© 2008 Macmillan Publishers Limited. All rights reserved.

© 2008 Macmillan Publishers Limited. All rights reserved.

NEWS & VIEWS

732 nature geoscience | VOL 1 | NOVEMBER 2008 | www.nature.com/naturegeoscience

Simon H. Brocklehurstis in the School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester M13 9PL, UK.

e-mail: shb@manchester.ac.uk

T raditionally, the movement of tectonic plates is assumed to build mountains, whereas rivers and glaciers

erode them away. Indeed, most of the large mountain ranges lie along current or former boundaries of tectonic plates. Insights from numerical modelling have recently challenged the idea that erosion merely carves picturesque landscapes from the edifi ces built by tectonics1. But fi eld-based evidence of a more dynamic role for erosion in mountain building has proved elusive. On page 793 of this issue, Berger and co-authors2 synthesize insights obtained from low-temperature thermochronometry (radioisotope determination of near-surface exhumation histories) and thermo-kinematic modelling, as well as off shore seismic refl ection and borehole data, to demonstrate that during the middle Pleistocene epoch, the advance of some Alaskan glaciers to the edge of the continental shelf changed the structural evolution of the St Elias orogen (Fig. 1).

Th e massive volumes of sediments pouring from tectonically active mountain ranges attest to the effi ciency of erosion in these settings3. In particular, the volume of material accumulating in the fj ords of Alaska demonstrates that, in certain settings, glaciers can erode even more effi ciently than the large rivers traversing the Himalayas4. But whether rock can be removed suffi ciently rapidly and in suffi cient volumes to infl uence lithospheric dynamics is unclear.

Numerical modelling results suggest that erosion does elicit a response from the lithosphere. For example, the unloading due to erosion either by the large rivers draining the Tibetan Plateau around each end of the Himalayan mountain chain5, or the rivers along the monsoon-drenched Himalayan front6, could cause ductile lower crustal material to be drawn towards

the Earth’s surface, thereby aff ecting the thermal structure and dynamics of this prominent mountain range. Precipitation, driven by prevailing winds, tends to fall on the upwind side of mountainous regions and could control the distribution of deformation within an orogen7. Changes in the rates of river erosion or the initiation of glacial erosion, both driven by climate change, could control the width of glaciated mountain ranges8,9.

Th e glacial cycles of the Northern Hemisphere began in earnest about 3 million years ago, but changed their characteristics in the middle Pleistocene epoch. Marine oxygen isotope records of global ice volume indicate that before about 1 million years ago, glacial–interglacial cycles had a period of about 40,000 years, were relatively symmetrical and had moderate temperature oscillations (Fig. 2). By contrast, the most recent glacial–interglacial cycles have been more extreme and asymmetrical, with a period of about 100,000 years that consists

roughly of an 80,000-year build-up of ice, 10,000 years of glacial maximum and a quick phase of deglaciation. Th is change is likely to have allowed larger glaciers to develop, but its implications for glacial erosion and mountain building have yet to be fully explored.

Berger and colleagues2 document a substantial advance of Alaskan glaciers around 1 million years ago, when smaller glaciers were replaced by large, highly erosive ice streams. And they interpret low-temperature thermochronological data as a coincident change in exhumation rates, just in the zone of the St Elias Range where glacial erosion would have been most effi cient. Th ey suggest that focused glacial erosion forced a tectonic response from the orogen.

Th e model for the evolution of the St Elias orogen during the Quaternary period proposed by Berger and colleagues is consistent with the predictions of the ‘critical Coulomb wedge’ theory8–10, in which the wedge-shaped cross-section of a mountain

The interactions between climate and tectonics in active mountain ranges are complex and important. Field and geophysical data from the St Elias Range of Alaska show that glacial erosion can infl uence the dynamics of the lithosphere in such settings.

GEOMORPHOLOGY

A glacial driver of tectonics

Figure 1 Infl uence of climate change on the St Elias orogen. Glaciers such as the Russell Glacier in the St Elias Range, Alaska, may have a more substantial role in the dynamics of the orogen than merely creating picturesque scenery.

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© 2008 Macmillan Publishers Limited. All rights reserved.

© 2008 Macmillan Publishers Limited. All rights reserved.

NEWS & VIEWS

nature geoscience | VOL 1 | NOVEMBER 2008 | www.nature.com/naturegeoscience 733

range is thought to be loosely analogous to a pile of sand that is pushed by a bulldozer. Th e width and surface gradient of the range are then determined by the frictional behaviour of the material that makes up the range, the rate of addition of material due to convergence, the rate of removal of material due to erosion, and the gradient of the fault that is treated as the base of the range. For example, an increase in erosion rate causes a

Coulomb wedge mountain range to narrow in order to maintain the surface gradient, while maximum elevations are reduced8,9. Despite its simplicity, the Coulomb wedge model has proved remarkably eff ective.

Berger and colleagues fi nd that the intensifi cation of glaciation during the middle Pleistocene epoch and concomitant increase in erosion rates caused a reduction in the width and maximum elevations

of the St Elias wedge. Th eir off shore seismic data reveal that enhanced glacial erosion of the subaerial part of the wedge coincided with a cessation of shortening and deformation of the submarine toe of the St Elias wedge. Th is suggests that the high infl ux of sediments resulting from increased onshore denudation buried the submarine toe of the wedge and reduced the critical taper, forcing the wedge to adjust structurally. In other words, a change in climate led to a change in tectonics.

Th e study by Berger et al.2 not only provides empirical evidence that glacial erosion has an important role in the evolution of active mountain ranges, but also highlights that the interaction between climate change and tectonic response may be more subtle than anticipated: it was not the onset of Northern Hemisphere glaciation that caused a tectonic response, but the intensifi cation of glacial coverage around 1 million years ago. Th is should encourage more detailed study of glacial history and tectonic response in other mountain ranges around the world, so that we can fully grasp the relationship between climate change and mountain building.

It should be borne in mind that the interpretation of thermochronological data is heavily dependent on the thermal model of the orogen. Th is is oft en diffi cult to constrain for the present, let alone when accounting for past changes in the thermal structure of an active orogen. Measurements of erosion rates from either sediment yields or various cosmogenic isotope methods are also diffi cult to extrapolate to the lifetime of an orogen. Detailed, reliable estimates of the topographic history of mountain ranges are highly desirable in this context, but tend to be elusive.

Nevertheless, Berger and colleagues2 have demonstrated that taken together, a diverse set of fi eld data can provide a compelling case for a substantial change in the dynamics of an active mountain range in response to a change in climate and erosion.

References1. Koons, P. O. Annu. Rev. Earth Planet. Sci. 23, 375–408 (1995).2. Berger, A. L. et al. Nature Geosci. 1, 793–799 (2008).3. Molnar, P. & England, P. Nature 346, 29–34 (1990).4. Hallet, B., Hunter, I. & Bogen, J. Global Planet. Change

12, 213–235 (1996).5. Zeitler, P. K. et al. GSA Today 11, 4–9 (2001).6. Beaumont, C., Jamieson, R. A., Nguyen, M. H. & Lee, B.

Nature 414, 738–742 (2001).7. Willett, S. D. J. Geophys. Res. 104, 28957–28982 (1999).8. Whipple, K. X. & Meade, B. J. J. Geophys. Res.

109, F01011 (2004).9. Tomkin, J. H. & Roe, G. H. Earth Planet. Sci. Lett.

262, 385–397 (2007).10. Dahlen, F. A., Suppe, J. & Davis, D. J. Geophys. Res.

89, 10087–10101 (1984).11. Berendsen, H. J. A. De Vorming Van Het Land, Inleiding in

de Geologie en de Geomorfologie, Vijfde Herziene Druk 5th edn (Van Gorcum IUGS, 2008).

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Figure 2 Glacial cycles. Marine oxygen isotope records indicate a change in the period of glacial cycles, from about 40,000-year cycles earlier than about 1 million years ago to the current 100,000-year cycles. This change may have prompted a substantial advance of Alaskan glaciers, and consequently a reorganization of the tectonics of the St Elias Range2. δ18O = [(18O/16O)sample/(18O/16O)standard] – 1, where the standard is Surface Mean Ocean Water (SMOW). Reproduced with permission from ref. 11.