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R.L. Sedlock

Publications and source records attributed to R.L. Sedlock.

3 recordsLinked to original sources
USGS1998

Remagnetization of Cretaceous forearc strata on Santa Margarita and Magdalena Islands, Baja California Sur: Implications for northward transport along the California margin

Paleomagnetic data for two sections of Cretaceous forearc strata with different structural attitudes on Santa Margarita and Magdalena Islands in Baja California Sur, Mexico, indicate that these rocks have been remagnetized, probably during the late Cenozoic. The in situ paleomagnetic directions, however, are similar to data from other Cretaceous rocks on peninsular California with unexpectedly shallow inclinations and easterly declinations. These data have been interpreted as indicating either northward tectonic transport (10°–15° of latitude) and clockwise rotation (>20°) or compaction shallowing of magnetic inclinations in sedimentary rocks combined with southwestward tilting of plutonic rocks. The available paleomagnetic data for Cretaceous forearc strata in southern and Baja California can be divided into three groups: (1) sections with normal-polarity magnetizations that fail fold tests and are remagnetized, (2) sections with normal-polarity magnetizations with no or inconclusive fold tests that may or may not be remagnetized, and (3) sections with both normal-and reversed-polarity intervals where pervasive remagnetization has not occurred. Other rocks of the Mesozoic Great Valley Group, Coast Range ophiolite, and Franciscan Complex in California also have secondary magnetizations with directions similar to younger geomagnetic field directions. Although these widespread remagnetizations could have variable local causes, we propose regional burial and uplift, related to changes in subduction parameters, as a possible explanation. Two episodes of remagnetization are apparent: one in the Late Cretaceous and a second in the late Cenozoic. On the other hand, the unremagnetized and apparently reliable data from sedimentary and plutonic rocks on the Baja Peninsula consistently indicate northward translation (14° ± 3°) and clockwise rotation (29° ± 8°) with respect to North America since the Late Cretaceous.

California↗
USGS1990

Remagnetization and northward translation of Mesozoic red chert from Cedros Island and the San Benito Islands, Baja California, Mexico

Samples for paleomagnetic analysis were collected from red radiolarian ribbon chert in a Franciscan-like subduction complex (Western Baja terrane) exposed along the western margin of the Baja California Peninsula. A 40-m-thick section of chert was sampled on Cedros Island, and a 30-m-thick and two smaller sections of chert were sampled on the San Benito Islands to the west. A site in pillow basalt underlying the chert also was sampled on Cedros Island. The sections of chert on Cedros Island and the San Benito Islands contain radiolaria ranging in age from Late Triassic to Early Cretaceous. All of these rocks have similar normal-polarity direction of magnetization, indicating that they were remagnetized long after deposition. Results from a chert site, including an intrastratal fold, also indicate that the magnetization is postdepositional. A significantly improved grouping (95% confidence level) of directions after correction for the structural attitudes of the Cedros and San Benito chert sections, however, implies that the remagnetization predates breaking up and incorporation of the chert into the subduction complex. Studies of modern subduction zones imply that the attitude of the ancient subducting slab, including the chert sections, was near horizontal (dip angles <10° at the time of remagnetization. Thus, assuming paleohorizontality, a paleolatitude of 12° ± 5° N can be estimated for the Western Baja terrane in mid-Cretaceous time. The paleomagnetic directions also indicate a subsequent poleward translation of 25° ± 7° in latitude (2,800 km) and a clockwise rotation of 56° ± 7° of the Western Baja terrane with respect to North America. The paleolatitude error limits at the 95% confidence level represent minimum values, because deviations of the bedding planes from horizontal during remagnetization would contribute to the error. A 10° NE dip of the subducting slab, however, would account for only 4° of the paleolatitude anomaly, and a ≃60° NE dip would be needed to account for the entire anomaly. Modern analogs, in combination with plate-reconstruction models, imply that the northward translation of the Western Baja terrane along the ancient convergent margin of North America could have been accomplished by arc-parallel strike-slip faulting that was driven by oblique subduction.

Baja California↗