Different Crust Failure Modes Controlled by Spreading Obliquity and Its Implication: Insight from Southwest Indian Ridge 46-52.5°E

Oblique spreading occurs when the direction of relative motion between two rigid plates is oblique to the mid-ocean ridge trend. The main oblique spreading ridges are the Southwest Indian Ridge (Dick et al., 2003), Sheba Ridge (Lepvrier et al., 2002), Reykjanes Ridge (Dauteuil and Brun, 1993), Mohns Ridge (Wijk and Blackman, 2007), and Knipovich Ridge (Okino et al., 2002). Recent works reveal significant discrepancy in topography, rock composition, lithospheric thickness, and crust failure modes between orthogonal and oblique supersegments along Southwest Indian Ridge 9-25°E (Dick et al., 2003; Standish et al., 2008; Cannat et al., 2008), illuminating the importance of spreading obliquity on mid-ocean ridge geodynamic process.

The very slow spreading Southwest Indian Ridge 46-52.5°E, which is composed of two orthogonal supersegments linked by one oblique supersegment (Tao et al., 2014), is expected to be an ideal area to research how spreading obliquity controls the spreading process. We processed the multibeam bathymetric data of Southwest Indian Ridge 46-52.5°E obtained in DY-cruises, and identified 1361 off-axis scarps. These scarps are interpreted as normal fault surfaces, thus are products of crust failure. Rose diagrams showing strike distribution of the off-axis scarps (Fig. 1) are compiled to illustrate different modes of crust failure controlled by spreading obliquity.

Results show systematic divergence of crust failure modes between orthogonal and oblique supersegments. For the orthogonal supersegements (S-I and S-III), scarp trends show one predominant direction, E-W, near perpendicular to the spreading direction. Exceptions are segment 1-2, which are possibly disturbed by Indomed transform fault, and segment 23-25, where considerable overlap of mid-ocean ridges happens. For the oblique supersegment S-II, scarp tends are more complex. There are overall two predominant directions of scarps: E-W (near perpendicular to spreading direction) and NE-SW (oblique to spreading direction and parallel to ridge tend). Exceptions occur in segment 10, 12-14, which are orthogonal segments, though developed in oblique supersegment.

As the E-W predominant directions of scarps are consistent with brittle extensional plate failure, implications are that they possibly represent crust failure from the top of the plate. Meanwhile, the NE-SW predominant directions of scarps, parallel to the mid-ocean ridge trend, are supposed to be initiated from the base of the plate. Thus a fundamental difference between orthogonal and oblique segments is that the latter effectively represent failure initiated from the base of the plate (Also refer to Dick et al., 2003). Further implications are that the base-to-top controlling effect may be the reason why transform faults disappear and a new class of accretionary plate boundary (oblique amagmatic segment) replaces them.