Sergio Llana-Fúnez

Metamorphic reaction rate controlled by fluid pressure not confining pressure: implications of dehydration experiments with gypsum

Pressure is a key control on the progress of metamorphic reactions. When fluids are present in rocks, the fluid pressure is commonly different to the load supported by the solid framework. Here, we show experimentally that, when the two pressures are varied independently, fluid pressure exerts the dominant control on reaction rate, even when the rock is compacting. We present 35 experiments on gypsum dehydration with independently controlled confining pressure, pore fluid pressure and temperature. Results show that a pore fluid pressure decrease at constant confining pressure has a strong effect on the average rate of the reaction. A decrease in confining pressure at constant pore fluid pressure has relatively little effect. Our results have implications for reaction kinetics: even though the product phase is supporting more and more load as reaction proceeds, that load does not appear to exert a chemical effect. On the large scale, our results imply that changes in fluid pressure will drive or stop the progress of metamorphic reactions. When estimating depth at which a metamorphic devolatilization reaction occurs, knowledge of the pore fluid pressure may be necessary rather than commonly used lithostatic pressure. This is relevant for basin diagenesis, mineralization in hydrothermal systems and chemical evolution after pore fluid pressure is perturbed by earthquakes.

Role of porosity and dehydration reaction on the deformation of hot-pressed serpentinite aggregates

Experimental deformation of hot-pressed powders of lizardite serpentinite was carried out to study the effect of the dehydration reaction to olivine + talc + water under controlled pore water pressure. Use of a porous aggregate ensured free movement of pore fluid into or out of the specimen in response to volume changes. The dehydration reaction further increases the porosity of the sample, causing weakening, but progressive pore collapse leads to strain hardening. At low strain rates, a transition to linear–viscous flow was inferred to be caused by the formation of transiently fine-grained olivine in the dehydration reaction. The inability of the rock to support high loads during dehydration at low strain rates means that the production of high-pressure water by dehydration and its subsequent expulsion will favour seismogenic failure in the surrounding rocks not directly involved in the dehydration reaction, rather than the serpentinite itself.

Quartz c-axis texture mapping of a Variscan regional foliation (Malpica–Tui Unit, NW Spain)

Abstract Mineral assemblages developed under high pressure and low to intermediate temperature conditions (H P –LI T ) are found in eclogites, orthogneisses and metasediments of the Malpica–Tui Unit (MTU), either defining or surrounded by a regional foliation that was mostly re-equilibrated under amphibolite to greenschist facies conditions. This regional tectonic foliation, tracking the exhumation path of the MTU in the context of the Variscan collision in SW Europe, has been studied using the microstructural record in quartz from the different rock types. Most of the quartz c -axis patterns show two distinct asymmetries, contained in perpendicular planes, which are related to two different non-coaxial components of the finite fabric (i.e. the regional foliation and the lineation). The asymmetry of deformation inferred from quartz c -axis textures suggests that fabric accumulation during exhumation occurred under 3-D general flow, departing strongly from the simple shear type. This has significant tectonic implications for the interpretation of regional stretching lineations, which in the present geodynamic setting of irregular collisional tectonics, are not parallel to the direction of general tectonic transport and, ultimately, to the plate convergence direction.

Structural record during exhumation and emplacement of high-pressure–low- to intermediate-temperature rocks in the Malpica-Tui unit (Variscan Belt of Iberia)

Upper crustal rocks of the Malpica-Tui unit were buried to high-pressure (P) and low- to intermediate-temperature (T) conditions during the Variscan orogeny in northwestern Iberia and subsequently exhumed in the same tectonic cycle. The structural record of the exhumation is presented in this chapter as a sequence of structures ordered according to superposing relations and decreasing grade of ductility. Synthetically, two different stages during the deformation process are identified corresponding to two major types of structures: a first general but heterogeneous ductile episode, seen in the rocks by the development of a regional tectonic fabric, and a second and subsequent ductile episode recorded in more discrete structures that include the bounding and discrete basal shear zone of the Malpica-Tui unit. The first event accounts for the exhumation of the high-P rocks to the middle crust and the second for their final emplacement, prior to the later Variscan intracontinental tectonics. From geological observations, consistent with the tectonic setting of indentation tectonics in the development of the Ibero-Armorican arc of southwestern Europe, we suggest that in both stages general shortening and associated tectonic transport are initially highly transverse to the orogenic trend. The data set presented here also emphasizes the relevance of internal deformation during ascent of high-P nappes.

Tectonic framework of the Cabo Ortegal Complex: A slab of lower crust exhumed in the Variscan orogen (northwestern Iberian Peninsula)

The Cabo Ortegal Complex is a composite allochthonous terrane that was thrust onto the western edge of Gondwana during the Variscan orogeny. It is formed of two main tectonic units: the Upper Tectonic unit, comprising rocks affected by high-pressure (P)-high-temperature (T) metamorphism, and the Lower Tectonic unit, which represents the resulting suture of the Variscan collision. The suture preserves remnants of strongly deformed and metamorphosed ophiolitic rocks overriding the parautochthon, and the lower Paleozoic sequence of the Ollo de Sapo antiform, regarded as the autochthonous sequence of the Iberian plate. The Upper Tectonic unit is formed by layered ultramafic, mafic, and quartzo-felspathic rocks that were buried at levels in excess of 50 km (-1.56 GPa) before the Variscan collision in a convergent plate boundary within the Rheic ocean domain ca. 490-480 Ma (Early Ordovician). They have been interpreted either (1) as an earlier thinned continental crust, underlain by a lithospheric mantle and oceanic spreading, or (2) as independent terranes, formed in different geodynamic settings (island arc, oceanic). Most structures observed in these rocks are ductile and are associated to their exhumation process. It started with the development of a persistent horizontal foliation in granulite facies conditions, which equilibrated in amphibolite facies conditions ca. 385 Ma, and ended in higher crustal levels with the progressive development of noncoaxial structures, such as east-verging asymmetrical isoclinal folds and thrusts, leading to the emplacement of the Upper Tectonic unit over the Lower Tectonic unit ca. 365 Ma (Late Devonian).

Contribution of crystallographic preferred orientation to seismic anisotropy across a surface analog of the continental Moho at Cabo Ortegal, Spain

The lithostratigraphic sequence of the Upper Allochthon of the Cabo Ortegal complex in northwestern Spain provides an excel lent analog for the direct study of petrophysical properties of the continental Moho transition. The various lithologies present were sampled for velocity measurements on minicores and determination of seismic veloc ities through microstructural analyses, with emphasis on the crustal part of the sequence. Here, we present data from electron backscatter diffraction (EBSD) to determine the crystallographic preferred orientation of all major rock-forming minerals. The orientation data are then combined with the elastic properties, densities, and modal fractions of the mineral phases to calculate P-wave and S-wave velocities. Calculated velocities coincide very well with direct measurements made on minicores of the same samples in the high-pressure laboratory. The threedimensional velocity patterns show the signifi cant contribution of the crystallographic preferred orientation of certain constituent minerals on the bulk properties of the rock. The bulk anisotropic signal is dominated by highly anisotropic phases that are susceptible to both shape and crystal preferred orientation. Factors of particular importance are small modal fractions of micas in gneisses, and amphiboles and clinopyroxenes in eclogites and high-pressure granulites. In the lower-crustal rocks, the direction of maximum P-wave velocities, although contained in the foliation plane, does not coincide with the orientation of the mineral and stretching lineation and therefore cannot be used as an indicator of the main strain direction in rocks. The maximum seismic birefringence is often contained within the foliation plane but very rarely coincides with the lineation. Our data set also illustrates the strong effect of the breakdown reactions of clinopyroxene and the appearance of plagioclase on the petrophysical properties of mafi c lower-crustal rocks. While it does not enhance anisotropy, it does produce a drop in velocity and as a consequence enhances the refl ectivity of the contact between lower-crustal rocks and ultra mafi cs from the mantle.

An example of low-Th/U zircon overgrowths of magmatic origin in a late orogenic Variscan intrusion: the San Ciprián massif (NW Spain)

Th/U values in zircon are commonly used to discriminate between metamorphic (Th/U < 0.1) and magmatic (Th/U > 0.1) origin for zircon overgrowths. We test this hypothesis in the San Ciprián massif, a late orogenic granitic intrusion in the hinterland of the Variscan orogeny. Zircon grains from this granite have cores with inherited Ediacaran ages and Th/U > 0.1, whereas zircon mantles yield an age of about 287 Ma, interpreted as the time of crystallization of the granite, and have Th/U < 0.1. Hence, the San Ciprián massif presents an uncommon but unambiguous example of magmatic zircon mantles with Th/U values typical of metamorphic zircon. The most likely causes for the unusually low-Th/U zircon values in the San Ciprián massif are a combination of a U-rich magma composition (owing to a fractionation process) and the absence of other U-enriched accessory minerals. Our work in determining Th/U ratios substantiates the warning previously made by some researchers and precludes the use of Th/U values in zircon as an unequivocal indicator of metamorphic origin in the absence of other chemical, zircon morphology or field-based independent criteria. Supplementary materials: Code and data to generate Figures 1 and 5 are available at http://www.geolsoc.org.uk/SUP18885.

Effect of strain geometry on the petrophysical properties of plastically deformed aggregates: experiments on Solnhofen limestone

Abstract Specimens of Solnhofen limestone were deformed under conditions where calcite deforms plastically using four experimental configurations: extension, torsion, direct shear and axisymmetric shortening. All experiments were run on dry specimens at the same temperature (600 °C), confining pressure (200 MPa) and comparable strain rates (c. 10−4 s−1). The different experimental settings and the heterogeneity of deformation within some of the specimens provided a large range of strain geometries. They allowed locally imposed strain geometries to be related to the crystallographic preferred orientation (CPO) patterns of calcite and the orientation of the shape fabric of calcite grains. CPO in calcite was measured using electron back-scattered diffraction (EBSD) in scanning electron microscopy. The development of CPO during deformation under the dominance of intracrystalline plasticity contains information about the strain geometry accumulated in rocks in 3D, although in nature the strain geometry can be modified by dynamic recrystallization that was not seen in the experiments. The different CPO patterns have a significant effect on the velocity structure of the deformed aggregates. Seismic properties inferred from CPO show that the orientation of the fastest Vp wave aligns with principal strain directions that are not equivalent in different strain geometries.

Tectonic thinning of a crust slice at high pressure and high temperature by ductile-slab breakoff (Cabo Ortegal Complex, northwest Spain)

The upper tectonic unit of the Cabo Ortegal Complex (northwest Spain) comprises an ordered rock sequence of ultramafic and mafic rocks and quartzofeldspathic gneisses. The entire assembly forms part of a transitional mafic crust. In an early Paleozoic subduction episode, most of this crust was metamorphosed at eclogite facies and high-pressure granulite facies conditions. Field and microstructural observations suggest that this metamorphism occurred simultaneously with deformation, producing a pervasive tectonic fabric that developed under a strain regime of bulk coaxial deformation. Downdip extension of the crust in the subduction zone is inferred to have thinned the rock sequence. The relatively high pressures and high temperatures (770-900 °C and >1.2-1.7 GPa) during deformation at peak conditions, in addition to the significant thinning of the units, suggest a tectonic setting of ductile slab breakoff as the cause of the intense and widespread deformation.

The seismogenic zone of the continental crust in Northwest Iberia and its relation to crustal structure

The distribution of seismicity at the western end of the Cantabrian mountain range (NW Iberia), reflecting current crustal dynamics, is investigated integrating seismically active structures, long-term structures, and the topographic features at the surface. The thickness of the seismogenic zone within the continental crust is established in 20 km. Two crustal domains can be distinguished in the study area in terms of the seismicity pattern, the style of Alpine structures and the relief. The presence of crustal fluids arises as a very likely contributing factor to the excessive thickness of the seismogenic zone in the study area. More importantly, a switch in tectonic style at the transition between crustal domains in coincidence with the lateral termination of orogenic frontal thrusts is envisaged as involving sufficient stress heterogeneity and amplification to explain the current concentration and characteristic pattern of historical seismicity in the region. Overall, the distribution of seismicity in the crust is sensitive to the type and style of crustal structures.