PRINCIPIO DE FUNCIONAMIENTO DEL SENSOR DE RESISTIVIDAD CENTRADO

Geometric relationships between fractures and fracture systems were mainly identified in the form of conjugate sets (Anderson, 1942) and Riedel-type shear fracture relationships (Eisbacher, 1991; van der Pluijm and Marshak, 1997).

Riedel shear fractures are subsidiary fractures significantly smaller than the main slip planes they are associated with. Ideally, they exist as conjugate pairs of syn- and antithetic fractures oriented at specific angles (usually less than 20° for synthetic, and about 70° to 80° for antithetic shears) to the main slip plane. In the field they are usually not recognizable as conjugate pairs but produce wedge-shaped breakouts on fault planes, of which the wedge points in the direction of displacement. The surfaces of the breakouts usually represent the planes of the synthetic shears and are usually too small to measure. Nonetheless, the shear sense can be determined qualitatively by their visual examination.

In the study area Riedel shears could only be determined on relatively fresh fault planes in predominantly granitic or highly homogenized metamorphic rocks. Highly weathered outcrops did not yield unambiguous information and thus were not examined with respect to Riedel-type shear sense indicators.

As can be seen in fig. 2-53 Riedel shear fractures are not unique to specific fracture sets. Only the very few datapoints for strike-slip faults (fig. 2- 53a and b) seem to restrict this type of displacement to the ± NE-SW direction. As a result, stress configurations similar to those obtained for slickensided strike- slip faults (i.e. subhorizontal ± NE-SW and ± NNW-SSE compression, respectively; see section 2.4.3.2) were obtained.

In contrast, the dip-slip faults do not show conspicuously regular patterns with respect to orientation. However, the dataset of normal faults (fig. 2-53c) contains dominant orientations similar to those of slickensided normal faults, i.e. NE-SW and NW-SE, (see section 2.4.3.2) as well as a group of N-S

striking fractures. The stress configurations estimated for the slickensided faults suggest an extensional regime with a subvertical maximum principal stress and extension in various subhorizontal directions. Similar conditions are also assumed for the faults containing subsidiary Riedel shear fractures. As will be detailed in the section on the study area’s brittle tectonic history this stress configuration is attributed to ongoing regional uplift associated with crustal extension, which in turn is evidenced by the presence and orientations of the extensional joint systems discussed in section 2.4.1.

Regarding reverse faults (fig. 2-53d) there are also similarities between slickensided faults and those associated with Riedel shears. Accordingly, the dominant strikes are NE-SW and NW-SE. In cases where the displacement arrows in fig. 2-53d do not parallel the dip direction the Riedel shears occur along with slickenlines on the same fracture plane, which is usually not the case for the other types of faults presented in fig. 2-53. Whether or not these subsidiary fractures formed contemporaneously with the slip lineations could not be determined. However, in case they did not, the Riedel shears must have predated the slickenlines since they are features linked to initial fault propagation. Nonetheless, the slip directions of slickensides were included in this image, because they allow for better

a b

d c

Figure 2-53: Fault planes associated with Riedel shear fractures. (a) Faults suggesting dextral strike- slip motion, (b) faults suggesting sinistral strike-slip motion, (c) faults suggesting normal motion, (d) faults suggesting reverse motion. In (a) and (b)

paleostress configurations were calculated: ○=σ1

◊=σ2□=σ3. nfaults = 2 in (a) and (b); nfaults = 12 and for

constraints on the latest directions of displacement than the frequently ambiguous orientations of Riedel-type breakouts on fracture planes. The attempt to deduce meaningful stress configurations for this group of faults must remain dissatisfactory due to the small amount and large scatter of datapoints. At best, gently to moderately inclined maximum principal stresses oriented ± NW-SE and ± NE-SW could be envisaged. Similar directions of compression, however with subhorizontal σ1, were determined for slickensided strike-slip

faults (section 2.4.3.2).

In the field conjugate relationships between individual fractures were noted in a

total of 14 sampling stations in all lithogroups. Frequently, they consisted only of two or three fractures per outcrop, while in other locations an entire fracture set showed a conjugate relationship with another one. This is especially the case for the

low-angle fractures conjugate to the foliation

parallel fractures.

Figure 2-54 depicts cumulative representations of their angular relationships. The raw dataset of conjugate fractures was separated

into fracture sets with similar orientations. The resulting subgroups consist of moderately inclined ± N-S striking sets (fig. 2-54a), moderately inclined ± NW-SE striking sets (fig. 2- 54b), gently inclined ± NW-SE and ± NNE-SSW striking sets (fig. 2-54c), and steeply inclined sets with varying strikes (fig. 2-54d). As assumed for the normal faults associated with Riedel shears an uplift regime in combination with various subhorizontal directions of extension is envisioned. Thus, according to the Anderson theory (fig. 2-51) E-W extension is attributed to

a b

c d

Figure 2-54: Conjugate fracture sets in the study area. (a) Fractures suggesting uplift and E-W extension. n = 8. (b) Fractures suggesting uplift and NE-SW extension. n = 11. (c) Fractures antithetic to those parallel to the metamorphic foliation, mainly suggesting subhorizontal NE-SW compression. Black planes are fractures conjugate to NE dipping foliation planes (red); green planes are fractures conjugate to NNW dipping foliation planes (blue). n = 25. (d) Conjugate fractures suggesting ENE-WSW compresison (black planes) and NNE-SSW compression (red planes). n = 6. Black crosses represent poles to the planes.

the ± N-S striking faults in fig. 2-54a, and NE-SW extension to the ± NW-SE striking ones in fig. 2-54b.

The gently inclined conjugate sets presented in fig. 2-54c constitute a rather interesting phenomenon. Here, sets of fractures cutting across the metamorphic foliation are related to those parallel to the fabric. In these cases the foliation parallel fractures usually dip in northerly directions while the cross fractures are inclined towards SSE to SW. The presence of unconsolidated fault breccia along these sets was frequently observed in the field, although no appreciable displacement could be detected. It is assumed that this type of deformation is the result of a northerly directed subhorizontal compression activating the metamorphic fabric as planes of weakness and creating additional fracture sets at conjugate angles.

Figure 2-54d depicts a very small sample of steeply inclined fractures. In keeping with the Anderson theory these conjugate sets are the results of strike-slip displacement in response to two different compressional stress fields. One (red planes in fig. 2-53d) suggests NNE-SSW compression and WNW-ESE extension, the other (bold black lines) is attributed to ENE-WSW compression and NNW-SSE extension. However, due to the scarcity of datapoints these results will not be credited with much further attention in the tectonic analysis of the area unless other evidence leads to comparable findings.

In conclusion of the discussion of subsidiary fractures and fault geometries it has to be mentioned that the kinematic information they provided often was not entirely clear and thus must be seen as fairly unreliable indicators for the deformation mechanisms in the study area. Thus, the more indicative shear sense indicators such as slickenlines and offset features will be given more weight in case of doubt.