Glacial-interglacial trench supply variation, spreading-ridge subduction, and feedback controls on the Andean margin development at the Chile triple junction area (45-48 degrees S)

During the Chile triple junction (CTJ) cruise (March-April 1997), EM12 bath ymetry and seismic reflection data were collected in the vicinity of the Ch ile triple junction (45-48 degrees S), where an active spreading ridge is b eing subducted beneath the Andean continental margin. Results show a contin ental margin development shaped by tectonic processes spanning a spectrum f rom subduction-erosion to subduction-accretion. The Andean continental marg in and the Chile trench exhibit a strong segmentation which reflects the sl ab segmentation and the Chile triple junction migration. Three segments wer e identified along the Andean continental margin: the presubduction, the sy nsubduction, and the postsubduction segments, from north to south. Both cli mate-induced variations of the sediment supply to the trench and the tecton ic reorganization at the Nazca-Antarctica plate boundary involving postsubd uction ridge jump are the two main factors that control the tectonic regime of this continental margin. Along the survey area we infer the succession of two different periods during the last glacial-interglacial cycle: a glac ial period with ice-rafted detrital discharges restricted to the shoreline area and low river output and a warmer period during which the Andean ice c ap retreat allowed the Andes to be drained off. During these warm periods, rapid increase in trench deposition caused the margin to switch from subduc tion-erosion or nonaccretion to subduction-accretion: (1) along the presubd uction segment after the last deglaciation and (2) along the postsubduction segment after the interglacial episode at 130-117 ka. Conversely, a nonacc retion or subduction-erosion mode characterized the presubduction and posts ubduction segments during glacial maximums. The major effects of subduction of the buoyant Chile ridge include a shallow trench which diverts trench s ediment supply and tectonic instabilities at the Nazca-Antarctica plate bou ndary. We suggest that a postsubduction westward jump of the Chile ridge oc curred during the past 780 kyr. It produced slab fragmentation and individu alization of an ephemeral microplate north of the 'Taitao fracture zone: th e Chonos microplate. In 780 kyr, two episodes of subduction-accretion separ ated by an episode of subduction-erosion occurred in relation with the Chon os microplate individualization and subduction. The current northward migra tion of the triple junction along the Chonos microplate-South America plate boundary introduces a sharp change in the tectonic mode from subduction-er osion to the north to subduction-accretion to the south. The data collected along the Taitao ridge have revealed the complex three-dimensional structu re of an accretionary wedge which includes a midslope thrust sheet exhibiti ng the characteristics of an ophiolite: the Taitao Ridge ophiolite. No conn ection exists between the Taitao Ridge ophiolite and the Bahia Barrientos o phiolite cropping out onland in the Taitao peninsula.