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In a continuing effort to better understand the subsurface nature and evolution of the North American continent, the Consortium for Continental Reflection Profiling (COCORP), recently recorded approximately 200km (124 mi) of deep seismic reflection profiles across the Ouachita Mountains in western Arkansas. The Late Paleozoic Ouachita orogenic belt forms a sinuous pattern across the south-central United States, stretching from central Mississippi to west Texas (Flawn, 1961). Paleozoic strata at the core of the belt are exposed only in the Ouachita Mountains of Arkansas and Oklahoma, and in the Marathon Mountains of West Texas (Figure 1). Borehole and geophysical data demonstrate that the remainder of the belt is buried beneath Mesozoic and Cenozoic strata of the Gulf Coastal Plain. Many recent workers (Briggs and Roeder, 1975; Viele, 1979; Walper, 1977) interpreted the Ouachita belt as the remnants of a collisional orogeny in which an exotic terrane ("Llanoria") was sutured to the North American continent in Late Paleozoic time.

The data shown on the accompanying panels are unmigrated time sections for the upper 8.5 secs of the north-south dip lines (that is, a composite of lines 1 and 3 shown on Figure 2). Detailed interpretations of the data, emphasizing regional geological and geophysical constraints, are found in Nelson et al (in prep.) and Lillie et al (in prep.). A brief discussion of gross structural features interpreted from the data is presented here. The line drawing (Figure 3a) schematically portrays prominent events observed on the unmigrated time sections for lines 1 and 3. Figure 3b is a preferred interpretation of the data in which tectonically thickened Paleozoic sediments (and metasediments) overlie crust of North American affinity. Alternative interpretations, in which crust exotic to North America extends as far north as the Southern Ouachitas or the Benton Uplift, are discussed in the Nelson et al, and Lillie et al papers. Major structural boundaries are shown in their approximate migrated time positions in Figure 3b. The approximate depth scales assume that 1.0 sec of two-way traveltime represents 2.5 km (or 8,000 ft) of section. Note, however, that depth conversions given below utilize stacking velocity functions and may differ from these approximations.

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