HR: 1340h
AN: T13B-1337    [Abstracts]
TI: Crustal anisotropy in the Archean Minnesota River Valley Subprovince and its significance
AU: * GEBELIN, A
EM: gebel003@umn.edu
AF: Aude GEBELIN, Dept. of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55414, United States
AU: FERRE, E C
EM: eferre@geo.siu.edu
AF: Eric C. FERRE, Dept. of Geology, Southern Illinois University, Carbondale, IL 62901-4324, United States
AU: TEYSSIER, C
EM: teyssier@umn.edu
AF: Aude GEBELIN, Dept. of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55414, United States
AB: The origin and evolution of the American continental lithosphere is a key question addressed by EarthScope. The Superior Province formed as an amalgamation of Archean/ Proterozoic terranes that subsequently acted as a stabilizing nucleus. This province is characterized by a strong seismic anisotropy (SWS = 1.3 s) of unknown origin. As suggested for the Archean Kaapvaal Craton (South Africa), this could be attributed (1) to current asthenospheric flow, or (2) to fossil lithospheric anisotropy, or (3) to the role of lithospheric keels on modern asthenospheric flow. The first hypothesis is not favored because SWS data for the Superior Province do not fit global mantle flow models. The second hypothesis would be compatible with obliquity between lithospheric mantle and crustal seismic anisotropies, possibly due to oblique docking. The third hypothesis would require asthenospheric flow to be controlled by lithospheric block geometry. The origin of seismic anisotropy and its spatial variations need to be determined to test these hypotheses. The deployment of USArray in the Superior Province in FY10, along with the prospect of deployment of a Flexible Array and the GeoFrame Superior focus area should provide a wealth of seismic data. Yet, the contribution of the Archean-early Proterozoic continental crust to seismic anisotropy is unknown. This study focusses on the Minnesota River Valley (MRV) Subprovince, part of the Superior Province. The MRV Subprovince consists of four juxtaposed blocks (Benson, Montevideo, Morton and Jeffers) of amphibolite to granulite grade migmatites, tonalites, granodiorites, diorites and pelitic rocks interlayered into each other. These blocks are separated by EW-dipping shear zones broadly parallel to SWS observations. In other parts of the world, the crustal seismic anisotropy is generally considered to be modest (SWS = 0.1-0.2 s), although experiments specifically designed to constrain it are scarce. The MRV represents a 200 km-wide, tilted cross-section of Archean-Proterozoic continental crust and is thought to be somewhat representative of the felsic crust at depth. We propose a multiscale methodology to quantify the magnitude/orientation of seismic anisotropy. This approach first establishes the directional relationship between crustal seismic anisotropy and anisotropy of magnetic susceptibility (AMS) measured on oriented specimens. The elastic properties of crustal rocks are calculated by forward modelling using EBSD-LPO measurements and published elastic constants for single crystals. The significance of layering seismic anisotropy can then be determined by in-situ experiments performed on meter-scale parallelepipedic blocks industrially extracted from quarries for dimension stones. The contribution of the continental crust [Sc] to seismic anisotropy can be subtracted from total seismic anisotropy [STOT] to yield the upper mantle only contribution [SUM]. The origin of seismic anisotropy in the continental crust is investigated by imaging the shape preferred orientation (SPO) of leucosomes in migmatites in 3-D. Preliminary data suggest that the leucosomes SPO is coaxial with the AMS fabric which supports the fact that the AMS tracks high-temperature ductile crustal flow. Crustal flow and crustal seismic anisotropy can then be interpreted, with reference to upper mantle anisotropy, either as a coupled or uncoupled crust-mantle system.
DE: 8103 Continental cratons
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8138 Lithospheric flexure
DE: 8160 Rheology: general (1236, 8032)
DE: 8194 Instruments and techniques
SC: Tectonophysics [T]
MN: 2007 Fall Meeting