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Submersible observations of Equatorial Atlantic mantle: The St. Paul Fracture Zone region

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Abstract

The St. Paul F.Z. is a large structural domain made up of multiple transform faults interrupted by several Intra-Transform Ridge (ITR) spreading segments. Two regions were studied in details by submersible: (1) The ITR short (<20 km in length) segment near 0° 37′N–25° 27′W and 1° N–27° 42′W and (2) The St. Peter and St. Paul's Rocks (SPPR) massif located at 29° 25′W (¡3700 m depth). (1) The short ITR segments consist of a magma starved rift valley with recent volcanic activities at 4700 m depth. A geological profile made along the rift valley wall showed localized volcanics (basalts and dykes) which are believed to overlay and intrude the ultramafics. The geological setting and the high ultramafic/volcanic ratio suggest an extremely low magmatic supply and crustal-mantle uplift during lithospheric stretching and denudation. (2) The St. Peter and St. Paul's Rocks (SPPR) massif consists of a sigmoidal ridge within the active transform zone. The SPPR is divided into two different geological domains called the North and the South Ridges. The North Ridge consists of strongly tectonized fault scarps composed of banded and mylonitized peridotite, sporadic gabbros (3900–2500 m) and metabasalts (2700–1700 m). The South Ridge is less tectonized with undeformed, serpentinized spinel lherzolite (2000–1400 m) and basalts. Extensional motion and denudation accompanied by diapirism affected the South Ridge within a transform domain. Instead, the North Ridge was formed during an important strike-slip and faulting motion resulting in the uplifted portion of the St. Paul F.Z. transverse ridge. There is a regional compositional variation of the volcanics where E-MORBs and alkali basalts are produced on the SPPR massif and are comparable to the adjacent northern segments of the Mid-Atlantic Ridge. On the other hand, N and T- MORBs collected from the eastern part of the St. Paul F.Z. (25° 27′ W IRT) are similar to the volcanics from the southern segments of the MAR. The peridotites exposed in these provinces (SPPR and ITR) are similar in their REE and trace element distribution. Different degrees (3–15%) of partial melting of a mixed composite mantle consisting of spinel and amphibole bearing lherzolite veined with 5–40% clinopyroxenite gave rise to the observed MORBs and alkali basalts.

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Authors and Affiliations

  1. Centre de Brest, IFREMER, 29280, Plouzané, France

    R. Hekinian, B. Sichler & R. Apprioual

  2. Institut Universitaire Européen de la Mer, Université de Bretagne Occidentale, Place Copernic, 29280, Plouzané, France

    T. Juteau

  3. Institut de Ciències de la Terra, Jaume Almera, Lluis Solé i Sabaris, 08028, Barcelona, Spain

    E. Gràcia

  4. Departamento de Geologia, Lagemar, 24210-340, Niteroi, R.J., Brazil

    S. Sichel

  5. Vernadsky Institute, 19 Kosygin, 117975, Moscow, Russia

    G. Udintsev

  6. CNR, Istituto di Geologia Marina, 101 via Gobetti, 40129, Bologna, Italia

    M. Ligi

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  1. R. Hekinian
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Hekinian, R., Juteau, T., Gràcia, E. et al. Submersible observations of Equatorial Atlantic mantle: The St. Paul Fracture Zone region. Marine Geophysical Researches 21, 529–560 (2000). https://doi.org/10.1023/A:1004819701870

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