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A paleomagnetic transect of the mid-Cretaceous Peninsular Ranges batholith, Baja California, Mexico

By
Roberto S. Molina-Garza
Roberto S. Molina-Garza
Centro de Geociencias, Universidad Nacional Autónoma de México–Campus Juriquilla, Querétaro 76230, Mexico
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Luis A. Delgado-Argote
Luis A. Delgado-Argote
Centro de Investigación Científica y Estudios Superiores (CICESE), Ensenada 22860, Baja California, Mexico
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Harald Böhnel
Harald Böhnel
Centro de Geociencias, Universidad Nacional Autónoma de México–Campus Juriquilla, Querétaro 76230, Mexico
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Elisa Ramírez
Elisa Ramírez
Centro de Investigación en Ciencia Aplicada y Tecnología Avanzada (CICATA), Ensenada 22860, Baja California, Mexico
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Amabel Ortega
Amabel Ortega
Instituto de Geología, Estación Regional del Noroeste, Hermosillo 83000, Mexico
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Rubén Contreras Flores
Rubén Contreras Flores
CICATA, Querétaro 76090 Mexico
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Published:
January 01, 2014

We report structural, paleomagnetic, and magnetic fabric data for mid-Cretaceous plutons of the Peninsular Ranges batholith along a transect at ~30°N latitude. Four plutons in the western sector are characterized by characteristic magnetizations residing in magnetite. In this sector, El Milagro, Aguaje del Burro, La Zarza, and San Telmo plutons yield a combined paleopole at 82.1°N, 169.7°E (K = 137.6, A95 = 7.9°; n = 4–38 sites), which, rotated for closure of the Gulf of California, falls at 79.3°N, 179.5°E, and it is concordant with the North America reference pole. Plutons in the transition zone, between the eastern and western sectors of the Peninsular Ranges, have magnetizations residing in hematite. El Potrero and San José plutons yield highly discordant paleopoles, indicating apparent clockwise rotation (R) and flattening (F) of 33.0° ± 5.1° and −27.6° ± 6.1°, respectively (San José), and 46.1° ± 5.9° and −31.0° ± 7.0° (El Potrero). The discordance is best explained by west-down tilt of the crustal block between the Main Mártir thrust and the Rosarito fault, which are major compressional structures parallel to the trend of the Peninsular Ranges. The San Pedro Mártir pluton, a large La Posta–type pluton on the eastern sector of the transect, has magnetizations that reside primarily in hematite. The mean paleomagnetic pole (71.3°N, 335.5°E; K = 40.7 and A95 = 7.2°) is slightly discordant, indicating westward tilt of ~15°. The different paleopoles obtained for individual plutons convincingly show that the Peninsular Ranges batholith has suffered internal deformation, which is more intense along the transition zone. The magnetic fabric for plutons representative of the western, eastern, and transitional sectors of the range show marked contrasts in the deformation recorded by anisotropy of magnetic susceptibility (AMS). Anisotropy is weakly developed in the western sector (El Milagro), very strongly developed in the transition zone (San José), and moderately developed in the eastern sector (Sierra San Pedro Mártir). Within the plutons, El Milagro fabrics record emplacement-related stress. In contrast, San José and San Pedro Mártir appear to record regional stress linked to evolution of the Main Mártir thrust. Overall, our data are consistent with rotation of the crustal block where Potrero and San José plutons are located; rotation was accommodated by major crustal faults in a compressional stress field, as the crustal block moved to occupy the space abandoned by the ascending (and westward expanding) San Pedro Mártir diapir batholith. The rotation could be related to interaction between the large Sierra San Pedro Mártir pluton and the Main Mártir thrust, or to mechanical controls such as wedging against a rigid salient.

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GSA Memoirs

Peninsular Ranges Batholith, Baja California and Southern California

Douglas M. Morton
Douglas M. Morton
U.S. Geological Survey and Department of Earth Sciences, University of California, Riverside, California 92521, USA
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Fred K. Miller
Fred K. Miller
U.S. Geological Survey, 904 West Riverside Ave., Spokane, Washington 99201, USA
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Geological Society of America
Volume
211
ISBN print:
9780813712116
Publication date:
January 01, 2014

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