Abstract

The tectonothermal history of the four major phases of the Sierra San Pedro Mártir pluton and surrounding metamorphic rocks of the Mesozoic Peninsular Ranges batholith of Baja California is presented on the basis of U/Pb, 40Ar/39Ar step-heating, and fission-track dating, in combination with Al-in-hornblende geobarometry. A previous model proposed up to 90° of east-side-up tilting of the pluton, exposing >20 km of crustal section to account for its crescent shape, asymmetrical zoning, internal structure, the eastward younging of K-Ar dates across the intrusion and eastward increase in the metamorphic grade of the country rocks, from greenschist to amphibolite facies.

The U/Pb data suggest that the different phases of the pluton were emplaced sequentially from west to east between 97.0 +4/−1 Ma and 93.8 +1/−1 Ma. All except one of the 105 40Ar/39Ar age spectra have well-defined plateaus and are interpreted as cooling ages. Samples from the pluton give hornblende and biotite 40Ar/39Ar plateau dates and apatite fission-track dates that young from west to east; thus, hornblende dates decrease from 95 to 91 Ma, biotite dates decrease from 94 to 88 Ma, and apatite dates decrease from 72 to 57 Ma. Muscovite, biotite, and plagioclase from the same rock sample collected at the easternmost phase of the pluton yield concordant 40Ar/39Ar dates of 88 Ma. The exposed part of the pluton underwent rapid cooling (approximately equal to 40 °C/Ma) down to ≈250 °C in the first 10 m.y. after intrusion. Modeling of track-length distribution in apatite is consistent with monotonic slow cooling from ca. 80 Ma to the present.

The data do not support a history that includes major tilting of the pluton. Eastward younging of 40Ar/39Ar and fission-track dates may be explained by approximately equal to 15° of east-side-up tilting of the pluton at or after 88 Ma about a north-south horizontal axis. Furthermore, the fission-track data suggest that part or all of this tilting may have taken place at or after 57 Ma, and therefore may be a consequence of regional-scale crustal extension associated with the opening of the Gulf of California in Neogene time. Such tilting is in agreement with the Al-in-hornblende geobarometry for the hornblende-biotite intrusive phase that yields pressures of 5.2 ± 0.6 kbar. An approximately equal to 15° northeast-side-up tilt of the crustal block containing this pluton would explain the apparent paleomagnetic inclination discrepancies with cratonic North America and militates against large-scale northerly transport of Baja California.

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