New limits on the motion between India and Australia since chron 5 (11 Ma) and implications for lithospheric deformation in the equatorial Indian Ocean

SUMMARY We present new aeromagnetic data from a 1990 survey across the Central Indian and Carlsberg ridges between 18°S and 5°N. The 86 new crossings of anomaly 5 fill a gap of more than 2000 km in previously identified crossings. We furthermore present 15 crossings from Project Magnet aeromagnetic data and 79 crossings from shipboard data to add to 56 previously identified crossings used in prior plate reconstruction work. These combine to give a total of 236 crossings of anomaly 5 flanking the Carlsberg Ridge and Central Indian Ridge. Fracture-zone crossings are extracted from satellite-derived gravity profiles from the Seasat, Geosat/ERM, ERS1 and Topex spacecraft-based altimeters giving 177 useful new crossings near anomaly 5 on 22 conjugate palaeotransform fault pairs; these data replace the 23 crossings used in prior plate reconstruction work. These and many other new altimetry crossings along other portions of the fracture zones permit all the fracture zones flanking the Central Indian and Carlsberg ridges to be recognized and delineated. We use these new plate-motion data to improve the reconstruction of the relative positions of the African, Indian and Australian plates at chron 5 (11 Ma). The improved quality and increased number of magnetic anomaly and fracture-zone crossings allow a great shrinking relative to prior work of the uncertainties in the relative rotations since chron 5 of the African, Australian and Indian plates. The volume of the 95 per cent confidence region is 98 times smaller than found before for the Africa-India rotation, 76 times smaller than found before for the Africa-Australia rotation, and 188 times smaller than found before for the Australia-India rotation. Unlike prior work, in which the Africa-Australia rotation depended strongly on estimates of Australia-Antarctica and Africa-Antarctica rotations, the Africa-Australia rotation can now be estimated accurately from data only along the Central Indian Ridge. The new small-confidence regions exclude all prior best estimates of these rotations. The much smaller uncertainties on the Australia-India rotation lead to specific predictions about where north-south divergence has been occurring, where north-south convergence has been occurring, and where the spatial integral of deformation is small. As has recently been shown to be true also for plate motion since chron 2A (3 Ma), the predictions are consistent with independent observations of the location, geographical limits, and style of deformation in the diffuse plate boundary between India and Australia. In particular, the pole of rotation lies in an apparently non-deforming region between the western diffuse plate boundary characterized by north-south stretching and the eastern diffuse plate boundary characterized by north-south to northwest-southeast shortening. The pole of rotation of the Australian plate relative to the Indian plate has moved insignificantly during the past 11 Myr, which suggests that information from the displacements and deformation at present, since 3 Ma, and since 11 Ma can be combined to obtain a more comprehensive kinematic model of how the relative motion between the Indian and Australian plates is and has been accommodated in the wide plate boundary. The 3 Myr average rotation of India relative to Africa is similar to that over 11 Myr, whereas the 3 Myr average rotation of Australia relative to Africa is insignificantly faster and indicates that Australia has changed to a slightly but significantly more eastward direction of motion than that over the past 11 Myr. We also present a new statistical test for determining whether a segment of plate-motion data reflects motion between a pair of rigid plates for which relative motion can be estimated independently from other plate-motion data assumed to lie along the same plate boundary. Application of this new test to the new data demonstrates that the boundary between the Indian and Australian plate along the Central Indian Ridge must have been (and presumably continues to be) at least several hundred kilometres in north-south extent and therefore is not a typical narrow oceanic plate boundary. The rigid Australian plate continues no farther north than an unnamed fracture zone that is immediately south of the Vema fracture zone and intersects the Central Indian Ridge near 10′s. The rigid Indian plate continues no farther south than just north of an unnamed fracture zone that is just south of the Vityaz fracture zone and intersects the Central Indian Ridge near 7′′s. Near the Central Indian Ridge, along a meridian at 70°, the diffuse plate boundary has accommodated 20 ± 4 km (1-D 95 per cent confidence limits) of north-south divergence since 11 Ma (chron 5). East of the pole of rotation, in the Central Indian Basin, the net north-south convergence along meridians at 80° and 90° is 31±7 km and 80 ± 12 km (1-D 95 per cent confidence limits), respectively. The indicated convergence along 78.8° is significantly larger, and that along 81.5° and 84.5° is insignificantly larger than estimated from prior analyses of long north-south seismic profiles. The discrepancy has several possible explanations, including flattening of thrust faults with depth, motions out of the vertical plane of the seismic profile, and initiation of motion before 7.5-8.0 Ma, which is the age of initiation of deformation estimated from seismic profiles and deep-sea drilling.