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

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

dating crystalline groundmass separates of altered cretaceous seamount basalts by the 40ar 39ar incremental heating technique abstract alteration of submarine basalts compromises geochronology using conventional k ar and 40 ar 39 ar techniques to help overcome these problems we re evaluated the potential of groundmass dating techniques incremental heating on acid leached groundmass samples following an overnight bakeout at 200 c and using high resolution heating schedules eliminated most of the low temperature alteration effects in the submarine basalts studied more than 75 of the groundmass analyses n 32 display accurate age plateaus consisting of 30 70 of the total amount of 39 ar k released more than 50 of the analyses have plateau ages concordant with their total fusion ages implying minor or proportional loss of radiogenic 40 ar and 39 ar k overall we could show a high degree of coherence between ages of groundmass separates and comagmatic phenocrysts this suggests that the dating of aphyric basalts which previously has proven problematic can be accomplished with increasing confidence as well adding these rock types to the list of datable submarine basalts significantly enhances our ability to understand the eruptive history of linear submarine volcanic chains

low temperature thermochronology of the northern cordillera real ecuador tectonic insights from zircon and apatite fission track analysis the post early cretaceous tectonic development of the northern andean segment north of 5 s is loosely constrained by a series of partially and fully reset k ar ages new apatite and zircon fission track data from three traverses across the cordillera real of ecuador reveal distinct periods of accelerated crustal cooling during the cenozoic fission track ages range from 65 to 14 ma zircon and 44 to 8 ma apatite with apatite mean track lengths ranging from 14 6 to 12 1 m these results record rapid cooling rates during 43 30 ma 23 15 ma and 10 0 ma occurring at temperatures below 275 c the events reflect up to 9 km of crustal exhumation in various regions of the cordillera in response to relative changes in the kinematics of the farallon nazca and south american plates the exhumation history of the cordillera real was not continuous but rather evolved during separate phases these phases persisted for 10 myr and are partly coincident with tectonic phases of the central andes although extend for longer time periods possibly as a result of isostatically driven crustal exhumation large spatial variations in the zircon and apatite fission track data within the cordillera real indicate that local fault bounded massifs have experienced different thermal histories since 43 ma major variations are not restricted to the regional scale and exist within individual terranes faulted slivers within terranes demonstrate highly variable exhumation histories suggesting that deformation was not dominated by terrane bounding faults additionally n s variations in exhumation depths suggest that ne sw trending faults have been active since 10 9ma

petrogenetic evolution of arc magmatism associated with late oligocene to late miocene porphyry related ore deposits in ecuador we present a data set on the geochemical and isotopic composition of mineralized porphyry intrusions se riate intrusions and arc volcanic formations spatially associated with late oligocene to late miocene porphyry related ore deposits in ecuador our aim is to constrain the petrogenetic evolution of these arc magmas to evaluate geodynamic and tectonic impact factors on arc magmatism and to explore the metallogenic signifi cance of these parameters we distinguish four petrogenetic groups associated with the investigated magmatic centers indicating dif ferent paths of low and high pressure crustal magma evolution tertiary ecuadorian arc magmas which dif ferentiated at higher crustal pressures are commonly characterized by adakite like compositions related to fractionation and or restite equilibration effects of amphibole garnet titanite particularly in hydrous silicic melts without concomitant or subsequent major plagioclase fractionation trace element distribution patterns of arc magmas through time are broadly systematic and indicate that crustal magma differentiation progres sively extends to higher pressures from the late oligocene to the late miocene possibly related to crustal thick ening locally adakite like features possibly related to high melt h2o contents or accessory phase fractiona tion are already observed in the early miocene in a regional setting otherwise dominated by low pressure magma evolution compositional differences between mineralized porphyry intrusions and spatially associated seriate precur sor intrusions in ecuador mostly reflect regional scale changes in magma composition through time and are not observed if the relative emplacement age difference is small where porphyry intrusions are associated with shallow crustal batholith systems in northern central ecuador their emplacement is commonly late with re spect to batholith magmatism intrusive compositions indicate that porphyry parental melts mostly evolved at higher crustal pressures where metallogenically favorable melt preconditioning might possibly occur

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

geodynamic controls on tertiary arc magmatism in ecuador constraints from u pb zircon geochronology of oligocene miocene intrusions and regional age distribution trends abstract we obtained u pb zircon ages of late tertiary intrusions in the northern andes to provide robust time calibration points for the intrusive geochronologic framework of ecuador which is mostly based on k ar data magmatism associated with intrusion emplacement ranges from 31 to 7 ma with ages mainly pooling in the late oligocene early miocene where both k ar and u pb data exist for a given intrusive system ages obtained by the different methods are usually concordant within 1 4 m y suggesting that k ar ages are accurate indicators for the timing of tertiary arc magmatism on a regional multi m y scale except for a single sample the investigated intrusions completely lack externally inherited zircons in agreement with dominantly zircon poor oceanic crustal basement domains spatio temporal distribution trends of tertiary arc magmatism inferred from screened u pb k ar and zircon fission track geochronologic data allow tracking of the progressive along arc broadening of a flat slab segment below southernmost ecuador in the mid to late miocene and of moderate slab shallowing in northern central ecuador in the late miocene these regional arc migration patterns correlate in time with the subduction of the buoyant inca plateau and the carnegie ridge seamount chain the temporal distribution of tertiary ecuadorian arc magmatism indicates a late oligocene early miocene arc magmatic flare up event comprising widespread ignimbrite eruption and batholith construction initiation of the flare up event coincides in time with accelerating less oblique farallon nazca south america plate convergence suggesting a positive feedback between convergence rates asthenospheric melt production mantle crust melt flux and upper crustal arc magmatic productivity in ecuador

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

initiation of subduction and the generation of slab melts in western and eastern mindanao philippines adakite found in both the eastern and western parts of mindanao island philippines is a rare rock type characterized by low heavy rare earth elements and y contents together with high sr y ratios and is considered to be the result of the melting of young subducted oceanic crust which leaves an eclogite residue pliocene quaternary adakites from western mindanao zamboanga peninsula are probably derived from the melting of the young miocene sulu sea crust which is currently subducting beneath zamboanga associated nb enriched basalts are thought to come from mantle metasomatized through interaction with adakitic melts in eastern mindanao pliocene quaternary cones and plugs of typical adakitic composition mark the trace of the philippine fault in surigao and north davao the underlying philippine sea crust is of eocene age and therefore cannot melt under normal subduction thermal conditions thermal models indicate that melting at the start of subduction can occur subduction of the philippine sea plate began 3 to 4 ma beneath eastern mindanao and probably accounts for the presence of adakites along the philippine fault

tectonic evolution of the caribbean and northwestern south america the case for accretion of two late cretaceous oceanic plateaus it is widely accepted that the thickened oceanic crust of the caribbean plate its basaltic accreted margins and accreted mafic terranes in northwestern south america represent the remnants of a single ca 90 ma oceanic plateau we review geologic geochemical and paleomagnetic evidence that suggests that the caribbean colombian oceanic plateau in fact represents the remnants of two different oceanic plateaus both dated as ca 90 ma the first of these plateaus the caribbean plateau formed ca 90 ma in the vicinity of the present day galapagos hotspot northeastward movement of the farallon plate meant that this plateau collided with the proto caribbean arc and northwestern south america 10 m y after the plateau s main phase of formation paleomagnetic evidence suggests that the second of these plateaus the gorgona plateau formed at 268 308s possibly at the site of the present day sala y gomez hotspot over the next 45 m y this plateau was carried progressively northeastward on the farallon plate and collided in the middle eocene with the proto andean subduction zone in northwestern south america the recognition of a second ca 90 ma pacific oceanic plateau strengthens the link between plateau formation and global oceanic anoxic events

simple bouguer gravity anomaly field and the inferred crustal structure of continental ecuador the simple bouguer gravity anomaly field of continental ecuador corresponds closely to the physiographic provinces of the country the sierra which includes the andes and their foothills is characterized by a pronounced low with values to 292 mgal which reflects the deep andean root bouguer anomalies over the oriente become less negative away from the sierra chiefly in response to progressive thinning of continental crust eastward the costa between the sierra and the pacific shore in the north has the most positive on land bouguer anomalies 162 mgal so far known in the western hemisphere this part of the costa is underlain by an ancient oceanic plate now welded to the northwestern corner of the otherwise continental south american plate

origin and cretaceous tectonic history of the coastal ecuadorian forearc between 1 n and 3 s paleomagnetic radiometric and fossil evidence the presence of at least one block pallatanga block derived from the caribbean colombian oceanic plateau has been proposed in the western cordillera of ecuador new 40ar 39ar dating of the basement of the coastal blocks 90 87 ma and fossil ages in the overlying calentura fm 89 84 ma argue for late turonian coniacian extrusion of the basement of the pinon block paleomagnetic inclinations indicate that the mafic basement of the pinon and san lorenzo blocks extruded at equatorial low southern latitudes no subsequent significant latitudinal drift can be observed during the cretaceous rapid changes in paleomagnetic declination between 73 and 70 ma in both the pinon and the san lorenzo blocks indicate that the large igneous province which originally hosted the coastal blocks experienced a major tectonic event during late campanian times similar late turonian coniacian basement ages and late campanian tectonic events are reported in the pallatanga block of the western cordillera we propose that the pinon san lorenzo and pallatanga blocks all derive from a common large igneous province the caribbean colombian oceanic plateau and were incorporated into the south american margin by collision during the late campanian 73 70 ma

global tectonic significance of the solomon islands and ontong java plateau convergent zone abstract oceanic plateaus areas of anomalously thick oceanic crust cover about 3 of the earth s seafloor and are thought to mark the surface location of mantle plume heads hotspot tracks represent continuing magmatism associated with the remaining plume conduit or tail it is presently controversial whether voluminous and mafic oceanic plateau lithosphere is eventually accreted at subduction zones and therefore 1 influences the eventual composition of continental crust and 2 is responsible for significantly higher rates of continental growth than growth only by accretion of island arcs the ontong java plateau ojp of the southwestern pacific ocean is the largest and thickest oceanic plateau on earth and the largest plateau currently converging on an island arc solomon islands for this reason this convergent zone is a key area for understanding the fate of large and thick plateaus on reaching subduction zones this volume consists of a series of four papers that summarize the results of joint us japan marine geophysical studies in 1995 and 1998 of the solomon islands ontong java plateau convergent zone marine geophysical data include single and multi channel seismic reflection ocean bottom seismometer obs refraction gravity magnetic sidescan sonar and earthquake studies objectives of this introductory paper include 1 review of the significance of oceanic plateaus as potential contributors to continental crust 2 review of the current theories on the fate of oceanic plateaus at subduction zones 3 establish the present day and neogene tectonic setting of the solomon islands ontong java plateau convergent zone 4 discuss the controversial sequence and timing of tectonic events surrounding ontong java plateau solomon arc convergence 5 present a series of tectonic reconstructions for the period 20 ma early miocene to the present day in support of our proposed timing of major tectonic events affecting the ontong java plateau solomon islands convergent zone and 6 compare the structural and deformational pattern observed in the solomon islands to ancient oceanic plateaus preserved in precambrian and phanerozoic orogenic belts our main conclusion of this study is that 80 of the crustal thickness of the ontong java plateau is subducted beneath the solomon island arc only the uppermost basaltic and sedimentary part of the crust 7 km is preserved on the overriding plate by subduction accretion processes this observation is consistent with the observed imbricate structural style of plateaus and seamount chains preserved in both precambrian and phanerozoic orogenic belts

the sonju lake layered intrusion northeast minnesota internal structure and emplacement history inferred from magnetic fabrics abstract the sonju lake intrusion sli in northeastern minnesota is a layered mafic complex of keweenawan age 1096 1 0 8 ma related to the midcontinent rift the cumulate paragenesis of the intrusion is recognized as broadly similar to the skaergaard intrusion a classic example of closed system differentiation of a tholeiitic mafic magma the sli represents nearly closed system differentiation through bottom up fractional crystallization geochemical studies have identified the presence of a stratabound 50 100 m thick zone anomalously enriched in au pge similar to the pge reefs of the skaergaard intrusion this pge enriched zone is hosted within oxide gabbro cumulates about two third of the way up from the base of the intrusion we present a petrofabric study using the anisotropy of magnetic susceptibility ams to investigate the emplacement and flow patterns within the sonju lake intrusion petrographic and electron microprobe studies combined with ams and hysteresis measurements indicate the primary source of the magnetic signal is pseudo single domain psd magnetite or titanomagnetite low field ams was measured at 32 sites within the sonju lake intrusion which provided information about primary igneous fabrics the magnetic fabrics in the layered series of the sonju lake intrusion are consistent with sub horizontal to inclined emplacement of the intrusion and show evidence that the cumulate layers were deposited in a dynamic environment well aligned magnetic lineations consistently plunging shallowly toward the southwest indicate the source of the magma is a vertical sill like feeder presumably located beneath the finland granite the finland granite acted as a density trap for the sonju lake magmas forcing lateral flow of magma to the northeast the strongly oblate magnetic shape fabrics indicate the shallowly dipping planar fabrics were enhanced by compaction of the crystal mush

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

vertical tectonics at a continental crust oceanic plateau plate boundary zone fission track thermochronology of the sierra nevada de santa marta colombia 1 the topographically prominent sierra nevada de santa marta forms part of a faulted block of continental crust located along the northern boundary of the south american plate hosts the highest elevation in the world 5 75 km whose local base is at sea level and juxtaposes oceanic plateau rocks of the caribbean plate quantification of the amount and timing of exhumation constrains interpretations of the history of the plate boundary and the driving forces of rock uplift along the active margin the sierra nevada province of the southernmost sierra nevada de santa marta exhumed at elevated rates 0 2 km my during 65 58 ma in response to the collision of the caribbean plateau with northwestern south america a second pulse of exhumation 0 32 km my during 50 40 ma was driven by underthrusting of the caribbean plate beneath northern south america subsequent exhumation at 40 25 ma 0 15 km my is recorded proximal to the santa marta bucaramanga fault more northerly regions of the sierra nevada province exhumed rapidly during 26 29 ma 0 7 km my further northward the santa marta province exhumed at elevated rates during 30 25 ma and 25 16 ma the highest exhumation rates within the sierra nevada de santa marta progressed toward the northwest via the propagation of nw verging thrusts exhumation is not recorded after 16 ma which is unexpected given the high elevation and high erosive power of the climate implying that rock and surface uplift that gave rise to the current topography was very recent i e 1 ma and there has been insufficient time to expose the fossil apatite partial annealing zone

distinguishing between in situ and accretionary growth of continents along active margins abstract active continental margins represent both sites of crustal loss by tectonic erosion and gain by juvenile magmatism and oceanic arc plateau accretion estimating the gain component depends on distinguishing between growth of margins by either accretion of oceanic crust or juvenile magmatic addition during extension distinguishing between these allochthonous and autochthonous models is frequently ambiguous due to difficulties in interpreting the tectonic origin of mafic rocks we present a detailed geochronological and isotopic study constraining the evolution of the continental margin of northwest south america during 190 113 ma semi continuous slab rollback resulted in crustal thinning juvenile magmatism and ultimately the removal of continental slivers negating previous hypotheses that the margin evolved by the accretion of an island arc however basic crustal growth estimates along with comparisons between rocks exposed in northern peru and the northern andes imply that uneven losses of continental crust occurred along northwest south america for mesozoic exposures and that the accretion of the caribbean large igneous province clip onto nw south america probably played a crucial role in maintaining and preserving mesozoic continental crust north of the huancabamba deflection

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

missing history 16 71 ma of the galapagos hotspot implications for the tectonic and biological evolution of the americas we present the results of volcanological geochemical and geochronological studies of volcanic rocks from malpelo island on the nazca plate 15 8 17 3 ma belonging to the gala pagos hotspot tracks and igneous complexes 20 8 71 3 ma along the pacific margin of costa rica and panama the igneous complexes consist of accreted portions of ocean island and seamount volcanoes and aseismic ridges representing the missing primarily subducted history of the gala pagos hotspot the age and geochemical data directly link the gala pagos hotspot tracks on the pacific ocean floor to the caribbean large igneous province ca 72 95 ma confirming a pacific origin for the caribbean oceanic plateau from the gala pagos hotspot we propose that emplacement of this oceanic plateau between the americas and interaction of the gala pagos hotspot tracks with the central american arc played a fundamental role in the formation of land bridges between the americas in late cretaceous paleocene and pliocene holocene time the land bridges allowed the exchange of terrestrial faunas e g dinosaurs mastodons saber tooth cats and ground sloths between the americas and served as barriers for the exchange of marine organisms between the central pacific ocean and the caribbean sea and the central atlantic ocean

geochemistry and tectonic setting of pre collision cretaceous and paleogene volcanic rocks of ecuador pre collision cretaceous lower tertiary volcanic rocks of western ecuador from the coastal area pinon and san lorenzo formations and western cordillera macuchi and celica formations consist of three rock suites the first suite includes tholeiites of ocean floor affinities that have distinct light ree depleted patterns and low la nb la hf and th hf ratios the island arc tholeiites of the second suite have lower contents of ti zr hf and nb the third suite comprises calc alkalic rocks mainly continental margin andesites and dacites the continental volcanic arc rocks crop out in an area east of the tholeiitic rocks the boundary between the two areas is the major suture separating continental south america from accreted terrenes to the west a model is suggested for the geodynamic evolution of what is now western ecuador during the late cretaceous it includes two east dipping subduction zones of turonian to santonian ages and the accretion of the oceanic island arc to the continent

low grade metamorphism and geotectonic setting of the macuchi formation western cordillera of ecuador the cretaceous eocene basic to intermediate marine volcanic rocks of the mucuchi formation constitute the western cordillera in northern ecuador their chemical features mostly correspond to those of tholeiitic basalts with some calc alkaline affinities and suggest an oceanic island arc setting the macuchi rocks are affected by low grade non deformative metamorphism characterized by zeolite prehnite pumpellyite and lower greenschist facies assemblages depth zonation is suggested by the downward mineral sequence i laumontite pistacitic epidote pumpellyite prehnite ii pumpellyite prehnite pistacitic epidote iii actinolite biotite pistacitic epidote chlorite this broad zonation and the chemistry of individual minerals point to an interaction between the volcanic rocks and sea water under a moderate to high thermal gradient 75 c km alteration appears to have been dependent primarily on fluid control volume pressure composition temperature and reaction kinetics which together partly overshadow the role of load pressure compositional variations of a mineral species at the scale of a contiguous flow or even at the scale of a thin section show that intensity of alteration was spatially uneven depending on rock permeability and consequently metastable equilibrium commonly exists however a progressive approximation to equilibrium as a result of p t control is shown by the mineralogy a high fo2 of the fluid phase is evident from the mineral chemistry the metamorphism of the macuchi volcanics is similar to the hydrothermal burial type produced during the development of a volcanic arc where lavas and volcanoclastics accumulated in a shallow marine environment however some of its characteristics point to a transition toward systems defined by a higher t p ratio such as those found in ocean floor metamorphism a model is proposed in which the macuchi volcanics are assigned to an oceanic island arc generated contemporaneously with a marginal basin which has opened as the outcome of progressive north south attenuation of the continental crust due to mantle diapirism

high temperature 350 c thermochronology and mechanisms of pb loss in apatite abstract natural processes driven by heat flow can be understood using quantitative reconstruction of the thermal history of accessory and common minerals that were formed or modified in these processes thermochronology assumes that isotopes are lost from minerals by thermally activated volume diffusion and forms the basis of many studies of the thermal evolution of the crust however some studies challenge this assumption and suggest that the mechanisms controlling isotope transport in minerals over geological time scales are dominated by aqueous fluid flow within mineral pathways here we test these contrasting hypotheses by inverse modelling apatite uranium lead u pb dates to produce theoretical t t solutions assuming pb was lost by volume diffusion these solutions are compared with independent geological constraints and intra grain apatite u pb dates which demonstrate that volume diffusion governed the displacement of pb this confirmation combined with an inverse modeling procedure that permits reheating and cooling paths to be distinguished between 375 and 570 c provides geologists with a unique tool for investigating the high temperature thermal evolution of accessory minerals using the u pb method

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

effects of aseismic ridge subduction on the geochemistry of frontal arc magmas abstract aseismic ridges are variably broad linear topographic features built by submarine volcanic activity that are thicker and more buoyant than the surrounding oceanic crust together with oceanic plateaus they constitute a non trivial 30 of the oceanic floor as such aseismic ridge subduction significantly affects subduction dynamics as well as deformation of the overriding plate indeed it is considered to exert major controls on volcanic activity the generation of juvenile earth s crust and the formation of porphyry cu au deposits previous studies claimed that magma chemistry changes associated with aseismic ridge subduction are mainly due to source processes such as melting of the subducting ridge melting of the edges of the oceanic plate following slab tear or subduction erosion of the continental margin with subsequent partial melting of the scraped and subducted continental material here we explore the role of the subducted aseismic carnegie ridge on modulating frontal arc magma chemistry in the ecuadorian southernmost colombian frontal arc we show symmetric changes of several geochemical indices th nd sm nb th la ba th and evolutionary paths na2o sio2 correlations in frontal volcanic arc rocks with respect to the equatorial latitude which coincides with the projection of the crest of the subducting carnegie ridge at the frontal arc we explore the systematic along arc changes of these geochemical indices through geochemical modelling using a monte carlo approach and conclude that the trends are controlled to a significant extent by intracrustal processes in particular we argue that magmatic systems associated with the central part of the ecuadorian volcanic arc evolve through fractionation partial melting of crustal rocks and mixing at average greater depths than those in the northern and southern sides we speculate about possible links between the systematic along arc geochemical changes and the subduction of the carnegie ridge our findings highlight a new facet of the controls that are exerted by the overriding plate on the geochemical variability of arc magmas

plate reconstructions aseismic ridges and low angle subduction beneath the andes low angle subduction segments beneath peru and chile are believed to represent interaction of the subduction zone with the aseismic nazca and juan fernandez ridges on the basis of symmetric sea floor spreading models and plate hotspot reconstructions predicted continuations of the relatively buoyant ridges correspond well with the limits of the low angle subduction segments further the history of interaction of the aseismic ridges with the subduction zone as predicted by plate reconstructions is not inconsistent with available information on volcanic episodes of the andes if the contemporary correlation of volcanic gaps and low angle subduction is also applicable to the rest of the late cenozoic the occurrence of deep seismicity beneath the eastern flanks of the andes is also believed to reflect the effects of low angle subduction the deep seismic segments are adjacent to segments which are presently or were recently experiencing low angle subduction thus the deep seismicity is interpreted as representative of remnants of normal oceanic lithosphere subducted before interaction of aseismic ridges with the subduction zone other geologic phenomena are associated with low angle subduction beneath south america including landward shift in loci of tectonism surficial subsidence and laramide style deformation the aseismic ridge buoyancy hypothesis appears to be a valuable predictive tool for the interpretation of such geologic effects

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

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

pre orogenic volcanic assemblages and structure in the western cordillera of ecuador between l 40 s and 2 20 s a major reverse fault with a separation of at least a few thousand meters is exposed in the western cordillera of ecuador near 2 s ree and other incompatible elements patterns show i that the rocks exposed in the upthrust block to the east of the fault aremorb that had not yet been identified and ii that a volcanic island arc assemblage extensively outcrops in the downthrust block west of the fault

andean trending mobile belts in the brazilian shield much of the phanerozoic andean mountain chain is floored by precambrian basement1 3 in places as old as 2 000 myr4 5 but relations between these rocks and those of the brazilian shield have long been obscure mainly because of the intervening cenozoic sedimentary cover we describe here recent work over the bolivian sector of the brazilian shield fig 1 which shows a pattern of proterozoic mobile belts approximately parallel with the andean chain to the west when the trends of these bolivian belts are projected towards the andes under the cenozoic cover their ages broadly accord with those of the andean basement our interpretation is that the two domains are linked in the form of a chain of five andean trending mobile belts with a combined width of 1 000 km becoming progressively younger to the west with ages ranging from 1 300 myr to the present

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

quaternary sanukitoid like andesites generated by intracrustal processes chacana caldera complex ecuador implications for archean sanukitoids high mg diorites enriched in incompatible elements and their extrusive equivalents are rare subduction related rock types that have been found in modern arc settings and in late archean sequences where they are associated with trondhjemite tonalite granodiorite ttg suites archean rocks with these geochemical features are known as sanukitoids and despite their limited abundance are considered to be the indicators of the onset of modern plate tectonics because of their similarities to modern subduction related high mg andesites and diorites understanding the genesis of sanukitoid rocks is thus an essential step towards understanding crustal growth processes the accepted petrogenetic models for modern enriched high mg andesites and their archean equivalents the sanukitoids consist of metasomatic enrichment of the mantle wedge by slab components and its subsequent partial melting or the modification of siliceous slab components through continuous reaction with mantle peridotite during their ascent through the mantle wedge we present new data on the petrography mineral chemistry and whole rock geochemistry major and trace elements and sr nd pb isotopes of andesitic rocks from an 30 ka lava flow yuyos flow from the chacana caldera complex eastern cordillera of ecuador these rocks show a remarkable geochemical affinity with archean sanukitoids including high magnesium numbers 0 58 0 63 accompanied by high contents of incompatible elements e g th 17 23 ppm u 6 7 5 ppm ba 1600 1800 ppm sr 1430 1565 ppm la 74 94 ppm additionally the sanukitoid like andesites of yuyos are associated with predominant silica rich adakite like andesites which are widespread throughout the quaternary arc of ecuador this makes the quaternary ecuadorian magmatic province a close equivalent of the archean ttg sanukitoid association the bulk rock geochemistry petrography and mineral chemistry data indicate that the sanukitoid like features of the andesites of the yuyos flow derive from intracrustal recycling of the felsic intermediate to mafic ultramafic roots of the quaternary volcanic arc of ecuador by normal mantle derived basaltic magmas with the geochemical characteristics of continental arc basalts or high alumina basalts in view of the similarities between the yuyos andesites and archean sanukitoids in terms of geochemistry and lithological association we suggest that genetic models should consider the possibility of intracrustal recycling as a process responsible for the peculiar signatures of both archean sanukitoids and modern enriched high mg andesites

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

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

evolution of the ecuador offshore nonaccretionary type forearc basin and margin segmentation abstract forearc basins have developed along accretionary and nonaccretionary convergent margins here we investigate the evolution of an offshore nonaccretionary type forearc basin that has developed along the ecuadorian convergent margin we used 2d multichannel seismic reflection profiles to recognize five seismic units overlying the acoustic basement the units were dated by correlation with wells and regional stratigraphy they recorded the structural evolution of the margin into north central north and central south margin segments identified from gravity free air anomalies intersegment transition zones accommodated deformation related to the margin segmentation contributing to the development of shelf forearc depocenters an offshore forearc basin subunit l1a developed on accreted oceanic terranes during the middle to late eocene the basin was affected by normal faulting during the oligocene to early middle miocene subunit l1b this extension stage initiated during the rifting and oblique subduction of the farallon plate beneath the ecuador margin the basin underwent an early middle miocene uplift outlined by unconformity u1 potentially associated with the subduction of the young nazca plate in a less oblique plate convergence environment during the early pliocene the central south margin segment individualized by uplift during the formation of unconformity u2 the manta anticline which developed in the intersegment transition zone accommodated the segmentation this event is associated with the arrival of the carnegie ridge in the subduction the north segment individualized by uplift during the pliocene early pleistocene formation of the pedernales structural high which may coincide with a reactivation of the canande fault synchronously unit l3 sediment recorded the subsidence of the central north segment and strike slip tectonics in the area of the jama bahia depocenter a mosaic of middle to late pleistocene unit l5 depocenters that alternate with uplifted zones along the shelf likely recorded the subduction of the topographically irregular carnegie ridge the study provides a remarkable example of the evolution of a non accretionary type forearc basin

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

contemporaneous paleogene arc magmatism within continental and accreted oceanic arc complexes in the northwestern andes and panama abstract the basement rocks of the northwestern andes are composed of accreted fault bound slivers of the cretaceous caribbean large igneous province clip previous geologic mapping geochronological and geochemical data show that paleogene arc like units make up part of the accreted clip in sw colombia timbiqui complex nw colombia santa cecilila la equis complex and at the boundary between colombia and panama acandi region similar paleogene igneous units have been reported in ecuador silante and macuchi arc and panama chagres bayano arc these arc like rocks are commonly interpreted to have formed as part of an intra oceanic arc before accretion to the continental margin alternative explanations suggest that the paleogene igneous rocks in ecuador were formed within a continental magmatic arc we compare paleogene igneous rocks in the san blas complex chagres bayano arc and acandi region santa cecilia la equis complex and the sw complex timbiqui complex silante and macuchi arcs using geochemical major and trace elements geochronological zircon u pb and isotopic zircon hf and whole rock pb sr nd isotopes data zircon u pb analyses yield ages of 46 42 ma for the acandi region 59 51 ma for the santa cecilia la equis complex and 44 42 ma for the timbiqui complex major and trace element data show low k to calc alkaline compositions with negative nb anomaly and lree hree enrichment typically attributed to arc like signatures for the three complexes pb sr and nd whole rock and zircon lu hf isotopic data for the santa cecilia la equis and the san blas complexes are consistent with a depleted mantle source mixed with pacific sediments however the presence of inherited proterozoic zircons elevated 207pb 204pb ratios in some evolved igneous rocks and crystallization under low pressure conditions suggest assimilation of continental derived sediments these data indicate that the santa cecilia la equis and the san blas complexes likely originated in an intra oceanic island arc setting in a close proximity to the south american margin before accretion to the andean margin in the case of the sw complex the absence of eu anomaly slightly depleted hree as well as increased sr y values with fractionation indicate that these magmas formed in a moderately mature arc within a mid shallow crust level moreover radiogenic pb and sr isotopic values for the less evolved igneous rocks of the sw complex evolving to less radiogenic values suggest involvement of continental sediments in the parental magma source and assimilation of the mafic basement crust this indicates that the paleogene magmatic rocks of the sw complex originated in a continental arc like setting within previously accreted slivers of the clip

provenance of the upper cretaceous to upper eocene clastic sediments of the western cordillera of ecuador geodynamic implications abstract the late cretaceous eocene clastic deposits of the western cordillera of ecuador record significant changes in the source areas grain size and location of the depocenters related to the accretion of oceanic terranes that constitute the present day western cordillera and coast major changes in the source areas occurred in the late maastrichtian and late middle eocene they are interpreted as corresponding to the accretion of the guaranda and macuchi oceanic terranes respectively major increases in the grain sizes occurred in the late maastrichtian late paleocene and late middle eocene and seem to coincide with the accretion of the guaranda pinon and macuchi terranes respectively the increasing occurrence of plutonic or metamorphic fragments and the westward shift of the depositional areas through the paleocene upper eocene interval indicate an increasing uplift and erosion of the cordillera real continuous although jerky uplift of the latter during the maastrichtian eocene period supports the idea that the accreted oceanic material contributed to the crustal thickening and relief creation of the ecuadorian andes

geochronology and stable isotope signature of alteration related to hydrothermal magnetite ores in central anatolia turkey hydrothermal iron ores at divrigi east central anatolia are contained in two orebodies the magnetite rich a kafa and the limonitic b kafa resources of 133 8 mt with 56 fe and 0 5 cu the magnetite ores are hosted in serpentinites of the divrigi ophiolite at the contact with plutons of the murmano complex hydrothermal biotite from the divrigi a kafa yield identical weighted mean plateau ages of 73 75 0 62 and 74 34 0 83 ma 2 this biotite represents a late alteration phase and its age is a minimum age for the magnetite ore similar magnetite ores occur at hasancelebi and karakuz south of divrigi there the iron ores are hosted in volcanic or subvolcanic rocks respectively and are associated with a voluminous scapolite amphibole biotite alteration at hasancelebi biotite is intergrown with parts of the magnetite and both minerals formed coevally the weighted mean plateau ages of hydrothermal biotite of 73 43 0 41 and 74 92 0 39 ma 2 therefore represent mineralization ages hydrothermal biotite from a vein cutting the scapolitized host rocks south of the hasancelebi prospect has a weighted mean plateau age of 73 12 0 75 ma 2 this age together with the two biotite ages from the hasancelebi ores constrains the minimum age of the volcanic host rocks syenitic porphyry dikes therein and the scapolite alteration affecting both rock types pyrite and calcite also represent late hydrothermal stages in all of these magnetite deposits the sulfur isotope composition of pyrite between 11 5 and 17 4 34s vcdt points towards a non magmatic sulfur source of probably evaporitic origin calcite from the divrigi deposit has 18o vsmov values between 15 1 and 26 5 and 13c vpdb values between 2 5 and 2 0 which are compatible with an involvement of modified marine evaporitic fluids during the late hydrothermal stages assuming calcite formation temperatures of about 300 c the presence of evaporite derived brines also during the early stages is corroborated by the pre magnetite scapolite alteration at divrigi and hasancelebi karakuz and with paleogeographic and paleoclimatic reconstructions the data are compatible with a previously proposed genetic model for the divrigi deposit in which hydrothermal fluids leach and redistribute iron from ophiolitic rocks concomitant with the cooling of the nearby plutons

structure of the ecuadorian forearc from the joint inversion of receiver functions and ambient noise surface waves the ecuadorian forearc is a complex region of accreted terranes with a history of largemegathrust earthquakes most recently amw7 8 megathrust earthquake ruptured the plateboundary offshore of pedernales ecuador on 16 april 2016 following this event an inter national collaboration arranged by the instituto geofisico at the escuela polit ecnica nacionalmobilized a rapid deployment of 65 seismic instruments along the ecuadorian forearc wecombine this new seismic data set with 14 permanent stations from the ecuadorian nationalnetwork to better understand how variations in crustal structure relate to regional seismichazards along the margin here we present receiver function adaptive common conversionpoint stacks and a shear velocity model derived from the joint inversion of receiver functionsand surface wave dispersion data obtained through ambient noise cross correlations for theupper 50 km of the forearc beneath the forearc crust we observe an eastward dipping slowvelocity anomaly we interpret as subducting oceanic crust which shallows near the projectedcentre of the subducting carnegie ridge we also observe a strong shallow positive conversionin the ecuadorian forearc near the borbon basin indicating a major discontinuity at a depth of 7 km this conversion is not ubiquitous and may be the top of the accreted terranes we alsoobserve significant north south changes in shear wave velocity the velocity changes indicatevariations in the accreted terranes and may indicate an increased amount of hydration beneaththe manab i basin this change in structure also correlates geographically with the southernrupture limit of multiple high magnitude megathrust earthquakes the earthquake record alongthe ecuadorian trench shows that no event with amw 7 4 has ruptured south of 0 5 sinsouthern ecuador or northern peru our observations along with previous studies suggestthat variations in the forearc crustal structure and subducting oceanic crust may influance theoccurrence and spatial distribution of high magnitude seismicity in the region

modification of an oceanic plateau aruba dutch caribbean implications for the generation of continental crust the generation of the continental crust may be connected to mantle plume activity however the nature of this link and the processes involved are not well constrained an obstacle to understanding relationships between plume related mafic material and associated silicic rocks is that later tectonic movements are liable to obscure the original relationships particularly in ancient greenstone belts studies of younger analogous regions may help to clarify these relationships on the island of aruba in the southern caribbean a sequence of partly deformed mafic volcanic rocks intruded by a predominantly tonalitic batholith is exposed the mafic lavas show geochemical and isotopic affinities with other basaltic picritic and komatiitic rocks that crop out elsewhere in the caribbean these are well documented as belonging to an 88 91 ma plume related oceanic plateau which is allochthonous with respect to the americas and is thought to have been formed in the pacific region the 85 to 82 ma tonalitic rocks share some geochemical characteristics high sr and ba low nb and y with archaean tonalite trondhjemite granodiorite ttg suites field relationships suggest that deformation of the plateau sequence possibly related to collision with a subduction zone was synchronous with intrusion of the aruba batholith new incremental heating 40ar 39ar dates combined with existing palaeontological evidence show that cooling of the batholith occurred shortly after eruption of the plateau basalt sequence sr nd isotopic data for both rock suites are uniform 87sr 86sri 0 7035 endi 7 whereas pb isotopes are more variable plateau sequence 206pb 204pb 18 6 19 1 207pb 204pb 15 54 15 60 208pb 204pb 38 3 38 75 aruba batholith 206pb 204pb 18 4 18 9 207pb 204pb 15 51 15 56 208pb 204pb 38 0 38 5 this suggests that there has been a minor sedimentary input into the source region of the batholith however the limited time interval between basaltic and tonalitic magmatism makes a normal subduction related origin for the tonalites improbable instead models involving derivation of tonalite from partial melting of the plateau sequence or alternatively genesis in an unusual subduction zone environment are investigated

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

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

implications of 187os isotopic heterogeneities in a mantle plume evidence from gorgona island and curacao recent work has suggested that the mafic ultramafic volcanism in evidence throughout portions of the caribbean central america and northern south america including the islands of gorgona and curacao was generated as part of a middle cretaceous large igneous province new re os isochron results for tholeiitic basalts from gorgona and curacao indicate crystallization ages of 89 2 5 2 and 85 6 8 1 ma respectively consistent with reported ar ages the gorgona ultramafic suite shows a large range in initial os isotopic composition with os values ranging from 0 5 to 12 4 this large range reflects isotopic heterogeneities in the mantle source similar to those observed for modern ocean island basalts in contrast to ocean island basalts however os isotopic compositions do not correlate with variations in nd sr or pb isotopic compositions which are within the range of depleted mid ocean ridge basalts the processes that produced these rocks evidently resulted in the decoupling of os isotopes from the nd sr and pb isotopic systems picrites from curacao have very uniform chondritic initial os isotopic compositions with initial os values ranging only from 0 4 to 1 4 basalts from curacao however define an isochron with a 187os enriched initial isotopic composition os 9 5 in contrast to the 187os enriched ultramafic rocks from gorgona the enrichment in these basalts could have resulted from lithospheric contamination if the gorgona and curacao rocks were derived from the same plume os results combined with sr nd and pb data indicate a heterogeneous plume with multiple compositionally and isotopically distinct domains the os isotopic results require derivation of os from a minimum of two distinct reservoirs one with a composition very similar to the chondritic average and one with long term enriched re os oceanic crustal recycling has been invoked to explain most of the 187os enrichments that have been observed in ocean island basalt sources and could potentially apply to the gorgona suite crustal recycling however requires large proportions of very ancient recycled basaltic crust in the sources of the 187os enriched ultramafic rocks to explain the magnitude of 187os enrichments observed for example addition of 20 oceanic crust to fertile mantle and nearly 3 billion years are necessary to generate a reservoir with the os isotopic composition of the most radiogenic komatiites if the recycled oceanic crust was added to basalt depleted mantle as may be indicated by nd values for the komatiites averaging about 10 even larger proportions of older crust are required large proportions of oceanic mafic crust in the sources of the 187os enriched komatiites although petrologically conceivable under certain melting conditions is unlikely here given the limited trace element and lithophile isotope system variations these results raise questions about the efficacy of using os isotopes to constrain the proportion of recycled oceanic crust in other plumes other possible mechanisms for generating 187os enriched mantle include invoking the existence of a 187os enriched lower mantle and minor outer core lower mantle interactions

correction to volcanic evolution in the galapagos the dissected shield of volcan ecuador by d geist w m white f albarede k harpp r reynolds j blichert toft and m kurz

rotated oceanic blocks in western ecuador a paleomagnetic study of over 250 cores from 21 sites sampled over a distance exceeding 250 km in the early cretaceous pinon formation in the costa of western ecuador indicates that the sampled lithologies essentially morb type basalts and dolerites carry a stable remanent magnetization whose direction significantly deviates from that of coeval formations of stable south america although both normal and reverse directions have been recorded the two polarities cannot easily be distinguished because of the low latitude of the ocean floor at the time of formation the most likely interpretation is that the entire area has undergone 70 clockwise rotation since its genesis these results thus yield new evidence for an allochtonous origin of this suspect terrane

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

the effect of intra crystal uranium zonation on apatite u pb thermochronology a combined id tims and la mc icp ms study abstract quantitative reconstruction of high temperature thermal histories 350 c has applications in metamorphic studies processes which exhume deep crust the evolution of active margins over hundreds of millions of years and the study of extra terrestrial rocks previous studies have demonstrated that apatite u pb data can be used to recover thermal history information at t 350 c from single crystal bulk dates we present u pb bulk grain id tims and intra grain la mc icp ms u pb data from apatites to evaluate the importance of parent isotope u distribution when extracting thermal history information from apatites that have experienced partial pb loss we find that parent u zonation can cause considerable scatter when single bulk grain dates are compared with their grain size and therefore u zonation must be taken into account when inverting u pb dates and grain sizes to seek thermal history solutions we show that accurate thermal history solutions can be obtained from apatites which have zoned concentrations when in situ variations in u concentration and u pb dates are modelled this observation validates the hypothesis that pb is lost by thermally driven volume diffusion in situ analytical techniques yield significantly more accurate thermal history solutions than bulk grain id tims analyses when investigating rocks which contain compositionally zoned apatites

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 ophiolitic bath dunrobin formation jamaica significance for cretaceous plate margin evolution in the north western caribbean two neighbouring areas of tholeiitic basalts dolerites isotropic gabbros and tonalites in eastern jamaica are recognized as the upper section 2 5 km thick of an ophiolite limestones associated with this bath dunrobin formation contain a fauna of upper campanian and lower maestrichtian age the basalts are overlain by a thick sequence of maestrichtian volcaniclastic rocks and are faulted against a blueschist terrain since its emplacement the ophiolite has been subjected to uplift and major left lateral strike slip tectonism beginning in the eocene recognition of this subduction zone complex and regional considerations of island arc polarity necessitates that ocean crust must have been consumed by a southward dipping subduction zone the overlying late cretaceous palaeogene arc stretched from guatemala through jamaica to oriente province cuba this arc separated the yucatan basin to the n where active spreading accompanied subduction from the colombian basin and the southern nicaraguan rise to the se where tholeiitic magmatism b penetrated older ocean crust the bath dunrobin formation probably represents ocean crust from the yucatan basin obducted during the maestrichtian the major ophiolite terrains of guatemala and oriente cuba are by inference also derived from a proto yucatan basin

volcanic stratigraphy and geochronology of the cretaceous lancones basin northwestern peru position and timing of giant vms deposits a 10 km thick sequence of basaltic to rhyolitic volcanic rocks forms the arc component of the cretaceous lancones basin in northwestern peru and underlies part of the huancabamba deflection the marine volcanic successions show markedly different compositional features and depositional facies consistent with a maturing arc within a shallowing marine basin the earliest volcanism accompanying rifting was dominated by basaltic pillow lava and breccia with lesser aphyric to feldspar quartz porphyritic felsic volcanic rocks these volcanic successions filled the lowest exposed portion of the basin and were accompanied by volcanogenic massive sulfide vms deposits which are inferred to have formed in a localized but relatively deep marine setting u pb zircon dating of felsic volcanic rocks associated with vms deposits at tambogrande indicates ages from 104 8 1 3 to 100 2 0 5 ma for the ore bearing volcanic sequence the timing of onset of rift related volcanism is not well constrained but is therefore of middle albian age or older subsequent latest albian to turonian volcanism is composed of successions of relatively more felsic rich volcaniclastic rocks and yields u pb zircon ages of 99 3 0 3 to 91 1 1 0 ma these later volcanic successions are intercalated and overlain by siliciclastic and carbonate sedimentary sequences prevalent in the western forearc section of the lancones basin finally the basin was intruded by late cretaceous to tertiary granitoids of the coastal batholith the genesis of the cretaceous lancones basin and other equivalent volcanic rift related marginal basins in western south america including the western peruvian trough is related tectonically to the break up of gondwana early volcanism and associated vms deposits formed in the lancones basin during the albian coincided with the initial rifting stage prior to active oceanic spreading between south america and africa during this time the relatively stationary western margin of continental south america was undergoing extension and rifting due to a westward and oceanward retreating arc resembling a mariana arc type setting the mochica orogeny marks the termination of rifting subsidence and related volcanism along the western margin of south america this orogenic event also broadly coincides with the onset of spreading of the south atlantic and westward drift of the south american continent subsequent volcanism in the lancones basin was more continental arclike under an andean type scenario

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

geochemical evolution of the quaternary chachimbiro volcanic complex frontal volcanic arc of ecuador abstract the chachimbiro volcanic complex cvc is a composite volcano located in the frontal part of the ecuadorian quaternary arc four periods of activity can be differentiated in chachimbiro 400 ka long volcanic history named ch1 405 7 20 0 298 6 32 9 ka ch2 121 75 23 2 36 08 2 8 ka ch3 36080 280 22730 120 ybp and ch4 5 760 30 ybp respectively the magmatic suite ranges in composition from andesites to rhyodacites and displays systematic increases of sio2 sr y la yb incompatible elements e g lree mree lile and depletion of hree with time these geochemical features coupled with radiogenic isotope data and textural observations e g several resorption and overgrowth cycles in plagioclase phenocrysts suggest an open system evolution for cvc including typical crustal magmatic processes such as recharge assimilation and fractional crystallisation to better constrain the magmatic crustal processes of the cvc we use a multi model approach including three open system models rfc recharge fractional crystallisation afc assimilation fractional crystallisation and ec rafc energy constrained recharge assimilation fractional crystallisation different simulations have been developed to best fit cvc s geochemistry including trace elements sr y ratios and sr isotopes which suggest predominant fractional crystallisation and variable assimilation as the main processes explaining the bulk of the geochemical data of cvc this is consistent with an overall thermal decline of the magmatic system through time the genetic model here presented involves the input of a magmatic recharge component matured in the low mid crust through dominant fractionation of amphibole garnet into intermediate crustal reservoir s where magma undergoes extensive fc

seismic stratigraphy and structure of the caribbean igneous province multichannel seismic reflection profiles reveal the tectonic and stratigraphic complexity of the caribbean province a smooth reflector b forms the top of widespread cretaceous volcanic flows drilled during dsdp leg 15 and odp leg 165 in addition we also show other seismic evidence of volcanism sills and dipping horizons the southern parts of the venezuela and colombia basins are underlain by rough basement that shows many characteristics of oceanic crust several thick volcanic plateaus separated by deep basins with thin crust comprise the caribbean volcanic province in the venezuela basin only normal oceanic crust is present but in other basins this crust is underlain by underplated volcanic material the crust of the venezuela basin is less than 5 km thick and was probably formed in the pacific during the jurassic refraction data indicate that a thin layer 2v is formed by original oceanic crust overlain by a thin volcanic layer an intra basement reflection sub b marks the top of original oceanic crust that is sandwiched between an upper volcanic layer and lower underplated material this underplated layer forms a very thick layer 3v beneath the beata and nicaragua volcanic plateaus the upper part of layer 3v is gabbroic and outcrops on the beata ridge the thickening occurs mainly in the high velocity lower part of layer 3v and is attributed to the presence of magnesian rocks picrites or ultramafic cumulates a highly reflective horizon r is located at the top of this layer we present a model where the galapagos plume is 2500 km wide but has narrow heads that formed the caribbean volcanic plateaus separated by deep basins the caribbean igneous province is compared to similar volcanic features in the indian ocean kerguelen plateau and in the western pacific ocean ontong java plateau nauru and pigafetta basins the thick plateaus of the caribbean province resemble the kerguelen and ontong java plateaus whereas the deep basins present some similarities with the nauru and pigafetta basins the caribbean igneous province is younger 90 75 ma than the indian and pacific plateaus 110 130 ma however several geological evidences from land and geophysical offshore observations indicate that the onset of volcanic activity in the caribbean province could be as old as 115 ma

a plate kinematic framework for models of caribbean evolution abstract we define the former relative positions and motions of the plates whose motions have controlled the geological evolution of the caribbean region newly determined poles of rotation defining the approximate spreading histories of the central north and the south atlantic oceans are given for the late jurassic early cretaceous anomaly sequence of the central north atlantic we have used previously published definitions of fracture zone traces and magnetic anomaly picks redetermining the pole positions and angular rotations for various isochrons on an evans and sutherland interactive graphics system for magnetic anomalies younger than the cretaceous quiet period in both oceans we 1 used seasat altimeter data to help define fracture zone traces and 2 identified and used marine magnetic anomalies to determine the positions of spreading isochrons along the flowlines indicated by the fracture zones by the finite difference method the relative paleopositions and the relative motion history between north and south america were computed this analysis defines the size and shape and the rate at which the size and shape changed of the interplate region between north and south america since the middle jurassic thus a plate kinematic framework is provided for the larger plates pertaining to the caribbean region in which can be derived more detailed scenarios for gulf of mexico and caribbean evolution north and south america diverged to approximately their present relative positions from late triassic to early campanian about 84 m y ago time this is the period during which the gulf of mexico and a proto caribbean seaway were formed since the campanian only minor relative motion has occurred from early campanian through to middle eocene times south america diverged only another 200 km and since the middle eocene minor n s convergence has occurred these very minor post early campanian motions have probably been accommodated by imperfect shear and compression along the atlantic fracture zones to the east of the lesser antilles and along the northern and southern borders of the caribbean plate accordingly it is suggested that from campanian time to the present the relative motions between the north and south american plates have had only minor effects on the structural development of the caribbean region primarily using the data of engebretson et al the convergence history of pacific plates with north america was calculated for two points near the western caribbean by completing finite difference solutions the convergence history of the pacific plates with the caribbean and south american plates can be approximated the direction and rate of convergence of the pacific plates with the americas may have controlled the style of subduction and possible microplate migration along the north american south american and western caribbean boundaries that define the eastern pacific plate margin

the early interaction between the caribbean plateau and the nw south american plate the determination of accurate and precise ages for the timing of collision between oceanic plateaus and continental crust requires an understanding of how the indenting and buttressing plates respond to the collision we present geochronological thermochronological geochemical and isotopic analyses of magmatic rocks from the ecuadorian andes which relate to the collision of the late cretaceous caribbean plateau and great arc sequence with nw south america the cessation of subduction magmatism during 65 64 ma beneath the eastern edge of caribbean plateau was synchronous with accelerated surface uplift and exhumation within the buttressing continental margin during 75 65 ma we interpret this as the collision of the leading edge of the caribbean plateau and arc sequence with the south american plate at 75 65 ma a u pb zircon shrimp age of 87 10 1 66 2 sigma ma yielded by an accreted fragment of the plateau precludes previous estimates of collision at 85 80 ma if the plateau erupted above the galapagos hotspot less