How have thick evaporites affected early seafloor spreading magnetic anomalies in the Central Red Sea?

Abstract

SUMMARYThe axial region of the Central Red Sea has been shown to be floored by oceanic crust, but this leaves the low amplitudes of off-axis magnetic anomalies to be explained. Furthermore, if seafloor spreading occurred in the late Miocene, it is unclear how that occurred as widespread evaporites were being deposited then and may have covered the spreading centre. In this study, we derive crustal magnetization for a constant-thickness source layer within the uppermost basement by inverting aeromagnetic anomalies using basement depths derived from seismic reflection and gravity data. Peak-to-trough variations in magnetization away from the axis are found to be slightly less than half of those of normal oceanic crust, but not greatly diminished, and hence the magnetic anomalies are mostly reduced by the greater depth of basement, which is depressed by isostatic loading by the evaporites (kilometres in thickness in places). There is no relationship between seafloor spreading anomalies and the modern distribution of evaporites mapped out using multibeam sonar data; magnetizations are still significant even where the basement lies several kilometres under the evaporites. This suggests that magnetizations have not been more greatly affected by alteration under the evaporites than typically exposed oceanic crust. A prominent magnetization peak commonly occurs at 60–80 km from the axis on both tectonic plates, coinciding with a basement low suggested previously to mark the transition to continental crust closer to the coasts. We suggest an initial burst of volcanism occurred at Chron 5 (at ∼10 Ma) to produce this feature. Furthermore, an abrupt change is found at ∼5 Ma from low-frequency anomalies off-axis to high-frequency anomalies towards the present axis. This potentially represents the stage at which buried spreading centres became exposed. In this interpretation, intrusions such as sills at the buried spreading centre led to broad magnetic anomalies, whereas the later exposure of the spreading centre led to a more typical development of crustal magnetization by rapid cooling of extrusives.