Chilean Margin Research Articles

Deep Structure of the Continental Plate in the South‐Central Chilean Margin: Metamorphic Wedge and Implications for Megathrust Earthquakes

AbstractWe study the deep structure of the continental wedge along the south‐central Chilean convergent margin based on a joint interpretation of wide‐angle reflection and refraction seismic phases acquired between the southern part of the Maule 2010 (36°S) and the northern part of the Valdivia 1960 (41°S) earthquake rupture areas. When combined with results of previous seismic studies, our results provide new insights into the deep structure (to 15–20 km) of the overriding, continental South American plate. We observe a latitudinal variation of P wave velocity, interpreted in terms of the distribution of Permo‐Triassic metamorphic complex near the coast, and deep reflectors at the base of these metamorphic units. A change in the observed deep structural style near 38°S suggests a segmentation in the interplate frictional properties between the Maule and Valdivia earthquakes rupture zones. This regional change in the continental wedge structure indicates a spatial correlation between the coseismic ruptures of large earthquakes (e.g., the 1960 and 2010 megathrust earthquakes) and the rheology/lithology along the interplate boundary.

Frictional Segmentation of the Chilean Megathrust From a Multivariate Analysis of Geophysical, Geological, and Geodetic Data

AbstractWe present a formal seismic segmentation model of the Chile Margin derived from a multivariate analysis of gravity anomalies, basal friction and interplate locking from GPS velocities. These independent proxies of the frictional structure along the megathrust integrate over timescales of 101–107 years. We analyze their covariance using Principal Component Analysis (PCA) and extracting the Empirical Orthogonal Functions (EOFs) and Principal Components (PCs). We find that (a) the first and first + second PCA modes for the triple combination of the proxies explain (respectively) 41.4% and 69.4% of the total variance and (b) the first‐order spatial pattern of each isolated proxy is largely reproduced when it is combined with the other two proxies. This suggests that the three proxies share a first order along‐trench tendency that must be connected through a common physical process. Comparing our results with rupture areas of earthquakes and using the rate‐and‐state friction theory, we infer that trench‐perpendicular PCs reflect the limits of the velocity‐weakening (VW) seismogenic zone. Polarity changes in along‐trench EOFs (mostly for gravity) mark changes between unstable and conditionally stable frictional regimes inside the VW zone and we used them to define the boundaries of 17 unitary segments. These segments correlate with first‐order tectonic features of the Andes at >103‐km scales as well as with zones of multi‐segment ruptures at 102‐km scale. We analyze the combined influence of subducting and upper‐plate geologic features on the nature of persistent seismic barriers. Our results have implications for understanding seismotectonic processes along the Andean margin and elsewhere.

Three-dimensional local earthquake tomography of pre-Cenozoic structures in the coastal margin of central Chile: Pichilemu fault system

Following the Mw 8.8 Maule megathrust earthquake that occurred on February 27, 2010, in central Chile, a sequence of normal faulting crustal earthquakes occurred close to the city of Pichilemu. This activity lasted several months and included two large events (Mw = 6.9 and Mw = 7.0) on March 11, 2010. An initial investigation of this activity analyzed a data set of about 630 earthquakes recorded locally by 8 short-period seismic stations and attributed this seismicity to the activation of the “Pichilemu fault system.” While there is no visible surface faulting associated with this system, it is inferred from discontinuities in lithofacies in the metamorphic complex of Pichilemu and from morphological breaks attributed to post-Pliocene neotectonic activity. There are no recorded shallow crustal earthquakes in the Pichilemu area prior to the 2010 events. In this study, we combine locally recorded earthquake data from the 2010 seismic deployment and data from 20 seismic stations (short period, three components, continuous recording) deployed in 2017 around Pichilemu, Chile, to create a more detailed characterization of the Pichilemu fault system through local earthquake tomography (LET). The combined data set composed of P- and S-wave arrival times from 3691 events was inverted to generate a 3D elastic wave speed model from the surface to about 50-km depth. One hundred twenty-two focal mechanisms for relocated earthquakes with M ≥ 1 were also generated. Relocated hypocenters show that most of the recorded seismicity is associated with the Pichilemu fault system; its main structure is oriented N145°E, and it is seismically active along about 50 km long. Normal faulting mechanisms predominate for events with M≥2, being similar to the mechanisms of the Mw 7.0 March 11, 2010, Pichilemu earthquakes. Low-velocity anomalies correlate with fracture zones associated with the Pichilemu fault, and a high contrast in Vp/Vs coincides with known structures of Paleozoic to Mesozoic age. A high Vp/Vs ratio is observed where a projection of the fault reaches the interplate contact, suggesting that this zone of the forearc crust is likely weakened by the presence of fluids from the slab. A high Vp anomaly is contiguous with the fault system and appears to be related to the presence of granitic rocks which belong to the Coastal Batholith within the Cordillera de la Costa. We suggest that the location of the Pichilemu fault system is governed by rheological contrasts inherited from the evolution of the subduction complex represented by the current Cordillera de la Costa, and we infer that the orientations of those structures, the possible hydration of them from the interplate contact and associated crustal blocks, play a key role in fault activation following the stressing by a great subduction earthquake.

Gas Hydrate System Offshore Chile

In recent decades, the Chilean margin has been extensively investigated to better characterize the complex geological setting through the geophysical data. The analysis of seismic lines allowed us to identify the occurrence of gas hydrates and free gas in many places along the margin and the change of the pore fluid due to the potential hydrate dissociation. The porosity reduction due to the hydrate presence is linked to the slope to identify the area more sensitive in case of natural phenomena or induced by human activities that could determine gas hydrate dissociations and/or leakage of the free gas trapped below the gas-hydrate stability zone. Clearly, the gas hydrate reservoir could be a strategic energy reserve for Chile. The steady-state modelling pointed out that the climate change could determine gas hydrate dissociation, triggering slope failure. This hypothesis is supported by the presence of high concentrations of gas hydrate in correspondence of important seafloor slope. The dissociation of gas hydrate could change the petrophysical characteristics of the subsoil triggering slopes, which already occurred in the past. Consequently, it is required to improve knowledge about the behavior of the gas hydrate system in a function of complex natural phenomena before the exploitation of this important resource.

A review of the gas hydrate distribution offshore Chilean margin

In last decades, the Chilean margin has been extensively investigated to better characterize the complex geological setting through the acquisition of geophysical data and, in particular, seismic lines. The analysis of seismic lines allowed identifying the occurrence of gas hydrates and free gas in many places along the margin. Clearly, the gas hydrate reservoir could be a strategic energy reserve for Chile, but, on the other hand, the dissociated of gas hydrate due to climate change could be an issue to face. Moreover, this region is characterized by large and mega-scale earthquakes that may contribute to gas hydrate dissociation and consequent submarine slides triggering. In this context, Chilean margin should be considered a natural laboratory to study the hydrate system evolution.

Gas hydrate versus seabed morphology offshore Lebu (Chilean margin)

Gas-hydrate occurrences along the Chilean margin have been widely documented, but the processes associated with fluid escapes caused by the dissociation of gas hydrates are still unknown. We report a seabed morphology growth related to fluid migration offshore Lebu associated with mud cones by analysing oxygen and deuterium stable water isotopes in pore water, bathymetric, biological and sedimentological data. A relief was observed at − 127 m water depth with five peaks. Enrichment values of δ18O (0.0–1.8‰) and δD (0.0–5.6‰) evidenced past hydrate melting. The orientation of the relief could be associated with faults and fractures, which constitute pathways for fluid migration. The benthic foraminifera observed can be associated with cold seep areas. We model that the mud cones correspond to mud growing processes related to past gas-hydrate dissociation. The integration of (i) the seismic data analysis performed in the surrounding area, (ii) the orientation of our studied relief, (iii) the infaunal foraminifera observed, (iv) the grain size and (v) the total organic matter and isotope values revealed that this area was formerly characterised by the presence of gas hydrates. Hence, this part of the Chilean margin represents a suitable area for investigating fluid-migration processes.

Basal Accretion Along the South Central Chilean Margin and Its Relationship to Great Earthquakes

AbstractThe south central Chilean margin regularly produces many of the world's largest earthquakes and tsunami, including the 2010 Mw 8.8 Maule and 1960 Mw 9.5 Valdivia events. In 2017, we acquired seismic reflection data along ~1,000 km of the margin using the R/V Langseth's 15 km long receiver array and 108.2 l (6,600 in3) seismic source to image structures associated with these ruptures. We focus on the Valdivia segment with the largest coseismic slip (~40 m). The outer 40 km of the forearc is an accretionary wedge constructed primarily of stacked sedimentary packages with irregular lengths and thicknesses and little along‐strike continuity. Forearc structures indicate that the accretionary wedge grows primarily through basal accretion of the downgoing trench fill. The décollement propagates along a weak boundary near the top of the trench fill but occasionally branches downward into the underthrust sediment along bedding horizons, peeling off slices that are underplated to the forearc. The shallow décollement level and the rarity of underplating events allow most of the trench sediment to subduct. As a result, only ~30% of the incoming sediment has been accreted since the Early Pliocene. This implies that, on average, ~1 km of sediment must subduct beyond the outer forearc, an inference that is supported by our seismic images. We propose that the thickness and great downdip and along‐strike extent of the underthrust layer, which separates the megathrust from the underlying roughness of the igneous ocean crust, ensures a smooth broad zone of strong coupling that generates the world's largest earthquakes and tsunami.

Triassic-Jurassic thermal evolution and exhumation of the western Gondwana foreland: Thermochronology and basalt thermobarometry from the Argentine Sierras Pampeanas

The geological record of the eastern Sierras Pampeanas province, in the modern Andean broken foreland of Argentina can be divided into four main events: (1) Proterozoic to early Paleozoic collisional tectonics, (2) middle-late Paleozoic anorogenic magmatism, relief generation and glacial paleovalley formation followed by (3) a classical foreland filling in the Permian and two proximal alluvial sedimentation associated with (3) Cretaceous rifting and (4) Neogene intermontane foreland accumulation. The region lacks Silurian, Triassic and Jurassic records, commonly associated with unknown deformation and/or no-sedimentation stages (bypass zone?). In this work, we analyzed the Mesozoic (Triassic-Jurassic) Pampean unconformity, using low-temperature thermochronological modelling. After a rapid Carboniferous cooling track, a Triassic reheating followed by a slow Jurassic to Cretaceous cooling. Considering that (1) no Triassic basins have been described to date in the eastern Sierras Pampeanas (i.e., reheating cannot be related to burial), (2) coeval surface heat flows are anomalously high in western Sierras Pampeanas, in the Ischigualasto basin, and (3) our petrogenetic modelling on Triassic basalts evidence mantle potential temperatures of ~1350–1400 °C (i.e., the heat source cannot be related with an anomalously high basal heat flows and/or mantle plumes); we interpreted the formation of the Mesozoic unconformity as a result of ridge collision and slab window formation, followed by slab rollback. Both processes might have affected not only the surface heat flow but also triggered a lithospheric thickness reduction, which drove isostatic rebound. In this context, the Jurassic history of the unconformity could be associated with cooling by erosion and exhumation until the Cretaceous, when the region was under extension. Our model agrees with other observations like the formation of back-arc hydrocarbon-productive Triassic-Jurassic depocenters to the west (Cuyo and Ischigualasto basins) and the magmatic evolution, from 28° to 34° SL, described along the Chilean margin.

Thick, strong sediment subduction along south-central Chile and its role in great earthquakes

The south-central Chile margin experienced the largest and sixth largest earthquakes ever recorded - the 1960 Mw 9.5 Valdivia and 2010 Mw 8.8 Maule megathrust earthquakes, respectively. In early 2017, we conducted a seismic survey along 1,000 km of south-central Chile to image these rupture zones using a 15.15-km-long multi-channel seismic streamer. We processed these data using pre-stack depth migration, which provides the best look at the shallow part of the south-central Chile margin to date. Relative to other sediment-dominated subduction zones, where sediment is typically accreted at the toe, an unusually large percentage of the thick trench sediments are consistently subducted beneath the slope with little thrust faulting or deformation. Analysis of the sediment P-wave velocities and structure in the trench and outer wedge leads us to conclude that most of south-central Chile contains well-drained, strong sediments. An exception in the vicinity of the subducting Mocha Fracture Zone (MFZ) has trench sediments that appear to experience localized delayed compaction, thus lowering their strength and allowing the development of protothrusts, similar to what is seen in other accretionary subduction zones. The very shallow décollement along the south-central Chile allows more sediment to pass beneath the lower slope than almost all other subduction zone, many of which have much thicker trench sections. We conclude that subduction of the strong, well-drained, thick sediment layer beneath the lower slope is typical for nearly all of the south-central Chile. Comparison to other thick-sedimented subduction zones worldwide reveals that the subduction of such a large fraction of the trench sediment is particularly unusual. This strong, thick, subducting sediment is likely a determining factor for developing a smooth plate interface located well above the subducted crust topography that ultimately becomes a broad megathrust with high, homogeneous frictional properties, and generates particularly large earthquakes along the south central Chile margin.

10Be/9Be Ratios Reveal Marine Authigenic Clay Formation

AbstractAs reverse weathering has been shown to impact long‐term changes in atmospheric CO2 levels, it is crucial to develop quantitative tools to reconstruct marine authigenic clay formation. We explored the potential of the beryllium (Be) isotope ratio (10Be/9Be) recorded in marine clay‐sized sediment to track neoformation of authigenic clays. The power of such proxy relies on the orders‐of‐magnitude difference in 10Be/9Be ratios between continental Be and Be dissolved in seawater. On marine sediments collected along a Chilean margin transect we chemically extracted reactive phases and separated the clay‐sized fraction to compare the riverine and marine 10Be/9Be ratio of this fraction. 10Be/9Be ratios increase fourfold from riverine to marine sediment. We attribute this increase to the incorporation of Be high in 10Be/9Be from dissolved biogenic opal, which also serves as a Si‐source for the precipitation of marine authigenic clays. 10Be/9Be ratios thus sensitively track reverse‐weathering reactions forming marine authigenic clays.

Timing, Kinematics, and Displacement of the Taltal Fault System, Northern Chile: Implications for the Cretaceous Tectonic Evolution of the Andean Margin

AbstractAn extensive system of NW striking faults constitutes a major tectonic feature of the Coastal Cordillera in northern Chile, but fundamental questions remain about timing and kinematics of these structures. We present new geologic mapping and geochronology that provide insight into the structural evolution and tectonic significance of the Taltal fault system (TFS). The TFS displaces the Early Cretaceous arc‐parallel Atacama fault system (AFS) with ~10.6 km cumulative offset across a ~15 km wide zone. Brittle fault data demonstrate that the TFS is vertical to steeply NE dipping with an average sinistral slip vector plunging 11° from the NW, compatible with E‐W shortening. Two late Early Cretaceous dikes cut the AFS but are cut by TFS faults, and synkinematic calcite on a TFS strand yielded a U‐Pb calcite date of 114.1 ± 7.0 Ma. These data demonstrate that the AFS was abandoned and deformation (re) initiated on the TFS between ~114–107 Ma, with continued slip after intrusion of the Tropezón (~110 Ma) and Librillo (106–101 Ma) plutonic complexes. Emplacement of a ~146 Ma rhyolite dike along the main Taltal fault and 141 ± 11 Ma calcite mineralization in the fault core suggests that a precursor structure influenced magma emplacement and fluid flow in the Late Jurassic/Early Cretaceous, supporting the hypothesis that the TFS reactivated long‐lived inherited crustal weaknesses. The Early Cretaceous shift from arc‐parallel shear to slip on the TFS and E‐W shortening shortly preceded migration of the magmatic arc and records a change in the Chilean margin subduction dynamics.

Calcification and latitudinal distribution of extant coccolithophores across the Drake Passage during late austral summer 2016

Abstract. Coccolithophores are globally distributed microscopic marine algae that exert a major influence on the global carbon cycle through calcification and primary productivity. There is recent interest in coccolithophore polar communities; however field observations regarding their biogeographic distribution are scarce for the Southern Ocean (SO). This study documents the latitudinal, as well as in depth, variability in the coccolithophore assemblage composition and the coccolith mass variation in the ecologically dominant Emiliania huxleyi across the Drake Passage. Ninety-six water samples were taken between 10 and 150 m water depth from 18 stations during POLARSTERN Expedition PS97 (February–April 2016). A minimum of 200 coccospheres per sample were identified in the scanning electron microscope, and coccolith mass was estimated with light microscopy. We find that coccolithophore abundance, diversity and maximum depth habitat decrease southwards, marking different oceanographic fronts as ecological boundaries. We characterize three zones: (1) the Chilean margin, where E. huxleyi type A (normal and overcalcified) and type R are present; (2) the Subantarctic Zone (SAZ), where E. huxleyi reaches maximum values of 212.5×103 cells L−1 and types B/C, C and O are dominant; and (3) the Polar Front Zone (PFZ), where E. huxleyi types B/C and C dominate. We link the decreasing trend in E. huxleyi coccolith mass to the poleward latitudinal succession from the type A to the type B group. Remarkably, we find that coccolith mass is strongly anticorrelated to total alkalinity, total CO2, the bicarbonate ion and pH. We speculate that low temperatures are a greater limiting factor than carbonate chemistry in the Southern Ocean. However, further in situ oceanographic data are needed to verify the proposed relationships. We hypothesize that assemblage composition and calcification modes of E. huxleyi in the Drake Passage will be strongly influenced by the ongoing climate change.

Shallow intraplate seismicity related to the Illapel 2015 Mw 8.4 earthquake: Implications from the seismic source

The September 16, 2015, Mw 8.4 Illapel, Chile earthquake is the first large event occurring in north-central Chile after the 1943 earthquake, filling a known seismic gap in the region. The earthquake took place in a complex tectonic region, nearby an area where transition from erosive to accretionary margin occurs due to the collision of Juan Fernandez Ridge (JFR) along the Chilean margin. We inverted the kinematic rupture process of the 2015 Mw 8.4 Illapel earthquake from the joint inversion of teleseismic body waves and near-field data. The relative weighting between datasets and the weighting of spatial/temporal constraints are objectively estimated by applying the Akaike's Bayesian Information Criterion. The coseismic slip model yields a total seismic moment of 4.92 × 1021 Nm occurred over ~120 s. The rupture shows both downdip and updip propagation with slip extending along the thrust interface from ~50 km depth to shallow near-trench depths (<15 km), with maximum slip of ~9 m located at shallow depths, where low average rupture speeds ~1.8–2 km/s are estimated. Outer-rise events, triggered within the oceanic Nazca plate after the mainshock, did not penetrate into the mantle and are related to preexisting faults due to both bending of Nazca plate and JFR uplift, which promotes a tensional stress regime in the surrounding area. This seismicity was triggered by static stress transfer from near-trench slip revealed by our source rupture modelling, suggesting outer-rise seismicity as a proxy for near-trench coseismic slip. Crustal seismicity within continental South American plate is observed prior to, and after, the mainshock, mainly related to extensional faulting within eroded and fractured wedge due to tectonic processes along erosive margins. We also show evidence of shallow seismicity after the mainshock associated with a long-lived crustal fault, which can represent a high seismic hazard for La Serena-Coquimbo conurbation.

Paleosensitivity of Hydrogen Isotope Ratios of Long‐Chain Alkenones to Salinity Changes at the Chile Margin

AbstractHydrogen isotope ratios of long‐chain alkenones (δ2HC37 ratios) have been shown to correlate with salinity in several culture studies. However, it is uncertain how applicable the δ2HC37‐salinity relationship is to reconstruct past salinity. The δ2HC37 ratios were measured on sediments from a drill core (ODP site 1234) at the Chilean Margin covering the last ~150 kyr. High δ2HC37 values corresponded to glacial time periods and interglacial periods are characterized by lower δ2HC37 values, aligning with δ18O ratios measured on planktonic foraminifera from the same core. Effects of parameters such as species composition, the δ2H‐δ18O relationship used for ice volume corrections, and nutrient‐ or light‐controlled growth rate, did not appear to significantly alter δ2HC37 ratios at ODP 1234. We used linear regression equations from batch culture experiments, marine surface sediments, and suspended particulate organic matter to quantitatively characterize salinity changes over the last ~150 kyr at ODP 1234. However, most of these equations yielded larger salinity shifts than previously suggested, a phenomenon also observed for other δ2HC37 records. This suggests that the paleosensitivity of δ2HC37 ratios to salinity was larger in the geologic record than has been observed in any modern environment or laboratory settings, or that glacial to interglacial salinity shifts might have been larger than currently believed.

Potential Instability of Gas Hydrates along the Chilean Margin Due to Ocean Warming

In the last few years, interest in the offshore Chilean margin has increased rapidly due to the presence of gas hydrates. We have modelled the gas hydrate stability zone off Chilean shores (from 33° S to 46° S) using a steady state approach to evaluate the effects of climate change on gas hydrate stability. Present day conditions were modelled using published literature and compared with available measurements. Then, we simulated the effects of climate change on gas hydrate stability in 50 and 100 years on the basis of Intergovernmental Panel on Climate Change and National Aeronautics and Space Administration forecasts. An increase in temperature might cause the dissociation of gas hydrate that could strongly affect gas hydrate stability. Moreover, we found that the high seismicity of this area could have a strong effect on gas hydrate stability. Clearly, the Chilean margin should be considered as a natural laboratory for understanding the relationship between gas hydrate systems and complex natural phenomena, such as climate change, slope stability and earthquakes.

Centennial‐Scale SE Pacific Sea Surface Temperature Variability Over the Past 2,300 Years

AbstractDetailed temperature reconstructions over the past 2,000 years are important for contextualizing modern climate change. The midlatitude SE Pacific is a key region in this regard in terms of understanding the climatic linkages between the tropics and southern high latitudes. Multicentennial timescale temperature variability remains, however, poorly understood, due to a lack of long, high‐temporal‐resolution temperature records from this region and from the southern high latitudes in general. We present a unique alkenone sea surface temperature (SST) record from 44°S on the southern Chilean margin in the SE Pacific spanning the last 2,300 years at decadal resolution. The record displays relatively large changes including a cooling transition from 14 to 12.5 °C between 1,100 and 600 cal yr BP, in line with other Chile margin SST records and coeval with Antarctic cooling. This cooling is attributable to reduced Southern Ocean deep convection, driven by a late Holocene sea‐ice increase in the Weddell Sea associated with increased El‐Niño Southern Oscillation variability. Superimposed on the late Holocene cooling, we observe multicentennial timescale SST variability, including relatively cool SSTs (12.5 °C) from 950 to 500 cal yr BP, corresponding to the Medieval Climate Anomaly, and warmer SSTs (13 °C) from 500 to 200 cal yr BP, corresponding to the Little Ice Age. These oscillations may reflect either multicentennial internal variability of the Southern Ocean deep convection and/or multicentennial variability in the phasing of El‐Niño Southern Oscillation and Southern Annular Mode events.

A Late Quaternary climate record based on long-chain diol proxies from the Chilean margin

Abstract. In this study we have applied different indices based on long-chain diols, i.e., the long-chain diol index (LDI) as a proxy for past SST, the diol index as an indicator of past upwelling conditions, and the nutrient diol index (NDI) as a proxy for nitrate and phosphate concentrations in seawater. The proxies were analyzed in marine sediments recovered at ODP Site 1234, located within the Peru–Chile upwelling system, with a ∼2 kyr resolution covering the last 150 kyr. We also generated TEX86H and U37K′ temperature and planktonic δ18O records, as well as total organic carbon (TOC) and accumulation rates (ARs) of TOC and lipid biomarkers (i.e., C37 alkenones, GDGTs, dinosterol, and loliolide) to reconstruct past phytoplankton production. The LDI-derived SST record covaries with TEX86H- and U37K′-derived SST records as well as with the planktonic δ18O record, implying that the LDI reflects past SST variations at this site. TOC and phytoplankton AR records indicate increased export production during the last interglacial (MIS 5), simultaneous with a peak in the abundance of preserved Chaetoceros diatoms, suggesting intensified upwelling during this period. The diol index is relatively low during the upwelling period, but peaks before and after this period, suggesting that Proboscia diatoms were more abundant before and after the period of upwelling. The NDI reveals the same trends as the diol index, suggesting that the input of nitrate and phosphate was minimal during upwelling, which is unrealistic. We suggest that the diol index and NDI should perhaps be considered as indicators for Proboscia productivity instead of upwelling conditions or nutrient concentrations.

High Gas Hydrate and Free Gas Concentrations: An Explanation for Seeps Offshore South Mocha Island

Recent studies have reported cold seeps offshore of Mocha Island. Gas hydrate occurrences along the Chilean margin could explain seeps presence. Gas-phase (gas hydrate and free gas) and geothermal gradients were estimated analysing two seismic sections. Close to Mocha Island (up to 20 km) were detected high (up to 1900 m/s) and low (1260 m/s) velocities associated with high gas hydrate (up to 20% of total volume) and free gas (up to 1.1% of total volume) concentrations, respectively. A variable and high geothermal gradient (65–110 °C/km) was obtained. These results are related to high supply of deep fluids canalised by faults and fractures. Faraway from Mocha Island (&gt;60 km), free gas concentrations decrease to 0.3% of total volume and low geothermal gradient (from 35 to 60 °C/km) are associated with low fluids supply. Finally, we propose gas hydrate dissociation processes as the main supply source for seeps in the vicinity of Mocha Island. These processes can be caused by: (a) active faults and seismic activity; and (b) warm fluid expulsion from deeper zones altering hydrate stability conditions. In both cases, gas hydrate dissociation could generate slope instability and landslides, as occurred in the past in this region and reported in the literature.

Modeling Trench Sediment‐Controlled Flow in Subduction Channels: Implications for the Topographic Evolution of the Central Andean Fore Arc

AbstractThe high elevation of the onshore fore‐arc platform in northern Chile cannot be accounted for by previously proposed tectonic mechanisms such as coastal underplating and coseismic deformation, whose topographic effects are restricted to the coastal zone. Subduction channels have been recognized both in modern and fossil noncollisional convergent margins, yet their role on fore‐arc surface elevation has not been sufficiently explored. Long‐term viscoelastic flow in a strengthened, finite‐thickness subduction channel promotes coupled offshore fore‐arc subsidence and onshore fore‐arc platform uplift. We propose that the onset of protracted hyperaridity in the coastal zone of northern Chile starved the trench of sediments, inducing a rise in shear stress at the top of the subduction channel sufficient to trigger hundreds of meters of uplift of the onshore fore‐arc basin surface, most of which took place by the middle Miocene. This is consistent with latitudinal correlations between coastal precipitation, trench sediment thickness and onshore fore‐arc topography along the Chilean margin, and with available paleoclimatic and paleotopographic evidence in northern Chile.

Subduction thermal structure, metamorphism and seismicity beneath north-central Chile

Massive M>8 earthquakes have repeatedly struck the coastal Chilean subduction megathrust at intervals of nearly ten years, and such tsunamigenic earthquakes have caused recurrent damage. A widely accepted mature seismic gap along the strongly coupled plate interface in northern Chile was filled unexpectedly by the occurrence of the 2014 Mw 8.2 Iquique earthquake, which occurred 130 years after the previous quake. This event revealed that our understanding of the state of seismogenesis along the Chilean margin is still poor. This study aims at investigating the temperature and water content distributions within the Nazca slab along a large portion of the Chilean margin. We employ 3-D time-dependent thermomechanical numerical models that incorporate up-to-date slab geometry and phase diagrams of hydrous minerals in the subducting plate to study the north-central Chilean (19 °S–33 °S) coupled megathrust, including the region where the slab is subhorizontal (28 °S–32 °S). Our modeling results show that most shallow off- and onshore interplate earthquakes in north-central Chile in the last decade have been mainly controlled by the phase transformation from lawsonite blueschist to amphibolite at shallow depths. On the other hand, some of the deep subduction seismicity is in good agreement with eclogitization within the oceanic crust and formation of harzburgite in the ultramafic rock in the lower slab. The M > 6 interplate earthquakes are estimated to occur where our models predict slab temperatures less than 400–600 °C and slab dehydration less than 0.03–0.05 wt%/km. Slow slip events tend to occur in regions where interplate temperatures are 400–600 °C (0.03–0.05 wt%/km). The activity level of regional interplate earthquakes and slow slip events is interpreted to be affected by the fluid release from and the temperature distribution in the Nazca slab.