Formation of cataclasites in shallow-subsurface settings - meteoric diagenetic processes control fault rock formation at seismogenic faults in the Abruzzi Apennines, Italy
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To understand the interaction of surface and tectonic processes during the formation of fault rocks, we studied two faults located in the Abruzzi Appenines NE of L’Aquila, that have been active in historical time. The south-dipping Assergi fault is at least 17 km long, with an offset of 2.5 km in its central part. Over most of its extent, the fault is evident by a scarp. Present day morphology is related to selective erosion, as the fault scarp is covered in some areas by lithified talus deposits. The talus is, however, in many places involved in the faulting. The Campo Imperatore fault is about 30 km long, with an offset of 2 km. The fault is located a few km north of the Assergi fault and has approximately the same orientation. It seems to be complimentary to the Assergi fault: where the offset across the Assergi fault diminishes, throw of the Campo Imperatore fault increases. The fault scarp of the Campo Imperatore fault is partly covered by active alluvial fans, but older lithified fans are offset by related antithetic faults. Both faults have several meters of fault rocks; The fault rocks of the Campo Imperatore fault are kakirites. Cataclasites of the Assergi fault vary in thickness between 15 and 3 meters, which is related to the presence of Riedel shears that offset the boundary between the host rock and the fault rock. Within the cataclasites diffuse Riedel planes crosscut the fault rocks and offset diffuse or sharp planes parallel to the main fault that can be closely spaced. Diffuse zones parallel to the main fault show karstic vugs produced by meteoric dissolution. The vugs may be lined or filled by calcite cement, and/or with internal sediments (e. g., lime mud, vadose silt, dissolution clasts of cataclasite). Meteoric dissolution guided by the main faults also resulted in large karstic pores filled with collapse breccias and flowstones; clasts of flowstones and flowstone-cemented breccias, in turn, locally became reworked into cataclasites. Presence or absence of solution and precipitation processes control the formation of cataclasites at the Assergi fault and kakirites at the Campo Imperatore fault, respectively. Processes shaping the fault rocks of the investigated faults are therefore not only tectonic processes controlling the crushing of rock, but also diagenetic processes. Under these conditions, which are probably widespread, cataclasites may form near the surface. Surface processes can control the appearance of fault rocks of seismogenic faults.Keywords:
Echelon formation
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Lithology
Fault gouge
Breccia
Fault trace
Normal fault
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Outcrop
Anticline
Extensional fault
Extensional tectonics
Normal fault
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Protolith
Transform fault
Fault gouge
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The main fault of Yangsan Fault Zone (YFZ) and Quaternary fault were found in a trench section with NW-SE direction at an entrance of the Sinheung village in the northern Eonyang, Ulsan, Korea. We interpreted the movement history of the southern part of the YFZ from the geometric and kinematic characteristics of basement rock's fault of the YFZ (Sinheung Fault) and Quaternary fault (Quaternary Sinheung Fault) investigated at the trench section. The trench outcrop consists mainly of Cretaceous sedimentary rocks of Hayang Group and volcanic rocks of Yucheon Group which lie in fault contact and Quaternary deposits which unconformably overlie these basement rocks. This study suggests that the movement history of the southern part of the YFZ can be explained at least by two different strike-slip movements, named as D1 and D2 events, and then two different dip-slip movements, named as D3 and D4 events. (1) D1 event: a sinistral strike-slip movement which caused the bedding of sedimentary rocks to be high-angled toward the main fault of the YFZ. (2) D2 event: a dextral strike-slip movement slipped along the high-angled beddings as fault surfaces. The main characteristic structural elements are predominant sub-horizontal slickenlines and sub-vertical fault foliations which show a NNE trend. The event formed the main fault rocks of the YFZ. (3) D3 event: a conjugate reverse-slip movement slipped along fault surfaces which trend (E)NE and moderately dip (S)SE or (N)NW. The slickenlines, which plunge in the dip direction of fault surfaces, overprint the previous sub-horizontal slickenlines. The fault is characterized by S-C fabrics superimposed on the D2 fault gouges, fault surfaces showing ramp and flat geometry, asymmetric and drag folds and collapse structures accompanied with it. The event dispersed the orientation of the main fault surface of the YFZ. (4) D4 event: a Quaternary reverse-slip movement showing a displacement of several centimeters with S-C fabrics on the Quternary deposits. The D4 fault surfaces are developed along the extensions of the D3 fault surfaces of basement rocks, like the other Quaternary faults within the YFZ. This indicates that these faults were formed under the same compression of (N)NW-(S)SE direction.
Detachment fault
Transform fault
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Synopsis Three main sets of faults have been identified in Eastern Ardgour, the NE–SW faults, the NW–SE to W–E faults and the N–S faults. Each set has formed in response to a specific tectonic regime. Cataclastic rocks associated with the faults fall into two main series, the protomicrobreccia–microbreccia–cataclasite–ultracataclasite series, and the fault breccia–fault gouge series. These rock types are abundant in the hills and along the north-west shore of Loch Linnhe, but their distribution is far from uniform. The most intense cataclasis occurs along or adjacent to the main faults of the area which themselves lie within broader zones of variably shattered rock. A history of fault activity has been established for the area; of particular interest are the NE–SW faults because they form part of the Great Glen fault zone. Study of these faults and their associated cataclastic rocks indicates that the Great Glen has been active, intermittently, as a brittle seismic fault zone since late Caledonian times.
Cataclastic rock
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The deformation associated with a number of kilometre‐scale strike‐slip fault zones which cut through outcropping carbonate rocks in the Southern Apennines was investigated at regional and outcrop scales. These faults trend roughly east‐west and were studied at the Gargano Promontory on the Adriatic Coast (in the Apulian foreland) and in the Matese Mountains, about 120 km to the west (within the Apenninic fold‐and‐thrust belt). The fault zones are 200–300 m wide and typically comprise a core surrounded by a damage zone. Within fault cores, fault rocks (gouges and cataclasites) typically occur along master slip planes; in damage zones, secondary slip planes and solution cleavage are the most important planar discontinuities. The protolith carbonates surrounding the fault zone at Gargano show little deformation, but they are fractured in the Matese Mountains as a result of an earlier thrust phase. Cleavage surfaces in the damage zone of the studied faults are interpreted to be fault‐propagation structures. Our field data indicate that cleavage‐fault intersection lines are parallel to the normals of fault slip‐vectors. The angle between a fault plane and the associated cleavage was found to be fairly constant (c. 40“) at different scales of observation. Finally, the spacing of the solution cleavage surfaces appeared in general to be regular (with a mean of about 22 mm), although it was found to decrease slightly near a fault plane. These results are intended to provide a basis for predicting the architecture of fault zones in buried carbonate reservoirs using seismic reflection and borehole data.
Thrust fault
Outcrop
Classification of discontinuities
Cleavage (geology)
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The More-Trondelag Fault Complex (MTFC) is a major fault zone, extending along the coast of mid-Norway. It has a prolonged history of reactivation. Three outcrops along parts of the MTFC, known as the Tjellefonna Fault have been described in this study. The focus has been the study of fault architecture and the related fault rocks. At Vik, more than three sets of fault lenses have developed, bounded by numerous faults in a complex pattern. The faults here are cutting the foliation at a high angle, resulting in the complex fault architecture. For the other localities of Rod and Mulvik, the faults are partly reactivating the Caledonian foliation, making the fault architecture more regular and systematic. All fault rocks sampled in this study are brittle, and are suggested to have formed at crustal depths of 9 km and shallower. The matrix of the fault rocks is dominated by elongated laths of zeolite. These are torn off from the crystals along the cleavage. At least 3 generations of zeoliteare found at Vik and Mulvik. The first generation pre-dates a zeolite-dominated cataclasite. At Mulvik this was followed by another generation of zeolite, two generations of cohesive breccia and a third zeolite generation. One of the last generations are found to be steep sets of extensional fractures filled with zeolite. This zeolite is analysed by XRD and found to be laumontite. The last activity of faulting is thought to be represented by the uncohesive breccia found at Mulvik. This is related to shears, interpreted as R-shears indicating an overall “northwest sidedown” sense of movement of this last activity along the fault.
Breccia
Outcrop
Brittleness
Extensional fault
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Bedrock
Thrust fault
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The Italian Central Apennines are one of the most seismically active areas in the Mediterranean (e.g., L’Aquila-Pizzoli Mw 6.7, 1703; Avezzano Mw 7.1, 1915; L’Aquila Mw 6.1, 2009). Most of this continuous seismicity is produced by earthquake ruptures propagating along normal faults hosted in carbonate rocks (dolostones and limestones).
Some of these active fault zones are well-exposed in the mountain belt within badlands exposures.
The most impressive structural feature of these exposed fault zones is the
occurrence of up to hundreds of meters thick in-situ shattered rocks (fault rocks reduced in fragments < 1 cm in size on average and affected by negligible shear strain, i.e. they may preserve original sedimentary fabrics such as bedding, laminations etc.). However, both the geometry of these shattered rock bodies and how they have been produced (during seismic
rupture propagation or other stages of the seismic cycle) remain largely unknown, also because of the lack of quantitative fault zone structural data (e.g., how shattered fault rocks are distributed along fault strike, how their thickness varies with fault length, displacement, geometry, etc.). A deep understanding of how in-situ shattered carbonate rocks are produced may impact our understanding of earthquake mechanics in carbonates and
seismic hazard studies.
Given the lack of quantitative data about in-situ shattered fault rocks in active fault zones in carbonates, the main goals of this thesis are:
1. the detailed field structural survey to quantify the distribution and thickness of in-situ shattered rocks and,
2. the remote-sensing analysis coupled with literature data review to determine, if any, scale relations between fault zone length, displacement, geometry and thickness of in-situ shattered fault rocks in carbonates.
Indeed, such dataset is at the base of any model about the formation of the in-situ shattered rocks. To achieve these goals:
1. I conducted detailed field structural geology survey of the Monte Marine fault zone (Central Apennines), whose damage zone is characterized by in-situ shattered dolostones,
2. I conducted Optical and Scanning Electron Microscopy microstructural investigations of both in-situ shattered fault rocks and fault slipping zones,
3. I produced a catalogue which includes six main active normal fault zones of the Central Apennines characterized by up to 100s m thick damage zones with in-situ shattered carbonates.
In particular, I mapped at 1:500 scale the Monte Marine fault zone (between thevillages of Pizzoli and Arischia, 10 km NE of the town of L’Aquila, Italy) where two fault strands overlap and collected data in 26 structural stations. Here, the fault core is ~ 30 m thick while the damage zone reaches ~ 1000 m in thickness and hosts in-situ shattered rocks, plus hundreds of minor synthetic and antithetic extensional faults, strike-slip and thrust faults. The latter are interpreted as Miocene to Pliocene structures reactivated during
the Quaternary extensional phase and that interfered with newly formed post-orogenic normal faults, thus increasing the cataclastic rock volume in the intersection areas. The geological cross sections provided in this study underlie structural complexities due to the linkage of different fault segments and to the inherited compressional-to-extensional tectonic inversion.
Based on the field observations, I propose that the extraordinary volumes of damage zone in the studied area of the Monte Marine Fault zone result from a combination of
1. geometrical complexities associated to the overstep sector,
2. presence of inherited compressional structures (thrust faults) and
3. seismogenic behaviour of the Master and minor faults.
Cataclasites and in-situ shattered rocks are inferred to be the result of shattering up to hundreds of meters far from the Master Fault due to the stress perturbations and the near-field elastic waves induced and released by the propagation of seismic ruptures both along the Master Fault (main shocks) and minor faults cutting the damage zone (aftershocks).
To produce the fault catalogue I used satellite images coupled with published
geological maps to recognize where badland-type exposures could be related to the presence of in-situ shattered rocks. The Middle-Aterno Valley, the Morrone, the Venere, the Campo Imperatore and the Pescasseroli fault zones were also selected for less detailed structural geology survey to determine the fault damage zone thickness.
The main results of the thesis include:
1. the first description of fault zone rocks distribution of the Monte Marine Fault and the reconstruction of the fault architecture in the overstep sector,
2. the first description of inherited compressional structures within the Monte Marine Fault zone,
3. exploiting the still limited fault catalogue, I find power-law relations between fault damage zone thickness, fault displacement and fault length. In particular, the thickness of the damage zone increases with fault displacement and slightly decreases with fault length suggesting that first order geometrical complexities (e.g., presence of step-overs) control the thickness of damage zones.
Bedding
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In order to outline the kinematics and movement history of a new Quaternary fault, Jingwan Fault in Dangjin, West Korea, we analyzed the geometry of the fault zone composed of a few gouge zones, and made ESR dating for fault gouge materials. The striking Jingwan Fault is a normal fault and exhibits a gradual change in dip (gentle in the lower part, steep in the upper part), indicating a listric fault. As for the fault gouge zone, its thickness varies and reaches 2~3 cm in the lower part or between basement rocks, and 20~30 cm in the middle-upper part or between the basement and Quaternary deposit. It is observed in the latter case that more than three gouge zones develop with different colors, and branch out and re-merge, or they are partly superimposed, indicating different movement episodes. The cumulative displacement is estimated to be about 10 m using the geological cross-sections, from which it is inferred that the total length of fault may be about 2.5 km on the basis of the empirical relation between cumulative displacement and fault length. Therefore, a more study would be needed to verify the entire fault length. The results of ESR dating for three gouge samples at different spots along the fault yields ages of , , and , indicating at least two movement episodes. Slickenlines observed on the fault planes indicate a pure dip slip (normal faulting), which suggests that the ENE-WSW trending Jingwan Fault was presumably moved under a NNW-SSE extensional environment.
Fault gouge
Merge (version control)
Detachment fault
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