Gouge-Bearing Faults Systems

Along sampled locations (Figs. 6.1-6.3) the characteristics and orientation of gouge bearing faults, fault surfaces and fault zones were examined. Due to outcrop conditions, most of the observations were restricted to road ravines. Approximately 1200 faults were examined.

Almost all observed faults show a well-developed clay gouge on the fault surface. Some gouges show small lenses of residual rock material within a clay matrix, consisting of either polymineralic rock-fragments or monomineralic grains (Fig. 6.4). Polymineralic rock-fragments show diameters of several millimetres to a few centimetres, consisting of host-rock material. The monomineralic grains are only a few milimeters in diameter and consist almost exclusively of quartz. Gouge thicknesses vary from several millimetres up to several decimetres, but mostly are between 1-5 cm. Most gouges have monochrome brownish, reddish, white or ochre-yellow colours but multi-coloured gouges with colours alternating in a centimetre-scale are also observed (Fig. 6.4).

The transition from the host-rock to the fault gouge is developed in several forms. In a mesoscopic scale some locations show a gradual transition from the undeformed host-rock to a damage zone (partially ultracataclasitic) and fault-gouge zone (see Chester and Logan 1986). In other locations the fault gouge is bounded directly by the undeformed host-rock (Fig. 6.4).

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Fig. 6.2: Simplified map of sample locations: a) Potrero de Los Funes; b) Nogolí, c) Merlo, d) Los Tuneles. RP = Ruta Provincial; RN = Ruta National; red triangles = fault-gouge sample location; orange lines = faults.

In the study area, kinematic indicators on fault surfaces are mainly developed in form of slickensides.

Most of them are built up by fibrous crystallites, consisting of quartz, calcite, chlorite as well as iron-oxides and iron-hydriron-oxides. Mineral lineation and slickenlines are generally well developed but almost exclusively lacking fault kinematic indictors (e.g. Doblas 1998) constraining the sense of movement along the lineation. Thus, collected data give only a bivalent sense of movement. Cross-cutting relationships of slickensides elucidating the temporal succession of slickenside development are only merely developed, prohibiting the decomposition of orientations in terms of a timely variation of movement directions.

Fig. 6.3: Simplified map of sample locations; a) Quines, b) Achala, c) Sierra de Los Gigantes. RP = Ruta Provincial; RN = Ruta National; red triangles = fault-gouge sample location; orange lines = faults.

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Fig. 6.4: Examples of gouge bearing faults zones from the study area; a-c) Nogolí (Sierra de San Luis); d) Los Tuneles (Sierra de Pocho); e) Merlo (Sierra de Comechingones). Left picture = overview, middle picture = detail of fault-gouge, right picture

= sketch of fault-zone setting.

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Fig. 6.5: Fault plane orientation data; Nogolí transect (a-d), Potrero de Los Funes (e-h) and compilation of all data from the Sierra de San Luis range (i). Sierra de Comechingones transects; Los Tuneles (j-l) and Merlo (m), compilation of all fault orientation data from the Comechingones ranges (q), additional locations; Achala (n), Los Gigantes (o), Quines (p).

Contour interval show multiples (n= 1 ,2 ,3…) of normal distribution. Plots drawn with OpenStereo.

Nogolí

A total of 542 fault plane orientations (Fig. 6.5 a-d) were measured along the transect from Nogolí to Rio Grande (Fig. 6.2a). Based on the fault plane orientations, the data set can be divided into five domains: 1) steep N or S dipping fault planes with mean fault planes dipping 165/85 and 003/79, respectively, 2) steep NW or SE dipping fault planes with mean planes dipping 306/79 and 137/84, 3) intermediate-to-steep ESE-WSW dipping with mean poles of 106/75 and 280/84, 4) steep SW dipping with a mean plane dipping 216/73 and 5) flat-to-intermediate ENE to dipping with a mean plane at 066/33.

Generally, the orientations of the fault planes show significant variation from the western to the eastern side of the transect. The western profile is dominated by steep SSE as well as SE dipping systems; associated NNW and NW dipping systems are only weakly developed (Fig. 6.5 a). Dip shows

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mainly steep, partially intermediate dip angles. In the central part of the profile (Fig. 6.5 b), the predominant SE dipping orientation of the faults change to more complex patterns with occasionally strong populations of steeply SW dipping planes, as well as an overall flattening of dip angles towards the easternmost parts in the area of the Rio Grande dam. Additionally, the distinct east-west striking fault system can be observed in the middle (La Escalerilla batholith) and the easternmost part (Fig. 6.5 c).

Potrero de Los Funes

A total of 338 fault planes were measured along the Potrero de Los Funes transect (Fig. 6.2b). Based on the orientation, four populations can be recognized: 1) steep N or S dipping with mean orientations of 004/86 and 181/86, 2) intermediate NNE dipping fault planes with a mean orientation of 066/59, 3) intermediate-to-steep SSE dipping population with a mean direction of 165/57 and 4) intermediate to steep NNW dipping population with a mean orientation of 026/70.

Some variation can be identified along the transect. The N and S dipping fault planes are present throughout the entire transect, except for the easternmost part (Fig. 6.5 e-h). The intermediate-to-steep SSE dipping population is not so dominant in the easternmost transect either. Observed dip angles in the Potrero de Los Funes region are almost exclusively intermediate to steep, and only occasional flat-lying faults were observed (Fig. 6.5 e-h). Interestingly, the strong NW-SE and ESE-WSW dipping systems in the Nogolí system are not developed in the Potrero area.

Los Tuneles A total of 139 faults were measured along the Los Tuneles transect (Fig. 6.2c). Fault plane orientation can be grouped into two major and several minor domains. NE dipping is predominant (Fig. 6.5 j-l): 1) intermediate ENE dipping faults, showing some scattering around a mean orientation of 065/30, 2) steep E dipping with a mean orientation of 088/70, 3) steep WSW dipping with a mean orientation of 249/72, 4) intermediate SE to ESE with a mean orientation of 126/53, 5) intermediate NNE dipping with a mean orientation of 017/50 and 6) intermediate NNE dipping with a mean orientation of 009/30.

From west to east, minor variations in the fault plane orientations are observed (Fig. 6.5 j-l). In the western section, intermediate SE and NNE populations can be found (Fig. 6.5 j), whereas the steep E dipping population is only observed in the eastern parts of the transect (Fig. 6.5 k). The dominant E-ENE dipping populations are found through the entire transect (Fig. 6.5 j-l).

Merlo

A total of 134 fault orientations were measured along the Merlo transect (Fig. 6.2d) with dip directions trending from ESE to NNE (Fig. 6.5m). Orientation data can be divided into two groups comprising intermediate E-ENE and steep ESE dipping faults. Mean orientations for the populations are 113/27 for the ESE-dipping population and 086/35 for the ENE-to-E-dipping population. No significant variation in fault orientation along the profile was observed.

112 Other Locations

Due to restricted outcrop conditions, only a minor amount of data (n≤ 20) could be collected in the area of the Sierra de Los Gigantes, San Martin and Achala (Fig. 6.1 and 6.3). Although the amount of measurements is not statistically significant, data fit with trends observed in the other transects. The Achala region shows a general north-south striking orientation (Fig. 6.5 n), as observed in the other Comechingones regions (Fig. 6.5 j-m). In the San Martin area, a general east-west striking is present (Fig. 6.5p), as is observed in the rest of the Sierra de San Luis (Fig. 6.5 i). Faults observed near La Calera in the northern part of the Sierra de Los Gigantes also show a general east-west striking trend with gentle-to-intermediate northward dipping (Fig. 6.5 o).

6.4.2 Fault Gouge Analyses

The following chapter discusses the results of fault-gouge analyses from 31 samples (Fig. 6.2+3;

Table 6.1) with three grain-size fractions each. Data for 21 samples are presented here for the first time, while data from 10 samples are reviewed from previous publications by Löbens et al. (2011) and Bense et al. (in review A). To give the reader a complete overview as well as to ease the traceability of the following discussion, data from both publications are integrated here (Fig. 6.2+3;

Table 6.1). Data published by Löbens et al. (2011) and Bense et al. (in review A) comprise K-Ar age, mineralogy, illite crystallinity and illite polytypism from six samples from the Merlo area (APG 82-09, 85-09, 89-09, 90-09, 91-09 and 92-09; Löbens et al. 2011) and four samples from the Nogolí area (APG 50-09, 51-09, 59-09 and 60-09; Bense et al. in review A). In addition to the data by Löbens et al.

(2011), detailed polytype quantification as well as I/S quantification for the Merlo samples is presented in this study. Data from Bense et al. (in review A) are completed by I/S quantification presented in this study.

Fig. 6.6: Summarised orientation data; rose diagrams showing strike direction. Histograms show dip angle. Plots were drawn with OpenStereo.

113 Mineralogy

Mineralogical analyses by XRD and TEM (Figs. 6.7 and 6.8), and checked by IR, CEC and DTA. Data show illite and smectite as most abundant clay mineral phases in all samples (Table 6.1, Fig. 6.7 and 6.8). Furthermore chlorite and kaolinite are frequent. Occasionally traces of halloysite (Fig. 6.8) are identified by TEM, XRD and IR analyses. In some 2-6 µm fractions, XRD observations reveal traces of quartz, K-feldspar and plagioclase, while smaller fractions (<2 µm) sporadically contain traces of quartz but are almost exclusively free of K-feldspar and plagioclase. The smallest grain-size fractions (<0.2 µm) consist of clay mineral mixtures with variable amounts of illite, smectite, chlorite and kaolinite. In some cases traces of quartz were found, but no feldspar was detected. Detection limits to quantify the amount of quartz, K-feldspar and plagioclase by XRD are around 5 %. Assuming a moderate K2O in K-feldspar of 12%, non-detected feldspar can contribute to the K2O content of illite by approx. 0.6%, introducing error to the age determination (see above). However, TEM observations show no trace of feldspar contamination of fractions which are indicated feldspar-free by XRD analyses. In addition TEM show no contamination of fractions with particles greater than the respective grain-size fraction limits, confirming an accurate grain-size separation technique.

Fig. 6.7: XRD pattern derived from randomly oriented samples with indicated positions of 2M1 and 1M polytype specific peaks. Other phases are indicated as follows: illite (ill), Illite–Smectite (i–s), quartz (qtz), kaolinite (kaol) K-feldspar (kfs) and hematite (hem).

Grain-size fractions bearing illite and smectite show variable amounts of irregular I/S mixed layers.

The occurrence of I/S mixed layers is mainly restricted to the fraction <2 µm and <0.2 µm. The percentage of illite in illite-smectite mixed layers range from <10 % to around 50 %, giving R0 (i.e.

unordered; e.g. Moore and Reynolds 1997) illite(0.1)/smectite to R0 illite(0.5)/smectite minerals. In addition to the illite in I/S mixed layers, almost all samples contain discrete illite (Table 6.1). No other mixed layer clays were found.

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CEC values ranging from 8 meq/100g to 80 meq/100g indicate smectite contents of up to 35 % in the 2-6 µm fraction and between 32 % and 84% in the smaller grain-size fractions, assuming normal layer charge density. Combined observations by CEC, DTA and XRF reveal the presence of several smectite member e.g. trans-vacant low-Fe montmorillonite, cis-vacant smectite as well as Fe-rich smectite.

Illite Crystallinity

The illite crystallinity (IC) expressed by the Kübler indices (KI values), range from 0.157 Δ°2ϴ to 0.920 Δ°2ϴ. Extreme values of 1.288 Δ°2ϴ and 1.433 Δ°2ϴ were measured (Table 6.1).The KI-values obtained represent diagenetic and anchizonal, as well as epizonal conditions. In general, KI-values increase with decreasing grain-size, reflecting mostly diagenetic conditions for the <0.2 µm fraction as well as the <2 µm fraction. Anchizonal conditions are mostly represented by KI-values from the

<2 µm fraction but have also been obtained for the 2-6 µm and <0.2 µm fractions. Epizonal conditions are mostly represented by KI-values from the 2-6 µm fractions but can be occasionally found in the other fractions.

Illite Polytypism

All 78 analysed powder samples show a low (002)/(020) peak ratio, documenting randomness in clay mineral orientation is assured. All analysed samples contain a mixture of polytypes with variable proportions in the respective grain-size fractions (Table 6.1). The dominating polytype (polytype content of >50 %) vary between fractions and correlates with grain-size, showing decreasing 2M1

content (increasing 1Md content) with decreasing grain-size. The 2-6 µm fraction is mostly dominated by 2M1 illite (20 of 26 fractions) only a small number of samples show predominance of the 1Md

polytype (Table 6.1,). The majority of the <2 µm fraction is controlled by 1Md illite (24 of 27 fractions) only very few samples show 2M1 content of >50 % in this grain-size fraction. The <0.2 µm fraction is exclusively dominated by the 1Md polytype (25 fractions). In general, the <2 µm fraction represents the most variable fraction analysed showing the highest variability in percental polytype composition, whereas the <0.2 µm fraction is the most invariable fraction in terms of polytype composition. The 2M1 and 1Md polytypes can be found in almost all analysed fractions but there occurrence show opposites trends in relation to grain size. 2M1 illite is lesser frequent in the smaller fractions, the 1Md polytype lesser frequent in the coarse grain-size fractions. The 1M polytype is less frequent and mainly linked to the fraction 2-6 µm and <2 µm (Table 6.1).

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Fig. 6.8: TEM images of samples APG 92-09 and 59-09, showing common clay mineral phases as well as accessorial minerals found in the clay fractions of fault-gouge samples.

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Fig. 6.9: Distribution of illite polytypes 2M1, 1M and 1Md in analyses samples and grain-size fractions.

K-Ar Age

A total of 88 K-Ar ages from 31 samples were obtained (Table 6.1, Fig. 6.9). Ages range from 382.1 Ma (Late Devonian) to 106.5 Ma (Middle Cretaceous). In general, ages increase with increasing grain-size fraction, whereby ages from fractions range between 297.6-106.5 Ma (<0.2 µm), 336.3-127.8 Ma (<2 µm) and 382.1 -193.7 Ma (2-6 µm; Fig. 6.9).

High content in radiogenic ⁴⁰Ar, ranging from 80 % to 99.6 %, confirm reliable analytical conditions for almost all analyses. Only 4 samples show ⁴⁰Ar contents between 60 % and 80 % (Table 6.1).

Potassium (K2O) content is less than 1 % for one samples, but ranges from 1.03 % to 8.41 % for the others (Table 6.1).

Considering the regional extend of the sampled fault gouges, significant differences in age can be observed. Generally, the samples from Nogolí, Potrero de los Funes and San Martin show younger ages, while the Achala, Los Tuneles and El Gigante and Merlo area reveal older ages.

Based on the age and calculated polytype compositions of the different grain-size fraction, we extrapolated the ‘end-member’ age of the 1Md polytype and the 2M1 polytype by plotting the age of each individual grain-size fraction of a fault gouge sample against the 2M1 illite content and linear extrapolation (see also Grathoff and Moore 1999; Grafhoff et al. 2001). Extrapolated ages represent hypothetical samples consisting of 0% 2M1 illite and 100 % 2M1 illite (Table 6.1) respectively.

Extrapolations show a coefficient of determination (R²) better than 0.9 for most of the samples. Six samples show an R² below 0.7. These values correspond to samples showing different attributes complicating the age extrapolation: (1) insignificant differences in age and/or (2) insignificant differences in 2M1 polytype content between two fractions and/or (3) an increase in grain-size which is not correlated with an increase in age (see Discussion; Table 6.1). However, in most cases, the high values for R² confirm a clear relationship between age and 2M1 polytype content (Table 6.1). A

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discussion of assumptions under which these calculation can give geological meaningful ages is given below.

6.5 Discussion