Cracking the Himalaya

driven erosion could govern tectonic deformation has instigated a decade of geodynamic models and geological studies 5 that explore potential climate–tectonic feedbacks. According to this idea, erosion of mass from Earth’s surface may determine where tectonic deformation is most rapid. Consequently, heavy precipitation, rapid erosion and active faulting are predicted to be spatially correlated in an active mountain belt, or orogen. Based on local variations in erosion within the Himalayan range, Wobus et al. (page 1008 of this issue) deduce the presence of a large, previously undocumented fault that ruptures the surface, and which is interpreted as a response to especially intense rainfall. One challenge in testing connections between climate and tectonics is that active faults are difficult to locate in the bedrock core of mountain belts: erosion commonly removes features, such as displaced river terraces, that record readily recognizable offsets. Although different types of bedrock may be juxtaposed across a fault, that in itself reveals little about how rapidly the fault slipped, or whether it last ruptured 100 million years ago or a decade ago. As a consequence, few active faults have been identified within the core of mountain ranges, even when it is known that rapid tectonic contraction is occurring across the range. Wobus et al. use an innovative combination of techniques to deduce that there is a major surface-breaking fault within the interior of the Himalayan range. Rather than examine observable offsets of the surface, they use two contrasting measures of erosion rates to demonstrate an abrupt change in rates across a narrow zone. Their approach involves measuring the concentrations of cosmogenic radionuclides and the ratio between argon isotopes (Ar/Ar) in sediments, which are interpreted to record variations in erosion rates on thousand-year and million-year timescales, respectively. An argon ‘cooling age’ measures the time since cooling below around 350 C for muscovite (the mineral dated by Wobus and colleagues), thereby defining an average cooling rate. When divided by a geothermal gradient (temperature change with depth), a cooling rate yields a mean erosion rate. In mountain ranges where contraction news and views