CHRONOLOGICAL CONSTRAINTS ON THE THERMAL AND TILTING HISTORY OF THE SIERRA-SAN-PEDRO-MATIR PLUTON, BAJA-CALIFORNIA, MEXICO, FROM U PB, AR-40/AR-39, AND FISSION-TRACK GEOCHRONOLOGY/

The tectonothermal history of the four major phases of the Sierra San Pedro Martir pluton and surrounding metamorphic rocks of the Mesozoic Peninsular Ranges batholith of Baja California is presented on the bas is of U/Pb, Ar-40/Ar-39 step-heating, and fission-track dating, in com bination with Al-in-hornblende geobarometry, A previous model proposed up to 90 degrees 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 c ountry rocks, from greenschist to amphibolite facies. The U/Pb data su ggest that the different phases of the pluton were emplaced sequential ly from west to east between 97.0 (+4)/(-1) Ma and 93.8 (+1)/(-1) Ma, All except one of the 105 Ar-40/Ar-39 age spectra have well-defined pl ateaus and are interpreted as cooling ages. Samples from the pluton gi ve hornblende and biotite Ar-40/Ar-39 plateau dates and apatite fissio n-track dates that young from west to east; thus, hornblende dates dec rease from 95 to 91 Ma, biotite dates decrease from 94 to 88 Ma, and a patite dates decrease from 72 to 57 Ma. Muscovite, biotite, and plagio clase from the same rock sample collected at the easternmost phase of the pluton yield concordant Ar-40/Ar-39 dates of 88 Ma, The exposed pa rt of the pluton underwent rapid cooling (approximate to 40 degrees C/ Ma) down to approximate to 250 degrees C in the first 10 m.y. after in trusion. Modeling of track-length distribution in apatite is consisten t 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, Ea stward younging of Ar-40/Ar-39 and fission-track dates may be explaine d by approximate to 15 degrees of east-side-up tilting of the pluton a t 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 r egional-scale crustal extension associated with the opening of the Gul f of California in Neogene time, Such tilting is in agreement with the Al-in-hornblende geobarometry for the hornblende-biotite intrusive ph ase that yields pressures of 5.2 +/- 0.6 kbar, An approximate to 15 de grees northeast-side-up tilt of the crustal block containing this plut on would explain the apparent paleomagnetic inclination discrepancies with cratonic North America and militates against large-scale northerl y transport of Baja California.