Is erosion important to the structural and petrological evolution of mountain belts? The nature of active metamorphic massifs colocated with deep gorges in the syntaxes at each end of the Himalayan range, together with the magnitude of erosional fluxes that occur in these regions, leads us to concur with suggestions that erosion plays an integral role in collisional dynamics. At multiple scales, erosion exerts an influence on a par with such fundamental phenomena as crustal thickening and extensional collapse. Erosion can mediate the development and distribution of both deformation and metamorphic facies, accommodate crustal convergence, and locally instigate high-grade metamorphism and melting.

Fission-track and 40Ar/39Ar cooling ages indicate that the late-Tertiary cooling history of the Himalayan ranges of northern Pakistan is largely a function of uplift and erosion. Interpretation of cooling ages which range from under 0.5 Ma to over 80 Ma suggests that during the late Tertiary, long-term uplift rates at least doubled, from under 0.2 mm/yr to in some cases well over 0.5 mm/yr. Uplift rates show strong and systematic regional variations as well which reflect the greater uplift of eastern and northern regions. The association of very rapid uplift and erosion with the Nanga Parbat-Haramosh Massif can be explained by a locally vigorous collision of India with Eurasia near a promontory of Indian crust. The resultant rapid uplift of the Nanga Parbat-Haramosh Massif reactivated the Main Mantle Thrust melange zone with a reversed sense of motion. Discontinuities in the coolingage distribution along the Main Mantle Thrust in the southern Swat-Hazara region may be the result of the thermal effects of overthrusting. 1Now at Research School of Earth Sciences,

Ongoing plate convergence between India and Eurasia provides a natural laboratory for studying the dynamics of continental collision, a fi rst-order process in the evolution of continents, regional climate, and natural hazards. In southeastern Tibet, the fast directions of seismic anisotropy determined using shear-wave splitting analysis correlate with the surfi cial geology including major sutures and shear zones and with the surface strain derived from the global positioning system velocity fi eld. These observations are consistent with a clockwise rotation of material around the eastern Himalayan syntaxis and suggest coherent distributed lithospheric deformation beneath much of southeastern Tibet. At the southeastern edge of the Tibetan Plateau we observe a sharp transition in mantle anisotropy with a change in fast directions to a consistent E-W direction and a clockwise rotation of the surface velocity, surface strain fi eld, and fault network toward Burma. Around the eastern Himalayan syntaxis, the coincidence between structural crustal features, surface strain, and mantle anisotropy suggests that the deformation in the lithosphere is mechanically coupled across the crust-mantle interface and that the lower crust is suffi ciently strong to transmit stress. At the southeastern margin of the plateau in Yunnan province, a change in orientation between mantle anisotropy and surface strain suggests a change in the relationship between crustal and mantle deformation. Lateral variations in boundary conditions and rheological properties of the lithosphere play an important role in the geodynamic evolution of the Himalayan orogen and Tibetan Plateau and require the development of three-dimensional models that incorporate lateral heterogeneity.

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We investigate the material fluxes in space and time as a result of exhumation and erosion processes at the ongoing Yakutat-North American collision in southeast Alaska. Many thermochronologic studies using a variety of sampling strategies are challenged by the widespread ice cover that limit field observations and accessibility. This paper reviews new and published low-temperature thermochronological data from southeast Alaska to give a comprehensive interpretation of the exhumation patterns through time and how they are influenced by surface processes and climate change. We find that the southeastern margin of Alaska was exhumed and eroded long before the late Miocene-Pliocene Yakutat collision, but since the beginning of the subduction of the Yakutat lithosphere in the Oligocene/early Miocene. Today there is a distinct pattern of exhumation in southeast Alaska with a localized very rapid and deep-seated exhumation at the Yakutat plate corner (St. Elias syntaxis), where strike slip motion changes to convergence. Exhumation is also rapid, but less deep along the dextral Fairweather fault, and in the evolving fold and thrust belt. We present a re-interpretation of the exhumation pattern in the fold and thrust belt and suggest that mass transport by exhumation is parallel to the observed active thrust faults and oblique to the suture zone and orogenic strike. The locus of most rapid exhumation migrated from northwest to southeast with Recent exhumation occurring near the St. Elias syntaxis. Exhumation of the Chugach terrane rocks is still active, however to a lesser degree than on the south side of the orogen where precipitation rates are much higher. The Wrangellia terrane to the north has experienced little exhumation and has essentially formed the backstop for terrane accretion in southeast Alaska since the Early Cretaceous. Apatite U-Th/He ages give the first evidence that rocks of the Wrangell Range have only been recently uplifted and eroded as a consequence of the continuing Yakutat collision. In general the thermochronology in southeast Alaska reveals that climate variations across the region as well as changes through time have a limited influence on the pattern of erosion and that the location of deep exhumation is primarily influenced by tectonic processes.

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The timing and role of exhumation in the St Elias orogen, the world's highest coastal mountain range, has been unclear. Sampling is limited to high mountain ridges that tower over widespread ice fields that sit in deeply eroded parts of the orogen. Existing bedrock studies 1-3 in the region are therefore prone to bias. Here we analyse detrital material of active sediment systems in the St Elias orogen to obtain age information from the inaccessible ice-covered valley bottoms. We present 1,674 detrital zircon fission-track ages from modern rivers that drain the glaciers. We find a population of very young ages of less than 3 Myr from the Seward-Malaspina glacier systems that is sharply localized in the area of the orogen's highest relief, highest seismicity and at the transition from transform to subduction tectonics. Our data provide evidence for intense localized exhumation that is driven by coupling between erosion and active tectonic rock uplift.The St Elias mountain belt originates from the collision of the Yakutat terrane with North America, at the corner formed by the dextral Fairweather transform and the Aleutian subduction zone (Fig. 1). Initiation of the Fairweather fault and northward transport of the Yakutat terrane started ∼30 Myr ago, but collision began at 10-5 Myr as the thickened crust of the Yakutat terrane accreted to the Aleutian trench 4,5 , stripping sedimentary cover from basement to construct a foreland fold and thrust belt 4,6 (Fig. 2). Along the orogenic belt, the youngest (5-0.5 Myr) low-temperature cooling ages of bedrock (60-110• C closure temperatures (T c ) for apatite (U-Th)/He (ref. 7) and fission track 8 ) are strongly correlated with those areas with the highest precipitation along the southern, seaward flanks of the orogen. Bedrock cooling ages are oldest in the drier northern side 1,2 (Fig. 2). Bedrock zircon fission-track (ZFT) ages (T c ∼ 250• C; ref. 9) give >10 Myr ages 3,10 ( Fig. 2), or are non-reset in the fold-thrust belt, because lateral transport of material into the orogenic wedge results in exhumation restricted to the upper 5 km (ref. 10). Similar to other active orogenic belts with high erosion rates, the St Elias range seems to have developed localized feedback between erosion and crustal strain [11][12][13] . Thus, it is puzzling that no evidence has emerged for locally enhanced exhumation and erosion in the form of localized young, higher-temperature cooling ages. However, the St Elias orogen is unique among active orogens in that more than 50% of its area is covered by glaciers hundreds of metres in thickness (Fig. 2). Besides reducing bedrock exposures in general, this glaciation also prohibits direct sampling of the low-elevation intensely glaciated valley bottoms where the most recently exhumed rocks and youngest cooling ages would be expected.To overcome this sampling obstacle, we analysed detrital zircons from rivers draining the main glacial systems to evaluate the cooling

Abstract. Within the syntaxial bends of the India-Asia collision the Himalaya terminate abruptly in a pair of metamorphic massifs. Nanga Parbat in the west and Namche Barwa in the east are actively deforming antiformal domes which expose Quaternary metamorphic rocks and granites. The massifs are transected by major Himalayan rivers (Indus and Tsangpo) and are loci of deep and rapid exhumation. On the basis of velocity and attenuation tomography and microseismic, magnetotelluric, geochronological, petrological, structural, and geomorphic data we have collected at Nanga Parbat we propose a model in which this intense metamorphic and structural reworking of crustal lithosphere is a consequence of strain focusing caused by significant erosion within deep gorges cut by the Indus and Tsangpo as these rivers turn sharply toward the foreland and exit their host syntaxes. The localization of this phenomenon at the terminations of the Himalayan arc owes its origin to both regional and local feedbacks between erosion and tectonics.