Deformation zone Jumps, in a young convergent setting; the Lengguru fold-and-thrust belt, New Guinea Island

The Lengguru fold-and-thrust belt in West Papua (Indonesia) has all the characteristics of a young orogen involved in a rapidly changing tectonic setting. The analysis of the young wedge shows however that its internal shortening has ceased recently, and that it is nowadays suffering severe extension. Recent topographic data, marine industrial seismic lines and drilling, were used with field observations and measurements to create detailed cross-sections and a new structural map. The study allows us to distinguish two superimposed prisms composed of stacked Mesozoic marine sediments of the Australian margin against a crustal buttress. The construction of these two wedges is younger than 11 Myr. The structures of the Lengguru belt external zones are sealed by an unconformable clastic series, indicating that the construction of the Lengguru prism had aborted suddenly due to a change in the way the Australian and Pacific plate convergence was accommodated. At that time, the internal zones probably started to exhume and the tectonic regime became extensional. Nowadays the internal part of the Lengguru fold-and-thrust belt is undergoing an active east-west extension. We believe that the extension observed in the Lengguru wedge is coeval with a transition from a compressive to a transtensional regime illustrated in the Central Range of Papua, and the onset of the Tarera-Aiduna and Paniai left-lateral faults. The structure of the Lengguru belt therefore results from events occurring over a very short time span; a previous Late Miocene northeast-southwest compression linked to the subduction process, a second from Middle Miocene to Early Pliocene and a Late Pliocene-Quaternary global extension in the whole range. The evolution is compared with that of the Seram wedge and the Misool-Onin-Kumawa continental ridge to the west; where deformation is accommodated at a localized zone which jumps as convergence between Australian and Pacific plate proceeds. This evolution of the belt reflects rapid changes in the accommodation oblique shortening, with the isolated orogenic wedge of Lengguru fold-and-thrust belt left to collapse. This example illustrates the way a long-lasting subduction terminates. At the lithospheric scale, the deformation remains rooted at the suture zone. However at the surface, the shortening is suddenly widespread over a large area during a very short time span (formation of the Lengguru belt) prior to being transferred to another plate boundary. (C) 2009 Elsevier B.V. All rights reserved.