The eastern Romanche transform-ridge intersection

Based on topography, the axis of the MAR segment between the Chain and the Romanche transforms is well constrained in its southern portion. Here a well formed axial rift valley can be clearly identified, where earthquake epicenters are clustered (Figs. 2, 3). However, the northern portion of the MAR, close to its impact against the Romanche, does not show a clear topographic signature (Figs. 3, 4, 5, 6). Oblique trends are observed. Based on epicenters and topography the presently active axis ot spreading is fragmented in short segments, some possibly with oblique strike. The intersection with the transform occurs probably at ahout 16:30W. No nodal deep is observed. This is surprising, because for a long offset, slow slip transform such as the Romanche, where the MAR impacts against $\sim$ 50 my old lithosphere, a well developed nodal deep is predicted by theory Fox1984.

An aseismic valley impacts against the Romanche at about 17:10W,i.e., 80 km west of the inferred RTI (Fig. 3, 4, 5, 6). The topography of this valley is consistent with the interpretation that it represents a former, presently inactive axial rift valley. If this interpretation is correct, a ridge axis jump to the east of about 80 km is called for some time within the last 5 my, increasing the offset length of the transform.

Figure 5: Detailed bathymetric map of an area close to the Romanche Eastern RTI. Areas deeper than 4000m are shaded. Circles indicate earthquake epicenters.

Petrological studies of basaltic and gabbroic rocks sampled from the eastern RTI area indicate a widespread magmatism with a strongly alkalic affinity, in contrast to normally prevaling MORB compositions (Peyve and Sushevskaja, in preparation). Alkalic magmatism has been found to be relatively common elsewhere along the Romanche transform BONATTI1979.

Figure 6: 3D view of the Romanche transform eastern intersection. View from SE.