Quaternary remagnetization of the Neoproterozoic limestone of Negash Synclinorium (Arabian–Nubian Shield, northern Ethiopia): With implications of no paleomagnetic testing for the proposed Snowball Earth events

Abstract

Eighty-one paleomagnetic cores were collected from 10 locations across a black limestone unit within the core of Negash Synclinorium, northern Ethiopia in order to test the proposed Snowball Earth events for the diamictite unit of the Tambien Group. Cores were cut into two standard paleomagnetic specimens and were subjected to stepwise demagnetizations using both Thermal (TH) and alternating field (AF) techniques. Rock magnetic analyses on representative specimens were done and results revealed goethite, pyrrhotite, titano-magnetite, and titano-hematite to be the major magnetic materials carrying the magnetizations with PSD (pseudo single domain) grain size range. In most cases paleomagnetic directions are defined by a single component of magnetization, where a viscous component is present it is usually removed by heating to a temperature of ∼200°C or an AF of ∼10mT. The high stability component isolated above temperature of 200°C or AF of 15mT, defined straight line trajectories directed towards the origin and considered as the Characteristic Remanent Magnetization Direction (ChRM). The direction of magnetization of the ChRM is determined for samples with stable straight line segments by the best-fit line using the least square technique of Kirschvink (1980). In the cases of overlapping spectra and unblocking temperatures, direction of magnetization is determined by remagnetization circles of Halls (1976, 1978). When site mean ChRM directions are plotted on stereogram, their distribution is relatively clustered in geographic coordinates and the overall mean direction is Decg=358.5°, Incg=16.6° (α95=3.8°, K=162.8, N=10). After a structural restoration to the horizontal is made the directions disperse and fail the fold test of both McElhinny’s and McFadden’s tests and the mean direction is Decs=353.5°, Incs=8.8° (α95=18.9°, K=7.5, N=10). This is interpreted to result from a later remagnetization of the black limestone. All directions are normal in polarity and have mean unrestored paleomagnetic directions comparable to the Quaternary paleomagnetic directions. Virtual Geomagnetic poles (VGP) in the unrestored position is used to calculate overall mean VGP position resulting long=235.7°E, latg=84.5°N (A95=3.0°, N=10). Comparison of the obtained pole with the apparent polar wander path (APWP) curve for Africa of Besse and Courtillot (1991, 2003) and with the 2Ma reference pole of stable Africa (Kidane et al., 2003) is found to be consistent with remagnetizations during the Quaternary period. Hence supporting evidence for the proposed Snowball Earth event of the Sturtian glaciation in the Negash rocks could not, unfortunately, be obtained from paleomagnetism.