Regional Constraints

4.6.4.1 Comparison of K-Ar Ages

K-Ar illite ages from fault gouges in the Sierra de Comechingones, east of the Sierra de San Luis (Fig. 4.1) show ages in the range of 340 Ma to 170 Ma (Löbens et al. 2011). In comparison, the oldest fault gouge ages in the Sierra de San Luis (SSL) are about 25 Ma younger (315-170 Ma, Table 4.1).

Thus, onset of brittle deformation started slightly earlier in the Sierra de Comechingones than in the SSL. The youngest illite ages, interpreted to document cooling below illite formation temperatures (110°C - 75°C, e.g. Hamilton et al. 1992), are synchronous for both ranges.

82

4.6.4.2 Comparison of (U-Th)-He and AFT Thermochronological Data

Single grain ages, as well as modelled cooling paths, show differential cooling for the Nogolí region (SSL) and the Yacanto area (Sierra de Comechingones; Löbens et al. 2011). On the basis of ZHe data, we conclude that cooling started earlier in SSL than in the Sierra de Comechingones. Over time, cooling of both ranges became subsequently more synchronous, as shown by overlapping AFT, K-Ar illite and AHe ages, although AHe ages also indicate that final cooling below PRZA temperatures occurred earlier in the Nogolí area.

Additional thermochronological data from the Sierra de Ancasti (Fig. 4.1; Sobel and Strecker 2003) in combination with preliminary data from the Sierras Pie de Palo, Aconquija and Pocho (Fig. 4.1;

Löbens et al. in review; Bense et al. in review B) document that the evolution of the Pampean ranges cannot be condensed in a chronologically or structurally uniform evolution, which would have been caused by a single event. Instead, a heterogeneous evolution must be considered to have been caused and affected by multiple events, resulting in distinct variations in cooling and exhumation history from north to south and east to west.

4.7 Conclusions

Based on the K-Ar illite fine-fraction ages and low-temperature thermochronological data presented in this study, as well as on the regional geologic evidence discussed, we draw the following conclusions:

Exhumation of the section of the Sierra de San Luis (SSL) studied here started during Permian times and is synchronous with an orogenic phase called San Rafael Orogeny further south. A direct link between this orogenic phase and the structural evolution of the SSL has not yet been developed. The coeval timing and the relative proximity of the proposed key area of the San Rafael Orogeny (San Rafael Massif, Mendoza province) should incite the idea of further studies to evaluate possible influences of this compressive phase for the San Luis region. In a more general framework, these exhumation processes might also record the tectonic imprint of a flat-slab subduction episode proposed at these latitudes (see e.g. Ramos and Folguera 2009).

These results point out that the lithologies currently exposed at the eastern and western parts of the SSL affected by regional Mesozoic planation processes have followed different exhumation paths.

The eastern part was already in a near-surface position and constituted the substratum of the Bajo de Véliz basin during Late Carboniferous-Permian times, whereas the rocks of the cross section analysed here were still below the PAZA by then.

The main exhumation phase can be bracketed between Middle Permian and Jurassic times. It is characterized by a strong differential exhumation with low exhumation rates in the east and higher rates in the west, which is interpreted to be caused by differential block faulting. The period from

83

Jurassic to Cretaceous times is characterized by further exhumation but also considered to be the time period when most of the remaining erosional surfaces were developed. However, erosional processes had also evolved previously.

The development of a distinct mountainous landscape in the SSL region probably started during Late Devonian to Early Carboniferous times. During the Carboniferous to Early Permian, the positive relief underwent erosion and development of intermountain basins, as evidenced by relictic Gondwanian deposits (e.g. Bajo de Véliz area). Planation processes giving rise to erosional surfaces have not been ruled out, but it is understood that their regional significance was achieved during Mesozoic times.

Cooling below 175 °C started differentially in the transect of the SSL and the Sierras de Comechingones studied but became more synchronous over time. There are arguments that final cooling to surface temperatures in the section of the SSL studied started significantly earlier than in the Sierra de Comechingones.

Cooling rates varied from slow to intermediate over time. The highest rates are observed during the Permian and Triassic periods (2-10°C/Ma). Post-Triassic cooling yields lower rates of 0.5-1-5 °C/km.

Data on the regional high temperature thermal history indicate a cooling propagating from west to east. Our data show that this trend is reversed during further cooling (T<180°C), with cooling propagation from east to west. As a consequence, the Bajo de Véliz region (east) cooled before the Nogolí region (west).

Cenozoic shortening and flattening of the subduction angle of the Nazca Plate due to the collision of the Juan Fernandez Ridge in the Miocene might have contributed to the exhumation of the SSL.

However, no Cenozoic event is evident in the presented data. Even if we cannot solve the post-Cretaceous uplift and exhumation story in full detail with our data, there is no doubt that shortening and range uplift took place after the Paleocene due to Andean shortening, which gives rise to neotectonic movement and uplift in the Sierras Pampeanas.

K-Ar illite fine-fraction dating on fault gouges revealed a long-lasting brittle deformation history for the SSL. Brittle deformation started during Carboniferous times in compliance with the ages of the regional cooling after the Achalian orogenic cycle. The youngest illite ages (ca. 130 Ma) represent cooling below illite formation temperatures but not the cessation of brittle deformation. This is in good accordance with the thermochronological data discussed here.

The oldest K-Ar ages are synchronous to an orogenic event defined as the Chanic Orogeny. If these ages have any connection to the Toco Orogeny, as is suggested for similar K-Ar fine-fraction ages from the San Luis Formation, it has not yet been clarified. However, a connection to the Toco Orogeny seems unrealistic due to the great distance between its key location and the study area.

84

As with the Sierras de Comechingones, the SSL also documents a long-lasting brittle deformation period. The onset of brittle deformation in the SSL started during the Carboniferous and is around 25 Ma younger than the onset of brittle deformation in the Sierra de Comechingones.

For the interpretation of K-Ar ages from fault gouge samples, we developed an interpretation scheme based on a number of constraints for cross-evaluating the data derived from K-Ar illite fine-fraction dating, illite polytype quantification and polytype age extrapolation with data derived from independent dating methods, such as K-Ar biotite and apatite fission track, as well as (U/TH)-He on apatite and zircon (Fig. 4.3). The schema developed combines and highlights concordant data sets derived from different data sources and eases the combination of all observations into a consistent regional evolution model. Presented ages are central ages ± 1 (Galbraith and Laslett 1993); ages were calculated using zeta calibration method (Hurford and Green 1983); glass dosimeter CN-5, and zeta value of S.L. is 323.16 ±10.1 a cm^-2; zeta error was calculated using ZETAMEAN software (Brandon 1996); n, number of dated apatite crystals; s/i, spontaneous/induced track densities (×10⁵ tracks cm^-2); Ns/Ni, number of counted spontaneous/induced tracks; Nd, number of tracks counted on dosimeter; P(²), probability obtaining chi-squared value (²) for n degree of freedom (where n is the number of crystals – 1), MTL, mean track length; SD, standard deviation of track length distribution; N, number of tracks measured; Dpar, etch pit diameter.

Table 4.4: Zircon and apatite (U-Th)/He data of the samples from the Nogolí – Rio Grande profile

Sample He

85

86

87