Abstract In this classic segment, many tectonic processes, like flat-subduction, terrane accretion and steepening of the subduction, among others, provide a robust framework for their understanding. Five orogenic cycles, with variations in location and type of magmatism, tectonic regimes and development of different accretionary prisms, show a complex evolution. Accretion of a continental terrane in the Pampean cycle exhumed lower to middle crust in Early Cambrian. The Ordovician magmatic arc, associated metamorphism and foreland basin formation characterized the Famatinian cycle. In Late Devonian, the collision of Chilenia and associated high-pressure/low-temperature metamorphism contrasts with the late Palaeozoic accretionary prisms. Contractional deformation in Early to Middle Permian was followed by extension and rhyolitic (Choiyoi) magmatism. Triassic to earliest Jurassic rifting was followed by subduction and extension, dominated by Pacific marine ingressions, during Jurassic and Early Cretaceous. The Late Cretaceous was characterized by uplift and exhumation of the Andean Cordillera. An Atlantic ingression occurred in latest Cretaceous. Cenozoic contraction and uplift pulses alternate with Oligocene extension. Late Cenozoic subduction was characterized by the Pampean flat-subduction, the clockwise block tectonic rotations in the normal subduction segments and the magmatism in Payenia. These processes provide evidence that the Andean tectonic model is far from a straightforward geological evolution.

Abstract New U–Pb detrital zircon ages are presented for the Tordillo Formation. The ages indicate that the most important source region of sediment supply was the Jurassic Andean arc (peaks at c. 144, 153 and 178 Ma), although two secondary sources were defined at c. 218 and 275 Ma. Temporal variation in the provenance indicates that at the beginning of the sedimentation, Carboniferous to Lower Jurassic magmatic rocks and Lower Palaeozoic metamorphic rocks were the most important sources. Towards the top, the data suggest that the Andean arc becomes the main source region. The comparison between provenance patterns of the Tordillo Formation and of the Avilé Member (Agrio Formation) showed some differences. In the former, the arc region played a considerable role as a source region, but this is not identified in the latter. The results permit a statistically robust estimation of the maximum deposition age for the Tordillo Formation at c. 144 Ma. This younger age represents a discrepancy of at least 7 Ma from the absolute age of the Kimmeridgian and Tithonian boundary (from the chronostratigraphic timescale accepted by the International Commission of Stratigraphy, IUGS), and has strong implications for the absolute age of the Jurassic–Cretaceous boundary. Supplementary material: Sample coordinates, values of the sandstone compositional framework and U–Pb (LAM-MC-ICP-MS) age measurements of zircons grains are available at http://www.geolsoc.org.uk/SUP18718

Abstract Isolated marine sedimentary Lower Cretaceous deposits crop out in the foreland of the Neuquén Basin, west-central Argentina. They are the result of an anomalous uplift of the Sierra de Chachahuén in the far foreland region. These outcrops are assigned to the Agrio Formation based on their rich fossil contents. In particular, the study reveals a unique outcrop of continental facies along the eastern proximal margin of the basin that were known only from core wells, and constitutes the first exposed evidence at the surface. These deformed deposits are 70 km from the Andean orogenic front and present 2 km of local uplift produced by high-angle basement reverse faults that reactivated a previous Early Mesozoic rift system. The increase in compression was related to the decrease in the subduction angle. This fact, together with the expansion of the magmatic arc, controlled the Chachahuén calc-alkaline Late Miocene volcanic centre and the uplift of the Mesozoic deposits in the foreland. This broken foreland was associated with localized heating of the Miocene volcanic centre that produced the rising of the brittle-ductile transitions. This fact weakens the foreland area, which was broken by compression during the development of the Payenia flat-slab.

The Cordillera of western North America occupies the central 5000 km of the circum-Pacific orogenic belt, which extends for 25,000 km along a great-circle path from Taiwan to the Antarctic Peninsula. The North American Cordillera is anomalous because dextral transform faults along its western flank have supplanted subduction zones, the hallmark of circum-Pacific tectonism, along much of the Cordilleran continental margin since mid-Cenozoic time. The linear continuity of the Cordilleran orogen terminates on the north in the Arctic region and on the south in the Mesoamerican region at sinistral transform faults of Mesozoic and Cenozoic age, respectively. The Cordilleran margin of Laurentia was formed initially by rift breakup of the supercontinent Rodinia followed by development of the Neoproterozoic to early Paleozoic Cordilleran miogeocline along a passive continental margin, but it was modified in California and Mexico by Permian to Triassic transform truncation of Paleozoic tectonic trends. Late Paleozoic and Mesozoic accretion of oceanic island arcs and subduction complexes expanded the width of the Cordilleran orogen both before and after Triassic initiation of ancestral circum-Pacific subduction beneath the Cordilleran margin. Mesozoic to Cenozoic extensions and counterparts of Cordilleran accreted terranes extend southward into the Caribbean Antilles and northern South America. The development of successive forearc and retroforeland basins accompanied the progress of Cordilleran orogenesis over time, and coeval Mesozoic to Cenozoic batholith belts reflect continuing plate consumption at subduction zones along the continental margin. The assembly of subduction complexes along the Cordilleran continental margin continued into Cenozoic time, but dextral strike slip along the Pacific flank of the Cordilleran orogen displaced elongate coastal segments of the orogen northward during Cenozoic time. In the United States and Mexico, Laramide breakup of the Cordilleran foreland during shallow slab subduction and crustal extension within the Basin and Range taphrogen also expanded the width of the Cordilleran orogen during Cenozoic time.

The Andes make up the largest orogenic system developed by subduction of oceanic crust along a continental margin. Subduction began soon after the breakup of Rodinia in Late Proterozoic times, and since that time, it has been intermittently active up to the present. The evolution of the Pacific margin of South America during the Paleozoic occurred in the following stages: (1) initial Proterozoic rifting followed by subduction and final re-amalgamation of the margin in Early Cambrian times, as depicted by the Puncoviscana and Tucavaca Basins and related granitoids in southern Bolivia and northern Argentina; (2) a later phase of rifting in the Middle Cambrian, and subsequent collisions in Middle Ordovician times of parautochthonous terranes derived from Gondwana, such as Paracas, Arequipa, and Antofalla, and exotic terranes originating in Laurentia, such as Cuyania, Chilenia and Chibcha; (3) final Permian collision between South America and North America to form Pangea during the Alleghanides orogeny, leaving behind rifted pieces of Laurentia as the Tahami and Tahuin terranes in the Northern Andes and other poorly known orthogneisses in the Cordillera Real of Ecuador in the Late Permian–Early Triassic; and (4) amalgamation of the Mejillonia and Patagonia terranes in Early Permian times, representing the last convergence episodes recorded in the margin during the Gondwanides orogeny. These rifting episodes and subsequent collisions along the continental margin were the result of changes of the absolute motion of Gondwana related to global plate reorganizations during Proterozoic to Paleozoic times. Generalized rifting during Pangea breakup in the Triassic concentrated extension in the hanging wall of the sutures that amalgamated the Paleozoic terranes. The opening of the Indian Ocean in Early Jurassic times was associated with a new phase of subduction along the continental margin. The northeastward absolute motion of western Gondwana produced a negative trench roll-back velocity that controlled subduction under an extensional regime until late Early Cretaceous times. The Northern Andes of Venezuela, Colombia, and Ecuador record a series of collisions of island arcs and oceanic plateaus from the Early Cretaceous to the middle Miocene as a result of interaction with the Caribbean plate. The remaining Central and Southern Andes record periods of orogenesis and mountain building alternating with periods of quiescence and absence of deformation as recorded in parts of the Oligocene. Based on the generalized occurrence of flat-slab subduction episodes through time, as recorded in most of the Andean segments in Cenozoic and older times, this paper presents a conceptual orogenic cycle that accounts for the sequence of quiescence, minor arc magmatism, expansion and migration of the volcanic fronts, deformation, subsequent lithospheric and crustal delamination, and final foreland fold-and-thrust development. These episodes are related to shallowing and steepening of the subduction zones through time. This conceptual cycle, similar to the Laramide orogeny in North America, may be recognized wherever a subduction system is or was active in a continental margin.

Along Pacific convergent margins of the Americas, high-standing relief on the subducting oceanic plate “collides” with continental slopes and subducts. Features common to many collisions are uplift of the continental margin, accelerated seafloor erosion, accelerated basal subduction erosion, a flat slab, and a lack of active volcanism. Each collision along America’s margins has exceptions to a single explanation. Subduction of an ~600 km segment of the Yakutat terrane is associated with >5000-m-high coastal mountains. The terrane may currently be adding its unsubducted mass to the continent by a seaward jump of the deformation front and could be a model for docking of terranes in the past. Cocos Ridge subduction is associated with >3000-m-high mountains, but its shallow subduction zone is not followed by a flat slab. The entry point of the Nazca and Juan Fernandez Ridges into the subduction zone has migrated southward along the South American margin and the adjacent coast without unusually high mountains. The Nazca Ridge and Juan Fernandez Ridges are not actively spreading but the Chile Rise collision is a triple junction. These collisions form barriers to trench sediment transport and separate accreting from eroding segments of the frontal prism. They also occur at the separation of a flat slab from a steeply dipping one. At a smaller scale, the subduction of seamounts and lesser ridges causes temporary surface uplift as long as they remain attached to the subducting plate. Off Costa Rica, these features remain attached beneath the continental shelf. They illustrate, at a small scale, the processes of collision.

Flat subduction of the Farallon plate beneath the western United States during the Laramide orogeny was caused by the combined effects of oceanic plateau subduction and unusually great suction in the mantle wedge, the latter of which was caused by the shallowing slab approaching the North American craton root. Once in contact with basal North America, the slab cooled and hydrated the lithosphere. Upon removal, asthenospheric contact with lithosphere resulted in magma production that was especially intense where the basal lithosphere was fertile (in what now is the Basin and Range Province), and this heating weakened the lithosphere and made it convectively unstable. Small-scale convection has since affected many areas. With slab sinking and unloading, the western United States elevated into a broad plateau, and the weak portion gravitationally collapsed. With development of a transform plate boundary, the western part of the weak zone became entrained with the Pacific plate, and deformation there is dominated by shear.

The Colorado Plateau is composed of Neoproterozoic, Paleozoic, and Mesozoic sedimentary rocks overlying mechanically heterogeneous latest Paleoproterozoic and Mesoproterozoic crystalline basement containing shear zones. The structure of the plateau is dominated by ten major basement-cored uplifts and associated monoclines, which were constructed during the Late Cretaceous through early Tertiary Laramide orogeny. Structural relief on the uplifts ranges up to 2 km. Each uplift is a highly asymmetric, doubly plunging anticline residing in the hanging wall of a (generally) blind crustal shear zone with reverse or reverse/oblique-slip displacement. The master shear zones are rooted in basement, and many, if not most, originated along reactivated, dominantly Neoproterozoic shear zones. These can be observed in several of the uplifts and within basement exposures of central Arizona that project “down-structure” northward beneath the Colorado Plateau. The basement shear zones, which were reactivated by crustal shortening, formed largely as a result of intracratonic rifting, and thus the system of Colorado Plateau uplifts is largely a product of inversion tectonics. The overall deformational style is commonly referred to as “Laramide,” and this is how we use this term here. In order to estimate the dip of basement faults beneath uplifts, we applied trishear modeling to the Circle Cliffs uplift and the San Rafael swell. Detailed and repeated applications of trishear inverse- and forward-modeling for each of these uplifts suggest to us that the uplifts require initiation along a low-angle shear zone (between ~20° and ~40°), an initial shear-zone tip well below the basement-cover contact, a propagation ( p ) to slip ( s ) ratio that is higher for mechanically stiffer rocks and lower for mechanically softer rocks, a planar shear-zone geometry, and a trishear angle of ~100°. These are new results, and they demonstrate that the basement uplifts, arches, and monoclines have cohesive geometries that reflect fault-propagation folding in general and trishear fault-propagation in particular. Expressions of the shear zones in uppermost basement may in some cases be neoformed shear zones that broke loose as “footwall shortcuts” from the deeper reactivated zones. Structural analysis of outcrop-scale structures permitted determination of horizontal-shortening directions in the Paleozoic and Mesozoic sedimentary cover of the uplifts. These arrange themselves into two groupings of uplifts, one that reveals NE/SW-directed shortening, and a second that reveals NW/SE shortening. Because the strain in cover strata is localized to the upward projections of the blind shear zones, and because the measured shortening directions are uniform across a given uplift (independent of variations in the strike of the bounding monocline), it seems clear that the regional stresses ultimately responsible for deformation were transmitted through the basement at a deeper level. Thus, the stresses responsible for deformation of the cover may be interpreted as a reflection of basement strain. The basement strain (expressed as oblique shear displacements into cover driven by reactivations of dominantly Neoproterozoic normal-shear zones) was a response to regional tectonic stresses and, ultimately, plate-generated tectonic stresses. The driving mechanism for the Sevier fold-and-thrust belt was coupled to subduction of the Farallon plate, perhaps enhanced by slab flattening and generation of higher traction along the base of the lithosphere. However, the disparate shortening directions documented here suggest that two tectonic drivers may have operated on the Colorado Plateau: (1) gravitational spreading of the topographically high Sevier thrust belt on the northwest side of the plateau adjacent to an active Charleston-Nebo salient of the Sevier thrust belt, which imparted northwest-southeast shortening; and (2) northeast-southwest shortening driven by the flat slab. The effect of the two drivers tended to “crumple” part of the region in a bidirectional vise, creating added complications to structural patterns. Testing of this idea will require, among other things, very precise age determinations of progressive deformation of the Colorado Plateau within the latest Cretaceous to early Tertiary time window, and sophisticated finite-element modeling to evaluate the nature of the deformation gradients that would be induced by the two drivers.

Contractional, basement-involved foreland deformation in the Rocky Mountains of the conterminous United States occurred during the latest Cretaceous to Paleogene Laramide orogeny. Current kinematic hypotheses for the Laramide orogeny include single-stage NE- to E-directed shortening, sequential multidirectional shortening, and transpressive deformation partitioned between NW-striking thrust and N-striking strike-slip faults. In part due to this kinematic uncertainty, the links between Laramide deformation and plate-margin processes are unresolved, and proposed driving forces range from external stresses paralleling plate convergence to internal stresses due to gravitational collapse of the Cordilleran thrust belt. To determine the tectonic controls on Laramide deformation, kinematic data from minor faults ( n = 21,129) were combined with a geographic information system (GIS) database quantifying Rocky Mountain structural trends. Minor fault data were collected from a variety of pre-Laramide units to calculate average Laramide slip (N67E-01) and maximum compressive stress (N67E-02) directions for the Rocky Mountains. These largely unimodal, subhorizontal slip and compression directions vary slightly in space; more E-W directions occur in the southern and eastern Rockies, and more NE-SW directions are found near the Colorado Plateau. This kinematic framework was extended to the entire orogen using map data for faults, folds, arches, and Precambrian fabrics from Wyoming, Colorado, northern New Mexico, southeastern Utah, and northeastern Arizona. Vector mean calculations and length-weighted rose diagrams show that Precambrian fabrics are at a high angle to most larger Phanerozoic structures but are commonly reactivated by smaller structures. Ancestral Rocky Mountain structures are subparallel to Laramide structures, suggesting similar tectonic mechanisms. Laramide faults, defined by their involvement of Mesozoic and Paleogene strata but not Neogene strata, are complex, and preexisting weaknesses and minor strain components commonly predominate. In contrast, Laramide fold (avg. N24W) and arch (avg. N23W) axis trends are oriented perpendicular to minor fault slip and compression directions due to their generation by thrust-related folding. Laramide deformation shows the primary external influence of ENE-directed shortening paralleling published convergence vectors between the North American and Farallon plates. Slightly radial shortening directions, from more NE-directed to the north to more E-directed to the south, suggest focused contraction originating near current-day southern California. A slight clockwise rotation of shortening directions going from west to east is consistent with proposed changes in Farallon–North American plate trajectories as the orogen expanded eastward. Additional complexities caused by localized preexisting weaknesses and impingement by the adjoining Cordilleran thrust belt have provided structural diversity within the Laramide province. Obliquities between convergence directions and the northern and southeastern boundaries of the Laramide province have resulted in transpressive arrays of en echelon folds and arches, not major through-going strike-slip faults.

In southwestern Mexico, Late Cretaceous to Early Tertiary deformation has been generally associated with the Laramide orogeny of the Cordillera. Several alternative models consider the deformation to result from the accretion of the Guerrero terrane, formed by the Zihuatanejo, Arcelia, and Teloloapan intraoceanic island arcs, to the continental margin of the North American plate. Here, we present a detailed geologic and structural study and new 40 Ar/ 39 Ar and U-Pb ages for a broad region in the central-eastern part of the Guerrero terrane that allow the accretion model to be tested. In the Huetamo–Ciudad Altamirano part of the region, an almost complete Cretaceous-Paleogene succession records the transition from an early Cretaceous shallow-marine environment to continental conditions that began in Santonian times, followed by the development of a major continental Eocene magmatic arc. Folding of the marine and transitional successions signifies a shortening episode between the late Cenomanian and the Santonian, and a subsequent, out-of-sequence, coaxial refolding event in Maastrichtian-Paleocene time amplified the previous structures. A major left-lateral shear zone postdates the contractional deformation, and it passively controlled the geographic distribution of Eocene silicic volcanism. Minor transcurrent faulting followed. Our results indicate that the Huetamo–Ciudad Altamirano region, which has been considered part of the Zihuatanejo subterrane, was in proximity to a continent during most of the Mesozoic. We found continental recycled material at various stratigraphic levels of the Huetamo Cretaceous succession and Grenvillian inherited ages in zircons from the ca. 120 Ma Placeres del Oro pluton. More importantly, detrital zircon ages from the pre-Cretaceous basement of the Huetamo succession (Tzitzio metaflysch) and the pre–Early Jurassic basement of the Arcelia subterrane (Tejupilco suite) yield very similar Late Permian and Ordovician age peaks. These ages are typical of the Acatlán complex, onto which the Guerrero terrane has been proposed to have been accreted in the Late Cretaceous. Similarly, Paleozoic and Precambrian ages are reported in detrital zircons from the volcano-sedimentary successions of the Zihuatanejo, Arcelia, and Teloloapan subterranes. Models considering this part of the Guerrero terrane as having formed by intraoceanic island arcs separated by one or more subduction zones cannot explain the ubiquitous presence of older continental material in the Mesozoic succession. We favor a model in which most of the Guerrero terrane consisted of autochthonous or parautochthonous units deposited on the thinned continental margin of the North American plate and where the Mesozoic magmatic and sedimentary record is explained in the framework of an enduring west-facing migrating arc and related extensional backarc and forearc basins. The results presented here exclude the accretion of allochthonous terranes as the cause for Laramide deformation and require an alternative driving force to explain the generation of the Late Cretaceous–early Tertiary shortening and shearing on the southern margin of the North American plate.

The geological development of Panama’s isthmus resulted from intermittent magmatism and oceanic plate interactions over approximately the past 100 m.y. Geochemical data from ~300 volcanic and intrusive rocks sampled along the Cordillera de Panama document this evolution and are used to place it in a tectonic framework. Three distinct trace-element signatures are recognized in the oldest basement rocks: (1) oceanic basement of the Caribbean large igneous province (CLIP basement) displays flat trace-element patterns, (2) CLIP terranes show enriched ocean-island basalt (OIB) signatures, and (3) CLIP rocks exhibit arc signatures. The Chagres igneous complex represents the oldest evidence of arc magmatism in Panama. These rocks are tholeiitic, and they have enriched but highly variable fluid-mobile element (Cs, Ba, Rb, K, Sr) abundances. Ratios of these large ion lithophile elements LILEs) to immobile trace elements (e.g., Nb, Ta, middle and heavy rare earth elements) have a typical, but variably depleted, arc-type character that was produced by subduction below the CLIP oceanic plateau. These early arc rocks likely comprise much of the upper crust of the Cordillera de Panama and indicate that by 66 Ma, the mantle wedge beneath Panama was chemically distinct (i.e., more depleted) and highly variable in composition compared to the Galapagos mantle material, from which earlier CLIP magmas were derived. Younger Miocene andesites were erupted across the Cordillera de Panama from 20 to 5 Ma, and these display relatively uniform trace-element patterns. High field strength elements (HFSEs) increase from tholeiitic to medium-K arc compositions. The change in mantle sources from CLIP basement to arc magmas indicates that enriched sub-CLIP (i.e., plume) mantle material was no longer present in the mantle wedge by the time that subduction magmatism commenced in the area. Instead, a large spectrum of mantle compositions was present at the onset of arc magmatism, onto which the arc fluid signature was imprinted. Arc maturation led to a more homogeneous mantle wedge, which became progressively less depleted due to mixing or entrainment of less-depleted backarc mantle through time. Normal arc magmatism in the Cordillera de Panama terminated around 5 Ma due to the collision of a series of aseismic ridges with the developing and emergent Panama landmass. Younger heavy rare earth element–depleted magmas (younger than 2 Ma), which still carry a strong arc geochemical signature, were probably produced by ocean-ridge melting after their collision.

The volcanic basement of the Ecuadorian Western Cordillera (Pallatanga Formation and San Juan unit) is made up of mafic and ultramafic rocks that once formed an oceanic plateau. Radiometric ages from these rocks overlap with a hornblende 40 Ar/ 39 Ar plateau age of 88 ± 1.6 Ma obtained for oceanic plateau basement rocks of the Piñon Formation in coastal Ecuador, and with ca. 92–88 Ma ages reported for oceanic plateau sequences in the Caribbean and western Colombia. These results suggest that the oceanic plateau rocks of the Western Cordillera and flat forearc in Ecuador are derived from the Late Cretaceous Caribbean-Colombia oceanic plateau. Intraoceanic island-arc sequences (Rio Cala Group) overlie the plateau in the Western Cordillera and yield crystallization ages that range between ca. 85 and 72 Ma. The geochemistry and radiometric ages of island-arc lavas from the Rio Cala Group, combined with the age range and geochemistry of their turbiditic, volcaniclastic products, indicate that the arc was initiated by westward subduction beneath the Caribbean Plateau. They are coeval with island-arc rocks of coastal Ecuador (Las Orquideas, San Lorenzo, and Cayo Formations) and Colombia (Ricaurte Arc). These island-arc units may be related to the Late Cretaceous Great Arc of the Caribbean. Paleomagnetic analyses of volcanic rocks of the Piñon and San Lorenzo Formations of the southern external forearc show that they erupted at equatorial or low southern latitudes. The initial collision between South America and the Caribbean-Colombia oceanic plateau caused rock uplift and exhumation (>1 km/m.y.) within the continental margin during the Late Cretaceous (ca. 75–65 Ma). Magmatism associated with the Campanian–early Maastrichtian Rio Cala Arc ceased during the Maastrichtian because the collision event blocked the subduction zone below the oceanic plateau. Paleomagnetic data from basement and sedimentary cover rocks in the coastal forearc reveal 20°–50° of clockwise rotation during the Campanian, which was synchronous with the collision of the oceanic plateau and arc sequence with South America. East-dipping subduction beneath the accreted oceanic plateau formed the latest Maastrichtian to early Paleogene (ca. 60 Ma) Silante volcanic arc, which was deposited in a terrestrial environment. Subsequently, Paleocene to Eocene volcanic rocks of the Macuchi unit were deposited, and these probably represent a continuation of the Silante arc. This submarine volcanism was coeval with the deposition of siliciclastic rocks of the Angamarca Group, which were mainly derived from the emerging Eastern Cordillera.

The proposed ages for the collision of the Carnegie Ridge with the South America trench, offshore Ecuador, range from 1 to 15 Ma. In this time frame, many geological features of Ecuador are commonly linked to the subduction of the Carnegie Ridge. (1) After the ridge collided with the trench at ca. 15 Ma, the subsequent interplate coupling produced high exhumation rates of volcanic materials at ca. 9 Ma. (2) The oblique convergence of the Carnegie Ridge would have caused the northward drift of the North Andean block and the opening of the Gulf of Guayaquil. (3) During the late Miocene, the subduction of the Carnegie Ridge would have triggered a regional tectonic inversion along the forearc. (4) Along the collision front of the ridge with the trench, subduction-related erosion is occurring, and the Ecuadorian continental margin is being uplifted in the present day. (5) The chemistry of the active volcanic arc is explained as resulting from the arrival of the Carnegie Ridge into the trench. For instance, the adakitic signal, which appears at 1.5 Ma, is thought to be ridge-induced. (6) The buoyancy of the subducted Carnegie Ridge would explain the flatness of the slab beneath Ecuador. In this paper, we review the geological evolution of the Northern Andes in order to establish which of these geological events may be related to the subduction of the Carnegie Ridge. This review suggests that there is no clear deformation linked with the subduction of the Carnegie Ridge or with its landward prolongation postulated at depth.

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 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.

Abstract The geologic field guides in this volume to the Andes of Argentina and Chile were written for the five field trips accompanying the 2006 Backbone of the Americas conference in Mendoza, Argentina, which was sponsored by the Geological Society of America and the Asociación Geológica Argentina. The meeting was organized around three processes influential in the evolution of the western margin and cordilleras of the Americas—ridge collision, shallowing and steepening subduction zones, and plateau and orogenic uplift. Designed for use in the office or the field, the field guides are to regions that highlight these themes and present up-to-date overviews with references. The trip in chapter 1 to southern Patagonia highlights the ridge-trench collision theme; the next three to different regions of the south-central Andes examine temporal and spatial issues related to shallowing subduction; and the trip in the last chapter to the central Andean Puna plateau highlights plateau uplift in the context of steepening subduction and lithospheric delamination.