Drainage Development in a Submarine Setting, Offshore Eastern Taiwan

The connectivity between the submarine and subaerial environments is intrinsic to landscape development in an emerging mountain belt. We present initial results from a study of onshore and offshore topography in Taiwan. Taiwan forms a linear mountain belt propagating from the north-east to south-west. The offshore bathymetry to the south-east is progressively uplifted and exposed subaerially, providing a template on which onshore topography develops. In turn the drainage network onshore transports material to the ocean controlling the distribution of sediment offshore and the position of river-fed submarine canyons. We have implemented a number of common geomorphic tools to compare onshore and offshore channel networks in eastern Taiwan. Onshore channels show typical concave profiles with many tributaries merging smoothly into the main channel. Local channel slope decreases systematically with increasing drainage area and there is a clear distinction between hillslope and channel processes at ∼1 km² upstream area. The slope frequency statistics peak at ∼30° , probably related to the threshold for slope failure. The main offshore channels are steeper implying the submarine slope has a steeper taper than the subaerial part of the mountain belt. The longitudinal profiles have large knickpoints suggesting less efficient cutting or more active faults offshore. The tributary catchments have constant slope with increasing upslope area and abruptly join the main channel. This constant slope, ∼20° , is considerably lower than onshore, possibly due to increased pore pressures, and suggests landslides dominate at smaller drainage areas. Despite similar slope area statistics at drainage areas greater than ∼1 km² our observations imply fundamental differences between the erosional processes in the submarine and subaerial domains.