Paleomagnetic study of thrust-sheet rotations in the Helena salient, southwest Montana

Conference · Tue Jan 01 04:00:00 EST 1985 · Geol. Soc. Am., Abstr. Programs; (United States)

OSTI ID:6876941

Eldredge, S; Van Der Voo, R

The Lower Cretaceous Kootenai Formation, sampled throughout the Helena Salient of southwestern Montana, contains components of remanent magnetization which are shown to be pre-deformational in age by one regional and two local fold tests. Thermal demagnetizations at temperatures up to 690/sup 0/C reveal much better isolation of characteristic remanence components than alternating-field demagnetization; hematite is indicated as the carrier of the remanence. Sixteen sites have characteristic directions that cluster well, and of these, six sites show significant rotations relative to stable foreland reference sites. The rotations within the southern portion of the salient are all in a clockwise sense with a maximum rotation of 54/sup 0/ seen near the Southwest Montana Re-entrant. These rotations appear to have been caused by buttressing effects of the syntectonically rising Rocky Mountain Foreland. Just to the northeast, in the Southwest Montana Transverse Zone, where the thrust traces run nearly perpendicular to the regional trend, only 23/sup 0/ of clockwise rotation is observed. There is considerable dextral as well as reverse slip along the faults of this region, with apparently less of a buttressing effect than is seen in the re-entrant. Local variations in amount of rotation imply that the thrust sheets have buckled and broken against the foreland margin, and it is unlikely that there were coherent, large-scale plate rotations in the interior of the salient.

• Research Organization: Univ. of Michigan, Ann Arbor (USA)
• OSTI ID: 6876941
• Report Number(s): CONF-8510489-
• Conference Information: Journal Name: Geol. Soc. Am., Abstr. Programs; (United States) Journal Volume: 17
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580100* -- Geology & Hydrology-- (-1989)
580201 -- Geophysics-- Seismology & Tectonics-- (1980-1989)
CRETACEOUS PERIOD
DEFORMATION
FEDERAL REGION VIII
GEOLOGIC AGES
GEOLOGIC FAULTS
GEOLOGIC FORMATIONS
GEOLOGIC FRACTURES
GEOLOGIC HISTORY
GEOLOGIC STRUCTURES
GEOMETRY
MAGNETISM
MATHEMATICS
MESOZOIC ERA
MONTANA
NORTH AMERICA
PALEOMAGNETISM
PLATE TECTONICS
TECTONICS
USA