Paleomagnetic and geochronological identification of the Réunion subchron in Ethiopian Afar

Abstract

This paper reports the paleomagnetic and geochronological analysis of a suite of samples from a basaltic fault‐related scarp of Pliocene age in the central part of the Afar depression (Ethiopia). Paleomagnetic work uncovers a clear characteristic remanent magnetization carried by (titano‐) magnetites, with high unblocking temperatures, close to the Curie temperature of pure magnetite. The characteristic direction, based on 27 non transitional flows, is D=8.1°, I=10.1° (α95=4.1°). This is consistent with earlier determinations of the clockwise tectonic rotation, due to rift propagation and overlap, of the block to which the section belongs. Magnetic stratigraphy consists of a succession from bottom to top of 16 reversed, 7 normal, and 4 reversed flows. One flow at the upper normal to reverse transition demonstrates complex behavior upon thermal and alternating field (AF) demagnetization, which is due to remagnetization by the overlying flow. K/Ar dating of five samples provides consistent determinations, averaging 2.07 ± 0.05 Ma. All ages agree with this average, given their rather large individual uncertainties (from 0.04 to 0.08 Ma at the 1σ level). Comparison with recent reference geomagnetic polarity timescales (GPTS) indicates that the normal subchron recorded in the Gamarri section must be linked with (one of) the Réunion event(s). The normal subchron and an earlier episode of large secular variation could provide the best volcanic record of a double Réunion event. Although uncertainties in ages are too large to allow unequivocal inferences, our age determination for the Réunion event is in better agreement with earlier determinations of McDougall and Watkins [1973] than with a more recent value proposed by Baksi et al. [1993] and used in the GPTS.