A thermal model for the distribution in space and time of the Himalayan granites

Abstract

In southern Tibet, crustal thickening due to the India-Asia collision has led to the formation of two granite belts. One is located at the southern edge of the accretionary wedge of Tethyan sedimentary rocks, close to the contact with basement gneisses of the Tibetan slab. The other is found within the wedge itself, close to the Kangmar thrust trace. Available ages suggest that the granites appeared first in the southern belt and then in the Kangmar belt. This sequence seems to violate the chronology of thrusting. Another feature of the Himalayas is that melting started only about 20 Ma after the onset of thickening, which is much less than the thermal time constant of thick crust. We give a thermal model, based on the assumption of conductive heat transfer, which explains these features. The model relies on the geometry of a sedimentary accretionary wedge bounded by low-angle thrust faults and on the existence of a thermal conductivity contrast between old basement and young sedimentary rocks. The wedge of sedimentary rocks acts as an insulating cap and its southern edge heats up along the contact with basement rocks. On a horizontal cross-section, there is a temperature maximum along this southern edge, which explains why melting starts there. The early thermal evolution is sensitive to local conditions and granites first appear in the vicinity of the most radiogenic parts of the basement. The distribution of granites in space and time is seemingly random, reflecting different melting events in different radiogenic environments in the heterogeneous basement. This model predicts a relationship between radioactivity and age which is compatible with available data. The results emphasize that there are large horizontal temperature variations across a thickened region and that granite ages are not related simply to the timing of tectonic phases.