Flat-slab Segment Research Articles

Neogene erosion of the Andean Cordillera in the flat-slab segment as indicated by petrography and whole-rock geochemistry from the Manantiales Foreland Basin (32°–32°30′S)

The Miocene Manantiales Foreland Basin is defined as a thick succession of sedimentary rocks belonging to the Chinches Formation in the Frontal Cordillera between 32°S and 32°30'°S, located over the flat-slab segment of the Andean Cordillera. It has been interpreted as a foreland basin that mainly received contributions from several mountain ranges such as the Principal Cordillera and Frontal Cordillera to the west. In this paper, we present a detailed study of its provenance based on the petrography and whole-rock geochemistry of sedimentary rocks from the Chinches Formation. Our results indicate that the Manantiales Basin was fed by igneous provinces in the hinterland that contain sub-alkaline intermediate rocks (andesitic) and acidic rocks (dacitic to rhyodacitic and granitic) arranged in a clear stratigraphic progression. These signatures may be assigned to Cenozoic rocks (Farellones Formation) in the Principal Cordillera and Permo-Triassic (Choiyoi Group) in the Frontal Cordillera. In contrast to a previous result, we found a continuous geochemical trend in the Chinches Formation between ca. 19 and 10Ma opposing the idea of a tectonic repetition in the sedimentary succession.

Temporal variations in the influence of the subducting slab on Central Andean arc magmas: Evidence from boron isotope systematics

The Pampean flat-slab segment in the southern Central Andes represents an ideal setting at which to investigate how changes in the tectonic configuration of a subduction zone (convergence angles and rates, seamount subduction and shallowing slab angle) affects the recycling of subducted components to arc magmas. To constrain sources, particularly of slab-derived fluids and their contribution to arc magmatism, boron isotope and select major and trace element compositions were determined for pyroxene- and zircon-hosted melt inclusions obtained from a suite of Paleocene to Miocene arc magmatic rocks, from the southern Central Andes. Considerable changes in δB11 values and boron concentrations are observed with time. Significantly lower δB11 values (average =−1.9±2.2‰(1σ)) and B/Nb ratios (average =3.3±1.3(1σ)) were obtained for melt inclusions from Oligocene arc rocks (∼24 Ma) compared to those from the Paleocene (∼61 Ma) (averages =+1.6±0.8‰ and 17.8±1.4(1σ), respectively) and the Miocene (∼18 Ma) (averages =+4.7±1.9‰ and 11.9±5.5 (1σ), respectively).A slab-derived fluid with a δB11 composition of +1.5‰, primarily derived from altered oceanic crust on the down-going slab, affected the source of the Paleocene arc magma. The source of the Oligocene arc magmas received less boron derived from the subducting slab (≲1% fluid addition) than the Paleocene and Miocene arc magmas (up to 3.5% fluid addition). This is consistent with a greater depth to the slab-mantle interface and is potentially related to the widening of the volcanic arc and more distal position of these samples relative to the trench during this time period. The higher δB11 values (up to ∼9‰) obtained for the Miocene melt inclusions record an increase in the influence of serpentinite-derived fluids on the source of arc magmas after ∼19.5 Ma. This is approximately coeval with the subduction of the Juan Fernandez Ridge (JFR), suggesting that the oceanic lithosphere associated with the subducting JFR in the Early Miocene was hydrated and serpentinised, similar to the present day ridge. As serpentinisation increases the buoyancy of the slab this finding supports the link between the intersection of the JFR with the Andean margin and the onset of flat-slab subduction.

Flat versus normal subduction zones: a comparison based on 3-D regional traveltime tomography and petrological modelling of central Chile and western Argentina (29°–35°S)

Our study compares the seismic properties between the flat and normal subduction regions in central Chile, to better understand the links between the slab geometry, surface deformation and the deeper structures. In comparison with previous studies, we show the most complete 3-D regional seismic tomography images for this region, in which we use (1) a larger seismic data set compiled from several short-term seismic catalogues, (2) a denser seismic array allowing a better resolution of the subduction zone from the trench to the backarc and into the upper ∼30 km of the slab and (3) a starting 1-D background velocity model specifically calculated for this region and refined over the years. We assess and discuss our tomography results using regional seismic attenuation models and estimating rock types on the basis of pressure and temperature conditions computed from thermomechanical models. Our results show significant seismic differences between the flat and normal subduction zones. As expected, the faster seismic velocities and increased seismicity within the flat slab and overriding lithosphere are generally consistent with a cooler thermal state. Our results are also consistent with dehydration of the mantle above the subducted Juan Fernandez Ridge at the eastern tip of the flat slab segment, indicating that the latter retains some fluids during subduction. However, fluids in the upper portion of the flat slab segment are not seismically detected, since we report instead fast slab seismic velocities which contradict the argument of its buoyancy being the cause of horizontal subduction. The forearc region, above the flat slab, exhibits high Vs and very low Vp/Vs ratios, uncorrelated with typical rock compositions, increased density or reduced temperature; this feature is possibly linked with the aftershock effects of the Mw7.1 1997 Punitaqui earthquake, the flat slab geometry and/or seismic anisotropy. At the surface, the seismic variations correlate with the geological terranes. The Andean crust is strongly reduced in seismic velocities along the La Ramada–Aconcagua deformation belt, suggesting structural damage. Slow seismic velocities along the Andean Moho match non-eclogitized hydrated rocks, consistent with a previous delamination event or a felsic composition, which in turn supports the extent of the Chilenia terrane at these depths. We confirm previous studies that suggest that the Cuyania terrane in the backarc region is mafic and contains an eclogitized lower crust below 50-km depth. We also hypothesize major Andean basement detachment faults (or shear zones) to extend towards the plate interface and canalize slab-derived fluids into the continental crust.

Magmatic evolution of pre-ore volcanics and porphyry intrusives associated with the Altar Cu-porphyry prospect, Argentina

Altar is a Cu-porphyry deposit related to several small plagioclase porphyry intrusions of the late Miocene formed on the margin of the Flat-Slab segment along the Andean Cordillera in north-west Argentina. New stratigraphic and structural mapping supported by geochemistry and geochronology of pre-ore volcanics at Altar has revealed that a period of ∼6–7 Ma of volcanism during the late Oligocene-early Miocene formed ∼4000 m of volcano-stratigraphic succession making up the Pachón Formation. It represents a period dominated by explosive to effusive eruption in a dynamic arc basin with local ash fall and flow deposition in lacustrine and fluvial sites. Volcanism is typified by medium- to high-K calc-alkaline arc magmatism with a shift from mafic compositions at the base to felsic rocks at the top of the formation containing zircons aged 21.9 ± 0.2 Ma (2 Std.Dev, U–Pb). A clear geochemical separation exists between early Miocene pre-ore volcanics that show signatures akin to non-adakitic, normal arc, extensional tectonic settings conducive of chemical differentiation at shallow crustal levels and correlate with intra-regional Abanico and Farellones Formations; and the middle to late-Miocene Cu-mineralised porphyry intrusions. After a break of ∼9 Ma in the geological record at Altar, these Cu-fertile bodies are emplaced entirely within the Pachón Rhyolite and represent adakite-like magmas with fractionation trends evolving from a lower crustal MASH zone. This distinction is controlled by a change from an extensional to compressive tectonic regime in the region during the middle Miocene in which magmas were stalled in the lower crust for an extended period, subsequently became enriched in metals and then formed several Cu-porphyry bodies which were emplaced during a relatively short period towards the late Miocene.

Dominant fault plane orientations of intermediate‐depth earthquakes beneath South America

AbstractThe South American subduction zone exhibits considerable variation: the subduction angle alternates between flat and steep; the subducting plate has complex structures; and arc volcanism in the overlying plate has gaps. I investigate the effect of these differences in incoming plate structure and slab geometry on intermediate‐depth earthquakes, specifically their fault orientations and rupture characteristics, and find that slab geometry has the largest impact on fault orientation. I use rupture directivity to estimate rupture direction and rupture velocity and to distinguish the fault plane from the auxiliary plane of the focal mechanism. From analysis of 163 large (Mw≥5.7) intermediate‐depth (60–360 km depth) earthquakes from along the length of South America, estimated rupture azimuths and plunges show no trends, appearing to be randomly distributed on the determined population of fault plane orientations, and a majority of earthquakes are made up of multiple subevents. As seen in other subduction zones, subduction segments descending at normal angles have predominantly subhorizontal faults. Flat slab segments also have a dominant fault orientation, but those earthquakes slip along the conjugate nodal plane of the focal mechanism. In strongly curved slab segments, such as at the downdip edge of flat segments where the slab resubducts, earthquakes may slip along either nodal plane orientation. While both fault orientations could be consistent with the reactivation of fossil outer rise faults, the fault orientations are also consistent with expectations for newly created faults in agreement with the ambient stress field. Fault reactivation alone does not explain why different fault orientations are active in segments with different geometries, so the preferred explanation for having regionally consistent fault orientations is that they minimize the total work of the system. The previously observed predominance of subhorizontal faults appears to be a consequence of slab geometry.

Petrogenesis of Miocene Volcanic arc rocks over the Chilean- Pampean flat-slab segment of the Centr

Miocene arc volcanism is manifested widely in the Valle del Cura-El Indio belt region (29°30´–30° South latitude), in the southern Central Andes of Argentina and Chile. The magmas that fed this volcanism are well represented by the Cerro de las Tortolas Formation, which is divided into two volcanic episodes based on petrographic, chemical and age differences: an older basaltic-andesitic event (16–14Ma) and a younger andesitic to dacitic (13–10Ma) one. Representative plagioclase, orthopyroxene, clinopyroxene and amphibole phenocrysts from rock samples already characterized from geochemical and isotopic viewpoints were selected for electron microprobe determination of mineral chemistry. Results indicate an overall homogeneous composition for each of the mineral phases. Equilibrium temperatures were estimated through two-pyroxenes, amphibole-plagioclase and amphibole geothermometers, which show a consistent temperature range between 970 to 850°C. Equilibrium pressure calculated using amphibole composition for volcanic suites produced the most comprehensive results for pressure equilibrium conditions, with results close to 4kb. Changes in the residual mineral assemblages and variations in isotopic signatures indicate that primary magmas were equilibrated at the lower crust with a gradual increase of crustal thickness. These melts evolved towards intermediate magma chambers, where crystallization of phenocrysts occurred at the same temperature and pressure conditions, hence, no increase in depth of intermediate magma chambers is registered although the increase of crustal thickness registered from Lower to Middle Miocene times.

Cross-strike structures controlling magmatism emplacement in a flat-slab setting (Precordillera, Central Andes of Argentina)

Abstract Detailed structural, kinematic and geophysical data on the foreland of the Pampean flat-slab segment (Hualilán area, Andean Precordillera of Argentina) have shown that cross-strike structures have had an important role in the evolution of this Andean segment since Miocene times. These structures represent pre-existing crustal fabrics reactivated during the Andean orogeny and could have controlled the emplacement of the Miocene arc-magmatism migrating into foreland due to the flattening of the slab. Likewise, kinematic results obtained for these structures support a similar stress frame to that obtained elsewhere in the Precordillera but showing different motions as a consequence of their high obliquity to the orogen trend. Moreover, they record a reorientation of kinematic axes during Late Miocene–Pliocene times.

El Oroclino del Maipo: Un rasgo estructural de primer orden en la evolución geodinámica Mioceno a Reciente en los Andes de Chile central.

In this paper we show the results of a paleomagnetic study carried out along the western Andes of cen- tral Chile from 30° to 36°S. Whereas paleomagnetic analysis from Jurassic to Late Cretaceous rocks in the Pampean flat slab segment shows small or non significant clockwise vertical-axis rotations, results obtained in Late Jurassic to Neogeone rocks to the south, on the normal subduction segment, show sistematically clockwise rotations up to 40o. Paleomagnetic rotations are coeval with thrusting along the High Andes in Late Miocene times. We propose a coupled Bolivian Orocline-Juan Fernandez Ridge (JFR) model where a far-field component of clockwise rotation related to the formation of the Bolivian Orocline was first acquired by the margin between 31o and 33oS. The same area was later rotated by a slight counterclockwise component related to the subduction of the JFR over the last 10 Ma. South of 33oS the clockwise rotations related to the Bolivian Orocline component were amplified by the subduction of the JFR.

Why has the Nazca plate slowed since the Neogene?

The classic example of the not-well-understood rapid change of tectonic plate motion is the increase and then decrease of the convergence rate between the Nazca and South America plates during the past 25–20 m.y. that coincided with the growth of the Andes Mountains. Currently, the decrease in convergence rate is explained either by the increasing load of the Andes or by the appearance of flat slab segments beneath South America. Here, we present an alternative view based on a thermomechanical self-consistent (gravity driven) model of Nazca plate subduction. We explain the changes in the convergence rate as a natural consequence of the Nazca plate penetration into the transition zone and lower mantle after long-term oblique subduction of the Farallon plate. The model is consistent with seismic tomographic images of the Nazca plate beneath South America. Our model also shows that the presence of the Andes does not significantly affect the convergence rate between the Nazca and South America plates.

Crustal structure of the Eastern Sierras Pampeanas of Argentina using high frequency local receiver functions

The Eastern Sierras Pampeanas are basement cored outcrops uplifted in the Andean foreland where the easternmost segment of the Pampean flat slab segment starts dipping more steeply into the deeper mantle. These ranges of central Argentina known as the Sierras de Córdoba have an enriched-quartz composition and are bounded by a series of reverse faults. Different models have been suggested to represent the style of the thick-skinned deformation in this area. However the overall structure linking the exposed faults and terrane boundaries with their probable continuation at depth is unknown. In this paper we present images of the crustal structure beneath the Sierras de Córdoba using the common conversion point stacking method of high frequency local receiver functions recorded by the ESP broadband seismic array. The work consists of two transects located around 31°S and 32°S across the Sierras de Córdoba. The results show a consistent sharp Moho signal associated with a high contrast in seismic velocities in good agreement with the granitic character of the crust lying above the mafic upper mantle. The Moho morphology varies exhibiting thicknesses of 38km in the west to 35km in the east with a vertical shifting under the Sierra Chica. We relate this variable character to the presence of the boundary between the Pampia terrane and the Rio de La Plata craton. Our results for the intra-crustal structure indicate the presence of three discontinuities in the northern transect and at least two discontinuities in the southern transect. These discontinuities appear vertically displaced beneath the surface traces of the major range bounding faults providing evidence for the continuation to mid-crustal depths of the exposed reverse faults. Thus, the reverse faults seem to have displaced several horizontal intra-crustal interfaces. Finally in the lower crust we found a region which seems to be aseismic.

Stable isotope composition of middle Miocene carbonates of the Frontal Cordillera and Sierras Pampeanas: Did the Paranaense seaway flood western and central Argentina?

The geographic extent, interconnectedness and chronology of the Miocene “Paranaense” epeiric seaway have been the subject of considerable debate. Understanding the timing and location of this marine incursion is significant for documenting sea level changes, paleoclimatic changes, and surface uplift or subsidence. Stable isotope analyses of carbonate strata in the flat-slab segment of the Sierras Pampeanas and high Andean Cordillera, previously purported to be of marine origin, provide evidence that the Paraná seaway did not inundate this portion of west and central Argentina. No unambiguously marine facies or isotopic signatures were recognized for five representative stratigraphic units: the Saguión Formation ESE of Salinas Grande in Córdoba Province; Anta Formation in the Quebrada de la Yesera and Lerma Valley of Salta Province; Del Buey and Del Abra Formations within the Famatina Ranges, La Rioja Province; and Chinches Formation in the Manantiales foreland basin in the Frontal Cordillera, San Juan Province. Paleontologic and lithologic features in support of a marine origin are reconsidered herein. Instead, a lacustrine origin is inferred for these formations, their contemporaneity perhaps related to concurrent global climate conditions and broad tectonic setting. Until more substantive evidence of marine deposition is found, we reject correlation of these units with the Paraná seaway, preserved in the Chaco-Pampean Plain subsurface, and discourage mapping of the seaway as extending into the central Sierras Pampeanas or Andean foreland. Our findings suggest the Miocene Andean foreland was elevated above sea level although it remained as a largely flat-lying area west of the Pampas and Chaco plains, which were inundated by the Paraná seaway.

Petrology of the Miocene igneous rocks in the Altar region, main Cordillera of San Juan, Argentina. A geodynamic model within the context of the Andean flat-slab segment and metallogenesis

The Altar porphyry Cu–(Au–Mo) deposit (31° 29′ S, 70° 28′ W) is located in the Andean Main Cordillera of San Juan Province (Argentina), in the southern portion of the flat-slab segment (28–33°S), 25 km north of the world-class porphyry Cu–Mo deposits of Los Pelambres and El Pachón. Igneous rocks in the area have been grouped into the Early Miocene Lower Volcanic Complex -composed of intercalations of lava flows and thin volcaniclastic units that grade upwards to a thick massive tuff- and the Middle-Late Miocene Upper Subvolcanic Suite that consists of a series of porphyritic stocks and dikes and magmatic and hydrothermal breccias. The Lower Volcanic Complex represents an Early Miocene arc (20.8 Ma ± 0.3 Ma; U–Pb age) erupted over a steep subduction zone. Their magmas equilibrated with an assemblage consisting of plagioclase- and pyroxene-dominated mineral residues, and experienced fractional crystallization and crustal contamination procesess. Their radiogenic signatures are interpreted to indicate conditions of relatively thickened continental crust in Altar during the Early Miocene, compared to the south and west. The Upper Subvolcanic Suite represents the development of a Middle-Late Miocene arc (11.75 ± 0.24 Ma, 10.35 ± 0.32 Ma; U-Pb ages) emplaced over a shallow subduction zone. A magmatic gap in Altar area betwen the Lower Volcanic Complex and Upper Subvolcanic Suite correlates with documented higher rates of compression in this period, that may have favored the storage of the USS magmas in cameras within the crust. Magmas of the Upper Subvolcanic Suite require a hornblende-bearing residual mineral assemblage that is interpreted to reflect their higher water contents. The relatively uniform radiogenic isotope compositions of the Upper Subvolcanic Suite magmas suggest a homogeneously mixed crust-mantle contribution in the source region. They have similar REE signatures as other fertile intrusives of the flat-slab. The differences observed in their isotopic signatures reflect an increase in the amount of crustal components incorporated into magmas from south (El Teniente) to north (El Indio) which correlate with an increase of crustal thickness. U–Pb ages of Altar rocks confirm a temporal connection between the ridge arrival and the magmatism associated with mineralization in this zone of the flat-slab segment. We argue that since the Middle-Late Miocene, mantle and lower crust may have been hydratated by fluids from the slab and from the Juan Fernández Ridge at the latitude of Altar, which favored the generation of the Middle-Late Miocene magmas. We also suggest that at this latitude the collision ridge-trench at ∼11 Ma and the subduction of the Juan Fernández Ridge beneath Altar region at ∼11–10 Ma may have promoted changes in the tectonic stress regime, allowing the USS magmas to rise to shallower levels in the crust. This may explain the location of a cluster of contemporaneous giant porphyry Cu–Mo deposits including El Pachón-Los Pelambres, the Altar Cu (Mo–Au) deposit, and other nearby recently discovered Cu prospects such as Piuquenes, La Coipa, Rincones de Araya and Los Azules.

Flat-Slab Thermal Structure and Evolution Beneath Central Mexico

Recent seismic and magnetotelluric experiments, aimed at better characterizing the shape and state of the subducting slab and continental crust beneath Central Mexico, exposed significant differences with conclusions of previous studies. A new slab geometry is revealed in which the subducting Cocos slab is perfectly flat between 120 to 290 km from the trench, after which it plunges into the asthenosphere at a dip angle of ~65°, in sharp contrast with the previously proposed ~20° dip angle. Seismic tomography studies show negative P-wave velocity anomalies (−2 to −4%) in the mantle wedge beneath the Mexican Volcanic Belt, and positive anomalies (+2 to +3%) for the subducted Cocos slab. Magnetotelluric experiments exposed a very low-resistivity area (1–10 Ωm) located within the continental crust just below the Mexican Volcanic Arc. Finally, several spots of non-volcanic tremors (NVTs) have been recorded inside the continental crust above the flat-slab segment. While all these experiments provide a better picture of the subduction system beneath Central Mexico, several key processes need further investigation. In this study, we take advantage of these new observations to better constrain the thermal structure beneath Central Mexico. Two different thermal models are computed for a mantle potential temperature (T p) of 1,350 and 1,450°C, respectively. The new thermal structures are then converted into P-wave velocity anomalies and compared with the observed V p anomalies. We found that a T p of 1,450°C produced larger V p anomalies that do not fit the observations. However, using a T p of only 1,350°C, our predicted V p anomalies are positive (+2 to +3%) for the cold slab and negative (−2 to −4%) in the mantle wedge. These V p estimates are consistent with the observed seismic tomography from P-wave arrivals, and therefore we conclude that a T p of 1,350°C is a better estimate for the mantle potential temperature beneath Central Mexico. The new thermal model, in conjunction with phase diagrams for sediments, hydrated basalt and lithospheric mantle, have been used to estimate the amount and location of fluids released from the subducting Cocos slab. Several dehydration pulses have been identified along the slab interface where most of the fluids stored in sediments and oceanic crust are released into the overlying continental crust above the flat-slab. We found a good correlation between the pattern of these dehydration pulses and the location of NVTs, suggesting that slab dehydration is responsible for triggering the tremors. We suggest that NVT bursts localized above the flat slab segment represent the manifestation of ongoing continental crust hydration and weakening, a process that has been going on since 15 Ma ago when the Cocos slab entered into a flat-slab regime. Such continuous weakening would have reduced the suction forces that kept the slab in a flat regime in the last 15 Ma, allowing the slab to easily roll back. The continuous low-resistivity region recorded beneath the volcanic front in Central Mexico might represent the evidence of slab dehydration and crust weakening over time.

Building the central Andes through axial lower crustal flow

The orogen‐scale morphology of the central Andes, which are longitudinally symmetric at ∼19°S, correlates with the modern geometry of the subducting Nazca slab but is largely independent of local bedrock geology or precipitation. Crustal thickness, as measured by the cross‐strike width of the orogen above 3000 m and the cross‐sectional area of the crust, is not well correlated with observed magnitudes of upper crustal shortening. In our interpretation, the large‐scale morphology is instead correlated with and controlled by lithospheric‐scale processes and subduction dynamics. Using a semianalytic, three‐dimensional Newtonian viscous flow model, we produce Andean‐like topography and distribution of upper crustal shortening during subduction of an oceanic lithosphere eastward beneath the Andes provided that (1) the more steeply dipping slab segment beneath the central Andes is overlain by the weak lower or middle crust, (2) the flat slab subduction segments to the north and south of this zone are overlain by the strong middle and lower crust, and (3) the steeply dipping central slab segment is overlain by a narrow, centrally localized zone of increased crustal shortening. The resulting model orogen displays substantial orogen‐parallel flow in the weak lower crust above the steep slab zone. Orogen‐parallel flow does not penetrate into the regions of stronger lower crust above the flat slab segments, resulting in a broad, plateau‐topped orogen above the central slab segment and narrow but high orogenic segments above the flat slab region. Redistribution of material through lower crust flow can account for the observed mismatch between crustal volume and upper crustal shortening in the central Andes and may explain young (<10 Ma) crustal thickening and surface uplift that has been argued for in some regions.

Tectonic and dynamic controls on the topography and subsidence of the Argentine Pampas: The role of the flat slab

We analyze the Pampean foreland (the Pampas) along the modern flat-slab segment of the south-central Andes between 31° and 33° South latitude and to the east of the Argentine “flat-slab” province, using flexural and gravity studies and computations of dynamic topography. Bouguer anomalies and flexural analysis predict a foredeep of ∼250km width and a peripheral bulge amplitude of ∼25m, which match the regional morphologies of the modern Argentine Pampean Plain. However, these studies do not account for the subsurface Miocene–Quaternary basin preservation, represented by sedimentary thicknesses >400m and with depocenters >400km eastward with respect to flexural models. The discrepancy suggests that two mechanisms, acting at different wavelengths, influence the Argentine Pampas. The basin preservation is likely the result of a large-scale geodynamic forcing. Models of mantle flow, driven by realistic flat-slab subduction geometry and density contrasts, reproduce the depocenter location and the wavelength of subsidence as well as most of the remaining amplitude. Nonetheless, more sophisticated studies (e.g. considering lateral viscosity variations in the mantle wedge) might help reduce the dynamic amplitudes and better reproduce the observed geological record.

Geodynamic controls on Tertiary arc magmatism in Ecuador: Constraints from U–Pb zircon geochronology of Oligocene–Miocene intrusions and regional age distribution trends

We obtained U–Pb zircon ages of Late Tertiary intrusions in the northern Andes to provide robust time calibration points for the intrusive geochronologic framework of Ecuador which is mostly based on K–Ar data. Magmatism associated with intrusion emplacement ranges from 31 to 7 Ma, with ages mainly pooling in the Late Oligocene–Early Miocene. Where both K–Ar and U–Pb data exist for a given intrusive system, ages obtained by the different methods are usually concordant within 1–4 m.y. suggesting that K–Ar ages are accurate indicators for the timing of Tertiary arc magmatism on a regional multi-m.y. scale. Except for a single sample, the investigated intrusions completely lack externally inherited zircons, in agreement with dominantly zircon-poor, oceanic crustal basement domains. Spatio-temporal distribution trends of Tertiary arc magmatism inferred from screened U–Pb, K–Ar, and zircon fission track geochronologic data allow tracking of the progressive along-arc broadening of a flat slab segment below southernmost Ecuador in the Mid- to Late Miocene, and of moderate slab shallowing in northern-central Ecuador in the Late Miocene. These regional arc migration patterns correlate in time with the subduction of the buoyant Inca Plateau and the Carnegie Ridge seamount chain. The temporal distribution of Tertiary Ecuadorian arc magmatism indicates a Late Oligocene–Early Miocene arc magmatic flare-up event comprising widespread ignimbrite eruption and batholith construction. Initiation of the flare-up event coincides in time with accelerating, less oblique Farallon/Nazca-South America plate convergence, suggesting a positive feedback between convergence rates, asthenospheric melt production, mantle–crust melt flux, and upper crustal arc magmatic productivity in Ecuador.

Andean flat-slab subduction through time

Abstract The analysis of magmatic distribution, basin formation, tectonic evolution and structural styles of different segments of the Andes shows that most of the Andes have experienced a stage of flat subduction. Evidence is presented here for a wide range of regions throughout the Andes, including the three present flat-slab segments (Pampean, Peruvian, Bucaramanga), three incipient flat-slab segments (‘Carnegie’, Guañacos, ‘Tehuantepec’), three older and no longer active Cenozoic flat-slab segments (Altiplano, Puna, Payenia), and an inferred Palaeozoic flat-slab segment (Early Permian ‘San Rafael’). Based on the present characteristics of the Pampean flat slab, combined with the Peruvian and Bucaramanga segments, a pattern of geological processes can be attributed to slab shallowing and steepening. This pattern permits recognition of other older Cenozoic subhorizontal subduction zones throughout the Andes. Based on crustal thickness, two different settings of slab steepening are proposed. Slab steepening under thick crust leads to delamination, basaltic underplating, lower crustal melting, extension and widespread rhyolitic volcanism, as seen in the caldera formation and huge ignimbritic fields of the Altiplano and Puna segments. On the other hand, when steepening affects thin crust, extension and extensive within-plate basaltic flows reach the surface, forming large volcanic provinces, such as Payenia in the southern Andes. This last case has very limited crustal melt along the axial part of the Andean roots, which shows incipient delamination. Based on these cases, a Palaeozoic flat slab is proposed with its subsequent steepening and widespread rhyolitic volcanism. The geological evolution of the Andes indicates that shallowing and steepening of the subduction zone are thus frequent processes which can be recognized throughout the entire system.

Tectonic response of the central Chilean margin (35–38°S) to the collision and subduction of heterogeneous oceanic crust: a thermochronological study

Along-strike geological segmentation in the Andean chain has been recognized at various scales and is usually attributed to changes in plate motion vectors, as well as the upper-plate expression of differing subducted slab age, strength and composition. We present new multi-phase 40 Ar/ 39 Ar, apatite fission-track and zircon and apatite (U–Th)/He data from a north–south-oriented traverse between 35 and 38°S along the Principal Andean Cordillera of Chile that reveal (1) rapid cooling at 18 and 15 Ma, which can be attributed to both thermal relaxation following magmatic intrusion and regional-scale exhumation, and (2) along-strike differences in the extent of exhumation since 7.5 Ma that may be a consequence of the subduction of the Juan Fernandez Ridge above the flat-slab segment at 32°, since 10 Ma. A comparison of the response of the South American Plate to the collision of the Carnegie, Nazca and Juan Fernandez ridges suggests that slab flattening is not the dominant driving force that exhumes the upper plate in these settings. Rather, the extent of exhumation is controlled by pre-existing structural weaknesses, the time duration of the dynamically supported topography, and climate.

Flat subduction dynamics and deformation of the South American plate: Insights from analog modeling

We present lithospheric‐scale analog models, investigating how the absolute plates' motion and subduction of buoyant oceanic plateaus can affect both the kinematics and the geometry of subduction, possibly resulting in the appearance of flat slab segments, and how it changes the overriding plate tectonic regime. Experiments suggest that flat subductions only occur if a large amount of a buoyant slab segment is forced into subduction by kinematic boundary conditions, part of the buoyant plateau being incorporated in the steep part of the slab to balance the negative buoyancy of the dense oceanic slab. Slab flattening is a long‐term process (∼10 Ma), which requires the subduction of hundreds of kilometers of buoyant plateau. The overriding plate shortening rate increases if the oceanic plateau is large enough to decrease the slab pull effect. Slab flattening increases the interplate friction force and results in migration of the shortening zone within the interior of the overriding plate. The increase of the overriding plate topography close to the trench results from (1) the buoyancy of the plate subducting at trench and (2) the overriding plate shortening. Experiments are compared to the South American active margin, where two major horizontal slab segments had formed since the Pliocene. Along the South American subduction zone, flat slab segments below Peru and central Chile/NW Argentina appeared at ∼7 Ma following the beginning of buoyant slab segments' subduction. In northern Ecuador and northern Chile, the process of slab flattening resulting from the Carnegie and Iquique ridges' subductions, respectively, seems to be active but not completed. The formation of flat slab segments below South America from the Pliocene may explain the deceleration of the Nazca plate trenchward velocity.

Cenozoic provenance history of synorogenic conglomerates in western Argentina (Famatina belt): Implications for Central Andean foreland development

In Famatina, at the modern northern flat-slab segment of the Central Andean belt, synorogenicconglomeratefaciesprovideaunique opportunity to reconstruct paleogeography and tectonic setting. Stratigraphically constrained provenance analyses record a multi-event history of the Andean foreland and a complex pattern of exhumed uplands, which, in turn, controlled the depocenter history. Western and eastern sources can be differentiated. The western domain was composed of equigranular granitoids and lower and upper Paleozoic volcano-sedimentary and sedimentary units, whereas the eastern domain was mainly formed by porphyrytic granitoids and minor slightly metamorphosed lower Paleozoic clastic rocks. The surrounding regions expose medium- to high-grade metamorphic rocks, which are not recorded in any Cenozoic conglomerate of Famatina. Unroofing sequences can be deduced from the compositional trends. Conglomerate composition and paleocurrent data show input from intraba-sinal sources and important participation of basement since the beginning of the Neogene. We infer an early Miocene broken foreland during deposition of the Del Creston Formation.A significant source inversion, with clasts supplied from the eastern basement domain, is recorded ∼2000 m from the base, within the middle Miocene Del Abra Formation; reactivation of western granitic thrust sheets occurred by the late middle Miocene and late Miocene, as indicated by provenance studies of the Santo Domingo and El Durazno Formations. In contrast to simple asymmetric foreland models, provenance analysis documents early participation of crystalline rocks, suggesting early development of intrafore-land basement highs within the southern Central Andes. At a regional scale, basement thrusting and associated intermontane basin development were diachronous. This interpretation is supported by broad propagation of basement thrusting from the northern Sierras Pampeanas (and Famatina) toward the central and eastern Sierras Pampeanas between early-middle Miocene and late Miocene–Pliocene time. Early Miocene reconstructions of the Juan Fernandez Ridge position on the Central Andean margin suggest that during this interval, shallow subduction was likely unrelated to oceanic plateau subduction under the Sierras Pampeanas.