The Romanche northern transverse ridge

Fracture zone transverse ridges, (i.e., ridges flanking slow-slip transforms and partly also their inactive fracture zone extensions) can be major topographic features BONATTI1978. The transverse ridge running on the northern side of the Romanche transform valley becomes particularly prominent in the eastern portion of the transform offset. The shallowest ($<$ 2000 m bsl) stretch of this transverse ridge is located roughly opposite the eastern RTI (Fig. 3, 4, 5). As noted by HONNOREZ1991 and MAMALOUKAS1992, the transverse ridge appears to be asymmetric in N/S sections, its N-facing slope being less steep than the S-facing one. Moreover, the S-facing slope is interrupted by the suspended valley described in the previous section. The suspended valley deepens towards the E and merges with the presently active transform valley at about the longitude of the eastern RTI. The shallowest ( $<$ 2000 m) portion of the transverse ridge displays a number of flat-top reliefs elongated roughly E-W. We identify them from W to E as reliefs A, B, C and D (Fig. 4). Samples were obtained from different sites on the transverse ridge during this and previous expeditions. A seismic reflection line was run roughly E-W along the crest of the transverse ridge (line ROM-2, Fig. 7) and three lines were run roughly normal to it (ROM-1, ROM-3 and ROM-5, Fig. 7).

Reliefs A, B and C show similar characteristics. Their summits range from a maximum of 930 m to a minimum of 1200 m bsl. A prominent more or less horizontal reflector is observed in all the three reliefs (Fig. 9 and 10). No clear reflections were detected below this horizontal reflector. Above this reflector a transparent, stratified unit is observed. This unit was sampled, and was found to consist in all the three reliefs of shallow water limestones in reef and lagunar facies displaying evidence of subaerial exposure BONATTI1979, BONATTI1994. Emersion as islands of these reliefs sometime in the past is confirmed by the recovery of ventifact basaltic pebbles from relief A by HONNOREZ1991 and from relief C by us. Seismic reflection profile over relief A (Fig. 9) shows what appears to be a sunken atoll, with a carbonate platform, a lagoon prograded by sediments and a fringing reef.

Figure 8: Portion of seimic reflectin profile ROM-3 showing the summit and the Southern Slope of the transverse ridge, including a section of the suspended valley. A: migrated timesection; B: interpreted line drawing. Location of this profile is shown in Fig. 7.

We propose the following interpretation of the seismic and lithological data. The units below the horizontal reflectors consist of blocks of oceanic crust and upper mantle. These lithtospheric blocks were uplifted and their tops reached above sea level. Subsidence followed. Erosion and wave truncation flattened the summit of the blocks at sea level during subsidence. Shallow carbonate banks, reefs and lagoons were implanted on the horizontal, erosional surfaces of the lithospheric blocks. These carbonate caps were exposed above sea level during minor reversals and oscillations of sea level and of the vertical motions of the lithospheric blocks. Overall subsidence continued, the carbonate reef formations sank rapidly and ceased to grow, up to the present situation when the tops ofthe reliefs lie between 1 and 1.5 km below sea level. This sequence of events is illustrated schematically in the cartoon of Fig. 11. Some time constraints of these events can be provided by paleontological ages that were obtained on carbonates sampled from the shallow water reef/lagunar limestone unit capping relief A BONATTI1979.

Figure 9: Portion of seismic reflection profile ROM-02 over relief A. Location of profile is shown in Fig. 7.

Figure 10: A, Portion of seismic reflection profile ROM-02 over relief C. Location of profile is shown in Fig. 7.

Age determinations. based on planctonic Foraminifera and on Stylophora sp. corals, date the shallow water carbonates sampled from relief A at the Miocene-Pliocene boundary, about 5 $\pm$ mybp. Assuming the summit of the relief was at sea level at that time, and that sea level at the end of the Miocene was within 50 m of present day sea level SHACKLETON1975, an average subsidence rate of 0.2 mm/y can be estimated, This rate is one order of magnitude faster than the thermal subsidence estimated for 50 my old crust BONATTI1979, BONATTI1981.

The casternmost relief of our survey on the transverse ridge (relief D) has a seismic signature very different from that of reliefs A, B and C. It lacks a carbonate cap and shows reflectors down to about 8 sec (Fig. 12). Igneous basement was not reached, and this stretch of transverse ridge appears to consist of a thick ($\sim$ 4 km) pile of sedimentary rocks affected by folds and overthrusts. An interpretation of this profile must wait for additional processing and for study of samples recovered from this area.

Figure 11: Cartoon illustrating a possible model of vertical motions on the transverse ridge giving rise to islands.