Palaeomagnetism of the Miocene Farellones formation (Chile)

We report on a detailed palaeomagnetic study of the Miocene Farellones volc anic formation in the Chilean Andes near Santiago (two sections, 37 sites, about 400 orientated cores). Petrological observations show evidence of low -grade metamorphism increasing downwards through the volcanic sequence. Opt ical observations of opaque minerals and magnetic experiments suggest that in many cases maghemitization is associated with hydrothermal alteration. H owever, thermal demagnetization data indicate that the low-grade metamorphi sm did not significantly modify the direction of the primary remanent magne tization recorded at the time of emplacement of the volcanic lava flows. Fo ur intervals of polarity with two intermediate palaeodirections were observ ed in the similar to 650-m-thick composite section. According to the disper sion of flow average directions, palaeosecular variation was slightly large r than that observed in general during the Upper Cenozoic. The site mean di rections obtained in this study differ significantly from the expected Mioc ene direction. Clockwise rotations of up to 20 degrees of small blocks are probably associated with the deformation of the Andean Cordillera since mid dle Miocene times. Geomagnetic palaeointensity data were obtained, using th e Thellier method, on 24 samples from eight distinct lava flows. The flow m ean VDM varies from 1.4 to 4.0 x 10(22) A m(-2). Altogether, our data seem to suggest the existence of a relatively low geomagnetic field undergoing l arge fluctuations. Although a linear relationship was observed between the natural remanent magnetization and the thermal remanent magnetization acqui red during the Thellier-Thellier experiments, undetected chemical alteratio n of the magnetic minerals during hydrothermalism may also explain the unus ually low palaeointensity obtained.