The largest earthquakes recorded on Earth (Mw>8) occur along low-dipping megathrusts at subduction plate margins. Shallow (< 5 km of depth) megathrusts are characterized by a spectrum of slip behaviours, including large earthquakes, tsunami earthquakes, slow slip and tremors. The physical and chemical processes controlling this complex behavior are still poorly constrained and, to investigate them, the study of fossil analogues exhumed by orogenic uplift represents an approach complementary to scientific drilling and geophysical investigations. My project has been aimed at studying in detail outcrops of megathrust fossil analogues by means of field, microstructural and geochemical analyses, with a particular focus on tectonic veins, to shed some light on the processes controlling fault slip style. I investigated two thrust fault zones, belonging to different fossil subduction complexes, to compare in detail the observed structural features and infer deformation mechanisms, helping to link the geological record to process operating on active subduction margins. The Sestola Vidiciatico tectonic Unit (SVU) in the Northern Apennines of Italy has been interpreted as a field analogue for the shallow portion (4-5 km, 150 °C maximum temperature) of subduction megathrusts. Throughout a multi-scale structural analysis of the SVU basal thrust cropping out in Vidiciatico (BO) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses performed on vein samples, the megathrust deformation history and interrelated fluid/stress variations during the seismic cycle has been reconstructed. Based on obtained results, the SVU basal thrust can be characterized as a weak fault, which progressively localized deformation to a cm-scale shear zone and which underwent a cyclical shifting of the principal σ1 and σ3 stresses, producing interrelated changes in permeability, fluid pressure and composition. The fault zone cropping out in SW Llŷn Peninsula, NW Wales (UK), is related to the ~ 600-500 Ma Pacific-type subduction-accretion Mona Complex. Meso- and microstructural analyses, fluid inclusion microthermometry and EBSD microscopy suggest that the fault zone deformed at conditions correspondent to the brittle-ductile transition, at T ≤ 300 °C. This occurred with a cyclical shifting from brittle to ductile deformation, mainly controlled by strain rate variations coupled with overpressured fluid pulses, producing repeated transient embrittlement. At low strain rate, the presence of fluid promoted hydrolitic weakening in vein crystals, thus facilitating the onset of low temperature quartz plasticity in high dislocation density regions. At higher strain rate, deformation localized in thin shear zones where fluid overpressures counteracted the principal stress, allowing dilatant fracturing to occur more easily and producing new shear veins. The results of my study of the described field megathrust analogues are consistent with what has been observed in several other fossil examples and with some of the recent data obtained from active subduction margins, and suggest that fluids are a crucial factor controlling the deformation style. They confirm that a multidisciplinary approach to the study of tectonic veins, involving field, microstructural and geochemical investigations, is a powerful tool to obtain information on the interrelations between fluid flow and deformation, enriching our knowledge of megathrust seismic behaviour.

I maggiori terremoti registrati sulla Terra (Mw>8) si verificano lungo i piani di contatto poco inclinati dei margini di subduzione di placca, i cosiddetti “megathrust”. Queste superfici sono caratterizzate a bassa profondità (meno di 5 km) da una gamma di stili di scivolamento lungo il piano, che include grandi terremoti (alcuni caus di tsunami), Slow Slip Events e tremori. I processi fisici e chimici alla base di questa molteplicità di comportamenti sismici sono ancora poco conosciuti, per cui, nell’ottica di comprenderli, lo studio di analoghi fossili, esumati in seguito a sollevamenti tettonici orogenetici, rappresenta un approccio complementare alle perforazioni oceaniche e alle tecniche di indagine geofisica. Il mio progetto ha avuto come obiettivo lo studio di dettaglio di affioramenti di megathrust fossili, per mezzo di indagini sul campo, microstrutturali e geochimiche focalizzate in particolare sulle vene tettoniche, in modo da fare luce sui meccanismi che controllano lo stile di scivolamento lungo il piano di faglia. Ho analizzato due zone di thrust appartenenti a complessi di subduzione fossili differenti, per confrontare e descrivere dettagliatamente le strutture osservate e fornire informazioni sui collegamenti tra record geologico e processi attivi ai margini di suduzione. L’Unità tettonica Sestola Vidiciatico (USV) negli Appennini Settentrionali, Italia, è stata interpretata come un analogo della porzione più superficiale (4-5 km, T massima = 150 °C) dei megathrust. Attraverso un’analisi strutturale multiscala del thrust alla base della USV affiorante a Vidiciatico (BO) e tramite spettrometrie di massa in laser ablation su campioni di vena, è stata ricostruita la storia deformativa del thrust e le variazioni correlate del campo di stress e della circolazine dei fluidi durante il ciclo sismico. Sulla base dei risultati ottenuti, il thrust basale della USV è stato caratterizzato come una faglia debole, che ha progressivamente localizzato la deformazione in una shear zone alla scala del cm ed è stata interessata da uno scambio ciclico tra le orientazioni degli stress principali massimo e minimo. Questo ha provocato variazioni correlate di permeabilità, pressione e composizione dei fluidi. La zona di faglia che affiora nella Penisola di Llŷn sudorientale, in Galles del Nord (UK), è collegata ad un complesso di subduzione-accrezione risalente a ~ 600-500 Ma. Analisi meso- e microstrutturali, dati da microtermometria delle inclusioni fluide e EBSD suggeriscono che la zona di faglia sia stata deformata in corrispondenza della transizione fragile-duttile, a una temperatura prossima a 300 °C. Questo è avvenuto tramite un passaggio ciclico da deformazione fragile a duttile e viceversa, controllato principalmente da variazioni di strain rate accoppiate a sovrappressioni dei fluidi, che producevano un embrittlement transitorio della zona di faglia. A bassa strain rate, la presenza di fluidi causava hydrolitic weakening dei cristalli di vena, permettendo l’innesco della plasticità del quarzo a bassa temperatura. Ad alta strain rate, la deformazione si localizzava in sottili shear zone dove la sovrappressione dei fluidi controbilanciava lo stress principale, permettendo più facilmente l’aprirsi di fratture dilatanti e la produzione di nuove vene di taglio. I risultati dello studio da me condotto sugli analoghi di megathrust descritti concordano con quanto è stato osservato in altri esempi fossili e con dati recenti provenienti dai margini di subduzione attivi. I partcolare, suggeriscono che i fluidi siano un fattore cruciale che contribuisce a controllare lo stile deformativo dei megathrust.

Meccanismi deformativi nella parte superficiale dei megathrust: studio meso e micro-strutturale di analoghi fossili / Anna Cerchiari , 2020 Feb 17. 31. ciclo, Anno Accademico 2017/2018.

Meccanismi deformativi nella parte superficiale dei megathrust: studio meso e micro-strutturale di analoghi fossili

CERCHIARI, ANNA
2020

Abstract

The largest earthquakes recorded on Earth (Mw>8) occur along low-dipping megathrusts at subduction plate margins. Shallow (< 5 km of depth) megathrusts are characterized by a spectrum of slip behaviours, including large earthquakes, tsunami earthquakes, slow slip and tremors. The physical and chemical processes controlling this complex behavior are still poorly constrained and, to investigate them, the study of fossil analogues exhumed by orogenic uplift represents an approach complementary to scientific drilling and geophysical investigations. My project has been aimed at studying in detail outcrops of megathrust fossil analogues by means of field, microstructural and geochemical analyses, with a particular focus on tectonic veins, to shed some light on the processes controlling fault slip style. I investigated two thrust fault zones, belonging to different fossil subduction complexes, to compare in detail the observed structural features and infer deformation mechanisms, helping to link the geological record to process operating on active subduction margins. The Sestola Vidiciatico tectonic Unit (SVU) in the Northern Apennines of Italy has been interpreted as a field analogue for the shallow portion (4-5 km, 150 °C maximum temperature) of subduction megathrusts. Throughout a multi-scale structural analysis of the SVU basal thrust cropping out in Vidiciatico (BO) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses performed on vein samples, the megathrust deformation history and interrelated fluid/stress variations during the seismic cycle has been reconstructed. Based on obtained results, the SVU basal thrust can be characterized as a weak fault, which progressively localized deformation to a cm-scale shear zone and which underwent a cyclical shifting of the principal σ1 and σ3 stresses, producing interrelated changes in permeability, fluid pressure and composition. The fault zone cropping out in SW Llŷn Peninsula, NW Wales (UK), is related to the ~ 600-500 Ma Pacific-type subduction-accretion Mona Complex. Meso- and microstructural analyses, fluid inclusion microthermometry and EBSD microscopy suggest that the fault zone deformed at conditions correspondent to the brittle-ductile transition, at T ≤ 300 °C. This occurred with a cyclical shifting from brittle to ductile deformation, mainly controlled by strain rate variations coupled with overpressured fluid pulses, producing repeated transient embrittlement. At low strain rate, the presence of fluid promoted hydrolitic weakening in vein crystals, thus facilitating the onset of low temperature quartz plasticity in high dislocation density regions. At higher strain rate, deformation localized in thin shear zones where fluid overpressures counteracted the principal stress, allowing dilatant fracturing to occur more easily and producing new shear veins. The results of my study of the described field megathrust analogues are consistent with what has been observed in several other fossil examples and with some of the recent data obtained from active subduction margins, and suggest that fluids are a crucial factor controlling the deformation style. They confirm that a multidisciplinary approach to the study of tectonic veins, involving field, microstructural and geochemical investigations, is a powerful tool to obtain information on the interrelations between fluid flow and deformation, enriching our knowledge of megathrust seismic behaviour.
Deformation processes of shallow subduction megathrusts: insights from field and microstructural analysis
17-feb-2020
REMITTI, Francesca
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11380/1199938
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