Boulder, Colo., USA - Two articles recently published online for the journal LITHOSPHERE investigate the influence of climate, erosion, and tectonics on the lay of the land in the Bolivian Andes. Nicole Gasparini of Tulane University and Kelin Whipple of Arizona State University tackle rainfall patterns, rock uplift, and the distribution of crustal deformation caused by tectonics. In both studies, they conclude that tectonics win out over rainfall when it comes to shaping Earth' surface in the area.
Other new articles cover (1) isotopic dating of volcanic rocks in the Eastern Sierras Pampeanas of northwest Argentina; (2) ties between two of the largest terranes in the North American Cordillera, the Alexander terrane and Wrangellia, along western British Columbia, southwest Yukon, and eastern Alaska; (3) investigation of preserved dune shapes from the ancient Gulf of Mexico, where large dunes overlie a deformable salt layer; and (4) estimate the oldest age for burial of the lower Indian continental crust beneath Tibet by dating the high-pressure component of garnet in the rocks to about 38 million years ago.
Abstracts are online at http://lithosphere.
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Diagnosing climatic and tectonic controls on topography: Eastern flank of the northern Bolivian Andes
Nicole M. Gasparini, Dept. of Earth and Environmental Sciences, Tulane University, New Orleans, Louisiana 70118, USA; and Kelin X. Whipple, School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, USA. Published online 12 May 2014; http://dx.
Geologists have long debated whether high rainfall rates can drive high erosion rates that in turn drive high rock uplift rates. This process could potentially contribute to building and sustaining high relief in mountains, and it has been previously hypothesized that high rainfall rates are a primary control responsible for the high relief and morphology of rivers on the Beni escarpment in the Northern Bolivian Andes, a steep topographic step of approximately 3.5 km over a distance of 45 km. This study explores that hypothesis using previously published erosion rate data, along with rainfall rate and topographic data, and a computational landscape evolution model. The results show that high rainfall rates do not drive high erosion rates on the Beni escarpment. Instead, the data suggest that rock uplift rates increase into the escarpment and this pattern in rock uplift rate controls the pattern in erosion rates, which is independent of the rainfall pattern. The rainfall pattern does appear to affect the shape of river profiles, but it is a secondary control, and the rock uplift pattern is the primary control. Rainfall and rock uplift patterns cannot explain all of the changes in topography on the Beni escarpment, and rock type may also influence the shape of some river channels, however it does not appear to be influential in all cases. The study also presents guidelines for deciphering the role of climate and tectonics on the morphology of landscapes in other settings.
Tectonic control of topography, rainfall patterns, and erosion during rapid post-12 Ma uplift of the Bolivian Andes
Kelin X. Whipple, School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, USA; and Nicole M. Gasparini, Dept. of Earth and Environmental Sciences, Tulane University, New Orleans, Louisiana 70118, USA. Published online 27 May 2014; http://dx.
Much controversy surrounds the uplift history of the Bolivian Andes and the interplay among climate, tectonics, topography, and proxy records of surface uplift. We present new geomorphic and geologic evidence that strongly corroborates 2 to 3 km of surface uplift of the Eastern Cordillera in Bolivia since about 12-million years ago. Despite the popular notion that the climatic contrast between northern (wetter) and southern (drier) Bolivia has contributed importantly to the marked differences in topography and exhumation histories, we show that patterns of topography and exhumation are primarily dictated by tectonics -- by the distribution of deformation. Moreover, there is no objective evidence for a tectonic response to rainfall patterns. Rather, rainfall patterns appear to be simply a response to tectonically produced topography. Concentrated rainfall in areas of high relief may further enhance local erosion rates, but such an influence is neither required by available erosion rate data nor required to explain along-strike contrasts.
Geochronology of igneous rocks in the Sierra Norte de Córdoba (Argentina): Implications for the Pampean evolution at the western Gondwana margin
W. von Gosen et al., Geozentrum Nordbayern, Krustendynamik, Friedrich-Alexander-Universität Erlangen-Nürnberg, Schlossgarten 5, D-91054 Erlangen, Germany. Published online 12 May 2014; http://dx.
The easternmost segment of the Eastern Sierras Pampeanas unit in northwest Argentina is bordered in the east by the western pre-Andean Gondwana margin. Isotopic dating of different igneous rocks from the Sierra Norte de Córdoba combined with results of structural analyses permit to interpret the different stages of the Pampean evolution of this unit through time. Deposition of a thick pile of clastic sediments at the western Gondwana margin in the Neoproterozoic to Early Cambrian time interval is related to a passive margin, which presumably was underlain by west Gondwanan crust. East- to northeast-directed subduction beneath the west Gondwana margin was combined with the formation of a magmatic arc and compressive deformation in the upper (eastern) plate. A second-stage deformation with dextral mylonitization also in the Early Cambrian was accompanied by synkinematic intrusive activity. It can be related to intraplate compression due to ridge subduction and/or collision, which might have been followed by accretion of a continental terrane in the west. This was directly followed by uplift still in the Early Cambrian whereas subsequent igneous intrusions and extrusions indicate that the final stage of the Pampean evolution was terminated in the Middle Cambrian.
New ties between the Alexander terrane and Wrangellia and implications for North America Cordilleran evolution
Steve Israel et al., Yukon Geological Survey, PO Box 2703(K-14), Whitehorse, Yukon, Y1A 2C6, Canada. Published online 12 May 2014; http://dx.
Early geologic research in the North American Cordillera identified several tectonic terranes that were considered to be fundamentally different from one another based upon lithologic and age characteristics. Many of these terranes were thought to have traveled great distances before separately accreting to the ancient North American margin. Two of the largest terranes, the Alexander terrane and Wrangellia, are found along western British Columbia, southwest Yukon, and eastern Alaska and are considered to be exotic to North America and each other. This paper looks at the relationships between Wrangellia and the Alexander terrane and suggests the two terranes shared a history since the Latest Devonian (about 364 million years ago), and that portions of Wrangellia are built upon a basement composed of the Alexander terrane. These conclusions are provocative in that they blur the definition of tectonic terranes, showing that many observations of early geologists can be attributed to evolving geologic processes rather than disparate geologic histories.
Sand on salt: Controls on dune subsidence and determining salt substrate thickness
Anastasia Piliouras et al., Dept. of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA. Published online 17 April 2014; http://dx.
In the ancient Gulf of Mexico, large dunes overlie a deformable salt layer. This allowed the dunes to partially sink into the salt, creating variable preserved dune shapes. Anastasia Piliouras and colleagues performed physical experiments and created a mathematical model to understand dune and salt deformation and to determine the controls on the final preserved dune geometry. They found that relative salt thickness and inter-dune spacing had strong controls on both the rate and amount that the dunes could sink into the salt. This understanding of how dunes deformed allows for predictions about dune and salt thickness from data obtained from seismic imaging.
Eocene deep crust at Ama Drime, Tibet: Early evolution of the Himalayan orogen
Dawn A. Kellett et al., Geological Survey of Canada, Ottawa, Ontario K1A 0E8, Canada. Published online 17 Apr. 2014; http://dx.
Rocks that were metamorphosed first at high pressure and then at high temperature can be found exposed in the Ama Drime massif, South Tibet, in the central-eastern part of the Himalayan collisional mountain belt. In this study, Ama Drime rocks were dated using two radiometric dating methods: Lu-Hf geochronology of garnet, and U-Pb geochronology of zircon. Garnet from the three samples analyzed yielded ages of approx. 37.5, 36.0, and 33.9 million years. Based on this, researchers Dawn A. Kellett and colleagues estimate that the high pressure component of garnet in the rocks grew about 38 million years ago. This is the oldest age for burial of the lower Indian continental crust beneath Tibet reported from the central-eastern Himalaya. High-temperature metamorphism followed at about 13 to 15 million years ago, as indicated by our U-Pb zircon ages. Unlike ultra-high pressure rocks found in the northwest Himalaya, the Ama Drime rocks were not formed by rapid burial and exhumation of a cold subducted slab at the leading edge of the Indian plate as it began to collide with Asia. The rocks instead were buried and metamorphosed as continental crust thickened during the early stages of the still ongoing continental collision between India and Asia. They then resided in the lower-middle crust for more than 20 million years before being re-heated and ultimately exhumed to the surface. These new data provide solid evidence for Indian crust involved in the Himalayan collision having already reached at least ~60 km thickness by Late Eocene.