west to east within the study area despite a uniform intermediate spreading rate 69 75 mm yr 1 the seir exhibits the range in axial morphology displayed by the east pacific rise and the mid atlantic ridge mar and usually associated with variations in spreading rate the spreading center is characterized by an axial high west of 102 45 e whereas an axial valley is prevalent east of this longitude both the deepening of the ridge axis and the general evolution of axial morphology from an axial high to a rift valley are not uniform a region of intermediate morphology separates axial highs and mar like rift valleys local transitions in axial morphology occur in three areas along the ridge axis the increase in axial depth toward the australian antarctic discordance may be explained by the thinning of the oceanic crust by 4 km and the change in axial topography the long wavelength changes observed along the seir can be attributed to a gradient in mantle temperature between regions influenced by the amsterdam and kerguelen hot spots and the australian antarctic discordance however local processes perhaps associated with an heterogeneous mantle or along axis asthenospheric flow may give rise to local transitions in axial topography and depth anomalies

forces driving ridge propagation recent multibeam bathymetric and geophysical data from the southeast indian ridge seir reveal eight propagating ridges prs moving down the regional bathymetry gradient toward the australian antarctic discordance aad these prs are propagating at a nearly uniform rate of 40 mm yr 1 despite dramatic variations in axial morphology and segment length and local reversals of the regional bathymetric gradient because existing dynamic rift propagation models do not explain the continued uniform propagation of the diverse seir prs we have developed a new model that includes variable crustal thickness and thermally driven along axis asthenospheric flow in addition to topographic gradients as positive contributions to the forces driving rift propagation along axis asthenospheric flow and variations in crustal thickness along the seir appear to provide the first order driving force required for ridge propagation even in the highly segmented magmatically starved region in and near the aad where previous models for ridge propagation fail further our model development requires that the rift valley at the propagating tip is a consequence of rifling in the cooler thermal regime of preexisting lithosphere rather than a consequence of viscous resistance to flow this explanation of pr tip topography is consistent with gravity data from the seir that suggest that the rift tip and associated pseudofaults are at least partially compensated by crustal thinning

a nonlinear rheology model for mid ocean ridge axis topography this paper describes a mechanical model of the mid ocean ridge axis based on a nonlinear rheology the oceanic lithosphere is modeled as a strongly temperature dependent power law material with the byerlee s friction law describing the brittle behavior of the upper oceanic lithosphere we examined two simple models a mantle only model predicted an axial rift valley 1 2 km deep and 10 15 km wide at slow half rates of 5 25 mm yr which is in fair agreement with observations however the rift valley did not completely vanish at fast spreading rates even though its width and relief became much smaller thus the mantle only model failed to explain the observed transition in axial topography with spreading rate by including a uniform thickness oceanic crust with theological properties different from the mantle our second model produced the first order variation of axial topography with spreading rate a pronounced rift valley with 1 2 km relief at slow rates a smaller relief 500 m rift valley at intermediate rates and no rift valley at fast rates there are two additional results of the nonlinear rheology model which are distinctively different from the previous uniform viscosity mantle models first horizontal velocity at the seafloor changes sharply in a very narrow zone at the ridge axis i e the stresses are concentrated in a very narrow zone for the nonlinear rheology this conclusion is remarkably independent of the boundary conditions at the seafloor second the computed width of the axial plastic failure zone decreases with spreading rate a result very different from that of a uniform viscosity model this is caused by the strong temperature dependence of the power law rheology and the fact that the temperature of the oceanic lithosphere depends greatly on the spreading rate beyond some distance from the ridge axis the brittle plate roughly the region above the 750 c isotherm deforms very little both the horizontal strain rate 11 and the maximum shear strain rate max are less than 10 17 s 1 which is in good agreement with the 10 18 s 1 average for oceanic brittle plate deformation inferred from seismic moment data within the ductile region area beneath the 750 c isotherm the strain rates are in the range of 10 14 10 16 s 1 except within a narrow band near the ridge axis where higher strain rates are located we found that the thermal structure of the oceanic lithosphere is not very sensitive to the details of the mantle flow pattern or the rheology but is very sensitive to the cooling by hydrothermal circulation near the ridge axis axial structure is also influenced by factors other than the spreading rate within the range of slow and intermediate rates our calculations show that the size of an axial rift valley decreases with an increase in either the thickness of the oceanic crust or the mantle temperature in other words a thinner crust or a cooler mantle temperature will produce a more pronounced rift valley in the vicinity of hotspots the absence of an axial rift valley even at slow spreading rates such as the reykjanes ridge can be well explained by the anomalously thicker crust and higher mantle temperatures in contrast at the australian antarctic discordance a deeper than normal section of an intermediate spreading ridge has very rough topography not present on the adjacent sections of the ridge this can be explained by our model as due to an anomalously low mantle temperature as has been suggested or perhaps also due to a thinner oceanic crust

on the spatial relationship between landslides and causative factors on lantau island hong kong abstract this paper presents a statistical approach to study the spatial relationship between landslides and their causative factors at the regional level the approach is based on digital databases and incorporates such methods as statistics spatial pattern analysis and interactive mapping firstly the authors propose an object oriented conceptual model for describing a landslide event and a combined database of landslides and environmental factors is constructed by integrating the various databases within such a conceptual framework the statistical histogram spatial overlay and dynamic mapping methods are linked together to interactively evaluate the spatial pattern of the relationship between landslides and their causative factors a case study of an extreme event in 1993 on lantau island indicates that rainfall intensity and the migration of the center of the rainstorm greatly influence the occurrence of landslides on lantau island a regional difference in the relationship between landslides and topography is identified most of the landslides in the middle and western parts of the island occurred on slopes with slope angles of 25 35 while in the eastern part the corresponding range is 30 35 overlaying landslide data with land cover reveals that a large number of landslides occurred in the bareland and shrub covered area and in the transition zones between different vegetation types the proposed approach can be used not only to analyze the general characteristics of such a relationship but also to depict its spatial distribution and variation thereby providing a sound basis for regional landslide prediction

subduction influence on magma supply at the east scotia ridge despite a spreading rate of 65 70 km ma 1 the east scotia ridge has along most of its length a form typically associated with slower rates of sea floor spreading this may be a consequence of cooler than normal mantle upwelling which could be a feature of back arc spreading at the northern end of the ridge recently acquired sonar data show a complex rapidly evolving pattern of extension within 100 km of the south sandwich trench new ridge segments appear to be nucleating at or near the boundary between the south american and scotia sea plates and propagating southwards supplanting older segments the most prominent of these north of 56 30 s has been propagating at a rate of approximately 60 km ma 1 for at least 1 ma and displays a morphology unique on this plate boundary a 40 km long axial high exists at the centre of this segment forming one of the shallowest sections of the east scotia ridge beneath it seismic reflection profiles reveal an axial magma chamber or amc reflector similar to those observed beneath the east pacific rise and valu fa ridge simple calculations indicate the existence here of a narrow 1 km wide body of melt at a depth of approximately 3 km beneath the sea floor from the topographic and seismic data we deduce that a localised mantle melting anomaly lies beneath this segment rates of spreading in the east scotia sea show little variation along axis hence the changes in melt supply are related to the unique tectonic setting in which the south american plate is tearing to the east perhaps allowing mantle flow around the end of the subducting slab volatiles released from the torn plate edge and entrained in the flow are a potential cause of the anomalous melting observed a southward mantle flow may have existed beneath the axis of the east scotia ridge throughout its history

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

topographic controls of landslides in rio de janeiro field evidence and modeling landslides are common features in the serra do mar located along the southeastern brazilian coast most of them associated with intense summer storms specially on the soil mantled steep hillslopes around rio de janeiro city where the favelas slums proliferated during the last few decades on february 1996 hundreds of landslides took place in city of rio de janeiro triggered by intense rainstorms since then many studies have been carried out in two experimental river basins in order to investigate the role played by the topographic attributes in controlling the spatial distribution of landslides inside them landslide scars and vegetation cover were mapped using aerial photographs and field observations a detailed digital terrain model 4 m 2 resolution of the basins was generated from which the main topographic attributes were analyzed producing maps for slope hillslope form contributing area and hillslope orientation by comparing these maps with the spatial distribution of the landslide scars for the 1996 event a landslide potential index lpi for the many classes of the different topographic attributes was defined at the same time field experiments with the guelph permeameter were carried out and a variety of scenarios were simulated with the shalstab model a process based mathematical model for the topographic control on shallow landslides the results suggest that most of the landslides triggered in the studied basins were strongly influenced by topography while vegetation cover did affect landslide distribution between the topographic attributes hillslope form and contributing area played a major role in controlling the spatial distribution of landslides therefore any procedure to be used in this environment towards the definition of landslide hazards need to incorporate these topographic attributes d 2003 elsevier b v all rights reserved

magnetic and tectonic studies of the dueling propagating spreading centers at 20 40 s on the east pacific rise evidence for crustal rotations we present the results of a magnetic study of a 225 km by 240 km area centered on the dueling propagating spreading centers located at 20 40 s on the east pacific rise a majority of the data used were collected during a cruise aboard the r v moana wave during which continuous seamarc ii coverage was obtained these data were combined with additional data to produce an anomaly map which extends to anomaly 2 aged crust a three dimensional inversion in the presence of bathymetry was carried out for the area the resulting magnetization distribution was interpreted and compared to side scan sonar and bathymetry data sets in order to determine the recent history of the discontinuity the results indicate consistent asymmetric spreading faster to the east discontinuous high magnetizations in the discordant zone associated with the discontinuity and southward migration of the feature at a rate of 90 100 mm yr between jaramillo and brunhes time 0 95 to 0 73 ma with slowing during the brunhes to less than 10 mm yr there is also an overlapping jaramillo isochron on the west flank and a gap in that isochron on the east flank indicating a transfer of crust during this time period from the nazca to the pacific plate in addition areas of oblique lineations possibly representing rotated crust were modelled using an inverse method which enables the specification of a nonuniform magnetization unit vector results from this second model support the presence of highly rotated pre brunhes nazca crust within brunhes pacific crust which has been deformed by bookshelf faulting this indicates at least two episodes of crustal transfer from the nazca plate to the pacific plate the discontinuity appears to mark the boundary between rigid plate tectonics to the north and deformation within the nazca plate between the discontinuity and the easter microplate to the south the detailed history of the discontinuity involves dueling propagation with a great deal of variation in the amount of overlap of the two ridges as well as inward and outward cutting and abandonment of the tips of both ridges

detailed tectonics near the tip of the galapagos 95 5 w propagator how the lithosphere tears and a spreading axis develops a model for propagation of an active spreading axis into preexisting lithosphere has been developed based on intensive detailed studies using seamarc ii sea beam deep tow and alvin near the tip of the propagator growing westward at 52 km m y along the east west trending galapagos spreading axis near 95 5 w initial lithospheric rifting appears to be accommodated along reactivated roughly east west abyssal hill faults at the tectonic tip extension leads to downdropping of a keystone block which is pervasively cut by normal faults and fissures the initial volcanism initial volcanic tip occurs about 6 5 km behind the tectonic tip as basalt erupts through these fissures covering most of the keystone block the neovolcanic axial tip occurs another 4 5 km back at the western termination of a well defined axial pillow ridge along which volcanism is localized spreading along the propagator axis accelerates to the full rate 29 mm yr half rate at the full rate tip interpreted to be 10 km behind the neovolcanic axial tip or 21 km behind the tectonic tip a complex pattern of strain in the tip area results from the interaction of plate spreading transform shear stress crack propagation dynamic depression of the western end of the axial valley and the transform zone and variations in the strength thickness heterogeneity and anisotropy of the lithosphere southwesterly curvature of the neovolcanic axis near its tip is inferred to result primarily from the response of young thin weak relatively homogeneous lithosphere to counterclockwise rotation of the maximum tensional stress direction relative to the spreading direction due to the influence of shear stress from the transform zone similar stress interactions likely occur at the tectonic tip but are inferred to be accommodated in this older thick strong heterogeneous and anisotropic lithosphere by oblique slip along a set of left stepping reactivated preexisting east west abyssal hill structures although on a time scale of 106 years the overall process of propagation is mostly continuous rifting at the tectonic tip appears discontinuous at a scale of 1500 m or 30 000 years based on the east west spacing of grabens formed as rifting begins the scale of discontinuity of propagation of the neovolcanic axis is 3 5 km or 50 000 100 000 years based on the distribution of northeast trending features interpreted as beheaded western terminations of paleoneovolcanic axes the extension involved in rifting is 10 20 based on summing the horizontal normal separation of all normal faults and fissures integrated and averaged over the zone of rifting these values are approximately doubled if simple shear listric faulting is dominant rather than pure shear normal faulting

transform zone migration implications of bookshelf faulting at oceanic and icelandic propagating ridges at a propagating ridge a migrating transform zone mtz offsets the propagating and retreating spreading axes we examine several kinematic models of mtz deformation to determine which models can generate observed seafloor fabric orientations vertical tectonics and faulting patterns within the 95 5 w galapagos propagating ridge pr system models that involve transform parallel simple shear within the mtz can fit observed seafloor fabric rotation patterns within the 95 5 w mtz however these models predict pervasive transform parallel strike slip faults within the mtz that are not observed and do not predict the dramatic shear concurrent deepening and post shear uplift that is seen in this propagating ridge system since we only measure the amount of structural rotation of the abyssal hill fabric within the mtz and do not measure extensional or compressional deformation perpendicular or parallel to abyssal hills this structural information can only determine the rotational component of the finite strain tensor thus this simple shear family of kinematic deformation models is actually only a small subset of mtz deformation patterns that can produce the same finite rotation patterns but with different amounts of shortening and or extension parallel or normal to abyssal hills there is strong seismic and surface fault break evidence in iceland that the south iceland seismic zone mtz propagates along an array of en echelon transform perpendicular strike slip faults a likely reason for this is that if deformation were accommodated through transform parallel strike slip faults then migration of the transform zone would require the creation of new transform parallel strike slip faults for each increment of transform migration instead if transform migration can be accommodated through slip on preexisting faults then transform migration would only require the incremental growth of and slip along preexisting faults for each increment of transform zone migration we examine the kinematic consequences of finite mtz deformation within this generalized bookshelf faulting deformation scenario we find that our kinematic realization of generalized bookshelf faulting predicts structural rotation and vertical tectonic patterns that are consistent with observations at the 95 5 w galapagos pr system vertical tectonics are an almost inevitable by product of bookshelf shearing on dipping faults because of the geometric strain incompatibility associated with this mode of deformation thus local extension within the abyssal hills of de steiguer deep may occur because these abyssal hills do not join a spreading axis where the geometric strain incompatibilities associated with bookshelf slip can be accommodated we conclude that generalized bookshelf faulting is a promising candidate for the dominant deformation process within a mtz since it can explain not only the structural rotations observed at the 95 5 w propagating ridge but can also relate these rotations to the uplift history recorded there

oceanic crustal thickness versus spreading rate a compilation of oceanic crustal thickness from seismic observations collected over the past two decades shows that the average crustal thickness away from plateaus is 6 km no systematic increase of crustal thickness with spreading rate is observed instead the data show large variations in crustal thickness at slow spreading ridges 3 8 km for half rates 30 mm yr the large variations at slow ridges and small variations at fast ridges are consistent with the results inferred from recent gravity studies of mid ocean ridges both data sets support the speculation of a transition from a 3 d structure of crustal accretion at slow ridges to a 2 d accretion pattern at fast ridges

systematics of ridge propagation south of 30 s abstract new high resolution geosat altimetry data south of 30 s reveal numerous propagating ridge wakes along intermediate and slow spreading ridges these new examples provide a large enough database to establish systematics of ridge propagation almost all active propagating ridges propagate down a regional along axis gravity or bathymetry gradient the sense of the propagating ridge offset right lateral vs left lateral is related to recent changes in spreading direction we find there is a significant difference between the propagation of ridges with an axial high morphology which propagate at greater than 50 of their full spreading rate and ridges with a median valley morphology which usually propagate at 25 of their spreading rate the axial high propagators leave behind an asymmetric wake the outer pseudofault appears as a continuous linear trough step while the sheared zone appears as a chain of small gravity bumps while we clearly see the propagating ridge wakes from offsets greater than 10 km at slow and intermediate spreading ridges at ridges spreading faster than 75 mm yr the amplitude of the wake topography decreases to the point where we no longer see these wakes in geosat altimetry data the systematics seen in this new data set support a fracture mechanics model for the dynamics of ridge propagation

east pacific rise from siqueiros to orozco fracture zones along strike continuity of axial neovolcanic zone and structure and evolution of overlapping spreading centers we have used the sea beam multibeam echo sounding system to survey the east pacific rise epr from 8 20 n to 18 30 n obtaining complete coverage of the epr axial neovolcanic zone and all intervening transform faults here we focus on the epr neovolcanic zone and the definition of overlapping spreading centers ocs s between the orozco and siqueiros transform faults the neovolcanic zone is narrow 0 5 2 0 km and continuous along strike and occurs within and near a strike continuous axial summit graben the neovolcanic zone and summit graben reside along the crest of a volcanic axial high which is 2 10 km wide and which continues uninterrupted along strike for 40 140 km within 20 50 km of an intersection with a transform fault the axial neovolcanic zone narrows and deepens and within only 3 6 km of the intersection the neovolcanic zone turns sharply into the transform valley at seven locations between the siqueiros and orozco transform faults the axial neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast with a classic ridge transform ridge plate boundary however offset rise terminations overlap each other by a distance approximately 3 times greater than their offset they curve toward each other and often one merges into the other along strike separating the osc s is an elliptical overlap basin up to 600 m deep whose long axis is subparallel to the adjacent spreading centers the overlap basin is characterized by volcanic constructional edifices a key difference between these offsets and small transform faults in the atlantic is that there are no transform fault structures in the overlap basin which link the offset rise axes a continuous axial depth profile reveals a long wavelength 20 60 km undulation of the rise axis which may be associated with variations in the magmatic budget along strike the lowest points in the axial depth profile occur near transform faults and at osc s suggesting that these features are associated with along strike minima in the magmatic budget we suggest that major phases of volcanic activity propagate episodically along the axis away from centers of magmatic inflation and that osc s develop where magmatic pulses fail to meet due to misalignment of fracture systems sinuous bends in the rise axis or other heterogeneities transform faults fail to develop on fast spreading centers where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate ridge transform fault pattern the osc geometry is unstable and evolves rapidly and the overlapping rises may propagate at rates faster than the local spreading rate one of the two osc s prevails while the other is abandoned while the locations of osc s may not be fixed relative to the rise axis they may tend to recur near the same place creating scars in older lithosphere which resemble fracture zones but whose fine scale structure is quite different there is excellent agreement between the observed structure of osc s and laboratory studies of osc s using wax models photoelastic studies of overlapping cracks and boundary element modeling of overlapping cracks seismic refraction and reflection near two osc s suggests that large offset osc s have separate magma chambers beneath the offset rise axes while small offset osc s may share a single axial magma chamber three dimensional inversion of magnetic anomalies suggests that significant petrologic anomalies may occur near some osc tips waxing and waning of overlapping magma chambers may give rise to the eruption of highly fractionated feti basalts and associated magnetic anomalies

evolution of the pacific antarctic ridge south of the udintsev fracture zone because of the proximity of the euler poles of rotation of the pacific and antarctic plates small variations in plate kinematics are fully recorded in the axial morphology and in the geometry of the pacific antarctic ridge south of the udintsev fracture zone swath bathymetry and magnetic data show that clockwise rotations of the relative motion between the pacific and antarctic plates over the last 6 million years resulted in rift propagation or in the linkage of ridge segments with transitions from transform faults to giant overlapping spreading centers this bimodal axial rearrangement has propagated southward for the last 30 to 35 million years leaving trails on the sea floor along a 1000 kilometer long v shaped structure south of the udintsev fracture zone

magnitude frequency characteristics and preparatory factors for spatial debris slide distribution in the northern faroe islands abstract the faroe islands in the north atlantic ocean are highly susceptible to debris avalanches and debris flows originating from debris slide activity in shallow colluvial soils to provide data for hazard and risk assessment of debris avalanches and debris flows this study aims at quantifying the magnitude and frequency of their debris slide origins as well as identifying which preparatory factors are responsible for the spatial debris slide distribution in the landscape for that purpose a debris slide inventory was generated from aerial photo interpretation api fieldwork and anecdotal sources covering a 159 km2 study area in the northern faroe islands a magnitude cumulative frequency mcf curve was derived to predict magnitude dependant debris slide frequencies while preparatory factors responsible for spatial debris slide distribution were quantified through gis supported discriminant function analysis dfa nine factors containing geological lithology dip geomorphological slope angle altitude aspect plan and profile curvature and land use infield outfield sheep density information were included in the multivariate analysis debris slides larger than 100 m2 with magnitude expressed as topographic scar area can be predicted from the power law function y 936 26x 1 277 r2 0 98 while a physical explanation is preferred for the roll over pattern of smaller slope failures the dfa is able to correctly classify app 70 of the modeled terrain units into their pre determined stable unstable groups preparatory factors responsible for the spatial debris slide distribution are aspect slope angle sheep density plan curvature and altitude while influence of the remaining factors is negligible

petrologic consequences of rift propagation on oceanic spreading ridges abstract the production of anomalously differentiated lava compositions at several mid ocean spreading centers can be attributed to magmatic processes associated with propagating rifts the degree of differentiation attained by magmas beneath oceanic spreading ridges depends mainly on the balance between cooling rate and the supply rate of new magma to shallow chambers low supply rates and moderate cooling rates allow advanced degrees of closed system fractionation to occur high supply rates result in open systems in which magma compositions are buffered by frequent replenishment with new hot magma propagating rift tips are a special class of ridge transform intersection in which the balance between cooling and supply rates is conducive to the development of advanced degrees of differentiation over an expanded length of ridge this balance is affected by the spreading rate the propagation rate of the rift the length of the bounding transform and proximity to hotspots maximum compositional variability and maximum degree of differentiation occur within 50 km of propagating rift tips and subsequently diminish with increasing distance rifts that propagate through plates in directions approximating their absolute motion relative to the lower mantle are characterized by the presence of anomalously differentiated lavas over longer ridge segments than are rifts that propagate against their absolute motion geochemical anomalies may persist though changing in degree and extent for several million years on ridge segments that stop propagating the concept of magnetic telechemistry is generally supported by our study but in the vicinity of hotspots magnetic anomaly amplitude may be controlled more by bathymetric and or thickened magnetic layer effects than by geochemistry

mantle temperature anomalies along the past and paleoaxes of the galapagos spreading center as inferred from gravity analyses to better understand the effects of hot spots on mid ocean ridge thermal structure we investigate the subsurface density structure of the galapagos spreading center and nearby lithosphere using shipboard gravity and bathymetry data we obtain maps of mantle bouguer anomalies mba by removing from the free air gravity the attractions of seafloor topography and a 6 km thick model crust comparison of observed and theoretical mba profiles along isochrons for ages 0 0 7 7 ma suggests that seafloor topography is isostatically compensated by mass anomalies primarily in the upper 100 km of the mantle this result is consistent with the notion that seafloor topography along the galapagos spreading center is supported by lateral changes of crustal thickness and upper mantle density both of which are controlled by temperatures in the upper mantle where decompression melting occurs along the ridge axis the mba decreases from the east and west toward the galapagos hot spot by 90 mgal reaching a minimum nearest the hot spot at 91 w seafloor topography mirrors the mba along axis increasing by 1 1 km toward the hot spot these variations in mba and bathymetry can be explained by crustal thickening and mantle density variations resulting from a gradual axial temperature increase of 50 25 c toward the hot spot the predicted crustal thickening of 2 4 km nearest the hot spot accounts for 70 75 of the along axis mba and bathymetry anomalies mantle density variations account for the rest of the anomalies from the crustal isochron of age 7 7 ma to the present day axis the along isochron amplitudes of mba decrease from 150 to 90 mgal the corresponding along isochron bathymetry anomalies decrease from 1 7 to 1 1 km these observations along the paleoaxes of the galapagos spreading center indicate that the axial temperature anomaly was 70 hotter in the past 86 25 c and has steadily decreased to 50 25 c as the ridge axis migrated away from the galapagos hot spot these along isochron temperature anomalies however have remained well below that estimated for the hot spot itself 200 c indicating that the lateral temperature gradient between the hot spot and the ridge axis has remained 10 20 times greater than that along the ridge axis over the past 7 7 m y

three dimensional flow beneath a slow spreading ridge axis a dynamic contribution to the deepening of the median valley toward fracture zones the axial valley along a slow spreading mid ocean ridge may be explained by a vertical pressure gradient due to viscous flow of asthenosphere upwelling in a relatively narrow conduit beneath the ridge axis along a ridge segment the axial valley floor deepens by as much as 2 3 km over distances of several tens of kilometers approaching a ridge transform intersection this deepening may be explained in part by a pressure gradient associated with horizontal flow in the ridge axis conduit vanishing vertical velocity on a conduit end wall at a ridge transform intersection results in reduced vertical flow within about one conduit width of the intersection horizontal flow along the conduit toward the intersection must occur to form lithosphere at a uniform rate along the ridge axis a simple model with a conduit of uniform width that terminates with vertical planar endwalls at ridge transform intersections is considered the model is based on an analytical solution for flow in a narrow conduit and boundary layer approximations for the flow structure near the vertical end walls this model indicates that the induced horizontal flow and pressure gradient extend for a distance along the ridge axis of several lithosphere thicknesses from an intersection for reasonable values of the conduit width and asthenosphere viscosity the model predicts that induced horizontal flow can contribute significantly to the deepening of an axial valley approaching an intersection

interaction of mantle plumes and migrating mid ocean ridges implications for the galapagos plume ridge system we investigate the three dimensional interaction of mantle plumes and migrating mid ocean ridges with variable viscosity numerical models numerical models predict that along axis plume width w and maximum distance of plume ridge interaction xmax scale with q u where q is plume source volume flux and u is ridge full spreading rate both w and xmax increase with buoyancy number b which reflects the strength of gravitational versus plate driven spreading scaling laws derived for stationary ridges in steady state with near ridge plumes are consistent with those obtained from independent studies of ribe 1996 in the case of a migrating ridge the distance of plume ridge interaction is reduced when a ridge migrates toward the plume because of the excess drag of the faster moving leading plate and enhanced when a ridge migrates away from the plume because of the reduced drag of the slower moving trailing plate given the mildly buoyant and relatively viscous plumes investigated here the slope of the lithospheric boundary and thermal erosion of the lithosphere have little effect on plume flow from observed plume widths of the galapagos plume migrating ridge system our scaling laws yield estimates of galapagos plume volume flux of 5 16 106km3 m y 1 and a buoyancy flux of 2 103 kg s 1 model results suggest that the observed increase in bathymetric and mantle bouguer gravity anomalies along cocos plate isochrons with increasing isochron age is due to higher crustal production when the galapagos ridge axis was closer to the plume several million years ago the anomaly amplitudes can be explained by a plume source with a relatively mild temperature anomaly 50 100 c and moderate radius 100 200 km predictions of the along axis geochemical signature of the plume suggest that mixing between the plume and ambient mantle sources may not occur in the asthenosphere but instead may occur deeper in the mantle possibly by entrainment of depleted mantle as the plume ascends from its source

history of rift propagation and magnetization intensity for the cocos nazca sspreading center analysis of magnetic anomaly profiles collected nearly parallel to tectonic flow lines allows detailed interpretation of the complicated tectonic history of the cocos nazca spreading center forward models of the magnetic anomalies accounting for spreading rate variations ridge jumps asymmetric spreading magnetization intensity variations and bathymetry show excellent agreement with observed anomalies spreading rates can be constrained to a common finite rotation history through anomaly 4a with three changes in rates rate changes at about 1 5 ma and 4 1 ma correspond to changes in rate gradients and occur during the well calibrated part of the reversal timescale so they can unquestionably be identified as true changes in plate motion a 15 rate decrease at about 5 2 ma could be interpreted either as a change in plate motion or as an artifact of poor calibration of the older part of the timescale the change at 4 1 ma is especially important because many timescales are based on the assumption of constant spreading rate for this plate pair for 0 6 ma rift propagation has played a dominant role in the continuous reorganization of the geometry of the ridge axis propagation has been predominantly away from the hotspot with jumps predominantly south ward propagation rates have ranged from 30 to 120 mm yr commonly near 70 mm yr origin of most propagation sequences is difficult to interpret but many appear to involve discrete southward ridge jumps forming a new segment near the hotspot magnetic anomaly amplitude appears to be a reliable tracer of fe content of lavas several generalizations can be drawn about along axis variations in magnetization intensities since 8 ma high magnetizations are only observed at the far ends relative to the galapagos hotspot of segments at least 150 km long offset at the end of a high magnetization segment is at least 15 km and there are no offsets larger than 30 45 km between high magnetization segments and the reconstructed position of the hotspot we interpret these patterns to indicate that fractionated lavas erupt where gradients in magma supply cause along axis flow of evolved magma the gradients in supply result from subaxial flow of hotspot derived asthenosphere in a narrow conduit this flow is only partly obstructed by an offset of 20 30 km but entirely blocked by an offset of 50 km

assessment of shallow landslide susceptibility by means of multivariate statistical techniques several multivariate statistical analyses have been performed to identify the most influential geological and geomorphological parameters on shallow landsliding and to quantify their relative contribution a data set was first prepared including more than 30 attributes of 230 failed and unfailed slopes the performance of principal component analysis t test and one way test allowed a preliminary selection of the most significant variables which were used as input variables for the discriminant analysis the function obtained has classified successfully 88 5 per cent of the overall slope population and 95 6 per cent of the failed slopes slope gradient watershed area and land use appeared as the most powerful discriminant factors a landslide susceptibility map based on the scores of the discriminant function has been prepared for ensija range in the eastern pyrenees an index of relative landslide density shows that the results of the map are consistent copyright 2001 john wiley sons ltd language en

a three dimensional gravity study of the 95 5 w propagating rift in the galapagos spreading center abstract seafloor at the galapagos 95 5 w propagating rift pr has a varied morphological expression that can be spatially correlated with the predicted kinematic history of the pr a median valley like depression occurs near the tip of the growing ridge axis to test if this bathymetry is a dynamic feature supported by mantle or lithosphere strength or if it is due to isostatically compensated crustal thickness variations we use three dimensional gravity modelling to constrain the crustal structure in this region from data collected by hey in 1979 and 1982 the gravity anomaly at the pr tip depression suggests that the tip depression is not caused by crustal thinning the data are consistent with a stress supported pr tip depression caused by asthenospheric along axis flow into the growing ridge axis phipps morgan and parmentier 1 in contrast to the tip depression seafloor in the sheared zone of material transferred through transform migration from the cocos to nazca plate is anomalously shallow and has a pronounced regional 300 400 m tilt towards the growing ridge axis over the 20 km width of the sheared zone the gravity data also suggest that the sheared zone is not compensated by crustal thickening

horizontal extent of anomalously thin crust near the vema fracture zone from the three dimensional analysis of gravity anomalies recent seismic refraction and reflection experiments have demonstrated that parts of oceanic fracture zones are characterized by very thin crust with total crustal thickness less than half that of normal oceanic crust the horizontal extent of this anomalous crust is not well known to map the lateral distribution of thin crust in a major fracture zone we have constructed free air anomaly bathymetry and sediment isopach maps of a 220 by 280 km region surrounding the western intersection of the vema fracture zone with the mid allantic ridge we isolate those components of the gravity field that are associated with variations in crustal thickness by first subtracting the attraction of the water crust interface the sediment layer and the crust mantle interface assuming constant crustal thickness and density these calculations are performed using a fully three dimensional model the remaining mantle bouguer anomaly is dominated by the effects of the cooling lithosphere variations in density of the crust and upper mantle associated with the cooling of the plate with increasing age of the seafloor are predicted for a thermal model that incorporates advection and the three dimensional conduction of heat the attraction of this three dimensional density structure is subtracted from the mantle bouguer anomaly to generate a map of the residual gravity anomaly which should directly reflect variations in crustal thickness or density the residual gravity anomaly map indicates that there is thin crust along much of the length of the vema fracture zone thin crust is concentrated primarily beneath the walls of the fracture zone rather than beneath the center of the fracture zone valley an interpretation confirmed by seismic refraction data the degree of thinning along the length of the fracture zone is variable ranging from one section with no appreciable thinning to others in which the crustal thickness is less than 3 km the excess depth of the fracture zone compared to surrounding seafloor is not isostatically compensated by local thinning of the crust other major topographic features such as the median valley and the elevated transverse ridge along the south wall of the fracture zone are also not locally compensated by changes in crustal thickness

migrating transform zone and lithospheric transfer at the galapagos 95 5 w propagator at the 95 5 w propagator system along the cocos nazca plate boundary the spreading axis is offset left laterally about 35 km 1 m y this offset migrates westward 52 km m y relative to points along the nearly east west trending ridge axis transferring lithosphere from the cocos to the nazca plate as it does so the required 58 km m y of right lateral motion across this migrating transform is apparently not accommodated by a throughgoing north south transform fault but rather by an approximately 15 to 20 km wide zone of shear deformation dominated by eastward curving of east west structures to a northwest southeast trend this pattern of curvature the coinciding zone of high seismicity and the abundance of talus in the area suggest that shear is distributed over this entire zone the deformation is inferred to be taken up by a bookshelf mechanism whereby faults oriented at high angles to the shear couple have left lateral slip antithetic to the right lateral shear couple rotating individual blocks clockwise these are interpreted as reactivated abyssal hill bounding normal faults probably dipping 45 60 northeast southwest shortening due to the rotation appears to be accommodated by reverse slip on the faults and may account for the uplift of de steiguer ridge in the zone of transferred lithosphere these faults have apparently had a very complex history initially normal faults formed as abyssal hills were created at the spreading axis and subsequently they had left lateral strike slip motion in the transform zone and reverse motion uplifting de steiguer ridge strain analysis based on observed structural curvature suggests that shear is distributed nonuniformly and lithospheric transfer requires 0 4 m y

topographic controls on evolution of shallow landslides in pastoral wairarapa new zealand 1979 2003 abstract this study aims to identify topographic influence on shallow landslides and their evolution during 1979 2003 in the pastoral lower north island new zealand landslide affected sites were mapped from aerial photographs and an aster satellite image the two landslide maps were overlaid in a geographic information system arcgis to detect the changes that have taken place in landslide affected areas the change map and the landslide distribution maps were overlaid with three topographic layers of slope gradient slope position and slope aspect that had been derived from a digital elevation model in arcgis the overlay analysis revealed that landslides affected area increased from 23 75 ha in 1979 to 110 97 ha in 2003 the distribution of landslide affected sites is inversely correlated with slope gradient but positively with slope position northern aspects are the most vulnerable to landsliding containing about 78 of the total affected sites moderate slope gradients suffered the most severe landsliding in the lower to middle slopes the overall magnitude and frequency of landsliding is also related to northern and eastern octants on the moderate slope gradients in the low to moderate slope positions although the same topographic setting is conducive to landsliding in both 1979 and 2003 the rate of landsliding varies with topographic setting hence topography is critical not only to landsliding but also to the evolution of sites already affected by landslides such an understanding over large spatial scales is possible only with the use of multitemporal remote sensing data and gis that are able to speed up data collection and analysis

factors affecting the magnitude of post glacial hillslope incision in japanese mountains abstract in japanese mountains increased post glacial rainfall triggered gullying in bedrock and thousands of shallow failures in regolith resulting in widespread hillslope incision to identify factors influencing the areal extent of the post glacial incision fifteen rugged river basins in central and northeast japan were surveyed topographical and geological properties of hillslopes were measured on geomorphological maps digital elevation data and geological maps analyses of ca 3 000 0 5 0 5 km morphometric samples have shown that the magnitude of regolith failure depends mainly on slope inclination and geology and subordinately on local storm intensity the magnitude of bedrock gullying also depends on these three factors but with greater dominance of storm intensity these observations reflect the differential stability of hillslopes with varying inclination erodibility of bedrock and sensitivity of regolith and bedrock to change in storm intensity the three factors also exert control on alluvial fan development in the lower reaches because they affect the amount of hillslope sediment supply and in turn the type of fluvial processes

buoyant decompression melting a possible mechanism for intraplate volcanism 1 volcanic activity away from plate boundaries occurs in a variety of settings linear age progressive volcanic chains have been explained as the manifestation of fixed hot spots possibly generated by plumes originating deep in the mantle however important examples of oceanic intraplate volcanism cannot be explained in this way including short lived chains and chains violating the predicted age distance behavior furthermore a significant fraction of volcanism does not occur in linear chains and observations suggest control by lithosphere structure in many cases in this study melting in mantle upwellings that result from the buoyancy associated with the melting itself is considered as a mechanism of intraplate volcanism numerical models of this buoyant decompression melting in a layer that is initially at its melting temperature over some depth range are used to determine the duration and amount of melting in upwellings which organize from an initially small random perturbation for a range of model parameters a predicted inverse correlation between the amount of melt produced and the duration of melting may be diagnostic of the buoyant melting process buoyant melting could occur in a number of geologic settings since the oceanic upper mantle may have previously melted beneath a spreading center and subsequently cooled spontaneous buoyant decompression melting may be possible only in regions where a large scale mantle upwelling can counteract conductive cooling keeping the mantle at its solidus temperature over some depth range where the mantle is at or sufficiently near its solidus buoyant decompression may spontaneously generate convective storms of melting this may explain the abundance of volcanism associated with the south pacific superswell where mantle is slightly cooler than its melting temperature buoyant melting may not occur spontaneously but may be triggered by some initial upwelling this may provide a physical explanation for volcanism that is controlled by lithosphere structure

gloria observations of the propagating rift at 95 50w on the cocos nazca spreading center in this paper we describe a gloria long range sidescan sonar survey of the area around 95 5 w on the cocos nazca rise where a propagating rift has been predicted to occur the sonar images clearly show a wedge of new crust that has been emplaced by propagation into 1 ma old crust the new crust is up to 800 m deeper than the surrounding seafloor although the elevation difference appears to decay with time the pseudofaults which bound this new crust have been observed from the rift tip back to crustal isochron 0 65 ma near the rift tip they consist of scarps several hundred meters high with narrow shallow valleys at their feet the sheared zone between the southern pseudofault and the trace of the failed rift to the south is characterized by abnormally shallow seafloor and nnw sse trending lineaments these may have been produced by the rotation of tectonic fabric formed at the dying rift perhaps accompanied by other deformation the youngest tectonic and volcanic lineaments on both spreading segments strike 273 but on the dying rift short spreading segments are offset en echelon so that the regional trend of the rise axis is 265 older lineaments throughout the area strike 265 the propagating rift is thus replacing an oblique rise axis by one which is orthogonal to the spreading direction and it is suggested that this is occurring in response to a recent change in spreading direction from 355 to 003 variations in the strikes of the pseudofaults indicate that the propagation direction has varied but that the rift has propagated most rapidly when following the 273 direction which we take to be normal to the new spreading direction

propagation and linkage of oceanic ridge segments we have investigated the propagation of spreading ridges and the development of structures that link ridge segments using an analogy between ridges and cracks in elastic plates the ridge propagation force and a path factor that controls propagation direction were calculated for echelon ridge segments propagating toward each other the ridge propagation force increases as ridge ends approach but then declines sharply as the ends pass so ridge segments may overlap somewhat the sign of the path factor changes as ridge ends approach and pass so the overlapping ridge ends may diverge and then converge following a hook shaped path the magnitudes of shear stresses in the plane of the plate and orientations of maximum shear planes between adjacent ridge segments were calculated to study transform faulting for different loading conditions simulating ridge push plate pull and ridge suction we identify a zone of intense mechanical interaction between adjacent ridge ends in which stresses are concentrated for all conditions the shear stress in the interaction zone increases as ends approach and remains large as the ends overlap thus crust in this zone may fracture and weaken in preparation for the formation of a through going transform fault the calculated shear planes rotate toward an orientation about 90 from the strike of ridges as the ends pass thus favoring the orthogonal arrangement of ridges and transforms the magnitudes of mean stresses in the plane of the plate and orientations of principal stress planes were also calculated the mean stress is tensile in the interaction zone so basins may form there except in the case of ridge push loading the planes across which the maximum tension acts are oblique to ridges thus favoring obliquely oriented normal faults bounding the transform valley

gravity anomalies flexure of axial lithosphere and along axis asthenospheric flow beneath the southeast indian ridge abstract mantle bouguer and residual gravity anomaly patterns consistent with two and three dimensional mantle flow and or crustal thickness variability are observed juxtaposed within the five first order segments of the southeast indian ridge seir surveyed between 98 and 112 e within each segment bull s eye or triangular patterns of mantle bouguer and residual gravity anomalies consistent with focused mantle upwelling and or three dimensional crustal thickness patterns occur systematically toward the western ends of segments which adjoin significant left stepping offsets in the ridge axis the eastern ends of these segments and those that abut smaller right stepping transforms display a more two dimensional anomaly character axial morphology also varies along strike axial highs coincide with focused gravity anomalies while rifted highs occur elsewhere these observations imply that more magmatically robust extension more vigorous mantle upwelling and or thicker crust characterize the western parts of those segments adjacent to significant ridge offsets these trends are mirrored east of the deepest portions of the australian antarctic discordance aad where more vigorous upwelling is located toward the eastern end of segment b5 in addition to the usual interpretations in terms of crustal thickness and or mantle temperature we suggest that the systematics of topographic style and gravity anomaly character provide further evidence for along axis asthenospheric flow beneath the seir in western portions of segments between 98 and 112 e sub axial flow toward the aad causes asthenosphere to be drawn from beneath older lithosphere to the west thereby increasing the region of mantle upwelling and melting supplying the upstream portion of the ridge axis

tectonics of the failing spreading system associated with the 95 5 w galapagos propagator the failing spreading system associated with the galapagos 95 5 w propagator system comprises a complex set of volcanic and structural provinces data from detailed surveys using gloria seamarc ii sea beam deep tow the submersible alvin magnetics and bottom cameras constrain the plate boundary geometry and evolution of this area although the failing spreading system may involve diffuse spreading the patterns of sedimentation volcanism and faulting suggest localization the failed spreading axis composed of a set of right stepping en echelon basins extends and ages southeastward from offset and overlapping doomed failing axes the failing axis is offset 13 km left laterally from the doomed axis although there may have been a preexisting offset in the doomed axis lack of structural evidence for this and lack of an offset of this magnitude in the brunhes matuyama magnetic reversal argue against this possibility a model whereby the offset is transient does not explain the pattern of failed spreading basins which leads directly away from the currently failing axis detailed structural and volcanic patterns suggest that this offset may be caused by a westward progression of northward jumps in the location of the failing axis like a secondary propagator probably in association with the migrating strain stress field of the migrating transform zone of the propagator system this is modelled as a migrating extensional relay zone merz similar to the relay zones observed at some fast slipping transform faults except that it propagates westward with the overall system bathymetric and magnetic evidence for tectonic elements associated with the merz supports this model although the expected extinct trace of the doomed axis is not obvious in the present incomplete data set unlike other models for the plate boundary geometry the merz model does not require serendipity for temporal and spatial juxtaposition of the primary propagator and the offset within the failing spreading system observation of a similar geometry in the lau basin supports this preferred model present data from the galapagos failing spreading system are inconclusive however and the merz model remains speculative if valid this model implies that in at least some cases paired propagators act together the merz is too short and deep to drive its own propagation according to standard cracking models

the tectonic fabric of the ocean basins 1 we present a global community data set of fracture zones fzs discordant zones propagating ridges v shaped structures and extinct ridges digitized from vertical gravity gradient vgg maps we use a new semi automatic fz tracking program to test the precision of our hand digitized traces and find a mean absolute deviation of less than 3 4 km from the raw vgg minima that most clearly delineate each feature and less than 5 4 km from the fz location predicted by fitting model profiles to the vgg data that represent the morphology of the individual fzs these offsets are small considering gravity data only provide an approximation for the underlying basement morphology we further investigate the origin of non fz seafloor fabric by combining published abyssal hill heights computed from gravity anomalies with global half spreading rates a residual abyssal hill height grid with spreading rate effects removed combined with our interpreted tectonic fabric reveals several types of seafloor fabric distinct from typical abyssal hills where discordant zones do not overprint abyssal hill signals residual abyssal hill height anomalies correspond to seafloor that accreted near mantle thermal anomalies or zones of melt depletion our analysis reveals several areas where residual abyssal hill height anomalies reflect pseudo faults and extinct ridges associated with ridge propagation and or microplate formation in the southern pacific ocean

the paradox of the axial profile isostatic compensation along the axis of the mid atlantic ridge mantle bouguer gravity anomalies mba and bathymetry on three profiles covering more than 1000 km along the axis of the mid atlantic ridge mar are highly correlated suggesting that along axis topographic relief is locally compensated by variations in crustal thickness and or mantle density structure the quantitative relationship between topography and gravity on these profiles could be explained by the flexure of a thin narrow elastic strip representing the response to isostatic loads of an inner rift valley isolated from the rest of the plate by weak bounding normal faults the paradox is that across axis profiles show that the median valley is an uncompensated feature apparently created by a dynamic mechanism new extensive off axis coverage of the mar from 31 to 36 s shows that the high correlation does not persist outside the axial zone we suggest that the on axis correlation exists because the mechanism creating the median valley is controlled by the mantle thermal structure and along axis variations in crustal thickness both of which contribute to the mba if the mechanism is extension of a brittle ductile lithosphere the critical parameter controlling topographic relief is the thickness of the relatively stiff mantle layer immediately beneath the crust at the ridge axis a three dimensional 3 d thermal model incorporating passive mantle flow hydrothermal circulation the plate boundary geometry and variable magmatic heating associated with observed variations in crustal thickness predicts variations in the thickness of the stiff mantle layer that correlate with the observed axial topography we model the expected topography using cross axis sections of the 3 d thermal model and a 2 d finite element model of an extending lithosphere that incorporates temperature and strain dependent rheology as well as the flexural response of a thickening plate the predicted topographic signal produced by the combined dynamic and isostatic effects matches the amplitude of the observed axial bathymetry at the 18 mm yr 1 spreading half rate in our south atlantic survey the 2 d models also reproduce the observed rapid transition along the axis between rift valley no rift valley morphology we conclude that extensional forces acting on segmented oceanic lithosphere with varying rates of crustal production produce the highly variable morphology of the southern mar and the along axis correlation between mba and bathymetry

the southeast indian ridge between 88 e and 118 e gravity anomalies and crustal accretion at intermediate spreading rates although slow spreading ridges characterized by a deep axial valley and fast spreading ridges characterized by an axial bathymetric high have been extensively studied the transition between these two modes of axial morphology is not well understood we conducted a geophysical survey of the intermediate spreading rate southeast indian ridge between 88 e and 118 e a 2300 km long section of the ridge located between the amsterdam hot spot and the australian antarctic discordance where satellite gravity data suggest that the southeast indian ridge seir undergoes a change from an axial high in the west to an axial valley in the east a basic change in axial morphology is found near 103 30 e in the shipboard data the axis to the west is marked by an axial high while a valley is found to the east although a well developed axial high characteristic of the east pacific rise epr is occasionally present the more common observation is a rifted high that is lower and pervasively faulted sometimes with significant 50 m throw faults within a kilometer of the axis a shallow axial valley 1200 m deep valley across a transform at 114 e the changes in axial morphology along the seir are accompanied by a 500 m increase in near axis ridge flank depth from 2800 m near 88 e to 3300 m near 114 e and by a 50 mgal increase in the regional level of mantle bouguer gravity anomalies over the same distance the regional changes in depth and mantle bouguer anomaly mba gravity can be both explained by a 1 7 2 4 km change in crustal thickness or by a mantle temperature change of 50 c 90 c in reality melt supply crustal thickness and mantle temperature are linked so that changes in both may occur simultaneously and these estimates serve as upper bounds the along axis mba gradient is not uniform pronounced steps in the regional level of the mba gravity occur at 103 30 e 104 e and at 114 e 116 e and correspond to the changes in the nature of the axial morphology and in the amplitude of abyssal hill morphology suggesting that the different forms of morphology do not grade into each other but rather represent distinctly different forms of axial structure and tectonics with a sharp transition between them the change from an axial high to an axial valley requires a threshold effect in which the strength of the lithosphere changes quickly the presence or absence of a quasi steady state magma chamber may provide such a mechanism the different forms of axial morphology are also associated with different intrasegment mba gravity patterns segments with an axial high have an mba low located at a depth minimum near the center of the segment at epr like segments the mba low is about 10 mgal with along axis gradients of 0 15 0 25 mgal km similar to those observed at the epr rifted highs have a shallower low and lower gradients suggesting an attenuated composite magma chamber and a reduced and perhaps episodic melt supply segments with a shallow axial valley have very flat along axis mba profiles with little correspondence between axial depth and axial mba gravity

evaluating debris slide occurrence using digital data paraglacial activity in chilliwack valley british columbia debris sliding is one of the most important processes acting to transport sediment within mountainous regions detailed study of debris slide activity at the basin scale typically involves landslide inventories generated from aerial photographs however it has been shown that some types of rapid mass movement can be accurately identified using a combination of high resolution satellite imagery and digital elevation data this approach is beneficial as the digital products allow for a more accurate and efficient data throughput into various types of geomorphic analysis here we demonstrate the use of an automated inventory in the geomorphometric evaluation of debris slide initiation for the chilliwack basin british columbia canada our results indicate that the occurrence of debris sliding is primarily determined by topographical controls for basins that are in equilibrium with the existing climate the frequency of debris sliding should demonstrate a strong relationship to bedrock geology as the production of unconsolidated materials available for failure is a function of weathering rates under these conditions the lack of bedrock control within the chilliwack basin suggests a state of paraglacial relaxation wherein glacial deposits dominate the sediment cascade within the area therefore topographic parameters can be used to discriminate the location of metastable slopes where debris slide erosion will be active the use of digital data in the characterization of debris slide occurrence would seem to be a viable alternative to the more traditional methods

morphology of the transition from an axial high to a rift valley at the southeast indian ridge and the relation to variations in mantle temperature the southeast indian ridge exhibits a transition in axial morphology from an east pacific rise like axial high near 100 e to a mid atlantic ridge like rift valley near 116 e but spreads at a nearly constant rate of 74 76 mm yr assuming that the source of this transition lies in variations in mantle temperature we use shipboard gravity derived crustal thickness and ridge flank depth to estimate the variations in temperature associated with the changes in morphological style within the transitional region seabeam 2000 bathymetry shows scattered instances of highs valleys and split volcanic ridges at the axis a comparison of axial morphology to abyssal hill shapes and symmetry properties suggests that this unorganized distribution is due to the ridge axis episodically alternating between an axial valley and a volcanic ridge axial morphology can then be divided into three classes with distinct geographic borders axial highs and rifted highs are observed west of a transform fault at 102 45 e rift valleys are observed east of a transform fault at 114 e and an intermediate style morphology which alternates between a volcanic ridge and a shallow axial valley is observed between the two one segment between 107 and 108 30 e forms an exception to the geographical boundaries gravity derived crustal thickness and flank depth generally vary monotonically over the region with the exception of the segment between 107 e and 108 30 e the long wavelength variations in these properties correlate with the above morphological classification gravity derived crustal thickness varies on average 2 km between the axial high and rift valley regions the application of previous models relating crustal thickness and mantle temperature places the corresponding temperature variation at 25 c 50 c depending on the model used the average depth of ridge flanks varies by 550 m over the study area for a variation of 25 50 c thermal models of the mantle predict depth variations of 75 150 m these values are consistent with observations when the combined contributions of crustal thickness and mantle density to ridge flank depth are considered assuming airy isostasy crustal thickness variations differ at the two transitions described above a difference of 750 m in crustal thickness is observed at the rift valley intermediate style transition suggesting small variations in crustal thickness and mantle temperature drive this transition at the axial high rifted high intermediate style transition crustal thickness variations are not resolvable suggesting that this transition is controlled by threshold values of crustal thickness and mantle temperature and is perhaps related to the presence of a steady state magma chamber

kinematics of overlapping rift propagation with cyclic rift failure existing kinematic models for propagation of oceanic spreading ridges that incorporate overlap between ridge segments fail to describe detailed observations of the failed segments i present a new model which discards the assumption of steady state behavior of the failing rift permitting inward curvature of both rift tips in the overlap region the shape of an inward curving failing rift must continuously change but is assumed to cyclically return to its original shape by discrete inward ridge jumps other assumptions of symmetric spreading and uniform simple shear deformation between the overlapping rift tips are retained from previous models inward curvature of failed rift structures provides much better agreement with observations and is consistent with tensile fracture theory if the offset between ridge segments is small enough the inward jumps of the failing rift will cut across deformed structures originally formed at the propagating tip possibly generating seafloor fabrics that crosscut each other at nearly right angles observations of such structures near the gorda ridge can be explained by a model incorporating variable cyclieity of the failing rift chrons curve inward toward the ridge faithfully recording the shape of the propagating tip in an undeformed part of the plate isochrons also curve toward the ridge along the inner pseudofault but in the original formulation of the rules of plate tectonics transform faults were assumed to be fixed relative to each other and the ridge segments they offset the propagating rift model 1 which discards this assumption has been very successful in describing oblique offsets in magnetic anomalies as a result of the lengthening of a ridge segment through time at the expense of the adjacent segment detailed observations of active propagating rifts suggest that the assumption that ridges are necessarily offset by discrete transform faults should also be discarded with the segments instead offset by zones of distributed shear with widths of 10 or more km 2 3

overlapping spreading centers implications from crack growth simulation by the displacement discontinuity method overlapping spreading centers osc s are a fundamental aspect of accretionary processes at intermediate and fast spreading centers and typically occur at deep points along the axial depth profile they have a characteristic geometry consisting of two en echelon overlapping curving ridges separated by an elongated depression the length to width ratio of this overlap basin is typically 3 1 we have been successful in reproducing the overlapping spreading center geometry by modelling the growth of two initially parallel elastic cracks of given length and offset in a tensile stress field at infinity a boundary element displacement discontinuity method was used to solve this problem our calculated results are compared with seafloor observations in terms of the size and shape of the overlap region for small osc s there is a very good agreement between calculations and observations but for large ones the overlap basin tends to be longer than our predicted results indicate this suggests that the assumptions made in the model i e perfectly elastic isotropic and homogeneous medium are perhaps valid for the brittle lid above the magma chamber that underlies osc s with small offsets 2 km but oversimplified for osc s with large offsets our modelling shows that the initial interaction of closely spaced surface ruptures along spreading centers is to deflect away from one another as they approach the deflection will be the greatest for small misalignments of the fracture systems thus even minor misalignments of the spreading centers may result in the development of osc s where the misalignment is less than the width of the cracking front the fracture systems may meet head on creating a saddle point along the axial depth profile our results support the hypothesis suggested by macdonald et al 1984 in which overlapping spreading centers develop where two magmatic pulses migrate toward each other along the strike of the spreading center following fracture systems and magmatic conduits which are imperfectly aligned

causes and rate limiting mechanisms of ridge propagation a fracture mechanics model all known propagating rifts pr s are growing away from shallow ridge axis seafloor we examine a model for rift propagation in which the ridge axis conduit like a crack in the lithosphere propagates in response to a stress concentration at its tip the stress concentration near the tip which causes progressive failure of the lithosphere is primarily due to excess gravity spreading stresses associated with anomalously shallow ridge axis along the growing ridge this stress concentration near the tip is characterized by a stress intensity factor k which must exceed a threshold value to cause failure of the plate a remotely applied tensile stress would make a positive contribution to k but would act equally on both ridge segments and not favor the propagation of either the pressure in the crack may depending on its distribution make either a positive or negative contribution to k for a high enough propagation rate viscous flow induced suction due to the growth of the ridge axis as well as the reduced vertical flow near the pr tip would make k 0 this simple model thus includes a mechanism to control the speed of propagation and is consistent with the axial morphology of the spreading segment associated with the well studied 95 5 w pr of the galapagos spreading center analytical solutions for the contribution to the stress intensity factor of viscous forces resisting ridge axis growth and causing the pr axial topography are consistent with the observed propagation rate for asthenosphere viscosities of about 1019 pa s 1020 p we also examine a more general model of a ridge transform ridge system on the basis of energy conservation in terms of energetics this more general model reduces to that of a single propagating ridge axis crack when both the work done by forces driving propagation and the viscous dissipation due to forces resisting propagation are generated along the growing ridge axis however the more general model also includes mechanisms that may stop ridge propagation propagation will stop either when the energy needed for transform migration is larger than that available for ridge axis growth or when the strain energy absorbed at the dying ridge axis is comparable to that released at the growing ridge axis i e if the stress intensity factors for both ridge axes are similar both of these possibilities may occur when the propagating rift completely propagates through a ridge segment as at 85 w in the galapagos finally in this model the slight systematic differences in ridge axis orientation of the growing and dying ridges are a consequence rather than a cause of ridge propagation

gravity and seismic study of crustal structure along the juan de fuca ridge axis and across pseudofaults on the ridge flanks variations in topography and seismic structure are observed along the juan de fuca jdf ridge axis and in the vicinity of pseudofaults on the ridge flanks left by former episodes of ridge propagation here we analyze gravity data coregistered with multichannel seismic data from the jdf ridge and flanks in order to better understand the origin of crustal structure variations in this area the data were collected along the ridge axis and along three ridge perpendicular transects at the endeavor northern symmetric and cleft segments negative mantle bouguer anomalies of 21 to 28 mgal are observed at the axis of the three segments thicker crust at the endeavor and cleft segments is inferred from seismic data and can account for the small differences in axial gravity anomalies 3 7 mgal additional low densities elevated temperatures within and or below the axial crust are required to explain the remaining axial mba low at all segments gravity models indicate that the region of low densities is wider beneath the cleft segment gravity models for pseudofaults crossed along the three transects support the presence of thinner and denser crust within the pseudofault zones that we attribute to iron enriched crust on the young crust side of the pseudofaults a 10 20 km wide zone of thicker crust is found reflection events interpreted as subcrustal sills underlie the zones of thicker crust and are the presumed source for the iron enrichment

analysis of propagators along the pacific antarctic ridge evidence for triggering by kinematic changes abstract we analyze the structure and evolution of two propagators along the pacific antarctic ridge par that we surveyed during the pacantarctic cruise of the n o l atalante a large propagator at 63 30 s 167 w shows a n50 e trending segment of the par propagating southwestward while the adjacent n45 e trending segment retreats the propagating and doomed ridges are offset by about 43 km they both curve into the offset to define an overlap zone about 25 km long the inner pseudofault is juxtaposed to a series of e w trending ridges inferred to represent the failed axes their direction and arrangement suggest an evolution as an overlapping propagator with cyclic rift failure the pseudofaults are 35 5 oblique to the propagating ridge which implies a rate of propagation of 44 8 mm yr using a 62 mm yr full spreading rate comparable to that of other propagators with similar morphology the age of the initiation of the propagation from the heirtzler fracture zone is estimated to be 5 6 ma which coincides with the age of a clockwise change in spreading direction a second smaller southwestward propagator is observed northeast of the major one at 63 15 s 165 10 w with a morphology very similar to that of the larger one it is inferred to have started near 1 ma again at the time of a clockwise change in spreading direction along the par these two propagators are likely to have evolved from extensional relay zones which developed within the heirtzler transform fault tf valley following clockwise changes in spreading direction the present day axial discontinuity is less than 40 km in offset and may not be a tf anymore the development of propagators in this area of the par appears to be triggered by kinematic changes rather than by thermal gradients along the ridge other propagators that have left similar signatures on the flanks of the par appear to have developed at similar spreading rates near 50 60 mm yr full rate as a result of kinematic changes

deformation of oceanic lithosphere near slow spreading ridge discontinuities abstract transform and non transform discontinuities that offset slow spreading mid ocean ridges involve complex thermal and mechanical interactions the truncation of the ridge axis influences the dynamics of spreading and accretion over a certain distance from the segment end likewise the spreading system is expected to influence the lithospheric plate adjacent to the ridge end opposite of the discontinuity tectonic effects of the truncated ridge are noticeable in for example the contrast between seafloor topography at inside corners and outside corners along axis variations in rift valley depth style of crustal accretion and ridge segment retreat and lengthening along such slow spreading discontinuities and their fossil traces oceanic core complexes or mega mullion structures are rather common extensional tectonic features in an attempt to understand deformation of oceanic lithosphere near ridge offsets the evolution of discontinuities and conditions that may favor oceanic core complex formation a three dimensional thermo mechanical model has been developed the numerical approach allows for a more complete assessment of lithosphere deformation and associated stress fields in inside corners than was possible in previous 3 d models the initial suite of results reported here focuses on deformation when axial properties do not vary along strike or with time showing the extent to which plate boundary geometry alone can influence deformation we find that non transform discontinuities are represented by a wide oblique deformation zone that tends to change orientation with time to become more parallel to the ridge segments this contrasts with predicted deformation near transform discontinuities where initial orientation is maintained in time the boundary between the plates is found to be vertical in the center of the offset and curved at depth in the inside corners near the ridge transform intersection ridge normal tensile stresses concentrate in line with the ridge tip extending onto the older plate across the discontinuity and high stress amplitudes are absent in the inside corners during the magmatic accretionary phase simulated by our models with the tested rheology and boundary conditions inside corner formation of oceanic core complexes is predicted to be unlikely during magmatic spreading phases additional modeling studies are needed for a full understanding of extensional stress release in relatively young oceanic lithosphere

development of the australian antarctic depth anomaly the oceanic australian antarctic discordance aad contains two unusual features 1 n s trending anomalously deep bathymetries and 2 rough basement morphologies in young 45 spreading obliquities

constraints on rift propagation history at the cobb offset juan de fuca ridge from numerical modeling of tectonic fabric abstract rift propagation has been proposed as the mechanism for the reorganization of the juan de fuca ridge during the cenozoic in particular magnetic anomalies in the cobb offset region of this ridge have been interpreted in terms of a complex dueling propagator history in which rift propagation has occurred alternately in opposing directions on adjacent ridge segments although the overall propagation history has been inferred from the magnetic data these data are limited in their resolution for the detailed history by the length of time during which magnetic polarity remains constant we thus examine whether more detailed resolution within polarity chrons can be obtained using seafloor tectonic fabric from acoustic imagery by modeling lineations as isochrons synthetic fabric can be generated for a given rift propagation history and the fit of the model to the data can be compared numerically for the cobb we used a propagation history model with a given timing and direction of propagation and generated synthetic fabric for different propagation rates the fabric is best fit for propagation rates between 400 and 900 mm yr slower 900 mm yr propagation rates yield slightly poorer fits the fabric results are thus in accord with the interpretation based on magnetics alone that the rates of rift propagation significantly exceeded the spreading rates an interesting feature of the model is that the complex tectonic fabric is reasonably well fit on this scale 1 km by simple rift propagation models which do not invoke shear zone behavior and thus deviate from rigid plate tectonics

ridge jumps and propagations in the south atlantic ocean the normal pattern of mid ocean ridge divergent plate boundaries comprises straight spreading segments with narrow zones of crustal accretion bounded by transform faults where these segments are offset normally the configuration of spreading segments and transform faults is stable for long periods but in places on fast and medium rate spreading ridges the transform offsets have propagated along the ridge axis as some spreading segments have grown at the expense of adjacent ones1 7 although such behaviour has hitherto not been well documented on slow spreading ridges we have mapped multiple ridge jumps and propagating rifts in two areas of the slow spreading 18 mm yr 1 half rate south atlantic ridge crest we attribute this anomalous behaviour in the south atlantic to the presence of hotspots near our study areas causing elevated asthenospheric mantle temperatures and lateral mantle flow with consequent excess volcanism and disruption of the normally well focused melt delivery processes at the ridge crest

identification of topographic settings conducive to landsliding from dem in nelson county virginia u s a the steep terrains in the blue ridge mountains of virginia are potentially vulnerable to landsliding hundreds of slope failures were initiated by a severe storm in 1969 in nelson county virginia the outlines of 87 landslide scars were delineated on aerial photographs within a 28 8 km2 area a digital elevation model dem of the same area was constructed from which three topographic variables slope orientation gradient and form were derived overlay analyses of the landslide data and the topographic data followed by chi square tests revealed that the topographic variables studied are statistically significant to the spatial distribution of the sites disturbed by landslide paths while the landsliding potential is low at very low and high elevations it is found to be high in the median range of elevation west northwest north and northeast facing slopes are consistently more vulnerable to landsliding than their east southeast south and southwest facing counterparts terrains with a gentle gradient are stable but the potential for landslides to develop increases greatly as the terrains become steeper the slopes with a concave cross section are more susceptible to landsliding than those having other shapes furthermore the landsliding potential does not vary significantly with slope forms in the profile direction it is concluded that the introduction of a dem to this study has provided renewed insight into topographic settings conducive to landsliding which could not have been achieved otherwise

assessing shallow landslide hazards using the trigrs and shalstab models serra do mar brazil the hillslopes of the serra do mar a system of escarpments and mountains that extend more than 1500 km along the southern and southeastern brazilian coast are regularly affected by heavy rainfall that generates widespread mass movements causing large numbers of casualties and economic losses this paper evaluates the efficiency of susceptibility mapping for shallow translational landslides in one basin in the serra do mar using the physically based landslide susceptibility models shalstab and trigrs two groups of scenarios were simulated using different geotechnical and hydrological soil parameters and for each group of scenarios a and b three subgroups were created using soil thickness values of 1 2 and 3 m simulation results were compared to the locations of 356 landslide scars from the 1985 event the susceptibility maps for scenarios a1 a2 and a3 were similar between the models regarding the spatial distribution of susceptibility classes changes in soil cohesion and specific weight parameters caused changes in the area of predicted instability in the b scenarios both models were effective in predicting areas susceptible to shallow landslides through comparison of areas predicted to be unstable and locations of mapped landslides such models can be used to reduce costs or to define potentially unstable areas in regions like the serra do mar where field data are costly and difficult to obtain

models of ocean ridge lithospheric deformation dependence on crustal thickness spreading rate and segmentation we use three dimensional 3 d temperature and rheology models to investigate the effects of crustal thickness and ridge segmentation on mid ocean ridge lithospheric structure we find that crustal thickness variations associated with focused magma accretion at a slow spreading ridge segment can cause significant along axis gradients in crustal temperature and a pita pocket shaped weak zone in the lower crust that decouples brittle upper crust from upper mantle in contrast fast spreading ridge segments with little crustal thickness variation are found to be uniformly weak along axis the overthickened crust produced by ridge hotspot interaction alters the heat balance between magma emplacement and hydrothermal cooling creating an extremely weak lithosphere in a steady state model we apply a simple two dimensional 2 d cyclic faulting model to across axis sections of temperature and rheological structure slow spreading segments are predicted to have relatively large along axis variations in fault height and spacing in contrast fast spreading and hotspot affected segments are predicted to have much smaller variations in faulting styles except in the immediate vicinity of a major transform fault the presence or lack of a steady state axial rift valley is also predicted to depend on crustal thickness spreading rate and the ability of rift bounding normal faults to propagate along axis for fast spreading and hotspot affected segments the sizes of axial rift valleys are predicted to be small and may not be distinguishable from those of neovolcanic or magma chamber supported isostatic features this is in contrast to most slow spreading segments where axial rift valleys are predicted to be a dominant morphological feature for a slow spreading segment of great length however the axial rift valley size is predicted to diminish toward the segment center because large faults formed at segment distal ends cannot propagate through a large decoupling zone created by locally thickened crust at the segment center

evidence for variable upper mantle temperature and crustal thickness in and near the australian antarctic discordance abstract the southeast indian ridge seir in and near the australian antarctic discordance aad exhibits at a constant spreading rate almost the full range of the many geophysical and geochemical parameters characteristic of the slow mid atlantic ridge and fast east pacific rise we used satellite derived gravity data in combination with seamarc ii bathymetry in and near the aad to examine regional density variations in the upper mantle beneath the aad through three dimensional gravity analysis we found that at least two end member models satisfy the gravity observations regional crustal thickness variations of at least 3 km along the seir near the aad or a temperature anomaly of the order of 150 c in the upper mantle beneath the seir these new observations combined with other geophysical and geochemical characteristics of the australian antarctic discordance provide further evidence that the temperature structure of a mid ocean ridge is a controlling factor in addition to spreading rate in the crustal accretionary process numerical models of mantle flow beneath mid ocean ridges offer one means of investigating the dynamic effect of a variable upper mantle temperature on the accretionary process our results indicate that temperature is important especially at intermediate and slower spreading rates where thermal effects can dominate mantle flow beneath a mid ocean ridge and result in increasing crustal production with decreasing spreading rate at the constant intermediate spreading rate of 37 mm yr characteristic of the seir in and near the aad our numerical models show that significant crustal thinning 2 4 km can occur with relatively small variations in upper mantle temperature all else being equal thus combined with our end member gravity models these observations and results suggest that both anomalously cool upper mantle and thin crust exist beneath the aad

rift valley no rift valley transition at mid ocean ridges a simple mechanical model for the axial topographic and gravity variations with spreading rate is developed the model emphasizes the rapid transition from one mechanism to another with changing spreading rate our model has a brittle layer over a ductile layer which are being pulled by forces away from the ridge axis there exists a potential failure zone of width lf where the stresses within the brittle plate exceeds its yield strength if the yield strength of the brittle plate is nearly constant this width is largely determined by the strain rate and the viscosity of the strong mantle the high ductility of oceanic crust at high temperatures relative to that of mantle at the same temperatures leads to the concept of a decoupling region a critical feature in this model the size of the decoupling region ld is determined by the thickness of the oceanic crust and temperatures within the crust if the decoupling region is small ld lf as shown to be the case for fast spreading ridges then each of the two layers responds separately to the horizontal stretching the brittle plate will be broken only at the axis and any buoyant force of the region would lift it up to form the rise crest high as a result of local isostasy the changing size of the decoupling region gives rise to the transition from atlantic type axial topography to pacific type as well as the transition from a high amplitude highly variable gravity signature to a low amplitude very uniform gravity signature parameters can be chosen so that such a transition occurs at a half rate of 35 mm yr which is the critical half rate suggested by both topographic and gravity data

major element constraints on melting differentiation and mixing of magmas from the galapagos 95 5 w propagating rift system projections in the pseudo quaternary system diopside plagioclase olivine quartz di pl ol q are used in this paper as a base on which to plot additional compositional petrologic or tectonic variables used in this manner the projections provide a sensitive means of displaying and evaluating the chemical variability of mid ocean ridge basalt morb suites such as that of the galapagos 95 5 w propagating rift system and allow mantle controls on the chemical variability of morb to be distinguished from the predominant effects of shallow level crystal fractionation and mixing primitive lavas mg 63 from the 95 5 w region form a broadly linear array within the di pl ol q tetrahedron parallel to the di ol join in terms of two recent high pressure studies of morb petrogenesis stolper 1980 takahashi and kushiro 1983 the primitive liquids in this array are not primary but have evolved by less than 10 olivine fractionation from an array of primary liquids which separated from residual mantle over a range of pressures between 10 20 kb with appropriate assumptions as to residual lithology and mg of residual olivine the nature and origin of 95 5 w primary magmas can be deduced 1 lavas of the normal rift are derived from a broad range of primary magmas that separated from compositionally uniform in terms of major elements mantle over a pressure range of 10 20 kb 2 beneath the newly forming propagating rift melting is initiated perhaps by pressure release at shallow depth 9 11 kb degree of melting and source composition appear similar to those for the normal rift within a few km of the propagating rift tip this shallow source is joined by a second deeper 11 12 kb more iron rich source both sources persist for at least 100 km along the propagating rift but the deeper source appears to become dominant farther behind the tip maximum pressure for melt separation anywhere beneath the propagating rift is less than 15 kb in addition to these source effects the projections allow a more complete description of the shallow level fractionation and mixing effects along the propagating rift primitive lavas of restricted composition erupted near the rift tip give way within 2 3 km to an increasingly diverse suite of lavas characterized by variable extents of fractionation and by numerous single mixing events consistent with magmatic evolution in small only occasionally interconnected magma bodies beyond 15 km the diversity gradually declines toward a restricted population of cpx saturated lavas which characterizes the propagating rift beyond 75 km and is consistent with the presence of a large well mixed magma chamber in this region

overlapping spreading centres new accretion geometry on the east pacific rise in a detailed seabeam investigation of the east pacific rise epr from 8 n to 18 n a new kind of volcano tectonic geometry associated with fast spreading centres has been discovered figs 1 2 at several locations along the rise axis the neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast to a classic ridge transform ridge plate boundary however the offset ridge terminations overlap each other by a distance approximately equal to or greater than the offset they curve sharply towards each other and often merge into one another along strike separating the overlapping spreading centres oscs is a closed contour depression up to several hundred metres deep which is sub parallel to the trend of the oscs the region between the oscs is a complex zone of both shear and rotational deformation with no obvious transform parallel structures based on wax model studies of spreading centres we suggest here that transform faults fail to develop at fast spreading centres where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate spreading centre transform fault pattern the osc geometry is unstable and evolves rapidly one of the two oscs prevails while the other is abandoned a significant area of sea floor created at fast spreading rates may bear the imprint of this newly observed process

seismic crustal structure and morphotectonic features associated with the chain fracture zone and their role in the evolution of the equatorial atlantic region oceanic transform faults and fracture zones fzs represent major bathymetric features that keep the records of past and present strike slip motion along conservative plate boundaries although they play an important role in ridge segmentation and evolution of the lithosphere their structural characteristics and their variation in space and time are poorly understood to address some of the unknowns we conducted interdisciplinary geophysical studies in the equatorial atlantic ocean the region where some of the most prominent transform discontinuities have been developing here we present the results of the data analysis in the vicinity of the chain fz on the south american plate the crustal structure across the chain fz at the contact between 10 and 24 ma oceanic lithosphere is sampled along seismic reflection and refraction profiles we observe that the crustal thickness within and across the chain fz ranges from 4 6 5 9 km which compares with the observations reported for slow slipping transform discontinuities globally we attribute this presence of close to normal oceanic crustal thickness within fzs to the mechanism of lateral dike propagation previously considered to be valid only in fast slipping environments furthermore the combination of our results with other data sets enabled us to extend the observations to morphotectonic characteristics on a regional scale our broader view suggests that the formation of the transverse ridge is closely associated with a global plate reorientation that was also responsible for the propagation and for shaping lower order mid atlantic ridge segmentation around the equator

propagation as a mechanism of reorientation of the juan de fuca ridge we present a revised model of the tectonic evolution of the juan de fuca ridge by propagating rifting the new model has three different relative rotation poles covering the time intervals 17 0 8 5 ma 8 5 5 0 ma and 5 0 ma to the present the rotation pole shifts at 8 5 and 5 0 ma imply clockwise shifts in the direction of relative motion of 10 to 15 at each of these shifts the pattern of propagation reorganizes and the new ridges formed by propagation are at an orientation closer to orthogonal to the new direction of motion than the orientation of the preexisting ridges the model containing a total of seven propagation sequences shows excellent agreement with the isochrons inferred from the magnetic anomaly data except in areas complicated by the separate explorer and gorda plates the agreement between model and data near the explorer plate breaks down abruptly at an age of about 5 ma indicating that the probable cause of the rotation pole shift at that time was the separation of the explorer plate from the juan de fuca plate

statistical analysis of factors affecting landslide distribution in the new madrid seismic zone tennessee and kentucky abstract more than 220 large landslides along the bluffs bordering the mississippi alluvial plain between cairo ill and memphis tenn are analyzed by discriminant analysis and multiple linear regression to determine the relative effects of slope height and steepness stratigraphic variation slope aspect and proximity to the hypocenters of the 1811 12 new madrid mo earthquakes on the distribution of these landslides three types of landslides are analyzed 1 old coherent slumps and block slides which have eroded and revegetated features and no active analogs in the area 2 old earth flows which are also eroded and revegetated and 3 young rotational slumps which are present only along near river bluffs and which are the only young active landslides in the area discriminant analysis shows that only one characteristic differs significantly between bluffs with and without young rotational slumps failed bluffs tend to have sand and clay at their base which may render them more susceptible to fluvial erosion bluffs having old coherent slides are significantly higher steeper and closer to the hypocenters of the 1811 12 earthquakes than bluffs without these slides bluffs having old earth flows are likewise higher and closer to the earthquake hypocenters multiple regression analysis indicates that the distribution of young rotational slumps is affected most strongly by slope steepness about one third of the variation in the distribution is explained by variations in slope steepness the distribution of old coherent slides and earth flows is affected most strongly by slope height but the proximity to the hypocenters of the 1811 12 earthquakes also significantly affects the distribution the results of the statistical analyses indicate that the only recently active landsliding in the area is along actively eroding river banks where rotational slumps formed as bluffs are undercut by the river the analyses further indicate that the old coherent slides and earth flows in the area are spatially related to the 1811 12 earthquake hypocenters and were thus probably triggered by those earthquakes these results are consistent with findings of other recent investigations of landslides in the area that presented field historical and analytical evidence to demonstrate that old landslides in the area formed during the 1811 12 new madrid earthquakes results of the multiple linear regression can also be used to approximate the relative susceptibility of the bluffs in the study area to seismically induced landsliding