The Owen Transform Fault (OTF) connecting the Sheba and the Carlsberg spreading ridges in the Indian Ocean currently forms the active plate boundary between India and Somalia plates. This 330-km-long transform fault is by far the longest transform fault along the India-Somalia plate boundary and its valley is buried under the thick distal turbidites of the Indus Fan with total thickness ranging from 1000 to > 5000m. A new set of seismic reflection and multibeam bathymetric data reveals remarkable transpressive structures along its entire length recorded as folds in the sedimentary cover, eruption of mud ridges at the seafloor, thrusts in the young oceanic lithosphere. Based on a new regional time-calibration of the seismic reflectors, we show that sediments in the transform valley (post 8.6 Ma) recorded a period of tectonic quiescence until the onset of a transpressive event around 1.5 − 2.4 Ma that we relate to a minor change in India-Somalia kinematics not captured by magnetic anomalies. This tectonic regime is still active based on compressive earthquakes and deformation of the most recent sediments. Transpression resulted in the formation of a proto-median ridge and the coeval propagation of the tip of the Carlsberg Ridge into the Somalian plate. These features are typically encountered at many other transform faults but rarely captured in their very early stage.

Key Points

A new set of seismic reflection lines allows time-calibration of the Indus Fan sediments deposited within the Owen transform valley

The recent tectonic regime includes a still active transpression since 1.5-2.4 Ma that followed a million-years-long period of quiescence

A nascent median ridge grows south of the fault, a feature found at many other transform faults but captured here in its very early stage

Plain Language Summary

Oceanic transform faults are tectonic plate boundaries along which two plates slide against each other. Often located far from sediment sources, oceanic transform faults are essentially sediment-free and their geological evolution can only be addressed using past Earth magnetic field inversions recorded in the seafloor basaltic rocks, leading to a time resolution no better than several million years. We report here new findings obtained at the active Owen Transform Fault, the longest oceanic transform faults between India and Somalia plates. Indus Fan sediments derived from the western Himalaya erosion have accumulated with time in a deep valley adjacent to the Owen Transform Fault, offering a unique high-resolution stratigraphic record of past regional tectonics. The analysis of seismic reflection profiles across this valley reveals that the Owen Transform Fault is under a transpressive regime since 1.5 − 2.4 Ma, unpredicted by magnetics-based India-Somalia kinematics. This unusual regime for a supposedly pure strike-slip fault is still active as shown by thrust earthquakes and deformation of the most recent sediments. We discuss the origin of similar structures found at long-lived oceanic transform faults in other oceans, and propose that subtle changes in plate motion may be responsible.