The ultimate composition of any sandstone is affected by a host of primary and secondary factors, including the lithologies present in source terranes, climate, depositional environment and diagenesis. In the case of a subduction complex, however, unequivocal identification of detrital provenance may be impossible because of the cumulative effects of tectonic and sedimentary transport. Long-distance sedimentary transport (> 1000 km) is common within trenches, and abyssal-plain turbidites can be tectonically transported for long distances as the underlying oceanic basement drifts towards a subduction front. Post-accretionary displacement can occur as a consequence of strike-slip faulting, and the total distance of tectonic dislocation may reach several thousand kilometers. The present-day Aleutian forearc region (North Pacific Ocean) illustrates many of the “problems” which typify subduction zones. Several petrologic suites can be identified, and there are significant variations in detrital modes in both time and space. The Aleutian region serves as a sobering modern analog for accreted rock units such as the Franciscan Complex of California, where intercalations of discrete sandstone suites have been noted. In the absence of paleomagnetic control, interpretations of sediment provenance within ancient subduction complexes probably should be restricted to the generic level.

Supplement to: Underwood, Michael B (1986): Sediment provenance within subduction complexes - an example from the Aleutian forearc. Sedimentary Geology, 51(1-2), 57-73