oceanic plateau formation a cause of mass extinction and black shale deposition around the cenomanian turonian boundary the cenomanian turonian boundary 90 4 ma represents a major period of worldwide environmental disturbance the physical manifestations of this are elevated atmospheric and oceanic temperatures a significant sea level transgression and a period of widespread anoxia leading to the formation of oceanic black shales and the extinction of 26 of all genera elevated 13 c values and enrichment of trace elements in cenomanian turonian boundary sediments combined with a reduction in 87 sr 86 sr also imply a severe environmental perturbation at this time oceanic crustal production rates reached their highest level of the last 100 million years this was principally caused by extensive melting of hot mantle plumes at the base of the oceanic lithosphere and the development of vast areas up to 1 10 6 km 2 of thickened oceanic crust in the pacific and indian oceans the anomalous volcanism associated with the formation of these oceanic plateaux may have been responsible for the environmental disturbances c 90 ma these eruptions would also have resulted in the emission of large quantities of co 2 into the atmosphere leading to global warming additionally the emission of so 2 h 2 s co 2 and halogens into the oceans would have made seawater more acidic resulting in the dissolution of carbonate and further release of co 2 this run away greenhouse effect was probably put into reverse by the decline of the anomalous volcanic activity and by increased co 2 driven productivity in oceanic surface waters leading to increased organic carbon burial black shale deposition anoxia and mass extinction in the ocean basins

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

plume overriding triggers shallow subduction and orogeny in the southern central andes abstract plate tectonic theory implies that mantle plumes may be eventually overridden by lithosphere during continental drift these events have particular tectonomagmatic consequences for active margins and hence related orogenic processes since the first documentation of plume overriding and the definition of the plume modified orogeny concept only few examples have been recognized in the geologic record in this study we analyze the neogene tectonic evolution of the southern central andes between 35 and 38 s and its potential relation to the subduction of the payenia plume as a recent analogue of this process through a series of tectonic reconstructions we show that progressive payenia plume overriding correlates with neogene arc front migrations linked to slab shallowing fold belt reactivation in the main cordillera and intraplate contraction in the san rafael block additionally nazca slab tear determined from tomographic analyses and subsequent diachronous steepening of the subducted plate may also be an aftermath of plume subduction as often described in the final stages of plume modified orogeny finally we propose a modern analogue for processes previously described dating back to the mesoproterozoic which provides further insights into these complex settings

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

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

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

plume mantle source heterogeneity through time insights from the duarte complex hispaniola northeastern caribbean 1 located in the cordillera central of hispaniola the duarte complex offers an opportunity to study an on land fragment of the caribbean colombian oceanic plateau geochemical sr nd isotope and 40ar 39ar radiometric age data combined with detailed mapping have shown that the duarte complex includes two lithostratigraphic units composed of four geochemical groups of metavolcanic rocks group ia low ti high mg basalts and group ib high ti picrites and primitive high mg basalts occur interlayered in the lowermost levels of the lower unit group ii light rare earth element lree enriched picrites ferropicrites and high mg basalts forms the main lava sequence of the lower unit and group iii lree enriched fe ti basalts is present exclusively in the upper unit nd isotope and incompatible trace element patterns are diverse in the duarte complex metavolcanics and are consistent with mantle sources related to a heterogeneous plume mantle melt modeling suggests that an early extensive melting 10 20 of shallow mantle resulted in the formation of relatively depleted group i and ii magmas whereas the late more enriched group iii magmas were the product of deeper low degree 96 ma hence an early cretaceous phase of the caribbean colombian oceanic plateau construction is recorded in hispaniola

ancient lithospheric source for quaternary lavas in hispaniola fragments of ancient continental lithosphere entrained in the shallow oceanic mantle have been found in a number of locations in the southern hemisphere isotopic analyses of material from quaternary volcanic centres in hispaniola indicate that the lavas are derived from an ancient lithospheric fragment with affinities to the supercontinent gondwana

gravity signatures of terrane accretion abstract in modern collisional environments accreted terranes are bracketed by forearc gravity lows a gravitational feature which results from the abandonment of the original trench and the initiation of a new trench seaward of the accreted terrane the size and shape of the gravity low depends on the type of accreted feature and the strength of the formerly subducting plate along the central american trench the accretion of gorgona island caused a seaward trench jump of 48 to 66 km the relict trench axes show up as gravity lows behind the trench with minimum values of 78 mgal n of gorgona and 49 mgal s of gorgona respectively these forearc gravity lows have little or no topographic expression the active trench immediately seaward of these forearc gravity lows has minimum gravity values of 59 mgal n of gorgona and 58 mgal s of gorgona respectively in the north the active trench has a less pronounced gravity low than the sediment covered forearc in the mariana arc two cretaceous seamounts have been accreted to the eocene arc the northern seamount is most likely a large block the southern seamount may be a thrust slice these more recent accretion events have produced modest forearc topographic and gravity lows in comparison with the topographic and gravity lows within the active trench however the minimum values of the mariana forearc gravity lows are modest only by comparison to the mariana trench 216 mgal their absolute values are more negative than at gorgona island 145 to 146 mgal we speculate that the forearc gravity lows and seaward trench jumps near gorgona island were produced by the accretion of a hotspot island from a strong plate the mariana gravity lows and seaward trench jumps or thrust slices were the result of breaking a relatively weak plate close to the seamount edifice these gravity lows resulting from accretion events should be preserved in older accreted terranes

clockwise block rotations in northern chile indications for a large scale domino mechanism during the middle late eocene the andean deformational pattern of the northern chilean precordillera between 26 30 and 27 00 s suggests significant influence of an orogen oblique northwest trending pre andean basement structure the left lateral reactivation of this structure and the associated deformation can be explained by clockwise vertical axis rotations of forearc wide crustal blocks during the middle late eocene ca 39 3 ma induced by the coupling stress due to oblique northeastward subduction of the oceanic farallon plate beneath the south american continent the compressive stress at the leading edges of the blocks may have been accommodated 200 km away from the peru chile trench in what is now the precordillera where the magmatic arc was then located the postulated large scale domino mechanism explains the paleomagnetically observed in situ block rotations as well as the orientation segmentation and shear sense of major structural elements within large parts of the present forearc in northern chile

70 m y history 139 69 ma for the caribbean large igneous province it is commonly accepted that large igneous provinces are formed through catastrophic volcanic events occurring over vast areas within a few million years at the initiation of hotspots mantle plumes new 40ar 39ar ages 111 139 ma and geochemical results from the nicoya peninsula costa rica extend the age range of volcanism in the caribbean large igneous province to 70 m y 69 139 ma our results are not consistent with the formation of this vast igneous province through a single plume head at the initiation of a mantle plume such as the galapagos instead we propose that multiple oceanic intraplate igneous structures such as plateaus and hotspot tracks were accumulated through the subduction process the igneous structures could be remnants of the earlier history of the galapagos hotspot making it one of the oldest active hotspots on earth alternatively they could have been derived from several spatially distinct mantle melting events that sampled similar source material e g oceanic lithosphere of similar age

detecting earliest shortening and deformation advance in thrust belt hinterlands example from the colombian andes low temperature thermochronometry and crosscutting relationships identified in newly released reflection seismic data reveal a previously unrecognized zone of early andean shortening in colombia apatite fission track data and thermal modeling help define a 60 50 ma onset of rapid exhumation along the present boundary between the magdalena valley hinterland basin and eastern cordillera thrust belt subsurface angular unconformities localized above fold thrust structures indicate paleogene deposition in a wedge top depozone containing doubly vergent reverse faults retrodeformation of a cross section based on interpreted seismic profiles and thermochronometric data indicates paleocene to early eocene shortening and exhumation occurred through simultaneous activation of east and west directed reverse faults across a broad orogenic front subsequent deformation focused along west directed inversion structures these relationships reveal that deformation operated in a disparate manner rather than following a systematic progression from hinterland to foreland the northern andes also exemplify the potential effects of hinterland sediment loading and fault strength on deformation advance in contractional orogens

how was the iapetus ocean infected with subduction because subduction in the iapetus ocean began only 35 m y after the end of rifting spontaneous foundering of mature passive margins is an unlikely subduction initiation mechanism subduction is more likely to have entered the iapetus from the boundary with the external paleo pacific similar to the incursion of the scotia caribbean and gibraltar arcs into the modern atlantic the subduction zone probably became sinuous entraining fragments of the gondwanan margin along its complex sinistral southern boundary where oblique collision caused monian penobscottian deformation following taconian grampian collision of part of the subduction system with laurentia remaining parts of the iapetus were progressively infected with subduction leading to silurian closure

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

deep slab dehydration and large scale upwelling flow in the upper mantle beneath the japan sea we present a three dimensional p and s wave tomography model beneath the central japan sea using arrival time data obtained by long term ocean bottom seismometer observations the tomographic model is determined down to over 300 km the resulting tomographic image has two features in the mantle wedge first a remarkable low velocity region is spread within the shallow part of the mantle wedge beneath the japan arc with a slight extension beneath the japan sea second an inclined low velocity anomaly exists in the deeper part beneath the japan sea and converges to the slab at 300 km depth moreover its anomaly amplitude is distinctly small compared to the shallow part of the low velocity region these low velocity anomalies are interpreted to represent fluids that were dehydrated from the pacific and philippine sea slabs and melt production affected by the fluid our observations suggest that the deep dehydration from the pacific slab is found at a depth of approximately 300 km and large scale upwelling flow mechanically induced by plate subduction exists beneath the japan sea our results indicate the importance of the deep heterogeneous mantle wedge structure beneath the japan sea with regards to better understanding the magmatism of the subduction dynamics in japan and the evolution of the opening of the japan sea

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

simple and efficient separation of magnetic minerals from speleothems and other carbonates abstract trace concentrations of iron oxide minerals in carbonate sediments can preserve fine details about earth processes from high resolution recordings of the earth s ancient magnetic field to microscopic remnants of extraterrestrial impacts this paper presents a novel flask extraction method which uses a neodymium magnet and an orbital shaker for simple and efficient separation of magnetic minerals from carbonate sediments a mineral assemblage of magnetic standards titanomagnetite magnetite goethite and hematite combined with other mineral standards kaolinite quartz and nanoscale tio2 was subjected to the extraction procedure and compared to a natural speleothem sample exposure of the magnetic standards to a mildly acidic acetate buffer ph 4 did not cause physical or chemical alteration the strongly magnetic minerals were reproducibly extracted with greater than 90 efficiency by mass from mixtures of the mineral standards xrd and low temperature magnetic characterization demonstrated phase purity of the extracts quantitative comparison with two commonly used literature methods showed that the flask extraction method was more reproducible and efficient the addition of surfactant na po3 6 did not significantly improve extraction efficiency sequential dissolution and flask extraction of a simulated speleothem containing magnetic particles resulted in consistent extraction efficiencies for samples containing large 1 m strongly magnetic grains but a reduction in efficiency was observed for smaller 1 m grain sizes no method successfully extracted the weakly magnetic goethite and hematite however unprecedented representative characterization of these minerals was possible through quantitative analysis of the remainder after the collection of the magnetic extract this approach may facilitate detailed characterization of a wide range of carbonates such as pelagic limestone dolostone unlithified carbonate ooze speleothems and freshwater and pedogenic carbonates such mineral extractions can lead to new insights into paleoenvironmental processes as well as an improved understanding of the recording of the earth s magnetic field

geochemical constraints on the origin of the late jurassic proto caribbean oceanic crust in hispaniola the nature of the oceanic crust produced through rifting and oceanic spreading between north and south america during the late jurassic is a key element for the caribbean plate tectonic model reconstruction located in the cordillera central of hispaniola the loma la monja volcano plutonic assemblage lma is composed of gabbros dolerites basalts and oceanic sediments as well as metamorphic equivalents which represent a dismembered fragment of this proto caribbean oceanic crust petrologic and geochemical data show that the lma have a relatively broad diversity in composition which represent the crystallization products of a typical low pressure tholeiitic fractionation of mid ocean ridge basalts morb type parental magmas ranging from n to e morb three geochemical groups have been distinguished in the volcanic sequence lree flat to slightly lree enriched basalts of groups ii and iii occur interlayered in the lower stratigraphic levels and lree depleted basalts of group i in the upper levels mantle melt modeling suggests that group iii magmas are consistent by mixing within a mantle melt column of low degree 15 melts of a shallow spinel source and groups ii and i magmas are explained with moderate to high 14 18 and very high 20 fractional melting degrees of a shallower spinel mantle source respectively thus upward in the volcanic sequence of the lma the magmas represent progressively more extensive melting of shallower sources in a plume influenced spreading ridge of the proto caribbean oceanic crust nb y versus zr y systematics combined with recent plate tectonic model reconstructions reveal that caribbean colombian oceanic plateau fragments in hispaniola formed through melting of heterogeneous mantle source regions related with distinct plumes during at least from aptian albian 96 ma to late campanian

ecology and paleoenvironmental application of testate amoebae in peatlands of the high elevation colombian paramo we investigated the ecology and paleoecology of testate amoebae in peatlands of the colombian paramo to assess the use of testate amoebae as paleoenvironmental indicators objectives were to 1 identify environmental controls on testate amoebae 2 develop transfer functions for paleoenvironmental inference and 3 examine testate amoebae in a holocene peat core and compare our findings with other proxy records results from 96 modern samples indicate that testate amoebae are sensitive to ph and surface moisture and cross validation of transfer functions indicates potential for paleoenvironmental applications testate amoebae from the triunfo peatland in the central cordillera provided a proxy record of ph and water table depth for the late holocene and inferred changes were correlated with peat c n measurements during most of the record comparison with a lake level reconstruction suggests that at least the major testate amoeba inferred changes were driven by climate our work indicates that testate amoebae are useful paleoenvironmental indicators in high elevation tropical peatlands

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

the 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

deposition and age of chicxulub impact spherules on gorgonilla island colombia the end cretaceous mass extinction 66 ma has long been associated with the chicxulub impact on the yucatan peninsula however consensus on the age of this impact has remained controversial because of differing interpretations on the stratigraphic position of chicxulub impact spherules relative to the mass extinction horizon one side argues that the impact occurred precisely at the cretaceous paleogene boundary thus coinciding with the mass extinction the other side argues that the impact predated the cretaceous paleogene boundary based on the discovery of primary impact spherules deposits in ne mexico and texas near the base of planktic foraminiferal zone cf1 dated at 170 k y before the cretaceous paleogene boundary a recent study of the most pristine chicxulub impact spherules discovered on gorgonilla island colombia suggested that they represent a primary impact deposit with an absolute age indistinguishable from the cretaceous paleogene boundary here we report on the gorgonilla section with the main objective of evaluating the nature of deposition and age of the spherule rich layer relative to the cretaceous paleogene boundary the gorgonilla section consists of light gray yellow calcareous siliceous mudstones pelagic deposits alternating with dark olive brown litharenites turbidites a 3 cm thick dark olive green spherule rich layer overlies an erosional surface separating maastrichtian and danian sediments this layer consists of a clast supported normally graded litharenite with abundant chicxulub impact glass spherules lithics mostly volcanic and maastrichtian as well as danian microfossils which transitions to a calcareous mudstone as particle size decreases mineralogical analysis shows that this layer is dominated by phyllosilicates similar to the litharenites turbidites that characterize the section based on these results the spherule rich layer is interpreted as a reworked early danian deposit associated with turbiditic currents a major hiatus 250 k y spanning the cretaceous paleogene boundary and the earliest danian is recorded at the base of the spherule rich layer based on planktic foraminiferal and radiolarian biostratigraphy and carbon stable isotopes erosion across the cretaceous paleogene boundary has been recorded worldwide and is generally attributed to rapid climate changes enhanced bottom water circulation during global cooling sea level fluctuations and or intensified tectonic activity chicxulub impact spherules are commonly reworked and redeposited into younger sediments overlying a cretaceous paleogene boundary hiatus of variable extent in the caribbean central america and north atlantic while primary deposits are rare and only known from ne mexico and texas because of their reworked nature gorgonilla spherules provide no stratigraphic evidence from which the timing of the impact can be inferred

geological tectonic framework of solomon islands sw pacific crustal accretion and growth within an intra oceanic setting the solomon islands are a complex collage of crustal units or terrains herein termed the solomon block which have formed and accreted within an intra oceanic environment since cretaceous times predominantly cretaceous basaltic basement sequences are divided into 1 a plume related ontong java plateau terrain ojpt which includes malaita ulawa and northern santa isabel 2 a normal ocean ridge related south solomon morb terrain ssmt which includes choiseul and guadalcanal and 3 a hybrid makira terrain which has both morb and plume plateau affinities the ojpt formed as an integral part of the massive ontong java plateau ojp at c 122 ma and 90 ma respectively was subsequently affected by eocene oligocene alkaline and alnoitic magmatism and was unaffected by subsequent arc development the ssmt initially formed within a normal ocean ridge environment which produced a morb like basaltic basement through which two stages of arc crustal growth subsequently developed from the eocene onwards the makira terrain records the intermingling of basalts with plume plateau and morb affinities from c 90 ma to c 30 ma and a contribution from late miocene present day arc growth two distinct stages of arc growth occurred within the solomon block from the eocene to the early miocene stage 1 and from the late miocene to the present day stage 2 stage 1 arc growth created the basement of the central part of the solomon block the central solomon terrain cst which includes the shortland florida and south isabel islands stage 2 arc growth led to crustal growth in the west and south the new georgia terrain or ngt which includes savo and the new georgia and russell islands both stages of arc growth also added new material to pre existing crustal units within other terrains the solomon block terrane collage records the collision between the alaska sized ojp and the solomon arc initial contact possibly first occurred some 25 20 ma but it is only since around 4 ma that the ojp has more forcefully collided with the solomon arc and has been actively accreting since that time continuing to the present day we present a number of tectonic models in an attempt to understand the mechanism of plateau accretion one model depicts the ojp as splitting in two with the upper 4 10 km forming an imbricate stack verging to the northeast over which the solomon arc is overthrust whilst deeper portions of the ojp beneath a critical detachment surface are subducted the subduction of young 5 ma hot oceanic lithosphere belonging to the woodlark basin at the ssts has resulted in a sequence of tectonic phenomena including the production of unusual magma compositions e g na ti rich basalts and an abundance of picrites an anomalously small

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

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

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

adakite like volcanism of ecuador lower crust magmatic evolution and recycling in the northern andes of ecuador a broad quaternary volcanic arc with significant across arc geochemical changes sits upon continental crust consisting of accreted oceanic and continental terranes quaternary volcanic centers occur from west to east along the western cordillera frontal arc in the inter andean depression and along the eastern cordillera main arc and in the sub andean zone back arc the adakite like signatures of the frontal and main arc volcanoes have been interpreted either as the result of slab melting plus subsequent slab melt mantle interactions or of lower crustal melting fractional crystallization and assimilation processes in this paper we present petrographic geochemical and isotopic sr nd pb data on dominantly andesitic to dacitic volcanic rocks as well as crustal xenolith and cumulate samples from five volcanic centers pululagua pichincha ilalo chacana sumaco forming a nw se transect at about 0 latitude and encompassing the frontal pululagua pichincha main ilalo chacana and back arc sumaco chains all rocks display typical subduction related geochemical signatures such as nb and ta negative anomalies and lile enrichment they show a relative depletion of fluid mobile elements and a general increase in incompatible elements from the front to the back arc suggesting derivation from progressively lower degrees of partial melting of the mantle wedge induced by decreasing amounts of fluids released from the slab we observe widespread petrographic evidence of interaction of primary melts with mafic xenoliths as well as with clinopyroxene and or amphibole bearing cumulates and of magma mixing at all frontal and main arc volcanic centers within each volcanic center rocks display correlations between evolution indices and radiogenic isotopes although absolute variations of radiogenic isotopes are small and their values are overall rather primitive e g end 1 5 to 6 87sr 86sr 0 7040 0 70435 rare earth element patterns are characterized by variably fractionated light to heavy ree la ybn 5 7 34 and by the absence of eu negative anomalies suggesting evolution of these rocks with limited plagioclase fractionation we interpret the petrographic geochemical and isotopic data as indicating open system evolution at all volcanic centers characterized by fractional crystallization and magma mixing processes at different lower to mid crustal levels as well as by assimilation of mafic lower crust and or its partial melts thus we propose that the adakite like signatures of ecuadorian rocks e g high sr y and la yb values are primarily the result of lower to mid crustal processing of mantle derived melts rather than of slab melts and slab melt mantle interactions the isotopic signatures of the least evolved adakite like rocks of the active and recent volcanoes are the same as those of tertiary normal calc alkaline magmatic rocks of ecuador suggesting that the source of the magma did not change through time what changed was the depth of magmatic evolution probably as a consequence of increased compression induced by the stronger coupling between the subducting and overriding plates associated with subduction of the aseismic carnegie ridge

from oceanic plateaus to allochthonous terranes numerical modelling abstract large segments of the continental crust are known to have formed through the amalgamation of oceanic plateaus and continental fragments however mechanisms responsible for terrane accretion remain poorly understood we have therefore analysed the interactions of oceanic plateaus with the leading edge of the continental margin using a thermomechanical petrological model of an oceanic continental subduction zone with spontaneously moving plates this model includes partial melting of crustal and mantle lithologies and accounts for complex rheological behaviour including viscous creep and plastic yielding our results indicate that oceanic plateaus may either be lost by subduction or accreted onto continental margins complete subduction of oceanic plateaus is common in models with old 40 ma oceanic lithosphere whereas models with younger lithosphere often result in terrane accretion three distinct modes of terrane accretion were identified depending on the rheological structure of the lower crust and oceanic cooling age frontal plateau accretion basal plateau accretion and underplating plateaus complete plateau subduction is associated with a sharp uplift of the forearc region and the formation of a basin further landward followed by topographic relaxation all crustal material is lost by subduction and crustal growth is solely attributed to partial melting of the mantle frontal plateau accretion leads to crustal thickening and the formation of thrust and fold belts since oceanic plateaus are docked onto the continental margin strong deformation leads to slab break off which eventually terminates subduction shortly after the collisional stage has been reached crustal parts that have been sheared off during detachment melt at depth and modify the composition of the overlying continental crust basal plateau accretion scrapes oceanic plateaus off the downgoing slab enabling the outward migration of the subduction zone new incoming oceanic crust underthrusts the fractured terrane and forms a new subduction zone behind the accreted terrane subsequently hot asthenosphere rises into the newly formed subduction zone and allows for extensive partial melting of crustal rocks located at the slab interface and only minor parts of the former oceanic plateau remain unmodified oceanic plateaus may also underplate the continental crust after being subducted to mantle depth u hp terranes are formed with peak metamorphic temperatures of 400 700 c prior to slab break off and subsequent exhumation rapid and coherent exhumation through the mantle along the former subduction zone at rates comparable to plate tectonic velocities is followed by somewhat slower rates at crustal levels accompanied by crustal flow structural reworking and syndeformational partial melting exhumation of these large crustal volumes leads to a sharp surface uplift

highly depleted oceanic lithosphere in the rheic ocean implications for paleozoic plate reconstructions the rheic ocean formed at ca 500 ma when several peri gondwanan terranes e g avalonia and carolinia drifted from the northern margin of gondwana and were consumed during the late carboniferous collision between laurussia and gondwana a key event in the formation of pangea several mafic complexes ranging in age from ca 400 330 ma preserve many of the lithotectonic and or chemical characteristics of ophiolites they are characterized by anomalously high end values that are typically either between or above the widely accepted model depleted mantle curves these data indicate derivation from a highly depleted hd mantle and imply that i the mantle source of these complexes displays time integrated depletion in nd relative to sm and ii depletion is the result of an earlier melting event in the mantle from which basalt was extracted the extent of mantle depletion indicates that this melting event occurred in the neoproterozoic possibly up to 500 million years before the rheic ocean formed if so the mantle lithosphere that gave rise to the rheic ocean mafic complexes must have been captured from an adjacent older oceanic tract the transfer of this captured lithosphere to the upper plate enabled it to become preferentially preserved possible mesozoic cenozoic analogues include the capture of the caribbean plate or the scotia plate from the pacific to the atlantic oceanic realm our model implies that virtually all of the oceanic lithosphere generated during the opening phase of the rheic ocean was consumed by subduction during laurentia gondwana convergence

the petrogenesis of gorgona komatiites picrites and basalts new field petrographic and geochemical constraints gorgona island colombia is remarkable not only because it contains the only phanerozoic komatiites but also because it has mafic to ultramafic lavas with a wide range of compositions from moderately enriched to extremely depleted relative to bulk earth the komatiite flows are in many respects similar to archaean komatiites they formed from mgo rich 18 liquids and have upper spinifex zones and lower cumulate zones the cumulate zones of archaean komatiites contain many solid grains in contrast more than 90 of the olivine in the gorgona cumulates is highly skeletal this combined with the fact that the gorgona cumulate zones are thinner than those in archaean komatiites suggests that the komatiite magma became strongly superheated en route to the surface the komatiites have trace element contents intermediate between those of the basalts and the ultramafic tuffs some basalts have isotope compositions indicative of long term enrichment in incompatible elements whereas other basalts and ultramafic volcanics have isotopic signatures that imply corresponding depletion it is apparent that the plume source region of the gorgona magmas was markedly heterogeneous with at least two source components contributing to the observed variation in composition this heterogeneity may have resulted from the incorporation of different components into the plume source or it may be the result of complex melting and melt extraction processes during the ascent of a heterogeneous plume despite earlier suggestions that there may have been a significant age gap between depleted komatiite and basalt flows and the enriched basal new 4oar 39ar dating of basalts and gabbros are more consistent with all being generated at 87 ma during formation of the caribbean colombian plateau possibly at the galapagos hotspot