Search

Seismic slip episodically occurring along shallow creeping faults in poorly lithified sediments represents an unsolved paradox, largely due to our poor understanding of the mechanics governing creeping faults and the lack of documented geological evidence showing how coseismic rupturing overprints creep in near-surface conditions. Here we describe the signature of seismic ruptures propagating along shallow creeping faults affecting unconsolidated forearc sediments. Field observations of deformation band-dominated fault zones show widespread foliated cataclasites in fault cores, locally overprinted by sharp slip surfaces decorated by thin (0.5-1.5 cm) black gouge layers (herein, black gouge). Compared to foliated cataclasites, black gouges have much lower grain size, porosity, and permeability. Moreover, they are characterized by distinct mineralogical assemblages compatible with high temperatures (180- 200 °C) due to frictional heating during seismic slip. Foliated cataclasites were also produced by laboratory experiments performed on host sediments at subseismic slip rates (⩽0.1 m/s), displaying high residual friction (μf = 0.65) and strain-hardening behavior. Black gouges were produced during experiments performed at seismic (1 m/s) slip rates, displaying low residual friction (μf = 0.3) due to dynamic weakening. Our results show that black gouges represent a potential diagnostic marker for seismic faulting in shallow creeping faults. These findings can help understanding the time-space partitioning between aseismic and seismic behavior of faults at shallow crustal levels. [ABSTRACT FROM AUTHOR]

The Neogene extensional province of southern Tuscany in central Italy provides an outstanding example of fossil and active structurally controlled fluid flow and epithermal ore mineralization associated with post-orogenic silicic magmatism. Characterization of the hydrodynamic regime leading to the genesis of the polysulphide deposit (known as Filone di Boccheggiano) hosted within the damage zone of the Boccheggiano Fault is a key target to assess modes of fossil hydrothermal fluid circulation in the region and, more generally, to provide inferences on fault-controlled hydrothermal fluid flow in extensional settings. We provide a detailed description of the fault zone architecture and alteration/mineralization associated with the Boccheggiano ore deposit and report the results of fluid inclusion and stable oxygen isotope studies. This investigation shows that the Boccheggiano ore consists of an adularia/illite-type epithermal deposit and that sulphide ore deposition was controlled by channelling of hydrothermal fluids of dominantly meteoric origin within the highly anisotropic permeability structure of the Boccheggiano Fault. The low permeability structure of the fault core compartmentalized the fluid outflow preventing substantial cross-fault flow, with focused fluid flow occurring at the hangingwall of the fault controlled by fracture permeability. Fluid inclusion characteristics indicate that ore minerals were deposited between 280° and 350°C in the upper levels of the brittle extending crust (lithostatic pressure in the order of 0.1 GPa). Abundant vapour-rich inclusions in ore-stage quartz are consistent with fluid immiscibility and boiling, and quartz ore vein textures suggest that mineralization in the Boccheggiano ore deposit occurred during cyclic fluid flow in a deformation regime regulated by transient and fluctuating fluid pressure conditions. Results from this study (i) predict a strongly anisotropic permeability structure of the fault damage zone during crustal extension, and (ii) indicate the rate of secondary (structural) permeability creation and maintenance by active deformation in the hangingwall of extensional faults as the major factor leading to effective hydraulic transmissivity in extensional terranes. These features intimately link ore-grade mineralization in extensional settings to telescoping of hydrothermal flow along the hangingwall block(s) of major extensional fault zones. [ABSTRACT FROM AUTHOR]

Natural fracture networks exert a firstorder control on the exploitation of resources such as aquifers, hydrocarbons, and geothermal reservoirs, and on environmental issues like underground gas storage and waste disposal. Fractures and the mechanical stratigraphy of layered sequences have been intensively studied to unravel the relationships between bed thickness and fracture spacing, but less attention has been paid to intrabed fracturing patterns due to the intrinsic local variability of sedimentary processes and products. Among sedimentary rocks, turbidites show great lateral and vertical variability of textural characteristics and depositional facies, which are expected to strongly influence the location and density of fractures. To better understand the contribution of stratigraphic, sedimentologic, and petrophysical properties on fracture patterns, we performed a high-resolution study on a selected stratigraphic interval of jointed foredeep turbidites in the Miocene Marnoso- Arenacea Formation (Northern Apennines, Italy). Cumulative statistical relations of field and laboratory structural, sedimentologic, and petrophysical data significantly improved when analyzed at the sedimentary facies scale. In particular, for facies recording different cross-flow (i.e., longitudinal to the paleocurrents) depositional conditions within the parent turbidity currents, we observed three-dimensional anisotropies of rock hardness (i.e., uniaxial compression) that were positively correlated with normalized fracture intensities, indicating a primary sedimentary control on fracture distribution. This type of intrabed joint distribution has crucial practical implications for the lateral prediction and evaluation of mesoscale fracture patterns in turbidite sequences. [ABSTRACT FROM AUTHOR]

The main issue addressed in this work is the process leading to fluid subsurface entrapment and pressure increase up to hydrofracturing and, possibly, to paroxysm in a hydro thermal setting, in order to envisage such processes and mitigate their effects in the volcanically active study area and elsewhere. A field and laboratory multidisciplinary approach is used in the fossil (late Pleistocene) portion of an active hydrothermal system (Colli Albani volcano, Rome, Italy). In this area, sulfate and sulfide mineralizations and strongly altered ignimbrites are exposed. The alteration acme occurs on top of a buried normal fault, where abundant degassing is still active, and fades away in 2-3 km. Based on pervasive versus discrete alteration styles, mineral assemblages, and further evidence, proximal and distal alteration domains are recognized. Both domains underwent steamheated advanced argillic alteration with likely temperatures up to ~400 °C in the proximal domain and less than 150 °C in the distal domain. The process of hydrothermal alteration progressively and severely depleted many elements from the most permeable rock units, whereas the lowest-permeable unit (Tufo Lionato ) underwent fracture and porosity healing accompanied by both mass and volume gain. In the proximal domain, the advanced argillic hydrothermal alteration eventually formed a substantial barrier to fluids. The hydrothermal fluids accumulated in and below this barrier, which was then suddenly hydrofractured when heat-driven hydraulic pressure overcame the effective stress, thus possibly leading to hydrothermal paroxysm. The decompression associated with hydrofracturing enhanced gas exsolution and mineral precipitation from the entrapped overpressured fluids. Mineral precipitation contributed, in turn, to fracture healing and to reinitiation of a new cycle of hydrothermal fluid entrapment. The key preconditions for the occurrence of the inferred processes are the contrasting compositions of K-alkaline host rocks and acidic alteration fluids, as also previously documented in other similar settings elsewhere. [ABSTRACT FROM AUTHOR]

Reconstructing the paleofluid evolution in mature fault zones, which typically have complex structural architectures, is a challenging task because reactivation of pre-existing deformation structures and dissolution-reprecipita tion processes are very abundant. Understanding why specific structural elements are preferentially mineralized and what are the factors leading to rapid fluid migration and accumulation, bears geological and economic implications, especially in seismically active fault zones. We studied the Compione Fault on the Tyrrhenian Sea side of the Northern Apennines orogenic wedge, Italy, which is a segment of the 30-km-long Northern Lunigiana high-angle extensional fault system still active today. The Compione Fault propagated from the metamorphic basement and accumulated about 1.5 km of displacement. We used structural, petrographic, isotopic, microthermometric, compositional, and organic matter analyses to constrain fluid and host rock properties during fault zone evolution. This approach allowed us to quantify the thermal anomaly in the fault zone and to infer the processes responsible for such a disequilibrium. Specifically, we show that in the fault process zone ahead of the upper fault tip, which is twice as wide as the damage zone, seismic pumping caused suprahydrostatic fluid pressures and that local dilation promoted the nucleation of a highly permeable mesh of conjugate extensional shear fractures hosting calc-silicate mineralization. The thermal difference between hydrothermal minerals in the conjugate fracture mesh and the host rock is 60-90 °C. The mineralizing fluids were deeply sourced from metamorphic reactions. Propagation of the upper fault tip caused process zone folding and incorporation into the fault damage zones. As the upper fault tip breached through shallower structural levels, it favored mixing between deep and meteoric fluids. [ABSTRACT FROM AUTHOR]

Hydrothermal silicification and deformation bands influence the physical properties of porous siliciclastic rocks. However, the impacts generated by the coexistence of these two processes on fault zone flow properties, such as porosity and permeability reductions, are still debatable. We integrated structural, geomechanical, and petrophysical data to investigate the influence of hydrothermal silicification on the physical properties of a fault zone. The fault affects Precambrian crystalline rocks and porous sandstones in the Rio do Peixe Basin, northeastern Brazil. The results indicate that quartz cementation is confined to a halo along the fault. Silicification decreases away from the fault zone toward the basin, which we subdivided into three main zones: (1) nonsilicified sandstone – Zone I, (2) moderately silicified sandstone – Zone II, and (3) intensely silicified sandstone – Zone III. The elongated geometry, the thickness of the silicified body, the proximity to the fault, and the high silica cement concentration indicate an external silica source. Nevertheless, we also propose internal sources related to feldspar dissolution. The primary porosity is rarely preserved in Zone III, while the secondary porosity is filled with silica cement. The primary sandstone porosity is well-preserved in Zone II, while many dissolved feldspar grains are not filled with cement. The petrographic and petrophysical analysis (cataclastic matrix amount) indicates that cement precipitation also occurred inside the deformation bands. Silicification increased the uniaxial compressive strength (UCS) by one order of magnitude in the fault damage zone relative to the protolith. On the other hand, the porosity and permeability in the silicified fault zone exhibit a reduction of two and four orders of magnitude, respectively, relative to the undeformed and nonsilicified protolith. This study indicates that silicification along a fault zone has crucial implications for the reservoir properties of siliciclastic rocks, strongly reducing the fluid flow properties and increasing the UCS values. [Display omitted] • We recorded hydrothermal silicification in a deformation band fault zone at the Rio do Peixe Basin, Brazil • We assessed how hydrothermal silicification affects the physical properties of arkosic sandstone in a fault zone • Hydrothermal silicification splits the fault zone into non-silicified, moderately silicified, and intensely silicified zones • Percolation of hydrothermal fluid and precipitation of silica cement occur within deformation bands and adjacent host rocks • Silicification increases UCS by 65% and decreases porosity-permeability by up to 2-4 orders of magnitude in fault zones [ABSTRACT FROM AUTHOR]

The Middle Jurassic limestones of the Vajont Formation, which constitute major parts of the flanks of the Vajont Canyon (Southern Alps, Italy), have been subjected to pervasive dolomitization. The dolomites are localized within fault damage zones. Based on field and petrographic characteristics, two dolostone groups can be differentiated. Group 1, being the most prevailing, corresponds to replacive matrix dolostones. They form non-stratabound plume-like geobodies. They also occur as infill in infrequent veins in the host limestones that occur adjacent to these replacive plume-like dolostone bodies. Group 2, that is less common, consists of saddle dolomite cements. It occurs in breccia and veins or makes up zebra dolostones. The latter cements frequently affect the replacive group 1 dolostones. Fluid inclusion analyses of both dolostone groups indicate comparable salinity values of 1.2–5.3 eq. wt.% NaCl with homogenization temperatures of 70–108 °C, which in addition of overlapping 87Sr/86Sr ratios (0.70811–0.70846) reflect similar parental fluids. The estimated salinities, in combination with δ18O values (−5.5 to −14.8‰ V-PDB), indicate significant contributions of 18O-depleted fluids postulated to be modified meteoric waters. This origin is supported by the low trace element contents measured in the studied dolostones. These data support that the fault-controlled dolostones in the studied Vajont area precipitated from hot hypo- to mesohaline fluids. These fluids could have circulated along inherited Mesozoic syn-rift fault and fracture corridors. However, another possibility is that fluids circulated along fault and fracture systems that developed during the Neo-Alpine Orogeny (Miocene). Both could have served as efficient deep-seated fluid migration conduits as well as relatively shallow ones facilitating the downward flow of topography-driven meteoric waters. • Dolomitization by hypo-to mesosaline fluids. • Likely candidate to deliver the dolomitizing fluids is the Upper Triassic Dolomia Principale. • Two similar dolomitization pulses were differentiated with similar salinity, stable carbon and oxygen and Sr87/Sr86 signatures. • Porosity enhancement occurred by dolomitization, while porosity reduction was recognized to be caused by over-dolomitization. [ABSTRACT FROM AUTHOR]

Fractured and karstified carbonate units are key exploration targets for the hydrocarbon industry as they represent important reservoirs. Furthermore, large water reserves and geothermal systems are hosted in carbonate aquifers. This paper documents the relationships between stratigraphy, structural patterns, silicification, and the spatial-morphological organization of a 3D multistorey cave system developed in a Neoproterozoic mixed carbonate-siliciclastic sequence. We found that the combination of lithology, silicification, fracture patterns (controlled by lithostratigraphic variability), and petrophysical properties control the formation of high or low permeability zones; their distribution was fundamental for the spatial organization of dissolution and the compartmentalization of the resulting conduit system in different speleogenetic storeys. We propose a deep-seated hydrothermal origin for the fluids involved in the main phases of karst formation. Warm and alkaline hydrothermal fluids caused silica dissolution, followed by chalcedony and quartz reprecipitation in pore space and fractures. Rising fluids concentrated along through-going vertical fracture zones in the lower storey, whereas sub-horizontal bedding-parallel fluid flow was focused on sedimentary packages containing highly silicified dolostones (SiO 2 >80 wt%) characterized by high permeability. The Calixto Cave is an enlightening example for the complex speleogenetic history affecting a mixed carbonate-siliciclastic succession where the combined effect of silicification and hydrothermal karst dissolution can potentially generate high-quality reservoirs. [Display omitted] • Stratigraphy and structures interplay in forming high-vs. low-permeability units. • High-permeability zones are focused in the silicified carbonates. • Mineral deposits and silica dissolution suggest a hypogene hydrothermal origin for the conduit system. • The 3D organization of the conduit system reflects the interplay among stratigraphy, fracture patterns and silicification. [ABSTRACT FROM AUTHOR]

Superposed events of rock exposure, fracturing, and the hydrological dynamics, are key elements that influence epigenic karstification in carbonate reservoirs. However, they are not fully identified in seismic or well data. One way forward is a multiscale characterization of the porous system to understand karst development, from the pore scale to large caves, confined to high-permeability layers that can reach the basin scale. The present study investigates superposed meteoric dissolution events in carbonate rocks by combining the analyses of depositional setting, diagenesis, stratigraphic architecture, tectonics, and hydrological zones. We performed analyses of 130 plugs collected in 11 outcrops using petrographic, petrophysical, mineralogical, and geochemical analyses. The study area is the Cretaceous Jandaíra Formation in the Potiguar Basin, Brazil, where we identified a regional subaerial unconformity-SU (sequence boundary-1). This surface is characterized by a change in facies and depositional system between dolomitic peritidal deposits (depositional sequence 1) and tight-cemented calcite subtidal deposits (depositional sequence 2). A concentration of caves occurs below the SU. We conclude that subaerial unconformities (vadose surfaces) can develop better reservoir qualities with superposed freshwater leaching and karstification. They control the lateral reservoir zonation, whereas faults and fractures provide vertical pore distribution within the vadose/phreatic hydrological zones. Water table fluctuations can develop multilevel caves and conduits along the epiphreatic zones, increasing the reservoir thickness, which reaches 50–100 m in the Potiguar Basin. We recognized two previous subaerial unconformities at the basin scale using well and seismic data. These findings may have important implications for assessing reservoir location and quality in large areas. [Display omitted] • We investigate high-permeability karst layers in carbonate units. • Caves systems concentrate below subaerial unconformities at the basin scale. • Early diagenesis affected peritidal deposits, with dolomitization and karstification. • Meteoric fluids cement the overlying subtidal deposits during burial diagenesis. • Fractures, unconformities, and hydrology control thickness-extension of reservoir. [ABSTRACT FROM AUTHOR]

Despite that many field studies on joint attribute statistics are available in the literature, there is still an ongoing debate on the most effective statistical treatment to properly describe the distribution of a fundamental parameter like joint spacing. This may also relate to the objective difficulty of collecting very large datasets from single linear scanlines directly in the field, because of limited exposures. To overcome such a limitation, we selected a spectacular outcrop of the Miocene Marnoso-Arenacea Formation turbidite sandstones located near the Coniale village, in the Northern Apennines (Italy), where a vertical cliff provides a 100% continuous outcrop about 90 m high and 280 m long, with well-accessible exposures at its base, along the Santerno River. This setting allows combining data acquisition on a photogrammetric DOM (digital outcrop model) covering a representative sector of the outcrop, with ground truthing by direct measurements in the field. More than 2000 joints were identified in 9 linear scanlines traced in the DOM along mechanical layers with different thickness. Comparison with field data indicates that both joint orientation and spacing data from the DOM are very reliable. The scanlines include between 549 and 37 joints, depending on layer thickness, allowing to perform robust statistical analysis. The Kolmogorov-Smirnov (K–S), Cramer-von Mises (CvM) and Anderson-Darling (A-D) nonparametric tests were used to identify the probability density function(s) that best fit spacing data among exponential, Gamma, log-normal and normal distributions. The Gamma distribution turned out as the most probable in most cases. In some specific sectors along all scanlines, joint density is significantly greater than the average for the presence of joint clusters. In some cases, joint clusters are vertically aligned with those in underlying and/or overlying scanlines, thus resulting in thoroughgoing joint corridors. Comparison of spacing statistics inside and outside joint corridors/clusters indicates a typical transition from Gamma distributions outside joint corridors/clusters, to log-normal distributions inside them. The presence of joint clusters and corridors, as well as FSR, mode/mean ratio, JSR, and FSI values, suggests that diffuse jointing in the sandstone layers of the turbiditic succession reached the saturation stage before a normal probability density function was produced, and then further strain was accommodated by localized deformation in clusters and corridors. • We study fold-related joint sets in the Marnoso-Arenacea Fm. using a digital outcrop model (DOM). • We characterized spacing statistics in 9 layers with different thickness. • We obtained several derived fracturing parameters and we discuss their reliability. • We discuss the maturity and saturation of joint sets, based on their statistical properties. [ABSTRACT FROM AUTHOR]

The reactivation of faults and shear zones is a widely documented process and represents a fundamental characteristic of deformation in the continental lithosphere. The Ross Sea in Antarctica mainly owes its present-day configuration to an extended period of early Paleozoic subduction-related crustal accretion and the subsequent Mesozoic-Cenozoic tectonics of the West Antarctic Rift System. It thus represents an ideal setting in which to study reactivation processes. This study uses the 40Ar-39Ar laserprobe technique in conjunction with mesostructural, microtextural, and nanotextural analyses to unravel the style and timing of a newly discovered mylonite-pseudotachylyte association from the rift shoulder in northern Victoria Land of Antarctica. Pseudotachylyte-bearing fault rocks overprint a reverse ductile shear zone developed in a Cambrian granite, within an overall transpressional right-lateral kinematic scenario. In situ 40Ar-39Ar analyses of biotite from the mylonite mainly yielded ages of 460-440 Ma, suggesting that ductile shearing occurred during the early Paleozoic orogenic cycle. In contrast, in situ data on the pseudotachylyte matrix yielded scattered and much younger ages of ca. 120-66 Ma. Younger ages of ca. 50 Ma were derived from step-heating experiments on matrix specimens obtained through microtexturally controlled microsampling. Taking into account the possible effects of analytical artifacts due to sample irradiation and of Ar loss due to the ultrafine grain of the matrix, coseismic faulting is assigned to the middle Eocene. Results indicate a minimum 390 m.y. period of tectonic quiescence and prove that brittle reactivation occurred in response to a totally different stress regime. Regionally, the study confirms a post-early Eocene age for the activation and propagation of intraplate dextral strike-slip tectonics in Victoria Land. [ABSTRACT FROM AUTHOR]

Caves developed in carbonate units have a significant role in fluid flow, but most of these subsurface voids are below seismic resolution. We concentrated our study on four caves to determine the roles of fractures and folds in the development of karst conduits that may form flow pathways in carbonate reservoirs. We performed structural field investigations, petrographic analyses, and geometric characterization using Light Detection and Ranging (LIDAR) for caves in Neoproterozoic carbonates of the Salitre Formation, central part of the São Francisco Craton, Brazil. We found that the conduit shape, usually with an ellipsoidal cross-section, reflects the tectonic features and textural variations. Carbonate layers containing pyrite and low detritic mineral contents are generally karstified and appear to act as favorable flow pathways. Our results indicate that the development of the karst system is related to fracture corridors formed along parallel and orthogonal sets of fold hinges, which provide preferential pathways for fluid flow and contribute to the development of super-K zones. This study provides insights into the prediction of subseismic-scale voids in carbonate reservoirs, with direct application for the hydrocarbon and hydrogeology flow and storage. [Display omitted] • We study fracture corridors and karst flow pathways on a subseismic scale. • We use LiDAR imagery to qualify and quantify deformation and karst geometry. • Hypogene karst development is related to localized rather than diffuse deformation. • Karst formed in carbonate units affected by multiple-direction fold hinges. [ABSTRACT FROM AUTHOR]

We propose a structural evolutionary model for deformation band (DB) development via microcrack propagation and coalescence process in a normal fault zone affecting arkosic sandstones. Based on structural analysis and reconstruction of the paleostress, we infer that a sub-horizontal minimum principal stress (σ 3) in the NNW-SSE direction controlled the multiscalar evolution of microcracks and DBs. Two main sets of coalesced microcracks and DBs are recognized: (1) NE-SW and (2) E-W, in which the coalescence pattern of microcracks is also observed among the DBs at meso-scale. Our results suggest that the DBs develop during three deformation stages: (1) pore collapse followed by initial grain breakage and the formation of NE-SW tensile microcracks favoured by cleavage planes in feldspars. These microcracks coalesce through linking NW-SE shear microcracks to form intermediary microplanes. An intense microcracking crush the grains, enhancing the cataclastic process. The further coalescence of these intermediary microplanes with other intermediary microplanes and/or microcracks might generate through-going microplanes inside the band, causing the formation of E-W DBs; (2) the E-W structures coalesce through NE-SW hard-linking DBs, which corresponds to the DBs clusters, generating NE-SW intermediary planes at meso-scale; (3) the NE-SW intermediary planes coalesce through an E-W hard-linking structure, which results in the DB fault zone. • We show a mechanic process at grain and meso-scale in deformation bands at arkosic sandstones of Rio do Peixe Basin. • The pore collapse and the feldspar's cleavages favor the microcrack formation. • The growth of deformation bands are products of microcracks development. • The microcracks and deformation bands grow perpendicular to the σ 3. • The deformation band fault zones are related to the hard linkage among deformation bands. [ABSTRACT FROM AUTHOR]

Aim of Study: To evaluate the outcome of laparoscopic surgery in patients affected by colonic diverticulitis., Materials and Methods: A prospective database was established in 1998 at our department to evaluate the results of laparoscopic treatment for diverticular disease. Contraindications to laparoscopic approach were: intestinal obstruction, faecal peritonitis, previuos major abdominal operations, presence of abdominal mass. Parameters evaluated were: indications to operation, age, sex, weight, type of operation, associated operations, operation time, conversion rate, mortality, post-operative complications, postoperative hospital stay, return to normal bowel function, time of removing nasogastric tube and time of restarting oral feeding., Results: From 1998 to 2005, 109 patients operated for diverticular disease were registered. The operation was performed in 17 cases (15.5%) with open technique, in 20 cases (18.5%) throught video-assisted approach with extracorporeal anastomosis and in 72 cases (66%) with laparoscopic approach making intracorporeal anastomosis. The conversion rate was 10.8%. Mortality was 0%. In videolaparoscopic group mean operating time was 177 minutes, major complications were 4 (6.3%), minor complications were 6 (9.5%) and mean hospital stay was 7.8 days. In videoassisted group minor operation time was 158 minutes, complications were 9 (47.3%) and mean hospital stay was 10.1 days., Conclusions: Laparoscopic approach to diverticular disease of the colon is, in selected cases, a safe, feasible and effective procedure.