CONCLUSIONS

1. Site 1277, near the crest of the Mauzy Ridge (near magnetic Anomaly M1), documents the formation of new seafloor by exhumation of subcontinental mantle lithosphere. It records part of the transition between exhumation of subcontinental mantle lithosphere and the initiation of "normal" seafloor spreading, and it is associated with minor MORB volcanism.
2. Serpentinized spinel harzburgite and minor intrusive gabbro were exhumed to the seafloor by means of a system of one or more extensional detachment faults. The brittle continental crust was completely removed, allowing mylonitic and cataclastic serpentinite (fault gouge) to be directly exposed on the seafloor above serpentinized ultramafic mantle.
3. The inferred detachment fault underwent submarine erosion to produce multiple debris flows dominated by lithoclasts of altered serpentinite and gabbro, occasionally larger than one meter. The mass flows ranged from cohesive matrix-supported debris flows with outsize blocks lower in the section to less cohesive pebbly debris flows including well-rounded clasts, generally higher in the section. A ferromanganiferous crust containing planktonic foraminifers then accumulated on the Mauzy Ridge. The ridge was much later covered by a relatively thin terrigenous deep-sea sedimentary succession.
4. Three intervals of basaltic lavas were erupted, interspersed with minor hyaloclastite (glassy chilled basalt). The middle lava interval consists of two sheet flows, whereas the upper flow includes lava breccia and possible pillow lavas. These flows, however, are small and basement at Site 1277 is dissimilar to "normal" oceanic crust.
5. Following eruption, high-angle fissures and fractures opened in the volcanic-sedimentary succession and were repeatedly infilled with calc-siltite, together with detrital grains of basalt and hyaloclastite, to form neptunian dykes. Hydrothermal calcite cementation and replacement of both clasts and matrix took place repeatedly, interrupted by fracturing and reopening of fissures in a setting of continuing extension. The fracturing may have resulted from continuing extension of the exhumed subcontinental mantle lithosphere until in-plane tensile stress was finally relaxed when subcontinental mantle lithosphere separated and "normal" seafloor spreading began; this may have taken place near the Aptian/Albian boundary.
6. The serpentinized harzburgite basement shows evidence of brittle high-angle fracturing and hydrothermal cementation like that of the overlying volcanic-sedimentary unit, suggesting that both the basement and its volcanic-sedimentary cover were deformed together in response to in-plane extension.
7. It is probable that the basement ridge (Mauzy Ridge) was uplifted by high-angle normal faulting in response to crustal extension after the initial exhumation of the subcontinental mantle lithosphere but prior to the onset of "normal" seafloor spreading.
8. The absence of terrigenous components in the volcanic-sedimentary sequence indicates that Site 1277 was isolated from margin-derived turbidites at the time the basement was exhumed. The most likely reason is the presence of a complex fault topography between the site and the adjacent rifted margin.
9. The basalts are similar to, or slightly enriched, relative to N-MORB composition. This suggests the existence of a relatively fertile mantle source at some depth below the highly depleted serpentinized spinel harzburgites that were cored at Site 1277.
10. An analytically significant negative Nb anomaly is present in MORB-normalized plots of the basalts from Site 1277. This anomaly is interpreted to be a subduction signature inherited from subcontinental mantle lithosphere that was involved in a preexisting subduction event, possibly related to Paleozoic closure of the Iapetus or Rheic oceans. Thus, care should be taken when interpreting similar negative Nb anomalies in ancient rift basalts exposed in orogenic belts (e.g., Tethys) as indicative of contemporaneous subduction, or arc volcanism, especially when the field relations are consistent with a rift setting that is unrelated to subduction.