MAGNETIC STRATIGRAPHY OF PERALKALINE VOLCANISM IN SIERRA LIBRE, SONORA, MEXICO

Middle Miocene (~12 Ma) magmatism in NW Mexico was dominated by the appearance of anorogenic liquids associated with the Proto-Gulf of California. These comprise a few occurrences of mafic volcanic rocks with transitional geochemical signatures, and a larger silicic volcanic event of peralkaline affinity. The silicic event is primarily composed of a large ignimbritic deposit widely recognized in Baja California as the Tuff of San Felipe, and in Sonora as the Hermosillo Ignimbrite. These are correlated by a number of characteristics including a unique low-inclination, reversed magnetization, probably associated with a field transition or a geomagnetic excursion within a reversed polarity subchron at 11.614-12.014 Ma (base of C5r.3r; Lourens et al., 2004). Thick sections of these peralkaline volcanic rocks crop out at Sierra Libre, geographically located ~45 km south of Hermosillo, Sonora. In this locality, a ~180 m thick stack of middle Miocene volcanic units (both pyroclastic and lavas) were sampled for paleomagnetic studies focusing on the magnetic stratigraphy of a set of 11 units (7 to 12 cores per unit) from El Galindro Canyon, which represents the thickest volcanic pile genetically related to Tuff of San Felipe and Hermosillo ignimbrite. Earth’s magnetic field wanders around an average geocentric axial dipole position over timescales of ~10 kyr and occasionally it “jerks” within this locus of normal geomagnetic secular variation, or excursions from it, spanning ~1’s to 10’s of degrees arc over shorter, centennial to millennial timescales. During geomagnetic polarity reversal transitions, Earth’s field changes at larger magnitude over these same, centennial to millennial timescales. Detailed paleomagnetic records of high-fidelity transitional sections can be used to better understand the behavior of the earth's magnetic field during reversals and excursions. Here we show that the Sierra Libre magnetizations wander erratically in declination and inclination, without following a simple sequential ‘‘Path’’, suggesting a rapid transition of the geomagnetic field rather than an excursion.