Alteration of ultramafic rocks plays a major role in the production of hydrocarbons and organic compounds via abiotic processes on Earth and beyond and contributes to the redistribution of C between solid and fluid reservoirs over geological cycles. Abiotic methanogenesis in ultramafic rocks is well documented at shallow conditions, whereas natural evidence at greater depths is scarce. Here we provide evidence for intense high-pressure abiotic methanogenesis by reduction of subducted ophicarbonates. Protracted (≥0.5–1 Ma), probably episodic infiltration of reduced fluids in the ophicarbonates and methanogenesis occurred from at least ∼40 km depth to ∼15–20 km depth. Textural, petrological and isotopic data indicate that methane reached saturation triggering the precipitation of graphitic C accompanied by dissolution of the precursor antigorite. Continuous infiltration of external reducing fluids caused additional methane production by interaction with the newly formed graphite. Alteration of high-pressure carbonate-bearing ultramafic rocks may represent an important source of abiotic methane, with strong implications for the mobility of deep C reservoirs.

International audienceMagma-poor rifted margins are being increasingly recognized in present-day Atlantic-type systems.However, findings of fossil areas floored by exhumed mantle or hyper-extended crust are comparativelyrare within orogenic belts that were originated through the inversion of pre-existing rifted margins. Thisdiscrepancy may be due to the common reactivation of lithological contacts during subduction/orogeny,potentially masking pre-orogenic relationships, and, most importantly, to the frequent lack of a preorogeniclayer-cake architecture, hindering retro-deformation of multiply deformed tectonic units. Thisstudy outlines a methodology to detect sections of magma-poor, hyper-extended rifted margins withouta layer-cake architecture in multiply deformed/metamorphosed terrains. This approach is defined by comparisonto well studied examples of fossil analogues preserved in weakly deformed parts of Alpine orogens.In the latter domains, continental basement and hydrated peridotites were exhumed at the basin floor duringJurassic rifting along long-offset detachment systems. Extensional geometries locally resulted in tectonicsampling of laterally discontinuous slivers of allochthonous continental basement and pre-rift sedimentsfromthe hanging wall blocks. Lithostratigraphic associations consisting of continental basement rocks directlyjuxtaposed with syn- to post-rift meta-sediments and/or serpentinized subcontinental mantle are widespreadwithin sections of Alpine-type orogenic belts that underwent high- to ultra-high-pressure metamorphism.However, similar associations may arise from a variety of processes other than rift-related lithospheric thinningin magma-poor environments, including subduction mélange dynamics or deposition of sedimentary mélangesalong convergent/divergent margins. The partial preservation of rift-related lithostratigraphic associations maystill be assessed, despite the lack of biostratigraphic evidence, by (1) the consistency of the lithostratigraphic architectureover large areas, despite pervasive Alpine deformation, which rules out chaotic mixing during subduction/exhumation, (2) the presence of clasts of basement rocks in the neighboringmeta-sediments, indicating the originalproximity of the different lithologies, (3) evidence of brittle deformation in continental basement and ultramaficrocks pre-dating Alpine metamorphism, indicating that theywere juxtaposed by fault activity prior to thedeposition of post-rift sediments, and (4) the similar Alpine tectono-metamorphic evolution of ophiolites, continentalbasement and meta-sediments.A re-assessment of basement–cover relationships in the North-Western Alps following this approach, combinedwith published studies on exhumedmantle domains sampled in the rest of theWestern Alps, indicates that severaltectono-metamorphic units fromthemost deformed/metamorphosed part of the belt, between the CanaveseLine and the Penninic Front, sample hyper-extended lithosphere related to the Jurassic opening of the WesternTethys. Relative plate motion during Cretaceous–Tertiary basin inversion was largely accommodated at the transitionbetween areas floored by hyper-extended crust or hydrated subcontinental mantle and domains consistingof thicker continental crust. As a result, distal margins were preferentially subducted, whereas the proximaldomains and the Briançonnais paleo-high underwent relatively minor deformation and metamorphism. Thehigh-pressure Alpine tectono-metamorphic units were probably detached from the downgoing lithospherealong a hydration front that is typically observed in present-day distal margins. The recognition of preserved pre-Alpine relationships between continental basement, post-rift sediments and/or serpentinized ultramaficrocks calls for a re-assessment of the relative role of subduction and rifting dynamics in establishing thepresent-day orogen architecture