ABSTRACT

Flores island, situated between latitudes 8°4' and 8°58' South, extends over a length of 360 kilometres, NW of Australia. It thus belongs to the South-eastern islands of the Indonesian archipelago (Nusa Tenggara province) with, in particular, the islands of Timor and Sumba.

From a geodynamical point of view, this island, flanked to the North by the Flores basin and to the South by the Savu basin, is volcanically active with at least 13 live volcanoes. It also corresponds to an area of intense seismic activity (12th December 1992 earthquake, magnitude 7.5).

Stratigraphic and magmatic (geochronology, geochemistry) analysis shows that Flores is a "young" island, emerging probably in the upper Oligocene, surely by mid-Miocene.

In detail we distinguish :

• a probably Oligocene cycle - middle to upper Miocene, characterised by :
• - an E-W volcanic axis (Kiro Formation) where 17 sampled lavas yield radiometric ages between 16 Ma and 8.4 Ma (end Burdigalian to mid Tortonian) and one lava between 27.7 Ma and 25.7 Ma (terminal Oligocene) ;
• - heterochronous lateral sedimentary and volcano sedimentary deposits, with excessively reworked microfaunas. We distinguish nonetheless, from base to top, the turbiditic tufacious mid-Miocene Nangapanda Formation, the neritic to reef limestone middle to upper Miocene Bari Formation, the chalky tufacious (with pumices) upper Miocene Laka Formation;

• an essentially volcanic terminal Miocene to Plio-Quaternary cycle where 13 samples including 2 granodiorites have been dated radiometrically at 6.7 Ma to 1.2 Ma.

From a regional and chronological point of view, volcanic activity starts on Flores when that of Sumba, having begun in the upper Cretaceous, ceases. Thus, during the Oligocene, Sumba island leaves its active arc position for the external forearc basin, while Flores takes over the volcanic activity.

During the Plio-Quaternary, the Australian continental plate, coming from the South, collides to the East with the Eurasian plate, at the Timor. Nonetheless, premises of collision are felt to the West, near the area of study, although still in the subduction system, with the net surrection of Sumba and the development of a relatively modest meridian compression facing Flores, marked by a network of active conjugate faults and, North of the island, the North verging "Flores sea" thrust, absorbing a part of generated shortening.

N° 12-1998 HENDARYONO : Géologie de l'ile de Florès. Apports à l'étude de la géodynamique de l'archipel indonésien oriental. 200 p. ISBN 2-904431-21-7. Résumé français, indonésien. - en vente au prix de 150 F ou  22,86 euros, + port  -

Cet ouvrages est diponibles auprès de Andre.Paillet@univ-savoie.fr