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Smeraglia, Luca
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- PublicationOpen AccessDolostone pulverization induced by coseismic rapid decompression of CO2-rich gas in nature (Matese, Apennines, Italy)(2023)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;; ;; ; ; ; ;South Matese, Apennines, is a hydrothermally and seismically active extensional area characterized by CO2outgassing and Mw≤7.1 earthquakes. There, meters-sized pockets of incohesive pulverized dolostone are hosted within Mesozoic carbonates at the hanging wall of seismically active normal faults. The aim of this paper is to understand the pulverization process. The pulverized dolostone is finely comminuted (down to a few microns), but primary structures, mainly bedding, are preserved. The grain size distribution is similar to that of previously studied pulverized rocks associated with active faults and dissimilar to that of carbonate cataclasites and fault gouges. The pulverized pockets are surrounded by zones (halos), in which the loose grains are cemented, in their original position, by microcrystalline calcite, resulting in a cemented micro-mosaic breccia. Stable isotopes from the cement are compatible with calcite precipitation from rapidly CO2-degassing shallow waters. Comparing our observations with results of laboratory experiments on carbonate pulverization through rapid decompression of pore-hosted CO2, the best explanation for the pulverized dolostone may lie on local accumulations of pressurized CO2-rich gas, suddenly decompressed during earthquakes. The limited permeability of the gas-saturated dolostone must have prevented a prompt escape of the gas from the rock, which was therefore anhydrously pulverized by the rapid expansion of the trapped gas. The sudden decompression must have suctioned bicarbonate-rich groundwaters, from which microcrystalline calcite rapidly precipitated, fossilizing the freshly pulverized dolostone. Calcite precipitation formed an impermeable shield around the pulverized pockets, which, therefore, remained internally uncemented. This process may have occurred over multiple cycles at depths shallower than the CO2subcritical–supercritical boundary (ca. -800m). Although hypothetical, the proposed mechanism is for the first time suggested for an active tectonic environment. The gas rapid decompression could have been triggered by coseismic processes (e.g., dynamic unloading or transient tensile pulses) previously proposed for the formation of other pulverized rocks. The presented case may improve our knowledge of possible chemical-physical processes connected with the subsurface storage of CO2in seismically active areas.186 48 - PublicationRestrictedThe role of trapped fluids during the development and deformation of a carbonate/shale intra-wedge tectonic mélange (Mt. Massico, Southern Apennines, Italy)(2020-05-18)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; Numerous studies exist on exhumed tectonic mélanges along subduction channels whereas, in accretionary wedge interiors, deformation mechanisms and related fluid circulation in tectonic mélanges are still underexplored. We combine structural and microstructural observations with geochemical (stable and clumped isotopes and isotope composition of noble gases in fluid inclusions of calcite veins) and U-Pb geochronological data to define deformation mechanisms and syn-tectonic fluid circulation within the Mt. Massico intra-wedge tectonic mélange, located in the inner part of the central-southern Apennines accretionary wedge, Italy. This mélange developed by shear deformation at the base of a clastic succession. Deformation was characterized by disruption of the primary bedding, mixing, and deformation of relicts of competent olistoliths and strata within a weak matrix of deformed clayey and marly interbeds. Recurrent cycles of mutually overprinting fracturing/veining and pressure-solution processes generated a block-in-matrix texture. The geochemical signatures of syntectonic calcite veins suggest calcite precipitation in a closed system from warm (108°-147 °C) paleofluids, with δ18O vlaues between þ9‰ and 14‰, such as trapped pore waters after extensive 18O exchange with the local limestone host rock and/or derived by clay dehydration processes at T > 120 °C. The 3He/4He ratios in fluid inclusions are lower than 0.1 Ra, indicating that He was exclusively sourced from the crust. We conclude that: (1) intraformational rheological contrasts, inherited trapped fluids, and low-permeability barriers such as marlyshaly matrix, can promote the generation of intra-wedge tectonic mélanges and the development of transient fluid overpressure; (2) clay-rich tectonic mélanges, developed along intra-wedge décollement layers, may generate low-permeability barriers hindering the fluid redistribution within accretionary wedges.790 9 - PublicationRestrictedLithological control on multiple surface ruptures during the 2016–2017 Amatrice-Norcia seismic sequenceOn August 24th 2016, a Mw 6.0 earthquake started the Amatrice - Norcia (Central Italy) seismic sequence, generated by the extensional tectonics along the Apennines, that had its apex with the Mw 6.5 October 30th mainshock. As a unique documented case reported in Italy, complex surface faulting occurred during both earthquakes along the Mt. Vettore fault. Multiple surface faulting was accompanied at depth by the development of a km-scale normal fault-propagation fold. This fold was characterized by breakthrough and by surface rupture within thick carbonatic layers only in the central and north-western area (Mt. Vettore). On the contrary, the fault remained blind where flexural slip was active in sandy-silty turbiditic deposits in the south-eastern area (Mt. Gorzano). We explain the different faulting behaviour with the occurrence of more rigid and competent lithologies in areas characterized by breakthrough and with the occurrence of weaker lithologies in areas characterized by blind faulting. Overall, the entire seismic sequence appears as a gradual gravitational adjustment of the hangingwall block, slipping along a NW-trending and 80 km long fault system. In particular, the following crustal blocks, partially overlapping and with different length (30, 40 and 22 km, respectively), progressively collapsed during the sequence: the Amatrice sector during the August 24th 2016, Mw 6.0 event, the Norcia-Visso sector during the October 26th 2016, Mw 5.9 and the 2016 October 30th Mw 6.5 event, and the Campotosto Lake sector during the four January 18th 2017, M>5 events. The progressive involvement of these three rock volumes, during the seismic sequence is here explained by the occurrence of a low angle detachment that limited the maximum potential depth of the mainshocks and consequently the dimensions of involved rock volumes, therefore limiting the magnitudes of the mainshocks.
187 9 - PublicationRestrictedCrustal-scale fluid circulation and co-seismic shallow comb-veining along the longest normal fault of the central Apennines, Italy(2018)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;; ; ;; ; ; ;The extensional Val Roveto Fault, which is the longest exhumed potentially-seismogenic structure of central Apennines, Italy, is examined to constrain earthquake-related fluid circulation and fluid sources within shallow carbonate-hosted faults. The study focuses on fault-related comb and slip-parallel veins that are calcite-filled and cut through the principal surface of the Val Roveto Fault. We observe multiple crack-and-seal events characterized by several veining episodes, probably related to different slip increments along the fault plane. We show that vein calcite precipitated in Late Pleistocene time below the present-day outcrop level at a maximum depth of ∼350 m and temperatures between 32 and 64◦C from meteoric-derived fluids modified by reactions with crustal rocks and with a mantle contribution (up to ∼39%). The observed warm temperatures are not compatible with a shallow (≤∼350m) precipitation depth, which, in this region, is dominated by circulation of cold meteoric water and/or shallow groundwater. Based on structural–geochemical data, we propose that deep-seated crust–mantle-derived warm fluids were squeezed upward during earthquakes and were hence responsible for calcite precipitation at shallow depths in co-seismic comb and slip-parallel fractures. As comb-and slip-parallel veins are rather common, particularly along seismogenic extensional faults, we suggest that further studies are necessary to test whether these veins are often of co-seismic origin. If so, they may become a unique and irreplaceable tool to unravel the seismic history of hazardous active faults.437 9 - PublicationRestrictedField- to nano-scale evidence for weakening mechanisms along the fault of the 2016 Amatrice and Norcia earthquakes, ItalyIn August and October 2016, two normal fault earthquakes (Mw 6.0 and Mw 6.5, respectively) struck the Amatrice-Norcia area in the central Apennines, Italy. The mainshocks nucleated at depths of ~ 7–9 km with the co-seismic slip propagating upward along the Mt. Gorzano Fault (MGF) and Mt. Vettore Fault System (MVFS). To recognize possible weakening mechanisms along the carbonate-hosted seismogenic faults that generated the Amatrice-Norcia earthquakes, the fresh co-seismic fault exposure (i.e., “nastrino”) exposed along the Mt. Vettoretto Fault was sampled and analyzed. This exposed fault belongs to the MVFS and was exhumed from ~ 2–3 km depth. Over the fresh fault surface, phyllosilicates concentrated and localized along mm- to μm-thick layers, and truncated clasts and fluid-like structures were found. At the nano-scale, instead of their common platy-lamellar crystallographic texture, the analyzed phyllosilicates consist of welded nm-thick nanospherules and nanotubes similar to phyllosilicates deformed in rotary shear apparatus at seismic velocities or altered under high hydrothermal temperatures (> 250 °C). Moreover, the attitude of the Mt. Vettoretto Fault and its kinematics inferred from exposed slickenlines are consistent with the co-seismic fault and slip vectors obtained from the focal mechanisms computed for the 2016 mainshocks. All these pieces of evidence suggest that the Mt. Vettoretto Fault slipped seismically during past earthquakes and that co-seismic slip was assisted and facilitated at depths of < 3 km by phyllosilicate-rich layers and overpressured fluids. The same weakening processes may also have been decisive in facilitating the co-seismic slip propagation during the 2016 Mw 6.0 Amatrice and Mw 6.5 Norcia earthquakes. The microstructures found along the Mt. Vettoretto Fault, which is certainly a seismogenic fault, provide a realistic synoptic picture of co-seismic processes and weakening mechanisms that may occur in carbonate-hosted seismogenic faults.
91 5 - PublicationOpen AccessMicrostructural evidence for seismic and aseismic slips along clay-bearing, carbonate faults(2017)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ;; ; ; ;In this multimethodological study, microstructural observations of fault rocks are combined with micromechanical property analyses (contact resonance atomic force microscopy (CR-AFM)) and with rotary friction experiments (Slow- to High-Velocity rotary-shear friction Apparatus apparatus) to find evidence of seismic to aseismic slip and understand the nanoscale rheology of clay-bearing, carbonate-hosted faults. Fluidized structures, truncated clasts, pores and vesicles, and phyllosilicate nanosized spherules and tubes suggest fast deformation events occurred during seismic slip, whereas clay-assisted pressure-solution processes, clumped clasts, foliation surfaces, and mantled clasts indicate slow deformation events occurred during postseismic/interseismic periods. CR-AFM measurements show that the occurrence of ~5 wt % of clay within the carbonate-hosted gouges can significantly reduce the fault core stiffness at nanoscale. In addition, during high-velocity friction experiments simulating seismic slip conditions, the presence of ultrathin phyllosilicate-bearing (≤3 wt %) layers within calcite gouges, as those observed in the natural fault, show faster dynamic weakening than that of pure calcite gouges. The weak behavior of such layers could facilitate the upward propagation of seismic slip during earthquakes, thus possibly enhancing surface faulting. Microstructural observations and experimental evidence fit some well-known geophysical and geodetic observations on the short- to long-term mechanical behavior of faults such as postseismic/interseismic aseismic creep, interseismic fault locking, and seismic slip propagation up to the Earth's surface. ©2017. American Geophysical Union. All Rights Reserved.163 49 - PublicationOpen AccessUltra-thin clay layers facilitate seismic slip in carbonate faults(2017)
; ; ; ; ; ; ; ; ; ; ;; ; Many earthquakes propagate up to the Earth's surface producing surface ruptures. Seismic slip propagation is facilitated by along-fault low dynamic frictional resistance, which is controlled by a number of physico-chemical lubrication mechanisms. In particular, rotary shear experiments conducted at seismic slip rates (1 ms-1) show that phyllosilicates can facilitate co-seismic slip along faults during earthquakes. This evidence is crucial for hazard assessment along oceanic subduction zones, where pelagic clays participate in seismic slip propagation. Conversely, the reason why, in continental domains, co-seismic slip along faults can propagate up to the Earth's surface is still poorly understood. We document the occurrence of micrometer-thick phyllosilicate-bearing layers along a carbonate-hosted seismogenic extensional fault in the central Apennines, Italy. Using friction experiments, we demonstrate that, at seismic slip rates (1 ms-1), similar calcite gouges with pre-existing phyllosilicate-bearing (clay content ≤3 wt.%) micro-layers weaken faster than calcite gouges or mixed calcite-phyllosilicate gouges. We thus propose that, within calcite gouge, ultra-low clay content (≤3 wt.%) localized along micrometer-thick layers can facilitate seismic slip propagation during earthquakes in continental domains, possibly enhancing surface displacement.134 55 - PublicationOpen AccessQuaternary geology and paleoseismology in the Fucino and L’Aquila basins(2016)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;; ;; ;; ; ;; ; ; ; ;; ;; ;; ;; This 2 days-long field trip aims at exploring field evidence of active tectonics, paleoseismology and Quaternary geology in the Fucino and L’Aquila intermountain basins and adjacent areas, within the inner sector of Central Apennines, characterized by extensional tectonics since at least 3 Ma. Each basin is the result of repeated strong earthquakes over a geological time interval, where the 1915 and 2009 earthquakes are only the latest seismic events recorded respectively in the Fucino and L’Aquila areas. Paleoseismic investigations have found clear evidence of several past earthquakes in the Late Quaternary to Holocene period. Active tectonics has strongly imprinted also the long-term landscape evolution, as clearly shown by numerous geomorphic and stratigraphic features. Due to the very rich local historical and seismological database, and to the extensive Quaternary tectonics and earthquake geology research conducted in the last decades by several Italian and international teams, the area visited by this field trip is today one of the best studied paleoseismological field laboratories in the world. The Fucino and L’Aquila basins preserve excellent exposures of earthquake environmental effects (mainly surface faulting), their cumulative effect on the landscape, and their interaction with the urban history and environment. This is therefore a key region for understanding the role played by earthquake environmental effects in the Quaternary evolution of actively deforming regions, also as a major contribution to seismic risk mitigation strategies.454 75