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The Cenozoic geologic history of west-central Sumatra is governed by the northward movement of the Indian plate with respect to Southeast Asia. The morphology and structure of the western Sumatran margin reflects the cumulative effects of the resulting subduction and right-lateral slip, especially of that since the late Oligocene. The characteristics of this margin are similar to those of other arc systems.

Nias, one of the islands of the trench slope break, consists of mid-Tertiary melange and younger, less deformed slope-basin strata. The trench slope break has been migrating westward since at least the mid-Miocene as shown by the Quaternary pattern of uplift and subsidence and by combined geological and geophysical data along the western flank of the forearc, or upper slope, basin. This forearc basin contains at least 4 km of Neogene sediments. On the western flank of the forearc basin, strata lie on a melange basement and are sharply flexed onto the trench slope break, but on the eastern flank the basin strata appear to lap onto an older continental slope and shelf. Seismic reflection profiles and drilling on the continental shelf delineate a marked unconformity that increases in depth from near zero at the coast to more than 2 km at the old shelf break. Subsidence and transgression of the old shelf by younger shelf sediments began in the early Miocene and may be continuing at present. Beneath the unconformity on the outer shelf there are Paleogene strata that may define an older forearc basin. The landward flank of this suspected basin, beneath the inner shelf, is probably underlain by Mesozoic and Paleozoic metamorphic and igneous rocks that are covered by littoral Paleogene strata. Scattered across the inner shelf and extending into the coastal mountains are numerous, probably Oligocene, andesitic intrusives and their associated extrusive debris. These igneous centers are anomalous in that they are closer to the trench than are the volcanic chains of younger and older ages. Both the uplift which cut the shelf unconformity and the andesites may have been related to the northward migration of a ridge-trench-trench triple junction along the Sunda arc.

The Paleogene littoral strata along the west coast show increasing intensity of folding toward the Barisan Range and at the mountain front are sharply flexed and sheared. The rate of deposition in the offshore basins and the lack of deformation of the shelf unconformity indicate that the major uplift of the Barisan block occurred during the late Miocene and Pliocene, although it may still be proceeding. There is no evidence of a mid-Miocene orogeny in central Sumatra, but instead, of continuous subduction since at least the late Oligocene which has been accompanied by westward migration of morphotectonic units.

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