Apparent boudinage in dykes

Intrusive rocks may be arranged in the form of strings of lenses or beads, as found on the Cap de Creus Peninsula, NE Spain, and in the South Finland Migmatite-Granite Belt. These structures first appear to be the result of stretching and boudinage of intrusive sheets or dykes. However, closer examination reveals that they are not boudins, but are instead primary intrusive structures. A detailed study was performed on a swarm of pegmatite intrusions at Cap de Creus. Layering is often continuous between beads, and, in some cases, individual beads exhibit a very irregular shape. These observations are shown to be incompatible with an origin by boudinage. Analogue experiments were used to test the effect of the two models (boudinage and emplacement) on the structures around beads, and show that an emplacement model of local expansion and collapse of magmatic sheets is the most compatible with field observations. Such structures can only form when dykes intrude hot rocks, when magma solidifies slowly enough to allow enough ductile flow of the wall rock to accommodate the formation of the beads. The fact that the pegmatite bead strings are not boudins has repercussions for the interpretation of the deformation at Cap de Creus and necessitates caution in the interpretation of deformation based on apparent boudins in intrusive rocks in other areas

Some misleading boudin-like structures

Las estructuras de boudinage son indicadoras de extensión paralela a las capas. Sin embargo, existen estructuras que poseen un alto grado de similitud con los boudins pero cuyo origen no está relacionado con un estiramiento significativo paralelo a las capas, venas o diques aparentemente boudinados. En el presente trabajo se discuten dos de estas estructuras: shear bands y venas en fracturas en zigzag. Las fallas o shear bands pueden cortar una capa separándola en bloques o «boudins» (de ahí el término clivaje de crenulación extensional). Sin embargo, en determinadas circunstancias, estas estructuras pueden formarse en capas paralelas al plano de no estiramiento en cizalla simple, pudiendo así dar un impresión falsa de extensión de la capa. Un segundo tipo de «falsos boudins» lo constituyen las venas formadas por la abertura de grietas a lo largo de una fractura, dando lugar a una ristra de cuerpos elongados cuya geometría presenta gran semejanza con los boudins reales formados por estiramiento

Numerical modelling of asymmetric boudinage

Asymmetric boudinage structures are commonly used as shear sense indicators but their development is incompletely understood. This paper describes the influence of initial shape and kinematic parameters on the evolution of boudin trains using a numerical approach based on the finite difference code FLAC. Boudin trains are simulated as a series of competent objects embedded in a soft matrix subjected to general monoclinic ductile flow. Deformation of boudin trains includes heterogeneous stretching, rotation of boudins and offset along the neck regions. The sense of relative boudin offset is mainly influenced by the initial orientation of the interboudin plane in the boudinaging layer, while kinematic vorticity number of the flow and the orientation of the boudin train with respect to the flow extensional eigenvector, usually the shear zone boundary, also play a role. Viscosity ratio and aspect ratio influence the magnitude of offset along the neck regions and the amount of rotation of boudins but not the sense of slip and rotation on the interboudin plane. Knowledge of the orientation of the interboudin plane itself is insufficient to use asymmetric boudins as independent shear sense indicators. Details of the boudin geometry such as the sense of deflection of marker horizons along the interboudin plane must be used for this purpose

Geoethical education: A critical issue for geoconservation

Some geological outcrops have a special scientific or educational value, represent a geological type locality and/or have a considerable aesthetical/photographic value. Such important outcrops require appropriate management to safeguard them from potentially damaging and destructive activities. Damage done to such rock exposures can include drill sampling by geologist for scientific purposes. In this work, we show how outcrops important to structural geology and petrology can be damaged unnecessarily by drill coring. Unfortunately, regulation and protection mechanisms and codes of conduct can be ineffective. The many resources of geological information available to the geoscientist community, e.g. via Internet, promote access to sites of geological interest, but can also have a negative effect on their conservation. Geoethical education on rock sampling is therefore critical for conservation of the geological heritage. Geoethical principles and educational actions are aimed to be promoted at different levels to improve geological sciences development and to enhance conservation of important geological sites

Shear partitioning across two parallel shear zones in Roses granodiorite (Eastern Pyrenees, Spain)

La granodiorite di Roses rappresenta uno degli affioramenti più esterni dei granitoidi tardo Varisici dei Pirenei orientali. E’ interessata da un’ importante deformazione milonitica varisica con cinematica sinistra che ha prodotto un reticolo di zone di taglio duttili in facies scisti verdi, correlabili con quelle di Cap de Creus. Localmente, tuttavia, sono state documentate cinematiche destre che non mostrano univoche relazioni di sovrapposizione con le zone di taglio caratterizzate da cinematica opposta Sono stati raccolti 9 campioni lungo un transetto ortogonale ai limiti delle zone di taglio stesse. Per indagare la deformazione l’analisi microstrutturale è stata affiancata dallo studio delle orientazioni dell’asse c del quarzo, dallo studio del flusso cinematico e dall’analisi di imagine

Analysis of a complex high-strain zone at Cap de Creus, Spain

The structural analysis of a high-strain zone developed in medium- to high-grade metamorphic micaschists from the Cap de Creus area, Spain provides an example of the complex relationships between geometry, strain and kinematics to be found in deep crustal shear zones. This high-strain zone is composed of E-W trending structural domains characterized by different strain intensities and associated with syntectonic emplacement of pegmatite dykes. The main phase of deformation discussed here, D-2, affects steeply dipping bedding, boudinaged quartz veins and S-1 developed parallel to bedding. D-2 deformation of these features and of syn-D-2 pegmatite dykes gives rise to fold/boudin structures. In map view, a D-2 high-strain zone coincides with a km-scale dextral flexure of S-0, S-1 and S-2, although all small-scale structures on outcrop surfaces close to horizontal indicate a prevalent sinistral shear sense. In addition, a subvertical stretching lineation is present approximately parallel to the axis of the foliation-deflection and thus normal to the apparent 'displacement' direction in the high-strain zone. It is proposed that this high-strain zone did not form by horizontal dextral simple shear with a flow plane parallel to the boundaries of high- and low-strain zones but acted as a zone of highly vortical sinistral non-coaxial flow with a strong vertical extension component. The dextral flexure of S-0, S-1 and S-2 can be explained by considering that sinistral non-coaxial flow in the low-strain zones was either slower or earlier that in the high-strain core. This example stresses the necessity of being careful when interpreting displacement directions from the deflection of older fabric elements in deep-seated high-strain zones

Geochemistry of metasedimentary rocks from the Eastern Pyrenees (Iberian Peninsula): Implications for correlation of Ediacaran terranes along the Gondwanan margin

The North African margin of Gondwana formed by subduction and accretion during Ediacaran to Cambrian times. Sections of this active margin were incorporated during the late Paleozoic to the Variscan and Appalachian orogens. In the Variscan realm, these sections define in some regions a Cadomian basement comprising thick metasedimentary series associated with coeval subduction-related magmatic rock dated in the range 650–500 Ma. In the Cap de Creus Massif (Eastern Pyrenean domain), the Cadomian metasedimentary rocks formed from sediments derived from mixed felsic and intermediate/mafic sources, intruded by granitoid and minor mafic igneous rocks. These series were formed in a peri-Gondwanan volcanic arc setting, like other Cadomian series described in the SW of the Iberian Massif, such as Serie Negra Group of the Ossa Morena Complex. However, the Nd model ages of the Cap de Creus metasedimentary samples range from 1431 to 1620 Ma and are younger than those of their SW Iberia equivalents. This result can be interpreted as an indication of an eastern location in the Ediacaran - Cambrian peri-Gondwanan magmatic arc. Furthermore, the Cadomian series in the Central Iberian Zone have even younger Nd model ages than those of SW and NE Iberia. We present two end-member paleogeographic models consistent with reconstruction of the initial location of the Cap de Creus Massif and its relations with other domains (Ossa Morena Complex and Central Iberian Zone) in this sector of the North African Gondwanan margin.We would like to express our gratitude to the Prof. G. Meinhold and another anonymous reviewer for the suggestions and recommendations that have helped to improve our manuscript. We appreciate their time and effort. We also thank to the editor Prof. Victoria Pease for her consideration of our research and her editorial assistance. Financial support has been provided by the Spanish projects PID2020-112489-GB-C21, PID2021-122467NB-C22 and PID2021-125585NB-I00, funded from the Ministerio de Ciencia, Innovación y Universidades/Agencia Estatal de Investigación/Fondo Europeo de Desarrollo Regional (FEDER), EU