Guido M. Gianni

Continental rifting reflects the balance between tectonic forces and lithospheric strength, yet the mechanisms of breakup remain debated. Rapid rifting of the Gulf of California oblique rift (GOCOR) ~6 million to 8 million years ago is a key example; it may have resulted from the lithosphere responding to external kinematics or acceleration of long-lived extension. The dynamics of either scenario remain incompletely understood. In this study, we highlight that the rapid rifting of the GOCOR coincides with the initiation of Western Pacific double subduction. Using three-dimensional geodynamic models, we show that such subduction reorganization can transmit extensional stresses of ~5 to 15 megapascals across the Pacific plate, loading the GOCOR and the southern San Andreas fault. Subduction reorganization may represent the missing driver for the initiation of rapid GOCOR rifting within a preconditioned lithosphere and may be an overlooked tectonic force behind other rifting events.

Mantle rocks undergoing serpentinization can generate significant amounts of natural hydrogen, yet the rates and controlling processes remain poorly understood. Here, we constrain the possible hydrogen generation rates in two distinct mantle rock types, the fertile lherzolites of the Western Pyrenees and the depleted harzburgites of Northern California, to relatively low rates of ~0.1 to ~0.5 tonnes H₂ yr⁻¹ km⁻³ of reactive rock. When integrated over the full reactive volumes, this corresponds to total production rates of ~300 to ~600 tonnes H₂ yr⁻¹. By combining three-dimensional geophysical inversion with numerical modelling of fluid-rock processes, we show that hydrogen generation rates are mainly limited by H₂ saturation in the fluid and reaction kinetics. Under these constraints, hydrogen generation in mantle-derived serpentinization systems proceeds slowly, making rapid large-scale replenishment unlikely and suggesting that large, economically relevant accumulations, would require timescales of thousands to tens of thousands of years to develop.

The Andean orogenic front between 31.5°S and 32.5°S represents a structurally complex and seismically active transitional zone of the southern Central Andes, where thin-skinned deformation of the Precordillera range interacts with thick-skinned tectonics of the Sierras Pampeanas broken foreland ranges. To date, a high-resolution three-dimensional structural model for this sector, which would allow a better understanding of the structural systems and their evolution, is lacking. To address this gap, we integrate 36 reprocessed 2D seismic lines, including, for the first time, N–S oriented profiles, together with gravity data, oil-well information, and detailed surface geological mapping. Interpretation of reprocessed seismic reflection lines depict a hybrid passive-roof structural architecture controlled by the uplift of a basement wedge, whose roof reaches depths of ~ 7 km and acts as the main detachment surface for an overlying thin-skinned thrust system. The basement wedge is emplaced beneath a thick Paleozoic-Neoproterozoic package (~4.5 km), reinterpreted as part of the Cuyania terrane carbonate platform, which was intensely deformed during pre-Andean tectonic events, possibly related to the Ordovician Ocloyic orogeny. The proposed kinematic model integrates at least eight evolutionary stages, ranging from Paleozoic deformation, through episodes of tilting, regional erosion, and syntectonic sedimentation, to Neogene-Quaternary eastward and southward migration of deformation. The identification of deeply rooted inherited structures and their differential reactivation explain the geometry of the main active folds and thrusts, as well as the spatial and temporal distribution of shortening. The lateral continuity of basement ramps and the large potential rupture areas imply a high seismogenic potential, with the capability of generating earthquakes of Mw ≥ 7. This study provides new constraints on the pre-Andean and Andean tectonic evolution of the region and establishes a robust structural framework for seismogenic potential assessment in a densely populated sector of west-central Argentina.

Establishing the frontal extent of ancient flat subduction events from the geologic record can be challenging. This difficulty arises because magmatic activity in the arc typically ceases during complete slab flattening, and other meaningful proxies are usually absent. To address this issue, we examine early Paleocene intraplate magmatic units in central Patagonia, specifically, the La Angostura and Pagasartundua basalts. These basalts erupted and were emplaced during the final phase of a significant flat subduction event, referred to as the Nalé flat slab. The small outcrops of these units are composed of metaluminous alkaline basalts, whose origin would be related to the decompression melting of the sub‐slab asthenosphere. This melting likely occurred due to local slab gaps in the frontal section of a flattened slab, where the oceanic lithosphere resumed a steep angle. The resulting mantle primitive melts would have caused partial melting of minor portions of the upper slab (including eclogitized components). The interaction of these two end‐member melts results in slight hybridization, as evidenced by positive anomalies in Cs, Pb, and Li, subtle Ta depletion, and local Th enrichment in the basalts. By recognizing these basalts and their geochemical characteristics in relation to the geodynamic context, we can establish the maximum frontal extent of a large‐scale Late Cretaceous‐Paleocene flat subduction event. Thus, this case represents the first ancient flat slab that is frontally constrained by slab gap‐related intraplate magmatism, a pattern that could be replicated in other similar settings worldwide.

The southern Central Andes forearc preserves extensive low-relief wave-cut platforms and fluvial terraces that record long-term margin uplift, yet the timing and driving mechanisms are still debated. Here we present eleven new in situ ¹⁰Be exposure ages from high fluvial terraces and four ages from a lower terrace, combined with geomorphic analyses across eight adjacent catchments, to reassess the age and tectonic significance of the degradational surfaces (pediplain) present between 29.5 and 32.5°S. Erosion-corrected exposure ages indicate that the high terrace formed during the Early Pleistocene, while a lower terrace records incision at Middle Pleistocene. Longitudinal terrace–channel profiles reveal systematically increasing relief toward the coast that terminates near the surface projection of the 50–60 km slab-depth contour, coincident with the downdip limit of megathrust domain-C earthquakes. This spatial relationship supports a regionally coherent uplift signal produced by the cumulative effect of deep coseismic deformation. In peninsular settings, notably the Altos de Talinay, this long-wavelength signal is overprinted by short-wavelength uplift consistent with localized underplating. Our results demonstrate that the high fluvial terraces and the shore wave-cut platform constitute a single, regionally continuous geomorphic marker recording an Early Pleistocene forearc uplift phase extending from ~16° to 42°S. This orogen-scale emergence implies a subtle but widespread change in subduction dynamics during the last Early Pleistocene, the causes of which are not clearly understood.

Plate-tectonic reorganizations are characterized by rapid shifts in plate motions, boundary forces, and margin tectonics, yet the role of mid-ocean ridge extinctions (MOREs) during these events remains poorly constrained. We address this gap by systematically examining a suite of MOREs occurring within plate reorganizations and by comparing them with strength-profile analyses of thermally evolving oceanic lithosphere beneath spreading axes following spreading shutdown. Thermal models and strength-envelope calculations show that extinct ridges undergo rapid lithospheric strengthening, developing sufficient mechanical resistance to transmit slab-pull stresses within ~2–10 Myr after spreading cessation. This rapid welding transforms formerly weak ridge–transform networks into coherent lithospheric blocks capable of mediating far-field stress transfer. We illustrate this process using three well-constrained Cenozoic MOREs. Extinction of the Wharton Ridge promoted Indo–Australian plate welding and enhanced slab pull along the Sunda–Java trench, accelerating plate motion and sustaining post-collisional India–Asia convergence. Progressive shutdown of Pacific–Farallon ridge fragments enabled coupling between the Pacific and North American plates and facilitated subsequent Pacific stress transmission, driving deformation in the Gulf of California rift and the San Andreas system. In the western Pacific, cessation of the Shikoku Ridge strengthened the Philippine Sea Plate, enabling efficient transmission of slab pull in the Ryukyu–Nankai/Izu–Bonin–Mariana double subduction system and triggering trench advance and regional compressional tectonics in northeast Japan. Our results indicate that MOREs act as active amplifiers of plate reorganizations by enhancing lithospheric coupling and facilitating far-field stress propagation. These findings support a cascading, plate-to-plate mode of tectonic reorganization (rather than mantle-driven) and highlight extinct ridges as critical nodes in the episodic reorganization of the global plate network. Building on these insights, we extend the discussion to the Paleozoic–Mesozoic Tethyan system, where successive terrane collisions and episodes of subduction initiation may likewise have involved MOREs acting as stress transmitters across ridge–transform networks.

Mantle rocks undergoing serpentinization at upper-crustal levels can generate significant amounts of natural hydrogen. Although the potential for substantial hydrogen generation during fluid-rock reactions of these rocks is documented by lab experiments and occurrences of hydrogen-rich natural gas seeps, the exact production rates and the factors limiting production remain poorly understood. Here, we demonstrate that in two mantle rock systems with distinct compositions, the fertile lherzolites of the Western Pyrenees and the depleted harzburgites of Northern California, continuous fluid infiltration may sustain hydrogen generation, but only at rates of approximately 500 tonnes per year. Furthermore, by integrating three-dimensional geophysical inversion with a semi-coupled thermo-hydro-chemical model, we show that the main limiting factors controlling hydrogen yield are H₂ saturation in the fluid, porosity evolution at the reaction front, and reaction kinetics. The generation rates determined within the physico-chemical limits suggest that, although natural hydrogen from mantle rock serpentinization is a clean energy source, it may not be renewable on a large scale. Instead, its generation times for substantial accumulations more closely resemble those of fossil-type resources.