T147 Structural Analysis of Polyphase Deformation from Orogen to Thin Section I: A Special Session in Honor of Sharon Mosher

Tuesday, 11 October 2022
8:00 AM - 12:00 PM
605

SHEETS AND RODS: STRAIN AND DUCTILE DEFORMATION OF THE STRATHAN CONGLOMERATE, MOINE NAPPE, NORTHERNMOST SCOTLAND

Paper number: T147-8
Program: Technical Sessions
Time: 9:50 AM – 10:05 AM

Authors

Abstract

Field, microstructural and crystallographic orientation data indicate that the Strathan Conglomerate within the Moine nappe in northern Scotland was deformed by WNW-directed noncoaxial shear and coaxial flattening under amphibolite facies conditions. Adjacent to the overlying Ben Hope thrust at Strathan Bay, the massive conglomerate displays extreme flattening (e.g. laterally extensive sheets of deformed pebbles with aspect ratios of 134:113:1; 88-92% estimated thinning) coupled with WNW-directed shear. Lower in the nappe package at Ben Hutig, polyphase folding of interlayered conglomerate and psammite dominated. There, early large-scale folds (F2) rotated into the transport direction and subsequent transport parallel (F3) folds and tubular sheath folds formed on the F2 limbs and became dismembered to form rods accompanied by minor flattening. The extreme constriction reported at Ben Hutig (e.g. rods with aspect ratios of 21:4:1; estimated extension of 1000%) is only apparent with no constriction having occurred. Strain partitioning is between intensity of strain and how deformation is manifested and was controlled by rheology, location relative to the Ben Hope thrust and temperature.

The extreme flattening and noncoaxial shear at Strathan Bay was accommodated by dislocation creep and GBM II, with subsequent SGR as temperatures cooled. The CPO fabrics associated with extensive flattening strains have been overprinted by significant shear strains. Emplacement along the Ben Hope Thrust of much hotter, thick nappes on top of the Strathan Conglomerate, maintained higher temperatures for a longer period of time, and allowed the mass of the overlying nappes to cause significant flattening in the underlying Moine nappe. At Ben Hutig, where temperatures were lower, dislocation creep and GBM I was strongly overprinted by elongate SGR recrystallized grains subparallel to F3 axial planes as temperatures cooled with CPO fabrics recording WNW-directed shear. Crystallographic vorticity analysis (CVA), which plots the poles along which each individual crystal lattices rotate, detects a combination of progressive coaxial and noncoaxial deformation not readily observed in CPOs and indicates that CVAs can record progressive deformation and retain evidence of earlier rotations.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-382360

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