Movement sense determination in sheared rocks

Conference · Tue Jan 01 00:00:00 EST 1985 · Geol. Soc. Am., Abstr. Programs; (United States)

OSTI ID:6322763

Simpson, C

Deformation within fault zones produces sheared rocks that range from cataclasites at high structural level, to mylonites and mylonitic gneiss at deeper levels. These rocks are easily recognized and mapped in the field and the strike and dip of the fault zone established. However, present-day geometry of the fault zone does not necessarily indicate relative motion - a zone dipping at 15/sup 0/ could represent a listric normal, thrust, oblique-slip or tilted strike-slip fault. Where offset stratigraphic or lithological markers are absent, the movement sense may be determined from meso- and micro-structural features within the sheared rocks. Of prime importance is the orientation of mineral elongation or stretching lineations which record the finite X direction of strain in the rock; this direction approaches the bulk movement direction with increase in strain. At mesoscopic scale the most reliable shear sense indicators are shear bands and associated features. Use of fold vergence requires caution. On a micro-structural scale, shear bands, mica fish, microfolds, rotated grains, asymmetrical augen structure and fiber growth patterns all give reliable results. Thin sections should be cut parallel to lineation and perpendicular to foliation in order to view maximum rotational component. Asymmetry of crystallographic fabric patterns gives consistent results in zones of relatively simple movement history. For high confidence shear sense determination, all structural elements should be internally consistent. If inconsistency occurs this may indicate a complex, multidirectional movement history for the fault zone.

• Research Organization: Virginia Polytechnic Institute and State Univ., Blacksburg (USA)
• OSTI ID: 6322763
• Report Number(s): CONF-8510489-
• Journal Information: Geol. Soc. Am., Abstr. Programs; (United States), Vol. 17; Conference: 98. annual meeting of the Geological Society of America, Orlando, FL, USA, 28 Oct 1985
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
GEOLOGIC FAULTS
GEOMETRY
ORIENTATION
METAMORPHIC ROCKS
MECHANICAL PROPERTIES
STRESS ANALYSIS
CLEAVAGE
DEFORMATION
GEOLOGIC HISTORY
METAMORPHISM
MINERALOGY
ROCK MECHANICS
STRAINS
TECTONICS
CRYSTAL STRUCTURE
GEOLOGIC FRACTURES
GEOLOGIC STRUCTURES
MATHEMATICS
MECHANICS
MICROSTRUCTURE
ROCKS
580201* - Geophysics- Seismology & Tectonics- (1980-1989)
580300 - Mineralogy
Petrology
& Rock Mechanics- (-1989)