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

boninitic and tholeiitic dykes in the lewis hills mantle section of the bay of islands ophiolite implications for magmatism adjacent to a fracture zone in a back arc spreading environment a detailed integrated field petrographic and geochemical study of the springers hill area of the bay of islands ophiolite exposed in the lewis hills was undertaken to explain the anomalously high abundance of veins and dykes of chromitite orthopyroxenite and clinopyroxenite and their associated dunites hosted by a refractory harzburgite dunite mixture a geodynamic situation is presented which is constrained by previous studies requiring formation of the springers hill mantle section at a ridge fracture zone intersection and the whole of the bay of islands ophiolite within a back arc spreading environment the veins and dykes formed during magmatism at the ridge fracture zone intersection and along the fracture zone as progressively hotter more fertile richer in clinopyroxene asthenosphere ascended and was channelled up and along the fracture zone wall shallow melting of refractory harzburgite in the presence of subduction derived hydrous fluids produced light rare earth element lree enric

quasi hydrodynamic lubrication effect of clay particles on sand grain erosion minor changes in the mass physical properties of submerged sand beds can have significant consequences relative to bed stability against erosion to examine the effect of small amounts of clay sized particles in bed pore water on the critical shear stress c for the erosion of sand grains flume experiments were carried out on the erosion of quartz sand beds impregnated with clay particles starting with no clay as the clay mass fraction was increased c was found to decrease below the value for pure sand co at m then reverted to co at r and continued to increase above co as was increased further post experimental analysis suggests that r is the pore space filling fine sediment fraction above which sand erosion is significantly influenced by clay in the range of m slider bearing type lubrication due to the viscosity of the clay laden interstitial fluid appears to govern the dependence of c on mimicking petroff s law of thick film lubrication when m as decreases lubrication is increasingly curtailed by grain asperities and c reverts ultimately to co at 0 an equation relating c to is proposed in analogy with the quasi hydrodynamic stribeck function for lubrication the observed effect of clay particles appears to be significant enough to require its consideration in coastal and estuarine sediment transport modeling it may also be a factor in the estimation of bed stability when biological activity in the benthic boundary layer introduces fine particles in clean sand beds

reconstruction of the geometry of accretion during formation of the bay of islands ophiolite complex two types of highly allochthonous ophiolitic terrains crop out adjacent to one another in the bay of islands region of southwestern newfoundland canada the first called the coastal complex cc is exposed as a long linear belt along the west coast it consists of a highly deformed ophiolitic assemblage interpreted as oceanic crust and upper mantle with a previous transform fault tectonic history the second called the bay of islands complex boic crops out to the east and consists of a series of four relatively undeformed ophiolitic massifs interpreted as oceanic crust and upper mantle with a more normal seafloor spreading tectonic history an autochthonous relationship can be demonstrated between the two types of ophiolitic terrains in the southernmost of these massifs indicating that each evolved along a single ridge transform system the internal structure of both the boic and the cc is consistent with the above interpretations the ne sw structural trends in the cc transform are oriented approximately parallel to the sea floor spreading direction inferred from the average orientation of the sheeted diabase dikes within the boic one way chilling statistics from the sheeted diabase dike unit growth directions of dendritic crystals along inclined comb layers in the plutonic section and the sense of shear along the cc transform all indicate that the boic was accreted on the ne side of a nw se trending spreading center present geographic reference frame juxtaposition of the boic with the ne sw trending cc occurred at a ridge transform intersection the transform exhibited a left lateral offset magma chamber profiles have been identified in the layered gabbroic sections of the boic by using the patterns outlined by the attitudes of fine scale igneous layering these patterns depict arcuate structures which appear to define roughly a cross sectional view of a systematically oriented half cylinder the long axis of which is oriented parallel with the plane of the sheeted dikes and major lithologic unit contacts thus we believe that a large cylindrical shaped crustal level subaxial magma chamber extended in a direction parallel with the ridge axial plane at the time of formation of the bolc the cc marks the abrupt truncation of this long linear magma chamber petrologic constraints demonstrate that the ultramafic and transition zone cumulates which form the base of the plutonic section crystallized at a range of high to intermediate pressures possibly along a long narrow conduit extending to 30 km depth beneath the crustal level magma chamber after formation these cumulates were deformed at high temperatures in the zone of mantle upwelling and divergence then tectonically transported to crustal or near crustal levels to form the basement upon which the remainder of the ophiolite stratigraphy was accreted a geometric and tectonic model is presented for the accretion process responsible for formation of the bay of islands ophiolite

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

global positioning system measurements of present day crustal movements in the arabia africa eurasia plate collision zone we present and interpret global positioning system gps measurements of crustal motions for the period 1988 1994 at 54 sites extending east west from the caucasus mountains of southern russia georgia and armenia to the aegean coast of turkey and north south from the southern edge of the eurasian plate pontus block to the northern edge of the arabian platform viewed from a eurasia fixed reference frame sites on the northern arabian platform move n38 13 w at 20 3 mm yr roughly consistent with the velocity implied by nuvel 1a circuit closure n23 7 w at 24 2 mm yr the motion of arabia appears to be transferred directly to the region of turkey north of the suture however eastern turkey is characterized by distributed deformation while central western turkey is characterized by coherent plate motion involving westward displacement and counterclockwise rotation of the anatolian plate internal deformation within the central part of the anatolian plate is less than 2 mm yr the anatolian plate is decoupled from eurasia along the right lateral strike slip north anatolian fault naf this different response in eastern and western turkey to the collision of arabia may result from the different boundary conditions the hellenic arc forming a free boundary to the west and the asian continent and oceanic lithosphere of the black and caspian seas forming a resistant boundary to the north and east we derive a best fitting euler vector for anatolia eurasia motion of 29 2 0 8 n 32 9 0 4 1 3 0 1 m y the mapped surface trace of the naf corresponds well to a small circle about this pole the new euler vector implies an upper bound for naf slip rate of 30 2 mm yr i e assuming all relative motion is accommodated along the naf using the nuvel 1a euler vector for arabia eurasia and the gps euler vector for anatolia eurasia we determine an arabia anatolia euler vector of 31 2 n 45 2 e 0 9 0 1 m y and an upper bound on the east anatolian fault slip rate of 15 3 mm yr the aegean trough region of western turkey deviates significantly from coherent plate rotation in addition to rotating with anatolia this region shows roughly n s extension at a rate of 14 5 mm yr taken together with satellite laser ranging results along the hellenic arc the contemporary pattern of deformation indicates increasing motions toward the arc suggesting that the westward displacement and counterclockwise rotation of anatolia is driven both by pushing from the arabian plate and by pulling or basal drag associated with the foundering african plate along the hellenic subduction zone

the role of strain heating in the evolution of megathrusts on the basis of elementary energy considerations and the nonlinear temperature dependence of experimental flow laws a model is developed for the interplay of thermal and mechanical processes along megathrust systems the model is based on the observation that thrust systems involving large portions of the lithosphere have a two part history during the early history of movement dry lower crustal upper mantle rocks are deformed in a shear zone where because of the nonlinear effect of temperature dependent diffusion strain heating rates are directly controlled by the initial thermal state that is heating is insignificant if initial temperatures are high but where temperatures are low large temperature increases are possible as a result strain heating acts as a thermal buffer such that upon arrival of upper crustal rocks at depths of 20 km or more the temperatures along the thrust would be controlled largely by the rheology of hanging wall rocks rather than the initial thermal state moreover the relatively high melting temperatures for dry lower crustal upper mantle rocks implies that the temperatures achieved along the thrust should be relatively high as thrusting progresses this early history is successively eliminated as wet upper crustal rocks are emplaced beneath the preheated hanging wall the granitic melts that characterize many natural megathrust systems are probably generated when these wet metasedimentary rocks move beneath the hanging wall either by hydrous fluxing of the hot hanging wall or by melting in the highest grade footwall rocks rapid uplift implied by both metamorphic studies and preservation of the inverted metamorphic sequence is interpreted as a late phase event created by tectonic pileup beneath the thrust thorough testing of the model will require thorough assessment of the time dependent problem however one dimensional steady state models together with energy considerations indicate that the general model is allowable the concept of thermal buffering by strain heating is important because it implies that megathrust systems are a natural system where metamorphic temperatures are controlled by rock rheology hence they represent real world systems where a thermal and mechanical system are intimately linked this rationale is used to extend the model by indicating a method in which metamorphic conditions along exhumed megathrusts might be used as a natural constraint on experimentally determined flow laws

kinematic model of active deformation in central asia the velocity field of present day deformation in central asia is modelled using a set of four rotating blocks siberia tarim tibet india on a spherical earth a best fit is inverted on the basis of estimated shortening rates across the main thrust zones himalayas tien shan and measured slip rates along the principal strike slip faults altyn tagh and karakorum separating those blocks the fit to the data implies that nearly all the present convergence between india and asia can be accounted for by slip partitioning on these four zones with as much as 50 absorbed by northeastwards extrusion of tibet this suggests that localised deformation governs the present mechanical behaviour of the central asian lithosphere

mantle crust transition zone and origin of wehrlitic magmas evidence from the oman ophiolite abstract abundant exposure and well preserved outcrops in the oman ophiolite allow detailed observation of the transition from mantle to crust in an oceanic environment the mantle crust transition zone shows large lateral variations in thickness and composition but consists essentially of residual dunites and a magmatic component present as clinopyroxene and plagioclase impregnations and gabbro lenses compositionally the transition zone marks a gradational passage from mantle to crust the moho may be placed at the base of the continuous gabbroic crustal section structures both within the transition zone and near the base of the crust are nearly parallel to the moho however the dominant flow mechanisms differ from the upper mantle to the lower crust plastic flow dominates in the upper mantle while viscous flow resulting in strong magmatic fabrics dominates at crustal levels wehrlitic bodies within the crustal gabbro section are intruded from the transition zone into which they are rooted the parent magma of the wehrlitic instrusions is in fact a crystal melt mixture composed of olivine and chromite probably including a component of mantle xenocrysts and xenoliths and of reequilibrated basaltic melt

microstructures and lattice fabrics in the hilti mantle section oman ophiolite evidence for shear localization and melt weakening in the crust mantle transition zone 1 experimental studies have shown that olivine aggregates with 4 melt are significantly weaker than melt free aggregates however questions remain as to the importance of melt weakening in nature in several studies melt weakening has been invoked to explain patterns of mantle flow in the oman ophiolite in this paper we reinvestigate evidence for melt weakening in the hilti mantle section using structural and microstructural methods the average olivine grain size increases with depth below the crust mantle boundary this is related to a change from equigranular to porphyroclastic microstructures at shallow levels to coarse porphyroclastic microstructures at depth a strong foliation and high degree of recrystallization in the upper part of the mantle section are interpreted as the strong imprint of localized deformation at stresses of 4 10 mpa lattice orientation data show that the high strain peridotites have recorded top to the west shear reversed shear senses were found deeper in the section since there is petrographic evidence for melt in the high strain zone strain localization was probably caused by a melt related weakening mechanism high melt contents required for melt weakening suggest that melt accumulated just below the crust mantle boundary we conclude that melt weakening is probably related to enhancement of grain boundary rather than intragranular deformation processes effective viscosities 2 1016 pa s may locally exist in the uppermost mantle beneath ridges if melt weakening occurs although our results agree with those of previous studies in the hilti massif we conclude that both the previously published active flow model and a ridge compression model can account for them

generation and obduction of ophiolites constraints from the bay of islands complex western newfoundland constraints provided by the bay of islands ophiolite complex boic western newfoundland suggest that its generation as well as its obduction is related to collision of the dunnage zone island arc system with the irregular continental margin of eastern north america the constraints include the supra subduction zone setting of the boic its extensional environment of generation the synchronous occurrence of ophiolite generation with initiation of drowning and deformation of the north american continental margin and the restriction of the boic as well as other appalachian ophiolites and associated taconian allochthons to reentrants in the north american margin during the early phases of collision of the dunnage arc with the north american margin subduction of old oceanic lithosphere trapped in reentrants will continue and allow the development of two potential sites of ophiolite generation the first develops through trench rollback leading to ophiolite generation in the overriding arc plate outboard of the reentrant whereas the second develops in a transtensional environment with spreading centers offset by strike slip faults which propagate through the overriding plate away from the indenter promontory the configuration of the boic spreading ridge normal to the continental margin and the direction of initial obduction parallel to the margin favor generation of the ophiolite in an overall strike slip regime with complete subduction of the oceanic remnant extension in the overriding plate ceases and the buoyant newly formed oceanic lithosphere is obducted onto the continental margin diachronous collision of the arc with the irregular continental margin accounts for the range of ages for ophiolite generation and obduction observed within the dunnage zone and for termination of arc volcanism along the north american margin

origin of complex upper mantle structures in the southern lewis hills bay of islands ophiolite newfoundland the springers hill area lewis hills bay of islands ophiolite may represent oceanic lithosphere created in close proximity to the nontransform segment of an oceanic fracture zone upper mantle rocks exposed on springers hill were investigated to establish whether their development was affected by the thermal and rheological changes associated with oceanic fracture zones harzburgites of the springers hill area reveal complex structural patterns on a small scale foliations defined by orthopyroxene grains intersect foliations defined by spinel grains at various angles olivine petrofabric work demonstrates that only the spinel foliation is related to the preserved flow plane the orthopyroxene foliation appears to be the result of pull apart of formerly larger grains during high temperature deformation on the larger scale orientation patterns of foliation lineation and dykes suggest that strike slip movement occurred parallel and at high angle to the fracture zone contact at various stages of a com

evidence for dynamic coupling of surface processes with isostatic compensation in the lower crust during active extension of western turkey western turkey in the eastern part of the aegean region is extending slowly with extension no faster than 0 5 mm yr 1 across any individual active normal fault up to 15 km of southward extension has occurred in the most active 100 km wide part of this region since 15 ma indicating a typical strain rate of 3 10 16 s 1 rivers carry a sediment flux of 4 106 m3 yr 1 westward to the aegean sea coast with offshore sedimentation at 1 mm yr 1 sediment loading causes coastal areas to subside much faster than would be expected from the extension alone most of this sediment is derived from regional erosion of inland areas to the east at a typical rate of 0 1 mm yr 1 this erosion being isostatically compensated by uplift it is suggested that the lower crustal material displaced by sediment loading near the coast flows east to isostatically compensate the erosion the average eastward velocity of this lower crustal channel flow beneath the aegean coast is estimated as 1 mm yr 1 the maximum velocity is 2 mm yr 1 this case study thus indicates the general point that isostatically compensated erosion and lower crustal channel flow can affect the evolution of sedimentary basins that are surrounded by eroding landmasses factors that are often neglected

basal lherzolites in the bay of island ophiolite origin by detachment related telescoping of a ridge parallel melting gradient decompression melting under a spreading centre should generate a residual upper mantle section with a weak vertical gradient in the degree of chemical depletion chemical data from the 6 km thick ophiolitic mantle section of the table mountain massif bay of islands ophiolite newfoundland reveal however a depletion gradient of 1 melting 100 m in the basal mantle section of the massif within the framework of a segmented ridge we suggest that during ophiolite detachment intermittent accretion of progressively less depleted peridotites occurred to the former base of the ophiolite amounting to a several hundred metre thick lherzolitic sole in this view the basal mantle section in table mountain extends a structural concept previously established for the metamorphic sole both metamorphic gradient in the sole and melting gradient in the basal mantle rocks are apparent and of tectonic nature like the sole the basal mantle sequence may thus expose mantle material derived from a scale larger than the preserved ophiolite complex

horizontal crustal motion in the central and eastern mediterranean inferred from satellite laser ranging measurements four campaigns to acquire satellite laser ranging slr measurements at sites in the mediterranean region have been completed these measurements to the lageos satellite made largely by mobile systems cover a time span beginning in november 1985 and ending in june 1993 the range data from 18 sites in the central and eastern mediterranean have been simultaneously analyzed with data acquired by the remainder of the global laser tracking network estimates of horizontal motion were placed into a regional northern europe fixed kinematic reference frame uncertainties are on the order of 5 mm yr for sites having at least four occupations by mobile systems and approach 1 mm yr for permanently located sites with long histories of tracking the resulting relative motion between sites in the aegean exhibit characteristics of broadly distributed pattern of radial extension but at rates that are about 50 larger than those implied from studies of seismic strain rates based on seismicity of magnitude 6 or greater or across the region the motion estimated for sites in turkey exhibit velocity components associated with the westward motion of the anatolian block relative to eurasia these results provide a present day snapshot of ongoing deformational processes as experienced by the locations occupiedmore by slr systems less

seismicity and tectonic deformation rate in soviet armenia implications for local earthquake hazard and evolution of adjacent regions the spitak armenia earthquake of december 7 1988 ms 6 7 mo 10 1018 nm was the largest this century in soviet transcaucasia in the northern part of the zone where the arabian and eurasian plates converge at 30 mm yr 1 field observations in its epicentral area and information from historical and recent earthquakes in and around armenia are used to determine shortening rates and hence to investigate how shortening is partitioned between the great caucasus armenia and other parts of soviet transcaucasia and eastern turkey and nw iran farther south this earthquake produced more than 20 km of surface faulting with up to 1 5 m reverse slip and 0 4 m right lateral slip on a fault with wnw strike and southward slip vector azimuth much of the local topography has formed by slip on faults with similar orientation there is no evidence of substantial recent folding elsewhere in armenia suggesting that shortening in armenia occurs mainly by slip on these faults shortening rate across armenia is 1 mm yr 1 at strain rate 2 10 16 s 1 with likely slip rate on each fault 0 2 mm yr 1 the local topography has formed in 1 myr shortening rate in the eastern great caucasus farther north can be estimated using quaternary uplift rate assuming uplift is caused by crustal thickening and occurs in isostatic equilibrium although quaternary uplift rates from 0 05 to 10 mm yr 1 have been previously suggested any rate above 1 mm yr 1 is too rapid to be explained by this process the remaining rates range from 0 05 to 0 2 mm yr 1 and imply 1 to 5 mm yr 1 local shortening rate suggesting overall shortening rate across the soviet part of the convergent zone is only 2 to 6 mm yr 1 historical seismicity suggests that at least 1 mm yr 1 of this shortening is coseismic most of the convergence between arabia and eurasia is thus taken up at present farther south in eastern turkey and nw iran along with probable substantial westward expulsion of crustal material and most of the topography and crustal thickness in the great caucasus probably already existed at least 10 myr ago

the east anatolian fault an oblique collisional belt abstract the tectonic setting for the area around the arabia africa anatolia triple junction is described from combined landsat spot satellite image analysis and field observations since the late miocene the motion along the east anatolian fault generated major structures corresponding to shortening with a sinistral slip component the 150 km wide area of the triple junction is highly deformed by folding and thrusting distributed mainly on the border of the anatolian block in addition kinematic reconstructions confirm that the arabia anatolia relative motion is both sinistral and convergent the sinistral strike slip faults of the east anatolian fault zone are of second order and local consequences of the n s arabia anatolia collision

cumulate recycling and crustal evolution in the bay of islands ophiolite the lower crustal section of the north arm mountain massif of the bay of islands ophiolite is composed of interlayered gabbroic and ultramafic cumulates despite pervasive ridge related deformation key contact relationships are preserved in low strain domains or late kinematic intrusions angular and crenulated gabbroic enclaves in ultramafic rocks and ultramafic dikes in gabbroic rocks imply that many of the ultramafic rocks intruded into older gabbroic hosts olivine gabbro gabbro gabbro norite norite and fe oxide amphibole gabbros the ultramafic intrusions are composed of dunite wehrlite harzburgite lherzolite and olivine websterite clino to ortho pyroxenite reaction rims line many peridotitic gabbroic contacts the voluminous pyroxenites of the lower crust are petrologically identical to in situ pyroxenite reaction rims and are thought to represent partially digested gabbros fine grained harzburgitic sills crosscut the foliation of wehrlitic host rocks with orthopyroxenite reaction rims

southeastern lewis hills bay of islands ophiolite geology of a deeply eroded inside corner ridge transform intersection the lewis hills massif bay of islands ophiolite newfoundland preserves a deeply eroded transform fault a major low angle extensional mylonite zone is next to the transform assemblage and probably merges laterally into it the mylonite developed in a high temperature environment and occurs at the top of relatively rigid lithospheric mantle the lithospheric mantle represents exhumed older arc type basement of the little port complex that is also exposed across the transform in the western lewis hills it differs dramatically from an asthenospheric mantle unit of the bay of islands complex exposed in the eastern lewis hills that formed in a spreading ridge environment the fossil ridge transform segment and associated low angle normal fault in the lewis hills formed at an inside corner structural setting similar to recently discovered core complexes at oceanic ridge transform intersections however the lewis hills segment developed in a setting where a spreading center propagated across a transform margin and rifted older arc type lithosphere factors that contributed to formation of the low angle detachment fault are the lithospheric nature of the mantle basement rheological weakening and strain focusing by intrusive sills and weak lateral coupling realized by the extensional transform assemblage

instability and surge development in debris flows 1 debris flows are often described as a succession of surges which are characterized by enhanced peak depth and velocity and therefore by a tremendous increase of their destructive power for given characteristics of the base flow if the channel is sufficiently long to allow an appreciable wave development the linear stability analysis in shallow streams is shown to provide a reasonable prediction of the critical flow condition and of the instability growth rate the one dimensional 1 d theory however does not allow the determination of the wave period of the fastest growing perturbations debris waves most frequently develop following a mechanism similar to water roll waves instabilities grow up becoming clearly distinguishable waves and then waves overtake one another with increasing wave period and amplitude the typical hydrograph of a multiple peak event is shown to be composed of a first surge which is usually characterized by the highest depth the longest duration the greatest erosive power and the most symmetrical shape and of secondary waves that burst on the flow tail in the recession phase the characteristics of the first surge can be explained by two different mechanisms all waves that rise up near the flood crest run faster than this first surge and coalesce into it causing its high depth and great volume moreover segregation during the flow induces the concentration of boulders at the fronts contributing to its depth enhancement erosive power and symmetrical shape when a debris surge impacts a structure the force pattern can be interpreted as the superposition of the reflection of the bouldery front and the formation of a vertical muddy jet due to the impact of the front wedge wave reflection can be described by a 1 d mass and momentum balance across the front whereas the pressure impulse due to the incompressibility of the interstitial fluid can be analyzed through inviscid formulations validated for the representation of tsunami forces

magma chamber profiles from the bay of islands ophiolite complex models of the oceanic lithosphere derived from constraints imposed by the geology of ophiolite complexes require a magma chamber to produce the cumulate and other plutonic portions of the oceanic crust and upper mantle evidence from the bay of islands ophiolite complex indicates that layered plutonic rocks have not formed solely due to differential crystal settling to the horizontal floor of a magma chamber but have formed largely by heterogeneous nucleation and in situ crystallization along gently inclined as well as steeply inclined bounding surfaces of a large magma reservoir coherent well defined and systematically oriented large scale arcuate patterns described by mesoscopic igneous layering have been identified in the plutonic sections of two ophiolite massifs in the bay of islands complex these layering patterns seem to outline the shape of a large continuously evolving steady state magma chamber that extended along a ridge axis and ended at a transform fault intersection

instability of concentrated free surface flows unexpected surface level and discharge oscillations have been reported in tributaries of the yellow river in china the oscillations typically occur during the declining part of long period flood waves when the flows carry extremely high concentrations of very fine sediment the oscillations reportedly cannot be explained by analysis techniques developed for flows of clear water an analysis is presented in which the flowing material is modeled as a bingham plastic the analysis is based on the depth integrated mass and momentum equations with a simplified representation of boundary friction with reasonable values of the parameters involved the analysis indicates that the existing qualitative observations of surface and discharge oscillations can be explained by a linear instability to surface waves although based on a bingham plastic model the qualitative conclusions can be extended to more general types of constitutive behavior

present day kinematics of the middle east and eastern mediterranean the middle east and eastern mediterranean regions contain three major strike slip fault zones the dead sea fault zone dsfz and the north and east anatolian fault zones nafz and eafz these accommodate northward motion of the african and arabian plates relative to eurasia and westward motion of the small turkish plate this study deduces an internally consistent solution for the present day kinematics of these plates which is accurate to better than 20 and applicable throughout time since 5 ma by constraining the slip senses and rates on these strike slip fault zones the 7 0 5 mm yr 1 left lateral slip rate on the dsfz is associated with counterclockwise rotation of arabia relative to africa at 0 33 m y 1 around an eulcr pole near 33 n 23 e the nafz takes up counterclockwise rotation of the turkish plate relative to eurasia at 0 83 m y 1 13 around a pole at 31 n 35 5 e with right lateral slip rate 15 mm yr 1 the resulting prediction for the eafz involves 14 mm yr 1 left lateral motion and 2 mm yr 1 of shortening between arabia and turkey the triple junction of africa arabia and turkey is near the city of kahramanmaras this locality contains a narrow promontory bounded by ssw trending left lateral faults at the edges of the arabian and turkish plates which is loosely attached to the african plate moving westward relative to its interior at 4 mm yr 1

discussion of depth averaged shear stress and velocity in open channel flows by shu qing yang the discussers thank the author for writing an informative and thought provoking paper predicting the lateral distribution of boundary shear stress in open channel flows is an intractable issue and any theoretical work in this area is to be welcomed the purpose of this discussion is twofold first to compare the analysis developed by the author to experimental data and second to seek clarification on several areas of potential confusion which may be due to typographical errors

evidence for variations in magma production along oceanic spreading centers a critical appraisal recent geologic geophysical and geochemical studies of the oceanic lithosphere near fracture zones have resulted in the proposal that the magma budget defined as the amount of magma delivered to magma chambers or conduits beneath a spreading center for a given amount of spreading decreases as fracture zones are approached seismic reflection and refraction studies indicate that the crust becomes anomalously thin near fracture zones but reference must be made to the boundary between residual upper mantle peridotites and overlying cumulate rocks in order to assess potential variations in the magma budget the position of this interface however generally is not constrained by geophysical studies geochemical variations in basaltic glasses collected near fracture zones are consistent with a decrease in partial melting as fracture zones are approached but they could also be produced by variations in open system mag matic processes with no change in the extent of partial melting although a decrease in the magma budget as fracture zones are approached is consistent with these data so are alternative models that incorporate a constant magma budget

instability of hyperconcentrated flow the instability of hyperconcentrated flow observed in nature is reproducible in laboratory flumes using suspensions of bentonite similar instabilities are observed in a much simpler hydraulic system consisting of a container feeding a closed conduit the behavior of two different clay suspensions kaoline and bentonite was investigated and the rheological properties were measured in a rotating viscosimeter the result of the analysis is that the instability is associated with some specific rheological properties by a simple mathematical model it has been explained why the bentonite suspension was unstable at small discharges and stable at sufficiently large discharges the kaoline suspensions were always stable the agreement between calculations and observations is encouraging

double thresholds in scour fill processes and some implications in channel adjustment abstract through an analysis of data collected from the yellow river and its tributaries on the loess plateau of china the phenomenon of double thresholds in scour fill processes of wide range water sediment two phase flows has been shown thresholds located in non hyperconcentrated flows may be called the lower threshold and that in hyperconcentrated flows the upper threshold this double threshold phenomenon leads to complicated sediment transport behavior of heavily sediment laden rivers with an increase in suspended sediment concentration the channel sediment delivery ratio increases initially and becomes higher than 1 followed by a decrease and finally becomes lower than 1 again controlled by the double thresholds in the scour fill processes channel adjustment of the lower yellow river is non linear and complex when the suspended concentrations were lower than the lower threshold or higher than the upper threshold scour or bed downcutting was the dominant channel forming process channel shape tends to be narrower and deeper and the channel thalweg became more sinuous when the suspended concentrations lay between the lower and upper thresholds deposition of sediment was the dominant channel forming process channel shape tended to be shallower and wider and channel thalweg became less sinuous

structure and petrology of peridotites clues to their geodynamic environment the first part of this paper is a review on the main mineralogical petrological and structural criteria which can be used to unravel the history of the various types of mantle peridotites met at the earth s surface these criteria are applied in the second part to show that most peridotites share a common history which is that of an asthenospheric uprise followed by specific fates corresponding to their lithospheric history the unifying concept is that of adiabatic ascent rate related to rifting and spreading rates rates lower than 0 5 cm yr in continental graben correspond to departure from adiabatic conditions at depths greater than 30 km this inhibits further melt extraction with as consequences a limited melt extraction and comparatively fertile spinel lherzolites as residue higher rates probably not exceeding 1 cm yr correspond to oceanic rifts with the main melt extraction completed around 15 km generating a thin oceanic crust and plagioclase lherzolites as residue finally rates greater than 1 2 cm yr correspond to oceanic ridges with melt being extracted at moho depth thus generating a 6 km thick crust and leaving depleted harzburgites as residue thus examination of the peridotite type and associated crustal formations give some clue to trace back the environment of origin this conclusion must however be tempered by the fact that the spreading rate envisioned here as the principal controlling parameter can change with space and time in a given environment mantle diapirism rifting seafloor spreading crust generation

the tectonic expression slab pull at continental convergent boundaries examination of five thrust belt systems developed at continental subduction boundaries suggests that they comprise two distinct groups that display pronounced and systematic differences in structural style topographic elevation denudation metamorphism postcollisional convergence and foredeep basin geometry and facies the distinctive geological features developed within each thrust belt group appear to be causally linked to the relative rates of subduction and convergence via the magnitude of horizontal compressional stress transmitted across the subduction boundary at subduction boundaries where the rate of overall plate convergence is less than the rate of subduction termed here retreating subduction boundaries the transmission of horizontal compressive stress across the plate boundary is small and regional deformation of the overriding plate is by horizontal extension the tectonic expression of these retreating subduction boundaries includes topographically low mountains little erosion or denudation low grade to no metamorphism little to no involvement of crystalline basement in shortening little to no postcollisional convergence anomalously deep foredeep basins and a protracted history of flysch deposition within the adjacent foredeep basin analysis of deflection and gravity data across three retreating subduction boundaries apennine carpathian and hellenic systems shows that subduction is driven by gravitational forces acting on dense subducted slabs at depths between about 40 and 80 km carpathians 50 and 150 km apennines and 50 and 250 km hellenides the total mass anomalies represented by the slabs are approximately 3 1012 6 1012 and 12 1012 n m respectively the slabs are partially supported by flexural stresses transmitted through the subducted lithosphere to the foreland and partially supported by dynamic viscous stresses in the asthenosphere at subduction boundaries where the rate of overall plate convergence is greater than the rate of subduction termed here advancing subduction boundaries the transmission of horizontal compressive stress across the plate boundary is large and regional deformation of the overriding plate is by horizontal shortening the tectonic expression of these advancing subduction boundaries includes topographically high mountains antithetic thrust belts large amounts of erosion and denudation exposure of high grade metamorphic rocks at the surface extensive deformation of crystalline basement to midcrustal depths protracted postcollisional convergence tens of millions of years and a protracted history of molasse deposition within the adjacent foredeep basins analysis of gravity and deflection data across two advancing subduction boundaries developed within the continental lithosphere western to eastern and southern alps and himalayas shows that the thrust sheets have been translated for great distances over the foreland lithosphere relative to the point at which the subduction forces are applied thus obscuring any flexural and gravity signals from the subducted slab however it appears that far field stresses presumably related to global plate motions drive most of the convergent motion across these subduction boundaries the concept that orogenic belts formed above retreating subduction boundaries have recognizable tectonic signatures that differ from those of orogenic belts formed above advancing subduction boundaries suggests that it may be possible to interpret the plate boundary settings in which ancient orogenic belts evolved appendix b is available with entire article on microfiche order from the american geophysical union 2000 floridaavenue n w washington d c 20009 document t92 004 2 50 payment must accompany order

bay of islands and little port complexes revisited age geochemical and isotopic evidence confirm suprasubduction zone origin the bay of islands complex of the humber arm allochthon west newfoundland contains the best exposed ophiolite in the appalachian orogen associated structural slices the little port and skinner cove complexes also contain rocks formed in an oceanic domain although their relationship to the bay of islands complex remains controversial to constrain the origin of the complexes and obtain a better understanding of the geology of the humber arm allochthon we have undertaken an integrated geochronological geochemical and isotopic study a u pb zircon age of ma for the little port complex and a zircon and baddeleyite age of 484 5 ma for the bay of islands complex have been obtained geochemical and isotopic data on trondhjemitic rocks from the two complexes indicate that petrogenetic models for these rocks must account for fundamental differences in source materials and mineralogy during differentiation the little port complex trondhjemites are characterized by initial end of 1 to 1 whereas those

obduction processes in ancient modern and future ophiolites geologic and tectonic processes of ophiolite obduction deduced from structural mapping timing of deformation and metamorphism and palinspastic reconstruction of ancient ophiolite complexes compare closely with modern processes operative in the western pacific many ongoing arguments about the origin of ophiolite complexes oceanic island arc or marginal basin and their emplacement subduction related or not are largely semantic and depend on exactly what individuals regard as island arcs marginal basins or subduction zones a review of tectonic processes currently active in the western pacific region shows a wide range in size shape and duration of arcbasin complexes and some of these show uniformitarian models similar to those proposed for emplacement of ancient ophiolites the tethyan ophiolites of the semail oman mountains and spontang western himalaya of ladakh zanskar and the ordovician western newfoundland ophiolites are compared with recent examples of the mariana basin yap trench banda arc luzon arc and papua new guinea it is concluded that they are all connected in some way with marginal basins and island arcs in varying stages of development and maturity emplacement of these ophiolites is related to continental underthrusting of a trapped marginal basin and accretionary prism comparable to present day examples of the n australian margin underthrusting timor and the banda sea and the se china margin underthrusting the luzon arc

transport of bed sediment in clay suspensions the applicability of einstein s and colby s sediment transport formulas is tested with flume data of total bed sediment discharge in clay suspensions the contact bed sediment discharge calculated using einstein s formula remains practically unchanged after correction for the fluid viscosity and density of fine sediment mixtures the increased total bed sediment discharge in hyperconcentrated flows is therefore attributable to the suspended sediment discharge the results of colby s method are similar to those of einstein without correction

the horizontal velocity field in the deforming aegean sea region determined from the moment tensors of earthquakes we use the spatial distribution of seismic moment tensors of earthquakes in the aegean region over the time interval 1909 1983 to recover a continuous horizontal velocity field that describes the overall deformation of the lithosphere at large length scales the calculated velocity field is dominated by two effects 1 an e w right lateral shear of the eastern aegean related to motion on the north anatolian fault becoming distributed as it enters the aegean and 2 a n s extension probably related to the sinking of the slab in the hellenic trench the southern part of the central aegean is found to be moving in a sw direction relative to europe at a rate of about 30 mm yr probably a lower bound with an error of around 10 mm yr and rotating clockwise in the seismogenic upper crust this velocity field is accommodated by right lateral strike slip faults in the eastern aegean and by normal faults that rotate clockwise in central greece a comparison of paleomagnetic declination data with the expected rotation of rigid elongate inclusions in the velocity field shows in most places an agreement in sense and approximate agreement in rate of rotation expected rotation rates of line elements are sometimes too low probably because our derived velocity field is smoothed and unable to match locally high strain rates there is only one part of the region where line elements are predicted to rotate in either clockwise or counterclockwise directions depending on their orientation this is in western turkey which coincident ally is the only place where paleomagnetic rotations in both directions have been observed this coincidence in particular suggests to us that the analogy of rigid elongate inclusions in the velocity field responding to forces on their bases may be useful in predicting the senses and approximate rates of rotation of crustal blocks in deforming continental regions the velocity field we obtain preserves the strike directions of the major faults as directions of zero length change in spite of considerable smoothing we use this observation to speculate that the interaction between the upper crust and the rest of the lithosphere beneath it may involve an interplay of effects on one hand the variation of strength with direction in the crust may control the strike directions of faults that form or become reactivated and may also limit the velocity fields that are allowable on the other hand the fault bounded blocks may rotate in the velocity field in response to forces on their bases which would require the velocity field to change with time if the directions of zero length change are fixed to the blocks

the role of deformation in the formation of metamorphic gradients ridge subduction beneath the oman ophiolite two tectonic scenarios have been proposed for genesis and emplacement of the oman ophiolite one suggests that the ophiolite was generated at a spreading center the other suggests generation within an intraoceanic arc an integrated thermal and kinematic model of the temperature stress rock type and displacement fields during early stages of the emplacement of the oman ophiolite was developed to test these two possibilities the thermal evolution was calculated by a finite difference algorithm for heat conduction considering heats of metamorphic reactions deformational heating heat advection by flowing rock mantle heat flow and radioactive heating the stress and displacement fields were calculated by an analytical model using a velocity boundary condition power law constitutive relations and a brittle frictional sliding relationship field observations in oman can be satisfied with the spreading center model but not with the arc model moreover simulations indicate that the ophiolite was probably 2 m y old at the time of intraoceanic thrusting and that shear stresses of 100 mpa were attained during thrusting in addition to satisfying currently available field constraints these simulations also indicate specific further work that will help resolve the controversy over thevolution of this unique orogen future work should be directed toward 1 using thermobarometry and thermochronology to constrain the spatial variation in ptt paths of the metamorphic sole 2 dating the igneous genesis of the ophiolite and 3 using quartzite and dunite recrystallized grain size paleopiezometers to infer stresses during thrusting these simulations point out the important effect synmetamorphic deformation can have on the formation of metamorphic field gradients in contractional fault zones metamorphic field gradients may be normal or inverted and contracted or extended relative to the initial thermobarometric gradient and adjacent fault zone rocks may display crossing pt paths unusually contracted or extended metamorphic field gradients may also develop in the footwalls of normal fault zones where portions of the heating hangingwall accrete to the uplifting cooling footwall

p t conditions during emplacement of the bay of islands ophiolite complex a high temperature contact is described between the basal pargasite bearing spinemherzolites of the bay of islands ophiolite complex and underlying garnet granulite facies metagabbros of its dynamothermal aureole three distinct high temperature hydrous assemblages occur in the basal mylonites of the peridotite and spinel and garnet bearing corona textures indicative of increase in pressure under constant or increasing temperature conditions are described for the first time from the uppermost part of the aureole on the basis of garnet clinopyroxene geothermometry and garnet forming reactions in metabasic rocks p t conditions of 7 11 kbar 750 850 c are estimated for rocks on both sides of the contact steep inverted gradients in both temperature and pressure of equilibration occur in the aureole which most likely represents a thinned overturned and metamorphosed section through an ophiolite sequence it is proposed that the aureole formed in a low angle shear zone cutting the oceanic crust and upper mantle age data shows that the bay of islands complex was 30 40 ma old and therefore relatively cold at the time of formation of the aureole prolonged 1 ma shear heating must therefore have occurred at high shear stresses and movement rates 1 kbar 10 cm yr to produce the high contact temperatures the displacement surface probably initiated as a discrete fault evolving into a viscous shear zone with time downward movement of the locus of shearing into weaker lithologies and finally thrusting of the ophiolite aureole complex over cold sediments accounts for the preservation of steep metamorphic gradients in the aureole the observed pressures at the ophiolite aureole contact are 3 7 kbar in excess of the expected load pressure from the present thickness of the ophiolite the cause of the pressure excess was removed before formation of lower grade parts of the aureole possible explanations are tectonic thinning of the ophiolite during displacement or more likely emplacement of nappes on top of the ophiolite before formation of the aureole a model involving detachment of the ophiolite slice from below a subduction zone can account for the high pressures rapid uplift and erosion during displacement and the coincidence of k ar ages of amphiboles from the aureole and the sheeted dyke complex of the ophiolite

space geodetic measurement of crustal deformation in central and southern california 1984 1992 we estimate the velocity field in central and southern california using global positioning system gps observations from 1986 to 1992 and very long baseline interferometry vlbi observations from 1984 to 1991 our core network includes 12 gps sites spaced approximately 50 km apart mostly in the western transverse ranges and the coastal borderlands the precision and accuracy of the relative horizontal velocities estimated for these core stations are adequately described by a 95 confidence ellipse with a semiminor axis of approximately 2 mm yr oriented roughly north south and a semimajor axis of approximately 3 mm yr oriented east west

the strain field in northwestern greece and the ionian islands results inferred from gps measurements abstract recent crustal movements detected by the analysis of repeated satellite geodetic measurements reflect the ongoing geodynamic processes in the alpine mediterranean area superimposed on the large scale counterclockwise rotation of the african plate complex dynamic processes are affecting the lithospheric fragments between the african and eurasian plates key features to better understand the driving forces and associated seismic activity in the africa eurasia collision zone are the calabrian and hellenic arcs in this paper geodynamic investigations along the west hellenic arc are discussed they are based on two epochs 1989 and 1993 of satellite geodetic measurements carried out using the us global positioning system gps the results are presented in terms of relative displacements and strain rates within the time span of 4 years southwestern greece has moved to the southwest relative to southeastern italy by an average of 120 mm increasing from 80 mm at lefkada in the center of the ionian islands to 160 mm at the peloponnesus the maximum strain rate is 0 18 strain a located in the vicinity of lefkada where anomalously high earthquake activity is observed the data provide strong evidence for dextral strike slip motion on the order of 25 mm a along the kephalonia fault zone kfz the deformation field of the kfz is interpreted as a transition zone between the kinematics of the apulian platform and the west hellenic fold and thrust belts

geologic and seismic velocity structure of the crust mantle transition in the bay of islands ophiolite complex geological investigations of the bay of islands ophiolite complex show that while a typical layered ophiolite suite is present the thickness of major lithologic units is extremely variable from place to place the composition and internal structure of map scale kilometers across lithologic units as well as the contacts that bound them are laterally variable inferred velocity depth functions of the crust mantle moho transition reconstructed for this terrane as oceanic lithosphere suggest an extremely complex internal seismic structure the crust mantle transition from mafic to ultramafic lithologies varies across the ophiolite from a sharp geologic and seismic velocity discontinuity to a complexly interlayered transition zone as much as 3 km thick the mafic ultramafic transition is characterized by laterally discontinuous lithologic units on the order of hundreds of meters to a few kilometers thick and up to several kilometers long these units megalenses may be composed of lithologies with higher or lower seismic velocities than those of the surrounding units solid state deformation has produced seismically anisotropic materials in the upper mantle and lower crustal units substantial relief and slopes of up to at least 12 over lateral distances of about 10 km occur on the top of the crust mantle transition zone as well as other geologic unit contacts although these types of geological features are striking to the field geologist working in ophiolites and are significant to the understanding of crustal accretion processes many of them occur on such a small scale that they might go undetected in different types of seismic experiments in contemporary oceanic lithosphere

recognition classification and mechanical description of debris flows abstract various types of flow or mass movement involving water and sediments occur on steep slopes in mountainous areas among them debris flows are peculiar events during which a large volume of a highly concentrated viscous water debris mixture flows through a stream channel throughout the world these phenomena cause considerable damage but remain poorly understood although a basic knowledge is already available concerning their recognition and propagation firstly a synthesis of the useful practical criteria of recognition is proposed debris flows must be seen as intermediate phenomena between hyperconcentrated flows intense bed load transport and landslides separated from them by sharp transitions of some characteristics celerity deposit nature and flow type two parameters solid fraction and material type thought to be appropriate for a sound and practical classification are brought out and the corresponding complete classification of flow and mass movements in mountain areas is presented two extreme debris flow types are thus distinguished muddy debris flows and granular debris flows a critical review of recent advances in debris flow dynamic is then proposed it is pointed out that adequate work must be carried out in the field of non newtonian fluid mechanics in particular one fundamental rheological property of debris flow materials is the yield stress which explains thick deposits on steep slopes and can be inferred from field measurements furthermore it can be used to estimate viscous dissipation within the bulk during flow relevant models predicting muddy debris flow dynamics are already available whereas further progress is needed concerning granular flows

mantle flow at oceanic spreading centers models derived from ophiolites abstract the internal structures of eleven ophiolite complexes have been structurally investigated the attitude of the foliation and the lineation in the peridotites is related to asthenospheric flow in the uppermost mantle at an oceanic spreading center from the orientation of these structures with respect to the paleohorizontal plane two oceanic situations are distinguished the horizontally spreading asthenosphere illustrated by the table mountain massif newfoundland is characterized by 1 a homogeneous regular and sub horizontal plastic flow pattern over large distances 2 b flat paleo mono and absence of large scale folding 3 c rare economical chromite deposits and depleted mantle the diapirically spreading asthenosphere illustrated by the acoje type philippines displays 1 a inhomogeneous and sub vertical plastic flow pattern 2 b deformed paleo moho and large scale folds 3 c important economical chromite deposits and less depleted mantle presumably due to secondary magmatic impregnation this leads to vogt s model of asthenospheric flow beneath an oceanic ridge the direction of which being equated with the dike swarm in this model the horizontally spreading asthenosphere is derived from the diapirically spreading one in the following way the vertical mantle flow at the diapir center diverges horizontally in every direction including that parallel to the ridge at about 50 km from the diapir center the mantle flow becomes progressively perpendicular to the ridge due to the damming effect of transform faults an important economical consequence of this model is that large chromite deposits are expected preferentially in the diapiric type of ophiolite because chromite is preferentially precipitated at the diapir center from pristine magma

microplate versus continuum descriptions of active tectonic deformation whether deformation of continents is more accurately described by the motions of a few small rigid plates or by quasi continuous flow has important implications for lithospheric dynamics fault mechanics and earthquake hazard assessment actively deforming regions of the western united states central asia japan and new zealand show features that argue for both styles of movement but new observations are necessary to determine which is most appropriate and at what scale the description applies geologic geodetic seismic and paleomagnetic measurements tend to sample complementary aspects of the deformation field so an integrated observation program can utilize the strengths of each method and overcome their separate spatial or temporal biases provided the total relative motion across each region is known and the distribution of active faults is well mapped determination of fault slip rates can provide potentially decisive constraints reconnaissance geological studies supply useful slip rate estimates but precise values depend upon detailed intensive investigation of individual sites geodetic survey measurements can determine the spatial pattern of contemporary movements and extract slip rate information but the sometimes elusive effects of cyclic elastic strain buildup and relief must be accounted for in relating current movements to the long term deformation pattern earthquake catalogs can be applied to determine seismic strain rates and relative velocities but must be averaged over large regions and are usually limited by the inadequate duration of historical or instrumental seismicity catalogs paleomagnetic determinations of vertical axis rotations provide estimates of block rotation rates but are often locally variable and averaged over many millions of years which of the two descriptions of continental tectonics is more nearly correct depends on the local rheological stratification of the lithosphere especially the strength and thickness of the elastic crust relative to the ductile lithosphere and dynamical models can provide contrasting forecasts of observable features with testable consequences

petrology and texture of the ultramafic rocks of the xigaze ophiolite tibet constraints for mantle structure beneath slow spreading ridges abstract the xigaze ophiolite southern tibet exhibits an unusual ophiolite sequence characterized by 1 1 the limited thickness less than 3 km of its crustal unit which is mostly composed of dolerite intrusives the gabbros being very scarce 2 2 the scarcity of harzburgites and dunites the ultramafic rocks consisting mostly of cr diopside bearing harzburgites and iherzolites although there are some textural and chemical variations from the basal iherzolites to the upper harzburgite and dunites no sharp discontinuity exists between the peridotite series of the xigaze ophiolite which all belong to the same peridotite suite the peridotites have been equilibrated at relatively low temperatures and pressures 1155 c 0 5 0 6 gpa for the cr diopside bearing harzburgites and 1255 c 1 0 1 1 gpa for the stratigraphically 2 km deeper iherzolites indicative of a low thermal gradient at the xigaze paleo ridge accordingly the peridotites have locally been deformed under low stress 0 2 gpa conditions and at decreasing temperatures from up to 1000 c to 900 800 c as evidenced by the activation of the 010 100 ok1 100 and 001 100 olivine glide systems the equilibrium temperatures and pressures estimated for the peridotites the inferred shape of the melting zone beneath the xigaze paleo ridge the low temperature low stress conditions inferred for the deformation of the peridotites and the pattern of the mantle structure determined on the basis of structural and microstructural data are in good agreement with a slow spreading ridge origin for the xigaze ophiolite

crustal thickening versus lateral expulsion in the indian asian continental collision since the beginning of the continental collision between india and asia there has been about 2500 km of convergence and the northward movement of india has been accommodated by major internal deformation of the asian lithosphere the crustal thickening in and around the tibetan plateau is clearly a direct consequence of this collision but there is considerable debate as to whether a large fraction of the indentation has been accommodated by eastward motion of the lithospheric blocks of southeastern asia and southern china numerical experiments described here test this hypothesis for a range of indentation geometries and rheological models of the lithosphere we employ a thin viscous sheet model of the lithosphere with a depth averaged nonlinear viscous rheology described by a stress strain rate exponent n and including gravitational buoyancy forces scaled by the dimensionless argand number ar the eastern boundary for the collision region is described as a lithostatic boundary the precollision normal stress is determined by static balance and that constant stress is applied throughout the collision the experiments show that during collision the eastern boundary is smoothly displaced to the east at a rate about 1 4 of the indentation rate with only minor variation due to geometry or rheology crustal thickening rates in the region of the plateau are reduced by between 10 and 25 of the corresponding rates determined for similar experiments with a rigid eastern boundary to generalize from the results of these experiments the total north south shortening strain produced by the collision is partitioned between crustal thickening and eastward displacement in the ratio of at least 3 1 and more probably 4 1

geochronology of ophiolites of the newfoundland appalachians precise u pb zircon ages 2 errors are reported for four ophiolite complexes from the newfoundland appalachians zircons from trondhjemite from blow me down mountain bay of islands complex give an age of five points those from gabbro from blow me down brook give a minimum 207pb 206pb age of 480 ma one point confirming that this is the age of the main plutonic episode these data refute previously determined latest cambrian u pb zircon and sm nd mineral rock isochron ages of 504 10 and 505 ma for the complex gabbro from the betts cove complex yielded zircon that gives an age of four points this refutes the interpretation of an earlier u pb zircon age of 463 6 ma for a trondhjemite dyke as the age of the ophiolite it was inconsistent with the presence of arenigian graptolites in the overlying snooks arm group two zircon ages from a pegmatitic and medium grained trondhjemite of the annieopsquotch complex are five points and two points zircon from trondhjemite of the pipestone pond c

why are there no clockwise rotations along the north anatolian fault zone mechanical analysis of lithospheric deformation associated with an intracontinental transform fault suggests that it should be associated with a pattern of vertical axis rotations governed by the length of the fault its displacement and the mechanical properties of the lithosphere paleomagnetic results from two transects across the north anatolian fault zone indicate that there have been no measurable clockwise rotations on a regional scale associated with neogene dextral strike slip motion along the fault the data come from two groups of volcanic rocks that have been dated by k ar whole rock geochronology and have yielded ages ranging from 45 3 3 1 m y to 41 8 1 3 m y eocene and 21 7 1 5 m y to 7 5 0 2 m y miocene the absence of clockwise rotations suggests one of the following explanations 1 in contrast to the assumptions underlying current mechanical analyses deformation in the upper crust may not directly reflect the flow in the lithosphere and is localized along a single fault trace 2 deformation is distributed but rotations have been prevented by the presence of elongate fault blocks subparallel to the trace of the main fault 3 displacement along the north anatolian fault may be too small or the rheologic properties of the lithosphere such that rotations about vertical axes cannot be resolved instead of the predicted clockwise rotations measured declinations show that there has been approximately 30 of anticlockwise rotation of both the eocene and miocene volcanic rocks this suggests that no significant rotation occurred between eocene and miocene times and that the anticlockwise rotation occurred either immediately before or concurrent with the plio pleistocene slip on the north anatolian fault

upper mantle peridotites in the bay of islands ophiolite newfoundland formation during the final stages of a spreading centre abstract the structure of upper mantle rocks in ophiolites reflects the high temperature flow beneath an oceanic spreading centre locally overprinted by detachment related strain the structure of the mantle section in the ophiolitic table mountain massif bay of islands complex boic newfoundland was investigated in detail it is correlated with other massifs in the boic and the spreading related mantle flow history is reconstructed six structural units can be recognized on the basis of changes in the attitude of the high temperature mineral stretching lineation the relative age relationships between the structural units can partly be established they support the plate thickening model for oceanic lithosphere formation the structural history of the mantle section suggests that early plate driven flow was followed by forced flow underplating of an off axis diapir to the base of the lithosphere and detachment this complex history is explained in a model of a decreasing spreading rate which tracks the waning stages of spreading before detachment of the ophiolite during this final history the rate of spreading was decoupled from the rate of mantle upwelling so that a large forced flow field could build up in the blow me down bmd massif of the boic all spreading related flow structures are parallel to the ridge suggesting that the structures and the massif were frozen in near the ridge itself the presence of several kilometres of massive dunites in the bmd massif might indicate that in the final stages of spreading melt production was higher than could be accommodated by spreading so that melt ponded at the base of the crust lherzolites occurring at the base of the boic are considered as product of non adiabatic mantle upwelling prevalent in the final stages of spreading

the structures of two ophiolite massifs bay of islands newfoundland a model for the oceanic crust and upper mantle abstract for a detailed analysis of their main structures accumulation planes foliation planes and lineations the peridotite gabbro units of table mountain and blow me down mountain bay of islands newfoundland have been divided into several structural units each one being characterized by specific microstructures of deformation or accumulation a fishbone pattern with the accumulation plane in the mafic cumulates dipping toward the ridge and the foliation plane in the ultramafic tectonites dipping away from it is postulated for the structure of the lithosphere formed at a spreading center the tectonic structures in the peridotites are related to asthenospheric flow in the uppermost mantle and the direction of the ridge is inferred from the lineation trend in the peridotites the structures in the cumulates are related to subsidence of the magma chamber floor during spreading at the base of the massifs the tectonic structures in the mafic metamorphic rocks are parallel to those observed the latter have been deformed at moderate temperatures under high stresses and display mylonitic textures at their contact with the mafic tectonites from these superimposed textures we infer a kinematic model for the oceanic thrusting of the lithosphere the direction of thrusting is given by the lineation trend and its sense by the analysis of shear all these data are combined into a paleogeographic scheme for the newfoundland caledonides

geochemical and geological evidence bearing on the origin of the bay of islands and coastal complex ophiolites of western newfoundland abstract suggestions that ophiolitic rocks of the bay of islands complex boic and coastal complex cc of southwestern newfoundland were formed at a seafloor spreading center are supported by 1 1 whole rock trace and major element data from diabases and basalts 2 2 spinel mineral chemistry from ultramafic rocks 3 3 the internal geology of the ophiolite terranes and 4 4 regional geologic and tectonic constraints in the western newfoundland appalachians diabase and basalt samples from the boic and the northern cc massifs are tholeiitic and geochemically indistinguishable from present day mid ocean ridge basalts morbs many diabases from the lewis hills massif the southernmost part of the cc are also identical to morbs but there is also an unusual suite of diabases from the lewis hills that is unlike any other suite yet reported from a variety of tectonic settings including island arcs ocean islands or spreading centers this suite is interpreted to have been produced by unusual magma generation processes in the deep structural levels of an oceanic ridge ridge transform fault approximately 90 of spinels from ultramafic rocks in the boic and cc have cr cr al ratios that are less that 0 60 comparable to those values reported from ultramafic rocks of the present day ocean basins these geochemical constraints in combination with previously described geological relationships within the ophiolite terranes and regional tectonic relationships in the western newfoundland appalachians suggest that the boic and cc ophiolites formed within a main ocean basin i e the early paleozoic iapetus ocean along a seafloor spreading center and ridge ridge transform fault respectively there is little evidence to suggest formation in either an island arc or back arc basin tectonic setting as such the boic and cc ophiolites are probably well suited for detailed petrologic structural and geophysical studies to determine the processes involved in the generation and evolution of contemporary oceanic crust and upper mantle

evaluation of upper mantle microstructures in the table mountain massif bay of islands ophiolite within the undisrupted 6 km thick mantle section of the table mountain massif bay of islands ophiolite newfoundland several distinct peridotite microstructures are recognized in the uppermost mantle section very high temperature t 1200 high strain microstructures with evidence for syntectonic melt infiltration are preserved remarkable among these is an orthopyroxene impregnated dunite at an intermediate level of the mantle section peridotites have been affected by a weak lower temperature t 1000 1100 c overprint they are underlain by a thin sliver of low strain high temperature peridotites all these peridotites are interpreted to have formed during spreading related strain lower temperature t 1000 c high stress microstructures with high accumulated strain are locally preserved at intermediate levels of the mantle section and are regionally present at its base they culminate in metasomatic ultramafic ultramylonites and are related to detachment of the ophiolite with the exception of the basal ultramylonitic peridotites the activated olivine slip system was 0kl 100 in the highest temperature peridotites a weak preference for the activation of distinct slip planes 010 001 011 was noted based on piezometry and olivine flow laws strain rate estimates for the basal detachment fabrics are 10 12 s 1 this strain rate is consistent with the observed width of detachment overprint in the ophiolite and reasonable plate tectonic convergence rates strain rate estimates for the very high temperature peridotites are even higher recent data about grain growth rates in olivine indicate that in order to preserve the peridotite microstructures olivine grain growth must have been highly impeded olivine flow law data suggest that even peridotites with lower temperature microstructures t 900 950 c could form within the rheological asthenosphere the detachment or spreading related origin of peridotites with a lower temperature overprint may thus only be concluded from the regional geological context

plate motions crustal and lithospheric mobility and paleomagnetism prospective viewpoint the shift in viewpoint from a static to a mobilistic solid earth as brought about by paleomagnetism opened many new directions of research that continue to be exciting today and hold prospects for an exciting future here five topics of current research on earth s mobilistic surface are reviewed and their future directions discussed first dividing earth s solid surface into two domains 1 nearly rigid plate interiors and 2 plate boundaries some of them very wide leads to a review and discussion of the kinematics of wide plate boundaries many more adjustable parameters will be needed to describe the complex kinematics of plate boundaries which cover 15 of earth surface than the few dozen parameters needed to describe the kinematics of plate interiors which cover 85 of earth s surface second the question of the degree of rigidity of plate interiors is discussed the integral of the velocity gradient across plates interiors is at least a few hundredths or tenths of millimeters per year and in several well determined cases less than 2 or 3 mm yr future investigations will no doubt be aimed at narrowing these limits and to expanding the area of earth s surface over which limits are known third space geodetic data mainly from very long baseline inter ferometry satellite laser ranging and the global positioning system have demonstrated a remarkable similarity of velocities of stable plate interiors averaged over a few to a dozen years with plate velocities averaged over a few million years already however significant differences of a few millimeters per year are emerging and the tectonic and dynamic significance of these differences need to be evaluated fourth despite more than two decades of investigation the question of how fast hotspots move relative to one another is still a contentious issue one group of researchers maintains that maximum speeds are 3 mm yr whereas another maintains that speeds are 10 20 mm yr or more the differences cannot be reconciled until the uncertainties in plate reconstructions relative to hotspots are systematically and properly incorporated into analyses of hotspot motion fifth paleomagnetists have estimated true polar wander over the past 200 myr by equating it with the apparent polar wander of hotspots however the possible motion among hotspots and the neglect of the uncertainties in reconstructions relative to the hotspots have left these analyses unconvincing further progress requires the incorporation of these uncertainties a possible exception is the apparent polar wander of the hotspots over the past 10 20 myr or less which requires only small adjustments for plate motion and has presumably smaller uncertainties due to plate reconstructions the cause of this apparently significant geologically recent shift of the pole is poorly understood compared with the rate and direction of the secular shift of the pole observed this century from the international latitude service and for 1976 to 1994 from space geodetic data the paleomagnetically observed shift is unlikely to be due to the removal of northern hemisphere midlatitude ice sheets which is the commonly accepted explanation for the shift observed over the past century i speculate instead that the apparently rapid uplift of the tibetan plateau sometime during the past 10 20 myr is at least partly responsible for the geologically recent shift of the pole finally i conclude that the theory of plate tectonics has provided a framework that leads naturally to further quantification of the kinematics and deformation of earth s solid surface chiefly because of the key assumption of the rigidity of plate interiors which permits specific predictions to be made this revolution in quantification still has far to go and holds exciting prospects for future tectonic studies

analysis of debris wave development with one dimensional shallow water equations the objective of this contribution is to analyze the formation of debris waves in natural channels numerical simulations are carried out with a 1d code based on shallow water equations and on the weighted averaged flux method the numerical code represents the incised channel geometry with a power law relation between local width and flow depth and accounts for all source terms in the momentum equation the debris mixture is treated as a homogeneous fluid over a fixed bottom whose rheological behavior alternatively follows herschel bulkley bingham or generalized viscoplastic models the code is first validated by applying it to dam break tests on mudflows down a laboratory chute and verifying its efficiency in the simulation of rapid transients then following the analytical method developed by trowbridge the stability of a uniform flow for a generalized viscoplastic fluid is examined showing that debris flows become unstable for froude numbers well below 1 applications of the code to real debris

global positioning system gps estimates of crustal deformation in the marmara sea region northwestern anatolia abstract in the marmara sea region the relative motion between the anatolian and eurasian lithosphere has been measured by means of the modern techniques of space geodesy using the global positioning system gps in order to resolve in detail the kinematic field within the active earthquake belts of northwestern anatolia two gps campaigns were carried out across a dense network consisting of 45 sites preliminary results reveal horizontal westward motion of anatolia relative to eurasia of 2 4 cm y on average that runs along the western end of the north anatolian fault zone e w oriented right lateral strike slip movement average 0 06 ppm y maximum 0 13 ppm y 1ppm y 1 strain y and ne sw extension average 0 06 ppm y maximum 0 21 ppm y can be obtained from the calculated strain field in the west strike slip motion trending wsw ene is observed these findings are compared to the stress pattern derived from seismology and neotectonics