Fluid pressure variations recorded by quartz vein geochemistry

<p>Veins that form contemporaneously with deformation are the best recorders of the fluids circulating in the depths of orogenic and subduction zones. We have analyzed syn-kinematic quartz veins from accretionary prisms (Shimanto Belt in Japan, Kodiak accretionary Complex in Alaska) and tectonic nappes in collisional orogens (Flysch à Helminthoïdes in the Alps, southern nappes of the variscan Montagne Noire), which formed at temperature conditions between 250 and 350°C, i.e. spanning the downdip limit of large subduction earthquakes and the generation of slow slip events. In all geological domains, veins hosted in rocks that have experienced the lower temperature conditions (~250-300°C) show quartz grains with crystallographic facets and growth rims. Cathodoluminescence (CL) imaging of these growth rims shows two different colors, a short-lived blue color and a brown one, attesting to cyclic variations in precipitation conditions. In contrast, veins hosted in rocks that have experienced the higher temperature conditions (~350°C), show a homogeneous, CL-brown colored quartz, except for some very restricted domains of crack-seal structures of CL-blue quartz found in Japan, Kodiak and Montagne Noire.</p><p>Based on laser ablation analysis and electron microprobe mapping, variations in CL colors appear correlated with the trace element content of quartz. The highly luminescent quartz contains high concentrations of aluminum (Al) and lithium (Li), up to 3000 and 400 ppm, respectively. Variations in Al and Li correlate well, so that Li appears as the main charge‐compensating cation for SiàAl substitution.</p><p>Due to their ubiquitous presence in various settings, the variations in CL colors in the lower temperature range reflect a common, general process. We interpret these cyclic growth structures as a result of deformation/fracturing events, which triggered transient changes in fluid pressure. The CL-blue growth rims delineate zones where quartz growth was rapid and crystals incorporated a large proportion of Al and Li. Crystal growth continued at lower pace after fluid pressure evolved to equilibrium conditions, leading to the formation of CL-brown quartz with fewer substitutions of tetrahedral Si. The variations in fluid pressure fluctuated at values close to lithostatic conditions, as indicated by growth in cavities that remained open.</p><p>The crack-seal microstructures have been interpreted as the result of slow-slip events near the base of the seismogenic zone (Fisher and Brantley, 2014; Ujiie et al., 2018). Our observations on quartz composition suggest that the quartz in crack-seal microstructures records episodic variation in fluid pressure, similar to vein quartz at T<~300 °C. In contrast to the cooler and shallower domain, the variations are significantly smaller, as recorded by the very limited extent of the CL-blue domains, and most if not all of the quartz growth occurred under constant physico-chemical conditions, including a near lithostatic fluid pressure. </p><p>We conclude that quartz trace element content might be a useful tool to track variations in fluid conditions. In particular, at seismogenic depths (i.e. near 250°C), fluid pressure varies significantly around a lithostatic value. In contrast, deeper, near the base of the seismogenic zone where slow slip events occur (i.e. near 350°C), the variations in fluid pressure are smaller.</p>

Paleotemperature investigation of the Variscan southern external domain: the case of the Montagne Noire (France)

The Montagne Noire located in the southern part of the French Massif Central represents the northern part of the South-Variscan Foreland. It is subdivided into three parts. The granite-migmatite Axial Zone dome is surrounded by non- or weakly metamorphosed Paleozoic sedimentary series. Both northern and southern flanks of the Montagne Noire dome are deformed by km-scale, south to southeast facing recumbent folds and thrusts sheets. The Raman Spectroscopy of Carbonaceous Material (RSCM) method, carried out in the low-grade metamorphic rocks of the southern flank of the Montagne Noire, yielded temperatures comprised between 400°C near the dome, and 230°C in the southern domain. Three Raman geothermometers were used to cover this temperature range. RSCM temperatures comply qualitatively with previous estimates based on illite crystallinity, conodont colour alteration, and fluid inclusions carried out in the same area, which document a metamorphic temperature increase towards the dome. The isotherms cut across the different nappe contacts and are oriented parallel to the southern margin of the Axial Zone. This temperature distribution supports the idea that the thermal structure was acquired during the Axial Zone dome emplacement. The thermal structure acquired during the recumbent folds emplacement and burial of the sedimentary series is totally overprinted by the doming. In addition, in a domain relatively remote from the Axial Zone dome, the RSCM measurements yielded significantly higher temperatures than illite crystallinity. This discrepancy points to a higher sensitivity of RSCM to short-lived thermal events than illite crystallinity, possibly because of more efficient kinetics of the carbonization reaction. On the other hand, high RSCM temperatures analysed far from the Axial Zone, between 300°C and 360°C, could be explained by the presence of granitic plutons under the foreland basin.

Reconstructing the pre-Variscan puzzle of Cambro-Ordovician basement rocks in the southwestern European margin of Gondwana

A Cambro-Ordovician palaeogeographical restoration of the southwestern European margin of Gondwana is proposed based on the relative positions of Variscan tectonostratigraphic units. Four palaeogeographical proximal–distal transects are recognized and comprise: (i) the Cantabrian, West Asturian-Leonese, Central Iberian/Central Armorican and Ossa-Morena/North Armorican zones and domains of the Iberian and Armorican massifs, respectively; (ii) the South Armorican Domain and its lateral prolongation into the Thiviers-Payzac unit and the Occitan Domain, including the transect from the Axial, southern and northern Montagne Noire, and the Albigeois-southern Cévennes unit; (iii) the southern and northern sides of the Canigó Massif in the Eastern Pyrenees; and (iv) the External Zone and the External and Internal nappes of Sardinia. Two geodynamic scenarios are recognized controlled by the presence/absence of: (i) the Furongian–Early Ordovician (Toledanian or ‘lacaune normande’) break-up unconformity across the Ossa-Morena/North Armorican and Central Iberian/Central Armorican belts; (ii) the Early–Late Ordovician (Sardic) Phase across the Occitan and Pyrenean domains and SW Sardinia; and (iii) the migration of peaks in trilobite and cinctan (echinoderm) diversity. Other similar palaeogeographical shifts are recognized in zircon provenance patterns, the occurrence of climatically sensitive subtropical facies and mineral indicators across platform–basinal transects along the Gondwana margin. This multidisciplinary framework is proposed as a preliminary step in the quest to produce more tightly constrained Early Paleozoic reconstructions along southwestern Europe.

Record by quartz veins of earthquakes and slow slip events

<p>Veins that form contemporaneously with deformation are the best recorders of the fluids circulating in the depths of orogenic and subduction zones. We have analyzed syn-kinematic quartz veins from accretionary prisms (Shimanto Belt in Japan, Kodiak accretionary prism in Alaska) and tectonic nappes in collisional orogens (Flysch à Helminthoïdes in the Alps, the southern domain of the variscan Montagne Noire), which formed at temperature conditions between 250 and 350°C, i.e. spanning the downdip limit of large subduction earthquakes and the generation of slow slip events. In all geological domains, veins hosted in rocks with the lower temperature conditions (~250-300°C) show quartz grains with crystallographic facets and growth rims. Cathodoluminescence (CL) imaging of these growth rims shows two different colors, a short-lived blue color and a brown one, attesting to cyclic variations in precipitation conditions. In contrast, veins hosted in rocks with the higher temperature conditions (~350°C), show a homogeneous, CL-brown colored quartz, except for some very restricted domains of crack-seal structures of CL-blue quartz found in Japan, Kodiak and Montagne Noire. Based on laser ablation and electron microprobe mapping, the variations in CL colors appear correlated with the trace element content of quartz, the short-lived CL-blue being associated with the substitution of Si<sup>4+</sup> by Al<sup>3+</sup>+Li<sup>+</sup>/H<sup>+</sup>.</p><p>Due to their ubiquitous presence in various settings, the variations in CL colors in the lower T range reflect a common, general process. We interpret these cyclic growth structures as a reflection of deformation/fracturing events, which triggered transient changes in (1) the fluid pressure through fluid flow and (2) the chemistry of the fluid due to enhanced reactivity of the fractured material. The CL-blue growth rims delineate zones where quartz growth was rapid and crystals incorporated a large proportion of Al and Li. Crystal growth continued at a lower pace after fluid pressure and composition evolved to equilibrium conditions, leading to the formation of CL-brown quartz with few substitutions of tetrahedral Si. The variations in fluid pressure fluctuated at values close to lithostatic conditions, as indicated by growth in cavities that remained open.</p><p>The crack-seal microstructures have been interpreted as the result of slow-slip events near the base of the seismogenic zone (Fisher and Brantley, 2014; Ujiie et al., 2018). Our observations on quartz composition suggest that the quartz in crack-seal microstructures records episodic variation in fluid pressure and composition, similar to vein quartz at T<~300 °C. In contrast to the cooler and shallower domain, the variations are significantly smaller, as recorded by the very limited extent of the CL-blue domains, and most if not all of the quartz growth occurred under constant physico-chemical conditions, including a near lithostatic fluid pressure. </p><p>We conclude that quartz trace element content is a useful tool to track variations in fluid conditions. In particular, at seismogenic depths (i.e. near 250°C), fluid pressure varies significantly around a lithostatic value. In contrast, deeper, near the base of the seismogenic zone where slow slip events occur (i.e. near 350°C), the variations in fluid pressure conditions are smaller.</p>