ocean drilling program leg 112 peru continental margin part 1 tectonic history studies of ocean drilling program leg 112 cores and geophysical data define early eocene quaternary normally faulted continental sediment sequences across the peru margin to the middle slope and a 15 km wide late miocene quaternary accretionary complex beneath the lower slope the eocene shelf and upper slope sea floor were eroded during the oligocene and in middle miocene the seaward part had subsided to lower slope depths this subsidence and the missing paleogene continental slope indicate tectonic erosion during normal plate convergence subduction of the nazca ridge locally accelerated tectonic erosion leaving a scarp but no associated compressional structure nearby late miocene sediment accreted against the erosional scarp local tectonic mechanisms of forearc basin subsidence are suggested by the rapid subsidence of a late miocene forearc basin and stability of the adjacent one despite similar histories of plate convergence

neogene tectonic evolution and exhumation of the southern ecuadorian andes a combined stratigraphy and fission track approach abstract coastal marine and continental sedimentary facies of middle to late miocene age are exposed in the andes of southern ecuador cuenca giron santa isabel loja malacatos vilcabamba and catamayo gonzanama basins the chronostratigraphy of the basin series was established by zircon fission track dating on a total of 120 tephra layers subsequently the timing of tectonic events was estimated through the well dated stratigraphic sequences and intervening unconformities sedimentation from 15 to 9 ma termed pacific coastal stage was dominantly of coastal marine type extending over an area far greater than the present basin perimeters it ended when a period of east west oriented compression at 9 5 8 ma exhumed the region and sedimentation was then restricted to smaller basins termed intermontane stage these late miocene continental sediments were for the first time sourced from the west in the rising western cordillera apatite fission track analysis was applied to some of the tephras in the cuenca basin and also to the older eocene 42 35 ma quingeo basin series in order to quantify the basin histories with respect to timing and amount of burial and later exhumation in the quingeo basin burial of the oldest sediments reached temperatures of 100 c at 18 ma when they started to cool down during a period of exhumation this process preceded the pacific coastal stage development of the other basins in the cuenca basin the oldest sediments were buried to temperatures of ca 120 c by 9 ma when a period of inversion began and a phase of erosion was dominant this timing correlates well with that estimated from structural evidence at ca 6 ma the cooling rate slowed down and maybe even reverted to a small increase in temperature until 3 ma when the final stages of exhumation took place assuming a geothermal gradient of 35 c km total uplift for this part for ecuador is about 6100 m over the last 9 million years assuming a steady state continuous movement this means a mean rock uplift rate of 0 7 mm yr and a surface uplift of 0 3 mm yr to the present

segmentation of the nazca and south american plates along the ecuador subduction zone from wide angle seismic profiles we describe the deep structure of the south colombian northern ecuador convergent margin using travel time inversion of wide angle seismic data recently collected offshore the margin appears segmented into three contrasting zones in the north zone affected by four great subduction earthquakes during the 20th century normal oceanic crust subducts beneath the oceanic cretaceous substratum of the margin underlined by seismic velocities as high as 6 0 6 5 km s in the central zone the subducting oceanic crust is over thickened beneath the carnegie ridge a steeper slope and a well developed high velocity cretaceous oceanic basement characterizes the margin wedge this area coincides with a gap in significant subduction earthquake activity in the south zone the subducting oceanic crust is normal the fore arc is characterized by large sedimentary basins suggesting significant subsidence velocities in the margin wedge are significantly lower and denote a different nature or a higher degree of fracturing even if the distance between the three profiles exceeds 150 km the structural segmentation obtained along the ecuadorian margin correlates well with the distribution of seismic activity and the neotectonic zonation

seabeam and seismic reflection imaging of the tectonic regime of the andean continental margin off peru 4 s to 10 s abstract marine geophysical surveys employing seabeam multi and single channel seismic reflection gravity and magnetic instruments were conducted at two locations along the continental slope of the peru trench during the seaperc cruise of the r v jean charcot in july 1986 these areas are centered around 5 30 s and 9 30 s off the coastal towns of paita and chimbote respectively these data indicate that 1 the continental slope off peru consists of three distinct morpho structural domains from west to east are the lower middle and upper slopes instead of just two as previously reported 2 the middle slope has the characteristics of a zone of tectonic collapse at the front of a gently flexured upper slope 3 the upper half of the lower slope appears to represent the product of mass wasting 4 thrusting at the foot of the margin produces a continuous morphologic feature representing a deformation front where the products of mass wasting are overprinted by a compressional tectonic fabric 5 a change in the tectonic regime from tensional to compressional occurs at the mid slope lower slope boundary the accretionary prism being restricted to the very base of the lower slope in the paita area the andean margin off peru is an extensional active margin or a collapsing active margin developing a subordinated accretionary complex induced by massive collapse of the middle slope area

tracking ancient magmatism and cenozoic topographic growth within the northern andes forearc constraints from detrital u pb zircon ages a comparison of detrital zircon ages from the cenozoic forearc basins in sw ecuador nw peru and those derived from the amotapes massif offers a singular opportunity to shed some light on the evolution of the nw andean margin of south america an extensive set of pre andean u pb zircon ages older than late cretaceous was obtained from 18 detrital samples coming from the talara zorritos progreso and gulf of guayaquil tumbes forearc basins as well as from two metasedimentary rocks of the amotapes massif u pb age analysis on detrital zircon grains allows a general source to sink scenario to be determined for this section of the andes these analyses give consistent insights into the early topographic growth during the modern andean orogeny the occurrence of proto andean magmatic periods in the western margin of gondwana and the evolution of the amotapes massif within the geodynamical setting of northwestern south america zircon ages at ca 320 ma and neoproterozoic clusters for the amotapes metasediments show significant similarities along the strike of the metamorphic belt thus defining a common metasedimentary origin and can be compared with previously dated rocks from the eastern cordillera of peru these strong similarities demonstrate the presence of a wide polyphased metamorphic belt that includes the western parts of the eastern cordillera the northern section of the occidental cordillera of peru and the amotapes massif detrital sediments of the forearc are marked by a major gap in early cretaceous jurassic ages and the minor presence of ordovician famatinian orogeny and carboniferous permian 340 285 ma ages the age spectrum is not compatible with sediment input from the eastern andes and suggests that the external areas of the andean chain e g the western cordillera were already uplifted prior to or immediately after forearc initiation during the paleocene thus representing a pervasive drainage divide throughout all of the cenozoic the amotapes massif or other terrains sharing the same age affinities e g the olmos terrane occidental cordillera of peru are likely to represent the best potential sources for pre andean zircon grains in the forearc age clusters in both source and sink at ca 600 ma 570 560 ma and 520 ma most likely define magmatic periods and may represent first order constraints for the difficult correlation between proto andean magmatic and orogenic periods to the north and south of the huancabamba deflection it is unlikely that the neoproterozoic zircon grains came from the pan african braziliano belts thus suggesting the onset of significant magmatic activity in this section of the andes at around 570 560 ma and probably ca 600 ma the 520 ma cluster is compatible with the most prominent magmatic period in the southern andes related to the end of the pampean orogeny thus suggesting the existence of a regional proto andean magmatic arc during early cambrian times

uplift history of the central and northern andes a review the elevation of the andean cordillera is a crucial boundary condition for both climatic and tectonic studies the andes affect climate because they form the only barrier to atmospheric circulation in the southern hemisphere and they intrigue geologists because they have the highest plateau on earth formed at a noncollisional plate margin the altiplano puna yet until recently few quantitative studies of their uplift history existed this study presents both 1 a review of the quantitative paleoelevation estimates that have been made for the central and colombian andes and 2 an examination of the source and magnitude of error for each estimate in the central andes paleobotanical evidence suggests that the altiplano puna had attained no more than a third of its modern elevation of 3700 m by 20 ma and no more than half its modern elevation by 10 7 ma these data imply surface uplift on the order of 2300 3400 m since the late miocene at uplift rates of 0 2 0 3 mm yr paleobotanical and geomorphological data suggest a similar uplift history for the eastern cordillera namely no more than half the modern elevation present by 10 ma no evidence exists for an exponential increase in uplift rate as has been interpreted from fissiontrack data these uplift rates mostly reflect mean surface uplift rather than rock uplift that is uplift of material points because little dissection of the western eastern cordillera has occurred south of lat 19 s and of the altiplano puna thus the central andean plateau appears to be young in the colombian andes paleobotanical data imply rapid uplift of the eastern cordillera between 2 and 5 ma at rates on the order of 0 6 3 mm yr however some of this uplift is likely rock uplift due to erosion driven isostatic rebound rather than mean surface uplift

ecuadorian andes volcanism a review of late pliocene to present activity abstract this article provides background information about ecuador s general volcanic setting as an introduction to the numerous articles in this special issue of the journal dealing with the many facets of ecuadorian volcanism it also provides a brief description of ecuador s physiography as well as its geodynamic setting a comprehensive bibliography is included as a guide to further reading the ecuadorian andes contain at least 20 volcanoes active during the holocene several reach altitudes of 5000 to 6000 m and are glacier clad eleven have erupted in pre columbian and historic times and have greatly affected local populations and caused fatalities

some geological implications of average quaternary glacial conditions analyses of quaternary landscape evolution tend to focus on events associated with culminations of glacial and interglacial ages but for most of quaternary time environments were intermediate in character average quaternary glacial conditions based on assessment of the marine isotope record approximated those near the stage 2 stage 1 transition and during substages 5b and 5d isotope stages 3 and 4 and substages 5a d lie within one standard deviation of the mean value under average glacial conditions ice sheets lying over northern north america and europe were much more contracted than their full glacial counterparts and the distribution of mountain glaciers reflected a snowline depression of some 500 m geomorphic processes operating under average quaternary conditions contributed importantly to landscape evolution examples of landscapes that may represent such average conditions include cirques and fluvial deposits of the pacific northwest the fjords and strandflat of western norway and atolls of the tropical oceans by examining the geologic record from the perspective of average conditions rather than those of climatic extremes added insight can be gained regarding the evolution of quaternary landscapes

fore arc deformation at the transition between collision and subduction insights from 3 d thermomechanical laboratory experiments 1 three dimensional thermomechanical laboratory experiments of arc continent collision investigate the deformation of the fore arc at the transition between collision and subduction the deformation of the plates in the collision area propagates into the subduction collision transition zone via along strike coupling of the neighboring segments of the plate boundary in our experiments the largest along strike gradient of trench perpendicular compression does not produce sufficiently localized shear strain in the transition zone to form a strike slip system because of the fast propagation of arc lithosphere failure deformation is continuous along strike but the deformation mechanism is three dimensional progressive along strike structural variations arise because coupling between neighboring segments induces either advanced or delayed failure of the arc lithosphere and passive margin the modeling results suggest that orogenic belts should experience deeper subduction of continental crust and hence higher pressure metamorphism where the two plates first collided than elsewhere along the plate boundary where collision subsequently propagated furthermore during the initial stage of collision the accretionary wedge is partially subducted which leads to lubrication of the interplate zone and a reduction of shear traction therefore a large convergence obliquity angle does not produce a migrating fore arc sliver rather the pressure force generated by subduction of the buoyant continental crust causes fore arc motion it follows that convergence obliquity during collision does not yield trench parallel deformation of the fore arc and its influence on the collision process is limited however convergence obliquity may control the geometry of the active margin during the oceanic subduction stage prior to collision and inherited structures may influence the propagation mechanism

imbabura volcano ecuador the influence of dipping substrata on the structural development of composite volcanoes during strike slip faulting abstract the geology and long term structural development of imbabura volcano ecuador have been discerned from field surveys as well as petrographical and geochemical studies the volcano is composed of an ancient edifice imbabura i which has experienced one major volcanic landslide directed n 330 340 after the emplacement of that debris avalanche deposit the petrographically and geochemically different edifices imbabura ii 1 and imbabura ii 2 developed on the n and ne flank of imbabura i as well as in its summit zone finally the smaller imbabura ii 3 edifice a dome complex locally known as huarmi imbabura was emplaced in the ssw flank of imbabura i the relatively complex structure of the edifice i e flank collapse direction and spatial distribution of the major stratigraphical units cannot be explained solely by volcano tectonic interactions with the regional left lateral rio ambi strike slip fault which traverses the edifice from nne to ssw instead the fact that the whole imbabura edifice stands on a substratum dipping to the nw i e the nw flank of the neighbor cubilche volcano is proposed to be the main factor influencing the departure of imbabura structure from previously known models of volcanic edifices interacting with strike slip faults this hypothesis was tested with scaled experiments during which the dip direction of the substratum was systematically changed with respect to the strike of the fault the structures developed by the experimental prototypes are highly coherent with those observed at imbabura volcano

the migration history of the nazca ridge along the peruvian active margin a re evaluation abstract the collision zone of the 200 km wide and 1 5 km high nazca ridge and the peruvian segment of the convergent south american margin between 14 s and 17 s is characterized by deformation of the upper plate and several hundred meters of uplift of the forearc this is evident by a narrowing of the shelf a westward shift of the coastline and the presence of marine terraces as the nazca ridge is oblique with respect to both trench and convergence direction of the nazca plate it migrates southward along the active plate boundary for reconstructing the migration history of the nazca ridge this study uses updated plate motion data resulting from a revision of the geomagnetic time scale the new model suggests that the ridge crest moved laterally parallel to the margin at a decreasing velocity of 75 mm a before 10 8 ma 61 mm a 10 8 4 9 ma and 43 mm a 4 9 ma to present intra plate deformation associated with mountain building in the peruvian andes since the miocene reduces the relative convergence rate between nazca plate and peruvian forearc taking an intra plate deformation at a rate of 10 mm a estimated from space geodetic and geological data into account does not significantly reduce these lateral migration velocities constraining the length of the original nazca ridge by its conjugate feature on the pacific plate yields a length of 900 km for the subducted portion of the ridge using this constraint ridge subduction began 11 2 ma ago at 11 s therefore the nazca ridge did not affect the northern sites of ocean drilling program odp leg 112 located at 9 s this is supported by benthic foraminiferal assemblages in odp leg 112 cores indicating more than 1000 m of subsidence since at least middle miocene time and by continuous shale deposition on the shelf from 18 to 7 ma recorded in the ballena industrial well at 11 5 s the model predicts the passage of the ridge crest 9 5 ma ago this agrees with the sedimentary facies and benthic foraminiferal stratigraphy of odp leg 112 cores which argue for deposition on the shelf in the middle and late miocene with subsequent subsidence of a minimum of several hundred meters onshore at 12 s the sedimentary record shows at least 500 m uplift prior to the end of the miocene also in agreement with the model

the current tectonic motion of the northern andes along the algeciras fault system in sw colombia abstract riedel synthetic and antithetic type faults principal displacement zones pdz pull apart basins such as lazy s shaped releasing bend extensive and rhomboidal shaped and releasing sidestep basins and minor folds located oblique to the main trace of the algeciras fault system afs are interpreted from landsat tm 5 images and geological mapping these tectonic features are affecting quaternary deposits and are related to major historical earthquakes and recent registered seismic events indicating neotectonic activity of the structure the afs is classified as a right lateral wrench complex structure with an important vertical component in which sedimentary cover and basement rocks are involved in addition the system represents a simple shear caused by the oblique convergence between the nazca plate and the northern andes the transpressive boundary in sw colombia was previously located along the eastern frontal fault system however this paper shows that the afs constitutes the actual boundary of the current transpressive regime along the northern andes which begins at the gulf of guayaquil in ecuador and continues into colombia and venezuela

trench parallel shortening in the forearc caused by subduction along a seaward concave plate boundary insights from analogue modelling experiments abstract three dimensional thermo mechanical analogue experiments are employed to test the hypothesis that oceanic subduction along a seaward concave plate boundary can generate trench parallel shortening in the forearc near the axis of curvature the model deformation is analyzed with a particle imaging velocimetry piv system that allows for comparison of forearc deformation along the oblique limbs of the curved plate boundary and near the axis of curvature moreover piv allows for separation of the trench parallel and trench perpendicular components of strain regardless of trench orientation the resulting deformation maps show a remarkable symmetry and indicate drag of the forearc above the interplate coupling area towards the axis of curvature trench perpendicular profiles show that along the oblique limbs of the plate boundary the forearc is submitted to trench normal shortening and trench parallel shearing but not trench parallel shortening or extension this contrasts with the situation near the axis of symmetry where the forearc is submitted to trench parallel and trench perpendicular normal shortening but is not sheared the experimental results confirm that trench normal thrusts observed in the fore arc of the central andes can be a mechanical consequence of subduction along a seaward concave plate boundary if the degree of interplate coupling is large

relative motion of the nazca farallon and south american plates since late cretaceous time by combining reconstructions of the south american and african plates the african and antarctic plates the antarctic and pacific plates and the pacific and nazca plates we calculated the relative positions and history of convergence of the nazca and south american plates despite variations in convergence rates along the andes periods of rapid convergence averaging more than 100 mm a between the times of anomalies 21 49 5 ma and 18 42 ma and since anomaly 7 26 ma coincide with two phases of relatively intense tectonic activity in the peruvian andes known as the late eocene incaic and mio pliocene quechua phases the periods of relatively slow convergence 50 to 55 30 mm a at the latitude of peru and less farther south between the times of anomalies 30 31 68 5 ma and 21 and between those of anomalies 13 36 ma and 7 correlate with periods during which tectonic activity was relatively quiescent thus these reconstructions provide quantitative evidence for a correlation of the intensity of tectonic activity in the overriding plate at subduction zones with variations in the convergence rate

a new type of active margin the convergent extensional margin as exemplified by the middle america trench off guatemala the results of dsdp legs 67 and 84 during which a transect of holes was drilled across the middle america trench off guatemala show no accretion at the front of that subduction zone since early eocene time the tectonic evolution of the trench includes extensional structure on the landward as well as the seaward trench slopes the guatemalan margin is proposed as the model of a new type of active margin the convergent extensional active margin ce margin

seismic strain rates and distributed continental deformation in the northern andes and three dimensional seismotectonics of northwestern south america abstract the seismic strain rates and configuration in the northern andes are estimated by summing the components of the centroid moment tensor cmt solutions for earthquakes with mw 5 0 and are compared with those predicted by various models of relative plate motions and gps data moment tensor components are summed linearly in four different regions with similar seismotectonics to infer changes in the regional strain patterns along the northern andes and to retrieve strain rates and relative plate velocities between this region and the stable south american plate the andean seismic strain rates are the result of east southeast directed contraction due to the collision of the nazca and caribbean plates with the south american plate and the collision of the panama block against central colombia with a component of northeast directed motion due to the obliquity of the eastern boundary of the northern andes with respect to the boundary of the nazca and caribbean plates with the south american plate along the colombia trench and the southern caribbean deformation zone respectively where the convergence is normal the present motion of the central andes towards the northeast does not require the plate boundary between the caribbean and south american plates to be along the eastern cordillera of colombia and the bocono fault in venezuela the seismic moment rates for the northern andes are small compared to the total expected seismic strain rates calculated from plate motion models and gps data suggesting that most of the strain is released aseismically and that is accommodated by folding fault creeping rigid block translation or other undetermined processes finally a three dimensional model of the cmt solutions and other seismicity in northwestern south america is presented where major subducting slabs including the peru flat slab and the ecuador and central colombia segments of the nazca and caribbean plates become more evident and easier to interpret the caribbean plate is interpreted to culminate at the location of the bucaramanga nest offering an explanation to this swarm of seismicity in the northern andes

history of rift propagation and magnetization intensity for the cocos nazca sspreading center analysis of magnetic anomaly profiles collected nearly parallel to tectonic flow lines allows detailed interpretation of the complicated tectonic history of the cocos nazca spreading center forward models of the magnetic anomalies accounting for spreading rate variations ridge jumps asymmetric spreading magnetization intensity variations and bathymetry show excellent agreement with observed anomalies spreading rates can be constrained to a common finite rotation history through anomaly 4a with three changes in rates rate changes at about 1 5 ma and 4 1 ma correspond to changes in rate gradients and occur during the well calibrated part of the reversal timescale so they can unquestionably be identified as true changes in plate motion a 15 rate decrease at about 5 2 ma could be interpreted either as a change in plate motion or as an artifact of poor calibration of the older part of the timescale the change at 4 1 ma is especially important because many timescales are based on the assumption of constant spreading rate for this plate pair for 0 6 ma rift propagation has played a dominant role in the continuous reorganization of the geometry of the ridge axis propagation has been predominantly away from the hotspot with jumps predominantly south ward propagation rates have ranged from 30 to 120 mm yr commonly near 70 mm yr origin of most propagation sequences is difficult to interpret but many appear to involve discrete southward ridge jumps forming a new segment near the hotspot magnetic anomaly amplitude appears to be a reliable tracer of fe content of lavas several generalizations can be drawn about along axis variations in magnetization intensities since 8 ma high magnetizations are only observed at the far ends relative to the galapagos hotspot of segments at least 150 km long offset at the end of a high magnetization segment is at least 15 km and there are no offsets larger than 30 45 km between high magnetization segments and the reconstructed position of the hotspot we interpret these patterns to indicate that fractionated lavas erupt where gradients in magma supply cause along axis flow of evolved magma the gradients in supply result from subaxial flow of hotspot derived asthenosphere in a narrow conduit this flow is only partly obstructed by an offset of 20 30 km but entirely blocked by an offset of 50 km

the 19 january 1995 tauramena colombia earthquake geometry and stress regime abstract the tauramena colombia earthquake m w 6 5 occurred on january 19 1995 in the andean eastern cordillera foothill region the so called piedemonte llanero the harvard cmt focal mechanism indicates an almost pure reverse fault rupture there was no surface faulting associated with this earthquake this event was located at the northern tip of a zone about 90 km in length with relatively low microseismic activity along the central segment of the piedemonte llanero in colombia a field expedition to the epicentral area was organized and a temporary portable network was installed for 1 month to register aftershock activity more than 800 events were recorded during this period a subset of the best located aftershocks 319 events shows epicenters extended over an area of 800 km 2 and suggests two antithetic planes on which most of the activity was concentrated the main event and a subset of 41 aftershocks occurred before the installation of the portable network but were recorded by the colombian national seismic network and were relocated they show an epicentral distribution similar to that of the subsequent events based on geologic information aftershock locations and focal mechanisms we built a model for the tauramena earthquake our results indicate that the tauramena earthquake was produced by reverse faulting along a steep dipping plane dip 50 nw associated with the guaicaramo system the fault plane cuts through basement rocks and folded mesozoic and cenozoic sedimentary rocks distribution of aftershocks in depth suggesting the presence of splay reverse faults and a backthrust is consistent with the hypothesis that the guaicaramo fault system was an old mesozoic normal fault reactivated during the andean compression as a reverse fault stress tensor inversion of p wave first motion polarities was performed and focal mechanisms for the best recorded aftershocks were calculated we found a well defined sub horizontal principal compression axis oriented in the nw se direction the predominant reverse faulting for the tauramena earthquake and the stability of 1 suggest that the tectonics in the central segment of the eastern cordillera frontal fault system is dominated by a compressive regime orthogonal to the cordillera the controlling processes being probably the caribbean subduction and the choco block collision

geochemical impact of injection of eagle ford brine on hosston sandstone formation observations of autoclave water rock interaction experiments abstract this study investigates water rock interactions induced by brine injection into the cretaceous clastic hosston formation in south central texas a potential host formation for disposal of produced water from the eagle ford shale during oil and gas extraction the primary objectives of this experimental study were to 1 predict the geochemical reactions between the injected brine and the host formation during disposal operation and 2 assess potential impact on flow properties of the host rock the results provide important information of potential impact to injectivity and available methods to alleviate the problems if they occur during brine disposal five batch autoclave experiments were conducted to react an argillaceous sandstone of the hosston formation with a series of synthetic brines and the eagle ford brine for 3 weeks at 193 bar and 80 c to better identify mineral dissolution and precipitation rock samples were ion milled and examined using scanning electron microscopy sem before and after the experiments for each experiment approximately 15 aqueous samples were taken with increasing time intervals from hours to days dissolution of anhydrite which results in a rapid increase of so4 and ca concentrations in the solution of 500 ppm and 200 ppm respectively is the most significant reaction small amounts of dolomite and k feldspar were also dissolved to release mg ca k and si small amounts of iron hydroxides and kaolinite precipitated geochemical modeling is able to reproduce the behavior of the major aqueous components and of the observed mineral dissolution and precipitation overall change in flow properties from the water rock interaction is insignificant mercury intrusion capillary pressure micp data show little difference in porosity permeability and pore throat size distribution between the pre and post experiment samples this study suggests that injection of eagle ford brine into the hosston formation will pose minimum risks for the brine disposal operation

transition from calc alkalic to adakitic magmatism at cayambe volcano ecuador insights into slab melts and mantle wedge interactions in volcanic arcs two main types of magmatism are recognized 1 widespread calc alkalic magmatism generated by hydrous partial melting of a metasomatized mantle wedge and 2 less common adakitic volcanism produced by subducted slab melting the cayambe volcanic complex in ecuador shows a progressive temporal change from an older calc alkalic volcano viejo cayambe to a younger adakitic edifice nevado cayambe this evolution may be related to the unusual geodynamic setting of the ecuadorian andes controlled mainly by the subduction of the nazca plate including the carnegie ridge thickened oceanic crust the viejo cayambe magmas appear to be generated from a mantle wedge source slightly metasomatized by slab derived melts conversely the nevado cayambe magmas imply either stronger and more advanced interactions between slab melts and mantle peridotite or that adakitic magma could reach the surface due to higher degrees of slab melting fractional crystallization assimilation and mixing processes also contributed to lava diversity we propose that the magmatic evolution of the cayambe volcanic complex is due to increasing efficiency of interaction between slab melts and the mantle wedge because of higher degrees of slab melting in response to the subduction of the carnegie ridge

a pliocene quaternary compressional basin in the interandean depression central ecuador summary the segment of the interandean depression of ecuador between ambato and quito is characterized by an uppermost pliocene quaternary basin which is located between two n s trending reverse basement faults the victoria fault to the west and the pisayambo fault to the east the clear evidence of e w shortening for the early pleistocene between 1 85 and 1 21 ma favours a compressional basin interpretation the morphology river deviations landslides folded and flexure structures demonstrates continuous shortening during the late quaternary the late pliocene quaternary shortening reached 3400 f 600 m with a rate of 1 4 f 0 3 mm yr the e w shortening is kinematically consistent with the current right lateral reverse motion along the ne sw trending pallatanga fault the quito ambato zone appears to act as a n s restraining bend in a system of large right lateral strike slip faults the compressive deformation which affects the interandean depression during the pliocene is apparently coeval to the beginning subduction of very young oceanic lithosphere north of the gulf of guayaquil the relatively buoyant new crust may have significantly increased the mechanical coupling in the subduction zone from pliocene to present

plate motions in the north andean region repeated geodetic measurements with the global positioning system gps provide direct measurements of displacements due to plate motions and active crustal deformation in central america and northern south america an area of complex interaction of the nazca cocos caribbean and south american plates the displacement rates for the period 1988 1991 obtained from the results of the first three central and south america casa gps campaigns are in general agreement with the predictions of the nuvel 1 plate motion model but there are differences in detail between the observations and the model the nazca north andes convergence rate vector measured by gps is different from the nuvel 1 vector at 95 confidence the difference implies that the north andes are moving northward relative to south america the measured convergence between the caribbean plate and the north andes suggests that the southern margin of the caribbean plate is located in the south caribbean deformed belt the april 1991 costa rica earthquake and the cocos caribbean convergence rate determined by gps suggest the possibility of significant ongoing deformation between central america and the stable interior of the caribbean plate our gps results are consistent with deformation of the overriding plates at the convergent margins of central and south america and confirm that active convergence is occurring around much of the southern margin of the caribbean plate from colombia west to costa rica costa rica and panama are not part of the stable caribbean plate instead the south caribbean deformed belt and the north panama fold belt probably represent the southern margin of the caribbean plate

implications for timing of andean uplift from thermal resetting of radiation damaged zircon in the cordillera huayhuash northern peru abstract the cordillera huayhuash is a north south oriented range along the drainage divide of the northern peruvian andes the range has high topography with peaks in excess of 5500 m and the second highest peak in peru nevados yerupaja 6617 m bedrock is dominated by folded mesozoic miogeoclinal rocks unconformably overlain by mid tertiary volcanics intruded by late tertiary granitic rocks and silicic dikes zircon fission track zft and u th he zhe dating of zircons along a west east transect elucidates the thermal evolution of exhumed and uplifted rocks the stability of fission tracks in zircons is a function of single grain radiation damage in samples with grain to grain variability in radiation damage resetting results in variable resetting and multiple age populations low retentive zircons lrzs which have a partly disordered crystalline structure have significant radiation damage and a low temperature of annealing ca 180 200 c high retentive zircons hrzs which are nearly cry

morphologic evidence of active motion of the zambapala fault gulf of guayaquil ecuador abstract the zambapala fault zone zfz is located at the link between the offshore structures of the gulf of guayaquil and the guayaquil caracas megashear gcm that accommodates the northeastward motion of the north andean block we use morphological observations of drainage offset to assess the active motion of the zambapala fault the relation between the horizontal offset amount d of the stream channel and the upstream length l from the offset segment and offset of beach morphology provide a measurement of the average slip rate of the motion of the fault to an accuracy of a fraction of millimeters per year the drainage network is short running down the southeastern slopes of the zambapala cordillera 297 m a quaternary dome uplifted along a positive flower structure we measure the d drainage offset along the fault l drainage length from the fault relation for the upper and more recent part of the drainage network the relation suggests that the fault is active at present capture occurs along the middle slopes and channel straightening near the littoral plain hiding part or most of the fault offset the fault trace crosses the littoral plain showing 35 40 m offset of the inner beach ridge and delimiting variations of the beach morphology the attribution of a maximum age of 5000 6000 years to the oldest beach ridge the postglacial transgression allows us to calculate a minimal mean slip rate of 5 8 8 mm year 1 this result confirms that the guayaquil caracas megashear extends to the gulf of guayaquil through the zambapala fault zone which accommodate at least 60 80 of the slip motion of the guayaquil carcas megashear

glacial interglacial trench supply variation spreading ridge subduction and feedback controls on the andean margin development at the chile triple junction area 45 48 s during the chile triple junction ctj cruise march april 1997 em12 bathymetry and seismic reflection data were collected in the vicinity of the chile triple junction 45 480s where an active spreading ridge is being subducted beneath the andean continental margin results show a continental margin development shaped by tectonic processes spanning a spectrum from subduction erosion to subduction accretion the andean continental margin and the chile trench exhibit a strong segmentation which reflects the slab segmentation and the chile triple junction migration three segments were identified along the andean continental margin the presubduction the synsubduction and the postsubduction segments from north to south both climate induced variations of the sediment supply to the trench and the tectonic reorganization at the nazca antarctica plate boundary involving postsubduction ridge jump are the two main factors that control the tectonic regime of this continental margin along the survey area we infer the succession of two different periods during the last glacial interglacial cycle a glacial period with ice rafted detrital discharges restricted to the shoreline area and low river output and a warmer period during which the andean ice cap retreat allowed the andes to be drained off during these warm periods rapid increase in trench deposition caused the margin to switch from subductionerosion or nonaccretion to subduction accretion 1 along the presubduction segment after the last deglaciation and 2 along the postsubduction segment after the interglacial episode at 130 117 ka conversely a nonaccretion or subduction erosion mode characterized the presubduction and postsubduction segments during glacial maximums the major effects of subduction of the buoyant chile ridge include a shallow trench which diverts trench sediment supply and tectonic instabilities at the nazca antarctica plate boundary we suggest that a postsubduction westward jump of the chile ridge occurred during the past 780 kyr it produced slab fragmentation and individualization of an ephemeral microplate north of the taitao fracture zone the chonos microplate in 780 kyr two episodes of subduction accretion separated by an episode of subduction erosion occurred in relation with the chonos microplate individualization and subduction the current northward migration of the triple junction along the chonos microplate south america plate boundary introduces a sharp change in the tectonic mode from subduction erosion to the north to subduction accretion to the south the data collected along the taitao ridge have revealed the complex three dimensional structure of an accretionary wedge which includes a midslope thrust sheet exhibiting the characteristics of an ophiolite the taitao ridge ophiolite no connection exists between the taitao ridge ophiolite and the bahia barrientos ophiolite cropping out onland in the taitao peninsula

tectonic control on rock uplift exhumation and topography above an oceanic ridge collision southern patagonian andes 47 s chile

cenozoic plate kinematics subduction and magmatism south american andes revised plate reconstructions and enlarged isotopic age data sets strongly support three mechanisms for controlling the configuration and associated volcanism and tectonism of the andean subduction zone these include age of subducted plate convergence rate and interaction with hotspot traces from 50 to 25 ma in the andes the age of oceanic plate subducted progressively decreased apparently resulting in a9gradual decrease in subduction angle and consequent eastward shift in the loci of magmatic activity a major reorganization of plate motions at 25 ma resulted in an increased convergence rate normal to the chilean andes and was accompanied by subsequent broadening of the locus of magmatic activity into adjacent portions of bolivia and argentina convergence rates changed only slightly across the peruvian margin and no clear effect on subsequent magmatism is indicated interaction of the andean subduction zone with the juan fernandez and easter nazca hotspot traces apparently resulted in lower angle subduction and consequent gaps in magmatic activity as predicted continuations of the traces correspond with regions lacking dated volcanic rocks the current gaps in igneous activity correspond with contemporary subduction of the two traces and this association apparently occurred in the past as well

latest pleistocene holocene tectonics of the ecuadorian andes abstract a structural study was carried out in the most tectonically active areas of the ecuadorian andes including part of the interandean valley the cordillera real and the subandean zone since the pleistocene the region has undergone complex wrench tectonics which are still active apparently two different stress states have coexisted within different parts of the andean range during the latest pleistocene and holocene 1 1 the external part of the chain including the cordillera real and the westernmost part of the subandean zone has been subject to a east northeast west southwest direction of greatest principal stress associated motions are partitioned between right lateral oblique thrusting and pure right lateral strike slip faulting along north south to north northeast trending structures as a whole these define a right lateral transpressive shear zone 2 2 in the interandean valley which lies above a major tertiary suture zone deformations are consistent with a north south direction of greatest horizontal principal stress associated with recent development of a left lateral transtensional shear zone in the first zone fault displacements are at least an order of magnitude larger than in the second zone the present tectonics of the area is explained by a kinematic model in which the wedge shaped cordillera real crustal block bounded to the east by a right lateral shear zone and to the west by a left lateral shear zone is being uplifted and extruded northward with respect to the cordillera occidental and to the stable south american foreland this motion is the result of oblique convergence between the nazca and south american plates

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

interactions between volcanism and geodynamics in the southern termination of the ecuadorian arc abstract this study focuses on the construction and evolution through time of volcanic edifices located in the southern part of the ecuadorian arc with the objective to contribute to a better understanding of the interactions between magmatism slab geometry and the activity of tectonic faults our new groundmass k ar ages obtained for a dozen volcanoes from the southern quaternary arc are rather young without ages older than 800 ka and highlight an increasing volcanic activity between 300 and 100 ka these new temporal constraints suggest that a southward migration of the ecuadorian arc occurred during the last 600 ka we interpret this evolution as the result of major and recent geodynamic reorganization in ecuador related to the activation of lithospheric faults and to the flexure of the slab at depth following the inland prolongation of the grijalva fracture zone both phenomena could have been induced by the oblique subduction of the nazca plate towards the convex margin of the south american continent and the coupling of these two plates along the wadati benioff zone progressively intensified by the carnegie ridge subduction southward migration of the slab flexure at depth could have changed pressure and temperature conditions favoring the partial melting of the mantle wedge and magma genesis thereby inducing a southward migration of the ecuadorian arc volcanoes these results contribute therefore to a better knowledge of the current arc dynamics in the northern andes and provide insights into the use of geochronological data applied to volcanic rocks for studies of past tectonic activity in ecuador

the effect of seamount subduction on seismic coupling it has long been speculated that the subduction of seamounts may have profound mechanical consequences particularly on the generation of large subduction zone earthquakes we consider this problem as a special case of the seismic coupling model of scholz and campos the model predicts that the subduction of a large seamount will increase the normal stress across the subduction interface and hence will enhance seismic coupling in the case of coupled arcs this increased coupling will result in a large increase in the recurrence intervals of earthquakes such events will therefore appear rare in the instrumental record as is observed for decoupled arcs the subduction of a large seamount should cause them to become locally coupled three of the latter localities have been found with satellite altimetry data two on the tonga kermadec trench and one on the izu bonin trench the diagnostic feature is a shallowing or obstruction of the trench axis adjacent to a chain or group of large seamounts on the subducting plate in each case the small to moderate plate interface seismicity typical of decoupled seismic zones is almost completely absent in a 100 200 km arc segment indicating that the subduction zone is locally locked as predicted by the model the occurrence of large interplate earthquakes at both tonga sites confirms that they are seismically coupled

paleoseismology and tectonic geomorphology of the pallatanga fault central ecuador a major structure of the south american crust the pallatanga fault pf is a prominent nne ssw strike slip fault crossing central ecuador this structure is suspected to have hosted large earthquakes including the 1797 riobamba event which caused severe destructions to buildings and a heavy death toll more than 12 000 people as well as widespread secondary effects like landsliding liquefaction and surface cracking the scope of this study is to evaluate the seismic history of the fault through a paleoseismological approach this work also aims at improving the seismotectonic map of this part of the andes through a new mapping campaign and finally aims at improving the seismic hazard assessment we show that the pf continues to the north of the previously mapped fault portion in the western cordillera rumipamba pallatanga portion into the inter andean valley riobamba basin field evidences of faulting are numerous ranging from a clear geomorphological signature to fault plane outcrops along the western side of the riobamba basin the strike slip component seems predominant along several fault portions with a typical landscape assemblage dextral offsets of valleys fluvial terrace risers and generation of linear pressure ridges in the core of the inter andean valley the main fault portion exhibits a vertical component along the c 100 m high cumulative scarp the presence of such an active fault bounding the western suburbs of riobamba drastically increases the seismic risk for this densely inhabited and vulnerable city to the east peltetec massif cordillera real the continuation of the pallatanga fault is suspected but not definitely proved yet based on the analysis of three trenches we state that the rumipamba pallatanga section of the pf experienced 4 maybe 5 holocene to historical strong events mw 7 the coseismic behavior of the fault is deduced from the occurrence of several colluvial wedges and layers associated with the fault activity and interbedded within the organic black soil sequence according to a series of 14c datings we document that these events occurred during the last 6500 years the clear deformation of the shallowest layer 14c 1633 ad is most likely associated with the 1797 riobamba earthquake after retrodeforming one of the 3 trenches we estimate coseismic vertical throws 0 70 to 0 90 m because of bad outcrop conditions we could not determine the horizontal component of slip and we used the slip vector determined in a previous work with a tectonic geomorphology study assuming this slip vector we obtain total coseismic offsets between 3 5 and 4 5 m indicative of earthquake magnitudes around c mw 7 5 the estimated recurrence time intervals range between 1300 and 3000 years indicating an average slip rate of c 2 5 mm a for the rumipamba pallatanga section of the fault

cenozoic stages of forearc evolution following the accretion of a sliver from the late cretaceous caribbean large igneous province sw ecuador nw peru

deep structures of the ecuador convergent margin and the carnegie ridge possible consequence on great earthquakes recurrence interval 1 the deep structure of the ecuador subduction zone and adjacent carnegie ridge cr was investigated using on shore off shore wide angle seimics a crustal model obtained by 2 d inversion of traveltimes reveals the overthickened 14 km oceanic crust of the cr that underthrusts the high velocity 6 km s basement of the upper plate margin wedge interpreted as part of the accreted oceanic terranes described on shore the plate interface dips 4 to 10 east from the trench to a depth of 15 km shadow zones observed on the margin obs records are interpreted as a low velocity zone consisting of a thin layer of underthrust sediments and the 3 km thick cr layer 2 whose average seismic velocity of 5 1 km s is slower than that of the margin wedge increased interplate coupling related to the subduction of the thick buoyant cr may account for an apparent local increased recurrence interval between great interplate earthquakes

syntectonic cenozoic sedimentation in the northern middle magdalena valley basin of colombia and implications for exhumation of the northern andes development of the colombian middle magdalena valley basin mmvb was determined by late cretaceous early eocene uplift of the central cordillera to the west and subsequent transferal of deformation to the eastern cordillera to the east these phases are separated in the tectono stratigraphic record by a major unconformity the middle magdalena valley unconformity mmvu paleocene coastal to alluvial facies underneath the mmvu were deposited in a foreland basin coupled to central cordillera kilometer scale uplift the middle eocene to neogene continental strata that onlap the mmvu document transformation of the mmvb into an interior basin due to eastern cordillera deformation recorded by growth strata in seismic lines and changing provenance and paleoflow patterns exhumation histories of the mmvb bounding ranges are further constrained by apatit fission track and vitrinite reflectance thermochronology and volcanic ash chronology this study indicates that the mmvb is fundamentally related to evolution of the entire andean margin of south america

flare shaped acoustic anomalies in the water column along the ecuadorian margin relationship with active tectonics and gas hydrates with hull mounted multibeam echosounder data we report for the first time along the active ecuadorian margin acoustic signatures of water column fluid emissions and seep related structures on the seafloor in total 17 flare shaped acoustic anomalies were detected from the upper slope 1250 m to the shelf break 140 m nearly half of the flare shaped acoustic anomalies rise 200 500 m above the seafloor the base of the flares is generally associated with high reflectivity backscatter patches contrasting with the neighboring seafloor we interpret these flares as caused by fluid escape in the water column most likely gases high resolution seismic profiles show that most flares occur close to the surface expression of active faults deformed areas slope instabilities or diapiric structures in two areas tectonic deformation disrupts a bottom simulating reflector bsr suggesting that buried frozen gas hydrates are destabilized thus supplying free gas emissions and related flares this discovery is important as it opens the way to determine the nature and origin of the emitted fluids and their potential link with the hydrocarbon system of the forearc basins along the ecuadorian margin

controls on architecture of the late cretaceous to cenozoic southern middle magdalena valley basin colombia the maastrichtian cenozoic southern middle magdalena valley basin of colombia contains a unique record of unconformities strata and structure from which we extract the histories of exhumation of the central cordillera to the west and evolution of the eastern cordillera fold and thrust belt to the east this study integrates field based analyses of stratigraphy laboratory analyses of provenance fission track thermochronology vitrinite reflectance data volcanic ash geochronology and studies of synorogenic geometries and structure displayed in seismic data a major unconformity the middle magdalena valley unconformity formed by eastward migration of central cordillera uplift during late cretaceous to early eocene time which transformed a maastrichtian marine basin into a paleocene depositional piedmont area this transformation is recorded by a coarsening upward sequence of marine shales to alluvial fan conglomerates which was partly eroded during further early eocene propagation of central cordillera deformation cessation of this phase of uplift led to formation of a pediment surface the middle magdalena valley unconformity which was buried by westward onlapping middle eocene to neogene alluvial deposits middle eocene to neogene facies paleoflow and unconformities were controlled by eastern cordillera deformation in the eastern cordillera foot hills growth strata and thermal history reveal two phases of folding of middle eocene oligocene and late miocene ages prior to intense pliocene pleistocene uplift two unconformities of early late miocene and pliocene pleistocene ages occur to the west of the eastern cordillera and record flexural tilting related to episodes of eastern cordillera loading

role of tectonic denudation in warping and uplift of low angle normal faults large displacement on a low angle normal fault results in isostatic uplift of the lower plate in response to tectonic denudation simple models of the denudation process predict warping of the lower plate into a broad antiform or antiform synform pair with axes perpendicular to the direction of extension the amount of warping is strongly influenced by initial fault geometry surface topography the amount of extension and the distribution of extension within the upper plate late stage processes that augment antiform growth include one sided denudation of the antiform reverse faulting due to concave upward flexure and wholesale detachment and reverse drag folding of the antiform antiformal uplifts now exhumed by erosion form mountain ranges in the southwestern united states where conditions were favorable for warping during mid tertiary crustal extension four major structural domains are apparent in transects across these uplifts and adjacent areas 1 unextended area 2 synformal upper plate 3 antiformal uplift and 4 wedge shaped upper plate each domain is separated from adjacent domains by the traces of a master low angle normal fault

seismological evidence on the geometry of the orogenic system in central northern ecuador south america analysis of the spatial distribution of seismicity beneath central ecuador from a temporary network gives new insights on two main points of the ecuadorian tectonics major structures in the ecuadorian andes are east dipping late jurassic to paleogene sutures reactivated by present day compression the oceanic plate is plunging continuously down to a depth of about 200 km with a dip of 25 35 we also show that the coastal plain acts as a buttress transmitting stresses to the andes beneath which deformation is concentrated

geological history of the cretaceous ophiolitic complexes of northwestern south america colombian andes abstract the western cordillera of colombia was formed by intense alpine type nappe forming folding and thrusting the cretaceous 80 120 ma b p tholeiitic material of the western cordilleran nappes has been obducted onto the paleozoic and precambrian polymetamorphic micaschists and gneiss of the central cordillera near yarumal the antioquia batholith 60 80 ma b p intrudes both obducted cretaceous oceanic material and the polymetamorphic basement rock of the central cordillera therefore nappe emplacement and obduction onto the central cordillera occurred during late senonian to early paleocene the nappes travelled from northwest to southeast so that the highest unit the rio calima nappe therefore has the most northwestern source whereas the lowest units originated from a more southeastward direction sedimentological analysis of the volcanoclastic and sandy turbidite material from each unit suggests a marginal marine environment during cretaceous times the opening of this marginal sea from now on called the colombia marginal basin probably originated by detachment of a block from the south american continent related to the farallon south america plate convergence in the popayan area southern colombia the central cordilleran basement exhibits glaucophane schist facies metamorphism this high pressure low temperature metamorphism is of early cretaceous 125 ma b p age and is related to an undated metaophiolitic complex the ophiolitic material originating from the western cordilleran is thrust over both the blueschist belt and the metaophiolitic complex these data suggest that the occidente colombiano suffered at least two phases of ophiolitic obduction during mesozoic time

tectonic segmentation of the north andean margin impact of the carnegie ridge collision the north andean convergent margin is a region of intense crustal deformation with six great subduction earthquakes mw 7 8 this century the regional pattern of seismicity and volcanism shows a high degree of segmentation along strike of the andes segments of steep slab subduction alternate with aseismic regions and segments of flat slab subduction this segmentation is related to heterogeneity on the subducting nazca plate in particular the influence of the carnegie ridge collision is investigated four distinct seismotectonic regions can be distinguished region 1 from 6on to 2 5on with steep ese dipping subduction and a narrow volcanic arc region 2 from 2 5on to 1os showing an intermediate depth seismic gap and a broad volcanic arc region 3 from 1os to 2os with steep ne dipping subduction and a narrow volcanic arc region 4 south of 2os with flat subduction and no modern volcanic arc the carnegie ridge has been colliding with the margin since at least 2 ma based on examination of the basement uplift signal along trench parallel transects the subducted prolongation of carnegie ridge may extend up to 500 km from the trench as suggested by the seismic gap and the perturbed broad volcanic arc these findings conflict with previous tectonic models suggesting that the carnegie ridge entered the trench at 1 ma furthermore the anomalous geochemical adakitic signature of the volcanoes in the broad ecuador volcanic arc and the seismicity pattern are proposed to be caused by lithospheric tears separating the buoyant shallowly subducting carnegie ridge from segments of steep subduction in regions 1 and 3 it is further suggested that carnegie ridge supports a local flat slab segment similar to that observed in peru the impact of the carnegie ridge collision on the upper plate causes transpressional deformation extending inboard to beyond the volcanic arc with a modern level of seismicity comparable to the san andreas fault system the pattern of instrumental and historical seismicity indicates 1 great earthquakes on the northern and southern flanks of the colliding ridge 2 a slight reduction in observed seismicity at the trench ridge intersection 3 increased stress far into the continent and 4 a nne displacement of the n andes block to be further effects of the collision 1999 elsevier science b v all rights reserved

duration and structure of the past four interglaciations reanalysis and additional dating of the devils hole 18o paleotemperature record confirm that the last interglaciation in the great basin the continental analog of marine isotopic substage 5e lasted 22 000 yr consistent with the vostok paleotemperature record which suggests a duration of 19 000 yr for this event in antarctica the three preceding interglaciations in the devils hole record analogs of marine isotopic substages 7e 9c and 11c range from 20 000 to 26 000 yr in duration a 20 000 yr duration for the last interglaciation is consistent with tims uranium series dated sea level high stands thus the widely held view that interglaciations were of 11 000 to 13 000 yr duration and constituted only about 10 of mid to late pleistocene climatic cycles needs reexamination the warmest portion of each interglaciation in the devils hole time series is marked by a 18o plateau signifying apparent climatic stability for periods of 10 000 to 15 000 yr duration

cretaceous and tertiary terrane accretion in the cordillera occidental of the andes of ecuador new field geochronological geochemical and biostratigraphical data indicate that the central and northern parts of the cordillera occidental of the andes of ecuador comprise two terranes the older pallatanga terrane consists of an early to late cretaceous oceanic plateau suite late cretaceous marine turbidites derived from an unknown basaltic to andesitic volcanic source and a tectonic melange of probable late cretaceous age the younger macuchi terrane consists of a volcanosedimentary island arc sequence derived from a basaltic to andesitic source a previously unidentified regionally important dextral shear zone named the chimbo toachi shear zone separates the two terranes regional evidence suggests that the pallatanga terrane was accreted to the continental margin the already accreted cordillera real in campanian times producing a tectonic melange in the suture zone the macuchi terrane was accreted to the pallatanga terrane along the chimbo toachi shear zone during the late eocene probably in a dextral shear regime the correlation of cretaceous rocks and accretionary events in the cordillera occidental of ecuador and colombia remains problematical but the late eocene event is recognised along the northern andean margin

quaternary state of stress in the northern andes and the restraining bend model for the ecuadorian andes inversion of shallow focal mechanisms in the northern andes together with a neotectonic analysis of the ecuadorian andes shows that the state of stress is homogeneous in most of the northern andes e w trending 1 however north of 5 n 1 is roughly nw se trending this difference in state of stress is due to the force balance between the nazca caribbean and south american plates south of 5 n the state of stress appears to be mainly controlled by the nazca south american plate interaction while north of 5 n it appears to be mainly controlled by the caribbean south american plate interaction this state of stress difference from south to north is consistent with dextral motions along faults trending parallel to the chain south of 5 n and sinistral motions along faults trending parallel to the chain north of 5 n this inversion of shallow focal mechanisms also illustrates the incomplete strain partitioning of the oblique convergence between the nazca and south american plates along the ecuadorian southern colombian trench the convergence obliquity that increases northward controls the active transcurrent fault slip rate in the upper plate finally this convergence obliquity is responsible for the location of the seismicity in the upper plate in addition the andean block is considered in this paper as a large triple zone under constriction with one of its borders caribbean behaving partly as a free border

temporal evolution of magmatism in the northern volcanic zone of the andes the geology and petrology of cayambe volcanic complex ecuador in the northern volcanic zone of the andes the cayambe volcanic complex consists of 1 a basal mostly effusive volcano the viejo cayambe whose lavas andesites and subordinate dacites and rhyolites are typically calc alkaline and 2 a younger essentially dacitic composite edifice the nevado cayambe characterized by lavas with adakitic signatures and explosive eruptive styles the construction of viejo cayambe began 1 1 myr ago and ended at 1 0 ma the young and still active nevado cayambe grew after a period of quiescence of about 0 6 myr from 0 4 ma to holocene its complex history is divided into at least three large construction phases angureal cone main summit cone and secondary summit cone and comprises large pyroclastic events debris avalanches as well as periods of dome activity geochemical data indicate that fractional crystallization and crustal assimilation processes have a limited role in the genesis of each suite on the contrary field observations and mineralogical and geochemical data show the increasing importance of magma mixing during the evolution of the volcanic complex the adakitic signature of nevado cayambe magmas is related to partial melting of a basaltic source which could be the lower crust or the subducted slab however reliable geophysical and geochemical evidence indicates that the source of adakitic component is the subducted slab thus the viejo cayambe magmas are inferred to come from a mantle wedge source metasomatized by slab derived melts adakites whereas the nevado cayambe magmas indicate a greater involvement of adakitic melts in their petrogenesis this temporal evolution can be related to the presence of the subducted carnegie ridge modifying the geothermal gradient along the wadati benioff zone and favouring slab partial melting

late quaternary kinematics of the pallatanga strike slip fault central ecuador from topographic measurements of displaced morphological features the northeast trending pallatanga right lateral strike slip fault runs across the western cordillera connecting n50 e n70 e trending normal faults in the gulf of guayaquil with n s reverse faults in the interandean depression over most of its length the fault trace has been partly obscured by erosional processes and can be inferred in the topography only at the large scale only the northern fault segment which follows the upper rio pangor valley at elevations above 3600 m is prominent in the morphology valleys and ridges cut and offset by the fault provide an outstanding record of right lateral cumulative fault displacement the fault geometry and kinematics of this particular fault segment can be determined from detailed topographic levellings the fault strikes n30 e and dips 75 to the nw depending on their size and nature transverse morphological features such as tributaries of the rio pangor and intervening ridges reveal right lateral offsets which cluster around 27 11m 41 5 4 m 590 65 m and 960 70 m the slip vector deduced from the short term offsets shows a slight reverse component with a pitch of about 11 5 sw the 41 5 4 m displacements are assumed to be coeval with the last glacial termination yielding a mean holocene slip rate of 2 9 4 6 mm yr 1 assuming a uniform slip rate on the fault in the long term the 27 m offset appears to correlate with an identified middle holocene morphoclimatic event and the long term offsets of 590 m and 960 m coincide with the glacial terminations at the beginning of the last two interglacial periods

the andean ophiolitic megastructures on the buga buenaventura transverse western cordillera valle colombia abstract a structural study of the buga buenaventura transverse in the central part of the western cordillera of colombia has shown the presence of three structural units which from bottom to top and from west to east are the rio dagua unit the rio calima unit and the loboguerrero window unit all three units comprise strata between 120 and 80 m y old overlying a basement of green rocks showing the characteristics of submarine flows the bolivar ultrabasic and basic massif is geographically linked to the rio calima unit in which green rocks predominate and is separated from it by a tectonic contact the upper part of the massif on the other hand shows high temperature metamorphic rocks formed during its emplacement the rio dagua unit shows two tectonic phases with isoclinal folding and development of schistosity the first phase is contemporaneous with low grade metamorphism equivalent to lower greenschist facies conditions the loboguerrero window unit shows a large recumbent fold oriented towards the southeast deformation in the rio calima unit is weaker and appears to correspond to a higher structural level than in the two other units as the attitudes of the s1 schistosity in the rio dagua unit and of the shear zones located at the green rocks sediments contact in this same unit are similar to that of the overthrust at the base of the rio calima unit we are led to postulate that the overthrust belongs to the first phase of deformation as also does the recumbent fold in the loboguerrero window unit we are thus led to propose a southeastward direction of emplacement for the nappes in the western cordillera of colombia in conclusion the authors extend their observations and propose a new structural interpretation of the occidente colombiano

nonlinear variations of the physical properties along the southern ecuador subduction channel results from depth migrated seismic data abstract we use two high quality pre stack depth migrated multichannel seismic profiles acquired to quantify physical properties variations of underthrust sediments along the first 30 km of subduction off the erosional southern ecuadorian margin seismic data show three zones along the subduction channel referred to as zones i ii and iii characterized by distinct velocity and velocity derived physical properties which are in agreement with values estimated from experimental results of deformation in granular media these three zones result from transformational changes of underthrust sediments governed by fundamentally different physical processes that control their mechanical behavior at increasing confining pressures based on our observations and its comparison with experimental results we argue that the transformations undergone by underthrust sediments as they dip into the subduction zone are the following within zone i progressively increasing velocity and decreasing velocity derived porosity indicates continuous sediment compaction which must be accompanied by effective fluid drainage along the decollement and or across the accretionary wedge the underthrust material is here unconsolidated from a mechanical point of view laboratory experiments indicate that the dominant processes at this range of pressures are grain rolling particle rotation and frictional slip at grain contacts within zone ii velocity and porosity remains constant for 16 km sis 72 and 12 km sis 18 this suggests undrained conditions resulting in growing fluid overpressure at the subduction channel grain deformation is similar to zone i within zone iii velocity increases and porosity falls rapidly indicating sediment compaction and subsequent release of over pressured fluids where grain deformation is likely to be elastic this might be the dominant process until the grains attain their crushing strength resulting in granular cataclasis and eventually in the collapse of the system we suggest that over pressured fluid release may induce hydrofracturation and it is likely to increase inter plate coupling down from zone iii

paleostress evolution of the northern andes eastern cordillera of colombia implications on plate kinematics of the south caribbean region 1 new tectonic field data and paleostress determinations in the eastern cordillera of colombia enable us to identify stress regimes that prevailed in the northern andes since late cretaceous times until present day these regimes were characterized by an e w to wsw ene active contraction from late cretaceous to late paleocene this direction subsequently changed to nw se and finally became wnw ese during the andean tectonic phase the characteristics and relative chronology of these regimes were deduced from field structures and stratigraphic criteria using stress inversion of fault slip data sets to evaluate our model we used the rotation poles of the south america caribbean and nazca plates relative to north america and paleogeographic reconstructions of the caribbean region to derive a kinematic model of the northern andes assuming a general correspondence between the regional convergence directions and the regional compression we built a theoretical model of the regional stress regimes based on kinematics of the northwestern south america since late cretaceous times this model reveals a major variation at the end of the paleocene when relative divergence shifted to convergence between the americas this is in agreement with our tectonic model based on independent tectonic field data inversion in the eastern cordillera this regional kinematic and tectonic change which affected the caribbean at the end of the paleocene was responsible for and consistent with the major change in stress patterns and structural style in the eastern cordillera it is also coeval with the development of a major regional unconformity in colombia

neogene stratigraphy and andean geodynamics of southern ecuador abstract the present paper reviews tertiary volcanic and sedimentary formations in the inter andean region of southern ecuador between 2 s and 4 20 s in order to develop a geodynamic model of the region the formations occur in the southern shallow prolongation of the inter andean valley between the cordillera real to the east and the cordillera occidental and amotape tahuin provinces to the west one hundred fifty zircon fission track analyses has established a detailed chronostratigraphy for the sedimentary and volcanic formations and several small intrusions the paleogene to early miocene formations are dominated by intermediate and acidic volcanic and pyroclastic rocks in addition relics of eocene continental sedimentary series have been identified the neogene sedimentary series lie unconformably on deformed and eroded metamorphic sedimentary and volcanic formations they were deposited in two stages which are separated by a major unconformity dated at 10 9 ma 1 during the middle and early late miocene 15 10 ma marginal marine deltaic lagoonal lacustrine and fluvial environments prevailed which we group under the heading pacific coastal sequences they presumably covered a greater surface area in southern ecuador than their present occurrence in small topographic depressions we suggest that they were deposited in the shallow marine cuenca and loja embayments deposition in a marginal marine environment is also supported by the occurrence of brackish water ostracods and other fauna 2 above the regional angular unconformity the coastal facies are overlain by late miocene 9 5 ma continental alluvial fan and fluvial facies which are in turn covered by mainly airborne volcanic material they represent the intermontane sequences of the basins of cuenca giron santa isabel nabon loja and malacatos vilcabamba sedimentologic and stratigraphic results are used to discuss the tectonic setting of neogene sedimentation in the forearc and arc domain of the ecuadorian subduction system during the pacific coastal stage northward displacement of the coastal forearc block along the calacali pallatanga fault zone has driven crustal collapse in the inter andean region as a result extensional subsidence drove the eastward ingression of shallow seas into the cuenca and loja embayments from the manabi and progreso basins to the west tectonic inversion in the forearc area during the early late miocene at 9 5 ma reflects the initiation of w e oriented compression and uplift in the inter andean region and the establishment of smaller intermontane stage basins which host the continental sequences coeval topographic rise of the cordillera occidental is indicated by the onset of clastic input from the west the small intermontane basin of nabon 8 5 7 9 ma formed during the period of maximum compression the present data prove that the neogene andean forearc and arc area in southern ecuador was a site of important but variable tectonic activity which was presumably driven by the collision and coupling of the carnegie ridge with the ecuadorian margin since 15 9 ma

deciphering the late cretaceous cenozoic structural evolution of the north peruvian forearc system the link between plate tectonics and the evolution of active margins is still an ongoing task to challenge since the acceptance of plate tectonic paradigm this paper aims at deciphering the structural architecture and uplift history of the north peruvian forearc system to better understand the evolution and the mechanics that govern the late cretaceous cenozoic building of this active margin in this study we report surface structural geology data interpretation of seismic reflection profiles apatite fission track data and the construction of two offshore onshore crustal scale balanced cross sections the structure of the north peruvian forearc system is dominated by an accretionary style with northwestward propagation of thrust related structural highs involving continental oceanic basement rocks and off scrapped sediments the thrust systems bound thick thrust top forearc depocenters mainly deformed by crustal normal to strike slip faults and thin skinned gravitational instabilities the sequential restoration of the margin calibrated with apatite fission track data suggests a correlation between uplift shortening and plate convergence velocity during late cretaceous and miocene pliocene quaternary shortening and uplift of the coastal zone is rather related to the subduction of asperities during convergence rate decrease the development of crustal normal to strike slip faulting and subsidence zones might be the consequence of slab flexure local basal erosion along subduction fault and or oblique subduction associated with sediment loading control we conclude that the evolution of the north peruvian forearc system was controlled by subduction dynamics strong sediment accumulation and recent ridge subduction and it recorded the orogenic loading evolution of the andes over the cenozoic

the nature and provenance of accreted oceanic terranes in western ecuador geochemical and tectonic constraints western ecuador consists of a complex tectonic melange of oceanic terranes accreted to the continental margin from late cretaceous to eocene time new geochemical data from these accreted terranes arising from a 5 year british geological survey mapping programme indicate that they comprise rocks from a variety of oceanic tectonic settings from thickened and relatively unsubductable oceanic plateau basalts through island arc tholeiites with occasional more calc alkaline lavas to back arc basin basalt sequences this study has enabled us to construct a new geodynamic model for the cretaceous tertiary evolution of the northern andes and has placed important new constraints on the extent of oceanic plateau sequences in colombia and around the caribbean the age and nature of sediments combined with evidence for the age of peak metamorphism suggests that a prolonged 15 20 ma accretionary event occurred in late cretaceous time and involved the collision of an oceanic plateau represented by the pallatanga unit with the continental margin this accreted unit can be correlated with similar oceanic plateau sequences from the western cordillera of colombia and those within and around the caribbean region the naranjal and macuchi island arcs and the associated la portada back arc basin developed along the accreted margin from late campanian to eocene time and these arcs accreted to the continental margin along with oceanic plateau material represented by the pinon unit and pedernales esmeraldas sequences during eocene time the development of island arcs which separate the two accretionary events implies that the most westerly coastal oceanic plateau sequences both in ecuador pinon and pedernales esmeraldas and in colombia gorgona and serrania de baudo cannot belong to the caribbean colombian oceanic plateau ccop it therefore appears that at least two different oceanic plateaux are preserved within the accreted oceanic terranes of the northern andes it is possible that the ccop formed over the galapagos hotspot as previously proposed but the more westerly coastal plateau was derived from a more southerly hotspot source region such as sala y gomez in the se pacific

particulate matter exchange across a rjord sill abstract recent studies have indicated the existence of a considerably higher planktonic biomass in the deep waters of the saguenay fjord as compared to corresponding depths in the adjacent st lawrence estuary on the other side of a shallow sill the hypothesis that has been put forward to explain this phenomenon is related to the advection of near surface estuarine waters at times very rich in particulate matter over the entrance sill into the deeper waters of the fjord mixing processes associated with the development of a density flow the presence of a hydraulic jump or other mechanisms are assumed to be responsible for the common occurrence of lower density subsurface water within the basin as compared to that penetrating over the sill the exchange processes between the estuary and the fjord are described and an estimate made of the estuarine water volume that penetrates into the lower layer of the fjord over a semidiurnal tide cycle from these calculations the replacement time for the outer basin was estimated to range between one and four days the biological characteristics of this water were used to establish a budget for particulate matter exchange which showed in early august a typical net input of 188 t of particulate organic carbon into the deep waters of the fjord over one tide cycle

development of the gulf of guayaquil ecuador during the quaternary as an effect of the north andean block tectonic escape 1 interpretation of industrial multichannel seismic profiles and well data are used to identify the main tectonic features of the gulf of guayaquil gg area these include two e w trending major detachments the posorja and the jambeli detachment systems which represent half grabens with oppositely dipping detachments to the south and to the north respectively the ne sw trending puna santa clara fault system developed as a transfer fault system between the posorja and the jambeli detachments the esperanza and the jambeli basins exhibit 3 4 km of sediment that accumulated during the past 1 6 1 8 myr the puna santa clara fault system bounds the esperanza and the jambeli basins evidencing that the evolution of these basins is tightly controlled by the two detachments at depth to the west the n s trending domito fault system bounding the posorja detachment system and the esperanza basin to the west acted as a transfer zone between the shelf area and the continental slope the pliocene series show no significant variations in thickness throughout the gulf of guayaquil area suggesting that no important tectonic deformation occurred from 5 2 to 1 8 1 6 ma the major period of tectonic deformation in the gulf of guayaquil area occurred during the pleistocene times three main tectonic steps are identified from early pleistocene to 180 ka major subsidence occurred along the esperanza and jambeli basins from 180 to 140 ka most of the gulf of guayaquil area was above sea level during the isotope substage 6 low stand from 140 ka to present tectonic activity is restricted along the normal faults bounding the esperanza basin the tenguel fault and the puna santa clara and domito fault systems a n s trending tensional stress regime characterizes the pleistocene times throughout the northward drifting of the north andean block is proposed to control the tectonic evolution and associated subsidence of the gulf of guayaquil area it is also accepted that the collision of the carnegie ridge with the trench axis has to play a major role in controlling the north andean block northward drift because the carnegie ridge subduction is possibly an ongoing process which began prior to the pliocene we postulate the along strike morphology of the ridge at the origin of interplate coupling variations the subduction of an along strike positive relief of the ridge is proposed at the origin of the major tectonic reorganization of the gg area occurring at 1 8 1 6 ma

forearc basin formation in the tectonic wake of a collision driven coastwise migrating crustal block the example of the north andean block and the extensional gulf of guayaquil tumbes basin ecuador peru border area trench parallel extensional strain resulting from the northward drift of the north andean block has controlled the tectonic evolution of the gulf of guayaquil tumbes basin at least for the past 1 8 1 6 ma industrial multichannel seismic and well data document that e w to ene low angle detachment normal faults the posorja and jambeli detachment systems to the north and the tumbes detachment system to the south accommodated the main subsidence step along the shelf area during late pliocene quaternary times 1 8 1 6 ma to present two tectonic regimes showing different styles and ages controlled the evolution of the southern ecuador and northern peru continental margin and shelf the n s extensional regime along the shelf area is related to north andean block drift whereas the e w extensional regime along the continental margin results from tectonic erosion at depth strain rotation takes place along a major n s trending transfer system formed by the inner domito fault and the inner banco peru fault which bound the detachment systems to the west the strike slip component along this transfer system roughly located at the continental margin shelf break evolved as a response to slip along the detachment systems bounding the basin to the north and to the south the tumbes detachment system is the master fault controlling basin evolution through time and it may represent the shallower expression of a reactivated obduction megathrust it connects landward with the continental structures assumed to be part of the eastern frontier of the north andean block for the past 2 ma the total lengthening calculated along a complete n s transect of the gulf of guayaquil tumbes basin ranges between 13 5 and 20 km this lengthening is compatible with the documented drift of the north andean block for the same period of time the gulf of guayaquil tumbes basin is not a classical pull apart basin it exemplifies a particular type of pull apart basin basically controlled by 1 detachments extending downward across the brittle crust and 2 plate coupling along the subduction decollement which controls the inward segmentation of deformation

the deep sea sediment record and the pliocene pleistocene boundary abstract the history of 18 o in planktonic and benthonic foraminifera from deep sea sediment cores is useful both as a tool for stratigraphic correlation and as a means of deriving the history of global ice volume it is also valuable as an indication of the pattern of climatic variability the reliability of this indicator has been demonstrated over the last few years as high resolution records for such diverse areas as the chinese loess plateau the highland of central america and the woodland of europe has been obtained kukla et al 1990 hooghiemstra et al 1993 wijmstra and groenhart 1983 follieri et al 1988 reille and de beaulieu 1990 for this reason any decision to erect new formal boundaries in the plio pleistocene should be based on an understanding of the appropriate part of the isotope record several 18 o records are now available that cover the entire plio pleistocene with the same temporal resolution that was until recently available only for the last few glacial cycles in addition a complete and robust astronomically calibrated timescale has been generated for the entire pliocene as well as the pleistocene both in open ocean deep sea sediments and in the classic pliocene and lower pleistocene sections that are well exposed in sicily and calabria shackleton et al 1990 1995a hilgen 1991a b the ultimate resolution and reliability of biostratigraphic datums is being developed and evaluated as these high resolution stratigraphic records become available raffi et al 1993 backman and shackleton 1983

recent crustal deformation and the earthquake cycle along the ecuador colombia subduction zone recent results from global positioning system gps measurements show deformation along the coast of ecuador and colombia that can be linked to the rupture zone of the earthquake in 1979 a 3d elastic boundary element model is used to simulate crustal deformation observed by gps campaigns in 1991 1994 1996 and 1998 deformation in ecuador can be explained best by 50 apparent locking on the subduction interface although there have not been any historic large earthquakes mw 7 south of the 1906 earthquake rupture zone 50 apparent elastic locking is necessary to model the deformation observed there in colombia only 30 apparent elastic locking is occurring along the subduction interface in the 1979 earthquake rupture zone mw 8 2 and no elastic locking is necessary to explain the crustal deformation observed at two gps sites north of there there is no evidence from seismicity or plate geometry that plate coupling on the subduction zone is reduced in colombia however simple viscoelastic models suggest that the apparent reduction in elastic locking can be explained entirely by the response of a viscous upper mantle to the 1979 earthquake these results suggest that elastic strain accumulation is occurring evenly throughout the study area but postseismic relaxation masks the true total strain rate

the geologic history of the caribbean cocos plate boundary with special reference to the nicoya ophiolite complex costa rica and d s d p results legs 67 and 84 off guatemala a synthesis abstract the pre upper senonian basement of costa rica crops out in the santa elena and nicoya peninsulas from south to north and from base to top the basement includes the esperanza matapalo and santa elena units the esperanza unit is albian santonian in age and consists mainly of pillow basalt and massive basalt flows the matapalo unit includes callovian to cenomanian radiolarite and includes massive basalt flows basalt and dolerite basement the santa elena unit contains ultramafic and mafic rocks in which harzburgite is the major component the most important tectonic features of the nicoya complex are the large santa elena and matapalo nappes nappe emplacement was from north to south during upper santonian time the sedimentary cover of the nicoya complex comprises 1 1 the campanian el viejo formation that consists of shallow water sediments in the north santa elena peninsula and the campanian maastrichtian sabana grande formation of deep water origin in the south nicoya peninsula 2 2 paleocene strata indicating deposition in a deep water environment comprises the rivas las palmas and samara formations 3 3 a post upper eocene sequence that consists of the shallow water barra honda and montezuma formations two unconformities are significant geological features of the upper senonian to tertiary history of costa rica the lower one is at the base of the sabana grande formation and marks a major change in the geologic conditions basalt is scarce in the campanian tertiary series the upper unconformity at the base of the barra honda and montezuma formations is not as major as the lower one during post campanian time normal faulting occurred in two stages separated by a strong erosional phase the geology of the landward slope of the adjacent middle america trench is outlined by interpreting multifold seismic reflection records off the west coast of costa rica and the dsdp legs 67 and 84 transects off guatemala the western caribbean plate boundary may have been under extensional stress for the last 75 m y the strong landward dipping reflectors of the middle america trench landward slope off guatemala could be equivalent to the on land pre campanian overthrusts of costa rica the available data are consistent with the convergent extensional margin concept

multiple plume events in the genesis of the peri caribbean cretaceous oceanic plateau province the oceanic crust fragments exposed in central america in north western south america and in the caribbean islands have been considered to represent accreted remnants of the caribbean colombian oceanic plateau ccop on the basis of trace element and nd sr and pb isotopic compositions we infer that cumulate rocks basalts and diabases from coastal ecuador have a different source than the basalts from the dominican republic the latter suite includes the 86 ma basalts of the duarte complex which are light rare earth element ree enriched and display relative to normal mid ocean ridge basalts nmorb moderate enrichments in large ion lithophile elements together with high nb ta pb and low th contents moreover they exhibit a rather restricted range of nd and pb isotopic ratios consistent with their derivation from an ocean island type mantle source the composition of which includes the himu high 238u 204pb component characteristic of the galapagos hotspot in contrast the 123 ma ecuadorian oceanic rocks have flat ree patterns and relative to nmorb are depleted in zr hf th and u moreover they show a wide range of nd and pb isotopic ratios intermediate between those of ocean island basalts and nmorb it is unlikely on geochemical grounds that the plume source of the ecuadorian fragments was similar to that of the galapagos in addition because of the nne motion of the farallon plate during the early cretaceous the ecuadorian oceanic plateau fragments could not have been derived from the galapagos hotspot but were likely formed at a ridge centered or near ridge hotspot somewhere in the se pacific

correlations between nazca farallon plate kinematics and forearc basin evolution in ecuador the tertiary evolution of the forearc basins of ecuador shows a close correlation between the changing convergence rate of the farallon and later nazca oceanic plates and continental south america the correlation occurs during the subduction of a relatively young slab and in the late miocene onset of the subduction of the carnegie aseismic ridge the ecuador forearc basins lie on a basement of oceanic crust known as the pinon terrane the accretion of this terrance occurred in the paleocene as the leading edge of the farallon plate the macuchi island arc collided with south america in the middle eocene this forearc terrane was the site of major pull apart basin formation and turbiditic sedimentation coincident with a phase of very rapid convergence between chron 21 and chron 13 48 37 ma this deformation was bounded by the trench and a major dextral strike slip fault zone and resulted in the northward translation of the forearc with respect to continental south america during the oligocene a phase of extension normal to the trend of the active margin occurred coincident with a phase of relatively slow convergence chron 13 to chron 6 37 20 ma this extension was followed in the middle miocene by inversion of most of the forearc basins coincident with a return to relatively fast convergence from chron 6 20 ma to the present day subduction of the carnegie aseismic ridge occurred during this period circa 8 ma to present and may have enhanced the compressive event further northward translation of the forearc silver accompanied this later deformation the relationships outlined for the forearc may be modeled in terms of a dynamic orogenic wedge which responds directly to changes in convergence rate at the subduction zone the convergence rate appears to be an important control on the coupling between the downgoing slab and overriding continental plate

geodynamics of the northern andes subductions and intracontinental deformation colombia new regional seismological data acquired in colombia during 1993 to 1996 and tectonic field data from the eastern cordillera ec permit a reexamination of the complex geodynamics of northwestern south america the effect of the accretion of the baudo panama oceanic arc which began 12 myr ago is highlighted in connection with mountain building in the ec the istmina and ibague faults in the south and the santa marta bucaramanga fault to the northeast limit an e se moving continental wedge progressive indentation of the wedge is absorbed along reverse faults located in the foothills of the cordilleras northward of 5 n and transpressive deformation in the santander massif crustal seismicity in colombia is accurately correlated with active faults showing neotectonic morphological evidences intermediate seismicity allows to identify a n ne trending subduction segment beneath the ec which plunges toward the e se this subduction is interpreted as a remnant of the paleo caribbean plateau pcp as suggested by geological and tomographic profiles the pcp shows a low angle subduction northward of 5 2 n and is limited southward by a major e w transpressive shear zone normal oceanic subduction of the nazca plate np ends abruptly at the southern limit of the baudo range northward the np subducts beneath the choco block overlapping the southern part of the pcp cenozoic shortening in the ec estimated from a balanced section is 120 km stress analysis of fault slip data in the ec northward of 4 n indicates an e se orientation of 1 in agreement with the pcp subduction direction northward near bucaramanga two stress solutions were observed 1 a late andean n80 e compression and 2 an early andean nw se compression

tension fractures and extensional tectonics a characteristic pattern of normal faults that we have studied in several regions illustrates successive geometries relating to increasing amounts of extension from 10 50 gulf of suez egypt to 50 100 western gulf of california mexico up to 200 southern basin and range usa tension fractures formed at an early stage of extension have an essential role the resulting structure is described as a multiple tilted packs of cards model in which rotational deformation within the major blocks is accommodated by normal faulting on previously formed tension fractures thus defining smaller blocks of second and third generation

dubious case for slab melting in the northern volcanic zone of the andes it was first suggested by r w kay that adakites may represent melts of subducted slab since then the term adakite has become synonymous with slab melts based on their unusual geochemical signature this contribution using the northern volcanic zone of the andes as an example aims 1 to expose the weakness in simply associating a geochemical signature with a genetic mechanism and 2 to underline the importance of using several integrated lines of evidence in assessing the viability of slab melting we conclude that although slab melting probably occurs in some arcs regional geochemical trends in the northern volcanic zone and their relationship to the subduction zone architecture are not indicative of slab melting and can be accounted for by normal arc magmatic processes acting on wedge derived basaltic magmas

along strike variations in the thermal and tectonic response of the continental ecuadorian andes to the collision with heterogeneous oceanic crust oblique to strike geological segmentation in the andean chain has been previously recognised at various scales and is commonly attributed to changes in the convergence vectors of the oceanic and continental plates as well as the upperplate expressions of differing along strike subducted slab age strength and composition we present new white mica and biotite 40 ar 39 ar and zircon and apatite fission track data from several traverses across the cordillera real of ecuador in the northern andes that reveal distinct along strike differences in the timing of accelerated crustal cooling during the cenozoic the data record elevated cooling rates from temperatures of v380 c during v65 55 and v43 30 ma from all sampled regions of the cordillera real and at v15 ma and since v9 ma in the northern cordillera real each cooling period was probably driven by exhumation in response to the accretion and subduction of heterogeneous oceanic crust elevated cooling rates of up to v30 20 c myr were initiated during the palaeocene and eocene early oligocene along the entire contemporaneous margin of ecuador and were driven by the accretion of the oceanic pallatanga terrane and pin on macuchi block respectively onto northwestern south america both of these geological provinces originated at the southern parts of the leading and trailing boundaries of the caribbean plateau and accreted onto the margin during the approximately northeastward migration of the plateau into its current position within ecuador the development of higher topography and elevated cooling rates of up to 50 c myr at v15 ma and since v9 ma are restricted to the region north of 1 30ps and is situated above the postulated subducted flatslab section of the aseismic carnegie ridge plate convergence rate calculations suggest the carnegie ridge collided with the ecuador trench at v15 ma which caused the pre existing coastal provinces to displace to the northeast subsequently driving extension and marine ingressions in southern ecuador and compression and uplift in northern ecuador fl 2001 elsevier science b v all rights reserved

spatial and temporal relationships between compression strike slip and extension in the central venezuelan andes clues for plio quaternary tectonic escape abstract the geometry of tectonic structures attributed to the neogene quaternary time interval is described in the active setting of the venezuelan andes our methodology is based on the analysis of radar satellite and digital elevation model imagery complemented by structural fieldwork and the compilation of seismotectonic data to make a structural analysis on a regional scale radar images provide first class data for morphostructural analysis in areas of dense vegetation and frequent cloud covering like the venezuelan andes we focused our analysis in the burbusay rio momboy and bocono faults corner located in the central part of the belt we have described three stages of deformation during the neogene quaternary the first one mio pliocene in age is a nw se compression responsible for the uplift of the venezuelan andes the second tectonic stage corresponds to a strike slip regime of deformation marked by shearing along the bocono burbusay and valera faults which separates two triangular wedges in the larger trujillo block this strike slip faulting dominated compressional extensional tectonic regime allowed the trujillo crustal block to move towards the ne wrenching has therefore started at some point between the pliocene and the quaternary these two tectonic events are consistent with ongoing strain partitioning in the venezuelan andes the third stage corresponds to extensional deformation limited to the trujillo block and is still active today extension is associated with the motion of crustal blocks moving relative to each other probably above the upper lower crust boundary such extensional deformation can be understood considering that the crust extends and stretches at the same time as it moves towards the ne the combination of both horizontal lateral motion and extension is characteristic of a tectonic escape process the northeastward escape of the trujillo block which belongs to the larger north andes block occurs as a result of the combination of the nw se intracontinental convergence between the south american plate and the maracaibo block and the presence to the north of the caribbean oceanic plate considered as a free boundary we have showed that the kinematics of the caribbean plate offers not only a favorable environment but may also be considered as the driving force of the tectonic escape of the north andes block

partitioning of oblique convergence in the northern andes subduction zone migration history and the present day boundary of the north andean sliver in ecuador along the ecuadorian margin oblique subduction induces deformation of the overriding continental plate for the last 15 ma both exhumation and tectonic history of ecuador suggest that the northeastward motion of the north andean sliver nas was accompanied by an eastward migration of its eastern boundary and successive progressively narrowing restraining bends here we present geologic data earthquake epicenters focal mechanisms gps results and a revised active fault map consistent with this new kinematic model all data sets concur to demonstrate that active continental deformation is presently localized along a single major fault system connecting fault segments from the gulf of guayaquil to the eastern andean cordillera although secondary faults are recognized within the cordillera they accommodate a negligible fraction of relative motion compared to the main fault system the eastern limit is then concentrated rather than distributed as first proposed marking a sharp boundary between the nas the inca sliver and the subandean domain overthrusting the south american craton the nas limit follows a northeast striking right lateral transpressional strike slip system from the gulf of guayaquil isla puna to the andean cordillera and with the north south striking transpressive faults along the eastern andes eastward migration of the restraining belt since the pliocene abandonment of the sutures and reactivation of north south striking ancient fault zones lead to the final development of a major tectonic boundary south and east of the nas favoring its extrusion as a continental sliver accommodating the oblique convergence of the nazca oceanic plate toward south america

plio quaternary extension in the venezuelan andes mapping from sar jers imagery abstract the geometry of extensional structures is described for the first time in the active setting of the venezuelan andes using remote sensing imagery we favored the use of a mosaic of synthetic aperture radar sar scenes of the japanese earth resources satellite 1 jers 1 assisted by complementary remote sensing devices landsat tm digital elevation models and field observations to make a structural analysis at regional scale radar images are particularly efficient in the venezuelan andes where dense vegetation and frequent cloud covering earlier lent difficulties to remote sensing studies we focused our analysis in the valera rio momboy and bocono faults corner and in the mucujun area we show that in an area where ongoing compression and strike slip deformations occur brittle extension can be detected independently from previous knowledge extensional structures correspond to elongated tilted blocks with dimension less than 10 km in width blocks are bounded by curved faults in plan view the concavity being turned towards the axial part of the belt the geometry and kinematics of such structures suggest that syn orogenic extension started together with initiation of right lateral strike slip motion along the bocono fault in the plio quaternary

tectonic record of strain buildup and abrupt coseismic stress release across the northwestern peru coastal plain shelf and continental slope during the past 200 kyr 200 kyr both segments uplifted at high rates of 10 to 20 mm yr 1 through tectonic pulses coeval with the eustatic deglacial sea level rises of isotope stage 1 and warm isotope substage 5e respectively the uplift and related extensive emersion of the coastal plain require high coupling along the subduction zone and or underplating at depth offshore industry acquired reflection seismic lines combined with em12 bathymetric data allow us to investigate the tectonic regime and deformation of the continental margin and shelf major dipping seaward detachments control the long term subsidence of this area these main tectonic features define a tectonic segmentation the talara paita and sechura segments are identified from north to south no clear tectonic correlation in time exists between the onshore and the continental margin segmentations or in space either the longterm subsidence of the offshore indicative of subduction erosion working at depth requires low coupling along the subduction channel at depth the distribution of permanent deformation along the northern peru forearc area includes long term uplift along the coastal plain and long term subsidence along the continental margin the neutral line being located within the 10 km seaward from the present coastline an extensive sequence of raised marine cliffs and associated notches evidences that the most recent uplift step 20 23 ka to present along the cabo blanco segment is related to a sequence of major earthquakes we infer that eustacy exerts important feedback coupling to the seismogenic behavior of the north peru subduction zone we speculate that during sea level fall pore fluid pressure diminishes along the subduction channel inducing a possible seaward migration of the locked zone i e migration of the updip limit reaching a maximum by the end of the eustatic low stand during eustatic sea level rise pore fluid pressure increases along the subduction channel this in turn is capable of weakening the previously locked zone along the plate interface beginning an earthquake sequence earth s orbital variations are a potential external cause that may control the physical processes at work along plate interface

thrust type subduction zone earthquakes and seamount asperities a physical model for seismic rupture a thrust type subduction zone earthquake of m sub w 7 6 ruptures an area of approximately 6 000 km sup 2 has a seismic slip of approximately 1 m and is nucleated by the rupture of an asperity approximately 25km across a model for thrust type subduction zone seismicity is proposed in which basaltic seamounts jammed against the base of the overriding plate act as strong asperities that rupture by stick slip faulting a m sub w 7 6 event would correspond to the near basal rupture of a approximately 2 km tall seamount the base of the seamount is surrounded by a low shear strength layer composed of subducting sediment that also deforms between seismic events by distributed strain viscous flow planar faults form in this layer as the seismic rupture propagates out of the seamount at speeds of kilometers per second the faults in the shear zone are disrupted after the event by aseismic slow viscous flow of the subducting sediment layer consequently the extent of fault rupture varies for different earthquakes nucleated at the same seamount asperity because new fault surfaces form in the surrounding subducting sediment layer during each fast seismic rupture

accreted fragments of the late cretaceous caribbean colombian plateau in ecuador the eastern part of the western cordillera of ecuador includes fragments of an early cretaceous c123 ma oceanic plateau accreted around 85 80 ma san juan unit west of this unit and in fault contact with it another oceanic plateau sequence guaranda unit is marked by the occurrence of picrites ankaramites basalts dolerites and shallow level gabbros a comparable unit is also exposed in northwestern coastal ecuador pedernales unit picrites have lree depleted patterns high endi and very low pb isotopic ratios suggesting that they were derived from an extremely depleted source in contrast the ankaramites and mg rich basalts are lree enriched and have radiogenic pb isotopic

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

subduction zone recycling processes and the rock record of crustal suture zonesthis article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent offshore observations at modern ocean margin subduction zones omszs reveal that bodies of accreted material are commonly volumetrically small or missing that crustal thinning and subsidence 3 5 km has occurred and that most trench axes lie close 5 30 km to the seaward tapering edge of coastal basement rock onshore mapping commonly documents missing or only narrow terranes of former forearc rock and the inboard migration of the arc magmatic front these observations are evidence that subduction is accompanied by the removal of sediment and crustal material from the submerged forearc by the kindred tectonic processes respectively of sediment subduction and subduction erosion subduction erosion truncates the margin migrates the trench inboard at 2 5 km ma onshore observations at ancient crust suturing subduction zones csszs imply that collisional suturing is accompanied by sediment subduction and truncation of both upper and lower plates during a protracted period of suturing 20 50 millio