EVOCOS · Evolution of Continental Strength from Rifting to Collision - A Journey through the Wilson Cycle

Evolution of Continental Strength from Rifting to Collision - A Journey through the Wilson Cycle

EVOCOS was designed to answer the following fundamental question in Tectonics and Geodynamics: under which geological conditions is the lower crust a mechanically weak or strong layer? Understanding the mechanical behaviour of the lower crust is one of the most relevant and challenging goals of tectonics research, and has important implications for: (1) the earthquake cycle and related hazards along active fault zones (some 20% of intracontinental seismicity nucleate in the lower crust at focal depths of 30-40 km) and (2) continental dynamics and the long-term evolution of continents. Despite numerous studies, it remains unclear under what geological conditions (tectonic setting, pressure, temperature, composition, water content, microstructure) the lower crust is mechanically weak and viscous or strong and seismogenic. EVOCOS has tackled this question with an innovative multidisciplinary approach, which has tracked the evolution of the mechanical properties of the lower crust through space and time using granulites as ‘strength proxies’ of the lower crust in different tectonic settings. The project has tested the idea of cycles of dehydration/hydration as the main factor determining the strength evolution of the lower crust. To test this idea, EVOCOS has pulled together expertise in the field of continental dynamics and crustal processes in an outstanding European network, and has investigated case studies from the Arctic Caledonides, from the Baltic Shield and from the Alps. A specific objective of the project was the quantitative determination of the intracrystalline water content in dry lower crustal rocks. Measurements collected in nominally anhydrous minerals (plagioclase, pyroxenes, quartz) from shear zones sampled in the three localities examined in the course of the project have invariably shown low intracrystalline water contents (< 80 ppm H2O by weight). Under such water-deficient conditions crystal plastic behaviour of minerals is inhibited, and the rocks instead deform by fracturing. Earthquakes in the lower crust produce specific rock types (pseudotachylytes: solidified frictional melt produced during seismic slip) and zones of intense brittle grain size reduction, which were studied in detail by EVOCOS. The ultrafine-grained mixtures formed during the fracturing events are subsequently capable to flow viscously by grain size sensitive creep processes at enhanced creep rates and, therefore, to localize strain. Although an original working hypothesis of EVOCOS was that fluid infiltration was necessary to promote strain localization in the dry lower crust, the project has demonstrated that fluid infiltration itself does not seem to be a necessary requisite for weakening of dry lower crustal rocks. Even “dry” lower crustal rocks contain minute quantities of aqueous fluids (typically < 0.4 wt %), and this fluid can be efficiently redistributed in fine-grained mixtures if sufficiently porosity is present. EVOCOS has demonstrated that fracturing and creep cavitation maintain a dynamic porosity in fine-grained mixtures, so that aqueous fluids are present at grain boundaries and can aid grain size sensitive creep processes. Thus, a key result of the project is that it is the aqueous fluid at grain boundaries to have a major rheological effect on lower crustal shear zones, by facilitating diffusion creep deformation and phase nucleation. Furthermore, EVOCOS demonstrated the fundamental role that two specific types of metamorphic reactions can play on strain localization in the mafic lower crust, namely (1) melt-rock reactions and (2) dehydration reactions. Strain localization due to the formation of weak metamorphic reaction products (i.e. reaction weakening) is commonly attributed to metamorphic hydration reactions in the presence of aqueous fluids. EVOCOS expanded this view by highlighting that also dehydration reactions can lead to strain localization, in that they (1) also produce ultrafine-grained material able to deform by grain size sensitive creep, and (2) make available water at grain boundaries. Additionally, EVOCOS demonstrated that melt-rock reactions in mafic granulites are potentially associated with up to three orders of magnitude increase in strain rate in the reaction products, thus showing that weakening by dramatic grain-size reduction through melt-rock reactions is a major rheological effect of melt infiltration during lower crustal shearing. All the case studies examined by EVOCOS have highlighted that brittle and viscous deformations overlap in space and time even at lower crustal conditions, thus calling into question the classic, static view of a brittle-viscous transition located at 10-15 km deep in the crust, and providing new insights into the earthquake distribution and cycle in the lower crust. In particular, EVOCOS has demonstrated that earthquakes are precursors of ductile shear zones in the dry and strong lower crust, thus highlighting the fundamental role of lower crustal earthquakes as agents of weakening in strong granulites. The results of EVOCOS are providing important links between tectonics, geodynamics, fluid-rock interaction, metamorphism, deformation and seismology. The key results will be used to help recognize the field, microstructural, and petrological manifestations in the geological record of the broad spectrum of fault slip behaviours in the lower crust recently highlighted by geodetic and seismic observations (e.g. aseismic creep, co-seismic slip, post-seismic slip, transient slow slip events). This will help interpret seismological and geodetic observations of the earthquake cycle, and design new generations of deformation experiments and numerical models aiming to address the strength evolution of the lower crust during the earthquake cycle. EVOCOS research has been based upon quantitative scanning electron microscopy analysis of deformed rocks, and some of the results can be relevant to the industry of micro-analytical systems, in particular to the on-going development of the electron backscatter diffraction analysis technique applied to ultrafine-grained geological materials. Furthermore, quantitative microanalysis of shear zones and faults have been conducted in collaboration with industrial partners in the field of energy resources and radioactive waste disposal. Thus, EVOCOS research has important impact also for the multi-scale structural characterization of geological sites chosen for the long-time storage of high-grade radioactive waste.

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