Masters Theses

Date of Award

8-1989

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Robert D. Hatcher

Committee Members

Nicolas B. Woodward, Kenneth Walker, Rick Williams

Abstract

The structural geometry of the Rocky Valley thrust system was controlled by the local stratigraphy and the amount of transport of the thrust sheet through an upper ramp hinge. The Rome-Conasauga structural lithic unit is interpreted as an incompetent unit that is easily faulted while the upper Conasauga-Knox structural lithic unit is a competent unit that is not readily faulted. Consequently, broad-hinged, gentle, asymmetric, upright, "box-fold" (fault-bend fold) geometries develop in the lower unit by kinking over a complex fault path behind the propagating thrust tip, whereas narrow-hinged, close to open, overturned to recumbent, asymmetric, chevron fold (fault-propagation fold) geometries develop in the upper unit by kinking in front of the propagating thrust tip. A fundamental increase in fault cutoff angles is suggested to occur from the lower to the upper structural lithic unit. There is a gradual decrease in the amount of transport onto the upper footwall flat of the Rocky Veilley thrust from northeast to southwest. There is also a general increase in complexity of structural geometry from northeast to southwest. In the northeast portion of the study area the entire Rocky Veilley thrust sheet consists of upper structural lithic unit strata folded into a single ramp anticline. Because cutoff angles in this region are anomalously low, however, the structure has the geometry of a gentle fault-bend fold. All strata in this region have been transported onto the upper footwall flat of the thrust. In the central portion of the study area the thrust sheet consists of a leading edge and trailing edge fold. Lower structural lithic unit strata comprise the trailing edge anticline and exhibit the typical broad-hinged, gentle fault-bend fold geometry. Only upper structural lithic unit strata are involved in the leading edge fold. This fold exhibits the common overturned, narrow-hinged, close geometry of a fault-propagation fold which has been transported through an upper ramp hinge and onto on upper footwall flat. All strata in this region have been transported onto the upper footwall flat of the thrust. In the southwest portion of the study area the Rocky Valley thrust sheet consists of leading-edge, trailing-edge and intrasheet folds. Only upper structural lithic unit strata are involved in the leading edge fold in this area, and the structure exhibits the typical transported fault-propagation fold geometry. The intrasheet Blue Spring Creek anticline is truncated by reverse faults and is believed to have formed out-of-sequence with respect to motion on the Rocky Valley thrust. This anticline exhibits a fault-propagation fold geometry and involves uppermost strata of the lower structural lithic unit and zill upper structural lithic unit strata. The Blue Spring Creek anticline is believed to have formed before motion on the Blue Spring Creek fault. The fault is suggested to have initiated as an "emptive thrust" from the overtightened core of the anticline. Only upper structural lithic unit strata are exposed in the trailing edge anticline in the southwestern portion of the study area. This anticline also exhibits the typical fault-propagation fold geometry. Only upper structural lithic unit strata have been transported through the upper ramp hinge and onto the upper footwall flat in the southwest portion of the study area, flanging wall cutoffs of all lower strata He on the footwall ramp of the Rocky Valley thrust. Dip-domain modelling of the Rocky Valley thrust sheet as a unified structure did not work well. Models constructed using constant initial step>-up angle and simple-step fault path assumptions only roughly approximated the dip spectra in the backlimb of the trailing-edge Rocky Valley anticline. The models did not account for the observed forelimb dips of the Rocky Valley anticline. Models which perfectly match the observed backlimb dip spectra of the Rocky Valley anticline can be constructed only after relaxing the initial assumptions to account for complex fault paths and variable initial step-up angles. These models still do not predict the observed geometry of the leading edge and interior of the thrust sheet. Modelling of individual folds in the Rocky Valley area with dip domain and buckle models worked very well. The geometry of the entire thrust sheet could be closely approximated in this manner. This modelling technique suggests a fundamental difference in initial fault step-up angle between the Rome-Conasauga and upper Conasauga-Knox structural lithic units. Map-scale folds in the Rocky Valley area are believed to have initiated in response to buckling instabilities. After a very small increment of buckling, however, layer parallel slip becomes the dominant deformation mechanism. The folds evolved as kinks from this point on. Layer-parallel slip was not confined to competent-incompetent layer boundaries, but is believed to have occurred on surfaces spaced more closely than the thickness of the thinnest recognized competent section In the regional stratigraphic column. Kinking was accomplished by progressive limb rotation about fixed anticlinal hinges, not by the migrating hinge model proposed by Suppe. Local balanced cross sections indicate the Rocky Valley thrust initiated in the region southwest of Strawberry Plains, Tennessee and propagated northwestward and laterally to the southwest and northeast. Displacement decreases northeast and southwest from the Strawberry Plains area. Cutoff line maps indicate the fault cut at increasingly shallower angles as it died out the the northeast. A less well constrained interpretation of the southwest end of the fault suggests it died out in this region by a combination of shallowing cutoff angles and a simple decrease in displacement. The three dimensional geometry of the thrust system is not compatible with a tip-fold hypothesis of thrust sheet evolution. There are fundamental differences in fault and fold geometry along strike in the thrust sheet. Therefore, it is not possible for geometries observed at the lateral tips of the thrust fault to have evolved into those observed in the more interior regions of the thrust sheet. Regional, balanced cross sections and cutoff line maps suggest the Rocky Valley thrust and Knoxville thrust are not the same fault. Ramp locations and displacements of the two faults are not compatible. The Mill Spring thrust is interpreted as the northeast extension of the Knoxville thrust because the two faults have very similar ramp geometries, offset the same stratigraphic sequence and have similar displacements. Regional fades trends in the Rome-Conasauga structural lithic unit are suggested as a major influence on the ease of thrusting and general thrust geometry in the east Tennessee Valley and Ridge.

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