Spatiotemporal Distribution of the Himalayan Leucogranite: Implications for Mountain-building as a Function of Indian Slab Dynamics

Abstract

The Himalayan orogen, as a natural laboratory for continental collision, has attracted intense research attention for decades. However, the question of how the orogen was built is still debated, and potential answers are few when considering how and why along-strike variations of the mountain-building processes occurred. Various tectonic models have been proposed to explain the kinematics of the mountain-building. These models include two dimensional models, such as wedge extrusion (Burchfiel and Royden, 1985; Grujic et al., 1996; Kohn, 2008), channel flow coupled to focused denudation (Beaumont et al., 2001; Hodges et al., 2001), tectonic wedging (Yin, 2006; Webb et al., 2007), duplexing (He et al., 2015; Larson et al., 2015), and a recently proposed three dimensional model in which lateral migration of Indian slab detachment controlled the mountain building (Webb et al., submitted). Here, these models are tested by examining which model(s) can explain the generation of the leucogranites that occur along the orogen. The two-dimensional models predict that leucogranite ages and distributions should not vary significantly along the length of the orogeny, whereas the three-dimensional slab detachment model predicts that leucogranite generation should vary along-strike in specific ways, most notably by showing increasingly young minimum ages of large leucogranite bodies towards the east-central Himalaya. We compiled the existing geochronological data sets and estimated the volume of Himalayan leucogranites, revealing (1) increasing volumes and younging of leucogranite bodies from the ends of the orogen towards the east-central Himalaya, and (2) that younger leucogranite bodies appear generally larger than older emplaced bodies in any given range sector. These findings are generally consistent with the predictions of the lateral migration of slab detachment model, indicating that this model offers a viable explanation for the spatiotemporal distribution of Himalayan leucogranite. This interpretation prompts a re-evaluation of pre-existing two-dimensional models and confirms that Himalayan mountain building proceeded largely via duplexing, as modulated in three dimensions and time by the dynamics of the subducting Indian plate.