A. Aranguren

The Los Pedroches Batholith (Southern Spain): Polyphase Interplay between Shear Zones in Transtension and Setting of Granites

The Los Pedroches batholith (LPB) in the Variscan belt of southern Spain is a 200 kilometres long magmatic body which has intruded low-grade metamorphic rocks of the Central Iberian Zone. The LPB is composed of granodiorites, a dyke swarm, and granites emplaced during three successive magmatic events. Field structural data from both the country rocks and the LPB indicate that the northern boundary of the LPB is a transtensional shear zone, the Conquista shear zone, dipping 50° to the NE. This shear zone is sealed by the Cerro Mogabar granite which was emplaced during the last magmatic event. A detailed magnetic susceptibility study, using 2236 oriented samples from 559 stations, of the main part of the batholith yields flat magmatic foliations, and magmatic lineations with a mean east-west trend. The structural, kinematic and magnetic susceptibility data indicate that the emplacement of the LPB was controlled by a crustal scale and dextral transtensional shear zone, parallel to the batholith elongation. The eastern part of the granodiorite takes the shape of a large laccolith prolonged westwards by four NW-SE elongate domes. These contrasted geometries reflect the upwelling of magma through the transtensional zone that vanishes towards the west. Intrusion of dykes along Riedel shears related to the regional transtension took place in a second magmatic event. The main transtensional shear zone is interpreted as a lateral ramp that accommodated the crustal thinning developed in the Central Iberian zone during the collapse of the Iberian Variscan belt.

The Guitiriz granite, Variscan belt of northern Spain: extension-controlled emplacement of magma during tectonic escape

Abstract The Ollo de Sapo domain of the northern part of the Variscan belt of Spain, contains Precambrian and Ordovician metamorphic rocks intruded by the Guitiriz granite. The domain is bounded by two NS transcurrent shear zones. Detailed structural and anisotropy of magnetic susceptibility studies of the Guitiriz pluton reveal a syntectonic emplacement, relative to movement on the two NS trending bounding shear zones, characterized by: (1) development of NS trending structural and magnetic fabrics: (2) concordant structures in granites and country rocks; and (3) development of shear zones along the east and west granite margins. Gravity data show two roots trending 110°E extending down to 4.5 km from an otherwise thin (

Composite magnetic fabrics from S-C mylonites

Magnetic anisotropy data are presented for samples of S-C mylonites from a shear zone developed in the Veiga granodiorite, a late-kinematic pluton located in the northern part of the Iberian Variscan belt (Spain). The magnetic susceptibility is geometrically represented by an ellipsoid whose principal axes, K,,, 2 Kint 1 Kmi,, often show a one-to- one correlation with the principal directions-X,Y and 2-f finite strain. We report an exception to this rule, since our results reflect composite magnetic fabrics arising from the contribution of both the C- and S-structures. The magnetic foliation, defined as the plane normal to K,i,, shows an intermediate orientation between the C- and S-planes, and rotates towards the orientation of the C-planes as finite strain increases. The magnetic lineation, parallel to K,,,, and the stretching lineation present a similar trend, but different values of plunge. Copyright 0 1996 Elsevier Science Ltd

Granite laccolith emplacement in the Iberian arc: AMS and gravity study of the La Tojiza pluton (NW Spain)

A laccolithic geometry is inferred from geological and geophysical data for the La Tojiza granitic body emplaced in an extensional regime. Nearly circular in outcrop, this 16 km diameter pluton, one of the so-called ‘post-tectonic granodioritic plutons’ of Galicia, is located in the northern Variscan arc of Iberia, within the Mondoñedo Nappe (Western Asturian–Leonese Zone). From anisotropy of magnetic susceptibility measurements and extensive microstructural data, the magnetic fabric of this pluton has been inferred to correlate with the magmatic fabric. Magnetic foliations strike NE–SW and dip gently SE. Magnetic lineations trend NW–SE and plunge gently SE. Gravity data suggest that the pluton has the shape of a sheet with a maximum preserved thickness of 1 km and a root zone located at its eastern border. This pluton is reinterpreted as late kinematic and emplaced during an extensional tectonic regime related to the ductile Vivero normal fault. It is suggested that the emplacement of most post-tectonic plutons of this part of the Variscan orogenic belt may have been controlled by the late extensional collapse of the Iberian Massif.

Structural evidence for the relationship between thrusts, extensional faults and granite intrusions in the Variscan belt of Galicia (Spain)

Abstract New structural and petrofabric data are presented from the Hombreiro pluton, a synkinematically emplaced granite located in the northern part of the Variscan belt of Spain. This pluton shows the imprint of two plastic deformations: the first is related to the motion of the Mondonedo Nappe, with a top-to-the-east shear sense, and the later one to the Vivero Fault, an extensional shear zone with a top-to-the-west motion. These deformations initiated at high temperatures, close to the granite solidus as shown by the preservation of ⋎c⋎-slip fabrics in quartz. This fact indicates that extensional faulting was activated soon after the thrusting of the Mondonedo Nappe during which the Hombreiro Granite was emplaced. We propose that the position of the Vivero Fault was controlled by a crustal instability induced by the intrusion of a number of plutons that delineate the footwall block of this fault.

Coalescence of lateral spreading magma ascending through dykes: a mechanism to form a granite canopy (El Hongo pluton, Sierras Pampeanas, Argentina)

This study deals with the structure of the El Hongo trondhjemite, a ENE–WSW elongate pluton belonging to the Famatinian magmatic arc that developed from Late Cambrian to Silurian times in the Sierras Pampeanas (Argentina). Anisotropy of magnetic susceptibility (AMS) measurements combined with structural and microstructural data permit the correlation of magmatic and magnetic fabrics within this ferromagnetic pluton. Magnetic foliations strike north–south and dip steeply west but in the marginal facies the foliation becomes parallel to the ENE–WSW pluton contacts. Magnetic lineations mostly trend north–south and plunge SE. Gravity data reveal that the pluton is in the form of a very thin horizontal sheet resting on two north–south-trending root zones detected below the central portion of the pluton. These root zones represent feeder dykes that exploited tension fractures. The emplacement of the El Hongo pluton took place during the action of a buried sinistral shear zone, parallel to the elongation of the pluton, corresponding to a transfer fault associated with the late extensional exhumation of the Famatinian domain. The emplacement of other Famatinian granites of the Sierras Pampeanas could be related to similar late shear zones.

Compressional granite-emplacement model: structural and magnetic study of the Trives Massif (NW Spain)

Abstract The Trives granodiorite is a syntectonically emplaced pluton belonging to the great Manzaneda batholith, which has intruded into the gneisses of the Ollo de Sapo Formation (northern part of the Iberian Variscan belt) during Carboniferous times. Combined metamorphic, petrofabric, structural and anisotropy of magnetic susceptibility studies reveal that interactions between regional metamorphism, deformation and magmatism played the main role in strain partitioning in and around the emplacing Trives pluton. Low-dipping S–C structures carrying N140°E-trending stretching lineations exist in both the roof-pendants which contain sillimanite bearing gneisses and in the subjacent granodiorites which are located in the central areas of the Trives pluton, whereas subvertical shear zones but with the same stretching lineation orientations are found along the pluton margins. At deeper levels of the Trives granodiorite, there are fabrics which are related to the crystallization of magma at the emplacement level. In these rocks, magnetic foliations are mainly steeply dipping and magnetic lineations are N140°E-trending. The anisotropy of magnetic susceptibility data here is consistent with field-observed macroscopic structures. Samples showing solid-state deformation display magnetic foliations and lineations subparallel to C-planes and their macroscopic stretching lineations, respectively. The foliations and lineations of magmatic origin are characterized by lower degrees of anisotropy. Structural maps of magmatic and solid-state deformation structures suggest that the emplacement of the Trives pluton was controlled by D 3 -deformational structures, N140°E-trending transcurrent shear zones and crustal detachments, which developed in response to a NE–SW compressional stress field.

Is there a time lag between the metamorphism and emplacement of plutons in the Axial Zone of the Pyrenees

Detailed petrographic and geochemical studies conducted on zircons from the Lys-Caillaouas pluton reveal their igneous and metamorphic affinities. The igneous zircons constrain the emplacement of the pluton to 300±2 Ma. By contrast, the metamorphic zircons yield an older age of 307±3 Ma, which probably dates the thermal peak of the HT/LP Variscan metamorphism. Therefore, a short time lag of c . 7 Ma emerges between the metamorphic climax and emplacement of the pluton in the Axial Zone (Pyrenees).

La faille de Conquista : caractérisation structurale et cinématique d'un cisaillement ductile dextre à composante normale dans le domaine sud de l'Arc ibéro-armoricain

The Conquista Fault is a high-temperature, ductile shear-zone developed along the northern contact of the Los Pedroches batholith in the southern part of the Central Iberian Zone (Variscan Belt, Spain). Structural and kinematic criteria show that the Conquista fault is a dextral oblique shear with a component of eastward extension. The motion of the Conquista fault is consistent with that of a transfer shear-zone subordinate to the eastward-directed late extensional evolution of the Central Iberian Zone.

Anisotropy of magnetic susceptibility of the Pyrenean granites

ABSTRACT In this paper, we report on a compilation of more than 2200 sites (more than 10,000 individual measurements) where anisotropy of magnetic susceptibility (AMS) was studied in granites from the Variscan Pyrenees. The standardization and homogenization of this information has allowed us to produce three Main Maps that synthesize all the information related with the AMS of the Pyrenean granites. We also describe the problems found during the construction of the database (variable geo-positioning, different published information, etc.). The information derived from 21 granite bodies, the database, and the synthesis maps (magnetic susceptibility, Km, and the orientation of the magnetic foliation, plane perpendicular to k3, and of the magnetic lineation, k1) allow us to see for the first time a complete image of this important kinematic and petrographic indicator.