KEYNOTE: CLIMATE AND TECTONICS IN THE CENTRAL NEPALESE HIMALAYA: IS THERE A CONNECTION?

New structural, thermochronologic, geomorphic, cosmogenic, and climatic data from central and western Nepal have been used to argue both for and against a dynamic coupling between climate and tectonics in the Himalayan orogen. Theoretical and numerical simulation studies strongly suggest that such feedbacks between erosion and rock deformation are inevitable. However, the strength and sensitivity of such feedbacks and the time and space scales over which they should operate – and therefore at which we should collect data to test the hypothetical coupling – are less clear. Field evidence remains equivocal. In an attempt to reconcile the disparate views on this important problem, we examine along strike differences between sites in central Nepal where evidence for both strong coupling and decoupling between climate and tectonics has been published. Detrital 40Ar/39Ar thermochronologic data from three trans-Himalayan drainages, and cosmogenic isotopic data from one drainage, delineate a sharp discontinuity in cooling history between 10 and 20 km south of the Main Central Thrust (MCT) that coincides with an abrupt physiographic transition morphologically reflective of a change in rock uplift rate. These data rule out transport of Lesser Himalayan rocks over a crustal ramp, and suggest the existence of Pliocene or younger out-of sequence thrusting at the foot of the High Himalaya. The co-location of this transition with peak precipitation rates hints at a tectonic response to climate-driven erosion patterns, consistent with geodynamic models. Notable along-strike variations in landscape morphology, thermal histories, the distribution of Quaternary thrust faulting, and their relation to precipitation patterns appear to reflect the proximity of the newly identified out-of-sequence thrust to the trace of the MCT. Where in close proximity, young deformation appears to reactivate the MCT and initiate new faults in a broad zone between the new and old structures, producing a less pronounced physiographic transition. These differences combined with different scales of observation may explain contradictory interpretations of the coupling between climate and tectonics in the Himalayan orogen.