Paleomagnetic directions and 40Ar/39Ar ages from the Tatara-San Pedro volcanic complex, Chilean Andes: Lava record of a Matuyama-Brunhes precursor?

[1] Lava flows within Quebrada Turbia valley of the Tatara San Pedro volcanic complex, in central Chile (36 degreesS, 289 degreesE), preserve a detailed record of what was presumed to be the Matuyama-Brunhes geomagnetic reversal. A sampling traverse in the central west wall of the valley yields reverse and transitional polarity flows of the Quebrada Turbia sequence. Two kilometers to the north, another section yields 17 transitional flows from the same volcanic sequence overlain by flows with normal polarity. The 40Ar/39Ar incremental heating experiments on lavas within the two sections provide nine independent age determinations and yield a weighted mean of 791.7 +/- 3.0 ka (+/- 2sigma) for the paleomagnetic transition. The sections are linked by geological mapping, the precise radioisotopic dating, and geochemical correlations. Alternating field and thermal demagnetization studies, rock magnetic analyses, and petrographic observations indicate that the magnetization is primary and carried by titanomagnetite. The polarity change is characterized by a jump from reverse poles to a quasi-stationary cluster of virtual geomagnetic poles over Australia, followed by a jump to normal polarity latitudes. Magnetization of these lavas is thus consistent with either a brief period when the field was dominated by a subequatorial dipole, or a more complex nondipolar field that may reflect the influence of a long-lived regional lower mantle control over a weakened dynamo. The Quebrada Turbia lavas are circa 16 kyr older than those, dated by exactly the same methods, which record a later more complex portion of the reversal at Haleakala volcano, Maui. Moreover, the 792 ka radioisotopic age of these Chilean lavas is older than most astronomical estimates for the Matuyama-Brunhes reversal suggesting that this section may, in fact, record a precursor to the actual field reversal, that is expressed by low paleointensities in more than a dozen well-studied marine sediment cores.