J. Escuder Viruete

Tectonothermal evolution associated with Variscan crustal extension in the Tormes Gneiss Dome (NW Salamanca, Iberian Massif, Spain)

Abstract The tectonothermal Variscan evolution of the Tormes Gneiss Dome is controlled by a subhorizontal ductile shear zone of crustal scale and extensional characteristics that induced a quick exhumation of the deep parts of the metamorphic complex during crustal thinning. The shearing affected a broad band of metamorphic rocks but, as the temperature decreased, became progressively concentrated in a low-grade ductile detachment that separates two distinct units, characterized by their lithology and different tectonothermal evolution. Kinematic indicators in non-coaxial fabrics show a displacement of the hanging wall to the southeast i.e., parallel to the trend of the foldbelt. The subhorizontal shearing is related to the extensional collapse of the variscan crust, previously thickened during the collision tectonics.

Variscan syncollisional extension in the Iberian Massif: structural, metamorphic and geochronological evidence from the Somosierra sector of the Sierra de Guadarrama (Central Iberian Zone, Spain)

Abstract The rocks affected by the Variscan orogeny in the Sierra de Guadarrama (Central Iberian Zone) can be separated into two domains: a high-grade Western Domain and a medium- to low-grade Eastern Domain. These domains are separated by the Berzosa–Riaza shear zone. A new integrated analysis of structural, metamorphic and U–Pb monazite age data, indicates that the Berzosa–Riaza shear zone is a major D2 extensional structure associated with oblique, crustal-scale extensional shearing and important decompression. Low-P/high-T metamorphism associated with the D2 event overprints earlier Barrovian metamorphic assemblages formed during the Variscan D1 compressional event. The kilometre-scale Berzosa–Riaza shear zone exhibits a S–C mylonitic front at its base and a crenulation front at its top, with successive fabric development under different P–T conditions indicating crustal thinning. Textural relationships and U–Pb dating of monazite indicate that the main D2 extensional event took place between 337±2 and 326±3 Ma (Early–Mid Carboniferous). A late-D2 extensional episode overprinted the major D2 ductile shear zone along narrow, low-grade, oblique-normal detachment zones. Kinematic indicators in both D2 and late-D2 fabrics indicate a displacement of the upper structural levels to the southeast. Field relationships, microstructures and qualitatively derived P–T–t–d paths suggest that the Early–Mid Carboniferous syncollisional extension in the Sierra de Guadarrama was associated with the major extensional collapse of the Variscan hinterland (Central Iberian Zone) and was contemporaneous with the earliest thrusting in the foreland.

Two-dimensional geostatistical modeling and prediction of the fracture system in the Albala Granitic Pluton, SW Iberian Massif, Spain

Abstract We use a fracture index distribution method of geostatistical modelling and prediction to characterize quantitatively the fracture system in two-dimensions (2D) in the Mina Ratones area, located in the Albala Granitic Pluton (SW Iberian Massif). The fracture index (FI) is a quantitative estimate of the fracture density in discrete domains. To validate the results of geostatistical modeling a detailed structural map of the area was also made on a scale of 1:1000. The resulting grids, expressed as pixel-maps, describe the continuous value of the FI in 2D for the whole Mina Ratones area. Based on the modelled distribution of the FI and their correlation with mapped faults, we distinguish two structural domains in the studied area: elongated bands of fracture zones with high FI values and romboidal blocks located between them with low FI values. The separation between both domains is gradual. Though a threshold value of the FI that separate both structural domains is not clearly defined, the fracture zones generally present FI>1 and the individualized blocks FI

Architecture of fault zones determined from outcrop, cores, 3-D seismic tomography and geostatistical modeling: example from the Albalá Granitic Pluton, SW Iberian Variscan Massif

Abstract The 3-D seismic tomographic data are used together with field, core and well log structural information to determine the detailed 3-D architecture of fault zones in a granitic massif of volume 500×575×168 m at Mina Ratones area in the Albala Granitic Pluton. To facilitate the integration of the different data, geostatistical simulation algorithms are applied to interpolate the relatively sparse structural (hard) control data conditioned to abundant but indirect 3-D (soft) seismic tomographic data. To effectively integrate geologic and tomographic data, 3-D migration of the velocity model from the time domain into the depth domain was essential. The resulting 3-D model constitutes an image of the fault zone architecture within the granitic massif that honours hard and soft data and provides an evaluation of the spatial variability of structural heterogeneities based on the computation of 3-D experimental variograms of Fracture Index (fault intensity) data. This probabilistic quantitative 3-D model of spatially heterogeneous fault zones is suitable for subsequent fluid flow simulations. The modeled image of the 3-D fault distribution is consistent with the fault architecture in the Mina Ratones area, which basically consists of two families of subvertical structures with NNE–SSW and ENE–WSW trends that displaces the surfaces of low-angle faults (North Fault) and follows their seismically detected staircase geometry. These brittle structures cut two subvertical dykes (27 and 27′ Dykes) with a NNE–SSW to N–S trend. The faults present high FI (FI>12) adjacent bands of irregular geometry in detail that intersect in space delimiting rhombohedral blocks of relatively less fractured granite (FI

3-D stochastic modeling and simulation of fault zones in the Albalá granitic pluton, SW Iberian Variscan Massif

A distribution of fracture index (FI) is obtained from stochastic modeling and simulation to characterize quantitatively the fault system of the Mina Ratones area in three-dimensions (3-D). FI is a quantitative estimate of the fracture intensity in discrete domains of the granitic rock massif. The resulting 3-D grids, expressed as block (cells) models or contoured isosurfaces of FI, show high and low FI zones. The correlation of these zones with mapped faults allows two structural domains to be distinguished: (1) variably irregular surfaces of high FI, and (2) rhomboidal blocks of low FI located inside them. High FI domains (FI>4.2 m−1) are interpreted as fault zones, since there is a good correlation at the surface between the domains and traces of the major fault zones. Low FI blocks (FI<2.5 m−1) correspond to less fractured granite. The contact between high and low FI domains is gradual. The high and low FI structural domains may correspond with the damage zone/fault core and the protolith in the model for fault zone architecture of Caine et al. (1996). Therefore, 3-D grids of the FI in granitic areas affected by strike-slip brittle tectonics, such as Mina Ratones, constitute an image of fault zone architecture.

Hornblende-bearing leucosome development during syn-orogenic crustal extension in the Tormes Gneiss Dome, NW Iberian Massif, Spain

Abstract The high-grade Lower Unit of the Tormes Gneissic Dome (TGD; NW sector of the Iberian Massif) contains tonalitic orthogneissic bodies that were migmatized during a heterogeneous ductile shearing, related to a major episode of syn-orogenic extensional deformation (D2). Both peak and retrograde P – T conditions were deduced from the analysis of reaction textures related to superimposed S2 fabrics developed during exhumation, analysis of mineral zoning and thermobarometric calculations in CaKFMASH and KFMASH systems. Syn-thermal peak partial melting of the tonalitic orthogneisses, via a biotite-dehydration melting reaction yielding hornblende, K-feldspar and melt, produced stromatic migmatites with leucosomes parallel to the S2 foliation of the mesosome as well as discordant patchy pegmatitic migmatites. The deduced post-thermal peak P – T path comprises an initial phase of decompression combined with cooling from 5.5–6 kbar at 750°C to ca. 4–4.5 kbar at 675°C, consistent with the sequence of initial retrograde mineral assemblages present in high- T mylonitic S2 fabrics. The later part of the syn-D2 P – T path indicates a significant cooling and is recorded by lower amphibolite to greenschist assemblages related to the later mylonitic S2 fabrics. The proposed melt reaction generating hornblende-bearing leucosomes may be important for crustal differentiation processes at mid-crustal levels during syn-orogenic extension, as well as for the production of certain types of high-level granites. The necessary conditions are large volumes of source material of intermediate composition and the production of a sufficient melt fraction.

One- and two-dimensional thermal modelling of orogenic crustal extension in the Tormes Gneissic Dome, NW Iberian Massif, Spain

Abstract Situated in the inner zone of the Variscan Iberian Massif, the Tormes Gneissic Dome offers a good opportunity for thermal modelling of orogenic crustal extension, because the P–T–t loops are well constrained by an extensive set of thermobarometric, structural and geochronological data. As an example of feedback between forward and inverse methods, the aim of this study was to establish one- and two-dimensional thermal models that reproduce the contrasting petrological P–T paths of two structural units separated by an extensional tectonic contact in the metamorphic complex, and to explain the spatial and temporary development of the low-pressure metamorphism in the rocks located just above this contact. In one dimension, the syn-extension path of the lower unit resulting from modelling is characterized by an isothermal decompression phase, followed by near isobaric cooling, which is typical of exhumed rocks. The upper unit path records a syn-extension near isobaric heating, more important in rocks just above the tectonic contact. Condensed isograds of low-pressure/high-temperature metamorphism in the basal upper unit are thus interpreted as a consequence of advective crustal extension and conductive upward heat transfer. In two dimensions, the delaminated simple shear geometric model of crustal extension explains the observed temperature rise in excess of 500  °C in the basal upper unit and is consistent with the spatial distribution of M2 low-pressure/high-temperature isograds. This demonstrates the important role of extensional structures produced during the collapse of the thickened crust in the thermal evolution. The heating phase, well explained with intermediate dip angle for extensional fault in the upper crust (45°) and finite extension of 75 km, is followed by cooling, thus reflecting normal erosional process.