Was the active Taiwan orogen one million years ago as high as today?

Abstract

Tectonic uplift drives erosion which limits the height of a mountain. Many scholars regard the main part of the Central Range in the Taiwan orogen, now 3000 – 4000 m in elevation, as a paradigm that erosion dominated by river/hillslope processes for millions of years can offset active tectonics by which the mountain range has obtained the topographic steady state. Our observations, however, challenge this belief: (1) the glacier/periglacial remains (cirques/U-shaped valleys with talus slopes) > 3300 m in elevation are well preserved (even showing dissolution grooves where they are made up of limestones) since the deglaciation (starting ~8 ka ago); (2) there are commonly barely eroded low-relief surfaces, capped by bamboo grass and characterized by low-gradient channels/depressions, stranding on the major drainage divide > 3000 m in elevation; (3) rivers originating from > 3000 m-high mountains commonly show prominent knickpoints (including > 100 m-high waterfalls) where flowing downward to elevations < 1000 m; (4) modern landslides and alluvial terraces (evidence of paleo-landslides) sourcing from the > 3000 m-high mountains are far less than those in the mid-elevation regions which typically originated from the sides of hillslopes. The combination of these data points out the relatively slow erosion in the > 3000 m-high mountain areas, although they are undergoing the rapidest tectonic uplift (based on GPS and leveling surveys). We attribute this apparent hillslope stability to the rarity of large earthquakes in the high mountains (as the major seismogenic faults are all distributed around the orogen). It is noted that incised low-relief surfaces similar to those exhibited in the high mountains are widely distributed in the lower parts of the orogen down to hilly regions. This configuration suggests that the low-relief landforms now preserved in the high mountains were originally created in the low-elevation regions when both river incision and tectonic rates were low; they were then uplifted (while dissected and eroded) to their present elevations, apparently associated with the acceleration of river incision and tectonic uplift (perhaps starting as late as 0.5 Ma). We consider that when the low-relief surfaces were raised to > 3000 m in elevation, they facilitated snow accumulation and thus, glaciation (i.e., all the glacier landforms preserved are inherited from the preexisting low-relief surfaces). In sum, we propose the elevation of the Central Range has been increasing with the acceleration of tectonic uplift since < 1 Ma ago, and it is the glacial erosion that could balance the tectonic uplift in the future.   (Keywords: River incision; landslides: glacial erosion; topographic steady state; the Taiwan orogen)