Crustal Anisotropy and Mantle Stratigraphy in the Tibetan Plateau and Central Andes

University Of Arizona, Tucson AZ

Investigators

Abstract

The largest mountains and highest plateaus on Earth are built when continents collide. Seismic converted phases (receiver functions) are used to investigate two key questions in continental collisions: does the lower crust flow beneath high plateaus, and does continental mantle subduct beneath mountain belts? If crustal flow occurs in a relatively thin channel, as some modeling predicts, it would develop a strong structural or mineral fabric that may be identified by seismic anisotropy. Unlike shear-wave splitting studies that provide only bulk anisotropy information between the source and receiver, azimuthal variations of converted phases can constrain anisotropy parameters for layers within the crust. Previous studies have generally found crustal anisotropies <5%, but highly anisotropic (>10%) layers within the crust have recently been identified in New Zealand and the Central Andes using the receiver function method. This new technique is being applied to receiver functions recorded across the highest elevations on Earth, the Tibetan Plateau, in order to search for evidence of large-scale crustal flow. The upper mantle beneath the stable interiors of continents is layered on a large scale. Some layer boundaries are observed regionally and denoted by seismologists as "H", "X", and "L". The origin of the stratification of the continental upper mantle is still poorly understood. In some locations the layering has significant dip. Receiver functions have been used to image modern subductions zones as well as identify dipping layers in the upper mantle beneath ancient cratons. Some of these layers beneath cratons appear to have anisotropic properties and may identify relict continental subduction zones. Even if the origin of the stratification is unknown, it can be used as a "tracer" to follow the subduction or destruction of an impinging strong plate beneath a continental collision zone, for example, to track the Indian plate beneath the Himalayas and Tibet. The primary goal of this project is to map out crustal anisotropy and upper mantle layering across the Tibetan Plateau, and use the results to improve our understanding of how high plateaus are built. This research will continue the development of the receiver function technique in the study of seismic anisotropy and mantle stratigraphy, and contribute to the technical education of two young scientists.