Carnegie Ridge Research Articles

Estimation of the Thermal Structure Beneath the Volcanic Arc of the Northern Andes by Coda Wave Attenuation Tomography

In the Northern Andes, the magmatic arc rises from a broad area of active volcanism in the South, at the border between Ecuador and Colombia, to a linear north-directed trend of active and inactive volcanic cones. A ~240-km-long west-east-striking slab tear (Caldas Tear) located approx. 5.5°N creates an offset in the volcanic arc. This tear looks like a significant controller of volcanic activity: its quasi-WE structure separates inactive magmatic bodies of late Miocene age or older in the North from Quaternary magmatic activity in the South. Coda wave attenuation tomography applied on seismic waveforms recorded between 1993 and 2018 illuminates the volcanic arc, which appears as a segmented structure derived from the complex process of subduction of the Nazca and Caribbean plates under South America. The attenuation measurements are transformed into thermal measurement using standard rock physics relationships, supported by thermal estimations and geothermal gradient observations measured in wells. Active and inactive magmatic belts are associated with a range of relatively low temperatures (~640°C – ~810°C in the depth range of 25 km – 100 km), which may be a consequence of the fluid content in hydrous minerals. Along the volcanic arc, the isotherms become shallower from South to North and are interrupted by a cold structure; this structure may reflect a lateral change of the mantle viscosity that prevents the continuity of the volcanic arc. Our estimations show an irregular depth-geometry of the isotherms, probably associated with recent slip events that have perturbed the thermal state along the study area. The isotherms also deepen to the West, probably due to the subduction process of the Nazca Plate (~0 – 5°N). In some areas, the isotherms follow a trend similar to that of a thrust fault-related folding geometry, which suggests a recent process of regional perturbation. We hypothesize that the Panama Arc collision at the north and the effect that imprint the Carnegie Ridge against the continent at the South are responsible for these thermal effects. The northern anomaly suggests thickening of the lithosphere that prevents the development of the volcanic arc at the north of the Caldas Tear.

The Esmeraldas Canyon: A Helpful Marker of the Pliocene‐Pleistocene Tectonic Deformation of the North Ecuador‐Southwest Colombia Convergent Margin

AbstractDeciphering the migration pattern of the Esmeraldas submarine Canyon (EC) and its history of cut‐and‐fill allows constraining the Pliocene‐Pleistocene tectonic evolution of the Ecuador‐Colombia convergent margin. Swath bathymetry, multichannel seismic reflection, and chronological data show that the EC is a 143‐km‐long, shelf‐incising, river‐connected canyon that started incising slope apron deposits in the Manglares fore‐arc basin ~5.3 Ma ago. The EC inception appears contemporaneous with the subduction of the Carnegie Ridge that is believed to have initiated 5–6 Myr ago and is considered an indirect cause of the EC formation. During its two‐stage left‐lateral migration, the EC upper‐half scoured deep incisions providing evidences for uplift episodes in the Manglares Basin that are correlated with mid‐Pliocene and Pleistocene regional tectonic events. Glacioeustatic variations contributed significantly to shape the EC and its upslope tributaries by increasing the rate of canyon incision during rapid sea level falls. Faults, folds, and diapirs have structurally controlled the location of the EC and of its tributary canyons, including the Ancon Canyon, which served as the main spillway of the Manglares Basin prior to be cut from its source ~170 kyr ago by the growth of a fault‐related anticline. The margin wedge that hosts the EC is highly unstable as it is cut by active faults and shaken by large subduction earthquakes. Several mass transport deposits have dammed the EC, one of them between >~65 and ~37 kyr causing an impoverishment of detrital material in the trench sedimentation and a possible interruption of the paleoseismological record.

1D-velocity structure and seismotectonics of the Ecuadorian margin inferred from the 2016 Mw7.8 Pedernales aftershock sequence

On April 16th 2016 a Mw 7.8 earthquake ruptured the central coastal segment of the Ecuadorian subduction zone. Shortly after the earthquake, the Instituto Geofisico de la Escuela Politecnica Nacional of Ecuador, together with several international institutions deployed a dense, temporary seismic network to accurately categorize the post-seismic aftershock sequence. Instrumentation included short-period and broadband sensors, along with Ocean Bottom Seismometers. This deployment complemented the permanent Ecuadorian seismic network and recorded the developing aftershock sequence for a period of one year following the main-shock. A subset of 345 events with ML > 3.5, were manually picked in the period of May to August 2016, providing highly accurate P- and S-onset times. From this catalogue, a high-quality dataset of 227 events, with an azimuthal gap <200°, are simultaneously inverted for, obtaining the minimum 1D velocity model for the rupture region, along with hypocentral locations and station corrections. We observe an average Vp/Vs of 1.82 throughout the study region, with relatively higher Vp/Vs values of 1.95 and 2.18 observed for the shallowest layers down to 7.5 km. The high relative Vp/Vs ratio (1.93) of the deeper section, between 30 km and 40 km, is attributed to dehydration and serpentinization processes. For the relocated seismicity distribution, clusters of events align perpendicular to the trench, and crustal seismicity is also evidenced, along with earthquakes located close to the trench axis. We also compute Regional Moment Tensors to analyze the different sources of seismicity after the mainshock. Aside from thrust events related to the subduction process, normal and strike-slip mechanisms are detected. We suggest that the presence of subducting seamounts coming from the Carnegie Ridge act as erosional agents, helping to create a scenario which promotes locking and allows seismicity to extend up to the trench, along zones of weakness activated after large earthquakes.

Ridge subduction and afterslip control aftershock distribution of the 2016 Mw 7.8 Ecuador earthquake

We characterise the aftershock sequence following the 2016 Mw=7.8 Pedernales earthquake. More than 10,000 events were detected and located, with magnitudes up to 6.9. Most of the aftershock seismicity results from interplate thrust faulting, but we also observe a few normal and strike-slip mechanisms. Seismicity extends for more than 300 km along strike, and is constrained between the trench and the maximum depth of the coseismic rupture. The most striking feature is the presence of three seismicity bands, perpendicular to the trench, which are also observed during the interseismic period. Additionally, we observe a linear dependency between the temporal evolution of afterslip and aftershocks. We also find a temporal semi-logarithmic expansion of aftershock seismicity along strike and dip directions, further indicating that their occurrence is modulated by afterslip. Lastly, we observe that the spatial distribution of seismic and aseismic slip processes is correlated to the distribution of bathymetric anomalies associated with the northern flank of the Carnegie Ridge, suggesting that slip in the area could be influenced by the relief of the subducting seafloor. To explain our observations, we propose a conceptual model in which the Ecuadorian margin is subject to a bimodal slip mode, with distributed seismic and aseismic slip mechanically controlled by the subduction of a rough oceanic relief. Our study sheds new light on the mechanics of subduction, relevant for convergent margins with a complex and heterogeneous structure such as the Ecuadorian margin.

The 2016 Mw 7.8 Pedernales, Ecuador, Earthquake: Rapid Response Deployment

The April 2016 Pedernales earthquake ruptured a 100 km by 40 km segment of the subduction zone along the coast of Ecuador in an Mw 7.8 megathrust event east of the intersection of the Carnegie ridge with the trench. This portion of the subduction zone has ruptured on decadal time scales in similar size and larger earthquakes, and exhibits a range of slip behaviors, variations in segmentation, and degree of plate coupling along strike. Immediately after the earthquake, an international rapid response effort coordinated by the Instituto Geofisico at the Escuela Politecnica Nacional in Quito deployed 55 seismometers and 10 ocean‐bottom seismometers above the rupture zone and adjacent areas to record aftershocks. In this article, we describe the details of the U.S. portion of the rapid response and present an earthquake catalog from May 2016 to May 2017 produced using data recorded by these stations. Aftershocks focus in distinct clusters within and around the rupture area and match spatial patterns observed in long‐term seismicity. For the first two and a half months, aftershocks exhibit a relatively sharp cutoff to the north of the mainshock rupture. In early July, an earthquake swarm occurred ∼100 km to the northeast of the mainshock in the epicentral region of an Mw 7.8 earthquake in 1958. In December, an increase in seismicity occurred ∼70 km to the northeast of the mainshock in the epicentral region of the 1906 earthquake. Data from the Pedernales earthquake and aftershock sequence recorded by permanent seismic and geodetic networks in Ecuador and the dense aftershock deployment provide an opportunity to examine the persistence of asperities for large to great earthquakes over multiple seismic cycles, the role of asperities and slow slip in subduction‐zone megathrust rupture, and the relationship between locked and creeping parts of the subduction interface.

Interactions between volcanism and geodynamics in the southern termination of the Ecuadorian arc

This study focuses on the construction and evolution through time of volcanic edifices located in the southern part of the Ecuadorian arc, with the objective to contribute to a better understanding of the interactions between magmatism, slab geometry and the activity of tectonic faults. Our new groundmass K-Ar ages obtained for a dozen volcanoes from the southern Quaternary arc are rather young, without ages older than 800 ka, and highlight an increasing volcanic activity between 300 and 100 ka. These new temporal constraints suggest that a southward migration of the Ecuadorian arc occurred during the last 600 ka. We interpret this evolution as the result of major and recent geodynamic reorganization in Ecuador related to the activation of lithospheric faults and to the flexure of the slab at depth, following the inland prolongation of the Grijalva fracture zone. Both phenomena could have been induced by the oblique subduction of the Nazca plate towards the convex margin of the South American continent, and the coupling of these two plates along the Wadati-Benioff zone, progressively intensified by the Carnegie ridge subduction. Southward migration of the slab flexure at depth could have changed pressure and temperature conditions, favoring the partial melting of the mantle wedge and magma genesis, thereby inducing a southward migration of the Ecuadorian arc volcanoes. These results contribute therefore to a better knowledge of the current arc dynamics in the Northern Andes, and provide insights into the use of geochronological data applied to volcanic rocks for studies of past tectonic activity in Ecuador.

Emergence/Subsidence Histories Along the Carnegie and Cocos Ridges and Their Bearing Upon Biological Speciation in the Galápagos

AbstractUsing plate motion reconstructions for the Nazca, Cocos, and South American plates in relation to the Galápagos hotspot, we find that age‐depth dependence of bathymetry, Galápagos plume dynamic topography, crustal relaxation, and magmatic production permit reasonable estimates for the subsidence of former islands along the Carnegie and Cocos Ridges. Our dynamic topography estimates are partially based upon geodynamic theory (fluid mechanics and elasticity) and also upon detailed bathymetric observations and analysis. For three different target areas over Carnegie Ridge and two over Cocos Ridge, we infer subaerial landmasses over the past ~20 Ma. We provide rough estimates of the surface areas of these paleo‐archipelagos, which were comparable to that of the present‐day Galápagos archipelago. We construct a history of emerged (paleo‐island) topography along the conjugate pair Carnegie‐Cocos hotspot tracks and infer that significant subaerial landscapes existed for at least the past ~20 Ma. The genetic ancestors of endemic faunal species in Galápagos (e.g., marine iguanas) migrated away from South America, genetically diverging from their land counterparts much earlier than the age of the present Galápagos Islands (~3 Ma). At 16.5 Ma, the total emerged landmass on the Carnegie Ridge alone may have been ~2.5 times that of the current Galápagos archipelago. (The conjugate landmass along the Cocos Ridge has been subducted.) We propose that this large landmass, at least one‐fourth the size of Iceland, be called Darwinia in recognition of its importance in the evolution of some Galápagos endemic flora and fauna made famous by Darwin's explorations.

A honeycomb seafloor morphology in carbonate sediment of the Carnegie Ridge (offshore Ecuador): Formation and potential geodynamic significance

Based on swath bathymetry, two-dimensional, high-resolution seismic reflection profiles, and Ocean Drilling Program/Deep Sea Drilling Project (ODP/DSDP) data, we describe a seafloor honeycomb pattern and propose a model for its formation in Pliocene–Miocene carbonate deposited on the uneven oceanic basement of the Carnegie Ridge (offshore Ecuador). Hydrothermal fluids derived from the basement aquifer fractured and dissolved carbonate sediment, creating seafloor pits above basements highs. Fluids expelled along polygonal faults may have assisted the nucleation of seafloor depressions. At the Pliocene-Pleistocene boundary, strong bottom currents scoured previously damaged sediments, enlarging the initial depressions and producing the seafloor honeycomb pattern. This regional erosive episode was contemporaneous with the final closing of the Isthmus of Panama and the clogging of the Ecuador Trench by the subduction of the Carnegie Ridge, so that the honeycomb pattern may be viewed as a regional marker of these two geodynamic events.

Geothermal Heating in the Panama Basin: 1. Hydrography of the Basin

AbstractThe Panama Basin serves as a laboratory to investigate abyssal water upwelling. The basin has only a single abyssal water inflow pathway through the narrow Ecuador Trench. The estimated critical inflow through the Trench reaches 0.34 ± 0.07 m/s, resulting in an abyssal water volume inflow of 0.29 ± 0.07 Sv. The same trench carries the return flow of basin waters that starts just 200 m above the bottom and is approximately 400‐m deeper than the depth of the next possible deep water exchange pathway at the Carnegie Ridge Saddle. The curvature of temperature‐salinity diagrams is used to differentiate the effect of geothermal heating on the deep Panama Basin waters that was found to reach as high as 2,200‐m depth, which is about 500 m above the upper boundary of the abyssal water layer.

Seismicity Distribution Near a Subducting Seamount in the Central Ecuadorian Subduction Zone, Space‐Time Relation to a Slow‐Slip Event

AbstractA temporary onshore‐offshore seismic network deployed during the 2‐year period of the Observación SISmológica en ECuador project provides a detailed and well‐focused image of the seismicity for magnitudes as low as 2.1 at the Central Ecuadorian subduction zone. During this 2‐year experiment, the shallow and locked subduction patch shows little evidence of background seismicity that instead occurred downdip of the coupled patch at ~20‐km depth. In this region, seismicity is possibly controlled by the crustal faults bounding the sedimentary basin of Manabí and the rheology of the upper plate. The dip angle of the interplate contact zone, defined by a smooth interpolation through the hypocenters of thrust events, is consistent with a progressive increase from 6° to 25° from the trench to 20‐km depth. Offshore, a seismic swarm, concomitant with a slow‐slip event rupturing the highly coupled subduction megathrust, highlights the reactivation of secondary active faults within the thickened crust of the subducting Carnegie Ridge at the leading edge of a large oceanic seamount.

Crustal motion and deformation in Ecuador from cGNSS time series

In this paper, we present the first velocity field from cGNSS (continuous GNSS) stations in the Continuous Monitoring GNSS Network (REGME) in Ecuador. We have analyzed data from 33 cGNSS REGME stations for the 2008–2014 period in order to characterize horizontal crustal motion and deformation in Ecuador. Prior to this, we analyzed the time series for the 33 REGME stations in order to determine their seasonality and the type of spectral noise. For most stations, we found a predominance of uncorrelated white noise with annual and semi-annual variations as the predominant first and second periods. Velocity was estimated by introducing the trend, seasonality and noise in each series in the general model, thus allowing us to improve accuracy as well as magnitude. The velocity and strain distribution correspond to the transpressive right-lateral slip of the westward-dipping faults of the Major Dextral System and the NNE movement of the North Andean Block (NAB) relative to the South American plate. The distributions of our deformation rate and velocity field indicate a differentiated tectonic behavior between northern, central and southern Ecuador. In northern Ecuador, there is an estimated right-lateral motion of 7.6 ± 0.5 mm/yr, consistent with the NNE movement of the NAB relative to the South American plate. In central Ecuador, the right-lateral motion decreases to 5.3 ± 0.4 mm/yr. In the southern region of Ecuador (from the Guayaquil Gulf to Peru) there is no strain accumulation, GNSS velocities decrease and turn to south. This zone belongs to the so-called Inca or Peru sliver.These results are consistent with the distinct behavior of subduction in Ecuador, with no coupling in southern Ecuador, and increased coupling towards the north, in the zone where megathrust earthquakes have occurred over the last century. The southern part of the Carnegie Ridge marks the limit between the two zones.We suggest that the main driving force responsible for the ongoing crustal deformation in Ecuador is the convergence between the Nazca and South American plates with the variable coupling pattern and the collision of the Carnegie Ridge. This results in different velocity patterns for the northern and the southern parts of Ecuador.

Gravity and magnetic anomalies of ecuadorian margin: Implications in the deep structure of the subduction of Nazca Plate and Andes Cordillera

To improve our knowledge of the deep structure of the Ecuadorian continental margin we combined satellite gravity and magnetic anomalies with the gravity field data obtained in land surveys, integrated as well as with seismicity and geological data. Satellite Bouguer anomalies are elongated, arched and parallel to the continental margin. The lows are related to the Andes Cordillera, while relative westward parallel highs correspond to the Cordillera Costanera and the Talara-Tumbes Cordilleras. Smooth and irregular positive anomalies characterize Oriente Basin. The oceanic Nazca Plate shows negative relative to the sounding normal oceanic crust, ENE-WSW gravity anomalies highlighting the Carnegie Ridge, surrounded by highs corresponding to standard oceanic lithosphere. New gravity field profiles at 1°S and 4°S show Bouguer anomalies comprised between -273 and + 75 mGal, giving higher resolution than satellite data. Gravity crustal models suggest that the continental crust in the Andean Cordillera is 70 km thick, while 20 to 40 km thick in the Coastal region and Oriente Basin. Seismic data show the oceanic crust, 8 km thick, extends beneath the continent to a depth of 200 km, with the highest dip of the subduction slab modelled at Profile 1ºS, which is the area of the Andes Cordillera having maximum elevation. Main sedimentary basins and intrusive bodies are related to the local low Bouguer anomalies, whereas high values correspond to allochthonous oceanic units. Reverse faults are widespread and contribute to crustal thickening. Normal faults are located in the forearc basin. Satellite magnetic data reveal anomalies related to the subducting slab below the South American margin, reaching the Andes Cordillera. The Carnegie Ridge is characterized by a roughly ENE-WSW minimum, corresponding to highly magnetized rocks located along the 1ºS profile. The E-W anomaly pattern changes sharply at the eastern edge of the seismicity related to subduction; further eastward the NW-SE anomalies are associated with the South American craton in Oriente Basin.

Mitigation of Ecuadorian Earthquake Impact

The mechanism of “Bing-Bang-Channeling-Cut” was proposed to reduce the volcanic energy (“Bing”) from the Galapagos Hot Spot (GHS) for the mitigation of Ecuadorian earthquake impact (“Bang”). The lag time between the volcanic eruptions in the GHS and Ecuadorian earthquake was probably caused by the undersea seamounts of the Carnegie Ridge (CAR) (“Channeling”). Experimental results from the water reservoir showed that the lag time decreased when increasing the number of drilled holes (“Cut”) in the bottom of reservoir. The present study showed that there was an additional vector initiating the Ecuadorian earthquake from the volcanic eruption in the GHS through the CAR. It was concluded that the harmful effects of Ecuadorian earthquakes could be mitigated by releasing the volcanic energy through the enlarged exit diameters in Holes of Site 1239 (A, B, C) with the presently productive upwelling chlorophyll system at the northeastern CAR.

Shear wave anisotropy in northwestern South America and its link to the Caribbean and Nazca subduction geodynamics

To investigate the subduction dynamics in northwestern South America, we measured SKS and slab-related local S splitting at 38 seismic stations. Comparison between the delay times of both phases shows that most of the SKS splitting is due to entrained mantle flow beneath the subducting Nazca and Caribbean slabs. On the other hand, the fast polarizations of local S-waves are consistently aligned with regional faults, which implies the existence of a lithosphere-confined anisotropy in the overriding plate, and that the mantle wedge is not contributing significantly to the splitting. Also, we identified a clear change in SKS fast directions at the trace of the Caldas Tear (∼5°N), which represents a variation in the subduction style. To the north of ∼5°N, fast directions are consistently parallel to the flat subduction of the Caribbean plate-Panama arc beneath South America, while to the south fast polarizations are subparallel to the Nazca-South America subduction direction. A new change in the SKS splitting pattern is detected at ∼2.8°N, which is related to another variation in the subduction geometry marked by the presence of a lithosphere-scale tearing structure, named here as Malpelo Tear; in this region, NE-SW-oriented SKS fast directions are consistent with the general dip direction of the underthrusting of the Carnegie Ridge beneath South America. Further inland, this NE-SW-trending mantle flow continues beneath the Eastern Cordillera of Colombia and Merida Andes of Venezuela. Finally, our results suggest that the subslab mantle flow in northwestern South America is strongly controlled by the presence of lithospheric tearing structures.

Subduction of seafloor asperities revealed by a detailed stratigraphic analysis of the active margin shelf sediments of Central Ecuador

The uplift of the coastal cordillera of central Ecuador is a likely consequence of the subduction of the Carnegie Ridge, a 400-km-long and 2-km-high topographic asperity. This study aims at analyzing the impact of the subduction of the Carnegie Ridge on the continental shelf sedimentation during the Quaternary. We interpret high-resolution (50–450Hz frequency) multi-channel (72 channels) and single-channel (Sparker) seismic data, piston cores, sediment profiles (3.5kHz), and high-resolution multibeam bathymetry acquired during the ATACAMES cruise with the RV L’Atalante in 2012. In the La Plata Island–Cabo San Lorenzo Peninsula region, the results show a full and detailed record of the last ten Quaternary sedimentary sequences deposited in intra-shelf basins, at and -just seaward of the shelf break. These 100-ka-scale sequences are tied to piston core data, to well-dated Quaternary onshore exposures and to a flight of coastal marine terraces on Cabo San Lorenzo Peninsula and La Plata Island. These sequences correlate with the global ice volume and deep marine temperature changes (d18O) for the last 0.7Ma, which points to a strict climatic control on depositional sequence development and preservation. However, the subsidence of the continental shelf acoustic basement, estimated by the stepwise backstripping of the sedimentary record, exhibits a complex deformation pattern with uplifting and subsiding regions. Deep marine seismic data, currently under processing, show evidence for a subducted seamount beneath La Plata Island and GPS data indicate an important interplate coupling in the same area with the potential to generate a Mw 7–7.5 earthquake. The pattern of the continental shelf deformation is consistent with the shape of this seamount. We tested the hypothesis of a link between the margin deformation and the subduction of the seamount, by comparing a stepwise subduction of the seamount to the palinspastic restoration of the deformation of the continental shelf for the last 1Ma. This comparison shows that the collision probably started c.500ka ago, together with the syntectonic sedimentation, and drastically slowed down by c.50ka, with the sealing of most of the deformation on the shelf.

A new view for the geodynamics of Ecuador: Implication in seismogenic source definition and seismic hazard assessment

AbstractA new view of Ecuador's complex geodynamics has been developed in the course of modeling seismic source zones for probabilistic seismic hazard analysis. This study focuses on two aspects of the plates' interaction at a continental scale: (a) age‐related differences in rheology between Farallon and Nazca plates—marked by the Grijalva rifted margin and its inland projection—as they subduct underneath central Ecuador, and (b) the rapidly changing convergence obliquity resulting from the convex shape of the South American northwestern continental margin. Both conditions satisfactorily explain several characteristics of the observed seismicity and of the interseismic coupling. Intermediate‐depth seismicity reveals a severe flexure in the Farallon slab as it dips and contorts at depth, originating the El Puyo seismic cluster. The two slabs position and geometry below continental Ecuador also correlate with surface expressions observable in the local and regional geology and tectonics. The interseismic coupling is weak and shallow south of the Grijalva rifted margin and increases northward, with a heterogeneous pattern locally associated to the Carnegie ridge subduction. High convergence obliquity is responsible for the North Andean Block northeastward movement along localized fault systems. The Cosanga and Pallatanga fault segments of the North Andean Block‐South American boundary concentrate most of the seismic moment release in continental Ecuador. Other inner block faults located along the western border of the inter‐Andean Depression also show a high rate of moderate‐size earthquake production. Finally, a total of 19 seismic source zones were modeled in accordance with the proposed geodynamic and neotectonic scheme.

Evidence and models for lower crustal flow beneath the Galápagos platform

AbstractThe volcanic Galápagos Islands are constructed upon a broad platform, with their active westernmost islands marking the current position of the hotspot. Built upon young oceanic lithosphere (&lt;15 Ma), this platform exhibits unique morphologic features including a system of stepped terraces on the southwestern escarpment with 3 km relief, contrasting with gentle slopes off the eastern platform toward the Carnegie Ridge. Considering horizontal lithostatic pressure differences associated with this relief, along with high temperatures within the young, hotspot‐affected lithosphere, it is likely that lower crustal flow contributes significantly to crustal deformation within the Galápagos platform. Using a 2‐D, isostatic, thin‐sheet approximation for the Stokes flow equation with (Newtonian) space‐time‐dependent viscosity, we suggest that the bathymetric rim along the eastern platform region (where gravimetry indicates Airy isostasy) near Española Island may be the expression of a mature lower crustal flow front developed over the last ∼3 Myr; horizontal mass displacements (∼50 km) associated with this crustal flow episode may have advected mantle plume geochemical signatures toward the southeast, and in directions not necessarily parallel to the hotspot track. Also, the stepped terraces along the southwestern platform may be explained by lower crustal flow‐associated backward tilting of the bathymetric surface that, although resulting in small angular changes (∼0.1°), effectively hinders the horizontal flow of lava sheets. This backward‐tilting process was likely restricted to the last ∼1 Ma or less, and may be a unique event involving extrusion of lavas from within the southwestward‐marching lower‐crustal flow front.

RECENT PLANKTONIC FORAMINIFERA FROM DEEP-SEA SEDIMENTS FROM THE EASTERN EQUATORIAL PACIFIC: PROXIES OF THE EQUATORIAL FRONT IN THE LATE QUATERNARY

Planktonic foraminifera recovered from 25 deep-sea sediment samples (core-lops) Irom the eastern Equatorial Pacific were analyzed tor thoir geographic distribution and possible environmental controls. Samples colloctod doopor than tho carbonato lysoclino (-2800 m) show significant signs of dissolution, - when compared to sediment-trap samples resulting in the increase of the solutionresistant species Neogloboquadrlna duterlrei. NeogOboquadnna pachyderma and Gioborotaha Cufrata and the reduction of tho soíuüon-suscoptiblo spocios Globigonmta glutmata. Globigennotdes ruber and Gtobigohnoidos saccuMor. Threo bioprovincos woro rocognizod by cluster analysis: (1) bioprovince I that occurs on tho Cocos Ridgo whore G. cultrata and N. pachyderma are dominant. (2) bioprovince II that occurs on the Carnegie Ridge where N. duterlrei, N. pachyderma and Globorolatia intlata aro dominant, and (3) bioprovince III that occurs in the Panama Basin whore G. saccuhtor and G. ruber aro dominant. Bioprovinces I and II reflect a shallow thormocllno inducod by upwelhng. although AOU, NO , and PO, and SiO, are significantly higher in tho lattor rogion. B-oprovinco III reflects a deeprrvxod layor and low nutriont contonts. Possiblo proxies ol tho Equatorial Front in the past are: (1) the Shannon diversity indox. ovonness and the number ot species that show a latitudinal break at -1.5*S and. (2J the G. cultrata / G. dutertrei ratio that decreases southward

FORAMINÍFEROS BENTÓNICOS RECIENTES EN LAS AGUAS PROFUNDAS DE LA CUENCA DE PANAMÁ: ECOLOGÍA Y SU POSIBLE RELACIÓN CON LAS CORRIENTES DE FONDO

The relationship between recent benthic foraminifera and bottom currents in the Panama basin (Colombian Pacific) is examined, and the main ecological variables which control the distribution of benthic foraminifera are discussed. The benthic foraminiferal study of 24 core top samples and the integration with previous reports, support the view that in upwelling areas or under high terrigenous influx, the assemblages are dominated by infaunal forms such as Uvigerina, Bolivina, Globobulimina, and Chilostomella, beside common forms such as Uvigerina peregrina Cushman and Epistominella spp., which are indicative of high productivity. In contrast, epifaunal forms such as Cibicidoides, Laticarinina, and Hoeglundina are more common on the flanks of the Cocos and Carnegie Ridges indicating a lower surface productivity and a larger content of dissolved oxygen on the sea floor. Infaunal foraminifera are dominated by Siphouvigerina proboscidea (Schwager). In addition, several proxy taxa of the intensity of deep sea currents were detected in the analyzed assemblages. From these taxa, Cibicides wuellerstorfi (Schwager) has the best proxy potential for the reconstruction of intense bottom currents in sediments from the Cocos and Carnegie Ridges for the Holocene.

Comparison of eastern tropical Pacific TEX86 and Globigerinoides ruber Mg/Ca derived sea surface temperatures: Insights from the Holocene and Last Glacial Maximum

The use of the TEX86 temperature proxy has thus far come to differing results as to whether TEX86 temperatures are representative of surface or subsurface conditions. In addition, although TEX86 temperatures might reflect sea surface temperatures based on core-top (Holocene) values, this relationship might not hold further back in time. Here, we investigate the TEX86 temperature proxy by comparing TEX86 temperatures to Mg/Ca temperatures of multiple species of planktonic foraminifera for two sites in the eastern tropical Pacific (on the Cocos and Carnegie Ridges) across the Holocene and Last Glacial Maximum. Core-top and Holocene TEX86H temperatures at both study regions agree well, within error, with the Mg/Ca temperatures of Globigerinoides ruber, a surface dwelling planktonic foraminifera. However, during the Last Glacial Maximum, TEX86H temperatures are more representative of upper thermocline temperatures, and are offset from G. ruber Mg/Ca temperatures by 5.8 °C and 2.9 °C on the Cocos Ridge and Carnegie Ridge, respectively. This offset between proxies cannot be reconciled by using different TEX86 temperature calibrations, and instead, we suggest that the offset is due to a deeper export depth of GDGTs at the LGM. We also compare the degree of glacial cooling at both sites based on both temperature proxies, and find that TEX86H temperatures greatly overestimate glacial cooling, especially on the Cocos Ridge. This study has important implications for applying the TEX86 paleothermometer in the eastern tropical Pacific.