HR: 0800h
AN: T11D-0420    [Abstracts]
TI: Crustal Anisotropy of the Archean Continental Lithosphere in the Minnesota River Valley Subprovince
AU: * Ferre, E C
EM: eferre@geo.siu.edu
AF: Department of Geology Southern Illinois University, 1259 Lincoln Dr., Carbondale, IL 62901 United States
AU: Gebelin, A
EM: gebelin@dstu.univ-montp2.fr
AF: Dept. of Earth Sciences University of Waterloo, Waterloo, Waterloo, ON N2L 3G1 Canada
AU: Teyssier, C
EM: teyssier@tc.umn.edu
AF: Dept. of Geology and Geophysics University of Minnesota, Pillsbury Hall, Minneapolis, MN 55455 United States
AB: The Minnesota River Valley [MRV] Subprovince belongs to the Superior Province and consists of four Archean crustal blocks separated by E-W north-dipping shear zones. These blocks, with contrasted aeromagnetic signatures, are characterized by a gradational, layered crust-mantle transition at a depth of 45-51 km. The northermost block hosts tonalites, quartz diorites, and granodiorites. This block is separated from the southern blocks by a Penokean age [2.45-1.75 Ga] shear zone. The 3 southern blocks consist of amphibolite- to granulite-grade migmatites with tonalitic, granodioritic, dioritic, and pelitic types, interlayered and grading into each other. Our petrofabric and microstructural investigations focused on the Morton Block, one of the three southern blocks. The Morton migmatites exhibit compositional layering formed by alternating layers (cm to 10's of cm thick) of pervasive pink Kfs-rich granitic leucosome, gray Plag-rich tonalitic leucosome and dark mafic melanosome at a cm scale. The layering and the foliation are parallel in these migmatites and dip about 20° to the east. The layering of the migmatite originated from solid-state mineral segregation, under high grade conditions, and from melt segregation.Amphibolite layers, a few tens of centimeters in thickness, occur as elongate boudins sub-parallel to the migmatitic layering and are interpreted as boudinaged tholeiitic basalt sills. Mineral stretching lineations are shallow plunging to sub-horizontal and trend ~N075°. Lineations could be measured in the field at six outcrops only. The scarcity of macroscopically visible lineations in the field was one of the main motivations to undertake a magnetic fabric study on the Morton Block. Multidomain to pseudo-single domain magnetite grains dominate the magnetic susceptibility and the AMS. The AMS is formed mostly by the magnetostatic (shape) anisotropy of magnetite grains, acquired at high temperature (600-800°C) by dislocation creep. The high temperature fabrics were not overprinted at lower temperature. This AMS is therefore interpreted as representative of lower crustal flow. Subhorizontal structures in the Morton Block rocks are consistent for several hundreds of meters of exposure, and are reminiscent of those observed, elsewhere, in most lower crustal terranes. Such a layering might result from high temperature plastic flow and might be typical of the continental lower crust, as recently suggested both by numerical modelling studies and by seismic anisotropy for the Tibetan plateau. It may develop during either collision-related thickening or alternatively during post-collisional collapse and extension. The AMS principal axes coincide with finite strain axes measured on the outcrops and therefore validate the AMS method as a powerful structural tool in high grade gneisses. The AMS structures are remarkably consistent over a wide area, suggesting crustal homogeneous flow.
DE: 1518 Magnetic fabrics and anisotropy
DE: 8100 TECTONOPHYSICS
DE: 8102 Continental contractional orogenic belts and inversion tectonics
DE: 8103 Continental cratons
DE: 8108 Continental tectonics: compressional
SC: Tectonophysics [T]
MN: Fall Meeting 2005