Magnetostratigraphic constraints on the Gondwanan origin of North China: Cambrian/Ordovician boundary results

Abstract

Summary A significant gap in the middle Palaeozoic apparent polar wander (APW) path precludes polarity definition of the early Palaeozoic palaeopole for North China. This problem can in part be resolved by the intercontinental correlation of magnetic polarity patterns across small time intervals. A magnetostratigraphic study was carried out on upper Cambrian to lower Ordovician sediments near Zhaogezhuang (long. 118.5°E, lat. 39.7°N), North China. After stepwise thermal or thermal and alternating field demagnetizations, a characteristic magnetic component with normal and antipodal reversed directions was identified. These data, drawn from 49 samples, yield a north palaeopole at long. 294.6°E, lat. 32.9°N (dp = 3.0°, dm = 5.3°). A concordant magnetic polarity pattern around the Cambrian–Ordovician boundary and lowest Ordovician obtained from different continents favours a Southern Hemisphere origin (∼17°) formation site. Using the Cambrian–Ordovician APW paths between North China and Gondwana, we suggest that the North China block (NCB) was part of Gondwana during the Cambro–lowest Ordovician, and started breaking away from Gondwana in the lower Ordovician. This finding is contrary to some palaeomagnetic models where the NCB was quite separate from Gondwana in the late Proterozoic, and was attached to the ‘Pacific’ side of Antarctica.

A third superchron during the Early Paleozoic

“…A few existing magnetostratigraphic studies can test our results for the Arenig and Llanvirn, which correspond to the time interval containing the "Second Stage", the "Third Stage" and the Darriwilian in the International Stratigraphic Chart (Gradstein et al, 2004a, b). All but one (Farr et al 1993) confirm the occurrence of a single reversed polarity interval during the Arenig (Torsvik and Trench, 1991;Trench et al, 1991;Torsvik et al, 1995b;Yang et al, 2002), while several short normal polarity intervals (up to three) have been suggested during the Llanvirn. It is actually possible that short magnetic intervals were missed in our records because of the magnetization lock-in process of hematite which is the predominant magnetic carrier in the Arenig to Llanvirn strata.…”

“…Meert, 1999; and references therein). Although magnetostratigraphic results were obtained from different Lower Ordovician sections from Siberia, China (Ripperdan et al, 1993;Yang et al, 2002) and Australia (Ripperdan and Kirschvink, 1992), the construction of a Tremadocian magnetic polarity time scale still remains a difficult exercise. For drawing correlation lines between the different sections, we use the time constraints provided by the numerous biostratigraphic data based on conodonts and trilobites available from the Batyrbay section in southern Kazakhstan (Rozova, 1986;Dubinina, 2000).…”

“…28, no. 2 (Pavlov and Gallet, 1998), China (Ripperdan et al, 1993;Yang et al, 2002) and Australia (Ripperdan and Kirschvink, 1992) sequences are indicated according to the biostratigraphic results available from the Batyrbay section (Rozova, 1986; Dubinina, 2000). These correlations allow one to propose a preliminary composite magnetic polarity time scale containing the Cambrian/Ordovician boundary.…”

Early orthoceratoid cephalopods from the Argentine Precordillera (Lower-Middle Ordovician)

“…In the widely accepted models of Niocaill et al (1997), Fortey and Cocks (2003), and Cocks and Torsvik (2004), the Precordillera drifted away from low-latitude Laurentia, which is distant from high-latitude West Gondwana and Avalonia, and toward the Argentine margin of Gondwana. In the paleomagnetic reconstructions of Huang et al (2000) and Yang et al (2002), the North China block separated from the eastern margin of Gondwana in the earliest Ordovician and moved eastward in the middle latitudes of the southern hemisphere toward the Argentinian margin of Gondwana. The position of North China is not given, but an entirely Northern Hemisphere position is supposed.…”

Geomagnetic reversal rates following Palaeozoic superchrons have a fast restart mechanism

“…Magnetic polarity data from the underlying late Cambrian are known from Siberia3940, Australia41 and China42 (Supplementary Fig. 2).…”