Tectonics and climate, and the rate of mountain erosion

It is rare for one issue of a “journal of record”, such as Nature to contain three papers on closely related topics, especially when they are geoscientific, but its 11 December issue of 2003 did.  All were about the way in which mountains erode, and attempted to measure the rates involved in three different settings.  Insofar as it is possible in Earth science, they try a reductionist approach in terms of the climatic and tectonic forces that are involved in denudation.  Getting useful timings is not as easy as it might seem with measuring fission tracks and the amount of radiogenic helium generated by decay of uranium and thorium isotopes in grains of apatite.  The principle lies in estimating when unroofed rocks rose and cooled below the temperatures at which apatite loses noble gases and the tracks in it formed by alpha particle emission heal up. In an exposed section subjected to erosion and isostatic uplift the higher rocks should record older ages than those lower down, the difference representing the pace of erosion and uplift.  There is, as yet, no way that periods less that 500 thousand years can be resolved by either method, and in terms of recent climate that can cover several glacial-interglacial cycles.

The simplest of the case studies was in the Cascade mountains of the NW USA, where there has been minimal tectonic activity, but a great deal of rain over the last few million years.  The crust has risen as material was stripped off the mountains. The average rates of erosion on time scales of millions to tens of million years closely follow the modern variation in precipitation over the area (Reiners, P.W. 2003.  Coupled spatial variations in precipitation and long-term erosion rates across the Washington Cascades.  Nature, v. 426, p. 645-647).  As a result, western parts of the range where rainfall is far higher than in the eastern rain shadow could be expected to be rising as much as three times faster, if a balance between erosion and isostatic uplift has been achieved. Since erosional power is expressed by rainfall and surface gradient, the fact that average erosion rates do not correlate well with topographic relief suggests that precipitation has outweighed the effects of slope steepness.  The opposite seems to hold in the Himalaya of central Nepal, which show the most gross variations in precipitation, due to monsoonal conditions (Burbank, D.W. and 7 others 2003.  Decoupling of erosion and precipitation in the Himalayas.  Nature, v. 426, p. 652-655), yet long-term erosion rates do not vary very much, except between the topographically distinct Lesser and Greater Himalaya ranges.  The Himalaya are altogether more geologically and tectonically complex than the NW USA, so finding such little variation is as interesting as it seems currently inexplicable.  The lack of correlation in the Greater Himalaya between precipitation (a five-fold decrease from south to north across the range) and erosion rates (more or less constant and high) suggests that tectonic uplift is the main driving force.  Much the same findings from the area immediately to the east in the Nepalese Himalaya, though using a mica Ar-Ar thermochronology method that spans a longer period, have been interpreted very differently (Wobus, C.W. et al. 2003.  Has focused denudation sustained active thrusting at the Himalayan topographic front?  Geology, v. 31, p. 861-864).  Wobus and his colleagues from MIT suggest that rapid rise of the Greater Himalaya (~10 km in the last 10 Ma) was induced by isostatic uplift driven by erosion, even maintaining movement on the huge bounding thrusts to the orogenic belt.  Altogether more complicated is the erosion of Taiwan, which is seismically active, has a complex tectonic history that affected rocks of very different strengths in different areas and is subject to a highly variable maritime climate (Dadson, S.J. and 11 others 2003.  Links between erosion, runoff variability and seismicity in the Taiwan orogen.  Nature, v. 426, p. 648-651).  They detect changing patterns of erosion as deformation has migrated.  Attempts at correlation between modern erosion rates and various factors came up with only two of significance, with recent seismicity and typhoons.  Each triggers landslips that instantaneously add debris to flowing rivers.  Precipitation rates, river discharge, slopes and stream power showed little link with erosion rates.  Of the four papers, only one (Wobus et al.) is able to relate differences in the erosive power of streams to the contrasting erosion rates of the Greater and Lesser Himalaya.

Such a hodge-podge of seemingly conflicting findings, based on studies that use supposedly revolutionising techniques, must worry agencies who have been induced to part with large funds to support fission-track and (U-Th/He) dating facilities supposedly to advance geomorphological studies.  Peter Molnar, who with Phillip England first reviewed the complex interplay between erosion, tectonics and uplift, and their counter-intuitive outcomes, made the following pithy comment, “The differences among these papers call attention to the inadequacy of current theory, without which one gropes for a way to plot data”.  Plainly, there has been over-excitement about techniques in the hope of empirically deriving theories, which has resulted in half-cocked research, and some gullibility among funding bodies.

See also:  Molnar, P. 2003.  Nature, nurture and landscape.  Nature, v. 426, p. 612-614.  New Scientist (31 January 2004) includes a 12 page special report on the technological issues involved in the Bush vision.

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