Nazca Plate Research Articles

Overlapping slabs: Untangling subduction in NW South America through finite-frequency teleseismic tomography

Both the Caribbean and Nazca plates subduct beneath northwestern South America. The configuration of the two subducted slabs and the nature of any interaction between them has long been a matter of debate. Based on the location of intermediate-depth seismicity and active and extinct volcanism, as well as on seismic imaging, several different tectonic scenarios have been proposed. In this paper, we use teleseismic data recorded by the Colombian National Network and the temporary CARMA array in Venezuela and Colombia to produce a finite-frequency tomography model for the region. Our results show several distinct subduction segments. Through synthetic tests, we show that our results require a zone of overlap between Nazca and Caribbean subduction north of the “Caldas Tear” as has been proposed by previous studies. Additionally, we find that the Bucaramanga Nest occurs within the Caribbean Plate and coincides with bending of the slab in two planes, where both the strike and the dip of the slab change. We infer that elevated stresses are an important factor in producing the very high rates of seismicity in the nest.

Increased megathrust shear force drives topographic uplift in the Colombian coastal forearc

The subsidence/uplift patterns of forearc basins are controlled by subduction dynamics and are strongly dependent on the nature of the margin (erosive vs. accretionary). Here, we explore the role that increased megathrust shear force plays in triggering and sustaining the short wavelength topography of coastal forearc regions at erosive margins, where subsidence is supposed to prevail. The northern Colombian coastal forearc contains a ~ 1000 m-high mountain range (Baudó Range) that abruptly stands-up for ~300 km along the northwestern Pacific margin of South America, where the buoyant Nazca plate is subducting at a shallow angle. Field observations and stratigraphic data of Cretaceous to late Neogene volcanic and sedimentary rocks, suggest that the Baudó Range has been uplifting since the middle-late Miocene. The uplift seems to continue nowadays as evidenced by the irregular cliffed and bluffed coastline, steep slopes and high topographic ruggedness in one of the wettest regions on Earth, where weathering and landscape smoothing is expected to be intense. Earthquake focal mechanisms, together with regional-scale N-S-trending sinistral reverse fault structures, west-verging folding, and NW- and NE-oriented topographic lineaments, seems to be related to a trench-perpendicular mostly compressional stress regime. The residual topography along the coastal range of the northern Colombian forearc indicates under-compensation, which may suggest that the relatively thin (~20 km-thick) crust cannot support isostatically the observed topography. We propose that increased shear force along a wide seismogenic zone linked to the shallow subduction of the Nazca plate and a poorly lubricated interface, due to limited sediment dispersal into the trench, drive the ongoing topographic uplift of the region. In contrast, the smoother relief and lower elevations in the southern Pacific coast of Colombia could be explained by a deficit of shear force available to support higher topography.

Effect of the cold Nazca Slab on the depth of the 660 km discontinuity in South America

The Nazca plate subducting beneath South America makes the 660 km discontinuity deeper and the mantle transition zone (MTZ) thicker under the continent. MTZ variations are often associated with mantle temperature and, therefore, can help confirm the slab position at greater depths. Recent P- and S-wave tomography results show the Nazca plate, near 20° S, being held below the MTZ for longitudes between 70° W and 55° W. We used 63,809 P-wave receiver functions from 1216 stations (using the LQT components of the incident ray system) to image the MTZ in South America. The receiver functions were corrected for move-out, stacked in cells of 3° x 3° degrees every 1° x 1°. We obtained 54,389 RF traces imaging the 410 km and the 660 km discontinuities. The discontinuity times were corrected using the SL2013 global tomography model to obtain depths. A thickened MTZ follows the trend of the Nazca plate beneath the sub-Andes. To the north of 18oS the thickened MTZ is only about 250 km wide; to the south, the thickened zone reaches up to a 1100 km width. This observation clearly indicates that the Nazca slab flattens close to the 660 km discontinuity lowering the mantle temperature and thickening the MTZ. The 660 km discontinuity is more affected than the 410 km, which is consistent with the Nazca slab being held just below the MTZ and not inside the MTZ in most of the region.

Exploring the Dynamics of Global Plate Motion Based on the Granger Causality Test

The geodynamic mechanism is the research focus and core issue of plate motions and plate tectonics. Analyzing the time series of earthquakes may help us understand the relationship between two plate boundaries and further explore movement mechanisms. Therefore, this paper uses earthquake event data and the Granger causality test method to quantitatively analyze the interaction and energy transfer relationship of plate boundaries from the viewpoint of statistics. The paper aims to explore the relationship between the pull effect and the push effect of plate motion and to provide knowledge to explore seismic energy transfer relationships, and even to predict earthquakes: (1) The directions of the global plate motion field are opposite to the directions of Granger causality between plate boundaries of the Pacific, Nazca, African, Australian, Eurasian, and Philippine plates. (2) The slab-pull force (not limited to the subduction force of the ocean plates) provides a main driving force for plate motions in the Pacific plate, Nazca plate, African plate, Australian plate, Eurasian plate, and Philippine sea plate. (3) The causality relationship and optimal lag length of energy release between plate boundaries may provide another view to forecasting earthquakes.

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.

Ground-motion model for subduction earthquakes in northern South America

Subduction ground motions in northern South America are about a factor of 2 smaller than the ground motions for similar events in other regions. Nevertheless, historical and recent large-interface and intermediate-depth slab earthquakes of moment magnitudes Mw = 7.8 (Ecuador, 2016) and 7.2 (Colombia, 2012) evidenced the vast potential damage that vulnerable populations close to earthquake epicenters could experience. This article proposes a new empirical ground-motion prediction model for subduction events in northern South America, a regionalization of the global AG2020 ground-motion prediction equations. An updated ground-motion database curated by the Colombian Geological Survey is employed. It comprises recordings from earthquakes associated with the subduction of the Nazca plate gathered by the National Strong Motion Network in Colombia and by the Institute of Geophysics at Escuela Politécnica Nacional in Ecuador. The regional terms of our model are estimated with 539 records from 60 subduction events in Colombia and Ecuador with epicenters in the range of −0.6° to 7.6°N and 75.5° to 79.6°W, with Mw≥4.5, hypocentral depth range of 4 ≤ Zhypo ≤ 210 km, for distances up to 350 km. The model includes forearc and backarc terms to account for larger attenuation at backarc sites for slab events and site categorization based on natural period. The proposed model corrects the median AG2020 global model to better account for the larger attenuation of local ground motions and includes a partially non-ergodic variance model.

Loreto Intermediate Depth Earthquake of 26 May 2019 (Northeast Peru): Source Parameters by Inversion of Local to Regional Waveforms and Strong-Motion Observations

AbstractThe Loreto earthquake of 26 May 2019 occurred below the extreme northeast part of Peru at a depth of 140 km within the subducting Nazca plate at a distance of 700 km from the trench Peru–Chile. The orientation of the seismic source was obtained from waveform inversion in the near field using velocity and strong-motion data. The rupture occurred in normal faulting corresponding to a tensional process with T axis oriented in east–west direction similar to the direction of convergence between the Nazca and South America plates. The analysis of the strong-motion data shows that the levels of ground shaking are very heterogeneous with values greater than 50 Gal up to distances of 300 km; the maximum recorded acceleration of 122 Gal at a distance of 100 km from the epicenter. The Loreto earthquake is classified as a large extensional event in the descending Nazca slab in the transition from flat-slab geometry to greater dip.

Full Waveform Inversion Beneath the Central Andes: Insight Into the Dehydration of the Nazca Slab and Delamination of the Back‐Arc Lithosphere

AbstractWe present a new seismic tomography model for the crust and upper mantle beneath the Central Andes based on multi‐scale full seismic waveform inversion, proceeding from long periods (40–80 s) over several steps down to 12–60 s. The spatial resolution and trade‐offs among parameters are estimated through the multi‐parameter point‐spread functions. P‐ and S‐wave velocity structures with spatial resolution of 30–40 km for the upper mantle and 20–25 km for the crust could be resolved in the central study region. In our study, the subducting Nazca slab is clearly imaged in the upper mantle, with dip‐angle variations from the north to the south. Bands of low velocities in the crust and mantle wedge indicate intense crustal partial melting and hydration of the mantle wedge beneath the frontal volcanic arc, respectively, and they are linked to the vigorous dehydration from the subducting Nazca plate and intermediate depth seismicity within the slab. These low‐velocity bands are interrupted at 19.8°–21°S, both in the crust and uppermost mantle, hinting at the lower extent of crustal partial melting and hydration of the mantle wedge. The variation of lithospheric high‐velocity anomalies below the back‐arc from north to south allows insight into the evolutionary foundering stages of the Central Andean margin. A high‐velocity layer beneath the southern Altiplano suggests underthrusting of the leading edge of the Brazilian Shield. In contrast, a steeply westward dipping high‐velocity block and low‐velocity lithospheric uppermost mantle beneath the southern Puna plateau hint at the ongoing lithospheric delamination.

Density structure, flexure, and tectonics of the Iquique Ridge, northern Chile

Kinematic paleo-reconstruction plate models show that the portion of the Iquique Ridge (IR) located next to the northern Chilean trench was formed 45–50 Ma onto the former oceanic Farallon (currently Nazca) plate near the intersection of the Foundation hotspot and the ancient Farallon-Pacific spreading center. The IR started to collide with South America >40 Ma off northern Peru, and the collision point started to migrate progressively southwards until present becoming northern Chile (20°-22°S) the current collision zone. In order to better understand the mechanical properties of the former oceanic Farallon plate during the IR formation, we used bathymetric and gravimetric data to characterize the hotspot swell and crustal structure of the IR. Results show an anomalous thick crust (10–15 km total thickness) that is capable of producing most of the swell topography (~200 km wide and 500–1000 m high) under nearly isostatic conditions (elastic thickness Te < 5 km, which is consistent with a hotspot track formed onto young oceanic lithosphere < 5 Ma). The swell and anomalous thick crust of the IR is heterogeneously distributed suggesting discontinuous magmatic pulses from the Foundation hotspot to the overlying oceanic lithosphere. As the 45–50 Ma oceanic Nazca plate approaches to the northern Chilean trench, a well pronounced fore-bulge (>200 km wide and >500 m in amplitude) develops accompanied by tensional faulting related to plate bending. By simultaneously modelling, the shape of the outer rise and the hotspot swell topography adjacent to the IR in the ridge-trench collision zone, we find a decrease in Te towards the trench axis (10–50%). This trenchward Te reduction is interpreted in terms of plate weakening caused by fracturing and hydration of the oceanic lithosphere as its yield strength is exceeded by plate bending under conditions of high plate curvatures (>10−7 m−1).

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.

Spatially and Geochemically Anomalous Arc Magmatism: Insights From the Andean Arc

AbstractWhile most volcanic arcs show a distinctive spatial relationship to subducting plates, there are many examples where volcanoes occur in anomalous locations. These are commonly also geochemically anomalous relative to the composition of more typical subduction‐related rocks. Using Holocene volcanoes in South America as a case study, we document the spatial and geochemical patterns along the Andean volcanic belt. To determine whether spatial variations are also geochemically anomalous, we assess a series of geochemical indices that provide information on the depth and degree of melting, and the role of metasomatic subduction inputs in melt generation. We use these parameters to develop a scoring system, with the lowest and highest scores indicating “typical” and “anomalous” arc melting processes, respectively. Typical arc magmatism is defined as melts generated in the sub‐arc mantle wedge through slab‐derived fluid metasomatism, with or without contributions from subducted sediments. In contrast, we show that anomalous volcanism in South America appears to relate to geometric anomalies in the subducting Nazca plate (e.g., beneath Sumaco, Laguna Blanca and Payun Matru), or to areas affected by variations in mantle flow due to the proximity to the slab edge (Crater Basalt Volcanic Field). By establishing relationships between anomalous magmatism and slab structure, we propose that similar geochemical fingerprints could be used to explore the magmatic response to slab deformation and/or tearing in older arc systems, particularly in cases where the three‐dimensional slab structure is no longer detectable.

Seismic source characteristics of the intermediate-depth and intraslab 2019 northern Peru earthquake (Mw 8.0)

On May 26, 2019, an unusual depth-intermediate focus earthquake, in the interior of the Nazca plate, shook the northern Peruvian jungle and caused the death of two victims. This earthquake was felt in almost all of Peru as well as Brazil, Ecuador and Colombia. The focal mechanism calculated is of the normal fault type (strike = 353∘, dip = 57∘, rake = − 99∘) for a focal depth of 114 km. The slip distribution was obtained through an inversion process of teleseismic waveforms. The maximum slip was 6.3 m, located around the central side of the fault plane. The moment magnitude calculated was Mw 8.0; the spectral analysis indicated a similar magnitude of Mw 8.0. The source time function indicates a multiple rupture earthquake, with the presence of several pulses of seismic energy release, with a total duration around 80 s. The rupture directivity was from the south to the north; we have assumed a propagation rupture velocity of 2.8 km/s. The modeled fault plane geometry has dimensions of 208×105 km2. The predicted vertical coseismic deformation field shows a pattern of mostly subsidence.

Late Quaternary Intraplate Deformation Defined by the Las Chacras Fault Zone, West‐Central Argentina

AbstractSeveral major (up to MW 7.5) earthquakes over the past 320 years have shaken the thick‐skinned Sierras Pampeanas region of Argentina, despite exhibiting much lower GPS‐shortening rates than across the thin‐skinned Precordillera region to its west. Whether geodetic shortening rates indicate an actual long‐term shortening gradient, and whether shortening rates translate to higher uplift rates due to steeper faults in the Sierras Pampeanas, remain uncertain due to the limited spatio‐temporal coverage and the inherently large error in the vertical component of deformation of GPS measurements. We measure geomorphic offsets and use 10Be terrestrial cosmogenic nuclide surface exposure dating to determine slip rates on the Las Chacras Fault Zone (LCFZ)—an ∼30 km long, NNW‐trending, steeply dipping (55–65°E) reverse fault that branches off of the longest, westernmost, thick‐skinned Valle Fértil range‐front fault in the western Sierras Pampeanas. Average shortening and uplift rates measured on the LCFZ are ∼0.2 and ∼0.3–0.4 mm/yr, respectively. Despite an uplift rate similar to most other faults in the region, the LCFZ shortening rate is lower than faults to its west; this is in agreement with the inferred west‐east decrease in shortening rates from GPS data, indicating consistent regional deformation patterns since the Late Pleistocene. The decrease in shortening to the east coincides spatially with the termination of the flat portion of the subducted Nazca plate between 67 and 68°W. From scaling relationships among magnitude, slip rate, and fault length, the LCFZ is capable of generating earthquakes of MW 6.7–7.1.

Neogene Patagonian magmatism between the rupture of the Farallon plate and the Chile Ridge subduction

Based on chemical composition and timing of late Paleogene and Neogene igneous rocks in southern Patagonia, six igneous suites are identified and correlated with subduction processes during the approaching and subduction of active oceanic ridges. Neogene magmatism took place after a period of decreased sub-alkaline magmatism during late Paleogene (LPg Suite). Early Miocene calc-alkaline rocks mark the reactivation of magmatism after subduction acceleration (Suite 1). These rocks are located at ca. 200 km from the trench and are typical products of continental magmatic arcs, where focused metasomatism of the mantle wedge results on high-degree of partial melting (>15%) in a restricted area expressed as a narrow volcanic arc. Contrarily, transitional sub-alkaline to alkaline rocks (Suite 2) were formed during the approaching of the Chile spreading ridge and the subduction of hot oceanic lithosphere (prior to the development of a slab window), where metasomatism encompassed a broader region of the mantle wedge, resulting in lower degrees of partial melting (<15%) and a wider volcanic arc, at ca 200–450 km from the trench. Intermediate igneous rocks with high Sr/Y ratio (Suite 3), located either within a magmatic arc setting, at ca. 270 km from the trench, or in the forearc region (less than 20 km from the trench), were formed during the subduction of the trailing edge of the Nazca plate. Alkaline basalts (Suite 4) are widely distributed, between 220 and 630 km from the trench. They are temporally and spatially related to a slab window generated after the subduction of the Chile Ridge. Andesites with high Sr/Y ratio (Suite 5) from the Quaternary Austral Volcanic Zone, at ca. 160 from the trench, were formed during the subduction of the leading edge of the Antarctica plate. The time and spatial distribution of these suites allow us to propose that the diverse chemical composition of Neogene igneous rocks in southern Patagonia was controlled by varying degrees of metasomatism and partial mantle melting controlled by the thermal structure of the mantle wedge prior, during and after the subduction of an active oceanic spreading ridge. Particularly, the chemical variability and the spatial distribution of Suite 2, which is not common in other magmatic arcs, could be used as a proxy for a warm mantle wedge structure.

The silent bending of the oceanic Nazca Plate at the Peruvian Trench

We predict the shape of the outer rise along the Peruvian Trench (6°-21°S) using flexural and gravity modelling in order to study the elastic properties of the subducting/oceanic Nazca Plate (NP). We include in our modelling the hotspot swell topography of the Nazca Ridge (NR) in the ridge-trench collision zone (~15°S). Results show an anomalous overthickening of the oceanic crust beneath the NR (from ~6 to 20 km), which is capable to produce most of the swell topography (500–700 km wide and up to 2.2 km high). The swell was formed likely under isostatic conditions (Te ~ 0 km) by the interaction of the NP with a hotspot-spreading center system. Despite the high buoyancy forces of the NR (0.5–4.0 × 1013 N/m) associated to the anomalous thick crust, the 45–50 Ma oceanic NP approaching the Peruvian Trench presents a well pronounced fore-bulge morphology with similar elastic thicknesses (Te = 35 ± 5 km) to those found for cold and old oceanic plate in the western Pacific. Consistently, our results do not show evidence for plate weakening (reduced Te values) in the NR-trench collision zone. We obtain similar results (Te = 40 ± 10 km) north of the NR implying that the oceanic NP is strong prior to subduction along most of the Peruvian convergent margin. This is consistent with the lack of great outer rise events (Mw ≥ 6) and the absence of reduced uppermost mantle velocities offshore Peru suggesting the presence of a poorly hydrated and rigid lithospheric mantle.

The influence of variations in crustal composition and lithospheric strength on the evolution of deformation processes in the southern Central Andes: insights from geodynamic models

Deformation in the orogen-foreland system of the southern Central Andes between 33° and 36° S varies in style, locus, and amount of shortening. The controls that determine these spatially variable characteristics have largely remained unknown, yet both the subduction of the oceanic Nazca plate and the strength of the South American plate have been invoked to play a major role. While the parameters governing the subduction processes are similar between 33° and 36° S, the lithospheric strength of the upper plate is spatially variable due to structures inherited from past geodynamic regimes and associated compositional differences in the South American plate. Regional Mesozoic crustal horizontal extension generated a < 40-km-thick crust with a more mafic composition in the lower crust south of 35°S; north of this latitude, however, a more felsic lower crust is inferred from geophysical data. To assess the influence of different structural and compositional heterogeneities on the style of deformation in the southern Central Andes, we developed a suite of geodynamic models of intraplate lithospheric shortening for two E–W transects (33° 40′ S and 36° S) across the Andes. The models are constrained by local geological and geophysical information. Our results demonstrate a decoupled shortening mode between the brittle upper crust and the ductile lower crust in those areas characterized by a mafic lower crust (36° S transect). In contrast, a more felsic lower crust, such as in the 33° 40′ S transect, results in a coupled shortening mode. Furthermore, we find that differences in lithospheric thickness and the asymmetry of the lithosphere–asthenosphere boundary may promote the formation of a crustal-scale, west-dipping detachment zone that drives the overall deformation and lateral expansion of the orogen. Our study represents the first geodynamic modeling effort in the southern Central Andes aimed at understanding the roles of heterogeneities (crustal composition and thickness) at the scale of the entire lithosphere as well as the geometry of the lithosphere–asthenosphere boundary with respect to mountain building.

Quaternary glaciolacustrine deposits around a Triple Junction site: Paleolakes at the foot of the Northern Patagonian Ice field (Argentina and Chile)

The area involved by the triple junction between the South American, Nazca and Antarctic plates activity was affected by Quaternary glaciations. Before 12,800 yrs BP an extended ice field occupied the top of the Patagonian Andes, irradiating glaciers towards the east and the west dominantly. Towards the east, the ice melted in piedmont lakes; towards the west, fjords melted into the Pacific Ocean. The Upper-Pleistocene climate amelioration caused the recession of those glaciers. Some piedmont lakes reversed their Atlantic outflow towards to the Pacific Ocean. The glaciers retreat caused the fluvial reactivations along crustal former faults that were located below the ice. The Patagonian ice field became therefore split into present Northern and Southern fields. At the second largest lake of South America, the Buenos Aires-General Carrera Lake, the water level dropped from about 500 m over present mean sea level to 230 m. Several glaciolacustrine deposits from this area are indicating significant variations caused by climatic changes, volcanism and tectonics, differing in spatial and temporal magnitudes. The triple junction activity involved subduction of the Chile Ridge below the continental South American plate, volcanic activity and faulting. During the glacier melting the Baker River captured three eastern-moving glacial systems towards the southwest, towards the Pacific Ocean. This rapid event is thought to occur 12,800 yrs BP. The lowering of these glaciolacustrine systems should be also interpreted in terms of the tectonic activity in the region and considering other processes operating in the lakes and within the watersheds.

From long-lived batholith construction to giant porphyry copper deposit formation: petrological and zircon chemical evolution of the Quellaveco District, Southern Peru

Porphyry Cu ore deposits are a rare product of arc magmatism that often form spatiotemporal clusters in magmatic arcs. The petrogenetic evolution of igneous rocks that cover the temporal window prior to and during porphyry Cu deposit formation may provide critical insights into magmatic processes that are key in generating these systems. This study documents the magmatic evolution of the Palaeocene–Eocene Yarabamba Batholith, Southern Peru, that was incrementally assembled between ~ 67 and ~ 59 Ma and hosts three, nearly contemporaneous, giant porphyry Cu–Mo deposits that formed at 57–54 Ma (Quellaveco, Toquepala and Cuajone). Whole-rock geochemistry, U–Pb geochronology and zircon trace element chemistry are reported from Yarabamba rocks that span the duration of plutonic activity, and from six porphyry intrusions at Quellaveco that bracket mineralisation. A change in whole-rock chemistry in Yarabamba intrusive rocks to high Sr/Y, high La/Yb and high Eu/Eu* is observed at ~ 60 Ma which is broadly coincident with a change in vector of the converging Nazca plate and the onset of regional compression and crustal thickening during the first stage of the Incaic orogeny. The geochemical changes are interpreted to reflect a deepening of the locus of lower crustal magma evolution in which amphibole ± garnet are stabilised as early and abundant fractionating phases and plagioclase is suppressed. Zircons in these rocks show a marked change towards higher Eu/Eu* (> 0.3) and lower Ti (< 9 ppm) compositions after ~ 60 Ma. Numerical modelling of melt Eu systematics and zircon-melt partitioning indicates that the time series of zircon Eu/Eu* in these rocks can be explained by a transition from shallower, plagioclase-dominated fractionation to high-pressure amphibole-dominated fractionation at deep crustal levels from ~ 60 Ma. Our modelling suggests that any redox effects on zircon Eu/Eu* are subordinate compared to changes in melt composition controlled by the fractionating mineral assemblage. We suggest that growth and intermittent recharge of the lower crustal magma reservoir from ~ 60 Ma produced a significant volume of hydrous and metallogenically fertile residual melt which ascended to the upper crust and eventually generated the three giant porphyry Cu–Mo deposits at Quellaveco, Toquepala and Cuajone from ~ 57 Ma. Our study highlights the importance of high-pressure magma differentiation fostered by strongly compressive tectonic regimes in generating world-class porphyry Cu deposits.

The 2014 Juan Fernández microplate earthquake doublet: Evidence for large thrust faulting driven by microplate rotation

On October 9, 2014, a Mw 7.1–6.7 seismic doublet occurred at the Juan Fernández microplate, close to the triple junction with Pacific and Nazca plates. The Mw 7.1 earthquake is the largest earthquake ever to have been recorded in the region. Its thrust focal mechanism is also unusual for the region, although the northern part of the microplate is expected to undergo compression. The region is remote and seismological data is limited to a seismic station at ~600 km distance on Easter Island and teleseismic observations for the largest events. We use a combination of advanced seismological techniques to overcome the lack of local data and resolve earthquake source parameters for the doublet and its aftershock sequence, being able to reconstruct the chronology of the sequence and the geometry of affected fault segments. Our results depict a complex seismic sequence characterized by the interplay of thrust and strike-slip earthquakes along different structures, including a second, reversed strike slip-thrust seismic doublet in November 2014. Seismicity occurred within the microplate and only in the late part of the sequence migrated northward, towards the microplate boundary. The first largest doublet, whose rupture kinematic is well explained by stress changes imparted by the first subevent on the second one, may have activated unmapped E-W and NE-SW faults or an internal curved pseudofault, attributed to the long-term rotation of the microplate. Few large, thrust earthquakes are observed within the sequence, taking place in the vicinity of mapped compressional ridges. We suggest that compressional stresses in the northern part of the microplate and at its boundary are partially accommodated aseismically. However, the occasional occurrence of large, impulsive thrust earthquakes, with a considerable tsunamigenic potential, poses a relevant hazard for islands in the South Pacific region.

Magnetotelluric study in the Los Lagos Region (Chile) to investigate volcano-tectonic processes in the Southern Andes

To analyze the process of subduction of the Nazca and South American plates in the area of the Southern Andes, and its relationship with the tectonic and volcanic regime of the place, magnetotelluric measurements were made through a transversal profile of the Chilean continental margin. The data-processing stage included the analysis of dimensional parameters, which as first results showed a three-dimensional environment for periods less than 1 s and two-dimensional for periods greater than 10 s. In addition, through the geomagnetic transfer function (tipper), the presence of structural electrical anisotropy was identified in the data. After the dimensional analysis, a deep electrical resistivity image was obtained by inverting a 2D and a 3D model. Surface conductive anomalies were obtained beneath the central depression related to the early dehydration of the slab and the serpentinization process of the mantle that coincides in location with a discontinuity in the electrical resistivity of a regional body that we identified as the Nazca plate. A shallow conductive body was located around the Calbuco volcano and was correlated with a magmatic chamber or reservoir which in turn appears to be connected to the Liquiñe Ofqui fault system and the Andean Transverse Fault system. In addition to the serpentinization process, when the oceanic crust reaches a depth of 80–100 km, the ascending fluids produced by the dehydration and phase changes of the minerals present in the oceanic plate produce basaltic melts in the wedge of the subcontinental mantle that give rise to an eclogitization process and this explains a large conductivity anomaly present beneath the main mountain range.