The Ouachita interior zone, arching through central Texas into the Big Bend region of the Rio Grande, has been defined chiefly on the basis of well samples and its associated gravity high. The only known exposure of rocks of the interior zone lies at the base of the Sierra del Carmen normal fault escarpment under approximately 1100 m of Lower Cretaceous carbonates in northern Coahuila, Mexico. These lower greenschist facies metamorphic rocks are thinly interlayered graphitic muscovite schist, graphitic marble, and minor quartzite with abundant quartz veins.
The dominant structures shown by the metamorphic rocks are isoclinal folds (F2) that have a well-defined axial planar foliation (S2). Earlier (F1) folds refolded by F2 are locally preserved. At the microscopic scale, S2 is a crenulation cleavage of the S1 foliation that has been enhanced by pressure solution. Feldspar porphyroblasts overgrow a folded graphitic S1 foliation. Throughout the area a second crenulation cleavage (S3) is present on the S2 foliation surface as an intersection lineation (L3). Subsequent Laramide regional- to outcrop-scale folds and associated thrusts overprint these ductile structures. Lower Cretaceous carbonates form a large, east-verging overturned to recumbent anticline-syncline fold pair with a northwest-trending fold axis. The S2 metamorphic foliations define an antiform that is nearly coincident in trend to that of the fold in the overlying carbonates, indicating that the metamorphic rocks were also folded by this regional structure. A Basin and Range normal fault is interpreted to have reactivated the Laramide thrust fault responsible for the contractional deformation. One to two normal faults cut the carbonates and metamorphic rocks and truncate the anticline-syncline fold pair. The maximum offset along the normal fault is 1600 m.
Rb/Sr data from the muscovite schist gives a six-point, mineral–whole-rock isochron age of 277 ± 10 Ma, indicating that polyphase deformation and metamorphism occurred during the late Paleozoic Ouachita orogeny. An initial Sr ratio of 0.721 suggests an ancient source for these metasediments. Common Pb ratios of the metamorphic whole rock and galena from the Puerto Rico Mine are isotopically distinct from those of the North American craton, suggesting that they were derived from a non–North American source and thus may be equivalent to unmetamorphosed Ouachita flysch sediments. This indicates that the metamorphic rocks of the Ouachita interior zone were derived from an ancient allochthonous source that was deformed and metamorphosed during the late Paleozoic Ouachita orogeny.