COLLISIONAL MELANGE DEVELOPMENT - GEOLOGIC ASSOCIATIONS OF ACTIVE MELANGE-FORMING PROCESSES WITH EXHUMED MELANGE FACIES IN THE WESTERN BANDA OROGEN, INDONESIA

Analysis of block assemblages, matrix clay mineral composition and mic rofauna, and offshore seismic reflection profiles reveal that the Bobo naro melange in the Timer region is sourced from mud-rich Australian c ontinental margin sequences that are remobilized during accretion and form various facies depending on the structural conditions of emplacem ent. Melange facies include broken formation, matrix-rich mud injectio ns, and classic mixed block-in-clay facies. Each of these are distingu ished by varying degrees of remobilization, mixing, and dispersion at different structural positions across the orogenic wedge. The most imp ortant structural control is whether melange was generated beneath or in front of upper plate Banda forearc basement (Banda Terrane). At the present collisional deformation bent in the Timer trough, seismic ref lection profiles show that melange forms mostly by stratal detachment and fluid-assisted remobilization above a basal decollement propagatin g laterally along overpressured Jurassic to Cretaceous clay-rich seque nces of the distal Australian continental margin. The broken, clay-ric h material injects upward through faults to form intrusive bodies at t he base of slope cover sediment to form mud ridges at the surface. Sim ilar patterns of stratal disruption are exposed onshore in the Pliocen e Kolbano fold and thrust wedge of southern Timer, which is structural ly contiguous with the Timer trough deformation front. Melange in the Kolbano Mountains is mostly broken formation and matrix-rich injection s of mud from Jurassic and Cretaceous units. Deformation mechanisms in clude intense layer-parallel extension associated with emplacement by mud diapirs that rise from near the decollement upward to the surface along fault conduits. In the hinterland of the orogenic wedge (East Ti mer and northern West Timer), melange is dominantly of mixed block-in- clay facies with large blocks derived from roof thrust sheets of inter mixed Banda Terrane and Maubisse Formation units. At the structural ba se of these thrust sheets is the Sonnebait Disruption Zone (SDZ), whic h formed the initial suture between the panda Terrane and structurally underthrust Australian continental margin sequences during the late M iocene to early Pliocene. The thickest accumulations of mixed block-in -clay melange in the Timer region are found at the southern edge (outl et) of the SDZ, near the Central and Viqueque synorogenic basins. The extent of block dispersion and mixing in the Sm is indicative of inten se shear strains perhaps induced by an oversupply of accretable materi al when the suture zone was clogged by underthrusting of the Australia n continental margin. Copyright 1998 by the American Geophysical Union .