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A 25-station magnetotelluric (MT) traverse across central Texas has been recorded and analyzed. The 84 line-mi (135 line-km), northwest-southeast-trending traverse begins 13 mi (21 km) southeast of Lampasas and ends 6 mi (10 km) north-northeast of Lexington, Texas. Geologic elements crossed are the Ouachita foreland facies, frontal and interior zones, and the rimming gravity high described by Flawn et al, Nicholas and Rozendal, and others.
Interpretation of the MT data, supported by 1- and 2-dimensional modeling, borehole data, and previously published gravity data, suggests northwest transport on the order of 22 mi (35 km) for the allochthonous Ouachita frontal zone.
A shallow resistive zone in the Precambrian and the overlying autochthonous Ouachita foreland facies is traced southeast 46 mi (74 km) from the sites nearest Lampasas, but become indeterminate north of Taylor, Texas. Data from the Shell 1 Purcell (Williamson County) suggest the
shallow Precambrian resistor to be correlative with granites outcropping in the Llano uplift. While indeterminate southeast of Taylor, autochthonous Ouachita foreland facies is interpreted to overly Precambrian basement of similar resistivity at least as far southeast as the rimming gravity high.
Several other resistive and conductive zones are indicated in the Precambrian basement, including a resistive zone which rises to the southeast from a depth of approximately 50,000 ft (15,240 m) near Georgetown to 10,000 ft (3,050 m) into the core of an antiformal basement structure coincident with the rimming gravity high.
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