shear wave splitting in northeast venezuela trinidad and the eastern caribbean abstract we present new measurements of splitting of core shear wave phases sks skks pks recorded at seven stations in the eastern caribbean region six stations in northeastern venezuela and trinidad constituted the secasa92 temporary broadband array the seventh station sjg is installed on puerto rico we also attempted to measure source side splitting of s waves from earthquakes in the lesser antilles subducted slab but with little success mean splitting parameters fast polarization direction and delay time t at the stations are consistently east west trending and over 1 s respectively delay times at the secasa92 array are amongst the highest yet measured reaching 2 1 s and generally over 1 5 s we note an approximately linear decrease in delay times here from maxima at the northernmost stations to a minimum at the southernmost the secasa92 array is situated in the wide south america caribbean sa ca plate boundary zone and sjg is within the north america caribbean na ca plate boundary zone in both cases trends are parallel to the plate boundary zones locally we interpret these results to indicate that the sa ca plate boundary deformation extends to depths of at least 200 km and that south american crust lithospheric mantle and asthenosphere are coherently deformed the extreme delay times we measure are probably the result of transpression in the sa ca plate boundary and overriding of the lesser antilles slab by south america compressional flattening results in extremes of foliation development and dextral shearing yields a strong lineation in upper mantle olivine by contrast the delay time at sjg 1 2 s is near the global average and reflects the fact that the na ca plate boundary zone is purely sinistral or includes minor transtension we note that recent counterclockwise rotation of puerto rico does not apparently affect upper mantle deformation beneath the island we suggest that this rotation is crustal driven by far field sinistral shear and partly decoupled from the mantle shear our results are consistent with the trench parallel mantle flow hypothesis russo and silver 1994 science 263 1105 1111 and indicate that asthenospheric mantle flow beneath the caribbean plate is eastward and probably drives the plate s eastward motion

mantle transition zone beneath the caribbean south american plate boundary and its tectonic implications abstract we analyzed receiver function data recorded by a temporary broadband array deployed as part of the bolivar project and the permanent seismic network of venezuela to study the mantle transition zone structure beneath the caribbean south american plate boundary and venezuela significant topography on both the 410 km and the 660 km discontinuities was clearly imaged in the ccp common conversion point stacked images beneath the southeastern caribbean the 410 km is featured by a narrow 200 km ew 25 km uplift extending in the ns direction around 63 west while the 660 km is depressed by 20 km in a narrow region slightly west to the uplift a scenario that is more consistent with westward descent of the oceanic south american plate rather than a break off of nnw dipping proto caribbean oceanic lithosphere along the el pilar fault we also found a thick transition zone beneath the falcon region in northwestern venezuela possibly associated with the subducted nazca plate a flat 410 km was observed beneath the guayana shield suggesting that the shield has a stable and moderately deep keel which has little effect on the underlying transition zone structure

undrained stress path of loose sand predicted from dry tests the constitutive properties of the soil skeleton of loose sand most relevant for predicting liquefaction flow slides are contraction and elastic compressibility they can be determined with dry triaxial tests a mathematical description of these idryi properties is presented and used to predict the specific stress path found in undrained triaxial tests a comparison between predicted and measured stress paths is discussed resume les propriotos de comportement du squelette de sable meuble les plus pertinentes pour prodire les couloes par liquofaction sont la contraction et la compessibilito olastique elles peuvent tre doterminoes par des essais triaxiaux secs une description mathomatique de ces propriotos sches est prosentoe et utilisoe pour prodire le cheminement de contraintes spocifique trouvo dans les essais triaxiaux une comparaison entre les cheminements mesuros et prodits est discutoe mots clos sable meuble liquofaction cheminement de contraintes non drainoes porosito mouilloe critique otat permanent traduit par la rodaction

high temperature elasticity of iron bearing olivines the first high temperature data on the nine adiabatic elastic moduli for iron bearing olivine are reported these measurements are on two single crystal specimens of natural olivine at ambient pressure and from room temperature to a maximum of 1500 k the two specimens contain 8 and 9 modal percent fayalite which required the oxygen fugacity be controlled at high temperature to preserve their chemical stability the rectangular parallelepiped resonance apparatus was adapted to buffer the specimens from the atmosphere with a mixture of co and co2 gas a small increase 1 2 gpa in the adiabatic bulk modulus of each specimen over that of end member forsterite was found the data are high quality to extreme temperatures with good agreement found when comparing the temperature derivatives of the elastic moduli of the two specimens neither specimen exhibits measurable nonlinear temperature dependence in the computed isotropic bulk and shear moduli which is in contrast to published forsterite data the temperature derivatives of the isotropic bulk modulus ks are 1 69 1 80 10 2 gpa k 1 for the two olivine specimens and the shear modulus g derivatives are 1 38 1 36 10 2 gpa k 1 these derivatives are only slightly larger in magnitude than ks t p 1 56 10 2 and g t p 1 30 10 2 gpa k 1 found previously for iron bearing olivine over a very small temperature range there are also no significant differences between the temperature derivatives found here and the average derivatives of end member forsterite from data retrieved over a slightly larger temperature range several dimensionless parameters have been calculated from these results and are discussed in view of systematics which bear on high pressure phases in earth s transition zone one result from these systematics related to the seismic velocities in the earth and especially the shear wave velocities is that an olivine content of less than 50 is implied at the 400 km discontinuity if earth s upper mantle is isochemical furthermore the substitution of almost 10 iron for magnesium at the forsterite end of the olivine solid solution series has little effect on the dimensionless parameters or on the temperature derivatives of the elastic moduli at high temperature

collision tectonics in the nw himalayas west himalayan tectonics involve the collision of microplates between the indian and asian plates the kohistan complex consists largely of tightly folded basic volcanics and sediments generated as late jurassic to late cretaceous island arcs these were intruded by post folding mid cretaceous eocene plutonics produced from continued subduction of the indian plate after closure of a suture between kohistan and the karakorum the himalayan structures show major thrust sheets and the kohistan arc is essentially a crustal pop up with southward upright and northward verging structures developed above a thick ductile decoupling zone the indus suture which can be traced for 100 km beneath kohistan on large reentrants this pop up formed by a two stage process closure of the northern suture followed by closure of the southern indus suture granitic rocks of the kohis tan ladakh batholith dated at 100 40 ma post date most of the structures related to the northern suture but were deformed and carried southwards on shear structures related to the indus suture postcollisional deformation carried this kohistan complex on deep decoupling zones over the indian plate on a series of imbricated gneiss sheets the thrusts climbing up section in the movement direction so that in the far s some override their own molasse debris folds above these deep decoupling zones deformed their overlying thrust sheets into large antiforms i e the nanga parbat and hazara syntaxes the nanga parbat syntaxis probably formed due to a shear couple near a branch line where one of the main himalayan thrusts joined the indus suture beneath kohistan crustal delamination to produce the imbricated gneiss sheets could not account for all the displacement of india into asia suggested by palaeomagnetic data there must also have been lateral displacement as demonstrated by the large oblique slip shear zone in the hunza valley n of kohistan

continental margin response to multiple arc continent collisions the northern andes caribbean margin abstract an integration of diverse strain markers into a palinspastic reconstruction of the northern andes provides a first order quantitative approximation to the response of a complex margin to two consecutive arc continent collisions a palinspastic reconstruction highlights margin parallel displacements with crustal blocks traveling 400 to 500 km northward while the margin normal shortening component excluding subduction is limited to 100 km the first arc continent collision closed a 500 km wide ocean basin where subduction of atlantic lithosphere took place under an intra oceanic arc 90 to 65 ma old originally 1100 km long and currently stretched 2200 km following collision eastward directed oblique subduction started at 65 ma with 800 km of proto caribbean lithosphere consumed under northwestern south america igniting a short lived magmatic arc along the post collisional margin two molasse deposits track deformation cooling exhumation and erosion of the margin the first heralds the arrival of the first intra oceanic arc to the southernmost northern andes in late campanian times younging northward as the collision propagated north this latest cretaceous molasse deposit records also the first altitudinal gain in the northern andean orogen along the central and real cordilleras the second molasse follows middle to late eocene magmatic arc shut down and regional exhumation and cooling of the margin formation rise and docking of the intra oceanic panama arc closed a 1200 km wide middle eocene seaway and renewed miocene magmatism in the northern andes as the nazca plate started subducting behind the docked arc

mantle flow deflected by interactions between subducted slabs and cratonic keels subducting slabs can influence mantle flow but the importance of neighbouring continental cratons is little understood geodynamical modelling constrained by seismic data that identify regions of mantle flow beneath the caribbean south american plate margin shows that the deep rooted south american craton acts to deflect and enhance mantle flow into a narrow channel

velocity field across the southern caribbean plate boundary and estimates of caribbean south american plate motion using gps geodesy 1994 2000 global positioning system gps observations between 1994 and 2000 at twenty two sites in the lesser antilles and northern south america indicate that the caribbean plate along its southern boundary slips at a rate of 20 5 2 mm a with an azimuth of n 84 2 e at 65 w relative to the south american plate east of 68 w 80 of the dextral slip is contained within a 80 km wide shear zone centered on the el pilar san sebastian fault system west of 68 w the plate boundary broadens to more than 300 km with dextral shear shared between the northeast trending bocono fault 9 11 mm a in western venezuelan and an offshore system near the northern coast

flow characteristics of 2 phase granular mass flows from model flume tests abstract changes in basal fluid pressure and normal flow depth in 2 phase granular mass flows the equivalent coefficient of friction and the equivalent coefficient of friction between gravity centres before and after failure were investigated for pumiceous gravel and a mixture of pumiceous gravel and volcanic ash by means of large scale model flume tests the proportion of finer soil particles the inclination of slope segments and the width of horizontal segments were selected as the controlled conditions no excess of basal fluid pressure was observed in the pumiceous gravel on the other hand an excess of pressure head was generated in the mixture of pumiceous gravel and volcanic ash this was due to the suspension of the fine particles in the pore fluid increasing its effective density and the quasi undrained shear deformation due to the low permeability of the mixture in the mass of the granular flow during its motion in tests with a slope of 30 both the pumiceous gravel and the mixture of pumiceous gravel and volcanic ash travelled less distance in the narrow flume than in the wide one this was likely because in the wide flume no clogging occurred where the width of the flume changed and less frictional collisions against side walls and resulting grains contact took place in tests with a narrow flume the pumiceous gravel clogged and most of material was deposited around the constriction however the mixture passed through the constriction smoothly this could be because less frictional resistance was generated in the mixture due to the lowering of effective stress between soil particles associated with the excess of pore fluid pressure although the test on a slope of 35 had the largest potential energy before the release of the materials and the test on a slope of 25 had the least the test with the 35 degree slope produced the largest equivalent coefficient of friction and the smallest value was obtained from the test with a 25 degree slope this finding suggests that the dissipation of kinetic energy especially in the vertical direction due to the collisions and the following abrupt deformation around the connection of the flume between the slope and horizontal segments may play an important role in the travel distance

s wave velocities of the lithosphere asthenosphere system in the lesser antilles from the joint inversion of surface wave dispersion and receiver function analysis abstract we present an overview of s wave velocities vs within the crust and upper mantle of the lesser antilles as determined with 19 seismic broadband stations receiver functions rf have been computed from teleseismic recordings of earthquakes and rayleigh wave group velocity dispersion relations have been taken from earlier surface wave tomographic studies in the caribbean area local smoothness optimization lso procedure has been applied combined with an h k stacking method the spatial distribution of hypocenters of local earthquakes and of the energy they released in order to identify an optimum 1d model of vs below each station several features of the caribbean plate and its interaction with the atlantic subducting slab are visible in the resulting models a relatively thick oceanic crust below these stations ranges from 21 km to 33 km being slight thinner in the middle of the island arc b crustal low velocity zones are present below stations saba seus ski smrt cbe dsd gcmp and tdba c lithospheric thickness range from 40 km to 105 km but lithosphere asthenosphere boundary was not straightforward to correlate between stations d the aseismic mantle wedge between the caribbean seismic lithosphere and the subducted slab varies in thickness as well as vs values which are in general lower below the west of martinique than below the west of guadeloupe e the depth of the subducted slab beneath the volcanic arc appears to be greater to the north and relatively shallower below some stations e g dlpl sam bim and fdf than was estimated in previous studies based on the depth distribution of seismicity f the wbz is 10 15 km deeper than the top of the slab below the central lesser antilles martinique and dominica where the presence of partial melt in the mantle wedge seems also to be more evident

contrasting metamorphic p t time paths of the kohistan granulites and tectonics of the western himalayas in the swat indus kohistan area lower crustal rocks of the kohistan arc including amphibolites granulites and ultramafic rocks are widely exposed detailed petrological structural studies reveal that the chilas jijal metamorphic complex comprises essentially intermediate to high pressure granulites with a granulite amphibolite transition controlled by extensive retrograde metamorphism and ductile deformation contrasting patterns of compositional zoning in garnet and orthopyroxene suggest complicated p t paths in the granulites geothermobarometry based on garnet bearing granulites identify two distinct p t time paths a high pressure anticlockwise path of the jijal complex and a low pressure retrograde path of the chilas complex these p t time paths represent the evolution of middle to lower crustal rocks of the kohistan arc the anticlockwise p t time path combined with published isotopic ages reveals that i the high pressure granulitefacies metamorphism is related to the kohistan asia collision and ii the crust of the kohistan arc had been thickened at least 55 km before the india asia collision

tectonic implications of tomographic images of subducted lithosphere beneath northwestern south america we used seismic tomography to investigate the complex structure of the upper mantle below northwestern south america images of slab structure not delineated by previous seismicity studies help us to refine existing tectonic models of subducted caribbean pacific lithosphere beneath the study area beneath western venezuela and colombia we distinguish two slabs a maracaibo and a redefined bucaramanga slab the maracaibo slab coinciding with most of the bucaramanga slab previously defined by w d pennington dips in a direction of 150 at an angle of 17 to a depth of 275 km and correlates to the subducted late cretaceous oceanic plateau of the caribbean plate farther south a second slab dips at an angle of 50 in a direction of 125 to a depth of at least 500 km and correlates to the subducted oceanic crust of the nazca plate and the downdip extension of the panama island arc we refer to this slab as the redefined bucaramanga slab because it is different from the bucaramanga slab segment defined by pennington the area of the south american plate overriding both slabs is characterized by the absence of an active volcanic arc an anomalously wide topographically uplifted and tectonically active area and the northward escape of the maracaibo block along active strike slip faults in support of earlier studies we attribute this to the underthrusting of the caribbean oceanic plateau our shallowly dipping maracaibo slab along the base of the south american lithosphere and to the recent collision of the panama island arc rafted in on more steeply dipping crust of the nazca plate our redefined bucaramanga slab

an insight into the bucaramanga nest abstract we used the local seismicity for the period of 1993 to 2001 in the northeast of colombia to show the existence of two slabs in the north and south of the bucaramanga nest the northern slab has a dip angle of about 25 and the southern slab has a 50 dip angle while the dip in the bucaramanga nest is about 29 in order to explain the nature of the bucaramanga nest we proposed the scenario of collision between these two slabs using a 3d finite element model fem we show that collision can concentrate modify and perturb the stress field the active process of dehydration embrittlement at intermediate depths and the concentrated stress field in the collision zone may explain the high rate of seismic activity inside the bucaramanga nest the perturbed and modified stress field resulting from the simultaneous effect of collision between two subducted slabs and subduction of the lithosphere under its own weight can explain the variation in the focal mechanism of micro earthquakes and the complexity in the source of the moderate size earthquakes in the bucaramanga nest

rbsr dating of the kohistan arc batholith in the trans himalaya of north pakistan and tectonic implications the kohistan arc batholith in northern pakistan is situated between the indus suture and the northern suture it belongs to the trans himalayan belt which continues eastwards as the ladakh arc batholith in northwest india and the kangdese batholith in southern tibet in kohistan the island arc consists upward sequence of the chilas stratiform complex of norites noritic gabbros and chromite layered dunites which formed in the sub arc magma chamber plutons of tonalite and diorite one of which has a rbsr isochron age of 102 12 ma the chalt volcanics of basaltic tholeiites succeeded by andesites to rhyolites and the yasin group sediments which formed in overlying intra arc basins and which contain albian aptian faunas all these rocks were deformed by major fold structures which are correlated with the formation of the northern suture ne sw trending basic jutal nomal dykes north of gilgit cross cut all the above structures and have a39ar 40ar hornblende age of 75 ma thus the northern suture must have formed in the period 102 12 to 75 ma with the island arc attached to the eurasian plate further northward subduction of the tethyan plate gave rise to an andean type batholith two plutons of which have rbsr isochron ages of 54 4 ma and 40 6 ma the dir utror group of calc alkaline lavas and sediments with eocene fossils are remnants of the volcanic cover of the andean type batholith we suggest that late cretaceous blueschists formed during subduction under the active continental margin and that continental collision and formation of the indus suture was in the eocene the batholith was intruded by layered aplite pegmatite sheets at 34 14 ma and 29 8 ma rbsr ages in post collisional times the87sr 86sr initial ratios of all the dated rocks range between 0 7039 and 0 7052

small scale laboratory flowslides loose cohesionless slopes are susceptible to liquefaction and subsequent flowsliding small scale sub aerial laboratory flowslides have been obtained by inducing the collapse of loose deposits of saturated fine quartz sand failure mechanisms resembled field cases and other laboratory experiments a detailed set of results is presented illustrating the temporal and longitudinal variation of both surface velocity and the elevations of flowing and static material initially the flowside accelerated under the influence of gravity movement was not arrested en masse but unsteady deposition of sand and pore fluid occurred being dependent on deceleration of the flowslide that propagated downstream as a result the slope over which the flowing material was moving increased with time excess pore water pressures were measured during flowslide motion the magnitudes of which were reasonably predicted using the coulomb failure criterion and assuming equilibrium of driving and resisting forces this was despi

aseismic continuation of the lesser antilles slab beneath continental south america 1 we present results of travel time inversions of teleseismic p and s waves recorded at the secasa92 southeast caribbean south america 1992 temporary broadband array in northeastern venezuela and trinidad the inversions reveal the unusual structure of the southern termination of the lesser antilles subduction zone a minimum 2 relatively high velocity anomaly trends wsw from the seismically defined lesser antilles slab beneath and nw of the paria peninsula to a point below the venezuelan serrania del interior well south of the caribbean coast resolution tests utilizing actual ray geometries and densities of the source data indicate that the regional scale structure beneath the study area is reasonably well resolved thus a detached and detaching subducted south american slab appears to lie beneath continental south america we infer that oceanic south american lithosphere has been overridden to a significant degree by continental south america the detached slab now lying beneath continental south america was driven into its current position after detaching from the former eastward striking mesozoic ocean continent passive margin as this margin entered the subduction zone because oceanic and continental south america are still attached without apparent relative motion between them along the atlantic passive margin southeast of our study region the slab must be the actively moving element during continental overriding thus the slab and its surrounding mantle both caribbean and south america beneath northeastern south america are mobile and have moved ese relative to the stable guyana shield craton

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

crustal structure of the caribbean northeastern south america arc continent collision zone 1 we present the results of a 568 km long regional wide angle seismic profile conducted in the southeastern caribbean that crosses an active island arc a remnant arc two basins possibly floored by oceanic crust an allochthonous terrane of forearc affinity and the passive margin of northern south america the velocity structures of the late cretaceous aves ridge remnant arc and miocene and younger lesser antilles arc are remarkably similar which implies that magmatic processes have remained moderately steady over time crustal thickness is 26 km at the aves ridge and 24 km at the lesser antilles arc in comparison to the izu bonin and aleutian arcs the lesser antilles arc is thinner and has no evidence for a lower crustal cumulate layer which is consistent with the estimated low magma production rates of the lesser antilles arc crustal thickness beneath the grenada and tobago basins is 4 10 km and the velocity structure suggests that these basins could be floored by oceanic crust a decrease of 1 km s in average seismic velocity of the upper crust is observed from nw to se across the north coast fault zone we argue that this marks the suture between the far traveled caribbean arc and the passive margin of the south american continent current strike slip motion between the caribbean and south american plates is located 30 km to the south and thus material originally deposited on the south american passive margin has now been transferred to the caribbean plate

metamorphism of an obducted island arc example of the kohistan sequence pakistan in the himalayan collided range in northern pakistan the extraordinary 40 km thick kohistan sequence of metamorphosed mafic ultramafic and calc alkaline layered plutonic and volcanic mainly andesitic to rhyodacitic rocks has been recently reinterpreted as the only complete vertical section of an intra oceanic island arc presently exposed anywhere in the world plate scale models have been suggested to explain the origin and the tectonic evolution of this arc in the himalayan collision range despite some noticeable differences in the models there is general agreement that the kohistan sequence represents the crust of an arc obducted onto the northern edge of the indian plate before the oligocene collision of india against asia new data on the tectonometamorphic evolution of the kohistan sequence have led to the conclusion that a first major d 1 event developed with increasing metamorphic grades oriented downward the kohistan pile but also toward ensandwiched basic ultrabasic granulitic rocks rare geochronological data and petrological evidence indicate that the latter granulites pyriclasites and various plagiopyrigarnites pyrigarnites metatroctolites metawebsterites metadunites etc were parts of enormous lower to middle cretaceous layered calc alkaline plutons emplaced during the arc building stages as the d 1 metamorphic event is correlated with the upper cretaceous obduction process of the kohistan onto india it is proposed that the d 1 thermal structure was strongly controlled by remnant magmatic heat source s within the obducted arc this interpretation fits nicely with recent theoretical thermal models since the obduction of the arc was probably shortly after 10 20 ma the emplacement of the former plutonic arc rocks a blueschist tectonic melange underlying the obducted arc was possibly synchronous to the obduction and not clearly linked to the older subductive process the oligocene collision of india against asia was associated with a barrovian overprinting metamorphism in the kohistan arc during this second event the dynamo thermal structure was much more classical and not controlled by the occurrence of the cretaceous metaplutonic rocks a tentative model of the thermal structures during the major events which affected the kohistan arc is proposed

north south extension within the convergent himalayan region recent work by burg et al 1984 indicates the presence of east west striking gently north dipping normal faults in the higher himalayas and southern tibet that formed during postcollisional convergence of india and tibet these faults extend for at least 800 km along strike we interpret these normal faults as probable late miocene extensional features with perhaps several tens of kilometres of down to the north displacement a simple elastic model suggests that these normal faults may have formed during gravitational collapse of the miocene topographic front between india and tibet in this interpretation gravitational collapse occurred by southward motion relative to india and tibet of a wedge of crustal rocks bounded above by gently north dipping normal faults and below by thrust faults that probably dip north north south extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere

subduction of the caribbean plate and basement uplifts in the overriding south american plate the new tectonic interpretations presented in this paper are based on geologic field mapping and gravity data supplemented by well logs seismic profiles and radiometric and earthquake data the present caribbean south american plate boundary is the south caribbean marginal fault where subduction is indicated by folding and thrusting in the deformed belt and a seismic zone that dips 30 to the southeast and terminates 200 km below the maracaibo basin the caribbean south american convergence rate is estimated as 1 9 0 3 cm yr on the basis of the 390 km length of the seismic zone and a thermal equilibration time of 10 m y the caribbean south american convergence has produced a northwest southeast maximum principal stress direction 1 in the overriding south american plate the mean 1 direction for the maracaibo santa marta block is 310 10 based on earthquake focal mechanism determinations and structural and gravity data on the overriding south american plate basement blocks have been uplifted 7 12 km in the last 10 m y to form the venezuelan andes sierra de perija and the colombian santa marta massif crystalline basement of the venezuelan andes has been thrust to the northwest over tertiary sediments on a fault dipping about 25 and extending to the mantle in the sierra de perija mesozoic sediments have been thrust 16 26 km to the northwest over tertiary sandstones along the cerrejon fault a thrust fault dipping 15 10 to the southeast is consistent with field mapping and gravity and density data the santa marta massif has been uplifted 12 km in the last 10 m y by northwest thrusting over sediments the basement block overthrusts of the perijas venezuelan andes and the santa marta massif are pliocene pleistocene analogs for laramide orogenic structures in the middle and southern rocky mountains of the united states the nonmagmatic basement block uplifts along low angle thrust faults reveal horizontal compression in the overriding plate over 500 km from the convergent margin present day east northeast west southwest 080 compression is indicated by earthquake focal mechanisms and strike slip motion on the bocono fault these earthquakes are intraplate deformation associated with east west 080 nazca south american convergence

tectonic evolution and mantle structure of the caribbean we investigate whether predictions of mantle structure from tectonic reconstructions are in agreement with a detailed tomographic image of seismic p wave velocity structure under the caribbean region in the upper mantle positive seismic anomalies are imaged under the lesser antilles and puerto rico these anomalies are interpreted as remnants of atlantic lithosphere subduction and confirm tectonic reconstructions that suggest at least 1100 km of convergence at the lesser antilles island arc during the past 45 myr the imaged lesser antilles slab consists of a northern and southern anomaly separated by a low velocity anomaly across most of the upper mantle which we interpret as the subducted north america south america plate boundary the southern edge of the imaged lesser antilles slab agrees with vertical tearing of south america lithosphere the northern lesser antilles slab is continuous with the puerto rico slab along the northeastern plate boundary this results in an amphitheater shaped slab and it is interpreted as westward subducting north america lithosphere that remained attached to the surface along the northeastern boundary of the caribbean plate at the muertos trough however material is imaged until a depth of only 100 km suggesting a small amount of subduction the location and length of the imaged south caribbean slab agrees with proposed subduction of caribbean lithosphere under the northern south america plate an anomaly related to proposed oligocene subduction at the nicaragua rise is absent in the tomographic model beneath panama a subduction window exists across the upper mantle which is related to the cessation of subduction of the nazca plate under panama since 9 5 ma and possibly the preceding subduction of the extinct cocos nazca spreading center in the lower mantle two large anomaly patterns are imaged the westernmost anomaly agrees with the subduction of farallon lithosphere the second lower mantle anomaly is found east of the farallon anomaly and is interpreted as a remnant of the late mesozoic subduction of north and south america oceanic lithosphere at the greater antilles aves ridge and leeward antilles the imaged mantle structure does not allow us to discriminate between an intra americas origin and a pacific origin of the caribbean plate key points tectonic reconstructions for the caribbean validated by seismic tomography cenozoic lesser antilles and mesozoic greater antilles subduction confirmed tomography gives insight into geodynamics of lesser antilles

constraints on mantle flow at the caribbean south american plate boundary inferred from shear wave splitting 1 we measured shear wave splitting from sks and skks data recorded by temporary stations deployed as part of the broadband onshore offshore lithospheric investigation of venezuela and the antilles arc region project and the national seismic network of venezuela approximately 3000 station event pairs yielded 300 with visible sks and or skks phases we obtained 63 measurements at 39 of the 82 stations in the network using the method of silver and chan 1991 and conventional quality criteria we combined our results with previous measurements made by russo et al 1996 the most prominent feature in the data is an area of large 2 0 s lag times with roughly east west fast axes in northeastern venezuela mineral physics models show split times this large are difficult to explain with horizontal foliation but are more feasible with anisotropy characterized by a coherent vertical foliation and an east west fast axis extending over most of the upper 250 km of the mantle we interpret the large split times in northeastern venezuela as a consequence of eastward translation of the atlantic slab which has left a strong vertical foliation in its wake parallel to the plate boundary the peak split times correspond closely with the point the slab intersects the base of the anisotropic asthenosphere at 250 km away from this area of large split times the measured times fall to more standard values but an east west fast axis still predominates we suggest this is linked to the rapidly varying strain field at the southern edge of the atlantic which quickly disrupts the coherent strain field that causes the very large split times in northeastern venezuela

instrumented laboratory flowslides flowslides have been induced in small instrumented coking coal stockpiles by slowly raising the water table these strongly resembled field cases showing sudden collapse rapid fluid like motion and very flat final slopes failure initiated under essentially static drained conditions and with a mobilized secant effective strength parameter d much less than the steady state value highspeed measurements showed that excess pore pressures were generated during rather than before movement and liquefaction was therefore a result of shear failure rather than the cause des glissements par liquefaction furent provogues dans de petites reserves de charbon cokeflant a instrumentation par elevations lentes de la nappe aquifere ils ressemblaient beaucoup a des cas sur chantier avec affaissement abrupt un mouvement rapide quasi fluide et des pentes finales tres plates la rupture commenca dans des conditions drainees essentiellement statiques et avec un parametre o de resistance effective secante mobilisee

wet subduction versus cold subduction 1 in situ x ray diffraction study of post spinel transformation in hydrous peridotite 2 wt h2o indicates that the phase boundary is shifted to higher pressures by 0 6 gpa relative to anhydrous peridotite at 1473 k whereas it shows no obvious shift at high temperature around 1873 k a linear equation for the boundary is p gpa 0 002 t k 26 3 which is applicable for temperatures below 1800 k the present data shows that a significant part of the depressions seen in the 660 km seismic discontinuity may be affected by existence of water it is also well resolved that the olivine wadsleyite phase transformation corresponding to the 410 km seismic discontinuity is shifted to lower pressures by 1 2 gpa by the addition of water thus the topography of seismic discontinuities in the transition zone associated with slabs can be attributed not only to cold subduction but also wet subduction

age and spreading history of the cayman trough as determined from depth heat flow and magnetic anomalies the opening of the cayman trough records aspects of relative motion between the north american and caribbean plates essential to accurate reconstruction of caribbean evolution assessment of opening rates based upon basement subsidence analysis as well as magnetic anomaly interpretation shows that the trough may have opened at one third to one half previously proposed rates depths to basement show that the crust has subsided away from the present axis in two stages a younger stage characterized by substantial and asymmetric greater to the east subsidence preceded by an older stage of slight increases of depth with distance depth to age relations calculated from subsidence curves which accommodate lateral as well as vertical cooling of the accreting slab indicate that the trough has opened at an overall rate of less than 15 mm yr since 25 30 ma and at 20 30 mm yr prior to 30 ma marine magnetic anomalies show lineations perpendicular to the trough including a distinct central anomaly but the pattern is neither well defined nor symmetrical about the present axis however partial anomaly sequences can be identified on both sides of the axis and the distribution of these sequences indicates that spreading may have involved frequent short ridge jumps mostly eastward the overall rate of opening as indicated by this interpretation of the magnetics is about 15 and 30 mm yr since and prior to 26 ma both subsidence and magnetics indicate that the trough opened by at least 45 50 ma

identification and tectonic implications of a tear in the south american plate at the southern end of the lesser antilles 1 in the southeast corner of the caribbean westward subduction of atlantic oceanic south america beneath the lesser antilles transitions to east west transform motion between continental south america and the caribbean plate this geometry requires negatively buoyant subducting oceanic south american lithosphere to progressively detach from positively buoyant continental south american lithosphere the most widely accepted model is slab break off with oblique arc continent collision and northwest dipping continental subduction precipitating narrow rifting in the subducting slab in contrast the subduction transform edge propagator step model conceptualizes progressive detachment along a vertical dip slip tear through the lithosphere with stress focused at the edge of the propagating transform boundary we present four types of seismic data to resolve the ongoing lithospheric detachment local seismicity receiver functions wide angle seismic velocity inversion and a regional balanced cross section constrained by petroleum industry data these four data sets image a near vertical tear extending through the entire lithosphere revealing a key mechanism for the structural evolution of venezuela

linking late cretaceous to eocene tectonostratigraphy of the san jacinto fold belt of nw colombia with caribbean plateau collision and flat subduction collision with and subduction of an oceanic plateau is a rare and transient process that usually leaves an indirect imprint only through a tectono stratigraphic analysis of pre oligocene sequences in the san jacinto fold belt of northern colombia we show the late cretaceous to eocene tectonic evolution of northwestern south america upon collision and ongoing subduction with the caribbean plate we linked the deposition of four forearc basin sequences to specific collision subduction stages and related their bounding unconformities to major tectonic episodes the upper cretaceous cansona sequence was deposited in a marine forearc setting in which the caribbean plate was being subducted beneath northwestern south america producing contemporaneous magmatism in the present day lower magdalena valley basin coeval strike slip faulting by the romeral wrench fault system accommodated right lateral displacement due to oblique convergence in latest cretaceous times the caribbean plateau collided with south america marking a change to more terrestrially influenced marine environments characteristic of the upper paleocene to lower eocene san cayetano sequence also deposited in a forearc setting with an active volcanic arc a lower to middle eocene angular unconformity at the top of the san cayetano sequence the termination of the activity of the romeral fault system and the cessation of arc magmatism are interpreted to indicate the onset of low angle subduction of the thick and buoyant caribbean plateau beneath south america which occurred between 56 and 43 ma flat subduction of the plateau has continued to the present and would be the main cause of amagmatic post eocene deposition

neogene block tectonics of eastern turkey and northern south america continental applications of the finite difference method continental plate boundary zones are generally wide diffuse zones of deformation within which in the upper 5 to 15 km may be recognized blocks which are bounded by fault zones within which strain is highly concentrated and along which slip rates are fairly high 2 30 m ma 1 these blocks may be irregular flakes defined by older crustal inhomogeneities of an upper brittle crust below which strain occurs in a ductile and more homogeneous lower crust with examples from the convergent plate boundary zone of eastern turkey and the southern caribbean plate boundary zone of northern south america we use plate slip rates and trends and slip rates of faults to construct block vector diagrams to deduce the sums of relative motion among block mosaics slip rates and trends of ill defined block boundaries and the extent to which blocks are internally strained

the origins of granulites a metamorphic perspective although many recent reviews emphasize a uniformity in granulite pressure temperature p t conditions and paths granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation a thorough survey of over 90 granulite terranes or occurrences reveals that over 50 of them record p t conditions outside the 7 5 1 kbar and 800 50 c average granulite regime preferred by many authors in particular an increasing number of very high temperature 900 1000 c terranes are being recognized both on the basis of distinctive mineral assemblages and geothermobarometry petrogenetic grid and geothermobarometric approaches to the determination and interpretation of p t histories are both evaluated within the context of reaction textures to demonstrate that the large range in p t conditions is indeed real and that both near isothermal decompression itd and near isobaric cooling ibc p t paths are important amphibolite granulite transitions promoted by the passage of co 2 rich fluids as observed in southern india and sri lanka are exceptional and not representative of fluid related processes in the majority of terranes it is considered on the contrary that fluid absent conditions are typical of most granulites at or near the time of their recorded thermal maxima itd granulites are interpreted to have formed in crust thickened by collision with magmatic additions being an important extra heat source erosion alone is not however considered to be the dominant post collisional thinning process instead the itd paths are generated during more rapid thinning 1 2 mm yr exposure related to tectonic exhumation during moderate rate or waning extension ibc granulites may have formed in a variety of settings those which show anticlockwise p t histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion with or without additional crustal extension ibc granulites at shallow levels very rapid 5 mm yr extensional thinning subsequent to collision it is suggested that the preservation of ibc paths rather than itd paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust and that hybrid itd to ibc paths should also be observed most ibc granulites and probably many itd granulites have not been exposed at the earth s surface as a result of the tectonic episodes which produced them but have resided in the middle and lower crust for long periods of time 100 2000 ma following these events the eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation

structure of the merida andes venezuela relations with the south america caribbean geodynamic interaction for over 50 years several models based on diverse geologic concepts and variable quality of data have been proposed to explain the major structure and history of the merida andes ma in western venezuela lately this chain growth and associated flexural basins deepening have been related to incipient type a subductions of either polarity accounting for the across chain asymmetry however these recent models have not well integrated the present tectonically active setting driven by neighboring major plate interactions at present this chain exhibits ongoing strain partitioning where cumulative right lateral slip along chain axis is as much as half of or about the same as the transverse shortening since late miocene thus implying that the nne directed maracaibo block extrusion with respect to the south america sa plate is not a secondary feature consequently this paper discusses some limitations exhibited by the se directed continental subduction models maracaibo crust underthrusting the merida andes in the light of available geological and geophysical data besides it is herein proposed that the merida andes structuration is related to a nw directed gently dipping incipient type a subduction where chain growth and evolution are similar to those of a sedimentary accretionary wedge i e barbados but at crustal scale and with ongoing strain partitioning this continental subduction is the se portion of a major orogenic float that also comprises the perija range and the santa marta block d 2002 elsevier science b v all rights reserved

controls on forearc basin formation and evolution insights from oligocene to recent tectono stratigraphy of the lower magdalena valley basin of northwest colombia abstract mechanisms of forearc basin evolution remain not very well understood mostly because of insufficient resolution of their temporal evolution and potential drivers the convergent margin of northwest colombia where the san jacinto and lower magdalena valley forearc basins are located offers a unique opportunity to study such mechanisms the formation of the lower magdalena amagmatic forearc basin occurred in a stable setting from the oligocene to the present characterized by the slow and nearly orthogonal low angle subduction of the caribbean plateau we use an exceptional regional database to reconstruct the subsidence extension sedimentation and paleo geographic history of the lower magdalena forearc basin we propose possible mechanisms controlling its evolution in the absence of major changes in plate kinematics and in a low angle subduction setting following the collapse of a pre oligocene magmatic arc late oligocene to early miocene fault controlled subsidence allowed initial basin fill at relatively low sedimentation rates after the connection of the lower and middle magdalena valleys the proto magdalena river in the north and the proto cauca river in the south both started delivering high amounts of sediments in middle miocene times fault controlled subsidence was gradually replaced by subsidence due to increased sedimentary load increase in sediment flux would have also caused the formation of an accretionary prism weakened the plate interface through lubrication with fluid rich sediment and initiated underplating with the development of forearc highs in the san jacinto area inherited basement structures allowed the tectonic segmentation of the basin with the formation its two depocenters plato and san jorge our results highlight the fundamental role of sediment flux of the basement structure and of flat subduction on the evolution of forearc basins such as the lower magdalena

transient quaternary erosion and tectonic inversion of the northern range trinidad abstract cosmogenic 10be measured in sediments from northern range catchments on the island of trinidad reveals low millennial scale rates of erosion average 40 mm ka that increase roughly eightfold from 11 to 92 mm ka eastward across the mountain range these results in conjunction with an analysis of mountain morphometrics are consistent with quaternary east side up tilting of the northern range which has occurred recently 100 ka the highest millennial scale erosion rates coincide spatially with quaternary east side surface uplift albeit not in magnitude high modern rainfall rates low topographic relief and convex longitudinal stream profiles indicating transient quaternary erosion we interpret that hillslope adjustment and erosion of the northern range is buffered from climatic and tectonic forcings by dense tropical vegetation cover thick saprolite and porous regolith compared with longer term thermochronology exhumation rates we document that an order of magnitude deceleration of and reversal in the locus of erosion and exhumation has occurred during the pliocene to the holocene we suggest that these combined data sets highlight distinct syn and post collisional phases of northern range development that are related to a major change in relative caribbean south american plate motion from oblique convergence to transform motion oblique collision during the mid miocene likely caused relatively higher rates of and asymmetric exhumation of the northern range post collisional mountain scale tilting is likely caused by a combination of crustal extension in the gulf of paria and by deep subsurface lithospheric detachment processes that drives dynamic topography

ejection landslides triggered by the 2008 wenchuan earthquake and movement modelling using aerodynamic theory and artificial disintegration collision technique many ejection landslides in the 2008 wenchuan earthquake exhibited unusually dramatic behaviour in which a huge sliding mass was launched into the air and supported for a long distance because of the aerodynamic effect in this paper the formation conditions and movement characteristics of ejection landslides are analysed the flight behaviour of the centroid of the ejection mass is studied by numerical solution with and without the aerodynamic effect the impact and movement processes of the sliding mass are simulated by incorporating an advanced discretisation approach into discontinuous deformation analysis dda simulation results of the dayanke rock avalanche induced by the 2008 wenchuan earthquake show that the aerodynamic effect significantly affects its flight behaviour before collision simultaneously the colluvium on the slope surface remarkably reduces its traveling speed and runout moreover the initial angle between the velocity of the detached ejection mass and the horizontal direction strongly affects the flight distance and final deposition pattern of the avalanche

subduction system and flat slab beneath the eastern cordillera of colombia seismicity at the northern terminus of the nazca subduction is diffused over a wide area containing the puzzling seismic feature known as the bucaramanga nest we relocate about 5000 earthquakes recorded by the colombian national seismic network and produce the first 3 d velocity model of the area to define the geometry of the lithosphere subducting below the colombian andes we found lateral velocity heterogeneities and an abrupt offset of the wadati benioff zone at 5 n indicating that the nazca plate is segmented by an e w slab tear that separates a steeper nazca segment to the south from a flat subduction to the north the flat nazca slab extends eastward for about 400 km before dip increases to 50 beneath the eastern cordillera where it yields the bucaramanga nest we explain this puzzling locus of intermediate depth seismicity located beneath the eastern cordillera of colombia as due to a massive dehydration and eclogitization of a thickened oceanic crust we relate the flat subducting geometry to the entrance at the trench at ca 10 ma of a thick buoyant oceanic crust likely a volcanic ridge producing a high coupling with the overriding plate sub horizontal plate subduction is consistent with the abrupt disappearance of volcanism in the andes of south america at latitudes 5 n

upper mantle structure beneath southern african cratons from seismic finite frequency p and s body wave tomography abstract we present a 3d high resolution seismic model of the southern african cratonic region from teleseismic tomographic inversion of the p and s body wave dataset recorded by the southern african seismic experiment sase utilizing 3d sensitivity kernels we invert traveltime residuals of teleseismic body waves to calculate velocity anomalies in the upper mantle down to a 700 km depth with respect to the ak135 reference model various resolution tests allow evaluation of the extent of smearing effects and help defining the optimum inversion parameters i e damping and smoothness for regularizing the inversion calculations the fast lithospheric keels of the kaapvaal and zimbabwe cratons reach depths of 300 350 km and 200 250 km respectively the paleo orogenic limpopo belt is represented by negative velocity perturbations down to a depth of 250 km implying the presence of chemically fertile material with anomalously low wave speeds the bushveld complex has low velocity down to 150 km which is attributed to chemical modification of the cratonic mantle in the present model the finite frequency sensitivity kernels allow to resolve relatively small scale anomalies such as the colesberg magnetic lineament in the suture zone between the eastern and western blocks of the kaapvaal craton and a small northern block of the kaapvaal craton located between the limpopo belt and the bushveld complex

the crustal structure of the guayana shield venezuela from seismic refraction and gravity data abstract we present results from a seismic refraction experiment on the northern margin of the guayana shield performed during june 1998 along nine profiles of up to 320 km length using the daily blasts of the cerro bolivar mines as energy source as well as from gravimetric measurements clear moho arrivals can be observed on the main e w profile on the shield whereas the profiles entering the oriental basin to the north are more noisy the crustal thickness of the shield is unusually high with up to 46 km on the archean segment in the west and 43 km on the proterozoic segment in the east a 20 km thick upper crust with p wave velocities between 6 0 and 6 3 km s can be separated from a lower crust with velocities ranging from 6 5 to 7 2 km s a lower crustal low velocity zone with a velocity reduction to 6 3 km s is observed between 25 and 25 km depth the average crustal velocity is 6 5 km s the changes in the bouguer anomaly positive 30 mgal in the west and negative 20 mgal in the east cannot be explained by the observed seismic crustal features alone lateral variations in the crust or in the upper mantle must be responsible for these observations

receiver function study of the crustal structure of the southeastern caribbean plate boundary and venezuela 1 we have investigated crustal thickness and composition across the southeastern caribbean plate boundary with the receiver function technique we used teleseismic data recorded by a temporary broadband array deployed under the bolivar project and the permanent national seismic network of venezuela we used the primary p to s conversion and crustal reverberations to estimate crustal thickness and average crustal vp vs ratio over the region we observe large variations in crustal thickness and poisson s ratio estimated moho depth ranges from 16 km beneath the southeastern caribbean sea to 52 km beneath northeastern venezuela and the venezuelan andes there is a good correlation between crustal structure and tectonic terranes data from the precambrian guayana shield suggest that the underlying crustal structure is relatively uniform with a moderate thickness 37 km and an intermediate composition a thick crust is found below the foreland basins the two mountain systems in northern venezuela the serrania del interior and the serrania del falcon have a thin crust with arc composition and are likely dynamically supported by elastic rebound or underthrusting of the oceanic plateau that characterizes the southern caribbean on the other hand the venezuelan andes and perija range on the western side of the country are probably isostatically balanced by thick crustal roots

the caribbean south american plate boundary at 65 w results from wide angle seismic data 1 we present the results of the analysis of new wide angle seismic data across the caribbean south american plate boundary in eastern venezuela at about 65 w the 500 km long profile crosses the boundary in one of the few regions dominated by extensional structures as most of the southeastern caribbean margin is characterized by the presence of fold and thrust belts a combination of first arrival traveltime inversion and simultaneous inversion of pmp and pn arrivals was used to develop a p wave velocity model of the crust and the uppermost mantle at the main strike slip fault system we image the cariaco trough a major pull apart basin along the plate boundary the crust under the southern caribbean deformed belt exhibits a thickness of 15 km suggesting that the caribbean large igneous province extends to this part of the caribbean plate the velocity structures of basement highs and offshore sedimentary basins imaged by the profile are comparable to those of features found in other parts of the margin suggesting similarities in their tectonic history we do not image an abrupt change in moho depth or velocity structure across the main strike slip system as has been observed elsewhere along the margin it is possible that a terrane of caribbean island arc origin was accreted to south america at this site and was subsequently bisected by the strike slip fault system the crust under the continental portion of the profile is thinner than observed elsewhere along the margin possibly as a result of thinning during jurassic rifting

upper mantle seismic discontinuities and the thermal structure of subduction zones the precise depths at which seismic velocities change abruptly in the upper mantle are revealed by the analysis of data from hundreds of seismometers across the western united states the boundary near 410km depth is locally elevated that near 660 km depressed the depths of these boundaries which mark phase transitions provide an in situ thermometer in subduction zones the observed temperature contrasts require at least moderate thickening of the subducting slab near 660 km depth in addition a reflector near 210 km depth may mark the bottom of the aesthenosphere

a model for the runout analysis of rapid flow slides debris flows and avalanches runout analyses are used for risk assessment and design of remedial measures against rapid landslides such as debris flows debris avalanches rockslide avalanches large scale liquefaction failure

a sliding consolidation model for flow slides a model is proposed for the behaviour of flow slides in loose cohesionless materials in which the debris spreads out as a uniform sheet excess pore fluid pressures are assumed to be generated for example by undrained loading arising from the collapse of metastable structure in the source area of the flow slide under the influence of these the leading element of the debris moves downslope by basal sliding consolidating as it does so by single upward drainage as a result the pore pressure at the base of the element eventually decays to a value that brings the element to rest hence defining the run out of the flow slide the model is applicable in principle to subaqueous and subaerial flow slides and to pore fluids consisting of water gas or both of these the predictions of the model are tested against the observed behaviour and measured properties of the 1966 flow slide at aberfan south wales the results show that a sliding consolidation model provides a possible mechanism for that event

post failure mobility of saturated sands in undrained load controlled ring shear tests the undrained shear behavior of soils with progress of shear displacement is essential to the understanding of liquefied slope failures with long travel distance in this paper using a newly devel

sub slab mantle flow parallel to the caribbean plate boundaries inferences from sks splitting abstract upper mantle deformation near the margins of the caribbean plate is investigated using observations of shear wave splitting in teleseismic and local shear phases the caribbean plate is almost stationary in the hot spot reference frame and is wedged between the north america south america nazca and cocos plates collisional belts and major shear zones encircle the plate data from seismic stations operated by iris geoscope the venezuelan seismological network and the british geological survey have provided nearly 2000 seismic records for analysis analysis of shear wave splitting in teleseismic core phases e g sks at stations reveals fast shear wave polarisations that are conformal to the plate boundary paralleling the major structural features the magnitude of the splitting is in general quite large 1 2 2 1 s in northern venezuela the magnitude of splitting increases towards the caribbean south american collisional front local shear phases from earthquakes up to 200 km deep beneath northeastern venezuela show very small amounts of splitting 0 1 0 3 s analysis of the depth dependence in the magnitude of the splitting suggests that most of the upper mantle wedge is isotropic and the splitting in the local phases is mostly accrued in the crust and uppermost mantle in ne venezuela fast shear wave polarisations in both the local and teleseismic phases closely parallel the fault systems in the region suggesting that the crust and mantle are coupled in this area stations on the island of montserrat and near bucaramanga columbia also show very small magnitudes in local s wave splitting but large amounts of sks splitting in general the small magnitude of the local s phase splitting suggests that the teleseismic phases accrue considerable splitting beneath the top of the slab we interpret the bulk of the splitting in terms of sub slab flow that is forced around the caribbean plate due to the convergence of the surrounding plates

mantle flow beneath northwestern venezuela seismic evidence for a deep origin of the merida andes abstract we measured shear wave splitting from sks data recorded by the national seismic network of venezuela and a linear broadband passcal rice seismic array across the merida andes the linear array was installed in the second phase of the passive seismic component of the bolivar project to better understand the complicated regional tectonics in western venezuela using the method proposed by wolfe and silver 1998 sks waveforms from 2 to 36 earthquakes mostly from the tonga subduction zone were selected for each of the 23 stations in the region in order to do splitting analysis the fast polarization directions can be divided into 3 zones all in agreement with local gps data the first zone comprises the stations north of the dextral strike slip oca ancon fault these stations show the largest split times 1 6 3 2 s oriented in a roughly ew direction and are similar to splitting observations made further to the east along the strike slip plate boundary growdon et al 2009 we attribute this to trench parallel mantle flow that passes around the northwest corner of the subducting caribbean plate and along the northern edge of south america as proposed by russo and silver 1994 forming an eastward flow beneath the southern caribbean plate zone two is the merida andes with the right lateral bocono fault in the center where split orientations are at n45 e suggesting that the observed seismic anisotropy is likely caused by lithospheric deformation parallel to the bocono fault zone three is east of the bocono fault inside the barinas apure basin where the measured split times are smaller 0 8 s with an ew fast direction that is consistent with those observed at the guarico basin maturin basin and the guayana shield in the east and are interpreted as orientation with the motion of the continent

representing anisotropic subduction zones with isotropic velocity models a characterization of the problem and some steps on a possible path forward despite the widely known fact that mantle flow in and around subduction zones produces the development of considerable seismic anisotropy most p wave tomography efforts still rely on the assumption of isotropy in this study we explore the potential effects of erroneous assumption on tomographic images and explore an alternative approach we conduct a series of synthetic tomography tests based on a geodynamic simulation of subduction and rollback the simulation results provide a self consistent distribution of isotropic thermal anomalies and seismic anisotropy which we use to calculate synthetic delay times for a number of realistic and hypothetical event distributions we find that anisotropy induced artifacts are abundant and significant for teleseismic local and mixed event distributions the occurrence of artifacts is not reduced and indeed can be exacerbated by increasing richness in ray path azimuths and incidence angles the artifacts that we observe are in all cases important enough to significantly impact the interpretation of the images we test an approach based on prescribing the anisotropy field as an a priori constraint and find that even coarse approximations to the true anisotropy field produce useful results using approximate anisotropy fields can result in reduced rms misfit to the travel time delays and reduced abundance and severity of imaging artifacts we propose that the use of anisotropy fields derived from geodynamic modeling and constrained by seismic observables may constitute a viable alternative to isotropic tomography that does not require the inversion for anisotropy parameters in each node of the model

intermediate depth earthquake faulting by dehydration embrittlement with negative volume change earthquakes are observed to occur in subduction zones to depths of approximately 680 km even though unassisted brittle failure is inhibited at depths greater than about 50 km owing to the high pressures and temperatures it is thought that such earthquakes particularly those at intermediate depths of 50 300 km may instead be triggered by embrittlement accompanying dehydration of hydrous minerals principally serpentine a problem with failure by serpentine dehydration is that the volume change accompanying dehydration becomes negative at pressures of 2 4 gpa 60 120 km depth above which brittle fracture mechanics predicts that the instability should be quenched here we show that dehydration of antigorite serpentinite under stress results in faults delineated by ultrafine grained solid reaction products formed during dehydration this phenomenon was observed under all conditions tested pressures of 1 6 gpa temperatures of 650 820 degrees c independent of the sign of the volume change of reaction although this result contradicts expectations from fracture mechanics it can be explained by separation of fluid from solid residue before and during faulting a hypothesis supported by our observations these observations confirm that dehydration embrittlement is a viable mechanism for nucleating earthquakes independent of depth as long as there are hydrous minerals breaking down under a differential stress

k ar and ar ar geochronology of the himalayan collision in nw pakistan constraints on the timing of suturing deformation metamorphism and uplift mica and hornblende k ar and ar ar data are presented from each of the three crustal components of the himalayan collision zone in north pakistan the asian plate the kohistan island arc and the indian plate together with u pb and rb sr data published elsewhere these new data 1 date the age of suturing along the northern suture which separates kohistan from asia at 102 85 ma 2 establish that the basic magmatism in kohistan which postdates collision along the northern suture predates 60 ma and that the later granite magmatism spanned a range of 60 25 ma 3 show that uplift amounts within kohistan are greater toward the nanga parbat syntaxis than away from it and that rate of uplift near the syntaxis increased over the last 20 ma to a current figure of about 5 5 mm a year 4 show that much of southern kohistan had cooled to below 500 c by 80 ma and that the major deformation which imbricated kohistan probably predated 80 ma and certainly predated 60 ma and was related to the kohistan asia collision rather than the kohistan india one 5 imply that uplift along the hunza shear in the asian plate together with imbrication of the metamorphics in its hanging wall took place at about 10 ma and was associated with breakback thrusting in the hanging wall of the main mantle thrust 6 suggest that the indian plate has a lengthy pre himalayan history with an early metamorphism at about 1900 ma major magmatism at 500 550 ma and early jurassic lithospheric extension or inversion and 7 show that the indian plate rocks were metamorphosed shortly after the collision within kohistan which occurred at circa 50 ma and subsequently cooled back through 500 c at circa 38 ma and 300 c at 30 35 ma with ages of cooling through 200 and 100 c as determined by fission track data locally controlled by nanga parbat related uplift tectonics

lithosphere tearing at step faults response to edges of subduction zones abstract slab edges are a relatively common feature in plate tectonics two prominent examples are the northern end of the tonga subduction zone and the southern end of the new hebrides subduction zone near such horizontal terminations of subduction trenches ongoing tearing of oceanic lithosphere is a geometric consequence we refer to such kinks in the plate boundary as a subduction transform edge propagator or step other steps are the north and south ends of the lesser antilles trench the north end of the south sandwich trench the south end of the vrancea trench and both ends of the calabria trench volcanism near steps is distinct from typical arc volcanism in some cases slab edges appear to coincide with mantle plumes using 3d mechanical models we establish that step faults are stable plate tectonic features in most circumstances in the probably rare cases that the resistance to fault propagation is high slab break off will occur relative motion along the transform segment of the plate boundary often is non uniform and the step is not a transform plate boundary in the rigid plate tectonics sense of the phrase step propagation may result in substantial deformation rotation topography and sedimentary basins with a very specific time space evolution surface velocities are substantially affected by nearby steps

pressure temperature time paths and a tectonic model for the evolution of granulites petrologic studies and application of well calibrated mineralogic thermometers and barometers reveal several important features common to many granulite terrains 1 peak metamorphic pressure and temperature conditions cluster around values of 7 5 pm 1 kbar and 800 pm 50 c implying average geothermal gradients of 30 35 c km 2 in comparison with upper amphibolite facies rocks especially in paired amphibolite granulite terrains it is evident that granulites are distinguished from amphibolites by the former s higher metamorphic temperatures but not higher pressure 3 initial cooling of granulite terrains from peak conditions is nearly isobaric for garnet closure temperatures of 600 pm 50 c the dp dt retrograde paths inferred from compositionally zoned garnet rims are 2 8 b c in most granulites 4 primary coarse sillimanite is found in most granulites and retrograde kyanite is known in a few these data suggest that granulites form as a result of anomalous thermal gradients

characterizing the caribbean south american plate boundary at 64 w using wide angle seismic data 1 we present wide angle velocity modeling results from profile 64 w of the broadband ocean land investigation of venezuela and the antilles arc region bolivar project line 64w is a 460 km long approximately north south onshore offshore reflection refraction transect located approximately at 64 w longitude the profile extends across the transform plate boundary between the southeastern caribbean car and south american sa plates east of the profile the plate boundary bends to the north and sa subducts beneath car we utilize first arrival tomography to resolve a smooth velocity field for the sediments and upper middle crust and then use a layered approach to resolve a sharp velocity contrast for the moho simultaneously inverting refracted pn and reflected pmp arrivals we image crustal and upper mantle structure across the plate boundary zone we interpret that the strike slip system that accommodates relative motion between car and sa extends near vertically through the entire crust and offsets the moho we see no evidence supporting a major component of convergence and rather than a wide boundary zone of overlapping lithospheric plates we interpret the plate boundary to be confined to the 33 km wide near vertical strike slip system previously interpreted thrust faults flanking the strike slip system appear to be confined to the upper middle crust and may be related to the detachment of subducting south american lithosphere at the southern terminus of the lesser antilles subduction zone east of 64 w

experimental study on the shearing behavior of saturated silty soils based on ring shear tests the shearing behavior of saturated silty soils has been examined extensively by performing undrained and partially drained the upper drainage valve of the shear box was open during shearing ring shear tests on mixtures of a sandy silt with different loess contents by performing tests at different initial void ratios the shear behavior of these silty soils at different initial void ratios is presented and discussed undrained shear test results showed that the liquefaction phenomena in ring shear tests were limited within the shear zone for a given void ratio or interfine void ratio both the peak and steady state shear strengths decreased with increase of loess content the partially drained shear tests revealed that a great reduction in the shear strength could result after the shear failure due to the buildup of excess pore water pressure within the shear zone the magnitude of reduction in shear strength after failure was affected by the initial void ratio the shear speed after failure as well as the loess content in the sample for a given void ratio or interfine void ratio with increase of loess content the drained peak shear strength became smaller while the brittleness index became greater it was also found that due to localized shearing the permeability of the soil within the shear box after drained shearing could be three orders of magnitude smaller than before shearing

mapping the subducting pacific slab beneath southwest japan with hi net receiver functions abstract we have used 4th root receiver function stacks and pre stack receiver function depth migrations to study the transition zone discontinuity structure beneath southwestern japan receiver functions were calculated from the quiet short period seismograms recorded by a recently deployed borehole network hi net we found that a relatively broad frequency band can be retrieved from a short period seismogram by a deconvolution of the instrument response the quality of the receiver functions formed from large earthquake recordings is comparable to those from broadband instruments we applied common conversion point gathering to the receiver function data to image the p to s conversion events beneath the network by stacking with a 4th root technique to improve lateral coherence we found that the topographic anomalies of the 410 and 660 km discontinuities beneath southwest japan have very different length scales the former is characterized by a narrow 150 200 km wide topographic high while the latter exhibits a broad 400 km wide moderate topographic low together with a small scale larger amplitude depression a 2 5d pre stack depth migration of the receiver functions shows the transition zone features clearly as well as images of a change of slope in the subducting slab at the 410 discontinuity and flattening of the slab onto the 660 km these observations show that the subducted pacific slab is deflected when it encounters the upper and lower boundaries of the transition zone and is flat lying either above or across the 660 km discontinuity the flat lying slab is however restricted to the bottom of the transition zone and probably experiences much less thickening than is suggested by some global tomographic images in which subhorizontal high velocity anomalies are seen throughout the transition zone between the two discontinuities

subduction driven recycling of continental margin lithosphere seismic images of the subducted atlantic slab under northeastern south america and the alboran slab beneath the gibraltar arc region show that subducting oceanic plates viscously entrain and remove the bottom of the continental thermal boundary layer from adjacent continental margins driving surface tectonics and pre conditioning the margins for further deformation alan levander and co authors present seismic images that show larger than expected volumes of high seismic velocity mantle associated with the subducted atlantic slab under northeastern south america and the alboran slab beneath the gibraltar arc region the anomalous regions are under and aligned with the continental margins at depths greater than 200 km with the lithospheric mantle under the continental margins significantly thinner than expected the authors conclude that subducting oceanic plates viscously entrain and remove the bottom of the continental thermal boundary layer from adjacent continental margins driving surface tectonics and preconditioning the margins for further deformation by creating topography along the lithosphere asthenosphere boundary whereas subduction recycling of oceanic lithosphere is one of the central themes of plate tectonics the recycling of continental lithosphere appears to be far more complicated and less well understood1 delamination and convective downwelling are two widely recognized processes invoked to explain the removal of lithospheric mantle under or adjacent to orogenic belts2 3 4 5 here we relate oceanic plate subduction to removal of adjacent continental lithosphere in certain plate tectonic settings we have developed teleseismic body wave images from dense broadband seismic experiments that show higher than expected volumes of anomalously fast mantle associated with the subducted atlantic slab under northeastern south america and the alboran slab beneath the gibraltar arc region6 7 the anomalies are under and are aligned with the continental margins at depths greater than 200 kilometres rayleigh wave analysis8 9 finds that the lithospheric mantle under the continental margins is significantly thinner than expected and that thin lithosphere extends from the orogens adjacent to the subduction zones inland to the edges of nearby cratonic cores taking these data together here we describe a process that can lead to the loss of continental lithosphere adjacent to a subduction zone subducting oceanic plates can viscously entrain and remove the bottom of the continental thermal boundary layer lithosphere from adjacent continental margins this drives surface tectonics and pre conditions the margins for further deformation by creating topography along the lithosphere asthenosphere boundary this can lead to development of secondary downwellings under the continental interior probably under both south america and the gibraltar arc8 10 and to delamination of the entire lithospheric mantle as around the gibraltar arc11 this process reconciles numerous sometimes mutually exclusive geodynamic models proposed to explain the complex oceanic continental tectonics of these subduction zones12 13 14 15 16 17

geo tectonic framework of the himalaya of n pakistan in the karakorum range there is a structurally complicated cretaceous are comprising the kohistan sequence on its northern side the northern suture consists of a mega melange and is bounded to the s by tightly folded pillow bearing volcanics and sediments to the s the kohistan plutonic belt consists of southwards a early foliated and late post tectonic tonalites and diorites b aplites and pegmatites up to 30 of rock volume c basic dykes up to 10 m thick d the chilas complex a stratiform cumulate body over 300 km long and 8 km thick chromite layered dunites gabbros and norites with a low pressure granulite facies mineral fabric of tectonic origin e an amphibolite belt with a complex mixture of other rocks and f the jijal complex a 200 km 2 tectonic wedge of high pressure granulites and chromite layered dunites cumulate graded units in the chilas complex show that it is folded by an isoclinal anticline f 1 the mid upper crust of the are is folded by a 50 km half wavelength f 2 syncline the whole kohistan sequence with its two phases of isoclinal folds was tilted during himalayan collision so that the structures are now subvertical the southern suture main mantle thrust has a wedge of glaucophane schists the indian plate contains a basement of psammites and schists intruded by cambrian granites and overlain by isoclinally folded and metamorphosed carbonates and shales

the tectonic history of kohistan and its implications for himalayan structure the tectonic history of kohistan northern pakistan involves two collisional events cleavage and folding developed at 90 100 ma along the northern suture between the kohistan island arc and the asian plate at the same time there was major folding and shearing of the lower part of the kohistan arc approximately 100 km south of the suture this deformation was followed by ocean subduction south of the kohistan arc generating the kohistan calc alkaline batholith with subsequent ocean closure during the eocene and obduction of the kohistan arc together with the adjacent part of the asian plate over the indian continental crust the construction of balanced cross sections through the imbricated upper part of the indian continental crust in the footwall to this southern suture indicates a minimum displacement of 470 km requiring the western himalayan hinterland to be underlain by a large wedge of indian middle to lower crust there is some shortening of the overriding kohistan and asian plates by thrusts and shear zones but it is insufficient to satisfy the palaeomagnetic data there must be major crustal shortening involving thrusts in the hindu kush and pamirs north of kohistan the post eocene thrust direction which for most of pakistan is towards 160 is almost perpendicular to that immediately to the east in the himalayan belt generating complex refolded thrust patterns in the hazara syntaxis and large scale folding and rapid uplift with associated brittle faulting and seismic activity adjacent to the nanga parbat syntaxis these different thrust trends indicate that major thrust movement as well as the folds and deformation fabrics cannot always be related to plate movement vectors but are modified by or develop from complex rotations during place collision or from the gravitational spreading of a thickened crust a regional approach is required to recognize and correctly attribute the various components in thrust displacements

seismic heterogeneity and small scale convection in the southern california upper mantle we invert traveltime residuals of teleseismic p and s phases for 3 d perturbations in vp vs and vp vs structures in the southern california upper mantle the tomographic inversion uses frequency dependent 3 d sensitivity kernels to interpret traveltime residuals measured in multiple frequency bands and recent advances in regional crustal thickness and velocity models to better isolate the mantle component of traveltime residuals in addition to separate vp and vs tomography we jointly invert the p and s data sets for vp vs perturbations by imposing a smoothness constraint on the lnvs lnvp field the regional upper mantle is very heterogeneous with the greatest amplitude of velocity and vp vs perturbations in the upper 200 km and mantle structures that correlate spatially with the major physiographic provinces of southern california our imaging improves the resolution of the geometry and amplitude of these features in addition to the major structures imaged and discussed in previous papers we find a large high velocity anomaly at depths between 340 and 500 km beneath the borderland transverse ranges and northern peninsular ranges this anomaly is separate from the well known uppermost mantle transverse ranges anomaly which extends no deeper than 175 200 km the strongest low velocity high vp vs anomaly is found beneath and west of the salton trough and is attributed to relatively high partial melt in the asthenosphere as a result of lithospheric thinning the magnitudes of vp vs and vp vs perturbations and knowledge of regional 1 d velocity and attenuation lead us to conclude that the asthenosphere contains up to 1 partial melt extending to depths as great as 150 200 km

intermediate depth seismicity in northern colombia and western venezuela and its relationship to caribbean plate subduction the recent intermediate depth seismicity in northern colombia and western venezuela was analyzed to understand its origin and its presumed relationship to a subducted lithospheric slab in northwestern south america the area included in this study is located to the north and east of the bucaramanga nest which is a particular region beneath bucaramanga northern colombia that presents a high concentration of intermediate depth earthquakes to the north of the nest the seismicity of the area is sparse and most of the events are of low magnitude mb 5 l thus only 23 earthquakes were large enough to be investigated using teleseismic data the focal parameters of the two largest events mb 5 4 were obtained from the formal inversion of long period body waves recorded at teleseismic distances the focal mechanisms of 10 more events were determined from first motion data in total the focal mechanisms of 12 events were determined from both the inversion of p and s h waveforms and the polarities of first arrivals for the smaller earthquakes the focal depths were calculated by measuring the observed pp p interval time and comparing it to the theoretical travel time tables the isodepth curves reflect a slab striking in a nne ssw direction and dipping approximately at 25 32 to the southeast this observation is corroborated by the direction and plunge of the t axes of the focal mechanisms which are generally parallel to the gradient of the slab defined by the spatial distribution of hypocenters these results indicate that the intermediate depth earthquakes in western venezuela and northern colombia are apparently related to the presence of a continuous lithospheric slab subducted near the northern coast of colombia the two largest earthquakes located at a significant distance from the bucaramanga nest present similar fault plane solutions moreover they also agree with those of the two largest earthquakes reported inside the nest this similarity suggests that the bucaramanga nest lies on the same subducted slab where the other earthquakes occur there is not enough shallow seismicity to define the location where the caribbean lithosphere subducted beneath the south american plate however the extension of the slab toward the surface inferred from the intermediate depth seismicity suggests that the subducted lithosphere may still be attached to the caribbean plate

seismicity and tectonics of the southeastern caribbean we present 33 new focal mechanisms for se caribbean earthquakes 1963 1988 we use these mechanisms in conjunction with 28 previously available mechanisms to distinguish between two models of plate boundary zone interaction in the se caribbean the trench trench transform and hinge faulting model and the right oblique collision model shallow 0 70 km and intermediate 70 200 km depth earthquakes occur in the study region we focus on the tectonic causes of these events and the motions they delineate the shallow earthquakes are in a broad linear zone which trends ne from the paria peninsula of venezuela towards barbados intermediate depth earthquakes cluster beneath and nw of the peninsula and deepen to the nw perpendicular to the ne trending shallow events the vertical distribution of the earthquakes suggests a slab with steep nw dip shallow dextral strike slip on e striking faults is restricted to a 60 km wide linear zone between the gulf of cariaco and the western margin of the gulf of paria dextral strike slip is active only as far east as the gulf of paria and not within or east of trinidad shallow thrust events with ene striking planes distributed between the araya peninsula and the gulf of paria indicate collision at crustal levels between south america and caribbean and that folding and thrusting are still active over a 60 km interval south of the araya paria isthmus active thrusting in venezuela corroborates predictions of transpression between caribbean and south america and discounts transtensional motions between the two plates in the se caribbean the conjunction of shallow thrust strike slip and normal earthquakes in the gulf of paria at around 62 3 may be the expression of unpartitioned oblique compressive deformation in the plate boundary zone intermediate 165 km h 70 km depth thrust and dip slip events within the nw dipping slab indicate that oceanic lithosphere probably originally attached to south america subducts to the nw beneath the caribbean plate shallow normal faulting events e and ne of trinidad are expressions of plate bending about near horizontal axes parallel to the lesser antilles subduction zone we conclude that the earthquake mechanisms provide strong support for the right oblique collision model of caribbean south american plate interaction

flowslides in stockpiled coal abstract occasional deep seated flowslides in coking coal stockpiles at the north queensland port of hay point are described the flowslides result from particularly loose placement of the light cohesionless material and saturation of the lower levels of the stockpile due to rainfall infiltration and initial moisture at placement consolidated undrained triaxial testing has been used to define the density stress conditions at which liquefaction occurs the flowslide mechanism field verification and preventative measures are briefly discussed

effect of irrigation induced densification on the post failure behavior of loess flowslides occurring on the heifangtai area gansu china abstract irrigation resulted in serious ground settlement on the heifangtai terrace in china and also repeatedly triggered a lot of loess flowslides on the margin of the terrace to study the possible effect of resultant loess densification accompanying the ground settlement on the sliding behavior of these landslides we took loess samples from the source area of a landslide in the heifangtai terrace and performed a series of undrained ring shear tests to examine the undrained shear behaviors of these samples at differing initial void ratios results showed that all these tested samples exhibited purely contractive behavior throughout the entire undrained shearing process the loose samples suffered collapse failure behavior while those relatively denser samples experienced gradual increase in pore water pressure after the shear failure occurred the peak and steady state shear strengths increased as void ratio decreased however the brittleness index decreased with reduction of void ratio until a certain value after that it increased with further reduction of void ratio therefore we concluded that irrigation induced densification may have affected the occurrence and mobility of landslides occurring on the terrace margin of the heifangtai area

calculation and application of clinopyroxene garnet plagioclase quartz geobarometers recently published thermodynamic and experimental data in a variety of chemical systems have been evaluated to derive gibbs free energies for hedenbergite and pyrope these were used to calculate the geobarometric equilibria

undrained shear behaviour of sands subjected to large shear displacement and estimation of excess pore pressure generation from drained ring shear tests undrained shear behaviour of fine silica and weathered granitic sand subjected to large shear displacement is examined parallel experiments using ring shear and the triaxial compression tests on soil specimens through a wide range of initial void ratios were conducted to investigate undrained shear strength as the key factor in the flow like motion of landslides the steady state undrained shear strengths achieved in ring shear tests were in general smaller than those in the triaxial compression tests probably because of the excess pore pressure generation by grain crushing within the shear zone that occurred in ring shear very low steady state shear strengths were achieved however in triaxial compression tests on the dense silica sand in which well defined shear surfaces developed in the cylindrical specimens in these triaxial compression tests shear deformation must have been concentrated on these surfaces to generate excess pore pressure similar to that found in ring shear tests an attempt wa

tectonic evolution of the caribbean the development of the caribbean is discussed in terms of modern tectonic theory the nature of the site on which the caribbean formed is examined and the development of the rifted margins of the caribbean is described constraints on caribbean evolution from the relative motions of north and south america are briefly examined and the caribbean oceanic plateau is discussed the great island arc system of the caribbean is addressed in detail emphasizing the way the great arc of the caribbean was segmented the role of central america in caribbean history is briefly considered

paleomagnetism and magnetic fabric of the eastern cordillera of colombia evidence for oblique convergence and nonrotational reactivation of a mesozoic intracontinental rift we report the paleomagnetic and magnetic fabric results of 58 sites from cretaceous miocene marine and continental strata from the eastern cordillera ec and the cucuta zone at the junction between the santander massif and the merida andes of colombia the ec is an intracontinental doubly vergent range inverting a triassic to early cretaceous rift zone twenty three sites reveal nonsystematic tectonic rotations including unrotated areas of the ec range with respect to stable south america our data show that the ec inverted a nne oriented rift zone and that the orientation of the mesozoic rift and the mountain chain roughly correspond interestingly magnetic lineations from anisotropy of magnetic susceptibility analysis do not trend parallel to the chain but rather are oblique to the main orogenic trend by also considering gps evidence of a 1 cm yr ene displacement of central western colombia accommodated by the ec we suggest that the miocene recent deformation event of this belt arises from ene oblique convergence reactivating a nne oriented rift zone oblique shortening was likely partitioned into pure dip slip shear characterizing thick skinned frontal thrust sheets well known along both chain fronts and by range parallel right lateral strike slip faults which have not been identified yet but likely exist in the axial part of the ec finally the 35 9 clockwise rotation observed in four post miocene magnetically overprinted sites from the cucuta zone reflects late cenozoic and ongoing right lateral strike slip displacement occurring along faults parallel to the bocono fault system possibly connected with the right lateral faults inferred to exist along the axial part of the ec

subduction in the southern caribbean images from finite frequency p wave tomography 1 the eastern boundary of the caribbean plate is marked by subduction of the atlantic under the lesser antilles the southeastern plate boundary is characterized by a strike slip margin while different configurations of subduction of the southwest caribbean under south america have been proposed we investigate the slab geometry in the upper mantle using multiple frequency teleseismic p wave tomography waveforms from p and pkpdf phases from 285 mb 5 0 events occurring at epicentral distances from 30 to 90 and greater than 150 were bandpass filtered and cross correlated to obtain up to three sets of delay times for each event the delay times were inverted using approximate first fresnel zone sensitivity kernels our results show the subducting atlantic slab as well as a second slab in the west of the study area that we interpret as a subducting fragment of the caribbean plate both slabs have steep dips where imaged and can be traced to depths greater than 600 km these results are consistent with transition zone boundary topography as determined by receiver function analysis the atlantic slab extends continent ward south of the plate bounding strike slip margin we interpret this extension as continental margin lithospheric mantle that is detaching from beneath south america and subducting along with the oceanic atlantic slab the steep subduction of the caribbean occurs 500 km landward from the trench implying an initial stage of shallow subduction as far to the east as the lake maracaibo merida andes region as has been inferred from intermediate depth seismicity

thermal modelling of the laramide orogeny testing the flat slab subduction hypothesis abstract the laramide orogeny is the late cretaceous to palaeocene 80 55 ma orogenic event that gave rise to the rocky mountain fold and thrust belt in canada the laramide block uplifts in the usa and the sierra madre oriental fold and thrust belt in mexico the leading model for driving laramide orogenesis in the usa is flat slab subduction whereby stress coupling of a subhorizontal oceanic slab to the upper plate transmitted stresses eastwards producing basement cored block uplifts and arc magmatism in the foreland the thermal models presented here indicate that arc magma generation at significant distances inboard of the trench 600 km during flat slab subduction is problematic this conclusion is consistent with the coincidence of volcanic gaps and flat slab subduction at modern convergent margins lawsonite eclogite xenoliths erupted through the colorado plateau in oligocene time are inferred to originate from the subducted farallon slab and indicate that the laramide flat slab subduction zone was characterised by a cold thermal regime thermal modelling indicates that this regime can be produced by flat slab subduction of old 50 myr oceanic lithosphere at high convergence rates in the canadian and mexican portions of the laramide orogen the coeval development of a magmatic arc within 300 km of the trench refutes the existence of flat slab subduction in these regions it is proposed that subduction of an oceanic plateau aseismic ridge may have overcome the negative buoyancy inherent in old oceanic lithosphere and resulted in a spatially restricted zone of flat slab subduction in the usa these findings cast doubt on the flat slab model as a primary means of driving laramide orogenesis along its entire length and instead point to the need for an alternative mechanism for cordilleran wide laramide orogenesis

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

subduction of the eastern panama basin and seismotectonics of northwestern south america spreading along the cocos nazca plate boundary since the breakup of the farallon plate in the miocene has resulted in the formation of the panama basin and a complex interaction of plates in and near northwestern south america current plate boundaries have been defined and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes considering only well located events and through focal mechanism determinations the existence of relict plate boundaries bathymetric features and the panamanian isthmus has affected the subduction process of the nazca plate beneath south america and determined the present day configuration of the subducting lithospheric plate there is no single triple junction separating the caribbean south american and nazca plates instead the panamanian isthmus and surrounding areas are accommodating east west compression and a lesser degree of north south compression along a series of thrust faults striking nw to ne and the andean ranges of ecuador colombia and venezeula are moving as a block nne relative to the rest of the south american plates along a system of faults following the front of the eastern cordillera the subducted portions of the panama basin and old farallon plate have become segmented into three pieces recognized in this study from north to south they are 1 a bucaramanga segment continuous with the caribbean seafloor northwest of colombia 2 a cauca segment continuous with oceanic crust nazca plate currently being subducted beneath south america at the colombia ecuador trench and 3 an ecuador segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern peru the segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted

liquefaction and flow failure during earthquakes two aspects of seismically induced liquefaction are discussed which are of vital engineering significance the triggering condition and the consequences of liquefaction the triggering condition is examined with respect to liquefaction analysis note being taken of the onset condition which is governed by cyclic strength consequences of liquefaction are discussed with respect to post seismic stability analysis in which the residual strength plays a major role procedures used for liquefaction analysis based on the results of in situ sounding tests are introduced and the applicability of this method for estimating associated ground settlements is discussed the evaluation of residual strength requires a better understanding of undrained sand behaviour results of extensive laboratory rests on japanese standard sand are examined and new index parameters are proposed to quantify undrained sand behaviour better the results of laboratory tests on silty sands are examined in the same way ah the results are

upper mantle structure beneath the caribbean south american plate boundary from surface wave tomography 1 we have measured shear wave velocity structure of the crust and upper mantle of the caribbean south american boundary region by analysis of fundamental mode rayleigh waves in the 20 to 100 s period band recorded at the bolivar geodinos stations from 2003 to 2005 the model shows lateral variations that primarily correspond to tectonic provinces and boundaries a clear linear velocity change parallels the plate bounding dextral strike slip fault system along the northern coast of venezuela illustrating the differences between the south american continental lithosphere the venezuelan archipelago and the caribbean oceanic lithosphere at depths up to 120 km beneath the venezuelan andes and the maracaibo block there is evidence of underthrusting of the caribbean plate but there is no other evidence of subduction of the caribbean plate beneath the south american plate in eastern venezuela linear crustal low velocities are associated with the fold and thrust belts whereas as higher crustal velocities are imaged in the guayana shield lithosphere the subducting oceanic part of the south american plate is imaged beneath the antilles arc the surface wave images combined with seismicity data suggest shear tearing of the oceanic lithosphere away from the buoyant continental south american plate offshore of northeastern venezuela the continental lithosphere south of the slab tear is bent down toward the plate boundary in response to the propagating tear in the lithosphere we interpret a nearly vertical low velocity column west of the tear centered beneath the cariaco basin with three dimensional asthenospheric flow around the southern edge of the subducting oceanic lithosphere with the asthenosphere escaping from beneath continental south america and rising into the plate boundary zone the complex plate boundary structure is best examined in three dimensions we discuss the new surface wave tomographic inversion in the context of results from other researchers including local seismicity teleseismic shear wave splits and interpretations from active source profiling