faulting pattern along slow spreading ridge segments a consequence of along axis variation in lithospheric rheology abstract we present here results from an analogue model designed to test the relative influence of along axis variations of the crustal thickness and of the thermal structure of the lithosphere on the geometry and on the faulting pattern of the axial rift of a slow spreading ridge rheologically calculated layered models are employed using quartz sand and silicone putty as analogues of the brittle and ductile components of the lithosphere respectively two parameters have been analysed in detail 1 the thickness of the brittle layer and 2 the viscosity of the ductile layer this study shows that the thickening of the brittle layer and the increase of the viscosity of the ductile layer bring about a widening of the axial valley associated with an increase of the number of faults a decrease of the depth of the axial valley and of the vertical throw of the faults is observed with an increase of the viscosity of the ductile layer and a decrease of the thickness of the brittle layer these results are consistent with observations along mid atlantic ridge segments for segment ends the fault pattern obtained in the models is similar to that described on both sides of the axis for segments bordered by a zero offset discontinuity and on outside corners for segments bordered by a lateral offset discontinuity our results suggest that the viscosity of the ductile layer plays a more important role in the fault pattern than the thickness of the brittle layer the influence of segment length offset length and temporal variation in thermal input could explain the more or less important along axis variation in the deformation pattern observed along segments of slow spreading ridges

bending thin lithosphere causes localized snapping and not distributed crunching implications for abyssal hill formation horizontal loads i e pushes or pulls on the lithosphere are thought to produce significant localized dip slip faults but vertical loads are generally considered to produce broadly distributed bending or flexure the same processes of weakening or yielding that lead to localization of deformation on faults during lithospheric stretching or shortening can produce highly localized faulting during lithospheric bending localized bending or snapping should occur when the bending moment begins to decrease with increasing plate curvature if yielding causes a reduction in cohesion then thin brittle layers may respond to bending in the snapping mode while thicker layers respond in a more distributed crunching mode for a mohr coulomb layer floating on an invicid substrate it is the ratio of the cohesion to the average shear stress needed to overcome friction that controls the mode of bending the average horizontal stress on the layer also affects the mode of bending for estimated rock properties a brittle layer has to be less than a few up to perhaps 10 km thick to break in a localized way lithosphere at many mid ocean ridges is thought to be only a few kilometers thick and also may be a site of vertical loading the localized snapping mode of bending may produce abyssal hills at fast spreading mid ocean ridges

a finite amplitude necking model of rifting in brittle lithosphere we formulate a mechanical model describing the formation of rifts as finite amplitude necking of an elastic plastic layer overlying a fluid substrate a perfectly plastic rheology is a continuum description of faulting in rift zones two important aspects of rift evolution are illustrated by this model the evolution of the rift width as extension proceeds and the finite strain that occurs a region at yield initially develops with a width determined by the thickness of the brittle layer and the internal deformation within this yield zone is proportional to the topographic slope as extension proceeds the surface within the rift subsides and the width of the subsiding yield zone decreases at any stage of rifting material in regions just outside the yield zone is deformed but no longer deforming the width of these deformed regions increases with increasing extension vertical forces due to the mass deficit of the rift depression will flex the elastic layer outside the yield zone creating flanking uplifts the external force required to maintain active rifting increases with the amount of lithospheric stretching indicating that rifting is a quasi static stable process because the yield zone will revert to elastic behavior if the external force causing extension is removed the model predicts that the rift depression and flanking uplifts will be preserved after extension stops our simple mechanical model demonstrates the inherent relationship among graben formation lithospheric thinning and rift shoulder uplift in rift zones

extending the vostok ice core record of palaeoclimate to the penultimate glacial period the ice core record of local temperature dust accumulation and air composition at vostok station antarctica now extends back to the penultimate glacial period 140 200 kyr ago and the end of the preceding interglacial this yields a new glaciological timescale for the whole record which is consistent with ocean records temperatures at vostok appear to have been more uniformly cold in the penultimate glacial period than in the most recent one concentrations of co2 and ch4 correlate well with temperature throughout the record

a physical explanation of the relation between flank uplifts and the breakup unconformity at rifted continental margins rift flank uplifts and the breakup or postrift unconformity are characteristic features of many rifted passive or atlantic type continental margins but are not predicted as primary features of simple lithospheric stretching models the explanations that have been proposed for their origin remain controversial either because they call upon special circumstances or because they are difficult to test plane strain finite element models are used in this paper to explore the dynamics of lithospheric necking during rifting and rupture the results agree with the conceptual interpretation that uplift of the rift boundaries to form flank mountains and uplift of the basin responsible for the breakup unconformity are related consequences of regional isostatic compensation of mass that is redistributed during the necking and rupture phases although the amplitudes of these uplifts depend on the model parameter values the relations between and relative signs of these two effects appear to be fundamental the explanation we propose may have been missed in recent studies because there has been a tendency to concentrate on kinematic stretching models which assume local isostatic equilibrium through out the rifting process such an approach is predicated on the assumption that the lithosphere has an insignificant strength or flexural rigidity during extension which is not true if our explanation is correct

microearthquake characteristics and crustal vp vs structure at the mid atlantic ridge 35 n we report on the results of a microearthquake experiment conducted at the along axis bathymetric high of the slow spreading ridge segment near 35 n on the mid atlantic ridge mar of a total of 255 microearthquakes recorded during the 43 day experiment 31 were located near the oceanographer transform at the northern end of the segment 79 occurred at the nontransform offset at the southern end of the segment and 145 were at the segment center at the segment center earthquake epicenters lay within the median valley inner floor and formed a 12 km long trend paralleling a steep scarp bounding the western wall of the inner valley focal depths were 3 4 km below the seafloor most 80 of the focal mechanisms for 32 segment center earthquakes are consistent with normal faulting on faults approximately parallel to the axial trend from a joint inversion for hypocenters and p and s wave velocity structures we determined a horizontally averaged v p v s ratio that decreases from 2 9 in the shallowmost 300 m to 1 7 at 2 km depth and we interpret this decrease as indicating a decreasing contribution of thin cracks to fracture porosity with depth the maximum depth of seismicity 4 km is anomalously shallow compared with other mar segments at which microearthquake experiments have been carried out cross axis relief is also anomalously low for this segment s center and on the basis of this and other mar microearthquake experiments there appears to be a correlation between cross axis relief and maximum depth of seismicity from the correlation of cross axis relief and inferred crustal thickness we suggest a relationship between thick crust high crustal temperatures and low cross axis relief in qualitative agreement with thermomechanical models for the depth of the axial valley

a simple model for the fault generated morphology of slow spreading mid oceanic ridges we postulate that fluctuations in magmatic activity at mid oceanic ridges perturb the horizontal least principal stress across rift bounding normal faults leading to alternating phases of magmatic accretion which increases valley width and tectonic extension which results in the growth of inner rift wall topography fine scale bathymetrie surveys and earthquake fault plane solutions show that active normal faults at slow spreading ridges are moderately dipping approximately 45 planar features throughout the seismogenic oceanic lithosphere a simple quantitative model that includes flexural deformation of a 10 km thick elastic plate by slippage on 45 dipping normal faults can match the bathymetrie profiles across several slow spreading ridge segments comparison among dip distributions of normal faulting earthquakes at mid ocean ridges in the trench outer rise region and on continents suggests that most events from these three tectonic environments initiated at dips close to 45 raising unanswered questions about the mechanical conditions under which the faults originated

magmatic controls on axial relief and faulting at mid ocean ridges abstract previous models do not simultaneously reproduce the observed range of axial relief and fault patterns at plate spreading centers we suggest that this failure is due to the approximation that magmatic dikes open continuously rather than in discrete events during short lived events dikes open not only in the strong axial lithosphere but also some distance into the underlying weaker asthenosphere axial valley relief affects the partitioning of magma between the lithosphere and asthenosphere during diking events the deeper the valley the more magma goes into lithospheric dikes in each event and so the greater the average opening rate of those dikes the long term rate of lithospheric dike opening controls faulting rate and axial depth the feedback between axial valley depth d and lithospheric dike opening rate allows us to analytically relate steady state values of d to lithospheric thickness h l and crustal thickness h c a two dimensional model numerical model with a fixed axial lithospheric structure illustrates the analytic model implications for axial faulting the predictions of this new model are broadly consistent with global and segment scale trends of axial depth and fault patterns with h l and h c

factors controlling normal fault offset in an ideal brittle layer we study the physical processes controlling the development and evolution of normal faults by analyzing numerical experiments of extension of an ideal two dimensional elastic plastic brittle layer floating on an inviscid fluid the yield stress of the layer is the sum of the layer cohesion and its frictional stress faults are initiated by a small plastic flaw in the layer we get finite fault offset when we make fault cohesion decrease with strain even in this highly idealized system we vary six physical parameters the initial cohesion of the layer the thickness of the layer the rate of cohesion reduction with plastic strain the friction coefficient the flaw size and the fault width we obtain two main types of faulting behavior 1 multiple major faults with small offset and 2 single major fault that can develop very large offset we show that only two parameters control these different types of faulting patterns 1 the brittle layer thickness for a given cohesion and 2 the rate of cohesion reduction with strain for a large brittle layer thickness 22 km with 44 mpa of cohesion extension always leads to multiple faults distributed over the width of the layer for a smaller brittle layer thickness the fault pattern is dependent on the rate of fault weakening a very slow rate of weakening leads to a very large offset fault and a fast rate of weakening leads to an asymmetric graben and eventually to a very large offset fault when the offset is very large the model produces major features of the pattern of topography and faulting seen in some metamorphic core complexes

flexural rotation of normal faults a conceptual model is proposed for the generation of low angle normal faults in metamorphic core complexes the model is based on three assumptions 1 the isostatic response to normal fault motion is of regional extent 2 when a fault segment is significantly rotated from the optimum angle of slip relative to the crustal stress field it is replaced by a new planar fault oriented in the optimum direction and 3 the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust the stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations active low angle normal faults are difficult to reconcile with rock mechanics theories earthquake focal mechanism studies and geochronologic results indicating rapid cooling of core complex rocks the model does not require active fault slip on low angle faults the flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation the anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust this can explain the observed short wavelength of topographic response to normal fault loads the model results in 1 a nearly flat lying abandoned normal fault or detachment below slices of upper plate rocks and sedimentary infill 2 a strong contrast in metamorphic grade across the abandoned detachment 3 rapid movement of lower plate rocks from midcrustal depths to shallower depths these results are qualitatively in agreement with geologic observations for core complexes

flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension we suggest that the uplift of rift flanks results from mechanical unloading of the lithosphere during extension and consequent isostatic rebound this mechanism is presented as an alternative to explanations for rift flank uplift involving thermal or dynamic processes and magmatic thickening of the crust our hypothesis is based on two critical concepts first the lithosphere retains finite mechanical strength or flexural rigidity during extension second isostatic rebound uplift of the lithosphere follows when the kinematics of extension produces a surface topographic depression that is deeper than the level to which the surface of the extended lithosphere would subside assuming local isostatic compensation we develop and analyze two kinematic models for instantaneous extension of the lithosphere to show that flexural rebound is a viable explanation for the uplift of rift flanks we first investigate the isostatic consequences of finite simple slip on an initially planar dipping normal fault cutting the entire lithosphere when the lithosphere retains flexural rigidity during extension the topography resulting from this model resembles a half graben and the footwall rift flank is flexurally uplifted this simple normal faulting model explains free air gravity anomalies and topography observed at rift flanks in oceanic lithosphere such as broken ridge in the eastern indian ocean the caroline ridges sorol trough in the western equatorial pacific and the coriolis trough behind the new hebrides island arc we then investigate a general kinematic model for lithospheric extension where simple slip on a surface of arbitrary shape is accompanied by pure shear extension in the upper and lower plates when the simple slip component is not zero or the distribution of pure shear in the upper and lower plates is not identical the surface of slip can be regarded as a detachment by simplification our general model accounts for pure shear extension of the lithosphere that is uniform with depth in this case detachments have no meaning in the geologic sense however the kinematics of depth independent pure shear may nevertheless be described in terms of a surface which we term a kinematic reference surface at some depth in the lithosphere we speculate that the depth of this surface may be rheologically controlled the magnitude of rift flank uplift by flexure depends critically on the depth of this reference surface in contrast if local isostasy is assumed when the lithosphere undergoes a given amount of depth independent pure shear the resulting topography will be the same regardless of how the kinematics of that extension are formulated the basin and rift flank topography and free air gravity anomaly over young continental rifts such as the rhine graben can be satisfied using our general extensional model with a small amount 5 km of extension along a listric shaped detachment soling into the crust mantle boundary because the flexural rebound mechanism explains the observed topography and gravity anomaly over both oceanic and continental extensional domains we suggest that rheological differences between the two lithospheric types may not be important in their overall response to extension

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

comment on mechanics of tidally driven fractures in europa s ice shell by s lee r t pappalardo and n c makris 2005 icarus 177 367 379 abstract we show lee pappalardo and makris 2005 icarus 177 367 379 argument that surface cracks in europa s icy shell penetrate 3 10 times deeper in the presence of subsurface ocean is not correct we use numerical calculations to demonstrate that there is at most 50 increase in penetration depth for a crack opening in a shell of finite thickness compared to a half space we also propose a simple equation based on force balances to estimate the maximum thickness of an ice shell that can be opened under tensile stress our calculations show that a crack can only penetrate 330 m thick ice shell under 200 kpa far field tensile stress and half of that if the stress is 100 kpa but the presence of water would allow crack penetrate 4 0 km into the ice shell with zero porosity

variations in the earth s oblateness during the past 28 years 1 analysis of satellite laser ranging slr data indicates that the earth s dynamic oblateness j2 has undergone significant variations during the past 28 years the dominant signatures in the observed variations in j2 are 1 a secular decrease with a rate of approximately 2 75 10 11 yr 1 2 seasonal annual variations with a mean amplitude of 2 9 10 10 3 significant interannual variations with timescales of 4 6 years and 4 a variation with period of 21 years and an amplitude of 1 4 10 10 with minimum in december 1988 two large interannual variations are related to the strong el nino southern oscillation events during the periods of 1986 1991 and 1996 2002 and it appears that another interannual cycle may have started in late 2002 the superposition of the decadal variation on the interannual signal makes the j2 fluctuation appear to be anomalously large during the 1996 2002 period contemporary models of the mass redistributions in the atmosphere ocean and surface water can explain a major part of the 4 to 6 year fluctuations however the cause of the decadal variation remains unknown

magmatic and tectonic extension at mid ocean ridges 2 origin of axial morphology 1 we investigate the origin of mid ocean ridge morphology with numerical models that successfully predict axial topographic highs axial valleys and the transition between the two the models are time dependent simulating alternating tectonic and magmatic periods where far field extension is accommodated by faulting and by magmatism respectively during tectonic phases models predict faults to grow on either side of the ridge axis and axial height to decrease during magmatic phases models simulate magmatic extension by allowing the axial lithosphere to open freely in response to extension results show that fault size and spacing decreases with increasing time fraction spent in the magmatic phase fm magmatic phases also simulate the growth of topography in response to local buoyancy forces the fundamental variable that controls the transition between axial highs and valleys is the rise sink ratio fm ft t m where fm ft is the ratio of the time spent in the magmatic and tectonic periods and t m is the ratio of the characteristic rates for growing topography during magmatic phases 1 m and for reducing topography during tectonic phases 1 t models predict the tallest axial highs when fm ft t m 1 faulted topography without a high or valley when fm ft t m 1 and the deepest median valleys when fm ft m t 1 new scaling laws explain a global negative correlation between axial topography and lithosphere thickness as measured by the depths of axial magma lenses and microearthquakes exceptions to this trend reveal the importance of other behaviors such as a predicted inverse relation between axial topography and spreading rate as evident along the lau spreading center still other factors related to the frequency and spatial pervasiveness of magmatic intrusions and eruptions as evident at the mid atlantic and juan de fuca ridges influence the rise sink ratio fm ft t m and thus axial morphology

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

effect of lithospheric geometry on rift valley relief 1 continental and mid oceanic rifts commonly show a rift valley bounded by normal faults and uplifted shoulders in this paper we numerically model the evolution of rift valley relief in a notched brittle lithosphere results show that the rift valley has a maximum valley relief which is controlled by the competition between isostasy and lithospheric geometry if the axial lithosphere is much thinner than the flanking lithosphere the isostatic force will break outward dipping faults on the axis and stop the deepening of the rift valley we call this the axial failure mode on the other hand if the thickness of axial lithosphere is close to that of the flanks flexural bending will break new faults on the flanks and stop the valley deepening we call this the flanking failure mode triggering of either mode gives the maximum valley depth and the selection is controlled by the interplay between isostasy and lithospheric geometry on the basis of these mechanisms we propose a scaling equation that relates the maximum valley depth to the geometry flanking thickness axial thickness and notch width and strength friction angle and cohesion of the lithosphere the model explains along axis variation of valley relief at mid oceanic ridges by changes of the axial lithosphere thickness

asymmetric lithospheric extension the role of frictional plastic strain softening inferred from numerical experiments plane strain thermomechanical finite element model experiments of lithospheric extension are used to investigate the effects of strain softening in the frictional plastic regime on the asymmetry of extension strain softening is considered in cases where the crust is either strongly or weakly coupled to the mantle and as the extension velocity varies from 0 3 to 30 cm yr in the absence of strain softening extension is symmetric ss mode when strain softening takes the form of a reduction in the internal angle of friction with increasing strain lithospheric extension may be asymmetric at a lithospheric scale aa mode or exhibit crustal asymmetry concomitant with mantle symmetry as mode the different styles depend on the relative control of the system by the frictional plastic and ductile layers which promote asymmetry and symmetry respectively high extension velocities and weak ductile crust mantle coupling tend to suppress the fundamental asymmetry induced by frictional strain softening this is because they respectively increase the effective strength of the lower lithosphere and decrease the control by frictional plasticity

high resolution holocene n2o ice core record and its relationship with ch4 and co2 1 nitrous oxide n 2 o concentration records exist for the last 1000 years and for time periods of rapid climatic changes like the transition from the last glacial to today s interglacial and for one of the fast climate variations during the last ice age little is known however about possible n 2 o variations during the more stable climate of the present interglacial holocene spanning the last 11 thousand years here we fill this gap with a high resolution n 2 o record measured along the european project for ice coring in antarctica epica dome c antarctic ice core on the same ice we obtained high resolution methane and carbon dioxide records this provides the unique opportunity to compare variations of the three most important greenhouse gases after water vapor without any uncertainty in their relative timing the co 2 and ch 4 records are in good agreement with previous measurements on other ice cores the n 2 o concentration started to decrease in the early holocene and reached minimum values around 8 ka 260 ppbv before a slow increase to its preindustrial concentration of 265 ppbv

finite extension and low angle normal faulting andersonian theory for infinitesimal strains predicts that normal faults will form at high angles to the surface in extensional terranes in contrast low angle normal faults are sometimes observed on which tens of kilometers of slip has occurred and moderate angle faults with slip of 5 to 15 km are common a simple extension of the theory to account for the stresses required to drive finite deformation shows that low angles are preferred for large extension on individual faults the formation of low angle faults may require inhomogeneities in stress or material properties but once slip begins on a low angle fault it is likely to continue whereas slip on high angle faults is likely to end after modest slip to be replaced by the formation of new high angle faults

generation of morphotectonic fabric on the mid atlantic ridge flanks 28 to 29 n implications for the limits of tectonic deformation and abyssal hill formation 1 high resolution multibeam bathymetry is used to analyze the morphotectonic fabric of a 35 000 km2 zone of the mid atlantic ridge crest between 28 and 29 n interpolated isochrons at 1 myr intervals derived from a kinematic reconstruction provide a temporal framework for the structural analysis the axial valley within the survey area is offset by three non transform discontinuities dividing the axis into en echelon segments the flanks display a predominantly ridge parallel fabric of elongate abyssal hills crosscut by several discordant zones throughout the zone faulting is the predominant factor in generating the morphotectonic fabric analysis of fault scarp distribution places the limit of active tectonic deformation between 2 and 3 ma 25 to 35 km on either side of the spreading axis fault scarp relief on the ridge flanks varies with a period of approximately 2 myr which corresponds to the wavelength of abyssal hills within the area we propose an interpretation in which abyssal hill formation occurs periodically 2 myr within the ridge mountains as the lithosphere transitions from the dynamic supported regime near the axis to the isostatic regime of the flanks at 2 3 ma 25 35 km from the axis

dynamic topography in rift zones implications for lithospheric heating extended abstract extension of the lithosphere in rift zones results in heating of the crust and mantle beneath the rift based on simple stretching models mckenzie 1978 the subsidence history of rifts has been attributed to crustal thinning during extension and subsequent conductive cooling of the lithosphere beneath the rift the amount of subsidence caused by cooling should provide an estimate of the amount of heat added to a column of lithosphere as it extends models of this type which assume local isostatic compensation and uniform extension over the whole thickness of the lithosphere underpredict the amount of post rift thermal subsidence royden keen 1980 and do not explain the flanking uplifts associated with rifts predictions of central subsidence from models that do not explain flanking uplifts should be suspect since the same mechanisms producing the uplift may also affect the central subsidence the origin of flanking uplifts may have important implications for the amount and horizontal distribution of lithospheric heating beneath a rift in a study of the gulf of suez rift steckler 1985 proposes that flanking uplifts are isostatically supported by thermal expansion due to heating because horizontal heat conduction occurs too slowly to explain the amplitude and width of flanking uplifts convection due to stretching induced horizontal temperature variations beneath the rift buck 1986 has been suggested as a heat transfer mechanism

a continuum mechanics model for normal faulting using a strain rate softening rheology implications for thermal and rheological controls on continental and oceanic rifting abstract we use a strain rate dependent visco plastic rheology in a finite element model to investigate the effects of crustal rheology and thermal structure on the development of normal faults in 2 d extending lithosphere strain rate softening in the brittle regime is used to simulate the rate dependence of frictional strength observed in laboratory studies results of numerical experiments show that efficient strain rate softening can result in localized zones of high strain rate which develop in response to the rheology and boundary conditions and are not imposed a priori we argue that these zones of localized shearing are analogous to faults and use the calculated deformation field to predict the preferred location of fault formation for a range of thermal and rheologic conditions when no regional temperature gradient is imposed deformation is predicted to be distributed between several sets of conjugate normal faults however in the presence of a horizontally varying temperature field faulting is calculated to focus where the lithosphere is thinnest forming rift like topography model results predict that when crustal thickness is sufficiently small that no ductile layer is present in the lower crust deformation is mantle dominated and rift half width is controlled primarily by the vertical geothermal gradient in contrast when crustal thickness is large enough that the stress accumulation in the upper crust becomes much greater than the stress accumulation in the upper mantle deformation becomes crust dominated and the calculated rift half width is a function of both crustal rheology and the vertical geothermal gradient this implies that deformation in the brittle upper crust does not behave independently of a strong upper mantle even in situations where ductile flow occurs in the lower crust

anelasticity explains topography associated with basin and range normal faulting topography associated with normal faulting in the basin and range western united states is usually modeled as a flexure of a broken elastic plate however modeled effective rigidities are usually 100 times lower than the rigidity deduced from upper crustal thickness this discrepancy may be related to a significant anelastic deformation which we explore through numerical modeling because experimental rock rheology evidences a pressure dependent yield stress beyond the elastic limit in crustal rocks we made a finite element model that accounts for such a crustal rheology and also for the frictional behavior of an embedded high angle fault resulting topography after extension is similar to that obtained from a thin elastic plate model however the corresponding strain pattern differs first the footwall rotates in a rigid fashion over a width of 15 km that matches the typical size of uplifted basin and range blocks second hanging wall subsidence results from a significant horizontal extension and rotation of the crust we suggest that plastic deformation in the deep part of the footwall could trigger the development of a new high angle fault with a fault spacing that matches basin and range structure

evidence for substantial accumulation rate variability in antarctica during the holocene through synchronization of co 2 in the taylor dome dome c and dml ice cores high resolution records of atmospheric co2 concentration during the holocene are obtained from the dome concordia and dronning maud land antarctica ice cores these records confirm that the co2 concentration varied between 260 and 280 ppmv in the holocene as measured in the taylor dome ice core however there are differences in the co2 records most likely caused by mismatches in timescales matching the taylor dome timescale to the dome c timescale by synchronization of co2 indicates that the accumulation rate at taylor dome increased through the holocene by a factor two and bears little resemblance to the stable isotope record used as a proxy for temperature this result shows that different locations experienced substantially different accumulation changes and casts doubt on the often used assumption that accumulation rate scales with the saturation vapor pressure as a function of temperature at least for coastal locations d 2004 elsevier b v all rights reserved

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

abyssal hills formed by stretching oceanic lithosphere tectonic plates are formed and move apart at mid ocean ridges some portion of this plate separation process can occur by stretching of the crust resulting in a complex pattern of extensional faults abyssal hills the most ubiquitous topographic features on earth1 are thought to be a product of this faulting2 3 here we report the results of a self consistent numerical model of lithospheric formation and stretching that includes spontaneous fault creation in this model an axial valley develops where the fault activity is most concentrated the frozen fault generated topography rafted out of the axial valley is visually and statistically similar to observed abyssal hills formed at many slower spreading ridges faults appear to be replaced by new faults because their offset changes the local stress field we accordingly need no temporal variation in magmatism as required by some previous models4 5 6 to control the spacing or offset of faults our model results suggest instead that the irregularity of abyssal hill relief may result from a self organized critical stress state at spreading centres

dating firn cores by vertical strain measurements we have developed a system for measuring a vertical strain rate profile in the firn on polar ice sheets using a readily available video camera to detect metal bands inserted in an air filled hole we used this system in 1995 and 1996 at taylor dome antarctica we use density measurements combined with our strain rates to infer vertical velocities from our velocities we calculate a steady state depth age scale for the firn at taylor dome the age of a visible ash layer from 79 1 m is 675 25 years this ash can be correlated with ash found at 97 2 m in a recent ice core at siple dome west antarctica

reconstructing past atmospheric co2 concentration based on ice core analyses open questions due to in situ production of co2 in the ice analysis of air extracted from bubbles of polar ice showed the anthropogenic increase of atmospheric co2 concentration during the past few hundred years preindustrial concentration 280 ppmv and unexpectedly that the concentration also increased due to natural causes from 200 to 280 ppmv during the transition from the last glacial epoch to the holocene however more detailed reconstructions based on ice core analyses suggested that some of the additional observed variations were actually due to the modification of co2 concentration in the bubbles by chemical reactions between impurities in the ice detailed analyses of acidity the carbonate concentration the concentration of oxidation agents like hydrogen peroxide h2o2 and organic compounds e g formaldehyde hcho along short intervals of ice cores representing only a few annual layers allowed us to investigate these chemical reactions the records are not conclusive but they indicate that the oxidation of organic compounds is at least as important as acid carbonate reactions ice containing a low mean carbonate and a low h2o2 concentration as well as a small scattering of co2 results obtained on several adjacent samples representing a few annual layers at most is at present the best guarantee for reliable results of the atmospheric co2 concentration

topographic controls on dike injection in volcanic rift zones abstract dike emplacement in volcanic rift zones is often associated with the injection of blade like dikes which propagate long distances parallel to the rift but frequently remain trapped at depth and erupt only near the tip of the dike over geologic time this style of dike injection implies that a greater percentage of extension is accommodated by magma accretion at depth than near the surface in this study we investigate the evolution of faulting topography and stress state in volcanic rift zones using a kinematic model for dike injection in an extending 2 d elastic viscoplastic layer we show that the intrusion of blade like dikes focuses deformation at the rift axis leading to the formation of an axial rift valley however flexure associated with the development of the rift topography generates compression at the base of the plate if the magnitude of these deviatoric compressive stresses exceeds the deviatoric tensile stress associated with far field extension further dike injection will be inhibited in general this transition from tensile to compressive deviatoric stresses occurs when the rate of accretion in the lower crust is greater than 50 60 of the far field extension rate these results indicate that over geologic timescales the injection of blade like dikes is a self limiting process in which dike generated faulting and topography result in an efficient feedback mechanism that controls the time averaged distribution of magma accretion within the crust

lithospheric necking a dynamic model for rift morphology abstract rifting is examined in terms of the growth of a necking instability in a lithosphere consisting of a strong plastic or viscous surface layer of uniform strength overlying a weaker viscous substrate in which strength is either uniform or decreases exponentially with depth as the lithosphere extends deformation localizes about a small imposed initial perturbation in the strong layer thickness for a narrow perturbation the resulting surface topography consists of a central depression and uplifted flanks the layer thins beneath the central depression the width of the rift zone is related to the dominant wavelength of the necking instability which in turn is controlled by the layer thickness and the mechanical properties of the lithosphere for an initial thickness perturbation with a width less than the dominant wavelength deformation concentrates into a zone comparable to the dominant wavelength if the initial perturbation is wider than the dominant wavelength then the width of the zone of deformation is controlled by the width of the initial perturbation deformation concentrates in the region of enhanced thinning and develops periodically at the dominant wavelength a surface layer with limiting plastic stress exponent n behavior produces a rift like structure with a width typical of continental rifts for a strong layer thickness consistent with various estimates of the maximum depth of brittle deformation in the continental lithosphere the width of the rift is essentially independent of the layer substrate strength ratio for a power law viscous surface layer n 3 the dominant wavelength varies with layer substrate strength ratio to the one third power and is always larger than for a plastic surface layer of the same thickness the great widths of rift zones on venus may be explained by unstable extension of a strong viscous surface layer

cretaceous accretionary collision complexes in central indonesia abstract the geology of cretaceous accretionary collision complexes in central indonesia is reviewed in this paper the author and his colleagues have investigated the cretaceous accretionary collision complexes by means of radiolarian biostratigraphy and metamorphic petrology as well as by geological mapping the results of their work has revealed aspects of the tectonic development of the sundaland margin in cretaceous time the cretaceous accretionary collision complexes are composed of various tectonic units formed by accretionary or collision processes forearc sedimentation arc volcanism and back arc spreading the tectonic units consist of chert limestone basalt siliceous shale sandstone shale volcanic breccia conglomerate high p t and ultra high p metamorphic rocks and ultramafic rocks dismembered ophiolite all these components were accreted along the cretaceous convergent margin of the sundaland craton in the cretaceous the southeastern margin of sundaland was surrounded by a marginal sea an immature volcanic arc was developed peripherally to this marginal sea an oceanic plate was being subducted beneath the volcanic arc from the south the oceanic plate carried microcontinents which were detached fragments of gondwanaland oceanic plate subduction caused arc volcanism and formed an accretionary wedge the accretionary wedge included fragments of oceanic crust such as chert siliceous shale limestone and pillow basalt a jurassic shallow marine allochthonous formation was emplaced by the collision of continental blocks this collision also exhumed very high and ultra high pressure metamorphic rocks from the deeper part of the pre existing accretionary wedge cretaceous tectonic units were rearranged by thrusting and lateral faulting in the cenozoic era when successive collision of continental blocks and rotation of continental blocks occurred in the indonesian region