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Morphostructural evolution of the Medeiros hills, Lermavalley, Cordillera Oriental, Northwestern Argentina

Vctor Hugo Garca1,2

*, Fernando Hongn2, Sergio Nico ls Gatica

1, Daro Rubn Vera

1, Martn Nazareno Parada

1

1Instituto de Investigacin en Paleobiologa y Geologa, Universidad Nacional de Ro Negro, Isidro Lobo y Belgrano (8332)

General Roca, Rio Negro, Argentina2Instituto de Bio y Geociencias del NOA (CONICET-UNSa), Mendoza 2 (4400) Salta, Argentina

*Contact email: victorg76@gmail.com

Abs tract . The Lerma valley is an intermontane tectonicdepression located in the Cordillera Oriental of NW

Argentina. Basement-cored thrust sheets are flanking thevalley and controlling its N-S elongated shape. The uplift ofthese basement blocks took place during late Miocene toPlio-Pleistocene, synchronously with the deposition offluvial-alluvial sequences. The Medeiros hills are located atthe northern extreme of this valley and represent thegeomorphic expression of the southward propagation of theVaqueros anticline. In these hills, middle to upperPleistocene fluvial conglomerates are uplifted more than150 meters with respect to its stratigraphic position in theundeformed basin. The pattern of six strath terracessculpted in the northern sector of the hills has beeninterpreted as evidences of the forced migration northwardof the Vaqueros river during recent uplifting events. Themain objectives of this research are to describe thestructure of the hills and interpret its tectonic evolutionalong the Quaternary using new structural, geomorphologicand topographic data.

Keywords: Neotectonics, deformed terraces, tectonicgeomorphology

1 Introduction

The Lerma valley is an intermontane tectonic depressionlocated between 2430` and 2535` SL into the CordilleraOriental geological province (Figure 1). N-S trendingbasement-cored thrust sheets are flanking the valley andcontrolling its elongated shape. According Monaldi et al.(1996), the uplift of these basement blocks took placeduring late Miocene to Plio-Pleistocene, synchronouslywith the deposition of fluvial-alluvial sediments in thedepression.The northernmost part of the Lerma valley is surrounded tothe east by the Mojotoro range, to the north by theVaqueros range and to the west by the Lesser range (Figure2). In this sector the Medeiros hills are the geomorphicexpression of the southward propagation of the Vaquerosanticline (Baudino, 1996; Garca, 2010). At these hills,middle to upper Pleistocene fluvial conglomerates areuplifted at least 150 meters above its stratigraphic positionin the undeformed valley (Baudino, 1996; GonzlezBonorino et al., 2003).The pattern of strath terraces (T1 to T6, from oldest toyoungest) sculpted in the northern sector of the hills(Figure 3a) indicates forced migration northward of the

Vaqueros river during recent uplifting events (Georgieffand Gonzlez Bonorino, 2005; Gonzlez Bonorino andAbascal, 2008; Garca, 2010, 2011). New structural,geomorphologic and topographic data have been collectedand analysed in order to describe the structure of the hillsand interpret its tectonic evolution along the Quaternary.

Figure 1. Location of the Lerma valley (red box) in the contextof the geological provinces of NW Argentina.

2 Geological setting

The neotectonic basement of the Lerma valley in theMedeiros sector is composed by deformedmetasedimentary rocks of Proterozoic age (PuncoviscanaFormation); Cambro-Ordovician marine sandstones andmudstones (Mesn Group and Santa Victoria Group); andMio-Pliocene continental red beds (Orn Group). The

Quaternary infill of the valley in this region begins withreddish conglomerates and siltstones of the uppermost partof the Orn Group (Piquete Formation), unconformablecovered by fluvial conglomerates and minor siltstones ofthe Calvimonte Formation (0,3 +/- 0,1 Ma, Malamudet al.,1996) and recent fluvial deposits of as much as 10 metersof thickness (Figure 2).At this latitude, the Cordillera Oriental is a thick-skinnedfold-and-thrust belt (FTB), involving basement rocks in thedeformation (e.g. Monaldi et al., 1996; Cristallini et al.,1997; Carrera and Muoz, 2008; Hongn et al., 2010;

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Carrapa et al., 2011). The dcollement level of the FTB islocated at depths between 15 and 25 km, dipping gently tothe west. Above the dcollement, both thrusts andbackthrusts compose the FTB. In the study area a mainthrust is uplifting the Mojotoro range to the east, while aseries of subparallel backthrusts uplift the Quijano andLesser ranges to the west. The Caldera and Vaquerosanticlines are depicted by Piquete Formation indicating a

shallower dcollement level probably located at the contactbetween Tertiary and Paleozoic units (Figure 2).

Figure 2. Geological and neotectonic map of the northern partof the Lerma valley (after Garca et al., 2011). The red boxindicates the studied area.

3 Structure and geomorphology of Medeiros

hills

3.1 Structural observations

Along some creeks at Medeiros hills, the unconformablerelationship between Calvimonte Formation and PiqueteFormation has been stablished. The conglomerates ofCalvimonte Formation are subhorizontal over the PiqueteFormation that is dipping around 60/090 at its eastern sideand between 30 to 10/270 progressively decreasing to itswestern side. These orientations define an assymetricalfault propagation anticline with eastward vergencedeveloped over a blind thrust dipping 30 to the west. Thedip decreasing in the backlimb are interpreted as

progressive unconformities controlled by backlimb trishearmechanisms (Cristallini and Allmendinger, 2002).Many minor faults were recognized. At the frontal flank aninterstratal backthrust developed in siltstones of PiqueteFormation is folding the overlying Quaternaryconglomerates. This fault has N-S trend and could be theresponsible for the topographic high located at the easternflank of the hills. In the axial zone of the anticline there aresome N-S normal faults developed in Piquete Formationwith evidences of syntectonic sedimentation, while anoblique backthrust dipping 45/125 is repeating Piquete

Formation and displacing the T1 terrace.From balanced cross-sections constructed for the Medeiroshills the total shortening and depth of the dcollement havebeen calculated. The total shortening measured for themiddle section of Piquete Formation is around 1 km andthe depth of dcollement is located at 1.35 km b.s.l.(around 2.7 km below the topographic surface).

3.2 Geomorphic markers

Topographic profiles of six terraces and the Vaqueros riverwere measured using the SRTM digital elevation model of30 meters of spatial resolution (Figure 3a). The profileswere re-projected over an E-W line, perpendicular to theaxis of the anticline in order to evaluate if the terraces arefolded. The Vaqueros river has a convex down profile,showing that erosion/sedimentation processes aredominating. The terraces profiles, instead, show convex upprofiles indicating that were affected by folding processes(Figure 3b).The altitude difference between the terraces and the

Vaqueros river was calculated to quantify the amount ofdeformation accumulated by each one (Figure 3c). Theprofiles of the terraces show an irregular design in frontallimb reflecting erosional degradation. The amplitude of thefolding is maximum for T1 (80 meters) and minimum forT6 (10 meters). The scarp developed toward the east of T1and T2 profiles is related with the oblique backthrust thatwas described above. The easternmost part of T3, T4 andT5 profiles shows the topographic high associated withinterstratal slip in the frontal limb. The small profile breakobserved to the west of T2 and T3 profiles could bereflecting reactivation of a blind thrust with east vergenceat the fold backlimb, similar to the documented at the

northern part of Vaqueros anticline.The design of the folded terraces can be modeled using thesame backlimb trishear algorithm applied to explain theprogressive unconformities in the Piquete Formation.

4 Discussion and conclusions

a. Supposing that the Medeiros anticline growing began atthe Pliocene-Pleistocene limit (ca. 2.6 Ma) the meanshortening rate of the structure is around 0.39 mm/yr.b. Synsedimentary normal faults recognized at the foldaxis and progressive unconformities interpreted at the

backlimb indicate growing of the anticline duringdeposition of Piquete Formation (Upper Pliocene to LowerPleistocene).b. Folding and faulting of T1, T2, T3 and T4 terraces areshowing tectonics activity during Middle to UpperPleistocene (Calvimonte Formation).c. Folding of T5 and T6 terraces could be interpreted as theresult of Upper Pleistocene to Holocene uplifting events.d. The uplift rate of Medeiros anticline for the last 300 Kacan be estimated in, at least, 0.5 mm/yr taking into account

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the altitude difference between T1 and the top ofCalvimonte Formation in the undeformed basin.

Acknow ledgements

This study was partially supported by the followingresearch projects: CIUNSa 1679, PICT 2008-381, PICT

2010-1441 and PICT-O UNRN 2010-175.

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Figure 3. A) Satellite image of the Medeiros terraces with the location of the topographic profiles measured. B) Topographic profiles ofthe terraces and the Vaqueros river using 30 m spatial resolution SRTM DEM. C) Altitude difference between the terraces and theVaqueros river. In both figures, the profiles have been projected along a line perpendicular to the N-S axis of the anticline.

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