Southern Patagonian Andes Research Articles

Neogene to Quaternary tectonic evolution of the Patagonian Andes at the latitude of the Chile Triple Junction

We conducted a preliminary structural and tectonic analysis of the region north and south of the Lake General Carrera–Buenos Aires (LGCBA, Southern Patagonian Andes, Chile and Argentina) in order to study the effects of the subduction of the active South Chile spreading Ridge (SCR) on the development of the tectonic front of the Patagonian Andes at the latitude of the Chile Triple Junction (CTJ). Here, one segment of the SCR entered the trench at 6 Ma and the corresponding slab window is now located below the study area. We emphasize the segmentation of the tectonic front which can be divided into four successive oblique segments (A to D) trending mainly N170 and right laterally offset. This geometrical pattern is consistent with a transpressive stress regime in relation to oblique subduction. The four segments of the tectonic front are separated and locally deformed by N170 oriented, more recent sinistral strike-slip fault zones. Kinematic analysis of minor fault and striaes confirms the abundance of strike-slip faults and suggests rotation of the stress tensor after at least 9 Ma. A consequence of the subduction of the SCR beneath the South American plate at the latitude of the LGCBA is a two-step history: (1) Pre-ridge subduction, oblique to the western margin of the South America plate showing contractional regime with its main axis trending NE–SW responsible for dextral transpressive deformation and (2) Post-ridge subduction more orthogonal convergent regime. Recent uplift occurred later, starting at 3.5 Ma, and could still be active. Neotectonic activity has been evaluated in the region from preliminary analysis of river patterns and fluvial–lacustrine terraces. Anomalies in stream development can be found. Although evidences for recent to active faulting can be shown, effects of syn-glacial and post-glacial-related processes cannot be ruled out. The evolution of this region during the Late Cenozoic can be divided into three distinct periods: (1) 20–15 Ma, deposition of foreland continental detrital sediments (Rio Zeballos Group), followed by a major contractional phase leading to the development of the main thrust front sometimes between 15 and 10–9 Ma. (2) 9–4 Ma, erosion, peneplanation and emplacement of voluminous plateau basalts coeval with fluvial–glacial sedimentation and, (3) 3–0 Ma, renewed orogenic activity, uplift and disruption of former erosional surfaces in relation to activation of strike-slip fault zones and north trending thrusts. Onset of the major contractional event in the region of study can be correlated with the initiation of the subduction of the Chile Ridge at 14–15 Ma in southernmost Patagonia, inducing a different stress regime in the frontal arc region.

Stable isotope composition of precipitation across the southern Patagonian Andes

A surface water and precipitation transect across the southern Patagonian Andes at 47°–48°S was conducted to assess how mountains affect the isotopic composition of precipitation. This westerly wind region derives its moisture that falls as precipitation from the Pacific. Orographic uplift of air over the Andes causes pseudoadiabatic cooling and orographic precipitation on the western side and a strong rain shadow on the eastern side of the mountains. These processes also produce a profound isotopic rain shadow, with δ18O values ∼4‰ lower in the east compared to the west. On the windward western side of the Andes, the isotope values of precipitation and surface waters show weak, although systematic, trends with elevation, but a pronounced correlation with distance from the main source of moisture (Pacific Ocean). A Rayleigh simulation of precipitation δ18O values as a function of condensation altitude matches our data well, suggesting that other processes, such as mixing of water sources and postcondensation evaporation, are negligible, and that topography is the dominant control on the isotope ratio of precipitation. In contrast, isotope ratios of waters on the leeward eastern side of the southern Patagonian Andes vary neither with elevation nor with distance from the mountains, consistent with much of the precipitation on the leeward side being derived from precipitation carried over the Andes by winds. Thus, paleoprecipitation isotopic composition on the leeward side of orogens in similar climatic settings (single moisture source and cool climate) yield useful estimates of the elevation of these orogens, but not local elevation.

The hydrological signal of the Perito Moreno Glacier damming of Lake Argentino (southern Andean Patagonia): the connection to climate anomalies

The periodic damming of Lake Argentino by the Perito Moreno Glacier (50°30′S, 73° 00′W) in Argentina's southern Patagonian Andes has been recorded seventeen times since the beginning of this century. Such events are significant factors controlling discharge anomalies (seasonal component removed) of the Santa Cruz River, the sole outlet of Lake Argentino. Power spectrum analysis of the deseasonalized discharge revealed significant period peaks in the 33- to 36-month range and in the 42- to 58-month range. The first frequency is probably determined by the anomalous position of the subtropical anticyclones in the Pacific (with 2–5 years recurrence intervals), whereas the remaining frequencies are coincidental with the multivariate ENSO index (MEI) frequency spectrum. Significant squared coherency (>0.78) between the Santa Cruz River discharge anomalies and the MEI suggests that there is a significant teleconnection between ENSO occurrences in the Pacific and the Perito Moreno Glacier dynamics. El Niño events, for example, appear to have fostered the advancement of the glacier's snout and influenced the recorded damming–rupture sequence.

Relative water fluxes and silicate weathering from the tributaries of a small glaciated watershed in the southern Patagonian Andes (Upper Manso watershed, Argentina)

Relative water fluxes and silicate weathering from the tributaries of a small glaciated watershed in the southern Patagonian Andes (Upper Manso watershed, Argentina)

Contents volume 53, 1992

Mountain building in the Andes, the longest continental mountain range on Earth, started in the Late Cretaceous but was highly diachronous. Reconstructing the timing of surface uplift for each of the different Andean regions is of crucial importance for our understanding of continental-scale moisture transport and atmospheric circulation, the origin and evolution of the Amazon River and Rainforest, and ultimately, the origin and evolution of species on the world most biodiverse continent. Here, I present (1) a compilation of estimates of paleoelevation for 36 geomorphological domains of the Andes from the literature, and (2) a paleoelevation reconstruction of the Andes since 80 Ma. In the northern Andes, uplift started in the Late Cretaceous (~70 Ma) in the Western and Central Cordilleras of Ecuador, while the northwestern corner of the continent was still covered by shallow seas. Mountain building migrated progressively northwards, with the Perija Range and Santander Massif uplifting since the Oligocene and the Eastern Cordillera, Garzon Massif and Mérida Andes since the Miocene. In the central Andes, uplift migrated from west to east, whereby the main phase of uplift in the Western Cordillera took place during the Late Cretaceous-Paleocene, in the western Puna plateau during the Paleocene, in the eastern Puna plateau during the early-mid Miocene, and in the Altiplano and Eastern Cordillera during the mid-late Miocene. In the southern Patagonian Andes, significant elevation was already in place at 80 Ma and in western Patagonia, modern elevations were reached in the early Eocene. A second pulse of uplift and eastward migration of the orogenic front occurred during the early-mid Miocene. The reconstruction developed here is made available as a series of raster files, so that it can be used as input for a variety of studies in the solid Earth, climate, and biological sciences, thereby being a stepping stone on the path towards a better understanding of the coevolution of the solid Earth, landscapes, climate, and life in South America.

Southern Patagonia: As Portrayed in Recent Literature

HIRTY years ago thousands of people who had rushed to the southern extremity of South America in search of gold turned to sheep raising as a more certain means of making a living. Today we perceive a tendency toward reversal of this trend. Not that new gold deposits have been found in this region-actually we know little more about them-but the world itself has changeda world that finds itself today with plenty of sheep and a shortage of the gold necessary to distribute those sheep, a world that needs gold far more than it needs mutton or wool or any other commodity. Indications are not wanting that this forgotten region is upon the threshold of return to public interest. Of all the regions of the earth's temperate zones it is doubtful whether there remains a single one of which man's knowledge is so incomplete as of that of the southern Patagonian Andes from the forty-fifth parallel of latitude to the outermost islands of the Tierra del Fuego archipelago. Yet we can hardly call this land of mystery remote: not one of its I8o,ooo odd square miles of mainland and islands is 200 miles distant from one of the New World's oldest trade routes. Nor has there been a dearth of interest in it. Between Magellan's discovery of the strait in I520 and the visit of the Beagle in I832 no fewer than 81 expeditions representing many nations have explored there. Between the voyage of the Beagle and the publication of Charles Darwin's Diary of the Voyage of H. M. S. 'Beagle' last year' scores of books and articles in various languages have appeared. Yet one needs only to glance over the most recent literature to realize how little is actually known, and even the latest maps disclose vast areas marked inexplorado. But, while the region has fallen into further economic obscurity with the replacement of the Strait of Magellan as a maritime channel by the Panama Canal, a new impetus seems to have been given to scientific exploration. To mention one event of importance: the most extensive of Patagonia's great inland ice fields was traversed for the first time by De Agostini in I93Itruly a noteworthy achievement. We may well begin our discussion with the problem of the present glaciation of Patagonia.