1,721,138 research outputs found
Nucleation, development and petrophysical properties of faults in carbonate grainstones: evidence from the San Vito Lo Capo peninsula (Sicily, Italy)
No full textDetailed field mapping and microstructural and textural analyses carried out in Lower Pleistocene grainstones in the San Vito Lo Capo
peninsula (in north-western Sicily) revealed document failure modes and fault development in porous carbonate grainstones. Individual com-pactive shear bands represent the simplest fundamental shear structures, while pressure solution processes commonly localize within previously developed compactive shear bands. In the Lower-Pleistocene carbonate grainstones of San Vito Lo Capo peninsula, composed of eroded car-bonate and marl fragments, pressure solution processes localize mostly grain-to-grain, leading to the formation of zones of weakness which facilitate slip and significant displacement. The transition from one deformation process to another is likely controlled by changing material properties and anisotropy within the bands. Finally, laboratory analyses of representative fault rock samples show that the structures described above have sealing capacity with respect to the host rock, and may compartmentalize any geofluid reservoir
Investigating mechanisms of stylolite formation
No full textThe Israel Science Foundation
Individual Research Grant
Research Grant Application no. 571/08
Pressure solution is considered the most important ductile deformation mechanism operating in the Earth’s upper crust. Although operating at the grain scale, pressure solution controls basic large-scale processes such as strength and healing of faults, compaction of sedimentary basins, and storage and flow of oil. Pressure solution is a process by which rock mass is dissolved at highly stressed regions, transported through the fluid phase, and re-precipitated at lower-stress regions. Pressure solution is macroscopically manifested either as pervasive dissolution in the rock, or as localized solution seams and stylolites. Stylolites play a crucial role in determining both the deformation and permeability of rocks, yet their evolution remains enigmatic. This is probably due to the fact that macroscopic localized dissolution was not yet reproduced in the lab nor fully modeled. Existing numerical and conceptual models of stylolites focus on different parts of the physical picture of stylolite evolution (e.g. porosity evolution feedbacks, stress or strain controlled dissolution), and a coherent understanding is still much needed. We propose a new visco- elasto model of pressure solution that attempts to consider a more complete picture of stylolite formation. In our model pressure solution is driven by both stress and strain-energy, and is enhanced by the presence of clays. The model is capable of studying growth of a single stylolite, as well as stylolite interactions. Recently we used this model, accompanied by analytical calculations, to show that the stress distribution around an initial dissolution defect, an ‘embryo stylolite’, does not promote a dissolution-stress feedback that spontaneously localizes pressure solution. Following many observations that clays enhance pressure solution, we thus propose to test localization by a new feedback – a feedback between clay content and pressure solution. In this feedback regions with high clay content undergo enhanced pressure solution, and thus accumulate even more insoluble clay residue and enhance their dissolution even further. Initial modeling results suggest that this feedback indeed produces stylolite morphology similar to that found in the field. Further analysis of this feedback is proposed. In addition to studying single stylolite growth, we propose to study stylolite - stylolite interactions, and stylolite-fracture interactions, which will require adding to our model the ability to model shear fracturing. Modeling results will be continuously compared with field and experimental observations
Revealing the secrets of an earthquake: physico-chemical constraints from a multidisciplinary study of exhumed faults
No full textUnderstanding the physico-chemical processes controlling faulting and earthquake generation is essential in seismic hazard assessment. Since dreadful earthquakes nucleate at depth (10-15 km), direct access to seismic sources is impossible and monitoring active faults from the Earth surface, or interpreting radiated seismic waves, yield limited information on earthquake mechanics. The indirect analysis of earthquakes is unable to provide tight constraints on fundamental mechanical parameters such as the dynamic fault strength and the energy budget of an earthquake. Here we propose (i) to investigate earthquake processes by studying fossil seismic sources (e.g., faults
containing solidified melts produced during seismic slip or pseudotachylytes) now exhumed at the Earth's surface and (ii) to analyze the fault rock materials in the laboratory by a novel
multidisciplinary approach involving the up-to-date techniques in microstructural analysis, mineralogy and petrology. In parallel with the study of natural faults we will develop a new apparatus to perform experiments under the extreme deformation conditions typical of earthquakes (e.g., slip velocities of ~ 1 m/s) simulating seismic slip in the lab. There is only one apparatus of this kind, located in Kyoto-Japan, currently operating in the world. Collaboration with the Kyoto University will serve to design the new high friction apparatus to be built in Padova. The experiments in Kyoto will be complemented by experiments at Brown University (RH, USA) with a
high-pressure apparatus to investigate the mechanical behavoiur of different fault rocks including fault gouges. Field work will investigate exceptional exposures of faults in Europe and Australia.
The study of natural faults will proceed together with theoretical modeling calibrated on the basis of the field data, the mechanical data from rock-friction experiments and the analytical data of natural
and experimental deformation products. The integration of all data is expected to yield estimates of seismic source parameters (e.g., dynamic shear stress, the seismic energy budget, all challenging
issues for Earth scientists) and to provide an insight into the mechanisms of earthquake nucleation.
The proposed study has implication also in the understanding of other friction-controlled processes important in Earth Sciences (e.g., rock landslides) as well as in industry. It is noteworthy that the
experimental results will find application to improve industrial milling techniques. The development of the new dedicated laboratory will allow Padova University to compete at top scientific level with the world’s leading institutions for the study of earthquake mechanics
L’analisi geologica finalizzata alla valutazione della pericolosità sismica: l’esempio del Bacino di Colfiorito: area epicentrale del terremoto del 26 Settembre 1997
No full textIn questo lavoro viene proposto un modello di segmentazione, applicato alle faglie attive e finalizzato alla valutazione della pericolosità sismica. Tale modello, verificato nell’area di Colfiorito a seguito della crisi sismica umbro-marchigiana, ha come conseguenza una revisione della terminologia utilizzata per lo studio delle faglie attive e necessita di una specifica procedura dell’analisi geologica.
I terremoti avvenuti negli ultimi decenni in Appennino centrale hanno mostrato che durante un singolo evento sismico si attivano in superficie più faglie. Il modello di segmentazione proposto è basato sul fatto che le faglie che si attivano in superficie durante un singolo evento sismico siano, nel loro insieme, l’espressione superficiale di un’unica struttura sismogenetica profonda. Tale modello si basa quindi sulla possibilità di riconoscere in superficie e per una data area, caratteristiche geologico-strutturali, morfostrutturali, storiche, geometriche e reologiche simili. La segmentazione viene applicata, quindi, ad un'area invece che ad un singolo segmento di faglia. Successivamente, sulla base delle caratteristiche specifiche dell’area individuata, è possibile valutare la geometria, la cinematica, lo slip-rate e le dimensioni della struttura sismogenetica profonda e quindi tutti quei parametri necessari alla valutazione del potenziale sismico
Geological analysis and seismic hazard in the Central Apennines
No full textThis study puts forward a segmentation model applied to active faults with the aim of assessing seismic hazard in the Central Apennines. The model, whose reliability had been ascertained during the 1997 Umbria-Marche earthquake sequence, also implies a review of the key terms used in the analysis of active faults and points out the need for a specific procedure in geological analysis for seismic hazard evaluation. Earthquake events which occurred in the Central Apennines in the past decades show that several surface faults can be activated throughout a single seismic event. As a result, the segmentation model proposed herein is based on the fact that surface faults are, on the whole, the superficial manifestation of a single deep seismogenic structure. Therefore, this model is based on the possibility to recognize similar geological, structural, historical, geometrical and rheological features at the surface, in order to infer the main properties of the fault zone at depth. Eventually, on the grounds of the specific features of the fault array, one may assess the geometry, kinematics, slip-rate and size of the deep seismogenic structure, i.e., all the required parameters to evaluate the seismic potential of the area
The resolution of geological analysis and models for earthquake faulting studies
No full textSee the preface of the volum
Faulting in porous carbonate grainstones
No full textIn the recent past, a new faulting mechanism has been documented within porous carbonate grainstones. This
mechanism is due to strain localization into narrow tabular bands characterized by both volumetric and shear
strain; for this reason, these features are named compactive shear bands. In the field, compactive shear bands are
easily recognizable because they are lightly coloured with respect to the parent rock, and/or show a positive relief
because of their increased resistance to weathering. Both characteristics, light colours and positive relief, are a
consequence of the compaction processes that characterize these bands, which are the simplest structure element
that form within porous carbonate grainstones. With ongoing deformation, the single compactive shear bands,
which solve only a few mm of displacement, may evolve into zone of compactive shear bands and, finally, into
well-developed faults characterized by slip surfaces and fault rocks. Field analysis conducted in key areas of Italy
allow us to documented different modalities of interaction and linkage among the compactive shear bands: (i) a
simple divergence of two different compactive shear bands from an original one, (ii) extensional and contractional
jogs formed by two continuous, interacting compactive shear bands, and (iii) eye structures formed by collinear
interacting compactive shear bands, which have been already described for deformation bands in sandstones.
The last two types of interaction may localize the formation of compaction bands, which are characterized by
pronounced component of compaction and negligible components of shearing, and/or pressure solution seams. All
the aforementioned types of interaction and linkage could happen at any deformation stage, single bands, zone of
bands or well developed faults.
The transition from one deformation process to another, which is likely to be controlled by the changes in the
material properties, is recorded by different ratios and distributions of the fault dimensional attributes. The results
of field analysis are consistent with length (L), displacement (D) and thickness (T) of single compactive shear
bands clustering around given values, peculiar to the individual lithologies, and does not point out to any scale
relationship among these parameters. On the contrary, in zones of shear bands and well-developed faults the
D values are maximum in the central portion of individual elements. Differently from what characterize the
well-developed faults, in which the slip increments are solved along the main slip surfaces, within zones of
compactive shear bands the displacement varies according to the number of individual single bands, so that an
increased displacement is related to an higher number of bands. As a consequence, the T-D plot concerning
zones of compactive shear bands and well-developed faults show two different populations, which suggest
that well-developed faults are much efficient to resolve displacement, with respect the zone of shear bands,
because they include sharp slip surfaces. The petrographical and petrophysical properties of the tectonic features
described above, which have been assessed by mean of detailed laboratory analyses, are consistent with the single
compactive shear bands and zones of shear bands behaving as seals for underground fluid flow with respect
to the host rock. These features, strongly present within the fault damage zones of well-developed faults, may
compartmentalize the fluid flow in faulted carbonate reservoirs
Faulting in carbonate rocks: new insights on deformation mechanisms, petrophysics, and fluid flow properties
No full textAlthough carbonate rocks form widespread oil, gas, and water reservoirs their deformation mechanisms are not well understood. The relative importance of the sedimentary fabric, diagenetic history, fluid content, and structural background of the carbonate protoliths in the formation of joints, pressure solution seams, and deformation bands are still object of debate.
In this session we propose to present state-of-the-art research on the processes of fault growth in platform, slope, and basinal carbonates. Our focus will be on the resulting fault architectures in these different types of carbonates. We aim to integrate the results of field-based studies with those of experimental and numerical works on the petrophysical and fluid flow properties of faults in carbonates
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