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

Geologic mapping of volcanic strata of the northern Puertecitos Volcanic Pr ovince (PVP) in northeastern Baja California, Mexico, performed in conjunct ion with Ar-40/Ar-39 analysis and petrochemical study, documents the Miocen e geologic history of a well-preserved volcanic succession within the north ern 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 San ta Isabel Wash region, span pre-17 to 15 Ma. Minor rift-related volcanism o ccurred at similar to 12.5 and similar to 9 Ma, prior to voluminous PVP-for ming volcanism at similar to 6-6.5 Ma. Isochron ages typically exhibit prec ision (1 sigma) for plagioclase of +/-2-7% and for anorthoclase and sanidin e of +/-2-5%, and replicate analysis of an internal anorthoclase standard i ndicate similar to 1-2% reproducibility within a given irradiation and simi lar to 2.5% for samples irradiated separately. Improved local correlations made possible by the rich stratigraphic section preserved in Santa Isabel W ash 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 geol ogic ties across the Gulf of California. The combined mapping and age resul ts 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 Matomi a ccommodation zone are also documented. Results support a transitional plate boundary model which implies that much of the Pacific-North America relati ve plate motion north of Delfin basin (i.e., the northernmost Gulf of Calif ornia) is accommodated on N- to NNW-striking faults developed during Late M iocene ENE-directed extension. The model predicts a zone of divergence east of the PVP which provides a structural mechanism for the positions and jum ps of nearby Gulf of California spreading centers (Upper and Lower Tiburon and Delfin basins) since 6 Ma, and relates major pulses of PVP volcanism at similar to 6 and similar to 3 Ma to these offshore spreading center adjust ments. Results also imply that most extensional deformation in Santa Isabel Wash is the result of incorporation of the PVP into the Gulf Extensional P rovince similar to 2-3 Ma due to northwestward propagation of the Guaymas f racture zone. Rotational deformation north of the PVP may have begun contem poraneously with this adjustment along the Gulf Extensional Province rift m argin. (C) 1999 Elsevier Science B.V. All rights reserved.