Regional polyphase deformation of the Eastern Sierras Pampeanas (Argentina Andean foreland): strengths and weaknesses of paleostress inversion

The Eastern Sierras Pampeanas of central Argentina are composed of a series of basement-cored ranges, located in the Andean foreland c. 600 km east of the Andean Cordillera. Although uplift of the ranges is partly attributed to the regional Neogene evolution (Ramos et al. 2002), many questions remain as to the timing and style of deformation. In fact, the Eastern Sierras Pampeanas show compelling evidence of a long lasting brittle history (spanning the Early Carboniferous to Present time), characterised by several deformation events reflecting different tectonic regimes. Each deformation phase resulted in further strain increments accommodated by reactivation of inherited structures and rheological anisotropies (Martino 2003). In the framework of such a polyphase brittle tectonic evolution affecting highly anisotropic basement rocks, the application of paleostress inversion methods, though powerful, suffers from some shortcomings, such as the likely heterogeneous character of fault slip datasets and the possible reactivation of even highly misoriented structures, and thus requires careful analysis. The challenge is to gather sufficient fault-slip data, to develop a proper understanding of the regional evolution. This is done by the identification of internally consistent fault and fracture subsets (associated to distinct stress states on the basis of their geometric and kinematic compatibility) in order to generate a chronologically-constrained evolutionary conceptual model. Based on large fault-slip datasets collected in the Sierras de Cordoba (Eastern Sierras Pampeanas), reduced stress tensors have been generated and interpreted as part of an evolutionary model by considering the obtained results against: (i) existing K\u2013Ar illite ages of fault gouges in the study area (Bense et al. 2013), (ii) the nature and orientation of pre-existing anisotropies and (iii) the present-day stress field due to the convergence of the Nazca and South America plates (main shortening oriented WSW-ENE). Although remarkable differences in reactivation mechanisms have been observed for the various studied lithological domains (schist, gneiss and granitic rocks), the brittle regional polyphase deformation of the Eastern Sierras Pampeanas appears to be dominated by two extensional episodes (sigma3 oriented NE/ENE and WNW, respectively), which can be associated with Middle-Late Permian to Early Cretaceous tectonism, followed by a compressional paleostress (sigma1 oriented ENE), which is compatible with the present day Andean convergence. Paleostress inversion techniques, despite all uncertainties involved, represent a robust approach to disentangle complex polyphase deformation histories both in term of reactivation mechanisms and strain partitioning

A likely geological record of deep tremor and slow slip events from a subducted continental broken formation.

open2noFluids in subduction zones play a key role in controlling seismic activity, drastically affecting the rheology of rocks, triggering mineral reactions, and lowering the effective stress. Fluctuating pore pressure is one important parameter for the switch between brittle and ductile deformation, thus impacting seismogenesis. Episodic tremor and slow slip events (ETS) have been proposed as a common feature of the geophysical signature of subduction zones. Their geological record, however, remains scanty. Only the detailed and further characterization of exhumed fossil geological settings can help fill this knowledge gap. Here we propose that fluctuating pore pressure linked to metamorphic dehydration reactions steered cyclic and ETS-related brittle and ductile deformation of continental crustal rocks in the subduction channel of the Apennines. Dilational shear veins and ductile mylonitic shear zones formed broadly coevally at minimum 1 GPa and 350 °C, corresponding to ~ 30–40 km depth in the subduction zone. We identify carpholite in Ca-poor metasediments as an important carrier of H2O to depths > 40 km in cold subduction zones. Our results suggest that the described (micro)structures and mineralogical changes can be ascribed to deep ETS and provide a useful reference for the interpretation of similar tectonic settings worldwide.This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 839779.openGiuntoli, F., Viola, G.Giuntoli, F., Viola, G

Cyclic Brittle‐Ductile Oscillations Recorded in Exhumed High‐Pressure Continental Units: A Record of Deep Episodic Tremor and Slow Slip Events in the Northern Apennines

open2noThe geological record of deep fossil seismogenesis in subduction zones is limited due to common rock overprinting during exhumation and only a few regions expose well-preserved deeply exhumed structures. We investigated a mesoscopic contractional duplex formed at blueschist facies conditions in continental metasediments in the Northern Apennines (Italy). Field observations reveal strain partitioning within the duplex between metapelite bands, corresponding to high-strain zones, and metaquartzarenite bands, which form low-strain, imbricated metric horses. Dilational shear veins occur in both lithotypes and are composed of quartz and carpholite fibers defining a stretching lineation parallel to the regional SW-NE transport direction. Geometrical, cross cutting and petrographic relationships suggest that veins formed broadly syn-mylonitization. Thermodynamic modeling constrains the formation of the mylonitic foliation to >0.7 GPa and ∼400°C and the dilational shear veins vein to ∼1.1 GPa and 350°C. Therefore, we document a top-to-the-E-NE thrust that formed at the deepest conditions recorded by this unit of the Northern Apennines. In the thrusted continental metasediments, aqueous fluid locally released by metamorphic dehydration reactions transiently increased pore pressure, in turn triggering brittleductile cyclicity. We propose that blueschist facies dilational shear veins and mylonitic foliation represent a geological record of deep episodic tremor and slow slip events. To the best of our knowledge, this is the first time that these events are genetically associated with brittle and ductile structures of a mesoscopic duplex. We suggest that these structures could be common features of the high-pressure metamorphic units of the Apennine orogenic belt.This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 839779. Open Access Funding provided by Universita di Bologna within the CRUI-CARE AgreementopenGiuntoli, Francesco; Viola, GiulioGiuntoli, Francesco; Viola, Giuli

Rock mechanical modelling of the Bentonite Rock Interaction Experiment, Äspö Hard Rock Laboratory, Sweden

Abstract The Bentonite Rock Interaction Experiment (BRIE) was performed in a tunnel at a depth of 420 m at the Aspo Hard Rock Laboratory in Sweden. The experiment focused on the hydraulic properties of rock and bentonite aiming at investigating the exchange of water across a bentonite-rock interface. The hypothesis for the mechanical modelling presented here was that changes in flow (observed in rock and on bentonite parcels) were due to local mechanical deformation. Induced stresses related to the construction (and experimental) stages for the BRIE site such as excavation of tunnels, drilling and over-coring of two vertical, tunnel-floor boreholes and, finally, installation and swelling of bentonite, were expected to be the main causes of these deformations. We assumed that this could be investigated using a step-wise rock mechanical modelling approach (with a relevant modelling sequence) and validated by using a transdisciplinary approach including field structural geological mapping (geometric, kinematic and dynamic interpretation of the exposed fracture sets) and hydrogeological investigations. For key fractures intersecting the boreholes, the modelled fracture normal and shear displacements were found to be local, small, and in line with field observations and measurements for BRIE. Results point at an agreement between the spatial locations of changes in flow identified from the bentonite parcels and the locations of inelastic deformation indicated by mechanical modelling for a reverse stress regime. Besides providing information about the key fractures, the structural mapping allowed to establish solid relationships between brittle structural features in the tunnel and in the cores, which were used as, or compared to, the main fracture input to the rock mechanical modelling. The identified fracture sets were found to be structurally reconcilable with the larger-scale tectonic picture of the area

Geology of the San Donato - Costa Thrust Zone, Belluno Thrust System, eastern Southern Alps, northern Italy

We present a 1:7500 scale geological map of part of the footwall of the south verging Belluno Thrust of the seismically active eastern Southern Alps of northern Italy. We report a previously unknown thrust zone, the “San Donato-Costa Thrust Zone”, that cuts across the local MesoCenozoic stratigraphic succession. 1:2500 and 1:5000 geological mapping, in combination with the revision and improvement of the local lithostratigraphy and detailed structural analysis, provided new insights into the thrust geometry and the setting of the greater area it deforms. We show that the recorded deformation style (e.g. folding vs. faulting) exhibits notable variations within the affected Meso-Cenozoic stratigraphic succession, with shortening accommodated by fold trains and several subparallel thrust splays with variable amounts of stratigraphic offset cumulated during repeated faulting. Our results contribute to a better definition of folding and faulting within the Mesozoic carbonate multilayer system of the eastern Southern Alps

The structural, metamorphic and magmatic evolution of Mesoproterozoic orogens

The Mesoproterozoic (1600–1000 Ma) is an Era of Earth history that has been defined in the literature as being quiescent in terms of both tectonics and the evolution of the biosphere and atmosphere (Holland, 2006, Piper, 2013b and Young, 2013). The ‘boring billion’ is an informal term that is given to a time period overlapping the Mesoproterozoic period, extending from 1.85 to 0.85 Ga (Holland, 2006). Orogenesis was not absent from this period however, with various continents featuring active accretionary orogenesis along their margins for the entire Mesoproterozoic (see Condie, 2013 and Roberts, 2013), and others featuring major continental collisional orogenesis that relates to the formation of the supercontinent Rodinia towards the end of the Mesoproterozoic. Looking at it another way, this period followed the formation of perhaps the first long-lived supercontinent, Columbia (a.k.a. Nuna), and then it prepared the ground for the momentous geological and biological events in the Neoproterozoic that paved the way for the Cambrian explosion of life. As such it is a very important period of Earth history to understand better. Do orogens formed in the Mesoproterozoic differ from those formed in the recent past, or those formed in early Earth history, and if so in what way? Do orogens in the Mesoproterozoic have distinct structural, metamorphic or magmatic characteristics? How are Mesoproterozoic orogens related geodynamically and kinematically? These are overarching questions that this collection of sixteen research papers aims to address. This introduction presents a brief discussion of the contribution of these papers to these questions and topics

Deformation Mechanisms of Blueschist Facies Continental Metasediments May Offer Insights Into Deep Episodic Tremor and Slow Slip Events

Abstract Exhumed fossil subduction zones are archives of the deformation conditions and mechanisms from depths not directly accessible. Microstructural analysis of samples exhumed therefrom offers insights into the micromechanics and deformation processes associated with subduction such as earthquakes, slow earthquakes, and aseismic creep. Subducted and exhumed continental metasediments of the Tuscan Metamorphic Units of the Italian Northern Apennines contain a mylonitic foliation and quartz and carpholite dilational hydroshear veins with crack-and-seal textures, both developed at blueschist facies conditions (350?400°C, ?1 GPa). As shown by overprinting relationships and mineral assemblages, these two structure types formed broadly coeval within the stability field of carpholite. Metaconglomerates and metaquartzarenites deformed mainly by dissolution-precipitation creep and secondary by dislocation creep. Microstructural and electron backscatter diffraction analyses of the veins suggest only limited recrystallization of quartz fibers by subgrain rotation recrystallization, with adjacent metapelite bands acting as decollement horizons, likely by slip on the basal plane of phyllosilicates. We estimated differential stresses of 43?55 MPa and strain rates between 10?13 and 10?14 s?1 from the vein recrystallized quartz fibers. We propose these microstructures and deformation mechanisms to represent a geological evidence of deep episodic tremor and slow slip events in subducted continental metasediments. Pore pressure cyclically reached supralithostatic values triggering tremors causing fracturing of all involved lithotypes. Likely, slow slip was accommodated preferentially by slip on phyllosilicate bands. Aseismic creep occurred mainly by dislocation creep with subgrain rotation recrystallization in vein quartz, slip on the basal plane of phyllosilicates, and dissolution and precipitation creep in the host rock