Alejandro Tassone

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 (Mlt;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.

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.

Erosional and depositional contourite features at the transition between the western Scotia Sea and southern South Atlantic Ocean: links with regional water-mass circulation since the Middle Miocene

The aim of the present study was to characterise the morpho-sedimentary features and main stratigraphic stacking pattern off the Tierra del Fuego continental margin, the north-western sector of the Scotia Sea abyssal plain (Yaghan Basin) and the Malvinas/Falkland depression, based on single- and multi-channel seismic profiles. Distinct contourite features were identified within the sedimentary record from the Middle Miocene onwards. Each major drift developed in a water depth range coincident with a particular water mass, contourite terraces on top of some of these drifts being associated with interfaces between water masses. Two major palaeoceanographic changes were identified. One took place in the Middle Miocene with the onset of Antarctic Intermediate Water flow and the enhancement of Circumpolar Deep Water (CDW) flow, coevally with the onset of Weddell Sea Deep Water flow in the Scotia Sea. Another palaeoceanographic change occurred on the abyssal plain of the Yaghan Basin in the Late Miocene as a consequence of the onset of Southeast Pacific Deep Water flow and its complex interaction with the lower branch of the CDW. Interestingly, these two periods of change in bottom currents are coincident with regional tectonic episodes, as well as climate and Antarctic ice sheet oscillations. The results convincingly demonstrate that the identification of contourite features on the present-day seafloor and within the sedimentary record is the key for decoding the circulation of water masses in the past. Nevertheless, further detailed studies, especially the recovery of drill cores, are necessary to establish a more robust chronology of the evolutionary stages at the transition between the western Scotia Sea and the southern South Atlantic Ocean.

GEOSUR: Mesozoic to Quaternary evolution of Tierra del Fuego and neighbouring austral regions II

This second thematic issue on Tierra del Fuego geology and geophysics, as the first one (Menichetti and Tassone, 2007) resulted from the new and profuse available information presented at the November 2004 Geosur International Symposium (Buenos Aires, Argentina). The aim of the organizing committee of Geosur 2004 was to provide sets of synthetic papers supplying a comprehensive review of the geological and geophysical knowledge on the Tierra del Fuego region; and also more specific papers focused on subjects still under study and debate, including original contributions from Tierra del Fuego and some neighbouring areas of the Patagonian Andes and Scotia plate.

GEOSUR 2004: Mesozoic to Quaternary evolution of Tierra del Fuego and neighbouring austral regions I

The impetus for the publication of thematic issues on Tierra del Fuego geology and geophysics born from the wealth of new information presented at Geosur International Symposium held in Buenos Aires, Argentina, in November 2004. The first aim of the organizing committee of Geosur 2004 was to provide sets of synthetic papers that could provide a rather comprehensive review of the geological and geophysical knowledge of the Tierra del Fuego region; a second aim was to publish more specific papers focused on subjects still under study and debate, including original contributions from Tierra del Fuego and neighbouring areas of the Patagonian Andes and Scotia plate.

Correction to “Western Scotia Sea margins: Improved constraints on the opening of the Drake Passage”

Figure 10. Magnetic anomaly distribution in the westernmost spreading corridors of the Scotia Sea, along magnetic profiles IT-41, IT-153, IT-244, and Shack71 (see Figure 1 for location), compared to a model (top of the figure) obtained assuming symmetrical spreading rates on both ridge flanks. Along line IT-153, the bathymetric profile is also displayed. The COB is the continent-ocean boundary determined from seismic data, reported here as line drawing along lines IT-244 and IT-41. TdF, Tierra del Fuego; SSR, South Scotia Ridge. See text for further details. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, B07199, doi:10.1029/2006JB004569, 2006

Geophysical survey in Lago Yehuin, Isla Grande de Tierra del Fuego, Argentina: origin and evolution

A new shallow multichannel seismic survey was carried out in the Llancanelo Lake region (Southern Mendoza Province, Argentina) in order to depict the major Neogene sedimentary-volcanic sequences and regional evolution of a tectonic-volcanic basin located in a key region at the eastern foot of the Andes cordillera. This research is settled on early studies that comprised seismic works reaching depths of 600/700 m as well as geoelectric and electromagnetic surveys reaching the uppermost 80-100 m of the sequences. The processing of the reflection seismic data enabled to obtain a stacked section useful for a preliminary geological interpretation. The obtained results indicated the presence of three major sedimentary units with increasing volcanic (basaltic layers) intercalations with depth that accommodate to the geometry of the depocenter. The entire sequence encompasses most of the Neogene. The previous research set the methodological basis for performing more detailed shallow seismic surveys and allowed defining the strategies for obtaining a better constrained geological model. Seismic transects located north of the lake were densified by adding new profiles. Deep seismic and borehole data from the industry were included in this study in order to provide a regional stratigraphic structure of the Cenozoic sequences and to contribute to the knowledge of the final stages of evolution of the Llancanelo basin.The Cuyania stratigraphic record of the Pygodus serra Zone, and the E. robustus and E. lindstroemi subzones, is discussed in this contribution. Three classical sections have been sampled for conodonts in Precordillera and the San Rafael Block. The studied successions are composed mainly by clastic rocks with variable amount of carbonate. The Ponon Trehue and La Cantera formations start with conglomerates and represent the input of coarse clastic deposits to the Cuyania basin. The Los Azules Formation, in Los Amarillitos section, exhibits a massive sandstone bed with carbonate nodules between the black shale. The key conodonts recovered indicate a late Darriwillian age for the bearing strata, these conodonts specimens from the Ponon Trehue region are compare with those from the Precordillera. Finally, the studied clastic sedimentary successions that record this biozone have been compared, concluding that eustacy did not control the sedimentary change in Cuyania for this time interval.In this contribution we present new specimens of Litopterna recovered during the last decade in Quebrada Fiera (Mendoza Province, Argentina), whose fossiliferous sediments, currently recognized as the base of Agua de la Piedra Formation, are assignable to Late Oligocene (Deseadan South American Land Mammal Age). Two remains mentioned in the first publication on this locality were neither detailed nor described, and they have not been located in the corresponding repository. The new material consists of postcranial fragmentary remains (astragali, calcaneum, and metapodials) of Macraucheniidae (Cramaucheniinae) and an incomplete upper molar (M3) of Proterotheriidae (Proterotheriinae). These few remains of litopterns contrast with the abundance of notoungulates at Quebrada Fiera. A comparative study was carried out with material from Patagonia (Argentina) and taxa recorded in Bolivia and Peru for the same temporal interval. The specimens of Cramaucheniinae are assigned to Coniopternium andinum and the molar of Proterotheriinae to cf. Lambdaconus suinus. This contribution allows us to extend the geographical range of Coniopternium, filling the gap between the Patagonian and lower latitude localities (Bolivia and Peru) in which this genus was found. The record of L. suinus in Quebrada Fiera expands the geographical range of this species outside from Patagonia.ABSTRACT. The aquatic sloth, Thalassocnus, is one of the most intriguing linage of mammal knew from the southern pacific coast of South America during the late Neogene. It was initially recognized in Peru, but recent paleontological surveys also demonstrate its presence in Chile. However, the determination of how many species of Thalassocnus were in Chile remains as an open question. Here, we provide a detailed morphological description of an isolated distal fragment of humerus recovered at the Mina Fosforita member (7-7.5 Ma), Bahia Inglesa Formation (Atacama Region, northern Chile), which shows affinities with Thalassocnus. Morphological comparisons with others forms from Chile and Peru allow us to attribute the CPUC/C/557 specimen to T. natans, though some degree of intraspecific variation is acknowledged. The assessment of the stratigraphic provenance of the materials with previously assigned to Thalassocnus from the Bahia Inglesa, Horcon and Coquimbo formations, permits us to propose that the taxonomic diversity of Thalassocnus in Chile is unequivocal conformed by T. carolomartini, and T. natans. We also admitted the likely presence of T. antiquus and a younger species than T. natans (probably T. yaucensis). Futures discoveries of more complete specimens, and/or the detailed analysis of undescribed specimens from Chile will undoubtedly contribute to illustrate the evolutionary history of this extraordinary and enigmatic clade of organisms.The uppermost Cretaceous continental deposits of the Neuquen Basin (west-central Argentina) have yielded a high diversity vertebrate assemblage, including numerous dinosaur species. The Neuquen Group is the richest dinosaur-bearing unit of the basin and comprises the Rio Limay, the Rio Neuquen and the Rio Colorado subgroups. In this group, there is abundance of isolated, disarticulated or partially articulated sauropods and theropods. However, little is known about the taphonomic history of fossil assemblages. In this study, dinosaur remains from the Rio Neuquen Subgroup (Plottier Formation) found in the Cerro Guillermo area in southern Mendoza are examined. The investigation of fossil occurrences within the study area revealed the existence of different taphonomic modes, from isolated bones until partially articulated skeletons preserved in floodplain and channel settings. SEM-EDS analysis showed the substitution of hydroxyapatite by francolite in the bone microstructure. The presence of fluorine –in one of the cases– suggested a link between the elemental composition and depositional environments: floodplain and fluvial channel. The survey of the vertebrate accumulation types and their sedimentary context allowed documenting a wide range of processes responsible for their genesis, operating within a fluvial-dominated environment. This contribution represents an holistic approach about taphonomic history of Cretaceous dinosaurs for assessing the differential preservation of fossil assemblages in fluvial environments.