Abstract

The aim of this work was to investigate the high-grade Moldanubian gneisses of the Oberpfälzer Wald and Ceský Les of north-eastern Bavaria and western Bohemia. This was achieved by using structural, strain and vorticity analyses, together with a separate study of the widespread migmatisation of the gneisses.
The deformation history can be divided into 5 separate stages, followed by minor local events. The third deformation was the most important event, with respect to the amount of strain and the development of new textures. It was clearly contemporaneous with the low pressure (3-4 kbar), high temperature (>750°C) metamorphism which has been dated at around 330-320 Ma. The last deformation occurred before the onset of late Variscan plutonism.
Two distinct fabrics can be shown to predate D3, but since it is impossible to determine whether the oldest fabric is bedding, it is known as S0/1. The deformations, D1 and D2, are combined in this work, because in most outcrops the differentiation of fabrics earlier than D3 is not possible. In a few outcrops, F1/2 folds have been preserved as structural "strain-free" enclaves. From these examples, it can be seen that D1/2 produced tight east-verging folds with sub-horizontal SSW plunging fold axes. By rotating F1/2 fold axes back to their position before D3 (since they were passively rotated during D3 towards the stretching lineation), it can be shown they originally plunged sub-horizontally NNE-SSE. The internal planar fabrics of pre-D3 garnets, the inclusions of which show evidence of a medium pressure (7-8 kbar) event, are also correlated to D1/2.
The "main phase" deformation, D3, was responsible for the regional, penetrative S3 foliation, which is near vertical, typically striking NW-SE to NE-SW, and the L3 stretching lineation, which plunges steeply NW. A microscopic mylonitic S3 fabric was caused by the new growth and reorientation of all the minerals present in the gneiss. Quartz fabrics indicate high temperature deformation (> 650°C), with the development of "chessboard" subgrain textures. In addition, the start of the migmatisation was coeval with the development of S3, and the fabric was enhanced by the production of foliation-parallel stromatitic leucosomes.
D4 caused large-scale folding of the S3 foliation (half wavelength = 4 km), although a new S4 fabric is only locally produced. F4 folds are tight, vertical folds with fold axes which plunge by 60° NW. F5 folds caused a minor refolding of F4, also with steep fold axes (plunging NW or NE depending on which F4 limb they occur) and vertical fold axial planes.
Small ductile flat-lying shearzones with no apparent sense of shear conclude the regional deformation. In the Zottbachtal, a 10 km section of river valley between Pleystein and Floß in the north of the area, a series of retrograde metamorphosed gneisses ("the diaphthorite zone") outcrop. The dominant S3 foliation of these gneisses was horizontally crenulated. Along the West Bohemian Shear Zone, the eastern contact between the Moldanubian and the Teplá-Barrandian zone, the S3 foliation is folded into vertically-plunging chevron folds.
The strain markers, mostly recording the strain only since the onset of D3, demonstrate different amounts of strain, but most of the data record near plane-strain deformation. This is important for later tectonic modelling of the working area.
Only three vorticity measurements were made, using a method based on the individual deformation history of a set of variously orientated veins. Measurements in the XZ and XY sections of a D3 fabric, as well as a XZ section through a D1/2 structure, show that in every case, the deformation contained a large if not entire component of simple shear.
The deformation of the large outcrops of retrograded eclogite and serpentinite at Winklarn and Niedermurach is similar, but distinct from the deformation history of the typical Moldanubian gneisses previously described. At Winklarn, a series of metabasites, with interbedded metapelites, are tightly folded together with migmatitic metapsammitic gneisses by vertical plunging folds. These were later faulted against unfolded serpentinites. At Niedermurach, orthogneisses and high-grade calc-silicate gneisses are thrust into a serpentinite body. No evidence of large-scale ductile or brittle emplacement structures of the high pressure rocks can be seen.
Five styles of migmatisation can be recognised. The simplest form is foliation-parallel stromatitic leucosomes. These leucosomes developed as the result of closed-system melting (with respect to melt), along a foliation-parallel shear fracture. The amount of melting adjacent to the leucosome is proportional to the distance from the tip of the fracture, but not symmetrical about the long axis of the leucosome. A model is envisaged where an elliptical proto-leucosome, under simple shear conditions, can develop along the foliation plane, which is also the direction of highest tangential stress on the surface of the melt body.
A derivation of the stromatitic leucosome form is the development of oblique leucosomes at 30° the foliation. These can also be explained by the state of stress developed between stromatitic leucosomes during simple shear.
Net type leucosomes form an open network of channels, and contain a larger volume of melt than the previous two leucosome types. Massive leucosomes are melt bodies, some of which represent channels of melt upto 10m thick.
In a number of examples, a fractal number of 1.81 ±0.1 describes the net-type leucosome texture over two orders of magnitude, irrespective of the orientation of the cut through the gneiss. This texture can be closely modelled using a Menger Sponge, a hypothetical three-dimensional model with a fractal dimension of 2.7.
It is demonstrated that the net-type leucosomes form a good transport system for melt migration, due to their fractal geometry. An example of a 5 x 5 km area between Waldmünchen and Furth im Wald was used to quantify the amount of melt migration in the Moldanubian. A vertical leucosome channel, 5m wide, and supported by a sufficient flux of melt from the surrounding area, was capable of supplying 2.2 km3 granitic magma in 10 Ma.
On the basis of the cross-section through the Moldanubian, between the its contacts with the ZEV and ZTT units, and by theoretically removing the effects of the last two main deformations (D4 and D3), it is possible to show that if the ZEV is the remnant of the Teplá-Barrandian nappe complex, then this unit was thrust over the Moldanubian before the low-pressure metamorphism.
The main deformation, D3, although a compressive event, did not cause significant thickening (or thinning) of the Variscan crust. Together with the fact that the coeval low pressure metamorphism only lasted less than 10 Ma, it is possible that the Variscan crust witnessed a near catastrophic mantle perturbation.
A model for the Variscan development of the Moldanubian and its neighbouring tectonic units is proposed, in which the Teplá-Barrandian and a possible high-pressure nappe complex were thrust over the Moldanubian before 380 Ma. The resulting collage was reworked 50 Ma later by a regionally penetrative high-temperature metamorphism and an intense deformation in a relatively high position in the Variscan crust.

Back to David Tanner's Homepage