Evolution Model

The following regional evolution model for the Sierra de San Luis (SSL) can be proposed. The topographic imprint of the SSL as a positive morpho-structural unit started during the Late Devonian to Early Carboniferous (Fig. 4.12a), as suggested by the onset of brittle deformation in the region of present-day SSL. Brittle deformation followed up the ductile to semi-brittle deformation associated with the Achalian orogenic cycle, which was mainly accommodated along some major shear zones (Rio Guzman, La Arenilla). Especially in the eastern part of SSL, it is likely that a mountainous landscape was developed during this time, after the cessation of magmatic activity.

77

During the Carboniferous to the Early Permian (Fig. 4.12b), a mountainous landscape developed, associated with the down-wearing of the Early Carboniferous surface (indicated by PTES in Fig. 4.12).

Between mountain ranges, several intermountain basins developed, some of which trapped and preserved sedimentary records of this time period (as in the outcrops of Bajo de Véliz, Agua Dorada, La Estanzuela and Suco region; Hünicken and Penza; 1980; Hünicken et al. 1981; Limarino and Spalletti 2006). However, the extent and dimensions as well as possible interconnections of these basins are unknown. In the Bajo de Véliz area, a fluvial-lacustrine environment with typical Gondwanian flora and fauna was preserved in a fault-bounded depression, which is considered to represent the south-easternmost extent of the Paganzo basin (which was widely developed in NW Argentina; e.g. Salfity and Gorustovich 1983; Limarino and Spalletti 2006). A similar situation with a Carboniferous landscape that is carved into older rocks and covered or filled by fluvial sediments can be found in the Chepes range (La Rioja province; pers. comment C. Carignano).

In addition to sedimentation and erosion, brittle deformation continued on within the entire region, as evidenced by K-Ar fault gouge data. Thermochronological data give no clear evidence of whether exhumation of the sampled area above the PRZZ temperatures took place during this time interval. , but it is more likely that this happened during Permian times. It can be concluded that a mountainous landscape had prevailed by then in this region, suggesting that the main planation processes which led to the present-day erosional surfaces had not yet started.

Data suggest that the middle Permian to Jurassic epoch (Fig. 4.12c) is the main exhumation and regional planation phase that affected the sampled area, maybe accompanied by Early Cretaceous processes. In addition, the data give indication of a strong, east-west directed differential exhumation history in the SSL. During Permian times, the eastern part was exhumed to a surface or a near-surface position and interacted with surficial processes, as evidenced by the conservation of Gondwanian sediments in the Bajo de Véliz region. In contrast, the western part (sampled locations) were at least below the PAZA depths and, thus, still at several kilometres in depth, as shown by our data.

We interpret this differential exhumation to be caused by differential block faulting with significant vertical components along different fault zones throughout the SSL. The latter is constrained by long-lasting fault activity, as it is traced by our K-Ar illite ages from fault gouges (Table 4.1).

Our thermochronological data do not explicitly constrain g the differential exhumation procedure.

However, our data clearly document the onset of exhumation during the Permian. In addition, it gives indication that PAZA temperatures were reached by the Latest Permian / Early Triassic (Fig. 4.11). This event comprises the main exhumation concerning the difference in depth from the PRZZ to the PAZA. Depending on the geothermal gradient, the latter may range from 1 km to 3km.

78

the ages obtained are highly synchronous to several compressional events in the region , although an explicitly association with a regional orogenic event is not definitely clear. Old K-Ar ages are synchronous to the latest Chanic orogenic phase of the Famatinian Orogen (defined e.g. Martino 2003) as well as to the Achalian Orogeny (Miller and Söllner 2005), as well as to the Toco Orogeny (Bahlburg and Breitkreuz 1991) in the NW-Argentina and N-Chile region. The Permian exhumation event is synchronous with the San Rafael Orogeny (e.g. Kleimann and Japas 2009) in the San Rafael region. However, until now, no link of the structural inventory in the Sierras de San Luis or the San Rafael Orogeny has been made.

Final exhumation to the surface level is less constrained by the data. However, thermal modelling (Fig. 4.11, Tables 4.4 and 4.5) indicates that exhumation above PAZA temperatures occurred in post-Permian times.

Concerning the time of development of planation surfaces, the Triassic epoch is not well suited for their regional development due to crustal rifting west of the SSL and the development of fault-bounded ranges and related half-graben basins. These processes have been well documented in southwestern San Luis and Mendoza province (Criado Roque et al. 1981; Ramos and Kay 1991;

Koslowski et al. 1993; Dellapé and Hegedus 1995). Evidence of these processes crop out at the Sierra de Varela area, 100 km to the south of the SSL (Costa et al. 1998). It is assumed that this prevailing tectonic unrest with a continuous base-level fall of the fluvial system did not favour the development of planation surfaces with regional significance. In addition, a Triassic emergence of the sampled area is not evidenced by AHe ages, which yield predominantly Jurassic and Cretaceous ages.

During Early Cretaceous times, rifting took place east and west of SSL. Rifting had probably already started in Late Jurassic times, as indicated by 150-160 Ma and 106 Ma old Ar-Ar ages of basalts in the Las Quijadas range, west of the SSL (González and Toselli 1973; Yrigoyen 1975). In addition to the Triassic epoch (see above), the time span from the Late Jurassic to Early Cretaceous is also less suited to development of regional planation surfaces because of the prevailing crustal tectonic activity.

79

Fig. 4.12: Exhumation model for the Sierra de San Luis. See text for details (BV = Bajo de Véliz; PTES = Previous Topo-graphic Eroded Surface; SSL = Sierra de San Luis; CD = Conlara Depression; CR = Comechingones range; SCR = Sierra Chica Range; white arrow indicating area affected by uplift or subsidence). For further details on the Sierra de Comechingones see Löbens et al. 2011.

80

It can be concluded that the final exhumation of the sample region, as well as regional planation processes, took place either during the Early-Middle Jurassic and/or during the Late Cretaceous. This idea is supported by the absence of Jurassic deposits in the Sierras Pampeanas, while Jurassic basins did develop in surrounding regions (e.g. the Neuquén basin, Howell et al. 2005). Thus, we assume that the Pampeanas region, or at least the southeastern Sierra Pampeanas, were had positive topography affected by erosion during that time.

AHe ages indicate that both time intervals contributed to the exhumation of the sampled area.

Samples from higher elevations show predominantly Jurassic ages and were probably also exhumed to surface conditions during this time. The lowermost sample, which represents the western footslope area of the SSL, passed through PRZA conditions in Middle Cretaceous times (AHe mean age 105 Ma, see Table 4.5). Whether or not emergence happened during this time cannot be concluded from our data. Further constraints may come from several basalts which were emplaced at higher altitudes in the SSL during the Cretaceous (Fig. 4.12d). Remnants of those basalts yield ages around 84 Ma (Pampa de La Invernada basalts; Solá 1981). If these basalts were emplaced atop the erosional surface, the emergence and formation of the erosional surface could be bracketed to be older than 84 Ma. This age is in contrast to AHe single grain ages of the lowermost sample from the western footslope (APM 49-08), which show a minimum age for passage through the PRZA of 72 Ma (Table 4.5), coinciding with a depth of about 2.4 km (considering a geothermal gradient of 25°C/km).

This favours the idea of diachronous development of the erosional surfaces in this part of the SSL, with an older surface on top of the range (>84 Ma) and a younger erosional surface (<72 Ma) in the footslope area of the westernmost SSL. In any case, the effusive character of the dated basalts is unclear because no robust field evidence for effusion above erosional surface exists. Basaltic remnants might also correspond to a sill or dyke which has no relationship to the erosional surface and, thus, cannot be used as a marker for emergence beyond doubt.

The geomorphologic situation in the Comechingones range (Fig. 4.12d) is also enigmatic and is still a matter of ongoing debate (e.g. Löbens et al. 2011) – especially in terms of whether the region represents a positive morphostructural unit that acted as a sedimentary source for Cretaceous basins to the east or, in contrast, was affected by subsidence and Cretaceous sedimentation itself. Findings of large blocks composed of red conglomerates along some creeks (Morla, pers. comm.), favour the existence of a sedimentary cover overlying the present Comechingones ranges. Similar findings are documented in the nearby Sierra Chica (Gordillo and Lencinas 1979; Bonalumi 2005).

Cenozoic events (Fig. 4.12e) are not evident in the data presented here. Thus, any Cenozoic contribution to the exhumation of the SSL can be concluded to be of minor magnitude when considering that the minimum total exhumation along a transect through the SSL equals the topographic difference between topmost and lowermost parts. This value must be smaller than the

81

depth of the Paleo-PRZ (2.4 Km for a gradient of 25°C/km). If the total exhumation since the record of the youngest age (Cretaceous) exceeds this difference, the samples from the footslope area of the SSL must yield younger, reset ages. Additionally, this would be represented by a break in slope in the age-elevation plot (see Gleadow and Fitzgerald 1987).

The evaluation of any exhumation postdating the youngest AHe ages depends on the interpretation of the 84 Ma basalts. If the basalts represent surficial lava flows above a paleolandsurface, exhumation had already finished by this time. In this case, final exhumation of the present day top regions of the SSL to the surface happened between the Jurassic and Upper Cretaceous. If, in contrast, the basalts represent dykes with no relationship to a paleosurface, no further constraints on the final exhumation can be made. Regardless, even in the latter case, the characteristic volcanic texture of the basalts can hint to a persisting exhumation between the passage of the samples through the PRZA and the emplacement of the basalts. Considering an AHe age for the region of the Pampa de La Invernada between the age of nearest samples APM 3308 (188 Ma) and 3408 (142 Ma), the age of the basalts (84 Ma), the maximum depth for the development of volcanic texture (1-1.5 km), and the depth of the PRZA (2.4 km with a geothermal gradient of 25°C/km), exhumation would be about 1 km between PRZA-passage, and basalt emplacement would be about 60 Ma later.

Even though we cannot solve the final, post-Cretaceous exhumation and uplift history (Fig. 4.12e), there is no doubt that range uplift occurred after the Paleocene due to Andean shortening, giving an imprint to the present day landscape (Costa 1992; Ramos et al. 2002). For instance, Piedmont landform association as well as morphotectonic parameters suggest that the SSL is currently characterized by moderate to low Quaternary uplift rates (< 0.1 mm/a, according to Costa 1992).

However, a significant change in the regional base level fall occurred from 6-4 Ma ago to present times. This is suggested by the behaviour of long-term neotectonic indicators and by the occurrence of Quaternary-active faulting on the main range bounding fault (Costa 1992; Costa et al. 1999; 2001).