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An age constraint on Gulf of California rifting from the Santa Rosalía basin, Baja California Sur, Mexico

Holt, John W. and Holt, Elizabeth W. and Stock, Joann M. (2000) An age constraint on Gulf of California rifting from the Santa Rosalía basin, Baja California Sur, Mexico. Geological Society of America Bulletin, 112 (4). pp. 540-549. ISSN 0016-7606. doi:10.1130/0016-7606(2000)112<0540:AACOGO>2.3.CO;2.

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Marine rocks of the Santa Rosalía basin, Baja California Sur, were sampled in a pilot study to determine their suitability for magnetostratigraphy and geochronology with the goal of providing an age constraint on Gulf of California rifting. Progressive demagnetization of samples from the Boleo Formation, the earliest marine sequence overlying the deeply eroded basement, reveals a high-coercivity characteristic remanent magnetization (ChRM) in addition to a low-coercivity overprint. The ChRM appears to be a primary magnetic remanence with stratigraphically bound normal- and reversed-polarity directions. A single ^(40)Ar/^(39)Ar isotopic age of 6.76 ± 0.90 Ma (2σ) was obtained for the cinta colorada, a tephra deposit of reversed paleomagnetic polarity within the Boleo Formation. The age of the cinta colorada is refined by calculating isotopic age probabilities for each of the reversed-polarity intervals of the geomagnetic polarity time scale (GPTS) in the ±2σ range 5.86–7.66 Ma. The interval with the highest probability is 6.57–6.94 Ma. In conjunction with the isotopic age, preliminary magnetostratigraphy of the Boleo Formation is correlated with the GPTS in order to further delineate the onset of marine sedimentation. The most likely correlation yields an age of 6.93–7.09 Ma (GPTS subchron C3Bn) for the base of the Boleo Formation and 6.14–6.27 Ma for the top. This correlation, indicating an average sedimentation rate of 28 ± 4 cm/k.y., could be significantly altered if a more thorough magnetostratigraphy proved the existence of additional polarity intervals in the Boleo Formation. However, even if the isotopic age of the cinta colorada is used as the only age constraint, the result is consistent with data from the northern Gulf of California and shows that rifting started much earlier than the ca. 3.6 Ma commencement of sea-floor spreading at the mouth of the Gulf of California. The deposition of the Boleo Formation was probably related to an early phase of gulf rifting caused by a change in Pacific–North American plate motions.

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Stock, Joann M.0000-0003-4816-7865
Additional Information:© 2000 Geological Society of America. Manuscript received by the society July 24, 1998. Revised manuscript received March 4, 1994. Manuscript accepted April 6, 1999. This is contribution 8637 or the Caltech Division of Geological and Planetary Sciences. This work was supported by National Science Foundation grants EAR-9019289, EAR-9419041, and EAR-9296102. We are pleased to thank Joseph L. Kirschvink for use of the Cal tech Paleomagnetics Laboratory, Mahmoud Chaudry for preparing mineral separates, Marty Grove at UCLA for performing the argon measurements, Elizabeth Nagy for performing initial argon statistics, Xavier Quidelleur for assisting in blank corrections and additional statistical analyses, and Rob Coe for the use of his rock magnetic facilities at University of California, Santa Cruz. We thank Laurie Brown, Bob Butler, and Kenneth Kodama for their careful reviews and suggestions. Also, muchas gracias to the hombre with the huge tractor who pulled our truck across the flooding river at Colonia San Vicente Guerrero.
Group:Seismological Laboratory, Division of Geological and Planetary Sciences
Funding AgencyGrant Number
Subject Keywords:geochronology; magnetostratigraphy; rifting; sedimentation; tephra
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Caltech Division of Geological and Planetary Sciences8637
Issue or Number:4
Record Number:CaltechAUTHORS:20140417-091012374
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Official Citation: John W. Holt, Elizabeth W. Holt, and Joann M. Stock An age constraint on Gulf of California rifting from the Santa Rosalía basin, Baja California Sur, Mexico Geological Society of America Bulletin, April, 2000, v. 112, no. 4, p. 540-549, doi:10.1130/0016-7606(2000)112<540:AACOGO>2.0.CO;2
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:44998
Deposited By: Ruth Sustaita
Deposited On:17 Apr 2014 16:51
Last Modified:10 Nov 2021 16:59

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