Abstract

Plate-tectonic interpretations of sandstone composition are made in the hope that "detrital framework modes of sandstone suites provide information about the tectonic setting of basins of deposition and associated provenances" (Dickinson et al. 1983). The underlying assumption that provenance tectonics are directly "associated" with tectonics of depositional and post-depositional sedimentary settings is not, however, universally true, as sandstones from two contemporary accretionary prism complexes (Nias Island, Indonesia, and Barbados, West Indies) illustrate. Mineralogically immature sandstones rich in feldspar and volcanic lithic fragments are expected in such tectonic settings, but both Nias Island and Barbadian sandstones are mineralogically very mature, dominated by quartz. In both cases this discrepancy between inferred and actual tectonic setting has been explained by postulating that sands from a provenance petrogenetically unrelated to the arc-trench system (e.g., continental or recycled orogen sands) have been introduced into the accretionary prism by sedimentary or tectonic processes. Sandstones which are transported (sedimentologically or tectonically) across the boundaries of sedimentary-tectonic provinces, and which therefore occur in tectonic settings petrogenetically unrelated to the tectonics of the sediment source area, are important components of at least some active accretionary complexes. The possibility of "trans-boundary" sedimentary and/or tectonic transport must be allowed for in making plate-tectonic interpretations based on sandstone petrography, especially in reconstructions of ancient orogenic belts.

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