Refined spreading history at the Southwest Indian Ridge for the last 96 Ma, with the aid of satellite gravity data

SUMMARY The spreading history of the oceans is modelled mostly by using magnetic anomalies and the fracture zone geometry. The high-quality, satellite-derived gravity data, that became available in recent years, reveal the details of fracture zones, which can be used as flow lines to control spreading models. We have applied this approach to the Southwest Indian Ridge (SWIR) in order to refine its spreading history. This is particularly useful for the period of complex spreading between magnetic anomalies 33 and 23, where the magnetic anomalies alone cannot resolve the detailed spreading history. We find four main stages in the spreading history of the SWIR since 96 Ma, including two that were not noted previously, between 96 Ma and anomaly 33 (76.3 Ma) and between anomalies 23o (51.7 Ma) and 18o (40.1 Ma; o denotes old boundaries of normal magnetization period). We also find that the start of the period of complex spreading was at anomaly 33, somewhat earlier than previously proposed. We discuss the characteristics of the extension that the old transform faults underwent during the complex spreading phase, in response to the counterclockwise rotation of spreading. New transform faults appeared at that time, considerably widening the transform zones.