Preliminary paleomagnetic evidence indicates that a significant part of Mexico rotated 130° counterclockwise during early Mesozoic time, coeval with the opening of the Gulf of Mexico, and that a major structural discontinuity must exist between Mexico and cratonic North America. Measurements made on samples from fourteen isolated outcrops of the Huizachal Group, a red-bed sequence exposed in northeastern Mexico indicate that these sediments were deposited in rifted basins and could range in age from post–Middle Permian to pre–Early Cretaceous. Of samples analyzed from the overlying Zuloaga Limestone (Oxfordian) and the Cuesta del Cura Formation (Aptian-Albian), the oldest yield a paleomagnetic pole position that implies that the sampling area was originally oriented about 130° clockwise relative to stable North America. Progressive counterclockwise rotation continued to Middle Jurassic time. Because of the limited geographical distribution of sampling sites, it is not possible to assess whether most of Mexico shared this rotation or whether the proposed left-lateral motion along major west-northwest–trending faults caused the rotation of a smaller crustal block. In either case, our results strongly suggest a major structural discontinuity in northern Mexico.

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