Late Precambrian Keweenawan asymmetric reversals

Abstract

Palaeopoles from the Keweenawan (1,200–1,000 Myr) rocks of the Lake Superior region form the western and eastern arms of the ‘Great Logan Palaeomagnetic Loop’^1,5 (Fig. 1a). This is defined by both normal (N) and reversed (R) poles which are the consequence of at least two geomagnetic reversals^4,6,7. The younger one (R→N) has been detected throughout the Lake Superior region from various rock types.^1–6 The older reversal (N→R) is only recorded in the lowermost Keweenawan lavas in the southern part of the lake.⁸ Wherever found, the reversals depart from the 180° symmetry (Fig. 1a)^2–6. Due to these asymmetries, the Logan Loop has large segments that are devoid of data and thus the geophysical interpretations of the Loop (or ‘hairpin’) critically depend on the cause of the reversal asymmetry.⁴ Four models¹¹ (secondary component⁶, apparent polar wander^1–5, Wilson's dipole offset^5,7,9 and non-dipole field configuration^1,3,10) have been put forward to explain the asymmetries. Here we demonstrate that the two-dipole field configuration model can explain the reversal asymmetries and discuss the global effects of this model in the light of worldwide late Precambrian palaeomagnetic data.