A Late Miocene acceleration of exhumation in the Himalayan crystalline core Cameron Wobus a, ⁎ , Malcolm Pringle b , Kelin Whipple c , Kip Hodges c a CIRES, Campus Box 216, University of Colorado, Boulder, CO 80309, United States b Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, United States c School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, United States Received 10 July 2007; received in revised form 7 January 2008; accepted 11 February 2008 Available online 4 March 2008 Editor: C.P. Jaupart Abstract Unraveling the relative roles of erosion and tectonics in shaping the modern topography of active orogens requires datasets documenting spatial and temporal patterns of exhumation, surface uplift and climatic forcing throughout orogenic growth. Here we report the results of biotite 40 Ar/ 39 Ar incremental heating and single-grain laser-fusion experiments from a nearly vertical, ∼ 1000 m age-elevation transect in the central Nepalese Himalaya. Age-elevation relationships constructed from these data suggest very slow cooling in this part of the Himalayan crystalline core during the Early Miocene, accelerating to only moderate rates at ∼ 10 Ma. If we assume purely vertical exhumation and a steady-state thermal structure, the exhumation rates implied by these data are ≪0.1 mm/yr prior to 10 Ma and ∼ 0.5 mm/yr from ∼ 10–7 Ma. The acceleration in cooling rate at 10 Ma requires a change in kinematics that may be linked to large-scale changes in climate, or to more local tectonic perturbations. Although we do not presently have enough data to assess the relative roles of regional vs. local drivers, these data provide a new constraint on exhumation through the Miocene that must be honored by any model of Himalayan evolution. © 2008 Elsevier B.V. All rights reserved. Keywords: Himalaya; argon thermochronology; tectonics; climate 1. Introduction The central Nepalese Himalaya reflects extremes in both tectonics and erosion: a series of thrust faults at the base of the range accommodates approximately 20 mm/yr of convergence between India and Eurasia, while the South Asian monsoon drops over 3 m of rain along the rangefront in a typical season (e.g., Bookhagen and Burbank, 2006). Because the tectonic and climatic signals are both strong in this region, the Himalaya has become a centerpiece in the debate surrounding the degree to which climate and tectonics may be coupled at the orogen scale (e.g., Burbank et al., 2003; Thiede et al., 2004; Wobus et al., 2005; Huntington et al., 2006). Most of these studies cite thermochronologic datasets in which spatial or temporal changes in cooling ages are used as proxies for spatial or temporal changes in exhumation over Pliocene–Recent time- scales. However, the strongest climatic signal to have affected the Himalaya is likely to have occurred much earlier (N 20 Ma) when recent evidence suggests that South and East Asian monsoon climates were established (Sun and Wang, 2005; Clift, 2006). In addition, a variety of evidence suggests there were substantial shifts in both climate and tectonics in the region in the Late Miocene (∼ 8–10 Ma) (e.g., Kroon et al., 1991; Molnar et al., 1993; Garzione et al., 2000). Pinpointing the timing of such regional “events”– and understanding the degree to which large climatic changes influence local and regional tectonics – requires an orogen-wide database documenting changes in exhumation rates through this period. Toward this end, we report here the results from a new 40 Ar/ 39 Ar age-elevation transect in the Langtang valley of cen- tral Nepal, documenting cooling of the Greater Himalayan Sequence through the ∼ 350 °C isotherm. Our data include Available online at www.sciencedirect.com Earth and Planetary Science Letters 269 (2008) 1 – 10 www.elsevier.com/locate/epsl ⁎ Corresponding author. E-mail address: cameron.wobus@colorado.edu (C. Wobus). 0012-821X/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2008.02.019