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2022 - Multidisciplinary investigations of a karst reservoir for managed aquifer recharge applications on the island of Vis (Croatia) [Articolo su rivista]
Patekar, Matko; Bašić, Mihaela; Pola, Marco; Kosović, Ivan; Terzić, Josip; Lucca, Alessio; Mittempergher, Silvia; Berio, Luigi Riccardo; Borović, Staša

2022 - Open-closed-open paleofluid system conditions recorded in the tectonic vein networks of the Parmelan anticline (Bornes Massif, France) [Articolo su rivista]
Berio, Luigi R.; Mittempergher, Silvia; Storti, Fabrizio; Bernasconi, Stefano M.; Cipriani, Anna; Lugli, Federico; Balsamo, Fabrizio

2022 - The timescale of solid-state deformation in the Northern Adamello igneous intrusive suite [Articolo su rivista]
Mittempergher, S.; Zanchetta, S.; Caldiroli, F.; Zanchi, A.; Bistacchi, A.; Hanchar, J. M.; Villa, I. M.

2021 - Fault reactivation and propagation in the northern Adamello pluton: The structure and kinematics of a kilometre-scale seismogenic source [Articolo su rivista]
Mittempergher, S.; Zanchi, A.; Zanchetta, S.; Fumagalli, M.; Gukov, K.; Bistacchi, A.

The northern Adamello batholith (European Southern Alps) is crosscut by E-W trending pseudotachylyte-bearing paleoseismic fault zones composed of multiple subparallel fault strands. The main faults are the Gole Strette Fault Zone (GSFZ), here discussed in detail, and the Gole Larghe Fault Zone (GLFZ). The western ending of the GSFZ intersects the propagation of the Gallinera Thrust, a regional structure of Late Cretaceous age, which was truncated and dismembered by the late Eocene - early Oligocene emplacement of the Adamello batholith. Fault slip analysis and paleostress reconstruction suggest that the GSFZ and the Gallinera Thrust were both active during dextral transpression related with Oligocene orogen-parallel shearing along the Periadriatic Fault System. The remnants of the Gallinera Thrust within the Adamello were reactivated as dextral-reverse faults and do not include pseudotachylytes. The pseudotachylyte-bearing GSFZ shows changing structural features from west to east: the fault dip angle decreases from 80° to 45°, the slip vector passes from strike-slip (pitch of 20°) to oblique (pitch of 35° to 45°), and the fault zone thickness increases (> 800 m at the intersection with the GLFZ). The GSFZ is an immature and strong fault as indicated by (i) the local geometry controlled by precursor joints; (ii) the along-strike segmentation; and (iii) the spread of seismogenic faults into wide fault zones.

2021 - Frictional Melting in Hydrothermal Fluid-Rich Faults: Field and Experimental Evidence From the Bolfín Fault Zone (Chile) [Articolo su rivista]
Gomila, Rodrigo; Fondriest, Michele; Jensen, Erik; Spagnuolo, Elena; Masoch, Simone; Mitchell, Tom; Magnarini, Giulia; Bistacchi, Andrea; Mittempergher, Silvia; Faulkner, Daniel; Cembrano, Josè; di Toro, Giulio

2021 - Structural Evolution of the Parmelan Anticline (Bornes Massif, France): Recording the Role of Structural Inheritance and Stress Field Changes on the Finite Deformation Pattern [Articolo su rivista]
Berio, L. R.; Storti, F.; Balsamo, F.; Mittempergher, S.; Bistacchi, A.; Meda, M.

2020 - Calcareous nannofossil biostratigraphy of the External Dinarides flysch (Vrčić-Staravasa Pag Island, Croatia): A key to an Eocene tectono-stratigraphic and paleoenvironmental interpretation [Articolo su rivista]
Persico, D.; Succo, A.; Mittempergher, S.; Storti, F.; Piccinini, E.; Villa, G.

It is largely accepted that in the Dinaric shallow foredeep basin, the onset of Cenozoic synorogenic sedimentation is diachronous along strike and shows orogen-parallel southward younging, creating a large uncertainty in the age of the Dalmatian flysch. Calcareous nannofossil biostratigraphy was studied to investigate a turbiditic succession exposed in Pag Island (Croatia) with the main purpose of constraining its age, which is still a matter of debate. The age assignment for the turbiditic deposition bears important implications for better understanding of the geodynamic and paleoenvironmental evolution of the External Dinarides. We logged a well-exposed sedimentary section in the south-western limb of the Pag anticline and performed calcareous nannofossil quantitative analyses revealing a high species diversity and abundant assemblage. The age diagnostic species indicate CNE14-CNE15 biozones, suggesting that flysch deposition occurred in the Lutetian–Bartonian. This age can be further restricted to the CNE14 (42.37–40.51 Ma) by means of the lowest occurrence of Reticulofenestra reticulata and the highest occurrence of Sphenolithus furcatolithoides. Vertical facies variation and paleoecological indications suggest an increase in paleobathymetric depth during deposition. Our data support a post-Lutetian age of folding and thrusting for the Pag Island region and suggest a relatively short time span during the middle-late Eocene period for the main contractional stage in this sector of the External Dinarides. This result has important implications for the evolution of the Dinaride foreland basin system in Cenozoic times.

2020 - Cyclical variations of fluid sources and stress state in a shallow megathrust zone mélange [Articolo su rivista]
Cerchiari, Anna; Remitti, Francesca; Mittempergher, Silvia; Festa, Andrea; Lugli, Federico; Cipriani, Anna

Differences in REE patterns of calcite from extensional and shear veins of the Sestola Vidiciatico Tectonic Unit in Northern Apennines suggest variations in fluid source during the seismic cycle in an ancient analogue of a shallow megathrust (Tmax ̴100°-150°C). In shear veins, a positive Eu anomaly suggests an exotic fluid source, likely hotter than the fault environment. Small-scale extensional veins were derived instead from a local fluid in equilibrium with the fault zone rocks. Mutually crosscutting relations between two sets of extensional veins, parallel and perpendicular to the megathrust, suggest a repeated shifting of the σ1 and σ3 stresses during the seismic cycle. This is consistent with: (i) a seismic phase, with brittle failure along the thrust, crystallization of shear veins from an exotic fluid, stress drop and stress rotation; (ii) a post-seismic phase, with fault-normal compaction and formation of fault-normal extensional veins fed by local fluids; (iii) a reloading phase, where shear stress and pore pressure are gradually restored and fault-parallel extensional veins form, until the thrust fails again. The combination of geochemical and structural analyses in veins from exhumed megathrust analogues represents a promising tool to better understand the interplay between stress state and fluids in modern subduction zones.

2020 - Damage zone characterization combining scan-line and scan-area analysis on a km-scale Digital Outcrop Model: The Qala Fault (Gozo) [Articolo su rivista]
Martinelli, M.; Bistacchi, A.; Mittempergher, S.; Bonneau, F.; Balsamo, F.; Caumon, G.; Meda, M.

Fault damage zones can act as a preferential corridor for fluid flow in the subsurface, and for this reason the characterization of their structure, including the attributes of the associated fracture network, is fundamental. In this work, we characterize the damage zone of the Qala fault, a normal fault developed in platform carbonates of the Gozo Island (Maltese Islands). We propose a new workflow that combines scanline and scan-area analysis applied on a high resolution DOM. Linear scanlines allow to characterize fracture spatial distribution, detect stationary area and identify damage zone width. Areal sampling permits to extract the fracture parameters matching the stationary 1D domains. This new approach allows us to: (1) univocally separate the damage zone from the background fractures, (2) identify fracture corridors, (3) collect fracture parameters (length, trend, density, intensity, spacing and topology), (4) identify the REV of the fracture density, intensity and topology and (5) characterize the fracture network connectivity.

2020 - On a new robust workflow for the statistical and spatialanalysis of fracture data collected with scanlines (or theimportance of stationarity) [Articolo su rivista]
Bistacchi, Andrea; Mittempergher, Silvia; Martinelli, Mattia; Storti, Fabrizio

2019 - 3D multi-scale characterization and modeling of three fractured carbonatic outcrop analogues: Pag Croatia, Parmelan France, Gozo Maltese Islands [Abstract in Atti di Convegno]
Meda, Marco; Martinelli, Mattia; Bistacchi, Andrea; Mittempergher, Silvia; Berio, Luigi; Balsamo, Fabrizio; Succo, Andrea; Storti, Fabrizio

2019 - Geological and structural map of the southeastern Pag Island, Croatia: field constraints on the Cretaceous - Eocene evolution of the Dinarides foreland [Articolo su rivista]
Mittempergher, Silvia; Succo, Andrea; Bistacchi, Andrea; Storti, Fabrizio; Bruna, Pierre Olivier; Meda, Marco

2018 - Does subduction of mass transport deposits (MTDs) control seismic behavior of shallow–level megathrusts at convergent margins? [Articolo su rivista]
Festa, Andrea; Dilek, Yildirim; Mittempergher, Silvia; Ogata, Kei; Pini, Gian Andrea; Remitti, Francesca

We present a critical appraisal of the role of subducted, medium (10–1000 km2) to giant (≥1000 km2) and heterogeneous, mud-rich mass transport deposits (MTDs) in seismic behavior and mechanisms of shallow earthquakes along subduction plate interfaces (or subduction channels) at convergent margins. Our observations from exhumed ancient subduction complexes around the world show that incorporation of mud-rich MTDs with a “chaotic” internal fabric (i.e., sedimentary mélanges or olistostromes) into subduction zones strongly modifies the structural architecture of a subduction plate interface and the physical properties of subducted material. The size and distribution of subducted MTDs with respect to the thickness of a subduction plate interface are critical factors influencing seismic behavior at convergent margins. Heterogeneous fabric and compositions of subducted MTDs may diminish the effectiveness of seismic ruptures considerably through the redistribution of overpressured fluids and accumulated strain. This phenomenon possibly favors the slow end-member of the spectrum of fault slip behavior (e.g., Slow Slip Events, Very Low Frequency Earthquakes, Non-Volcanic Tremors, creeping) compared to regular earthquakes, particularly in the shallow parts (T < 250 °C) of a subduction plate interface.

2018 - From soft sediment deformation to fluid assisted faulting in the shallow part of a subduction megathrust analogue: the Sestola Vidiciatico tectonic Unit (Northern Apennines, Italy) [Articolo su rivista]
Mittempergher, Silvia; Cerchiari, Anna; Remitti, Francesca; Festa, Andrea

The Sestola Vidiciatico tectonic Unit (SVU) accommodated the early Miocene convergence between the subducting Adriatic plate and the overriding Ligurian prism, and has been interpreted as a field analogue for the shallow portion of subduction megathrusts. The SVU incorporated sediments shortly after their deposition and was active down to burial depth corresponding to temperatures around 150 °C. Here, we describe the internal architecture of the basal thrust fault of the SVU through a multi-scale structural analysis and investigate the evolution of the deformation mechanisms with increasing burial depth. At shallow depth, the thrust developed in poorly lithified sediments which deformed by particulate flow. With increasing depth and lithification of sediments, deformation was accommodated in a meter scale, heterogeneous fault zone, including multiple strands of crack-and-seal shear veins, associated with minor distributed shearing in clay-rich domains and pressure solution. In the last stage, slip localized along a sharp, 20 cm thick shear vein, deactivating the fault zone towards the footwall. The widespread formation of crack-and-seal shear veins since the first stages of lithification indicates that failure along the thrust occurred at high fluid pressure and low differential stress already at shallow depth. Progressive shear localization occurs in the last phases of deformation, at temperatures typical of the transition to the seismogenic zone in active megathrusts.

2017 - Production of nanoparticles during experimental deformation of smectite and implications for seismic slip [Articolo su rivista]
Aretusini, S.; Mittempergher, S.; Plümper, O.; Spagnuolo, E.; Gualtieri, A. F.; Di Toro, G.

Nanoparticles and amorphous materials are common constituents of the shallow sections of active faults. Understanding the conditions at which nanoparticles are produced and their effects on friction can further improve our understanding of fault mechanics and earthquake energy budgets. Here we present the results of 59 rotary shear experiments conducted at room humidity conditions on gouge consisting of mixtures of smectite (Ca-montmorillonite) and quartz. Experiments with 60, 50, 25, 0 wt.% Ca-montmorillonite, were performed to investigate the influence of variable clay content on nanoparticle production and their influence on frictional processes. All experiments were performed at a normal stress of 5 MPa, slip rate of , and at a displacement of 3 m. To monitor the development of fabric and the mineralogical changes during the experiments, we investigated the deformed gouges using scanning and transmission electron microscopy combined with X-ray powder diffraction quantitative phase analysis. This integrated analytical approach reveals that, at all slip rates and compositions, the nanoparticles (grain size of 10–50 nm) are partly amorphous and result from cataclasis, wear and mechanical solid-state amorphization of smectite. The maximum production of amorphous nanoparticle occurs in the intermediate slip rate range (), at the highest frictional work, and is associated to diffuse deformation and slip strengthening behavior. Instead, the lowest production of amorphous nanoparticles occurs at co-seismic slip rates (), at the highest frictional power and is associated with strain and heat localization and slip weakening behavior. Our findings suggest that, independently of the amount of smectite nanoparticles, they produce fault weakening only when typical co-seismic slip rates (>0.1 ms−1) are achieved. This implies that estimates of the fracture surface energy dissipated during earthquakes in natural faults might be extremely difficult to constrain.

2015 - Frictional properties of fault zone gouges from the J-FAST drilling project (Mw9.0 2011 Tohoku-Oki earthquake) [Articolo su rivista]
Remitti, Francesca; Smith, S. A. F.; Mittempergher, Silvia; Gualtieri, Alessandro; Di Toro, G.

Smectite-rich fault gouges recovered during Integrated Ocean Drilling Program Expedition 343 (Japan Trench Fast Drilling Project (J-FAST)) from the plate boundary slip zone of the 2011 Mw 9.0 Tohoku-Oki earthquake were deformed at slip velocities of 10 µm s−1 to 3.5 m s−1 and normal stresses up to 12 MPa. Water-dampened gouges (1) are weaker (apparent friction coefficient, μ* <0.1) than room-humidity gouges (apparent friction coefficient, μ* ~0.1–0.35) at all slip velocities, (2) are velocity insensitive to velocity weakening at all slip velocities, unlike room-humidity gouges that are velocity strengthening at intermediate velocities (V = 0.001–0.1 m s−1), and (3) have negligible peak μ* at high slip velocities (V > 0.1 m s−1). A significant amount of amorphous material formed in room-humidity experiments at low- and high-slip velocities, likely by comminution and disordering of smectite. Our results indicate that the frictional properties of water-dampened gouges could have facilitated propagation of the Tohoku-oki rupture to the trench and large coseismic slip at shallow depths.

2014 - Origin of hydrous fluids at seismogenic depth: Constraints from natural and experimental fault rocks [Articolo su rivista]
Mittempergher, S.; Dallai, L.; Pennacchioni, G.; Renard, F.; Di Toro, G.

Fluids control the mechanical behavior of fault zones during the seismic cycle. We used geochemical, mineralogical, microstructural, hydrogen isotope compositions and Fourier Transform Infrared (FTIR) investigations to characterize the origin of hydrous fluids involved in ductile and brittle shear zones at the bottom of the seismogenic crust. Natural samples were collected from exhumed mylonitic shear zones and cataclasite-pseudotachylyte bearing faults in the northern Adamello (Italian Southern Alps), which were active at 9-11 km depth. Pseudotachylytes, solidified coseismic friction-induced melts, testify to ancient seismogenic behavior of the faults. Natural pseudotachylytes were compared with artificial pseudotachylytes produced in high velocity friction experiments simulating seismic slip.Mylonites have mineralogical, elemental and hydrogen isotope compositions (-80‰ &lt; δD &lt; - 78‰) similar to the host tonalite (-77‰ &lt; δD &lt; - 73‰), within the analytical error of ±5‰. Cataclasites have instead mineralogical (chlorite, epidote, K-feldspar, no biotite), major and trace elements (enrichment in K2O, Ba, Rb; depletion in CaO, Na2O, SiO2) and hydrogen isotope (-69‰ &lt; δD &lt; - 60‰) compositions suggesting interactions with a crustal metamorphic fluid. Pseudotachylytes are composed of high temperature minerals (plagioclase, biotite, dmisteinbergite, cordierite, and scapolite) and have elemental compositions resulting from mixing of tonalite and cataclasite. Pseudotachylytes have complex microstructures, including: (i) microlitic domains, with well crystallized micrometric biotite, which have hydrogen isotope composition (-81‰ &lt; δD &lt; - 59‰) similar to cataclasites and tonalite; and (ii) cryptocrystalline domains, with poorly crystallized biotite, which have very high water content, release water upon heating at T &gt; 50°C and have low δD value (-93‰). The hydrogen isotope composition of bulk samples is dominated by the composition of cryptocrystalline domains (-103‰ &lt; δD &lt; - 88‰), where most of the water is hosted. Their hydrogen isotope composition is compatible with adsorption of present day rainfall water (δD = - 95‰). Artificial pseudotachylytes have the same hydrogen isotope compositions of the starting tonalite (-76‰ &lt; δD &lt; - 74‰) or cataclasite (-68‰ &lt; δD &lt; - 62‰), with a slight decrease of the δD values in some samples (-85‰ &lt; δD &lt; - 81‰).The first ingression of a crustal metamorphic fluid occurred in cataclastic faults. Natural pseudotachylytes, when not contaminated by present day rainfall water, have a hydrogen isotope composition similar to tonalite and cataclasite, as reproduced in dry high velocity friction experiments. The fluids dissolved in coseismic melts are most likely derived from the breakdown of hydrous minerals of cataclasite and tonalite undergone melting, and we could not identify the infiltration of an external fluid during earthquakes. © 2013 Elsevier B.V.

2013 - The structure of an exhumed intraplate seismogenic fault in crystalline basement [Articolo su rivista]
Smith, S. A. F.; Bistacchi, A.; Mitchell, T. M.; Mittempergher, S.; Di Toro, G.

The 600. m-thick Gole Larghe Fault Zone (GLFZ) is hosted in jointed crystalline basement and exposed across glacier-polished outcrops in the Italian Alps. Ancient seismicity is attested by the widespread occurrence of cataclasites associated with pseudotachylytes (solidified frictional melts) formed at 9-11. km depth and ambient temperatures of 250-300. °C. Previous work focused on the southern part of the fault zone; here we quantitatively document fault zone structure across the full width of the GLFZ and surrounding tonalite host rocks by using a combination of structural line transects and image analysis of samples collected across fault strike. These new datasets indicate that the GLFZ has a broadly symmetric across-strike damage structure and contains distinct southern, central and northern zones distinguished by large variations in fracture density, distribution of pseudotachylytes, volume of fault rock materials, and microfracture sealing characteristics. The c. 100. m wide central zone is bound by two thick (~. 2. m) and laterally continuous (&gt; 1. km) protocataclastic to ultracataclastic horizons. Within and immediately surrounding the central zone, fracture density is relatively high due to cataclastic fault-fracture networks that reworked earlier-formed pseudotachylytes. The fault-fracture networks were associated with pervasive microcracking and fluid-rock interaction, resulting in the development of a c. 200. m thick alteration zone delimited by lobate fluid infiltration fronts. In the c. 250. m thick southern and northern zones, fracture densities are much lower and pseudotachylytes systematically overprint cataclastic faults that exploited pre-existing magmatic cooling joints. Analysis of the structure of the GLFZ suggests that it shares certain characteristics with the seismogenic source responsible for the 2002 Au Sable Forks intraplate earthquake sequence in the northeastern USA, including seismicity distributed across a fault zone 500-1000. m thick and large (&gt; 100. MPa) static stress drops associated with frictional melting. © 2013 Elsevier B.V.

2012 - A microstructural study of fault rocks from the SAFOD: Implications for the deformation mechanisms and strength of the creeping segment of the San Andreas Fault [Articolo su rivista]
Hadizadeh, J.; Mittempergher, S.; Gratier, J. -P.; Renard, F.; Di Toro, G.; Richard, J.; Babaie, H. A.

The San Andreas Fault zone in central California accommodates tectonic strain by stable slip and microseismic activity. We study microstructural controls of strength and deformation in the fault using core samples provided by the San Andreas Fault Observatory at Depth (SAFOD) including gouge corresponding to presently active shearing intervals in the main borehole. The methods of study include high-resolution optical and electron microscopy, X-ray fluorescence mapping, X-ray powder diffraction, energy dispersive X-ray spectroscopy, white light interferometry, and image processing.The fault zone at the SAFOD site consists of a strongly deformed and foliated core zone that includes 2-3 m thick active shear zones, surrounded by less deformed rocks. Results suggest deformation and foliation of the core zone outside the active shear zones by alternating cataclasis and pressure solution mechanisms. The active shear zones, considered zones of large-scale shear localization, appear to be associated with an abundance of weak phases including smectite clays, serpentinite alteration products, and amorphous material. We suggest that deformation along the active shear zones is by a granular-type flow mechanism that involves frictional sliding of microlithons along phyllosilicate-rich Riedel shear surfaces as well as stress-driven diffusive mass transfer. The microstructural data may be interpreted to suggest that deformation in the active shear zones is strongly displacement-weakening. The fault creeps because the velocity strengthening weak gouge in the active shear zones is being sheared without strong restrengthening mechanisms such as cementation or fracture sealing. Possible mechanisms for the observed microseismicity in the creeping segment of the SAF include local high fluid pressure build-ups, hard asperity development by fracture-and-seal cycles, and stress build-up due to slip zone undulations. © 2012 Elsevier Ltd.

2012 - Fault roughness evolution with seismogenic slip in the gole larghe fault zone: 3D imaging, spectral analysis and forward modeling [Articolo su rivista]
Bistacchi, A.; Griffith, W. A.; Spagnuolo, E.; Nielsen, S.; Toro, G. D.; Smith, S. A.; Jones, R.; Mittempergher, S.

2012 - Fault zone structure and seismic slip localization in dolostones, an example from the Southern Alps, Italy [Articolo su rivista]
Fondriest, M.; Smith, S. A. F.; Di Toro, G.; Zampieri, D.; Mittempergher, S.

Fault zones cutting limestones and dolostones represent significant seismogenic sources worldwide. The structure of an exhumed strike-slip fault zone hosted in dolostones, the Borcola Pass Fault Zone (BPFZ, Italian Southern Alps), was studied by means of field and microstructural analysis. Ambient conditions of faulting were ca. 1.6-1.7 km and 50 °C. The BPFZ consists of a &gt;80 m wide damage zone cut by three systems of sub-vertical secondary faults striking approximately N-S, E-W and NW-SE. N-S and E-W striking faults reactivated pre-existing Jurassic-Paleogene joints with spacing between 0.2 and 0.5 m, whereas NW-SE striking faults were newly formed during post-Paleogene activity associated with movements along the nearby Schio-Vicenza Line. The core of the BPFZ consists of dolostone fault rock lenses bound by slip zones up to 10 cm thick. Both the principal and secondary slip zones consist of cement-supported dolomitic cataclasites and dolomite-filled veins. Some slip zones contain a sub-centimeter thick "vein-like" cataclastic layer (Layer-A) located immediately beneath the slip surface that truncates another cataclasite below (Layer-B). Detailed microstructural and clast size distribution analysis suggests that Layer-A experienced fluidization (cuspate-lobate boundaries, injection structures, strong grain sorting: D &lt; 1 for clast diameters smaller than 300 μm) possibly related to fast fault slip following seismic ruptures. In light of these observations a conceptual model is proposed for the formation of Layer-A, and the structure of the BPFZ is compared to that of an active seismogenic fault cutting carbonates. © 2012 Elsevier Ltd.

2012 - The contribution of structural geology, experimental rock deformation and numerical modelling to an improved understanding of the seismic cycle. Preface to the Special Volume "Physico-chemical processes in seismic faults" [Articolo su rivista]
Di Toro, G.; Mittempergher, S.; Ferri, F.; Mitchell, T. M.; Pennacchioni, G.

2011 - Aseismic sliding of active faults by pressure solution creep: Evidence from the San Andreas fault observatory at Depth [Articolo su rivista]
Gratier, J. -P.; Richard, J.; Renard, F.; Mittempergher, S.; Doan, M. -L.; Di Toro, G.; Hadizadeh, J.; Boullier, A. -M.

Active faults in the upper crust can either slide steadily by aseismic creep, or abruptly causing earthquakes. Creep relaxes the stress and prevents large earthquakes from occurring. Identifying the mechanisms controlling creep, and their evolution with time and depth, represents a major challenge for predicting the behavior of active faults. Based on microstructural studies of rock samples collected from the San Andreas Fault Observatory at Depth (California), we propose that pressure solution creep, a pervasive deformation mechanism, can account for aseismic creep. Experimental data on minerals such as quartz and calcite are used to demonstrate that such creep mechanism can accommodate the documented 20 mm/yr aseismic displacement rate of the San Andreas fault creeping zone. We show how the interaction between fracturing and sealing controls the pressure solution rate, and discuss how such a stress-driven mass transfer process is localized along some segments of the fault. © 2011 Geological Society of America.

2011 - Evidence of transient increases of fluid pressure in SAFOD phase III cores [Articolo su rivista]
Mittempergher, S.; Di Toro, G.; Gratier, J. P.; Hadizadeh, J.; Smith, S. A. F.; Spiess, R.

The San Andreas Fault Observatory at Depth (SAFOD) in Parkfield, central California, has been drilled through a fault segment that is actively deforming through creep and microearthquakes. Creeping is accommodated in two fault strands, the Southwest and Central Deforming Zones, embedded within a damaged zone of deformed shale and siltstone. During drilling, no pressurized fluids have been encountered, even though the fault zone acts as a permeability barrier to fluid circulation between the North American and Pacific plates. Microstructural analysis of sheared shales associated with calcite and anhydrite-bearing veins found in SAFOD cores collected at 1.5m from the Southwest Deforming Zone, suggests that transient increases of pore fluid pressure have occurred during the fault activity, causing mode I fracturing of the rocks. Such build-ups in fluid pressure may be related to permeability reduction during fault creep and pressure-solution processes, resulting in localized failure of small fault zone patches and providing a potential mechanism for the initiation of some of the microearthquakes registered in the SAFOD site. Copyright © 2011 by the American Geophysical Union.

2010 - Evidence of dmisteinbergite (hexagonal form of CaAl2Si2O8) in pseudotachylyte: A tool to constrain the thermal history of a seismic event [Articolo su rivista]
Nestola, F.; Mittempergher, S.; Toro, G. D.; Zorzi, F.; Pedron, D.

The determination of the maximum temperature achieved by friction melt (Tmelt) in pseudotachylyte-bearing faults is crucial to estimate earthquake source parameters (e.g., earthquake energy budgets, co-seismic fault strength) on a geological basis. Here we investigated the mineralogy of a pseudotachylyte from the Gole Larghe Fault (Italian Alps) by using X-ray powder diffraction, micro-Raman spectro-scopy, and EDS-equipped field emission scanning electron microscopy. In particular, we report the presence of the hexagonal polymorph of CaAl2Si2O8(dmisteinbergite) in a pseudotachylyte. Published experimental work shows dmisteinbergite can crystallize at 1200-1400 °C by rapid quenching. Therefore, the presence of dmisteinbergite in pseudotachylyte could be a reliable geothermometer for friction melts for which Tmelthas only as yet been estimated.

2009 - Involvement of pore fluids in frictional melting from stable isotopes study of pseudotachylytes [Relazione in Atti di Convegno]
Mittempergher, S.; Dallai, L.; Di Toro, G.; Pennacchioni, G.

2009 - The effects of fault orientation and fluid infiltration on fault rock assemblages at seismogenic depths [Articolo su rivista]
Mittempergher, S.; Pennacchioni, G.; Di Toro, G.

The factors controlling the development of different types of fault rock assemblages and, more specifically, the formation of friction melts are still not fully understood. In this study we compared two exhumed strike-slip faults in the Adamello batholith (Southern Alps): the Gole Larghe and the Passo Cercen fault zones, active at 9-11 km depth and temperatures of 250-300 °C. Each fault zone consists of hundreds of sub-parallel strands exploiting pre-existing joints. The Gole Larghe fault strikes N105 ± 5° and is dextral; the fault rocks are cataclasites and widespread, centimetre-thick pseudotachylytes. The Passo Cercen fault strikes on average N130° and is formed by multiple fault horizons: fault segments striking N105°-N130° are mainly dextral, whereas faults striking N135°-N140° are mainly sinistral. Microstructural, mineralogical and geochemical investigations show that the fault rocks are cataclasites associated with thick epidote + K-feldspar + quartz veins and rare, millimetre-thick pseudotachylytes. Field evidence suggests that in both fault zones, the direction of the maximum horizontal stress σ1 was N135°. The Gole Larghe fault strikes at about 30° to σ1 and is favourably oriented for reactivation. By contrast, the Passo Cercen fault strikes at low angles to σ1 and is unfavourably oriented for reactivation, therefore requiring the development of high pore pressures, as suggested by the occurrence of extensive epidote veining and hydraulic breccias. It is proposed that frictional melting in the Passo Cercen fault zone was inhibited by the development of high pore pressures and low effective normal stresses. © 2009 Elsevier Ltd. All rights reserved.

2007 - Effects of fluids on the seismicity of a fault. Analysis of exhumed structures in the Northern Adamello (Southern Alps, Italy) [Articolo su rivista]
Mittempergher, S.; Di Toro, G.; Pennacchioni, G.

A comparison between two major cataclastic faults within the northern composite Adamello batholith (Southern Alps) is attempted. The two investigated faults have strike-slip kinematics and are related to the slip along the adjacent Tonale Line, a segment of the Periadriatic Lineament bordering to the north the Adamello. The exhumed fault sections were active 30Ma ago at 9-11km depth and 250-300°C ambient temperature. The Gole Larghe Fault Zone (GLFZ) has an E-W strike and cut across the Avio tonalites. The Passo Cercen Fault Zone (PCFZ) is located, a few kilometres north of GLFZ, within the Presanella pluton. Both faults consist of a swarm of subparallel green indurated cataclasites exploiting precursor sets of joints developed during the early stages of pluton cooling (PENNACCHIONI et alii, 2006); pseudotachylytes (frictional melts formed during coseismic slip on fault planes) are abundant along the PCFZ, but almost absent along the PCFZ. The field work arid the laboratory (geochemical and microstructural) analyses have highlighted the following main differences between the two faults: (i) Orientation: the PCFZ has direction N130°, almost parallel to the direction of the regional σ1 during the faulting; the GLFZ has orientation N105° and forms an angle of about 30° with σ1. (ii) Fault rocks: a) The GLFZ includes abundant pseudotachylytes, which are very rare along the PCFZ; b) Within the PCFZ, cataclasites are associated to diffuse macroscopic veins of epidote + Kfeldspar + quartz, which are rare along the GLFZ. Our hypothesis is that both faults were seismic (small amount of pseudotachylytes are present within the PCFZ), but processes inhibiting frictional melting were active along the PCFZ. The different orientation in the regional stress field determines a different component of normal stress on fault planes affecting the degree of fluid infiltration along the two fault zones. The orientation of the PCFZ to the regional stress field is favourable to exploitation of the precursor joints as hybrid mode of fractures. Hybrid shear-extensional fractures developing at depth of 9-11km imply high pore pressure and low differential stress; both factors act reducing the effective normal stress, and thus the shear strength along the fault planes. In condition of low shear strength, frictional heating is reduced; furthermore, fluid-rich conditions trigger processes such as fluid pressurization that inhibit frictional melting.