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[1] Quantifying the degree to which tectonic and erosive processes shape landscapes is key to understanding the evolution of tectonically active mountain belts. Here we explore the interplay of these two processes along the humid, eastern flank of the South American Andes. We use high-resolution Tropical Rainfall Measurement Mission (TRMM) and SRTM data to characterize elevation, relief, and hillslope angle of peak rainfall at orographic barriers. Over a distance of more than 3500 km along the eastern flanks, we find that peak rainfall (>3.5 m/yr) occurs at a mean elevation of 1.3 ± 0.17 km, a mean relief of 0.95 ± 0.08 km, and at moderate mean hillslope angles of 18.3 ± 1.7°. We suggest that topographic relief is the best first-order rainfall predictor and we demonstrate how relief changes along strike south of the Andean orocline alter rainfall distribution. Changes in climatic, sedimentary, and tectonic processes prevent the formation of high relief amounts at the mountain front and these areas are not characterized by pronounced rainfall peak. Citation: Bookhagen, B., and M. R.Strecker (2008), Orographic barriers, high-resolution TRMM rainfall, and relief variations along the eastern Andes, Geophys.

The history of the southern central Andes, including the world's second largest plateau and adjacent intermontane basins and ranges of the Eastern Cordillera and the northern Sierras Pampeanas of Argentina and Bolivia, impressively documents the effects of tectonics and topography on atmospheric circulation patterns, the development of orographic barriers, and their influence on erosion and landscape evolution at various timescales. Protracted aridity in the orogen interior has facilitated the creation and maintenance of the Puna-Altiplano plateau. Contraction and range uplift, filling of basins, and possibly wholesale uplift of the plateau increased gravitational stresses in the orogen interior, which caused the eastward migration of deformation into the foreland and successive aridification. The uplift of the Andean orogen has also had a far-reaching influence on atmospheric and moisture-transport patterns in South America. This is documented by the onset of humid climate conditions on the eastern side of the Andes in late Miocene time, which was coupled with the establishment of dramatic precipitation gradients perpendicular to the orogen, and changes in tectonic processes in the Andean orogenic wedge.

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[1] Erosion in the Himalaya is responsible for one of the greatest mass redistributions on Earth and has fueled models of feedback loops between climate and tectonics. Although the general trends of erosion across the Himalaya are reasonably well known, the relative importance of factors controlling erosion is less well constrained. Here we present 25 10 Be-derived catchment-averaged erosion rates from the Yamuna catchment in the Garhwal Himalaya, northern India. Tributary erosion rates range between~0.1 and 0.5 mm yr À1 in the Lesser Himalaya and~1 and 2 mm yr À1 in the High Himalaya, despite uniform hillslope angles. The erosion-rate data correlate with catchment-averaged values of 5 km radius relief, channel steepness indices, and specific stream power but to varying degrees of nonlinearity. Similar nonlinear relationships and coefficients of determination suggest that topographic steepness is the major control on the spatial variability of erosion and that twofold to threefold differences in annual runoff are of minor importance in this area. Instead, the spatial distribution of erosion in the study area is consistent with a tectonic model in which the rock uplift pattern is largely controlled by the shortening rate and the geometry of the Main Himalayan Thrust fault (MHT). Our data support a shallow dip of the MHT underneath the Lesser Himalaya, followed by a midcrustal ramp underneath the High Himalaya, as indicated by geophysical data. Finally, analysis of sample results from larger main stem rivers indicates significant variability of 10 Be-derived erosion rates, possibly related to nonproportional sediment supply from different tributaries and incomplete mixing in main stem channels.

[1] Low internal relief, aridity, and internal drainage characterize the Puna-Altiplano Plateau and the Tibetan Plateau and the Tarim basin. Structurally, these areas are reverse fault bounded terrains with intervening wedge-top basins that store thick accumulations of sediment for millions of years. Orographic barriers along the margins of these basins are oriented normal to moisture bearing winds, resulting in regional aridity. The combination of this aridity, rapid shortening and uplift rates, and resistant exposed lithologies coincides with the location of these internally drained basins. We hypothesize that these large, internally drained areas persist indefinitely when uplift overwhelms the fluvial systems and defeats the channel network. To test this hypothesis, we developed models of channel defeat to examine the threshold conditions required to fragment the channel network. The results of these models suggest that low-erodibility rocks, moderate to high uplift rates, and low precipitation favor defeat of orogen-traversing channels. Most coupled tectonic-landscape development models prevent the development of internal drainage and so may overestimate the voracity of erosion in regimes that favor internal drainage, profoundly influencing predicted orogenic growth patterns.

[1] This work explores the control of fore-arc structure on segmentation of megathrust earthquake ruptures using coastal geomorphic markers. The Arauco-Nahuelbuta region at the south-central Chile margin constitutes an anomalous fore-arc sector in terms of topography, geology, and exhumation, located within the overlap between the Concepción and Valdivia megathrust segments. This boundary, however, is only based on $500 years of historical records. We integrate deformed marine terraces dated by cosmogenic nuclides, syntectonic sediments, published fission track data, seismic reflection profiles, and microseismicity to analyze this earthquake boundary over 10 2 -10 6 years. Rapid exhumation of Nahuelbuta's dome-like core started at 4 ± 1.2 Ma, coeval with inversion of the adjacent Arauco basin resulting in emergence of the Arauco peninsula. Here, similarities between topography, spatiotemporal trends in fission track ages, Pliocene-Pleistocene growth strata, and folded marine terraces suggest that margin-parallel shortening has dominated since Pliocene time. This shortening likely results from translation of a fore-arc sliver or microplate, decoupled from South America by an intra-arc strike-slip fault. Microplate collision against a buttress leads to localized uplift at Arauco accrued by deep-seated reverse faults, as well as incipient oroclinal bending. The extent of the Valdivia segment, which ruptured last in 1960 with an M w 9.5 event, equals the inferred microplate. We propose that mechanical homogeneity of the fore-arc microplate delimits the Valdivia segment and that a marked discontinuity in the continental basement at Arauco acts as an inhomogeneous barrier controlling nucleation and propagation of 1960-type ruptures. As microplate-related deformation occurs since the Pliocene, we propose that this earthquake boundary and the extent of the Valdivia segment are spatially stable seismotectonic features at million year scale.Citation: Melnick, D., B. Bookhagen, M. R. Strecker, and H. P. Echtler (2009), Segmentation of megathrust rupture zones from fore-arc deformation patterns over hundreds to millions of years, Arauco peninsula, Chile,