Possible Early Triassic location of the Caborca Block
Maureen B. Steiner, Oleg Pinos, Spencer G. Lucas, John E. Marzolf, John W. Estep, 2005. "Possible Early Triassic location of the Caborca Block", The Mojave-Sonora Megashear Hypothesis: Development, Assessment, and Alternatives, Thomas H. Anderson, Jonathan A. Nourse, James W. McKee, Maureen B. Steiner
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Shallow-marine Triassic red sedimentary rocks and diabase intrusives were investigated on the Caborca Block in Sonora, Mexico. The lower 212 m half of the sequence was sampled as a magnetostratigraphic section. Samples exhibit exceedingly linearly decaying remanent magnetization and pass a fold test. Unblocking temperatures suggest that the remanence is carried by magnetite. The beds are inferred to be Early Triassic in age because they overlie Permian strata and are overlain by late Early Triassic (Spathian) Tirolites-bearing strata. The red bed samples exhibit an apparently reversed polarity (northern-hemisphere) remanence. Comparison of this polarity to a global compilation of Early Triassic magnetostratigraphy, combined with the age of the superposed beds and the sequence stratigraphic framework, suggests that the age of these beds and their magnetization may be middle Early Triassic (Dienerian). The remanence suggests a paleolatitude of magnetization of 21° N (±4°), so that in the Early Triassic, the Caborca Block may have lain off of western North America near the present location of Seattle, Washington. The overlying red sedimentary rocks containing Spathian ammonites have been remagnetized in a recent geomagnetic field direction. The entire sedimentary section has been intruded by diabase sills; yet oddly, diabase samples gave only widely scattered directions.
The sampling site and Caborca Block are bordered by the left-lateral Mojave-Sonora megashear, but the paleopole is rotated clockwise relative to the North America Early Triassic reference pole, compatible with transport of the terrane in conjunction with right-lateral strike-slip faulting. Many terranes along the western North American margin have been shown to exhibit a history of Jurassic left-lateral transport followed by Cretaceous-Tertiary right-lateral movement (Beck, 1991). The current location of Caborca relative to its inferred Early Triassic paleolocation and the clockwise displacement of the Early Triassic paleopole may stem from a Jurassic left-lateral transport as postulated for the Mojave-Sonora megashear, followed by post-Early Cretaceous right-lateral motion, as observed in numerous other western North American terranes. The important point is that because of the multiplicity of terrane histories, e.g., northward then southward motion relative to cratonic North America, the inference of right-lateral transport for the Caborca Block does not, and cannot, disprove the existence of the left-lateral Mojave-Sonora megashear.