Paleomagnetism, paleointensity and geochronology of Miocene basalts and baked sediments from Velay Oriental, French Massif Central

Paleomagnetic study of Miocene basalts and baked sediments from Velay Oriental, France, yields a paleomagnetic pole with coordinates 84.1 degrees N, 171.2 degrees E (A(95) = 8.6 degrees, K = 29, N = 11). The pole is supported by a positive reversal test, and it corresponds well with the apparent polar wander path for Eurasia for 10 Ma. Paleosecular variation, estimated as the angular standard deviation of the VGP distribution 14.9 degrees +6.5 degrees/-3.5 degrees, is close to expected, suggesting that the paleomagnetic pole represents a time-averaged field. Moreover, five new 40Ar/39Ar plateau ages, falling between 9.20 and 13.55 Ma, were obtained. Thellier paleointensity experiments were carried out on both basalts and baked sediments. Almost all basalts are chemically unstable during the laboratory heatings and of the 36 preselected samples only three yield usable results. In contrast, the baked sediments are ideal for Thellier experiments with 11 out of 13 samples giving reliable paleointensity estimates. Rock magnetic experiments show that the main ferromagnetic minerals in the baked sediments are fine-grained magnetite and hematite with a large proportion of superparamagnetic grains. In addition to the standard partial thermoremanent magnetization (pTRM) check, used in Thellier experiments to detect chemical/crystalline alteration, we introduce a second "pTRM tail" check designed to test the basic prerequisite of the method, namely, the equality of unblocking and blocking temperatures. For the baked sediments, unblocking and blocking temperatures are found to be identical, which indicates that the remanence is carried by single-domain grains.