Flat-slab Segment Research Articles

Adakite-like signature of Late Miocene intrusions at the Los Pelambres giant porphyry copper deposit in the Andes of central Chile: metallogenic implications

Adakite-like features are recognized in the Late Miocene (~10 Ma) porphyritic intrusions of the Los Pelambres giant porphyry copper deposit, central Chile (32°S). Located within the southern portion of the flat-slab segment (28–33°S) of the Chilean Andes, the Al- and Na-rich porphyries of Los Pelambres display distinctly higher Sr/Y (~100–300) and LaN/YbN (~25–60) ratios than contemporaneous and barren magmatic units (e.g., La Gloria pluton, Cerro Aconcagua volcanic rocks) of the same Andean magmatic belt. Strong fractionation of heavy rare earth elements (HREE), absence of Eu anomalies, high Sr/Y and Zr/Sm and low Nb/Ta ratios suggest melt extraction from a garnet-amphibolite source. The Late-Miocene adakite-like porphyritic intrusions at Los Pelambres formed closely related in time and space to the subduction of the Juan Fernández Ridge (JFR) hotspot chain along the Chilean margin. Current tectonic reconstructions reveal that, at the time of formation of the Los Pelambres rocks, a W-E segment of the JFR started to subduct beneath them, producing a slow-down of a previously rapid southward migration of a NE-ridge—trench collision. These particular tectonic conditions are favorable for the origin of the Los Pelambres porphyry suite by melting of subducting young hotspot rocks under flat-slab conditions. The incorporation of crustal components into the oceanic lithopheric magma source by subduction erosion is evidenced by the Sr-Nd isotope composition of the Los Pelambres rocks different from the MORB signatures of true adakites. A close relationship apparently exists between the origin of this adakite-like magmatism and the source of the mineralization in the Los Pelambres porphyry copper deposit.

Thermal models of flat subduction and the rupture zone of great subduction earthquakes

In subduction zones, the size of the seismogenic zone that ruptures during a great thrust earthquake may be thermally controlled. We have constructed finite element thermal models of six transects across three different subduction zones in order to determine the temperature distribution along the plate interface and to predict the size of the seismogenic zone. These models incorporate the complex plate geometries (variable dip downsection) necessary to model a flat slab subduction style. We focus on the rupture zones of the great earthquakes of Nankai (SW Japan) 1946 M8.3 and Alaska 1964 M9.2 as well as on the Cascadia margin. Subduction zone segments with moderate to steep dips exhibit rupture zones of 100–150 km downdip width, consistent with earlier elastic dislocation models and thermal models. For shallow dipping flat slab segments, our models predict larger locked zones, of 150–250 km width, in good agreement with aftershock and geodetic studies. The wider seismogenic zone predicted for flat slab segments results from an uncommonly wide, cold, forearc region, as corroborated by surface heat flow observations. A global analysis of great M > 8 interplate earthquakes of the 20th century reveals that more than a third of these events occurred in flat slab segments, whereas these segments represent only 10% of modern convergent margins. This implies substantially higher interplate coupling for flat slab segments, likely due to the increased downdip extent of the seismogenic zone, and suggests that the seismic risk near such regions may be higher than previously thought.

On the role of subducting oceanic plateaus in the development of shallow flat subduction

Oceanic plateaus, aseismic ridges or seamount chains all have a thickened crust and their subduction has been proposed as a possible mechanism to explain the occurrence of flat subduction and related absence of arc magmatism below Peru, Central Chile and at the Nankai Trough (Japan). Their extra compositional buoyancy could prohibit the slab from sinking into the mantle. With a numerical thermochemical convection model, we simulated the subduction of an oceanic lithosphere that contains an oceanic crustal plateau of 18-km thickness. With a systematic variation, we examined the required physical parameters to obtain shallow flat subduction. Metastability of the basaltic crust in the eclogite stability field is of crucial importance for the slab to remain buoyant throughout the subduction process. In a 44-Ma-old subducting plate, basalt must be able to survive a temperature of 600–700 °C to keep the plate buoyant sufficiently long to cause a flat-slab segment. We found that the maximum yield stress in the slab must be limited to about 600 MPa to allow for the necessary bending to the horizontal. Young slabs show flat subduction for larger parameter ranges than old slabs, since they are less gravitationally unstable and show less resistance against bending. Hydrous weakening of the mantle wedge area and lowermost continent are required to allow for the necessary deformation of a change in subduction style from steep to flat. The maximum flat slab extent is about 300 km, which is sufficient to explain the observed shallow flat subduction near the Nankai Trough (Japan). However, additional mechanisms, such as active overthrusting by an overriding continental plate, need to be invoked to explain the flat-slab segments up to 500 km long below Peru and Central Chile.

The impact of the South‐American plate motion and the Nazca Ridge subduction on the flat subduction below South Peru

Flat subduction near Peru occurs only where the thickened crust of the Nazca Ridge subducts. Furthermore, the South‐America continent shows a westward absolute plate motion. Both the overriding motion of South‐America and the subduction of the Nazca Ridge have been proposed to explain the flat slab segment below South Peru. We have conducted a series of numerical model experiments to investigate the relative importance of both mechanisms. Results suggest that the average upper mantle viscosity should be about 3.5 × 1020 Pa s or less and basaltic crust should be able to survive 600 to 800°C ambient temperature before transforming into eclogite to explain the slab geometry below Peru. The effect of the overriding plate is estimated to be as large or twice as large as that of the plateau subduction.

Geological setting of the Argentine Frontal Cordillera in the flat-slab segment (30°00′–31°30′S latitude)

The Argentine Frontal Cordillera, between latitudes 30°20′ and 31°20′S, presents an eastward-verging polycyclic structure. Two large groups of rocks with markedly different structural nature can be distinguished: a Paleozoic basement with characteristic thin-skinned structure and the Andean cover showing remarkable extensional Mesozoic structures inverted by Tertiary compressional tectonic events. These groups are divided by a large tectonic unconformity separating not only rocks with different style and degree of deformation but also different paleogeographic settings. The Cenozoic tectonic inversion is linked with the subhorizontal subduction of the Nazca Plate in the Pampean flat-slab segment and subsequent uplift of the Andean Cordillera. The Paleozoic basement is composed of Devonian and Permo-Carboniferous marine sedimentary rocks, separated by an angular unconformity. These rocks were deposited in siliciclastic and mixed sedimentary platforms, respectively, and intruded by Permo-Triassic granitoid rocks. The Devonian rocks were deformed by the Famatinian orogeny in Late Devonian times. The most important Paleozoic structures are related to the Gondwanic orogeny, of Permian age. These include east-vergent thrusts and related folds that record a large shortening component. The Andean cover, lying unconformably over the Paleozoic basement, is primarily volcanic and volcanoclastic, with some interbedded continental sedimentary rocks. Two main stratigraphic groups can also be defined here: an older sequence linked to an extensional tectonic event and a synorogenic sequence connected to a compressional tectonic event that produced an inversion of previous extensional features. The lower unit (Choiyoi Group: Permian and Triassic) is affected by normal faults with downthrown western blocks and intruded by Jurassic granodioritic rocks. The faults in this unit involve the Gondwanic basement in a typical thick-skinned tectonic style and are grouped in bands with N–S strike. The uppermost units (Vizcachas, Melchor, and Olivares groups: Tertiary and Plio-Quaternary) also lie unconformably over the latter. The normal faults were inverted in the upper Miocene by the uplift of the western block, deforming the lower and upper units in a compressional context during the Andean orogenic cycle. Displacement on reverse faults, measured in the Tertiary synorogenic rocks, is usually less than 1 km. The Tertiary crustal shortening calculated from cross-sections is about 10%, in contrast with the estimated 50% shortening of the Precordillera unit. All this shows that most of the crustal shortening of the Andean Cordillera during the compressional stage has been transferred to the Precordillera unit through the lower detachment fault. These facts also show that the Cordillera Frontal unit is an uplifted block in which the extensional structures have been preserved, and that the Rodeo–Upsallata basin is a piggy-back type.

Andean subduction styles and their effect on thermal structure and interplate coupling

The variability in subduction styles along the Andean margin is investigated with regard to its probable causes and likely effects. It is shown that there is no single Andean or Chilean subduction style. The alternation of segments of flat-slab subduction and steep subduction argue against mechanisms involving continental scale boundary conditions. The best candidate for explaining flat subduction style appears to be buoyancy related to thickened oceanic crust of moderate to young age. A high degree of interplate coupling and great subduction zone earthquakes can occur along both flat-slab segments and steeply subducting segments. In general interplate coupling is higher for flat-slab segments as demonstrated by a statistical analysis of deformation in the upper plate. A flat subduction style is shown to have an important thermal impact resulting in a much colder plate interface and may succeed in explaining the apparent increase in coupling.

Evidence for Cenozoic extensional basin development and tectonic inversion south of the flat-slab segment, southern Central Andes, Chile (33°–36°S.L.)

The mainly volcanic Cenozoic deposits that make up much of the western part of the Principal Cordillera in Central Chile are generally subdivided into two major units: an older Abanico or Coya-Machalı́ Formation and a younger Farellones Formation. Difficulty in differentiating these units has led to considerable debate. On the basis of the wide distribution, great thickness, and presence of sedimentary intercalations, it has been postulated that these arc volcanics were deposited in an intermontane basin; more recently, it has been proposed that this basin developed under extensional conditions and underwent subsequent tectonic inversion. We present field, geochronologic, geochemical, and thermal maturity data that support the latter interpretation. Collectively, this new information clarifies the stratigraphic, tectonic, and paleogeographic evolution of these deposits. The vast geographic extent of the Abanico Formation and lateral equivalents, which reach from at least 32°30′ to 44°S along the Principal Cordillera, its great thickness, and the presence of repeated thick fluvial and lacustrine intercalations all indicate deposition in a large, strongly subsident, and probably north–south oriented basin, developed between middle to late Eocene and Oligocene. The unconformable contact with underlying Mesozoic units observed at several localities indicates that deposition followed a substantial erosional episode during late Cretaceous and/or early Cenozoic time. Basal deposits of the Abanico Formation near Termas del Flaco increase rapidly in thickness to the west. Still further to the west, a thick Abanico section contains, in its upper part, mammal fossils older than those found in the basal deposits near Termas. This evidence indicates a major space of deposition west of this locality, which had been filled before deposition took place at Termas. The east-vergent, high-angle El Fierro thrust fault on the east side of the westward-growing deposits is interpreted as an inverted normal fault associated with initial basin development and deposition. High-angle thrust faults observed elsewhere on the eastern outcrop margin of the Abanico Formation (i.e. the Chacayes-Yesillo Fault in the Maipo section and the Espinoza Fault in the Cachapoal–Las Leñas section) also have been interpreted as inverted normal faults. The irregular folding style of the Abanico Formation, with its highly variable amplitude, longitude, tightness, and vergency, suggests that deformation is attributable to the inversion of faults associated with basin development. Geochemical characteristics of the Abanico Formation indicate a relatively thin crust during early basin development. Thermal maturity data reflect a deep burial of the deposits during accumulation, and thermal modeling indicates high heat flow conditions during burial. These data support a major extensional episode of the crust and the development of a large depositional space (basin) in this region. On the basis of this evidence, we suggest that deposition of the Abanico Formation is related mostly to crustal extension and its deformation to tectonic inversion. In the western Las Leñas river valley, a growth structure indicates that deformation occurred between 20.8 and 16.1 Ma, while the Abanico Formation was still being deposited. Deformation apparently did not occur coevally throughout the region; however, sedimentation and volcanic deposition in the basin apparently occurred uninterrupted. This argues against a single, obvious unconformity separating the Abanico and Farellones Formations. Instead, it supports the existence of local angular unconformities where fault inversion affected the basin fill. Comparison of the timing of extensional basin development and subsequent contraction (inversion) with the convergence rates between the Nazca and South American plates during the Cenozoic period shows a correspondence with periods of decreasing and increasing convergence rates, respectively. Tectonic and volcanic events on the east versant of the Andes [Journal of South American Earth Sciences 15 (2002)], which are coeval with the basin inversion and crustal thickening episodes presented herein and, therefore, with the previously mentioned period of increasing convergence rate, are assumed to correspond with the same episode of major tectonic accommodation of the crust in this Andean region. It is not yet possible to determine if the collisional event of the Juan Fernández ridge at approximately 15 Ma in the flat-slab segment region had a local or a more regional effect on the late Cenozoic tectonic evolution of the continental margin in the Central Andes.

The Challenger–Juan Fernández–Maipo major tectonic transition of the Nazca–Andean subduction system at 33–34°S: geodynamic evidence and implications

The late Tertiary tectonic and geological evolution of the Southern Andes at 33–34°S has been strongly controlled by the Challenger–Juan Fernández–Maipo (CH–JF–M) structural system. The present configuration of a flat slab between 28–33°S may be explained by a series of favorable conditions evolving with time since the breakup of the Farallon Plate at 25 Ma. The dramatic shift of the pole of rotation and the rapid eastward propagating rift along the Challenger Fracture Zone induced an unbalanced slab pull force, south of the CH–JF–M, that may have triggered the detachment of the subducting slab. The upwelling of a warmer asthenospheric material and the partial melts of the slab are likely consequences that are consistent with the anhydrous tholeiitic late Oligocene volcanism and the anomalous adakite-type magmatism of the early Miocene, respectively. The present seismogenic zone across the CH–JF–M tectonic boundary shows a continuity for more than 600 km along the flat slab segment, in contrast with the much shorter slab southward. Such a tectonic configuration is probably a quasi-steady condition since 25 Ma. Gravity modeling along the JF chain and the broadly located focal mechanisms at the locus of the already subducted JF chain indicate a thick (>25 km), wide (100 km), and continuous belt of lighter oceanic crust, which is the major contribution to the positive buoyancy of the slab. The decoupling of the subducted slab at intermediate depths further contributes to the flattening of the slab, focusing the buoyancy forces associated with the thickened oceanic crust along the already subducted JF chain. The absolute plate reconstruction during the Miocene shows that the JF collision against the margin migrates southward, in agreement with geological and tectonic observations that further support the causative relationship between the flattening of the slab and the subduction of the JF chain. Preliminary deformation models for the indentation of the JF ridge against the continental lithosphere is consistent with the particular east–west trend of the Maipo deformation zone, connecting in this way the CH–JF–M major tectonic boundary in the ocean-continent lithosphere system.

Latent heat effects of the major mantle phase transitions on low-angle subduction

Very low to zero shallow dip angles are observed at several moderately young subduction zones with an active trenchward moving overriding plate. We have investigated the effects of latent heat for this situation, where mantle material is pushed through the major mantle phase transitions during shallow low-angle subduction below the overriding plate. The significance of the buoyancy forces, arising from the latent heat effects, on the dynamics of the shallowly subducting slab is examined by numerical modeling. When a 32-Ma-old slab is overridden with 2.5 cm/yr by a continent, flat subduction occurs with a 4–5 cm/yr convergence rate. When latent heat is included in the model, forced downwellings cause a thermal anomaly and consequently thermal and phase buoyancy forces. Under these circumstances, the flat slab segment subducts horizontally about 350 km further and for about 11 Ma longer than in the case without latent heat, before it breaks through the 400-km phase transition. The style of subduction strongly depends on the mantle rheology: increasing the mantle viscosity by one order of magnitude can change the style of subduction from steep to shallow. Similarly, an overriding velocity of less than 1 cm/yr leads to steep subduction, which gradually changes to flat subduction when increasing the overriding velocity. However, these model parameters do not change the aforementioned effect of the latent heat, provided that low-angle subduction occurs. In all models latent heat resulted in a substantial increase of the flat slab length by 300–400 km. Varying the olivine–spinel transition Clapeyron slope γ from 1 to 6 MPa/K reveals a roughly linear relation between γ and the horizontal length of the slab. Based on these results, we conclude that buoyancy forces due to latent heat of phase transitions play an important role in low-angle subduction below an overriding plate.

Magnetic anomaly interpretation across the southern central Andes (32°–34°S): The role of the Juan Fernández Ridge in the late Tertiary evolution of the margin

Marine and terrestrial magnetic surveys have been integrated to study the tectonic structure of the convergent margin of Chile between 32°–34°S. Three magnetic domains have been identified: oceanic, continental margin, and subaerial. The oceanic domain has seafloor spreading anomalies (16 (∼37 Ma) to 18 (∼39.5 Ma)) disturbed by anomalies of the Juan Fernández hot spot chain. In the continental margin, the most prominent fabric are E‐W anomalies in the upper slope corresponding to onshore E‐W anomalies of large intrusive bodies. Onshore, a N‐S lineament of short‐wavelength anomalies defines the roots of a Cretaceous volcanic arc. A resembling lineament offshore indicates a submerged older volcanic arc and that continental basement extends to ∼50 km landward of the trench. Absolute Cenozoic plate motion for Nazca and South American plates and dating of the Juan Fernández chain provide a kinematic model of ridge‐continent collision. The reconstruction indicates rapid southward migration of the collision point along ∼1400 km of the margin from 20 to 11 Ma (∼20 cm yr−1). From 11 Ma to present the collison point has migrated at a slower rate along ∼375 km of the margin (3.5 cm yr−1). The predicted location of the subducted portion of the Juan Fernández chain coincides with the south edge of the southward migrating flat slab segment of the subducted lithosphere and with a cluster of deep earthquakes indicating a causal relationship. In the last ∼10 Myr the ridge has separated a sediment starved trench to the north where subduction erosion may dominate from a sediment filled trench to the south where recent sediment accretion dominates. These observations indicate that subduction of the Juan Fernández chain plays a major role in arc‐forearc tectonics.

Neogene deformation of the flat-subduction segment of the Argentine-Chilean Andes: Magnetostratigraphic constraints from Las Juntas, La Rioja province, Argentina

Substantial Neogene deformation occurred in the part of the Andes Mountains and the Sierras Pampeanas that overlie a subhorizontal segment of the subducting Nazca plate in northern Argentina. The history of the volcanic arc suggests that the Nazca plate attained the present low-angle inclination between about 17 and 10 Ma, with the region underlain by the flat Nazca plate widening eastward thereafter. We compare the history of deformation, provided by magnetostratigraphy of Miocene strata in foreland and intermontane basins, to the history of the subducting plate. At Las Juntas, in the Precordillera thin-skinned thrust belt, 4,780 m of Neogene nonmarine strata record the initiation and termination of sedimentation in a segment of a foreland basin. Magnetostratigraphic data, complemented by fission-track dating of an interbedded tuff, indicate that the strata span from 18 to ∼9 Ma. The earliest strata correlate to the end of a time of volcanic activity in the magmatic arc and the beginning of reverse faulting in the Frontal Cordillera. Subsequently, the deformation front migrated eastward, generating the Precordillera thrust belt, and finally the Sierras Pampeanas basement uplifts. The rate of strata accumulation varied systematically as new uplifts arose, with a maximum rate exceeding 1.0 mm/a between ∼15 and 12 Ma. A correlation between the deformational history and the geometric history of the subducted slab suggests that thrusting migrated eastward as the angle of subduction shallowed. Although large-scale factors affecting the plate margin beyond the flat-slab segment may have partly induced the compressional deformation, this correlation suggests that processes related to flat subduction are of importance in deforming the continental plate.