EVIDENCE FOR RELATIVE MOTIONS BETWEEN THE INDIAN AND AUSTRALIAN PLATES DURING THE LAST 20 MY FROM PLATE TECTONIC RECONSTRUCTIONS - IMPLICATIONS FOR THE DEFORMATION OF THE INDO-AUSTRALIAN PLATE

We use plate tectonic reconstructions to establish whether motions between India and Australia occurred since chron 18 (43 Ma). We test the Africa/Antarctica/Australia/India plate circuit closure at chrons 5 (10 Ma), 6 (21 Ma) and 13 (36 Ma) using a compilation of magnetic anomalies and fracture zone traces from the Southeast, Southwest, Central Indian and the Carlsberg ridges. Additional reconstructions at chrons 23 (55 Ma) and 26 (61 Ma) are used to estimate the overall motion between India and Australia. Relative motions between the Indian and Australian plates are estimated using the plate circuit India --> Africa --> Australia. A new statistical approach, based on spherical regression analyses, is used to assess the uncertainty of the "best-fitting" finite rotations from the uncertainties in the data. The uncertainty in a rotation is described by a covariance matrix directly related to the geometry of the reconstructed plate boundary, to the distribution and estimated errors of the data points along it. Our parameterization of the rotations allows for simple combination of the rotation uncertainties along a plate circuit path. Results for chron 5 are remarkably consistent with present-day kinematics in the Indian Ocean, except that the Arabian and Indian plates are found to be separate plates. Comparisons of the motions between the Indian and African plates across the Carlsberg Ridge with that between the Australian and African plates across the Central Indian Ridge evidence a significant counterclockwise rotation of the Australian plate relative to the Indian plate about a pole located in the Central Indian Basin. The determinations are consistent for chrons 26, 13, 6 and 5. Determination at chron 23 is different but questionable due to the small number of available data. We propose two alternative solutions that both predict convergence within the Wharton and Central Indian basins and extension in the vicinity of the Chagos-Laccadive Ridge. The first solution assumes that all the deformation in the equatorial Indian Ocean started 7 Ma ago as found by Ocean Drilling Program Leg 116. Hence all the determinations at different times represent the total motion between Indian and Australia. The averaged India/Australia Euler vector (chrons 5, 6, 13, and 26: 11.1-degrees-S, 78.0-degrees-E, omega = 3.54-degrees) lies within the Central Indian Basin and yields a N-S contraction of 46 +/- 52 km at 85-degrees-E, and 80 +/- 63 km at 90-degrees-E. However, the difference of the India/Australia Euler vectors at chrons 5 and 6 suggests that the India/Australia convergence started between 10 and 21 Ma, following the continent-continent collision of India with Asia in the Early Mioceno. The second averaged solution (chrons 6, 13, and 26: 5.2-degrees-S, 74.3-degrees-E, omega = 5.93-degrees) predicts a total N-S contraction of 123 +/- 73 km at 85-degrees-E, and 178 +/- 91 km at 90-degrees-E. Both models are compatible with the deformation pattern observed in the equatorial Indian Ocean.