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J. G. Muncey (1), W. J. Ehni (2)
A 25-station, 84 line-mile, remote reference magnetotelluric (MT) traverse across a portion of central Texas has been recorded and analyzed. Among the geological and geophysical elements crossed are the Ouachita foreland, frontal and interior zones, and the rimming gravity maximum described by Flawn et al., (1961), Nicholas and Rozendal (1975), and others.
MT signatures of the allochthonous Ouachita facies have been established with the aid of well control and are correlated along the traverse. Both conductive foreland facies and the underlying resistive Precambrian can be traced beneath the resistive frontal thrust zone but become indeterminate along the central part of the traverse, due to rapid thinning of the Precambrian resistor. Well control suggests that this resistive basement is correlative with the Grenville-age granitic basement of the nearby Llano Uplift. A thick conductive interval of metasedimentary (?) basement material is observed to underlie the Paleozoics along the central portion of the traverse. This conductive basement appears to dip steeply beneath the "Llano-type" resistive basement and subcrops on the foreland flank of a basement antiform coincident with the rimming gravity maximum.
Still another resistive basement interval appears deep within the basement near the central portion of the traverse, and shallows rapidly to the southeast, to form the core of the basement antiform coincident with the rimming gravity maximum. Stratigraphic superposition within the Precambrian basement suggests that the resistive basement which cores the basement antiform may be older than the conductive metasedimentary (?) and the resistive "Llano-type" basement, and that the basement beneath the Ouachita trend is of preorogenic North American affinity at least as far south and east as the rimming gravity maximum.
The MT signature of the subthrust foreland facies is truncated on the crest of the basement antiform coincident with the rimming gravity maximum. The geometry of the truncation suggests that the frontal thrust zone may have detached from the age-equivalent foreland facies near the present crest of the basement uplift, and that the distance between the foreland facies truncation and the foreland facies-frontal thrust zone boundary may serve as a crude minimum estimate of frontal thrust zone translation (about 60 mi, 100 km).
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