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[1] High topography in central Asia is perhaps the most fundamental expression of the Cenozoic Indo-Asian collision, yet an understanding of the timing and rates of development of the Tibetan Plateau remains elusive. Here we investigate the Cenozoic thermal histories of rocks along the eastern margin of the plateau adjacent to the Sichuan Basin in an effort to determine when the steep topographic escarpment that characterizes this margin developed. Temperaturetime paths inferred from 40 Ar/ 39 Ar thermochronology of biotite, multiple diffusion domain modeling of alkali feldspar 40 Ar release spectra, and (U-Th)/He thermochronology of zircon and apatite imply that rocks at the present-day topographic front of the plateau underwent slow cooling (<1°C/m.y.) from Jurassic times until the late Miocene or early Pliocene. The regional extent and consistency of thermal histories during this time period suggest the presence of a stable thermal structure and imply that regional denudation rates were low (<0.1 mm/yr for nominal continental geotherms). Beginning in the late Miocene or early Pliocene, these samples experienced a pronounced cooling event (>30°-50°C/m.y.) coincident with exhumation from inferred depths of $8 -10 km, at denudation rates of 1 -2 mm/yr. Samples from the interior of the plateau continued to cool relatively slowly during the same time period ($3°C/m.y.), suggesting limited exhumation (1 -2 km). However, these samples record a slight increase in cooling rate (from <1 to $3°C/m.y.) at some time during the middle Tertiary; the tectonic significance of this change remains uncertain. Regardless, late Cenozoic denudation in this region appears to have been markedly heterogeneous, with the highest rates of exhumation focused at the topographic front of the plateau margin. We infer that the onset of rapid cooling at the plateau margin reflects the erosional response to the development of regionally significant topographic gradients between the plateau and the stable Sichuan Basin and thus marks the onset of deformation related to the development of the Tibetan Plateau in this region. The present margin of the plateau adjacent to and north of the Sichuan Basin is apparently no older than the late Miocene or early Pliocene ($5 -12 Ma).

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Although most data suggest that the India-Eurasia continental collision began ∼45-55 Myr ago, the architecture of the Himalayan-Tibetan orogen is dominated by deformational structures developed in the Neogene period (<23 Myr ago). The stratigraphic record and thermochronometric data indicate that erosion of the Himalaya intensified as this constructional phase began and reached a peak around 15 Myr ago. It remained high until ∼10.5 Myr ago and subsequently slowed gradually to ∼3.5 Myr ago, but then began to increase once again in the Late Pliocene and Pleistocene epochs. Here we present weathering records from the South China Sea, Bay of Bengal and Arabian Sea that permit Asian monsoon climate to be reconstructed back to the earliest Neogene. These indicate a correlation between the rate of Himalayan exhumation-as inferred from published thermochronometric data-and monsoon intensity over the past 23 Myr. We interpret this correlation as indicating dynamic coupling between Neogene climate and both erosion and deformation in the Himalaya.

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The geomorphic character of major river drainages in the Himalayan foothills of central Nepal suggests the existence of a discrete, west-northwest-trending break in rock uplift rates that does not correspond to previously mapped faults. The 40 Ar/ 39 Ar thermochronologic data from detrital muscovites with provenance from both sides of the discontinuity indicate that this geomorphic break also corresponds to a major discontinuity in cooling ages: samples to the south are Proterozoic to Paleozoic, whereas those to the north are Miocene and younger. Combined, these observations virtually require recent (Pliocene-Holocene) motion on a thrust-sense shear zone in the central Nepal Himalaya, ϳ20-30 km south of the Main Central thrust. Field observations are consistent with motion on a broad shear zone subparallel to the fabric of the Lesser Himalayan lithotectonic sequence. The results suggest that motion on thrusts in the toe of the Himalayan wedge may be synchronous with deeper exhumation on more hinterland structures in central Nepal. We speculate that this continued exhumation in the hinterland may be related to intense, sustained erosion driven by focused orographic precipitation at the foot of the High Himalaya.

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