Collisional melange development: Geologic associations of active melange‐forming processes with exhumed melange facies in the western Banda orogen, Indonesia

Abstract

Analysis of block assemblages, matrix clay mineral composition and microfauna, and offshore seismic reflection profiles reveal that the Bobonaro melange in the Timor region is sourced from mud‐rich Australian continental margin sequences that are remobilized during accretion and form various facies depending on the structural conditions of emplacement. Melange facies include broken formation, matrix‐rich mud injections, and classic mixed block‐in‐clay facies. Each of these are distinguished by varying degrees of remobilization, mixing, and dispersion at different structural positions across the orogenic wedge. The most important structural control is whether melange was generated beneath or in front of upper plate Banda forearc basement (Banda Terrane). At the present collisional deformation front in the Timor trough, seismic reflection profiles show that melange forms mostly by stratal detachment and fluid‐assisted remobilization above a basal decollement propagating laterally along overpressured Jurassic to Cretaceous clay‐rich sequences of the distal Australian continental margin. The broken, clay‐rich material injects upward through faults to form intrusive bodies at the base of slope cover sediment to form mud ridges at the surface. Similar patterns of stratal disruption are exposed onshore in the Pliocene Kolbano fold and thrust wedge of southern Timor, which is structurally contiguous with the Timor trough deformation front. Melange in the Kolbano Mountains is mostly broken formation and matrix‐rich injections of mud from Jurassic and Cretaceous units. Deformation mechanisms include 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 Timor and northern West Timor), melange is dominantly of mixed block‐in‐clay facies with large blocks derived from roof thrust sheets of intermixed Banda Terrane and Maubisse Formation units. At the structural base of these thrust sheets is the Sonnebait Disruption Zone (SDZ), which formed the initial suture between the Banda Terrane and structurally underthrust Australian continental margin sequences during the late Miocene to early Pliocene. The thickest accumulations of mixed block‐in‐clay melange in the Timor region are found at the southern edge (outlet) of the SDZ, near the Central and Viqueque synorogenic basins. The extent of block dispersion and mixing in the SDZ is indicative of intense shear strains perhaps induced by an oversupply of accretable material when the suture zone was clogged by underthrusting of the Australian continental margin.