C. Guivel

New geochemical constraints for the origin of ridge-subduction-related plutonic and volcanic suites from the Chile triple junction (Taitao peninsula and Site 862, LEG ODP141 on the Taitao ridge)

Abstract Several features of Neogene and Quaternary magmatism in the region south of the present-day Chile Triple Junction (CTJ) at 46°12′S are directly related to the migration of the triple junction. Due to the obliquity of the ridge orientation with respect to the subduction front, the triple junction migrated from South to North during the last 14 Ma. The Taitao Peninsula — the westernmost promontory of the Chile coast — and the Taitao Ridge — a submarine promontory north of the Taitao Peninsula — provide the most complete collection of ridge subduction-related magmatic products in the region. The emplacement of near-trench volcanics, the intrusion of a variety of plutonic rocks and the related hydrothermal activity at these two sites have been interpreted as resulting from magma interactions between subducted ridge segments of the Chile spreading centre and the continental crust. We present new field observations and geochemical data that help to better constrain the problem of the sources and evolution of the Taitao magmas. The new geochemical data were obtained on samples collected from the Taitao Peninsula during a field expedition in 1995, and from samples of the Taitao Ridge during Leg ODP141, Site 862, which have been re-sampled in 1996 by one of us. Selected major- and trace-element compositions of 20 volcanic rocks from the Taitao Ridge are discussed together with 53 analyses from different rock types from the Taitao Peninsula including 24 unpublished analyses. Nd and Sr isotopic compositions were obtained from 5 whole rocks and separated minerals of the Taitao Peninsula together with the oxygen isotope composition of four separated clinopyroxenes. Six main magmatic types are identified: (1) N-type MORB; (2) E-type MORB; (3) LREE-depleted MORB showing some trace-element features typical of arc basalts; (4) moderately Nb-depleted E-MORB; (5) calc-alkaline andesites, dacites and rhyolites; and (6) andesites and dacites with adakitic signature. Chemical similarities exist between some forearc magmas of the Taitao Ridge and the Taitao Peninsula and magmas emplaced at the Chile active spreading ridge. One important result, based on isotope data, is that the lavas emplaced over the continental crust (Taitao Peninsula) did not originate from melting of continental crust nor from extensive assimilation of such a crust by mantle-derived magmas. The likely source of these basalts could be the hot convective oceanic mantle of the southern Chile spreading ridge buried at moderate depth (10–30 km).

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)

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-erosion 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.

Very shallow melting of oceanic crust during spreading ridge subduction: Origin of near‐trench Quaternary volcanism at the Chile Triple Junction

2Nd = +2.9 to +3.8, and d 18 O values = +6.4 to +6.9% respectively, consistent with mixed magma sources that include a MORB-type component and sediments. We propose that the high-Si dacites are derived from the hydrous melting of a mixture of MORB and sediments at high temperatures (800� –900� C) under low pressures (<0.8 GPa). The low-Si dacites originate from the melting of a similar source under higher pressures consistent with depths of 25–45 km. Two scenarios accounting for the near-trench position of these latter rocks are envisioned. The first invokes rapid tectonic erosion and changes in sedimentary wedge geometry. The second one postulates that parts of the slab are subducted rapidly to depths of 20–30 km right under the trench. INDEX TERMS: 3640 Mineralogy and Petrology: Igneous petrology; 8120 Tectonophysics: Dynamics of lithosphere and mantle— general; 9360 Information Related to Geographic Region: South America; KEYWORDS: slab melting, Chile trench, ridge subduction, forearc magmatism, geochemistry, calc-alkaline magmatism

Newly identified segments of the Pacific–Australia plate boundary along the North Fiji transform zone

Abstract The North Fiji transform zone, a 1500 km long and 200 km wide transform segment of the Pacific–Australia plate boundary, is one of the major transform fault systems of the Earth. New data collected during the ALAUFI cruise (March 2000) on board the R/V L’Atalante make it possible to define more accurately the geometry and kinematics of this transform plate boundary. Three spreading centers or extensional zones (the North Cikobia spreading center, the Futuna spreading center and the southeast Futuna volcanic zone) and a strike-slip fault zone (the Futuna transform fault) have been discovered over a distance of 500 km along the eastern North Fiji transform zone, from the north of the Fiji platform to the east of the Futuna archipelago. The Futuna transform fault oriented 100° has been mapped over a distance of 250 km. It must be considered to be an important tectonic element of the transform plate boundary. Pure strike-slip as well as transpression and transtension motions are responsible for the complex morphology of this feature. The uplifted Futuna–Alofi ridge represents a major compressional relay along the Futuna transform fault. The Futuna spreading center trending 20–30° is composed of a series of en echelon left-stepping spreading segments. It represents a 200 km long extensional relay between the Futuna transform fault and the western part of the North Fiji transform zone, the Fiji transform fault, which bounds the Fiji platform to the north. The opening rate at the Futuna spreading center is estimated at 4 cm/yr. Although the North Cikobia spreading center and the southeast Futuna volcanic zone have been only partly mapped, bathymetric and reflectivity data clearly reveal that active extension also takes place along these two features. A spreading rate of 2 cm/yr is inferred at the North Cikobia spreading center. Therefore, the North Fiji transform zone appears to be composed of two main overlapping transform segments relayed by parallel extensional zones. The three active extensional zones have an ENE–WSW to NNE–SSW orientation, while compressive features along the Futuna transform fault are NW–SE to NNW–SSE oriented, in accordance with the present-day left-lateral transform motion along this part of the Pacific–Australia plate boundary.

Giving birth to hotspot volcanoes: Distribution and composition of young seamounts from the seafloor near Tahiti and Pitcairn islands

Apart from being popular holiday destinations, oceanic-island volcanoes such as Hawaii, Tahiti, or the Canaries provide magmas that yield valuable information about the interior of our planet. Until recently, studies have concentrated on the easily accessible, subaerial parts of the volcanoes, largely ignoring their earlier-formed, submarine parts. These submarine parts, however, provide critical information about how the mantle begins to melt and about the lowest-melting-point mantle components-information not available from the subaerial volcanoes but highly relevant for the chemical evolution of the whole mantle. We present here compositional information from small (<500 m) volcanoes on the seafloor near Tahiti and Pitcairn Islands and show that these small volcanoes erupt only highly differentiated magmas. These early melts are derived exclusively from the most trace element-enriched, isotopically extreme mantle component, evidence that this component has the lowest melting temperature and is the first product of melting of a new batch of mantle. The geochemical mantle components (enriched mantle EM-I, EM-II) proposed in the 1980s to explain the compositional variations among oceanic volcanoes worldwide appear in reality to represent distinct rock masses in the mantle.