tracking exhumation of andean ranges bounding the middle magdalena valley basin colombia the shortening history of the andes is important for understanding retroarc deformation along convergent margins and forcing mechanisms of cenozoic climate however the timing of uplift in the northern andes is poorly constrained with estimates ranging from cretaceous to pliocene detrital zircon u pb ages from the middle magdalena valley basin in colombia reveal two provenance shifts during cenozoic time the fi rst shift occurs between early and late paleocene strata where u pb results show a switch from proterozoic dominated to phanerozoic dominated age spectra we attribute this change to uplift related exhumation of the central cordillera the second shift occurs between middle late eocene and late oligocene strata where increased grenville ages and diminished mesozoic ages can be linked to uplift of the eastern cordillera our results show that signifi cant pre neogene deformation affected the northern andes underscoring the potential importance of andean uplift on the dynamics of paleogene climate

geochronology geochemistry and tectonic evolution of the western and central cordilleras of colombia abstract autochthonous rocks of the pre cretaceous continental margin of nw south america the tahami terrane are juxtaposed against a series of para autochthonous rock units that assembled during the early cretaceous allochthonous oceanic crust of the caribbean large igneous province collided with and accreted onto the margin during the late cretaceous we present the first regional scale dataset of zircon u pb la icp ms ages for intrusive and metamorphic rocks of the autochthonous tahami terrane early cretaceous igneous para autochthonous rocks and accreted oceanic crust the u pb zircon data are complemented by multiphase 40ar 39ar crystallization and cooling ages the geochronological data are combined with whole rock major oxide trace element and ree data acquired from the same units to constrain the tectonic origin of the rock units and terranes exposed in the western cordillera cauca patia valley and the central cordillera of colombia the tahami terrane includes lower paleozoic orthogneisses 440 ma that may have erupted during the active margin stage of the rheic ocean basement gneisses were intruded by permian continental arc granites during the final assembly of pangea triassic sedimentary rocks were subsequently deposited in rift basins and partially melted during high t metamorphism associated with rifting of western pangea during 240 220 ma continental arc magmatism during 180 145 ma is preserved along the whole length of the central cordillera and was followed by an early cretaceous out board step of the arc axis and the inception of the quebradagrande arc that fringed the continental margin back stepping of the arc axis may have been caused by the collision of buoyant seamounts which were coeval with plateau rocks exposed in the nicoya peninsular of costa rica rapid westward drift of south america closed the quebradagrande basin in the late aptian and caused medium high p t metamorphic rocks of the arquia complex to exhume and obduct onto the continental margin subduction beneath hot spot derived rocks of the caribbean plateau 100 92 ma formed an intra oceanic arc 92 75 ma which collectively comprise the late cretaceous caribbean large igneous province the remnant ocean basin located between south america and the caribbean large igneous province was partly consumed via continental subduction forming the large antioquia batholith the caribbean large igneous province collided and accreted to south america during 75 70 ma along the cauca almaguer fault resulting in the cessation of both arcs and the paleocene onset of subduction beneath the accreted oceanic crust

controls on petroleum composition in the llanos basin colombia implications for exploration physical and geochemical characteristics of produced petroleum from the central region of the llanos basin colombia were analyzed to understand the petroleum charge history and alteration processes petroleum properties in the study area are the result of the complex charge history of the oil fields the amount of gas in fluids is controlled by the migration distance from the late or possibly the current generation kitchen located beneath the foothill deformation zone gas influx decreases toward the foreland domain as indicated by lower values of the gas oil ratio and saturation pressure the api gravity of the oil samples is mainly controlled by the intensity of biodegradation marine sourced oils accumulated in shallow reservoirs of the foreland prior to the onset of andean deformation those fluids were subjected to different levels of biodegradation depending on the time they remained at reservoir temperatures lower than 80 c 176 f and before being buried to their maximum depth geochemical data suggest multiple charge pulses from different source kitchens of two main types of source rocks as well as different biodegradation levels the proposed petroleum charge and alteration model allows prediction of the temperature history of a reservoir and the most likely physical properties of the petroleum at a specific location the model can be used as an exploration tool to assess the risk of charge prior to drilling in unexplored areas of the basin

tectonic reconstruction of the northern andean blocks oblique convergence and rotations derived from the kinematics of the piedras girardot area colombia abstract a detailed kinematic study in the piedras girardot area reveals that approximately 32 km of ene wsw oblique convergence is accommodated within a northeast trending transpressional shear zone with a shear strain of 0 8 and a convergence factor of 2 early campanian deformation is marked by the incipient propagation of northeast trending faults that uplifted gentle domes where the accumulation of sandy units did not take place maastrichtian unroofing of a metamorphic terrane to the west is documented by a conglomerate that was deformed shortly after deposition developing a conspicuous intragranular fabric of microscopic veins that accommodates less than 5 extension this extensional fabric distortion of fossil molds and a moderate cleavage accommodating less than 5 contraction developed concurrently but before large scale faulting and folding paleogene folding and southwestward thrust sheet propagation are recorded by syntectonic strata neogene deformation took place only in the western flank of this foldbelt the amount direction and timing of deformation documented here contradict current tectonic models for the cordillera oriental and demand a new tectonic framework to approach the study of the structure of the northern andes thus an alternative model was constructed by defining three continental blocks the maracaibo cordillera central and cordillera oriental blocks oblique deformation imposed by the relative eastward and northeastward motion of the caribbean plate was modeled as rigid body rotation and translation for rigid blocks derived from published paleomagnetic and kinematic data and as internal distortion and dilation for weak blocks derived from the piedras girardot area this model explains not only coeval dextral and sinistral transpression and transtension but also large clockwise rotation documented by paleomagnetic studies in the caribbean northern andean region

fluvio lacustrine sedimentation and volcanism in a late carboniferous tensional intra arc basin northern chile abstract extensive outcrops of late carboniferous to triassic volcanoplutonic magnetic arc complexes occur in the andes of northern chile in the salar de atacama area terrestrial volcanosedimentary successions include a 200 600 m thick fluvio lacustrine sequence miembro medio the terrestrial basin which accommodated this sequence formed during the latest carboniferous on top of the deposits of the pre existing carboniferous composite volcanoes the lower part of the miembro medio consists of green limnic and multicoloured alluvial fan deposits the upper part is made up of red fluvial sedimentary rocks basic volcanic rocks occur locally throughout the sequence the climate is inferred to have been warm and humid the limnic freshwater environment had a minimum extension of 300 100 km limnic facies is inferred from the existence of fine parallel bedded sediments ooliths and a limnic benthic fauna lake shore deposits prevail in the outcrops in some sections the green limnic sequence is followed by red fine grained floodplain or alluvial plain and fluvial channel deposits the framework composition of the miembro medio is dominated by volcanogenic detritus presumably eroded from the successions of the previous late carboniferous volcanic arc another volcanogenic clast type was provided by syndepositional basalto andesitic phreatomagmatic activity which also produced peperitic sills dykes and lavas the limnic alluvial fan facies association the predominance of volcanogenic detritus and the accompanying intrabasinal basic calc alkaline volcanism together with the considerable size of the basin point to a nw se trending half graben setting controlled by trans tensional arc tectonics with regard to the late carboniferous triassic volcanosedimentary successions of northern chile this latest carboniferous tensional setting is the only feature in this area supporting those models which inferred an extensional magmatic arc for the entire central and southern andes during the permo triassic

kinematic evolution of andean fold thrust structures along the boundary between the eastern cordillera and middle magdalena valley basin colombia 1 surface and subsurface data support a kinematic reconstruction of cenozoic fold thrust deformation along the eastern cordillera magdalena valley transition in colombia the la salina fault lsf marks the boundary between west vergent eastern cordillera structures and hinterland deposits of the middle magdalena valley basin apatite fission track and u th he thermochronological results for the west directed lsf reveal initial hanging wall exhumation during middle eocene early oligocene 45 30 ma shortening renewed exhumation in the early middle miocene 18 12 ma and accelerated late miocene pliocene 12 3 ma exhumation vitrinite reflectance data suggest maximum burial of 4 6 km helping constrain cenozoic basin architecture mapping of the lsf reveals hanging wall cretaceous eocene rocks in a broad anticline syncline pair with limited faulting and footwall eocene quaternary basin fill in a complex series of tight thrust related folds limited displacement along the westernmost frontal thrust suggests that shortening is largely accommodated by east directed thrusting within a broader triangle zone of a passive roof duplex and probable minor strike slip deformation in the preferred kinematic restoration the most recent phase of shortening to transpressional deformation represents out of sequence reactivation of the lsf consistent with irregular crosscutting relationships among footwall structures earliest exhumation by 45 30 ma in the eastern cordillera fold thrust belt is correlated with increased sedimentary lithic fragments and high compositional maturity in sandstones of the adjacent magdalena valley basin exhumation since 15 ma coincided with decreased compositional maturity and elevated accumulation rates for the real group the compositional provenance shifts are attributed to westward advance of fold thrust deformation into the proximal eastern segments of the magdalena valley basin

an integrated approach to define new plays in mature oil basins the example from the middle magdalena valley basin colombia an integrated approach to detect new areas of potential interest associated with stratigraphic traps in mature basins is presented the study was carried out in the middle magdalena valley basin colombia the workflow integrates outcrop and subsurface interpretations of facies activity of faults and distribution of depocenters and paleocurrents and makes use of them to construct a three dimensional exploration scale geocellular facies model of the basin the outcrop and well log sedimentological analysis distinguished facies associations of alluvial fan overbank floodplain and channel fill the last one constituting the reservoir rock the seismic analysis showed that tectonic activity was coeval with the deposition of the productive units in the basin and that the activity ended earlier before the middle miocene along the western margin than along the eastern margin paleogeographic reconstructions depict transverse and longitudinal fluvial systems alluvial fans adjacent to the active basin margins and floodplain facies dominating the structural highs and the southwestern depositional limit these reconstructions provided statistical data lateral variograms to construct the model the exploration scale facies model depicts the complete structure of the basin in three dimensions and the gross distribution of the reservoir and seal rocks the predictive capability of the model was evaluated positively and the model was employed to detect zones of high channel fill facies probability that form bodies that are isolated or that terminate upward in pinchouts or are truncated by a fault our approach can prove helpful in improving general exploration workflows in similar settings

the origin of the upper palaeozoic chanaral melange of n chile the fragmented and mixed strata of the chanaral melange extend for some 220 km from n to s in the coastal ranges of n chile 26 20 to 28 20 s the melange was produced in an accretionary wedge resulting from the ne subduction of the ancestral pacific oceanic plate beneath the gondwana land margin during carboniferous times the melange consists essentially of blocks of sandstone in a pelitic matrix these sediments were deposited as deep sea basin plain turbidites prior to their fragmentation other blocks in the melange include mafic volcanic rocks with the geochemical characteristics of oceanic island basalts and rare shallow marine limestones structures in the melange suggest an origin by two distinct deformation processes involving unlithified sediments initial boudinage and break up of the strata was apparently accomplished by intrastratal movement resulting from imbricate thrusting within the accretionary wedge the second process produced cross cutting zones of breccia which are interpreted as fluid escape conduits resulting from the high pore pressures produced during underthrusting

an accreted continental terrane in northwestern peru abstract a paleomagnetic study of over 250 cores from 26 sites sampled in early to late cretaceous and paleogene volcanic plutonic and sedimentary formations of the lancones basin in the piura province of northern peru indicates that most of these lithologies carry a stable primary remanent magnetization whose direction is significantly different from that of coeval formations of stable south america a clockwise rotation ranging from 90 for the lowermost units to 35 for the uppermost ones has been documented although the lack of precise chronology has not allowed a detailed temporal description four sites from late carboniferous pennsylvanian formations in the amotape tahuin range also show a 110 clockwise rotation and yield evidence for a northward displacement when considered together with previous geological studies these data are consistent with the hypothesis of the accretion of an amotape tahuin continental terrane to the peruvian margin in neocomian times the accretion was followed by in situ rotation suggesting a dextral shear regime these results indicate that the geodynamical evolution of northern peru is more closely related to the processes observed in ecuador than to those classically assumed for the central andes of peru

mode and timing of terrane accretion in the forearc of the andes in ecuador the volcanic basement of the ecuadorian western cordillera pallatanga formation and san juan unit is made up of mafi c and ultramafi c rocks that once formed an oceanic plateau radiometric ages from these rocks overlap with a hornblende 40 ar 39 ar plateau age of 88 1 6 ma age obtained for oceanic plateau basement rocks of the pinon formation in coastal ecuador and with ca 92 88 ma ages reported for oceanic plateau sequences in the caribbean and western colombia these results suggest that the oceanic plateau rocks of the western cordillera and fl at forearc in ecuador are derived from the late cretaceous caribbean colombia oceanic plateau intraoceanic island arc sequences rio cala group overlie the plateau in the western cordillera and yield crystallization ages that range between ca 85 and 72 ma the geochemistry and radiometric ages of island arc lavas from the rio cala group combined with the age range and geochemistry of their turbiditic volcaniclastic products indicate that the arc was initiated by westward subduction beneath the caribbean plateau they are coeval with island arc rocks of coastal ecuador las orquideas san lorenzo and cayo formations and colombia ricaurte arc these island arc

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

variation in bioturbation with water depth on marine slopes a study on the little bahamas bank abstract reconstructing the paleoceanography of intermediate depth waters is dependent on sedimentary records preserved on marine slopes but knowledge of bioturbation processes in such slope environments is much poorer than for the deep sea in this study 210 pb profiles were measured for the upper 20 cm of sediment from five box cores taken on the slopes of the bahamas in water depths ranging from 400 to 1500 m these were compared with data from the bahamas bank tops and from elsewhere to assess the rate and depth of mixing in the slope environment excess 210 pb inventories are too high to explain by water column decay of 226 ra and are derived largely from atmospheric fallout down core 210 pb xs profiles show an upper zone where 210 pb xs activities decrease exponentially with depth suggesting uniform mixing and a lower zone with patchy 210 pb values reflecting isolated burrows in contrast to bank top sediment which shows 210 pb xs mixing to depths of 40 cm and mixing rates of 1000 cm2 kyr mixing of 210 pb on the slopes penetrates to only 8 cm with mixing rates of 210 pb derived mixing activity in ocean margin sediments diffusive versus nonlocal mixing j mar res 54 1207 1227 a simple model is used to assess the effects of mixing on typical bahamian slope sediments which have deposition rates and compositions which vary dramatically in response to sea level change proxy records such as 18 o are expected to be displaced by up to 0 8 kyr in bulk sediment and individual sediment constituents may be moved by up to 2 kyr relative to one another these mixing effects are reasonably small and may be safely ignored for some studies but for high resolution records or studies where precise age control is important bioturbation of slope sediments should be considered

210 pb and 14 c as indicators of callianassid bioturbation in coral reef sediment abstract callianassid shrimps are the dominant endofauna of reef top sediments on john brewer reef a small platform reef in the central section of australia s great barrier reef province sediment reworking by these shrimps was investigated using the 210pb radioisotope in conjunction with 14c radiometry and textural data 210pb analyses of cores from two reef top sites revealed a classical tripartite subdivision characteristic of depositional settings in which sediment reworking dominates over sediment accumulation 1 a vertical surface mixed layer sml extending to a depth of 50 cm beneath the sediment surface and representing a zone of intensive callianassid reworking 2 an intermediate region of radioa tive attenuation in which excess 210pb activity decreases logarithmically to a depth of slightly more than one meter beneath the sediment surface and interpreted as representing a region utilized less frequently by callianassa for coarse grain storage and predator avoidance 3 a basal region beyond callianassid influence of consistently low background levels of 210pb activity sediment turnover rates in excess of 120 cm2 yr 1 in the sml and up to 32 cm2 yr 1 in the region of radioactive attenuation were high compared to those typical of siliciclastic continental shelf and deep sea deposits emphasizing the intensity of callianassid reworking in shallow coral reef environments 210pb geochronology is unsuitable for determining sediment accumulation rates in such settings because the disruptive impact of callianassa gives rise to unacceptably high figures the depth of callianassid influence established by 210pb radiometry corresponds to points of inflection indicating the bases of vertical surficial sections in 14c subsurface age profiles for the two john brewer reef sites inflection points in other 14c profiles may similarly indicate sediment reworking depths at locations across the reef where 210pb data were not obtained

structure of the eastern cordillera of colombia implications for trap styles and regional tectonics the northwest margin of south america has undergone multiple phases of compressional deformation beginning in the late cretaceous and culminating in the pliocene pleistocene the latest phases miocene to pleistocene of this deformation created the eastern cordillera of colombia compressional structures in the eastern cordillera formed traps that have petroleum exploration potential these are generally in the form of fault bend and fault propagation folds containing top sealed reservoir and mature source rocks trap delineation although difficult to achieve because of the poor seismic quality obtained in the region can be accomplished by supplementing the seismic data with geometric analysis of surficial structures the eastern cordillera is asymmetrical with the dominant tectonic transport direction towards the east west directed thrust faults are localized along the western flank of the eastern cordillera and are interpreted to be back thrusts splaying off a deep mid crustal detachment a regional retrodeformable cross section across the eastern cordillera shows that basement detached deformation of late miocene to pliocene age was followed by pliocene to holocene uplift of the cordillera on high angle basement involved reverse faults these two structural styles were superimposed on preexisting jurassic and lower cretaceous grabens whose normal faults were partially inverted during the later stages of compressional deformation a palinspastic restoration of this regional cross section indicates that approximately 150 km of shortening 40 shortening has occurred 105 km eastward directed towards the llanos basin and 45 km westward directed towards the middle magdalena basin this calculated shortening is very close to that calculated by summing microplate motions for the northwest corner of south america shortening in the sedimentary cover rocks exceeds that in the basement rocks by about three times this shortening can be regionally balanced with a gently dipping mid crustal detachment that extends beneath the middle magdalena basin and central cordillera and which roots in the benioff zone beneath the western cordillera the presence of a mid crustal detachment as postulated here provides a mechanism to transmit the shortening from the convergent plate margin in the pacific to the andean foreland and is interpreted to have provided the structural link between the three ranges of the colombian andes the detachment also provides a mechanism to explain the imbalance of shortening seen between eastern cordillera basement and the overlying strata and implies that the western and central colombian andes have been tectonically transported eastwards a distance of at least 150 km

an autochthonous geological model for the eastern andes of ecuador abstract we describe a traverse across the cordillera real and sub andean zone of ecuador poorly known areas with very little detailed mapping and very little age control the spine of the cordillera comprises deeply eroded triassic and jurassic plutons the roots of a major arc emplaced into probable palaeozoic pelites and metamorphosed volcanic rocks the w flank comprises a jurassic submarine basaltic andesitic volcanic sequence which grades up into mixed jurassic cretaceous volcanic and sedimentary rocks of the inter andean valley the sub andean zone on the e flank of the cordillera comprises a newly recognized cretaceous basin of cleaved mudrocks quartz arenites and limestones east of the syndepositional cosanga fault the cretaceous basin thins into a condensed sequence that is indistinguishable from the rocks of the adjacent hydrocarbon bearing oriente basin the principal penetrative deformation of the cordillera real was probably latest cretaceous palaeocene it telescoped the magmatic belts but shortening was largely partitioned into the pelites between plutons the plutons suffered inhomogenous deformation some portions completely escaped tectonism the pelites conserve two foliations the earliest comprises slaty cleavage formed under low or sub greenschist conditions the later is a strong schistosity defined by new mica growth it largely transposed and obliterated the first both foliations may have developed during a single progressive deformation we find inappropriate recent terrane models for the cordillera real and sub andean zone of ecuador instead we find remarkable similarities from one side of the cordillera to the other including a common structural history in place of sutures we find mostly intrusive contacts between major plutons and pelites triassic to cretaceous events occurred on the autochthonous western edge of the archaean guyana shield the latest cretaceous paleocene deformation is interpreted as the progressive collision of an oceanic terrane s with the south american continent young fault movements have subsequently juxtaposed different structural levels through the cordillera real orogen

new geological and geochronological data from the colombian andes continental growth by multiple accretion the regional geology of colombia consists of a composite palaeozoic to cenozoic sedimentary basin overlying precambrian shield rocks which are exposed in the eastern cordillera magdalena valley and at least part of the extensive llanos plains a palaeozoic metamorphic schist belt with abundant post metamorphic igneous rocks comprises the central cordillera a mesozoic volcanic suite extends from the western flank of the central cordillera and accounts for the bulk of the western cordillera a cenozoic sedimentary sequence probably overlying a late cretaceous oceanic crust and fore arc complex forms the pacific coastal plain the continental edge of central and southern colombia is interpreted as a composite margin made up of successively accreted oceanic island arc related sequences of palaeozoic early mesozoic and late cretaceous ages the present day cycle which began in the miocene is not associated with island arc volcanism

la isla de gorgona colombia a petrological enigma a wide range of intrusive wehrlite dunite gabbro and olivine gabbro and extrusive komatiites picrites and basalts igneous rocks are found on the small pacific island of gorgona the island is best known for its 90 ma spinifex textured komatiites the only true phanerozoic komatiites yet discovered early work led to suggestions that the rocks of the island formed at a mid ocean ridge however more recent research supports an origin as part of a hot mantle plume derived oceanic plateau one of the main lines of evidence for this origin stems from the inferred high mantle source temperatures required to form the high mgo n15 wt komatiites and picrites another remarkable feature of the island considering its small size 8 2 5 km is the degree of chemical and radiogenic isotopic heterogeneity shown by the rocks this heterogeneity requires a mantle source region with at least three isotopically distinctive source regions two depleted and one enriched although these mantle source regions appear to be derived in significant part from recycled oceanic crust and lithosphere enrichments in 187os 186os and 3he in gorgona lavas and intrusive rocks suggest some degree of transfer of material from the outer core to the plume source region at dw modelling reveals that the komatiites probably formed by dynamic melting at an average pressure of 3 4 gpa leaving residual harzburgite trace element depletion in gorgona ultramafic rocks appears to be the result of earlier deeper melting which produced high mgo trace element enriched magmas the discovery of a trace element enriched picrite on the island has confirmed this model gorgona accreted onto the palaeocontinental margin of northwestern south america in the eocene and palaeomagnetic work reveals that it was formed at 26 8s it has been proposed that gorgona is a part of the caribbean colombian oceanic plateau ccop however the ccop accreted in the late cretaceous and was derived from a more equatorial palaeolatitude this evidence and differing geochemical signatures strongly suggests that gorgona and probably other coastal oceanic plateau sequences in colombia and ecuador belong to a completely different oceanic plateau to the ccop

late cenozoic uplift in southern asia and the american west rationale for general circulation modeling experiments geologic evidence suggests significant increases in uplift rates and absolute elevation in several areas during the late cenozoic especially in the last 10 m y regions of major uplift include the tibetan plateau and himalayan mountains of southern asia and the sierra nevada colorado plateau basin and range rocky mountains and high plains of the american west uplift occurred at a scale large enough to affect regional and global climate in a number of ways several of which we evaluate by general circulation model experiments in accompanying papers

mantle diversity beneath the colombian andes northern volcanic zone constraints from sr and nd isotopes in order to provide mantle and crustal constraints during the evolution of the colombian andes sr and nd isotopic studies were performed in xenoliths from the mercaderes region northern volcanic zone colombia xenoliths are found in the granatifera tuff a deposit of cenozoic age in which mantle and crustal derived xenoliths are present in bombs and fragments of andesites and lamprophyres compositions garnet bearing xenoliths are the most abundant mantle derived rocks but websterites garnet free xenoliths and spinel bearing peridotites are also present in minor amounts amphibolites pyroxenites granulites and gneisses represent the lower crustal xenolith assemblage isotopic signatures for the mantle xenoliths together with field petrographic mineral and whole rock chemistry and pressure temperature estimates suggest three main sources for these mantle xenoliths garnet free websterite xenoliths derived from a source region with low p and t 16 kbar 1065 c and morb isotopic signature 87sr 86sr ratio of 0 7030 and 143nd 144nd ratio of 0 5129 garnet bearing peridotite and websterite xenoliths derived from two different sources in the mantle i a source with intermediate p and t 29 35 kbar 1250 1295 c conditions similar to that of sub oceanic geotherm with an oib isotopic signature 87sr 86sr ratio of 0 7043 and 143nd 144nd ratio of 0 5129 and ii another source with p and t conditions similar to those of a sub continental geotherm 38 kbar 1140 1175 c and oib isotopic characteristics 87sr 86sr ratio 0 7041 and 143nd 144nd ratio 0 5135

an integrated analysis of an orogen sedimentary basin pair latest cretaceous cenozoic evolution of the linked eastern cordillera orogen and the llanos foreland basin of colombia the integration of restored basin geometry and internal features of syntectonic units e g stratal architecture thickness sandstone composition with fl exural modeling of the lithosphere constrains the evolution of a basin and its fl exural history related to orogenic growth spatial temporal loading confi guration using this approach we determined the maastrichtian cenozoic polyphase growth of the eastern cordillera of colombia an inverted mesozoic extensional basin the record of this growth occurs in an andean post middle miocene thrust belt the eastern cordillera and in adjacent foreland basins such as the llanos basin to the east this approach permitted the identifi cation of fitectono stratigraphic sequences in the foreland basin and fi ve phases of shortening for the eastern cordillera thermochronological and geochronological data support the spatial and temporal evolution of the orogen foreland basin pair tectono stratigraphic sequences were identifi ed in two restored cross sections one located at a salient and the other in a recess on the eastern fl ank of the eastern cordillera the lower two sequences correspond to late maastrichtian paleocene fl exural events and record the eastward migration of both tectonic loading and depositional zero in the llanos basin these sequences consist of amalgamated quartzarenites that abruptly grade upward to organic rich fi ne grained beds and to the top light colored mudstones interbedded with litharenites in isolated channels amalgamated conglomeratic quartzose sandstones of the third sequence record 15 m y of slow subsidence in the llanos basin and llanos foothills during early to middle eocene time while shortening was taking place farther west in the magdalena valley the fourth sequence of late eocene middle miocene age records a new episode of eastward migration of tectonic loads and depositional zero in the llanos basin this sequence begins with deposition of thick fi ne grained strata to the west whereas to the east in the llanos basin amalgamated quartzarenites unconformably overlie cretaceous and older rocks former forebulge apatite fi ssion tracks in the axial zone of the eastern cordillera growth strata in the

petrology and geotectonic development of the western ecuadorian andes the basic igneous complex abstract the basic igneous complex bic consists of several volcanic formations which form the ecuadorian western cordillera and coastal cordillera the rocks differ significantly from other cordillerian successions and form an isolated tectnic block the upper cretaceous to lower tertiary volcanic series of the ecuadorian basic igneous complex are of island arc type according to their petrography and geochemistry an outer tholeiitic island arc series parts of the pinon formation can be distinguished from a central calc alkalic series depleted in potassium macuchi celica and silante formations the chontal yunguilla and zapotillo formations as well as the upper cayo formation guayaquil cherts are interpreted to represent back arc sediments and sediments of small intra arc basins within a largely submarine island arc the cayo formation consists almost entirely of volcaniclastic and epiclastic rocks deposited on the oceanward side of the island arc which was situated close to the south american continent volcanic activity continued into early eocene times as revealed by k ar dating of the cayo and silante formations 52 ma a small back arc basin was filled with the sediments of the yunguilla formation similar to present day volcanically active island arcs e g aleutians and kuriles kamchatka the bic island arc was mostly underlain by oceanic crust although parts of it built on continental crust celica formation southern ecuador in the early tertiary the island arc was accreted to the south american continent cessation of volcanic activity and accretion to the continent may be explained by increased drift velocities of the lithospheric plates accompanied by a change in the geometry and stress in the subduction zone

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

k ar ages of basic rocks in the patia valley southwest colombia abstract the patia valley situated between the western and central cordilleras of the southwest colombian andes contains two areas in which mesozoic basic and ultrabasic rocks crop out in abundance late cretaceous diabase group pillow basalts which make up much of the western cordillera are at least 81 5 ma in the e1 tambo e1 penol area 105 97 ma hornblende and whole rock dates from the ophiolitic los azules complex indicate an albian age of formation although many dates are lower 65 62 ma owing the low grade ocean floor metamorphism the metamorphic age distribution here supports an end of cretaceous emplacement for the complex rather than the early mid cretaceous emplacement suggested for north colombian ophiolites similarly aligned along the romeral fault system tertiary dacites intruding the low azules complex are 36 15 ma

a tectonostratigraphic synthesis of the sub andean basins implications for the geotectonic segmentation of the andean belt the tectonic structural and depositional history of 19 sub andean foreland and eight southern caribbean basins has been synthesized and is summarized by regional geohistory charts these charts recapitulate the subsidence regimes recognized and have been used to evaluate correlations between genetically related stratigraphic sequences based on spatial and temporal changes in palaeo depositional setting the sub andean region can be subdivided longitudinally into several tectonostratigraphic domains the differential amount of subsidence between two adjacent tectonostratigraphic provinces or sub provinces relies on the presence of a transverse zone of structural accommodation the location of these transfer zones correlates with a number of known and several newly identified zones of intracontinental deformation combined with changes in the structural geometry of the andean fold and thrust belt and the relative dominance of basement fault systems along the length of the andean foreland it is suggested that the andean belt can be separated into five tectonic domains which are themselves bounded by transverse structural accommodation zones representing the multiphase reactivation of pre existing basement fault systems the influence of these broad zones of transcontinental deformation is most apparent during the triassic jurassic period suggesting that they were intimately associated with accommodating intraplate stresses during the breakup of the gondwana supercontinent

the occurrence and timing of high pressure metamorphism on margarita island venezuela a constraint on caribbean south america interaction the metamorphic rock sequences exposed on the island of margarita venezuela located in the southeastern corner of the caribbean plate margin are composed of a high pressure lowtemperature hp lt nucleus subducted to at least 50 km depth now structurally overlain by lower grade greenschist facies units lacking any sign of high pressure subduction zone metamorphism the hp lt nucleus involves protoliths of both oceanic metabasalts and intimately associated carbonaceous schists of the la rinconada unit peridotite massifs and continental affinity metapelites marbles and gneisses of the juan griego unit all hp lt units were joined together prior to the peak of high pressure metamorphism as shown by their matching metamorphic pressure temperature evolution the metamorphic grade attained produced barroisite as the regional amphibole glaucophane is not known from margarita contrary to a widely propagated assumption there are no major nappe structures post dating

thermochronology of allochthonous terranes in ecuador unravelling the accretionary and post accretionary history of the northern andes the western cordilleras of the northern andes north of 5 s are constructed from allochthonous terranes floored by oceanic crust we present 40ar 39ar and fission track data from the cordillera occidental and amotape complex of ecuador that probably constrain the time of terrane collision and post accretionary tectonism in the western andes the data record cooling rates of 80 2 c my from temperatures of 540 c during 85 to 60 ma in a highly tectonised melange pujili unit at the continent ocean suture and in the northern amotape complex the rates were highest during 85 80 ma and decelerated towards 60 ma cooling was a consequence of exhumation of the continental margin which probably occurred in response to the accretion of the presently juxtaposing pallatanga terrane the northern amotape complex and the pujili unit may have formed part of a single regional scale tectonic melange that started to develop at 85 ma part of which currently comprises the basement of the interandean depression cooling and rotation in the allochthonous continental amotape complex and along parts of the continent ocean suture during 43 29 ma record the second accretionary phase during which the macuchi island arc system collided with the pallatanga terrane distinct periods of regional scale cooling in the cordillera occidental at 13 and 9 ma were synchronous with exhumation in the cordillera real and were probably driven by the collision of the carnegie ridge with the ecuador trench finally late miocene pliocene reactivation of the chimbo toachi shear zone was coincident with the formation of the oldest basins in the interandean depression and probably formed part of a transcurrent or thrust system that was responsible for the inception and subsequent growth of the valley since 6 ma

allochthonous terranes in northwestern ecuador abstract formations and lithostratigraphic units of the ecuadorian northwestern cordillera can be divided into three major groups the first group comprises lower cretaceous e morb basalts conformably overlain by tholeiitic to boninitic components of an ensimatic island arc the second group consists mainly of tholeiitic to calc alkaline volcanic and volcano sedimentary island arc deposits unconformably on top of the first group and of eocene to early oligocene age the third group is represented by calc alkaline continental volcanics ranging from late oligocene to recent two main tectonic events occur during early eocene and middle oligocene times the first event can be interpreted as an accretion of a slab of oceanic basalt with its overlying arc material the suture zone is marked by slices of morb type material delineating the southward extension of the colombian cauca patia fault zone the second event is related to a major rearrangement of plate tectonic regimes in the eastern pacific during oligocene times its nature is most probably transpressional accompanied by thrusting

thermochronology and tectonics of the central and western cordilleras of colombia early cretaceous tertiary evolution of the northern andes abstract new thermochronological data record a complex cooling history in the central and western cordilleras of colombia that is a function of early cretaceous to late miocene tectonic events alkali feldspar 40ar 39ar cooling ages of 138 130 ma immediately post date the cessation of jurassic arc magmatism and a major unconformity within the retro foreland region of the northern andes we interpret these ages as cooling driven by exhumation in response to either compression driven by subduction of a seamount or extension and oceanward migration of the slab during the earliest cretaceous giving rise to the lower cretaceous quebradagrande arc sequence biotite and alkali feldspar 40ar 39ar data from the palaeocontinental margin reveal the presence of a younger cooling event at 117 107 ma which was contemporaneous with hornblende 40ar 39ar cooling ages obtained from medium high p t metamorphic relicts of a late jurassic early cretaceous subduction channel this cooling event is attributed to exhumation driven by the collision and accretion of a fringing arc against the continental margin and obduction of the subduction channel onto the forearc inverse modelling of zircon and apatite fission track and u th he data from throughout the central and western cordilleras reveals three periods of rapid cooling since the late cretaceous the earliest phase is recorded by jurassic and cretaceous granitoids that cooled rapidly during 75 65 ma we attribute cooling to exhumation of the continental margin during 75 70 ma 1 6 km my which was forced by the collision and accretion of the caribbean large igneous province in the campanian the central cordillera exhumed at moderate rates of 0 3 km my during 45 30 ma which are also observed over widely dispersed regions along the andean chain and were probably caused by an increase in continent ocean plate convergence rates exhumation rates drastically increased in the middle late miocene with the greatest amount occurring in southern colombia as a consequence of the collision and subduction of the buoyant carnegie ridge at 15 ma

tobago west indies a fragment of a mesozoic oceanic island arc petrochemical evidence the pre cenozoic rocks of tobago west indies represent an exceptional cross section through a portion of a composite mesozoic arc system other parts of which may be found in the villa de cura klippe the aruba blanquilla island chain and perhaps in the western cordillera of colombia and ecuador the geochemical data from tobago presented in this paper provide a basis from which a regional view of the petrologic character of the accreted mesozoic arc of the southern caribbean mountain system can be deduced the mesozoic rocks of tobago can be divided into four main groups the north coast schist the ultramafic to mafic plutonic complex the tobago volcanic group and late mafic dykes rocks of the north coast schist served as wallrock for the plutonic volcanic dyke complex which evolved during the mid cretaceous geochemical and nd and sr isotopic data presented demonstrate the oceanic island arc character of all the pre cenozoic rocks of the island all suites span the calc alkaline and tholeiitic regions on discrimination diagrams rare earth element ree patterns for the north coast schist tobago volcanic group and plutonic suite are essentially identical and are moderately lree enriched la yb 1 1 2 7 the dykes exhibit the most unevolved ree patterns and are interpreted as most closely representative of parental liquid compositions initial nd 6 6 to 9 4 and 87 sr 86 sr 0 70303 0 70364 for all samples except north coast schist and tobago volcanic group argillites are similar in value and overall range to other oceanic island arcs limited isotopic data from argillite samples from tobago suggest an important change in the continental derived sediment contribution during mesozoic arc growth nd isotopic data from north coast schist siliceous argillite indicate virtually no continental detritus in this neocomian sediment while nd isotopic data from the tobago volcanic group argillite suggest a definite continental contribution during albian arc growth

joint inversion of multichannel seismic reflection and wide angle seismic data improved imaging and refined velocity model of the crustal structure of the north ecuador south colombia convergent margin 1 improving seismic imaging of the crust is essential for understanding the structural factors controlling subduction zones processes we developed a processing work flow based on the combined analysis of multichannel seismic reflection mcs and wide angle wa reflection refraction data to derive both shallow and deep velocities suitable for prestack depth migration and to construct a blocky velocity model integrating all identifiable seismic phases contained in mcs and wa data we apply this strategy to the study of the north ecuador sw colombia subduction margin to improve the imaging and geostructural interpretation of a splay fault and surrounding outer and inner margin wedges results show improvements over tomographic inversion of wa data only such as 1 sediment velocity variation across the trench and margin slope that correlates with lateral lithologic changes tectonic compaction and effect of mass wasting processes 2 a two layer velocity structure of the inner wedge basement that is consistent with the crust of an oceanic plateau 3 a complex velocity structure of the outer wedge basement that consists of a deep high velocity 5 0 5 5 km s 1 core and a low velocity zone 3 8 5 0 km s 1 associated with the major splay fault 4 a 1 3 km thick low velocity 3 5 4 0 km s 1 subduction channel that extends beneath the margin outer wedge both the splay fault and subduction channel are expected to direct fluid flows and 5 downdip velocity increase 5 6 km s 1 in the subducting oceanic crust associated with a low 7 8 km s 1 upper mantle velocity possibly reflecting changes in rock nature or properties

basin development and tectonic history of the llanos basin eastern cordillera and middle magdalena valley colombia the middle magdalena valley eastern cordillera and llanos basin constituted a major regional sedimentary basin from the triassic to the middle miocene basin development began during the triassic to the earliest cretaceous with a synrift megasequence related to the separation of north and south america in the proto caribbean the synrift megasequence began with deposition in a continental environment that became paralic and shallow marine in the early cretaceous basin development continued into the cretaceous in a back arc setting east of the andean subduction zone the back arc megasequence was dominated by shallow marine sedimentation and produced an excellent regional source rock during the turonian coniacian marine deposition was abruptly terminated during the ear y maastrichtian due to the final accretion of the western cordillera accretion of the western cordillera created the early pre andean foreland basin megasequence of late maastrichtian to early eocene age this depositional episode consists of coal rich alluvial plain coastal plain and estuarine deposits throughout the middle magdalena valley eastern cordillera and eastern llanos basin the megasequence was terminated by middle eocene deformation in the magdalena valley which ended sediment deposition throughout colombia loading effects of this deformation reestablished the basin in which the late pre andean foreland basin megasequence was deposited until the early miocene this megasequence also consists of alluvial plain coastal plain and estuarine deposits including the primary reservoir in the llanos foothills the upper eocene mirador fo mation the megasequence also includes a series of four major grossly coarsening upward cycles in the llanos basin these cycles correspond to changes in sea level sediment supply and foreland basin loading the mudstone in the lowermost of these cycles is the regional seal in the llanos basin and foothills the middle miocene onset of andean deformation in the eastern cordillera isolated the middle magdalena valley from the llanos basin the deformation was dominated by inversion of the basin controlling faults the resultant loading of the lithosphere created the accommodation space for the andean foreland basin megasequence a major transgression into the llanos basin coeval with this deformation caused deposition of marine mudstones in the lower part of the megasequence however the majority of the andean foreland basin megasequence consists of the guayabo formation a classic molasse sequence deposited in a high energy coarse grained bed load dominated fluvial system that was supplied by the developing mountains of the eastern cordillera

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

geometrical control of subduction related magmatism the mesozoic and cenozoic plutonic history of western colombia radiometric age data from western colombia combined with geological evidence allow five main plutonic episodes ranging in age from triassic to tertiary to be recognized it is suggested that variation in the convergence angle of the oceanic plate relative to the continental edge was an important factor affecting the timing and spatial distribution of plutonic activity on a regional scale major breaks in activity are probably best attributed to either low angle parallel convergence or periods of accretion along the convergent margin

high mg extrusive rocks from the romeral zone ophiolites in the southewestern colombian andes abstract in the southwestern colombian andes high mg extrusive rocks occur within the mesozoic late cretaceous ophiolitic fragments from the romeral fault zone a major suture between the continental domain of the central cordillera and the oceanic domain of the western cordillera the high mg rocks include picrites and picritic basalts and are associated in places with peridotites showing transitional to extrusive textures plagioclase free picrites characterized by quench textures have been found in two localities and occur as pillow lava rio boloblanco or as clasts of monogenic and polygenic volcanic breccias el encenillo they show similar olivine phyric micro spinifex textures with groundmass consisting of hopper and plate olivine or ca poor pyroxene at el encenillo skeletal clinopyroxene and glass hyaloporphyritic texture occurs in the rio boloblanco picrites at pillow rims chemically these picrites are characterized by mgo contents of 20 and are distinguished by the content of tio 2 and incompatible elements in a high ti type rio boloblanco and a very high ti type el encenillo the latter particularly remarkable for the high contents of ti zr and nb which give evidence of an alkaline tendency phase chemistry is consistently different between the two picrite types but comparably high al contents up to 14 of skeletal pyroxenes suggest that the magmas were erupted and cooled in similar conditions olivine phyric picritic rocks characterized by microlitic non quenched groundmasses occur as clasts of volcanic breccias at el encenillo picritic basalts and are present with comparatively high frequency within major los azules and minor upper chapungo valley ophiolitic fragments from the romeral zone either as lavas and volcanigenic breccias these rocks are mostly richer in olivine than the quench textured picrites contain modal plagioclase and relatively low al pyroxenes as regards chemistry a high ti and very low al group los azules upper chapungo valley and a very high ti group el encenillo can also be distinguished within these picrites a comparison with the ultramafic volcanic rocks from gorgona island in the nearby pacific domain off colombia among which the unique phanerozoic komatiites were discovered indicates close similarities between the rio boloblanco picrites and some gorgona island komatiites olivine phyric komatiites with micro spinifex groundmass regarding their petrochemical features it is suggested that they record a unique event related to the same tectonic environment and consequently that they belong to the same structural unit developed in the late cretaceous and extending from continental colombia to the pacific domain this is in contrast with previous subdivisions of the ophiolitic complexes from southwestern colombia into distinct age groups progressively younger from the central cordillera toward the pacific ocean

geodynamic evolution of the northern and central andes during early to middle mesozoic times a tethyan model the early to middle mesozoic sedimentary and magmatic history of the northern and central andes i e the nne trending colombian ecuadorian segment and the nw trending peruvian segment exhibits a spatially contrasted evolution involving several successive tectonic and geodynamic settings the late triassic to late liassic period began with a widespread marine transgression from the latest triassic the colombian marine shelf was progressively destroyed by southward propagating extensional tectonic activity but marine sedimentation continued in peru during this time no significant magmatic activity is recorded except in the emerging colombian area this period is interpreted as the result of rifting of a tethyan oceanic arm which separated the colombian and palaeo mexican margins from latest liassic an important calc alkaline magmatic arc developed along the emergent colombian segment further south the north peruvian shelf probably emerged but marine sedimentation continued in southern peru during middle and early late jurassic times the colombian segment was characterized by important magmatic activity and by coarse clastic continental sedimentation along the peruvian segment a turbiditic trough emergent areas and continental basins were created and the scarcity of calc alkaline magmatism suggests that only very local subduction took place this period is regarded as one of the southeastward subduction beneath the colombian segment and of tethyan oceanic crust originating in the newly formed colombian oceanic arm this pattern would have induced a chiefly left lateral transform motion along the peruvian segment by kimmeridgian tithonian times the palaeogeographic framework had drastically changed along the colombian segment magmatic activity ceased and continental accretions occurred along dextral strike slip sutures in peru tectonic activity was recorded by the creation of a new turbiditic trough and by the resumption of detrital sedimentation in the coastal area arc related volcanism indicated that subduction took place beneath this segment this geodynamic change is interpreted as the result of a sharp decrease in the spreading activity of the tethyan ridges and replacement by pacific spreading centres inducing a roughly northeastward convergence direction

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

late cenozoic tectonics and paleomagnetism of north cauca basin intrusions colombian andes dual rotation modes abstract further paleomagnetic results are reported from 18 sites in late tertiary hypabyssal igneous rocks adjacent to the romeral fault system which forms the eastern boundary of the north cauca basin the paleomagnetic directions in these massive igneous intrusions are distributed in vertical planes parallel to the adjacent romeral suture zone two modes of rotation involving non coherent rotations about horizontal rotation axes and coherent rotations about vertical rotation axes can explain the observed remanence patterns the paleovectors have been rotated variably non coherently both upwards and downwards through up to nearly 90 about horizontal axes which are perpendicular to the prevailing structural trend these horizontal axis rotations are believed to be associated with shear gradients operating on rough projections or asperities on otherwise approximately lenticular bodies these give rise to torques which cause variable horizontal axis rotation along splay faults of the romeral and adjacent systems in addition the north segment of the zone of intrusions has been rotated coherently i e through uniform angles of about 30 counterclockwise about vertical axes relative to the southern zone the change in declination trend corresponds to a major change in trend of the romeral suture zone the vertical axis rotations are believed to represent regional scale kinking of vertical lenti laminar fault panels of crust accreted to the paleocontinental margin along the romeral suture it is hypothesized that oblique transpression has pressed the zone of intrusions into the caldas re entrant in the paleocontinental margin

the formation and fate of large oceanic igneous provinces abstract large igneous provinces are conspicuous features of late phanerozoic geology and include continental flood basal rifted continental margin volcanic sequences and oceanic plateaus the latter are formed in an environment which typically recycles back into the mantle on a time scale of 200 m y but because comparisons have recently been made between oceanic plateaus and precambrian greenstone belt sequences new questions arise about their formation their fate and their preservation here we review some critical aspects of three oceanic plateaus ontong java kerguelen and the caribbean colombian abducted plateau and comment on their make up and the factors governing their preservation with particular relevance to ancient terranes many large igneous provinces can be linked to mantle plumes where plumes ascend beneath spreading ridges their energy is transformed into a large melt volume producing over thickened plateau crust where the spreading rate is low in relation to magma supply the plateau may become subaerial e g iceland but with fast spreading the plateau remains submarine thicker lithosphere may result in plume incubation before magma extrusion and there are many intermediate situations where plumes could readily break through thin lithosphere oceanic or continental because magma supply exceeds extension rate plateaus may be characterised by thick sequences of flows and sills rather than the sheeted dykes typical of phanerozoic ophiolites precambrian greenstones could represent imbricated oceanic plateaus or plumes penetrating thin continental lithosphere the initial high temperature and the buoyant nature of the depleted refractory keel of plateaus contributes to their preservation relative to normal oceanic crust when they collide with active margins they choke the subduction zone causing subduction flip or backstep and the development of extensive talc alkaline arc volcanism on top of the plateau sequences however after x 100 m y they are potentially negatively buoyant so if fluids become available to promote tmnsfotmation of the deeper zones to eclogite they may be able to spontaneously subduct

the late jurassic to present evolution of the andean margin drivers and the geological record 1 uncommonly long lived subduction and variable plate geometry along the south american andean plate margin resulted in diverse relationships between magmatic flux and extensional and contractional deformation as recorded by the overriding continental plate convergence velocities absolute overriding plate velocities and subducting slab ages were resolved along the trench from 170 ma to the present using a recently developed kinematic global plate model to identify any relationship between subduction conditions deformation style and magmatic features in the overriding plate key correlations reflect the dependence of macroscopic crustal strain style on subduction mechanism and relative plate vectors extensional back arc basins involving mafic oceanic crust developed only when the overriding plate velocity of south america was directed away from the trench and the modeled age of the subducting slab was older than 50 myr the development of fold and thrust belts and uplift of major plateaus was accompanied by trench normal convergence velocities in excess of 4 cm yr parameters investigated in this study revealed no correlation with the timing of major magmatic events nor was any correlation observed with the structural style of fold and thrust belts

concept of continuum as opposed to periodic tectonism in the andes abstract results of a study of various andean sedimentary basins that are distinct in terms of age and geodynamic position allow us to postulate the idea of a compressional tectonic continuum as opposed to the short tectonic events which have so far been considered as one of the main features of the dynamic evolution of the andean belt in these basins the development of kilometre scale synsedimentary folded structures implies the existence of compressional stress over a period of several million years the existence of tectonic events of long duration allows us to modify the interpretation of the classic andean tectonic model the timing of the continuum tectonism is perhaps dependent on the variation in the convergence rate between the farallon nazca and south american plates

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

destruction of luzon forearc basin from subduction to taiwan arc continent collision abstract along offshore to the east of southern taiwan different stages of subduction and collision occur simultaneously along strike of the convergent boundary as a result the evolution of the luzon arc and its forearc basin can be studied from the younger subduction zone to the south to the collision zone to the north examining more than 8000 km of seismic lines we analyzed the seismic stratigraphy of strata in a forearc basin and its successive basins in the collision zone to study the processes related to arc collapse and forearc basin closure the study area presents three evolutional stages intra oceanic subduction initial arc continent collision and arc continent collision we divided 9 seismic sequences in the forearc basin and found older sub parallel basin fill sequences 4 9 and younger divergent sequences 1 3 isochron maps of the sequences were used to interpret different deformation modes and their areal extends on the arc side of the basin of the subduction and initial collision zones we found relatively undisturbed strata showing little arc deformation on the trench side the growth strata in sequences 1 through 3 are the result of recent tectonic wedging along the rear of the accretionary prism tectonic wedging and back thrusts incorporate the forearc strata into the rear of the accretionary prism until they close the forearc basin at a region with a 2200 m basement relief this relief is not caused by active deformation as young flat forearc strata lap onto it and mark the transition from initial collision to collision where many growth strata to the north suggest abrupt increase in active arc basement deformation the 1 deforming basement 2 back thrusts and 3 other sedimentary processes affect the architecture of the successive basins in the collision zone until the arc is juxtaposed to the rear of the fold and thrust belt on land

the geology and mesozoic collisional history of the cordillera real ecuador abstract the geology of the metamorphic rocks of the cordillera real of ecuador is described in terms of five informal lithotectonic divisions we deduce that during the mesozoic repeated accretionary events occurred and that dextral transpression has been of fundamental importance in determining the tectonic evolution of this part of the northern andes the oldest event recognised of probable late triassic age may be related to the break up of western gondwana and generated a regional belt of s type plutons during the jurassic major calc alkaline batholiths were intruded following this in latest jurassic to early cretaceous time a volcano sedimentary terrane of possible oceanic or marginal basin origin the alao division and the most westerly gneissic chaucha arenillas terrane were accreted to continental south america the accretion of the oceanic western cordillera took place in latest cretaceous to earliest tertiary time this latter event coincided with widespread thermal disturbance as evidenced by the large number of young k ar mineral ages recorded from the cordillera real

plateaux ancient and modern geochemical and sedimentological perspectives on archaean oceanic magmatism unequivocal examples of oceanic plateaux older than about 2 1 ga have yet to be identified from the geological record the most likely explanation for this is the partial dismemberment of ancient plateaux at convergent plate margins and their subsequent juxtaposition in accretionary complexes with arc volcanic sedimentary and plutonic rocks a review of cretaceous plateaux in the eastern indian ocean suggests that certain geochemical isotopic criteria e g positive nb anomalies relative to primitive mantle employed by petrologists to identify modern oceanic plateau lavas are of little practical use in distinguishing ancient plateau lavas from arc and volcanic rifted margin basalts the extreme chemical and isotopic heterogeneity of indian ocean plateaux together with large variations in area and volume suggest that these edifices represent one extreme of a spectrum of oceanic plateau compositions the other extreme represented by iceland and ontong java is characterised by relatively homogeneous melt compositions reflecting steady state melting conditions and the absence of a continental lithosphere contaminant to plateau magmas crustal thickness and compositional estimates for archaean oceanic plateaux using the mckenzie bickle method give a mean thickness of 43 km and mean mgo content of 19 wt archaean plateaux were thus compositionally distinct from surrounding ocean floor basaltic komatiite and notably more magnesian than plateaux such as iceland mean for tertiary basalts 6 3 wt mgo and ontong java mean for core samples 6 9 wt mgo by analogy with cretaceous tertiary ultramafic complexes in western colombia the deeper portions of these plateaux may have consisted of noritic rocks underlain by iherzolite pyroxenite gabbronorite and dunite the general absence of such rocks in archaean terrains suggests that these portions of plateaux are only rarely preserved the search for remnants of ancient oceanic plateaux has identified examples of archaean submarine lavas in the canadian shield 2 7 ga southern abitibi belt superior province and kaapvaal craton of southern africa 3 5 3 3 ga barberton and pietersberg belts chemical and isotopic data suggest that komatiite tholeiite units from these areas were derived by partial melting of a plume or asthenospheric mantle source but do not distinguish adequately the tectonic environment rifted continental margin ocean floor oceanic plateau in which these rocks were erupted however the uplift histories of these putative plateau fragments as recorded in sediments bounding and intercalated with the komatiite basalt sequences appear to be broadly comparable to those of the best documented phanerozoic oceanic plateaux although further stratigraphical investigations of presumed plateaux ancient and modern are required to test this hypothesis we believe that the sedimentary record offers the best evidence yet for the formation of oceanic plateaux as far back as the mid archaean 3 5 ga

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

current states of stress in the northern andes as indicated by focal mechanisms of earthquakes abstract systematic inversion of double couple focal mechanisms of shallow earthquakes in the northern andes reveals relatively homogeneous patterns of crustal stress in three main regions the first region presently under the influence of the caribbean plate includes the northern segment of the eastern cordillera of colombia and the western flank of the central cordillera north of 4 n it is characterized by wnw ese compression of dominantly reverse type that deflects to nw se in the merida andes of venezuela where it becomes mainly strike slip in type a major bend of the eastern thrust front of the eastern cordillera near its junction with the merida andes coincides with a local deflection of the stress regime sw ne compression suggesting local accommodation of the thrust belt to a rigid indenter in this area the second region includes the sw pacific coast of colombia and ecuador currently under the influence of the nazca plate in this area approximately e w compression is mainly reverse in type it deflects to wsw ene in the northern andes south of 4 n where it is accommodated by right lateral displacement of the romeral fault complex and the eastern front of the northern andes the third and most complex region is the area of the triple junction between the south american nazca and caribbean plates it reveals two major stress regimes both mainly strike slip in type the first regime involves sw ne compression related to the interaction between the nazca and caribbean plates and the panama micro plate typically accommodated in an e w left lateral shear zone the second regime involves nw se compression mainly related to the interaction between the caribbean plate and the north andes block which induces left lateral displacement on the uramita and romeral faults north of 4 n deep seismicity about 150 170 km concentrates in the bucaramanga nest and cauca valley areas the inversion reveals a rather homogeneous attitude of the minimum stress axis which dips towards the e this extension is consistent with the present plunge of the nazca and caribbean slabs suggesting that a broken slab may be torn under gravitational stresses in the bucaramanga nest this model is compatible with current blocking of the subduction in the western northern andes inhibiting the eastward displacement of slabs which are forced to break and sink in to the asthenosphere under their own weight

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

the internal structure of oceanic plateaus inferences from obducted cretaceous terranes in western colombia and the caribbean although the structure of mantle plume derived oceanic plateaus has recently been assessed using remote geophysical techniques combined with petrological modelling it is nevertheless desirable to test whether these theoretical rock types dunites gabbros and basalts actually exist and to establish their geochemical nature oceanic plateaus may have initially formed above or near sea level during a short vigorous pulse and thereafter commonly subside to abyssal depths as the lithosphere cools thus making sampling of their deeper levels extremely difficult however the cretaceous age colombian caribbean oceanic plateau was partially accreted against the south american continent so making the imbricated segments available for study during the process of plateau accretion and imbrication it is predominantly the basaltic layers which are obducted but parts of the sequence down to layered and banded gabbros with associated pyroxenites and dunites sometimes foliated can be exposed where the imbricate thrusting brings up deeper levels most of the upper crustal sequence in western colombia is composed of basaltic pillowed and massive flows and sills that are chemically uniform and undepleted relative to normal mid ocean ridge basalts komatiites and more abundant picrites are found at intervals and appear to occur near the base of the sequence in these zones both depleted and moderately enriched basalt and komatiite compositions occur and may result from dynamic partial melting and mixing processes associated with the high temperature part of the plume it is possible to integrate these compositional characteristics into a general model for oceanic plateau structure where the rate of magma supply is in excess of that which can be accommodated by normal spreading processes thus leading to extrusion of flows and the emplacement of sills and high level magma chambers but relatively few dykes the dense and chemically heterogeneous ultramafic magmas intrude the base of the pile and undergo fractionation to form ultramafic cumulates whereas the well mixed basaltic magmas are erupted to form the homogeneous plateau basalts based on these observations a possible structure of the caribbean colombian plateau is proposed which is compatible with geophysical models for other less well exposed oceanic plateaus

the geological history of northwestern south america from pangaea to the early collision of the caribbean large igneous province 290 75ma abstract northwestern south america preserves a record of the assembly of western pangaea its disassembly and initiation of the far western tethys wilson cycle subsequent pacific margin magmatism and ocean plateau continent interaction since the late cretaceous numerous models have been presented for various time slices although they are based on either spatially restricted datasets or dates that are inaccurate estimates of the time of crystallisation here we review a very large quantity of geochronological geochemical thermochronological sedimentological and palaeomagnetic data that collectively provide tight constraints for geological models these data have been collected over a trench pacific parallel distance of 1500 km colombia and ecuador and reveal important temporal trends in rifting and subduction the temporal framework for our model constraints are obtained from robust concordant zircon u pb ages of magmatic rocks during 290 75 ma the late cretaceous thermal history of the margin

four phase tectonostratigraphic development of the southern caribbean abstract although the stratigraphy of northern south america and the southern caribbean has been studied in detail in many areas a clear understanding of the sequence of tectonic events and their relation to regional stratigraphy has not been forthcoming since northern south america collided with southern north america in the late palaeozoic we identify four discrete tectonic phases from the stratigraphy of the area that is now the southern caribbean these phases derived from a synthesis of regional stratigraphy were 1 panama arc collistion with colombia in the late miocene early pliocene which drove a complex triangular wedge bounded by strike slip faults northward into the caribbean 2 right lateral strike slip faulting associated with post eocene eastern motion of the caribbean ocean floor into the gap between the separating americas 3 island arc development and collision associated with the entry of the present caribbean ocean floor an area of buoyant ocean crust into the gap between the separating americas and 4 continental rifting and passive margin development in the late jurassic early cretaceous associated with the separation of north and south america approximately along the palaeozoic suture zone overprinting of younger structures on inherited structures from previous phases is responsible for the overall complexity of the region

high sr y magma petrogenesis and the link to porphyry mineralization as revealed by garnet bearing i type granodiorite porphyries of the middle cauca au cu belt colombia porphyry style cu au mo mineralization is widely associated with igneous suites with sr y 40 and low heavy rare earth element hree content this geochemical signature is commonly attributed to garnet or amphibole fractionation in the lower crust where mantle derived melts evolve into intermediate composition arc suites however the presence of a residuum related to mineralized porphyry systems where amphibole and garnet are stable is generally inferred from geochemistry and has nowhere been documented directly we here report on two occurrences of late miocene garnet bearing granodiorite porphyry at the tesorito au cu prospect in the quinchia district and el poma au cu prospect in the middle cauca au cu porphyry belt in colombia these rocks provide proof that garnet was stable in the lower crust at the time of porphyry mineralization at both studied sites garnet bearing i type granodiorite porphyries were intruded by intramineralization garnet free plagioclase phyric granodiorite porphyries that were cut by quartz and k feldspar veins and affected by biotite alteration garnet bearing porphyries predate intramineralization porphyries by 0 4 to 2 m y garnet occurs as unzoned grains commonly included in large plagioclase phenocrysts that in some cases also contain inclusions of igneous epidote the latter indicating pressures of 1 gpa for early crystallized phases large plagioclase phenocrysts exhibit two growth stages separated by a resorption boundary high al amphibole phenocrysts as well as titanite and magnetite formed after the first stage but prior to the second stage of plagioclase growth at pressures of ca 0 9 gpa whole rock geochemical compositions of garnet bearing porphyries are consistent with only limited garnet fractionation whereas some of the intramineralization porphyries have y 40 which is together with other trace element indicators taken as evidence for significant garnet fraction at depth during porphyry mineralization hree content of zircon in garnet bearing porphyries are higher than that of intramineralization porphyries which supports the interpretation that garnet fractionation in the lower crust was significant during the formation of the intramineralization porphyries the phase relationships and geochemical data reflect a geodynamic change from an environment that permitted rapid ascent of oxidized garnet bearing porphyries prior to au cu mineralization to a regime that favored melt evolution in the lower crust where garnet was a residual phase and at the same time allowed the formation of large mid to upper crustal magma chambers the latter are widely inferred to be essential for porphyry mineralization to occur

the metamorphic basement of ecuador the metamorphic rocks of ecuador once thought to constitute a uniform and ancient basement belong to several distinct units of widely differing ages compositions and metamorphic histories the largest area of metamorphic rocks constitutes an unbroken belt on the eastern andean slope from the colombian to the peruvian border the bulk of the rocks in this belt are of cretaceous age and were metamorphosed in the greenschist facies under a barrovian facies series rocks in the extreme north and those south of lat 3 15 s are different and may be of paleozoic age a smaller terrain on the western andean slope in southwestern ecuador is yet more varied it includes four metamorphic terrains the polymetamorphic piedras group of precambrian age the low pressure tahuin group of paleozoic age the high pressure raspas formation of cretaceous age and a low pressure terrain of uncertain age north of the la palma fault several isolated outcrops of metamorphic rocks in ecuador are of uncertain significance

active mountain building along the eastern colombian subandes a folding history from deformed terraces across the tame anticline llanos basin active crustal shortening and surface uplift are vividly expressed in the foothills of the eastern cordillera of colombia quaternary alluvial fan and terrace deposits are folded and uplifted tens to hundreds of meters above the local base level recording deformation and incision of the landscape we used geomorphic observations and structural analysis from a 300 km 2 three dimensional seismic reflection survey over an actively growing anticline to investigate the relationship between finite shortening and surface uplift and its effects on landscape evolution a quantitative description of finite shortening of the pregrowth strata from excess area and line length calculations were compared with uplift and shortening estimates obtained from geomorphic strain markers dated using in situ 10 be cosmogenic nuclide depth profiles this combined approach provides the first estimates of quaternary surface uplift and horizontal shortening rates along the foothills of the northeastern colombian andes finite shortening for the tame anticline was calculated using two different approaches excess area shortening varies from 530 36 m 2 at its southern end decreasing northward to 404 72 m 2 whereas line length shortening is typically an order of magnitude smaller using the maximum horizontal shortening our results for the tame anticline reveal a quaternary uplift rate of 2 8 1 5 mm yr and a shortening rate of 2 1 1 2 mm yr 2 with the older terraces located at the highest elevations and recording the highest rotation from the horizontal indicating growth by limb rotation based on these rates the initiation of folding is estimated to be 0 25 0 09 0 05 ma 2 we hypothesize that the discrepancy between shortening estimates from line length and excess area calculations can be reconciled by accounting for internal deformation likely developed during the initial stages of folding which can be represented by strain hardening for example the shortening rates we obtain are comparable to geodetically derived shortening rates but are slightly lower than long term geologic shortening rates slower shortening rates and the presence of a youthful structure at the thrust front suggest that the orogen has been migrating eastward since the late pleistocene probably due to reorganization of a supercritical thrust wedge farther west in the hinterland to attain critical taper

geochemical window into subduction and accretion processes raspas metamorphic complex ecuador the high pressure low temperature p 1 3 2 gpa t is 600 c raspas metamorphic complex is an exhumed fragment of the partially accreted partially subducted amotape chaucha terrane in southwest ecuador comparative analysis of major and trace elements plus sr nd and pb isotopes in bulk lithologies and individual crystalline phases shows that the complex includes one to three layers of ordinary oceanic crust and underlying mantle lithosphere together with oceanic plateau fragments subduction and exhumation of oceanic lithosphere resulted in selective bulk trace element geochemical changes rb ba and sr have been lost in amounts from approximately 85 50 from the high p low t metamorphosed pelites and basalts whereas pb is enriched in mafic rocks during formation of the eclogite u pb and rare earth elements rees were immobile high p low t metamorphosed terranes form the basement of active ecuadorian arc volcanoes partial melting of this basement by mantle wedge derived basalt is a likely source of adakitic components

tropical chemical weathering of hillslope deposits and bedrock source in the aburra valley northern colombian andes abstract tropical chemical weathering produces extensive lateritization and formation of deep weathering profiles both processes are fundamental to landscape evolution and slope instability the aburra valley of the northern colombian andes is characterized by tropical conditions the valley slopes are mostly covered by hillslope deposits originating from four basement rock suites which comprise contrasting granitoid volcanic sedimentary ophiolitic and metamorphic sources respectively tropical chemical weathering of the aburra hillslope deposits and their respective bedrock were examined using x ray fluorescence and x ray diffraction analysis to document and quantify their chemical weathering profiles compositions and mineralogical properties the chemical index of alteration cia loss on ignition loi and the mobiles index imob were used to quantify the degree of weathering of hillslope deposits and bedrock source weathering trends were analyzed using a cn k and a cnk fm diagrams the material mantling the slopes in the aburra valley records an intense weathering history chemical weathering is characterized by increased development of clay minerals kaolinite halloysite and iron and aluminum sesquioxides lateritization characterizes the final stage of the weathering profiles concentrations of cao na2o k2o decrease markedly in the weathering products compared to the fresh bedrock source whereas concentrations of al2o3 fe2o3 and mgo increase significantly cia ratios of matrix slope deposits derived from all four sources near 100 whereas those of boulder slope deposits and saprolites are lower but exceed source rock values different a cn k weathering paths are evident for each lithotype validating the correlation established between the hillslope deposits and their various parents chemical weathering indices in some samples are strongly influenced by the presence of sesquioxides as reflected by high loi anomalously low cia and varying enrichment trends on the a cnk fm diagrams consequently different chemical indices based on different criteria need to be combined to obtain best results as illustrated here by the combination of loi cia and imob the overall results suggest that tropical conditions have dominated for a long time in the northern colombian andes leading to uniformly high weathering indices in matrix slope deposits irrespective of parent lithotype prolonged warm and humid conditions could thus be responsible for the weathering and remobilization of extensive old hillslope deposits during the quaternary however in addition to the influence of climatic factors tectonism has also undoubtedly influenced slope evolution in the aburra valley

cretaceous basaltic terranes in western colombia elemental chronological and sr nd isotopic constraints on petrogenesis accreted terranes comprising mid to late cretaceous picrites basalts and dolerites occur in three north south trending belts in western colombia in the central cordillera western cordillera and along the pacific coast the geochemistry of these rocks is consistent with an oceanic plateau plume related origin and they most probably formed in the pacific as part of the caribbean oceanic plateau these igneous rocks display small but significant inter cordillera variations being younger and more depleted in incompatible trace element ratios and with more positive end values to the west the igneous rocks of the pacific coast serrania de baudo are dated at 73 78 ma 40ar 39ar and those of the western cordillera at 90 ma whereas the volcanics of the central cordillera are believed to be older than 100 ma most of the igneous rocks are basaltic and it is suggested that they have fractionated from picritic primary magmas generated by partial melting within a hot mantle plume variable and positive end values reveal that the plume must have been heterogeneous originating from a mantle source with a long term history of depletion partial melt modelling suggests that the composition of the basalts requires at least some input from a mantle source region containing garnet and that the extent of partial melting required to reproduce the composition of the erupted basalts is of the order of 20 mixing of melts from different depths either in the mantle melting column or during fractionation in lithospheric magma chambers can explain the relative homogeneity of basaltic lavas erupted to form this and other oceanic plateaux the caribbean colombian oceanic plateau may have formed at an oceanic spreading centre and valuable comparisons can be made between iceland and the caribbean colombian plateau

evaluation of magmatic processes for the products of the nevado del ruiz volcano colombia from geochemical and petrological data abstract trace element concentrations of the eruptive products from the nevado del ruiz volcano indicate that at least for the last 1 ma the major controlling factor in the evolution of the magma has been a simple fractional crystallization the zr hf ratios suggest that the source material is mantle like in origin and whilst subduction related contamination of the source material has taken place there is no evidence that assimilation of continental crust has affected the series no convincing geochemical evidence for magma mixing was found and data on banded pumices of pleistocene historic and the 1985 eruptions strongly support a crystal fractionation model to explain the compositional variation of their glasses therefore if mixing has taken place it can only be a mixing of layers within a stratified or partially stratified magma chamber modeling of fractional crystallization using a stepwise program has been undertaken and least squares approximations with small residuals are commensurate with analyzed samples crystal fractionation models with plagioclase dominating two pyroxenes and iron oxides give good fits for all calculations we conclude that simple crystal fractionation is the process most consistent with major and trace element variations in the ruiz series this closed system model requires at least 77 crystal fractionation of a basaltic parent to generate the observed compositions of dacites

episodic orogenic front migration in the northern andes constraints from low temperature thermochronology in the eastern cordillera colombia 1 new thermochronometric data from the eastern cordillera of the colombian andes reveal diachronous exhumation associated with cenozoic contractional deformation in this sector of the northern andes we present a comprehensive account of exhumation patterns along a 150 km long across strike transect between 4 and 6 n by integrating 29 new apatite fission track aft ages and 17 new zircon fission track zft ages with sparse published thermochronological data from this area our data reveal episodic eastward migration of the orogenic front at an average rate of 2 5 2 7 mm a during the late cretaceous cenozoic we identify three major stages of orogen propagation 1 slow propagation 0 5 3 1 mm a until early eocene 2 rapid orogenic advance 4 0 18 0 mm a during middle late eocene which accounts for 86 of the orogen s present width and 3 slow orogen propagation 1 2 2 1 mm a from oligocene to holocene times our data demonstrate that in the course of changes in plate kinematics the presence of inherited crustal anisotropies such as the former rift bounding faults of the eastern cordillera favor a nonsystematic progression of foreland basin deformation through time by preferentially concentrating accommodation of slip and thrust loading along these zones of weakness

ophiolite complex from la tetilla southwestern colombia south america a disrupted ophiolitic complex of cretaceous age from the romeral fault zone in the southwestern colombian andes occurs in the la tetilla area near popayan it represents a peculiar association of low ti and high ti tholeiitic basalts among the ophiolitic complexes from the romeral zone it includes 1 a main sequence with incipient metamorphism under prehnite pumpellyite to greenschist facies conditions consisting in ascending order of cumulus wehrlite and gabbro massive gabbro and basaltic and doleritic lavas and breccias the basalts are characterized by the crystallization of clinopyroxene prior to plagioclase cpx phyric basalt group and have a primitive low ti tholeiite composition mg numbers between 70 6 and 65 4 the clinopyroxene chemistry is consistent with this magmatic affinity the plutonic rocks can be related to a similar low ti tholeiitic magma 2 a sequence of pervasively metamorphosed basalts with greenschist facies metamorphism of oceanic type structurally underlies the mai

devonian plants from colombia with discussion of their geological and palaeogeographical context plant fossils are described from the cuche formation eastern cordillera colombia in the area of floresta those identified as colpodexylon cf deatsii banks and cf archaeopteris sp suggest an earliest late devonian frasnian age for the formation these or similar taxa are also found in contemporaneous deposits in western venezuela and other elements of the venezuelan flora are found in a geographically intermediate locality all three devonian plant localities in the northwest of south america are within the colombian eastern cordillera and its northern extension the venezuelan perija range an area that has been integrated as a part of the so called eastern andean terrane or central andean province supposedly accreted to the autochthonous block of the guyana shield during the early jurassic or before although both invertebrates and plants from this terrane have strong affinities to north american and european assemblages and might be interpreted as implying a laurussian origin for the eastern andean terrane the evidence is not yet unequivocal with some authors postulating an in situ development of this province

neogene and quaternary development of the neotropical rain forest the forest refugia hypothesis and a literature overview abstract the upheaval of the northern andes in miocene and pliocene time changed the drainage system in northern south america significantly and caused the present day rain forest areas of choco and the lower magdalena valley became separated from amazonas plant diversity may have reached the highest level in the miocene or pliocene and excessive present day phytodiversity may be regarded as a legacy of the tertiary rather than an evolutionary product of the quaternary in the quaternary strong temperature oscillations related to the series of ice ages were superposed on the late tertiary forest dynamics which included river displacement and latitudinal migrations of the equatorial rain belt caloric equator with the rhythm of the precession cycle of orbital climate forcing the hypothesis that claims a permanent rain forest cover all over the amazon basin during the last glacial is in contrast with the forest refugia hypothesis which accepts replacement of rain forest by savanna or savanna forest during dry climatic intervals both scenarios have been evidenced by pollen records in this paper it is suggested that both hypotheses are not necessarily conflicting and apparently did occur in different parts of the amazon basin and in different periods depending on the climatological constraints a compilation of the most important literature concerning the vegetational climatic and environmental history of the rain forest areas of amazonas and choco and surrounding dry ecosystems has been included

global tectonics since the breakup of pangea 180 million years ago evolution maps and lithospheric budget abstract pangea the earth s youngest single supercontinent broke up 180 million years ago tectonic plates were subsequently formed by dispersal of the continental fragments and accretion of new oceanic lithosphere the configurations of all the major lithospheric plates at 0 20 65 95 140 180 ma bp are reconstructed on six globes of the earth each with a radius of 10 cm it appears that plate boundaries maintain a remarkably close fit on model globes of constant radii if the reconstructions include the recovery of subducted spreading patterns this is illustrated with maps in equatorial orthographic oblique orthographic and transverse hammer aitoff projections the snug fit of the tectonic plates at every tested time since the breakup of pangea 180 ma bp is consistent with the theory of plate tectonics on a non expanding earth and contradicts rapidly expanding earth models the areas of oceanic lithosphere produced and consumed during the past 180 ma bp are estimated from surface measurements of the globes reconstructed on the basis of particular assumptions these measurements suggest a consistent increase in the production rate of oceanic lithosphere during the past 140 ma it was decided to revise the assumptions and see if alternative reconstructions of the ancient spreading patterns on the floors of the tethys and eo pacific oceans could avoid implying an increase of lithospheric production rates with time this appeared to be possible the revised maps suggest that ophiolites older than 180 ma bp may have been obducted in cenozoic collision zones of the himalayas andes rockies and the western part of the banda arc timor new guinea estimates of the ocean floor production and consumption budget appear to be quite similar for both map series and only the possible ranges are summarized here time averaged for the past 180 ma world wide production and consumption of oceanic lithosphere appears to have varied between 2 6 and 3 5 km2 a 1 at most the mean of world wide spreading velocities ranges between 2 4 and 3 5 cm a 1 and world wide means of subduction velocities lie between 4 2 and 6 3 cm a 1 more accurate estimates are possible on the basis of particular assumptions and have been specified for the various time intervals distinguished i e the neogene 0 20 ma bp palaeogene 20 65 ma bp upper cretaceous 65 95 ma bp lower cretaceous 95 140 ma bp and middle and upper jurassic 140 180 ma bp

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

geochronological control for the main tectonic magmatic events of ecuador abstract a chronological outline of the principal tectonic magmatic events of ecuador is presented based upon approximately 100 k ar and rb sr age determinations eight magmatic episodes are recognized in southwestern ecuador evidence of plutonic and metamorphic activity in the late precambrian and in the middle to late triassic is found large granitic batholiths of middle to late jurassic age associated with widespread acidic volcanism are traceable along the eastern flank of the andes into colombia early cretaceous volcanism and plutonism is recognized only in southern ecuador although apparently equivalent units continue southwards into the coastal cordillera of peru a great orogenic belt characterized by metamorphism plutonism and volcanism was established in the late cretaceous and continued through the paleogene it is traceable into peru and colombia the macuchi fm of the western cordillera along with the pin on fm and associated sediments of the coastal zone represent the remnants of an island arc and the adjacent oceanic floor respectively that collided with the south american plate during the eocene feininger 1981 in the neogene quartz dioritic intrusions were emplaced in the western cordillera as well as in the cordillera de muelleturo although apparently at different times in the late oligocene and miocene acidic volcanism is noted in the southern ecuadorian andes but not in the north in the quaternary andesitic volcanism and large stratovolcanoes characterize the northern andes while widespread rhyolitic ashflow activity is noted in the southern andes in ecuador the post triassic magmatic episodes show a westerly migration similar to that found in colombia but opposite to that noted in peru

clastic deposition provenance and sequence of andean thrusting in the frontal eastern cordillera and llanos foreland basin of colombia sedimentological provenance and detrital thermochronological results for basin fill at the modern deformation front of the northern andes 6 n latitude provide a long term eocene to pliocene record of foreland basin sedimentation along the eastern cordillera llanos basin boundary in colombia lithofacies assemblages and paleocurrent orientations in the upward coarsening 5 km thick succession of the nunchia syncline reveal a systematic shift from craton derived shallow marine distal foreland back bulge accumulation in the mirador formation to orogen sourced deltaic and coastal influenced sedimentation of the distal to medial foreland foredeep in the carbonera and leon formations to anastomosing fluvial and distributive braided fluvial megafan systems of the proximal foreland foredeep to wedge top basin in the lower and upper guayabo formation these changes in depositional processes and sediment dispersal are supported by up section variations in detrital zircon u pb and u th he ages that record exhumation of evolving compartmentalized sediment source areas in the eastern cordillera the data are interpreted in terms of a progressive eastward advance in fold and thrust deformation with late eocene oligocene deformation in the axial zone of the eastern cordillera along the western edge of floresta basin soapaga thrust early miocene reactivation inversion of the eastern margin of the mesozoic rift system pajarito and guaicaramo thrusts and middle late miocene propagation of a footwall shortcut fault yopal thrust that created the nunchia syncline in a wedge top piggyback setting of the eastern foothills along the transition from the eastern cordillera to llanos foreland basin collectively the data presented here for the frontal eastern cordillera define a general in sequence pattern of eastward advancing fold and thrust deformation during cenozoic east west shortening in the colombian andes

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

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

the paleogene arcs of the northern andes of colombia and panama insights on plate kinematic implications from new and existing geochemical geochronological and isotopic data abstract the northern andes of colombia comprise two non cogenetic paleogene arcs formed in contrasting geodynamic settings including continental and oceanic domains new whole rock geochemistry and isotopic constraints together with a review of 332 geochemical 76 isotopic and 204 geochronological data from paleocene to eocene volcanic and plutonic rocks exposed in central and western colombia and panama are used to evaluate cause effect relations between regional plate kinematics and the spatio temporal distribution of the circum caribbean magmatic arcs short lived 60 45 ma arc like magmatism in the central cordillera of colombia was emplaced in a thickened continental crust due to the oblique subduction of the caribbean oceanic plate underneath south america as suggested by the high sr y ratios conversely the panama arc and its poorly explored extension in the northwestern segment of the western cordillera of colombia document a major phase of tholeiitic to calc alkaline arc magmatism between 71 ma and 34 ma mostly derived from a hydrated mantle wedge and emplaced in an oceanic plateau crust this arc record a major phase of magmatic activity between 40 ma and 50 ma that can be associated to changes in the convergence direction and subduction rates of the farallon plate farther to the south in the pacific region of colombia eocene arc related rocks from the timbiqui complex show a geochemical signature that suggests a magmatic origin from melting of both a subduction modified mantle and lower tectonically thickened crust a possible interpretation for the pacific and western colombia panama arc systems and its analogous in ecuador recall an aleutian type convergent margin in which the continental arc laterally switches to a purely oceanic system along the trailing edge of the caribbean plate as consequence of eastward subduction of the farallon plate

kinematic reconstruction of the caribbean region since the early jurassic abstract the caribbean oceanic crust was formed west of the north and south american continents probably from late jurassic through early cretaceous time its subsequent evolution has resulted from a complex tectonic history governed by the interplay of the north american south american and paleo pacific plates during its entire tectonic evolution the caribbean plate was largely surrounded by subduction and transform boundaries and the oceanic crust has been overlain by the caribbean large igneous province clip since 90 ma the consequent absence of passive margins and measurable marine magnetic anomalies hampers a quantitative integration into the global circuit of plate motions here we present an updated quantitatively described kinematic reconstruction of the caribbean region back to 200 ma integrated into the global plate circuit and implemented with gplates free software our reconstruction includes description of the tectonic units in terms of euler poles and finite rotation angles our analysis of caribbean tectonic evolution incorporates an extensive literature review to constrain the caribbean plate motion between the american continents we use a novel approach that takes structural geological observations rather than marine magnetic anomalies as prime input and uses regionally extensive metamorphic and magmatic phenomena such as the great arc of the caribbean the clip and the caribbean high pressure belt as correlation markers the resulting model restores the caribbean plate back along the cayman trough and major strike slip faults in guatemala offshore nicaragua offshore belize and along the northern andes towards its position of origin west of the north and south american continents in early cretaceous time we provide the paleomagnetic reference frame for the caribbean region by rotating the global apparent polar wander path into coordinates of the caribbean plate interior cuba and the chortis block we conclude that formation of the caribbean plate west of the north and south americas as a result of panthalassa pacific spreading leads to a much simpler plate kinematic scenario than proto caribbean atlantic spreading placing our reconstruction in the most recent mantle reference frames shows that the clip originated 2000 3000 km east of the modern galapagos hotspot and may not have been derived from the corresponding mantle plume finally our reconstruction suggests that most if not all modern subduction zones surrounding the caribbean plate initiated at transform faults two of these along the southern mexican and nw south american margins evolved diachronously as a result of migrating trench trench transform triple junctions

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

setting characteristics and age of the andean porphyry copper belt in colombia three distinct porphyry copper subbelts are defined in the colombian andes 1 a western subbelt emplaced during the early to middle eocene in an oceanic domain underlain by ophiolitic basement along the western side of the western cordillera 2 a composite eastern subbelt generated during the middle jurassic to earliest cretaceous in a continental domain underlain by elements of the guyana shield along the eastern side of the central cordillera and the western side of the eastern cordillera and between the two 3 a central subbelt formed in the early to late miocene at the contact of the oceanic and continental domains along the cauca patia depression and its flanks k ar geochronometry undertaken as part of this study provided preferred minimum ages of 54 7 or 1 3 m y for murindo 48 1 or 1 0 m y for acandi and 42 7 or 0 9 m y for pantanos pegadorcito in the western subbelt 166 or 4 m y for mocoa 166 or 4 m y for dolores 144 or 3 m y for california and 131 or 2 m y for infierno chili in the eastern subbelt and 17 4 or 0 4 m y for piedrasentada in the central subbelt the porphyry copper prospects are associated with dacite to tonalire porphyry stocks which in the western and eastern subbelts intruded batholiths emplaced perhaps 15 to 30 m y earlier but which in the central subbelt are isolated within metamorphic or sedimentary host rocks the porphyry copper systems exhibit a variety of alteration patterns with potassium silicate and sericitic alteration important in the highest grade prospects and propylitization predominant in low grade occurrences none of the prospects displays significant supergene enrichment although porphyry coppers in the continental domain tend to be more molybdenum rich than those in the oceanic domain gold rich prospects occur in both settings as do major lode gold deposits the jurassic and miocene subbelts are interpreted to have been emplaced during eastward subduction at andean type continental margins whereas the western eocene subbelt could have been generated in either a modified andean type setting or in an intraoceanic island arc prior to its accretion to the continental edge

mixing depths and sediment accumulation rates on an arid tropical shelf based on fine fraction 210pb analysis abstract continental shelves in arid tropical settings present particular challenges to 210pb based analysis of sedimentation rates and surface mixing owing to the combination of coarse sediment deep and year round bioturbation and lower atmospheric flux of 210pb the modern continental shelf at the northern terminus of the hyper arid gulf of eilat aqaba goe receives flood runoff of siliciclastics producing a mixed seabed with 20 carbonate that is dominated by larger benthic foraminifera and mollusks focusing analysis on only the fine fraction of deep penetrating cores 70 cm from 15 to 40 m water depths yields reliable 210pb profiles in contrast to analysis of bulk sediment graphical abstract sedimentation rates increase offshore five fold from 0 01 0 04 cm y in 15 m water depth to 0 21 0 27 cm y in 30 40 m depths reflecting offshore redistribution of flood delivered siliciclastic sediments away from the wadi mouth a result also supported by an offshore increase in the inventory of excess 210pb in contrast the thickness of the surface mixed layer sml decreases from 30 cm to 20 cm with proximity to anthropogenic stressors channelized flood runoff historic release of sewage and operation of fish cages which we attribute to the suppression of macrobenthic burrowers the rate of sedimentation on the gulf shelf away from dynamic bypassing in the 15 m shoreface is comparable to other tropical carbonate shelves rich in large benthic foraminifera and is higher than rates documented on the adjacent slope increasing confidence in this approach to 210pb analysis using only the fine fraction analysis of the fine fraction rather than bulk sediment would be a useful adjustment to 210pb methodology in any area with scarce fine grained sediment