The tectonic significance of veins of irregular fold-like shape (contorted veins)
Elena Druguet: Dept. de Geologia, Universitat Aut˜noma de Barcelona, 08193 Bellaterra, Spain; firstname.lastname@example.org
Lina M. Casta–o: Dept. de Geologia, Universitat Aut˜noma de Barcelona, 08193 Bellaterra, Spain; email@example.com
Dyanna M. Czeck: Dept. of Geosciences, University of Wisconsin-Milwaukee, Milwaukee, WI 53201, USA; firstname.lastname@example.org
Peter J. Hudleston: Dept. of Geology and Geophysics, University of Minnesota, Minneapolis MN 55455, USA; email@example.com
Jordi Carreras: Dept. de Geologia, Universitat Aut˜noma de Barcelona, 08193 Bellaterra, Spain; firstname.lastname@example.org
Magmatic veins with complex irregular geometries are commonly found in lithologically heterogeneous metamorphic and migmatitic domains (Fig. 1a and b). These veins have some structural features, i.e. ptygmatic folds and boudinage, that can be related to heterogeneous ductile deformation of competent veins in relatively less competent host rocks (e.g. Ramberg, 1959; Ramsay, 1967). However, there are features such as jogs or sharp deflections that do not seem to be caused by ductile deformation, but could instead be the result of segmentation at the time of intrusion. Indeed, veins commonly form with a non-planar, zigzagging segmented geometry when they are injected into anisotropic layered rocks. The phenomenon, similar to strain refraction (Treagus, 1988), can be attributed to stress refraction, and it is not exclusive to magmatic veins or dykes (Druguet et al., 2008) but is also associated with some varieties of cleavage (Foster and Hudleston, 1986), hydrothermal mineral veins (Shelley, 1968; Lafrance, 2004), clastic intrusions (Truswell, 1972) and faults (Peacock and Sanderson, 1992).
To explore the significance of these striking but common veins in ductily deformed rocks, field structural studies of natural competent leucocratic veins from Cap de Creus in the Eastern Pyrenees (Fig. 1a) and from Rainy Lake in Canada (Druguet et al., 2008, Fig. 1b) have been integrated with a series of deformation experiments with the prototype BCN-Stage to test the progressive development of refracted veins (Casta–o, 2010; Fig. 1c). Four factors are identified as the main controllers in the development of the observed vein geometries: (1) zigzag vein/buckle fold interaction; (2) angular relations among vein, host rock layering and deformation axes; (3) competence contrasts among the host rocks and vein, and (4) strain intensity.
The veins described here likely originated as extension and/or hybrid extensional-shear fractures. Strong fracture refraction is favored by the presence of lithological boundaries with competence contrasts and also by high fluid pressures and low differential stress. Veins that at the time of intrusion are deflected across layer boundaries in banded rocks (e.g. zigzagging veins) can experience complex post-emplacement ductile strain. A variety of structures can arise from strain partitioning across layers of different rheology.
The structures described here are of tectonic relevance in areas where veins are potential tools to estimate regional strain, kinematics and/or competence contrast. The widespread assumption of original planar veins and the mistaking of zigzags for folds may lead to imprecise or incorrect strain and kinematic inferences such as an overestimation of shortening. This is comparable with the overestimation of extension from originally segmented veins that resemble boudins but that recorded little or no extension (Bons et al., 2004).
This contribution highlights the importance of recognizing primary structures in veins. Some insights can be used to distinguish between intrusion refraction and the effects of post-intrusion ductile deformation. Sharp jogs in veins form at rheological boundaries because of intrusion refraction. By contrast, smoothly curving sinusoidal folds of class 1B or 1C shape (Ramsay, 1967) result from buckling of competent veins. Post-intrusion folding and refraction can also be inferred from features in the host rocks. Although this study focuses on magmatic veins, the results can also be extended to hydrothermal and sedimentary veins.
Fig. 1. Examples of irregular contorted veins and sketches of models for their development. (a) Horizontal view of folded zigzagging veins of pegmatite in heterogeneous schists from the Cap de Creus peninsula (Catalunya, Spain). (b) Horizontal view of folded irregular veins of leucogranite crosscutting banded metavolcanic rocks from the Rainy Lake zone (Ontario, Canada). (c) Synthetic sketches of the deformation of zigzagging veins by shortening parallel to the vein envelope for three different aspect ratios of the initial segments. In all models, the veins are much more competent that the enclosing materials. The sketches are based on field structures and on the results from the deformation experiments. ÒcÓ and ÒiÓ refer to relative competent and incompetent, respectively. Z and X are main shortening and extension directions, respectively.
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