Journal of Conference Abstracts

Contrasted Volcano-tectonic Activities at 34°N and 35°N on the Mid-Atlantic Ridge: A Submersible Study

Daniel Bideau Département Géosciences Marines, IFREMER, Centre de Brest, B.P. 70,

29280 Plouzané Cédex, France

dbideau@ifremer.fr

Roger Hékinian Département Géosciences Marines, IFREMER, Centre de Brest, B.P. 70,

29280 Plouzané Cédex, France

Claire Bollinger CNRS URA 1278, DRO-GM, IFREMER, Centre de Brest, B.P. 70, 29280 Plouzané Cédex, France

Marc Constantin Marine Geol. Res. Lab., Univ. of Toronto, 22 Russell St., Toronto, Ontario, M5S 3B1, Canada

Eulalia Gracia Département des Sciences de la Terre, UBO, 6 av. Le Gorgeu, 29275 Brest, France

Christèle Guivel Département des Sciences de la Terre, UBO, 6 av. Le Gorgeu, 29275 Brest, France

Bertrand Sichler Département Géosciences Marines, IFREMER, Centre de Brest, B.P. 70,

29280 Plouzané Cédex, France

Introduction

During the OCEANAUT cruise (RV Nadir, September 1995), two segments of the Mid-Atlantic Ridge (MAR), located between the Oceanographer (35°15'N-36°10'W) and Hayes (33°30'N-38°W) transform faults, were explored with the submersible Nautile. These segments exhibit contrasted morphostructural and gravity features (FARA-SIGMA geophysical survey, RV l'Atalante, 1991). The central high of the northern segment is characterized by the presence of a median ridge (2000 to 1700 m) and a band of off-axis volcanoes extending perpendicularly on both sides of the ridge axis (1750 to 1000 m). The southern segment is deeper (3000 to 3500 m), intensively faulted and devoid of median ridge. A total of 20 submersible dives (up to 300 rock samples) and detailed surface ship magnetic surveys were carried out on both areas during the cruise. Magnetic anomaly maps representing about 3000 km of profiles including transverse and closely spaced (1 mile) profiles perpendicular to the ridge axis (30-23 miles long, 10 Kt ship velocity) were made.

1. Main Geological Differences

The off-axis volcanoes show increasing weathering with distance from the axis and look as old as their surrounding seafloor. In the northern area, the volcanic activity was episodic but intense during the last two million years, whereas normal faulting was prominent during at least one million years in the south. The episodicity of the northern volcanism is characterized by the alternance of extrusions of viscous porphyritic and/or highly vesicular lavas (pillow flows), and the extrusion of fluid aphyric or microporphiritic flows (lava lakes, sheet flows, ropy lavas and draperies). The neovolcanic zone is located on the inner floor of the rift valley east of the median ridge. The freshest lavas (shiny glass on sheet flows) were observed at the foot of the eastern wall of the valley. This part of the eastern valley is nearly untectonized, but fissuring and faulting rapidly increases north and south of the central high. The highly tectonized southern segment is mainly characterized by recent fissuring and faulting, but very fresh pillow flows were also sampled on the inner floor. This recent volcanic activity was also observed on the east side of the valley floor. This near amagmatic segment exhibits serpentinite massifs at the inside corners of its intersections with the second order discontinuities at both extremities. These ultramafic edifices have a topographic signature which contasts strongly with the north-south tectonic fabric of the ridge flanks. The serpentinites are associated with dolerites and metadolerites and outcrops of in situ material are uncommon.

2. Preliminary Petrological Results

The distribution of the major element contents (microprobe analyses) of glassy margins of 30 selected representative samples shows two populations of basalts. The southern basalts are primitive (MgO generally less than 8 wt%) depleted N-MORBs (K/Ti less than 0.2), whereas the northern basalts are slightly more evolved E-MORBs (MgO 7 to 9 wt%) to N-MORBS (K/Ti up to 0.6). The average content in alkalies (Na₂O, K₂O) and titanium of the northern samples is also higher than those from the southern area. This compositional distribution is related to the mantle regional chemical anomaly of the Azores. The T-MORBs (K/Ti 0.2 to 0.4) might represent either the result of mixing between parental melts issued from the co-existing depleted and enriched mantle sources, or might have been produced by multistage segregation of the melt during melting of a composite source. If this latter hypothesis is correct, the most depleted melts are delivered at the end of a magmatic cycle.

Conclusion

The similar degree of seawater weathering and the lack of magnetic anomaly variation between the seamounts and the surrounding seafloor suggests that they are contemporaneous. It is likely that the off-axis volcanoes were built on the ridge axis during successive volcanic cycles and moved away during spreading. On the northern segment, the magmatic cycles extend over a period of time averaging one million years. Each volcanic cycle consists of a period of fissure eruptions building elongated abyssal hills topped by small (100-300 m high) circular volcanoes (focused volcanism), and a period of crustal collapse followed by extrusion of fluid flows in the valleys. Assuming that the distribution of basalt composition is the result of a multistage segregation process during melting of a composite source, the production of N-MORBs in the north should represent the end of a magmatic cycle. The distribution of N-MORBs on the northern segment is restricted to the fluid lavas in the eastern trough of the rift valley. The E-MORBs are mainly vesicular pillows occurring on the median ridge and on the walls. Thus it is believed that the extrusion of fluid flows represents the end of a magmatic cycle after the reheating of the feeding column by earlier extrusions of viscous enriched flows. In the southern area, the occurrence of fresh pillow flows suggests persistence of limited volcanic activity or beginning of a new volcanic episode. The absence of E-MORBs and T-MORBs on the southern segment limits the influence of the regional mantle anomaly of the Azores farther north. The location of the neovolcanic zone at the east side of the rift valley floor is consistent with the asymmetry observed in the magnetic anomalies of both study areas