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GEOREF RECORD

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

Jonathan T. Hagstrum and Richard L. Sedlock
Remagnetization and northward translation of Mesozoic red chert from Cedros Island and the San Benito Islands, Baja California, Mexico
Geological Society of America Bulletin (July 1990) 102 (7): 983-991

Abstract

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 degrees at the time of remagnetization. Thus, assuming paleohorizontality, a paleolatitude of 12 degrees + or - 5 degrees N can be estimated for the Western Baja terrane in mid-Cretaceous time. The paleomagnetic directions also indicate a subsequent poleward translation of 25 degrees + or - 7 degrees in latitude (2,800 km) and a clockwise rotation of 56 degrees + or - 7 degrees 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 degrees NE dip of the subducting slab, however, would account for only 4 degrees of the paleolatitude anomaly, and a nearly equal 60 degrees 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.


ISSN: 0016-7606
EISSN: 1943-2674
Coden: BUGMAF
Serial Title: Geological Society of America Bulletin
Serial Volume: 102
Serial Issue: 7
Title: Remagnetization and northward translation of Mesozoic red chert from Cedros Island and the San Benito Islands, Baja California, Mexico
Affiliation: U. S. Geol. Surv., Menlo Park, CA, United States
Pages: 983-991
Published: 199007
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 57
Accession Number: 1990-045661
Categories: StratigraphySolid-earth geophysics
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables, geol. sketch map
N23°00'00" - N32°30'00", W117°00'00" - W109°00'00"
Secondary Affiliation: Stanford Univ., USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 1990
Program Name: USGSOPNon-USGS publications with USGS authors
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