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Early (pre–8 Ma) fault activity and temporal strain accumulation in the central Indian Ocean

¹National Institute of Oceanography, Council of Scientific and Industrial Research, Dona Paula, Goa 403004, India
²School of Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton, Southampton SO14 3ZH, UK
³School of Geosciences, University of Edinburgh, Edinburgh EH9 3JW, UK

View larger version (52K): [in this window] [in a new window]   Figure 1. Locations of seismic-reflection profiles within central Indian Ocean from which fault throw data have been derived. ANS and NER indicate Afanasy Nikitin seamount and Ninety-east Ridge, respectively. Shading (yellow) shows position of diffuse plate boundary separating Capricorn (CAP), Indian (IND), and Australian (AUS) plates (Royer and Gordon, 1997). Approximate spatial extents of long-wavelength folding at three different phases (8.0–7.5, red; 5.0–4.0, light blue; 0.8 Ma, green; Krishna et al., 2001) are superimposed on this area. Inset map shows regional plate geometry (Royer and Gordon, 1997). Star shows approximate location of India-Capricorn pole of rotation (Gordon et al., 1998), which predicts compression in central Indian Ocean and extension around Chagos Bank (Henstock and Minshull, 2004). DSDP—Deep Sea Drilling Project.

View larger version (64K): [in this window] [in a new window]   Figure 2. Three interpreted north-south seismic-reflection profiles illustrating how strain has been accumulated on reverse faults with time over spatially separated regions. In all three sections, Pleistocene (blue), Pliocene (pink), and Miocene (yellow) unconformities are visible. In all sections shown, earlier motion can be demonstrated by greater vertical separation of reflectors on either side of faults at depths greater than Miocene unconformity. Labels on faults indicate those discussed in main text, and those for which activity history is described under each seismic section. For faults F2 and F4, there is clear evidence of early normal motion before reactivation. Depth-dependent velocity law of Bull and Scrutton (1990b) was used to determine displacements in meters. TWT—two-way time.

View larger version (31K): [in this window] [in a new window]   Figure 3. Correlation of basement structure with reverse fault throws measured at four intervals (20 to 8.0–7.5; 8 to 5.0–4.0; 5.0–4.0 to 0.8; and 0.8–0 Ma) along seismic profiles at 81.4°E, 83.7°E, and 87°E. Strain distribution along each profile is shown above, calculated for 100 km bins with a rolling window of 10 km. TWT—two-way time; DSDP—Deep Sea Drilling Project; ODP—Ocean Drilling Program.

View larger version (9K): [in this window] [in a new window]   Figure 4. Strain budget calculated from fault throws plotted against age. Strain is normalized by deformation extent on each profile. Fault throw data derived from seismic profiles (Fig. 1) along 81.5°E, 84.5°E, and 78.8°E (Chamot-Rooke et al., 1993; Jestin, 1994; Van Orman et al., 1995), and 81.4°E, 83.7°E, and 87°E (this study) were used for calculation of strain. Lithospheric shortening rate in central Indian Ocean initiated slowly, but increased significantly at 8 Ma and continues, at variable rates, to present.