continental oceanic crustal transition off southwest africa more than 1 426 mi 2 300 km of 48 trace 12 fold seismic reflection profiles were used to examine the nature of the continent ocean boundary off southwest africa south of the orange river faulted blocks which we interpret as rifted continental crust can be traced seaward to the 3 km isobath the transition to oceanic basement occurs in a zone 19 to 25 mi 30 to 40 km wide beneath the continental rise although structural details are obscured by a thick seaward dipping wedge of pre aii pre late early cretaceous overburden oceanic crust at the presumed contact is not older than magnetic anomaly m9 126 to 121 m y b p north of the orange river a pronounced hinge in continental crust correlates with magnetic anomaly g of rabinowitz 1976 seaward of the hinge a complex fault block terrane is evident oceanic basement can not be traced with any certainty landward of anomaly m4 intracontinental stretching and associated volcanism appear to have been important in the early history of the cape basin rifting and local dike intrusion may explain the presence of some lineated magnetic anomalies previously attributed to sea floor spreading related extrusives form at least part of the pre aii wedge just seaward of the hinge zone our interpretation of the seismic data suggests that the initiation of normal spreading in the cape basin postdates by 4 to 9 m y the valanginian age derived from prevailing plate tectonic reconstructions of the south atlantic

plate tectonic development of the south west indian ocean a revised reconstruction of east antarctica and africa a revised sequence of reconstructions for east antarctica relative to africa is generated from published seafloor spreading and fracture zone anomalies these date from the cretaceous tertiary boundary to the late jurassic and lead to a revised predrift reconstruction for east and west gondwanaland euler pole 1 67 s 35 99 e rotation angle 53 43 this reconstruction derived wholly from marine data places east antarctica adjacent to the lebombo mountains and south of and subparallel to the sabi acid volcanic suite supporting the postulate that these rocks represent mesozoic plate boundaries this refit implies that part of the coastal plain of mozambique is underlain by oceanic or very highly extended continental crust the reconstruction is supported by the alignment of 1 the cape fold belt with its antarctic equivalents and 2 the western limit of pan african reactivation of the mozambique mobile belt with the western limit of the sverdrupfjella metamorphic suite antarctica initially moved along a transform fault lying immediately east of and subparallel to the lebombo mountains the eastern face of the tugela cone the southeastern face of the falkland plateau in its reconstructed position and the eastern flank of the agulhas plateau in its reconstructed position at m10 time a triple junction formed near the tip of the falkland plateau implying that the oldest lineated magnetic anomalies in the eastern weddell sea associated with this triple junction cannot predate m10 at m2 time the mozambique ridge rifted from the astrid ridge in a ridge jump episode such an event may also explain the presence of magnetic anomalies in the southern mozambique basin gunnerus ridge conrad rise and del cano rise are all tectonically associated with the madagascar ridge

geomorphic characterization of four shelf sourced submarine canyons along the u s mid atlantic continental margin abstract shelf sourced submarine canyons are common features of continental margins and are fundamental to deep sea sedimentary systems despite their geomorphic and geologic significance relatively few passive margin shelf breaching canyons worldwide have been mapped using modern geophysical methods between 2007 and 2012 a series of geophysical surveys was conducted across four major canyons of the us mid atlantic margin wilmington baltimore washington and norfolk canyons more than 5700 km2 of high resolution multibeam bathymetry and 890 line km of sub bottom chirp profiles were collected along the outer shelf and uppermost slope depths of 80 1200 m the data allowed us to compare and contrast the fine scale morphology of each canyon system the canyons have marked differences in the morphology and orientation of canyon heads steepness and density of sidewall gullies and the character of the continental shelf surrounding canyon rims down canyon axial profiles for washington baltimore and wilmington canyons have linear shapes and each canyon thalweg exhibits morphological evidence for recent relatively small scale sediment transport for example washington canyon displays extremely steep wall gradients and contains 100 m wide 5 10 m deep v shaped incisions down the canyon axis suggesting modern or recent sediment transport in contrast the convex axial thalweg profile the absence of thalweg incision and evidence for sediment infilling at the canyon head suggest that depositional processes strongly influence norfolk canyon during the current sea level high stand the north walls of wilmington washington and norfolk canyons are steeper than the south walls due to differential erosion though the underlying cause for this asymmetry is not clear furthermore we speculate that most of the geomorphic features observed within the canyons e g terraces tributary canyons gullies and hanging valleys were formed during the pleistocene and show only subtle modification by holocene processes active during the present sea level high stand

strain rate and temperature dependence of omori law scaling constants of ae data implications for earthquake foreshock aftershock sequences 1 little is known about the temperature and strain rate dependence of acoustic emission ae activity ae hence we carried out a preliminary series of flow through triaxial compression tests on porous sandstones at different temperatures and strain rates the ae data exhibits clear foreshock and aftershock sequences with respect to the dynamic failure of the test specimen significant ae activity starts less than 5 min before sample failure irrespective of the strain rate the increase in the ae event rate is steeper and the foreshock exponent p is smaller in the slow strain rate tests it could be the reason why there are no easily recognisable foreshock sequences for most individual earthquakes the aftershock decay parameter p is a linear function of test temperature as it has also been inferred for natural seismicity the seismic b value decreases systematically with increasing deformation rate suggesting a greater proportion of small cracks in the slow strain rate tests hence the ae activity is a function of both strain rate and temperature

mantle plumes and continental tectonics abstract it is now recognised that plumes originate from a thermal boundary layer deep in the mantle a new plume consists of a large head with a diameter of approximately 800 to 1200 km and it is followed by a smaller tail which has a diameter of 100 to 200 km at the top of its ascent the head flattens to form a disk 2000 to 2500 km in diameter that may melt to produce a continental flood basalt or oceanic plateau the tail melts to produce chains of volcanic activity such as the hawaiian emperor chain the ascent of a new plume leads to surface uplift of 500 to 1000 m over a length scale of 2000 km this places the lithosphere under tension but the horizontal forces produced do not result in runaway extension unless the continental lithosphere is already anomalously hot and therefore weak if the lithosphere is cold and strong slow extension leading to a new spreading ridge may postdate plume arrival by 10 to 40 m y the time difference being the time taken to conduct heat from the plume to the overlying lithosphere plume heads may control the timing and position of new ocean ridges but are not the fundamental force that drives plate tectonics the heat from plume tails may weaken the overlying plate and pin the position of spreading the ascent of a mantle plume can also have an important influence on subduction large oceanic plateaus such as the ontong java plateau are more buoyant than normal oceanic crust because they have a thicker basaltic layer and because they are underlain by a 200 km thick layer of hot material from a mantle plume head as a consequence oceanic plateaus are more difficult to subduct than normal oceanic crust this can lead to jamming of plates at subduction zones resulting in a flip in the sense of subduction as appears to have happened in the caribbean region or to obduction of the plateau finally when a plume head is emplaced beneath continental crust heat released from the plume may lead to melting of the overlying crust the granites associated with many greenstone belts and intercontinental magmatic provinces such as the paleozoic lachlan fold belt of eastern australia and the middle proterozoic anorogenic granites of the central and southwestern united states may have formed in this way

forces associated with mantle plumes this study addresses first order aspects of the physics of continental lithosphere extension above mantle plumes developing a testable model that predicts extension rates from flow parameters radial flow in the head of a plume causes a viscous force and uplift over this head causes a buoyancy force which both drive extension the viscous force is roughly proportional to the plume s volume flux and given my best estimates for flow parameters predominates for the most vigorous plumes extension is opposed by a viscous force beyond the plume head and its rate is thus affected by the dimensions of the plate on which this force acts extension rates are 2 mm yr 1 above yellowstone 1mm yr 1 above baikal and 0 7 mm yr 1 above the east african plume volume fluxes for all these plumes are 50 m3 s 1 the extension rates depend mainly on plate dimensions

submarine canyons in the bathtub abstract submarine megafans and their associated canyons have long attracted the attention of a large community in the earth sciences because they record the history of past erosional processes as such their morphology and sedimentary infill may be of use to unravel past climatic and tectonic evolution yet despite an important corpus of research on their structural characteristics we lack a well accepted quantitative description for the dynamics of the formation and growth of these sedimentary systems one of the main reasons for this lies in our inability to successfully understand reproduce and predict the dynamics of the channel systems tens to hundreds of meters deep several kilometers in width and hundreds of kilometers in length that build submarine fans here we report on a series of small scale laboratory experiments on the formation of subaqueous channels and lobes our experiments show that steady flow of a dense current on a bed of light particles can induce both spontaneous formation of channels longer than a few channel widths and spontaneous meandering

paleomagnetism of jurassic and cretaceous rocks in central patagonia a key to constrain the timing of rotations during the breakup of southwestern gondwana abstract a paleomagnetic study in jurassic and cretaceous rocks from the canadon asfalto basin central patagonia indicates the occurrence of about 25 30 clockwise rotation in upper jurassic lowermost cretaceous rocks whereas the overlying mid cretaceous rocks do not show evidence of rotation this constrains the tectonic rotation to be related to a major regional unconformity in patagonia which in turn seems to be close in time with the early opening of the south atlantic ocean the sense and probably the timing of this rotation are similar to those of other paleomagnetically detected rotations in different areas of southwestern gondwana suggesting a possible relationship between these and major tectonic processes related with fragmentation of the supercontinent on the other hand the mid cretaceous rocks in the region yield a paleopole located at lat 87 south long 159 east a95 3 8 this pole position is consistent with coeval high quality paleopoles of other plates when transferred to south american coordinates implying it is an accurate determination of the aptian circa 116 ma geomagnetic field in south america

experiments on self channelized subaqueous fans emplaced by turbidity currents and dilute mudflows the passage of turbidity currents over submarine fans often results in intense channelization the channels are typically bounded by levees and they vary from mildly to strongly meandering the process of self channelization remains however somewhat obscure preliminary experiments reported here demonstrate for the first time that self channelization of subaqueous fans by turbidity currents can be reproduced at laboratory scale the same experiments also illustrate self channelization by dilute mudflows the resulting weakly sinuous channels can be predominantly depositional predominantly erosional or some combination of the two the channels can elongate to the length of the entire reach available for their formation they can show both gradual shift and avulsion a necessary condition for the formation of intricate channelization in the laboratory experiments appears to be a turbidity current that is insufficient to cover the entire area of the fan at any given time loose field analogs to the experimental channelized fans i e analogs that are imprecise but contain many similarities can often be found at the distal end of submarine fans two possible examples are cited one from the pochnoi system bering sea and one from the gulf of cadiz

new paleomagnetic data from the antarctic peninsula and their tectonic implications new paleomagnetic data presented here further constrain the relative motion of the antarctic peninsula relative to east antarctica during the mesozoic development of the southern ocean basins the antarctic peninsula ap is one of four crustal blocks that define west antarctica the others being the ellsworth whitmore mountains ewm thurston island eights coast ti and marie byrd land mbl a jurassic pole 155ma 124 e 64 s a95 7 1 n 10 vgps was obtained from the ap block which suggests that the ap block rotated clockwise between 175 and 155 ma due to significant early opening in the weddell sea basin a new early cretaceous paleomagnetic pole 182 e 74s a95 5 9 n 6 site mean vgps indicates that the ap block was in or near to its present day position with respect to east antarctica by 130 ma between 155 and 130 ma counterclockwise rotation of the ap ti blocks together with the southward motion of east gondwanaland probably resulted in subduction of weddell sea ocean floor beneath the southern ap block and initiated the palmer land deformational event a 130 ma pole from the ti block requires clockwise rotation of the ti and possibly the ewm blocks between 130 and 110 ma producing sinsistral strike slip motion between the ewm block and east antarctica and dextral transpressional motion between the ti ewm blocks and the ap block new ap block 110 ma and 85 ma poles from this study 199 e 74 s a95 6 9 n 13 vgps 152 e 86 s a95 7 5 n 6 vgps respectively are similar to equivalent age poles from east antarctica and suggest little or no relative motion between the antarctic peninsula and east antarctica northern and southern antarctic peninsula mid cretaceous poles are very much alike suggesting that the s shape of the antarctic peninsula is not due to oroclinal bending since 110 ma

reservoir infill by hyperpycnal deltas over bedrock 1 we propose a simple morphodynamic theory of reservoir deltas prograding over bedrock under the influence of turbid underflows the theory models the geomorphic actions of fluvial and hyperpycnal flows as diffusion processes and treats bedrock alluvial and river lake transitions as moving internal boundaries it yields self similar analytical solutions for the co evolving river and lake bed profiles under different slope and influx conditions these include a special case in which delta deposits are swept underwater to prograde into a subaqueous turbid pool the theoretical predictions are in good agreement with microscale experiments and should help interpret and anticipate longitudinal sedimentation patterns in mountain reservoirs

stream power controls the braiding intensity of submarine channels similarly to rivers we use physical experiments to investigate the response of submarine braided channels driven by saline density currents to increasing inflow discharge and bed slope we find that similarly to braided rivers only a fraction of submarine braided networks have active sediment transport we then find similar response to imposed change between submarine and fluvial braided systems 1 both the active and total braiding intensities increase with increasing discharge and slope 2 the ratio of active braiding intensity to total braiding intensity is 0 5 in submarine braided systems regardless of discharge and slope and 3 the active braiding intensity scales linearly with dimensionless stream power thus braided submarine channels and braided rivers are similar in some important aspects of their behavior and responses to changes in stream power and bed slope in light of the scale independence of braided channel planform organization these results are likely to apply beyond experimental scales

survival of a submarine canyon during long term outbuilding of a continental margin net depositional submarine canyons are common in continental slope strata but how they survive and prograde on constructional margins is poorly understood in this study we present field evidence for the coevolution of a submarine canyon and the adjacent continental slope using a three dimensional seismic data cube that images the ebro margin northwest mediterranean we identify a preserved canyon on a middle pleistocene paleosurface and relate it directly to its expression on the present day seafloor a subparallel stacking pattern of seismic reflectors similar to that seen between prograding clinoforms in intercanyon areas is observed between the modern and paleocanyon thalwegs the concavity of the modern long profile differs from the convex concave long profile on the middle pleistocene surface suggesting a long term change in canyon sedimentation we interpret this change as a shift to a canyon dominated by turbidity currents from one strongly influenced by the pattern of sedimentation that built the open slope canyon interfluves we find support for our interpretation in previous studies of the ebro margin

reply to discussion on early tectonic extension between the agulhas bank and the falkland plateau due to rotation of the lafonia microplate

two diffusion description of hyperpycnal deltas 1 alluvial deltas formed upstream of lakes and reservoirs often exhibit concave foresets with maximum inclinations smaller than the angle of repose in the present work we test whether this morphology could be attributed to bed load sediment transport by turbid underflows acting along the foreset beds before their fine sediment load settles out of suspension under hyperpycnal conditions both the topset and foreset of the delta would thus be subject to the geomorphic influence of the dense river inflow to describe this joint geomorphic action by subaerial and subaqueous currents we derive a two diffusion theory and modify it to account for inclination thresholds different diffusion strengths apply to the topset and foreset on either side of the moving shoreline for a channel of uniform initial slope and a constant water level in the body of standing water we show that the resulting moving boundary problem admits exact similarity solutions to test the theory the analytical solutions are compared with small scale laboratory experiments in which turbid underflows are replaced by brine currents the curved profiles predicted by the theory and measured in the experiments resemble those of surveyed deltaic deposits in lakes known to be prone to turbidity currents

experimental investigation on self channelized erosive gravity currents submarine canyons are the main conduits for sediment transfer from continental shelves to deep abyssal plains a large number of bathymetric and seismic surveys provide detailed information on their morphology and structure shedding light on the mechanisms involved in their formation however because of the difficulty in deploying instruments in deep marine environments little is known about the processes by which turbidity currents erode and incise the canyon floors in this study we use a micro scale tank experiment to investigate the incision of a mobile sediment bed by a gravity current we inject a brine at the top of a ramp covered with low density plastic sediments conditions for channel formation are investigated through systematic exploration of the slope vs discharge phase space measurements of the sediment bed topography and the gravity current velocity profile allow us to characterize the channel incision dynamics a channel forms when the bed shear stress exceeds the sediment critical shear stress in this case a channel starts forming near the upstream end of the incline and propagates down slope while slowly deepening after a transient state of about 1 2 hours the channel reaches steady state experimental results suggest that the slope controls the rate of erosion and the speed of channel incision while the brine discharge controls the channel geometry the erosional morphologies created by our experimental gravity currents show strong analogies with the axial incision of canyons the primary mechanism by which canyons connected to continental shelves or large rivers evolve

structure and tectonics of the agulhas falkland fracture zone abstract the 1200 km long agulhas falkland transform developed during the early cretaceous break up of west gondwanaland on the african plate the continent bordering the agulhas fracture zone extends between the agulhas bank and the south tugela reentrant it is divided into four distinct parts mallory trough segment i diaz ridge segment ii east london segment iii and durban segment iv from southwest to northeast each segment differs from the ohters in its physiography and in the nature of the continent ocean crustal boundary in segment i a wedge like eastward narrowing deep water basin separates the steep continental slope from a marginal fracture ridge of probable volcanic origin along the transform fault trace along segment ii the south african continental slope is deeply embayed and a great thickness of sedimentary strata has been ponded behind a buried ridge along the northern side of the fracture zone trace above a probable fragment of oceanic crust representing a northerly extension of the jurassic falkland plateau basin in segment iii the continental margin is steep especially along the middle slope it lies directly along the transform trace but conspicuously rugged along the continental slope due to the effects of submarine canyoning and sediment slumping in segment iv the continent ocean boundary also lies directly along the transform fault trace and the margin shows evidence of seaward tilting above landward dipping faults along the middle or lower slope the segmentation is inherited from jurassic structural fabrics formed prior to early cretaceous transform motion when tectonic rotation of microplates caused rifting and partial oceanization along the future agulhas falkland megafault trajectory

cyclone induced hyperpycnal turbidity currents in a submarine canyon 1 density currents such as turbidity currents are major transport agents in various terrestrial lacustrine and marine environments worldwide however a gap exists between those who study the deposits by turbidity currents turbidite on a field scale and those who study turbidity currents using small scale laboratory experiments and theoretical numerical models we report two typhoon triggered hyperpycnal turbidity current events observed in a submarine canyon our findings verify turbidite sequences with the characteristics of suspended sediment carried by passing turbidity currents that displayed distinct waxing and waning phases our study also confirms the direct link between typhoon triggered hyperpycnal flows in a small mountainous river and turbidity currents in a nearby submarine canyon that transport sediment to the deep sea efficiently

hypocenter distribution and focal mechanism of ae events during two stress stage creep in yugawara andesite more than a thousand acoustic emission ae hypocenters were determined in a cylindrical andesite specimen under two stage uniaxial creep at stresses of 204 and 214 mpa strains were monitored for 6 peripheral points at the middle part of the cylindrical specimen s wall the strain data indicate gradual increase of nonuniform deformation during steady creep and strong intensification of the nonuniformity during acceleration creep and therefore biased stress distribution within the specimen the correlation between dilatant strain and ae hypocenters was investigated for whether or not tensile cracks emit ae the region with high ae activity shows only a small dilatant strain this negative correlation between ae and the dilatant region may eliminate tensile cracks as possible ae sources a composite focal mechanism solution of local ae events covering a wide solid angle of the focal hemisphere indicates that shear fractures emit ae waves the direction of the compressional axis in this solution shows a significant deviation from that inferred from the applied external force suggesting that the local stress field is governed by preexisting weak zones that are presumably produced by tensile cracks within the specimen ae hypocenters tended to form clusters during steady creep under the constant compressional stress during acceleration creep caused by a small step increase of the external stress the preceding clusters disappeared while a new cluster appeared in an incipient fault plane this suggests that changes in seismicity pattern such as migrations or quiescences of swarm important clues for earthquake predictions may be caused by an instantaneous change in the tectonic stress levels

analysis of slope failures in submarine canyon heads an example from the gulf of lions to improve understanding of evolution of submarine canyons a three dimensional slope stability model is applied to bourcart canyon in the western gulf of lions in the mediterranean sea the model builds on previous work by chen and others and it uses the upper bound theorem of plasticity to calculate the factor of safety of a kinematically admissible failing mass examples of three dimensional failure surfaces documented in the literature were used to test the model formulation model application to bourcart canyon employed the results of a detailed stratigraphic analyses based on data acquired by swath bathymetry sub bottom profiling high resolution seismic reflection surveys and piston coring the sediment layers were also characterized using in situ geotechnical measurements and laboratory tests the effects of three loading scenarios were analyzed 1 earthquake shaking 2 hemipelagic sedimentation and 3 axial incision these three mechanisms influenced the predicted volumes and shapes of slope failures along the flanks of bourcart canyon and comparison of these predictions with failure geometries inferred from seafloor morphology showed that mass failures could account for the observed morphology along the canyon walls as well as a mechanism of canyon widening

southern ocean hotspot tracks and the cenozoic absolute motion of the african antarctic and south american plates abstract a detailed analysis based on an antarctica africa finite reconstruction at chron c29 64 ma an assumption of no relative wander between the marion prince edward and tristan hotspots and on recently revised bathymetric maps of the southern ocean region shows that the fixed hotspot reference frame is tenable for absolute plate motions bouvet hotspot and probably trinidade as well also shows little or no cenozoic relative motion contrary to previous models bouvet hotspot is unrelated to the meteor rise cape rise seamount chain instead the bathymetric data when compared with the predicted hotspot tracks indicate another hotspot exists near the southernmost south atlantic spreading ridge segment new geochemical evidence from the latter region supports this hypothesis in showing the effects of plume enrichment from a source that is compositionally distinct from bouvet the peculiar zig zag shape of the cape rise meteor rise lineament is the result of this hotspot crossing the active transform segment of the falkland agulhas fracture zone in late mesozoic times followed by an early cenozoic ridge jump to the pre weakened trace on the then south american plate from the averaged cenozoic absolute motions of the african antarctic and south american plates it is evident that antarctica has been most nearly stationary in an absolute motion sense

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

creation and destruction of weddell sea floor in the jurassic new paleomagnetic data invite reassessment of the breakup history of parts of the pacific margin of gondwana due to the mesozoic opening of the southern ocean basins these data indicate clockwise rotation of the antarctic peninsula relative to east antarctica between 175 and 155 ma that would have created as much as 1000 km of pre anomaly m25 ocean floor in the weddell sea basin the paleomagnetic data also suggest that between 155 and 130 ma the antarctic peninsula rotated counterclockwise relative to east antarctica rotation that along with the southward motion of east antarctica would have caused subduction of the newly created weddell sea ocean floor beneath the southern antarctic peninsula east vergent folding and thrusting of jurassic back arc basin rocks along the southeastern antarctic peninsula known as the palmer land event probably resulted from deformation along this newly created convergent margin the presence of jurassic weddell sea floor would have isolated eastern gondwana east antarctica australia and india from the antarctic peninsula and western gondwana africa and south america encouraging increased marine faunal connections between the pacific and weddell sea mozambique somali ocean basins during the 175 to 130 ma interval after 130 ma the antarctic peninsula would have been connected to east antarctica via the other west antarctic terranes thereby facilitating land migrations between eastern and western gondwana

subduction zone earthquakes and stress in slabs the pattern of seismicity as a function of depth in the world and the orientation of stress axes of deep and intermediate earthquakes are explained using viscous fluid models of subducting slabs with a barrier in the mantle at 670 km 670 km is the depth of a seismic discontinuity and also the depth below which earthquakes do not occur the barrier in the models can be a viscosity increase of an order of magnitude or more or a chemical discontinuity where vertical velocity is zero longn versus depth wheren is the number of earthquakes shows 1 a linear decrease to about 250 300 km depth 2 a minimum near that depth and 3 an increase thereafter stress magnitude in a subducting slab versus depth for a wide variety of models shows the same pattern since there is some experimental evidence thatn is proportional toe where is a constant and is the stress magnitude the agreement is encouraging in addition the models predict down dip compression in the slab at depths below 400 km this has been observed in earlier studies of earthquake stress axes and we have confirmed it via a survey of events occurring since 1977 which have been analysed by moment tensor inversion at intermediate depths the models predict an approximate but not precise state of down dip tension when the slab is dipping the observations do not show an unambiguous state of down dip tension at intermediate depths but in the majority of regions the state of stress is decidedly closer to down dip tension than it is to down dip compression chemical discontinuities above 670 km or phase transitions with an elevation of the boundary in the slab predict when incorporated into the models stress peaks which are not mirrored in the profile of seismicity versus depth models with an asthenosphere and mesosphere of appropriate viscosity can not only explain the state of stress observed in double benioff zones but also yield stress magnitude profiles consistent with observed seismicity models where a nonlinear rheology is used are qualitatively consistent with the linear models

microstructural based time dependent failure mechanism and its relation to geological background abstract the time dependent hydro mechanical behavior of rocks is important in assessing the long term stability of deep underground excavation sites meant for long term storage of radioactive waste while evaluating the manner in which damage growth progresses at an excavation site it is important to represent the failure criteria and microcrack evolution from the viewpoint of factors pertaining to microcrack geometry such as the crack density we performed creep tests on inada granite samples to examine the effects of water confining pressure and weathering on time dependent failure the crack density of the samples which was damaged at three characteristic stages of creep behavior i e primary secondary and tertiary creep was measured by means of a longitudinal wave velocity test the microcrack evolution and failure criteria are discussed on the basis of the crack tensor determined in the test the following conclusions were reached from the test results 1 the onset of tertiary creep corresponds to the peak stress under static loading conditions and the tertiary creep stage corresponds to the post failure region 2 at the onset of tertiary creep the crack density can be the failure criterion for time dependent failure and crack density can be estimated using the longitudinal wave velocity 3 the time to failure for weathered granitic rock is approximately ten times faster than that for intact granitic rock 4 the ratio of the mica group mineral in the granite may affect the time to failure a fact that offers insight into the time dependent stability of a rock mass on the field scale

a study on the time dependent microfracturing and strength of oshima granite abstract uniaxial compressive loading experiments where axial strain rate or dilatant volumetric strain rate was kept constant were carried out using a servo controlled hydraulic testing machine the rock studied was oshima granite in order to investigate the effect of the strain rate on the strength a theory based on the assumption of redistribution of the pre existing microcracks due to subcritical crack growth was examined results expected from the theory were well borne out by the observations the strength was expressed by power functions of both axial strain rate and dilatant strain rate the stress corrosion index of oshima granite appeared to be 31 3 which agreed well with the values reported previously when the axial strain rate was held constant the acoustic emission rate increased exponentially and the b value the exponent in the frequency magnitude relation of the acoustic emissions decreased with increasing applied stress when the dilatant volumetric strain rate was kept constant the acoustic emission rate and the b value remained constant and independent of the stress increase furthermore when the dilatant strain rate was held constant at a higher value the b value was constant at a lower level as the decrease in the b value indicates the higher dilatant strain rate the b value is one of the most useful key parameters in the fracture predictions

early tectonic extension between the agulhas bank and the falkland plateau due to the rotation of the lafonia microplate abstract along the southeast african continental margin a deep sedimentary basin and marginal ridge bordering the agulhas fracture zone formed coevally with south american counterparts underlying the falkland plateau extension in both the southern outeniqua and the falkland basin is related to large angle 100 rotation of the lafonia falkland islands block in middle late jurassic times during attempted propagation of the proto indian ocean across the active circum pacific margin of gondwanaland in the early cretaceous the jurassic microplate boundaries were partly reactivated and partly transected by a right lateral shear along the 1200 km long falkland agulhas transform fault preceded by substantial vorticity of the surrounding crustal blocks this boundary segment separating the south american and african plates was probably generated by complex geodynamic interactions between oceanic plate subduction continental to back arc rift propagation and the impingement of deep mantle plumes on the lithosphere

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

relation of fracture resistance to fabric for granitic rocks double torsion specimens of two granitic rocks were prepared in several directions with reference to microcracks fabric even for the same rock and at the same stress levels the observed crack velocities in two granitic rocks were dependent on both the propagation direction and the opening direction the maximum difference by several orders of magnitude was found for both rocks the highest crack velocity was observed when the subcritical crack was parallel to most of the preexisting cracks the maximum critical stress intensity factor was about twice as high as the minimum one in different directions an analysis for a thin plate having anisotropic elasticity under torsional load showed that the observed difference in the crack velocity and the critical stress intensity factor was not an error due to conventional equations derived on the assumption of isotropic elasticity but the true material s property as the preferred orientation of microcracks has been pointed out for many granitic rocks we can conclude that the anisotropic nature of the fracture resistance of the two granitic rocks used in this study was not exceptional a region of a transport limited velocity was not found for rocks even at the velocity of 10 2 m s that is almost equal to the theoretical limit of the stress corrosion cracking

conditions for the ignition of catastrophically erosive turbidity currents abstract the basic impediment to a clear understanding of eroding and depositing turbidity currents has been the lack of a proper formulation of bed sediment entrainment this problem is addressed herein a detailed analysis indicates that the bagnold criterion is necessary but insufficient for self sustaining turbidity currents the analysis reveals two possible equilibrium states a relatively low velocity ignitive state and a high velocity dense catastrophic state a stability analysis indicates that the ignitive state is unstable flows below it invariably die out and flows above it ignite i e accelerate and entrain sediment to the catastrophic state which is stable the ignitive state thus defines the criterion for a self sustaining turbidity current estimates of the catastrophic state suggest that it is highly erosive and competent to scour out submarine canyons

temporal variations in seismicity during quasi static and dynamic rock failure abstract a comprehensive model is presented which can explain temporal fluctuations in seismic b values in the period leading to mechanical failure in terms of the underlying physical processes of time varying applied stress and stress corrosion enhanced crack growth under conditions of constant strain rate the form of the b value anomaly in the period leading to failure depends on the form of the stress time relationship for the case where dynamic failure occurs at peak stress after a period of strain hardening the model predicts a single cusp like b value anomaly reaching a critically low value of 0 5 at failure for the physically most realistic case where dynamic failure is preceded by a period of precursory strain energy release during strain softening the model predicts two minima in the b value separated by a temporary maximum or inflection point these fluctuations in the b value are consistent with reported intermediate term and short term earthquake precursors separated by a period of seismic quiescence for the case of quasi static cataclastic flow the b value mirrors the stress and never falls to the critical value because there is no critical rupture new results from a series of controlled laboratory experiments are presented in which microseismic event rates and b value were monitored contemporaneously with stress time data for all variants of the stress time relationship recent field observations of temporal changes in seismicity rates and b value preceding major earthquakes are also reported both data sets exhibit b value anomalies which are consistent with the model predictions

caribbean tectonics and relative plate motions during the last century three different ways of interpreting the tectonic evolution of the gulf of mexico and the caribbean have been proposed taking into account the bailey willis school of a permanent pre jurassic deep sea basin the edward suess school of a subsided continental terrain and the alfred wegener school of continental separation the present investigation is concerned with an outline of an interpretation which follows that of pindell and dewey 1982 an attempt is made to point out ways in which the advanced hypotheses can be tested the fit of africa north america and south america is considered along with aspects of relative motion between north and south america since the early jurasic attention is given to a framework for reconstructing caribbean plate evolution the evolution of the caribbean the plate boundary zones of the northern and southern caribbean and the active deformation of the caribbean plate

fracture nucleation process in intact rocks abstract yukutake h 1992 fracture nucleation process in intact rocks in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 247 257 we produced tomographic images for cross sections of granite samples by measuring the elastic wave velocity along 20 ray paths at high sampling rates during triaxial compression tests the dynamic fracturing process just before ultimate fracture was investigated from the rapid changes in the associated tomographic images it was found that microcracks concentrated locally in several regions as axial load was increased and the velocity then recovered in some of the intensively microcracked regions 6 60 s before fracture subsequently the amplitude of the elastic waves through the most intensively microcracked region also recovered in a few cases the frequency of the elastic wave through the most intensively microcracked region became extremely high we consider that fracture nucleated in the intensively microcracked regions and infer that several macrocracks occur due to an abrupt coalescence of microcracks in the fracture nucleated region a crack length in the process zone for brittle fracture was evaluated from the stress drop observed just before fracture and the published value of the shear fracture energy for westerly granite we expect from the results of both tomography and stress drops that subsequent instability occurred when one of larger macrocracks formed by the coalescence of the macrocracks reached a critical length in the most intensively nucleated region