Eduardo A. Rossello

Loess geochemistry and its implications for particle origin and composition of the upper continental crust

Abstract Chemical and Nd Sr isotopic compositions of loess samples from Argentina, Europe and Spitsbergen were analyzed to examine the nature of source terrains, the origin of silt-size particles and the suitability of using loess as starting material for estimating the average chemical composition of the upper continental crust. From the relations between Na2O/Al2O3 and K/2O/Al2O3 ratios and CIA values (chemical index of alteration), the loess protoliths must have undergone previous sedimentary differentiation and subjected to moderate chemical weathering. REE patterns are remarkably uniform with (La/Yb)N ≈ 10, which is characteristic of the upper continental crust (UCC). Negative Eu anomalies, expressed in Eu/Eu* ratios, vary from 0.65 for European loess to 0.8 for Pampean loess from Argentina. All loess deposits have nearly constant La/Th or Th/U ratios, which are very similar to those of the average UCC or post-Archean shales. These ratios are not fractionated in size-fractions relative to the whole-rock values. Nd and Sr isotopic compositions clearly distinguish Argentinean loess (87Sr86Sr = 0.706–0.709, eNd(0) = −6to−1.5) from all other loess deposits (87Sr86Sr = 0.712–0.730, eNd(0) = −13to−8). The REE and isotopic results clearly indicate a significant contribution of young Andean volcanics to the Pampean loess deposits, whereas multi-recycled and well-mixed ancient sediments are principal sources for the other deposits. The present results reinforce the earlier conclusion reached by S.R. Taylor, S.M. McLennan and M.T. McCulloch [Geochemistry of loess, continental crustal composition and crustal model ages, Geochim. Cosmochim. Acta 47 (1983) 1897–1905] that the average chemical composition of UCC can be obtained from eolian deposits as well as from fine-grained clastic sediments.

Style and history of Andean deformation, Puna plateau, northwestern Argentina

Topographically, the Puna plateau of northwestern Argentina is the southern continuation of the Bolivian Altiplano. Its thickening and consecutive uplift result from the Andean orogeny. To better constrain the structural style and its progressive development, we have studied field data, topographic and satellite imagery, balanced cross sections, seismic reflection data, kinematic analysis of fault slip data, anisotropy of magnetic susceptibility (AMS), paleomagnetic data, and apatite fission track (AFT) data. Across the Puna plateau, Precambrian and Paleozoic basement ranges, bounded by high-angle reverse faults (dips ≥ 60°), alternate with Cenozoic intermontane basins. Major thrusts trend NNE-SSW and do not show a preferred vergence. Intermontane basins have various degrees of symmetry, depending on the geometries and attitudes of associated thrusts as well as on the magnitudes of their offsets. There is a close correlation between the surface expression of a basin and the amount of internal deformation. A line-balanced cross section of the Puna at 25°S has yielded a Cenozoic shortening of 10–15%, in a direction subperpendicular to the orogen. By kinematic analysis of Cenozoic fault slip data we have obtained principal directions of strain rate across the Puna. Shortening axes are subhorizontal and trend on average WNW-ESE (∼N110°), stretching axes are subvertical, and intermediate axes are subhorizontal and trend on average NNE-SSW. Strain ellipsoids are dominantly of plane strain type, and they represent dip-slip thrusting. From paleomagnetic and AMS data, shortening axes form a radial pattern around the eastern edge of the central Andes. The pattern is attributed to an inhomogeneous stress field, reflecting the eastward convex shape of the central Andean thrust front. From the history of burial and uplift, Andean shortening reached the northeastern part of the Puna in the late Eocene and the adjacent Eastern Cordillera in the late Eocene or early Oligocene. This shortening was presumably due to the Incaic phase of the Andean orogeny. In the eastern part of the orogen the onset of shortening was probably guided by preexisting Paleozoic and Mesozoic structures, so that Andean deformation propagated unevenly eastward.

Cenozoic crustal thickening, wrenching and rifting in the foothills of the southernmost Andes

Abstract The southernmost Andes form an orocline, between the Patagonian cordillera, trending N–S, and the Fueguian cordilleras, trending E–W. On the foreland side is the Magellan Basin. The area has a history of Paleozoic compression, Triassic to Early Cretaceous rifting and Late Cretaceous to Quaternary compression, in response to changing plate tectonics. Major structures of Late Cretaceous and Cenozoic age vary along the strike. In the Patagonian cordillera and foothills, folds and thrusts trend NNW, slightly oblique to the orogen, whereas strike–slip faults are parallel to the orogen and right-lateral. In the Fueguian cordilleras and foothills, folds and thrusts trend ESE, slightly oblique to the orogen, whereas strike–slip faults are parallel to the orogen and left-lateral. In the axial zone of the Magellan Basin, folds and thrusts are parallel to the orogen and rifts are sub-perpendicular to it. To a first approximation, the pattern of structures has mirror symmetry about the axis of the Magellan Basin. In detail, however, wrenching appears to be more prevalent in the Fueguian cordillera and foothills, than it is in the Patagonian cordillera and foothills. Minor faults of Cenozoic age are common in the foothills. From a kinematic analysis of fault–slip data: (1) shortening and stretching directions are mostly sub-horizontal; (2) shortening directions vary in trend, from ENE in the Patagonian foothills, to NE in the Fueguian foothills; and (3) stretching directions are sub-parallel to traces of major thrusts. In the Fueguian cordillera and foothills, strike–slip faulting is prevalent; in the Patagonian foothills, crustal thickening is prevalent over strike–slip faulting. The kinematics reflect a combination of thrusting and wrenching and they are consistent with the major structures. To investigate the origin of the Cenozoic structures, we used analogue models on a fully lithospheric scale, where an oceanic plate subducted beneath a continental corner. The corner was an area of transition, from frontal subduction, to transcurrent motion. The boundary conditions may not have been fully realistic, but the experiments did account for the major elements of the structural pattern in southernmost South America, including rifts that are perpendicular to the orogen and counterclockwise block rotations.

Partitioning of transpressive motions within a sigmoidal foldbelt : the Variscan Sierras Australes, Argentina

Abstract The Sierras Australes (Buenos Aires province, Argentina) form a sigmoidal foldbelt, about 150 km long. The last major orogenic event was Variscan, as shown by synsedimentary folds in upper Palaeozoic sediments and by K-Ar or Rb-Sr ages on cleavage-forming minerals of low metamorphic grade. Folds in the Paleozoic cover are associated with cleavage, stretching lineation and kinematic indicators (shear bands and sigmoidal tails around porphyroclasts). Reworking of the granitic Precambrian basement resulted in shear zones and faults. We distinguish three structural domains within the Sierras Australes: a Northwestern Arc, a Southeastern Basin and a Central Belt. The Northwestern Arc is a simple fold-and-thrust belt, verging towards the NE. The Southeastern Basin is a foreland basin, where an underthrust towards the SW is overshadowed by a right-lateral wrench along strike. Finally, the Central Belt has resulted from a combination of right-lateral wrenching and overthrusting, both in a northerly direction. All three domains may have resulted from a uniform state of transpressive stress. Although the deformation is strongly partitioned at outcrop, it may be simpler at depth. This is suggested by experiments in which sandpacks are subjected to transpression, by reactivating a single basement fault in oblique (right-lateral reverse) slip. We describe one such experiment where fault blocks rotate counterclockwise about vertical axes, producing arcuate thrusts at the surface. By analogy, we suggest that the Sierras Australes formed in a transpressive context, as a result of oblique-slip reactivation of a deep fault zone. Strong partitioning in the upper crust was facilitated by upward splaying of faults.

Cenozoic dextral transpression and basin development at the southern edge of the Puna Plateau, northwestern Argentina

Abstract In northwestern Argentina, the southern edge of the Altiplano-Puna is the most remarkable positive topographic anomaly (average altitude > 3700 m) of the Andean chain. To the south are the Sierras Pampeanas, a less elevated domain made of alternating compressive basins and ranges developed during the Cenozoic. At about 27°S, the transition between the Puna and the Sierras Pampeanas coincides with a substantial narrowing of the high chain and with a change in the dip of the Pacific subduction zone (Nazca plate), from 30°E below the plateau to subhorizontal below the Sierras Pampeanas. We argue that this structural transition, which can be identified at the scale of the Andean topography, is a major dextral transpressional transfer zone, the Tucuman Transfer Zone. In this region, ENE-WSW- to E-W-directed subhorizontal shortening related to bulk convergence between the Nazca and South American plates interferes with NW-SE-directed subhorizontal shortening strains, due to lateral influence of changes in the amount of crustal thickening on intraplate kinematics. Structural observations, digital mapping, SPOT satellite images and a microtectonic analysis of fault populations provide general insights into the Neogene kinematic evolution of transpressional basins and ranges at the edge of the Puna.

Distribution, timing, and causes of Andean deformation across South America

Abstract The Andean Orogeny in South America has lasted over 100 Ma. It comprises the Peruvian, Incaic and Quechuan phases. The Nazca and South American plates have been converging at varying rates since the Palaeocene. The active tectonics of South America are relatively clear, from seismological and Global Positioning System (GPS) data. Horizontal shortening is responsible for a thick crust and high topography in the Andes, as well as in SE Brazil and Patagonia. We have integrated available data and have compiled four fault maps at the scale of South America, for the mid-Cretaceous, Late Cretaceous, Palaeogene and Neogene periods. Andean compression has been widespread since the Aptian. The continental margins have registered more deformation than the interior. For the Peruvian phase, not enough information is available to establish a tectonic context. During the Incaic phase, strike-slip faulting was common. During the Quechuan phase, crustal thickening has been the dominant mode of deformation. To investigate the mechanics of deformation, we have carried out 10 properly scaled experiments on physical models of the lithosphere, containing various plates. The dominant response to plate motion was subduction of oceanic lithosphere beneath continental South America. However, the model continent also deformed internally, especially at the margins and initial weaknesses.

Paleomagnetic study along the southeastern edge of the Altiplano - Puna Plateau: Neogene tectonic rotations

In northwestern Argentina the transition zone between the Puna and the Sierras Pampeanas was deformed in Late Miocene and Pliocene time. Structural observations suggest that local clockwise block rotations might be observed associated with dextral transpression. This study contributes new paleomagnetic results from four sites in Cretaceous rocks and 19 sites in Neogene sedimentary sequences. When compared with reference paleomagnetic data for stable continental South America, the results show an average inclination shallowing error of 10.5° and a pattern of clockwise rotations up to 29°. Study of the anisotropy of magnetic susceptibility (AMS) on 278 specimens provided, for most sites, AMS tensors with an oblate shape controlled by sedimentation and compaction, explaining the observed inclination shallowing. A slight AMS lineation was measured and we argue that this magnetic lineation reflects Late Miocene-Pliocene compression. A correlation observed between magnetic lineations, deduced from the AMS analysis, and rotation estimates from remanent magnetizations, demonstrates that the inferred shortening directions have also been rotated clockwise.

Magellan Strait: Part of a Neogene rift system

The Magellan Strait joins the Atlantic and Pacific Oceans, separating Tierra del Fuego from southernmost continental South America. The strait cuts both the Andean Cordillera and the Magellan foreland basin. Other arms of the sea intrude the axial zone of the Magellan basin. These depressions have long been interpreted as glacial valleys. On the basis of Landsat images, digital topography, and field data, we interpret the depressions as rifts or half-rifts. In general, active rifts developed within foreland basins are unusual, but in Patagonia they are consistent with regional deformation and its plate-tectonic setting during the Neogene.

Late Paleozoic transpression in Buenos Aires and northeast Patagonia ranges, Argentina

Paleozoic sediments are present in three regions in eastern central Argentina: (1) the Sierras Australes of Buenos Aires, (2) Sierras Septentrionales of Buenos Aires and (3) Northeast Patagonia. All of these deposits share a common deformational imprint imparted by late Paleozoic Gondwanan deformation. Exposures of these rocks are scattered, variably deformed, and isolated by younger sediments deposited in basins related to the Mesozoic through Tertiary opening of the South Atlantic such as the offshore Colorado Basin. The Sierras Australes of Buenos Aires outcrops are the best preserved. They are mostly located along the Sierras Australes foldbelt, with minor outliers distributed in the adjacent Claromec-basin. The Tunas Formation (early-early late? Permian) is the uppermost unit of the Pillahuinco Group (late Carboniferous-Permian) and is crucial to the understanding of the tectono-sedimentary evolution of the region during the late Paleozoic. The underlying units of the Pillahuinco Group (Sauce Grande, Piedra Azul and Bonete Formations) exhibit a depositional and compositional history characterized by glaciomarine sedimentation and postglacial transgression. They are also characterized by rather uniform quartz-rich compositions indicative of a cratonic provenance from the La Plata craton to the NE. In contrast, the sandstone-rich Tunas Formation has low quartz contents, and abundant volcanic and metasedimentary fragments; paleocurrents are consistently from the SW. Glassrich tuffs are interbedded with sandstone in the upper half of the Tunas Formation. The age of the deformation in the Sierras Australes is Permian and early-middle Triassic. This is based on metamorphic events indicated by formation of illite at 282 ± 3 Ma, 273 ± 8 Ma, 265 ± 3 Ma, and 260 ± 3 Ma (KAr illite) in the Silurian Curamalal Group. Evidence of syntectonic magmatism is provided by a radiometric date of 245 ± 12 Ma (KAr hornblende) for the Lopez Lecube Granite, immediately west of the Sierras Australes. In the Sierras Septentrionales of Buenos Aires, Precambrian through early Paleozoic deposits of La Tinta, Sierras Bayas, Las Aguilas and Balcarce Formations rest on Precambrian crystalline basement of the La Plata craton. These exposed rocks are affected by subordinate, right lateral wrench faulting; some thrusting indicates tectonic transport toward the NE. In northeast Patagonia (Sierra Grande region) synkinematic deformation of early Permian (261 ± 5 Ma, RbSr whole rock) age has been identified in Silurian metasediments of the Sierra Grande Formation. Bands of deformation in Sierra Grande quartzites indicate right lateral wrenching in a N-S direction. Contraction in a NE-SW direction is evidenced by folding. Three stages of tectonic evolution can be discerned for the above regions: (1) Early Paleozoic platform sedimentation, punctuated by episodes of accelerated subsidence during the Silurian and early Devonian, as shown by transgressive episodes, (2) late Paleozoic sedimentation and deformation, and (3) Meso-Cenozoic extensional inversion due to the South Atlantic opening. The late Paleozoic sedimentation and deformation (stage 2) includes late Carboniferous-earliest Permian glacial deposits of the Sierras Australes and Colorado offshore basin, deposited during an initial phase of extension, and cratonward foreland subsidence triggered sedimentation of the synorogenic deposits of the Permian Tunas Formation. Tuffs are intercalated in the upper half of this unit. These tuffs are associated with the silicic volcanism along the Andes and Patagonia (Choiyoi magmatic province) that peaked between the late early Permian and late Permian. Likewise, the first widespread appearance of tuffs in the Karoo basin is in the Whitehill Formation, of late early Permian (260 Ma) age. The deformation described in this paper can be considered as part of a large scale intracontinental deformation in SW Gondwanaland inboard of an Andean-type compressive margin. This deformation is characterized by transpression (right lateral wrenching) combined with overthrusting to the NE and N-S horizontal contraction.

Neogene dextral transpression due to oblique convergence across the Andes of northwestern Patagonia, Argentina

Abstract Between Bariloche (41°S) and El Bolson (42°S), Neogene sediments of the Nirihuau foreland basin and Paleogene volcanoclastic rocks have been thrust westward beneath basement rocks of the Andean cordillera. North of Bariloche (40°–41°S), Paleogene volcanoclastic rocks within the main cordillera show Neogene deformation. The large-scale Neogene tectonics of the area are revealed by superimposing geological maps with digital topographic data. Fault-slip data provide information on the relative amount of crustal thickening and strike-slip faulting. Throughout the area, major reverse faults and thrusts trend northwest, forming the edges to Cenozoic basins of foreland or ramp styles. Some of these are inverted grabens of Mesozoic age. The dominant strike-slip faults are right-lateral and trend nearly north, parallel to the cordillera. Conjugate left-lateral faults trend nearly east. At a regional scale, based on the fault-slip data, the principal direction of shortening is northeast, in areas where thrusts predominate, but swings around to the north in areas where strike-slip faults predominate. Thus the results indicate a degree of strain partitioning, but they are broadly compatible with the oblique direction of convergence between the Nazca and South American plates. This tectonic style seems to have lasted throughout the Neogene.