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Subduction initiation is a critical link in the plate tectonic cycle. Intraoceanic subduction zones can form along transform faults and fracture zones, but how subduction nucleates parallel to mid-ocean ridges, as in e.g., the Neotethys Ocean during the Jurassic, remains a matter of debate. In recent years, extensional detachment faults have been widely documented adjacent to slow-spreading and ultraslowspreading ridges where they cut across the oceanic lithosphere. These structures are extremely weak due to widespread occurrence of serpentine and talc resulting from hydrothermal alteration, and can therefore effectively localize deformation. Here, we show geochemical, tectonic, and paleomagnetic evidence from the Jurassic ophiolites of Albania and Greece for a subduction zone formed in the western Neotethys parallel to a spreading ridge along an oceanic detachment fault. With 2-D numerical modeling exploring the evolution of a detachment-ridge system experiencing compression, we show that serpentinized detachments are always weaker than spreading ridges. We conclude that, owing to their extreme weakness, oceanic detachments can effectively localize deformation under perpendicular far-field forcing, providing ideal conditions to nucleate new subduction zones parallel and close to (or at) spreading ridges. Direct implication of this, is that resumed magmatic activity in the forearc during subduction initiation can yield widespread accretion of suprasubduction zone ophiolites at or close to the paleoridge. Our new model casts the enigmatic origin of regionally extensive ophiolite belts in a novel geodynamic context, and calls for future research on three-dimensional modeling of subduction initiation and how upper plate extension is associated with that.

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[1] Expeditions 304 and 305 of the Integrated Ocean Drilling Program cored and logged a 1.4 km section of the domal core of Atlantis Massif. Postdrilling research results summarized here constrain the structure and lithology of the Central Dome of this oceanic core complex. The dominantly gabbroic sequence recovered contrasts with predrilling predictions; application of the ground truth in subsequent geophysical processing has B071031 of 25 produced self-consistent models for the Central Dome. The presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100-220 kyr, and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth. Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, igneous contacts, and to an interval of locally intense serpentinization in olivine-rich troctolite. Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies, but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized, despite paleomagnetic indications that the dome has undergone at least 45°rotation, presumably during unroofing via detachment faulting. Both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent Southern Ridge of the massif. There, the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises ∼70% of recovered samples. We develop a working model of the evolution of Atlantis Massif over the past 2 Myr, outlining several stages that could explain the observed similarities and differences between the Central Dome and the Southern Ridge.

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Three-quarters of the ocean crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the seafloor.Despite the significance of these rocks, sampling them in situ is extremely challenging due to the overlying dikes and lavas. This means that our models for

Formation of new subduction zones represents one of the cornerstones of plate tectonics, yet both the kinematics and geodynamics governing this process remain enigmatic. A major subduction initiation event occurred in the Late Cretaceous, within the Neo‐Tethys Ocean between Gondwana and Eurasia. Suprasubduction zone ophiolites (i.e., emerged fragments of ancient oceanic lithosphere formed at suprasubduction spreading centers) were generated during this subduction event and are today distributed in the eastern Mediterranean region along three ~E‐W trending ophiolitic belts. Several models have been proposed to explain the formation of these ophiolites and the evolution of the associated intra‐Neo‐Tethyan subduction zone. Here we present new paleospreading directions from six Upper Cretaceous ophiolites of Turkey, Cyprus, and Syria, calculated by using new and published paleomagnetic data from sheeted dyke complexes. Our results show that ~NNE‐SSW subduction zones were formed within the Neo‐Tethys during the Late Cretaceous, which we propose were part of a major step‐shaped subduction system composed of ~NNE‐SSW and ~WNW‐ESE segments. We infer that this subduction system developed within old (Triassic?) lithosphere, along fracture zones and perpendicular weakness zones, since the Neo‐Tethyan spreading ridge formed during Gondwana fragmentation would have already been subducted at the Pontides subduction zone by the Late Cretaceous. Our new results provide an alternative kinematic model of Cretaceous Neo‐Tethyan subduction initiation and call for future research on the mechanisms of subduction inception within old (and cold) lithosphere and the formation of metamorphic soles below suprasubduction zone ophiolites in the absence of nearby spreading ridges.

Serpentinization of ultramafic rocks during hydrothermal alteration at mid-ocean ridges profoundly changes the physical, chemical, rheological, and magnetic properties of the oceanic lithosphere. There is renewed interest in this process following the discovery of widespread exposures of serpentinized mantle on the seafloor in slow spreading oceans. Unroofing of mantle rocks in these settings is achieved by displacement along oceanic detachment faults, which eventually results in structures known as oceanic core complexes (OCCs). However, we have limited understanding of the mechanisms of serpentinization at the seafloor and in particular their relationship with the evolution of OCCs. Since magnetite is a direct product of serpentinization, the magnetic properties of variably serpentinized peridotites can provide unique insights into these mechanisms and their evolution in the oceanic lithosphere. Here we present new results from an integrated, rock magnetic, paleomagnetic, and petrological study of variably serpentinized peridotites from the first fossil OCC recognized in an ophiolite. Integration with existing data from mid-ocean ridge-related abyssal peridotites recovered from several scientific ocean drilling sites yields the first magnetic database from peridotites extending across the complete range (0-100%) of degrees of serpentinization. Variations in a range of magnetic parameters with serpentinization, and associated paleomagnetic data, provide: (i) key constraints on the mechanism(s) of serpentinization at mid-ocean ridges; (ii) insights on the potential for serpentinized peridotites to contribute to marine magnetic anomalies; and (iii) evidence that leads to a new conceptual model for the evolution of serpentinization and related remanence acquisition at OCCs.