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Paper No. 6
Presentation Time: 8:00 AM-6:00 PM

GARNET PORPHYOBLASTS AS MARKERS FOR STRAIN PARTITIONING IN THE HANGING-WALL OF A MAJOR OUT-OF-SEQUENCE THRUST


ANDERSON, Mark W., School of Geography, Earth and Environmental Sciences, University of Plymouth, Drake Circus, Plymouth, PL4 8AA, United Kingdom and STOKES, Alison, Experiential Learning Centre for Excellence in Teaching and Learning (CETL), University of Plymouth, 3-15 Endsleigh Place, Drake Circus, Plymouth, PL4 8AA, United Kingdom, manderson@plymouth.ac.uk

Within the stack of Caledonian crystalline thrust sheets of northern Scandinavia, a single amphibolite facies lithotectonic unit, the Småtinden nappe, is identified as a major, basement-coupled (“stretching”) shear zone. This dominantly pelitic unit achieved peak metamorphic conditions of 535-550°C and 8-9kbars, and the stretching geometry suggests that this most likely occurred in response to overthrusting of a hot, pre-assembled Caledonian thrust stack. Porphyroblast-fabric relationships in the shear zone, especially involving garnet, show marked variations along strike and indicate the role of strain partitioning during late-stage decoupling of the Småtinden unit from the basement.

Southern sections through the shear zone are characterised by relatively late, static growth preserving concordant Si-Se relationships, and typically symmetrical external fabrics consistent with formation under dominantly pure shear conditions. The north of the area is characterised by early garnet growth, with discordant Si-Se relationships and asymmetric external fabric geometries consistent with formation during ESE-directed simple shear. Further, geometrical relationships between internal (Si) and external (Se) fabric domains indicate that the curved inclusion trails characteristic of the Småtinden nappe represent overgrowths of ESE-vergent microfolds, formed during bulk ESE-directed non-coaxial deformation. This model is supported by chemical zonation patterns within garnet porphyroblasts which are consistent with nucleation controlled by local dilation along the short limbs of evolving asymmetric crenulations.

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