Opening and Spreading Processes of the Eastern Paleo-Tethys Ocean: Paleomagnetic Insights from the South China Block

The scarcity of high-quality Paleozoic paleomagnetic data from the South China Block (SCB) has rendered the kinematic history of its rifting and drifting from Gondwana highly debated, consequently leaving the timing of opening and subsequent spreading of the Ailaoshan-Songma eastern Paleo-Tethys Ocean (ASePTO) poorly constrained. Here we report new paleomagnetic results from Givetian (~385 Ma) limestones of Dushan County, Guizhou, yielding a characteristic remanent magnetization (ChRM-B). Although the ChRM-B predominantly shows single polarity and an inconclusive fold test, multiple lines of evidence support its primary origin: (i) the corresponding paleomagnetic pole (29.5° N, 229.8° E; dp/dm = 2.0°/3.8°) differs substantially from all post-Middle Devonian SCB poles; (ii) rock magnetic analyses combined with high-resolution transmission electron microscopy indicate that nanoscale detrital magnetite and maghemite are the main remanence carriers; and (iii) statistical consistency between the 7 site-mean directions of the ChRM-B and 34 coeval site-mean directions from the published Yuntaiguan Formation red bed records in western Hunan. In the light of the compatible ages and geological settings of the sampling strata from the two sampling areas, the two datasets were combined to derive a robust Middle Devonian paleomagnetic pole for the SCB. The merged dataset (41 sites) yields mean directions of Dg/Ig = 37.2°/−18.9° (kg = 46.4, α95 = 3.3°) before, and Ds/Is = 43.1°/−23.8° (ks = 54.4, α95 = 3.1°) after the tilt-correction with a B-class reversals test and a positive fold test. The new robust paleomagnetic pole for the SCB, at 33.2° N, 234.7° E with A95 = 2.4° and reliability index R = 7, when integrated with other reliable paleomagnetic records, allows the construction of an updated apparent polar wander path (APWP) for the SCB. Comparison with the synthetic APWP of Gondwana enables the reconstruction of the opening of the ASePTO at ~410–400 Ma, followed by the spreading of the ocean basin to a N-S width of ~1200 km by ~385 Ma.