La croûte océanique et les points chauds de la Polynésie française (Océan Pacifique central)

Abstract

Using a compilation of available marine geophysical data and a new free-air anomaly gravimetric grid computed from satellite altimetry, we propose a new synthesis for the structure and evolution of the oceanic trust in the French also identtfi the main seamounts from the gravimetric grid and compile the available age data in the region. The results are shown on a tectonic map drawn up at a scale of l:l0,000,000. Crust generation in the French Polynesia region is due to three distinct spreading processes. TO the west, the trust was generated during the Cretaceous (between Il8 and 84 Ma) at the axis of the Pacific-Antarctic Ridge. It is separatedfrom the trust resulting from spreading at the axis of the Pacific-Farallon Ridge by the trace of a triple junction (Pacific, Aluk, and Farallon). The trace was first identified between 35” and 45” S by Cande and Iiaxby (l99l) and we propose to extend it towards the north as jàr as 22” S. TO the east of l60” W, three spreading segments of the Pacific-Farallon Ridge are identified, separated by three fracture zones striking N 80” E: Galapagos, Marquesas and Austral. Almost complete sequences of magnetic anomalies are identified in the three segments from anomaly 34 (84 Ma using the magnetic time scale of Berggren et al. (l98.5)) to anomaly 7 (26 Ma). The spreading rate increases progressively from 23 km/Ma (anomaly 34) to 82 km/Ma (anomaly 7). At about 26 Ma, the ridge orientation changes from N 80” to N ll5” E. The Nll5” E trend corresponds to spreading at the axis of the East Pacific Rise with a spreading rate of about ll0 km/Ma. This event is followed by a major jump of the East Pacific Rise towards the west (Mammerickx et al., 2980; Goff and Cochran, l996): l500 km in the Marquesas segment, 600 km in the Tuamotu Segment, and 250 to 400 km in the Austral segment. It is also possible to identyfy a microplate similar to the Easter or Juan Fernandez microplates, located at 3-8” S and l24-l30” W to the north of the Marquesas Fracture Zone. Numerous oceanic islands are obsewed in the French Polynesia region and many seamounts have also been identified. Using the gravimetric grid, we have identified and mapped 42l circular or elongate positive anomalies with an amplitude greater than 30 mgal. Comparison with bathymetric data shows that this value corresponds to a seamount with an elevation of about l000 m above the seafloor at a mean water depth of 4500-5000 m. Thus, the mapped seamounts correspond to elevations of more than l000 m. Oceanic islands are grouped in a number of volcanic lineaments showing a progression of formation ages. Only half of the Line Islands (Winterer et al., l973; Schlanger et al., l976; Saito and Ozima, l977; Haggerty et al., l982; Schlanger et al., l984) are consistent with a single hot-spot model with an age progression of 96 km/Ma. TO the south of the Galapagos Fracture Zone, we observe two volcanic lineaments with the western lineament yielding an age progression of l40 km/Ma. The Marquesas Islands (Duncan and Mc Dougal, l974; Duncan et al., l986; Brousse el al., l990; Diraison, l99l; Ielsch et al., l997) were built up 5.6l to l.33 Ma ago on trust ranging in age,from 55 to 49 Ma. The main orientation is N l40” E, which dixfers ,from the N ll5” E direction of the absolute motion of the Pacifie Plate. No age data are available,fbr the Tuamotu Islands (Okal and Cazenave, l985; McNutt and Fischer, l987; Ito et al., l995) which are coral atolls with volcanic pedestals built on a wide oceanic plateau whose origin and age are unknown. The Tuamotu Islands seem to have been formed along two distinct volcanic lineaments, which could correspond to the southeast prolongation of the two Line Islands lineaments. The limit between the Line Islands and Tuamotu lineaments corresponds to a change of direction in the age progression, which could be related to a similar change observed along the Emperor-Hawaii volcanic chain at 43 Ma (Norton, l99.5). The Society Islands (Duncan and McDougall, l976; Gisbert, l989; Roperch and Duncan, l990; White and Duncan, l996; Blais et al., l997; Guillou et al. l998), between l5’and l8” Sand l47” and l58” W, show ages which decrease smoothly towards the southeast with a migration rate of ll0 km/Ma. The Pitcairn-Gambier Islands (Duncan et al., l974; Guillou et al., l994) make up a l000 km long lineament with ages ranging from Il Ma in the northwest to 0 Ma in the southeast. The Cook-Austral islands (Jarrard and Clague, l977; Turner and Jarrard, l982; Duncan and Clague, l985; Baudry et al., l988; Stoffers et al., l989; McNutt et al., l997) form one of the longest lineaments at more than 2000 km long. In addition to the currently active Macdonald seamount at the southeastern end of the lineament, recent volcanism has occurred ut three other islands belonging to the lineament (Aïtutaki, Rarotonga, and Rurutu). Moreover, during a recent cruise of R/V Maurice Ewing, several seamounts were dredged around the Macdonald seamount, yielding ages ranging between 25 and 33 Ma. At least four hot-spot tracks are needed to explain the variability of the Cook-Austral islands.