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Earthquakes, Rivers and Ice: Scientific Research at the Laguna San Rafael, Southern Chile, 1986

1988; Wiley; Volume: 154; Issue: 3 Linguagem: Inglês

10.2307/634611

1475-4959

Denise J. Reed, Robert M. Wood, Jim Best,

Cryospheric studies and observations

The Laguna San Rafael, Aisen Province, southern Chile, is a tidal lagoon which receives the San Rafael glacier?the nearest tidewater glacier to the equator. The glacier is sustained by very high precipitation and pronounced topographic relief on the tectonically active western Andean margin. This area formed the focus for three research projects undertaken as part of an expedition mounted by Operation Raleigh in early 1986: investigating and dating subsidence events in association with past great subduction zone earthquakes; analysing the sedimentation of braided streams emerging from the North Patagonian ice field; and studying the tidal and meltwater dynamics of the lagoon. glaciers, southern Chile. THE WESTERN MARGIN of South America is underlain by a subduction zone, where Pacific Ocean crust passes down towards the east beneath the Americas. At the southern end of this subduction zone, in southern Chile, a mid-ocean spreading ridge intercepts the zone just offshore from the Taitao Peninsula (at around 47?S). This triple junction, where the Nazca, Antarctica and South American plates meet, is an area of massive earthquakes and very rapid crustal deformation (Forsythe and Nelson, 1985). This great tectonic activity is reflected by a correspondingly fast-evolving landscape with the steep fault-controlled margin of the Andes in southern Chile rising from sea level to over 1400 metres in less than 5 kilometres (Plate I). Where the Taitao Peninsula connects with the South American mainland, lies the Laguna San Rafael (46?40'S, 74?00'W) (Fig. 1), a 130 square kilometre tidal lagoon into which the San Rafael Glacier calves (Plate II). The glacier, classified by Lliboutry (1956) as the fifth largest in Chile, flows down the steep margin of the Andes from the North Patagonian Icefield at rates of up to 17 metres per day (Nakajima, 1985). This rapid velocity for a non-surging glacier is a product of the steep western slope of the tectonically active mountains and high snowfall in the accumulation area with annual precipitation at the Laguna San Rafael totalling more than 4000 mm (Enomoto and Nakajima, 1985). Although the San Rafael Glacier is the only branch of the North Patagonian Icefield to reach the sea, the meltwaters from other glaciers source large braided streams with high sediment loads and rapidly changing channel patterns. Various components of this dynamic landscape around the Laguna San Rafael provided the focus for scientific research on the Chile 4B phase of Operation Raleigh in January 1986. The Laguna San Rafael connects with the open fiord, Golfo Elefantes, to the north via a 30-metre deep channel, the Rio Tempanos. At present this is the only exit from the Laguna although in 1940 an attempt was made to open up a second exit via a 2-kilometre long canal through the moraine to the south-west, interconnecting with the Rio Negro. The Rio Negro flows from the bogs of the Taitao Peninsula and joins a series of sediment-laden rivers from the St Quintin Glacier before flowing into the Golfo de Penas. Completion of this canal would provide a sheltered route for boats, avoiding the exposed outer coast of the Taitao Peninsula. The canal was partially completed and there is, at present, renewed interest in finishing the project. The future construction of roads into the region is also planned in order to open up this remote area of the southern Andes to both settlers and tourists. In order for all these schemes to be successfully planned and their environmental impact monitored more information is needed on the morphological evolution of the region and possible local hazards.