Forearc deformation and tectonic significance of the ultramafic Molucca Central Ridge, Talaud islands (Indonesia).

Anne Gaëlle Bader ⁽¹⁾, Manuel Pubellier ⁽²⁾

(1) : Institut Francais du Petrole, 1 & 4 Av. de Bois Preau, 92852 Rueil Malmaison Cedex, now at (2). Email gael@geologie.ens.fr

(2) : CNRS, UMR 8538, Laboratoire de Géologie, Ecole Normale Supérieure, 24 rue Lhomond 75231 Paris Cedex 05 France.Email: Manuel.Pubellier@geologie.ens.fr

Abstract

The Molucca Sea is a narrow basin located south of Mindanao (Philippines) and underlined by a N-S ophiolitic ridge. This ridge represents the outer ridge of the Sangihe subduction zone and emerges by uplift in the central part of the basin, in the Talaud islands. Field studies indicate that forearc sediments rest unconformably on a (1) dismembered ophiolitic series and (2) thick melanges. Structural analysis indicates two deformation events, one of which is oriented E-W coaxial with the present state of strain. We interpret the earlier (N20E) direction as a thrusting event that affected an ophiolitic basement associated with the edge of the Celebes Sea. Thrusting within the oceanic crust and sediments also generated olistostromes (melanges). The style of deformation is characterised by flattened rhombs of peridotites which exist in situ in the upper section of the crustal sequence and were also found inside the melange. Incipient Sangihe subduction around 15 Ma uplifted the deformed crust and buried the melanges beneath the forearc sediments. Recent E-W shortening during subduction of the Snellius Plateau reactivated the melanges within thrusts cutting the forearc series.

Key words : Talaud island, ophiolite, melanges, forearc deformation, kinematic reconstruction.