Structural and magmatic features of northern Sumatra are reviewed, based on a continuing programme of reconnaissance mapping. Distinction is drawn between Tertiary sedimentation on the W and that in the centre and E, the dividing line being the main outcrop of the Sumatran Fault System (SFS) which traverses the length of the island. Large transcurrent movements on the SFS are indicated by (a) regional slivers of oceanic crust trapped at the leading junction of the western continental plate of Sumatra as it moved NW against the main mass of the island; (b) palaeomagnetic evidence (N. S. Haile) here interpreted as showing eastern Sumatra as part of the Malaya block and occupying its equatorial position since the Cretaceous in contrast to the NW tip of Sumatra, W of the SFS, for which more southerly palaeolatitudes have been determined; (c) juxtaposition of Li-rich and Li-poor geochemical provinces along the SFS.
The Sumatran magmatic arc commenced at least in the Mesozoic. The offset of the current volcanic arc to the E at Lake Toba is ascribed to a change in the angle of the Benioff Zone, the dividing line suggested being a split in the descending plate which is coincident with prolongation of the Investigator transform fault. The SFS continues N of Sumatra as a transform of the Andaman spreading complex, and terminates in the S in the Sunda Strait area either at the edge of the continental block or against a major plate junction indicated by the commencement of deep focus earthquakes E of Sumatra.