Carnegie Ridge Research Articles

Seismic observations of Nazca-plate crustal thicknesses providing constraints for a first-order asthenospheric-mantle potential-temperature anomalies assessment

The Nazca plate is populated with several magmatic tracks exhibiting complex morphologies responding to time-dependent positioning of spreading centers (East Pacific Rise, Cocos-Nazca). Mantle plumes beneath the Nazca plate are/were located on- or off-ridge (with ridge being spreading center), modulating the resulting hotspot-tracks’ crustal thickness accordingly. We use published seismic observational constraints of Moho depths to study asthenospheric-mantle potential-temperature anomalies (ΔTp) beneath the Nazca plate. We use a simple thermodynamics formulation for adiabatic cooling and decompression melting, and a depth-dependent, static analytical description presenting different options for hydrated-solidus-liquidus curves up to 8 GPa-lithostatic pressure as a function of bulk water content in the peridotite source. The calculated Tp-anomalies are relative to an average Nazca crust (∼6 km thick) formed at the East Pacific Rise with source hydration levels of about 0.01 wt%. As active-upwelling perturbing passive MOR environments, on-ridge plumes added mass (lower crustal intrusion, surface extrusion) to young crust creating OIB hotspot-tracks whose mean crustal thicknesses (≤18 km) are consistent with inferred ΔTp range-values of [-100, 75] °C (Iquique Ridge), [-50, 130] °C (Nazca Ridge), and [-20, 150] °C (Carnegie Ridge south), considering conservative bulk water contents of 0.005–0.08 wt%. The Juan Fernández OIB hotspot track was formed by an off-ridge active-upwelling plume impinging under a 27 Myr-old oceanic lithosphere with ∼7 km-thick pre-existing crust. Currently, the track presents 4-5 km-high isolated volcanic edifices and about 1 km crustal root, totaling about 12–13 km aggregate melt thickness, and suggesting ΔTp range-values of [-20, 160] °C.

Seismic imaging of the Northern Andean subduction zone from teleseismic tomography: a torn and fragmented Nazca slab

SUMMARY The Nazca-South America subduction zone in Ecuador is characterized by a complicated along-strike geometry as the slab transitions from flat slab subduction in the south, with the Peruvian flat slab, to what has been characterized as ‘normal’ dipping subduction beneath central Ecuador. Plate convergence additionally changes south to north as the trench takes on a convex shape. Highly heterogeneous bathymetry at the trench, including the aseismic oceanic Carnegie Ridge (CR), and sparse intermediate-depth seismicity has led many to speculate about the behaviour of the downgoing plate at depth. In this study, we present a finite-frequency teleseismic P-wave tomography model of the northern Andes beneath Ecuador and Colombia from 90 to 1200 km depth. Our model builds on prior tomography models in South America by adding relative traveltime residuals recorded at stations in Ecuador. The complete data set is comprised of 114 096 relative traveltime residuals from 1133 stations across South America, with the added data serving to refine the morphology of the Nazca slab in the mantle beneath the northern Andes. Our tomography model shows a Nazca slab with a fragmented along-strike geometry and the first teleseismic images of several proposed slab tears in this region. At the northern edge of the Peruvian flat slab in southern Ecuador, we image a shallow tear at 95–200 km depth that appears to connect mantle flow from beneath the flat slab to the Ecuadorian Arc. Beneath central Ecuador at the latitudes of the CR, the Nazca slab is continuous into the lower mantle. Beneath southern Colombia, the Malpelo Tear breaks the Nazca slab below ∼200 km depth.

Timing of Quaternary volcanism and its relationship with tectonics in the central segment of the Ecuadorian Andes

The unusually high number of volcanoes in the Ecuadorian Arc, located in the deformation zone of the continental North Andean Sliver, coincides with the projection of the major oceanic structures observed in the Nazca Plate, such as the Carnegie Ridge and the Grijalva fracture zone. Although the relationship between this tectonic setting and volcanism has been widely discussed in the literature, their temporal relationship has not been thoroughly investigated due to the lack of geochronological data. We present here 20 new KAr and 2 40Ar/39Ar ages obtained for 7 volcanoes of the central segment of the Ecuadorian arc, which together with previous data show that volcanism in this area started at ∼1.3 Ma. A notable increase in volcanic activity occurred since ∼0.6 Ma, when the formation of a dozen volcanoes occurred in a relatively small area of the central segment. While this arrangement of volcanoes, here referred to as a “volcanic cluster”, appears to be controlled by crustal tectonic structures, the order of onset of these volcanoes and their eruptive activity does not show clear migration patterns over time. However, the presence of older volcanoes in the north of the central segment suggests a possible southward extension of volcanism between ∼1.3 and ∼ 0.6 Ma. Finally, based on the cumulative bulk volumes calculated for the volcanic edifices over time, we infer that the magmatic productivity rate has been roughly constant during the last ∼550 kyr in this area.

A conceptual model for the Tufiño-Chiles-Cerro Negro (TCCN) geothermal system (Ecuador-Colombia): New insights into geothermal exploration from chemical and isotopic composition of hydrothermal fluids

The Tufiño-Chiles-Cerro Negro (TCCN), located on the border between Colombia and Ecuador, is one of the most promising geothermal systems in the Andean Northern Volcanic Zone, presenting an estimated geothermal potential of 130–138 MWe associated with two reservoirs located at 500–1000 m, and > 1500 m of depth, respectively. In this work, we present a geochemical conceptual model for TCCN to explore the physicochemical conditions acting on the reservoir, based on a comprehensive dataset of water, gas geochemistry, and stable isotopes of the hydrothermal manifestations of the geothermal system, integrated with previously published data. Four new water samples and two bubbling gas samples were collected from the TCCN area during fieldwork in January 2015. Waters from TCCN, which show temperatures up to 54.7 °C, are near-neutral to slightly acidic and have variable TDS values (687 to 3095 mg/L) that generally increase at increasing temperature. All the samples fall far from the chloride-mature waters field, being characterised by SO42− or HCO3−dominated composition, suggesting steam-heated and peripheral water characteristics. The 3He/4He ratios (R/Ra) of gas samples range between 2.4 and 4.6, indicating an important mantle He contribution diluted by the addition of radiogenic and atmospheric 4He. Based on their CO2/3He and δ13C–CO2 ratios, gas samples show a carbon origin prevailing from subducted limestone, likely connected to the presence of the subducting Carnegie ridge. A mixing between a thermogenic and biogenic origin for methane is suggested, consistent with the dominant inputs from microbial methanogenesis and thermal maturation of sedimentary organic matter in the TCCN system. The high δ34S-H2S values of gas (+6.7 ‰ and 10.7 ‰ vs. V-CDT) and δ34S-SO4 of water (+19.2 ‰ and + 19.3 ‰ vs. V-CDT) samples coupled with the high values of S/3He indicate that the sulphur signature could be mostly controlled by the incorporation of subducted sulphate. Gas geothermometric estimations in the CO2-H2 system indicate a temperature between 245 and 250 °C for the geothermal reservoir. The gas samples appear to be in disequilibrium concerning the redox conditions related to the FeO/FeO1.5 buffer typical for hydrothermal systems, implying that these samples represent immature vapours. This could be explained by the input of magmatic fluids into the roots of the TCCN geothermal systems, possibly related to the seismic crisis linked to the unrest of the volcanic complex at the end of 2014.

Late Miocene Exhumation of the Western Cordillera, Ecuador, Driven by Increased Coupling Between the Subducting Carnegie Ridge and the South American Continent

AbstractThe subduction of bathymetric highs, such as aseismic ridges, leads to far‐reaching changes in the dynamics of subduction zones with increased plate coupling and deformation in the upper plate. Subduction of the submarine Carnegie Ridge on the Nazca Plate has fundamentally impacted late Cenozoic magmatism and tectonic activity in the northern Andes. However, the timing of onset of Carnegie Ridge subduction has been a matter of debate. Time‐temperature inverse modeling of new thermochronological data from the Western Cordillera of Ecuador reveals two phases of cooling separated by isothermal conditions. The first cooling phase postdates early and middle Miocene magmatism in the Western Cordillera and is attributed to post‐magmatic thermal relaxation. The second cooling phase started after 6 Ma. Inferred to record the onset of tectonically controlled rock uplift and exhumation in the Western Cordillera, this phase is coeval with the last cooling phase recorded in the Eastern Cordillera. Based on these findings, we suggest that the onset of subduction of the Carnegie Ridge at ∼6–5 Ma increased plate coupling at the subduction interface, promoting shortening, regional rock uplift, and exhumation in the northern Andes. Overall, our results highlight the essential role of bathymetric highs in driving regional upper‐plate deformation at non‐collisional convergent plate margins.

Mega‐Depressions on the Cocos Ridge: Links Between Volcanism, Faults, Hydrothermal Circulation, and Dissolution

AbstractHigh‐resolution bathymetry and three‐dimensional seismic data along the Cocos Ridge reveal a 245 km2 field of ∼1–4 km in diameter seafloor depressions. The seafloor depressions are part of a two‐tiered honeycomb pattern. The lower‐tier depressions have steep faults that truncate strata with chaotic internal reflections consistent with sediment collapse into the depression. These extend into a lens shaped interval just above igneous basement. Overlying these depressions is a second broader set with rough seafloor morphology with gently dipping boundaries defined by pinch‐out stratigraphic patterns. Drilling results indicate that the lens‐shaped zones that host the deeper depressions represent anomalous regions of high porosity, low velocity, and low density within calcareous rich sediment. Analysis of nannofossils from IODP Site U1414 suggests the collapse structures formed during the late Miocene, whereas the younger shallower depressions likely formed between the early Pliocene and the Pliocene‐Pleistocene boundary. Geochemical and petrological analysis at Site U1414 suggests that hydrothermal circulation during the late Miocene led to carbonate dissolution and collapse. Following collapse, focused fluid‐flow and bottom current scouring resulted in formation of the overlying set of depressions and a honeycomb seafloor morphology. Similar sets of depressions along the Carnegie Ridge to the south support the hypothesis that two‐tiered depressions formed in response to processes that occurred broadly across the Panama Basin between the late Miocene and the Pliocene‐Pleistocene transition. Geochemical results at Site U1414, combined with geophysical data, suggest this two‐tiered system of depressions currently guides ongoing fluid outflow.

From Long‐ to Short‐Term Inter‐Plate Coupling at the Subducted Carnegie Ridge Crest, Offshore Central Ecuador

AbstractWe investigate the relationship between the long‐term (Quaternary) interplate coupling and the short‐term geodetically derived interseismic coupling at the Central Ecuador subduction zone. At this nonaccretionary margin, the Cabo Pasado shelf promontory and coastal area are associated with two inter‐plate geodetically locked patches. The deepest patch ruptured co‐seismically during the Mw7.8‐2016 Pedernales earthquake, while the shallowest underwent dominantly after‐slip. Marine geophysical and chronostratigraphic data allow reconstructing the Quaternary tectonic evolution of the shelf promontory and substantiating variation of the long‐term inter‐plate coupling that led to the geodetically locked patches. Prior to ∼1.8 Ma, the outer‐wedge inter‐plate coupling was strong enough to activate trench‐subparallel strike‐slip faults. Then, between ∼1.8 and 0.79 Ma, shortening and uplift affected the shelf promontory, implying a locally increased inter‐plate coupling. After a short, post‐0.79 Ma period of subsidence, shortening and uplift resumed denoting a high inter‐plate coupling that endured up to the present. The synchronicity of the structural evolution of the shelf promontory with the subduction chronology of two reliefs of the Carnegie Ridge crest suggests that the locked patches are caused by a geometrical resistance to subduction that propagates landward causing permanent deformation. In 2016, the deepest subducted relief localized stress accumulation and high seismic slip, while the shallowest relief, which is associated with a weakened outer‐wedge, prevented updip rupture propagation. Thus, at nonaccretionary margins, active outer‐wedge strike‐slip faults might be considered a proxy of near‐trench coupling, and subducted relief a cause of plate coupling but an obstacle to the tsunami genesis when the relief is shallow.

Seismic and thermo-compositional insights into the uppermost mantle beneath the Northern Andes magmatic arc

The mantle wedge beneath the Northern Andes magmatic arc is expected to be latitudinally heterogeneous as suggested by the highly variable composition of recovered xenoliths, a discontinuous pattern of low seismic velocity zones, and unusually high Vp/Vs ratios below volcanic centers. Such heterogeneity might be controlling the development of the modern arc which currently shows a narrow northern volcanic segment between ∼4° and 6°N (<75 km wide), a volcanic gap of hundreds of km in its central portion (∼2°-4°N), and a wider southern volcanic expression between ∼2°S and 2°N (>120 km wide). Through an analysis of Pn and Sn inter-station wave speed estimates, the seismic structure dissimilarity and latitudinal thermo-compositional insights were constrained within the uppermost mantle beneath the arc. Results, integrating wave speed estimates, anisotropy, and thermo-compositional inferences obtained by wave velocity comparison using physical properties of minerals and phase equilibria modeling, suggest a seismically slower structure, higher anisotropy, and warmer conditions beneath the northern region (>4°N), whereas a faster, less anisotropic, and colder mantle towards the south (<2°N). We interpret the warmer and higher degree of anisotropy in the northern uppermost mantle as influenced by the Caldas tear, prompting hot mantle influx. Beneath the gap region, seismic speeds are similar to those in the north, yet a colder thermal state is suggested. We interpret the colder temperatures and the abrupt reduction in surface volcanism as reflecting an absence, or a reduced amount of magma pounding within the Moho vicinity. The southern upper mantle is seismically faster and colder compared to the northern portions of the arc. We interpret the fast velocities, and a wider volcanic expression on the surface, as influenced by the Carnegie ridge interaction prompting a shallower subduction with respect to the north. Our wave speed estimate and thermo-compositional inferences draw an increase in temperature and seismic anisotropy in the uppermost mantle from south to north along the Northern Andes magmatic arc. The main controlling factor of the general anisotropy below the entire arc seems to be the preferred orientation of olivine and pyroxene rather than an aligned melt fabric.

Seismic and Aseismic Cycle of the Ecuador–Colombia Subduction Zone

The Colombia–Ecuador subduction zone is an exceptional natural laboratory to study the seismic cycle associated with large and great subduction earthquakes. Since the great 1906 Mw = 8.6 Colombia–Ecuador earthquake, four large Mw &amp;gt; 7.5 megathrust earthquakes occurred within the 1906 rupture area, releasing altogether a cumulative seismic moment of ∼35% of the 1906 seismic moment. We take advantage of newly released seismic catalogs and global positioning system (GPS) data at the scale of the Colombia–Ecuador subduction zone to balance the moment deficit that is building up on the megathrust interface during the interseismic period with the seismic and aseismic moments released by transient slip episodes. Assuming a steady-state interseismic loading, we found that the seismic moment released by the 2016 Mw = 7.8 Pedernales earthquake is about half of the moment deficit buildup since 1942, suggesting that the Pedernales segment was mature to host that seismic event and its postseismic afterslip. In the aftermath of the 2016 event, the asperities that broke in 1958 and 1979 both appears to be mature to host a large Mw &amp;gt; 7.5 earthquakes if they break in two individual seismic events, or an Mw∼7.8–8.0 earthquake if they break simultaneously. The analysis of our interseismic-coupling map suggests that the great 1906 Colombia–Ecuador earthquake could have ruptured a segment of 400 km-long bounded by two 80 km wide creeping segments that coincide with the entrance into the subduction of the Carnegie ridge in Ecuador and the Yaquina Graben in Colombia. These creeping segments share similar frictional properties and may both behave as strong seismic barriers able to stop ruptures associated with great events like in 1906. Smaller creeping segments are imaged within the 1906 rupture area and are located at the extremities of the large 1942, 1958, 1979, and 2016 seismic ruptures. Finally, assuming that the frequency–magnitude distribution of megathrust seismicity follows the Gutenberg–Richter law and considering that 50% of the transient slip on the megathrust is aseismic, we found that the maximum magnitude subduction earthquake that can affect this subduction zone has a moment magnitude equivalent to Mw ∼8.8 with a recurrence time of 1,400 years. No similar magnitude event has yet been observed in that region.

A preliminary seismic travel time tomography beneath Ecuador from data of the national network

The subduction of the Nazca plate has specific features that impact the geophysical structure beneath Ecuador. In addition to the convexity of the trench between Peru and Ecuador, the Carnegie ridge and the Grijalva scarp are associated with strong heterogeneities in the plate that interfere with the subduction process. We have taken advantage of the large amount of manually picked P and S wave arrival times accumulated over time thanks to the national RENSIG network to perform an inversion for both event locations and seismic velocities over Ecuador within lat. 1.5∘N and 5∘S and long. 77∘W and 82∘W. After data filtering, the model presented in this article results from the inversion of 335,498 P and 111,457 S arrival times, corresponding to 25,410 events between 1988 and 2016. The intermediate depth seismicity outlines a continuous Wadati-Benioff zone in southern Ecuador that clearly defines the topography of the Farallon plate and its plunge towards the north-east at the Puyo nest, whereas in the northern part of the slab its pattern is more heterogeneous and is dominated by three nests at depths ranging between 75 km and 110 km, the Guayaquil, La Maná and Maldonado nests. The velocity model reveals a discontinuity of the Nazca plate along an axis oriented N110∘E, starting in the seismogenic zone at the southern limit of the occurrence of large earthquakes. This tear is associated with an overlap of the southern Farallon part of the slab by its younger northern part. We propose that it is the result of plate buckling due to lateral compression at depth caused by the sharp bend of the trench line between Peru and Ecuador.

GNSS Constraints to Active Tectonic Deformations of the South American Continental Margin in Ecuador.

GNSS observations constitute the main tool to reveal Earth’s crustal deformations in order to improve the identification of geological hazards. The Ecuadorian Andes were formed by Nazca Plate subduction below the Pacific margin of the South American Plate. Active tectonic-related deformation continues to present, and it is constrained by 135 GPS stations of the RENAGE and REGME deployed by the IGM in Ecuador (1995.4–2011.0). They show a regional ENE displacement, increasing towards the N, of the deformed North Andean Sliver in respect to the South American Plate and Inca Sliver relatively stable areas. The heterogeneous displacements towards the NNE of the North Andean Sliver are interpreted as consequences of the coupling of the Carnegie Ridge in the subduction zone. The Dolores–Guayaquil megashear constitutes its southeastern boundary and includes the dextral to normal transfer Pallatanga fault, that develops the Guayaquil Gulf. This fault extends northeastward along the central part of the Cordillera Real, in relay with the reverse dextral Cosanga–Chingual fault and finally followed by the reverse dextral Sub-Andean fault zone. While the Ecuadorian margin and Andes is affected by ENE–WSW shortening, the easternmost Manabí Basin located in between the Cordillera Costanera and the Cordillera Occidental of the Andes, underwent moderate ENE–WSW extension and constitutes an active fore-arc basin of the Nazca plate subduction. The integration of the GPS and seismic data evidences that highest rates of deformation and the highest tectonic hazards in Ecuador are linked: to the subduction zone located in the coastal area; to the Pallatanga transfer fault; and to the Eastern Andes Sub-Andean faults.

Marine terraces of the last interglacial period along the Pacific coast of South America (1° N–40° S)

Abstract. Tectonically active coasts are dynamic environments characterized by the presence of multiple marine terraces formed by the combined effects of wave erosion, tectonic uplift, and sea-level oscillations at glacial-cycle timescales. Well-preserved erosional terraces from the last interglacial sea-level highstand are ideal marker horizons for reconstructing past sea-level positions and calculating vertical displacement rates. We carried out an almost continuous mapping of the last interglacial marine terrace along ∼ 5000 km of the western coast of South America between 1∘ N and 40∘ S. We used quantitatively replicable approaches constrained by published terrace-age estimates to ultimately compare elevations and patterns of uplifted terraces with tectonic and climatic parameters in order to evaluate the controlling mechanisms for the formation and preservation of marine terraces and crustal deformation. Uncertainties were estimated on the basis of measurement errors and the distance from referencing points. Overall, our results indicate a median elevation of 30.1 m, which would imply a median uplift rate of 0.22 m kyr−1 averaged over the past ∼ 125 kyr. The patterns of terrace elevation and uplift rate display high-amplitude (∼ 100–200 m) and long-wavelength (∼ 102 km) structures at the Manta Peninsula (Ecuador), the San Juan de Marcona area (central Peru), and the Arauco Peninsula (south-central Chile). Medium-wavelength structures occur at the Mejillones Peninsula and Topocalma in Chile, while short-wavelength (&lt; 10 km) features are for instance located near Los Vilos, Valparaíso, and Carranza, Chile. We interpret the long-wavelength deformation to be controlled by deep-seated processes at the plate interface such as the subduction of major bathymetric anomalies like the Nazca and Carnegie ridges. In contrast, short-wavelength deformation may be primarily controlled by sources in the upper plate such as crustal faulting, which, however, may also be associated with the subduction of topographically less pronounced bathymetric anomalies. Latitudinal differences in climate additionally control the formation and preservation of marine terraces. Based on our synopsis we propose that increasing wave height and tidal range result in enhanced erosion and morphologically well-defined marine terraces in south-central Chile. Our study emphasizes the importance of using systematic measurements and uniform, quantitative methodologies to characterize and correctly interpret marine terraces at regional scales, especially if they are used to unravel the tectonic and climatic forcing mechanisms of their formation. This database is an integral part of the World Atlas of Last Interglacial Shorelines (WALIS), published online at https://doi.org/10.5281/zenodo.4309748 (Freisleben et al., 2020).

3D Local Earthquake Tomography of the Ecuadorian Margin in the Source Area of the 2016 Mw 7.8 Pedernales Earthquake

AbstractBased on manually analyzed waveforms recorded by the permanent Ecuadorian network and our large aftershock deployment installed after the Pedernales earthquake, we derive three‐dimensional Vp and Vp/Vs structures and earthquake locations for central coastal Ecuador using local earthquake tomography. Images highlight the features in the subducting and overriding plates down to 35 km depth. Vp anomalies (∼4.5–7.5 km/s) show the roughness of the incoming oceanic crust (OC). Vp/Vs varies from ∼1.75 to ∼1.94, averaging a value of 1.82 consistent with terranes of oceanic nature. We identify a low Vp (∼5.5 km/s) region extending along strike, in the marine forearc. To the North, we relate this low Vp and Vp/Vs (&lt;1.80) region to a subducted seamount that might be part of the Carnegie Ridge (CR). To the South, the low Vp region is associated with high Vp/Vs (&gt;1.85) which we interpret as deeply fractured, probably hydrated OC caused by the CR being subducted. These features play an important role in controlling the seismic behavior of the margin. While subducted seamounts might contribute to the nucleation of intermediate megathrust earthquakes in the northern segment, the CR seems to be the main feature controlling the seismicity in the region by promoting creeping and slow slip events offshore that can be linked to the updip limit of large megathrust earthquakes in the northern segment and the absence of them in the southern region over the instrumental period.

Composition and diversity patterns of deep sea benthic foraminifera from the Panama basin, eastern equatorial Pacific

Environmental controls affecting the composition and distribution of deep sea benthic foraminifera were analyzed in the Panama Basin (Eastern Equatorial Pacific). For this study, results of the analysis of a new set of 33 core-top samples from the upper-lower slope (714–3819 mbsf) were integrated with previous data from this area.The composition of the foraminiferal assemblages resembles a typical deep sea association, illustrating a close relationship with dissolved oxygen at the seafloor and indirectly with surface productivity. Low oxygen content and organic matter influx, as inferred in the eastern Panama Basin, are connected to terrigenous supply and seasonal upwelling on the Panama Bight and southern Costa Rica. Buliminids, bolivinids, and Epistominella spp. are common elements of the continental slope. In addition, physiographic aspects such as the type of substrate, turbidity fluxes, and bottom currents also exert control over some of the described taxa and defined morphogroups. A broad exploration of the infaunal/epifaunal relationships in the basin does not suggest a clear relationship with dissolved oxygen content. However, there are fluctuations in the proportion of some of the defined epifaunal morphogroups and selected taxa (e.g. Cibicidoides wuellerstorfi, Chilostomella oolina, and Hoeglundina elegans, among others) along the Carnegie Ridge. These changes are explained by the variable physiographic features (surface upwelling, strong bottom currents) that coalesce in this part of the Panama Basin.Furthermore, re-evaluation of the foraminiferal morphogroups in two deep sea cores that comprised the last 25 ky, indicate that some taxa of the recent samples (Cassidulina spp. and Siphouvigerina peregrina, among others) reacted to paleoproductivity and bottom current fluctuations at the end of the Last Glacial Maximum and the Younger Dryas.Despite the lack of ecological data in the foraminiferal assemblages for a large proportion of the Panama Basin, the application of selected taxa or morphogroups as proxies for paleoceanographic conditions is promising.

First timing constraints on the Ecuadorian Coastal Cordillera exhumation: Geodynamic implications

In this study, we provide the first detrital apatite (U–Th-Sm)/He (AHe) and zircon U–Pb ages to establish a detailed short-term chronology of the burial and exhumation history, which occurred in the Coastal Cordillera along the forearc domain of Ecuador. First, our results allowed us to define a range of maximum deposition ages for the Angostura Formation from 9.6 ± 0.2 Ma to 11.5 ± 0.6 Ma, which records high enough temperatures to partially reset AHe ages. QTQt thermal inverse modeling of the AHe dataset reveals three main periods of exhumation along the Costal Cordillera at ∼2 Ma, ∼5–6 Ma and ∼8–10 Ma, independent of the sample geographic locations. We discuss the origin of these periods of exhumation in relation to the geodynamic frame. The oldest exhumation event, at ∼8–10 Ma, evidenced locally along fault systems, could be related to overriding plate kinematic changes or early arrival of Carnegie Ridge. The intermediate exhumation period, at ∼5–6 Ma, could be explained by a later arrival of the Carnegie Ridge or to the subduction of an along-strike positive relief of the ridge. Later, subduction of sea-floor asperities could be responsible for heterogeneous uplift of the Coastal Cordillera during the Pleistocene (∼2 Ma) that induced exhumation as supported by our models. These results are corroborated by previous studies and demonstrate that AHe data are sensitive enough to provide reliable constraints in sedimentary domains.

Structure of the Ecuadorian forearc from the joint inversion of receiver functions and ambient noise surface waves

SUMMARYThe Ecuadorian forearc is a complex region of accreted terranes with a history of large megathrust earthquakes. Most recently, a Mw 7.8 megathrust earthquake ruptured the plate boundary offshore of Pedernales, Ecuador on 16 April 2016. Following this event, an international collaboration arranged by the Instituto Geofisico at the Escuela Politécnica Nacional mobilized a rapid deployment of 65 seismic instruments along the Ecuadorian forearc. We combine this new seismic data set with 14 permanent stations from the Ecuadorian national network to better understand how variations in crustal structure relate to regional seismic hazards along the margin. Here, we present receiver function adaptive common conversion point stacks and a shear velocity model derived from the joint inversion of receiver functions and surface wave dispersion data obtained through ambient noise cross-correlations for the upper 50 km of the forearc. Beneath the forearc crust, we observe an eastward dipping slow velocity anomaly we interpret as subducting oceanic crust, which shallows near the projected centre of the subducting Carnegie Ridge. We also observe a strong shallow positive conversion in 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 also observe significant north–south changes in shear wave velocity. The velocity changes indicate variations in the accreted terranes and may indicate an increased amount of hydration beneath the Manabí Basin. This change in structure also correlates geographically with the southern rupture limit of multiple high magnitude megathrust earthquakes. The earthquake record along the Ecuadorian trench shows that no event with a Mw &amp;gt;7.4 has ruptured south of ∼0.5°S in southern Ecuador or northern Peru. Our observations, along with previous studies, suggest that variations in the forearc crustal structure and subducting oceanic crust may influance the occurrence and spatial distribution of high magnitude seismicity in the region.

Evolution of the Ecuador offshore nonaccretionary-type forearc basin and margin segmentation

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 Canandé fault. Synchronously, unit L3 sediment recorded the subsidence of the Central-North segment, and strike-slip tectonics in the area of the Jama-Bahía 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.

Volcanic history reconstruction in northern Ecuador: insights for eruptive and erosion rates on the whole Ecuadorian arc

In the northern Andes, the Ecuadorian arc presents a large number of Quaternary volcanoes, spread over a rather restricted area. The origin of this volcanic clustering is not well understood, and only a few chronological data older than the Holocene are available in northern Ecuador to document the arc development stages. In this study, we present new K-Ar ages obtained on lava flow and pumice samples for Cushnirumi, Mojanda, Fuya Fuya, Imbabura, Cubilche, and Cusin volcanoes, located about 40 km north of Quito, the Ecuador’s capital city. Our results show that the volcanic activity in the northern part of the Ecuadorian arc started at least at ~ 1 Ma and that construction of volcanoes mainly occurred during the last 500 ka. Together with the radiometric data, numerical reconstructions of the paleomorphology of the volcanoes are used to estimate the volume of emitted magmas and the amount of eroded material in order to quantify their eruptive and erosion rates. Emission rates of Ecuadorian volcanoes range between < 0.2 and 3.6 ± 2.1 km3/kyr. Highest rates are obtained for volcanoes constructed over time periods shorter than 100 kyr by sporadic eruptive pulses, whereas lowest rates are calculated over longer periods that include quiescence phases. Erosion rates range between 0.02 ± 0.01 and 0.14 ± 0.09 km3/kyr and highlight that volcanic edifices whose activity ended recently are rapidly dismantled by physicochemical processes. Finally, the spatial distribution of Quaternary volcanoes as well as the spatio-temporal evolution of lava geochemistry may reflect the progressive influence of the Carnegie Ridge at depth.

Upper-plate structure in Ecuador coincident with the subduction of the Carnegie Ridge and the southern extent of large mega-thrust earthquakes

SUMMARYThe Ecuadorian convergent margin has experienced many large mega-thrust earthquakes in the past century, beginning with a 1906 event that propagated along as much as 500 km of the plate interface. Many subsections of the 1906 rupture area have subsequently produced Mw ≥ 7.7 events, culminating in the 16 April 2016, Mw 7.8 Pedernales earthquake. Interestingly, no large historic events Mw ≥ 7.7 appear to have propagated southward of ∼1°S, which coincides with the subduction of the Carnegie Ridge. We combine data from temporary seismic stations deployed following the Pedernales earthquake with data recorded by the permanent stations of the Ecuadorian national seismic network to discern the velocity structure of the Ecuadorian forearc and Cordillera using ambient noise tomography. Ambient noise tomography extracts Vsv information from the ambient noise wavefield and provides detailed constraints on velocity structures in the crust and upper mantle. In the upper 10 km of the Ecuadorian forearc, we see evidence of the deepest portions of the sedimentary basins in the region, the Progreso and Manabí basins. At depths below 30 km, we observe a sharp delineation between accreted fast forearc terranes and the thick crust of the Ecuadorian Andes. At depths ∼20 km, we see a strong fast velocity anomaly that coincides with the subducting Carnegie Ridge as well as the southern boundary of large mega-thrust earthquakes. Our observations raise the possibility that upper-plate structure, in addition to the subducting Carnegie Ridge, plays a role in the large event segmentation seen along the Ecuadorian margin.

Effects of aseismic ridge subduction on the geochemistry of frontal arc magmas

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.