Journal of Conference Abstracts

Volcanic and Morphological Features at the Mid-Atlantic Ridge Axis South of the Azores (37°N - 39°N): A Study at the Submersible Scale

Hélène Ondréas DRO-GM, IFREMER, Centre de Brest, B.P. 70, 29280 Plouzané Cédex, France

hondreas@ifremer.fr

Yves Fouquet DRO-GM, IFREMER, Centre de Brest, B.P. 70, 29280 Plouzané Cédex, France

Isabel Costa University of Lisboa, 1700 Lisboa, Portugal

Nuno Lourenço University of Lisboa, 1700 Lisboa, Portugal

Meg Tivey Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

Gérard Auffret DRO-GM, IFREMER, Centre de Brest, B.P. 70, 29280 Plouzané Cédex, France

Introduction

Three different areas of the Mid-Atlantic Ridge (MAR), near the Azores Triple Junction, were explored during the DIVA1 diving cruise, carried out in May 1994 on board the NO Le Nadir with the French submersible Nautile. The first area corresponds to the Lucky Strike hydrothermal field and is located at 37°18'N. The second area is named "Menez Gwen" and is located at 37°50'N. The third explored area is located at 38°20'N and named as such. These three segments are located in shallow waters. The two northernmost segments (Menez-Gwen and "38°20'N") show unusual morphological features for the MAR; the rift valley is absent and the present-day magmatism is focused on shallow axial volcanoes. These volcanoes are split into two even halves by an axial graben.

Segment morphostructure

Detailed bathymetry and side-scan imagery of this area were obtained using a SIMRAD EM12 swath-bathymetric multibeam system during the FARA-SIGMA cruise (Needham et al., 1992). The axial depth decreases toward the north from 3000 m to 1650 m for the Lucky Strike segment, from 1050 m to 700 m for Menez Gwen and from 930 m to 400 m for "38°20'N".

The Lucky Strike area is characterised by a 16 km wide, 2 km deep rift valley. A volcanic structure 6 km in diameter is present in the centre of the rift valley. This structure is split by a 1 km wide, 2 km deep graben. A circular lava lake 300 m in diameter is located in the middle of the segment, surrounded by all the Lucky Strike hydrothermal vents. Fresh lava (pillow, draped or lobated lavas) has been found on the floor of the axial graben.

On Menez Gwen, no rift valley exists. The main volcanic system of this segment is an edifice 16 km in diameter located in the central part of the segment. This volcano is 800 m high and split in its central part by a 2 km wide and 350 m (maximum) deep graben. A small volcano 700 m in diameter and 120 m high is present at the northern part of this graben.

Fresh lavas, including a lava lake, cover most of the graben floor. Explosive volcanic ejecta cover the top of the two halves of the split volcano. These deposits are more than 300 m thick and appear as different layers of ash and lapilli. The Menez Gwen segment is active volcanically and tectonically.

The "38°20'N" segment is composed of a 22 km wide, 40 km long and 1200 m high axial volcano. This volcano is split by a 30 km long graben widening towards the south from 2 km to 10 km. A new volcanic cone is located at the northern part of the graben. This edifice, 1200 m in diameter, culminates in less than 500 m water depth. At the central part of the graben there are two small volcanoes at a depth of less than 600 m. The 38°20'N segment is characterised by explosive volcanic ejecta like the Menez Gwen segment.

For these two segments volcanic dynamics oscillates between effusive and explosive activity.

Segment evolution

Simple reconstructions using an average spreading rate of 2 cm/year (DeMets et al., 1990) show that the "38°20'N" segment was the locus of intense explosive volcanic activity prior to 200,000 years ago, until the axial graben opened. This volcanic stage started at least 1 Ma. Considering an average width of 4 km for this graben, the opening started around 200,000 Ma.

Explosive activity also occurred on Menez-Gwen during the volcanic predominance between 800,000 and 100,000 years ago. This volcanic stage could have started with an effusive stage as shown by the massive flows on both flanks of the graben. Then, as shown by all the volcanic ejecta deposits on the two halves of the split volcano, an explosive stage occurred. We are not yet able to determine the exact age of the start of this explosive phase. We can speculate it took place when the top of the volcano reached a shallow depth. The 2 km wide axial graben represents a tectonic predominance starting 100,000 years ago. A new volcanic effusive stage, shown by the presence of fresh lavas on the central volcano in the northern part, may have started 40,000 years ago.

At Lucky Strike, we estimate the beginning of a tectonic stage at 800,000 years. Thus a volcanic effusive stage which built the main volcanic system could have started at least 300,000 years ago. A new axial graben 1 km wide and 2 km deep opened within this last volcanic system. This graben formed around 50,000 years ago.

Results

Even if the Menez Gwen and "38°20'N" segments have been dominated by focused magmatic accretion, each of the three segments may have experienced a specific accretionary history with a succession of volcanic and tectonic stages.

The recent evolution of each segment shows: - a temporal variability of volcanic or tectonic predominance which governs the morphostructural pattern of the ridge. As regards our analysis, the duration of the volcanic stages for the three areas is suggested, and seems to decrease from north to south from 800,000 years at "38°20'N", 700,000 years at Menez Gwen to 250,000 years at Lucky Strike. We suggest that this decrease in duration may be a consequence of the decrease in influence of the Azores hot spot. At a given time, two contiguous segments can be in two opposite stages. For example, 710,000 years, a tectonic stage may have started on Lucky Strike segment while a volcanic stage started on Menez Gwen segment. - explosive and effusive eruption phases alternate during the volcanic stage. They are linked to the presence of a hot domain located above the middle of the segment and influenced by changes in water depth. We have estimated, from simple reconstructions considering a spreading rate of 2 cm/yr (DeMets et al., 1990), the depth of the volcano tops on the two northern segments at the end of the volcano building stage. We calculate a depth of less than 700 m for the Menez Gwen segment and less than 300 m for the "38°20'N" segment.

References

DeMets, C., Gordon, R.G., Argus, D.F. & Stein, S., Geophys. J. Roy. Astr. Soc. London 101, 425-478 (1990).

Needham, H.D., Voisset, M., Renard, V. & Bougault, H., Eos, Trans., AGU, 73, 43 (1992).