Rigid plate accretion in an inter-arc basin: Mariana trough.

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. It is verified that an oblique rigid opening results in good fit of the rifted portions of arc on each side of the Mariana trough. The good fit suggests that there has been rigid plate accretion and that a plate 200km wide and 1500 km long can stay undeformed between active consuming and accreting plate boundaries. Small ocean basins, which often exist behind island arcs, have been called basins. KARIG (1 970, 197 1 b) showed that the Lau-Havre trough between the Tonga-Kermadec ridge (frontal arc) and the Lau-Colville ridge (third arc), and the Mariana trough between the Mariana ridge (frontal arc) and the West Mariana ridge (third arc) are the sites of active extension. Using Karig's terminology, a new inter-arc basin is formed by relative migration of the third arc (an inactive rifted portion of the frontal arc) away from the frontal arc-trench system. Some authors such as VAN DER LINDEN (1969) and MILSOM (1970) have implied that the extension results from rigid plate accretion, as on mid-ocean ridges. However, it is now often assumed, following KARIG (1971a) that the process of rifting and creation of new lithosphere in marginal basins is different from the process of rigid plate accretion at mid-ocean ridge crests (PACKHAM and FALVEY, 1971). The actual process of opening is not clearly defined but is assumed by Karig to result from the rise of a hot diapir from the asthenosphere. It is also assumed that, during the opening of the marginal basin, the frontal arc did not remain rigid but was deformed, probably through block faulting, along a series of faults both parallel and perpendicular to the arc (KARIG, 1970; 1971a and b) Westward from the Mariana trench, there is a poorly developed midslope basement-high on the eastern flank of the frontal arc (Mariana ridge) which carries the raised limestone islands: Guam, Saipan (Fig. 1). The back of the frontal arc is defined by a westward facing scarp with 2-3 km of relief t Contribution no 271 du Departement Scientifique du Centre Oceanologique de Bre