Oblique foliation
Oblique foliation or inclined structure is a special type of foliation that occurs during the tectonic deformation process . This structure occurs mainly in quartz-rich layers of mylonites . It is used in microtectonics , a branch of structural geology , to determine the respective sense of shear.
Description of the structure
Oblique foliation is encountered in shear zones . The structure is created by shear movements within these deformation areas. As the name suggests, oblique foliation forms an acute angle with the edge of the shear zone (the so-called structure atractor , in English fabric attractor ) or with any existing internal layers. The acute angle is usually between 40 ° and 20 °. In some ultramylonitic shear zones only around 5 ° are measured. However, angles> 45 ° also occasionally occur. On closer inspection it can be seen that the foliation is made up of a multitude of small, roughly equal, elongated, parallel grains.
Oblique foliation is consequently a structure that has a preferred regulation of the grain shapes ( shape preferred orientation - SPO).
In its geometrical configuration is similar to the oblique foliation (type I) SC-structure , but in which the grains by a true ausgelängten foliation be replaced or foliation. SC structure also contains shear surfaces (C surfaces). Occasionally, mica fish are incorporated into the fabric. This structure is then referred to as Type II SC structure (after Lister and Snoke, 1984).
Origin of the structure
Inclined foliation structures have indeed reached a dynamic state of equilibrium, but they in no way reflect the overall deformation. This is explained as follows:
It is assumed that the structure arises from the interaction of two opposing processes, namely on the one hand from the process of passive flattening and rotation of the affected grains in a non-coaxial deformation field and on the other hand from the process of grain boundary migration , which partially re-establishes the resulting adjustment destroyed. The shear deformation causes an alignment of the grains having the maximum Auslängungsrichtung the incremental Verformungsellipsoids (eng. Strain ellipsoid ) and with the instantaneous extension axes (ISA), the dynamic whereas recrystallization (indicated by the uniform, regular grains) to the new means of formation is equal to works against large and deformation-free grains. The latter process can only take place by destroying existing grains.
As the deformation progresses, the foliation therefore remains relatively stationary in its orientation with respect to the kinematic frame of reference and is not completely rotated into the structural attraction. As a result, the oblique foliation lags behind the total deformation ellipsoid and only reflects the last section in the deformation process.
Occurrence of the structure
Oblique foliation is tied to shear zones and is mainly found in monomineral rocks, but can also occur in polymineral rocks. All metamorphic grades are covered by the structure, from low grade to high grade metamorphic rocks. Main occurrences are mono-mineral layers of quartz, muscovite and calcite in stratified mylonites. Descriptions of the structure are available for quartz in quartzites , for calcite in carbonate rocks and for olivine in peridotites . Oblique foliation is also known from rock analogues such as ice or man-made octachloropropane .
Theoretical considerations
The angle between the oblique foliation and the structure tractor is a function of the following parameters:
- dynamic vorticity W k .
- Deformation rate dγ / dt.
- Recrystallization rate (and indirectly the temperature T).
Conclusions were made about the vorticity W k by means of an angle measurement ; however, this is problematic since all other parameters are not taken into account.
A different kind of problem is represented by angles> 45 °, which cannot be explained with the current theory (e.g. the ISA cannot exceed 45 °). It is possible that the influence of transtension shear zones is noticeable here, which by means of its distensive component brings the usual oblique foliation <45 ° into a steeper position.
Importance of structure
The main purpose of oblique foliation is to determine the sense of shear in the respective shear zone. The foliation lies in the shear direction, thus follows the flow direction, ie in a dextral shear zone it lies to the right, and vice versa in a sinistral shear zone to the left. In conjunction with other indicators, such as δ objects , the oblique foliation provides a very reliable statement about the shear direction in question.
Individual evidence
- ^ Means, WD (1981). The concept of steady-state foliation. Tectonophysics, 78, p. 179-199
- ^ Ree, JH (1991). An experimental steady-state foliation. Journal of Structural Geology, 13, p. 1001-1011
- ^ Dell Angelo, LN & Tullis, J. (1989). Fabric development in experimentally sheared quartzites. Tectonophysics, 169, p. 1 - 21
- ^ De Bresser, JHP (1989). Calcite c-axis textures along the Gavarnie thrust zone, central Pyrenees. Geol. Mijnb., 68, p. 367-376
- ^ Van der Wal, D., Vissers, RMD & Drury, MR (1992). Oblique fabrics in porphyroclastic Alpine peridotites: a shear sense indicator for upper mantle flow. Journal of Structural Geology, 14, p. 839-846
literature
- Passchier, CW & Trouw, RAJ (1996). Microtectonics. Springer publishing house. ISBN 3-540-58713-6
- Trouw, RAJ, Passchier, CW & Wiersma, DJ (2010). Atlas of Mylonites - and related Microstructures. Springer publishing house.
- Vernon, RH (2004). A practical guide to rock microstructure. Cambridge University Press. ISBN 0-521-89133-7