the Malpica-Lamego Line

e-Terra
http://e-terra.geopor.pt
ISSN 1645-0388
Volume 23 – nº 2
2010
Revista Electrónica de Ciências da Terra
Geosciences On-line Journal
GEOTIC – Sociedade Geológica de Portugal
VIII Congresso Nacional de Geologia
Simpósio do Grupo de Geologia Estrutural e Tectónica
_________________________________________________________
Zonas de cisalhamento de escala cortical: o Lineamento
Malpica-Lamego
Crustal-scale shear zones: the Malpica-Lamego Line
S. LLANA-FÚNEZ – [email protected] (Universidad de Oviedo, Departamento de Geología)
A. MARCOS – [email protected] (Universidad de Oviedo, Departamento de Geología)
RESUMO: Os cisalhamentos à escala crustal, afectando rochas tanto da crusta superior como da inferior,
são comuns em orógenos colisionais. São estruturas chave para a compreensão da reologia da litosfera,
uma vez que se formam em consequência da cedência litosférica durante a convergência das placas. O
núcleo da Orogenia Varisca, aflorante no Oeste da Ibéria, mostra várias destas zonas de cisalhamento de
escala crustal, sendo uma delas o Lineamento de Malpica-Lamego.
PALAVRAS-CHAVE: zonas de cisalhamento, deformação não planar, fluidos e deformação, transição frágildúctil, reología.
ABSTRACT: Crustal-scale shear zones, affecting rocks both in the upper and in lower crust, are common
in collisional orogens. They are key structures to understand the rheology of the lithosphere since they
form as a consequence of lithosphere failure during plate convergence. At the surface in Western Iberia,
the core of the Variscan Orogeny is exposed and this part of the ancient orogen shows several of these
crustal-scale shear zones, one of them is the Malpica-Lamego Line, which gives the title to the present
symposium.
KEYWORDS: shear zones, non-plane strain, fluids and deformation, brittle-ductile transition, rheology.
1. INTRODUCTION
Internal deformation processes in the Earth have a direct impact at the surface, however the
study of these sites is limited in presently active scenarios to the use of indirect techniques. The
present level of erosion of the Variscan Belt in Iberia allows direct observations in rocks that
were deformed at middle to lower crustal depths during the development of the Variscan
Orogeny. Access to the microstructures in relation to tectonic activity will allow to establish
rheological behaviour during deformation.
2. GEOLOGICAL SETTING
The core of the Variscan Orogen in Western Iberia is characterised by a number of large shear
zones with a strike parallel to the grain of the orogen. All of these large structures have
magmatic activity associated to them in various stages of the their development. The length of
some of these shear zones exceeds few hundred kilometers and some have igneous rocks with
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e-Terra
Volume 23 – nº 2 | 2010
VIII Congresso Nacional de Geologia
affinities with the upper mantle, so that they are considered of crustal-scale since they affect all
crustal levels. Most of these shear zones were active in the late stages of the Variscan collision as
deformation affects the numerous granitic intrusions in Western Iberia, however, others show
evidences of being formed earlier in the development of the orogen, and register a more complex
structural record.
3. RESULTS
The Malpica-Lamego Line runs approximately parallel to the grain of the orogen from the
northern Spanish coast to disappear southwest of Lamego. The shear zone modified substantially
the tectonic pile of allochthnous complexes in northern Iberia, for most of its length it preserves
in its footwal the Malpica-Tui Unit, which recorded an early Variscan high pressure low
temperature metamorphic episode. In its northern sector and based on geological observations on
either side of the fault, an estimate of the vertical and the horizontal separations was inferred to
be around 15 and 28 km, respectively, for a length of 275 km and was thus considered of crustalscale (Llana-Fúnez and Marcos, 2001). These offsets decrease towards the SE.
From the regional point of view an added significance of this structure resides in its long
tectonic history. According to our previous work, and based also on the geological record or
neighbouring rock units, the active life of the structure exceeded the 30 Ma and it is likely to be
around 40 Ma. The size of the structure and the duration in time of its tectonic activity indicates
that the mechanics of a significant proportion of the crust in Variscan times was controlled or
modulated by this structure.
Besides the regional importance of the Malpica-Lamego Line, the present level of erosion
exposes the middle crust, in general thought to represent the base of the seismogenic zone in the
continental crust (Sibson 2007). Work in progress in the MLL will aim to identify the presence
of fluids associated to deformation structures and metamorphism early in the development of the
shear zone that may have a direct impact on the rheology of the MLL and ultimately in the
subsequent tectonic history.
Acknowledgements
Funding from the Spanish Ministry of Science and Innovation grant CGL2006-08822/BTE and
Fellowship RYC-2008-02067 is acknowledged.
References
Llana-Fúnez, S., Marcos, A. (2001) – The Malpica-Lamego Line: a major crustal-scale shear zone in the Variscan
belt of Iberia. Journal of Structural Geology, 23, pp. 1015–1030.
Sibson, R.H. (2001) – Au-quartz mineralization near the base of the continental seismogenic zone. In: Ries, A.C.,
Butler, R.W.H. and Graham, R.H. (Edts.), Deformation of the Continental Crust: The Legacy of Mike Coward,
Geological Society, Special Publications, 272, pp. 519–532.
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