Inactive transform valley

A major suspended valley has been previously identified on the northern wall of the transform valley HONNOREZ1991, MAMALOUKAS1992, This suspended valley is clearly displayed in the 3-D elaboration of topography shown in Fig. 6, where it is seen to merge with the active transform valley in the general area of the RTI. The angle between the active transform valley and the suspended valley is about 10°.

A roughly N/S seismic reflection profile across the summit of the transverse ridge and the suspended valley at about 17W (ROM-3, Figs. 7, 8) shows that the suspended valley contains a sediment pile about 500 msec or $\sim$ 450 m thick (assuming sound velocity of 1.8 cm/sec). Moreover, the basement below the suspended valley appears to be affected by sets of low angle listric faults and smaller antithetic faults dipping in the opposite direction, suggesting N-S extentional tectonics.

It is important to note that the suspended valley can be traced as a continuous feature westwards for several hundreds kms, up to about 150 km from the western RTI (Fig. 3, 4). A possible interpretation is that this aseismic valley represents the trace of a former location of the Romanche transform boundary. Given that this inactive valley appears only about 150 km from the western MAR segment (Fig, 3, 4), and assuming a one way spreading rate of 1.75 cm/y. we estimate that the valley ceased to be an active transform boundary between 8 and 10 my ago.

Figure 7: Tracks of seismic reflection profiles carried out during cruise S-13. The portions of the profiles shown in Figs. 8, 9, 10 and 12 are marked with a thicker line. Sites where rock samples were recovered are also indicated, with different symbols for different rock types.