Subgrain rotation and dynamic recrystallization of olivine, upper mantle diapirism, and extension of the basin-and-range province |
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| Authors: | Hans GAv Lallemant |
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| Institution: | Hans G.Avé Lallemant |
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| Abstract: | On the basis of an abrupt change of olivine grain size at certain depths in the upper mantle beneath the Basin-and-Range province in the western U.S.A., Mercier (1980b) proposed that subgrain rotation (SGR) recrystallization had taken place above the grain size discontinuity, and grain boundary migration (GBM) recrystallization at depths greater than the discontinuity.The rotation of subgrains and the characteristics of dynamically recrystallized neoblasts of olivine were analyzed in a series of dunite samples deformed experimentally to compressive strains of about 15–60%. Whereas the rotation angle between adjacent subgrains increases with strain, the mean rotation angle did not exceed 2° even in the most heavily deformed samples. Rotation angles between kink bands, which formed at all experimental conditions, also increase with strain; at the highest strains rotations of up to 110° are not uncommon. At low strains the dominant rotation axis between kinks is 001] and less frequently ovw]; at high strains uvw] rotation axes are common. Neoblasts did form mainly on old grain boundaries and less frequently within old grains. Rotation axes between intragranular neoblasts and their hosts are of the type uvw] and the rotation angles are always large, features which seem to be inconsistent with SGR recrystallization and suggest GBM recrystallization. Neoblasts may have formed in highly strained regions of old grains where slip occurs with uvw] rotation axes.The olivine textures and fabrics of lherzolite nodules originating in the uppermost mantle above the grain size discontinuity are similar to those of nodules from the Dreiser Weiher, Germany (Mercier, 1980b). A study of Dreiser Weiher samples shows, however, that they have features difficult to reconcile with the SGR mechanism.It is proposed here that GBM recrystallization occurred throughout a rising upper mantle diapir beneath the Basin-and-Range extension zone. As the upward flow crossed the regional lithosphere-asthenosphere boundary at about 65 km depth, it diverged causing flow velocities to decrease abruptly and the strain rate dropped approximately by an order of magnitude. As a consequence the differential stress decreased by about a factor of two and the olivine grain size increased by a factor of two. |
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