A miocene polarity transition (R‐N) from the Agno Batholith, Luzon

Abstract

A geomagnetic reversal record has been obtained from the Agno Batholith (16°15′N, 120°39′E) in Luzon Island, the Philippines. This body is a biotite‐hornblende quartz diorite dated at between 14.5 and 15.0 m.y. by the K‐Ar whole rock method. Hysteresis properties show that the magnetic remanence is dominated by multidomain, low‐titanium titanomagnetite. A stable component of magnetization is isolated upon both thermal and alternating field demagnetization. Normal and reversed polarity zones within the batholith give tightly grouped results which, when compared to other Miocene poles from the Baguio area, allow a tilt correction to be made. The resulting reversed‐to‐normal reversal sequence has a far‐sided VGP path and a decrease in intensity of magnetization of approximately a factor of 5, with a longer duration than the directional change. The VGP path is confined in a longitudinal band 120° from the site and so differs from the characteristic reversed‐to‐normal transition paths in that it is not near‐sided. Intermediate field geometry cannot therefore always be the same for individual reversals. The Agno reversal record is consistent with a quadrupole field and reversal initiated in the northern hemisphere. An octupolar intermediate field would require simultaneous flooding from both polar latitude bands.