La deriva continental cenozoica y mesozoica en el Atlántico Sur

La información paleomagnética muestra que la fragmentación del bloque América del Sur—África se produjo con posterioridad al Triásico Superior pero antes del Cretácico Medio, y sugiere que, probablemente, dicho episodio habría comenzado antes del Jurásico Medio (161 millones de años). Los datos actualmente conocidos de la propagación del fondo del océano Atlántico Sur pueden ser extrapolados sólo hasta la transición Cenozoico-Mesozoico (alrededor de 70 millones de años). Recientemente, ha sido sugerido que la fragmentación del bloque América del Sur- África y la formación del Atlántico Sur, son el resultado de un episodio continuo, que se inició en el Mesozoico Medio y se prolongó durante todo el Cenozoico. Este trabajo tiene por finalidad demostrar que la información paleomagnética de América del Sur y África, y la proveniente de la propagación del fondo del Atlántico Sur, actualmente disponibles, sugieren la presencia de dos episodios de deriva continental independientes: uno ocurrido en el Mesozoico, que originó la fragmentación del entonces bloque América del Sur-África y la formación de un océano abierto ya en el Cretácico Medio (115—125 millones de años) y otro acaecido en el Cenozoico, que completó la formación del Atlántico Sur, llevando a América del Sur y África a sus posiciones geográficas actuales. Entre ambos procesos se produjo un cambio en la dirección de las fuerzas impulsoras y una probable subsidencia del fondo oceánico ya formado. La semejanza en la dirección, el sentido y la velocidad de los movimientos relativos entre América del Sur y África durante ambos episodios, sugerirían que los mecanismos que los originaron fueron muy similares.Palaeomagnetic data show that the fragmentation of the South America-African block occurred after Upper Triassic but before the Middle Cretaceous, and suggests that this episode would have started before Middle Jurassic Times (161 m.y.). The available South Atlantic-floor spreading data can be extrapolated only until the Cenozoic- Mesozoic transition (about 70 m.y.). Recently it has been suggested that the fragmentation of the South America-African block and the formation of the South Atlantic Ocean are the results of a continous drift episode which started in the Middle Mesozoic and lasted during all the Cenozoic. In this paper it is shown that the palaeomagnetic data of South America and Africa an the South Atlantic-floor spreading data suggest that two independent continental drift episodes would have occurred: i) a Mesozoic episode, during which the fragmentation of the South America-African block started and an open ocean was already formed for Middle Cretaceous times (115—125 m.y.), and ii) a Cenozoic episode during which the formation of the South Atlantic Ocean was completed and South America and Africa drifted until their present geographic positions. Between both episodes a change in the direction of the driving forces and a probably subsidence of the ocean-floor would have occurred. The similitude of the direction, sense and velocity of the relative movements between South America and Africa during both episodes would suggest that their driving mechanisms were similar.Asociación Argentina de Geofísicos y Geodesta

El magnetismo remanente de los estratos rojos : Su aporte al conocimiento de la Cuenca de Paganzo

Los datos paleomagnéticos obtenidos del estudio de tres secuencias de estratos rojos y rocas ígneas asignadas al lapso carbónico tardío-triásico son analizados con miras a definir aspectos de la evolución de la Cuenca de Paganzo. Estos datos indican que algunos de los sedimentos se depositaron durante un lapso en que América del Sur tuvo un rápido desplazamiento respecto del polo, descartan la posibilidad de que hayan ocurrido rotaciones entre las áreas de recolección y sugieren que la cuenca estuvo sometida a movimientos verticales durante el Carbónico tardío - Pérmico temprano.PalaeomagnetIc data from sequences of red beds and Igneous rocks of late carboniferous - triassic age are analyzed to define episodes of the evolution of the Paganzo Basin. These data indicate that: l) some of the red beds were formed during a rapid polar shift episode of South America and II) no local rotations have occurred between the sampling sites. They also suggest that vertical movements have occurred during the late Carboniferous - early Permlan.Asociación Argentina de Geofísicos y Geodesta

Structural geology of the Fuegian Andes and Magallanes fold-and-thrust belt - Tierra del Fuego Island

A synthesis of the structural geology of the Tierra del Fuego Island, which integrates a new data set derived from field surveys and literature data of the last few years, is presented here. The main geological features of the region developed during the Mesozoic-Cenozoic Andean orogenic cycle that started in the Middle to Late Jurassic with a back-arc extension, crustal stretching and widespread volcanism, related to the break-up of Gondwanaland. An extensional fault system deriving from the mechanical and thermal subsidence led the evolution of the Rocas Verdes marginal basin, which hosts the upper Jurassic volcanoclastic rocks, the lower Cretaceous turbiditic sequences and few isolated elongated ophiolitic complexes. From the Late Cretaceous onward, the orogenic cycle of the Fuegian Andes continued with the shortening and inversion of the back-arc margin through horizontal contraction and crustal thickening. The uplift of the Cordillera, the emplacement of plutonic rocks, and the intercontinental polyphase deformation resulted from thick-skinned tectonics. The thrust system developed from its deeper roots, where the Palaeozoic basement was involved in compressive deformation, and propagated to the shallower stratigraphic levels of the northward verging Magallanes fold-and-thrust belt. The Magallanes foreland basin developed in front of the orogenic wedge that records at least four syntectonic angular unconformities from Late Cretaceous to Lower Miocene. During the Late Cretaceous Andean compression, three distinct phases of penetrative ductile deformation defined by low-greenschist facies assemblages took place, both in the basement and in the cover units. These deformations are related to a single metamorphic event with foliation development, as observed from microscopic analysis of the schist in the Ushuaia area. The first foliation S1 is preserved either as relic sericite microfolds between microlithons of the dominant S2, or as early refolded veins of recrystallized quartz. The S2 foliation is defined by oriented white mica. The crenulation of S2, which is related to D3 and occurs in most strained zones, becomes a pressure solution S3 spaced foliation, lined by opaque minerals. From the Palaeogene to the present, EW sinistral wrench tectonics affected the region as a component of the relative motion between South America and the Antarctic Peninsula. This strike-slip activity is well documented from the Carbajal valley to the Canal de Beagle region south of the Magallanes-Fagnano transform fault system. Restraining bends and overlapping step-over geometry characterize few sectors of the strike-slip faults with pop-ups, pressure ridges and uplifted slivers of crust. Releasing step-over along the transform fault system, both in on-shore and off-shore zones, formed several elongated pull-apart basins with many tens of km in length and a few km in width. The Lago Fagnano represents the main morphotectonic expression of this structural setting. A N-S geological cross-section through the Fuegian Andes synthesizes all the geological and geophysical data. The major stacks of internal thick skinned basement involved in the thrusting are high-grade Upper Palaeozoic to Lower Tertiary metamorphic rocks. The geometry of the thrust complex is an upright, south plunging monocline of moderately tilted sedimentary cover strata, as well as related thrusts, faults and chevron folds involving the Upper Jurassic and Cretaceous rocks. The orogenic shortening of the Fuegian Andes, including the Cordillera and the Magallanes fold-and-thrust belt, reaches few hundred kilometres with a left-lateral wrenching component of many tens of meters. The Tierra del Fuego Island is characterized by low seismicity (M<3.5) and shallow crustal earthquakes. The southern part presents strong morphological evidence of the Quaternary activity of the E-W left-lateral strike-slip faults, with a present deformation pattern (DGPS) with a horizontal slip component of about 6 mm/year. Moreover, the northern sector of the Island is affected by extensional tectonics related to the normal fault systems of the eastern arms of the Magallanes Strait

Seismostratigraphic and structural setting of the Malvinas Basin and its southern margin (Tierra del fuego Atlantic offshore)

New multichannel seismic reflection profiles acquired off the Tierra del Fuego Atlantic margin, from the southern part of the Malvinas foreland basin to the inner sector of the Magallanes fold and thrust belt, combined with available commercial profiles and exploration wells, allowed to outline the sedimentary architecture of the foreland basin and the structure of its deformed southern margin. Five major unconformities were differentiated within the sedimentary fill of the southern Malvinas basin, which neighbours the offshore extension of the Magallanes basin in Tierra del Fuego. The unconformity-bounded units record the corresponding major evolutionary tectonostratigraphic phases of the southern part of the Malvinas basin, and the development of the Magallanes fold-and thrust belt during Mesozoic and Cenozoic times: Unit 1 - Pre-Jurassic basement; Unit 2 - Rift phase (Middle - Upper Jurassic); Unit 3 - Sag phase (Lower - Upper Cretaceous); Unit 4 - Foredeep transitional phase (Upper Cretaceous - Middle Eocene); Unit 5 - Foreland phase (Middle Eocene - Pleistocene). The southern edge of the Malvinas basin corresponds to the imbricate basement wedges of the Fuegian Cordillera, which shows a thick-skin structural style developed as a consequence of the Middle Tertiary Andean compressional tectonic phase. Large folds, with low angle NE-verging thrusts propagated the shortening basin-ward at shallow structural levels. These structures are superimposed by an array of left-lateral strike-slip lineaments pertaining to the EW trending Magallanes-Fagnano fault system. In the Tierra del Fuego region these structures represent the western segment of the South America - Scotia plate boundary. Several Neogene pull-apart basins were formed along the principal deformation zone in correspondence of step-overs and releasing bends. These basins show an evident asymmetry in the sedimentary architecture, and are bounded by sub-vertical faults that in some cases reach the sea-floor. Other transtensional features were also recognized in the inner sector of the fold-and-thrust belt together with the formation of restricted pull-apart basins

Gravity map of the Isla Grande de Tierra del Fuego, and morphology of Lago Fagnano

A complete Bouguer gravity map of the central-eastern part of the Isla Grande de Tierra del Fuego, and a general bathymetric chart of the Lago Fagnano have been realized, on the basis of a series of field geophysical surveys carried out on the Island since 1998. The regional gravity anomaly trend onshore shows a progressive negative gradient from N to S. Distinct, broadly E-W-trending gravity minima are superimposed on this regional negative gradient. They follow the main trace of the Magallanes-Fagnano fault system, which represents the western segment of the left-lateral South America-Scotia transform plate boundary. The gravity minima reflect the presence in the subsurface of restricted and elongated basins developed within the principal displacement zone of the fault system. A relative positive gravity maximum is located just at the SE corner of the Lago Fagnano, and represents the response of a partially exposed crystalline body, occupying an area 3 x 3 km wide. A 2D vertical crustal model has been constructed, combining gravity data inversion and geological information available for the central-eastern region of Lago Fagnano. The bathymetric map of the Lago Fagnano delineates the main morphological features of this 110-km-long, 7-km-wide lake, the largest of Isla Grande. The floor is divided into distinct parts, which suggests that the basin is composed of different sub-basins. In most areas, the basin floor is highly asymmetric in shape, with flat depocentral areas. The most pronounced asymmetry of the basin is seen in the eastern end of the lake, where there is also the deepest depression. The steeper slope of the basin, along the northern shore of the Lago Fagnano, also coincides with the most pronounced regional topographic gradient. The general gravimetric and morphological features of the investigated region are here discussed

Did Patagonia collide with Gondwana in the Late Paleozoic? Some insights from a multidisciplinary study of magmatic units of the North Patagonian Massif

The origin of Patagonia and its relations with the South American crustal blocks to the north have been a matter of debate for decades. We report results from a multidisciplinary study centered on Paleozoic granitoids exposed in the northeastern corner of the North Patagonian Massif. Microstructural and magnetofabric studies reveal two suites of granitoids. Late Carboniferous (?) granitoids (Yaminué Complex, Tardugno Granodiorite, Cabeza de Vaca leucogranite) were emplaced and subsequently deformed in a major NNE-SSW compressive stress regime that also provoked top-to-the-SW thrust deformation in shallow crustal levels. Gravity and geobarometric studies show that the same major deformation event has been recorded at different crustal levels. The age and type of deformation of this event recorded across the northern boundary of Patagonia strongly supports a Late Carboniferous - Early Permian frontal collision between Patagonia and Gondwana. This major deformation event ceased by 281 Ma when the Navarrete Plutonic Complex, which shows mainly magmatic fabrics, was emplaced under a far-field WNW-ESE stress regime. Crustal continuity between the North Patagonian Massif and the Pampia and Arequipa- Antofalla terranes is suggested by similar Late Paleoproterozoic crustal model ages, comparable detrital zircon ages in Early Paleozoic successions, the apparent continuity of an Early Ordovician continental magmatic arc and paleomagnetic data. Reconciliation of this evidence with the Late Paleozoic frontal collision is obtained in a tectonic model that suggests that the North Patagonian Massif is a parautochthonous crustal block

Análisis de las variaciones paleoseculares del campo magnético terrestre sobre la base de estudios paleomagnéticos y radimétricos en sedimentos lacustres

Se analizan los resultados de los estudios paleomagnéticos y radimétricos de sedimentos extraídos del fondo de lagos del sudoeste argentino (Lagos El Trébol, Moreno y Nahuel Huapi, provincia de Río Negro) con el objeto de definir las variaciones paleoseculares del campo magnético terrestre en tiempos postglaciales. Los perfilajes de declinación e inclinación del magnetismo remanente estable de los sedimentos en función de su profundidad, muestran oscilaciones que pueden ser utilizadas tanto para su correlación entre los diferentes lagos, como para la construcción de curvas de referencia de variaciones paleoseculares para esta área de América del Sur. Estas curvas son comparadas con las obtenidas para Australia, notándose una similitud en las correspondientes a las anomalías de la inclinación, lo cual sugiere que la velocidad de la deriva hacia el Oeste de las fuentes del campo no dipolar para este parámetro, fue del mismo orden que la actual.The results of paleomagnetic and radiometric studies of sediments obtained from the botton of lakes of southwestern Argentina (Lakes el Trébol, Moreno and Nahuel Huapi, province of Rio Negro) are analyzed. The logs of declination and inclination of stable remanent magnetization vs depth show oscillations that may be used for correlation among different lakes, and also for the construction of master curves of paleosecular variation for this area of South America. These curves are compared with those obtained for Australia, a similitude can be observed in the curves which show the anomalies of the inclination, this suggests that the westward drift rate of the sources of the non dipolar field for this parameter was of the same magnitude as today.Asociación Argentina de Geofísicos y Geodesta