Microbathymetry data, in situ observations, and sampling along the 13°20′N and 13°20′N oceanic core complexes (OCCs) reveal mechanisms of detachment fault denudation at the seafloor, links between tectonic extension and mass wasting, and expose the nature of corrugations, ubiquitous at OCCs. In the initial stages of detachment faulting and high‐angle fault, scarps show extensive mass wasting that reduces their slope. Flexural rotation further lowers scarp slope, hinders mass wasting, resulting in morphologically complex chaotic terrain between the breakaway and the denuded corrugated surface. Extension and drag along the fault plane uplifts a wedge of hangingwall material (apron). The detachment surface emerges along a continuous moat that sheds rocks and covers it with unconsolidated rubble, while local slumping emplaces rubble ridges overlying corrugations. The detachment fault zone is a set of anostomosed slip planes, elongated in the along‐extension direction. Slip planes bind fault rock bodies defining the corrugations observed in microbathymetry and sonar. Fault planes with extension‐parallel stria are exposed along corrugation flanks, where the rubble cover is shed. Detachment fault rocks are primarily basalt fault breccia at 13°20′N OCC, and gabbro and peridotite at 13°30′N, demonstrating that brittle strain localization in shallow lithosphere form corrugations, regardless of lithologies in the detachment zone. Finally, faulting and volcanism dismember the 13°30′N OCC, with widespread present and past hydrothermal activity (Semenov fields), while the Irinovskoe hydrothermal field at the 13°20′N core complex suggests a magmatic source within the footwall. These results confirm the ubiquitous relationship between hydrothermal activity and oceanic detachment formation and evolution.

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To better address how Mid-Ocean Ridge Basalt (MORB) sulfur isotope composition can be modified by assimilation and/or by immiscible sulfide fractionation, we report sulfur (S), chlorine (Cl) and copper (Cu) abundances together with multiple sulfur isotope composition for 38 fresh basaltic glasses collected on the Pacific-Antarctic ridge. All the studied glasses-with the exception of 8 off-axis samples-exhibit relatively high Cl/K, as the result of pervasive Cl-rich fluid assimilation. This sample set hence offers an opportunity to document both the upper mantle S isotope composition and the effect of hydrothermal fluids assimilation on the S isotope composition of erupted basalts along segments that are devoid of plume influence. Δ33 S and Δ 36 S yield homogenous values within error of Canyon Diablo Troilite (CDT), whereas δ 34 S are variable, ranging between-1.57±0.11‰ and +0.60±0.10‰ with a mean value of-0.64±0.40 ‰ (1σ, vs. V-CDT). The geographic distribution of δ 34 S follows a spike-like pattern, with local 34 S-enrichments by up to +1.30‰ compared to a low-δ 34 S baseline. As hydrothermal massive sulfides are characterized by relative 34 S-enrichments, such first-order variability can be accounted for by hydrothermal sulfide assimilation, a process that would occur for a subset of samples (n=10). Excluding these particular samples, the mean δ 34 S is significantly less variable, averaging at-0.89±0.11‰ (1σ, n=28), a value that we suggest to be representative of the average MORB source value for Pacific-Antarctic basalts. Weak trends between δ 34 S and 206 Pb/ 204 Pb are displayed by such uncontaminated samples suggesting the recycled oceanic crust to have a modest impact on the S budget of mantle. Their positive signs, however, suggest the depleted mantle to have a δ 34 S of-1.40±0.50 ‰. The sub-chondritic 34 S/ 32 S value that was previously observed for the South-Atlantic mantle is here extended to the Pacific-Antarctic domain. Such feature cannot originate from oceanic crust recycling and substantiates the concept of a core-mantle fractionation relict. 2 Antarctic domain. Such feature cannot originate from oceanic crust recycling and substantiates the concept of a core-mantle fractionation relict. 1. Introduction Sulfur is one of the light elements entering the core composition thereby satisfying a part of its 5-10% density deficit (Birch, 1964). More than 97% of terrestrial sulfur would reside in the Earth's core (Dreibus and Palme, 1996), providing an upper estimate of 1.7 wt% S in the core (see also McDonough, 2003). Any S isotopic fractionation between silicate and metal would lead the silicate reservoir to exhibit a 34 S/ 32 S shift with respect to chondrites. Since the landmark study of Thode et al. (1961), however, the sulfur (S) isotope composition of the Earth's mantle is thought to be homogeneous with a mean 34 S of 0.0‰, a value indistinguishable from chondrites that display an average 34 S of +0.04±0.31 ‰ (1 , n=24, Gao and Thiemens, 1993a,b, Orgueil excluded) , n=24, Gao and Thiemens, 1993a,...

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