True polar wander, a supercontinental legacy : Evans D.A., Earth and Planetary Science Letters, 1998, 157/1–2 (1–8)

Abstract

A palaeomagnetic study is reported of reddened facies (Fengtai Formation) of Neoproterozoic glacial rocks which underlie Early Cambrian rocks with disconformity in the Huabei (North China/Sino-Korean) Block. The diamictite (preferred age 620-600 Ma) carries a dual polarity remanence residing in hematite of the red matrix. The mean direction derived from 62 samples is DI = 205.9− 32.4° (α95 = 3.9°) yielding a pole position at 233°E, 62°N. Tests on the matrix deformed beneath dropstones suggest that remanence was fixed before full compaction, although clasts near the base of the formation are largely overprinted. Overlying Early Cambrian sediments of the Houjiashan and Yutaishan formations have a similar remanence also of dual polarity (DI = 205.5− 32.1°, α95 = 3.9°, 32 samples). The magnetisation in the diamictite is therefore interpreted to have been acquired during loading by the ice sheet and/or the overlying Cambrian succession. Red shales from the Liulaobei Formation (∼890-840 Ma) near the base of the Neoproterozoic succession in the Huabei Block yield a contrasting remanence of DI = 59.475.3° (α95 = 7.1°, 19 samples) equivalent to a pole position at 150°E, 43°N. Pole positions from this study accord with Lower-Middle Cambrian poles from the Australian and South China Blocks with North China sited adjacent to northeastern Australia in accordance with recent biogeographic and palaeogeographic models. Although the palaeolatitude derived from the Fengtai diamictite (17°) is Cambrian in age and probably later than the glaciation, the correlation with Australia implies that glaciation in North China took place in low palaeolatitudes (∼20°) and reinforces the view that late Neoproterozoic glaciation, at least at this perimeter of Gondwana, occurred at low latitudes. Neoproterozoic-Cambrian poles from North China, South China and Australia fail to conform to the popular Rodinia reconstruction with the latter blocks adjacent to western North America and also fail to support the view that Gondwana formed by the fusion of widely separated eastern and western segments during Pan African orogeny. Instead they accord to poles from Africa and South America in a precursor of Gondwana (Protogondwana). The subsequent Palaeozoic derivation of this supercontinent was probably achieved by intracontinental strike-slip motion during Pan African deformation following extinction of the Afro-Arabian arc.