The proposal is written in English to ease to the contribution of foreign co-workers 1-General scientific context of the VARUNA project The Aden-Owen-Carlsberg (AOC) triple junction in the NW Indian Ocean is located at the contact between Arabia, Somalia and India (Fig. 1). The AOC triple junction was initially described as a Ridge-Fault-Fault triple junction (Wilson, 1965; McKenzie et al., 1970; Gordon and DeMets, 1989), which involves the Sheba Ridge, the Owen Transform connecting the Carlsberg Ridge, and the Owen Fracture Zone connecting the Makran subduction zone. In details, the present-day configuration of the AOC triple junction is more complex: the Owen Fracture Zone connects an area of diffuse extension instead of directly connecting the spreading center (Fig. 1). The present-day configuration of the AOC triple junction therefore corresponds to a transient stage from an unstable Ridge-Fault-Fault configuration to a more stable Ridge-Ridge-Ridge configuration (Fournier et al., 2001; 2008 a,b). The Owen Transform and the Owen Fracture Zone are therefore two distinct, active plate boundaries (India-Somalia and India-Arabia, respectively). Several oceanographic cruises have been carried in this area by our group during the last decade, including the AOC (2006), the OWEN 1 (2009) and OWEN 2 cruises (2012). The AOC cruise led to the acquisition of multibeam and magnetic data over the northeastern part of the Sheba Ridge. The OWEN 1 cruise led to the full-multibeam mapping of the Owen Fracture Zone, and the acquisition of sub-bottom profiles along its strike. Finally, the OWEN 2 cruise allowed the acquisition of seismic profiles crossing the Owen Fracture, the Owen Basin, and the eastern Oman margin. The VARUNA Project is associated to another cruise proposal- CARLMAG, PI Nicolas Chamot-Rooke- which aims to better constrain the India- Somalia kinematics since the Oligocene from multibeam and magnetic data. The Owen Transform System, located between the Sheba and Carlsberg spreading center, and the area between the termination of the Owen Fracture Zone and the Sheba Ridge, have not been surveyed since some dredgings in the late 70’s (Hamlyn and Bonatti, 1980). The acquisition of multibeam, magnetic, gravimetric and seismic data over the area of the Owen Transform will complete the dataset acquired during the previous cruises. The long-term objective is to provide a full-geological history of a transform boundary, from the spreading centers to the subduction zone, to investigate how it has been affected by kinematic changes related to the growth of the Himalaya. 2-Scientific question: Sensitivity of oceanic transform faults to kinematic changes Transform faults were discovered in the 1960’s based on horizontal offsets of magnetic anomalies lineaments in the oceanic domain (Wilson, 1965). These conspicuous offsets, reaching several hundreds of kilometers in some places, reflect the segmentation of oceanic spreading centers. In terms of morphology, transform faults are characterized by steep bathymetric offsets resulting from the age difference of the oceanic lithospheres on each side of the transform. Transform systems juxtapose oceanic lithospheres with different physical properties due to their different ages (formation of bimaterial interfaces/ establishment of strength contrasts). Except a few places where intra-plate stresses reactivate fracture zones (Abercrombie et al., 2003; Storti et al., 2007; Delescluse et al., 2012), the seismicity is restricted to the segment of the transform limited by the spreading centers (Sykes, 1967). Kinematic changes are recorded by changes in trend and curvature of fracture zones, which express changes in direction of seafloor spreading. The trend of transform faults and fracture zones is therefore commonly used to determine the pole of rotation of a couple of plates at a given geological time (DeMets et al., 2010). Kinematic changes locally induce transpressive or transtensive stresses, resulting in the uplift of elongated, several kilometers-high transverse ridges (Weissel et al., 1992; Bonati et al., 2005; Rodriguez et al., 2014; Maia et al., 2016). At the lithospheric scale, transverse ridges correspond to positive flower structures (St Paul transform; Maia et al., 2016) or tilted panels of oceanic lithosphere (Vema transform; Bonati et al., 2005), where mantle exhumation is observed. Kinematic changes may also trigger migration events (or jumps) of successive transform faults (Sclater et al., 2005; Rodriguez et al., 2016; Maia et al., 2016). However, unknowns remain about: 1-The precise duration of the adjustment of a transform system to a kinematic change. 2-The mode of development of the structural features emplaced during a kinematic change (transverse ridges, releasing bends) and their potential contribution to mantle exhumation. 3-The mode of migration of the transform system and the mode of emplacement of transient structures during these episodes. These questions require both a detailed kinematic framework from magnetic anomalies and a detailed tectono- FIGURE 1: Simplified tectonic map of the stratigraphic framework of the Arabian Sea. The inset represents a simplified deformation. structural framework of the Aden-Owen- Carlsberg (AOC) triple junction. 3- Why do we need to the study the Owen Transform System? 3-1-The development of the Owen Transform system has been affected by several kinematic changes since its initiation, and as such provides a new, detailed record of the geodynamic history of the Indian Ocean. The complex history of India and Arabia motion towards Eurasia has affected seafloor accretion at the Carlsberg and Sheba spreading centers. These spreading centers experienced dramatic reorganization events, expressed by changes in trends and rates of spreading, in the wake of major geological events at the convergence zone. The initiation of the Owen Transform in the Early Miocene follows a major kinematic change 24 Ma (Patriat et al., 2008) linked to the collision of Arabia with Eurasia. It marks the initiation of the Sheba Ridge and the beginning of its propagation towards the Afar Hotspot (Fournier et al., 2010; d’Acremont et al., 2006; 2010). Since the 24 Ma episode, the magnetic anomaly record documents a gradual decrease of spreading rates at both the Sheba and Carlsberg Ridges until a new plate reorganization event occurred at 8-10 Ma. Since then, seafloor spreading rates have remained steady at both spreading centers (DeMets et al., 2005; Merkouriev and DeMets, 2006, Fournier et al., 2010; DeMets et al., 2015; 2016).This 8-10 Ma kinematic change corresponds to the separation of Somalia from Nubia related to the East African Rift system (DeMets et al., 2005; 2016). It is also expressed by the onset of a diffuse deformation area in the Central Indian Ocean separating India from Australia (Weissel et al., 1980; Wiens et al., 1985; Bull and Scrutton, 1990, 1992; Chamot-Rooke et al., 1993; Delescluse et al., 2008; Bull et al., 2010). Two additional kinematic changes may be suspected and remain to be confirmed in the area: -A kinematic change is recognized in the Central Indian Ocean 15 Ma. It is interpreted as the consequence of the increase in gravitational potential energy gradient induced by the rise of the Himalaya (Molnar and Stock, 2009; Bull et al., 2010). Evidence for this event remains to be discovered in the Arabian Sea. -The last major reorganization of the Aden-Owen-Carlsberg Triple junction, and the onset of the Owen Fracture Zone, occurred only 2.4 Ma. However, no coeval kinematic change has been detected so far in the Indian Ocean (DeMets et al., 2017). FIGURE 2: Left: Multibeam bathymetry of the Aden-Owen-Carlsberg (AOC) triple junction. OFZ: Owen Fracture Zone. Right: free-air gravimetry of the study area, highlighting the location of the Varuna Ridge. FIGURE 3: W-E Seismic profiles crossing the Owen Fracture zone, where sediments are calibrated down the Base Oligocene (Rodriguez et al., in prep.). 3-2-The Owen Transform system is related to conspicuous but poorly investigated tectonic structures (Fig. 2). The detailed structure of the present-day active segment of the Owen Transform remains unknown, despite its 250-km-length, and its seismic activity with frequent earthquakes Mw>5. Whether transverse ridges or releasing bends formed in response to kinematic changes remains unknown. A ~200-km-long series of oceanic ridges, referred as the Varuna Ridge, is well imaged on the free-air gravity anomaly map and may constitute remnants of the Owen Transform prior to the triple junction reorganization (Fig. 2; Rodriguez et al., 2017). Peridotites have been dredged on the top of the Varuna Ridge (Exon et al., 2011) and along the Owen Transform (Hamlyn and Bonatti, 1980). How mantle has been exhumed in these areas? Is it related to transform tectonics? 3-3 - The Owen Transform system may have experienced migration events since the Miocene. The trend of the fossil Varuna Ridge is different from the present-day trend of the Owen Transform (Fig. 2). This strongly suggests an episode of migration of the transform system somewhere in the Miocene-Pliocene, with the associated transfer of oceanic silvers between Somalia and India. Episodes of transform migration and transfer of lithospheric silvers have been evidenced in the Arabian Sea in the Late Cretaceous-Paleogene period (Rodriguez et al., 2016). 3-4-The Owen Transform formed in an intra-oceanic context (North Somali and Owen Basins) after a period of ultraslow spreading 42-24 Ma, which did not form any transform faults at the Carlsberg Ridge. The Owen Transform therefore represents a rare case where the initiation of a new transform system within an intra-oceanic context can be investigated and compared to models (Gerya, 2010). This case study will lead to new perspectives compared to the investigation of transform nucleated in the frame of continental break-up (Taylor et al., 2009, Gerya, 2012). 3-5-The sedimentation of the distal Indus turbiditic fan provides a good stratigraphic record of the tectonic events (Fig.