ACTIVE SLIVERING OF OCEANIC CRUST ALONG THE MOLUCCA RIDGE (INDONESIA-PHILIPPINE). GEODYNAMIC IMPLICATION.

Anne Gaëlle BADER (1), Manuel PUBELLIER (1), Claude RANGIN (2), Christine DEPLUS (3), and R. LOUAT (3,4)

(1) : URA 1759 CNRS, Département de Géotectonique, T 26-0 E1, Université Pierre et Marie Curie, 4 Pl. Jussieu, 75252 Paris France. (e-mail : gael@peridot.ens.fr).

(2) : Laboratoire de géologie ENS, 24 rue Lhomond, 75005 Paris.

(3) : Département de Geophysique, Institut de Physique du Globe, T26-16 E3, 4 Pl. Jussieu, 75252 Paris France.

(4) : ORSTOM,

ABSTRACT

Recent marine geophysical survey in the Northern Molucca Sea showed the structure to be that of a classical active margin. We observe from west to east; a volcanic arc (Sangihe), a forearc basin resting on an outer ridge (the Molucca ridge) which serves as a buttress for an accretionary wedge and a composite downgoing plate (Snellius Ridge and Philippine Sea Basin). Gravity modeling indicate a strong negative anomaly above the wedge, which cannot be explained with reasonable density values. Modeling imposes a basement deepening, and rupture from the 700 km-long subducting lithosphere. This process individualizes the lithospheric slab from the Snellius Ridge, which in turn was separated recently from the South Philippine Basin by the incipient Philippine Trench. This induces a deformation of the forearc region with backthrusting of the outer ridge and forearc basin, visible on bathymetry and seismic data. We extrapolate the tectonic emplacement of such oceanic blocks to the Oligocene times in order to explain the origin of the Pujada Miangas outer ridge as a similar sliver accreted to the Eurasian margin.