SURFACE PROCESSES IN THE ACCRETIONARY PRISM SOUTH OF TAIWAN: RESPONSE TO TECTONIC FORCING AND OCEANIC ‘CLIMATE’

In the accretionary prism south of Taiwan, tectonic controls on uplift patterns are likely comparable to those operating along the active Taiwan mountain belt, but surface processes that modify the tectonically induced morphology are quite different. Rates at which these processes operate remain largely unconstrained offshore, but the resulting underwater “mountain” belt can be compared directly to the mature orogen onland to the north.

The submarine prism widens to the north, as the frontal fold-and-thrust belt incorporates thick Asian passive-margin strata and overlying collisional deposits, and as the arcward flank of the accretionary prism incorporates forearc-basin strata to the east. Surface processes in the submarine orogen are dominated by deposition but also include local erosion, with turbidity currents in canyon systems and debris flows along steep slopes. Structural control is strong, with numerous examples of the interplay of tectonic uplift and sediment transport systems. Mass wasting features are particularly conspicuous along the steep eastern slope of the submarine accretionary prism. Seismicity is likely as important as it is onshore in inducing mass wasting, and may be even more effective, in light of the lack of vegetative cover to stabilize slopes. Hemipelagic sedimentation mantles large regions with fine sediment, reflecting the tremendous sediment production from the Taiwan orogen, but is apparently inactive in some regions, suggesting entrainment of suspended sediment by the strong Kuroshio current. Although weathering is presumably not an issue, numerous submarine outcrops of strata exhibit apparent truncation by erosional processes. The Kuroshio current may be responsible for submarine erosion of the crest of the uplifted accretionary prism.

Rates of erosion, sediment transport, and deposition have not yet been investigated offshore, and the best current estimates are based on analogous strata that have been exposed onland to the north. Sequences that show very rapid deposition, such as Pleistocene marine strata now exposed on both sides of Taiwan, likely accumulated in areally restricted troughs between tectonic culminations, strongly affecting estimates useful for mass balance calculations.