Fabrizio Balsamo

Pliocene–Pleistocene HT–LP metamorphism during multiple granitic intrusions in the southern branch of the Larderello geothermal field (southern Tuscany, Italy)

This work presents the results of a multidiscipinary study carried out in the southern branch of the Larderello geothermal field (Travale–Montieri area), which was based on the integration of field data with information on the deeper structures as derived from interpretation of seismic reflection profiles and the P–T–t history of the metamorphic substratum as reconstructed from borehole data. Our data document that the structural and metamorphic signature of the metamorphic substratum is chiefly related to a prograde HT–LP metamorphic overprint, ranging in age from c. 2.8 to 0.7 Ma. The metamorphic climax was attained within the K-feldspar zone and equilibrated at P–T conditions of about 0.2 GPa and 650 °C. We interpret these new findings as evidence of Pliocene–Pleistocene thermal metamorphism associated with multiple granite intrusions at shallow levels in the crust. These results (1) add strength to the interpretations emphasizing the contribution of Neogene thermal metamorphism to the tectonometamorphic signature of the basement rocks found at depth in the Larderello geothermal field, and (2) provide insights on the thermal structures of the crustal section hosting the geothermal field and on the modes through which the long-lived positive thermal anomaly operated in southern Tuscany.

Structural control on the formation of iron-oxide concretions and Liesegang bands in faulted, poorly lithified Cenozoic sandstones of the Paraíba Basin, Brazil

Iron-oxide coloration and deposits in sandstone are significant indicators of the mobility of solutes (Fe 2+ and O 2 ) in groundwater, mainly controlled by host-rock porosity and permeability. We describe the occurrence and geometry of different types of iron-oxide deposits developed within the vadose zone along faults affecting poorly lithified, quartz-dominated, heterolithic sands in the Paraiba Basin, NE Brazil. The development of highly permeable damage zones (10 0 −10 2 Darcy) and low-permeability fault–core-mixed zones (10 −3 -10 1 Darcy) promotes the physical mixing of Fe 2+ -rich waters and oxygenated groundwater. This arrangement favors iron-oxide precipitation as meter-scale sand impregnations, centimeter- to decimeter-scale concretions, and well-cemented decimeter- to meter-thick mineral masses. The formation of hydraulically isolated compartments along hard-linked strike-slip faults promotes: (1) the development of Liesegang bands in a reaction zone dominated by pore-water molecular diffusion of O 2 into Fe 2+ -rich stagnant water, and (2) the precipitation of iron-oxide impregnations and concretions in the fault-core-mixed zone boundaries, likely by O 2 diffusion in flowing Fe 2+ -rich waters. Late-stage fault reactivation provides preferential pathways for the circulation of gravity-driven reducing fluids, resulting in localized dissolution of iron and bleaching along fractures and iron remobilization. These relationships reveal the roles of tectonic activity and near-surface sandstone diagenesis in determining preferential hydraulic pathways for the physicochemical interaction between oxygenated groundwater and iron-rich fluids. Structural setting, fault-zone architecture, and related grain-size–permeability structures determine the dominant mode of solution interaction, leading to the formation of iron-oxide Liesegang bands where O 2 diffuses into stagnant Fe 2+ -rich water, and concretions when diffusion is complemented by Fe 2+ advective flow.

Fluid flow within the damage zone of the Boccheggiano extensional fault (Larderello–Travale geothermal field, central Italy): structures, alteration and implications for hydrothermal mineralization in extensional settings

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.

A way to hydrothermal paroxysm, Colli Albani volcano, Italy

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 hydrothermal 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 steam-heated 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.

Hydraulic properties of fault zones in porous carbonates, examples from central and southern Italy

We present the results of in situ permeability measurements performed, using a portable field permeameter, on normal and strikeslip fault zones that crosscut high-porosity carbonate grainstones. The measurement sites expose in the Cretaceous Orfento Formation of the Majella Mountain (Abruzzo, Italy), and the Lower Pleistocene deposits of the Favignana Island (Sicily, Italy). Nine small-displacement, compactive shear banding-based fault zones have been tested in the field. The fault offset ranges between 10 and 200 centimeters. The acquired permeability data indicate a two orders of magnitude decrease of porosity and permeability from the host rock to the cataclastic fault cores. A clear dependence of the fluid circulation paths through porous carbonates is therefore inferred at depth due to orientation, density and connectivity of the fault zones. Moreover, this study indicates the key role played by the pore network characteristics (pore dimensions above all) of undeformed host rocks on determining extremely different permeability values of the faulted porous carbonate grainstones. Accordingly, the results presented in this study may be helpful in applications such as geofluids management for improving the forecasting of carbonate reservoir quality and understanding the extent of reservoir compartmentalization.

The signature and mechanics of earthquake ruptures along shallow creeping faults in poorly lithified sediments

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.

Fault-related fluid flow history in shallow marine sediments from carbonate concretions, Crotone basin, south Italy

We present the results of a multidisciplinary study on the relationships between the structural architecture of an extensional fault system in poorly lithified shallow marine sands and the associated pattern of diagenetic carbonate concretions. Based on their shape, spatial distribution, cement texture and chemistry, and isotopic signature, carbonate concretions are grouped into (1) tabular (encompassing also nodular and lens-shaped) concretions developed within the fault zones, and (2) elongate and coalescent strata-bound concretions formed adjacent and mostly parallel to fault zones. Cement chemistry and morphology, as well as stable carbon–oxygen isotopic signatures, indicate that tabular concretions formed during early diagenesis in the vadose mixing marine–meteoric zone, possibly during coseismic rupture propagation and, in the interseismic periods, as a result of slow capillary suction along the low-permeability fault zones. On the other hand, elongate concretions formed after regional uplift and during telodiagenesis by precipitation from meteoric, phreatic water flowing parallel to the fault zones, which acted as a hydraulic barrier. The concretion patterns record the evolution of fluid flow orientation and the changing chemistry of fault-zone fluids, which were fundamentally driven by fault system propagation and resulting fault architecture during two major diagenetic stages.

Photogrammetric digital outcrop reconstruction, visualization with textured surfaces, and three-dimensional structural analysis and modeling: Innovative methodologies applied to fault-related dolomitization (Vajont Limestone, Southern Alps, Italy)

Different remote sensing technologies, including photogrammetry and LIDAR (light detection and ranging), allow collecting three-dimensional (3D) data sets that can be used to create 3D digital representations of outcrop surfaces, called digital outcrop models (DOM). The main advantages of photogrammetry over LIDAR are represented by the very simple and lightweight field equipment (a digital camera), and by the arbitrary spatial resolution, that can be increased simply getting closer to the outcrop or by using a different lens. The quality of photogrammetric data sets obtained with structure from motion (SFM) techniques has shown a tremendous improvement over the past few years, and this is becoming one of the more effective ways to collect DOM data sets. The Vajont Gorge (Belluno Dolomites, Italy) provides spectacular outcrops of jurassic limestones (Vajont Limestone Formation) in which mesozoic faults and fracture corridors are continuously exposed. Some of these faults acted as conduits for fluids, resulting in structurally controlled dolomitization. A 3D DOM study, based on a photogrammetric SFM data set, was carried out, aimed at enabling interdisciplinary characterization and reconstruction of coupled brittle deformation and fluid flow processes. For this study we used a DOM (730 m × 360 m × 270 m) consisting of continuous triangulated surfaces representing the outcrop, textured with high-resolution images. Interpretation and modeling performed on this data set include (1) georeferencing of structural measurements and sampling stations; (2) tracing of stratigraphic boundaries, structural surfaces, and dolomitization fronts (ground-truthed); (3) correlation and extrapolation of realistic 3D surfaces from these traces; and (4) development of a 3D geological model at the scale of the Vajont Gorge, including stratigraphy, faults, dolomitization fronts, and volumetric meshes suitable for the statistical analysis of structural, diagenetic, and geochemical parameters. The DOM study highlighted the close relationship between faults and dolostone geobodies, demonstrating that dolomitization was guided by fluid infiltration along Mesozoic normal faults. In order to explore the uncertainty associated with the 3D model of irregularly shaped dolostone bodies, three different 3D dolostone geobody realizations have been modeled, providing a minimum, intermediate, and maximum estimate of the dolostone/limestone volumetric facies ratio, while honoring the field constraints.

Anatomy and paleofluid evolution of laterally restricted extensional fault zones in the Jabal Qusaybah anticline, Salakh arch, Oman

The E-W−trending Jabal Qusaybah anticline, at the western termination of the Salakh arch, Oman Mountains, is characterized by a complex fault network that developed in layered Cretaceous carbonates. This network includes NE-SW left-lateral, N-S extensional, and subordinate E-W extensional fault zones. The N-S−striking extensional faults zones are roughly perpendicular to the fold axis and are best developed in the longitudinally bulged central sector of the anticlinal crest. They are likely due to along-strike outer-arc extension associated with positive fault inversion and salt migration. These extensional fault zones are confined within, and locally abut, major NE-SW left-lateral strike-slip fault zones. Extensional fault displacements range between a few decimeters and ∼60 m, whereas the maximum exposed trace lengths range between a few meters and ∼800 m. Narrow (∼1−15-cm-thick) cataclastic fault cores are surrounded by vein-dominated damage zones as thick as tens of meters. Moreover, fault zones show widespread evidence for substantial dilation in the form of (1) dilation breccias, (2) infilling by large columnar calcite crystals and aggregates, and (3) centimeter- to meter-thick veins. Dilation breccias and calcite infillings are primarily localized at fault tips, fault overlaps, and interaction zones between strike-slip and extensional fault segments. Displacement profiles along the N-S−striking extensional fault zones indicate that they are one order of magnitude shorter than values predicted by most published displacement-length scaling laws. By analyzing fault abutting geometries, detailed vein relative chronology, δ13C and δ18O signatures, and fluid inclusion data from calcite veins and calcite fault infillings, we propose a model whereby a deep-seated, regionally sized, left-lateral strike-slip fault system that was active during anticline growth inhibited the lateral propagation of late-stage transversal extensional fault zones. Our findings show that, in this geological setting, the structural position, rather than fault displacement, is the parameter controlling the location of the more dilatants (and permeable) fault segments. Results of the present work suggest that fault intersections may be more useful than fault throw for predicting zones of enhanced vertical fluid flow in structurally complex carbonate reservoirs.

Complex fault-fold interactions during the growth of the Jabal Qusaybah anticline at the western tip of the Salakh Arch, Oman

The Jabal Qusaybah anticline is located at the western end of the Salakh Arch, a major salient in the foothills of the Oman Mountains. We performed a structural and petrographical-geochemical study of vein sets and fault zones associated with the development of this anticline. Our data illustrate a complex deformation pattern both in space and time, characterized by the unusual presence of widespread NE-SW left-lateral strike-slip fault zones trending oblique to the E-W fold axial strike, and of abundant and well-developed N-S fold-perpendicular extensional fault zones associated with axial bulging and dilation, well developed in the central region of the anticlinal crest. We propose a three-stage evolution for the Jabal Qusaybah anticline, starting with prefolding jointing in the foreland of the late Cretaceous Oman Mountains, and followed by development of extensional faulting in Campanian times. Positive inversion of the Qusaybah Fault, possibly in Miocene times, caused development of a layer-parallel shortening fabric and amplification the Jabal Qusaybah Anticline, in concomitance with the activity of NE-SW left-lateral strike-slip fault zones that triggered N-S, fold-perpendicular extensional faulting, particularly in the axial bump of the anticline. The final evolutionary stage was characterized by further amplification of the axial bump and related N-S extensional fracturing and by uplift and exhumation. To explain the complex noncylindrical fault-fold interactions in the study anticline, we tentatively propose that they were triggered by near foredeep-parallel tapering of the sedimentary/tectonic overburden of the Ara evaporites.