Eric Salomon 1,2, Silke Schmidt-Mechernich 1,3, Ralf Hetzel 1, Francisco Mingorance 4, Victor A. Ramos 5

1 Institut für Geologie und Paläontologie, Westfälische Wilhelms-Universität Münster, Germany.2 Institut für Geowissenschaften, Johannes Gutenberg-Universität Mainz, Germany.3 Institut du Physique du Globe, CNRS Strasbourg, France. 4 Instituto de Mecánica Estructural y Riesgo Sísmico, Universidad Nacional de Cuyo, Mendoza, Argentina.5 Laboratorio de Tectónica Andina, Universidad de Buenos Aires, Argentina.

References

5. Conclusions

Acknowledgements

(1) The Peñas and Cal thrusts appear to have ruptured in characteristic Mw ~7 earthquakes that produced vertical offsets of ~1.0 m.

(2) The Holocene horizontal shortening rates of both thrusts are 1-2 mm/a.

(3) The geological shortening rates (3-6 mm/a) and the GPS rates (2-5 mm/a) are only slightly higher indicating (i) that the two range-bounding thrust faults account for a large amount of the deformation at the Andean mountain front, and (ii) that the shortening rates at the eastern Andean mountain front are similar since ~10 Ma.

Brooks, B.A. et al. (2003). Crustal motion in the Southern Andes (26°-36°S): Do the Andes behave like a microplate? G3 4, 1085, doi: 10.1029/2003GC000505.

Hilley, G.E., Strecker, M.R., Ramos, V.A. (2004). Growth and erosion of fold-and-thrust belts with an application to the Aconcagua fold-and-thrust belt, Argentina. J. Geophys. Res. 109, B01410, doi: 10.1029/2002jb002282.

Kendrick, E. et al. (2006). Active orogeny of the south-central Andes studied with GPS geodesy. Rev. Asoc. Geol. Arg. 61, 555-566. Ramos, V.A. et al. (2004). The Andean thrust system- Latitudinal variations in structural styles and orogenic shortening, in Thrust tectonics and hydrocarbon systems: AAPG Memoir, edited by K.R. McClay, pp. 30-50. Salomon, E. et al. (in press). Repeated folding during late Holocene earthquakes on the La Cal thrust fault near Mendoza city (Argentina). Bull. Seismol. Soc. Am. 103, doi: 10.1785/0120110335.

The financial support of the German Research Foundation (DFG, HE 1704/6-1) is gratefully acknowledged.

AGU2012: T43A-2639

Schmidt, S. et al. (2011). A note of caution on the use of boulders for exposure dating of depositional surfaces. EPSL 302, 60-70. Schmidt, S. et al. (2011). Coseismic displacements and Holocene slip rates for two active thrust faults at the mountain front of the Andean Precordillera (~33°S). Tectonics 30, TC5011, doi:10.1029/2011TC002932. Schmidt, S. et al. (2012). Optical dating of alluvial deposits at the orogenic front of the Andean Precordillera (Mendoza, Argentina). Geochronometria 39, 62-75. Walcek, A.A. & Hoke, G.D. (2012). Surface uplift and erosion of the Southernmost Argentine Precordillera. Geomorphology 153-154, 156-168.

Shortening rates at the mountain front of the Andean Precordillera (Argentina) on timescales of millions, thousands and a few years

contact: mechernich.s@gmail.com

Slip rates of active reverse faults remain largely unknown at the eastern Andean mountain front, although large historical earthqua-kes illustrate the serious threat - e.g. a magnitude 7 earthquake deva-stated Mendoza in 1861 (Fig. 1).

According to GPS measure-ments, the present day E-W shortening rate in the Andean back-arc region (29.3 - 33.5°S) is 2-5 mm/a (Brooks et al., 2003; Kendrick et al., 2006; Fig. 2).In the extension of the Cal fault a local shortening rate of 2.8 ± 1.3 mm/a was calcu-lated using GPS data of Brooks et al. (2003) (Salomon et al., in press).

Peñas thrust

32.788°S

32.790°S

68.836°W

N

50 m

suburb of Mendozasuburb of Mendoza

Calthrust

Calthrust

0.77±0.15 ka0.77±0.15 ka

CT1CT1

CT2CT2

T4T4

T3T3

T2T2T3T3

T2T2

T0T0

T2T2T3T3

T1T1

T2T2

T4T4

T0T0

trenchtrench

c

CT3

CT4

CT3

CT4

23.2±1.0 ka23.2±1.0 ka

12.1±0.6 ka12.1±0.6 ka13.7±0.6 ka13.7±0.6 ka

21.9±1.0 ka21.9±1.0 ka

27.2±1.3 ka27.2±1.3 ka16.4±1.0 ka16.4±1.0 ka

fan surface

fan surface

active surfaceactive surface

roadroad

200 m

T4T4

T4T4

T4T4

T3T3

T3T3

T2T2

T2T2

T2T2T1T1

T3T3

N

3.84±0.14 ka

68.85°W68.86° W

32.755°S

32.76°S

active Cal thrustscarp profiles10Be age (ka) of boulders12.112.114C age (ka) of wood3.83.8

CT1CT1

fluvial terraces

T1 - T4

OSL age (ka) of quartz

0.770.77

b

Fig. 3: (a) Google Earth image showing northern Mendoza, the trace of the Cal thrust, and the study sites. (b,c) Terraces at the Cal thrust with the location of fault scarp profiles. 10Be ages from sandstone boulders are maximum ages (Schmidt et al., 2011 EPSL), hence terrace T4 has an age of ≤ 12.1 ka. (d) Profiles on terraces T1, T2, T3 and T4 indicate vertical displacements of ~0.9, ~2.6, ~3.6 and ~7 m, respectively (Schmidt et al., 2011 Tectonics).

Fig. 4: (a) Trench across the 2.5-m-high fault scarp of the Cal thrust (Salomon et al., in press). OSL ages after Schmidt et al. (2012). (b) Photograph of fold F1. (c) Retrodefor-mation of the folds using the area-balancing method (here example for F1).

2.6°

2.0°0.9 m

T1

T1

CT1

3.1°

4.6°

3.6 m

T3

T3

CT3

CT4

3.2°

3.7°

6.9 m

T4

T4

20 m5m

VE=3

T3: 3.84 ± 0.14 ka

T4: ≤12.1 ka

W E

T2: 0.77 ± 0.15 ka trench in Fig. 4

d

1.9°

2.8°

2.5 m

T2

T2

CT2

2. Cal thrust (millennial timescale)

Cal thrustCal thrust

MendozaMendoza5 km

68°58'W 68°50'W

32°46'S

32°46'S

a

Pre

cord

ill e

ra

NLa Cal

mountain1065 m

a

b

Fig. 5: Sequential restoration of the trench section (Salomon et al., in press).

PT3

E

50 m10 m

VE=3

T3 (12.6 ± 0.2 ka)vertical offset: 11.0 ± 1.0 m

1.9°

2.2°

10.6 mT3

PT1

T1vertical offset:0.9 ± 0.1 m

W

50 m10 m

VE=32.6°

2.6° 1.9 mT2 T2

T2 (3.3 ± 0.6 ka)vertical offset:1.9 ± 0.2 m

2.9°

0.9 m2.7°

T1

T15 m

1 m

VE=3

PT2

PT3PT3

WSW ENE3. Peñas thrust (millennial timescale)

active Peñas thrustterraces

view of Fig. 7

scarp profiles

14C age (cal) of wood

10Be age of sand

13.010Be age of pebbles

12.4 12.6

PT1T1 - T3

T2

20

Escondida creek

PT2

PT3

PT1

3.2 ± 0.6 ka

5.4 ± 1.0 ka

68.79°W

32.52°S

12.4 ± 1.5 ka 13.0 ± 1.9 ka

12.6 ± 0.2 ka

100 m

T3T3

T3T3

T2T2

T2T2

T1T1T1T1

N

b

11.1 ± 1.4 ka

3.3 ± 0.6 ka 3.3 ± 0.6 ka

500 m

68.78°W68.79°W

32.52°S

Peñas thrust

Escondida creek

a

bN

Fig. 7: Terraces T1, T2 and T3 have been dis-placed vertically by ~0.9, ~2 and ~11 m, respectively (Schmidt et al., 2011 Tec-tonics).

Fig. 6: (a) QuickBird image of an alluvial fan cut by the Peñas thrust. (b) Dated terraces at the Peñas thrust with the lo-cation of fault scarp profiles. In case ofthe 10Be depth profiles we calculated two ages: a minimum age by neglecting erosion and an age using a constant erosion rate of 20 mm/ka.

1. GPS and geological slip rates

Fig. 1: Map of the central Andes showing M ≥ 7 earthquake epicen-ters at the eastern mountain front.

AT35

AT37

AT39AT40

AT32

AT31

Jocoli

Mendoza

UspallataUspallata

32.5°

33°S

68.5°W69°69.5°69.5°

N

20 km

Pr e

c or d

i ll e

r a

Pr e

c or d

i ll e

r a

Peñas thrust

Cal thrust

Elevation5940 m

500 m

2.8 ± 1.3 mm/a

1920Ms 6.3

AT32

road

study sites of millennial rates

GPS station of Brooks et al. (2003) with velocity

earthquake epi- center, year, and magnitude

10 mm/a

Borbollón anticline

Capdeville anticline

1782Ms 7

1920Ms 6.3

2006Ms 5.6

1929Ms 5.7

Ms 7.01861

1917Ms 6.5

1985Ms 6.0

1967Ms 5.7

2001Ms 5.6

Fig. 6Fig. 6

Fig. 3Fig. 3

Pacific Ocean

125 Depth contours of Benioff zone (km)

SanJuanPr

ecor

dille

raPr

ecor

dille

ra

26°S

30°

34°

70°W 65°

N

100 km

Mendoza

1000 km

1861Ms 7.0

1944Ms 7.4

1977Ms 7.4

1782

Ms 7

1984Ms 7.5

Normal sub-

duction

Flat-slab sub-

duction

Normal sub-

duction

5075 100 150

200

64 mm/a

Santiago

Mendoza

Fig. 2: Tectonic map of the study area near Mendoza. GPS station velocities are taken from Brooks et al. (2003).

GPS shortening rates

Geological shortening ratesFor geolgical timescales, the rate of deformation is estimated by balanced crus-tal cross sections and different dating methods (mainly Ar-Ar of tuff). In the Precordillera the shortening rate illustrates 3-6 mm/a at 33°S (<30 km shorten-ing in ~10-5 Ma; compilation from Hilley et al., 2004; Ramos et al., 2004; Walcek and Hoke, 2012).

4. Millennial shortening rates4. Millennial shortening rates

Fig. 8: Horizonral shortening rates of the Cal and Peñas thrust faults in the Holocene.

Peñas thrust fault

T2

T3

00 2 4 6 8 10 12

age (ka)

5

10

15

20

25

horiz

onta

l offs

et (m

)

2.0 mm/a

2.2 mm/a

constant shortening rate

18611-2 EQs

1 EQ

Cal thrust fault

00 2 4 6 8 10 12

horiz

onta

l offs

et (m

)

age (ka)

2

4

6

8

10

12

T2

T3

T4

0.5 mm/a

maximum age

maximum age

variable shortening rate

≥0.9 mm/a

5.3 m

m/a

1.5 m

m/aunit (a)

deposition of unit (a): 770 ± 150 years ago(a)

(b)

axial plane of future fold F1

vertical displacement: 1.47 m

formation of folds F2 and F3 during the penultimate event

future wedge II

fold F2shortening:2.39 m

shortening: 1.71 m

vertical displacement: 1.08 m

formation of fold F1 during the last event

futurewedge I

fold F1

(c)

present-day

wedge Iwedge II

fold F3

fold F2

fold F1

C2

C1

(d)

W E

1 m

1 m

Fold F3

Fold F2

Terrace T2

1m

1m

W E

unit a

unit b

unit a

wedge I

wedge II

Fold F1

Terrace T2

1861 Ms~7.0 earthquake

1-2 M6.5-7.0 earthquakes between 861 AD and 770 BP

C2

C1OSL:12.3±2.4 ka

OSL:12.3 ± 2.3 ka

OSL: 11.7 ± 2.2 ka

OSL:770 ± 150 a

ba

Fold F1

WE

W ERetrodeformation of fold F1:

Fold F2: V=1.05 m, H=2.10 m, T=2.35 mFold F3: V=0.42 m, H=0.29 m, T=0.51 m

0.47 m

3.46 m

3.38 m2.049 m2

H = 1.71 mV = 1.08 m

T = 2.02 m

3.26 m

3.58 m

2.049 m2

0.76 m

cb