The orogenic front of the Mendoza front of Precordillera: Neotectonics in the western margin of Sierras Pampeanas Objectives Objectives::

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The orogenic front of the Mendoza Precordillera: Neotectonics in the western margin of

Ramos Instituto de Estudios Andinos Don Pablo Groeber Universidad de Buenos Aires CONICET

Precordillera and Sierras Pampeanas that coincides with the most active seismogenic zone of

The wedge-top zone is characterized by active faults and growth strata, bounded to the west

playa-lake deposits, main depocenter of the foreland basin.

Precordillera and can be identified by their different structural trends (northeast

1 The orogenic front of the Mendoza Precordillera: Neotectonics in the western margin of Sierras Pampeanas Victor A. Ramos Instituto de Estudios Andinos Don Pablo Groeber Universidad de Buenos Aires CONICET Objectives: To identify the wedge-top developed along the orogenic front between Precordillera and Sierras Pampeanas that coincides with the most active seismogenic zone of the Pampean flat-slab segment of the Central Andes. The wedge-top zone is characterized by active faults and growth strata, bounded to the west by the fold and thrust belt orogenic front, and to the east by the foredeep undeformed playa-lake deposits, main depocenter of the foreland basin. The basement structures of Sierras Pampeanas interact with the thinskinned tectonics of the Precordillera and can be identified by their different structural trends (northeast basement structures vs. north trending Andean thrusts t in Precordillera). To caharacterize the different structural styles along the wedge-top zone and the seismogenic structures that concentrate the main crustal earthquakes along the orogenic front. 1

Precordillera Foothills The wedge-top Long lasting

2 Precordillera Foothills The wedge-top Long lasting deformation during 10 My at the low shortening rate of 0.33 mm/a. Periods of slow uplift rates alternate with basement exhumation and rapid uplift. Changes in sub- sidence rates triggered by tectonic loading of shifting thrust front. Foreland Basin System Playa-lake deposits Mutti et al. (2009) 2

3 San Juan Pie de Palo Cerro Salinas Montecitos Río Las Peñas Jocolí Foredeep Mendoza Precordillera and Sierras Pampeanas Interaction: Basement wedges with east vergence. Main east-dipping fabrics of Pampean basement controls the surface ruptures by backthrusts. Cominguez and Ramos (1991) Basement wedges Verges et al

4 Eastern Precordillera: Sierra de Villicum thin-skinned thrust 4

5 Main structures of Sierra de Pie de Palo The region is seismically active as shown by the large Recent intraplate earthquakes Caucete San Juan La Rinconada Villicúm Las Chacras Bermejo Matagusanos Zonda Cerro Salinas... 5

6 6

7 The San Juan earthquake of 1941 LA LAJA FAULT arthquake of

8 San Juan Co. Salinas Jocolí Fordeep Río Las Peñas Cerro Salinas Precordillera de Mendoza 8

9 Precordillera of northern Mendoza STRUCTURAL SECTION OF OJO DE AGUA Cegarra (1997) Seismogenic zones associated with the orogenic front: Wedge-top zone NEOTECTONICS In the OROGENIC FRONT La Laja Sierra de Zonda Cerro Salinas Río Las Peñas Cerro La Cal Borbollón Mendoza Barrancas 9

10 Neotectonic faults Cerro Salinas 10

11 11

12 Vergés et al. (2007) Deep seismic reprocessing Comínguez and Ramos (1991) 12

13 Cerro Salinas Southern sector Seismic line Vergés et al. (2007) Cerro Salinas: Line Growth strata Tobas Angostura Pre-growth go strata Cominguez and Ramos (1991) Metamorfic basement 13

14 Cerro Salinas: Line Vergés et al. (2007) 14

15 South of Cerro Salinas Montecitos anticline Rinconada Montecitos Anticline Long lasting structure of slow growth Late Pliocene growth strata Los Mogotes Fm.< 2.6 Ma Pleistocene growth strata Western flank 27ºNW to 68ºNW The Present surface is gently deformed Costa and Cortés (2000) 15

FTB zone Orogenic front Wedge-top zone Foredeep zone Bajada del

Orogenic front Wedge-top zone: synorogenic Miocene deposits and

Miocene synorogenic deposits thrust on Pleistocene terrace 2.

16 FTB zone Orogenic front Wedge-top zone Foredeep zone Bajada del río de Las Peñas Kilometers Sierra de las Peñas: Precordillera Orogenic front Wedge-top zone: synorogenic Miocene deposits and Quaternary growth strata Miocene deposits 4 1. Miocene synorogenic deposits thrust on Pleistocene terrace 2. Miocene and early Pleistocene deposits thrust on middle Pleistocene deposits 3. Miocene and middle Pleistocene deposits thrust on late Pleistocene deposits 4. Miocene deposits out-of-sequence thrust on Holocene deposits 16

17 E W Sierra de las Peñas Main thrust (3) W Holocene deposits E 17

18 Out-of-sequence thrust: 4 Miocene deposits Holocene deposits Sierra de las Peñas: Precordillera Orogenic front (northern side) 18

19 Sierra de las Peñas: Precordillera Orogenic front (northern side) T Q T Q T Quaternary growth srata Sierra de las Peñas: Precordillera Orogenic front (northern side) 19

20 Growth strata Miocene playa-lake lake deposits Sierra de las Peñas: Precordillera Orogenic front (northern side) T Q T Q T 20

Sierra de las Peñas Fault (48 km long) Synorogenic topography in

(2015) Thick Pleistocene deposits folded and thrust by the older

The upper levels are monoclinal folded. Vertical slip rates: 0.

Cerro La Cal Borbollón Barrancas Wedge-top zone Cerro Salinas:

west-vergence controlled by early Paleozoic fabrics.

21 Sierra de las Peñas Fault (48 km long) Synorogenic topography in the orogenic front Costa et al. (2015) Thick Pleistocene deposits folded and thrust by the older synorogenic Miocene deposits. The upper levels are monoclinal folded. Vertical slip rates: 0.9 to 0.6 mm/a in the last 13 ka Horizontal shortening rate: mm/a Schmidt at al. (2011) Precordillera Orogenic front Cerro Salinas Río Las Peñas Cerro La Cal Borbollón Barrancas Wedge-top zone Cerro Salinas: Basement wedge similar to Sierras Pampenas structures with west-vergence controlled by early Paleozoic fabrics. Río Las Peñas: Interaction of thin-skinned structures with basement wedge of opposite vergence. Cerro La Cal to Barrancas: East vergence basement faults associated with inversion of Triassic rifts. 21

22 Co. La Cal Mendoza Barrancas Olgiati (2002) Cerro La Cal Fault (31 km long) Vertical slip rate: 0.6 to 0.9 mm/a in the last 12 ka. It was active during the 1861 earthquake. Schmidt at al.,

23 Cerro La Cal Active Fault (1861 earthquake) Cerro La Cal Active fault Active fault Cerro La Cal Active Fault (1861 earthquake) Schmidt et al. (2011) 23

24 Cerro La Cal Active Fault (1861 earthquake) Salomon et al. (2014) Cerro La Cal Active Fault (1861 earthquake) Salomon et al. (2014) 24

25 Cerro La Cal Active Fault in Mendoza City Ramos (2013) Mendoza Earthquake: ~ 7 City of Mendoza 20th. of March,

La Cal Fault Falla La Cal Sitio Fundacional Active fault that crosses the City of Mendoza. It is related to the 1861 earthquake.

thrust: La Laja active fault backthrust. At Sierra de Las Peñas is dominated by tectonic inversion of a Triassic rift system: Las Peñas active fault.

26 La Cal Fault Falla La Cal Sitio Fundacional Active fault that crosses the City of Mendoza. It is related to the 1861 earthquake. 2,95 m vertical uplift along 31 km in the last 10,000 years. MENDOZA Estimated magnitud between 6.7 to 7. Estimated shortening rate mm/a in the last 10,000 years. Bastías et al. (1993) Concluding remarks: The orogenic front has different structural styles from north to south: At San Juan the Eastern Precordillera is a basement structure with a n west- vergence thrust: La Laja active fault backthrust. At Sierra de Las Peñas is dominated by tectonic inversion of a Triassic rift system: Las Peñas active fault. At the city of Mendoza la Cal Fault crosses the entire city intersecting the growth strata of Cerro la Gloria. The rate of shortening of the different described structures is very slow, at least in the wedge-top zone which allows the preservation of the growth strata. Active faults and sedimentation characterize all the wedge-top zone, contrasting with the playa-lake facies of the undeformed foredeep zone. and finally as Bodenbender more than 100 years ago said 26

27 Final remarks No es necesario asustarse por lo terremotos, ni ponerse a discutir sobre la conveniencia de trasladar la ciudad a otro sitio. Trasladando la ciudad al poniente se la retira de una grieta para acercarla a otra. Para ampararse de estos fenómenos de la naturaleza no hay otro medio que el de edificar convenientemente las habitaciones (*) Guillermo Bodenbender, (It is not necessary to be frightened by the earthquakes, nor to argue about the convenience of moving the city to another place. Moving the city to the west, it is removed from a fissure to bring it closer to another. To guard against these phenomena of nature there is no other way than to properly build the rooms) (*) Boletín de la Academia Nacional de Ciencias, XIV(2):

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