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Tectonic Domains in the Rio Grande / Rio Bravo Border Region, Texas and Mexico: Laramide Structures Suggest Earlier History
Regional trends of Laramide structures define five tectonic domains and four northwest-trending discontinuities in northeastern Mexico and adjacent Texas. The domains are, from northeast to southwest: two open foldbelts (Rio Grande domain); a series of uplifted blocks with open folds (Salado domain); high relief evaporite-cored folds (Sabinas-Monterrey domain); gently folded, uplifted blocks (Coahuila Block domain); and a partly allochthonous foldbelt (Parras-Peregrina domain). The gently folded domains contain thin Mesozoic strata overlying Paleozoic and Triassic basement, representing thick, less-extended crustal blocks. Strongly folded domains contain thick Mesozoic strata and salt, which overlie thin, highly-extended crustal blocks.
Northwest-trending discontinuities transect the area. The four most significant are, from northeast to southwest: the Frio River zone (bounding the San Marcos arch); the La Babia–Zapata zone (bounding the Burro block, offsetting the Salado blocks); the San Marcos–San Carlos zone (bounding the Coahuila block and San Carlos block); and the Parras-Galeana zone (the southern edge of the Monterrey salient). The discontinuities parallel the Atlantic opening direction, and may represent left-lateral transforms coeval with Middle Jurassic crustal stretching prior to seafloor spreading of the Gulf of Mexico. Offsets in Salado blocks indicate motion up to 300 km. Northwest-trending extensional features (Sabinas basin, Chittim rift) are inconsistent with left-lateral motion, and may represent pre-transform interarc extension.
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