Rapid long-term erosion in the rain shadow of the Shillong Plateau, Eastern Himalaya

Geodynamic models of collisional orogens suggest that precipitation gradients profoundly influence spatial patterns of exhumation and deformation in active collisional mountain ranges. A basic tenet of this hypothesis is that in unglaciated areas, spatial patterns of long-term precipitation, erosion and exhumation should be correlated. A correlation of this type has been observed in the Eastern Himalaya, where uplift of the Shillong Plateau by Pliocene time drastically reduced monsoonal rainfall in the Himalayan range downwind. Existing apatite fission-track data suggest that the resulting precipitation gradient caused a twofold gradient in long-term erosion rates across an area with similar geology, suggesting a strong influence of climate on the region's geomorphic and tectonic evolution. We extend this dataset by presenting 53 new bedrock apatite and zircon fission-track ages from deeper within the rain shadow. We expected latest Miocene to Pliocene apatite ages, similar to previously published ages from neighboring areas in the rain shadow. Instead, apatites as young as 1.3 ± 0.2 Ma and zircons as young as 4.5 ± 1.0 Ma (2σ) demonstrate that spatial gradients in precipitation do not correlate with variations in long-term erosion and crustal strain as predicted by geodynamic models. Thermal–kinematic modeling of these data suggests that local exhumation patterns reflect gradients in rock uplift dictated by fault kinematics in this rapidly deforming area, despite a dramatic precipitation gradient. These findings both highlight the need to better understand how erosive processes scale with precipitation amount and intensity in such settings, and suggest a disconnect between the predictions of orogen-scale geodynamic models and the relationship between erosion and tectonics at the regional scale.

► We report 53 bedrock apatite and zircon fission-track ages from the Eastern Himalaya. ► Ages and inferred long-term erosion rates do not correlate with precipitation patterns. ► Correlation of precipitation and erosion was expected based on previous models. ► Thermal-kinematic model suggests structural control may explain cooling ages. ► Results highlight need to understand how erosion scales with rain amount & intensity.