Structural control of the Cueva Huerta Natural Monument (Cantabrian Mountains, Spain)

El Monumento Natural de Cueva Huerta está constituido por un sistema de galerías kársticas que exceden los 20 km acumulados desarrollados en calizas carboníferas. En este trabajo analizamos las estructuras geológicas que determinan la orientación de los sectores principales de la cueva. Las observaciones realizadas en el interior de la cueva permiten reconocer varias superficies de deslizamiento de falla como la principal estructura determinando el desarrollo y orientación del sistema kárstico en Cueva Huerta. La orientación de la galería principal es paralela a la traza de una falla asociada a la Falla de León, mientras que varios sectores de la cueva siguen estructuras de dirección noroeste-sureste, probablemente alpinasThe Cueva Huerta Natural Monument is formed by more than 20 km of karstic conduits hosted by carboniferous limestones. Here we study the geological structures that determine the orientation of the major galleries in the cave. The observations made inside the cave point to several fault slip surfaces as the main candidates in the structural control of orientation of karstic conduits in Cueva Huerta. The main gallery follows closely a splay from the Leon fault, while the other major sectors in the cave run with a northwesterly orientation, following minor faults, likely alpine in ag

Geomorphological evolution of the calcareous coastal cliffs in North Iberia (Asturias and Cantabria regions)

This paper presents an analysis of the main morphologies observed in the coastal cliffs of northern Spain (Asturias and Cantabria regions). The objective of this contribution is to establish a hypothesis on the origin and evolution of this rocky coast, as well as to present a detailed inventory, to characterise quantitatively and qualitatively singular morphologies and to highlight the geological heritage of this protected coast. The evolution process starts with the formation of an ancient coastal planation surface characterised by a flat morphology caused by regional mainly uplift and to relative sea level falls. Afterwards, wave erosion processes would have started eroding the cliff foot and simultaneously, karst activity produced some exokarst morphologies (sinkholes, karren, etc.) through stratification and fracturing network, while the underground drainage systems produced some caves and chasms. In the following step, corresponding to the last glaciation from the paleoclimatic point of view, sea level fall together with a deepening of the fluvial network caused the preservation of the existing caves and chasms and the generation of new ones at a lower level. On the other hand, dissolution processes on limestones created sinkholes in those areas characterised by alternating layers of limestones and marls, generating collapses. When the sea level reached the maximum height during the Holocene a new erosion cycle of the coastal cliffs began. As a consequence, new landforms and processes were produced, like bays, caves fillings, and intrusion of new sediments in small confined estuaries. In these areas, other types of morphologies associated with the last sea level rise can be observed, such as closed beaches, uncommon closed estuaries developed inside a sinkhole, blowholes produced by mixed wave action and widening of prevailing vertical pipes inside the limestones (including the second largest in the world), total or partial sedimentary filling of small confined estuaries, as well as a tombolo deposit. It is important to point out, that some sites described are included in the Spanish Inventory of Sites of Geological Interest (IELIG). Due to the evolution model here proposed, a portion of the coastal sector described are included in the Global Geosites Project

Radon concentration in caves as a proxy for tectonic activity in the cantabrian mountains (Spain)

Radon (Rn) constitutes a good geochemical tracer for neotectonic activity in faults since associated fracturing near the surface favours fluid escape to the atmosphere. In this contribution, we measured the Rn concentration in the air inside karst caves to constraints the recent fault activity in the Cantabrian Mountains (N Spain). Rock formations exhumed during the uplifting of the Cantabrian Mountains record a long history of fracturing, which has the potential to connect deeper sources of Rn with the surface. In this regional study, we correlate Rn measurements with cave survey data and geological structures using a Geographic Information Systems. Thirty-four Rn average concentration was recorded by CR-39 detectors during 8 integrated months. The method is applied to the central part of the Cantabrian Mountains that is built on sedimentary and low-grade metamorphic rocks relatively poor in U. Dominant tectonic structures and Rn concentration are examined in 28 cavities. The concentration of Rn values is higher than 0.5 kBq·m-3 in caves developed preferably following fractures with the direction N30oW, being the concentration greater than 0.8 kBq·m-3 in cavities located less than 200±50 m from subvertical faults with such orientation. Rn anomalies point to relative high connectivity along subvertical fault zones NW-trending, preserving fracture connectivity in the most recent structures in the Cantabrian Mountains. Finally, in the study area there is a low but significant radioactive hazard which is associated to fault zones in a fractured rock massif. It contrasts with other active tectonic settings where the radioactive hazard may come from fault movements

Insights on high-grade deformation in quartzo-feldspathic gneisses during the early Variscan exhumation of the Cabo Ortegal nappe, NW Iberia

High-grade, highly deformed gneisses crop out continuously along the Masanteo peninsula and constitute the upper part of the lower crustal section in the Cabo Ortegal nappe (NW Spain). The rock sequence formed by migmatitic quartzo-feldspathic (qz-fsp) gneisses and mafic rocks records the early Ordovician (ca. 480–488 Ma) injection of felsic dioritic/granodioritic dykes at the base of the qz-fsp gneisses, and Devonian eclogitization (ca. 390.4 ± 1.2 Ma), prior to its exhumation. A SE-vergent ductile thrust constitutes the base of quartzo-feldspathic gneissic unit, incorporating mafic eclogite blocks within migmatitic gneisses. A NW-vergent detachment displaced metasedimentary qz-fsp gneisses over the migmatites. A difference in metamorphic pressure of ca. 0.5 GPa is estimated between both gneissic units. The tectono-metamorphic relationships of the basal ductile thrust and the normal detachment bounding the top of the migmatites indicate that both discrete mechanical contacts were active before the recumbent folding affecting the sequence of gneisses during their final emplacement. The progressive tectonic exhumation from eclogite to greenschist facies conditions occurred over ca. 10 Ma and involved bulk thinning of the high-grade rock sequence in the high pressure and high temperature (HP–HT) Cabo Ortegal nappe. The necessary strain was accommodated by the development of a widespread main foliation, dominated by flattening, that subsequently localized to a network of anastomosing shear bands that evolved to planar shear zones. Qz-fsp gneisses and neighbouring mafic granulites were exhumed at > 3 mm yr−1 , and the exhumation path involved a cooling of ∼ 20 ◦C/100 MPa, These figures are comparable to currently active subduction zones, although exhumation P–T trajectory and ascent rates are at the hotter and slower end in comparison with currently active similar settings, suggesting an extremely ductile deformation environment during the exhumation of qz-fsp gneisses within a coherent Cabo Ortegal nappe

Insights on high-grade deformation in quartzo-feldspathic gneisses during the early Variscan exhumation of the Cabo Ortegal nappe, NW Iberia

High-grade, highly deformed gneisses crop out continuously along the Masanteo peninsula and constitute the upper part of the lower crustal section in the Cabo Ortegal nappe (NW Spain). The rock sequence formed by migmatitic quartzo-feldspathic (qz-fsp) gneisses and mafic rocks records the early Ordovician (ca. 480–488 Ma) injection of felsic dioritic/granodioritic dykes at the base of the qz-fsp gneisses, and Devonian eclogitization (ca. 390.4 ± 1.2 Ma), prior to its exhumation. A SE-vergent ductile thrust constitutes the base of quartzo-feldspathic gneissic unit, incorporating mafic eclogite blocks within migmatitic gneisses. A NW-vergent detachment displaced metasedimentary qz-fsp gneisses over the migmatites. A difference in metamorphic pressure of ca. 0.5 GPa is estimated between both gneissic units. The tectono-metamorphic relationships of the basal ductile thrust and the normal detachment bounding the top of the migmatites indicate that both discrete mechanical contacts were active before the recumbent folding affecting the sequence of gneisses during their final emplacement. The progressive tectonic exhumation from eclogite to greenschist facies conditions occurred over ca. 10 Ma and involved bulk thinning of the high-grade rock sequence in the high pressure and high temperature (HP–HT) Cabo Ortegal nappe. The necessary strain was accommodated by the development of a widespread main foliation, dominated by flattening, that subsequently localized to a network of anastomosing shear bands that evolved to planar shear zones. Qz-fsp gneisses and neighbouring mafic granulites were exhumed at > 3 mm yr−1 , and the exhumation path involved a cooling of ∼ 20 ◦C/100 MPa, These figures are comparable to currently active subduction zones, although exhumation P–T trajectory and ascent rates are at the hotter and slower end in comparison with currently active similar settings, suggesting an extremely ductile deformation environment during the exhumation of qz-fsp gneisses within a coherent Cabo Ortegal nappe

Petrophysical properties of Barrios Fm quartzarenites (Cantabrian Zone, N Spain)

La Formación Barrios es una de las unidades litoestratigráficas más características de las zonas externas del orógeno Varisco. Constituida por cuarzoarenitas muy continuas que pueden exceder el centenar de metros de potencia, está presente en casi todas las unidades tectónicas de la Zona Cantábrica. La composición mineralógica determina la alta resistencia mecánica de la mayor parte de la sucesión de estratos que forman la unidad, especialmente las cuarzoarenitas. En este trabajo se describen las propiedades físico-mecánicas determinadas en seis bloques de muestras de esta litología, en general poco alteradas, de las que se han extraído y ensayado 64 probetas. Se han determinado densidad aparente y la porosidad abierta, realizado diferentes ensayos mecánicos sin confinamiento (compresión uniaxial y tracción indirecta), y se han caracterizado las propiedades elásticas mediante ultrasonidosThe Barrios Formation is one of most characteristic lithostratigraphic units in the external parts of the Variscan orogen. It is constituted by continuous quartzarenites that can exceed a hundred meters in thickness. These rocks are present in almost all tectonic units within the Cantabrian Zone. The mineralogy determines its strength for most part of the unit, particularly for the beds made of quartzarenites. In this contribution, basic physical and mechanical properties are described in samples from six blocks, generally fresh, from which 64 cores have been tested. The properties determined are apparent density, open porosity, strength to uniaxial compression, tensional strength (brazilian testing), and elastic properties using ultrasonic testin

Particle quantification from image analysis: application to a quartz-rich sand injection sample at the basal shear zone of the Esla Nappe (Cantabrian Zone, Variscan orogen)

Image analysis of rock samples in thin sections or polished surfaces allows the characterisation of the sizes and shapes of the particles that conform the rock. In order to test the effectiveness of different parameters in discriminating particles and grains of different origin and evolution within a rock, we use one sample from a quartz grain-rich injection at the base of the Esla Nappe (Cantabrian Zone, Variscan Orogen). We characterise particles using the parameters area (A), perimeter (P), sphericity (Es), solidity (So), normalised perimeter-area (PAn) and fractal dimension (DF). The analysis allows to discern rounded sand grains of detrital sedimentary origin from grains that have been partially or totally fractured. However, the parameters used could not discriminate sufficiently grains partially corroded by fluids during their injection perhaps due to the effect of subsequent overgrowths by cementsEl análisis de imagen en muestras de roca en lámina delgada o superficie pulida permite la caracterización de los tamaños y formas de los granos o partículas que constituyen una roca. Para comprobar la eficacia de diferentes parámetros en la discriminación de granos y partículas con diferente origen y evolución en una misma roca, utilizamos una muestra de una inyección arenosa en la base del manto del Esla (Zona Cantábrica, orógeno Varisco). Caracterizamos las partículas con los parámetros de área (A), perímetro (P), esfericidad (Es), solidez (So), perímetro-área normalizado (PAn) y dimensión fractal (DF). Este análisis permite diferenciar los granos redondeados de origen sedimentario detrítico, de aquellos que han sufrido fracturación parcial o total. Sin embargo, el análisis no permitió discriminar suficientemente los granos parcialmente expuestos a la acción corrosiva de fluidos durante la inyección debido quizá a una posible cementación posterio

On equilibrium in non-hydrostatic metamorphic systems

Metamorphic geology has accumulated a huge body of observation on mineral assemblages that reveal strong patterns in occurrence, summarized, for example, in the idea of metamorphic facies. On the realization that such patterns needed a simple explanation, there has been considerable a posteriori success from adopting the idea that equilibrium thermodynamics can be used on mineral assemblages to make sense of the patterns in terms of, for example, the pressure and temperature of formation of mineral assemblages. In doing so, a particularly simple implicit assumption is made, that mineral assemblages operate essentially hydrostatically. Structural geologists have studied the same rocks for different ends, but, remarkably, the phenomena they are interested in depend on non-hydrostatic stress. We look at the effect of such behaviour on mineral equilibria. With adoption of some plausible assumptions about how metamorphism in the crust works, the consequence of minerals being non-hydrostatically stressed is commonly second order in equilibrium calculations