Differential stress determined from deformation-induced microstructures of the Moine thrust zone

Journal Article · Mon Dec 10 04:00:00 EST 1979 · J. Geophys. Res.; (United States)

DOI:https://doi.org/10.1029/JB084iB13p07495· OSTI ID:5486441

Weathers, M S; Bird, J M; Cooper, R F; Kohlstedt, D L

The dislocation structure, recrystallization, and elongation of quartz grains in tectonites from three localities along the Moine thrust fault have been analyzed by transmission electron and optical microscopy. The dislocation density, 5 x 10/sup 8/ cm/sup -2/, and the grain size after recrystallization, 15 ..mu..m, in the basal quartzite unit at the Stack of Glencoul are independent of distance from the fault, to the maximum sampling distance of approx.100 m. The differential stress level determined from the deformation-induced microstructures is on the order of 100 MPa. The magnitude of the differential stress at the fault decreases by a factor of 2 from Knockan Crag to Loch Eriboll, a distance of 50 km. At the Stack of Glencoul, grains in the basal quartzite are progressively elongated and recrystalliZed approaching the fault, corresponding to a progressive increase in strain. At 100 m from the fault, the aspect ratio of relict quartz grains is approx.1:a; at 0.01 m it is 85:1. A quantitative estimate of the strain, based on the aspect ratios of the relict quartz grains, as a function of distance from the fault has been used to calculate strain rate and temperature profiles. For a creep activation energy of 0.19 MJ mol/sup -1/ the temperature decreases away from the fault with a gradient of 7.5 x 10/sup -3/ /sup 0/C cm/sup -1/. For a differential stress of 100 MPa, a steady thrust sheet velocity of 11 cm yr/sup -1/ would be required to produce this gradient. At 100 m from the fault, 5% of the quartzite is recrystallized from 1-mm to 15-..mu..m grains; at 0.01 m it is 100% recrystallized to 15-..mu..m grains. Systematic changes in the elongation and recrystallization of quartz grains in Lewisian Gneiss and Moine Schist are obscured by the constraints imposed by other mineral phases. Very little static recovery followed the plastic deformation event.

• Research Organization: Department of Geological Sciences, Cornell University, Ithaca, New York 14853
• OSTI ID: 5486441
• Journal Information: J. Geophys. Res.; (United States), Journal Name: J. Geophys. Res.; (United States) Vol. 84:B13; ISSN JGREA
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580100* -- Geology & Hydrology-- (-1989)
580300 -- Mineralogy
Petrology
& Rock Mechanics-- (-1989)
CRYSTAL DEFECTS
CRYSTAL STRUCTURE
DEFORMATION
DISLOCATIONS
GEOLOGIC FAULTS
GEOLOGIC STRUCTURES
LINE DEFECTS
METAMORPHIC ROCKS
MICROSTRUCTURE
QUARTZITES
ROCK MECHANICS
ROCKS
STRAIN RATE
STRESSES