Peru-Chile Trench Research Articles

Two-Dimensional Geothermal Model of the Peruvian Andes above the Nazca Ridge Subduction

The aseismic Nazca Ridge produces localized flat-slab subduction beneath the South American margin at latitudes 10° to 15° S. The geological evolution and the spatio-temporal pattern of deformation of the upper plate have been strongly influenced by the presence of the flat slab. In this study, we investigated the lithospheric thermal structure of this region by elaborating a 2D geothermal model along a section across the top of the Nazca Ridge, the Peru–Chile trench, the Andean Cordillera, and the Amazonian Basin, for a total length of 1000 km. For the sake of modelling, the crust of the overriding plate was subdivided into two parts, i.e., a sedimentary cover (including the entire lithostratigraphic sequence) and a crystalline basement. Applying an analytical methodology, we calculated geotherms and isotherms by setting (i) thickness, (ii) density, (iii) heat production, and (iv) thermal conductivity for each geological unit and considering (v) heat flux at the Moho, (vi) frictional heating produced by faults, and (vii) plate convergence rate. The resulting model could make a significant advance in our understanding of how flat slab geometry associated with the Nazca Ridge subduction affects the thermal structure and hence the tectonic evolution of the region.

Apatite (U–Th)/He thermochronology and Re–Os ages in the Altar region, Central Andes (31°30′S), Main Cordillera of San Juan, Argentina: implications of rapid exhumation in the porphyry Cu (Au) metal endowment and regional tectonics

Altar is a large porphyry Cu (Au) deposit located in the Main Cordillera of Argentina, 20 km to the north of the giant Los Pelambres–El Pachon porphyry copper cluster, at the southern portion of the Pampean flat-slab segment of the Andes. Although this region hosts telescoped porphyry-epithermal deposits, the precise temporal relationship between porphyry emplacement, mineralization, cooling, and regional orogenic uplift are still poorly understood. New Re–Os molybdenite ages indicate that Altar orebodies are associated with two magmatic hydrothermal centers: Altar East (11.16 ± 0.06 Ma) and Altar Central (10.38 ± 0.05 Ma) formed at temporally distinct periods. New (U–Th)/He ages from the Early Permian and Late Eocene plutons, and the Middle Miocene subvolcanic stocks associated with Cu–Au mineralization of the Altar region reflect a rapid cooling pulse during the Middle Miocene (15.02 to 10.66 Ma) coeval with a major phase of tectonic shortening and regional uplift. The main pulse of rapid cooling and related tectonic uplift in the Altar region was synchronous with the formation of the hydrothermal systems and resulted in an increased focused metal endowment (Au–Cu grades) due to the telescoping of epithermal mineralization over the rapidly uplifted porphyry system. This 11–10 Ma tectonically triggered exhumation event coincides with the collision of the E-trending segment of the Juan Fernandez Ridge with the Peru–Chile trench, at this latitude. Collision and ensuing ridge subduction may have driven a localized pulse of rapid cooling and exhumation of the Main Cordillera that has not been well documented to the north or south of the Altar–Los Pelambres region.

Geomorphological modelling and mapping of the Peru-Chile Trench by GMT

Abstract The author presents a geospatial analysis of the Peru-Chile Trench located in the South Pacific Ocean by the Generic Mapping Tool (GMT) scripting toolset used to process and model data sets. The study goal is to perform geomorphological modelling by the comparison of two segments of the trench located in northern (Peruvian) and southern (Chilean) parts. The aim of the study is to perform automatic digitizing profiles using GMT and several scripting modules. Orthogonal cross-section profiles transecting the trench in a perpendicular direction were automatically digitized, and the profiles visualized and compared. The profiles show variations in the geomorphology of the trench in the northern and southern segments. To visualize geological and geophysical settings, a set of the thematic maps was visualized by GMT modules: free-air gravity anomaly, geoid, geology and bathymetry. The results of the descriptive statistical analysis of the bathymetry in both segments show that the most frequent depths for the Peruvian segment of the Peru-Chile Trench range from -4,000 to -4,200 (827 recorded samples) versus the range of -4,500 to -4,700 m for the Peruvian segment (1,410 samples). The Peruvian segment of the trench is deeper and its geomorphology steeper with abrupt slopes compared to the Chilean segment. A comparison of the data distribution for both segments gives the following results. The Peruvian segment has the majority of data (23%) reaching 1,410 (-4,500 m to -4,700 m). This peak shows a steep pattern in data distribution, while other data in the neighbouring diapason are significantly lower: 559 (-4,700 m to -5,000 m) and 807 (-4,200 m to -4,400 m). The Chilean segment has more unified data distribution for depths of -6,000 m to -7,000 m. This paper presents GMT workflow for the cartographic automatic modelling and mapping deep-sea trench geomorphology.

Microplastics and synthetic particles ingested by deep-sea amphipods in six of the deepest marine ecosystems on Earth.

While there is now an established recognition of microplastic pollution in the oceans, and the detrimental effects this may have on marine animals, the ocean depth at which such contamination is ingested by organisms has still not been established. Here, we detect the presence of ingested microplastics in the hindguts of Lysianassoidea amphipod populations, in six deep ocean trenches from around the Pacific Rim (Japan, Izu-Bonin, Mariana, Kermadec, New Hebrides and the Peru-Chile trenches), at depths ranging from 7000 m to 10 890 m. This illustrates that microplastic contaminants occur in the very deepest reaches of the oceans. Over 72% of individuals examined (65 of 90) contained at least one microparticle. The number of microparticles ingested per individual across all trenches ranged from 1 to 8. The mean and standard error of microparticles varied per trench, from 0.9 ± 0.4 (New Hebrides Trench) to 3.3 ± 0.7 (Mariana Trench). A subsample of microfibres and fragments analysed using FTIR were found to be a collection of plastic and synthetic materials (Nylon, polyethylene, polyamide, polyvinyl alcohol, polyvinylchloride, often with inorganic filler material), semi-synthetic (rayon and lyocell) and natural fibre (ramie). Notwithstanding, this study reports the deepest record of microplastic ingestion, indicating that anthropogenic debris is bioavailable to organisms at some of the deepest locations in the Earth's oceans.

Geographic and bathymetric comparisons of trace metal concentrations (Cd, Cu, Fe, Mn, and Zn) in deep-sea lysianassoid amphipods from abyssal and hadal depths across the Pacific Ocean

Spatial patterns in trace metal (Cd, Cu, Fe, Mn, and Zn) bioavailability were in analysed in deep-sea lysianassoid amphipods (Eurythenes gryllus and Hirondellea spp) from three subduction trenches; the Izu-Bonin (8000–9000 m), Kermadec (3000–10,000 m) and Peru-Chile trenches (4500–6000 m). Geographical differences in metal concentrations were evident. Iron and Mn concentrations were higher in H. gigas from the Izu-Bonin Trench compared to the H. dubia from the Kermadec Trench. Copper and Cd were higher in the E. gryllus in the Peru-Chile Trench compared to Kermadec Trench. There were significant interactions between trench and depth for a number of metals. This was evident in the tissues of the genus Hirondellea where there was an interaction between trench and depth for Cu and Zn. Both these metals were found in higher concentrations at approximately 8000 m in the Izu-Bonin Trench compared to the same depth in the Kermadec Trench. At deeper depths, however, the opposite occurred. An interaction between trench and depth also occurred for Fe and Mn in E. gryllus where Fe and Mn were found in higher concentrations at approximately 4500 m in the Kermadec Trench compared to the Peru-Chile Trench but the opposite was true at deeper depths. This indicated that the relation between metals and depth were not consistent over the depth ranges sampled. Cadmium in H. dubia from Kermadec Trench was the only metal that decreased in concentration across the depths sampled whereas Mn and Zn increased in concentrations with depth within this species and trench. The high concentrations of Cd within these amphipods suggested that the Cd-anomaly observed in polar amphipods could potentially be extended to deep-sea amphipods. Furthermore the low levels of Cu in E. gryllus may indicate Cu-deficiencies. These results indicated a complex relationship between depth and trench and metal concentrations in amphipods with Fe, Mn and Zn largely reflecting the concentrations in the environment.

Analysis and validation of the PTVA tsunami building vulnerability model using the 2015 Chile post-tsunami damage data in Coquimbo and La Serena cities

Abstract. Chile is highly exposed to tsunami hazard from large earthquakes often occurring along the Peru–Chile trench, like the 16 September 2015 event. However, only recently has tsunami hazard been considered in the land-use policies of the Chilean coast. These new regulations must enforce the identification of the most vulnerable sectors of the Chilean coastal cities. This paper analyses and validates the two latest versions of the Papathoma Tsunami Vulnerability Assessment (PTVA) model in the 2015 tsunami reconstructed scenario in the cities of La Serena and Coquimbo. Both models result in a similar number of very high and high relative vulnerability index (RVI) scores. However, the less vulnerable categories do not show a similar trend and the PTVA-4 model obtains a larger number of minor and average RVI scores. When compared to the damages caused by the tsunami, the PTVA-3 shows a more similar distribution to the actual damages than that obtained by the PTVA-4 model, which shows a more concentrated distribution of the RVI scores. These results suggest this version of the model should be used in Chilean coastal cities in future land-use or mitigation planning.

Community structure and diversity of scavenging amphipods from bathyal to hadal depths in three South Pacific Trenches

There are few biological datasets that span large bathymetric ranges with sufficient resolution to identify trends across the abyssal and hadal transition zone, particularly over multiple trenches. Here, scavenging Amphipoda were collected from three trenches in the South Pacific Ocean at bathyal to hadal depths. Diversity and community structure were examined from stations within the Kermadec Trench (1490–9908m) and New Hebrides Trench (2000–6948m) and additional data were included from the South Fiji Basin (4000m) and Peru-Chile Trench (4602–8074m). The hadal community structure of the Kermadec and New Hebrides trenches were distinct from the surrounding abyssal and bathyal depths and correlated to hydrostatic pressure and POC flux. Low POC flux in the New Hebrides Trench and South Fiji Basin best explained the dissimilarity in abyssal community structure from those of the disparate Kermadec and Peru-Chile trenches. POC flux also best explained patterns in hadal community structure with the Kermadec and New Hebrides Trench communities showing greater similarity to each other than to the eutrophic Peru-Chile Trench. Hydrostatic pressure was the strongest driver of intra-trench assemblage composition in all trench environments. A unimodal pattern of species diversity, peaking between 4000 and 5000m, was best explained by hydrostatic pressure and temperature.

Shallow and deep-sea chitons of the genus Leptochiton Gray, 1847(Mollusca: Polyplacophora: Lepidopleurida) from Peruvian and Chilean waters.

The Southeast region off Chile and Peru has yielded a very rich diverse fauna of basal chitons of the genus Leptochiton. The present contribution is based on the study of 1055 specimens of chitons. Thirteen leptochitonid species are reported of which seven species are new, namely Leptochiton lascrusesi n. sp., L. linseae n. sp., L. longibranchiae n. sp., L. peruvianus n. sp., L. macleani n. sp., L. sigwartae n. sp., and L. mutschkeae n. sp. Five species were found on the shelf, but only one of those is limited to this zone. The other eight species live in bathyal or abyssal depths. Thus, 12 of 13 species were found deeper than 200 m. The genus Leptochiton originated in the late Palaeozoic in shallow waters. It probably shifted to deep-waters because of competition by more advanced genera of chitons. The leptochitonid fauna of the Peru-Chile Trench turned out to be rich. Five species were studied from this trench system. Two of them-L. longibranchiae n. sp. and L. peruvianus n. sp.-are characterized by an unusually high number of gills and an accordingly wider distribution in the mantle cavity, reaching in an anterior direction to valves V and III. This morphological peculiarity is unusual for Lepidopleurida and resembles the conditions found in the order Chitonida.

Geological evidence for an unusually large tsunami on the Pacific coast of Aomori, northern Japan

To assess long-term tsunami inundation history, we studied a wetland on the Pacific coast of Aomori, Japan, at the northern end of the Japan Trench. We found five sand sheets interbedded in freshwater mud and peat, three of which contained brackish diatoms indicating deposition by marine inundation. We identified the youngest sand sheet, deposited AD 1480–1770, as a tsunami deposit based on its lateral extent (>1 km) from the shoreline at the time of its deposition. Although this area has been struck by many tsunamis generated by earthquakes along the Japan Trench as well as along the Kuril and Peru–Chile trenches, no tsunami recorded at least in the past 120 years has inundated the coast as far inland as this tsunami deposit is distributed. We therefore infer that it was deposited by an unusually large tsunami. Historical and geological evidence for tsunamis in north-east Japan suggests that possible sources are the AD 1611 Keicho earthquake along the Japan Trench, a 17th-century earthquake along the Kuril Trench, or an unknown large earthquake.

The giant coastal landslides of Northern Chile: Tectonic and climate interactions on a classic convergent plate margin

Documented for the first time are an extensive suite of late Neogene giant terrestrial coastal landslides along the classic convergent margin of western South America (18° to 24° south). These are remarkable in terms of their unusual abundance and atypical setting, such failures previously being linked with oceanic volcanic edifices or over-steepened glaciated coastlines. Located within the hyper-arid Coastal Cordillera of the Atacama Desert of Northern Chile we report the presence of more than 60 individual large-scale landslides with individual volumes up to 9 km3 developed over a horizontal coastline distance of some 650 km. These landslides were emplaced as a combination of rock avalanches and multiple rotational failures. The majority terminated directly into the Pacific – likely generating significant tsunami hazard to the Chilean and south Peruvian coastline in a region which is today considered to be part of a notorious seismic gap. The proliferation and scale of these Late Neogene giant landslides in this actively uplifting, hyperarid terrain suggests they are the main geomorphic agent for relief reduction, probably triggered by megathrust earthquakes and potentially providing a unique palaeoseismic archive. The temporal and spatial distribution of these giant landslides corresponds with a period of surface steepening of the forearc wedge in the Central Andes and south to north differential uplift associated with factors such as aseismic ridge subduction. The resulting surface gradient increases, combined with the persistent climatic aridity of the region, have served to limit effective relief-reducing geomorphic processes in this oversteepened terrain to large-scale landsliding. The phenomena documented here geospatially link previously recognised large-scale slope failures from the off-shore environment and higher altitude areas of the Andean forearc, suggesting that large-scale landsliding is capable of transferring sediment on a regional scale to the off-shore Peru–Chile trench. This has implications for the friction of the subducting Nazca plate and associated seismicity and uplift.

Chilean flat slab subduction controlled by overriding plate thickness and trench rollback

How flat slab geometries are generated has been long debated. It has been suggested that trenchward motion of thick cratons in some areas of South America and Cenozoic North America progressively closed the asthenospheric wedge and induced flat subduction. Here we develop time-dependent numerical experiments to explore how trenchward motion of thick cratons may result in flat subduction. We find that as the craton approaches the trench and the wedge closes, two opposite phenomena control slab geometry: the suction between ocean and continent increases, favoring slab flattening, while the mantle confined within the closing wedge dynamically pushes the slab backward and steepens it. When the slab retreats, as in the Peru and Chile flat slabs, the wedge closure rate and dynamic push are small and suction forces generate, in some cases, flat subduction. We model the past 30 m.y. of subduction in the Chilean flat slab area and demonstrate that trenchward motion of thick lithosphere, 200–300 km, currently ∼700–800 km away from the Peru-Chile Trench, reproduces a slab geometry that fits the stress pattern, seismicity distribution, and temporal and spatial evolution of deformation and volcanism in the region. We also suggest that varying trench kinematics may explain some differing slab geometries along South America. When the trench is stationary or advances, the mantle flow within the closing wedge strongly pushes the slab backward and steepens it, possibly explaining the absence of flat subduction in the Bolivian orocline.

On the relation between lithospheric strength and ridge push transmission in the Nazca plate

The ridge push force and the total lithospheric strength of the Nazca plate are compared along an East-West transect from the East Pacific Rise to the Peru–Chile trench at latitude 12°S. The thermal structure of the plate is estimated from the plate cooling model and constrained by heat flow, bathymetry, and geoid height data. The best fitting thermal model has a basal temperature of ∼1600 K and an asymptotic plate thickness (not reached because of the relatively young age of the plate at the trench) of ∼101 km. The ridge push force, also determined from the plate cooling model, is of the order of 1.5 TN m −1 at the trench. The total lithospheric strength as a function of age is estimated for a possible range of conditions (compressional/extensional intraplate tectonic regime, wet/dry rheology). A comparison of ridge push force with lithospheric strength, extended beyond the Nazca plate by considering different spreading rates and ages, shows that oceanic plates with dry rheology have strengths higher than the ridge push force at any age if the tectonic regime is compressional, and comparable if the regime is extensional. On the other hand, oceanic plates with wet rheology have strengths lower than the ridge push force, especially if the tectonic regime is extensional. Therefore, if the rheology is wet and mantle drag at the base of the plate is sufficiently strong, the ridge push force may result in intraplate deformation and be partly dissipated within the plate.

Deep ocean species discovered

One of the world's deepest ocean trenches has revealed surprises. Nigel Williams reports. One of the world's deepest ocean trenches has revealed surprises. Nigel Williams reports. A new species has been discovered in one of the most remote and inaccessible parts of the ocean previously thought to be entirely free of fish. The new type of snailfish was found living at a depth of 7,000 metres in the Peru-Chile trench of the south-east Pacific Ocean. Mass groupings of cusk-eels and large crustacean scavengers were also discovered living at these depths for the first time, the researchers said. The findings, in one of the deepest parts of the ocean on the planet, were made by a team of marine biologists from the University of Aberdeen, New Zealand and Japan. The team used deep-sea imaging technology to take 6,000 pictures at depths between 4,500 metres and 8,000 metres within the trench. Alan Jamieson, who led the expedition said: “Our findings, which revealed diverse and abundant species at depths previously thought to be void of fish, will prompt a rethink into marine populations at extreme depths.” This expedition was prompted by findings in 2008 and 2009 off Japan and New Zealand when the researchers discovered a new species of snailfish inhabiting trenches there with each trench home to its own species of the fish. “To test whether these species would be found in all trenches, we repeated our experiments on the other side of the Pacific Ocean, some 6,000 miles from our last observations,” said Jamieson. During the three-week expedition of the research vessel Sonne, the researchers used state-of-the-art deep-sea imaging technology, including an ultra-deep free-falling baited camera system, to take images deep into the trench. A species of cusk eel gathered at the camera and fed on the bait for the full day of deployment. “Further research needs to be conducted to decipher whether this is an entirely new species of cusk eel that we have discovered,” the researchers said. The expedition was the seventh to take place as part of Hadeep, a collaborative research project between the University of Aberdeen's Oceanlab, the University of Tokyo's Ocean Research Institute and with support from New Zealand's National Institute of Water and Atmospheric Research (Niwa). The Hadeep team has been investigating extreme depths across the globe for three years. The results have been photographs of the world's deepest-living fish for the first time. The team also discovered a species of amphipod crustacean scavengers which they previously did not know existed at such depths and in such great numbers. Niamh Kilgallen, an amphipod expert from Niwa said: “The sheer abundance of these big amphipods was overwhelming, particularly at 7,000–8,000 metres, which is much deeper than they have been found in any other trench. It begs the question of why and how they can live so deep in this trench but not in any other.” These findings “prompt a re-evaluation of the diversity and abundance of life at extreme depths,” said Jamieson. “Each of the deep trenches across the globe hosts a unique assembly of animals which can differ greatly from trench to trench. The immense isolation of each trench draws parallels with island evolution popularised by Darwin's finches,” he said.

Origin of the regional stress field along the Liquine-Ofqui Fault Zone (LOFZ), Southern Chilean Andes by means of FE simulation

The Liquine-Ofqui Fault Zone (LOFZ) of southern Chilean Andes is one of the largest active strike-slip fault zones. There is an ongoing debate regarding the origin of the stress field along the LOFZ due to its complex geometry. This paper represents a study of the origins of the LOFZ regional stress field. Stress fields are calculated by finite element (FE) analysis. The two possible stress origins, i.e., oblique plate convergence and ridge collision/indenter tectonics of Chile ridge against Peru-Chile trench, have been emphasized in the present study. Three types of boundary conditions for the three particular models have been applied to calculate stress fields. Models are assumed to be elastic and plane stress condition. Modeling results are presented in terms of four parameters, i. e., orientation of maximum horizontal stress (sigma (H max)), displacement vector, strain distribution, and maximum shear stress (tau (max)) contour line within the model. The results of the first model with oblique plate convergence show inconsistency between the geometric shape of the LOFZ and the distribution of the four parameters. Although more realistic results are obtained from the second model with normal ridge collision, there are few coincident in the LOFZ geometry and regional stress field. The third model with normal and oblique ridge collision is reasonable in understanding the origin of stress field and geometrical condition in the lithosphere of the LOFZ.

Dextral shear, terrane accretion and basin formation in the Northern Andes: best explained by interaction with a Pacific-derived Caribbean Plate?

Abstract The structure, stratigraphy and magmatic history of northern Peru, Ecuador and Colombia are only adequately explained by Pacific-origin models for the Caribbean Plate. Inter-American models for the origin of the Caribbean Plate cannot explain the contrasts between the Northern Andes and the Central Andes. Persistent large magnitude subduction, arc magmatism and compressional deformation typify the Central Andes, while the Northern Andes shows back-arc basin and passive margin formation followed by dextral oblique accretion of oceanic plateau basalt and island arc terranes with Caribbean affinity. Cretaceous separation between the Americas resulted in the development of a NNE-trending dextral–transpressive boundary between the Caribbean and northwestern South America, becoming more compressional when spreading in the Proto-Caribbean Seaway slowed towards the end of the Cretaceous. Dextral transpression started at 120–100 Ma, when the Caribbean Arc formed at the leading edge of the Caribbean Plate as a result of subduction zone polarity reversal at the site of the pre-existing Trans-American Arc, which had linked to Central America to South America in the vicinity of the present-day Peru–Ecuador border. Subsequent closure of the Andean Back-Arc Basin resulted in accretion of Caribbean terranes to western Colombia. Initiation of flat-slab subduction of the Caribbean Plate beneath Colombia at about 100 Ma is associated with limited magmatism, with no subsequent development of a magmatic arc. This was followed by northward-younging Maastrichtian to Eocene collision of the trailing edge Panama Arc. The triple junction where the Panama Arc joined the Peru–Chile trench was located west of present-day Ecuador as late as Eocene time, and the Talara, Tumbes and Manabi pull-apart basins directly relate to its northward migration. Features associated with the subduction of the Nazca Plate, such as active calc-alkaline volcanic arcs built on South American crust, only became established in Ecuador, and then Colombia, as the triple junction migrated to the north. Our model provides a comprehensive, regional and testable framework for analysing the as yet poorly understood collage of arc remnants, basement blocks and basins in the Northern Andes.

Transcurrencia a lo largo de la Falla Sierra de Varas (Sistema de fallas de la Cordillera de Domeyko), norte de Chile

Strike-slip along the Sierra de Varas Fault (Cordillera de Domeyko Fault-System), northern Chile. The north-south trending Cordillera de Domeyko Fault System in northern Chile considered herein is parallel to the Peru-Chile trench. The displacement history of the Sierra de Varas Fault, a master fault of the mentioned system, was examined in detail in the Aguada del Hornito-Aguada del Cerro Alto de Varas segment. Upper Paleozoic granitoids of the same composition, internal structure and age were cut and displaced by the fault. A sinistral horizontal separation of 15.6±1 km with a vertical component of 4.9±0.1 km, suggests a sinistral-reverse net displacement of 16.4±1 km. This is consistent with the local stratigraphic section that was eroded from the eastern block. A kinematic and dynamic analysis of mesofaults spatially related to the SVF displacements was conducted to identify the different fault populations and to obtain the stress tensor. Two structural systems were identifi ed: an early reverse-strike-slip system and a late dextral superposed system. The fi rst one ocurred during late middle Eocene, and the second is post-Miocene with an horizontal displacement of 0.6 km. The presence of coeval strike-slip displacements along the Sierra de Varas Fault and reverse displacements in a 'reverse fl ower' in the studied segment show that the structural evolution of the Sierra de Varas was dominated by a bulk transpression during the late middle Eocene. The left-lateral displacement here demonstrated for the Sierra de Varas Fault and its infl ection to the SE, south of the Aguada del Cerro Alto de Varas are compatible with the westward vergence of the folds and reverse faults in the El Profeta fault-and-thrust belt, which should be also the result of the transpression.

Structural control of the emplacement of the Portrerillos porphyry copper, central Andes of Chile

The Potrerillos porphyry is located in the Late Eocene–Oligocene metallogenic belt of porphyry copper deposits of Chile, Cordillera de Domeyko, central Andes. It is formed of a porphyritic biotite–hornblende monzonite that was emplaced at less than 1000 m depth. The intrusion has a cylindrical vertical form at depth, whereas its uppermost section resembles a mushroom whose stem is inclined 20–25° to the northwest. The location of the Potrerillos porphyry coincides with a major facies and east–west thickness change in Jurassic to Late Cretaceous marine sedimentary and Tertiary volcanic host rocks. Structurally, the porphyry was emplaced at the intersection between the northeast-oriented Potrerillos fold-and-thrust belt, and a system of northwest-trending strike-slip faults of the Ciénaga fault system. Both structural elements are related to a sinistral regional transpression that was active during the Middle to Late Eocene. The north–northeast trending Sierra Castillo-Agua Amarga fault system was trench-linked with an oblique subduction along the Peru–Chile trench in the southwestern part of South America at this time. One of the main structural elements of the Potrerillos fold-and-thrust belt is the eastward verging Potrerillos Mine reverse fault. We demonstrate a syntectonic relationship between the Potrerillos porphyry emplacement and the onset of the Potrerillos Mine fault. The control at the shallowest levels of the Potrerillos porphyry emplacement was studied by means of a structural analysis of folds, cleavage and deformed oolites in the country rock. This analysis suggests that the porphyry occupied the core of an anticline. The form of this anticline was modeled as a fault-propagation-fold taking into account a 200 m displacement along the basal fault and a local shortening of 45.5% occurred. This model is compatible with a limited displacement along the Potrerillos Mine fault under hot, ductile conditions. The remaining displacement of 1600 m would have taken place afterwards under cold, brittle conditions. The Potrerillos porphyry was emplaced during a transpressional regime within the Potrerillos fold-and-thrust belt. The fact that the cleavage does not show a concentric distribution around the Potrerillos porphyry, discounts the possibility of a forced emplacement of the porphyry, pushing aside its wall rocks. This is reinforced by the deformed oolites which do not show a strain gradient from outside towards the edge of the body. The clockwise transection of the fold axes by the cleavage and the southeast direction of displacement along the Potrerillos Mine fault are interpreted as related to the regional sinistral transpression related to the Sierra Castillo Fault movement.

Sedimentation and preservation of the Miocene Atacama Gravels in the Pedernales–Chañaral Area, Northern Chile: Climatic or tectonic control?

In recent years, longitudinal changes on the thin/thick-skinned tectonic styles of the Central Andes has been intensively discussed while other studies have considered the role of mass transfers on the unloading of the orogen, and on the stress regime along the plate interface arising from changes on the volume of sediment arriving into the Peru–Chile trench. The search for paleo-climate records is therefore crucial for our understanding of the history of the Central Andes. In this paper, we focus on the Atacama Gravels, an extensive blanket of Miocene continental deposits filling a Neogene paleo-valley system along the southern Atacama Desert in northern Chile. An east–west transect, between Pedernales and Chañaral (26°30′S), enabled us to carry out a sedimentological and tectonic study of the Atacama Gravels, based on logging and field observations along the Rio Salado canyon. New 39Ar– 40Ar ages obtained on intercalated and overlying ignimbrites date the beginning of the Atacama Gravels sedimentation at around the Oligocene–Miocene boundary and cessation of sedimentation in the Late Miocene. Thirteen lithofacies, included within five facies associations (A1 to A5) were identified. Depositional environments vary from proximal alluvial fan (A1, A2) in the Precordillera through ephemeral fluvial (A3, A4) to distal playa lake (A5) in the Coastal Cordillera. No evidences of synsedimentary deformation have been found, showing that the change from sediment removal to sediment preservation cannot be explained by tectonic causes, and climate change appears to be the dominant controlling factor of sediment preservation. A progressive change from semi-arid towards hyper-arid climatic conditions during the Miocene, led to a reduction on the transport capacity of the fluvial system and sediment preservation along the paleo-valley system formed during the Oligocene.

Relocation of a seafloor transponder—Sustaining the GPS-Acoustic technique

Abstract Rigid seafloor arrays of three to four precision acoustic transponders have been repeatedly positioned with the GPS-Acoustic technique to measure horizontal plate motion. In the event that one transponder becomes inactive, a replacement transponder must be precisely located relative to the existing array. Here we present a technique to determine the geodetic azimuth and baseline between the inactive and replacement transponders. We include three examples of relocations between 2002 and 2003 on the Juan de Fuca plate and near the Peru-Chile trench, which add ±16–29 mm uncertainty to the GPS-Acoustic estimated position. A simulation of optimal network geometry shows that the relocation’s contribution to uncertainty can be as low as ±10 mm.

Magma evolution of Quaternary minor volcanic centres in southern Peru, Central Andes

Minor centres in the Central Volcanic Zone (CVZ) of the Andes occur in different places and are essential indicators of magmatic processes leading to formation of composite volcano. The Andahua–Orcopampa and Huambo monogenetic fields are located in a unique tectonic setting, in and along the margins of a deep valley. This valley, oblique to the NW–SE-trend of the CVZ, is located between two composite volcanoes (Nevado Coropuna to the east and Nevado Sabancaya to the west). Structural analysis of these volcanic fields, based on SPOT satellite images, indicates four main groups of faults. These faults may have controlled magma ascent and the distribution of most centres in this deep valley shaped by en-echelon faulting. Morphometric criteria and 14C age dating attest to four main periods of activity: Late Pleistocene, Early to Middle Holocene, Late Holocene and Historic. The two most interesting features of the cones are the wide compositional range of their lavas (52.1 to 68.1 wt.% SiO2) and the unusual occurrence of mafic lavas (olivine-rich basaltic andesites and basaltic andesites). Occurrence of such minor volcanic centres and mafic magmas in the CVZ may provide clues about the magma source in southern Peru. Such information is otherwise difficult to obtain because lavas produced by composite volcanoes are affected by shallow processes that strongly mask source signatures. Major, trace, and rare earth elements, as well as Sr-, Nd-, Pb- and O-isotope data obtained on high-K calc-alkaline lavas of the Andahua–Orcopampa and Huambo volcanic province characterise their source and their evolution. These lavas display a range comparable to those of the CVZ composite volcanoes for radiogenic and stable isotopes (87Sr/86Sr: 0.70591–0.70694, 143Nd/144Nd: 0.512317–0.512509, 206Pb/204Pb: 18.30–18.63, 207Pb/204Pb: 15.57–15.60, 208Pb/204Pb: 38.49–38.64, and δ 18O: 7.1–10.0‰ SMOW), attesting to involvement of a crustal component. Sediment is absent from the Peru–Chile trench, and hence cannot be the source of such enrichment. Partial melts of the lowermost part of the thick Andean continental crust with a granulitic garnet-bearing residue added to mantle-derived arc magmas in a high-pressure MASH [melting, assimilation, storage and homogenisation] zone may play a major role in magma genesis. This may also explain the chemical characteristics of the Andahua–Orcopampa and Huambo magmas. Fractional crystallisation processes are the main governors of magma evolution for the Andahua–Orcopampa and Huambo volcanic province. An open-system evolution is, however, required to explain some O-isotopes and some major and trace elements values. Modelling of AFC processes suggests the Charcani gneisses and the local Andahua–Orcopampa and Huambo basement may be plausible contaminants.