HERRAMIENTAS CONVENCIONALES

The tectonic significance of subvertical joints in general is the implication that stress fields exerting a subhorizontal extension on the study area must have existed during its brittle deformation history. According to general models (van der Pluijm and Marshak, 1997) the minimum principal stress (σ3) lies normal to the joint plane while the maximum principal

stress (σ1) is oriented parallel to its plane. Consequently, subvertical joints formed in

response to a horizontal or vertical compression. The direction of compression can usually be deduced from the joints’ propagation directions, which in turn are indicated by the orientations of arrest lines and twist hackles. Unfortunately, in most cases the plumose structures detected in the study area were very faint, so clear propagation directions could not be obtained for the majority of joints. In addition, certain problems arose from the analysis of plumose structures. For one, joints exposing these structures on their planes could also have been created by blasting. Since most of them were found in granite quarries an anthropogenic origin must be considered. Thus, the tectonic origin of a fracture had to be

ensured, which was achieved by the exclusion of curved joint planes. These planes were assumed to have originated as a result of blasting and were interpreted as structures similar to the impact shatter cones described by Eisbacher (1991). Additionally, only joint planes containing hydrothermal minerals were examined, because hydrothermal alteration is a sure sign for a relatively old, and therefore non-anthropogenic, age of formation. Another complication for the analysis of plumose structures is that most of them belong to the curvy variety (van der Pluijm and Marshak, 1997), and only parts of a structure have been preserved in almost all cases. Thus, a clear propagation direction was very difficult to obtain. However, on a few fracture planes very vague propagation directions could be determined. There, the orientations of arrest lines and twist hackles suggest a subvertically oriented compressional stress suggesting that the joints formed during uplift.

Additionally, several other indicators suggest joint formation during regional uplift: (1) The presence of normal faults with strikes similar to those of the joints (see section 2.4.2.1) indicates a subvertical σ1 with two more or less orthogonal σ3 directions. Similar extension

directions can also be obtained from fig. 2-49. Furthermore, several joints were later reactivated by dip-parallel shearing, which can only result from a subvertical σ1. (2) The

presence of approximately orthogonal fracture sets in several sampling stations, which do not show any appreciable cross-cutting relationships or offsets, suggests a more or less contemporaneous formation due to an uplift-related σ1 as well as a σ2 and σ3 of a relatively

equal magnitude. This stress configuration can lead to a switch in the orientations of σ2 and σ3, resulting in the formation of joints at right angles to each other. However, these

orthogonal sets must have formed at times when the northward directed push from the alpine orogeny, which, despite its present-day distance of about 200 km, had an influence on the regional stress field. According to Bergerat and Geyssant (1982; 1983) this push was comparatively minor such that σ2 and σ3 could have similar magnitudes.

Yet certain complications with regard to the orthogonal fracture sets in metamorphic regions can arise. The distribution of discontinuities in well-foliated rocks is frequently dominated by those parallel to the metamorphic foliation, which represent preexisting planes of weakness preferentially used for fracturing. Thus, extensional strain is predominantly accommodated by these planes, which are not necessarily oriented at right angles to the regional minimum principal stress. As a result, joint sets, which would otherwise form at right angles to each other due to the mechanisms described above, are oriented at various angles. In such cases outcrops are frequently dominated by a set of fractures parallel to the foliation, and one cutting across at high angles. Depending on the inclination of the foliation planes shearing motion can occur along the foliation parallel fractures, because only vertical joints can respond to subhorizontal extension without any displacement. Thus, instead of

orthogonal joint sets different combinations of joints and faults with varying angles with respect to each other can occur in metamorphic lithologies in response to an uplift-related vertical σ1.

2.4.2 Faults

In the study area 258 faults sensu stricto have been documented. These fractures clearly show signs of displacement along their planes. These signs comprise offsets of tectonic markers, slickensides, or the presence of fault rocks such as loose or compacted breccia seams. In the latter case, however, only fractures labeled as faults in the field notes are included in this dataset, which is presented in fig. 2-50a. Adding all fractures not labeled as faults in the field notes, but associated with breccia seams, which are very likely to have been caused by shearing motion (i.e. faults sensu lato), to the dataset of faults amounts to a total of 471 structures (fig. 2-50b).

2.4.2.1 Orientations and spatial distribution

The examination of the plots in fig. 2-50 yields similar orientations of both faults sensu strictu and sensu lato. Thus, the majority of fractures possess conspicuously steep inclinations, although fig. 2-50b also contains two gently inclined populations. The moderately to steeply SW dipping concentrations of poles in both plots trend subparallel to the metamorphic foliation planes. Approximately 84 (i.e. 5 out of 31 faults in fig. 2-50a) and 78 (i.e. 13 out of 59 faults in fig. 2-50b) per cent of the faults of this orientation were recorded in metamorphic lithologies.

The other dominant populations trend in northerly directions, NW-SE and NE-SW. Thus, they are in part subparallel to the joints plotted in fig. 2-49. Only the northerly strikes occur predominantly in the group of faults. About half of the outcrops (i.e. 7 out of 14) in which these strikes were recorded are situated in the vicinity of major topographic lineaments trending in similar directions. A connection between northerly striking faults and lithologic units could not be established. The NW-SE and NE-SW trending faults with their conspicuous subparallelism to the dominant joint sets will be discussed in the following section.

Lithogroup Total Number of

Outcrops Faults sensu stricto Outcrops with Faults sensu lato Outcrops with

1 Granitic rocks 8 8 (100 %) 8 (100 %)

2 Layered metamorphic rocks 12 8 (67 %) 11 (91 %)

3 Strongly folded and migmatitic rocks

21 12 (57 %) 17 (82 %)

4 Hydrothermal and pneumatolytic rocks

5 4 (80 %) 5 (100 %)

Faults in general are relatively evenly distributed over the study area. Of a total of 49 sampling stations 32 possess faults sensu stricto and 41 faults sensu lato. Classified according to their occurrence in the particular lithogroups shows that all outcrops situated in granitic rocks contain faults s. str. while only about two thirds of the exposures located in strongly folded and migmatitic rocks do so (table 2-7). Layered metamorphic rocks show slightly higher values and hydrothermal/pneumatolytic rocks rank second. Faults s. l. are an ubiquitous feature existing in 80 to 100 % of the outcrops in a specific lithogroup.

Table 2-7: Sampling stations, in which faults were measured, classified according to their associated lithogroups.

a b Figure 2-50: Cumulative Plots of faults in the study area. (a) Plot of planes explicitly called "fault" in

the field. (b) Data from (a) plus all fractures associated with breccia and/or gouge suggesting fault motion. n = 257 in (a); n = 470 in (b), contours represent integer mrd.

2.4.2.2 Tectonic significance

Faults are the direct results of displacement between two rock blocks in response to deviatoric stresses. Thus, in case the direction of displacement is obvious, faults can identify paleo-stress fields. As will be detailed in the following sections several kinematic indicators such as slickenlines, offsets and the presence of fault rocks were used to elucidate the region’s tectonic history.

A very striking property of many faults in the database is their extremely steep dip, even though a large number of them could be identified as dip-slip faults, which usually tend to be rather moderately inclined. Also, their orientations are very similar to those of the extensional joints. In one of the preceding sections the joints were related to uplift, in which probably also the dip-slip faults have their origin. In keeping with the predictions of the Mohr- Coulomb criterion (van der Pluijm and Marshak, 1997) and the Anderson theory of faulting (Marshak and Mitra, 1988) fig. 2-51 illustrates how the angular relationships between joints and faults in an uplift-regime should exist. In this model vertical joints form along with faults inclined at angles of approximately 60°. In the reality of this study area this is obviously not the case. Instead, since in a number of incidents slickensides and plumose structures were found on the same fracture planes, it is suspected that shear reactivation followed joint formation. Thus, the joints acted as preexisting planes of weakness, which were easier to reuse for shear displacement than to create new faults. Also, since major offsets along fault planes have not been found the throw is expected to have been relatively minor.

A relatively small number of faults is moderately to gently inclined (fig. 2-50a) or even gently inclined to subhorizontal (fig. 2-50b). The subhorizontal fractures (inclined at up to 20°) were exclusively recorded in outcrops belonging to lithogroups 1 and 3 (i.e. granitic rocks and strongly folded/migmatitic rocks) and are characterized as faults by the presence of breccia seams. Offsets and slickensides are generally absent. Especially in the granites these discontinuities appear to be reactivated unroofing joints, where uplift initially created flat-lying extensional fractures, which were later reactivated by subhorizontal compression. For the fractures in the lithogroup of strongly folded/migmatitic rocks the interpretation becomes more difficult, because in these cases the fractures do not have the appearance

Figure 2-51: Conceptual 2-D model of the orientations of principal stresses, faults, and joints based on the Anderson theory of faulting (Marshak and Mitra, 1988). σ1, σ2, and σ3 represent principal stresses. Vertical gray tapered line represents a joint opening normal to σ3. σ2 stands normal to this page. Diagonal black lines represent a conjugate set of faults. Half-arrows point in the direction of displacement.

typical for unroofing joints. They rather exist as densely fractured and brecciated zones, which show no signs for having resulted from the reactivation of formerly extensional joints (fig. 2-52). In fig. 2-50b this group of NNE to E dipping faults is represented by the steeply to subvertically NNW to W plunging concentration. The strikes of these fractures are noticeably similar to those of the metamorphic foliation, even though a macroscopically detectable fabric – with the occasional exception of preferentially oriented porphyroblasts - is widely absent in these rocks. Nonetheless, the belt of moderately to steeply SW inclined poles in fig 2-50a definitely parallels the metamorphic foliation, even more so because many faults belonging to this concentration were actually recorded in well-layered rocks.

Thus, it can be assumed that the majority of faults in the study area came into existence by the reactivation of preexisting planes of weakness such as vertical joints, unroofing joints, or foliation planes. Still, occasional primary formations are possible.

2.4.3 Kinematic indicators

In the preceding section on faults in the study area an overview regarding their orientations and general properties was presented. Now, the kinematic aspects thereof shall be concentrated on. At this point it must be mentioned that clear and unambiguous kinematic indicators are extremely rare in this region. Outcrop-scale offsets and cross-cutting relationships usable for paleostress analyses are virtually absent and even if such features were found the magnitude of displacement indicated usually did not exceed a few decimeters. The reasons for this lie either in the frequent absence of tectonic markers in the mostly homogenized or plutonic rocks, or in very minute rates of displacement along individual faults, which only amount cumulatively to throws of greater magnitudes.

Thus, in order to decipher the tectonic history of the study area one primarily has to fall back on the interpretation of geometric relationships between fracture systems,

Figure 2-52: Gently inclined fault in a poorly foliated rock (Körnelgneis). Location: Viechtach/ Alterberg, 6943-14.

slickensides, and the distribution of brittle fault rocks. These are ubiquitous features in the study area, although not all outcrops could be used for their sampling. Especially the slickensides are dependent on the degree of weathering in a particular exposure and are thus restricted to relatively fresh outcrops, such as the granite quarries in the southern study area. In the gneissic regions they are usually more rare, most likely because intense weathering has frequently worn off the top part of the fault planes or obscured the slickenlines with seams of weathered material. Nonetheless, sufficient data could be collected to obtain an impression of the brittle tectonic mechanisms that have acted on the study area since the late Mesozoic.