Frictional Heating Processes and Energy Budget During Laboratory Earthquakes

International audienceDuring an earthquake, part of the released elastic strain energy is dissipated within the slip zone by frictional and fracturing processes, the rest being radiated away via elastic waves. While frictional heating plays a key role in the energy budget of earthquakes, it could not be resolved by seismological data up to now. Here we investigate the dynamics of laboratory earthquakes by measuring frictional heat dissipated during the propagation of shear instabilities at stress conditions typical of seismogenic depths. We estimate the complete energy budget of earthquake rupture and demonstrate that the radiation efficiency increases with thermal-frictional weakening. Using carbon properties and Raman spectroscopy, we map spatial heat heterogeneities on the fault surface. We show that an increase in fault strength corresponds to a transition from a weak fault with multiple strong asperities and little overall radiation, to a highly radiative fault behaving as a single strong asperity. Plain Language Summary In nature, earthquakes occur when the stress accumulated in a medium is released by frictional sliding on faults. The stress released is dissipated into fracture and heat energy or radiated through seismic waves. The seismic efficiency of an earthquake is a measure of the fraction of the energy that is radiated away into the host medium. Because faults are at inaccessible depths, we reproduce earthquakes in the laboratory under natural in situ conditions to understand the physical processes leading to dynamic rupture. We estimate the first complete energy budget of an earthquake and show that increasing heat dissipation on the fault increases the radiation efficiency. We develop a novel method to illuminate areas of the fault that get excessively heated up. We finally introduce the concept of spontaneously developing heat asperities, playing a major role in the radiation of seismic waves during an earthquake

Elephant Moraine 96029, a very mildly aqueously altered and heated CM carbonaceous chondrite: Implications for the drivers of parent body processing

Elephant Moraine (EET) 96029 is a CMcarbonaceous chondrite regolith breccia with evidence for unusually mild aqueous alteration, a later phase of heating and terrestrial weathering. The presence of phyllosilicates and carbonates within chondrules and the fine-grained matrix indicates that this meteorite was aqueously altered in its parent body. Features showing that water-mediated processing was arrested at a very early stage include a matrix with a low magnesium/iron ratio, chondrules whose mesostasis contains glass and/or quench crystallites, and a gehlenite-bearing calcium- and aluminium-rich inclusion. EET 96029 is also rich in Fe,Ni metal relative to other CM chondrites, and more was present prior to its partial replacement by goethite during Antarctic weathering. In combination, these properties indicate that EET 96029 is one of the least aqueously altered CMs yet described (CM2.7) and so provides new insights into the original composition of its parent body. Following aqueous alteration, and whilst still in the parent body regolith, the meteorite was heated to ~400–600 °C by impacts or solar radiation. Heating led to the amorphisation and dehydroxylation of serpentine, replacement of tochilinite by magnetite, loss of sulphur from the matrix, and modification to the structure of organic matter that includes organic nanoglobules. Significant differences between samples in oxygen isotope compositions, and water/hydroxyl contents, suggests that the meteorite contains lithologies that have undergone different intensities of heating. EET 96029 may be more representative of the true nature of parent body regoliths than many other CM meteorites, and as such can help interpret results from the forthcoming missions to study and return samples from C-complex asteroids

Signatures of the post-hydration heating of highly aqueously altered CM carbonaceous chondrites and implications for interpreting asteroid sample returns

The CM carbonaceous chondrites have all been aqueously altered, and some of them were subsequently heated in a parent body environment. Here we have sought to understand the impact of short duration heating on a highly aqueously altered CM through laboratory experiments on Allan Hills (ALH) 83100. Unheated ALH 83100 contains 83 volume per cent serpentine within the fine-grained matrix and altered chondrules. The matrix also hosts grains of calcite and dolomite, which are often intergrown with tochilinite, Fe(Ni) sulphides (pyrrhotite, pentlandite), magnetite and organic matter. Some of the magnetite formed by replacement of Fe(Ni) sulphides that were accreted from the nebula. Laboratory heating to 400 °C has caused partial dehydroxylation of serpentine and loss of isotopically light oxygen leading to an increase in bulk δ18O and fall in Δ17O. Tochilinite has decomposed to magnetite, whereas carbonates have remained unaltered. With regards to infrared spectroscopy (4000–400 cm-1; 2.5–25 µm), heating to 400 °C has resulted in decreased emissivity (increased reflectance), a sharper and more symmetric OH band at 3684 cm-1 (2.71 µm), a broadening of the Si—O stretching band together with movement of its minimum to longer wavenumbers, and a decreasing depth of the Mg—OH band (625 cm-1; 16 µm). The Si—O bending band is unmodified by mild heating. With heating to 800 °C the serpentine has fully dehydroxylated and recrystallized to ∼Fo60/70 olivine. Bulk δ18O has further increased and Δ17O decreased. Troilite and pyrrhotite have formed, and recrystallization of pentlandite has produced Fe,Ni metal. Calcite and dolomite were calcined at ∼700 °C and in their place is an un-named Ca-Fe oxysulphide. Heating changes the structural order of organic matter so that Raman spectroscopy of carbon in the 800 °C sample shows an increased (D1 + D4) proportional area parameter. The infrared spectrum of the 800 °C sample confirms the abundance of Fe-bearing olivine and is very similar to the spectrum of naturally heated stage IV CM Pecora Escarpment 02010. The temperature-related mineralogical, chemical, isotopic and spectroscopic signatures defined in ALH 83100 will help to track the post-hydration thermal histories of carbonaceous chondrite meteorites, and samples returned from the primitive asteroids Ryugu and Bennu

Post-Metamorphic Thermal Anomaly across the Nacimiento Block, Central California: a Hydrothermal Overprint?

International audienceThe thermal history of the Nacimiento block located within the Franciscan Complex (California, USA) has been previously proposed based on both vitrinite reflectance (Rm) and illite cristallinity methods (Underwood et al., 1995). These authors suggest that the Nacimiento block is locally perturbed by a thermal anomaly (up to 300ºC), probably caused by post-metamorphic hydrothermal activity linked to the emplacement of an Au-deposit: the Los Burros Gold deposit. Although both thermal anomaly and deposit seem spatially correlated, their relationship is still poorly constrained. Detailed geological and structural mapping within the Los Burros Mining District (LBMD) coupled with a thermal study was conducted to better understand processes responsible for the anomalous temperatures recorded near the deposit. The regional maximum temperature reached by metasediments from the Nacimiento block have been first investigated using the Raman Spectroscopy of Carbonaceous Materials (RSCM) method. In addition, through careful fluid-inclusion and stable isotopes (O and C) studies on the deposit, the temperature and the potential source of the fluid responsible for the Los Burros Au-deposit emplacement were investigated. RSCM technique confirms the presence of a thermal anomaly in the range 260-320ºC near LBMD. However, our structural and petrographic results suggest that the thermal anomaly is not correlated to a post-metamorphic hydrothermal overprint but rather to a late, transpressive deformation uplifting buried metamorphic rocks

Post-Metamorphic Thermal Anomaly across the Nacimiento Block, Central California: a Hydrothermal Overprint?

International audienceThe thermal history of the Nacimiento block located within the Franciscan Complex (California, USA) has been previously proposed based on both vitrinite reflectance (Rm) and illite cristallinity methods (Underwood et al., 1995). These authors suggest that the Nacimiento block is locally perturbed by a thermal anomaly (up to 300ºC), probably caused by post-metamorphic hydrothermal activity linked to the emplacement of an Au-deposit: the Los Burros Gold deposit. Although both thermal anomaly and deposit seem spatially correlated, their relationship is still poorly constrained. Detailed geological and structural mapping within the Los Burros Mining District (LBMD) coupled with a thermal study was conducted to better understand processes responsible for the anomalous temperatures recorded near the deposit. The regional maximum temperature reached by metasediments from the Nacimiento block have been first investigated using the Raman Spectroscopy of Carbonaceous Materials (RSCM) method. In addition, through careful fluid-inclusion and stable isotopes (O and C) studies on the deposit, the temperature and the potential source of the fluid responsible for the Los Burros Au-deposit emplacement were investigated. RSCM technique confirms the presence of a thermal anomaly in the range 260-320ºC near LBMD. However, our structural and petrographic results suggest that the thermal anomaly is not correlated to a post-metamorphic hydrothermal overprint but rather to a late, transpressive deformation uplifting buried metamorphic rocks

Early weakening processes inside thrust fault

International audienceObservations from deep boreholes at several locations worldwide, laboratory measurements of frictional strength on quartzo-feldspathic materials, and earthquake focal mechanisms indicate that crustal faults are strong (apparent friction μ ≥ 0.6). However, friction experiments on phyllosilicate-rich rocks and some geophysical data have demonstrated that some major faults are considerably weaker. This weakness is commonly considered to be characteristic of mature faults in which rocks are altered by prolonged deformation and fluid-rock interaction (i.e., San Andreas, Zuccale, and Nankai Faults). In contrast, in this study we document fault weakening occurring along a marly shear zone in its infancy (<30 m displacement). Geochemical mass balance calculation and microstructural data show that a massive calcite departure (up to 50 vol %) from the fault rocks facilitated the concentration and reorganization of weak phyllosilicate minerals along the shear surfaces. Friction experiments carried out on intact foliated samples of host marls and fault rocks demonstrated that this structural reorganization lead to a significant fault weakening and that the incipient structure has strength and slip behavior comparable to that of the major weak faults previously documented. These results indicate that some faults, especially those nucleating in lithologies rich of both clays and high-solubility minerals (such as calcite), might experience rapid mineralogical and structural alteration and become weak even in the early stages of their activity

Peak metamorphic temperatures in the Alpine tectonic wedge, south Cottian Alps, Italy

International audienceIn the south Cottian Alps, the metamorphic wedge of the Western Alps is widely exposed. It includes, fromSW to NE, the Briançonnais internal border (Acceglio units), the ophiolitic Schistes lustrés nappes (Piemonte-Liguria Ocean) and the southern Dora-Maira units derived from the Briançonnais s.l. basement (Michard et al.,2022, and references therein). Located between the Acceglio and Dora-Maira units, the Val Maira-Sampeyreand Val Grana Allochthons (MGA) consist of a Permo-Triassic to Mid Jurassic “Prepiemonte”-type sequencedetached from the Briançonnais s.l. basement. The peak metamorphic grade of these tectonic complexesevolves from blueschist- to coesite-eclogite (Brossasco-Isasca unit of Dora-Maira). Here we report new peaktemperatures by carrying the RSCM geothermometry approach (see Lahfid et al., 2019) on most of the southCottian transect with emphasis on the MGA and on the intervening mélange shear zones. TRSCM valuesrange from ~400°C to >500°C, going from the most external Val Grana unit and overlying Queyras schiststo the uppermost Dora-Maira unit. We present a new metamorphic map and profiles based on published andnew data, including our new thermometric data. We note an overall fit between our TRSCM results and thoseinferred from the mineral assemblages, where available. The thermal anomaly linked to the rifting evolutionthat gave birth to the Piemonte-Liguria Ocean was recognized in the poorly metamorphic Adriatic margin SEof the Periadriatic line (Beltrando et al., 2015), but cannot be evidenced in the studied Briançonnais-derivedunits due to their high metamorphic grade.Beltrando M., Stockli D.F., Decarlis A. & Manatschal G. (2015) - A crustal-scale view at rift localization along the fossilAdriatic margin of the Alpine Tethys preserved in NW Italy. Tectonics, 34, 1927-1951.Lahfid A., Baidder L., Ouanaimi H., Soulaimani A., Hoepffner C., Farah A., Saddiqi O. & Michard A. (2019) - Fromextension to compression: high geothermal gradient during the earliest Variscan phase of the Moroccan Meseta; a firststructural and RSCM thermometric study. Eur. J. Mineral., 31, 695-713.Michard A., Schmid S.M., Lahfid A., Ballèvre M., Manzotti P., Chopin C., Iaccarino S. & Dana D. (2022) - The Maira-Sampeyre and Val Grana Allochthons (south Western Alps): review and new data on the tectonometamorphic evolutionof the Briançonnais distal margin. Swiss J. Geosci. (in press)

Peak metamorphic temperatures in the Alpine tectonic wedge, south Cottian Alps, Italy

International audienceIn the south Cottian Alps, the metamorphic wedge of the Western Alps is widely exposed. It includes, fromSW to NE, the Briançonnais internal border (Acceglio units), the ophiolitic Schistes lustrés nappes (Piemonte-Liguria Ocean) and the southern Dora-Maira units derived from the Briançonnais s.l. basement (Michard et al.,2022, and references therein). Located between the Acceglio and Dora-Maira units, the Val Maira-Sampeyreand Val Grana Allochthons (MGA) consist of a Permo-Triassic to Mid Jurassic “Prepiemonte”-type sequencedetached from the Briançonnais s.l. basement. The peak metamorphic grade of these tectonic complexesevolves from blueschist- to coesite-eclogite (Brossasco-Isasca unit of Dora-Maira). Here we report new peaktemperatures by carrying the RSCM geothermometry approach (see Lahfid et al., 2019) on most of the southCottian transect with emphasis on the MGA and on the intervening mélange shear zones. TRSCM valuesrange from ~400°C to >500°C, going from the most external Val Grana unit and overlying Queyras schiststo the uppermost Dora-Maira unit. We present a new metamorphic map and profiles based on published andnew data, including our new thermometric data. We note an overall fit between our TRSCM results and thoseinferred from the mineral assemblages, where available. The thermal anomaly linked to the rifting evolutionthat gave birth to the Piemonte-Liguria Ocean was recognized in the poorly metamorphic Adriatic margin SEof the Periadriatic line (Beltrando et al., 2015), but cannot be evidenced in the studied Briançonnais-derivedunits due to their high metamorphic grade.Beltrando M., Stockli D.F., Decarlis A. & Manatschal G. (2015) - A crustal-scale view at rift localization along the fossilAdriatic margin of the Alpine Tethys preserved in NW Italy. Tectonics, 34, 1927-1951.Lahfid A., Baidder L., Ouanaimi H., Soulaimani A., Hoepffner C., Farah A., Saddiqi O. & Michard A. (2019) - Fromextension to compression: high geothermal gradient during the earliest Variscan phase of the Moroccan Meseta; a firststructural and RSCM thermometric study. Eur. J. Mineral., 31, 695-713.Michard A., Schmid S.M., Lahfid A., Ballèvre M., Manzotti P., Chopin C., Iaccarino S. & Dana D. (2022) - The Maira-Sampeyre and Val Grana Allochthons (south Western Alps): review and new data on the tectonometamorphic evolutionof the Briançonnais distal margin. Swiss J. Geosci. (in press)

Peak metamorphic temperatures in the Alpine tectonic wedge, south Cottian Alps, Italy

International audienceIn the south Cottian Alps, the metamorphic wedge of the Western Alps is widely exposed. It includes, fromSW to NE, the Briançonnais internal border (Acceglio units), the ophiolitic Schistes lustrés nappes (Piemonte-Liguria Ocean) and the southern Dora-Maira units derived from the Briançonnais s.l. basement (Michard et al.,2022, and references therein). Located between the Acceglio and Dora-Maira units, the Val Maira-Sampeyreand Val Grana Allochthons (MGA) consist of a Permo-Triassic to Mid Jurassic “Prepiemonte”-type sequencedetached from the Briançonnais s.l. basement. The peak metamorphic grade of these tectonic complexesevolves from blueschist- to coesite-eclogite (Brossasco-Isasca unit of Dora-Maira). Here we report new peaktemperatures by carrying the RSCM geothermometry approach (see Lahfid et al., 2019) on most of the southCottian transect with emphasis on the MGA and on the intervening mélange shear zones. TRSCM valuesrange from ~400°C to >500°C, going from the most external Val Grana unit and overlying Queyras schiststo the uppermost Dora-Maira unit. We present a new metamorphic map and profiles based on published andnew data, including our new thermometric data. We note an overall fit between our TRSCM results and thoseinferred from the mineral assemblages, where available. The thermal anomaly linked to the rifting evolutionthat gave birth to the Piemonte-Liguria Ocean was recognized in the poorly metamorphic Adriatic margin SEof the Periadriatic line (Beltrando et al., 2015), but cannot be evidenced in the studied Briançonnais-derivedunits due to their high metamorphic grade.Beltrando M., Stockli D.F., Decarlis A. & Manatschal G. (2015) - A crustal-scale view at rift localization along the fossilAdriatic margin of the Alpine Tethys preserved in NW Italy. Tectonics, 34, 1927-1951.Lahfid A., Baidder L., Ouanaimi H., Soulaimani A., Hoepffner C., Farah A., Saddiqi O. & Michard A. (2019) - Fromextension to compression: high geothermal gradient during the earliest Variscan phase of the Moroccan Meseta; a firststructural and RSCM thermometric study. Eur. J. Mineral., 31, 695-713.Michard A., Schmid S.M., Lahfid A., Ballèvre M., Manzotti P., Chopin C., Iaccarino S. & Dana D. (2022) - The Maira-Sampeyre and Val Grana Allochthons (south Western Alps): review and new data on the tectonometamorphic evolutionof the Briançonnais distal margin. Swiss J. Geosci. (in press)