Possible Uses of POLARIS Data: From Subasthenospheric Mantle Flow Beneath Plate Boundary Zones to the Fabric of Archean Lithosphere
Abstract
The POLARIS program provides an opportunity to explore a variety of first order problems, making use of both seismic and geodetic networks that are either installed or in the planning stages. The area that POLARIS will cover ranges from the actively deforming plate boundary zone in the west to the long-stable terrains of the Archean Canadian Shield. I will discuss a few studies that have made use of techniques that could be successfully utilized in the analysis of POLARIS data. In the case of actively deforming regions, we have shown (Silver and Holt, 2002) that the simultaneous inversion of surface deformation observations from GPS and mantle deformation data inferred from seismic anisotropy, provides an estimate of the mantle flow velocity field at the base of the asthenosphere, under certain simplifying assumptions. In that study, the mantle beneath westernmost North America south of the Mendocino triple junction was found to be moving to the east at 5cm/yr in a hotspot reference frame, and likely due to the sinking of the Farallon slab through the mantle. The nature of this flow field north of the triple junction and into Canada, in particular the influence of the subducting Juan de Fuca slab on this flow field, constitutes an important scientific problem that could be attacked with data from POLARIS. A second study, relevant to the Canadian Shield, involves the analysis of the anisotropic properties of the lithosphere beneath cratonic southern Africa. That study of 80 broadband transportable stations from the Southern African Seismic Experiment revealed the existence of anisotropy restricted to the mantle portion of the lithosphere, based on the study of both splitting in core phases and radial anisotropy in surface waves. This anisotropy was generated by Archean tectonic events and the corresponding mantle fabric appeared to have controlled the subsequent magmatic evolution of the region over the next subsequent billion years (Silver et al., 2004). The strength of mantle anisotropy (based on delay-time size) beneath the Superior Province of the Canadian Shield is greater than any stable continental region globally, and is at least double that found for the mantle beneath Southern Africa. Yet the size of the relative contributions of lithosphere and asthenosphere in the mantle beneath the Canadian Shield remain unresolved at this point. The combined study of splitting and surface wave radial anisotropy provides a means of placing more precise limits on depth, as in southern Africa, and thus a stronger constraint on the underlying physical process.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2004
- Bibcode:
- 2004AGUSM.S23B..01S
- Keywords:
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- 7218 Lithosphere and upper mantle