Crustal shortening on the Lombard thrust: A detailed study along the southwest Montana Transverse Zone
Conference
·
· Geological Society of America, Abstracts with Programs; (United States)
OSTI ID:6906230
- Western Michigan Univ., Kalamazoo, MI (United States). Geology Dept.
Recent drilling of the Devil's Fence Anticline on the Helena Salient of the Cordilleran thrust belt in SW Montana shows that the Lombard thrust sheet has been translated eastward by as much as 70 km, several times greater than all previous estimates. In the southern part of the salient the principal frontal thrust becomes a transverse ramp with significant dextral strike slip separation estimated at about 20 km. Part of the discrepancy between the 70 km of thrust slip and 20 km of strike slip may be manifested as internal shortening of the thrust sheet. The study area, situated between the Devil's Fence Anticline and the southwest Montana Transverse Zone, shows significant internal shortening within the thrust sheet. The internal shortening is partitioned among tight folds and subsequent faulting, minor decollements and homogeneous layer parallel deformation. At least 9 major N to Ne plunging folds, about 1 km in wavelength with an average plunge of 20 degrees, account for a total shortening of approximately 40% over a map distance of less than 5 km. Local fault movement accounts for 5--15% shortening. Layer parallel shortening in the form of cleavage and calcite twinning that occurred prior to thrusting may account for an additional 10%. The amount of internal shortening within this study area alone may provide an explanation for the discrepancies in the two estimates of regional shortening.
- OSTI ID:
- 6906230
- Report Number(s):
- CONF-9404217--
- Conference Information:
- Journal Name: Geological Society of America, Abstracts with Programs; (United States) Journal Volume: 26:5
- Country of Publication:
- United States
- Language:
- English
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