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ScienceWeek
EARTH SCIENCE: CLIMATE AND TECTONIC DEFORMATION
The following points are made by Douglas W. Burbank (Nature 2005 434:963):
1) The provocative idea that climatically driven erosion could govern tectonic deformation has instigated a decade of geodynamic models[1-3] and geological studies[4,5] that explore potential climate-tectonic feedbacks. According to this idea, erosion of mass from Earth's surface may determine where tectonic deformation is most rapid. Consequently, heavy precipitation, rapid erosion and active faulting are predicted to be spatially correlated in an active mountain belt, or orogen. Based on local variations in erosion within the Himalayan range, new work[6] has deduced the presence of a large, previously undocumented fault that ruptures the surface, and which is interpreted as a response to especially intense rainfall.
2) One challenge in testing connections between climate and tectonics is that active faults are difficult to locate in the bedrock core of mountain belts: erosion commonly removes features, such as displaced river terraces, that record readily recognizable offsets. Although different types of bedrock may be juxtaposed across a fault, that in itself reveals little about how rapidly the fault slipped, or whether it last ruptured 100 million years ago or a decade ago. As a consequence, few active faults have been identified within the core of mountain ranges, even when it is known that rapid tectonic contraction is occurring across the range.
3) Wobus et al[6] used an innovative combination of techniques to deduce that there is a major surface-breaking fault within the interior of the Himalayan range. Rather than examine observable offsets of the surface, they use two contrasting measures of erosion rates to demonstrate an abrupt change in rates across a narrow zone. Their approach involves measuring the concentrations of cosmogenic radionuclides and the ratio between argon isotopes (40Ar/39Ar) in sediments, which are interpreted to record variations in erosion rates on thousand-year and million-year timescales, respectively. An argon "cooling age" measures the time since cooling below around 350 deg C for muscovite (the mineral dated by Wobus et al), thereby defining an average cooling rate. When divided by a geothermal gradient (temperature change with depth), a cooling rate yields a mean erosion rate.
4) In mountain ranges where contraction results from a long-lived continent-to-continent collision, erosion rates are commonly considered as proxies for rates of rock uplift. Consequently, the differential uplift implied by the spatially abrupt change in erosion rates, as identified by Wobus et al[6], suggests that active faulting has persisted in the core of the Himalaya for millions of years. Moreover, noting that the highest monsoon rainfall occurs near the inferred fault, Wobus et al link fault slip to climatically driven erosion, thereby coupling tectonics with climate.
References (abridged):
1. Beaumont, C., Fullsack, P. & Hamilton, J. in Thrust Tectonics (ed. McClay, K. R.) 1-18 (Chapman & Hall, London, 1992)
2. Beaumont, C., Jamieson, R. A., Nguyen, M. H. & Lee, B. Nature 414, 738-742 (2001)
3. Willett, S. D. J. Geophys. Res. 104, 28957-28982 (1999)
4. Dadson, S. J. et al. Nature 426, 648-651 (2003)
5. Burbank, D. W. et al. Nature 426, 652-655 (2003)
6. Wobus, C., Heimsath, A., Whipple, K. & Hodges, K. Nature 434, 1008-1011 (2005)
Nature http://www.nature.com/nature
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EARTH SCIENCE: ON WATER AND PLATE TECTONICS
The following points are made by Rob Evans (Nature 2004 429:356):
1) In places such as the Pacific Rim, the sea floor collides with and is pushed below the land masses that border the ocean. This process of "subduction", and its consequences in the form of earthquakes and volcanism, are the result of plate tectonics, in which the formation of sea floor at mid-ocean ridges is accommodated by the recycling of tectonic plates in the Earth's interior. But the recycling is a complex process: the interaction of the subducting plate, or slab, with the surrounding material itself produces volcanism and, in some settings, new sea floor.
2) Water plays an important role in subduction recycling: its release from the oceanic plate can affect the rheology of the rock as it moves into the mantle, below the Earth's crust, and also increases rock melting(2). Yet the amount of water released, the depth of release and the pathways it takes are poorly defined, as are the depths of melting and the route that the melt takes as it moves from close to the subducting plate to its eruption in the overlying volcanic arc.
3) Seismic techniques for looking into the Earth are familiar, but there are other approaches, one being the magnetotelluric (MT) method, which offers promise for locating melt and identifying the distribution of water in the mantle. This technique uses naturally occurring electric currents in the ionosphere, created by the capture of charged particles by the planet's magnetic field, to measure Earth's electrical conductivity. That conductivity depends partly on composition and temperature. But it can be dramatically increased by small amounts of partial melt, provided that the melt forms an interconnected network(3). And it can also be affected by water in the mantle, in the form of dissolved hydrogen, both above and below the transition that occurs globally at around 410 km, where the mantle composition undergoes a change from one mineral phase (olivine(4)) to another (wadsleyite(5)). MT has been used only infrequently to address subduction-zone processes, mostly because these systems are close to the ocean and require complex onshore-offshore investigations.
4) Booker et al(1) have presented dramatic models of the subduction system beneath the Andes that suggest that the recycling processes run deeper than previously thought. Their interpretation depends on new data, which in themselves are striking but which also allow the authors to make some inferences about the influence of water at depth. Not only are the results of Booker et al(1) dramatic in their own right, they also represent a significant advance in MT imaging of subduction systems.
References (abridged):
1. Booker, J. R., Favetto, A. & Pomposiello, M. C. Nature 429, 399-403 (2004)
2. Hirth, G. & Kohlstedt, D. L. Earth Planet. Sci. Lett. 144, 93-108 (1996)
3. Roberts, J. J. & Tyburczy, J. A. J. Geophys. Res. 104, 7055-7066 (1999)
4. Karato, S. Nature 347, 272-273 (1990)
5. Xu, Y. et al. Science 280, 1415-1418 (1998)
Nature http://www.nature.com/nature
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ON CONTINENTAL DRIFT AND PLATE TECTONICS
The following points are made by Naomi Oreskes (citation below):
1) Since the 16th century, cartographers have noticed the jigsaw-puzzle fit of the continental edges. Since the 19th century, geologists have known that some fossil plants and animals are extraordinarily similar across the globe, and some sequences of rock formations in distant continents are also strikingly alike. At the turn of the 20th century, Austrian geologist Eduard Suess (1831-1914) proposed the theory of Gondwanaland to account for these similarities: that a giant supercontinent had once covered much or all of the Earth's surface before breaking apart to form continents and ocean basins.
2) A few years later, German meteorologist Alfred Wegener (1880-1930) suggested an alternative explanation: continental drift. The paleontological patterns and jigsaw-puzzle fit could be explained if the continents had migrated across the Earth's surface, sometimes joining together, sometimes breaking apart. Wegener argued that for several hundred million years during the late Paleozoic and Mesozoic eras (200 million to 300 million years ago), the continents were united into a supercontinent that he labeled "Pangea" -- all Earth. Continental drift would also explain paleoclimate change, as continents drifted through different climate zones and ocean circulation was altered by the changing distribution of land and sea, while the interactions of rifting and drifting land masses provided a mechanism for the origins of mountains, volcanoes, and earthquakes. Continental drift was not accepted when first proposed, but in the 1960s it became a cornerstone of the new global theory of plate tectonics. The motion of land masses is now explained as a consequence of moving 'plates' -- large fragments of the Earth's surface layer in which the continents are embedded.
Adapted from: Naomi Oreskes (ed.): Plate Tectonics: An Insider's History of the Modern Theory of the Earth. Westview Press, Cambridge MA 2001, p.3.
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