The dynamics of the erosive central Andean forearc vary significantly along strike. In northern Chile at 20°S–27°S, and particularly at 22°S–25°S, the forearc in the Coastal Cordillera has been undergoing extension since at least the Pliocene, reactivating steep E-dipping faults of the Mesozoic Atacama fault system. This has been explained by forearc uplift driven by underplating, shallow slab dip, subduction of bathymetric features, and elastic rebound during the earthquake cycle. These processes, however, are active over a much wider area of the central Andean forearc than Coastal Cordillera extension and therefore cannot explain why extension is localized to the northern Chilean onshore outer forearc. We compiled crustal seismicity and the depth of lithospheric boundaries from existing studies to investigate other possible explanations for onshore forearc extension. At 22°S–25°S, seismicity increases above the background subduction-related level present to the north and south. Extensional focal mechanisms, consistent with steep E-dipping faults active at depths up to ~40 km, are also present onshore within this latitude range, but absent to the north and south; this is consistent with the distribution of mapped active fault scarps. The Salar de Atacama crust is seismically active at depths up to ~40 km. Thick lithosphere is present beneath the forearc, and the longitudinal axis of thickest lithosphere is deflected to the east at the latitude of the Salar de Atacama. To the east, the Puna Plateau lithosphere has been thinned by lithospheric removal events. The most robust correlation with onshore forearc extension is the thick, cold, strong crust and mantle lithosphere beneath the anomalous Salar de Atacama in the inner forearc of northern Chile. The combination of underplating-driven outer forearc uplift, the presence of the preexisting structure of the Atacama fault system favorable for reactivation, and the negative buoyancy beneath the Salar de Atacama is inferred to drive Coastal Cordillera normal faulting at this latitude. Recent (<10 Ma) lithospheric removal beneath the Puna Plateau to the east may have enhanced the effect of the negatively buoyant Salar de Atacama lithosphere on the forearc. This implies that both preexisting lithospheric structure and lithospheric processes in the hinterland may influence forearc dynamics.

Low-temperature thermochronological ages of samples from the central Andes correlate with major tectonic events during Late Cretaceous and Cenozoic times. Apatite fission-track (AFT) ages show prominent clusters during the Early–Late Cretaceous in the Coastal Cordillera and the Cordillera de Domeyko; Paleocene–Oligocene ages in the western Puna Plateau and Cordillera de Domeyko; and latest Eocene–Pliocene ages in the Eastern Cordillera. These ages track the expansion of the Andean orogenic edifice, the eastern front of which migrated rapidly eastward ~200 km and ~150 km during late Eocene and Pliocene times, respectively. During the intervening time interval, ca. 35–5 Ma, the orogenic strain front migrated slowly eastward through the Eastern Cordillera. A second cluster of Cretaceous ages in the Eastern Cordillera and Santa Bárbara Ranges documents exhumation related to extension in the Salta rift. The highly unsteady pace of orogenic wedge propagation suggests that kinematics controlled local climate, rather than vice versa. The frequency of AFT ages is anticorrelated with magmatic production in the central Andean arc and the rate of convergence between the Nazca and South American plates. We propose a link between AFT bedrock cooling ages in the central Andes and exhumation related to cyclical processes of shortening, wedge propagation, magmatism, and removal of dense roots from beneath the magmatic arc and thickened hinterland region. In particular, periods of sustained exhumation associated with local crustal shortening alternate with periods of rapid eastward wedge propagation during which exhumation was more spatially diffuse across the high-elevation hinterland. Episodes of spatially confined exhumation are correlated with periods of relatively low magmatic production in the central Andean arc and relatively slow or declining plate convergence rates. We speculate that shortening in the upper crust was contemporaneous with underthrusting of lower crust and mantle lithosphere beneath the magmatic arc. Because of thermal inertia, melting of these underthrusted rocks lagged behind the shortening events themselves, thus producing the observed temporal anticorrelation between rapid shortening-induced exhumation and arc magmatism.

Integrated magmatic, structural, and geophysical data provide a basis for modeling the Neogene lithospheric evolution of the high Central Andean Puna-Altiplano Plateau. Reconstruction of three transects south of the Bolivian orocline in the Altiplano and Puna Plateau shows processes in common, including subduction characterized by relatively shallow and changing slab dips, crustal shortening, delamination of thickened lower crust and lithosphere, crustal melting, eruption of giant ignimbrites, and deep crustal flow. Temporal similarities in events in the three transects can be correlated with changes in the rate of westward drift of South America and slab rollback. Temporal differences between the three transects can be attributed to variations in Nazca plate geometry in response to southward subduction of the aseismic Juan Fernandez Ridge. Subduction of the north-south arm of the ridge can explain an Oligocene flat slab under the Altiplano, and subduction of a northeast arm of the ridge can explain a long period of relatively shallow subduction characterized by local steepening and shallowing. Major episodes of ignimbrite eruption and delamination have occurred over steepening subduction zones as the ridge has passed to the south. Late Miocene to Holocene delamination of dense lithosphere is corroborated by published seismic images. The southern Altiplano transect (17°S–21°S) is notable for high, structurally complex Western and Eastern Cordilleras flanking the Altiplano Basin, the eastern border of which is marked by late Miocene ignimbrites. The broad Subandean fold-and-thrust belt lies to the east. The Neogene evolution can be modeled by steepening of a shallowly subducting plate, leading to mantle and crustal melting that produced widespread volcanism including large ignimbrites. Major uplift of the plateau at 10–6.7 Ma was dominantly a response to crustal thickening related to Subandean shortening and peak lower-crustal flow into the Altiplano from the bordering cordilleras as the ignimbrites erupted, and partly a response to delamination along the eastern Altiplano border. A smaller ignimbrite volume than in the northern Puna suggests the Altiplano lithosphere never reached as high a degree of melting as to the south. An Oligocene flat-slab stage can explain extensive Oligocene deformation of the high plateau region. The northern Puna transect at ~21°S–24°S is notable for voluminous ignimbrites (>8000 km 3 ) and a narrower Subandean fold-and-thrust belt that gives way southward to a thick-skinned thrust belt. The evolution can be modeled by an early Miocene amagmatic flat slab that underwent steepening after 16 Ma, which led to mantle melting that culminated in widespread ignimbrite eruptions beginning at 10 Ma, peaking in the backarc at ca. 8.5–6 Ma, restricted to the near arc by 4.5 Ma, and ending by 3 Ma. The formation of eclogitic residual crust caused periodic lower-crustal and lithospheric mantle delamination. Late Miocene uplift was largely due to crustal thickening in response to crustal shortening, magmatic addition, and delamination. Crustal flow played only a minor role. The high degree of mantle and crustal melting can be explained as a response to steepening of the early Miocene flat slab. The southern Puna transect at ~24°S–~28°S is notable for eastward frontal arc migration at 8–3 Ma, intraplateau basins bounded by high ranges, long-lived Miocene stratovolcanic-dome complexes, voluminous 6–2 Ma ignimbrites, 7–0 Ma backarc mafic flows, and the latest Miocene uplift of the reverse-faulted Sierras Pampeanas ranges to the east. Its evolution can be modeled by a moderately shallow slab producing widespread volcanism with subsequent steepening by 6 Ma, leading to delamination of dense lithosphere culminating in the eruption of the voluminous Cerro Galan ignimbrite at 2 Ma.

The Sierras Pampeanas in the west-central part of Argentina are a modern analog for Laramide uplifts in the western United States. In this region, the Nazca plate is subducting beneath South America almost horizontally at about ~100 km depth before descending into the mantle. The flat-slab geometry correlates with the inland prolongation of the subducted oceanic Juan Fernández Ridge. This region of Argentina is characterized by the termination of the volcanic arc and uplift of the active basement-cored Sierras Pampeanas. The upper plate shows marked differences in seismic properties that are interpreted as variations in crustal composition in agreement with the presence of several Neoproterozoic to Paleozoic accreted terranes. In this paper, we combine the results from the CHile-ARgentina Geophysical Experiment (CHARGE) and the CHile-ARgentina Seismology Measurement Experiment (CHARSME) passive broadband arrays to better characterize the flat-slab subduction and the lithospheric structure. Stress tensor orientations indicate that the horizontal slab is in extension, whereas the upper plate backarc crust is under compression. The Cuyania terrane crust exhibits high P-wave seismic velocities (Vp ~6.4 km/s), high P- to S-wave seismic velocity ratios (Vp/Vs = 1.80–1.85), and 55–60 km crustal thickness. In addition, the Cuyania terrane has a high-density and high-seismic-velocity lower crust. In contrast, the Pampia terrane crust has a lower Vp value of 6.0 km/s, a lower Vp/Vs ratio of 1.73, and a thinner crust of ~35 km thickness. We integrate seismic and gravity studies to evaluate crustal models that can explain the unusually low elevations of the western Sierras Pampeanas. Flat-slab subduction models based on CHARGE and CHARSME seismic data and gravity observations show a good correlation with the predicted Juan Fernández Ridge path beneath South America, the deep Moho depths in the Andean backarc, and the high-density and high-seismic-velocity lower crust of the Cuyania terrane. The Cuyania terrane is also the region characterized by more frequent and larger-magnitude crustal earthquakes.

Abstract The southern Patagonian Cordillera south of the present location of the Chile Triple Junction (46.5°S) preserves distinctive deformational and backarc magmatic features that are a consequence of a series of northward-propagating ridge collision events that started at ca. 14 Ma. An abrupt increase of ~2000 m of topographic elevation and the exhumation and uplift of mid-Miocene to Pliocene plutons within the cordillera south of the Chile Triple Junction is accomplished by horizontal compressive deformation (both thin- and thick-skinned) within the Patagonian fold-thrust belt. Ridge-trench collisions have formed asthenospheric slab windows beneath the southern Patagonian Cordillera. Backarc magmatism associated with slab window formation includes a distinctive suite of adakites and extensive outpourings of oceanic island basalt (OIB)-like plateau basalts. The adakites formed from the partial melting of the young, hot trailing edge of the Nazca plate that preceded slab window opening, whereas the OIB-like plateau basalts formed from dynamic asthenospheric flow as the slab windows opened up beneath the backarc.

Abstract This field guide provides an opportunity to examine the central Andes between 31° and 32°S latitude in a segment characterized by flat-slab subduction. The field trip road was chosen to observe the westernmost contact between the basement uplift of Sierras Pampeanas and Precordillera, the early Paleozoic stratigraphy, and the Andean structure of the Precordillera, as well as a complete section of the Frontal and Principal Cordilleras in Argentina and Chile. The trip ends in the Coastal Cordillera along the Pacific margin. This road log discusses a complete early and late Paleozoic history of the central Andes with their typical Famatinian and Gondwanan orogenic rocks and the accretionary evolution of the Pacific margin at these latitudes. Superimposed on this framework, the structure of the Andes is viewed through the examination of the Precordillera and the Aconcagua fold-and-thrust belts, together with the observation of the Andean volcanic history, will allow reconstructing the shallowing of the subduction zone through the Neogene and the final formation of the Pampean flat-slab.

To compare magmatic crystallization temperatures between ocean island basalt (OIB) proposed to be plume-related and normal mid-ocean ridge basalt (MORB) parental liquids, we have examined and compared in detail three representative magmatic suites from both ocean island (Hawaii, Iceland, and Réunion) and mid-ocean ridge settings (Cocos-Nazca, East Pacific Rise, and Mid-Atlantic Ridge). For each suite we have good data on both glass and olivine phenocryst compositions, including volatile (H 2 O) contents. For each suite we have calculated parental liquid compositions at 0.2 GPa by incrementally adding olivine back into the glass compositions until a liquid in equilibrium with the most-magnesian olivine phenocryst composition is obtained. The results of these calculations demonstrate that there is very little difference (a maximum of ∼20 °C) between the crystallization temperatures of the parental liquids (MORB 1243–1351 °C versus OIB 1286–1372 °C) when volatile contents are taken into account. To constrain the depths of origin in the mantle for the parental liquid compositions, we have performed experimental peridotite-reaction experiments at 1.8 and 2.0 GPa, using the most magnesian of the calculated parental MORB liquids (Cocos-Nazca), and compared the others with relevant experimental data utilizing projections within the normative basalt tetrahedron. The mantle depths of origin determined for both the MORB and OIB suites are similar (MORB 1–2 GPa; OIB 1–2.5 GPa) using this approach. Calculations of mantle potential temperatures (T P ) are sensitive to assumed source compositions and the consequent degree of partial melting. For fertile lherzolite sources, T P for MORB sources ranges from 1318 to 1488 °C, whereas T P for ocean island tholeiite sources (Hawaii, Iceland, and Réunion) ranges from 1502 °C (Réunion) to 1565 °C (Hawaii). The differences in T P values between the hottest MORB and ocean island tholeiite sources are ∼80 °C, significantly less than predicted by the thermally driven mantle plume hypothesis. These differences disappear if the hotspot magmas are derived by smaller degrees of partial melting of a refertilized refractory source. Consequently the results of this study do not support the existence of thermally driven mantle plumes originating from the core-mantle boundary as the cause of ocean island magmatism.

New and recycled intermediate to felsic crust in island arcs and continental margins is generated by magmatism surmounting convergent plate junctions. Paired Pacific-type orogens develop adjacent sites of long-lived subduction of oceanic lithosphere. They consist of (1) an outboard accretionary prism deposited in and continentward from the oceanic trench, the filling of which is derived mainly from (2) an inboard calc-alkaline volcanic-plutonic arc. The trench assemblage includes arc-sourced clastic mélanges, minor but widespread deepwater cherts and carbonates, and tectonically disaggregated ophiolites. These relatively incompetent sections recrystallize under high-pressure (HP) conditions and decouple from the descending oceanic plate at depths of ∼15–50 km. A massive, slightly older to coeval andesitic-granodioritic arc dominates the landward belt where new sialic crust is added from I-type magmas, and preexisting continental materials are recycled as S-type melts; high temperature (HT) characterizes local-regional metamorphism of the wall rocks. In contrast, Alpine-type orogens result from the underflow of an oceanic plate that transports island arcs, microcontinents, and/or continental salients into the subduction zone. Reflecting lithospheric coherence, the sialic crust may be carried down as much as 90–140 km, becomes thermally softened, and decouples from the sinking plate. Metamorphism of deeply subducted parts of Alpine belts ranges from high pressure to ultrahigh pressure (UHP), and is not paired with a HT calc-alkaline arc. In both types of subduction complex, outboard thrust faults dip landward and fold vergence is seaward, reflecting the polarity of underflow (and rollback) of the oceanic lithosphere. Antithetic thrusting typifies some contractional continental realms. Propelled by buoyancy, allochthonous nappes conduct relatively low-density HP and UHP sections to midcrustal levels. At convergent syntaxes (plate-margin cusps) of overthickened arcs, tectonic aneurysms may produce domical uplifts, further exhuming UHP units surfaceward. The regional crustal thickness of an active mountain belt partly reflects climate as well as the main convergent-transform plate processes producing the orogen. The volcanogenic Chilean Cordillera parallels the seaward convergent plate junction and the eastward-sinking Nazca plate. The highest ranges and thickest continental crust (∼70 km) occur in the compressed north-central Andes at ∼25°S, a region of extremely low rain- and snowfall. Aridity and low erosion rates help account for the high-standing calc-alkaline volcanic-plutonic contractional arc and the elevated, internally drained Altiplano downwind. Thus erosional degradation is weakly developed. A very different climate typifies the orogen at ∼45°S, where abundant moisture-laden westerlies bring abundant precipitation to the Chilean margin. The rugged mountain belt is of lower elevation compared with the north-central Andes and is supported by a crust of only moderate thickness (35–40 km). Vigorous erosion supplies voluminous detritus to the offshore Chile-Peru Trench as well as eastward, so a plateau is lacking in the lee of the arc. The Himalayas and Tibetan Plateau, the Sierra Nevada and Colorado Plateau, the Japanese island arc, and New Zealand exhibit somewhat similar relationships between crustal thickness and precipitation-linked erosion.

Teleseismic earthquakes recorded within the ISSA (Integrated Seismological Experiment in the Southern Andes) temporary seismic experiment in the Southern Andes between 36° and 40°S latitude have been used to construct receiver function images of the crust and upper mantle. The oceanic Moho of the subducted Nazca plate is observed down to a depth of ∼100 km, corresponding well with the Wadati-Benioff zone seismicity and wide-angle seismic reflections. Beneath the volcanic arc, the slab begins to be invisible with P-to-S converted waves, implying the completion of the gabbroeclogite transformation in the oceanic crust at that depth. The continental Moho has been imaged at depth of ∼40 km beneath the main cordillera and shallows toward the eastern end of the profile beneath the Neuquén Basin to ∼35 km depth. Beneath the Loncopué graben, the Moho is locally uplifted to 30 km depth, possibly resulting from the backarc spreading beginning in the Pliocene-Pleistocene. An anomalously high Poisson's ratio beneath the volcanic arc may indicate partial melting in the upper-plate crust.

The Neuquén Embayment, which developed along the eastern foothills of the southern Central Andes, has a complex history of intraplate deformation. The Paleozoic basement fabrics exerted a major influence in Mesozoic and Cenozoic deformation. The most important feature is an E-W–striking fault system that is related to a late Paleozoic fabric and is associated with the Huincul basement high, which truncates the basin. This fabric is interpreted as being the result of the accretion of the Patagonia terrane with Gondwana during the Early Permian. Two-dimensional (2D) and three-dimensional (3D) seismic coverage and subsurface information identify different sectors in the Neuquén Embayment that record alternating episodes of contraction and extension during the Jurassic and Cretaceous. The deformation history east of the thrust front of the Agrio fold-and-thrust belt is characterized by periods of (1) transpression and almost orthogonal contraction to the continental margin, (2) extension, and (3) relative quiescence, which alternates in different sectors. The earliest shortening occurred in the Early Jurassic when the main stress was oriented in the N-NW sector. The stress rotated to the northwest up to Valanginian times, when a more orthogonal orientation to the continental margin became dominant and prevailed after the Cenomanian. After a period of quiescence in the Neuquén Embayment associated with very oblique subduction during the Paleogene, the final contractional deformation took place in the late Miocene, with a west-east orientation of the main stress, and was followed by Pliocene extension. The changing stress patterns correlate with differences in convergence vectors between the Aluk, Farallon, and Nazca oceanic plates and the Gondwana or South American continental plates. The Aluk stage from the Jurassic to the Valanginian was characterized by tectonic inversion that is shown by shortening and right-lateral strike-slip structures that are concentrated in the Huincul system and more subtle deformation in the Chihuidos and Entre Lomas systems. The early Farallon stage was distinguished by reduced inversion and displacement in the Huincul system and a general retreat of deformation after the Valanginian. The change to late Farallon stage was characterized by a prominent tectonic inversion of the Entre Lomas system, which resulted from the inception of the formation of the Agrio fold-and-thrust belt in the retroarc area. This belt developed during most of the Late Cretaceous, when the embayment showed a general quiescence. The Nazca stage was characterized by the main episode of uplift, tectonic inversion of the older half-grabens, and important strike-slip faulting that was followed by local collapse of some structures during the Pliocene.

New 40 Ar/ 39 Ar, major and trace element, and isotopic data for ca. 24–15 Ma backarc volcanic rocks from the Sierra de Huantraico, Sierra Negra, and Sierra de Chachahuén–La Matancilla regions (36°S–38°S) in the Neuquén Basin shed light on the early Miocene evolution of the south-central Andes. A model calling for incipient shallowing of the sub-ducting slab under the northern Neuquén Basin and an increase in the rate of westward motion of South America relative to the underlying mantle at ca. 20 Ma can explain many regional features. Early Miocene magmatism in the Neuquén Basin began with the eruption of ca. 24–20 Ma alkali olivine basalts from monogenetic and simple polygenetic centers located up to 500 km east of the trench. Their characteristics (Ta/Hf > 0.45, ε Nd = +3.6–+4.2; La/Ta < 14; Ba/La < 16; 87 Sr/ 86 Sr = 0.7037–0.7040) indicate a backarc mantle devoid of arc-like components. These basalts erupted at a time of extension all along the margin during a period of rapid, near-normal Nazca–South America plate convergence when spreading ridges between the Pacific, Nazca, and Antarctic plates were becoming more parallel to the Chile Trench. Ridge rotation along with slab roll-back in response to slow relative motion between South America and the underlying mantle can explain why isotopically enriched magmas erupted far to the east of the trench in a generally extensional regime. Subsequently, 19–15 Ma basaltic to trachyandesitic backarc lavas with weak arc-like La/Ta (15–26), Ba/La (15–32), and Ta/Hf (0.2–4.5) ratios and a more depleted isotopic signature (ε Nd = +3.9–+4.7; 87Sr/86Sr = 0.7033–0.7037) erupted in a contractional regime. Their chemical features fit with incipient shallowing of the Nazca plate under the northern Neuquén Basin. A contractional regime that extended all along the margin can be explained by westward acceleration of South America over the underlying mantle as Nazca–South America plate convergence slowed.