Overlooked high-elevation gentle slopes/rivers in the rapidly uplifting Taiwan orogen

Many scholars believe that the core part of the Taiwan orogen, with rapid tectonic uplift for millions of years but only 3000 - 4000 m high, has obtained a topographic steady state, in which the tectonic uplift is balanced by erosion. Steady-state topography is expected to be dominated by steep slopes reaching the angle of repose. With this new notion, the existence of gentle slopes/rivers in the core part of the orogen, which has been noted by Japanese scholars a century ago, was recently overlooked, and those gentle landforms were regarded as “transient” features. We thoroughly examined the topography of the orogen and confirmed the observations by the Japanese scholars: (1) the gentle slopes/rivers, surrounded by steep slopes/knickzones downward, are mainly exhibited on or near drainage divides; (2) the gentle rivers show a great range of width and sinuosity; those flowing in wide valleys are commonly associated with ponds or wetlands; (3) the slopes drained by the gentle rivers can be gentle or steep; the gentle slopes are typically underlain by thick, heavily weathered colluviums; (4) many major drainage divides follow rounded ridges with gentle slopes on both sides; (5) the gentle slopes/rivers are widely exhibited throughout the orogen, from north to south, from west to east, and from low to high elevations.

The juxtaposition between the observed gentle slopes/rivers and the steep slopes/knickzones around them has suggested the change of river behavior from incision-limited to incision-dominated, associated with an increase in landslide activity. The spatial configuration of these landforms further shows re-organization of drainage systems, including river capture and its resulting contraction/expansion of catchments, adjusting to the inferred geomorphic change. Through these analyses, we have confidence that the observed gentle slopes/rivers are better treated as relict landforms preserved on/near ridge tops, not “transient” features developed after the loss of catchment areas as suggested by some scholars.            

Our inferred geomorphic change fits well with the known thermochronological data and the data from the sediment-hydrogen-isotope-based paleo-topographic studies, which jointly show an acceleration of both crustal denudation and surface uplift starting later than 2 Ma. We propose that the orogen had long been dominated by hills when both tectonic uplift and denudation were slow (< 2 mm/yr). The uplift then accelerated (up to 6 - 8 mm/yr), triggering river incision/landsliding which progressively eroded the gentle slopes/rivers created earlier. Since then, the combined river/hillslope erosion has not been able to balance the tectonic uplift, which allowed the preexisting gentle landforms to be raised to high elevations (as long as they are preserved). Once these gentle landforms were raised to elevations > 3000 m, they facilitated snow accumulation and thus, glaciation, during the last glacial period (i.e. the relict cirques and U-shape valleys currently preserved in the orogen were strongly inherited from the preexisting gentle landforms).