Hydrofractures, or hydraulic fractures, are fractures where a significantly elevated fluid pressure played a role in their formation. Natural hydrofractures are abundant in rocks and are often preserved as magmatic dykes or sills, and mineral-filled fractures or mineral veins. However, we focus on the formation and evolution of non-igneous hydrofractures. Here we review the basic theory of the role of fluid pressure in rock failure, showing that both Terzaghi’s and Biot’s theories can be reconciled if the appropriate boundary conditions are considered. We next discuss the propagation of hydrofractures after initial failure, where networks of hydrofractures may form or hydrofractures may ascend through the crust as mobile hydrofractures. As fractures can form as a result of both tectonic stresses and an elevated fluid pressure, we address the question of how to ascertain whether a fracture is a hydrofracture. We argue that extensional or dilational fractures that formed below c. 2–3 km depth are, under normal circumstances, hydrofractures, but at shallower depth they may, but must not be hydrofractures. Since veins and breccias are often the products of hydrofractures that are left in the geological record, we discuss these and critically assess which vein structures can, and which do not necessarily, indicate hydrofracturing. Hydrofracturing can suddenly and locally change the permeability in a rock by providing new fluid pathways. This can lead to highly dynamic self-organization of crustal-scale fluid flow.
Presentamos nuevos datos sobre la evolución pre-varisca, varisca y alpina de las rocas pre-Silúricas de los alrededores de Gréixer, en La Cerdanya, Pirineo oriental. El análisis estructural y cartografía de los meta-sedimentos de bajo grado metamórfico de edad cambro-ordovícica y ordovícica superior, sugieren la existencia de tres episodios de deformación así como el emplazamiento de cabalgamientos y de una vena de cuarzo de tamaño hectométrico. La comparación con otros sectores próximos sugiriere que todos ellos forman el flanco sur (invertido) de un antiforme “D2” kilométrico E-O con vergencia hacia el sur, de edad varisca. Este pliegue fue deformado por pliegues “D3” posteriores, de orientación NO-SE y vergencia sur, cuyos flancos buzan moderadamente hacia el norte y cuya edad es desconocida, varisca o alpina. Además, el análisis microestructural y la disposición de las meso-estructuras D2 sugieren la existencia de un evento de deformación “D1”, anterior al Ordovícico Superior, que afectaría tan solo a la sucesión Cambro-Ordovícica. Por comparación con zonas vecinas, se sugiere una posible edad alpina para el emplazamiento de la vena de cuarzo y para el desarrollo de los cabalgamientos.
Abstract. Evaporites flow in the solid state under relatively low differential stress, and have unique mechanical properties compared to other sedimentary rocks. Worldwide, they control the structural and stratigraphical architecture of many basins and orogens in ancient and active tectonic settings. Moreover, they host mineral deposits and play key roles in petroleum systems because they typically act as seals due to their low-permeability, and their flow produce structural hydrocarbon traps such as folds and faults in their encasing rocks. Additionally, evaporite structures can be used as subsurface storage sites for Geo-energy applications and nuclear waste. The systematic characterization of subsurface evaporite structures is thus key for the development of geoscience-based technologies to address societal challenges. Owing to their value, massive amounts of surface and subsurface information about (among others) the stratigraphy, structure, geochemistry, and petrophysical properties of evaporite structures and their surrounding rocks have been acquired by earth scientists, petroleum and mining exploration companies, and geological surveys. However, the data often appear segregated (i.e., in the form of database fragments, scientific articles and unpublished reports), not systematically organized, and sometimes not fully accessible. This contribution presents the Iberian Evaporite Structure DataBase (IESDB), the first comprehensive assessment that focuses on evaporite structures carried out in any region of the world. The IESDB includes information and figures for 150 evaporite structures and their surrounding rocks inventoried in Spain and Portugal, and is sourced from other six thematic databases and more than 1,500 published and unpublished scientific documents. The database targets undeformed to slightly deformed evaporite successions, outcropping and buried diapirs, evaporite-cored anticlines, evaporite-detached thrusts, and allochthonous evaporite bodies. Collated data include information about the structure, stratigraphy, event chronology, surface and subsurface data availability, mining activity, and key bibliographic references. The IESDB follows the FAIR principles of database management (Findable, Accessible, Interoperable and Reusable) and is presented as an interactive webpage and an open-access database, where indexed structures can easily be selected from a map or browser and filtered by a search engine. The IESBD intends to be a useful resource for teaching (i.e., pointing out examples of exceptional evaporite outcrops), academic and industry research (i.e., identifying knowledge deficits on specific structures or tectonic settings), and for the sustainable exploration and appraisal of mineral resources and Geo-Energy applications (i.e., representing a terminus a quo for site selection and suitability assessment). The framework provided by the IESDB is an opportunity to enhance the scientific research on Iberian evaporite structures in Spain and Portugal and to take advantage of their scientific and economic potential to tackle important societal challenges faced by these countries. The IESDB is freely available at https://iesdb.eu and the datasets can be downloaded from https://doi.org/10.20350/digitalCSIC/14586 (González-Esvertit et al., 2022).
<p>Evaporite rocks are one of the most mined mineral commodities and act as a seals or shaping traps in some of the major hydrocarbon provinces worldwide. In addition, they are also considered as suitable sites for the storage of energy and nuclear waste, being a key asset for the energy transition. Due their historic, present and future value, vast amounts of surface and subsurface information about evaporite structures have been generated by earth scientists, mining and exploration companies or geological surveys in the last century. However, this information is often scarcely useful due to access issues, segregation and scarce dissemination. Here we present the Iberian Evaporite Structure DataBase (IESDB), the first overall assessment focused on evaporite structures developed in any region of the world. The IESDB includes information and figures of 150 outcropping and buried evaporite structures and their surrounding rocks inventoried in Iberia. The Iberian Evaporite Structure Database (IESDB) includes information about the stratigraphy, structure, evolution, geophysical and petrophysical data availability, and mining activity, including a complete set of geological maps, sketches and geological cross-sections. The database targets different evaporite structures, such as undeformed successions, diapirs, evaporite-cored anticlines, evaporite-detached thrusts or allochtonous evaporite bodies. The IESDB is sourced from six different databases and more than 1,500 published and unpublished references, and includes information and figures for each of the 150 evaporite structures inventoried. The IESDB follows the FAIR principles of data management (Findable, Accessible, Interoperable, Reusable) and aims to be a resource for earth science teaching, academic research and resource exploration and appraisal. The IESDB is freely available at https://iesdb.eu</p> <p>This research was performed within the framework of DGICYT Spanish Project PID2020-118999GB-I00 funded by the Ministerio de Ciencia, Innovaci&#243;n y Universidades/Agencia Estatal de Investigaci&#243;n/Fondo Europeo de Desarrollo Regional. Grants RYC2021-033872-I (Juan Alcalde) and RyC-2018-026335-I (Enrique Gomez-Rivas) funded by MCIN/AEI/10.13039/501100011033 and ESF &#8220;Investing in your future&#8221;.</p>
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