| 英文摘要: | Much remains to be learned about the cratonic lithosphere, including the physical and compositional properties that distinguish its mantle from the underlying asthenosphere, how its motion couples to flow in the asthenosphere, and the processes by which it formed. To gain insight on these topics, we will combine the complementary resolving power of scattered waves (Sp and Ps) and anisotropic surface wave tomography in the interior and across the margin of the Baltic Shield, where dense arrays of permanent and temporary broadband stations exist, and the central U.S. craton and its margins, where the EarthScope Transportable Array and other stations provide excellent sampling. We will image crust and mantle discontinuities with Sp and Ps phases, constrain crust and mantle velocities and attenuation with Love and Rayleigh wave tomography, and carry out joint inversions that integrate surface wave, Sp, Ps and other data to derive 3D azimuthally anisotropic models of the lithosphere and asthenosphere. This work will help to constrain whether the vertical gradient in shear velocity between the lithosphere and asthenosphere (the seismological lithosphere-asthenosphere boundary) fundamentally differs between cratonic and non-cratonic continental lithosphere. We will interpret these results in terms of contrasts in temperature, bulk composition, volatiles, melt content and grain size between the lithosphere and asthenosphere. Within the body of the cratonic lithosphere, negative velocity gradients at mid-lithospheric depths have been observed with Sp and Ps phases, often coinciding with the "8˚ discontinuity" first seen in long-range seismic profiles, and, in North America, with a vertical gradient in the fast direction of azimuthal anisotropy from long-period-waveform tomography. We will obtain new resolution of the internal structure of the cratonic lithosphere in the central U.S. and Baltic shield, and we will use these results to test models for the processes that formed the cratonic mantle or contributed to its subsequent evolution. Finally, shearing in the asthenosphere relative to the lithosphere will cause vertical gradients in the orientation of azimuthal anisotropy. This work will constrain the distribution of azimuthal anisotropy in the mantle beneath the Baltic Shield and central U.S. cratons, allowing us to assess the geometry of shear in the sub-continental asthenosphere and how continental plate motion is coupled to the asthenosphere across the lithosphere-asthenosphere boundary.
Cratons represent ancient, stable regions of the continental lithosphere that have not undergone major tectonic activity for the last ~550 million years or more. Geophysical and geological evidence has shown most cratons are underlain by layers of mantle with anomalously high seismic wavespeeds that represent mantle lithosphere that is cold and chemically distinct from the asthenosphere, and whose thickness is much greater than the surrounding, younger lithosphere. However, much remains to be learned about the physical and chemical properties of the cratonic mantle lithosphere and how it differs from the underlying, weaker asthenospheric mantle. In this research we are improving models for the structure of the cratonic mantle and asthenosphere beneath cratonic regions in northern Europe and the central U.S.; in the latter region we are employing data from the EarthScope Transportable Array as well as other stations. Our approach combines seismic waves that are sensitive to localized gradients in seismic velocity structure with those that reflect volume-averaged velocity structure. The improved constraints on mantle structure will help to test models for how the cratonic mantle lithosphere formed, how it has evolved over time, and how its plate motion relates to flow and deformation in the asthenosphere. |