Allochthonous Terranes of the Southwest Pacific and Indonesia [and Discussion]

Rift-drift processes associated with the Mesozoic break-up of Gondwana and subsequent collisional events involving rifted crustal blocks from Gondwana in Tethys led to the formation and emplacement of allochthonous terranes in fold and thrust mountain belts. These terranes include small allochthonous continental blocks (ca. 1000 km2) and allochthonous exotic blocks that may exceed 200 km2. Other processes consequent upon the break-up of Gondwana and subsequent plate convergence between Gondwana and Asia include the emplacement of suprasubduction zone ophiolites as allochthonous terranes superimposed on the continental margin. The accretion of oceanic plateaus to continental and arc terranes in the Western Pacific have resulted from trenches and strike-slip faults being deflected by crustal heterogeneities. Indonesian and Western Pacific regions display allochthonous terranes forming and being emplaced at rates similar to plate movements. Many allochthonous terranes of the SW Pacific and eastern Indonesia have been accreted during the past 3 Ma and, being so young, have not suffered overprinting by later tectonic and thermal events. Furthermore, we can trace some of the very young nappes into their roots and trace some accretionary processes into zones where, at present plate convergence rates, the collision and hence accretion will not occur for about another 1 Ma. These active regions reveal the importance of local events affecting less than about 1000 km of plate boundary over a period of 1-2 Ma.