a self consistent model of melting magma migration and buoyancy driven circulation beneath mid ocean ridges numerical modeling and analysis are used to investigate the processes leading to the eruption of mantle derived magma at mid ocean ridges our model includes the following effects melting due to decompression magma migration by percolation and circulation of the mantle driven by both the oceanic plates and the distribution of buoyancy beneath the ridge the distribution of buoyancy is due to both the low density of the liquid and the difference in the density of the residual solids relative to unmelted mantle material the calculation of densities is based on a simple petrological model in which garnet spinel lherzolite melts to form a basaltic liquid and a harzburgite residue we find that the system spontaneously evolves to a state in which a rapid upwelling beneath the ridge axis faster than the plate velocity is confined laterally by stably stratified residual material beneath the newly formed plates this effect is exaggerated if a modest decrease in the shear viscosity of the solid upon melting is included our results provide a simple explanation for the narrowness of the zone of crustal formation at mid ocean ridges the model also predicts a transition from steady state to episodic crustal formation as the spreading velocity is reduced perhaps giving rise to along axis variations in the character of seafloor spreading the narrow rapid upwelling gives rise to substantial porosities at depths that are a large fraction of the depth to the solidus this may allow the liquid at depth to segregate into macroscopic channels which would account for the consensus from experimental petrology that the liquids parental to morb are derived from well below the base of the crust

impact of halite dissolution subsidence on quaternary fluvial terrace development case study of the huerva river ebro basin ne spain abstract this paper analyses the control of evaporite dissolution subsidence on the evolution of the lower 30 km long reach of the huerva river up to its confluence with the ebro river in the study area the huerva river flows across different interfingered lithofacies of the subhorizontally lying ebro basin fill 1 shales and sandstones upstream of cadrete 2 evaporites dominated by gypsum between cadrete and cuarte 3 gypsum with glauberite and halite units in the subsurface downstream of cuarte the halite shows a marked downvalley increase in thickness twelve terrace levels and seven pediment levels correlative to some of the terraces have been mapped in the studied area upstream of cadrete the terrace deposits with a relatively constant thickness less than 4 m and overlying unsoluble bedrock remain undeformed between cadrete and cuarte the deposits of some terraces show local thickening 60 m thick terrace alluvium that fills a 5 km long trough generated by synsedimentary subsidence phenomena caused primarily by the interstratal dissolution of halite the older terraces t1 to t4 show slight thickening 18 m and locally truncate paleocollapse structures ascribed to the interstratal karstification of halite beds the intermediate terraces t5 to t7 correspond to degradation surfaces that grade downstream of cuarte into aggradation flights underlain by the abruptly thickened deposits that fill this 5 km long dissolution trough subsidence migrated episodically downstream during the generation of these terraces and the subsidence aggradation rate was probably large enough to induce a base level drop and knickpoint migration upstream generating strath terraces with convergent longitudinal profiles the sedimentological changes that show the thickened terrace deposits in the subsidence area high proportion of overbank fines 60 including palustrine facies gravel channels with lower width depth ratio multiple fining upward cycles decrease in the calibre of the channel gravels indicate a change in the channel pattern from braided to meandering due to the lowering of the valley slope in the reach affected by subsidence some of the anomalous features of the lower terraces t8 to t12 attributable to dissolution subsidence include 1 steepening of the t8 terrace profile 2 change of the t10 terrace from degradational to aggradational downstream cuarte and 3 thickening recorded beneath the t11 terrace surface

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

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

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

some theoretical predictions on the relationships among spreading rate mantle temperature and crustal thickness a series of numerical experiments on mantle flow and melting predict a positive relation between mantle temperature and crustal thickness the models also demonstrate that crust formed at slow spreading rates is more sensitive to variations in mantle temperature than crust formed at fast rates so that the range of calculated thicknesses is much greater for crust formed at slower rates an instantaneous mantle temperature increase results in a transient pulse of melt production that is also more pronounced at slower spreading rates the predicted behavior is caused by the interplay between mantle flow driven by plate separation and that driven by thermal compositional and melt related buoyancy a temperature increase results in a decrease in mantle viscosity and an increase in the depth at which melting begins a lower viscosity leads to stronger buoyancy driven flow that carries more mantle to shallow depths below the ridge thermal buoyancy effects which may result in cooling and mixing of depleted and undepleted material under the ridge appear to be of greater importance at slower spreading rates the steady state results are broadly consistent with global compilations of oceanic crustal thickness that show larger variations in crustal thickness at slower spreading rates than at faster rates thicknesses estimated from seismic refraction data from crust formed within a single segment of the mid atlantic ridge but at different spreading rates 1 0 to 1 9 cm yr are consistent with but do not prove the model results the transient pulse of melt production associated with a rapid increase in mantle temperature might occur when a ridge becomes proximal to a hot spot

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

volcanic episodicity and a non steady state rift valley along northeast pacific spreading centers evidence from sea marc i sea marc i side looking sonar images and sea beam bathymetry along a 400 km stretch of the juan de fuca ridge crest provide evidence that excessive extrusive volcanism periodically builds a crestal ridge along the axis of seafloor spreading an elongate summit depression esd or rift valley is commonly observed in the spine of this crestal ridge the crestal ridge volcanic landform has a distinctive shape that is recognized in bathymetric contours both along the spreading axis and at least up to 30 km away from the axis the landform has a plan form shape that resembles a side view of an archer s bow with the long dimension of the bow form parallel to the strike of the ridge in cross section the bow form is flat on top and has steep flanks these bow form shapes can be explained by magma that rises into the crust at a discrete center and flows laterally into belts of ridge parallel dikes similar to icelandic fissure eruptions both the variable dimensions of the esd along axis of the juan de fuca ridge and the relationship among volcanic flow morphologies within and beyond the esd suggest the four different segments of the juan de fuca ridge presented in detail here display different stages in a cycle of oceanic crust accretion this cycle includes episodes in which there is 1 extrusive volcanic construction which widen the crestal ridge prior to the collapse of the summit depression 2 collapse within the summit region of the crestal ridge to form an esd during a phase of volcanic inactivity and 3 renewed magmatism in the esd as its floor widens by extension and brittle fracture of the upper crust this episodic model implies that the width of the young seafloor affected by volcanic extrusion or dominated by tectonic stretching varies through time

the present rate of uplift of fennoscandia implies a low viscosity asthenosphere the rate of displacement in fennoscandia has been intensively discussed for many years it is now widely accepted to be an isostatic response of the glacial history of the area the earth s present response to deglaciation in fennoscandia is simulated using a three dimensional 3d viscoelastic model in which the asthenosphere and mantle viscosity are allowed to vary so that the maximum rate of present uplift matches its observed value the deglaciation history is considered to be known and the c14 datings are converted to sidereal years the pattern of the present uplift gives a firm match with the observed data when a low viscosity asthenosphere is introduced assuming a 15 000 years load cycle i e the glacier was applied to the surface for 15 000 years before the melting started the best fitting earth viscosity model is a 1024 nm lithosphere overlying a 75 km thick 2 0 1019 pas asthenosphere and a 1 2 1021 pas mantle the simulations suggest a remaining maximum uplift of 40 m

late cenozoic uplift of mountain ranges and global climate change chicken or egg the high altitudes of most mountain ranges have commonly been ascribed to late cenozoic uplift without reference to when crustal thickening and other tectonic processes occurred deep incision and recent denudation of these mountain ranges abundant late cenozoic coarse sediment near them and palaeobotanical evidence for warmer climates where high mountain climates today are relatively cold have traditionally been interpreted as evidence for recent uplift an alternative cause of these phenomena is late cenozoic global climate change towards lower temperatures increased alpine glaciation a stormier climate and perturbations to humidity vegetative cover and precipitation

morphology and tectonics of the kane transform from sea beam bathymetry data high resolution sea beam data have been used to construct an accurate regional scale bathymetric map of the kane transform a large offset slowly slipping 30 mm yr central north atlantic transform the kane transform trends about 100 nearly orthogonal to the regional trend of the adjacent mid atlantic ridge mar rift valley the transform valley walls are composed of an alternating series of ridges and troughs which parallel the regional trend of the mar the ridges narrow and deepen toward the transform valley over distances of 10 20 km and the intervening troughs widen into distinct closed contour depressions that appear to be relict nodal basins the morphologic elements found on either side of the transform valley exhibit along strike continuity and represent undisrupted displacement markers the swath of terrain sandwiched between these markers is only several kilometers wide and is made up of elongate basins and ridges that integrate to create a distinctive transform parallel grain this interval the transform tectonized zone ttz marks the time averaged location of strike slip tectonism as parcels of crust move from each intersection along the transform interface the ttz is widest 6 km near the eastern ridge transform intersection rti and narrows to a strand less than 3 km wide along the central and western portions of the transform along the eastern portion of the transform valley a narrow 3 km median ridge several hundred meters high can be traced for more than 40 km along the valley floor and a similar but shorter feature is also present near the western rti at both rtis the neovolcanic zone from the adjacent rift valley can be traced as a series of constructional edifices across the entire intersection area to the opposing transform valley wall at the eastern rti the neovolcanic zone wraps around the eastern edge of the nodal basin while at the western rti it appears to bisect a paired nodal basin located in the intersection area a line drawn connecting the ends of each neovolcanic zone lies within the narrow ttz and has a strike of 098 using the bathymetric map of the kane transform as a record of the time integrated tectonic evolution of this large slowly slipping ridge transform ridge plate boundary we have constructed a simple model for the evolution of rtis our model emphasizes the interplay between temporal variations in crustal production rates i e variations in the volume of basaltic melt production along oceanic spreading centers and extensional tectonism this model can explain the existence of nodal basins at rtis and relict nodal basins along transforms and fracture zones as well as the occurrence of regularly spaced ridge and trough topography on the ridge flanks

hydrogeological controls on post fire moss recovery in peatlands summary wildfire is the largest disturbance affecting boreal peatlands however little is known about the controls on post fire peatland vegetation recovery while small scale variation in burn severity can reduce post fire moss water availability high water table wt positions following wildfire are also critical to enable the re establishment of keystone peatland mosses i e sphagnum thus post fire moss water availability is also likely a function of landscape scale controls on peatland wt dynamics specifically connectivity to groundwater flow systems i e hydrogeological setting for this reason we assessed the interacting controls of hydrogeological setting and burn severity on post fire moss water availability in three burned sphagnum dominated peatlands in alberta s boreal plains at all sites variation in burn severity resulted in a dichotomy between post fire surface covers that 1 exhibited low water availability regardless of wt position and had minimal i e lightly burned feather mosses and severely burned sphagnum fuscum or 2 exhibited high water availability depending on wt position and had substantial 50 moss re establishment i e lightly burned s fuscum and where depth of burn was 0 05 m notably hydrogeological setting influenced the spatial coverage of these post fire surface covers by influencing pre fire wts and stand characteristics e g shading because feather moss cover is controlled by tree shading lightly burned feather mosses were ubiquitous 25 in drier peatlands deeper pre fire wts that were densely treed and had little connection to large groundwater flow systems moreover hydrogeological setting also controlled post fire wt positions thereby affecting moss re establishment in post fire surface covers that were dependent on wt position e g lightly burned s fuscum accordingly higher recolonization rates were observed in a peatland located in a groundwater flow through system that had a shallow post fire wt therefore we argue that hydrogeological setting influences post fire recovery in two ways 1 by influencing vegetation structure prior to wildfire thereby controlling the coverage of post fire surface covers and 2 by influencing post fire wt positions these results suggest that post fire moss recovery in peatlands isolated from groundwater flow systems may be particularly susceptible to droughts and future climate change

relationship between axial morphology crustal thickness and mantle temperature along the juan de fuca and gorda ridges we analyze gravity data over the juan de fuca and northern gorda ridges to understand the lithospheric structure of two ridges with contrasting axial morphologies spreading at the same intermediate rate 28 mm yr half rate the cleft segment at the south end of the juan de fuca ridge has an axial high morphology while the northern segment of the gorda ridge has a rift valley residual mantle bouguer anomalies rmba on the northern gorda ridge are elevated relative to the cleft segment by 10 20 mgal indicating thinner crust and or a colder mantle the minimum value 50 mgal of the rmba along the juan de fuca ridge is over axial seamount and gradually increases south toward the blanco transform the observed rmba are interpreted to result from along axis variations in crustal thickness and mantle density both of which are controlled by temperatures in the upper mantle where decompression melting occurs we estimate that mantle temperatures are elevated by 30 40 c beneath axial seamount resulting in an excess crustal thickness of 1 7 km the cleft segment is associated with crust that is estimated to be only 300 700 m thicker and mantle temperatures are only 10 15 c higher than beneath the northern gorda ridge however even these small differences in crustal thickness and mantle temperature appear to be sufficient to produce a major change in lithospheric strength and axial morphology these results are consistent with the predicted sensitivity of recent thermo mechanical models for rift valley formation to small changes in crustal thickness and mantle temperature at intermediate spreading rates we attribute the systematic differences in axial morphology crustal thickness mantle temperature and lithospheric strength along the juan de fuca gorda ridge system to the presence of the cobb thermal anomaly at axial seamount the hotter mantle beneath the juan de fuca ridge results in greater amounts of decompression melting and the formation of a thicker crust and a thinner weaker lithosphere than along the gorda ridge

heat flow from the southeast indian ridge flanks between 80 e and 140 e data review and analysis 1 we analyze available heat flow data from the flanks of the southeast indian ridge adjacent to or within the australian antarctic discordance aad an area with patchy sediment cover and highly fractured seafloor as dissected by ridge and fracture parallel faults the data set includes 23 new data points collected along a 14 ma old isochron and 19 existing measurements from the 20 to 24 ma old crust most sites of measurements exhibit low heat flux from 2 to 50 mw m 2 with near linear temperature depth profiles except at a few sites where recent bottom water temperature change may have caused nonlinearity toward the sediment surface because the igneous basement is expected to outcrop a short distance away from any measurement site we hypothesize that horizontally channelized water circulation within the uppermost crust is the primary process for the widespread low heat flow values the process may be further influenced by vertical fluid flow along numerous fault zones that crisscross the aad seafloor systematic measurements along and across the fault zones of interest as well as seismic profiling for sediment distribution are required to confirm this possible suspected effect

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

accelerated uplift resulting from anglian glacioisostatic rebound in the middle thames valley uk evidence from the river terrace record abstract a relatively simple analysis of the terraces of the middle thames valley reveals a complex sequence of responses to late anglian late middle pleistocene drainage disruption a decrease in the amounts of vertical incision separating the later post anglian terraces strongly suggests a major change in the rate of incision in the middle thames area in the post anglian a late anglian and early post anglian incision rate of 1 45 m ka 1 was quickly replaced by incision at a rate of 0 089 m ka 1 in the later post anglian period the lower incision rate of the later post anglian compares favourably with previous estimates of regional uplift two plausible explanations for the enhanced incision in the late anglian and early post anglian are recognized one related to ponding of the river by anglian ice and the other to glacio isostatic rebound in the latter case the mismatch between expected and actual heights of late anglian deposits would suggest glacio isostatic uplift of the order of 22 25 m

alluvial terraces on the ionian coast of northern calabria southern italy implications for tectonic and sea level controls abstract in this paper we present the results of an integrated geomorphological pedological and stratigraphical study carried out along the ionian coast of northern calabria southern italy this area is characterised by the occurrence of five orders of alluvial terraces that are striking features of the landscape where large and steep catchments debouch from the mountain front to the hilly coastal belt field investigations indicate that the deposits of all five terraces are suggestive of shallow gravel bed braided streams on the basis of the age of the pleistocene substratum and morphostratigraphic correlation with marine terraces cropping out in the nearby areas each order has been associated to specific marine oxygen isotope stages consequently we focused on the interplay of allocyclic factors influencing stream aggradation degradation soil features and other climatic proxies suggest that climate didn t play an important role with respect to tectonic and base level changes in controlling fluvial dynamics in particular we recognised that during the middle pleistocene the study area experienced a period of subaerial landscape modelling as suggested by the thick and complex alluvial sequence of the highest terrace t1 the onset of regional uplift marks a change in the geomorphic scenario with tectonic and eustatically driven changes in base level working together in causing switches in fluvial aggradational erosional phases t2 t5 terraces because of the uplift river dissection occurred during phases of sea level fall whereas aggradation phases occurred during periods of climate amelioration sea level rise just before highstands were attained as a consequence the stepped terraces in the study area reflect the interplay between tectonics uplift and sea level changes in which terraces define episodes of relative sea level fall during the late quaternary

constraints on the mantle temperature gradient along the southeast indian ridge from crustal structure and isostasy implications for the transition from an axial high to an axial valley summary axial and ridge flank depths on the southeast indian ridge seir systematically increase for 2500 km from 90 e to 120 e approaching the australian antarctic discordance the seir also experiences an abrupt change in ridge axis morphology near 103 30 e with axial highs found to the west and axial valleys to the east since the spreading rate is constant throughout this region these variations have been ascribed to an along axis gradient in mantle temperature a seismic refraction experiment provides information on the crustal thickness and seismic velocity structure of two segments with differing axial morphology segment p2 centred near 102 e with an axial high has a mean crustal thickness of 5 9 0 2 km while the mean crustal thickness is 5 3 0 3 km at segment s1 with an axial valley and centred near 109 45 e isostatic compensation of the difference in crustal thickness and density structure between the two segments only accounts for 33 m of the 198 m difference in average ridge flank depth between the two segments the remaining depth difference must result from a difference in mantle density melt production models imply a mantle temperature difference of 11 13 5 c to produce the observed difference in crustal thickness isostatic compensation of the two segments requires that the resulting density difference must extend to about 300 km in the mantle the transition in axial morphology along the seir is very abrupt occurring over a narrow zone within a single segment in which the transition is complete if a linear mantle temperature gradient is assumed the temperature difference across the transition segment is only 2 4 c the change in axial morphology is accompanied by abrupt changes in other parameters including abyssal hill height magnetic anomaly amplitude layer 2a thickness and the presence or absence of an axial magma lens the abrupt coincident change in a number of parameters with a very small change in mantle temperature strongly suggests a threshold change between two distinctly different modes of crustal accretion the trigger for the transition appears to be whether a steady state crustal magma lens can be maintained

volcanic growth faults and the origin of pacific abyssal hills the topographic features known as abyssal hills characterize 30 of the ocean floor and yet their origin has been the subject of vigorous debate for over 40 years submersible based investigations show that pacific abyssal hills are created on the flanks of the east pacific rise as horsts and grabens which lengthen with time hills are bounded on one side by ridge facing scarps produced by normal faulting and on the other by more gentle slopes produced by volcanic growth faulting

constraints on melt production rate beneath the mid ocean ridges based on passive flow models we present a model for computing the total melt production rate from the decompression partial melting region beneath a mid ocean ridge and the maximum oceanic crustal thickness created at the ridge axis assuming an ideal melt migration mechanism the calculations are based on a self consistent numerical model for the thermal structure and steady state mantle flow field at a mid ocean ridge the model includes the effect of decreasing the melt production rate within the partial melting region by melt extraction as the residual mantle matrix becomes increasingly difficult to melt thus the melt fraction depends not only on temperature and pressure determined by the location beneath the ridge axis the eulerian description but also on the accumulated melt extraction since the upwelling mantle matrix enters the partial melting region determined by the location along the flow line path the langrangian description this effect has been neglected by previous models the model can predict the size of the melting region and the locations of the boundaries between mantle residual mantle and the partial melting region for a given spreading rate also the distribution of the melt depletion and the mean melting depth given the observed average thickness of oceanic crust 6 km which is relatively independent of spreading rate the model results also provide a constraint on the overall efficiency of melt migration to the ridge axis the efficiency must decrease from 100 at 10 mm yr to about 60 at fast spreading rates 50 mm yr although this reduction may be partially due to the increasing size of the melting region with increasing spreading rate it still requires less efficient melt migration near the ridge axis at fast spreading rate we found that the calculated crustal thickness is very sensitive to the mantle temperature for a normal mantle temperature of 1350 c the model can generate the observed 6 km oceanic crust over the global range of spreading rates while the anomalous thicker crusts of the iceland hotspot and the reykjanes ridge are related to higher mantle temperatures associated with the hotspot finally by comparing our model results with previous ones we found that neglecting variations of the melting relations of the residual mantle matrix with melt removal will overestimate the crustal thickness by at least a factor of 1 7

valley incision and the uplift of mountain peaks simple end member models describe the effect of isostatically compensated valley incision on the elevation of mountain peaks these models allow calculation of the maximum portion of mountain peak elevations attributable to the development of observed relief and predict general downstream patterns of ridgetop uplift in response to erosion by fluvial processes analyses of topographic profiles across major drainages of the central sierra nevada and the tibetan plateau indicate that isostatically compensated valley incision could account for at most 5 10 of the present elevation of mountain peaks in contrast analyses of topographic profiles across the himalaya show that as much as 20 30 of the present elevation of himalayan peaks could be explained by isostatically compensated valley incision the degree to which valley incision is compensated locally however depends on the flexural rigidity of the lithosphere the size of the range and its tectonic boundary conditions these results show that while a tectonic control is required for late cenozoic surface uplift of the sierra nevada a significant portion of the elevation of himalayan peaks may reflect late cenozoic valley incision on the margin of the tibetan plateau

thermal anomalies beneath the australian antarctic discordance abstract significant variations in shear wave velocity along the southeast indian ridge are delineated by ss s differential travel times the australian antarctic discordance shows ss s residuals that are 3 8 s faster than the adjoining segments 1500 2000 km away the large variability suggests that thermal structure in the upper mantle as opposed to crustal structure is the major cause of the velocity anomaly to quantify this inference we invert both geoid and topography into temperature perturbations beneath the ridge within specific depth extents assuming that the thermal sources and the geophysical observations are dynamically related in a viscous earth the joint inversion of these two sets of data is robust demanding a negative temperature anomaly of 80 and 250 c beneath the discordance for layers extending from the surface to depths of 300 and 100 km respectively in a forward sense the temperature so constrained predicts well the trend of the along axis variations in ss s residuals but with a smaller amplitude especially for the 8 s contrast between the discordance and the eastern end of the ridge where unusually high residuals sample a broad shallow region of ridge segments considering the proximity of the ss bounce points to the ridge axis and the possibility of sampling a partial melt zone the discrepancy in magnitude may be partially reconciled by deducting a presumed 2 3 s melt effect from the high residuals in the eastern end the resolved thermal perturbations drive asthenospheric flow along the ridge the flow converging and descending in the vicinity of the discordance this simple thermal viscous flow fmodel is consistent with the idea of pipe flow for the southeast indian ridge the source of the pipe flow may be the mantle plumes at the two ends of the ridge

the axial summit graben and cross sectional shape of the east pacific rise as indicators of axial magma chambers and recent volcanic eruptions abstract the axis of the east pacific rise epr undulates up and down hundreds of meters over distances of 30 200 km along strike the deep areas occurring at transform faults and other ridge axis discontinuities such as overlapping spreading centers oscs we have suggested that systematic variations in depth and cross sectional shape of the rise are indicators of the changes in the local axial magmatic budget along a given ridge segment 1 a comparison of recently collected multichannel seismic mcs data 2 with our sea beam and seamarc ii data have allowed us to test and advance this hypothesis along the epr from 9 to 13 n there is an excellent correlation between three parameters that are all directly related to the phase of a magmatic cycle along a given ridge segment the cross sectional shape of the rise the presence or absence of an axial summit graben and the presence or absence of a shallow axial magma chamber as interpreted from mcs data where the axial magma chamber is present the cross sectional shape of the ridge is broad and an axial summit graben is recognized along the axis in contrast where the cross sectional shape of the rise is narrow and triangular an axial magma chamber is not detected and an axial summit graben is absent these ridge axis characteristics tend to occur along deeper portions of a given ridge segment often near ridge axis discontinuities we suggest that these systematic variations in ridge axis morphology cross sectional shape and structure presence or absence of an axial graben reflect spatial and temporal variations in the magmatic budget of the ridge axis where the magmatic budget is waxing shallow level magma reservoirs in the crust and underlying upper mantle swell creating a broad axial bulge with a summit graben where the magmatic budget is diminished the crustal magma chamber is small this proposed correlation of shape structure and magmatic parameters fails along only two short portions of the ridge in these areas there is evidence for an axial magma chamber and the rise has a broad cross sectional shape but there is no summit graben bottom photographs and submersible results however show that in these areas the rise crest is covered with very fresh lavas undisrupted by faulting suggesting that the summit graben has been recently filled in by lava flows and the development of a summit graben or a linear caldera by volcano tectonic collapse has not yet occurred

buoyancy effects on mantle flow under mid ocean ridges results of a series of two dimensional numerical experiments of mantle flow melting and melt migration under a spreading center are reported the model predicts the distribution of melt in the subridge mantle the width over which most melt is delivered to the crust and the thickness of crust the sources of buoyancy considered are thermal expansion compositional variation caused by melt extraction and the phase change of solid to melt we infer that the steady state average viscosity of the mantle below a ridge cannot be much less than about 1019 pa s for a lower average viscosity thermal convection causes rapid cooling of a large region under a ridge raising the viscosity results imply that transient increases in mantle temperature should lead to larger increases in the oceanic crustal thickness for slow spreading ridges than for fast spreading ridges we assume that the viscosity is proportional to exp 0 where is melt fraction we parameterize the permeability in terms of the reference velocity for percolation of the melt vr where the relative velocity of melt to solid is vr analytic approximations are used to extrapolate the model results to large values of permeability and small values of 0 if vr is less than 1 m yr then more melt would be retained in the subridge mantle than is estimated from analysis of topography and gravity data at fast spreading centers for vr greater than about 100 m yr so little melt would be retained in the mantle that it is difficult to explain the gravity data and the low shear wave velocity structure close to the east pacific rise estimated from seismic surface waves buoyancy effects can lead to a region of mantle upwelling and melting that is as narrow as the observed zone of oceanic crustal accretion for most melt to be added to the crust within a few kilometers of a fast spreading center requires that 0 be less than 0 015 if vr equals 1 m yr and less than 0 003 if vr is 100 m yr

the rare holsteinian mazovian interglacial limnic deposits in the ksiąznica outcrop at krzczonow near świdnica sudetic foreland abstract the article presents the results of research on peats muds and sands filling a palaeocavity within alluvial fans of the bystrzyca river valley in the sudetic foreland of sw poland in the 534 cm deep research profile in the central part of a palaeolake 267 samples were collected at an interval of 2 cm and lithologically structural and textural studies palaeobotanically studies of diatoms pollen plant macrofossils and fossil wood palaeozoologically cladocera mollusca and ostracoda geochemically chemical elements and isotopes and palaeomagnetically analyzed the profile consists of three parts organic with mineral layers lower mineral middle and organic upper the palynological diagram was divided into three zones that represent local sets of pollen zones this diagram although incomplete is clearly indicative of the younger part of the holsteinian mazovian pollen iii and iv periods interglacial the upper part of the diagram consists of four local pollen zones that are correlated with stadial and interstadial periods analysis of several samples of fossil wood shows the widespread presence of pinus remains and numerous fragments of rhytidome bark of betula development of the peat layer 1 36 0 68 m depth started with paludification of the forest with the betula forest analysis of cladocera shows the presence of chitin armor components species chydorus sphaericus alona rectangula and ephippia of alona rectangula in three samples lithological cladocera and palaeo magnetic research supported by geochemical studies indicate the existence of at least three main phases the lake originated during the younger part of the holsteinian mazovian intergalcial as an oxbow within the valley where the deposition of variously thick up to 1 5 m depth peats organic and minerogenic silts the first stage in the second phase a closed fairly deep late holsteinian lake continued to exist under relatively warm conditions with mainly mineral deposition from the slope the lake became shallower and wider and then more open and better oxygenated with through flow in the final phase the lake was completely filled by organic sediments under interglacial conditions the absence of diatom molluscan and ostracod communities indicates an acid lacustrine environment at this time the fluvial and limnic deposits are sealed by overlaying saalian odranian till

a tectonic model for ridge transform ridge plate boundaries implications for the structure of oceanic lithosphere abstract the first order geologic and morphologic relationships at along and proximal to ridge transform ridge plate boundaries are used to construct an empirical and speculative tectonic model the distinctive but variable morphotectonic fabric and crustal structure of the transform domain are the product of a tectonic continuum that ranges from very slow rates of strain associated with very thick edges of lithosphere slowly slipping ridge transfrom ridge plate boundaries to extremely high rates of strain associated with very thin edges of lithosphere fast slipping ridge transform ridge plate boundaries the geometry of a ridge transform intersection necessitates the juxtaposition of a relatively cold thick edge of lithosphere against the truncated end of an accreting plate boundary the cold face of lithosphere cools the adjacent wedge of asthenosphere rising beneath the axis of accretion and restricts the amount of partial melting thus attenuating the volume of basaltic melt segregated from the asthenosphere per unit time the shallow level manifestation of this cold edge effect is a thinner oceanic crust at depth the evolving lithosphere thickens rapidly as the cold edge is approached creating changes in the properties of the young lithosphere and forming a weld of upper mantle material against the cold edge of truncating lithosphere the ridge transform weld of upper mantle material creates a shear couple in the lithosphere underlying the intersection resulting in the progressive reorientation of the maximum tensile stress from normal to the ridge axis at some distance from the ridge transform intersection to an oblique angle near the boundary the brittle carapace of oceanic crust that overlies the mantle weld will deform accordingly with the development of oblique dip slip faults the model predicts that the geologic expressions of this cold boundary effect will become more dramatic with increasing thickness of the truncating edge at very slow rates of accretion 100 km juxtapose thick 30 50 km edges of lithosphere against the accreting plate boundary all but nullifying the processes that lead to the emplacement of normal oceanic lithosphere in this environment a relatively strong mantle weld will form at the ridge transform intersection and the thick edges of opposing lithosphere along the length of the transform will tend to confine the strike slip tectonism to a relatively narrow and temporally stable principal transform displacement zone in contrast at very fast rates of accretion large offset transforms 100 km place relatively thin edges of lithosphere

an abrupt change in ridge axis gravity with spreading rate the global mid ocean ridge system shows a marked change in morphology and isostatic compensation as a function of spreading rate fast spreading ridges have axial highs with little bathymetric relief and low amplitude gravity signatures indicating that they are nearly in local isostatic equilibrium slow spreading ridges have large axial valleys bounded by rugged topography macdonald 1982 and large axial gravity troughs indicating that they are dynamically maintained while this variation in ridge axis morphology with spreading rate has been observed it has not been analyzed in a comprehensive manner moreover it is not known whether the transition from axial valley to axial high is a continuous function of spreading rate or whether it occurs abruptly at a particular rate such observations would provide important constraints on models of ridge axis dynamics vertical deflection profiles collected by the geosat radar altimeter have sufficient accuracy and resolution to reveal the change in ridge axis gravity with spreading rate in this study we have analyzed 44 geosat profiles over ridges with spreading rates ranging from 14 to 155 mm yr in agreement with previous studies we find that slow spreading ridges 70 mm yr are characterized by low amplitude ridge axis highs 15 rad unexpectedly we find that the transition from axial trough to axial high occurs abruptly at a spreading rate of 60 70 mm yr ridge axis gravity signatures are highly variable for rates less than 65 mm yr and very uniform at higher rates the transition of the gravity signature appears to be more abrupt than the transition of the topographic signature suggesting an abrupt change in the style of isostatic compensation with spreading rate published models of ridge axis dynamics do not explain this sharp transition

bathymetric roughness of the southeast indian ridge implications for crustal accretion at intermediate spreading rate mid ocean ridges the nature of the transition from axial highs to axial valleys at mid ocean ridges and the physical processes involved in the transition are important for understanding how axial morphology changes with spreading rate mantle temperature and lithospheric strength at mid ocean ridges in order to provide observational constraints on the nature of the changes in axial morphology we examined the regional and segment scale variations in axial and flank morphology at the intermediate spreading southeast indian ridge seir using newly collected geophysical data an empirical orthogonal function analysis was used to separate regional and local components of the topography field and to estimate bathymetric roughness three distinct types of axial morphology were identified from the regional component of ridge topography in our area axial highs shallow axial valleys and mid atlantic ridge type deep axial valleys axial depth increases by 2100 m from 88 e and 118 e while off axis depth only increases by 500 m in addition except for one segment with a deep axial valley there is little change in off axis depth within segments in contrast to the large intrasegment variations in axial depth these observations indicate that the overall and intrasegment variations in crustal thickness are much smaller than would be predicted from the variations in axial depth and that the major portion of the variations in ridge axis depth are dynamically supported there are step like increases in bathymetric roughness as axial morphology changes from an axial high to a shallow axial valley and from a shallow axial valley to a deep axial valley the step changes in roughness imply that the change from one mode of axial morphology to another is accompanied by an abrupt change in the strength of the lithosphere the abrupt changes in lithospheric strength may be due to the existence of a threshold mantle temperature or crustal thickness about which the lithospheric strength is very sensitive to small fluctuations systematic intrasegment variations in roughness are also observed roughness shows v shaped patterns within segments with axial highs but no clear pattern within segments with axial valleys the different patterns in roughness at axial highs and axial valleys on the seir may result from the presence or absence of a magma chamber the presence of a magma chamber at a ridge segment with an axial high implies weaker axial lithosphere and hence lower roughness near the center of segments relative to the segment ends

drainage evolution in the polish sudeten foreland in the context of european fluvial archives detailed study of subsurface deposits in the polish sudeten foreland particularly with reference to provenance data has revealed that an extensive preglacial drainage system developed there in the pliocene early pleistocene with both similarities and differences in comparison with the present day odra oder system this foreland is at the northern edge of an intensely deformed upland metamorphosed during the variscan orogeny with faulted horsts and grabens reactivated in the late cenozoic the main arm of preglacial drainage of this area at least until the early middle pleistocene was the palaeo nysa klodzka precursor of the odra left bank tributary of that name significant preglacial evolution of this drainage system can be demonstrated including incision into the landscape prior to its disruption by glaciation in the elsterian sanian and again in the early saalian odranian which resulted in burial of the preglacial fluvial archives by glacial and fluvioglacial deposits no later ice sheets reached the area in which the modern drainage pattern became established the rivers incising afresh into the landscape and forming post saalian terrace systems issues of compatibility of this record with the progressive uplift implicit in the formation of conventional terrace systems are examined with particular reference to crustal properties which are shown to have had an important influence on landscape and drainage evolution in the region

mechanisms for the origin of mid ocean ridge axial topography implications for the thermal and mechanical structure of accreting plate boundaries to better understand the implications of ridge axis morphology for the thermal and mechanical structure of accreting plate boundaries we examine mechanisms for the creation of stress supported axial topography and its dependence on parameters such as spreading rate asthenosphere viscosity and plate thickness near the ridge axis three basic mechanisms are considered 1 asthenospheric upwelling in a narrow steady state conduit beneath a ridge axis 2 mantle flow stresses due to plate spreading extending ridge axis lithosphere or melt segregation and associated mantle compaction and 3 topography supported by moments due to horizontal extensional stresses in a thickening lithosphere if mid ocean ridge topography is due to asthenosphere rising in a narrow conduit at the ridge axis then the conduit structure cannot be thermally controlled as the associated thermal structure has relatively broad flat isotherms instead of a conduitlike shape mantle flow associated with plate spreading alone induces no stress supported topography thus the intuitive notion that diverging flow at the ridge axis due solely to plate spreading produces the axial valley is incorrect vertical mantle flow at the base of a stretching accreting lithosphere and mantle flow associated with melt segregation can however lead to stress supported axial topography however for these mantle flow related processes to produce significant amounts of axial relief the mantle viscosity beneath the ridge axis must be on the order of 1020 pa s 1021 p poise significantly higher than generally accepted values of 1018 pa s 1019 p horizontal extensional stresses in a strong brittle lithosphere that thickens with distance from the ridge axis can produce axial topography of the form and amplitude observed on slow spreading ridges a continuum idealization of deformation due to faulting is used to examine this mechanism this model predicts the axial valley width is controlled by the plate thickness near the ridge axis an 8 km thick plate near the ridge axis would produce the 30 km wide axial valley observed at slow spreading ridges an increased lithosphere thickness of only several kilometers over the half width of the axial valley can produce kilometer scale relief to examine whether a plate 8 10 km thick at the ridge axis that thickens by several kilometers within 15 km of the axis is plausible at a slow spreading ridge thermal models that include both the heat of magmatic crustal accretion on the ridge axis and upwelling mantle flow beneath the axis are formulated hydrothermal cooling that increases the effective conductivity by about a factor of 10 is required for an 8 km thick plate to exist at a slow spreading ridge axis however even with this enhanced hydrothermal cooling the plate thickness at a fast spreading ridge axis and the resulting horizontal force in the plate are too small to result in appreciable topography while mid ocean ridge axial topography is strongly influenced by spreading rate other factors also shape axial structure for example the anomalously shallow slow spreading reykjanes ridge has an axial structure more like that of a fast spreading ridge and the anomalously deep intermediate spreading australian antarctic discordance has a median valley axial structure typical of a slower spreading ridge in addition within a given spreading segment there is often a systematic deepening of the ridge axis toward transform offsets these non spreading rate dependent axial topographic variations may be produced by variations in magmatic heat input associated with crustal emplacement which shapes the thermal structure and lithosphere thickness near the ridge axis

the origin of tensile fracture lineaments abstract fracture controlled lineaments commonly seen where brittle basement is exposed at the earth s surface are generally restricted to a small number of sets with angles of 45 90 between sets the length frequency distribution of lineaments in each set follows a truncated poisson function such lineaments usually show almost no shearing offset suggesting a tensile origin a simple mechanical model of tensile fracturing is used to explain the spacing directions and length of lineaments as well as their depth frequency distribution results suggest that the penetration depth of tensile fractures which produce lineaments at the earth s surface is directly related to their length and that the fracture density is inversely proportional to fracture depth finally the angles between lineament sets may be controlled by the ratio of strength of unfractured rock to that of pre existing fractures which might heal with time the most likely source of tension is tectonic uplift fractures due to typical uplifts of 0 5 1 km over distances of 10 100 km may penetrate as brittle fractures to several kilometres into the crust perhaps to the depth at which seismic activity ceases

dependence of ridge axis morphology on magma supply and spreading rate why do some mid ocean ridge axes have as their topographic expression a median valley 1 2 km deep and 15 30 km wide whereas others have an axial high 100 200 m high and 1 2 km wide in general median valleys exist at the axes of ridges spreading at half rates less than 20 25 mm yr 1 whereas axial highs are found at faster spreading ridges1 2 a recent crustal genesis model3 incorporating hydrothermal cooling and crustal accretion by means of a magma lens agrees well with observations of the spreading rate dependence of axial morphology1 2 the depth of the axial magma chamber3 4 and the thickness of the ridge axis lithosphere5 6 but there are several notable exceptions to the correlation between spreading rate and axial morphology such as the reykjanes ridge which spreads at l0mm yr 1 but has an axial high here we show that by extending the model of ref 3 to include variations in crustal thickness we can explain the observed dependence of axial morphology on crustal thickness and spreading rate our results suggest that the ultimate control on axial morphology is the thermal structure at the ridge axis which is a function of both spreading rate and magma supply

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

crustal uplift in southern england evidence from the river terrace records abstract much of the past work on the quaternary rivers of northwest europe has been concerned with river terraces which characterise almost every valley while these terraces are undoubtedly striking features of the landscape the incision achieved by quaternary rivers is equally significant in terms of river behaviour and for an understanding of the factors affecting landform development during the quaternary this paper examines the incision achieved during the quaternary by the thames in both its upper and lower catchments and by the hampshire avon in southern england valley incision rates of ca 0 07 0 10 m ka 1 have been identified although in the lower catchment of the thames these have been enhanced by additional incision in response to glacio isostasy and valley shortening a model is proposed in which regional uplift is recognized as the primary cause of incision by these quaternary rivers possible mechanisms for regional uplift are considered

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

neotectonically controlled fluvial features walbrzych upland middle sudeten mts southwestern poland abstract the landscape of the walbrzych upland sudeten mts southwestern poland has developed over a long time involving three main stages i the oligocene early pliocene ii the late pliocene early pleistocene and iii the middle late pleistocene stage iii is neotectonically controlled and is discussed in detail the entire walbrzych upland was covered by a continuous ice cover during the early saalian and at least in its marginal part during the elsterian the pre early saalian fluvial landscape shows no evidence of tectonic activity in contrast with that of post early saalian time the post early saalian valleys have been formed during the intensive and short term uplift of the mountain upland with simultaneous re activation of fault zones the main geomorphological effects of this uplift are abandonment of valleys formation of deep river valleys cut within bedrock varying shapes of newly incised valleys highly irregular longitudinal profiles of channels varying numbers of terraces and their heights along the valleys tilting of terraces and formation of fault scarps at some fault lines the main geological effects are variable thicknesses of alluvial deposits caused by rotation of bedrock blocks synsedimentary increases of thickness of alluvial deposits at some fault lines and breaking of the continuity of some alluvial surfaces at the major fault lines the uplift of the walbrzych upland was controlled by glacio isostatic rebound after the early saalian glaciation supplemented by localised extensional tectonics total tectonic uplift was about 40 50 m

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

mechanics of graben formation in crustal rocks a finite element analysis small 1 10 km wide grabens are ubiquitous tectonic structures that have been observed on the surfaces of all the terrestrial planets and satellites in spite of their frequent occurrence there is no fundamental understanding of how they form other than that they are characteristic of extensional terranes this paper examines the mechanics of the initial stages of graben formation and demonstrates that the configuration of a pair of antithetically dipping normal faults a graben is the energetically most favorable fault configuration in elastic brittle rocks subjected to pure extension a corollary to this hypothesis is that the occurrence of synthetic normal faults is indicative of a stress field other than purely extensional or of nonhomogeneous characteristics of the rock the analysis proceeds by computing the stress field in the vicinity of a single initial normal fault using a two dimensional finite element model it is shown that slip on this fault substantially lowers the proximity to failure of the surrounding rocks except for two small regions one near the surface of the downthrown block and the other near the fault tip the region on the downthrown block is located a horizontal distance from the initial fault slightly less than the fault s depth the proximity to failure actually increases here and a secondary fault develops an antithetic secondary fault would link this region with the other failure zone near the fault tip to demonstrate the preference for antithetic secondary faults more rigorously we show that this fault configuration results in a smaller total strain energy than the synthetic configuration in addition a two layer model is used to show that the graben s width is only weakly dependent upon the presence of mechanically distinct layers in the crust both weak over strong and strong over weak configurations are investigated the depth of the initial fault and the thickness of the top layer are varied and the width of the resulting graben is evaluated by computing the distance between the initial fault and the region of highest proximity to failure near the surface this analysis indicates that the depth of the initial fault is the major factor controlling graben width

the spreading rate dependence of three dimensional mid ocean ridge gravity structure we analyze over 1300 km of high resolution along axis gravity profiles at ridges with half spreading rates ranging from 1 2 to 5 5 cm yr the results show consistently higher along axis gradients of mantle bouguer anomaly at the slow spreading mid atlantic ridge mar 0 3 1 2 mgal km than at the intermediate to fast spreading cocos nazca ridge and east pacific rise epr 0 1 0 2 mgal km the regional peak to trough amplitude of mantle bouguer anomaly is also greater along the mar 30 60 mgal than the cocos nazca ridge and the epr 10 20 mgal with increasing spreading rate the regional peak to trough amplitude of axial seafloor depth decreases from 1000 1700 m to 200 700 m 3 d numerical experiments suggest that mantle contributions to the gravity can be significant only near large offset transforms at the more commonly observed non transform offsets gravity anomalies will reflect crustal thickness variations the along axis gravity data thus indicate that the amplitude of along axis crustal thickness variation decreases with increasing spreading rate we propose that this spreading rate dependent crustal accretion style may originate in the mantle finite amplitude mantle upwelling is intrinsically plume like 3 d beneath a slow spreading ridge but more sheet like 2 d beneath a fast spreading ridge such a transition in mantle upwelling may occur if the relative importance of passive upwelling over buoyant upwelling increases with increasing spreading rate small amplitude 3 d upwellings may occur at a fast spreading ridge but their effects on crustal thickness variations will be significantly reduced by along axis melt flows along a persistent low viscosity crustal magma chamber in contrast the large crustal thickness variations due to 3 d mantle upwellings will be maintained at a slow spreading ridge because less along axis melt flows can occur in the colder and more rigid crust there

microearthquakes beneath the median valley of the mid atlantic ridge near 23 n hypocenters and focal mechanisms we report hypocenters and focal mechanisms of microearthquakes located by an ocean bottom seismic network deployed in the median valley of the mid atlantic ridge near 23 n during a 3 week period in early 1982 the network consisted of seven ocean bottom hydrophones and three three component ocean bottom seismometers the instrument coordinates were acoustically determined to within 25 m at the 1 standard deviation level of confidence the hypocentral parameters of the 26 largest microearthquakes are reported 18 of these events have epicenters arid focal depths which are resolvable to within 1 km formal error at the 95 confidence level microearthquakes occur beneath the inner floor of the median valley and have focal depths generally between 5 and 8 km beneath the seafloor composite fault plane solutions for two spatially related groups of microearthquakes beneath the inner floor indicate normal faulting along fault planes that dip at angles of 30 or more these solutions are similar to the mechanisms of nearby large earthquakes microearthquakes also occur beneath the steep eastern inner rift mountains the rift mountain earthquakes have nominal focal depths of 5 7 km and epicenters as distant as 10 15 km from the center of the median valley but these hypocenters have larger uncertainties because of the possible effects of large topographic relief and associated lateral heterogeneity in velocity structure we interpret the depth distribution and source mechanisms of these microearthquakes to indicate that this segment of ridge axis is undergoing brittle failure under extension to a depth of at least 7 8 km we infer that the entire crustal column has been cooled to temperatures within the brittle field of behavior and that significant time has elapsed since the most recent episode of volcanic or shallow magmatic activity along this ridge segment

the structure and segmentation of the southeast indian ridge abstract the southeast indian ridge seir spreads at a relatively narrow range of intermediate rates 59 75 km ma but exhibits the full range of slow to fast spreading morphology and segmentation satellite gravity data reveal transitions in the structure of the spreading center where it is influenced by the amsterdam and kerguelen hotspots and at the australian antarctic discordance aad although the spreading rate between the hotspots and the aad is nearly constant the ridge exhibits a variety of distinct styles of morphology and segmentation not observed at fast or slow spreading centers recently collected multibeam bathymetry data reveal a transition from east pacific rise style overlapping axial highs near 92 e to mid atlantic ridge style axial valleys with non transform offsets near 116 e the intervening segmentation is characterized by propagating offsets coexisting with stationary transforms which exhibit different degrees of temporal stability currently there are 10 transform offsets between the hotspots and the aad but only five of these have persisted since seafloor spreading stabilized at 35 ma the other five appear to have formed since 35 ma and several more have disappeared by transform shortening or coalesced by along axis propagation there is a transition from monotonic offset propagation near the hotspots to oscillatory propagation approaching the aad this change in offset stability corresponds to transitions in depth axial morphology and offset structure through much of the transitional region higher order segmentation is characterized by en echelon offsets of a diffuse spreading axis that generally lacks a well defined neovolcanic zone since the spreading rate is nearly constant the regional variation in axial morphology and segmentation appears to be controlled by an upper mantle thermal gradient possibly a result of flux of asthenosphere from the hotspots to the aad this is consistent with the gradual increase in average ridge flank depths along this part of the plate boundary but segment scale changes in axial depth reveal spatio temporal variability in the dynamic topography that are not preserved on older lithosphere intrasegment transitions in axial morphology and en echelon offsets within first order segments suggest that local variations in mantle thermal structure introduce short lived instabilities in higher order segmentation and dominate the short term evolution of the plate boundary

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

gravity anomalies and crustal thickness variations along the mid atlantic ridge between 33 n and 40 n multibeam bathymetry and gravity data have been obtained along an 800 km long section of the mid atlantic ridge from just south of the hayes fracture zone at 33 n to the northern edge of the azores platform near 40 n a three dimensional analysis of these gravity and topography data combined with results from earlier seismic refraction studies in this area reveal two different scales of crustal heterogeneity a systematic along axis segment scale 8 9 km near the middle of spreading segments and the thinnest crust 3 4 km near segment offsets the magnitude of this along axis variation in crustal thickness is proportional to the length of the spreading segment and the size of the adjacent ridge offset there also is a distinct asymmetry in crustal structure across the rift valley near large segment offsets with gravity highs inferred to be thinner crust beneath the inside corner highs adjacent to these offsets this segment scale crustal heterogeneity is similar to that reported from the kane to atlantis section of the mid atlantic ridge and from parts of the intermediate spreading southern mid atlantic ridge it is superimposed on a second longer wavelength variation in gravity and crustal thickness associated with the azores hot spot the most pronounced effect of the azores hot spot on the mid atlantic ridge occurs between 38 n and 40 n where the ridge axis rapidly shoals by more than 1000 m the crust thickens by over 2 km and the rift valley largely disappears the absence of a deep axial rift valley on the azores platform and near the midpoints of some ridge segments along the mid atlantic ridge to the south agrees well with the predictions of recent ridge crest thermal and rheological models that suggest a dependence of axial morphology on both crustal thickness and spreading rate the transition from a rift valley to an axial high morphology at these spreading rates 11 mm yr half rate occurs at a crustal thickness of about 9 1km

stochastic analysis of seafloor morphology on the flank of the southeast indian ridge the influence of ridge morphology on the formation of abyssal hills in this study we estimate the statistical properties of abyssal hill morphology adjacent to the southeast indian ridge in a region where the axial morphology changes from axial high to axial valley without a corresponding change in spreading rate we explore the influence of axial morphology on abyssal hills and place these results within the context of response to spreading rate two cruises aboard the r v melville collected sea beam 2000 multibeam data along the southeast indian ridge providing continuous multibeam coverage of the axis from 89 w to 118 w and 100 coverage within four survey regions extending out to 45 km 1 2 ma from the axis sempere et al 1997 cochran et al 1997 we apply the statistical modeling method of goff and jordan 1988 to gridded data from the four survey areas examining in particular estimates of abyssal hill rms height characteristic width and length aspect ratio and skewness two analyses are performed 1 comparison of the along axis variation in abyssal hill characteristics to ridge segmentation and 2 a calculation of population statistics within axial high intermediate and axial valley data populations of this study and comparison of these results to population statistics derived from studies adjacent to the mid atlantic ridge and east pacific rise we find that abyssal hills generated along axial high mid ocean ridges are very different from those generated along axial valley mid ocean ridges not only with respect to size and shape but also in their response to such factors as spreading rate and segmentation

geophysical study of the east pacific rise 15 n 17 n an unusually robust segment bathymetric side scan sonar magnetic and gravity data from the east pacific rise epr between 15 and 17 n are used to establish the spreading history and examine melt delivery to an unusually robust spreading segment the axial ridge between the orozco transform fault 15 30 n and the 16 20 n overlapping spreading center osc has an average elevation of 2300 m which is 300 m shallower than typical epr depths and its cross sectional area is double the average value for the northern epr the total opening rate is 86 km myr but the inflated segment appears to have spread faster to the east by more than 20 since 0 78 ma the orientation of magnetic isochrons and lineaments in the side scan sonar indicates a 3 counterclockwise rotation of the spreading direction since 1 8 ma c2 and reflects a change in the pacific cocos plate motion the side scan lineaments also show that the percentage of inward facing faults 83 and the spacing between faults 1 5 km are consistent with the spreading rate dependence shown by carbotte and macdonald 1994 however the mean fault length 4 8 km is 1 5 km shorter than expected for the spreading rate and suggests that extensive off axis volcanism has draped the faults gravity analysis shows that the inflated segment has a 12 mgal bull s eye shaped low in residual mantle bouguer anomaly we offer several possible end member models for the anomaly including a prism of 10 partial melt in the mantle and lower crust or a crustal thickness anomaly of 2 25 km kinematic modeling that is based on structure and magnetic data suggests that two large magmatic pulses occurred at approximately 0 8 ma and 0 3 ma and have reshaped the plate boundary geometry and inflated the segment

crustal thickness variations along the southeast indian ridge 100 116 e from 2 d body wave tomography 1 axial morphology along the southeast indian ridge seir systematically changes from an axial high to a deep rift valley at a nearly uniform intermediate spreading rate between 100 116 e west of the australian antarctic discordance aad basalt geochemistry has a consistent indian mid ocean ridge basalt morb type isotopic signature so changes in axial topography are attributed to variations in both mantle temperature and melt supply wide angle seismic refraction lines were shot to four ocean bottom hydrophones within seir segments p1 p2 s1 and t where each segment is characterized by a different morphology we constructed 2 d crustal velocity models by jointly inverting hand picked p wave refraction pg and moho reflection pmp traveltime data using a top down minimum structure methodology the results show a 1 5 km eastward decrease in crustal thickness across the study area with segment averages ranging from 6 1 km at p1 to 4 6 km at t melt generation models require a 30 c decrease in mantle temperature toward the aad to account for the crustal thickness trend significant changes in axial morphology accompany small scale variations in crustal thickness consistent with models of crustal accretion where ridge topography is determined by a balance between mantle temperature melt supply and cooling from hydrothermal circulation layer 3 thins by 3 0 km as layer 2 thickens by 1 4 km between segments p1 and t reflecting the eastward decrease in melt supply and increase in melt lens depth the trade off in seismic layers may be explained by models relating the increase in overburden pressure on a deepening melt lens to the volume of magma erupted into the upper crust rather than cooling at depth to form new lower crustal material

climate and tectonic controls on pleistocene sequence development and river evolution in the southern vienna basin austria abstract the impact of climate and tectonism on sedimentation and lithofacies development is demonstrated for austria s largest pleistocene sedimentary basin situated within the vienna basin the coarse grained massive facies is associated with high energetic flood events reflecting a distinct seasonality effect of discharge during glacial times in contrast fine grained well stratified sediments mark periods of rather high discharge to sediment supply ratios during warmer periods preservation of depositional sequences was facilitated by subsidence rates of approx 0 5 1 mm a high subsidence rates in the basin preserve a relatively large record of sequences compared to fluvial terraces which reflect mainly sediments deposited during and shortly after glacials 14c and relative ages demonstrate the strong impact of oscillations in climate on fluvial stratigraphy covering times from the upper period of mis 3 to mis 1 abundant sediment supply and the associated increase in accumulation space as well as tectonic factors affect not only the sequence development but also the geomorphology of two mountain front alluvial fans which toe out into the mitterndorf basin high discharge to sediment supply ratios leads to fan incision and to abandonment of fan surfaces during phases of reduced sediment supply the effect of subsidence results in headcut erosion and if sufficient time is available may lead to complete through trenching abandoned fan surfaces are exposed to widespread soil formation which mark sequence boundaries to the following coarse grained massive facies reflecting high sediment supply to discharge ratios