<p>The Esla Nappe is located in the external foreland and thrust belt of the Variscan Orogen in the NW Iberian Massif (Cantabrian Zone, NW Iberia). It is formed by a near-complete Palaeozoic sedimentary succession. With a displacement of around 19 km, the nappe was emplaced along a thin (<2&#8211;3 m) basal shear zone (ENBSZ) located at an estimated minimum depth of 4 km. Emplacement took place during the Moscovian (ca. 312 Ma). Fault-rock assemblages record a variety of alternating deformation mechanisms and processes, including cataclastic flow,&#160; pressure solution and hydrofracturing and vein precipitation. All these processes are considered evidence of an aseismic stable behaviour of the ENBSZ, where deformation was influenced by secular variations in the fluid pore pressure.</p><p>Following emplacement, the ENBSZ was breached by clastic dykes and sills which were intruded following re-opened previous anisotropies, including bedding planes, thrust surfaces, joints and stylolites. Together, they constitute an interconnected network of quartz sand-rich lithosomes reaching structural heights occasionally exceeding 20 m above the ENBSZ. The orientation of the dykes suggests that the injection process took place under low differential stress conditions in the hangingwall, and near-lithostatic fluid pore overpressure conditions in the footwall. The injected slurry consisted of overpressured pore fluid, quartz-sand grains derived from the footwall and entrained host-derived fragments. Depending on fracture aperture and slurry composition, a variety of fluid velocities can be inferred in the order of 15&#8211;30 cm/s. Thin pure injections of quartz grains (ca. <1 cm) were characterised by a laminar flow (Re<2100), whereas the thickest quartz and host-derived mixed injections (~1 m) displayed a fully turbulent flow (Re~2 x 10<sup>4</sup>).</p><p>The causes for the fluids to reach near-lithostatic fluid overpressures within the uppermost footwall remain unknown. It is not possible to rule out a seismic trigger, but the absence of extreme shear localization structures typical of seismic slip suggests that the injection process was driven by fluid progressive accumulation, possibly related with clay dehydration reactions, tectonic loading, pore compaction or fluid migration from underlying formations. Actual breaching and injection may have been allowed by a decrease in bedding-parallel compressive stresses in the Esla Nappe associated with the subsequent evolution of the thrust-wedge.</p>
<p>The size and shape of rock constituent particles can provide substantial information about the environment in which rocks are formed and also about their evolution during their geological history. There are several geological processes that generate specific particle shapes. We focus on three processes and their effects on particles as end members: sediment transport in water producing sub-rounded particles, tectonic fracturing producing angular fragments and chemical corrosion at grain boundaries increasing their rugosity. In this work we test several shape morphological parameters in natural rock specimens with the ultimate goal of quantifying the proportion of different typologies of particles in a rock, all of which can be related to specific geological processes. The main aim of this work is to distinguish different typologies of quartz particles according to the quantitative and qualitative evaluation of shape parameters by using several shape parameters in grains and/or particles.</p><p>The procedure followed includes: i) the petrographic characterization of rock specimens in thin section, visually establishing the different typologies of quartz grains present, ii) the acquisition and segmentation of outlines of quartz particles and iii) the quantification of size and shape parameters such as area (A), perimeter (P), fractal dimension (FD), solidity (So), normalized perimeter-area (PoA), Wadell roundness (Rw), Drevin roundness (RD), Pg/Pe roundness (RP), sphericity (S) and a regularity indicator (RBC). A total of 293 particles were studied by means of ImagePro-Plus, ImageJ and Roussillon Toolbox software.</p><p>We have used two rock specimens from the base of the Esla nappe, a thrust sheet emplaced in the foreland fold and thrust belt of the Variscan orogen in NW Iberia (Cantabrian Zone). The first phase of this work was to identify the petrographic characteristics of the samples. One specimen was sampled from a quartz sand injection at the base of the thrust sheet. The other is from a sandstone in the footwall, the likely source for the quartz grains injected in the hanging wall. There are some evidence of fracturing and corrosion of the injected quartz grains during the injection process at the base of the Esla nappe. In summary, the first sample contains quartz grains with distinctive shapes that can be directly related to very specific geological processes affecting particle shape in a rock.</p><p>The result of the analysis completed allowed the definition of: i) the parameters that best represent the grain shape variations and ii) the range of values for each parameter that are characteristic of each process, thus allowing the classification of the grain shapes. Furthermore, the analysis allowed distinguishing sub-rounded quartz grains of detrital sedimentary origin from grains that have been partially or totally fractured. However, the used shape parameters do not allow a univocal identification of grains corroded by fluids.</p><p>Acknowledgments: The Spanish National Plan (CGL2017-86487-P PETROCANTABRICA Project) funded this research.</p>
<p>The Esla Nappe is located in the foreland and thrust belt of the Variscan Orogen (Cantabrian Zone, NW Iberia). It is formed by a near-complete Palaeozoic sedimentary succession. With a displacement of around 19 km, the nappe was emplaced along a thin (<2&#8211;3 m) basal shear zone (ENSZ) at a minimum depth of 4 km during the Moscovian (ca. 312 Ma). Fault-rock assemblages record a variety of alternating deformation mechanisms and processes, including cataclastic flow, pressure solution and hydrofracturing and vein precipitation.</p><p>Following emplacement, the ENSZ was breached by clastic dykes and sills injected within re-opened previous anisotropies such as bedding planes, thrust surfaces, joints and stylolites. Together, they constitute an interconnected network of quartz sand-rich lithosomes that reach structural heights in excess of 20 m above the ENSZ. The orientation of the dykes suggests that the injection process took place under low differential stress conditions in the hangingwall and near-lithostatic fluid pressure conditions in the footwall. The injected slurry consisted of overpressured pore fluid, quartz-sand grains derived from the footwall and entrained host-rock fragments. The temperature of the fluids estimated from the clumped isotope composition of calcite cements is 71&#8211;86 &#176;C, with an average of 80 &#177; 4 &#176;C. The calcite isotopic composition (&#948;<sup>13</sup>C = -0.15, &#948;<sup>18</sup>O = -9.53, both VPDB) is well within the typical values of the host L&#225;ncara Fm., which suggests that the fluids achieved equilibrium with the host prior to calcite precipitation. Using this calculated temperature and depth estimates for the base of the Esla Nappe, the geothermal gradient during deformation is estimated to be in the order of 16&#8211;24 &#176;C/km, a relatively low value.</p><p>Flow conditions within the injections have been inferred from properties such as the particle drag coefficient, morphology, diameter and concentration, and the fluid density and viscosity, necessary for the calculation of the terminal fall velocity of the particle array. Thin injections formed of pure quartz, with a thickness <1 cm, are consistent with flow velocities of 0.01&#8211;0.35 m/s and a laminar flow (Reynolds number (Re) <800). Thicker pure quartz injections (<10 cm), on the other hand, required faster flow velocities (0.35 m/s) and transitional to turbulent flows (800 < Re < 8000). The thicker injections (&#8776;1 m) that entrained larger host-derived fragments would require transitional to turbulent flows (1200 < Re < 1.2&#215;10<sup>4</sup>) at fast velocities (0.35 m/s).</p><p>The estimated geothermal gradient is consistent with the lower estimations for current foreland basins, and very similar to ocean trenches. The velocities and Reynolds numbers derived for the Esla Nappe are larger than usually estimated for deep seated injections without hydraulic connection with the surface, where the vertical pressure gradient driving them is limited. In those cases, laminar flow conditions are usually invoked, but our results suggest that turbulent flow is possible in the thicker injections. Nonetheless, the values are lower than those reported for shallow injections in connection with the surface.</p>
<p>The isotopic carbon and oxygen isotope composition of carbonates (&#948;<sup>13</sup>C and &#948;<sup>18</sup>O), determined by temperature and the relative abundances of stable isotopes of both elements in water at the time the carbonate is precipitated, can be modified subsequently during geological processes that involve the recrystallization of carbonate. Temperature changes mostly affect &#948;<sup>18</sup>O, while additional sources of carbon have a greater impact on &#948;<sup>13</sup>C. Amongst the various processes that may alter the original isotopic signature of carbonate rocks are deformation processes, which can lead the dissolution and reprecipitation of carbonates during deformation, or the involvement of fluids of various origin during younger tectonic events.</p><p>Here, we present the results of isotope analysis in fault rocks from two distinct faults in the Cantabrian Zone (CZ) in northern Spain. It represents the foreland fold and thrust belt of the Variscan orogen in Iberia and is characterized by numerous and large thrust sheets that were emplaced during the Carboniferous. Subsequent rifting episodes in the Mesozoic and more recently Alpine North-South convergence produced the overprinting of some of the earlier Variscan structures. In all cases, brittle processes produced often similar-looking rocks as the fracturing occurred under upper crustal conditions, relatively close to the surface. Fluids involved during deformation on both cycles are likely to differ, so to evaluate alternative tools to distinguish the different cycles of fracturing in carbonates, a stable isotope analysis on carbon and oxygen was undertaken in two well-known structures in the region: the Somiedo nappe and the Ventaniella fault.</p><p>The Somiedo nappe is one of the largest thrust sheets in the Cantabrian Zone, with an estimated offset close to 20 km. The base of the thrust sheet is characterized by well-developed cataclasites and ultracataclasites that formed on Cambrian fine-grained dolostones. It has relatively minor vein activity associated, although the dolostones have been partially recrystallized. The Ventaniella fault is a dextral strike-slip structure cutting obliquely the Cantabrian Mountains. It runs for tens of kilometres inland and has an estimated offset of approximately 5 km. The fault zone in the studied area is characterized by the fracturing and dextral offset of Carboniferous micritic limestones and, more importantly, a relatively strong vein activity that formed a distributed network of calcite veins.</p><p>Cataclasite matrix and fragments, and associated veins were sampled for isotope analysis in the two fault zones. In both cases, the matrix has a signature which is intermediate between the undeformed rock and that of the veins. The fragments have a signature which is indistinguishable from the matrix, suggesting the reworking of the fault rock. The veins have a distinct pattern in both faults, but different from each other. Those related to the Ventaniella fault are mostly hydrothermal, with limited range in &#948;<sup>18</sup>O and &#948;<sup>13</sup>C, while the veins from the base of the Somiedo nappe have a larger range of &#948;<sup>13</sup>C, but limited &#948;<sup>18</sup>O variation.</p>
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.