Eocene thickening without extra heat in a collisional orogenic belt: A record from Eocene metamorphism in mafic dike swarms within the Tethyan Himalaya, southern Tibet

Knowledge of the nature of the earliest metamorphism experienced by collisional orogenic belts is essential for reconstruction of tectonic processes that build high mountain chains and their environmental consequences. Understanding the metamorphic nature of Eohimalayan-phase orogeny of the Himalayan orogen, one of the typical examples of orogenic belts worldwide, could provide some important constraints to test different tectonic models (shallow continental subduction vs. slab breakoff) for the early phases of the development of large-scale orogenic belts. As exhumed middle-to lowercrustal rocks in the Kangmar gneiss dome, the garnet amphibolites with a protolith age of 176.4 +/- 3.6 Ma experienced a phase of metamorphism at 47.2 +/- 1.8 Ma with an increase in pressure as well as temperature from 3-5 kbar and 550-600 degrees C to over similar to 11 kbar and 650 degrees C. This suggests that the middle- to lower-crustal rocks experienced heating at least by similar to 50 degrees C while they underwent compression and thickening. Heat-flow estimation further demonstrates that the self-produced heat was high enough to achieve the observed pressure-temperature conditions recorded by the garnet amphibolite. Therefore, an additional heat supply is not required during early Eocene metamorphism. A breakoff of the leading part of the subducting Indian continental slab, if it occurred, should be younger than ca. 47 Ma.

Automated summary

Knowledge of the earliest metamorphism in collisional orogenic belts, such as the Himalayan orogen, is crucial for understanding the tectonic processes and environmental consequences of building high mountain chains. In this study, garnet amphibolites from the Kangmar gneiss dome in the Himalayas were found to have undergone a phase of metamorphism, experiencing increased pressure and temperature. The heating of these rocks suggests compression and thickening, and the self-produced heat was sufficient to achieve the observed pressure-temperature conditions.