Victor A. Ramos

Andean tectonics related to geometry of subducted Nazca plate

Seismological and geological data show that tectonic segmentation of the Andes coincides with segmentation of the subducted Nazca plate, which has nearly horizontal segments and 30° east-dipping segments. Andean tectonics above a flat-subducting segment between 28°S to 33°S are characterized by (from west to east): (1) a steady topographic rise from the coast to the crest of the Andes; (2) no significant Quaternary, and possibly Neogene, magmatism; (3) a narrow belt of eastward-migrating, apparently thin-skinned, Neogene to Quaternary shortening of the Andes; and (4) Plio-Pleistocene uplift of the crystalline basement on reverse faults in the Pampeanas Ranges. From about 15° to 24°S, over a 30°-dipping subducted plate, a west to east Andes cross section includes: (1) a longitudinal valley east of coastal mountains; (2) an active Neogene and Holocene andesitic volcanic axis; (3) the Altiplano-Puna high plateau; (4) a high Neogene but inactive thrust belt (Eastern Cordillera); and (5) an active eastward-migrating Subandean thin-skinned thrust belt. Tectonics above a steeply subducting segment south of 33°S are similar west of the volcanic axis, but quite different to the east. Early Cenozoic tectonics of western North America were quite similar to the Neogene Andes. However, duration of segmentation was longer and the width of deformation was greater in the western United States. Patterns of crustal seismicity are systematically related to Plio-Quaternary structural provinces, implying that current deformational processes have persisted since at least the Pliocene. Horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench. Above flat-subducting segments, crustal seismicity occurs over a broad region, whereas over steep segments, it is confined to the narrow thrust belt. Strain patterns in the forearc region are complex and perhaps extensional, and a broad region of the Altiplano-Puna and Eastern Cordillera appears to be aseismic.

Tectonic and Magmatic Evolution of the Andes of Northern Argentina and Chile

Abstract Two orogenic cycles, both with different evolution, are developed in the western margin of the South American continent in northern Argentina and Chile: the Paleozoic “Hercynic” cycle and the Meso-Cenozoic “Andean” cycle. The Hercynic cycle. A wide marine basin extending westward of the Cordillera Oriental which developed in Cambrian-Ordovician times marks the beginning of this cycle. In contrast to Peru and Bolivia where this basin developed between two Precambrian blocks, the western margin of this basin in northern Argentina and Chile is still unknown. The Ordovician sedimentation and accompanied volcanism ends with the Ocloyic deformation phase and its synkinematic granitic plutonism. Two basins were developed in the Silurian-Devonian separated by the Puna arc, uplifted during this ocloyic phase. The shallow-water marine terrigeneous sediments which were deposited in them were deformed by a new tectono-magmatic associated phase (Chanic phase). Carboniferous to Lower Permian marine carbonates were deposited west of the Puna arc and red beds east of it. Later on, during the Permian to Triassic, a magmatic belt was developed along the Cordillera Occidental. The rhyolites, ignimbrites and the granitic to granodioritic related intrusives are well represented in Chile. Although the overall geologic history of this period is known, many problems concerning its origin and relations to plate tectonics are still unsolved. The Andean cycle. During this cycle, a series of magmatic-arc systems related to the subduction of the Pacific crust was built up along the western margin of South America. Huge volumes of calc-alkaline lavas and related plutons were emplaced since the Jurassic, along belts parallel to the present coastline, showing a general eastward migration trend. Up to the Lower Cretaceous, an ensialic back-arc basin was formed east of this magmatic arc. Thousands of meters of marine and continental sediments were deposited in it. This basin disappeared during the Middle Cretaceous, probably as a result of the final opening of the Atlantic and the active westward movement of the South American Plate. Since Middle Cretaceous times, the magmatic-arc has been the fundamental paleogeographic element. The arc progressively migrated stepwise eastward, each step marked by a tectonic phase. Subduction-related crustal erosion could explain the lack of fore-arc petrotectonic assemblages. The eastward magmatic polarity which characterizes this section of the Andes, could also be explained by such a mechanism.

Late Paleozoic to Jurassic silicic magmatism at the Gondwana margin: Analogy to the Middle Proterozoic in North America?

A vast region of upper Paleozoic to Middle Jurassic (300-150 Ma) silicic magmatic rocks that erupted inboard of the Gondwana margin is a possible Phanerozoic analogue to the extensive Middle Proterozoic (1500-1350 Ma) silicic magmatic province that underlies much of the southern mid-continent of North America. Like the North American rocks, the Gondwana silicic magmas appear to be melts of crust that formed about 200-300 m.y. earlier. In the North American case, this older crust formed and was accreted to the continent during a major period of crustal formation (1700-1900 Ma), whereas in the Gondwana case, the crust that melted consisted mainly of magmatic are terranes accreted to the continental margin during the Paleozoic. In both cases, basic to intermediate magmatic rocks are extremely rare and magmatism is less abundant in regions that contain older (and previously melted) crust. The similarities between the North American and Gondwana silicic rocks suggest that both suites formed in extensional settings where basaltic magmas, ponded at the base of the preheated crust, caused extensive crustal melting that inhibited upward passage of the basalts. In both cases, silicic volcanism occurred after major assembly of a supercontinent by subduction and accretion processes, and before breakup of the supercontinent. By analogy with the polar wander curves for Gondwana, the granite-rhyolite provinces may have formed during a period of very slow motion of the supercontinents relative to the poles.

Southern Patagonian plateau basalts and deformation: Backarc testimony of ridge collisions

Abstract The distribution and volume of Tertiary Patagonian plateau basalts and the evolution of the Patagonian fold and thrust belt between 46° and 49°S appear to be closely tied with ridge-trench interactions along the continental margin to the west. Eocene plateau basalts south of 43°S are associated with a gap in the Eocene arc and can be related to a proposed collision of the Aluk-Farallon ridge against the trench at this time. Eruptions of Late Miocene to Recent plateau basalts between 46° and 49°S can be related to time-transgressive slab windows in the underlying mantle generated by the collision of segments of the Chile ridge at 10–14 Ma and 6 Ma. The principal deformation in the Patagonian fold and thrust belt predates the eruption of the basalts and also appears to be related with Late Miocene ridge collision. Comparison of plateau basalts related to the collision of the three distinct ridge segments suggests that the volume of basalt erupted increases with the length of the collided ridge segment. Almost all Eocene to Recent plateau basalts between 46° and 49°S have OIB-like (ocean island basalt) trace element signatures showing that the mantle source region changed little with time. Plateau basalts with more arc-like signatures appear to be contaminated by continental crust. Relative melting percentages inferred from the trace element chemistry of the plateau basalts correlate with volumes of erupted basalt implying spatial and temporal changes in temperatures in the mantle source that can be correlated with slab windows.

Cuyania, an Exotic Block to Gondwana: Review of a Historical Success and the Present Problems

Abstract A review of the early history of the Cuyania terrane and the numerous pioneering works of the past century provides the present robust framework of evidence supporting a derivation from Laurentia, travel towards Gondwana as an isolated microcontinent, and final amalgamation to the protomargin of western Gondwana in Middle to Late Ordovician times. The major remaining uncertainties and inconsistencies, such as the time of deformation and collision with Gondwana, the lack of evidence of Famatinian-derived zircons, the effects of strike-slip displacements proposed along the suture, as well as the potential sutures defined by ophiolite assemblages, are discussed. The precise boundary along the northern and southern limits is not yet well defined. The most suitable hypothesis based on present data is that Cuyania originated as a conjugate margin of the Ouachita embayment, south of the Appalachian platform during Early Cambrian times. The subsequent travel toward the Gondwana protomargin is clearly depicted by the changing faunal assemblages in the carbonate platform. New geochemical and age data on K-bentonites presented by several authors reinforce the strong connection between Cuyania ash-fall tuffs and Famatina volcanics by 468–470 Ma, indicating Cuyania and Gondwana were in close proximity at that time. Extension related to flexural subsidence, preceded by the drowning of the carbonate platform in early Llanvirnian times, is recorded by abrupt facies changes in the sedimentary cover during late Llanvirnian and early Caradocian times. This episode marked the beginning of contact between Cuyania and Gondwana. The subsequent evolution of the foreland basin indicates that deformation lasted until latest Silurian-Early Devonian times. The time of collision is tracked by the cessation of arc-related magmatic activity in the upper plate (Gondwana protomargin), at about 465 Ma in western Sierras Pampeanas, and ages around 454 Ma corresponding to syncollisional and postcollisional magmatism. The age of the collision is also preserved in the lower plate (Cuyania), where both angular unconformities in the sedimentary cover and the ages of peak of regional metamorphism in the basement rocks point to 460 Ma as the most probable age for the beginning of the collision. Evidence from the upper plate is essentially identical with an age of 463 Ma. Thermal gradients along this suture vary from 13°C/km in the lower plate, to 18°C/km in the fore arc upper plate, reaching more than 30°C/km along the Famatinian arc. Based on these data, a Llandelian-Caradocian age for the collision can be postulated on firm grounds. Deformation continued through most of the early Paleozoic until amalgamation of the Chilenia terrane by the Late Devonian.

Neogene Patagonian plateau lavas: Continental magmas associated with ridge collision at the Chile Triple Junction

Extensive Neogene Patagonian plateau lavas (46.5° to 49.5°S) southeast of the modern Chile Triple Junction can be related to opening of asthenospheric “slab windows” associated with collisions of Chile Rise segments with the Chile Trench at ≈ 12 Ma and 6 Ma. Support comes from 26 new total-fusion, whole rock 40Ar/39Ar ages and geochemical data from back arc plateau lavas. In most localities, plateau lava sequences consist of voluminous, tholeiitic main-plateau flows overlain by less voluminous, 2 to 5 million year younger, alkalic postplateau flows. Northeast of where the ridge collided at ≈12 Ma, most lavas are syncollisional or postcollisional in age, with eruptions of both sequences migrating northeastward at 50 to 70 km/Ma. Plateau lavas have ages from 12 to 7 Ma in the western back arc and from 5 to 2 Ma farther to the northeast. Trace element and isotopic data indicate main-plateau lavas formed as larger percentage melts of a garnet-bearing, oceanic island basalt (OIB) -like mantle than postplateau lavas. The highest percentage melts erupted in the western and central plateaus. In a migrating slab window model, main-plateau lavas can be explained as melts that formed as upwelling, subslab asthenosphere which flowed around the trailing edge of the descending Nazca Plate and then interacted with subduction-altered asthenospheric wedge and continental lithosphere. Alkaline, postplateau lavas can be explained as melts generated by weaker upwelling of subslab asthenosphere through the open slab window. Thermal problems of high-pressure melt generation of anhydrous mantle can be explained by volatiles (H2O and CO2) introduced by the subduction process into slab window source region(s). An OIB-like, rather than a mid-ocean ridge basalt (MORB) -like source region, and the lack of magmatism northeast of where ridge collision occurred at ≈13 to 14 Ma can be explained by entrainment of “weak” plume(s) or regional variations in an ambient, OIB-like asthenosphere.

Cenozoic tectonics of the High Andes of west-central Argentina (30–36°S latitude)

Abstract The structure of the Central Andes shows three distinctive segments characterized by different geometries. These geometries are superimposed on the present large-scale plate tectonic setting characterized by distinct subduction segments. The northern La Ramada segment is a thick-skinned fold and thrust belt formed by tectonic inversion of a Late Triassic rift. The central Aconcagua segment consists of a thin-skinned fold and thrust belt while the southern Malargue segment like the first one is a thick-skinned fold and thrust belt developed by tectonic inversion of a Late Triassic-Early Jurassic rift system during late Cenozoic times. The amount of shortening gradually decreases from north to south, as indicated by the crustal roots of the Central Andes. The different geometries along the Principal Cordillera controlled the abrupt changes in the shortening among segments. The structure of Precordillera and Sierras Pampeanas has also been considered in order to account for the total shortening. In the La Ramada segment the main shortening occurred in the Precordillera; in the Aconcagua segment in the Principal Cordillera while in the Malargue segment the shortening is widely distributed in a broader Principal Cordillera, because south of the flat-slab subduction segment the Precordillera and Sierras Pampeanas are missing.

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.

Andean Foothills Structures in Northern Magallanes Basin, Argentina

The foothills of the northern segment of the Magallanes basin between 47° and 49°S latitudes record the development of a complex fold-and-thrust belt in the Patagonian Cordillera during the late Miocene. Upper Paleozoic sedimentary and metasedimentary rocks and thick sequences of Lower Cretaceous deposits with well-defined source and reservoir rocks are deformed by the Andean events. The structure is characterized by a triangle zone and a series of related underthrusts, which define a nonemergent backthrust system at the mountain front. Differences in the degree of deformation from south to north reveal the evolution of the triangle zone. The stratigraphy of the molasse deposits constrains the beginning of the deformation at least to the early Tertiary, although the final structure was developed during the late Miocene. These deformed deposits are covered by extensive late Miocene horizontal retro-arc basalts. Analysis of the evolving subduction zone located west of and beneath the study area shows that the formation of a subsequent volcanic gap in the magmatic arc and the development of extensive retro-arc basalts are related to the collision of a segment of the Chile Ridge during the late Miocene. The final deformation and uplift of the Patagonian Cordillera fold-and-thrust belt at these latitudes are closely linked to this collision.

Tectonic evolution of the Andes of Neuquén: constraints derived from the magmatic arc and foreland deformation

Abstract The Andes of the Neuquén region (36° − 38°S latitude) of the Central Andes have distinctive characteristics that result from the alternation of periods of generalized extension followed by periods of compression. As a result of these processes the Loncopué trough is a unique long depression at the foothills parallel to the Principal Cordillera that consists of a complex half-graben system produced during Oligocene times and extensionally reactivated in the Pliocene-Pleistocene. Its northern sector represents the present contractional orogenic front. The nature and volume of arc-related igneous rocks, the location of the volcanic fronts, expansions and retreats of the magmatism, and the associated igneous activity in the foreland, together with the analyses of the superimposed structural styles, permit the constraint of the alternating tectonic regimes. On these bases, different stages from Jurassic to Present are correlated with changes in the geometry of the Benioff zone through time. Periods of subduction-zone steepening are associated with large volumes of poorly evolved magmas and generalized extension, while shallowing of the subduction zone is linked to foreland migration of more evolved magmas associated with contraction and uplift in the Principal Cordillera. The injection of hot asthenospheric material from the subcontinental mantle into the asthenospheric wedge during steepening of the subduction zone produced melting and poorly evolved magmas in an extensional setting. These periods are linked to oceanic plate reorganizations in the late Oligocene and in the early Pliocene.