Crustal Anisotropy Across Tibet: Implications for the Existence of Partial Melt and the Vertical Coherence of Deformation

点击翻译按钮获取中文摘要

The proposed work aims to improve understanding of the deformation of Tibet by modeling seismic anisotropy. This approach is not entirely new, but current observations are either highly localized or poorly resolved so that insight remains incomplete. The proposed research will produce new, high-resolution information about radial and azimuthal anisotropy in the Tibetan crust and uppermost mantle based on data from PASSCAL experiments and, through Chinese collaborators, the Chinese Earthquake Array. Three specific types of seismic observations will be made. (1) High resolution Love wave phase velocity measurements obtained from ambient noise and earthquakes across all of Tibet will be obtained to infer Vsh in the crust. These observations will help to determine whether the observed crustal LVZs result entirely or only partially from mineral alignment. (2) Azimuthal anisotropy observed for the crust from ambient noise and for the lower crust and uppermost mantle from earthquake records constrain vertical continuity of strain. These observations will help to adjudicate between channel flow and vertically coherent deformation. (3) Joint inversion of receiver functions with surface wave dispersion will help to calibrate the model parameterization in the crust, providing information about potential velocity jumps at the top and bottom of the crustal LVZ and about a potential eclogitized layer at the base of the crust. This will improve the estimate of the strength of mid-crustal radial anisotropy, which is crucial to determine whether mineral alignment suffices to produce the observed LVZs or if partial melt may be required. All inversions leverage recent advances in surface wave tomography methods (e.g., eikonal and Helmholtz tomography) to improve the fidelity of estimated wave speeds. A key component of this work is the estimation of uncertainties using Monte Carlo and related Bayesian statistics from primary measurements to the final seismological models.Tibet presents a natural laboratory to study continent- continent collision, a process that has shaped and scarred the Earth throughout its history but is at present unique to Tibet. The proposed research aims to illuminate the nature of deformation in the Tibetan crust and is motivated particularly by the following questions. To what extent is deformation of the middle crust localized in a channel (channel flow) and hence different from that above and perhaps below it? Does extensive partial melt facilitate channel flow? In particular, are the low velocity zones (LVZs) observed across the Tibetan middle crust caused by the extensive presence of partial melt, or do they dominantly reflect the alignment of anisotropic minerals caused by previous or on-going deformation? At the opposite extreme, to what extent is deformation of the entire lithosphere vertically coherent? These question will be addressed through the development of new high resolution models of directionally-dependent seismic wave speeds (anisotropy) in the crust and uppermost mantle beneath Tibet. New seismic data resources installed by the Chinese and collaboratively between the US and Chinese will form the basis for the research and new methods of tomography will be to produce much higher resolution and reliable images than were possible previously.