ALBERT

Description: Intraplate strike-slip deformation belts are typically steeply-dipping structures that develop in both oceanic and continental lithosphere where they form some of the largest and most spectacular discontinuities found on Earth. In both modern and ancient continental settings, intraplate strike slip deformation belts are of major importance in accommodating horizontal displacements where they additionally form very persistent zones of weakness that substantially influence the rheological behaviour of the lithosphere over very long time periods (up to 1 Ga or more). These deformation zones provide a fundamental geometric, kinematic and dynamic link between the more rigid plate-dominated tectonics of the oceans and the non-rigid, complex behaviour of the continents. During convergence, they help to transfer major displacements deep into the plate interiors. During divergence, they act as transfer zones that segment rifts, passive continental margins and, ultimately, oceanic spreading ridges. Such belts are also of great economic importance, controlling the location of many destructive earthquakes, offshore and onshore hydrocarbon deposits and metalliferous ore deposits. In the oceans, intraplate strike-slip movements are relatively minor along transform-related fracture zones, but there are an increasing number of documented examples that may reflect spatial and temporal variations in spreading rate along individual active ridge segments.

Description: The West Antarctic Rift System is one of the largest areas of crustal extension in the world. Current interpretations on its driving mechanisms mostly rely on the occurrence of one or more mantle plumes, active during the Cenozoic or the Mesozoic. Recent studies of structural-chronological relationships between emplacement of plutons, dyke swarms, and volcanic edifices since middle Eocene in northern Victoria Land imply that magma emplacement is guided by strike-slip fault systems that dissect the western rift shoulder in Victoria Land. These studies led to a critical re-examination of the arguments used to support plume models. In Victoria Land, the linear geometry of the uplift and the relative chronology of uplift and extension are inconsistent with the traditional concepts of lithospheric evolution above a mantle plume. The geochemical signature of the mafic rocks is equivocal, because both OIB and HIMU features cannot be exclusively interpreted in terms of plume activity. From a thermal point of view, magma production rates are low compared with the core part of plume-related provinces. Additionally, the hot mantle below the West Antarctic Rift System is not documented as deep as expected for mantle plumes and the shape of thermal anomaly is related to lithospheric geometry, being linear rather than having circular symmetry. The lack of any decisive evidence for plume activity is contrasted by evidence that large-scale tectonic features guide magma emplacement: the Cenozoic fault systems reactivated inherited Palaeozoic tectonic discontinuities and their activity is dynamically linked to the Southern Ocean Fracture Zones. As an alternative to both active, plume-driven rifting and passive rifting, we propose that lithospheric strike-slip deformation could have promoted transtension-related decompression melting of a subplate mantle already decompressed and veined during the late Cretaceous amagmatic extensional rift phase. Magma ascent and emplacement occurred along the main strike-slip fault systems and along the transtensional fault arrays departing from the master faults.

Description: 〈span〉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-reprecipitation 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.〈/span〉

Description: Classical frictional fault reactivation models indicate that slip along misoriented fault planes is not possible under most conditions. Nevertheless, active or exhumed low-angle normal faults have been described in many settings worldwide. This discrepancy is addressed by contrasting models: (1) those proposing that low-angle normal faults result from postkinematic passive rotation of former high-angle extensional faults; and (2) those proposing that specific conditions can promote slip along misoriented fault planes. This paper describes the Tellaro detachment, a mid–late Miocene low-angle normal fault that was responsible for ~500 m of tectonic vertical thinning in the carbonate-dominated Triassic to Lower Miocene succession of the Northern Apennines, Italy. By integrating structural, petrographic, isotopic, and fluid inclusion data, we show that: (1) the main kinematic activity of the Tellaro detachment occurred between ~8 and 4 km depths and peak temperature ~190 °C; (2) dilational breccias, tens of cubic meters in volume, are frequently associated with major low-angle fault segments; (3) slip along misoriented planes was favored by elevated fluid pressures and low differential stress; and (4) the fault system was characterized by transient permeability pulses and overpressure buildups, associated with multiple fracturing and cementation events that caused the downward migration of master slip surfaces. Results presented in this study show that: (1) in a fluid-active regime, continental crustal thinning can occur for shallow values of fault dip; (2) low-angle normal faults have a great influence on fluid circulation within the upper crust; and (3) episodic permeability enhancement and destruction in detachment faults can promote overpressure buildups, triggering deformation episodes.

Description: 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 x 360 m x 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.

Description: 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.

Description: We studied a structurally oversimplified, extensional fault zone developed in poorly lithified, quartz-rich, high-porosity sandy sediments of the seismically active Crotone Basin (southern Italy). The fault zone consists of a cm-thick, discrete fault core embedded in virtually undeformed wall sediments. By combining grain size, shape, and microstructural analyses with mineralogical analyses and permeability measurements, we investigated the influence of initial sedimentological characteristics of sands on the final faulted granular products and related hydrologic properties. Faulting produces a general grain-size and porosity reduction by changing both the grain-size and shape distributions. We document a combination of intragranular fracturing, spalling, and flaking of grain edges in the fault core, which do not depend on grain mineralogy. The dominance of cataclasis, also confirmed by fractal dimensions 〉2.6, is generally not expected at a deformation depth

Description: 〈span〉〈div〉Abstract〈/div〉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-reprecipitation 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.〈/span〉