Neogene to Present teetonic and orographie evolution of the Beni Subandean Zone

1 Neogene to Present teetonic and orographie evolution of the Beni Subandean Zone M. Strub 1, G. Hérail 2, J. Darrozes 1, R. Garcia-Duarte 3, &...

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6th International Symposium on Andean Geodynamics (ISAG 2005, Barcelona), Extended Abstracts: 709-713

Neogene to Present teetonic and orographie evolution of the Beni Subandean Zone M. Strub 1

1,

G. Hérail

J. Darrozes 1, R. Garcia-Duarte

2,

LMTG, Toulouse, France;

2

IRD, Santiago , (hile;

3

3,

& G. Astorga

3

UMSA , La Paz, Bolivia

Introduction / GeologicaI setting

The Tertiary evo lution of the Central Andes is characterised by the Eastward progression of the deformation, from the Western Cordillera to the Interandean Zone, and to the Subandean zones (Sempéré et al., 1990; Suarez et al., 1983) (fig. 1).

- = - -'-'

ARGENTI NA

Central Andes Map, localisation of the studled reg/on.

Sind th rust projection

(:..

Paree-stream direction ln Tutumo fromation Meêln thrust

_ ~"".1.".1,,1

Syntetlsed column o( Neogene Formatlons ln the HlIdldl Syndlne Hodl(led (rom Baby 1995.

1

1

Fig. 1: Locali sation o f the studied area : North Sub an dean Zon e of Bolivia. Loca lisation o f the three co nglomerates Form ation s, Cangalli in the Map iri Region. Mayaya in the Ben i Syncline and Tutumo in the Madidi Synclin e.

This deformation spreads by thin tectonic of fold and thrust propagation. ln the North Subandean Zone of Bolivia , the deformation expresses itself as large piggyback synclines (Alto Beni and Madidi ). The y are separated by localised thrust propagation anticlines (Baby et al., 1993). During the Neogene, the Subandean synclines passed from foredeep zone to piggy-back basin, and actually they are incised. A new dating of a tertiary tuff deposition helps to understand the propagation of the deformation from early Miocene to Present.

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6th International Symposium on Andean Geodynamics (ISAG 2005, Barcelona), Extended Abstracts: 709-713

The crustal shortening of the No rth Subandean zone of Bolivia is more important than in the l'est of the Subandean Zones of the North Central Andes (Gil Rodriguez, 2001 ), but nowadays the shortening rate see ms to be the weakest (Be vis et al., 200 J). To understand the evolution of the shortening velocity in th is area we observe the transition al phases between distal fluvial sedimentation and coarse conglomeratic deposit in the different synclines as testimonies of relief deveJopment. We also describe the present deformation to compare it with the deformation rate given by gps measurements of (Bevis et al., 2001).

Geochronology and stratigraphie data In the Mapiri Region, the Cangalli (fluvial conglomerate formation) presents a tuff layer (Chontalorni tuff) . This tuff gives an Ar/Ar age on biotite of 7.96±0.6 Ma (Hérail et al., 1994). In the Chargui Fm (fluvial sand and clay formation of the Madidi synclinc, fig.l), an interlayed tuff gives an Ar/Ar age in biotite of 8.7 +/- 0.9 Ma. It confirms the synchronous deposit of Chargui and Cangalli Fms. The Chargui Fm is therefore the distal facies of the Cangaili Fm as it was suggested (H érail et al., 1994). The stratigraphie evolution (stratigraphie column fig.I) is very similar in the three piggyback basins of the Subandean Zone; but the facies transition is diachronie along the basin. The Tipuani region has been hardly incised during middle Miocene. Rivers dug

~ I Km

deep vaJleys. During the Upper Miocene, these valleys were

infilled by the conglornerates of the Cangalli Fm (H érail et al ., 1994) . Nowadays, this region is dug again;

~lkm

deep valley cuts the Cangalli Fm and the Palaeozoic substratum. The Mapiri region gives evidences of regional tilt : the actual talweg slopes lesser than the Miocene paleo-talweg(Hérail et al ., 1994). ln the Beni syncline, the Quendegue Fm begins with distal fluvial facies and ends with Mayaya conglomerates deposition . This tran sition occurred during the Beni Syncline deformation . It is characterized by growth strata progressive discordances in this piggyback basin, observable on seismic cross section (Baby et al. , 1995 ). In the Madidi Basin, the Quendegue Fm is a di stal fluvial facies. Il evolves to a more proximal facies, including sorne thin channels of thin conglomerates, in the Chargui Fm and then, the deposit of the Tutumo con glomerate (Oller, J 986). Chargui Fm is dated in its middle by a tuff at 8.7+/- 0.9 Ma (Ar/Ar). The Tutumo Fm crop out in the Madidi Syncline; it is slightly deformed. The measured paleo-currents indicate that the paleo-rivers l'an along the frontal fold and thrust bend . But the Tutumo Fm does not show discordance with Chargui Fm. The tectonic activity of the frontal fold and thrust belt was just starting growing during the Tutumo deposition (fig .l) . ln the Beni fiat plain the transition between sand and conglomeratic fluvial deposits is buried . Nowadays, conglomerates do not arrive to the foredeep zone; they stop in the plain sorne km after the Susi Snia (fig.2), that means in the guaternary frontal piggy-back basin. Indeed, the deformation seems to spread also to the Beni plain. Slight deformation could be observed ahead of the frontal thrust, in the Beni plain (see fig.l). In the Ixiamas area, the rivers are c1early deviated by a feature paraileI to the Subandean Front. This feature is interpreted as the result of a blind thrust propagation fold. South-East to Rurrenabague, the fans coming from the Frontal bend has been translated so rne km to the North-East, following a belt parailel discontinuity. This migration occurred after 4ka J4C BP . Quaternary evolution tectonic activity is also registered by the fluvial terraces. Evidences of Quaternary thrusting are observed in two terraces . In the San Miguel syncline (fig. 2), the terrace accumulation was dated at 4 ka 14C BP ; this terrace is thrusted .

710

6th International Symposium on Andean Geodynamics (ISAG 2005, Barcelona), Extended Abstracts: 709-713

..... 7,96

ORIENTAL

CORDILLERA

sw

+/- 0.58 Ma

INTERNALSUBANDEANZONE

nPUANI REGION C8ngam con",lomerate..c;;

EXTERNAL SUBANDEAN ZONE

8.7

,

+/- 0.9 Ma

FQRELANO

MADIOl SYNCllNE

SAN MIGUEL

SVNCLINE

Thtumo

conglom.rate. Snl"

Srua Cllqulllhual'...it

6 . 1..

susl

thrv'>t

. ~ - * ,-

7/

Rc)'e...

o

NE

" t Okrn ... _-_. ConglolTM!lt. l. C o n O.l o lc

Hu r l a n ·Dbvonlan O r d o", l~utn

Sc p.

e c se m e nr

Jur a » l...: P otor1't10 C d r b o n lf e r o u !
o e vco too

Fig.2: cross section of the Beni Subandean Zone (Baby et al., 1993), Ar/Ar ages and location of the tuff layer.

In the external f1ank of the Snia Susi fold and thrust bend, one terrace accumulation was dated at 10 ka 14C BP, this terrace is also thrusted. These two thrusts show a recent activation of the frontal thrusts.

Discussion on Neogene and Quaternary evolution of the Beni Subandean Zone The stratigraphie evidences and the recent deformation point out the deformation propagation from the Interandean Zone to the Beni Plain during the Neogene: The incision of the paleao-Tipuani valley took place during the Middle Miocene. The corresponding proximal sedimentation occurred in the Beni Syncline by deposition of Mayaya conglomerates. The progressive discordance present in the older formations in the Beni syncline shows that this syncline played as a piggyback basin (Baby et al., 1995). Furthermore the Mayaya conglomerates are limited to the upper part of the Beni syncline, they did not deposit downstream. The depositional zone couJd have been limited by the rise of the central thrust of the Beni Syncline (Snia Marimonos and Toregua, fig. 2). So, the Subandean zone was already deforming in its internai part, during the Middle Miocene. The infilling of the paleo-Tipuani valley by the Cangalli conglomerates occurred around 8Ma. It was produced by the blockage of the sedimentation flow, due to the growing of the relief in relation with the activation of the CFP (Mean Frontal Thrust). The thinner sediments go through this gate. Actually, Cangalli formation is essentially coarse and does not presents lake deposit (Hérail et al., 1994). The thinner sediments went straight to the Madidi syncline to form the Charqui Fm. Occasional powerful fluvial episodes provide to carry centimetres pebbles of quartzite to the Madidi syncline. These incursions managed ribbon of pebbly sandstone in the Charqui Fm. The deformation going eastward and up, the regional slope increased and the Mapiri region was re-incised. The slope increase provided to carry coarse sediments deposited in the Madidi Syncline, the Tutumo Fm. When the conglomerate deposited in the Madidi syncline, the frontal fold and thrust belt was already slightly constructed and the rivers had their present-day geometry. The deformation is still active as we can see it in with the birth of blind thrusts on the Beni plain. Two thrusts moved after 4ka Bp in the Frontal bend. Horizontal displacement associated with these faults is

~2m,

so the

averaged displacement velocity since 4ka, is higher than 0.5 mm/year. Using the balanced cross section, horizontal shortening in the external Subandean Zone (Snias Susi and Baia fold and thrust bend) from 8 Ma to present is around lA mm/yr. The two rates are around 1mm/yr to the North-East. Gps deformation data do not

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6th International Symposium on Andean Geodynamics (ISAG 2005, Barcelona), Extended Abstracts: 709- 713

indicate major rates but they are in the opposite direction. While the margin of error is very high for the gps data (Bevis et al., 2001), the quaternary thru sts put in evidence the North-Eastward millimetres deformation.

00000

'rutumc

cco çto-oe-arcs . . . . . Cangalh

conçrc rnera tes 00000

Mavav~

conçt orneraœs

--

Upper Mloune/PlIocene

Fig.3: Evolution of the North Subandean Zone of Bol ivia during the Neogene. Unfolding was realised by 2Dmove program from the Baby cross section (Baby et al., 1993).

Tutumo conglomera tes deposin on . hternal Scbenoeen Zone derorm anon .

Upper Mlocene Cha rqul a nd ca nga lll rulf

n puanJ rtvers JilUng. I nte rna t Suba ndean Zone deforma tion.

c

Middle Mf.ocene Tlpua nl reg ion rnds .on . Mayay conçlomerares cepostnon .

Internat Subandean Zone c ercemauon.

Conclusion Ar/Ar dating of the Charqui Fm tuf provides a new age for the Tertiary series of the Beni Subandean zone. lt confirrns that, at 8 Ma , the Charqui Fm was the distal facies of the Cangalli Fm (Hérail et al., 1994). This new age enables to con str ain the evolution of the Subandean zone. The thrust and fold belt of the Internai Subandean Zone develop at least during the Middle Miocene, with the sedimentation on the Beni piggyback syncJine. This sedimentatio n ended with the deposit of the Mayaya conglomerates. The development of this internai belt locked the coarse sed ime nts in the Tipuani region around 8 Ma. At the same time, the second piggy back basin took place in the frontal Subandean Zone, in the Madidi basin with the sedimentation of the Charqui Fm. As in the Internai Zone, the filling of the Madidi basin fini shed with the conglomeratic facies, Tutumo Fm. Quaternary deformation evidences s ho w that the deformation s preads to the North-East, affecting the Beni plain. It explains why the GPS result on the Reyes statio n (Be vis et al. , 2001) do not correspond to a s table craton station. Reyes see ms to be now in the front of the Subandine zone (fig.l). Horizontal shortening, in the order of 1 mm/yr in the frontal fold and thrust bent, appears very slow compared to the cru stal shortening of the Subandean fold and thrust belt averaged over the past 25 Ma: 8-13 mm/yr (Liu et al. , 2000). The deformation spreads to the North-East, but it should be accommodated also in the Internai part of the Subandean zone. Furthermore, it couJd cast doubt on the acceIeration of the deformation rates during the Neogene (Benjamin et al., 1987; Gregory-Wodzicki, 2000).

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6th International Symposium on Andean Geodynamics (ISAG 2005, Barcelona), Extended Abstracts: 709-713

Bibliographie Baby, P., Colleta, B. and Zubieta, D., 1995. Etude géomtrique et expérimentale d'un bassin transporté: exemple du synclinoriurn de l'Alto Beni. Bull. Soc. France, 166(6): 797-811. Baby, P. et al., 1993. Structural Synthesis of Bolivian Subandean zone, Second ISAG, Oxford (UK), pp. 159162. Benjamin, MT, Johnson, N.M. and Naeser, C.W., 1987. Recent uplif in the Bolivian Andes: evidence from fission-track dating. Geology, 15: 680-683. Bevis, M. et al., 2001. On the strength of interplate coupling and the rate of back ars convergence in the central Andes: An analysis of the interseismic velocity field. Geochemistry Geophysics Geosysterns, 2. Gil Rodriguez, W., 2001. Evolution latérale de la déformation d'un front rogénique: Exemple des bassind subandins entre 0° et 16° S., Université Paul Sabatier, Toulouse. Gregory-Wodzicki, K., 2000. Uplift history of the Central Andes: A review. GSA Bulletin, 112(7): 1091-1105. Hérail, G., Sharp, W., Viscarra, G. and Fornari, M., 1994. La edad de la formacion Cangalli: nuevos datos geocronologicos y su significado geologico, Memorias dei XI Congreso Geologico de Bolivia. Liu, M., Yang, Y., Stein, S., ZHu, Y. and Engeln, J., 2000. Crustal shortening in the Andes: Why do GPS rates differ from geological rates? Geophysical Research Letters, 27( 18): 3005-3008. Oller, J., 1986. Consideraciones generales sobre la geologia de la Faja Subandina norte., Universidad mayor de San Andreas, La Paz, Bolivia, 120 pp. Sempéré, T., Hérail, G., Olier, J. and Bonhomme, M.G., 1990. Late Oligocène-early Miocene major tectonic crisis and related basins in Bolivia. Geology, 18: 946-949. Suarez, G., Molnar, P. and Burchfiel, c., 1983. Seismicity, fault plane solutions, depth of faulting, and active tectonics of the Andes of Peru, Ecuador, and southern Colombia. J.G .R., 88(BI2): 10,403-10,428.

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