We document phyllosilicates occurrence along five shallow (exhumed from depths < 3 km) carbonate-... more We document phyllosilicates occurrence along five shallow (exhumed from depths < 3 km) carbonate-hosted extensional faults from the seismically-active domain of the central Apennines, Italy. The shallow portion of this domain is characterized by a sedimentary succession consisting of ~5e6 km thick massive carbonate deposits overlain by ~2 km thick phyllosilicate-rich deposits (marls and siliciclastic sand-stones). We show that the phyllosilicates observed within the studied carbonate-hosted faults derived from the overlying phyllosilicate-rich sedimentary deposits and were involved in the faulting processes. We infer that, during fault zone evolution, the phyllosilicates downward injected into pull-aparts (i.e., dilational jogs) that were generated along staircase extensional faults. With further displacement accumulation, the clayey material was smeared and concentrated into localized layers along the carbonate-hosted fault surfaces. These layers are usually thin (a few centimeters to decimeters thick), but can reach also a few meters in thickness. We suggest that, even in tectonic settings dominated by high frictional strength rocks (e.g., carbonates), localized layers enriched in weak phyllosilicates can occur along shallow fault surfaces thus reducing the expected fault strength during earthquakes, possibly promoting co-seismic slip propagation up to the Earth's surface.
Keywords: fluid circulation seismic cycle fluid pressure stable isotopes fault zone evolution cen... more Keywords: fluid circulation seismic cycle fluid pressure stable isotopes fault zone evolution central Apennines We examine the potentially-seismic right-lateral transtensional–extensional Tre Monti Fault (central Apennines, Italy) with structural and geochemical methods and develop a conceptual evolutionary model of extensional faulting with fluid involvement in shallow (≤3 km depth) faults in carbonate rocks. In the analysed fault zone, multiscale fault rock structures include injection veins, fluidized ultracataclasite layers, and crackle breccias, suggesting that the fault slipped seismically. We reconstructed the relative chronology of these structures through cross-cutting relationship and cathodoluminescence analyses. We then used C-and O-isotope data from different generations of fault-related mineralizations to show a shift from connate (marine-derived) to meteoric fluid circulation during exhumation from 3 to ≤1 km depths and concurrent fluid cooling from ∼68 to &lt;30 • C. Between ∼3 km and ∼1 km depths, impermeable barriers within the sedimentary sequence created a semi-closed hydrological system, where prevalently connate fluids circulated within the fault zone at temperatures between 60 • and 75 • C. During fault zone exhumation, at depths ≤1 km and temperatures &lt;30 • C, the hydrological circulation became open and meteoric-derived fluids progressively infiltrated and circulated within the fault zone. The role of these fluids during syn-exhumation seismic cycles of the Tre Monti Fault has been substantially passive along the whole fault zone, the fluids being passively redistributed at hydrostatic pressure following co-seismic dilatancy. Only the principal fault has been characterized, locally and transiently, by fluid overpressures. The presence of low-permeability clayey layers in the sedimentary sequence contributed to control the type of fluids infiltrating into the fault zone and possibly their transient overpressures. These results can foster the comprehension of seismic faulting at shallow depths in carbonate rocks of other fold-thrust belts involved in post-collisional seismogenic extensional tectonics.
The Central Apennine are one of the most seismically active regions in the Mediterranean area and... more The Central Apennine are one of the most seismically active regions in the Mediterranean area and is affected by moderate to large shallow earthquakes that enucleate in and propagate through carbonate rocks. In this work we present a detailed fieldwork and microstructural analysis to define the architecture and deformation mechanisms of an exhumed fault zone in carbonates, the Tre Monti fault, at the northern boundary of the Fucino Basin. Fault rocks assemblages show differences in deformation mechanisms between the main and external fault planes, and subsidiary fault planes developed within the damage zone. We infer that this variety of fault rocks represents different deformation processes acting during different stages of fault development and fluid circulation. The multidisciplinary but field-based study of fault surfaces and fault rocks is fundamental to reveal the geological record of past earthquakes and seismic cycles and is strongly complementary to the seismological-based ...
Natural faults produce granular wear material, known as gouge or cataclasite, as a function of sh... more Natural faults produce granular wear material, known as gouge or cataclasite, as a function of shear and grinding along the slipping surfaces. The characteristics of fault gouge have been studied extensively in the field, laboratory, and numerical simulations in order to gain a better understanding of fault mechanics (e.g., Marone and Scholz, 1989). However, observations of natural fault gouges in active fault zones can still provide precious information about fault activity and mechanical processes acting during fault evolution. Here, we report detailed microstructural observations (optical and electronic microscopy) on natural fault rocks from the scarp of an active fault in carbonate rocks: the Tre Monti fault, in the Lazio-Abruzzi Apennines. This area is one of the most seismic regions in the Mediterranean area (e.g., L’Aquila Earthquake, Mw 6.3, 2009). We revealed, for the first time in this area, the occurrence of very comminute localization zones enriched with exotic material...
The measurement of crustal stress magnitude is always challenging and generally poorly constraine... more The measurement of crustal stress magnitude is always challenging and generally poorly constrained. This is particularly significant in active fault zones where the knowledge of stress magnitude is crucial for understanding fault mechanics during earthquakes nucleation. In this work we propose a workflow using laboratory and field data as a proxy for quantitative paleostress reconstruction along active fault zone. We studied the exhumed Olevano-Antrodoco Thrust Fault (OATF) in Central Italy consisting of a SW-dipping thrust fault that juxtapose middle Miocene carbonates in the hangingwall above upper Miocene foredeep sandstones, W-SW-dipping, in the footwall. We collected 26 samples of footwall sandstones approaching progressively the OATF, from the undeformed deposits (1 km away to the E) to the tectonically deformed sandstones close (50 m far) to the OATF. Field data highlighted that the footwall sandstones dips towards W-SW, thus moving towards the OATF, shallower strata progress...
The Central Apennines are characterized by a direction of maximum stretching oriented NE-SW. Desp... more The Central Apennines are characterized by a direction of maximum stretching oriented NE-SW. Despite this extension is mostly accommodated by normal slip on NW-trending fault segments, several faults have different orientation. In this study we present a detailed structural and geological survey to define the kinematics and architecture of an exhumed fault zone in Mesozoic carbonates, the WSW-trending and 8 km long Tre Monti-Celano fault zone, both by field and remote sensed methods (e.g. satellite pictures, aerial images, digital earth model). The Tre Monti-Celano fault zone is the northern boundary of the Fucino Basin, an intramontane half-graben filled by Plio-Quaternary alluvial and lacustrine deposits located in the central part of the Apennines chain, which was formed in Upper Pliocene and in Quaternary time by the extensional tectonic activity. The main fault plane consists of SSE-dipping normal fault cutting lower to middle Cretaceous limestone in the footwall and upper Pliocene to middle Pleistocene lacustrine deposits and subaerial slope debris in the hangingwall. Kinematics indicators, fracture orientation, and geometry of fault splays along the fault zone recorded mainly dextral transtension movement. The main fault is splitted into smaller segments (from 700 m up to 2 km in length) arranged with dextral en-echelon geometry. Inversion of kinematics indicators on fault segments oriented respectively ENE-WSW, NE-SW and NNE-SSW indicate a direction of maximum extension oriented NE-SW, accordingly to the direction of the maximum regional extension. These data suggest that the Tre Monti fault zone acted as a dextral transfer zone between two left-stepping major normal faults, oriented NW-SE and characterized by normal or slightly dextral oblique slip: the Venere-Serrone Fault in the southeast and the Velino fault in the north-west. Detailed mapping along the fault zone allowed to recognize different types of fault rocks generated by different lithologies involved in deformation and different amount of fault displacements. Further work will focus on a microstructuralmineralogical and geochemical characterization of natural fault rock samples to understand the mechanism of deformation and the role of fluid within these processes.
Journal of Geophysical Research: Solid Earth, 2014
Petrophysical properties of rocks and their applicability at larger scale are a challenging topic... more Petrophysical properties of rocks and their applicability at larger scale are a challenging topic in Earth sciences. Petrophysical properties of rocks are severely affected by boundary conditions, rock fabric/microstructure, and tectonics that require a multiscale approach to be properly defined. Here we (1) report laboratory measurements of density, porosity, permeability, and P wave velocities at increasing confining pressure conducted on Miocene foredeep sandstones (Frosinone Formation); (2) compare the laboratory results with larger-scale geophysical investigations; and (3) discuss the effect of thrusting on the properties of sandstones. At ambient pressure, laboratory porosity varied from 2.2% to 13.8% and P wave velocities (V p ) from 1.5 km/s to 2.7 km/s. The P wave velocity increased with confining pressure, reaching between 3.3 km/s and 4.7 km/s at 100 MPa. In situ V p profiles, measured using sonic logs, matched the ultrasonic laboratory measurement well. The permeability varied between 1.4 × 10 À15 m 2 and 3.9 × 10 À15 m 2 and was positively correlated with porosity. The porosity and permeability of samples taken at various distances to the Olevano-Antrodoco fault plane progressively decreased with distance while P wave velocity increased. At about 1 km from the fault plane, the relative variations reached 43%, 65%, and 20% for porosity, permeability, and P wave velocity, respectively. This suggests that tectonic loading changed the petrophysical properties inherited from sedimentation and diagenesis. Using field constraints and assuming overburden-related inelastic compaction in the proximity of the fault plane, we conclude that the fault reached the mechanical condition for rupture in compression at differential stress of 64.8 MPa at a depth of 1500 m.
We document phyllosilicates occurrence along five shallow (exhumed from depths < 3 km) carbonate-... more We document phyllosilicates occurrence along five shallow (exhumed from depths < 3 km) carbonate-hosted extensional faults from the seismically-active domain of the central Apennines, Italy. The shallow portion of this domain is characterized by a sedimentary succession consisting of ~5e6 km thick massive carbonate deposits overlain by ~2 km thick phyllosilicate-rich deposits (marls and siliciclastic sand-stones). We show that the phyllosilicates observed within the studied carbonate-hosted faults derived from the overlying phyllosilicate-rich sedimentary deposits and were involved in the faulting processes. We infer that, during fault zone evolution, the phyllosilicates downward injected into pull-aparts (i.e., dilational jogs) that were generated along staircase extensional faults. With further displacement accumulation, the clayey material was smeared and concentrated into localized layers along the carbonate-hosted fault surfaces. These layers are usually thin (a few centimeters to decimeters thick), but can reach also a few meters in thickness. We suggest that, even in tectonic settings dominated by high frictional strength rocks (e.g., carbonates), localized layers enriched in weak phyllosilicates can occur along shallow fault surfaces thus reducing the expected fault strength during earthquakes, possibly promoting co-seismic slip propagation up to the Earth's surface.
Keywords: fluid circulation seismic cycle fluid pressure stable isotopes fault zone evolution cen... more Keywords: fluid circulation seismic cycle fluid pressure stable isotopes fault zone evolution central Apennines We examine the potentially-seismic right-lateral transtensional–extensional Tre Monti Fault (central Apennines, Italy) with structural and geochemical methods and develop a conceptual evolutionary model of extensional faulting with fluid involvement in shallow (≤3 km depth) faults in carbonate rocks. In the analysed fault zone, multiscale fault rock structures include injection veins, fluidized ultracataclasite layers, and crackle breccias, suggesting that the fault slipped seismically. We reconstructed the relative chronology of these structures through cross-cutting relationship and cathodoluminescence analyses. We then used C-and O-isotope data from different generations of fault-related mineralizations to show a shift from connate (marine-derived) to meteoric fluid circulation during exhumation from 3 to ≤1 km depths and concurrent fluid cooling from ∼68 to &lt;30 • C. Between ∼3 km and ∼1 km depths, impermeable barriers within the sedimentary sequence created a semi-closed hydrological system, where prevalently connate fluids circulated within the fault zone at temperatures between 60 • and 75 • C. During fault zone exhumation, at depths ≤1 km and temperatures &lt;30 • C, the hydrological circulation became open and meteoric-derived fluids progressively infiltrated and circulated within the fault zone. The role of these fluids during syn-exhumation seismic cycles of the Tre Monti Fault has been substantially passive along the whole fault zone, the fluids being passively redistributed at hydrostatic pressure following co-seismic dilatancy. Only the principal fault has been characterized, locally and transiently, by fluid overpressures. The presence of low-permeability clayey layers in the sedimentary sequence contributed to control the type of fluids infiltrating into the fault zone and possibly their transient overpressures. These results can foster the comprehension of seismic faulting at shallow depths in carbonate rocks of other fold-thrust belts involved in post-collisional seismogenic extensional tectonics.
The Central Apennine are one of the most seismically active regions in the Mediterranean area and... more The Central Apennine are one of the most seismically active regions in the Mediterranean area and is affected by moderate to large shallow earthquakes that enucleate in and propagate through carbonate rocks. In this work we present a detailed fieldwork and microstructural analysis to define the architecture and deformation mechanisms of an exhumed fault zone in carbonates, the Tre Monti fault, at the northern boundary of the Fucino Basin. Fault rocks assemblages show differences in deformation mechanisms between the main and external fault planes, and subsidiary fault planes developed within the damage zone. We infer that this variety of fault rocks represents different deformation processes acting during different stages of fault development and fluid circulation. The multidisciplinary but field-based study of fault surfaces and fault rocks is fundamental to reveal the geological record of past earthquakes and seismic cycles and is strongly complementary to the seismological-based ...
Natural faults produce granular wear material, known as gouge or cataclasite, as a function of sh... more Natural faults produce granular wear material, known as gouge or cataclasite, as a function of shear and grinding along the slipping surfaces. The characteristics of fault gouge have been studied extensively in the field, laboratory, and numerical simulations in order to gain a better understanding of fault mechanics (e.g., Marone and Scholz, 1989). However, observations of natural fault gouges in active fault zones can still provide precious information about fault activity and mechanical processes acting during fault evolution. Here, we report detailed microstructural observations (optical and electronic microscopy) on natural fault rocks from the scarp of an active fault in carbonate rocks: the Tre Monti fault, in the Lazio-Abruzzi Apennines. This area is one of the most seismic regions in the Mediterranean area (e.g., L’Aquila Earthquake, Mw 6.3, 2009). We revealed, for the first time in this area, the occurrence of very comminute localization zones enriched with exotic material...
The measurement of crustal stress magnitude is always challenging and generally poorly constraine... more The measurement of crustal stress magnitude is always challenging and generally poorly constrained. This is particularly significant in active fault zones where the knowledge of stress magnitude is crucial for understanding fault mechanics during earthquakes nucleation. In this work we propose a workflow using laboratory and field data as a proxy for quantitative paleostress reconstruction along active fault zone. We studied the exhumed Olevano-Antrodoco Thrust Fault (OATF) in Central Italy consisting of a SW-dipping thrust fault that juxtapose middle Miocene carbonates in the hangingwall above upper Miocene foredeep sandstones, W-SW-dipping, in the footwall. We collected 26 samples of footwall sandstones approaching progressively the OATF, from the undeformed deposits (1 km away to the E) to the tectonically deformed sandstones close (50 m far) to the OATF. Field data highlighted that the footwall sandstones dips towards W-SW, thus moving towards the OATF, shallower strata progress...
The Central Apennines are characterized by a direction of maximum stretching oriented NE-SW. Desp... more The Central Apennines are characterized by a direction of maximum stretching oriented NE-SW. Despite this extension is mostly accommodated by normal slip on NW-trending fault segments, several faults have different orientation. In this study we present a detailed structural and geological survey to define the kinematics and architecture of an exhumed fault zone in Mesozoic carbonates, the WSW-trending and 8 km long Tre Monti-Celano fault zone, both by field and remote sensed methods (e.g. satellite pictures, aerial images, digital earth model). The Tre Monti-Celano fault zone is the northern boundary of the Fucino Basin, an intramontane half-graben filled by Plio-Quaternary alluvial and lacustrine deposits located in the central part of the Apennines chain, which was formed in Upper Pliocene and in Quaternary time by the extensional tectonic activity. The main fault plane consists of SSE-dipping normal fault cutting lower to middle Cretaceous limestone in the footwall and upper Pliocene to middle Pleistocene lacustrine deposits and subaerial slope debris in the hangingwall. Kinematics indicators, fracture orientation, and geometry of fault splays along the fault zone recorded mainly dextral transtension movement. The main fault is splitted into smaller segments (from 700 m up to 2 km in length) arranged with dextral en-echelon geometry. Inversion of kinematics indicators on fault segments oriented respectively ENE-WSW, NE-SW and NNE-SSW indicate a direction of maximum extension oriented NE-SW, accordingly to the direction of the maximum regional extension. These data suggest that the Tre Monti fault zone acted as a dextral transfer zone between two left-stepping major normal faults, oriented NW-SE and characterized by normal or slightly dextral oblique slip: the Venere-Serrone Fault in the southeast and the Velino fault in the north-west. Detailed mapping along the fault zone allowed to recognize different types of fault rocks generated by different lithologies involved in deformation and different amount of fault displacements. Further work will focus on a microstructuralmineralogical and geochemical characterization of natural fault rock samples to understand the mechanism of deformation and the role of fluid within these processes.
Journal of Geophysical Research: Solid Earth, 2014
Petrophysical properties of rocks and their applicability at larger scale are a challenging topic... more Petrophysical properties of rocks and their applicability at larger scale are a challenging topic in Earth sciences. Petrophysical properties of rocks are severely affected by boundary conditions, rock fabric/microstructure, and tectonics that require a multiscale approach to be properly defined. Here we (1) report laboratory measurements of density, porosity, permeability, and P wave velocities at increasing confining pressure conducted on Miocene foredeep sandstones (Frosinone Formation); (2) compare the laboratory results with larger-scale geophysical investigations; and (3) discuss the effect of thrusting on the properties of sandstones. At ambient pressure, laboratory porosity varied from 2.2% to 13.8% and P wave velocities (V p ) from 1.5 km/s to 2.7 km/s. The P wave velocity increased with confining pressure, reaching between 3.3 km/s and 4.7 km/s at 100 MPa. In situ V p profiles, measured using sonic logs, matched the ultrasonic laboratory measurement well. The permeability varied between 1.4 × 10 À15 m 2 and 3.9 × 10 À15 m 2 and was positively correlated with porosity. The porosity and permeability of samples taken at various distances to the Olevano-Antrodoco fault plane progressively decreased with distance while P wave velocity increased. At about 1 km from the fault plane, the relative variations reached 43%, 65%, and 20% for porosity, permeability, and P wave velocity, respectively. This suggests that tectonic loading changed the petrophysical properties inherited from sedimentation and diagenesis. Using field constraints and assuming overburden-related inelastic compaction in the proximity of the fault plane, we conclude that the fault reached the mechanical condition for rupture in compression at differential stress of 64.8 MPa at a depth of 1500 m.
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Papers by Luca Smeraglia