SECONDARY MAGNETIZATION SUGGESTS A LATE JURASSIC DEFORMATION IN CENTRAL TEXAS

Early and Middle Permian red sedimentary rocks from central Texas exhibit well-defined Permian paleomagnetic directions after thermal demagnetization to temperatures of 550 to 650 degrees C. Mean directions from each of six formations yield paleomagnetic poles that are statistically the same (at the alpha-95 confidence level) and correspond well with other Permian paleopoles from cratonic North America. However, an equally well-defined paleomagnetic direction is observed in the temperature unblocking range of 350 to 475 degrees C. The secondary magnetization is observed in about 60% of the approximately 300 samples. A mean direction and paleopole derived from the secondary magnetization is statistically identical (at alpha-95) to the Late Jurassic (Kimmeridgian-Tithonian) upper Morrison Formation paleopole. Paleomagnetic poles of Permian through Late, but not latest, Jurassic ages diverge between cratonic North American sites and sites on the Colorado Plateau. Late Late Jurassic upper Morrison Formation paleopoles are statistically identical before correction for Colorado Plateau rotation, suggesting rotation of the plateau between Oxfordian and Kimmeridgian times (Steiner, 2003). Moreover, the formation of the Gulf of Mexico also falls in the Oxfordian-Kimmeridgian time period. The similarity in inferred age of the Texas secondary magnetization, opening of the Gulf of Mexico, and the inferred timing of the Colorado Plateau rotation may all be manifestations of a single plate tectonic event affecting the North American plate in latest Jurassic time. The mechanism for the imprint of the Texas secondary magnetization has not yet been investigated, and could be thermally-induced magnetization or new mineral growth resulting from fluid migration.