From thrusting to back-arc extension: seismic structure and field evidence of the Apennine Tyrrhenian margin (Central Italy)

The Apennine Tyrrhenian margin records the evolutionary steps of the back-arc basin developed at the rear of a E-ward migrating fold-and-thrust belt. As well-documented in literature, the counterclockwise rotation of the Apennines is related to the southward increase of the roll back-related subduction of the Adria slab. This led first to the progressive incorporation of thrust sheets within the Apennine prism in the upper plate and later to its subsequent back-arc extension that is contemporaneous with the continuate inarching of the Apennine front towards the Adriatic and Ionian seas. Uncertainties arise on the structural style and timing in the internal Apennines between the orogenic and post-orogenic stages, that are respectively represented by thrust-sheet implacement, and crustal thinning

Understanding the Origin and Mixing of Deep Fluids in Shallow Aquifers and Possible Implications for Crustal Deformation Studies: San Vittorino Plain, Central Apennines

Expanding knowledge about the origin and mixing of deep fluids and the water–rock–gas interactions in aquifer systems can represent an improvement in the comprehension of crustal deformation processes. An analysis of the deep and meteoric fluid contributions to a regional groundwater circulation model in an active seismic area has been carried out. We performed two hydrogeochemical screenings of 15 springs in the San Vittorino Plain (central Italy). Furthermore, we updated the San Vittorino Plain structural setting with a new geological map and cross-sections, highlighting how and where the aquifers are intersected by faults. The application of Na-Li geothermometers, coupled with trace element and gas analyses, agrees in attributing the highest temperatures (>150 °C), the greatest enrichments in Li (124.3 ppb) and Cs (>5 ppb), and traces of mantle-derived He (1–2%) to springs located in correspondence with high-angle faults (i.e., S5, S11, S13, and S15). This evidence points out the role of faults acting as vehicles for deep fluids into regional carbonate aquifers. These results highlight the criteria for identifying the most suitable sites for monitoring variations in groundwater geochemistry due to the uprising of deep fluids modulated by fault activity to be further correlated with crustal deformation and possibly with seismicit

From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy)

International audienceThe Apennines are a low-temperature accretionary prism generated by the west-directed subduction of the Adriatic–Ionian plate, whose structural origin is still to be fully understood. The highest and best-exposed seg-ment of the Apennines, the Gran Sasso range is here documented to unravel the tectonic history of the northern tip of Gondwana. It is located along a NE-verging salient of thrust sheets decoupling the sedimentary cover of the subducting Adriatic lithosphere. Field mapping and structural analysis along the E–W trending left-lateral transpressive segment of the salient highlight the interplay of the inherited Mesozoic passive margin stratigraph-ic and tectonic framework with the Neogene contraction. The rheological differences between the massive carbonate platform and the well-bedded turbiditic and pelagic limestones determined along-strike undulations of the thrusts geometries and fold styles during shortening. Heterogeneities are due to inherited syn-and post-rift Mesozoic tectonics. The Gran Sasso overturned anticline shows a backlimb anomalously tilted toward the foreland and we infer this dip as being related to a deeper back-thrust of a triangle zone. The pinching out of the foredeep sequence on the growth anticline forelimb dates the contractional phases of the region to the late Messinian. From the late Pliocene to Present, the area has been uplifted and extended about 2 km by oblique normal faults cross-cutting the accretionary prism. Some of them are seismically active, as shown by the 2009 Mw 6.3 L'Aquila earthquake

Cretaceous syn-sedimentary faulting in the Wildhorn Nappe (SW Switzerland)

International audienceDuring Cretaceous time, the area of the future Helvetic nappes (Central Alps, south-western Switzerland) was part of a large ramp-type carbonate depositional system on the European margin, in which the area of the Wildhorn Nappe was transitional to the more distal and relatively deeper Ultrahelvetic basin. The Wildhorn Nappe includes an Upper Cretaceous succession bearing clear evidence for syn-sedimentary normal faulting, such as syn-sedimentary geometries related to well oriented NE-striking faults, sedimentary dykes, lateral variations in the thickness and facies of formations, anomalous and discordant contacts corresponding to palaeo-escarpments, and slump folds. Four stages of syn-sedimentary fault activity have been recognized. (1) Post-Cenomanian disruption and exhumation of the Schrattenkalk platform related to distributed normal faulting, which contributed to the initiation of karst erosion on topographic highs and sedimentation in topographic lows. (2) Turonian–Santonian marine transgression accompanied by localized normal faulting, creating growth-fault structures, differential subsidence and slope instability. A transition from distributed to more localized faulting is observed, related to a final stage in the evolution of the Cretaceous extensional process. (3) Early Maastrichtian faulting. The facies and thickness of subsequent sediments reflect a passive adaption to the pre-existing topography of the sea floor, established during the earlier tectonic movements. (4) Post-Maastrichtian north-directed tilt and erosion. In the Wildhorn Nappe, palaeo-fault activity most probably ended in the Early Maastrichtian rather than continuing into the Eocene. Until now, the regional importance and magnitude of Late Cretaceous extension has not been recognized in the Helvetic domain. This widespread event may be related to post-breakup extensional tectonics along the European margin or, alternatively but less likely, to lateral gravitational collapse of the margin

Veining and post-nappe transtensional faulting in the SW Helvetic Alps (Switzerland)

International audienceIn the Rawil Depression of the south-westernHelvetic Alps, oblique (normal plus dextral strike slip) faultsare common but their relative age, regional role and theprocesses leading to their development are not yet fullydetermined. This field study establishes the orientation anddistribution of these faults and associated veins, the faultgeometries and kinematics, and the relationship betweenveining and faulting. Three post-nappe sets of faults can bedistinguished on the basis of their strike: (1) NNW/NWstriking;(2) WNW/W-striking; and (3) WSW-striking ones.Faults sets (1) and (2) generally dip at moderate angle to theSW and typically develop domino-like structures, with aspacing of around 1 km. Fault set (3) is steeper, the strikeslipcomponent is larger, and it is directly associated with themain regional-scale branch of the Simplon–Rhoˆne Fault.Although these faults are broadly coeval, there are clearexamples of set (2) cross-cutting set (1), and set (3) crosscutting(1) and (2), which establishes, at least locally, a relativechronological succession. This transtensional faultinglargely post-dates folding related to nappe formation becausefold geometry can be matched across the obliquely crosscuttingfaults. Regional dextral-transtensional fault developmentis related to differential exhumation of the External Crystalline Massifs over the last 15–17 Ma, coeval withrelated movement on the Simplon–Rhoˆne Fault. Locally there is a transition from an initial more ductile myloniticfabric to cataclasite, accompanied by brittle-ductile veiningand intense pressure solution. This progressive embrittlementduring faulting is due to exhumation and cooling duringfaulting, higher strain rates, or increased pore-fluid pressures.Faults of sets (1) and (2) developed across the brittleductiletransition and may represent fossil seismogenic zonesin rocks with high pore-fluid pressure, providing exposedexamples of seismic faults in similar rocks currently active atdepth north of the Rhoˆne Valley

Brittle-ductile deformation and kinematics during exhumation of metamorphic complexes below detachments: examples from Sifnos and Syros Islands (Greece)

International audienceExhumation of metamorphic core complexes is accompanied by progressive strain localization within large-scale shear zones, which may evolve into long-lived bounding detachments affected by ductile to brittle deformation. Despite the well-studied P-T-t patterns of individual nappes, their relative timing, mode and kinematics of exhuma-tion are debated. In this study, in the frame of the Mediterranean syn-and post-orogenic deformation, examples of shear zone hierarchization and strain localization from Sifnos and Syros islands (Cyclades, Greece) are documented in detail in order to explain 3D-geometries and regional kinematics and are here tentatively related to the Ar/Ar ages available in literature. During the Eocene syn-orogenic uplift, the degree of strain localization increases progressively from blue-to green-schists deformation. Some of these shear zones where then reworked during the Oligo-Miocene post-orogenic deformation in different, usually warmer P-T conditions and a new episode of strain localisation, and an evolution toward brittle faulting, either along the main detachments or along newly created faults (as in Sifnos). Such shear zones demonstrate long-lived efficiency, especially where fluid circulation enhance retrograde metamorphic reactions. During Neogene, the final shape and exhumation of domes is the result of crustal thinning and brittle-ductile deformation in the whole Cycladic region. Although stretching directions along individual kilometric scale shear zones may be complex in the details, a simple general picture is shown for the Oligo-Miocene episode, less so for the Eocene one. Most Cycladic islands show a top-to-the-North sense of ductile shear from the syn-orogenic to the post-orogenic stage, this is the case of Sifnos for instance. The syn-orogenic stretching is however often more E-W trending, as exemplified by Syros and Tinos. The top-North or Top-East sense of shear is attributed to the NCDS for the post-orogenic stage and to a syn-orogenic detachment (Vari D.) for the syn-orogenic stage. However the southwestern Cyclades show clear top-South post-orogenic kine-matic indicators related to the WCDS, with a complete ductile to brittle evolution. As for the NCDS the WCDS leads to intense ductile deformation in its footwall. To the east of Serifos this ductile top-South sense of shear is lost. Only top-South brittle faults are found in Folegandros and Sifnos where they cut across the older foliation. The complexity of the distribution of shear sense on Syros and Sifnos, both in the ductile and brittle regimes, during the post-and syn-orogenic stages, is discussed in the framework of the transition in time from syn-orogenic exhumation faults to post-orogenic detachments and of the interactions between different sets of detachments, such as the NCDS and the WCDS

Strain localization in shear zones during exhumation: a graphicalapproach to facies interpretation

International audienceStrain localization is a fundamental process determining plate tectonics. It is expressed in the ductile field by shearzones where strain concentrates. Despite their worldwide distribution in most metamorphic units, their detailedcharacterization and processes comprehension are far to be fully addressed. In this work, a graphic approach totectono-metamorphic facies identification is applied to the Delfini Shear Zone in Syros (Cyclades, Greece), whichis mostly characterized by metabasites displaying different degree of retrogression from fresh eclogite to prasinite.Several exhumation mechanisms brought them from the depths of the subduction zone to the surface, fromsyn-orogenic exhumation to post-orogenic backarc extension. Boudinage, grain-size reduction and metamorphicreactions determinate strain localization across well-deformed volumes of rocks organized in a hierarchic frameof smaller individual shear zones (10-25 meters thick). The most representative of them can be subdivided in 5tectono-metamorphic (Tm) facies, TmA to E. TmA records HP witnesses and older folding stages preserved withinlarge boudins as large as 1-2 m across. TmB is characterized by much smaller and progressively more asymmetricboudins and sigmoids. TmC is defined by well-transposed sub- to plane-parallel blueschist textures crossed bychlorite-shear bands bounding the newly formed boudins. When strain increases (facies TmD-E), the texture isprogressively retrograded to LP-HT greenschist-facies conditions. Those observations allowed us to establish a sequenceof stages of strain localization. The first stage (1) is determined by quite symmetric folding and boudinage.In a second stage (2), grain-size reduction is associated with dense shear bands formation along previously formedglaucophane and quartz-rich veins. With progressively more localized strain, mode-I veins may arrange as tensiongashes that gradually evolve to blueschist shear bands. This process determinates the formation of smaller asymmetricsyn-blueschist boudins and is associated with epidote grain-size reduction, synonym of more intense strainlocalization. In the latest stage (3), newly formed chlorite-rich shear bands bound new boudins of the previouslyproduced terms having blueschist fine-grained heritage. Sense of shear is consistently top-to-the-ENE through theentire localization process. Greenschist-related boudinage is associated with quartz precipitation along the mainshear bands and at the necks of boudins, places of strain localization. Metamorphic reactions and transpositionsgradually obliterate the former texture, being progressively more active towards the top, where minor shear zoneshave been deforming at the expenses of similar folds and epidote-rich levels. The resulting qualitative analysismeans to be a proxy for further numerical models and a comparison to similar field-study areas