Tectonic Regime as a Control Factor for Crustal Fault Zone (CFZ) Geothermal Reservoir in an Amagmatic System: A 3D Dynamic Numerical Modeling Approach

Crustal fault zones provide interesting geological targets for high-temperature geothermal energy source in naturally deep-fractured basement areas. Field and laboratory studies have shown the ability of these systems to let fluid flow down to the brittle–ductile transition. However, several key questions about exploration still exist, in particular the fundamental effect of tectonic regimes on fluid flow in fractured basement domains. Based on poro-elasticity assumption, we considered an idealized 3D geometry and realistic physical properties. We examined a model with no tectonic regime (benchmark experiment) and a model with different tectonic regimes, namely a compressional, an extensional and a strike-slip tectonic regime. Compared to the benchmark experiment, the results demonstrate that different tectonic regimes cause pressure changes in the fault/basement system. The tectonic-induced pressure changes affect convective patterns, onset of convection as well as the spatial extent of thermal plumes and the intensity of temperature anomalies. Driven by poro-elastic forces, temperature anomalies around vertical faults in a strike-slip tectonic regime have a spatial extent that should be considered in preliminary exploratory phases

Advanced 3D TH and THM Modeling to Shed Light on Thermal Convection in Fault Zones With Varying Thicknesses

Fault zones exhibit 3D variable thickness, a feature that remains inadequately explored, particularly with regard to the impact on fluid flow. Upon analyzing an analytic solution, we examine 3D thermal-hydraulic (TH) dynamical models through a benchmark experiment, which incorporates a fault zone with thickness variations corresponding to realistic orders of magnitude. The findings emphasize an area of interest where vigorous convection drives fluid flow, resulting in a temperature increase to 150°C at a shallow depth of 2.7 km in the thickest sections of the fault zone. Moreover, by considering various tectonic regimes (compressional, extensional, and strike-slip) within 3D thermal-hydraulic-mechanical (THM) models and comparing them to the benchmark experiment, we observe variations in fluid pressure induced by poroelastic forces acting on fluid flow within the area of interest. These tectonic-induced pressure changes influence the thermal distribution of the region and the intensity of temperature anomalies. Outcomes of this study emphasize the impact of poroelasticity-driven forces on transfer processes and highlight the importance of addressing fault geometry as a crucial parameter in future investigations of fluid flow in fractured systems. Such research has relevant applications in geothermal energy, CO2 storage, and mineral deposits

Magmatic to solid-state evolution of a shallow emplaced agpaitic tinguaite (the Suc de Sara dyke, Velay volcanic province, France): implications for peralkaline melt segregation and extraction in ascending magmas

In the last decades the mush model has been generalized to the complete trans-crustal magmatic system in which differentiation would be driven by segregation and extraction of trapped melts from crystal-rich mushes. Melt extraction processes involved are porous flow and strain localization, the latter being regarded as the main process acting during transfer through dykes and necks along which high differential stresses are acting on. We combine structural measurements together with petrological analyses and textural observations to constrain the model of emplacement and finally emphasize how shear deformation and strain localization structures promoted the residual melt segregation that occurred in a shallow silica-undersaturated peralkaline intrusion (Suc de Sara, Velay volcanic province, French Massif Central). In this study, we demonstrate that segregation and subsequent extraction of the CO2-rich residual melt occurred during magma ascent and final emplacement of the Suc de Sara tinguaite. Contrasting features of shear deformation between the margins that exhibited different permeabilities highlight that melt segregation started by compaction as a loose packing of emerging microlites and continued with melt filling of an anastomosed C/C′ band network developing in the crystal-rich mush subjected to high shear strain. Subsequent melt extraction throughout the country rock was controlled by the permeability of the hanging wall. Along the western hanging wall of the intrusion, extraction of the residual melt was prevented by the 15 cm thick chilled margin. In contrast, segregated melt circulated through the highly porous and permeable eastern margin, causing the fenitization of the country rock.</p

Decrypting magnetic fabrics (AMS, AARM, AIRM) through the analysis of mineral shape fabrics and distribution anisotropy

The fieldwork was supported by the DIPS project (grant no. 240467) and the MIMES project (grant no. 244155) funded by the Norwegian Research Council awarded to O.G. O.P.'s position was funded from Y-TEC.Anisotropy of magnetic susceptibility (AMS) and anisotropy of magnetic remanence (AARM and AIRM) are efficient and versatile techniques to indirectly determine rock fabrics. Yet, deciphering the source of a magnetic fabric remains a crucial and challenging step, notably in the presence of ferrimagnetic phases. Here we use X-ray micro-computed tomography to directly compare mineral shape-preferred orientation and spatial distribution fabrics to AMS, AARM and AIRM fabrics from five hypabyssal trachyandesite samples. Magnetite grains in the trachyandesite are euhedral with a mean aspect ratio of 1.44 (0.24 s.d., long/short axis), and > 50% of the magnetite grains occur in clusters, and they are therefore prone to interact magnetically. Amphibole grains are prolate with magnetite in breakdown rims. We identified three components of the petrofabric that influence the AMS of the analyzed samples: the magnetite and the amphibole shape fabrics and the magnetite spatial distribution. Depending on their relative strength, orientation and shape, these three components interfere either constructively or destructively to produce the AMS fabric. If the three components are coaxial, the result is a relatively strongly anisotropic AMS fabric (P’ = 1.079). If shape fabrics and/or magnetite distribution are non-coaxial, the resulting AMS is weakly anisotropic (P’ = 1.012). This study thus reports quantitative petrofabric data that show the effect of magnetite distribution anisotropy on magnetic fabrics in igneous rocks, which has so far only been predicted by experimental and theoretical models. Our results have first-order implications for the interpretation of petrofabrics using magnetic methods.Publisher PDFPeer reviewe

Prograde Shearing In The Lower Crust: Seismic Consequences

Continental thickening can be associated at depth with strain localization into anas- tomosing shear zones. These shear zones, formed during metamorphism, are often obliterated by later thermal and tectonic events possibly related with the exhumation of mountain roots under retrograde conditions. Anastomosing shear zones exposed in the lower crust of the Kohistan arc, in Pakistan, are rather exceptional because they have parageneses indicating increasing pressure during progressive deformation. We investigated their seismic properties on rock samples with constant bulk chem- istry. The compressional wave velocity was experimentally determined at high con- fining pressure and temperature. We observed that density, average Vp and acoustic impedance increased from the protoliths through a gradient zone to the mylonitic rock, whilst seismic anisotropy reached a maximum in the gradient zone. These findings will be presented and discussed in terms of reflectivity of the lower crust in collision zones

Prograde Shearing In The Lower Crust: Seismic Consequences

Continental thickening can be associated at depth with strain localization into anas- tomosing shear zones. These shear zones, formed during metamorphism, are often obliterated by later thermal and tectonic events possibly related with the exhumation of mountain roots under retrograde conditions. Anastomosing shear zones exposed in the lower crust of the Kohistan arc, in Pakistan, are rather exceptional because they have parageneses indicating increasing pressure during progressive deformation. We investigated their seismic properties on rock samples with constant bulk chem- istry. The compressional wave velocity was experimentally determined at high con- fining pressure and temperature. We observed that density, average Vp and acoustic impedance increased from the protoliths through a gradient zone to the mylonitic rock, whilst seismic anisotropy reached a maximum in the gradient zone. These findings will be presented and discussed in terms of reflectivity of the lower crust in collision zones

Experimental observations on the effect of interface slip on rotation and stabilization of rigid particles in simple shear and a comparison with natural mylonites

For axial ratios R> 3c3, porphyroclasts from three mylonites all show a very strong SPO with the long axis of the best fit ellipse at an antithetic angle of 5\u201310\ub0 to the shear direction. This is more consistent with a stable end orientation than with the transient fabrics predicted by theory for elliptical rigid particles in simple shear. The cause of this divergence is investigated in a series of high simple shear strain (\u3b3>15) analogue experiments, performed in a ring-shear machine (couette flow) using a linear viscous matrix (PDMS). The rotational behaviour of elongate (R= 3c5) rigid particles with elliptical and rhomboidal shapes, comparable with the natural examples, is modelled for both coherent and slipping particle\u2013matrix interfaces. Interface slip causes a dramatic reduction in the rotation rate of the elliptical particle compared with theory when the long axis is close to the shear direction, but not stabilisation. Interface slip does result in stabilisation of the rhomboidal particle, with the long diagonal oriented at a small antithetic angle to the shear direction. For monoclinic particles, mirror image shapes (referred to here as Types 1 and 2) show different rotational behaviour. For the Type 1 particle (with a shape comparable to \u3c3 porphyroclast systems and mica fish), the long side rotates asymptotically into parallelism with the shear direction. Natural examples of Type 1 particles, such as hornblende and olivine porphyroclasts measured from the Finero mylonites (Southern Alps), show a very strong preferred orientation (for R> 3c3), with the long side parallel or at a small (<5\ub0) antithetic angle to the mylonitic foliation. For Type 2 particles, the short side stabilises close to the shear direction, or at a small synthetic angle, as also observed for sillimanite porphyroclasts from the Mont Mary mylonites (Western Alps). In this natural case, stabilisation of the short sides is against an extensional crenulation cleavage rather than the mylonitic foliation. The analogue experiments establish that interface slip is one mechanism for stabilisation of elongate rhomboidal particles. In natural examples, decoupling from the matrix may be affected by extensional crenulation cleavage or C-planes in S\u2013C fabrics

Prograde Shearing In The Lower Crust: Seismic Consequences

Continental thickening can be associated at depth with strain localization into anas- tomosing shear zones. These shear zones, formed during metamorphism, are often obliterated by later thermal and tectonic events possibly related with the exhumation of mountain roots under retrograde conditions. Anastomosing shear zones exposed in the lower crust of the Kohistan arc, in Pakistan, are rather exceptional because they have parageneses indicating increasing pressure during progressive deformation. We investigated their seismic properties on rock samples with constant bulk chem- istry. The compressional wave velocity was experimentally determined at high con- fining pressure and temperature. We observed that density, average Vp and acoustic impedance increased from the protoliths through a gradient zone to the mylonitic rock, whilst seismic anisotropy reached a maximum in the gradient zone. These findings will be presented and discussed in terms of reflectivity of the lower crust in collision zones