Middle Neoproterozoic (Tonian) polar wander of South China:
Paleomagnetism and ID-TIMS U-Pb geochronology of the Laoshanya Formation
Abstract
Paleomagnetic records of middle Neoproterozoic (820-780 Ma) rocks
display high amplitude directional variations that lead to large
discrepancies in paleogeographic reconstructions. Hypotheses to explain
these data include rapid true polar wander, a geomagnetic field geometry
that deviates from a predominantly axial dipole field, a hyper-reversing
field (> 10 reversals/Ma), and/or undiagnosed
remagnetization. To test these hypotheses, we collected 1057 oriented
cores over a 85 m stratigraphic succession in the Laoshanya Formation
(Yangjiaping, Hunan, China). High precision U-Pb dating of two
intercalated tuff layers constrain the age of the sediments between 809
and 804 Ma. Thermal demagnetization isolates three magnetization
components residing in hematite which are not time-progressive but
conflated throughout the section. All samples possess a north and
downward directed component (in geographic coordinates) at temperatures
up to 660°C that is ascribed to a Cretaceous overprint. Two components
isolated above 660°C reveal distinct directional clusters: one is
interpreted as a depositional remanence, while the other appears to be
the result of a mid-Paleozoic (460-420 Ma) remagnetization, which is
likely widespread throughout South China. The high-temperature
directions are subtly dependent on lithology; microscopic and rock
magnetic analyses identify multiple generations of hematite that vary in
concentration and distinguish the magnetization components. A comparison
with other middle Neoproterozoic paleomagnetic studies in the region
indicates that the sudden changes in paleomagnetic directions, used
elsewhere to support the rapid true polar wander hypothesis (ca. 805
Ma), are better explained by mixtures of primary and remagnetized
components, and/or vertical axis rotations.