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Structure and neotectonics of the Jaboncillo and Del Peral anticlinesNew evidences of Pleistocene to Holocene deformation in the AndeanpiedmontVictor H. Garcia 1,4 , Ernesto O. Cristallini 1,4 , Jos M. Corts 2.4 , ft Cecilia Rcdriguez :1 Laboratorio de Modelado Geol6gico (LaMoGe), Departamento de Ciencias Geol6gicas, FCEyN UBA, Pabe1l6nZ, Ciudad Universitaria , C14Z8EHA Ciudad de Aut6noma de Buenos Aires, Argentina; victorg@gl.fcen.uba.ar,ernesto@gl.fcen.uba.ar2 rea Neotectonica, Departamento de Ciencias Geologlcas, FCEyN , UBA3 Laboratorio de Bioestratigrafia de Alta Resolucin, Departamento de Ciencias Geologicas, FCEyN UBA4 ON 1 T

INTRO U TIONThe neote ctoni c featur es developed in the northern region of Tunuyan Gra ben ( figure 1a) have been previouslystudied by field works and photo-interpretation (Polanski, 1963 ; Yrigoyen, 1993, Corts , 1993, Costa el al.2000). We integrate surface and subsur face data to develop an or iginal schematic structural sec tion and weanalyze detailed topo graphie profiles and aer ial photos to evaluate the structure and neotectonics of the area.This is located at 3320 SL near the southern end of the central segment of Central Andes , in the Cord 6n delPlata foothills ( figure 1a). The structural units recognized in the region are: Cordi llera Frontal , CerrilladasPedemontanas and Tunuy n Graben (figure la).The Cordil lera Frontal was uplifted as a rigid block du ring the Plie-Pleistocene and is composed by aCarboniferous sedimentary basement covered with volcanic rock s associated with Choiyoi Group and intruded

by Upper Paleozoic and Tr iass ic granitoids (Polanski, 1958). In the study reg ion this mountain belt can bedivided in two segments: Cordon dei Plata and Cordon deI Porti llo (figure 1a). The Cord6n dei Plata thrust-frontis repre sented by La Carrera fault- system which was acti ve unt il Pleistocene times Caminos, 1979 ; Corts,1993). Nevertheless , the Cord6n del Portillo thrust-front doesn t emerge and is covered by alluvialconglomerates indicating a decrease in neotectonic activity towards the south (Corts el al. 1999In the Cerrilladas Pedernontanas region Miocene to Pleistocene synorogenic deposits crops out in three NNW-

trending belts of low altitude hills (figure 1a). These hills are the superficial expression of compressivedeformation of Triassic depocenters of Cuyo basin that began in Late Pliocene and is sti active (Jrigoyen el al.2000. 2002). Previous resea rchers have ident ified both thin- and thick-skinned tectonic styles that coexist in acomplex arrangement (Sarewi tz, 1988, Kozlowski el 1 1993, Cristall ini el 01. 2000 Chiaramonte el 01.2 .The Tunuy n Graben (figure 1a) was defined by Polanski (1963), and comprises a La te Pl iocene? to Holocene

interrnountain basin located between the Cordillera Frontal to the west and the Cerrilladas Pedemontanas to theeast. Neogene synorogenic depos its and thin Triassic layers are pre sents in subsurface. Upper Pliocene to MiddlePleistocene coarse allu vi al sediments were accumulated as response to latter orogenie pulses in the Cordi lleraFrontal. Folds and scarps developed in Pleistocene alluvial deposits are evidences of active tectonics in this zone(Corts el al. \999).

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STRUCTUREIntegration of seismic lines with new field data made possible the structural modeling of Jaboncillo and Del

Peral ant iclines f igure lb) . Seismic Iines across both folds show that they were formed above an eas t-vergentvery low-angle thrust Garcia, 2004). Two detachment levels are out lined within the Marino Fm. and they arelinked by a low-angle ramp located below Jaboncil lo ant ic line figure 1b). Del Pera l antic line is compound byMiddle Pleistocene sediments and Neogene uni ts f igure 1c). This structure is interpreted as a fault -propagationfold developed by the eastward migration of the shallower thrust, loosing displacement to the south andinterfering with the Totoral fau It to the north.

.Iaboncillo hills are composed by Middle Pleistocene coarse alluvial sediments La Invernada Fm. ) and littleNeogene outcrops, that are observed in the bedrock of some cross-cut ting creeks near the fold hinge f igure lc .Jaboncillo anticline is modeled as a fault-bend fold formed above the l inking ramp. As it can be interpreted fromthe s tructura l sections , this s tructure is a doubly-plunging anti cline with a curved fold axis NNE-t rend in thesouth and NW-trend in the nor th) f igure lc . Both folds define a thin-skinned structural style that contrasts withthe dominant one in the Cerrilladas Pedemontanas, where basement is involved in the deformation.

NEOTECTONICSThree scarps developed in Jabonci llo hills were ident ified. These are regularJy spaced and have a sub-paral lel

map des ign with a genera l NNW- trending figure 1c). Another common feature is that the eastern block is theelevated one, indicating, together with its straight map design, that these are fault-scarps. We interpret that thesehave been formed by very shallow backthrusts detached within incompetent layers of La Pilona Fm Late MiddleMiocene). ln the NE sector, the easternmost fault-scarp affects Upper Pleistocene terraced alluvial deposits LasTunas Fm.), alJowing us to infer Late Pleistocene tectonic activi ty. The map design of the scarps respec t to thegeneral trend of the fold axis also indicates that dextral str ike-slip displacements probably occurred in this regionfigure Ic). A local pediment-shaped surface developed above both folds was recognized. Th is is better

preserved in the backlimbs and it was formed dur ing an erosive period near the Midd le to Late Ple is tocene . Theoriginal slope of the pediment was of about 4_5 to the east, but at the present is nearly horizonta l to 1 dippingto the west; therefore a little counter -slope rotat ion is interpreted. This rotat ion is linked with folding act ivity inboth anticlines during Late Pleistocene times.

Field work mainly consis ted in the survey of ten detai led topographie profi les across both hills , fol lowingcreeks and hilltops. The hiJJtops profiles have been used to verify the presence of the scarps in sec tors wherethese features are poorly preserved. Likewise, an accurate recognition of the pediment sur face was possible inthese profiles. The creeks equilibrium-profiles were analyzed in detail. Convex up profiles of Jaboncillo creeksare founded and it was poss ible to define several segments with different dips. Some changes in the s treamsinuosi ty are associated with these dip-changes, increasing the sinuosi ty with dipping. Streams equil ibriumprofiles character ize an unbalanced zone associated with Jabonci l o ant icline. Using ail creek profiles the SLstream-Iength) index was calculated Hack, 1973). The results were represented in a map of SL index valueswhich indicates sectors with per turbat ions in the equil ibrium profi les high SL values) . These perturbations areassociated with: fault-scarps activity, modem folding and Jithological variations.

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69 2

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= ~CF l. )io ,.,,, .E Cil u:. FM ~ l ~ ~

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E-G:aFigure 1: a Study zone location, b Schematic structural section, c Geological map.

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ON LUS ONS1. Jaboncillo and Del Peral anti clines are fault -related folds devel oped abo ve a v ery low-angle thrust with

shallow detachment levels within the Marino Fm . Middle Miocene) defining a thin-skinned tectonic style.2. The fault scarps and pediment-shaped surface affects Middle to Late Pleistocene deposits La Invemada and

Las Tunas Formations) indicating tectonic activity in these times .3. The zone with convex-up creek equilibrium profiles and some perturbations determined by high SL values

are strong evidences of Late Pleistocene to I-Iolocene? deformation .

ReferencesC am in os , R., 1979. C or di ll er a Front al. In lC.M . Turner E d.) Segundo Simposio de Geologia regional

Argentina, Academia Nacional de Ciencias 1: 397-453 .Corts, J.M., 1993 . El frente de c or ri mie nt o de la Cor dil lera Frontal y el e xt remo sur del valle de Uspallata,Mendoza. XII Congreso Geol6gico Argentino y II Congreso de Exploraci6n de Hidrocarburos, Actas Ill: 168178

Corts, J.M., Vinciguerra, P., Yarnin, M. and Pasini, M.M ., 1999. Rasgos estructurales dei territorio argentino. 2.Tect6nica cuaternaria. A) Te ct n ic a cuaternaria de la regi6n a ndi na dei nu evo Cuyo 28 - 38 LS). In: R.Caminos Ed.): Geologia Argentina. Servicio Geol6gico Minero Argentino. Anales 29: 760-778.

Costa c M.N. Machette, 1. Dart .R.1. , H.E. Bas tia s, J.D. P ar ede s, 1.P. Per ucca, G.E. Tel lo y K.M. Haller,2000. Map and database of Quatemary faults and folds in Argentina. U.S. Ge olo gi ca l Survey, Op en Fi leReport, 76 pp, Reston.

Cristallini, E.O., Boggeti, D., Regazzoni, c. , Anzulovich, 1. , Cerdan, J., Ayala, M., Scolari, J.c. and Le ir 6, F.,2000. Cuenca Cuyana. lnterpretacin estructural regional. Repsol-YPF, CONICET, Universidad de BuenosAires informe indito).

Chiaramonte, 1. , R am os , V.A . and A ra uj o, M., 2 00 0. E st ru ct ur a y s is mo te ct 6n ic a dei an ti cl ina l B ar ra nc as ,cuenca Cuyana, provincia de Mendoza. Revista de la Asociaci6n Geol6gica Argentina, 55 4): 309-336.Ga rc ia , V.H., 2 00 4. An lisis e st ru ct ur al y n eo te ct 6n ic o de las l om as J ab on ci ll o y deI P eral , d ep ar ta me nt o de

Tupungato, provincia de Mendoza.Licenciatura thesis, Universidad de Buenos Aires. Unpublished. 100 p.Hack, J. L, 1973. Stream profile analysis and s tr ea m- gra di en t index. U.S. Geo log ic al S ur vey Journal of

Research, 1: 421-429.Irigoyen, M.V ., Buchan, K .1. and Brown, R.1. , 2000. Magnetostratigraphy of Neogene Andean foreland-basin

strata, lat. 33S, Mendoza province, Argentina. Geological Society of America Bulletin, 112: 803-816.Irigoyen, M.V ., Buchan, K.1. , V illeneuve, M .E. and Brown, R.L., 2002. Cronologia y significado tect6nico de

los e st ra to s s in or og n ic os n e6 ge no s a fl or an te s en la r eg i6 n de C ac he ut a- Tu pu ng at o, P ro vi nc ia de M en do za.Revista de la Asociaci6n Geol6gica Argentina, 57 1): 3-18.

Kozlowski, E., Ma nc ed a, R. and Ramos, V.A., 1993. Estructura. In : V.A . Ramos Ed.): Geologa y recursosnat ura le s de la p ro vi nc ia de M en do za. XII Co ngr es o G eo l6g ico Ar ge nt ino y Il Co ng re so de Expl oraci6n deHidrocarburos, Relatorio, Capitule 1-18: 235-256.

Polanski , J. , 1958. El bloque vari scico de la Cordillera Frontal de Mendoza. Revista de la Asociaci6n Geol6gicaArgentina, 12 3 1957) : 165-193.

Polansk i J., 1963. Estratigrafia, neotect6nica y geomorfologia deI Pleistoceno pedemontano, en tre los dosDiamante y Mendoza. Revista de la Asociaci6n Geol6gica Argentina, 17 3-4) 1962): 127-349.

Sarewitz, D., 1988. High rate of Late Cenozoic crustal shortening in the Andean foreland, Mendoza province,Argentina . Geology 16 12): 1138-1142.

Yrigoyen, M.R. , 1993. Los d ep 6s it os s in or og n ic os te rci ari os . En: R am os , V.A . Ed.) : G eo lo gi a y re cur so snaturales de la provincia de Mendoza. ReJatorio 12 Congreso Geol6gico Argentino y 2 Congreso deExploraci6n de Hidrocarburos, 1 II : 123-148 .

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