The Banda

Figure 7.26 (a) Track of the easternmost cape of Puerto Rico in the period 64 to 76 Ma. (b) Detail of track shown for the period 72-73.6 Ma.
Figure 7.27 General sketch-map of the East Indies, indicating the setting of the Banda Arc.

The view one takes of the Banda Arc depends upon the scale of the map one studies. When viewed on the grand scale (Figure 7.27), the Banda Arc looks as though it may be a natural continuation of the main island chain that starts with Sumatra. Alternatively, the curvature of the Banda Arc could possibly be attributed to the influence of the E-W compression of the Pacific plate. However, when viewed at a more detailed level (which is permitted by the map of the SW quadrant of the Plate-Tectonic Map of the Circum-Pacific Region, Doutch, 1981, a modified detail of which is given in Figure 7.28), it can be inferred that the amount of slip along the Molucca-Sorong fault has been insufficient to account for the degree of curvature. Moreover, one would expect that, were the curvature to be attributed to the influence of this major strike-slip fault complex, then there would be smaller, parallel or sub-parallel strike-slip faults cutting and displacing the Banda Arc; thereby providing a stepped, apparent curvature of the arc. As will be seen from Figure 7.28, such faults are conspicuous by their absence.

It is suggested here that the Java Trench extends only as far east as Sumba, and that the triangular Savu Basin separates Timor and Sumba from the islands of Flores to Wetar. It may also be noted that the volcanic islands of the Banda Arc are minuscule compared with the islands of the Sumatra-Timor chain.

Figure 7.28 Modified detail of the SW quadrant of the Circum-Pacific Plate-Tectonic Map, showing structural elements about the Banda Arc (courtesy of Institute of American Association Petroleum Geology).

Consequently, one is left with the impression that the Banda Arc is a more recent, coincidental, appendage to the main island chain, and that its origin is not linked with the development of the main island chain.

It will be seen that the Banda Arc is different from both the Scotia and the Caribbean structures, in that it exhibits no trench, at least, where one would expect one to be. An obvious circular arc of the Banda structure is the dashed line in Figure 7.28, which marks the subsurface limit of continental crust. It will be noted that the circular arc AA' approximates very closely to the inferred subsurface limit. The contours of the subducted slab are emphasised by the figures which indicate the depth of the slab in hundreds of kilometres. It can be inferred that the line AA' approximates to the submarine outcrop of the upper surface of the subducted slab. At depth, the contours of the slab become progressively less circular, thereby defining an E-W line of symmetry at about 5° 45'S. The arc of volcanoes occurs above the 150 km contour of the slab, which, in the south and west, appears to elide with the Weter-Flores chain of islands.

In normal plate-tectonic situations, where the lithosphere is under sufficient compression to give rise to subduction, one would expect the break to occur at the weakest point or line, which may occur at or near the junction between continental and oceanic lithosphere. This gives rise to the situation where the oceanic lithosphere becomes the down-going slab and disappears beneath the continent. This is what happened when the Indian Ocean plate encountered the Asian plate along the Sumatra-Flores island chain. The section shown in Figure 7.29 represents the reverse situation. It may be inferred, therefore, that a break occurred within the oceanic lithosphere. The eastern side subducted and carried continental cover westward, until these continental rocks encountered the trough, leading one to infer that subduction of the continental rocks proved too difficult, so that an underthrust developed and an embryo trough (the Seram-Uru Trough) formed behind the wedge of continental rocks.

Subsurface limit Weber Continental crust Volcanic Basin (below sea level)

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