Glacial-interglacial trench supply variation, spreading-ridge subduction, and feedback controls on the Andean margin development at the Chile triple junction area (45-48°S)
Résumé
During the Chile triple junction (CTJ) cruise (March–April 1997), EM12 bathymetry and seismic reflection data were collected in the vicinity of the Chile triple junction (45‐480S), where an active spreading ridge is being subducted beneath the Andean continental margin. Results show a continental margin development shaped by tectonic processes spanning a spectrum from subduction‐erosion to subduction‐accretion. The Andean continental margin and the Chile trench exhibit a strong segmentation which reflects the slab segmentation and the Chile triple junction migration. Three segments were identified along the Andean continental margin: the presubduction, the synsubduction, and the postsubduction segments, from north to south. Both climate‐induced variations of the sediment supply to the trench and the tectonic reorganization at the Nazca‐Antarctica plate boundary involving postsubduction 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 glacial period with ice‐rafted detrital discharges restricted to the shoreline area and low river output and a warmer period during which the Andean ice cap retreat allowed the Andes to be drained off. During these warm periods, rapid increase in trench deposition caused the margin to switch from subductionerosion or nonaccretion to subduction‐accretion: (1) along the presubduction segment after the last deglaciation and (2) along the postsubduction segment after the interglacial episode at 130–117 ka. Conversely, a nonaccretion or subduction‐érosion mode characterized the presubduction and postsubduction segments during glacial maximums. The major effects of subduction of the buoyant Chile ridge include a shallow trench which diverts trench sediment supply and tectonic instabilities at the Nazca‐Antarctica plate boundary. We suggest that a postsubduction westward jump of the Chile ridge occurred during the past 780 kyr. It produced slab fragmentation and individualization of an ephemeral microplate north of the Taitao fracture zone: the Chonos microplate. In 780 kyr, two episodes of subduction‐accretion separated by an episode of subduction‐erosion occurred in relation with the Chonos microplate individualization and subduction. The current northward migration of the triple junction along the Chonos microplate‐South America plate boundary introduces a sharp change in the tectonic mode from subduction‐erosion to the north to subduction‐accretion to the south. The data collected along the Taitao ridge have revealed the complex three‐dimensional structure of an accretionary wedge which includes a midslope thrust sheet exhibiting the characteristics of an ophiolite: the Taitao Ridge ophiolite. No connection exists between the Taitao Ridge ophiolite and the Bahia Barrientos ophiolite cropping out onland in the Taitao peninsula.
Domaines
Tectonique
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