Geophysical analysis in a Quaternary compressive environment controlling the emplacement of travertine, eastern piedmont of Argentine Precordillera

Highlights

• Quaternary reverse faults affect eastern Precordillera piedmont deposits.

• A gravimetric analysis was performed to define blind neotectonic structures.

• 2D gravity profiles show N–S and NW structures over the Precordillera piedmont.

• Travertine bodies' spatial continuity seem to be controlled by NW structures.

• The understudy area could be a potential seismogenic region.

Abstract

This work carries out the analysis, through a geophysical (gravity) method, of an area in the eastern Piedmont of Eastern Precordillera, Central Andes of Argentina. Geological evidence shows several neotectonic reverse fault scarps affecting Quaternary alluvial deposits located to the East of the Sierra de Villicum. Topographic features observed in the understudy region suggest an important Holocene historical seismic activity, one of them the 15th January 1944 San Juan earthquake (Mw 7.0), the worst natural disaster in Argentina. Several N–S springs and carbonate deposits (travertine) are found aligned along the traces of these active faults, disappearing abruptly to the north and south, possibly due to a tectonic control. Travertine deposits are located along fault traces and have been traditionally considered to be contemporaneous with active faults. The existence of travertine bodies deposited by springs, and their low preservation potential, has for the understudy region a neotectonic significance. The eastern Piedmont of the Sierra de Villicum is characterized by the ~N–S La Laja west-vergent east-dipping reverse fault and several subparallel faults (e.g., Cantera and Museo faults) that uplift Neogene sedimentary rocks over Quaternary (Late Pleistocene-Holocene) alluvial deposits. Gravity data derived from the global Earth gravity model WGM 1.0 were used in the regional geophysical analysis. The analysis aims to delimit geological structures, which characterized the interest area. Three detailed gravimetric profiles were measured to analyze local anomalies related to the main structures. The gravimetric and geological analysis confirms the existence of north-south faults parallel to La Laja fault related to travertine deposits. On the other hand, a set of southeast-northwest structures that limit and control the travertine bodies both northward and southward are recognized. Employing the gravity method associated with structural analysis, the spatial continuity of these neotectonic structures was defined in those sectors where field evidence is not so clear or blind structures are present. Our study confirms the existence of probable cross strike structures trending NW. We suggest they would represent preexisting crustal fabrics reactivated during the Andean orogeny in the Precordillera province, segmenting this orogen-parallel fold and belt system. One of the main hypotheses is that these fault systems related to basement-involved strike-slip faults perform as control structures in fluid migration during the travertine bodies' emplacement. Thus, our work points to the relevance of travertine location in the analysis of the seismotectonic configuration of a region.

Introduction

Several hydrothermal springs characterized by bicarbonate-supersaturated fluids are related to active reverse faults in the eastern piedmont of the Sierra de Villicum, Precordillera of San Juan, Argentina (Fig. 1a and b). These active faults seem to control tectonically the carbonates masses of travertines, which cover an area of ~27 km². It is possible to observe Late Pleistocene-Holocene travertine layers overlapping Neogene deposits. N–S structures control the existence of many rising water points at different topographic positions; at the same time WNW structures bound travertine bodies both north and south, recognizing these kinds of deposits in Precordillera only at the understudy region, (Fig. 1c). Presently, there are several descriptions about the seismogenic potential of the sources located on the study area (Groeber, 1944; Harrington, 1944; Costa et al., 2000; Perucca and Paredes, 2000, 2003; Siame et al., 2002; Meigs et al., 2006; Vergés et al., 2007; Rockwell et al., 2014). However, the close relationship between travertine occurrence and active faults has not been fully explored yet, in spite of Late Quaternary travertines deposited from springs can reveal important information about the neotectonic structures. Astini et al. (2016) pointed out that La Laja travertines were placed in a tectonic setting quite different from classic extensional basins travertines. However, these authors did not explain the cause of their emplacement. We considered La Laja travertine bodies could reflect several aspects of the regional tectonics because of the direct association between carbonate deposits and active faults as conduits along which travertine-depositing waters may rise. According to several studies (e.g. Aydın, 2000; Person et al., 2012), faults are extremely efficient conduits for fluid migration by promoting the circulation and upwelling of hydrothermal solutions triggered by CO₂ pressure fluctuation (Uysal et al., 2009) and seismic activity (Gratier et al., 2002). Furthermore, De Filippis et al. (2013) pointed out that these channels that allow the calcium bicarbonate-rich fluids to ascend toward the surface are the main scientific targets on different geofluids, and hence their outstanding importance.

From a structural point of view, travertines could be emplaced in the hanging wall of normal faults (Brogi and Capezzuoli, 2009; De Filippis et al., 2013), in shear belts (Faccenna, 1994), above fault tips or near their lateral end (Çakır, 1999). However, the largest masses are placed in strain-releasing step-overs and along relay ramps developed between margin-bounding faults (Çakır, 1999; Hancock et al., 1999; Brogi and Capezzuoli, 2009).

Moreover, Hancock et al. (1999) remarked that Late Quaternary travertines deposited from hot springs can reveal much about the neotectonic attributes and histories of structures. Their low preservation potential indicates that where travertines are preserved, they are recent and thus potentially of neotectonic significance.

According to several authors (e.g., Groeber, 1944; Kadinsky-Cade et al., 1985; Smalley et al., 1993; Ramos et al., 1997; Costa et al., 2000; Perucca and Paredes, 2000, 2002; 2003; Siame et al., 2002; Meigs et al., 2006; Vergés et al., 2007; Rockwell et al., 2014), the most active zone of crustal deformation in central-western Argentina is located in the transition area between Eastern Precordillera and Western Sierras Pampeanas provinces. The active faulting, folding and high level of seismic activity support that.

On 15 January 1944, took place the Mw 7.0 San Juan earthquake, considered the worst natural disaster in Argentina (Groeber, 1944; Harrington, 1944; Castellanos, 1945; INPRES, 1977, Alvarado and Beck, 2006; Perucca and Paredes, 2002, 2003, among others) (Fig. 2a). Most of the researchers have located the 1944 earthquake with its epicenter in the study area. Some of them (Harrington, 1944; Groeber, 1944) analyzed the sector a few months after the seismic event and recognized surface deformation. Harrington (1944) described a step of 0.22 m crossing La Laja Street formed during the earthquake and reported a 0.25 m horizontal right-lateral strike-slip displacement on the same segment. He observed that the fault was visible both to the north and south of the La Laja Street (Fig. 2b), either as an open crack or as a darker belt. This author determined a rupture length of at least 7 km and assumed that the fault could have had a longitude of at least twice as visible, hidden by urbanization and crops. About 1000 m farther west of this fault, he described another fault, also reverse and dipping 55° E. Also, Groeber (1944) described a vertical co-seismic displacement of 0.3 m during the earthquake, which crossed the street and extended towards the north and the south. Perucca and Paredes (2003) pointed out that several meters to the north of La Laja Street, the fault scarp height is ~3.50 m, which trends N32°E (Fig. 2c). These authors identified almost 600 m farther west, a subparallel fault trending N42°W, crossing the landing track of the Albardón airfield. The fault shows a 30 cm counter-slope scarp with aligned grasses and shrubs (Fig. 2d).

Siame et al. (2002) considered that the seismic event source fault was located to the east, which ruptured up-dip to the Villicum – Pedernal thrust system, and caused secondary slip on La Laja fault. Alvarado and Beck (2006) estimated the depth of the hypocenter at ~12 km, and the epicenter to the southeast of the La Laja fault trace, and inferred a west-vergence to the source fault.

Bastías (1985), Perucca and Paredes (2000, 2002), Siame et al. (2002, 2006), Alvarado and Beck (2006), among others, suggested three or four east-dipping faults that extend into the basement as the seismogenic sources of the earthquake of 1944. These authors described reverse faults trending N30-40° E, generally in coincidence with the Neogene layers that dip near the surface between 30 and 45° E and affecting at least three Quaternary alluvial levels and Late Pleistocene to Holocene travertine deposits. The length of the different sections varies between 6 and 8 km (Fig. 1b).

According to Costa et al. (1999) and Meigs et al. (2006), La Laja fault is a secondary flexural-slip fault and even though a local flexural slip faulting with minor vertical offset is registered, the range front thrust system does not cut surficial deposits.

Besides, Meigs et al. (2006) described in the eastern piedmont of the Sierra de Villicum, Neogene deposits forming a southeast-facing and southwest-plunging monocline. These authors considered that co- and post-seismic surface rupture occurred along roughly 6 km of the La Laja fault in 1944 in the limb of this monocline. Evidence shows that surface deformation in the 1944 earthquake was dominated by folding including a terraces fold geometry, which is consistent with kink-band models for fold growth and bedding-fault relationships that indicate that the La Laja fault is a flexural slip fault.

Schiffman (2007) recognized a suite of progressively abandoned fluvial strath terraces recording Pleistocene to Holocene deformation, while the underlying bedrock of Neogene and Paleozoic deposits thrust sheets recorded earlier deformational events. The combination of these different structural levels in space and time shows how deformation has progressed from thin-skinned back-thrusting associated with the development of a foreland fold and thrust belt, to block uplift and fold-growth associated with basement faults uplifting and deforming previously folded and faulted strata.

Ragona (2007) remarked that the local structure was poorly known in 1944, and so, surficial ruptures were only identified where they cut a road, leaving open the possibility that other similar ruptures occurred during the earthquake.

Vergés et al. (2007) and Meigs and Nabelek (2010) interpreted for the Eastern Precordillera, a two-layer system of faults with thin-skinned thrusts and west-vergence at depths shallower than ~5 km. These authors suggested that below 5 km, a system of seismically active planar reverse faults extends through the crust. Meigs and Nabelek (2010) placed the earthquake at a depth of ~25 km, with the epicenter located to the west of La Laja fault. They considered that the east-vergent rupture extended up-dip to ~8 km depth. They concluded that, whereas thin-skinned thrust sheets master the shallow-crustal structure, seismological and geological data show planar reverse faults and pure-shear deformation -involving more than 75% of the crust-that characterize this thick-skinned structural province.

Rockwell et al. (2014) examined the earthquake chronology from La Laja fault, which ruptured with the 1944 San Juan earthquake, to understand the nature of active blind thrust faults. These authors considered La Laja fault is a secondary fault that is likely either a backthrust or a fold-related flexural-slip fault. The fault moves in association with the earthquake on a primary, mainly, or completely blind reverse fault.

One of the essential questions addressed in this paper concerns the relationship of travertine emplacement with active faulting. Intending to contribute to a better understanding of the origin of the travertine and its relationship with active faults, we made a detailed geophysical and morphostructural approach to provide a structural model of formation. Our fieldwork has focused between the Salado River to the north and the Loma de Las Tierritas to the south and east (Fig. 1b).