Abstract

Dr G. K. Westbrook asked Dr Karig where he would place the edge of the crystalline crust beneath the fore-arc basin/trench-slope break region, whether it might not be a significant factor in controlling the position of the trench-slope break, and what he thought was the likely origin of crust beneath the fore-arc basin.

In reply, Dr Karig writes: As mentioned in the paper, some form of crystalline basement, with a seismic velocity structure suggestive of continental crust was found to underlie the palaeo-shelf edge. We also felt it quite likely that oceanic crust was trapped beneath the larger re-entrants in that palaeomargin, so that the boundary or transition probably occurs beneath the slope between these two crustal types. Our reconstruction of the Sumatran fore-arc region and recent evidence from other young contemporary arc systems would suggest that the initial trench slope break position is closely related to the initial trench location, which in turn appears to follow the continental–oceanic crustal interface where it is not too irregular. However, we have also presented evidence that the trench slope break off Sumatra has migrated trenchward during growth of the accretionary prism. As a result of that migration, the seaward flank of the Sumatran fore-arc basin should be underlain by accreted material.

Dr D. J. C. Laming enquired whether Nias Island would have been emergent had there been no deep-sea fan sediments on the adjacent Indian Ocean Plate.

Despite most of the exposed sedimentary rocks on the island being derived from the

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