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

ridge segmentation faulting and crustal thickness in the atlantic ocean the mid atlantic ridge mar between the kane and atlantis fracture zones consists of segments 20 85 km in length1 bull s eye patterns in the mantle bouguer gravity anomaly field2 centred on several segments associated with narrow rift valleys3 have been interpreted as centres of strong mantle upwelling and thick crust here i present a map of normal faults inferred from 5 104 km2of sea beam bathymetry4 along the mar between 28 n and the atlantis fracture zone faults are mapped both on and off ridge using a criterion that distinguishes them from volcanic topography abyssal hill lineations seem to form at the rift valley walls through the growth of the normal faults towards ridge segment ends these faults are more widely spaced with larger throws than those at segment centres and often define the boundaries of the gravity bull s eyes these two different faulting styles which probably reflect changes in lithospheric strength are preserved into the rift mountains large throw faults are highly correlated with the mantle bouguer highs suggesting that amagmatic extension on the large faults contributes to the crustal thinning inferred towards the segment ends

satellite radar interferometry 1993 1999 suggests deep accumulation of magma near the crust mantle boundary at the krafla volcanic system iceland 1 deep magma accumulation near the crust mantle boundary 21 km depth at the krafla volcanic system is suggested from insar observations a best fit model derived from four interferograms covering 1993 1999 comprises an opening dike representing plate spreading and post rifting deformation and two mogi sources a mogi source deflating at a rate of 0 3 106 m3 yr coincides with the shallow krafla magma chamber while a deeper inflating mogi source further north at 21 km depth inflates at a rate of 26 106 m3 yr the inflating source is at or near the crust mantle boundary as identified by seismic studies and is interpreted as accumulating magma

evidence of a recent magma dike intrusion at the slow spreading lucky strike segment mid atlantic ridge 1 mid ocean ridge volcanic activity is the fundamental process for creation of ocean crust yet the dynamics of magma emplacement along the slow spreading mid atlantic ridge mar are largely unknown we present acoustical seismological and biological evidence of a magmatic dike intrusion at the lucky strike segment the first detected from the deeper sections 1500 m of the mar the dike caused the largest teleseismic earthquake swarm recorded at lucky strike in 20 years of seismic monitoring and one of the largest ever recorded on the northern mar hydrophone records indicate that the rate of earthquake activity decays in a nontectonic manner and that the onset of the swarm was accompanied by 30 min of broadband 3 hz intrusion tremor suggesting a volcanic origin two submersible investigations of high temperature vents located at the summit of lucky strike seamount 3 months and 1 year after the swarm showed a significant increase in microbial activity and diffuse venting this magmatic episode may represent one form of volcanism along the mar where highly focused pockets of magma are intruded sporadically into the shallow ocean crust beneath long lived discrete volcanic structures recharging preexisting seafloor hydrothermal vents and ecosystems index terms 3035 marine geology and geophysics midocean ridge processes 7280 seismology volcano seismology 8419 8149 tectonophysics planetary tectonics 5475 4259 oceanography general ocean acoustics 9325 information related to geographic region atlantic ocean keywords mid atlantic ridge earthquake hydroacoustic

elastic thickness control of lateral dyke intrusion at mid ocean ridges abstract magmatic accretion at slow spreading mid ocean ridges exhibits specific features although magma supply is focused at the centre of second order segments melts are episodically distributed along the rift toward segment ends by lateral dyke intrusions it has been previously suggested that an along axis downward topographic slope away from the magma source is sufficient to explain lateral dyke propagation however this cannot account for the poor correlation between dyke opening and surface elevation in the 2005 2010 series of 14 dyke intrusions of afar ethiopia using mechanical arguments constrained by both geodetic and seismological observations we propose that the large dykes that initiate near the mid segment magma source are attracted toward segment ends as a result of a thickening of the elastic brittle lithosphere in the along rift direction this attraction arises from the difference of elastic resistance between the segment centre where the lithosphere is thermally weakened by long term focusing of melts and comparatively colder hence stronger segment ends the axial topographic gradient in magmatic rifts may be more likely explained as an incidental consequence of these variations of along axis elastic brittle thickness rather than the primary cause of lateral dyke injections

rift jump process in northern iceland since 10 ma from 40ar 39ar geochronology due to the westward american eurasian plate boundary migration the rift zone location in northern iceland imposed by the stable icelandic hotspot has jumped eastward the present day north volcanic zone of iceland is thus shifted about 100 km to the east with respect to the kolbeinsey ridge a miocene paleo rift location was proposed along the hunafloi skagi axis unconformities inside the lava pile on both sides of the present day rift as a result of the new rift formation have been used to date the rift jump but timing is still controversial whether extinction of the paleo rift was synchronous with initiation of the new rift or the two rifts were active during some time period is unknown using the 40ar 39ar radiochronology method 37 dykes were dated along a 350 km profile parallel to the divergent plate motion direction i e ese wnw crossing the present rift the unconformities and the inferred paleo rift our results reveal that the paleo rift axis was not located along the hunafloi skagi axis but 60 km to the east this previously unknown paleo rift is named the skagafjordur paleo rift it remained active until approximately 3 ma the activity of the present north volcanic zone started about 8 8 5 ma intruding 9 9 5 myr rocks located at least 10 km east of the skagafjordur paleo rift axis consequently the two rifts were simultaneously active for 5 5 5 myr during this time period their accretion rates were nearly equal but with asymmetrical opening the largest rates occurring on external flanks these results are coherent in the icelandic context and permit a first reconstruction of the evolution of northern iceland since 10 ma

groundwater level fluctuation in the waimea plains new zealand changes in a coastal aquifer within the last 30 years data for the waimea plains new zealand indicate that the lower confined groundwater aquifer is hydraulically homogeneous and that shallow groundwater levels inland are affected mostly by anthropogenic processes while those near the coast are affected more by sea level variation analysis of long term data for new zealand indicates that sea level has increased continuously but trends are not spatially uniform results from non parametric trend analysis show that rising trends for groundwater levels are predominant in the shallow aquifer both inland on the waimea plains and for recent years near the coast while decreasing trends are evident in the underlying confined aquifer near the coast groundwater level change in the shallow aquifer appears to be more affected by climate change than the lower confined aquifer correlation analysis indicated that groundwater levels are more affected by rainfall during the rainy season than the dry season and more influenced by rainfall inland than near the coast groundwater level declines in the lower confined aquifer near the coast which has its major recharge area inland in the catchment may be substantially affected by groundwater abstraction in inland areas as well as sea level variation but there are little evidences of seawater intrusion meanwhile groundwater recharge over the catchment area has great influence on rising groundwater levels in the shallow aquifer and its recharge is estimated to be 417 8 mm year using chloride concentrations of precipitation and groundwater

neutral buoyancy and the structure of mid ocean ridge magma reservoirs the mechanical structure of the regions for shallow storage and lateral intrusion of mid ocean ridge basalt morb is explored in part by examining the density range appropriate for picritic to tholeiitic liquids and liquid crystal mixtures and its relationship to the nonlinear density depth structure of the ridge magma density spans the range 2 6 to at least 2 82 mg m 3 and incorporates the effects of fractional crystallization on the density evolution the incorporation of olivine phenocrysts and the contributions of water in melt phase density reductions the melt and melt crystal density band crosses the in situ rock density curve over the 1 to 3 km depth interval beneath the volcanic surface within that averaged interval picritic to tholeiitic melt and melt crystal mixtures are in approximate mechanical equilibrium with their surroundings and the interval is referred to as the morb horizon of neutral buoyancy hnb the detailed locations of the transition regions between the negative and neutral buoyancy and between the neutral and positive buoyancy regimes are dependent on melt phase composition as well as the presence of suspended mineral phases and their identity in the east pacific rise the hnb has a 1 1 correspondence with the inferred location of the sheeted dike complex and the seismically detected minimum compressional wave velocity region in ophiolite complexes the region corresponds to the sheeted dike complex and the uppermost isotropic gabbros above the hnb is the region of negative buoyancy seafloor to an averaged 1 km depth within which negative buoyancy forces may induce the descent of nonvesiculated melt this region corresponds to the pillow basalt sediment sequence the region of positive buoyancy extends from 3 km to the depth of melt generation and picritic melt ascends in dikes and veins of finite height 500 m to 11 km and relatively narrow width 20 mm to 100 cm the finite strengths of high temperature fluid weakened rocks mandate finite height melt parcels along the ascent pathway the magmatic source region is therefore never hydraulically connected with the surface nor with the near surface reservoir thus relatively locally integrated buoyancy has a natural role in the ascent of melt from the source region to the region of neutral buoyancy during the final stages of magma ascent hydrothermal fluid circulation enshrouds the hnb in a three dimensional envelope as suggested by the depth extent of alteration mineralogy in ophiolite complexes while hydrothermal cooling may play an important role in arresting the ascent of magma in very low spreading rate environments gravitational equilibrium represents the effectively dominant theme in intermediate and high spreading rate regions such as the east pacific rise elastic crack penetration of rheological transition zones near the hnb identify modes of crack tip morphologic changes as a function of young s modulus ratios e2 e1 above and beneath the transition and affect the style of intrusion while melt buoyancy determines the ultimate equilibrium resting place elastic crack stability considerations relate the vertical magma pressure gradients for tholeiitic pt and picritic pp magmas to the vertically varying gradient in the horizontal stress component h and suggest further subdivision into two horizons of neutral buoyancy one for tholeiitic magma hnbt and one for the picritic liquid olivine mixtures hnbp these intervals are very approximate but are estimated as 600 m to 1400 m and 1400 to 3000 m depth respectively beneath the east pacific rise axis as oceanic lithosphere is created at the rise axis and spreads laterally the horizons of negative buoyancy and neutral buoyancy ride with it and may continue to modulate the subsurface magma dynamics that influence off axis volcanism

effects of serpentinization on the lithospheric strength and the style of normal faulting at slow spreading ridges rock deformation experiments indicate that serpentinization can strongly influence the strength and tectonics of the oceanic lithosphere strength versus depth profiles calculated for conditions appropriate for slow spreading ridges indicate that the presence of serpentinite can reduce the integrated strength of the lithosphere by up to 30 results from flexural fault models indicate that if serpentinization is isolated to fault zones strain localization should be enhanced providing an explanation for the variations in the style of normal faulting along slow spreading ridge segments at segment centers where serpentinites are scarce deformation is accommodated on closely spaced faults with small throws at the segment ends where serpentinites are most abundant faults are widely spaced and have large throws q 1997 elsevier science b v

thermally driven exchanges between a coral reef and the adjoining ocean abstract in this paper hydrographic observations made over a fringing coral reef at the northern end of the gulf of aqaba near eilat israel are discussed these data show exchange flows driven by the onshore offshore temperature gradients that develop because shallow regions near shore experience larger temperature changes than do deeper regions offshore when subjected to the same rate of heating or cooling under heating conditions the resulting vertically sheared exchange flow is offshore at the surface and onshore at depth whereas when cooling dominates the pattern is reversed for summer conditions heating and cooling are both important and a diurnally reversing exchange flow is observed during winter conditions heating occupies a relatively small fraction of the day and only the cooling flow is observed when scaled by v the observed profiles of the cross shore during cooling velocity collapse onto a single curve the value of v depends on the convective velocity scale uf and the bottom s

broad accommodation of rift related extension recorded by dyke intrusion in saudi arabia volcanic fields at the eastern margin of the red sea rift were regarded as seismically quiet until a swarm of 30 000 earthquakes struck in 2009 geological analyses reveal the intrusion of a volcanic dyke and indicate that rift related extension is spread over a broad region far from the rift axis

fluid controlled faulting process in the asal rift djibouti from 8 yr of radar interferometry observations the deformation in the asal rift djibouti is characterized by magmatic inflation diking distributed extension fissure opening and normal faulting an 8 yr time line of surface displacement maps covering the rift constructed using radar interferometry data acquired by the canadian satellite radarsat between 1997 and 2005 reveals the aseismic behavior of faults and its relation with bursts of microseismicity the observed ground movements show the asymmetric subsidence of the inner floor of the rift with respect to the bordering shoulders accommodated by slip on three of the main active faults fault slip occurs both as steady creep and during sudden slip events accompanied by an increase in the seismicity rate around the slipping fault and the fieale volcanic center slip distribution along fault strike shows triangular sections a pattern not explained by simple elastic dislocation theory these observations suggest that the asal rift faults are in a critical failure state and respond instantly to small pressure changes in fluid filled fractures connected to the faults reducing the effective normal stress on their locked section at depth

the afar depression transition between continental rifting and sea floor spreading abstract detailed geophysical and geological studies were undertaken in the 1970 s in the afar depression ethiopia in order to study the processes of continental break up occurring in this unique setting the afar is located between the ethiopian and somali plateaus and the red sea we have re evaluated the available seismic refraction and gravity data using modern interpretation techniques and incorporated new information derived from magnetic gravity seismicity and seismic refraction data the results of the re evaluation indicate that the afar depression is a transition zone between the continental rifts of kenya and the present ongoing sea floor spreading of the red sea and the gulf of aden in the depression itself the crust is greatly attenuated and is underlain by a low velocity 7 4 7 5 km s high temperature upper mantle material however it is still a continental crust which thins to the northeast and east towards the red sea and the gulf of aden it is not a newly formed oceanic crust as has been suggested in the past the trends of the areas of crustal thinning are indicated by the gravity anomalies the trend of the ethiopian rift reaches south afar where the trend of crustal thinning changes and becomes parallel to the red sea trend it is offset en echelon from the spreading area of the red sea which is marked by an alignment of gravity maxima the gulf of aden trend continues west towards the ethiopian rift the gravity trends coincide with areas of maximum crustal thinning and may thus suggest a possible triple junction in development

rift zone development around a plume related magma centre on the isle of skye scotland a model for stress inversions using the example of the isle of skye in scotland we investigate the influence of pressure variations in upper crustal magma reservoirs on the development of rift type normal faulting around central volcanoes the regional synmagmatic stress regime is of strike slip type in scotland during the lower tertiary during a prolonged period of overall high pressure in the skye magma reservoir gabbro intrusion stage crustal extension results from the injection of basaltic dykes parallel to the trend of the far field maximum stress during a subsequent period of pressure decrease in the reservoir granites intrusion stage normal faults trending parallel to the dykes are initiated these faults tilt the upper crustal blocks along with the former dyke swarm and associated lava pile finite element modelling shows that a decrease of magma pressure in a circular cavity may lead as in skye to a change from a regional strike slip to a local rift type normal stress regime

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

strain accommodation by slow slip and dyking in a youthful continental rift east africa continental rifts begin and develop through repeated episodes of faulting and magmatism but strain partitioning between faulting and magmatism during discrete rifting episodes remains poorly documented in highly evolved rifts tensile stresses from far field plate motions accumulate over decades before being released during relatively short time intervals by faulting and magmatic intrusions1 2 3 these rifting crises are rarely observed in thick lithosphere during the initial stages of rifting here we show that most of the strain during the july august 2007 seismic crisis in the weakly extended natron rift tanzania was released aseismically deformation was achieved by slow slip on a normal fault that promoted subsequent dyke intrusion by stress unclamping this event provides compelling evidence for strain accommodation by magma intrusion in addition to slip along normal faults during the initial stages of continental rifting and before significant crustal thinning

emplacement of dikes sills and crustal magma chambers at divergent plate boundaries abstract a model of dike emplacement at divergent plate boundaries is developed which predicts that below the uppermost 1 3 km of the oceanic crust the number and lengths of dikes in any particular swarm should increase but that the thickness should decrease with depth in the crust these predictions are supported by field observations it is proposed that rapid injections of dikes at divergent plate boundaries may temporarily alter the normal stress field in such a way that at a certain level in the crust subsequent dikes change into sills such sills may develop into crustal magma chambers either when many nearby sills combine into a large one or when an initial sill absorbs the magma of many dikes that enter it in a rapid succession while the sill is liquid for the fastest spreading ridges the initial thickness of a sill that is to develop into a crustal chamber need only attain 20 m whereas at the slowest spreading ridges the initial thickness must exceed 170 m because thin sills are much more likely to form than thick sills the probability of sills developing into crustal chambers increases with dike intrusion frequency hence with spreading rate furthermore the model predicts first that with increasing spreading rate the length of the chamber along the ridge axis should increase and second that the most likely site for magma chambers is at depths of 2 4 km both these predictions are supported by available data

what controls the along strike slopes of volcanic rift zones we investigate the dynamics of viscous pressure losses associated with lateral magma transport in volcanic rift zones by performing 1 coupled elastic hydrodynamic simulations of downrift magma flow in dikes and 2 analog experiments mimicking lateral dike propagation in the presence of an along rift topographic slope it is found that near source eruptions are likely to be favored by shallow slopes while distant downrift eruptions may be encouraged by steeper slopes provided that along rift variations in the tectonic stress are negligible or uncorrelated on the timescale of multiple dike intrusions this implies the existence of a critical slope to which a volcanic rift zone would naturally evolve such behavior is produced by three dimensional 3 d elastic effects and is controlled by the ratio of the driving pressure gradient due to the along strike topographic slope to the vertical gradient in the excess magma pressure in the dike this model may be viewed as complementary to commonly cited mechanisms that appeal to magma viscosity and the dynamics of freezing of lava flows at the surface to explain the low profiles of basaltic shield volcanoes our estimated values of the critical slopes are in general agreement with observations in hawaiian rift zones but further development of fully 3 d models is required for more accurate predictions

the relationship between buoyant mantle flow melt migration and gravity bull s eyes at the mid atlantic ridge between 33 n and 35 n abstract crustal thickness and mantle bouguer anomaly mba patterns along the mid atlantic ridge mar between the oceanographer and hayes transforms 33 35 n are predicted from a three dimensional mantle flow model driven by plate spreading and mantle density variations assuming a two or three dimensional model of melt extraction our two dimensional melt extraction model focuses melt to the nearest ridge segment by assuming that melt moves only vertically and parallel to the spreading direction in contrast the three dimensional model assumes melt migrates vertically to the top of the melting region and then up the local slope of this surface the shape of this surface which is controlled by enhanced conductive cooling at ridge offsets directs melt toward the center of segments our results suggest that buoyant mantle flow driven by temperature variations and a small retained melt fraction

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

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

effects of the crustal stress on the growth of dikes conditions of intrusion and extrusion of magma dike growth is analyzed on the basis of the fluid dynamical behavior of incompressible newtonian magma in a spatially uniform fissure of temporally variable width the length and width of the dike vary with the magma pressure at the dike entrance connected to the magma chamber and with the mean stress working normal to the dike walls the mean stress consists of the regional crustal stress and the mean elastic stress which is approximately evaluated using an analytic solution of the crack with a constant aperture the length and width of the dike are determined as a function of time by simple ordinary differential equations the dike can grow when the initial pressure of the entering magma is above the lithostatic pressure and high enough to suppress compression of the crustal stress the assumed size of the initial crack affects the growth only in the duration needed for an initial adjustment stage after this stage the width and length first grow exponentially and then the growth slows as magma is released from the chamber the growth in the later stage is classified into two types when the initial magma pressure is high enough the dike elongates unlimitedly while the width finally decreases this extrusive growth is likely to lead to eruptions the other intrusive growth in which the dike finally stops its growth with a finite length and width occurs when the crustal stress component normal to the fissure is of sufficiently strong extension buoyancy of magma is not very effective in the earlier growth stage but it shifts the marginal condition separating the extrusive and intrusive growths the extrusive and intrusive events that recently happened in izu oshima volcano and izu tobu volcano group japan are interpreted as the examples of these two growth types

sequence of rifting in afar manda hararo rift ethiopia 2005 2009 time space evolution and interactions between dikes from interferometric synthetic aperture radar and static stress change modeling thirteen dike intrusions in the manda hararo rift afar ethiopia from september 2005 to june 2009 studied using an extensive interferometric synthetic aperture radar insar data set provide insight into the mechanics of a major active rift kinematic inversions of insar data reveal that dikes opened by 0 8 3 5 m at an average 5 km depth with volumes of 0 04 0 2 km 3 with up to 12 m opening and a volume greater than 1 km 3 for the september 2005 megadike dikes have their source in a midsegment magma reservoir which induces a local shallowing of the brittle ductile boundary presumably due to thermal weakening of the lithosphere the smaller dikes in 2006 2009 were emplaced in regions of minimum opening of the september 2005 megadike above the central magma reservoir in contrast the most voluminous dike intrusions in 2006 2009 occurred near the locus of the peak of maximum opening of the september 2005 megadike 10 km north of the magma source this may suggest that tension on the plate boundary was highest there both prior to 2005 and possibly also after 2005 evolution and distribution of normal stress on the plate boundary throughout the rifting episode may indicate that tension near to the magma reservoir is lower than toward segment ends average relief of normal stresses of tectonic origin coeval to dike intrusions is comparable with shear stress drops for earthquakes presumably because dikes in the manda hararo rift are intruded at low magma pressure and high tectonic stress time space evolution and interactions between dikes from interferometric synthetic aperture radar and static stress change modeling

a comparison of rift zone tectonics in iceland and hawaii observations of eroded volcanic rift zones indicate that dikes in iceland are typically several times thicker than those in hawaii geodetic and seismic observations of active rifts however suggest that dike heights in the two regions are similar provided the elastic properties of the rift zones are the same this implies that dikes are intruded with higher driving pressures magma pressure minus compressive stress perpendicular to the dike plane in iceland than hawaii a second major difference between the two regions is the greater prevalence of large normal fault scarps in rift zones in iceland from this it can be infered that a lower percentage of dikes breach the surface in iceland than in hawaii thus although dikes in iceland are intruded with higher driving pressures they possess lower absolute magma pressures than in hawaii these differences can be interpreted in terms of the tectonic settings in the two regions in iceland a steady remote extension reduces the horizontal stress perpendicular to the rift zone allowing dikes to be intruded with low absolute pressures but high driving pressures when magma becomes available in hawaii a more continuous magma supply on the timescale over which the dike induced stresses are relaxed and perhaps a greater role for intrusions in driving long term rift extension ensure that the rift compressive stress is not relaxed significantly before the next dike is intruded thus the magma pressure must be nearly sufficient for eruption in order for intrusion to occur if the mechanism for relaxing the rift compressive stress were less efficient still then an even higher percentage of dikes would erupt and at times the rift zone trend could become an unfavorable orientation for dike intrusion such might be the case at mauna loa which lacks large rift zone faults and fissures and possesses numerous radial vents outside its two main rift zones

seismicity during lateral dike propagation insights from new data in the recent manda hararo dabbahu rifting episode afar ethiopia seismicity released during lateral dike intrusions in the manda hararo dabbahu rift afar ethiopia provides indirect insight into the distribution and evolution of tensile stress along this magma assisted divergent plate boundary in this paper 5 dike intrusions among the 14 that form the 2005 present rifting episode are analyzed with local and regional seismic data during dike intrusions seismicity migrates over distances of 10 15 km at velocities of 0 5 3 0 km h away from a single reservoir in the center of the rift segment confirming the analogy with a slow spreading mid ocean ridge segment comparison with geodetic data shows that the reservoir is located 7 km down rift from the topographic summit of the axial depression dikes emplaced toward the north are observed to migrate faster and to be more voluminous than those migrating southward suggesting an asymmetry of tension in the brittle elastic lithosphere seismicity during dike injections is concentrated near the propagating crack front in contrast faults and fissures in the subsurface appear to slip or open aseismically coeval with the intrusions the seismic energy released during dike intrusions in the manda hararo rift appears to be primarily modulated by the local magnitude of differential tensile stress and marginally by the rate of stress change induced by the intrusion the low level of seismic energy accompanying dike intrusions despite their significant volumes is likely an indicator of an overall low level of tension in the lithosphere of this nascent plate boundary

crustal structure and magmato tectonic processes in an active rift asal ghoubbet afar east africa 2 insights from the 23 year recording of seismicity since the last rifting event 1 the asal ghoubbet ag rift has sustained a major volcano tectonic rifting episode in 1978 and has been subsequently monitored with continuous geodetic and seismological surveys it is therefore an ideal place to study the transient magmato tectonic processes that operate after a rifting episode we examine the space time evolution of 2500 md 2 8 earthquakes recorded in the rift from 1979 to 2001 we focus on the relationships between this seismic activity and both the three dimensional structure of the rift and its postrifting behavior depicted from geodesy the results highlight the major role of the central magmatic system fieale shark bay on the structure seismic activity and overall behavior of the rift from 1978 to 1986 the rift opens at a fast rate yet mainly aseismically the opening is magmatically driven and accommodated since 1986 when the opening rate abruptly decreased the seismicity is concentrated in the central part of the rift and reveals pulses of activity of the central volcanic system these pulses result from that magmatic zone undergoing alternating stretching and inflating episodes thus while the plate driven induced stresses have been rebuilding in the rift since 1987 the rift opening is still essentially accommodated in the axial magmatic zone the ag rift has thus sustained a postrifting unsteady opening over more than 23 years following its stretching episode that transient opening has essentially occurred aseismically and most tectonic faults remain relaxed and locked