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ABSTRACT: Melange genesis in the Banda Arc subduction-collision transition, Indonesia

Harris, Ron , Brigham Young University, Provo, UT

Analysis of the age, structure and composition of melange uplifted along the active Banda arc-continent collision zone reveals it is derived mostly from lower plate mud-rich sequences that are remobilized by decollement propagation and diapirism. Mappable melange facies include broken formation, matrix-rich mud injections, and classic mixed block-in-clay facies. Each of these is distinguished by varying degrees of remobilization, mixing and dispersion at different structural positions across the orogenic wedge. The major structural control of facies variation is whether melange was generated beneath or in front of upper plate forearc basement (Banda Terrane).

Seismic reflection profiles across the deformation front at the toe of the accretionary wedge show mud diapirs rising from a basal decollement that propagates laterally along Jurassic to Cretaceous clay-rich sequences of NW Australia, which have high fluid over-pressures. Field studies of uplifted parts of this zone reveal clay-rich injections that intrude upward through faults to form diapirs and mud ridges that feed mud volcanoes at the surface. The melange facies is mostly broken formation and matrix-rich mud injections with few exotic blocks.

In the orogenic hinterland of East Timor and northern West Timor, melange is dominantly of mixed block-in-clay facies with large blocks derived mostly from roof thrust sheets. Offshore seismic reflection profiles across the retrowedge and field studies on shore show mud diapirs intrude into and incorporate large pieces of roof thrust sheets into a mixed block-in-clay facies melange. Accumulations of this facies are at least 3000m thick near the south coast of East Timor, which is regarded as the outlet of a subduction channel clogged when Australian continental margin slope then shelf sequences entered the trench.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia