Geochronological constraints on the tectonic history of the Peninsular Ranges Batholith of Alta and Baja California: Tectonic implications for western México

Abstract

A compilation of existing age data and new age determinations (U/Pb zircon, 4 0 Ar/ 3 9 Ar step-heating, K/Ar, Rb/Sr, and apatite fission-track dates) for the Peninsular Ranges batholith of Alta and Baja California provides geochronological constraints on the tectonic history of the Peninsular Ranges batholith and tectonic implications for western Mexico. The plutons of the Peninsular Ranges batholith ofAlta and Baja California between the 28°N and 34°N parallels were emplaced from west to east between ca. 140 to 80 Ma (U/Pb zircon dates) and display 4 0 Ar/ 3 9 Ar hornblende and biotite plateau dates that range from 118 to 83 Ma and 116 to 80 Ma, respectively, and biotite K/Ar dates as young as 65 Ma. Rapid cooling is indicated by the small differences in U/Pb zircon and cogenetic 4 0 Ar/ 3 9 Ar dates. Mineral pairs having mainly concordant dates yield 4 0 Ar/ 3 9 Ar cooling ages that also systematically decrease from southwest to northeast. Also, monotonic eastward younging of ages across various plutons was found as was previously reported for the Sierra San Pedro Martir pluton. This systematic regional and local eastward younging may be explained by the systematic tilting of pluton-sized crustal blocks containing individual plutons. The northeastward decrease in ages across the Peninsular Ranges batholith and across some of its plutons is therefore attributed to regional eastward migration of granitic intrusion foci combined with rapid differential exhumation histories and superimposed east-side-up tilting of crustal-sized blocks containing some of the plutons and beautifully exemplifies the short time lapse between batholith intrusion and unroofing. Additionally, apatite fission-track dates from samples collected across the Peninsular Ranges, from Ensenada to San Felipe, Baja California, establish that there is also an eastward younging of ages from 104 Ma to 51 Ma from west to east and also across plutons. The fission-track dates indicate that from west to east, present exposures of much of the western Peninsular Ranges batholith have been within «3 km of the Earth's surface (assuming a normal geothermal gradient) since Early Cretaceous time, and that by early Tertiary time, much of the eastern Peninsular Ranges had cooled to temperatures of <110 °C, indicating rapid exhumation histories and superimposed east-side-up tilting shown by younger ages at higher elevations within plutons. The batholith was sufficiently uplifted during the Late Cretaceous such that 10-15 km of material was eroded off the eastern part of the batholith by early Paleocene time. Furthermore, a regional compilation of the available age data for western Mexico provides geochronological constraints for the testing of controversial paleomagnetic and geological models for the tectonic evolution of the Mesozoic Peninsular Ranges of Alta and Baja California. The controversy involves different perceptions of the geologic development of western Mexico and southernmost California as derived from paleomagnetic studies and regional geology. Whereas the geological data and plate tectonic reconstructions seem to indicate a northward motion between 300 and 500 km with respect to the rest of North America since the Cretaceous, paleomagnetic data suggest much greater movement involving at least 2500 km of northward translation for the Baja peninsula during the same period.