The anticorrelated velocities of Africa and India in the Late Cretaceous and early Cenozoic

Abstract

We present a revised interpretation of magnetic anomalies and fracture zones on the Southwest Indian Ridge (SWIR; Africa-Antarctica) and the Southeast Indian Ridge (SEIR; Capricorn-Antarctica) and use them to calculate 2-plate finite rotations for anomalies 34 to 20 (84 to 43 Ma). Central Indian Ridge (CIR; Capricorn-Africa) rotations are calculated by summing the SWIR and SEIR rotations. These rotations provide a high-resolution record of changes in the motion of India and Africa at the time of the onset of the Reunion plume head. An analysis of the relative velocities of India, Africa and Antarctica leads to a refinement of previous observations that the speedup of India relative to the mantle was accompanied by a slowdown of Africa. The most rapid slowdown of Africa occurs around Chron 32Ay (71 Ma), the time when India's motion relative to Africa notably starts to accelerate. Using the most recent Geomagnetic Polarity Timescale (GTS12) we show that India's velocity relative to Africa was characterized by an acceleration from roughly 60 to 180mmyr-1 between 71 and 66 Ma, a short pulse of superfast motion (~180mmyr-1) between 66 and 63 Ma, an abrupt slowdown to 120mmyr-1 between 63 and 62 Ma, and then a long period (63 to 47 Ma) of gradual slowing, but still fast motion (~100mmyr-1), which ends with a rapid slowdown after Chron 21o (47 Ma). Changes in the velocities of Africa and India with respect to the mantle follow a similar pattern. The fastest motion of India relative to the mantle, ~220mmyr-1, occurs during Chron 29R. The SWIR rotations constrain three significant changes in the migration path of the Africa-Antarctic stage poles: following Chron 33y (73 Ma), following Chron 31y (68 Ma), and following Chron 24o (54 Ma). The change in the migration path of the SWIR stage poles following Chron 33y is coincident with the most rapid slowdown in Africa's motion. The change in the migration path after Chron 31y, although coincident with the most rapid acceleration of India's northward motion, may be related to changes in ridge push forces on the SWIR associated with the onset of extension along the Bain transform fault zone. The initial slowdown in India's motion relative to Africa between 63 and 62 Ma is more abrupt than predictions based on published plume head force models, suggesting it might have been caused by a change in plate boundary forces. The abrupt change in the migration path of the SWIR stage poles after Chron 24o is not associated with major changes in the velocities of either Africa or India and may reflect Atlantic basin plate motion changes associated with the arrival at the Earth's surface of the Iceland plume head. The abruptness of India's slowdown after Chron 21o is consistent with a collision event.