Dating transitionally magnetized lavas of the late Matuyama Chron: Toward a new 40Ar/39Ar timescale of reversals and events

The K-Ar based geomagnetic polarity timescale was constructed using data from lavas and tuffs that bracketed, but rarely dated, the transitions between polarity intervals. Subsequent 40Ar/39Ar dating indicated that the ages of some polarity transitions had been underestimated by about 6%. Although the accepted ages of the polarity chron boundaries have increased, their precise temporal definition remained uncertain. We have taken a different approach and used incremental-heating techniques to obtain 18 new 40Ar/39Ar ages from basaltic lavas within flow sequences at Punaruu Valley, Tahiti, and Haleakala volcano, Hawaii. These lavas record transitional paleomagnetic directions corresponding to four mid-Pleistocene polarity reversals or events. Three lavas from Punaruu Valley previously thought to record the Cobb Mountain Normal Polarity Subchron (CMNS) gave a mean age of 1.105 +/- 0.005 Ma, indicating that they were erupted about 76 kyr after the CMNS; this period of transitional field behavior is designated the Punaruu event. In addition, seven new 40Ar/39Ar ages from the Punaruu Valley indicate that the Jaramillo Normal Polarity Subchron (JNS) lasted about 67 kyr, starting at 1.053 +/- 0.006 Ma and ending 0.986 +/- 0.005 Ma. This agrees with astronomical estimates but conflicts with JNS ages proposed by Spell ann McDougall [1992] and Izett and Obradovich [1994] on the basis of 40Ar/39Ar dating of rhyolite domes in the Valles Caldera. Indistinguishable 40Ar/39Ar ages of seven lavas, including one from Punaruu Valley and six from Haleakala that record broadly similar intermediate paleodirections, suggest that the Kamikatsura event occurred at 0.886 +/- 0.003 Ma. Moreover, these data indicate that the Kamikatsura event occurred 20-40 kyr after another geomagnetic event, most probably taking place at 0.92 Ma. We designate this earlier field behavior the Santa Rosa event, adopting its name from that of a transitionally magnetized rhyolite dome which happened to figure prominently in the original definition of the end of the JNS in the 1968 study of Doell et al. [1968]. The discovery of these new short-lived polarity events during the Matuyama reversed chron suggests that the 400 kyr period between 1.18 and 0.78 Ma experienced no less than 7 and perhaps more than 11 attempts by the geodynamo to reverse. This newly determined higher frequency of geomagnetic activity illustrates vividly the importance of obtaining precise age control directly from transitionally magnetized rocks.