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

Abstract

Geologic mapping of volcanic strata of the northern Puertecitos Volcanic Province (PVP) in northeastern Baja California, Mexico, performed in conjunction with 40Ar/39Ar 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 exhibit precision (1s) for plagioclase of ą 2-7% and for anorthoclase and sanidine of ą 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 Matomí accommodation zone are also documented. Results support a transitional plate boundary model which implies that much of the Pacific-North America relative plate motion north of Delfín basin (i.e., the northernmost Gulf of California) is accommodated on N- to NNW-striking faults developed during Late Miocene ENE-directed extension. The model predicts a zone of divergence east of the PVP which provides a structural mechanism for the positions and jumps of nearby Gulf of California spreading centers (Upper and Lower Tiburón and Delfín 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 Guaymas fracture zone. Rotational deformation north of the PVP may have begun contemporaneously with this adjustment along the Gulf Extensional Province rift margin.

Appendix A: Petrographic Descriptions & Stratigraphic Associations (download as *.rtf format)

Appendix B: Mineral Compositions from Electron Microprobe Analyses (download as *.rtf format)

	Figure 1. Simplified geologic map of a portion of northeastern Baja California, Mexico (modified from Gastil et al., 1975) showing location of the Santa Isabel Wash study area (Figure 2). The San Pedro Mártir fault (SPMf) marks the western edge of the Gulf Extensional Province north of the Puertecitos Volcanic Province. The pre-6 Ma Matomí accommodation zone (A-A’ after Stock (in press) and A'-A" after Nagy (1997, in review)) separates a region of greater extension to the north from a less extended area to the south. CNf (Cuervo Negro fault); MC (Mesa Cuadrada); MT (Mesa El Tábano); SIf (Santa Isabel fault); SSFf (Sierra San Felipe fault); VSFf (Valle de San Felipe fault). Inset: GEP (Gulf Extensional Province); MGE (Main Gulf Escarpment); SBf (San Benito fault); SJ (Sierra Juárez); SLT (Sierra Las Tinajas); T-Af (Tosco-Abreojos fault).
	Figure 2. Simplified geologic map (after Nagy, 1997) of Santa Isabel Wash in the northern Sierra Santa Isabel (location in Figure 1). Informal names given here include Santa Isabel Wash, Arroyo Oculto, and Cuervo Negro and Santa Isabel faults. Stratigraphic column includes 40Ar/39Ar ages determined in this study (see Tables 1 and 2). The labels for the Miocene units indicate whether the rock is classified as a rhyolite, dacite, andesite, basalt, or volcaniclastic sediment by an "r", "d", "a", "b", or "vs", respectively, following the "T" (Tertiary) and "m" (Miocene). Subscripts are abbreviations of informal unit names. One anomalously young sample age for Tmrsiw, interpreted to be the result of glass contamination, is not listed. The stratigraphic relationship between group 7 lava flows is not apparent in the field except that Tmrcan clearly overlies Tmagem. Fault symbols: dashed faults are approximately located, dotted faults are concealed (inferred), and faults with the letter “s” along them are features in the Quaternary alluvium interpreted to be fault scarps. A’-A” and B’-B” mark the positions of two NE-facing paleo-topographic slopes across which ~ 6-6.5 Ma tuffs (group 6) thicken considerably from SW to NE. The paleo-topographic slopes are interpreted to be fault-controlled and associated with the southwestern margin of the pre-6 Ma Matomí accommodation zone.v
	Figure 3.Isochron plots of 21 rock samples from Santa Isabel Wash dated by 40Ar/39Ar geochronology. Error bars on individual data points are ą 1s. Isochron ages (ą 2s) are also given. Samples labeled ”2W” were subjected to a low temperature fusion step (see Appendix 1) and those labeled “gamma” were exposed to a 137Cs g-radiation source to facilitate mineral separation. Letters in plots (q) and (r) refer to different splits of our INF-94-53 anorthoclase internal standard that were distributed throughout sample tubes for both irradiations (see Table 1). Sample labeled ”MIT” (q) is the same anorthoclase separate (INF-94-53) measured independently at the MIT Cambridge Laboratory for Argon Isotopic Research. All samples of the internal standard from irradiation UofM#81 and the MIT analysis were also exposed to g-radiation prior to neutron bombardment.
	Figure 4. Stratigraphic columns showing preferred correlations between group 2, 3, 4, and 5 rocks defined in Santa Isabel Wash and those described by other workers in nearby localities. Stratigraphy from other studies has been simplified by listing only the lithologic units relevant to the discussion. All ages are ą 2s. Solid lines indicate preferred correlations; dashed lines show unresolved ambiguities.
	Figure 5. Stratigraphic columns showing preferred correlations between group 6 and 7 rocks defined in Santa Isabel Wash and those described by other workers in nearby localities. The study area of Martín-Barajas and others (1995) includes the region between Arroyo Matomí and Arroyo Los Heme from the coast to ~ 10 km west (Figure 1). In some cases the stratigraphy from other studies has been simplified by listing only the lithologic units relevant to the discussion. Ages are ą 2s. The date with the ”” is from the same Tmr3* mineral separate analyzed here measured in a different 40Ar/39Ar laboratory (Appendix C available from http://oro.ess.ucla.edu/PVP/santa_isabel.html; see also Nagy, 1997). Solid lines indicate preferred correlations; dashed lines show unresolved ambiguities. The overall stratigraphic order of the various cooling units in the Tmr3 position from different areas is unclear; however, paleomagnetic studies suggest that deposits in Santa Isabel Wash are not the same as those from the identical stratigraphic position in the nearby regions. Note that units t3, t4, t2u, and t2l in Arroyo Matomí are also defined as Mpru by Stock (1989, 1993). Stratigraphic order of some group 7 units in Santa Isabel Wash is ambiguous.
	Figure 6. Schematic block models of pre-6 and post-6 Ma deformation in Santa Isabel Wash in the northern PVP. Top: Prior to 6 Ma most of the northern Sierra Santa Isabel was located southwest of the NW-striking Matomí accommodation zone (simplified; see Figure 2 for segmentation) which separated the Gulf Extensional Province to the northeast from the unextended regions to the southwest. Elevation differences across the accommodation zone of various lithologic contacts imply 300-350 meters NE-side-down displacement. A strike-slip component of motion along the zone is not evident in this area. NE- to ENE-directed extension is inferred from the orientation of the Matomí accommodation zone and its structural relationship with the San Pedro Mártir fault (see Nagy (in review) for details). Bottom: Most faults in the study area displace all mapped units and are thus post-6 Ma features. About 500 meters of E-side-down displacement of ~ 12.5 Ma (group 4) and ~ 6.3-6.6 Ma (group 6) pyroclastic flow deposits is evident across the Cuervo Negro and Santa Isabel faults. Slickenlines and fault orientations suggest ENE- to E-directed extension. Individual ~ 6.3-6.6 Ma (group 6) tuffs thicken from 1-2 meters southwest of the Matomí accommodation zone to 40-80 meters to the northeast.
	Figure 7. Present-day Pacific-North America plate boundary in the northern Gulf of California region. Solid/striped patterns represent active/extinct spreading centers. Plate motion direction after Atwater and Stock (1998). The Wagner Transition Zone is proposed as a region of diffuse plate boundary deformation which includes ENE-directed extension (double-headed arrow), clockwise vertical-axis rotations, and dextral, perhaps oblique, strike-slip motion along N- to NNW-striking faults (depicted schematically through Wagner Basin). Boundaries (dashed lines) are oriented perpendicular to Pacific-North America relative plate motion. A zone of divergence created at the southeast margin of the Wagner Transition Zone offers one explanation for the positions and jumps of nearby spreading centers (Upper and Lower Tiburón and Delfín basins) since ~ 6 Ma and for the timing of incorporation of the Puertecitos Volcanic Province into the Gulf Extensional Province in Pliocene time. Santa Isabel Wash is presently within the GEP (open circle). See Nagy and Stock (in review) for structural evolution of the Wagner Transition Zone since the late Miocene.

*Group*	*Units*
1	Pz, Mzg
2	Tmvs, Tmr_bio
3	Tmb_kc, Tmb_lol, Tmd_tomb
4	Tmr_sf
5	Tma_toro
6	Tmr_siw, Tmb_new, Tmr3, Tmr_3-4, Tmr4, Tmr_ao, Tmr_ec, Tmr_bs, Tmr_fp
7	Tma_gem, Tmr_can, Tma_ugl, Tma_hem

*Group*	*Unit*
3	Tmb_kc, Tmb_lol, Tmd_tomb
4	Tmr_sf
5	Tma_toro
6	Tmr_siw, Tmb_new, Tmr3, Tmr_ec
7	Tma_gem, Tmr_can, Tma_hem

*Stratigraphic Group*	*Sample*	*Mineral*
7	CAN-95-11	Oligoclase
7	CAN-95-12	Oligoclase
6	CHO-94-45	Oligoclase
6	INF-94-33	Oligoclase
6	INF-94-47	Oligoclase
6	INF-94-52	Andesine
6	INF-94-68	Labradorite
6	INF-94-75	Anorthoclase
5	INF-95-24	Andesine
4	INN-94-73	Andesine
3	KC-95-17	Andesine
3	KC-95-19	Andesine
3	LCR-94-110	Oligoclase
3	LCR-95-08	Andesine
3	LCS-94-72	Anorthoclase
3	PHO-94-10	Andesine
3	PHO-94-86	Andesine
3	SUN-94-30	Oligoclase
2	WIN-94-32	Andesine