ALBERT

Description: Oxygen isotope geochemistry is a powerful tool for investigating rocks that interacted with fluids, to assess fluid sources and quantify the conditions of fluid-rock interaction. We present an integrated modelling approach and the computer program PTLOOP that combine thermodynamic and oxygen isotope fractionation modelling for multi-rock open systems. The strategy involves a robust petrological model performing on-the-fly Gibbs energy minimizations coupled to an oxygen fractionation model both based on internally consistent databases. This approach is applied to subduction zone metamorphism to predict the possible range of δ18O values for stable phases and aqueous fluids at various pressure-temperature (P-T) conditions in the subducting slab. The modelled system is composed by a sequence of oceanic crust (mafic) with sedimentary cover of known initial chemical composition and bulk δ18O. The evolution of mineral assemblage and δ18O values of each phase is calculated along a defined P-T path. Fluid-rock interactions may occur as consequence of (1) infiltration of an external fluid into the mafic rocks or (2) transfer of the fluid liberated by dehydration reactions occurring in the mafic rocks into the sedimentary rocks. The effects of interaction with externally-derived fluids on the mineral and bulk δ18O of each rock are quantified for two typical compositions of metabasalts and metasediments with external fluid influx from serpentinite. The dehydration reactions, fluid loss and mineral fractionation produce minor to negligible variations in bulk δ18O values, i.e. within 1 ‰. By contrast, the interaction with external fluids may lead to shifts in δ18O up to one order of magnitude larger. Such variations can be detected by analysing in-situ oxygen isotope in key metamorphic minerals such as garnet, white mica and quartz. The simulations show that, when the water released by the slab infiltrates the forearc mantle wedge, it can cause extensive serpentinization within fractions of a Myr and significant oxygen isotope variation at the interface. This technique opens new perspectives to track fluid pathways in subduction zones, to distinguish porous from channelized fluid flows, and to determine the P-T conditions and the extent of fluid/rock interaction.

Description: Anelasticity may decrease the shear modulus of the asthenosphere by 8–10 % at semi-diurnal tidal periods compared with the reference 1 s period of seismological Earth models. We show that such anelastic effects are likely to be significant for ocean tide loading displacement at the M2 tidal period around the East China Sea. By comparison with tide gauge observations, we establish that NAO99Jb is the most accurate numerical ocean tide model in this region, and that related errors in the predicted M2 vertical ocean tide loading displacements will be 0.2–0.5 mm. In contrast, GPS observations on the Ryukyu Islands (Japan), with uncertainty 0.2–0.3 mm, show discrepancies of over 1.5 mm with respect to ocean tide loading displacements predicted using the purely elastic radial Preliminary Reference Earth Model. We show that the use of an anelastic PREM-based Earth model reduces these discrepancies to no more than 0.8 mm, which is of the same order as the sum of the remaining errors due to uncertainties in the ocean tide model and the GPS observations. Use of a regional Earth model based on the laterally-varying S362ANI, with or without further empirical tuning, results in minor additional improvements in fit.

Description: Soil position in the landscape reveals its history of formation and genesis. Therefore, the landscape is the combination of features of the surface of the earth with subsurface components (parent material), while the soil is a three-dimensional, dynamic natural body inserted in the landscape. This research aimed to study the soil-landscape relationship in a sandstone-gneiss topolithosequence in Amazonas, Brazil. The study was carried out along a 9.253-meter transect from the top downwards the softer slope. Soil profiles were selected in five landscape compartments (top, upper third, lower third, transport foothill, and deposition foothill). Morphological, mineralogical, physical, chemical, and ray diffraction characterizations were performed. Soils had different morphological, physical, chemical, and mineralogical attributes due to the variations of the geological substrate and landscape position. The mineralogy of the clay fraction is composed of kaolinite, goethite, hematite, and gibbsite, with goethite being the predominant iron oxide. A sand fraction dominance was observed in relation to the other fractions in all the profiles, being related to the alluvial nature of the parent material, with the highest values occurring in the lower third. The separation of the landscape into geomorphic surfaces and identification of the parent material were effective for understanding the variation of soil attributes along the landscape.

Description: Tectonic nappes are observed for more than a hundred years. Although geological studies often refer to a nappe theory, the physical mechanisms of nappe formation are still incompletely understood. We apply two-dimensional numerical simulations of shortening of a passive margin, to investigate the thermo-mechanical processes of detachment, transport and stacking of nappes. We use a visco-elasto-plastic model with standard creep flow laws and Drucker-Prager yield criterion. We consider tectonic inheritance with two initial mechanical heterogeneities: (1) lateral heterogeneity of the basement-cover interface due to half-grabens and horsts and (2) vertical heterogeneities due to layering of mechanically strong and weak sedimentary units. The model shows detachment and horizontal transport of a thrust nappe and stacking of this thrust nappe above a fold nappe. The detachment of the thrust sheet is triggered by stress concentrations around the sediment-basement contact and the resulting brittle-plastic shear band formation. The horizontal transport is facilitated by a basal shear zone just above the basement-cover contact, composed of thin, weak sediments. Fold nappe formation occurs by a dominantly ductile closure of a half-graben and the associated extrusion of the half-graben fill. We apply our model to the Helvetic nappe system in Western Switzerland, which is characterized by stacking of the Wildhorn thrust nappe above the Morcles fold nappe. The modeled structures and temperature field agree with data from the Helvetic nappe system. The mechanical heterogeneities must generate contrasts in effective viscosity (i.e. ratio of stress to strain rate) of four orders of magnitude to model nappe structures similar to the ones of the Helvetic nappe system.

Description: Modern geodetic and seismic monitoring tools are enabling the study of moderate-sized earthquake sequences in unprecedented detail. Here we use a variety of methods to examine surface deformation caused by a sequence of earthquakes near Lake Muir in southwest Western Australia in 2018. A shallow MW 5.3 earthquake on the 16th of September 2018 was followed on the 8th of November 2018 by a MW 5.2 event in the same region. Focal mechanisms for the events suggest reverse and strike-slip rupture, respectively. Interferometric Synthetic Aperture Radar (InSAR) analysis of the events suggests that the ruptures are in part spatially coincident. Field mapping, guided by the InSAR results, reveals that the first event produced an approximately 3 km long and up to 0.5 m high west-facing surface rupture, consistent with slip on a moderately east-dipping fault. Double difference hypocentre relocation of aftershocks using data from rapidly deployed seismic instrumentation confirms an east-dipping rupture plane for the first event, and shows a concentration located at the northern end of the rupture where the InSAR suggests greatest vertical displacement. The November event resulted from rupture on a northeast-trending strike-slip fault. UAV-derived digital terrain models (differenced with pre-event LiDAR) reveal a surface deformation envelope consistent with the InSAR for the first event, but could not discern deformation unique to the second event. New rupture length versus magnitude scaling relationships developed for non-extended cratonic regions as part of this study allow for the distinction between “visible†surface rupture lengths (VSRL) from field-mapping and “detectable†surface rupture lengths (DSRL) from remote sensing techniques such as InSAR, and suggest longer ruptures for a given magnitude than implied by commonly used scaling relationships.

Description: Nappe assembly in the Köli Nappe Complex, Jämtland, Sweden, has been associated with in- and out-of-sequence thrusting. Kinematic data from shear zones bounding the Köli Nappe Complex are compatible with this model, but direct evidence from fault zones internally subdividing the nappe complex does not exist. We studied a series of pseudotachylyte exposures in these fault zones for deciphering the role seismic faulting played in the assembly of the Caledonian nappe pile. To constrain the fault kinematics, microstructural and magnetic fabrics of pseudotachylyte in foliation-parallel fault veins have been investigated. Because the pseudotachylyte veins are thin, we focused on small (c. 0.2 cm3) samples for measuring the anisotropy of magnetic susceptibility. The results show inverse proportionality between specimen size and anisotropy of magnetic susceptibility degree, which is most likely an analytical artifact related to instrument sensitivity and small sample dimensions. This finding implies magnetic anisotropy results acquired from small specimens demand cautious interpretation. However, analysis of structural and magnetic fabric data indicates that seismic faulting occurred during exhumation into the upper crust but yield no kinematic in-formation. Structural field data suggest that seismic faulting was postdated by brittle E–W extensional deformation along steep normal faults. Therefore, it is likely that the pseudotachylytes formed late during out-of-sequence thrusting of the Köli Nappe Complex over the Seve Nappe Complex.

Description: Residual pressure can be preserved in mineral inclusions, e.g. quartz-in-garnet, after exhumation due to differential expansion between inclusion and host crystals. Raman spectroscopy has been applied to infer the residual pressure and provides information on the entrapment temperature and pressure conditions. However, the amount of residual pressure relaxation cannot be directly measured. An underestimation of pressure relaxation may lead to significant errors between calculated and actual entrapment pressure. This study focuses on three mechanisms responsible for the residual-pressure relaxation: 1) viscous creep; 2) plastic yield; 3) proximity of inclusion to thin-section surface. Criteria are provided to quantify how much of the expected residual pressure is relaxed due to these three mechanisms. An analytical solution is introduced to demonstrate the effect of inclusion depth on the residual pressure field when the inclusion is close to thin-section surface. It is shown that for quartz-in-garnet system, the distance between thin-section surface and inclusion centre needs to be at least two times the inclusion radius to avoid pressure relaxation. In terms of viscous creep, representative case studies on quartz-in-garnet system show that viscous relaxation may occur from temperatures as low as 600∼700 °C depending on the particular P-T path and various garnet compositions. For quartz entrapped along the prograde P-T path and subject to viscous resetting at peak T above 600∼700 °C, its residual pressure after exhumation may be higher than predicted from its true entrapment conditions. Moreover, such a viscous resetting effect may introduce apparent overstepping of garnet nucleation that is not related to reaction affinity.

Description: Subglacial water influences the dynamics of ice masses. The state of subglacial pore water, whether liquid or frozen, is associated with differences in electrical resistivity that span several orders of magnitude, hence liquid water can be inferred from electrical resistivity depth profiles. Such profiles can be obtained from inversions of time domain electromagnetics (TEM) soundings, but these are often non-unique. Here, we adapt an existing Bayesian transdimensional algorithm (MuLTI) to the inversion of TEM data constrained by independent depth constraints, to provide statistical properties and uncertainty analysis of the resistivity profile with depth. The method was applied to ground-based TEM data acquired on the terminus of the Norwegian glacier Midtdalsbreen, with depth constraints provided by co-located ground penetrating radar data. Our inversion shows that the glacier bed is directly underlain by material of resistivity 102 Ωm ± 100 %, with thickness 5–40 m, in turn underlain by a highly conductive basement (100 Ωm ± 15 %). High resistivity material, 5 × 104 Ωm ± 25 %, exists at the front of the glacier. All uncertainties are defined by the interquartile range of the posterior resistivity distribution. Combining these resistivity profiles with co-located seismic shear-wave velocity inversions to further reduce ambiguity in the hydro-geological interpretation of the subsurface, we propose a new 3D interpretation of the Midtdalsbreen subglacial material partitioned into partially frozen sediment, frozen sediment/permafrost and weathered/fractured bedrock with saline water.

Description: The Patos-Marinza oil field in Central Albania (40.71° N,19.61° E), operated since 1939, is one of the largest onshore fields in Europe. More than 7 millions oil barrels are extracted per year from the Messinian sandstone formations of the Durres Basin in the Albanian Peri-Adriatic Depression by the Bankers company operating the field since 2004. In the region, the background seismicity culminated in December 2016, when a shallow seismic swarm developed in the oil field, damaging houses and triggering the opening of a public inquiry. However, because of the lack of a dense local seismic array and incompleteness of historical catalogues for such moderate events, understanding whether this seismicity could be induced by the extraction/injection activities is an arduous task. In this study, we take advantage of the new Sentinel-1 radar images acquired every 6 to 12 days over Albania to measure the surface displacement in the Myzeqeja plain and in the Patos-Marinza oil field in particular. Images from two ascending and descending tracks covering the area are processed through a radar interferometry (InSAR) time-series analysis over the 2014 to 2018 time-span, providing consistent average Line-Of-Sight velocity maps and displacement time-series. The regional deformation field exhibits a slow subsidence of the entire sedimentary basin relative to the highlands (at rates of 2.5 mm/yr), that we interpret as a combination of natural and man-induced compaction. This broad picture is complicated by a very strong local subsidence signal with rates as high as 15 mm/yr that spatially correlates with the Patos-Marinza oil field and is maximal in the zone holding most of the operating horizontal wells, where Enhanced Oil Recovery techniques are used. The striking spatial correlation between the maximum subsidence area and the active wells, as seen from optical images, argues in favor of an oil-extraction induced surface deformation. The observed surface deformation is well reproduced by elastic models mimicking the basin and reservoir compaction using planar crack dislocations. Such modeling provides a first-order estimation of the volumetric deflation rate in the oil reservoir (~ 0.2 Mm3/yr). This strong subsidence signal, together with the increase of the background seismicity since the oil field reactivation, are evidences of significant man-induced stress changes in the basin that should be further monitored and taken into account for seismic hazard assessment.

Description: Progresses in understanding the sedimentary dynamic of the Western Alboran Basin lead us to propose a model of evolution of its tectonic inversion since the Pliocene to present-time. Extensive and strike-slip structures accommodate the Miocene back-arc extension of the Alboran Basin, but undergo progressive tectonic inversion since the Tortonian. Across the Alboran Basin, the Alboran Ridge becomes a transpressive structure accommodating the shortening. We map its southwestern termination: a Pliocene rhombic structure exhibiting series of folds and thrusts. A younger structure, the Al-Idrissi fault zone (AIF), is Pleistocene to present-day active strike-slip fault zone. This fault zone crosses the Alboran Ridge and connects southward to the transtensive Nekor Basin and the Nekor fault. In the Moroccan shelf and at the edge of a submerged volcano, we date the inception of the local shelf subsidence from the 1.81–1.12 Ma. It marks the propagation of the AIF toward the Nekor Basin. Pliocene thrusts and folds and Quaternary transtension appear at first sight as different tectonic periods but reflects the long-term evolution of a transpressive system. Despite a constant direction of Africa/Eurasia convergence since 5 Ma at the scale of the southern margin of Alboran Basin, the Pliocene-Quaternary inversion evolves from transpressive to transtensive on the AIF and the Nekor Basin. This system reflects the expected evolution of the deformation of the Alboran Basin under the indentation of the African lithosphere.