Motion between the Indian, Antarctic and African plates in the early Cenozoic

We used a three-plate best-fit algorithm to calculate four sets of Euler rotations for motion between the India (Capricorn), Africa (Somali) and Antarctic plates for 14 time intervals in the early Cenozoic. Each set of rotations had a different combination of data constraints. The first set of rotations used a basic set of magnetic anomaly picks on the Central Indian Ridge (CIR), Southeast Indian Ridge (SEIR) and Southwest Indian Ridge (SWIR) and fracture zone constraints on the CIR and SEIR, but did not incorporate data from the Carlsberg Ridge and did not use fracture zones on the SWIR. The second set added fracture zone constraints from the region of the Bain fracture zone on the SWIR which were dated with synthetic flowlines based on the first data set. The third set of rotations used the basic constraints from the first rotation set and added data from the Carlsberg Ridge. The fourth set of rotations combined both the SWIR fracture zone constraints and the Carlsberg Ridge constraints. Data on the Indian Plate side of the Carlsberg Ridge (Arabian Basin) were rotated to the Capricorn Plate before being included in the constraints. Plate trajectories and spreading rate histories for the CIR and SWIR based on the new rotations document the major early Cenozoic changes in plate motion. On the CIR and SEIR there was a large but gradual slowdown starting around Chron 23o (51.9 Ma) and continuing until Chron 21y (45.3 Ma) followed 2 or 3 Myr later by an abrupt change in spreading azimuth which started around Chron 20o (42.8) Ma and which was completed by Chron 20y (41.5 Ma). No change in spreading rate accompanied the abrupt change in spreading direction. On the SWIR there was a continuous increase in spreading rates between Chrons 23o and 20o and large changes in azimuth around Chrons 24 and 23 and again at Chron 21. Unexpectedly, we found that the two sets of rotations constrained by the Carlsberg Ridge data diverged from the other two sets of rotations prior to anomaly 22o. When compared to rotations for the CIR that are simultaneously constrained by data from all three branches of the Indian Ocean Triple Junction, there is a progressively larger separation of anomalies on the Carlsberg Ridge, with a roughly 25 km misfit for anomaly 23o and increasing to over 100 km for anomaly 26y. These data require that there was previously unrecognized convergence somewhere in the plate circuit linking the Indian, Capricorn and Somali plates prior to Chron 22o. We quantify this motion by summing our new Capricorn–Somalia rotations with previously published rotations for Neogene India–Capricorn motion and for early Cenozoic Somali–India motion based solely on Carlsberg Ridge data. The most likely possibility is that there was motion within the Somalia Plate due to a distinct Seychelles microplate as young as Chron 22o. The sense of the misfit on the Carlsberg Ridge is consistent with roughly 100–150 km of convergence across a boundary passing through the Amirante Trench and extending north to the Carlsberg Ridge axis between anomalies 26y and 22o. Alternatively, there may have been convergence within the Indian Plate, either along the western margin of Indian or east of the CIR in the region of the current Capricorn–Indian diffuse plate boundary. Our work sharpens the dating of the two major Eocene changes in plate motion recognized in the Indian Ocean.