Dynamic Implication and Constraint of seismic anisotropy in Central Tibetan Lithosphere: insights from the Mantle Xenoliths

The tectonic evolution of the Tibetan Plateau is still debated. Two predominant hypotheses have been put forth: one posits a northward subduction of the Indian plate, coupled with a concurrent southward subduction of the Eurasian plate; the other suggests a unidirectional northward subduction of the Indian plate alone.

In this study, we introduce new data derived from peridotite mantle xenoliths, which were exhumed by Eocene volcanoes in the Qiangtang terrane. The systematic lateral and radial variations in the petrological, geochemical, and microstructural characteristics of these xenoliths reveal a heterogeneous structure within the lithospheric mantle beneath central Tibet. The uppermost portion of the lithospheric mantle is refractory and displays an AG-type olivine fabric, characterized by a point maximum of the [010] axes perpendicular to the foliation plane, and a girdle distribution of the [100] and [001] axes within the foliation plane. In contrast, the lower segment has been re-fertilized and exhibits a distinct fabric, marked by the dominant activation of the 001 slip system. We infer that the fabric of the lower part of the lithospheric mantle was transformed from an AG-type to a B-type fabric during melt-related deformation associated with re-fertilization triggered by asthenosphere upwelling. The most plausible scenario driving this re-fertilization in the lower sections of the lithospheric mantle is the convective removal of the lowermost lithosphere. Concurrently, the refractory ‘ghost lithosphere’ residing in shallower regions beneath the Qiangtang terrane has preserved the earlier AG-type fabric, potentially representing a residual subcontinental lithospheric mantle that remains within the current lithospheric mantle. This vertical dichotomy of the mantle generates multiple seismic interferences, which align well with deep seismic observations and substantiate the model of a single northward subduction of the Indian plate.