Stochastic Modeling of Small-Scale, Anisotropic Structures in the Continental Upper Mantle,
Abstract
New methods for the analysis of three-component seismograms have been applied to data from both continental and oceanic regions. Polarization anisotropy, manifested as the splitting of surface, guided, and shear body waves, has been observed in all regions, but no significant azimuthal anisotropy has been detected. This is expected if the local orientation of olivine crystals in the upper mantle is incoherent on the scale of the path lengths used in these experiments (Delta = 35 deg - 50 deg). From a detailed study of trans-Australia paths, we have shown that the shear-wave anisotropy in the western Australia craton averages 3-4% in the uppermost mantle, but it terminates abruptly at a Lehmann (L) discontinuity near a depth of 250 km. Our physical interpretation is that the L discontinuity beneath the ancient continental cratons marks the transition from an anisotropic mechanical boundary layer to a more isotropic region of a thick continental tectosphere. Above L, the temperatures have evidently remained cold enough to freeze-in the small-scale anisotropic structures and tectonic fabrics generated in episodes of orogenic compression associated with tectospheric stabilization. Below L, such structures were either never generated or were annealed out subsequent to their formation. These hypotheses about subcontinental structure have considerable bearing on a diverse set of geological and geodynamical problems, ranging from formation of the cratons to kimberlite vulcanism.
Document Details
- Document Type
- Technical Report
- Publication Date
- Aug 14, 1995
- Accession Number
- ADP204455
Entities
People
- James B. Gaherty
- Thomas H. Jordan
Organizations
- Massachusetts Institute of Technology