1,354,179 research outputs found
Association of cone sheets and radial dykes on Ascraeus Mons (Mars): structural analysis and modelling
No full textAscraeus Mons was one of the first of the Martian volcanoes to be imaged by the High Resolution Stereo Camera
(HRSC) experiment onboard the ESA Mars Express spacecraft. These images show much of the volcano at a
higher resolution than previously (12 m/px), and details of its lava flows, sinuous rilles, flank vents and tectonic
features. Concentric fractures systems (pit chains, grabens) around the volcano, changing transitionally into radial
structures systems have been recognized and cartographed using a HRSC mosaic. The structural interpretation
showed strong analogies with concentric dykes (cone sheets) on many terrestrial volcanoes such as Isla Fernandina
(Galapagos), Tejeda Complex (Canary Islands) and Cuillins Complex (Isle of Skye, Scotland). In particular this
last terrestrial analogue has been studied in detail and with the use of a Finite Element Method (FEM) modeling the
shape of the magma chamber that originated the cone sheets on the Cuillins Complex was discovered (Bistacchi
et al. 2010). An oblate inflating magma chamber is responsible for the formation of those structures. By analogy,
we tested the presence of an oblate magma chamber below Ascraeus Mons. We measured the diameter of the
transition zone between concentric and radial structures on Ascraeus Mons flanks, that is strongly related to the
diameter of the deep magma chamber (1-1.2 magma chamber diameters). With a FEM model built for Ascraeus
we have been able to discover the average depth of the oblate magma chamber, which could have originated the
concentric structures. The presence of a plume with an oblate summit instead of the magma chamber has also
been tested. Moreover an additional oblate shallow magma chamber that likely originated the summit caldera has
been verified. In addition, the deformation event that originated the structures on Ascraeus flanks has been dated
by crater counting, resulting very recent
Fault zone hydraulic properties provide an independent estimate of coseismic fracturing at 8 km depth (Gole Larghe Fault Zone, Italian Southern Alps)
No full textThe Gole Larghe Fault Zone (GLFZ, Italian S Alps) was exhumed from c. 8 km, where it was characterized by seis-
mic activity (pseudotachylytes) but also by hydrous fluid flow (alteration halos and precipitation of hydrothermal
minerals in veins and cataclasites). The fault zone has previously been quantitatively characterized (Bistacchi 2011,
PAGEOPH; Smith 2013, JSG) providing a rich dataset to generate 3D Discrete Fracture Network (DFN) models
and simulate fault hydraulic properties. A fundamental parameter that cannot be directly evaluated in the field is
the fraction of fractures-faults that were open over a certain time period in the evolution of the fault zone. Based
on field and microstructural evidence, we infer that the opening and closing of fractures resulted in a toggle-switch
mechanism for fluid flow during the seismic cycle: higher permeability was obtained in the syn- to post-seismic
period, when the largest number of fractures was (re)opened by off-fault deformation, then permeability dropped
due to fracture cementation.
Postseismic permeability has been evaluated in a few cases in the world thanks to seismological evidence of fluid
migration along active fault systems. Therefore, we were able to develop a parametric hydraulic model of the GLFZ
and calibrate it to obtain the fraction of faults-fractures that were open in the postseismic period to obtain realistic
fluid flow and permeability values. This fraction is very close to the percolation threshold of the DFN, and it can
be converted to fracture intensity (fracture surface per unit volume in the fault zone), which could be integrated to
obtain the fracture energy due to off-fault fracturing. Since the fracture energy due to on-fault processes has already
been estimated for the GLFZ (Pittarello, 2008, EPSL), this also allows us to estimate the total fracture energ
Three-dimensional characterization of a crustal-scale fault zone: The Pusteria and Sprechenstein fault system (Eastern Alps)
No full textThe characterization and representation of fault zones is of paramount importance for studies of fault and earthquake mechanics, since their rheological and geometric complexity controls seismic/aseismic behaviour and fluid circulation at depth. We present a 3D geological model of a fault system, created by integrating borehole and surface structural data, which allows us to bridge the gap between outcrop-scale
descriptions and large-scale geophysical models. The model integrates (i) fault geometry and topology, (ii) fault-rock distribution, and (iii) characterization of fracturing in damage zones at the km-scale. The dextral-reverse Pusteria and Sprechenstein-Mules Faults (Italian Eastern Alps) provide an
opportunity to study fault rocks and damage distribution as a function of host-rock lithology and fabric, and of fault geometry. A first-order control is exerted by the composition of protoliths (quartzo-feldspathic vs. phyllosilicate-rich) and/or by the presence of an inherited anisotropic fabric (massive vs. foliated), resulting in a marked asymmetry of damage zones. Interestingly, the pervasive foliation typical
of some protoliths may explain both this asymmetry and the relative weakness of one of the faults. The importance of geometrical factors is highlighted when the damage zone thickness increases five times in proximity to a km-scale contractional jog. On the other hand, the type of fault rock present within the fault core does not show a direct relationship with damage intensity. In addition, the thickness of damage zones along planar fault segments does not appear to grow indefinitely with displacement, as might be
envisaged from some scaling laws. We interpret both of these observations as reflecting the maturity of
these large-displacement faults
Slip-tendency analysis as a tool to constrain the mechanical properties of anisotropic rocks
No full textThe mechanical strength of foliated rocks is typically anisotropic because it varies with the orientation of the
foliation relative to the applied principal stresses and commonly depends on phyllosicate content and phyllosicate
physical interconnectivity. We constrain the degree of mechanical anisotropy associated with pre-existing
planar discontinuities, such as metamorphic foliations and inherited faults, by combining paleostress analysis
and meso- and microscale characterization of brittle failure modes in different phyllosilicate-bearing rocks
outcropping in the Sierras de Córdoba (SDC) of Central Argentina. The SDC show evidence of a long brittle
deformation history from Early Triassic – Present with three distinct brittle deformational events. Each phase
caused new strain increments accommodated by the formation of newly-formed faults or by the reactivation of
inherited discontinuities. Structural investigations reveal that gneisses and phyllites deformed by different
failure modes during the different events. Therefore, we were able to use a conceptual field-scale triaxial experiment
by applying a stress model based on normalised slip-tendency analysis. We constrained the friction
coefficient for slip along the foliations (μs) and along pre-existing faults (μf) to 0.2 to 0.3 and 0.4, respectively.
These values fit independent estimates for similar rocks confirming the potential of our approach for other case
studies
Misoriented faults in exhumed metamorphic complexes: Rule or exception?
No full textLow angle normal faults and other weak faults are common in the metamorphic core of collisional orogens.
They frequently show a phyllosilicate-rich mylonitic foliation that was reactivated under brittle conditions.
Experimental and theoretical works indicate that mechanical anisotropies exert a substantial influence on
shear failure and frictional sliding, eventually inhibiting the nucleation and propagation of new Andersonian
shear fractures and favoring the localization of brittle failure along the pre-existing foliations. Metamorphic
phyllosilicate-rich rocks may show a friction coefficient varying from 0.6, at high angles to the foliation, to
0.2–0.4, for shear along the inherited foliation. To test the influence of mechanical anisotropies on the
development of non-Andersonian faults, we have applied a modified slip tendency analysis to three
misoriented phyllosilicate-rich faults of the European Alps. The analysis accounts for anisotropy in friction
coefficients, and has been named “Anisotropic Slip Tendency analysis”. Here we show that brittle deformation
along misoriented phyllosilicate-rich foliations is more probable than the development of new Andersonian
faults. The presence of a well developed network of weak, phyllosilicate-rich faults may influence the overall
structural style and mechanical properties of the brittle lithosphere in collisional orogens
Fluid-rock interactions and their implications on carbonate reservoir characterization
No full textThe objective of my research has been to define the interactions between fluid and rock properties at different environmental conditions and observation scale to reduce the uncertainty in the carbonate reservoir characterization.
Here, I integrate field observations, subsurface data, petrophysical and frictional laboratory measurements focusing on carbonate-bearing rocks to better constrain factors controlling fluid-rock interactions with applications to active petroleum systems. In particular, I focus on the Burano-Bolognano carbonate petroleum system that extends from the northern sector of the Majella mountain to the Cigno/Vallecupa oil fields, in Abruzzo Region (Central Italy).
This area is of particular interest because of the following reasons:
It has received great attention by oil companies in the past for its structural, stratigraphic, and geodynamic evolution, which led it to be an important target for hydrocarbon exploration during the past century. For this reason, it is characterized by a public dense dataset of wells, reports, maps, etc.
It allows to study all petroleum system elements (with the exception of the source rock), such as: reservoir, seal, traps, and migration pathways at field scale.
It allows to understand the influence of subsurface fluids on the petrophysical properties of carbonate reservoir rocks since the outcropping portions of reservoir interval are both clean and hydrocarbon-saturated. This allows measuring and comparing the variations of petrophysical properties between hydrocarbon-bearing and hydrocarbon-free samples.
It is regarded as an analogue of a faulted and fractured reservoir for other carbonate petroleum systems worldwide and in particular in the Adriatic area.
The results of my research quantify the influence of fluid properties in changing of the petrophysical and frictional properties of the bearing-carbonate rocks. The presence of viscous fluids, such as heavy hydrocarbons, at ambient temperature defines an increase of the wave velocities respect to those of the unsaturated samples and determines a possible strengthening and stiffening of the reservoir rock. With increasing temperature, distinct downward trends are recorded, especially for the P-wave velocities. Moreover, the presence of fluids along faults promotes a slow slip behaviour, which was more marked with the presence of clay minerals along fault surface.
Finally, starting from these results, I simulate the evolution of the Burano-Bolognano petroleum system and the related fluids movements, inferring that within this petroleum system the vertically hydrocarbon migration is driven by fractures/faults and the subsequently lateral migration determinates a gradual oil biodegradation with an increase of its density
The timescale of solid-state deformation in the Northern Adamello igneous intrusive suite
No full textThe Late Cenozoic Adamello batholith in the Southern Alps records solid-state deformation structures including, in order of decreasing relative age, cooling joints, mylonitic shear zones and cataclasite-pseudotachylyte faults. We constrained the age and the duration of each phase with 40Ar/ 39Ar geochronology. Host-rocks of the Avio granodiorite, sheared cooling joints, mylonites, pseudotachylytes and cataclasites were sampled and characterized through microstructural, mineralogical, µCT and EPMA analyses. The dated K-bearing phases are: (i) magmatic biotite, (ii) biotite and K-feldspar in joints and mylonites, (iii) pseudotachylytes, (iv) hydrothermal K-feldspar in cataclasites. The wall-rock biotite is 33.2 ± 0.2 Ma old, independently of grainsize, overlapping with the age of cooling joints. Bulk biotite-rich mylonites feature agesbetween 32.4 ± 0.5 and 30.8 ± 0.08 Ma. The K-feldspar cementing cataclasite is 26.4 ± 0.6 Ma old. Four pseudotachylyte matrices cluster between 29.7 ± 0.4 and 32.1 ± 1 Ma, one is 25.3 ± 0.2 Ma old. The resolvable difference in age between magmatic biotite and mylonites indicate that biotite is not a thermochronometer, as its age is mostly controlled by deformation and fluid-rock interactions. 40Ar/39Ar ages mostly confirm the relative ages determined from field relations, with mylonites active within a time window of 1.6 Ma and subsequent seismic faulting protracting for more than 6 Ma
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