Age and stratigraphic relationships of pre- and syn-rift volcanic deposits in the northern Puertecitos Volcanic Province, Baja California, Mexico

Ž . Geologic mapping of volcanic strata of the northern Puertecitos Volcanic Province PVP in northeastern Baja California, Mexico, performed in conjunction with ArrAr analysis and petrochemical study, documents the Miocene geologic history of a well-preserved volcanic succession within the northern Sierra Santa Isabel and its relationship to the evolving Pacific–North America plate boundary. Subduction-related volcanic deposits, well-exposed in profile along the northern margin of the PVP in the informally named Santa Isabel Wash region, span pre-17 to 15 Ma. Minor rift-related volcanism occurred at ;12.5 and ;9 Ma, prior to voluminous PVP-forming volcanism at ;6–6.5 Ma. Isochron ages typically Ž . exhibit precision 1s for plagioclase of "2–7% and for anorthoclase and sanidine of "2–5%, and replicate analysis of an internal anorthoclase standard indicate ;1–2% reproducibility within a given irradiation and ;2.5% for samples irradiated separately. Improved local correlations made possible by the rich stratigraphic section preserved in Santa Isabel Wash help constrain the relationships of several widespread pyroclastic flow deposits in northeastern Baja California. These correlations are important for both paleomagnetic studies within the region and for establishing geologic ties across the Gulf of California. The combined mapping and age results imply that most extensional deformation in the study area is post-6 Ma, although some earlier faulting and the development of the pre-6 Ma Matomı accommodation zone are also documented. ́ Results support a transitional plate boundary model which implies that much of the Pacific–North America relative plate Ž . motion north of Delfın basin i.e., the northernmost Gulf of California is accommodated on Nto NNW-striking faults ́ developed during Late Miocene ENE-directed extension. The model predicts a zone of divergence east of the PVP which Ž provides a structural mechanism for the positions and jumps of nearby Gulf of California spreading centers Upper and . Lower Tiburon and Delfın basins since 6 Ma, and relates major pulses of PVP volcanism at ;6 and ;3 Ma to these ́ ́ offshore spreading center adjustments. Results also imply that most extensional deformation in Santa Isabel Wash is the result of incorporation of the PVP into the Gulf Extensional Province ;2–3 Ma due to northwestward propagation of the ) Corresponding author. Laboratoire de Geochronologie, Universite Paris 7, 2 Place Jussieu, tour 24-25, 1er etage, 75251 Paris, Cedex 05, ́ ́ ́ France. Tel.: q33-01-4427-2823; fax: q33-01-4427-8148; E-mail: [email protected] 0377-0273r99r$ see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0377-0273 99 00080-3 ( ) E.A. Nagy et al.rJournal of Volcanology and Geothermal Research 93 1999 1–30 2 Guaymas fracture zone. Rotational deformation north of the PVP may have begun contemporaneously with this adjustment along the Gulf Extensional Province rift margin. q 1999 Elsevier Science B.V. All rights reserved.