Complex magnetization of a 15.6 my old transitionally-magnetized lava flow: geomagnetic field change during cooling or lightning-induced remagnetization?

Abstract

A paleomagnetic and rock magnetic study of a lava flow (VC 9 in the Sheep Creek range of North Central Nevada) suspected of recording extremely rapid field change demonstrates that its inhomogeneous remanence is more likely the result of a lightning-induced remagnetization. Detailed thermal demagnetization to 570?C of 14 specimens from a vertical profile through the flow shows that all but the top two samples have a multicomponent remanent magnetization. All specimens shared a NNW and up component and the secondary ESE and down component was smallest at the base of the flow, grew toward the middle of the flow, and tapered off toward the top of the flow completely disappearing at the top of the flow. Rock magnetic studies of selected specimens, however, shows that there are significant vertical inhomogeneities in the magnetic properties of the lava flow. AF demagnetization of ARM, MDFIRM, and susceptibility-temperature experiments all indicate that the magnetic mineralogy of the two groups of specimens is significantly different. In particular, these parameters suggest that the specimens displaying the ESE overprint would be more susceptible to remagnetization by lightning. Additionally, the flow directly above (flow VC 9.5) displays a remanent magnetization of NNW and up, the direction the field was pointing when flow VC 9 first started cooling below 570?C. Thus, it is unlikely that there was significant field change between the emplacements of flows VC 9 and VC 9.5.