April 2007 Meeting

Monday, April 9, 2007
6:00 - 7:30 pm

Pellissippi State Technical Community College
10915 Hardin Valley Road, Knoxville
Ned R. McWherter Technology Building
Room 302


Structural Geology and Fold-related Joints of the Chattanooga Area

Dr. Jonathan W. Mies
Robert Lake Wilson Associate Professor of Geology
University of Tennessee at Chattanooga


Relationships among stratigraphy, geologic structures, and topography are well displayed near Chattanooga, Tennessee, at the transition between the Cumberland Plateau and the Valley and Ridge. Under the plateaus, lower to middle Paleozoic strata are relatively undeformed, whereas under the Valley and Ridge, these same strata have been folded and are displaced by numerous thrust faults. From northwest to southeast, the transition includes the Sequatchie Valley thrust and anticline, Waldens Ridge (plateau), and the Chattanooga thrust and associated structures. In northern Georgia, the transition also includes the Lookout Valley anticline, Lookout Mountain (plateau), the McLemore Cove anticline, and Pigeon Mountain (plateau). Continuing into the Valley and Ridge, prominent structures include the Chattanooga, Missionary Ridge, Kingston and Clinchport thrusts, which have a collective stratigraphic separation of more than 15,000 feet.

Joints in the Lookout Valley and McLemore Cove anticlines are differentiated by orientation into two sets that are geometrically related to their respective folds. In each case, cross-fold joints are orthogonal to the fold axis, whereas axial joints are parallel to the fold axis and perpendicular to layering (bedding). Interpreted as mode I (dilational) fractures formed perpendicular to the extension direction (&sigma3), cross-fold and axial joints indicate layer-parallel extensions parallel and normal to the fold axis.

The present geomechanical model for fold-axis-parallel extension, as required by cross-fold joints, is based in the similarity between an upright horizontal anticline and a vault, i.e. an arch with length. Such structures redistribute stress so efficiently that an anticline in the subsurface need only carry a small part of the total vertical load as a vault to develop significant layer-parallel compressive stress normal to the fold axis. The combination of stress thus derived and vertical compression due to support from underlying layers will result in a general state of stress (&sigma1&ge&sigma2&ge&sigma3) in which &sigma3 is parallel to the fold axis. By this model, the geometric relationship between large folds and cross-fold joints does not require that these structures are temporally related. Cross-fold joints may have formed in response to fold-guided stress due to the load of overburden, in which case they may be more recent structures.


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