k-ar dates from the altiplano and cordillera oriental of ... · plegamientos pre-miocenos....

Pergmon

08959811(95)oooo3-8

hunal of South American Eunh Sciences. Vol. 8, N o. 2, pp. 163-186, 1995 Cqyight 0 1995 Blwicr Science L4d dr Euth Scieaes & Ruarcu htihue

Rinted in Great Britain. All riphc revved

0895-9811195 59.50 + 0.00

K-Ar dates from the Altiplano and Cordillera Oriental of Bolivia: implications for Cenozoic stratigraphy and tectonics

‘L. KENNAN and S. LAMB 2C. RUNDLE

‘Department of Earth Sciences, Parks Road, Oxford OX1 3PR, UK 2NERC Isotope Geosciences Laboratory, Keyworth, NG12 5GG, UK

(Received February 1994; Revisions Accepted November 1994)

Abstract - New K-Ar dates from tuffs, lavas and granites, collected in the Bolivian Altiplano and Cordillera Oriental, constrain the evolution of the Bolivian Andes. A phlogopite megacryst from a post-cleavage kimberlite dike near Independencia gave an age of ca. 98 Ma. In the central Altiplano, a widespread suite of basaltic to andesitic lavas, sills and rhyolitic tuffs has yielded dates of 25-22 Ma, coeval with the first major influx of conglomerates in the central Altiplano basin. Rapid early Miocene erosion of the Cordlllera Oriental is suggested by biotites from the Quimsa Cruz Granite in the Cordillera Oriental, which vary in age between 32-22 Ma, and also by zircon fission track data. Gentle folding in the Cordillera Oriental near Potosi pre-dates the early Miocene Mondragon Formation which contains a ca. 19 Ma ignimbrite near its base. Further folding here occurred prior to the eruption of the flat-lying ca. 7 Ma ignimbrites of the Condor Nasa Meseta and ca. 12 Ma ignimbrites from the central Los Frailes Meseta. Intense folding in the central Altiplano also occurred between 9 Ma and 5 Ma, and younger tuffs are only slightly tilted. In the Cochabamba region, sedimentary infill of the Parotani Basin contains a ca. 20 Ma tuff horizon, and folded tuffs from the nearby Sacaba Basin have been dated at ca. 2.2 Ma. These ages suggest a protracted history for associated basin-margin ESE sinistral strike-slip and normal faults. However, regional folding pm-dates the San Juan de1 Oro Surface, which formed an extensive flat- lying peneplain, preserved at ca. 3000 m in the central and eastern Cordillera Oriental. The age of valley-fill tuffs suggest that dissection of this surface had commenced in the Cochabamba region by 6.5 Ma. However, near Sucre, flat-lying tuffs, dated between 3.5 and 1.4 Ma, mantle the surface and predate the deep Pleistocene dissection of the central Cordillera Oriental. Deformation has been concentrated in the Subandean foreland thrust belt since the Pliocene.

Resumen - Se ha delimitado la evoluci6n de 10s Andes Bolivianos con la ayuda de nuevas dataciones K-Ar de tobas, lavas y granitos, colectados en el Altiplano sur-central y en la Cordillera Oriental. Un rinico megacristal de flogopita, procedente de un dique post-clivaje, cerca a Independencia, di6 una edad de 98 Ma. En el Altiplano, un conjunto de lavas basalto-andesiticas, sills y tobas riolfticas tienen edades entre 25 y 22 Ma, coincidente con la llegada de 10s ptimeros conglomerados a las cuencas de1 Altipl- ano. Las edades en cristales de biotita procedentes de1 granito de Quimsa CNZ en la Cordillera Oriental varIan entre 32 y 22 Ma. Estos datos, junto con edades de trams de fisi6n en zircon, indican una rapida erosi6n de la Cordillera Oriental. En las cercanias de Potosi, las capas rojas de la Formaci6n Mondragdn que contienen una ignimbrita de 19 Ma en su base, descansan sobs suaves plegamientos pre-miocenos. Plegamientos posteriores son m&s j6venes que las ignimbritas de 7 Ma de la Meseta de Condor Nasa y ellas de 12 Ma de la meseta central de Los Frailes, que no se encuentran plegadas. El plegamiento intensa en el Altiplano tuvo lugar entre 9 Ma y 5 May tobas m&s j6venes se encuentran ~610 suavemente plegadas. En la regi6n de Cochabamba, la cuenca de Parotani contiene una toba de 20 Ma y tobas en la cuenca de Sacaba dan edades de ca. 2.2 Ma. Estas edades indican que las fallas con rumbo ESE associadas se encontraban activos desde el Miocene Temprano. Sin embargo, el plegamiento regional pm-data la Superhcie San Juan de1 Oro, una meseta ancha desarrollada a ca. 3000 m en la parte Central y Este de la Cordillera Oriental. Dat- aciones de tobas en valles que cortan esta superficie indican que fue disectada en la regi6n de Cochabamba a 10s 6.5 Ma. En las cercanIas de Sucre tobas de 3.5 a 1.4 Ma cubren la superficie y pre-data a la intensa disseci6n de esta parte de la Cordillera Orien- tal. Estas edades indican que en el Pliocene el plegamiento se habia movido totalmente hacia el Este, dentro de la faja cabalgada de1 antepa& subandino.

INTRODUCTION

THE WAY THAT THE CONTINENTAL LITHOSPHERE responds to forces generated by relative plate motions is an important problem in geology. It is clear that many active plate-boundary zones through continental lithosphere are extremely wide. For instance, deformation in Central Asia occurs in a zone which extends for thousands of kilometers both in length and width, including the great ranges of Tibet and the Himalayas. The Andes form part of another large and active plate-boundary zone, extending for over 5000 km along the entire western margin of South

America and reaching 700 km in width in northern Chile and Bolivia with topography over 6000 m. Here, the oce- anic Nazca plate is being obliquely subducted beneath the South American continent.

Unlike the great mountain ranges of Central Asia, the Central Andes in Bolivia are well populated and easily accessible. For this reason, they are an extremely good place to study the development of wide zones of continental deformation. Also, thick continental red bed sequences have accumulated in both foreland and intramontane basins during the tectonic evolution of the Bolivian Andes.

Address all correspondence and reprint requests to Simon Lamb or Lorcan Kennan, Department of Earth Sciences, Parks Road, OX1 3PR, UK. Telephone: 44 1865 274582. Fax: 44 1865 272072. email: lorcan or [email protected].

Oxford

163

164 L. KENNAN, S. LAMB and C. RUNDLE

t^-*“l Salars I=[ Miocene lgnimbrite: s 11 Tertiary Strata

17

I] 11

~ Pre-Tertiary

- Major Faults 1

I Major Towns M Volcanoes

140

50

6O

70 l i

Santa cruz -18 0

l ! , - lgc

2( -2 !O"

1 '

2 ' -2 !I"

$ 2: I p .

4? - 2: z"

2 4 I

2ookms ' 23 I0

q-- 660 65O 64O 63O

Fig. 1. Outline map of the Central Andes showing major physiographic divisions, outcrops of pre-Tertiary and Tertiary strata, Late Miocene ignimbrite shields, Quatemary salars and major fault zones discussed in the text. The C.T.L. (Cochabamba-Tapacati Lineament System) trends ESE and has modest, ca. 10 km scale, sinistral displacement. The Aiquile fault trends N-S and has unknown dextral displacement. Numbered boxed regions are covered by detailed maps in this paper.

0 660 65O 640 63" I

K-Ar dates from the Altiplano and Cordillera Oriental of Bolivia 165

Dating these sediments helps to constrain both the timing and distribution of deformation. The ultimate aim is to build up a detailed picture of the geological development of such a wide mountain belt, which can then be compared with theoretical models of the large scale deformation of continental lithosphere (England and Jackson, 1989; Wdowinski et al., 1989; Wdowinski et al. 1994a,b).

Tertiary sedimentary sequences in the Bolivian Andes frequently contain pyroclastic and volcaniclastic deposits, and are sometimes cross-cut by intrusive rocks. Unlike fossil faunas, which have been described from some localities, the volcanic and plutonic rocks can be accurately dated using isotopic techniques. Indeed, much of the previous geochronological work in this part of the Andes has been aimed at constraining the ages of the fossil faunas. In this paper we report new K-Ar ages, together with descriptions of the structure and stratigraphy of local Tertiary sedimentary sequences. We use these to refine our understanding of the timing and nature of Andean mountain-building.

Bolivian Andes

The Bolivian Andes comprise several distinct physiographic provinces (Fig. 1). These provide a convenient way of describing the geology of this part of the Andes, because the tectonic evolution of each province has been generally distinct.

The Cordillera Occidental comprises the active volcanic arc along the international border between Bolivia and Chile, consisting of spaced Miocene and Quaternary dacitic-andesitic volcanoes overlying thick ignimbrite sheets, lava flows and associated calderas (Avila, 1991). These erupted through a poorly known sequence of Tertiary, Cre- taceous and older rocks. Volcanic cones rise over 2000 m above the general land surface, reaching elevations over 6000m.

The Altiplano forms a ca. 200 km wide region of subdued topography, east of the Cordillera Occidental, at an average altitude of ca. 3800 m. It is essentially a region of internal drainage bounded by the Cordilleras to the west and east which supplied sediment to several basins within the Altiplano throughout the Tertiary. Near La Paz, drainage has locally broken through to the Amazon Basin and the present topography mainly reflects the levelling of deformed Paleozoic to Tertiary hills. The vast salars of Uyuni and Coipasa, and also Lake Poopo, are the remnants of once extensive lakes whose ca. 15-40 Ka terraces and beach deposits are found throughout the Altiplano (Ser- vant and Fontes, 1978).

The Cordillera Oriental, consisting of deformed Paleo- zoic-Tertiary strata and igneous rocks, rises abruptly from the eastern side of the Altiplano, reaching elevations of 6500 m and forming the high eastern spine of the Bolivian Andes which slopes off towards the Amazon Basin in a zone 200 to 400 km wide. The eastern margin of the Cor- dillera Oriental, where the Andes merge with the Amazon Basin, is referred to as the Subandean zone. This is pres- ently the most tectonically active part of the Bolivian

Andes, where up to ca. 140 km of shortening has been accommodated since the Late Miocene (Roeder, 1988; Herail et al., 1990 and Baby et al., 1992, 1993).

Geochronological Sampling

We have dated 30 rhyolitic airfall and reworked tuffs which either mantle topography or form part of sedimentary sequences in the Altiplano and Cordillera Oriental of Bolivia in a region 300 km by 300 km (Fig. 1 and Table 1). They contain variable amounts of biotite, feldspar, horn- blende, quartz and glass. We have also dated two samples from a volcanic complex in the central Altiplano. In addition, eight samples from the Quimsa Cruz granite in the Cordillera Oriental were dated in parallel with fission track dating of zircon and apatite (Lamb and Hurford, in preparation). Finally, we have dated a sample from a recently discovered kimberlite in the Cordillera Oriental (Matos, 1990).

All the samples were dated using the K-Ar method (see Appendix, results in Table 2). The work was carried out at the NERC Isotope Geosciences Laboratory at Keyworth. The discussion of sample sites and results is divided into two main sections. The first section deals with samples from the central Altiplano, and the second section deals with those from the Cordillera Oriental. The overall implications of the new age data for the Tertiary tectonic evolution of this part of the Andes is discussed in a final section.

CENTRAL ALTIPLANO

The Bolivian Altiplano has acted as a major intramontane basin for much of the Tertiary, where as much as 10 km of Tertiary terrestrial sedimentary deposits have accumulated. However, both facies changes and lateral variations in stratigraphic thickness suggest that, in detail, the Altiplano consisted of a number of sub-basins. Correlation of lithological units over large distances is sometimes dif- ficult. Unconformities between units cannot be correlated across the entire region and can often be traced into conformable sequences elsewhere in the basins.

Recently, there have been a number of attempts to iso- topically date Tertiary sedimentary sequences in the northern Bolivian Altiplano, where important mammal fossils have been found (Swanson et al. 1987; Lavenu et al., 1989 and Marshall et al., 1992). However, only a very few isotopic dates have been published for Tertiary sequences in the central and southern Altiplano region (Kussmaul et al., 1975; Evemden et al, 1977 and Grant et al., 1979). It is in this latter region that we have concentrated our work. Although geological maps exist for much of this region (Bolivian Geological Survey-GEOBOL 1: 100,000 Geo- logical Map Series), there is some confusion about both stratigraphic nomenclature and the correlation of stratigraphic units within the region.

The central part of the Bolivian Altiplano contains an exceptionally thick Tertiary sedimentary sequence (Fig. 2). Both stratigraphic measurements of exposed sequences, and seismic reflection profiles (unpublished oil company reports) show that locally, as much as 10 km of red beds

166 L. KENNAN, S. LAMB~~~C.RUNDLE

Sample

Table 1. Location of Samples Dated for this Study

Locality Topographic Map Sheet UTM Grid Altitude Geological Name Name Number Reference Meters Context

Tambillo Tuffs

s3

Rod-91

Chiar Kollu

TAM-E

Corque Tuffs

CORlO

COR22

COR28

COR26

Parotani Basin

PAROTlNV

2018A

2029

Sacaba Basin

2033A

20348

1334

Valley Fills

2079A

RODEO1

2050

Sucre Airport Tuff

SUC/Sucre

TUFFB

Cuchi Vilque RIO MULATOS SE 19-16 FU 786 391 3700 Azurita Fm.’

Rodeo RIO MULATOS SE 19-16 FU 834 393 3800 Tambillo “Basalt”

Chiar Koliu RIO MULATOS SE 19-16 FU 697 503 3740 Chiar Kollu sill

Tambo Tambillo RIO MULATOS SE 19-16 FU 743 516 3780 Tambillo Fm.

Corque CORQUE SE 19-11 FV 714 381 4850 Huayllamarca Fm.

Choquecota CORQUE SE 19-l 1 FV 152 961 4850 Umala Fm, Toba 76

Opoqueri CORQUE SE 19-11 FV 167 506 3780 Huayllapucara Fm.

Opoqueri CORQUE SE 19-11 FV 172513 3780 Huayllapucara Fm.

Huaycha Loma Parotani 6341 III GA 823 544 2500 Parotani Fm.

Chara Mokho Parotani 6341 III GA 868 517 2450 Parotani Fm.

Huirquini Parotani 6341 Ill GA 894 521 2420 Parotani Fm.

Lawa Lawa Alto

Lawa Lawa Alto

Colque Mayu

Colomr

Colomi

Colomi

6441 IV

6441 IV

6441 IV

JF818687

JF 822 691

JF 860 716

2945 Sacaba Fm.

2920 Sacaba Fm.

2930 Sacaba Fm.

Jatun Chutu Tarata 6341 II GA 984 603 2875 Valley fill

Rodeo PUNATA SE 20-5 JF 245 457 3400 Valley fill

Parotani Parotani 6341 III GA 817 567 2560 Valley fill

Airport Rd.

Km9

SUCRE 6536 IV KD 603 946 2840 Sucre basin fill

Sucre to Tarabuco Road

CKH0.L Ckochis

VILAITARAl TUFF2 Tarabuco

TUFF2

TH.PH.flUFFl Thiju Phujru

Khoha Paya lgnimbrite

KPAAUFF4 Khofia Paya

Tinguipaya lgnimbrite

T6P Tinauioava Road

Sucre 6536 I KD 687 834 2840 Mantling tuff

SUCRE SE 20-l 3 LD 000 810 3200 Valley fill

Sucre

SUCRE

SUCRE SE 20-13 KD705 022 3725

6536 I

SE 20-13

KD 722 774

KD 309 481

3140

3050

Valley fill

Khoria Paya lgnimbrite

Mondragon Fm.


Table 1. Location of Samples Dated for this Study (Continued)

Sample Locality Name

Topographic Name

MapSheet UTM Grid Number Reference

Altitude Geological Meters Context

Los Fraiies ignimbrite

LFTRLE Yocalla- SUCRE

LFrRSL Tinguipaya Road SUCRE

LFPUR Potosi-uyuni SUCRE Road

SE 20-13

SE 20-13

SE 20-l 3

Quimsa Cruz Granite

QCl

QC2

QC3

All samples from All samples 6143 Ill the Mina from the Mina Argentina to Caracoles map 6143 “I Choquetanga Road

sheet. 6143 Ill

QC4

QC5

QC6

QC7

QC8

Kholioaqui Kimberiite

AR-1 Laguna Kholloaqui

6143 III

6143 III

6143 III

6143 III

6143 III

lndependencia 6242 IV

JD 980 642

JD 920 585

JD 972 182

4100 Condor Nasa

3800 lgnimbrite

3850 Los Frailes lgnimbrite

FB 773 276 4580

FB 787 297 ?

FB 786 283 4095

FB 787 277 4265

FB 787 275 4400

FB 786 272 4550

FB 790 269 4680

FB 792 261 4855

GB 277 178 4200

Block not in situ

All samples from the Mina Argentina Granite

Kholloaqui Kimberlite Dike

* Upper case indicates 1:250,000, lower case 1:50,000; Indicates proposed correlation.

Table 2. K-Ar Dates from the Cordillera Oriental and Altiplano

Sample Material Grain-size K (%) Dated (pm)

Tambiiio volcanic horizon and tuffs

Weight for Atmos. Rad. Ar. Age (Ma) Ar (g) Ar (%) (nug) +2-sigma

s3

s3

Rod-91

Chiar Kollu

TAM-E

Corque tuffs

CORlO

CORlO

COR22

COR22

COR28

COR26

Biotite

Biotite

W.R.

W.R.

Biotite

Biotite

Biotite

Biotite

Biotite

Biotite

Biotite

125-250 7.19 0.1533 43.69 6.986 24.8 f 0.8

250-500 7.2 0.1492 50.25 6.8696 24.4 I!Z 0.9

250-500 2.15 0.8058 68.56 1.9392 23.1 f 1.2

250-500 1.09 0.7144 48.63 1.0756 25.2 f 0.9

250-500 7.14 0.2059 41.74 4.4129 15.8 + 0.5

250-500 6.67 0.0866 69.54 6.2466

500-l 000 6.78 0.2076 51.68 6.0895

250-500 5.9ttt 0.218 89.43 1.9201

500-l 000 6.03 0.2238 94.66 1.1201

250-500 5.9t 0.2424 67.42 3.542

250-500 5.785 0.0895 90.98 3.0812

23.9 f 1.3

23 _+ 0.8

8.35 + 1.49

4.77 ?I 1.84

15.4 + 0.9

13.7 * 2.9

168 L.KENNAN,S.LAMB~~~C.RUNDLE

Table 2. K-Ar Dates from the Cordillera Oriental and Altiplano

Sample Material Dated

Grain-size K (%) (pm)

Weight for Atmos. Rad. Ar. Age (Ma) Ar (g) Ar (W) (nug) &Bsigma

Parotani Basin

ParotW

Parotl

Parotl-Rl*

Parotl -R2*

2018A

2029

Sacaba Basin

2033A

20338

20348

20348

1334

1334-R’

Valley fills

2079A

20798

20798

RODEO1

RODEO1

2050

2050

Biotite

Biotite

Biotite

Biotite

Biotite

Biotite

Biotite

Biotite

Biotite

Biotite

Biotite

Biotite

Biotite

Sucre Airport luff

TUFFB* Biotite

Sucre’ Biotite

Sucre-R’ Biotite

SUC.A Biotite

SUC.B Biotite

Sum to Tarabuco Road

CKH0.L Biotite

TARAl Biotite

TUFFZ’ Biotite

VILA.B Biotite

VILA.C Biotite

TUFFl Biotite

TH.PH.l Biotite

TH.PH.2 Biotite

TH.PH.2 Biotite

125-500 7.15 0.1354 41.19 5.3103 19kO.9

125-500 7.11 0.089 87.61 5.3328 19.2 + 2.9

125-500 7.11 0.2114 26.61 5.3602 19.3 f 0.7

125-500 7.11 0.0563 73.6 5.1751 18.6k 1.3

250-500 7.25 0.2051 34.24 5.7961 20.5 f 0.6

125-250 7.31 0.1535 53.29 5.8571 20.5 + 0.7

250-500 7.58f 0.1204 93.62 0.87

125-500 7.7 0.2826 83.35 0.9187

125-250 7.575 0.2333 89.29 0.6431

250-500 7.625 0.3311 75.01 0.6484

500- 1000 3.50tt 0.2552 91.02 0.9423

500-l 000 3.58tt 0.1055 95.67 0.6424

2.95 f 0.92

3.07 f 0.33

2.18 + 0.38

2.19 + 0.15

6.76 f 1.44

4.61 + 2.12

125-250 7.53 0.1801 90.16 0.6483 2.21 kO.42

125-250 7.64 0.3587 94.27 0.4296 1.45 f 0.53

250-500 7.75 0.1812 92.97 0.5577 1.85 f 0.51

125-250 7.25 0.2389 57.14 2.79 9.87 f 0.39

250-500 7.3 0.2701 66 2.4951 8.77f 0.42

250-500 7.57 0.2123 75 1.9501 6.62 f 0.44

500-l 000 7.53 0.3718 72.9 1.8634 6.36 + 0.39

125-500 7.51 0.4959 54.36 0.9321 3.19 f 0.12

125-500 7.47 0.2099 77.33 0.9793 3.37 f 0.25

125-500 7.47 0.2801 76.25 0.9713 3.34 + 0.24

250-500 7.61 0.2776 66.42 1.0684 3.61 f 0.18

250-500 7.7 0.19 82.13 1.026 3.43 k 0.33

250-500 7.61 0.3453 66.34 1.0895

125-500 7.1t 0.216 79.81 0.8238

125-500 6.96tt 0.4988 52.11 1.0282

250-500 7.55 0.3576 82.37 0.9921

250-500 7.72 0.39 80.58 0.9872

125-500 7.7t 0.5135 95.41 1.315

250-500 7.71 0.3239 92.26 0.5152

250-500 7.7 0.4664 89.77 0.4636

500- 1000 7.65 0.3901 94.59 0.4316

3.68 + 0.18

2.98 f 0.28

3.8 k 0.22

3.38 -+ 0.33

3.29 f 0.29

4.39 + 2.49

1.72 f 0.42

1.55 f 0.32

1.45 f 0.54


Table 2. K-Ar Dates from the Cordillera Oriental and Altiplano (Continued)

Sample Material Dated

~$n-size K (%) Weight for Atmos. Rad. Ar. Ar (g) Ar (%)

Age (Ma) (nug) f2-sigma

Khoiia Paya lgnimbrite

KPA Biotite

TUFM’ Biotite

Tinguipaya lgnimbrite

T6P’ Biotite

TGP-w Biotite

Los Frailes lgnimbrites

LFTRLE Biotite

LFTRSL Biotite

LFTRSL Biotite

LFPUR Biotite

Quimsa Cruz Granite

QCl Biottte

QC2 Biotite

QC3 Biotite

QC4 Biotite

QC5 Biotite

QC6 Biotite

QC7 Biotite

QC8 Biotite

Kholloaqui Kimberlite

AR-1 Phlog.

250-500 7.6 0.3715 84.99 0.9685 3.28 f 0.39

125-500 7.4 0.2327 74.19 0.8889 3.09 f 0.2

125-500 6.98 0.1051 50.19 5.3805 19.7 f 0.8

125-500 6.98 0.1057 35.09 5.1698 19.0 + 0.7

250-500 7.11 0.3096 60.83 1.9733 7.13 f 0.3

125-250 6.97 0.355 45.61 1.8443 6.8 f 0.23

250-500 6.8 0.2461 78.52 1.9608 7.4 f 0.6

250-500 7.36 0.2739 62.86 3.4183 11.9+ 0.6

500-l 000 6.95 0.1972

500-l 000 6.21 0.2123

500- 1000 6.8lt 0.1999

500-l 000 7.29 0.2005

500-l 000 7.46 0.1846

500- 1000 6.65 0.085

500- 1000 6.24 0.0825

500- 1000 6.98 0.2226

Megacrysf 7.87 0.1005

43.99 6.7097 24.7 + 0.8

40.76 5.529 22.8 f 0.7

21.16 6.3636 23.9 + 0.9

44.49 6.8027 23.9 Z!I 0.8

50.89 7.0054 24 + 0.8

62.38 7.0587 27.1 + 1.2

75.31 8.3691 34.2 f 2.3

23.02 7.0119 25.7 k 0.7

26.65 30.7235 97.7 rt 2.8

All constants as in Steiger and JBger, 1977. t all K determinations have c. 1% error except t = 1.5%, tt = 2% and ttt indeterminate, due to insufficient samples for repeat analysis. Dated during 1992 pilot. Rl , R2 etc. refer to replicate analysis from the same prepared sample.

overlie Cretaceous strata. These are now folded into a NNW-trending regional syncline, the Corque Syncline (Fig. 3), which is truncated to the east by a major fault zone (Fig. 3). A thick sequence of Tertiary red beds outcrop farther south in the Tambo Tambillo area (Fig. 4). Here, the transition between Tertiary and Cretaceous sequences is very well exposed.

We report new ages which constrain the ages of these unusually thick Tertiary sequences. In the following descriptions we generally adopt the stratigraphic nomenclature of the available published geological maps (Paz et al., 1965, 1966; Ponce er af., 1966).

Tambo TambiUo Region

A stratigraphic summary and the positions of dated samples are shown in Fig. 2. Uppermost Cretaceous El Molino Formation sandstones, shales, oolites and stromatolitic limestones are overlain conformably by as much as 4 km of Potoco Formation red shales, siltstones and fine sandstones with some cross-bedded medium sandstone members. These are overlain conformably by a prominent 30 m sandstone-conglomerate unit, containing granitoid, gneissic and volcanic clasts. followed by about 350 m of sandstones, tuffs and tuffaceous sandstones.


Corque Syncline

Western Limb Eastern Limb Umala Fm. Umala Fm.

Fine sandstones,

and medium sandstone members

Base not exposed

Additional symbols:

- Angular unconformity

vvvvvw Lavas, volcaniclastic rocks and tuff s

Tambo Tambillo Region

oolitio and stromatolltlc

- limestones

Dated samples .._ . . . . . . . . .I. Proposed correlations

S Generalised palaeocurrent directions based on field measurements of cross-beds, ripples etc

Fig. 2. Lithostratigraphic logs of sequences in the central Altiplano showing stratigraphic positions of the samples dated and the proposed correlations across the Corque Syncline, and with the Tambo Tambillo region.

The overlying ea. 120 m thick Tambo Tambillo Volcanic Horizon (TVH) is a laterally variable complex of basaltic to andesitic intrusive and extrusive volcanics. Red sandstones above this volcanic horizon rapidly coarsen up into the Tambillo Formation conglomerates which contain clasts up to 50 cm across of all the underlying formations. This sequence, at least 3 km thick, contains occasional biotite-bearing tuffs and is overlain unconformably by Rosa Pata and Quehua Formation red beds and ignimbrites. Figure 4 shows the locations of the dated samples.

Geochronological Results. S3 is a very fresh, biotite- rich tuff about 150 m stratigraphically below the TVH outcropping near Cuchi Vilque village, SSE of Tambo Tam- billo. Two size fractions of biotite from S3 gave ages of 24.8 +_ 0.8 (0.125-0.25 mm fraction) and 24.4 +_ 0.9 Ma (0.25-0.5 mm fraction). The high potassium contents of these biotites (ca. 7.2%) suggest that there has been no significant alteration. In addition, a whole rock sample from a very fresh basaltic andesite (Rod-91), collected from the TVH in the nose of the Tambillo Syncline to the east, yielded an age of 23.1 + 1.2 Ma.


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10km

Seismic section

Umala Fm ““conformity m

Crucero/Pomata Fm ““conformity r

Totora/HuavllaDucara Fm - . Volcaniclastic horizon

Huayllamarcflurco Fm

Andamarca H

Azurita conglomerates

Cretaceous - lowest Tertiary

Gypsum diapirs

Fault 4 A Syncline axis *

Fig. 3. Sketch geological map of the Corque Syncline. The syncline is bounded to east and west by major faults which are clearly imaged on seismic sections. The map shows the major stratigraphic divisions discussed in the text. An angular unconformity is clear at the base of the CrucerolPomata and Umala formations. Also shown are major towns, line of seismic section and the locations of sample sites. “Line of section” refers to Fig. 5.

A sill (sample Chiar Kkollu), intruded into steeply dipping Tertiary red beds about 500 m stratigraphically below the TVH (Hoke et al., 1993 and Fig. 4), was also dated. This is an extremely fresh medium to fine-grained basalt with mm-sized subhedral olivine (forsterite-rich) and aug- ite phenocrysts in a labradorite and titanomagnetite-rich matrix and gave an age of 25.2 f 0.9 Ma.

Sample Tam-E, outcropping beside the road east of Tambo Tambillo village, is a welded biotite and pumice bearing tuff within the Tambillo Formation conglomerates

about 2.5 km stratigraphically above the TVH. A coarse biotite fraction (0.25-0.5 mm) yielded an age of 15.8 + 0.5 Ma. The biotites contain 7.14 wt% K, and also have a low atmospheric Ar content (41.74%), suggesting that they have not been significantly altered despite their rust-red appearance.

Discussion. Our K-Ar dates are consistent with the observed stratigraphy. Thus, the age of a basaltic andesite from the TVH (ca. 23 Ma) is bracketed by both the age of immediately underlying ca. 25 Ma and overlying ca. 16


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Sample Sites

lijiiiiijjiiiijjl

Tam billo Conglomer ‘ate

Tam billo Volcanics Potoco Fm

Cretaceous

Sketch Cross-section

SW NE

10 kms Fig. 4. Sketch geological map and cross-section of the Tambo Tambillo region. Sample sites at and below the Tambo Tambillo Volcanic horizon and within the Tambillo conglomerates are shown. For clarity, the unconformably overlying, flat-lying <IO Ma Quehua Forma- tion red beds and ignimbrites are not shown.

Ma tuffs. This is also consistent with an age of ca. 22 Ma (no error bars) reported by Evernden et al. (1977) for a basalt from the TVH south of Tambo Tambillo village. The age of the nearby basaltic sill at Chiar Kkollu (25.2 t 0.9 Ma) suggests that it was emplaced at about the same time as the eruption of the overlying TVH.

Assuming a constant sedimentation rate above the TVH, the ages suggest that the preserved thickness of Tambillo Formation conglomerates represents sedimentation up to at least 14 Ma. This suggests that the folding affecting these took place after 14 Ma and is consistent with the fossil evidence from the unconformably overlying Rosa Pata and Quehua Formations (Marshall and Sem- pere, 1990) which suggests an age of 10 Ma or younger.

The finer-grained part of the Potoco Formation, beneath the Tambillo volcanic horizon, which is as much as 5 km thick and forms the lower part of the Tertiary sequence

exposed in this part of the Altiplano (Fig. 2), may be at least 40 Ma old. These formations could be significantly older if sedimentation rates in the finer-grained facies were slower than in the younger, coarser-grained sequences.

Corque Syncline

Figure 2 shows the lithostratigraphy exposed on both limbs of the Corque Syncline, which dip between 30” and 90”. The stratigraphy on the eastern limb is complicated by faulting. However, a thick unfaulted sequence, up to 10 km thick, outcrops near and northwest of the village of Corque. At Corque, well-indurated and alternating medium cross-bedded sandstones, siltstones and shales of the Huayllamarca Formation, coarsen upwards and the sequence becomes conglomeratic. Near the base of these conglomerates there is a distinctive 5 m thick greenish vol-


caniclastic breccia-conglomerate with abundant volcanic fragments (shown as a “dacitic sill” by Paz et al., 1966). About 150 m stratigraphically above this volcanic horizon, the first of several biotite-bearing tuffs and tuffaceous sandstones, each up to 50 cm thick, outcrops along a prominent ridge, which essentially marks the western limit of well-indurated rocks on the eastern limb of the Corque Syncline.

About 2 km west of Corque, there is a transition into the less indurated Totora Formation, which outcrops throughout the core of the syncline. In the Corque region, this is as much as 4.5 km thick and is locally overlain with angular unconformity (unpublished oil company data) by the prominent Crucero Formation conglomerate which includes the ca. 9 Ma Callapa tuff at its base (Marshall et al., 1992). The Crucero Formation is overlain with slight angular unconformity by the poorly consolidated sandstones and conglomerates of the Umala Formation (equivalent to “Mauri Formation” of Paz et al., 1966) which has a thick pumice-fall tuff (Toba 76) near its base (Marshall et al., 1992).

The sequence on the western limb of the syncline is slightly different than on the eastern limb near Corque. The lowest exposed stratigraphic unit is the medium- grained Turco Formation. Above this is a well exposed, prominent 40 m thick conglomerate, called the Azurita Formation, which contains metamorphic, granitoid and volcanic clasts. In the overlying sandstone sequence, about 50 m stratigraphically above the Azurita Formation conglomerate, there is a ca. 40 m thick rubbly agglomerate unit (shown as a “basaltic sill” by Paz et al., 1966), containing rounded and angular clasts up to 50 cm across of porphyritic basaltic to dacitic volcanics. This is overlain by a ca. 5-6 km thick conformable sequence mapped as Hua- yllapucara and Totora Formations. Above this, the 500 m Pomata Formation conglomerates contain predominantly andesitic volcanic clasts up to 15 cm across. In contrast to the eastern limb, the sandstones and tuffs of the Umala

CORQUE SYNCLINE

Formation overlie these older units with angular unconformity (Hoffstetter et al., 1972). Figure 3 shows the locations of the dated samples.

Cross-sections through the Corque Syncline, based on field mapping and a network of good quality seismic sections (unpublished oil company data, Fig. 5), clearly dem- onstrate that the syncline has a relatively simple concentric structure and that no faults disrupt the core of the syncline. The units on the western limb are direct lateral equivalents of those on the eastern limb. We can clearly correlate the first appearance of conglomerates and the volcaniclastic horizons at Corque, on the eastern limb, with those at Opo- queri, on the western limb. Also, the Crucero Formation, in the east, and the Pomata Formation, in the west, are clearly equivalent.

Geochronological Results. We were unable to find tuffs suitable for dating in the lower part of the Huayllama- rca Formation. CORlO comes from a white tuff 1 km west of Corque, about 200 m stratigraphically above the volcaniclastic agglomerate. Two size fractions of fresh biotite gave ages of 23.9 + 1.3 Ma (0.25-0.5 mm fraction) and 23.0 + 0.8 Ma (0.5-1.0 mm fraction). We also dated two biotite separates from COR22, a weathered ignimbrite at the base of the Umala Formation. The sample is from the same locality as the one dated by Marshall et al. (1992, sample LGM23, dated at 5.34 Ma), referred to as Toba 76. Our samples gave inconsistent dates of 8.35 + 1.49 Ma and 4.77 f 1.84 Ma.

Two samples from the west limb of the Corque Syn- cline, near Opoqueri, were also dated. A single biotite separate from COR28, a biotite-rich tuffaceous sandstone from about 200 m above the agglomerate, contained only 5.9% K, suggesting some alteration had occurred. It gave an apparent age of 15.4 * 0.9 Ma. A biotite separate (COR26) from a white tuff layer, ca. 500 m stratigraphi- tally higher in the section, had a marked reddish color (oxidized) with only 5.79% K and yielded an apparent age of 13.7 f 2.9 Ma.

Azurita Formation COR28

Pomata Crucero Formati CORlO Volcaniclastic

NE

horizon

Vertical = Horizontal Fig. 5. Cross-section, based on exposed strata and unpublished oil company data, of the Corque Syncline. Note the thick sequence in the core of the syncline. Sequences at Opoqueri, on the west limb, and at Corque, on the east limb, can be easily correlated.

v&s &Z-D

174 L. KBNNAN, S. LAMB and C. RUNDLE

Discussion. An early Miocene age for the upper Huayl- lamarca Formation near Corque is consistent with the discovery of a middle Miocene (12-15 Ma) fossil mammal fauna (Hoffstetter et al., 1972) in the Totora Formation near Choquecota (Fig. 3), well above the Huayllamarcal Totora transition. The ca. 23.5 Ma date for the CORlO tuff, on the eastern limb of the Corque Syncline, suggests that this stratigraphic level is nearly chronostratigraphically equivalent to the Tambo Tambillo Volcanic Horizon. The cross-section (Fig. 5) clearly shows that the distinctive green-colored breccia-conglomerate, ca. 250 m stratigraphically below CORlO, and the agglomerates near Opoqueri are equivalent and we would correlate both of these directly with the Tambo Tambillo Volcanic Horizon. This horizon is clearly of regional significance, and can be traced for at least 150 km along strike.

We would also correlate the first appearance of coarse conglomerates in the Tambo Tambillo region, just below the Tambo Tambillo Volcanic Horizon, with the distinctive first appearance of conglomerate below CORlO, on the eastern limb of the Corque Syncline, and with the base of the Azurita Formation, on the western limb. Therefore, we can define an important regional episode of coarse-grained sedimentation in this part of the Altiplano, starting at about 25 Ma.

Thus, our ages for biotites in tuffs on the western limb of the syncline (COR26 and COR28) seem rather young, given that the tuffs are only a few hundred meters stratigraphically above the 23-24 Ma agglomerates. The low potassium contents of the biotites (< 6% K) suggest significant alteration. We suggest that alteration may have pro- moted greater loss of Ar than of K, thus causing the “dates” to be too young. The conclusion that these tuffs are actually older than the dates obtained is supported by dates reported by Swanson et al. (1987) of 18.8 f 0.5 Ma and 24.5 f 0.6 Ma for sanidines in tuffs which are probably lateral equivalents of COR26 and COR28.

The ca. 9 Ma Pomata and Crucero Formation conglomerates can also be clearly correlated on seismic sections across the core of the syncline. They locally lie with low angle unconformity on the Totora Formation on the east limb of the syncline. This unconformity defines the end of the marked north-south strike ridges of indurated sandstones near Corque but dies out rapidly northwards. Clearly uplift and erosion on the eastern limb of the southern part of the Corque Syncline was initiated between 15 and 9 Ma, prior to the deposition of the Pomata/Crucero Formations, resulting in up to 20” of tilting on the eastern limb of the Corque Syncline.

Marshall et at. (1992) also found a wide variation in ages for biotites from a thick pumice-fall tuff west of Choquecota (COR22, also referred to as Toba 76), ranging between 5.4 and 9.8 Ma. Our ages range between 4.8 and 8.4 Ma. The Marshall et al. preferred age of 5.34 Ma was based on repeated laser dating of individual biotite crys- tals. They considered the older ages to date xenocrystic biotite. If we accept this age, then the underlying ca. 7 to 8 km thick conglomeratic Tertiary sequence between COR22 and CORlO (Crucero, Totora and top of the Huay- llamarca Formations) represents about 15 Ma of sedimentation.

Finally, the age of the base of the Umala Formation (sample COR22) also constrains the timing of the main folding in the Corque Syncline. Although on the eastern limb the Umala Formation overlies the Crucero Formation with relatively low-angle unconformity, there is locally marked angularity at its base on the western limb (Hoff- stetter et al., 1972). This suggests that the bulk of the tilting took place sometime in the interval 9 to 5 Ma. A further small amount of tilting (< 10”) took place after the deposition of the Umala Formation.

CORDILLERA ORIENTAL

The Cordillera Oriental consists mainly of folded Pale- ozoic flysch sequences, up to 10 km thick (Martinez and Tomasi, 1978). Thin (generally < 2 km) Cretaceous fluvio- lacustrine and possibly marine deposits overlie folded and faulted Paleozoic rocks with marked angular unconformity. The Cretaceous sequences are generally only exposed in isolated synclinal cores, mainly between Coch- abamba and Sucre in the central parts of the Cordillera.

Tertiary red bed deposits are found throughout the Cor- dillera Oriental. Sandstone, shale and conglomerate sequences, up to 2 km thick, are found either conformable with Cretaceous sequences, or resting with angular unconformity on Paleozoic and Cretaceous rocks. Thin sandy and conglomeratic deposits, with intercalated tuffs, locally mantle topography or infill small valleys. On the western margin of the Cordillera Oriental, in the Potosi region, there is a regionally extensive sequence of ignimbrites, including those of the Los Frailes Formation. The latter are generally flat-lying and appear to post-date significant compressive deformation in this part of the Cordillera Ori- ental. Further north, the western margin of the Cordillera Oriental contains a number of granitic bodies which intrude Paleozoic and Cretaceous rocks.

Dating these sequences and intrusives constrains both the timing and distribution of Tertiary deformation in the Cordillera Oriental, and also the development of the present-day deeply dissected topography.

Cochabamba Region

Tuff-bearing post-Eocene sedimentary strata are found in the Cochabamba area in a series of regular fault- bounded basins (Fig. 6). The basins are related to ESE- trending strike-slip and normal faults in the bend region of the Bolivian Andes, where there is a marked change in the general structural trend. The fault systems cut across folded Lower Paleozoic to Eocene rocks and may play an important role in the later Tertiary kinematics of the central Bolivian Andes (Sheffels and Klepacki 1985; Sempere et al. 1990; Dewey and Lamb 1992). The new geochronological results are given below for each basin, and their significance is discussed.

Results from the Parotani Basin. The Parotani Basin (Fig. 7) is a shallow half-graben bounded on its northern margin by the Tapacari fault. The basin fill consists of about 200-300 m of fine red marls with locally thin conglomerate lenses along the basin margins, which are folded into a series of NNW-trending synclines and anti-


Fig. 6. Outline geological map of the Cochabamba region, showing major strike-slip faults and associated Miocene and Pliocene- Pleistocene basins. The NW-SE trending Colomi fault is an active sinistral fault at the back of the active Subandean forelaud fold and thrust belt. Active regional shortening does not occur west of this fault. Sample sites and locations of more detailed maps are also shown.

clines with limb dips up to 30”. We have dated biotite separates from the same folded white airfall tuff at three separate localities (Fig. 7, samples Parotl, ParotW, 2018A and 2029). The tuff is about 100 m stratigraphically above the base of the red marl basin fill, and also contains l-5 mm clasts of fine-grained friable pumice, The biotites have K contents between 7.11% and 7.31%, suggesting little significant leaching.

Parotl and ParotW come from the top and bottom respectively of a ca. 1 m thick tuff. ParotW gave an age of 19.0 + 0.9 Ma while three repeat runs on Parotl gave ages of 18.6 + 1.3 Ma, 19.2 + 2.9 Ma and 19.3 f 0.7 Ma. Sam- ple 2018A comes from the coarse basal bed of a composite 3 m thick package of tuffs and gypsiferous red marls. The sample gave an age of 20.5 + 0.6 Ma. Sample 2029 comes from the same layer exposed on the northern margin of the basin. The tuff is very biotite-rich and disturbed by plant rootlets. A single biotite separate gave an age of 20.5 f 0.7 Ma. The ages of all the dated biotites are identical within error, with a mean age of 19.5 f 0.5 Ma.

Results from the Sacaba Basin. The Sacaba Basin (Fig. 8) lies to the east of Cochabamba with a present-day exposed surface at an elevation of ca. 2600 m. It is a half- graben basin, bounded on the northern margin by the major ESE-trending Tunari Fault. It contains at least 600 m of sedimentary fill which is gently folded (generally dipping < 40“). Coarse-grained alluvial fan deposits on the eastern and northern margins pass laterally into fine- grained lacustrine deposits in the central and western parts. We have dated biotite separates from two separate tuffs in a flat-lying conglomerate sequence on the southern margin of the basin (Fig. 8 and Table 1).

Samples 2033 and 2034 are different parts of a ca. l-2 m thick grey tuff unit which outcrops near Lawa Lawa Alto (Fig. 8). The lower part of the unit (sample 2033A and B) is an airfall tuff, while the upper part (sample 2034B) has been reworked and shows a marked concentration of biotite, fine-grained material having been washed out. Biotites have 7.5 to 7.7% K, suggesting little or no leaching of K. Two size fractions from sample 2034 gave dates of 2.18 f 0.38 Ma (0.125-0.25 mm fraction) and 2.19 + 0.15 Ma (0.25-0.5 mm fraction) while two size fractions from 2033 gave dates of 2.95 + 0.92 Ma (0.25-0.5 mm fraction) and 3.07 + 0.33 Ma (0.125-0.5 mm fraction). Therefore, an estimate for the age of this tuff would be 2.60 k 0.48 Ma.

Repeat runs on a coarse biotite fraction (0.5-l .O mm) from a tuff further east (Fig. 8, sample 1334) yielded apparent ages of 4.61 f 2.12 Ma and 6.76 f 1.44 Ma. These ages have large uncertainties and may be unreliable since the very low potassium (3.58%) and the high atmospheric argon content suggest substantial alteration.

Valley Fill Deposits. Biotite-bearing tuffs are occasion- ally found in sand and gravel valley-fill sequences in the Cochabamba region (Fig. 6), which post-date the main phase of deposition in the adjacent basins. However, in some cases, these sequences are cut by reactivated faults related to the major basins.

Samples 2079A and B are part of a valley-fill between the Santibafiez and Cochabamba Basins which is cut by ESE and NE-SW faults. A single composite tuff band has well developed bed-parallel lamination suggesting an airfall origin. Some thin beds are certainly reworked and contain freshwater mollusc shells. The three dated biotite se-


- Fault 0 SamDIes W Parotani x3 Vertical Exaggeration

Fig. 7. Sketch map of the Miocene Parotani Basin showing outcrops of the Parotani Formation and the location of sample sites. The cross-section shows the gentle folding of the tuff horizon, ca. 100 m from the base of the Parotani Formation, which can be traced throughout the basin. The Parotani Formation locally unconformably overlies basin-margin faults and earlier folds near Parotani village and west of sample 2029. Sample 2050 is from a younger valley fill lying at 2560 m.

parates all have high K contents between 7.53 and 7.75%. and yielded ages of 2.21 f 0.42 Ma (0.125-0.25 mm fraction), 1.85 + 0.5 1 Ma (0.25-0.5 mm fraction) and 1.45 + 0.53 Ma (0.125-0.25 mm fraction). The mean of these overlapping ages is 1.84 + 0.44 Ma.

Southeast of the Punata Basin greenish lacustrine shales and tuffs fill a series of valleys. Outcrops at ca. 3500 m in the southeastern part of the valley near Rodeo village (Fig. 6) contain tuff bands up to 50 cm thick intercalated with poorly consolidated gravels and sands. Two size fractions of biotite from the lowest tuff (RODEO-l) yielded ages of 9.87 f 0.39 Ma (0.125-0.25 mm fraction) and 8.77 * 0.42 Ma (0.25-0.5 mm fraction), with K contents of 7.25% to 7.3%.

Sample 2050 lies at an elevation of ca. 2560 m on the NE side of the low hill west of Parotani village (Fig. 7). It is a very coarse unlaminated pumice-rich tuff which rests unconformably on the red marl sequence of the Parotani Basin. Biotites have a K content of about 7.6%. Two biotite size fractions gave ages of 6.62 + 0.44 Ma and 6.36 + 0.39 Ma.

Discussion. The above results suggest that the basins in the Cochabamba area, though morphologically similar, have very different ages. The age of tuffs in the Parotani Basin suggest that sedimentation here began in the Early Miocene and not the Pliocene as has been previously thought (Lavenu and Ballivian, 1979). However, the basin- fill rests unconformably on Cretaceous and Paleozoic rocks, suggesting pre-Early Miocene deformation in this region. The basin-fill is also folded, and this deformation must be older than the remnants of ca. 3000 m regional peneplanation surfaces (see below) preserved in the vicin-

ity. Substantial dissection of these predates ca. 6 Ma, when a younger tuff was deposited in a valley-fill sequence which unconformably overlies the Parotani Basin (sample 2050).

Likewise, erosion and the development of the landscape southeast of the Punata Basin is older than ca. 8.5 Ma, when tuffs in local valley-fill deposits were deposited (Rodeo-l). Sedimentation in the Sacaba Basin appears to be young, and postdates the 3000 m surfaces. We prefer a late Pliocene age for the upper parts of the Sacaba basin fill, consistent with the reports of Glyptodont fossils on the southern margin in the Lawa Lawa area (Marshall et al. 1992) and the discovery of Pliocene fossil deer vertebrae in correlative strata in the central parts of the basin (Ken- nan, D. Phil. thesis, 1994).

The protracted history of basin development in the Cochabamba region can be related to movement on the major ESE-trending basin bounding faults. Movement on some of these faults appears to have begun prior to the early Miocene, since the fill of the Parotani Basin truncates faults and associated en echelon folds. Gentle folding in the Sacaba Basin, probably related to sinistral strike-slip movement on the Tunari fault, must also be younger than late Pliocene. Faults cutting the valley fill near samples 2079A and B suggest faulting probably continued around the more westerly basins while the Sacaba Basin fill onlaps the basin margins at Lawa Lawa Alto and is unfaulted.


. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~;;~iy~~~~, ::::::::::::::::::::66’OOriiiiiii :::::::: Pabeo*olo ..*i?

.:::::::: ::Ll”aof iiiiiii ~~~~~~~

:

........ ........

........

........

........

........

........

........

........

........

........

........

.......

66”05’ : ....... .+. ..... : : : Qmbrada Fierro Churu .................. .

::::I::::::: . ... . . . . ... . . ............ III::::I:::III::3::

Fig. 8a. Sketch map of the Sacaba Basin, showing the present drainage networks and the major faults along the northem basin margin. All the dated samples come from a single, prominent tuff band that can be traced throughout the basin. Deer fossils (F) of the same age have been found in the center of the basin. Quatemary erosional terraces are cut into the folded basin fill. “Line of section” refers to Figure 8b.

Sucre Region

Throughout the Cordillera Oriental, at altitudes between 2800 m and 3300 m, there are remnants of a regional peneplain (Pig. 9) which formed in a strip about a 100 km wide, extending nearly 500 km from near Cocha- bamba, in the north, all the way to the Argentinian border. In southern Bolivia, this peneplain has been referred to as the San Juan de1 Oro surface (Servant er al., 1989; Gub- bels er al., 1993). It is usually an erosional surface which cuts across deformed Paleozoic, Cretaceous and Tertiary rocks, and is virtually flat lying. However, the surface is locally defined by the top of flat-lying sequences of poorly consolidated fluvial deposits, which in southernmost Bolivia have yielded Middle to Late Miocene vertebrate faunas (Marshall and Sempere, 1991; MacFadden et al.; 1990, 1993).

5000 metres

The surfaces may well be all that remains of a wide Fig. 8b. Sketch cross-section across the Sacaba Basin, showing coarse alluvial fans passing laterally into fine, lacustrine

region of drainage. Such a system of low gradient drainage has the effect of smoothing out the topography by eroding the high ground and depositing sediment in the low areas. This would continue until the topographic gradients become too low for the rivers to have significant erosive power. This process is similar to that which sculpted the present region of subdued topography at an average elevation of ca. 3800 m in the Altiplano, which is mainly a region of internal drainage. The remains of another region of subdued topography are found to the east of the San Juan de1 Oro surface and its lateral equivalents, at a lower altitude of 2000 to 2500 m. The peneplains in the Cordillera Oriental are now deeply dissected by the present drainage system, which has cut more than 1000 m down through the surfaces. Thus, dating the peneplains

sequences in the basin center.

places constraints on both the timing of the last phase of folding in this region and the initiation of the development of the present landscape in this part of the Andes.

In the Sucre region, there are well preserved remnants of the ca. 3000 m surface which are either mantled by thin sequences of fluvial deposits and intercalated tuffs, or contain tuffs in shallow valleys cut into the surface. We have dated biotite separates from a number of these tuffs. All the biotites are glossy and black and appear extremely fresh with potassium contents between 7.47% and 7.72%. Figure 9 shows a general location map for the dated samples.


6~7 '15 ' 65"OO '

19000 '

Vila Vila Tuff

Tarabuco

Surface c.3000 m

0 Sample sites

m Major Towns

\ u e Major Roads

19045 '

65"15 ' 65"OO

19000 '

19015 '

19030

19045 '

Fig. 9. Map of the Sucre area showing remnants of the 3000-3200 m San Juan de1 Oro Surface and major present-day drainage systems. Shallow erosional bowls cut into the surfaces contain tuffs and ash flows which were sampled at four sites. Late Pliocene to Recent dissection by the Rio Pilcomavo and its tributaries has removed most of the lower parts of the surface. The surface overlies the southern end of the A&tile dextral fauh (Fig. 1).

Sucre Airport Road. Near Sucre Airport, a ca. 40 m thick sequence of gravels is capped by two tuffs interbed- ded with pebbly sands and silts (Fig. 9 and 10). The sequence lies in a shallow bowl at an altitude of ca. 2840 m and is folded into a gentle syncline cut by several N- trending dextral strike-slip faults. Samples come from the upper biotite-rich tuff. Biotite separates (0.125-0.5 mm size fraction) from sample Sucre yielded ages of 3.37 + 0.25 Ma and 3.34 f 0.24 Ma (repeat run) and sample TUFF3 yielded 3.19 + 0.12. Biotite separates (0.25-0.5 mm size fraction) from samples SUCA and SUC.B gave ages of 3.43 f 0.33 Ma and 3.61 f 0.18 Ma. The ages for SUC.A and TUFF3 do not overlap, possibly reflecting differential weathering or analytical problems. The mean of the five ages is 3.39 f 0.14 Ma.

Tarabuco Region. The road to Tarabuco, southeast of Sucre, crosses an extensive remnant of a peneplain surface at an altitude of ca. 3ooO m. Here, a number of nearly horizontal tuff deposits either rest directly on folded Paleozoic rocks or are part of thin valley-fill sequences in shallow depressions within the surface.

Sample CKH0.L is part of a ca. 1 m thick and pumice- bearing airfall deposit, which mantles the remains of a broad valley at an altitude of ca. 2880 m, slightly lower than the extensive planar surfaces to the southeast (ca.

3000 m). A biotite separate (0.25-0.5 mm size fraction) yielded an age of 3.68 + 0.18 Ma.

Sample V1LA.A. and V1LA.B come from a dry stream about 5 km north of Tarabuco cut into a gently undulating plain at an altitude of ca. 3200 m. Two to three meters of cross-bedded tuffaceous sandstones, locally very biotite- rich tuffs, and occasional thin pebbly or soil horizons are exposed. Two biotite separates from well-separated sample sites (0.25-0.5 mm size fraction) were analyzed in 1992. Sample VILA.B yielded an age of 3.38 + 0.33 Ma, while V1LA.C gave 3.29 k 0.29 Ma. Two weathered samples with lower K content from the same site yielded ages of 2.98 + 0.28 Ma and 3.8 + 0.22 Ma. The mean of the two high-K ages is 3.34 + 0.22 Ma.

TH.PH.l, TH.PH.2 and TUFF 1 all come from a tuff which caps the fill of a shallow valley cut into an extensive, very flat plain at an altitude of ca. 3000 m near Yam- paraez. The tuff is about 1 m thick and reworked, is very biotite-rich and contains some fresh, probably primary, muscovite. TH.PH. 1 gave an age of 1.72 f 0.42 Ma (0.25- 0.5 mm size fraction) while TH.PH.2 gave ages of 1.55 + 0.32 Ma (0.25-0.5 mm fraction) and 1.45 If: 0.54 Ma (0.5- 1 .O mm fraction). TUFFI gave a poorly constrained age of 4.39 It 2.49.


Biotite enriched placer

10 Metres Vertical scale is twice horizontal Lower tuff

Fig. 10. Sketch of the Sucre Airport road tuff sample site. All the dated samples come from the upper, biotite-rich, reworked tuff. The other sediments are mainly rootleted silts, sands and slopewash breccia. The gentle fold is cut by minor N to NNE trending dextral faults and E to ESE trending sinistral faults.

Khor?a Paya Ash Flow. At Khofia Paya bridge (Fig. 9), there is a composite, unwelded ash-flow tuff about 10 m thick, with a greeny bentonitic matrix. This overlies a few meters of valley fill gravels which are resting on Paleozoic bedrock. The tuff is restricted to the floor of a valley lying at about 3050 m, about 150 m lower than an extensive very flat plateau region to the east and south. Biotite separates from the middle and base of the flow yielded ages of 3.09 It 0.2 Ma (TUFF4,0.125-0.5 mm size fraction) and 3.28 + 0.39 Ma (KPA, 0.25-0.5 mm size fraction) respectively.

Discussion. The above ages suggest that the ca. 3000 m regional peneplain in the Sucre region certainly existed prior to ca. 3.5 Ma when the older dated tuffs were deposited. However, this surface must have had a topographic relief of a few hundred meters at that time, forming essentially a series of broad depressions with occasional high standing ridges. Limited dissection of the surface must have begun by ca. 3.5 Ma, producing the valleys, such as at Khoiia Paya, beneath the surrounding dominant surfaces. Much of the surface was still substantially intact at 1.5 Ma, when the youngest tuff that we have dated in this region was deposited in a ca. 20 m deep paleovalley. All these tuffs and associated deposits are cut by the major valleys, such as that of the Rio Pilcomayo, which form part of the present drainage system and have deeply dissected the surface. This suggests that all the main topographic features in this part of the Cordillera Oriental, at altitudes lower than ca. 3000 m, are younger than ca. 3.5 Ma and perhaps younger than 1.5 Ma.

An age of 3.36 f 0.3 Ma has been reported for a tuff which mantles a lower peneplain surface near Padilla, at an altitude of ca. 2100 m (Marshall and Sempere, 1991). The relation of this lower surface to the San Juan de1 Oro surface remains unclear. There is no evidence for significant Plio-Pleistocene vertical movements in this region that could have displaced vertically a once-continuous surface. Thus, it appears that prior to ca. 3 Ma the Cordillera Oriental had a step-like topography, with two general levels at ca. 3000 m and ca. 2100 m. Though we believe that these general levels formed as a consequence of erosion in

a system of low gradient drainage, it is still unclear why such internal drainage basins should form in the first place. They are clearly a significant feature of the late Miocene to Pliocene development of the Cordillera Oriental.

The ca. 3000 m surface post-dates regional folding in this part of the Cordillera Oriental, and possibly significant movement on major lineaments such as the Aiquile fault zone. However, the deformation of the tuff near Sucre airport suggests that there has been limited N-trending dextral-strike slip movement in this region since ca. 3.5 Ma.

Potosi Region

The Potosi region has a particular concentration of volcanic and pyroclastic rocks. The most prominent is the Los Frailes Formation, comprising a series of regionally extensive and subhorizontal ignimbrite sheets which unconformably cap the underlying geological formations. Beneath the Los Frailes a number of lava and tuff units occur within Tertiary red bed sequences which themselves unconformably overlie deformed Cretaceous strata. Figure 11 shows the location of dated samples.

Results from the Mondragon Formation. The Mon- dragon Formation (Fig. 12) is exposed beneath the Los Frailes and Morococala ignimbrite mesetas along their eastern margins. A possible correlative is exposed along the Potosi to Oruro road in an erosional window beneath the Los Frailes. The sequence of continental sandstones and conglomerates is several hundred meters thick and unconformably overlies the Cretaceous La Puerta Forma- tion, overstepping onto Paleozoic strata. The formation appears to have been deposited in a number of isolated basins parallel to regional strike. About 100 m from the base of the formation there is a composite package of three welded ashflow tuffs, each about 10 m thick. They contain both flattened pumice fragments up to 10 cm across and xenoliths of Paleozoic rocks. A biotite separate (T6P) from outcrops along the Tinguipaya road yielded ages of 19.7 f 0.8 Ma and 19.0 I!Z 0.7 Ma (duplicate run).


t 6

I

66”OO’ Los Frailes

m Fm.

Condor -

-20°00 20”00’-

17”OO 66”OO L

Fig. 11. Geological sketch map of the late Miocene Los Frailes ignimbrite shield near Potosi, showing the Central Meseta (I 2 Ma and younger) and the satellite Livi Chuco (undated) and Condor Nasa (ca. 7 Ma) Mesetas. The map also shows the outcrop of the underlying, deformed, early Miocene Mondragon Formation and the location of the sample sites.

Los Frailes Zgnimbrites. The Cordillera Los Frailes (Fig. 11) is the larger of two ignimbrite shields in the western part of the Cordillera Oriental. The Central Meseta is about 100 km across and rises in a broad dome to about 4800 m in the center. There are satellite mesetas further east, including the Condor Nasa Meseta (Schneider, 1985) which unconformably overlies the Mondragon Formation near Tinguipaya. We have dated fresh-looking biotite separates (K between 6.8% and 7.36%) from welded ignimbrites in the Condor Nasa Meseta (samples LFTRSL and LFTRLE) and from the southern margin of the Central Meseta (sample LFPUR). LFTRLE yielded an age of 7.13 &- 0.3 Ma. Two size fractions from LFTRSL gave ages of 7.4 + 0.6 Ma (0.25-0.5 mm fraction) and 6.8 _+ 0.23 Ma (0.125-0.25 mm fraction). Sample LFPUR yielded an age of 11.9f0.6Ma.

Discussion. The results from sample T6P support an Early to Middle Miocene age for the Mondragon Forma- tion. Evernden et al. (1977) report an age of 20.1 Ma for biotite from a volcanic flow...“close to the base of the formation” at a similar locality to T6P. Also, volcanic flows and ashes in the thick Canteria and Aqua Dulce Forma- tions near Potosi have been dated at between 20.3 and 2 1.9 Ma (Evemden et al., 1977 and Grant et al., 1979) and may be lateral equivalents of the Mondragon volcanic rocks. The Mondragon Formation rests with angular unconformity on Cretaceous and older rocks, indicating pre-early Miocene folding in this region. However, the Mondragon Formation is also folded with dips up to 20”, and is unconformably overlain by the subhorizontal Los Frailes ignim-

brites. Thus, our results also suggest further folding in this region between ca. 20 Ma and ca. 12 Ma (apparent age of sample LFPUR).

The younger ages for the ignimbrites in the Condor Nasa Meseta are similar to those of 7.4 f 0.23 and 7.5 k 0.17 reported by Grant et al. (1979) who obtained an age of 6.3 f 0.1 Ma for ignimbrites in Morococala Meseta further north. This suggests a regional episode of ignimbrite eruption in this part of the Cordillera Oriental between 6 and 8 Ma. Slightly younger ages for cones constructed on the Morococala flows are reported by Lavenu et al. (1989).

Intrusive Rocks from the Cordillera Oriental

The high part of the Cordillera Oriental in northern Bolivia is often referred to as the Cordillera Real. This consists of a series of high peaks, some reaching 6500 m, made up of granitoids and their metamorphic aureoles. Topographic relief is locally several kilometers and so the granitoid bodies and associated metamorphic rocks offer a good opportunity for radiometric cooling studies. Zircon U-Pb dates of ca. 225 Ma suggest the granitoids of the Zongo valley, north of La Paz, are Triassic (Farrar et al., 1990). Fission track, K-Ar and Ar-Ar, and Rb-Sr studies do not provide unequivocal evidence for age of intrusion but reveal an extended history of Triassic to Tertiary reheating and cooling (Farrar et al., 1988; McBride et al., 1983 and 1987). A combination of both fission track and K-Ar dating on a vertical section offer the possibility of tightly constraining both the cooling and erosion history in this part of the Cordillera Oriental.

Los Frailes lgnimbrite

Yondragon Formation Miocene Sandstones and silts

La Puerta Fm Cretaceous Sandstones

Ordovician Shales and Slates

-7 Ma

dark lava m thick)

Fig. 12. Schematic log of the stratigraphy exposed at site T6P above Tinguipaya village, showing the pre-Cretaceous, pre-early Miocene and pre-late Miocene unconformities and the positions of the dated samples. Elsewhere the Mondragon Formation oversteps the Cretaceous La Puerta Formation to lie directly on Paleozoic basement.

K-Ar dates from the Altiplano and Cordillera Oriental of Bolivia

Quimsa Cruz Granitoids

The southernmost granitoids in the Cordillera Real outcrop in the Quimsa Cruz range (Fig. 13). We follow the terminology of Miller (1988) who defined two major granitoid bodies with different ages of intrusion and observed several small-scale intrusive bodies. The “Mina Argentina Granite” (MAG) consists of equigranular monzogranite and granodiorite and the “Mina Viloco Granite” (MVG) porphyritic monzogranite.

Miller also reports marked chemical and isotopic simi- larities between the MAG and the definitely Triassic Zongo-Yani granitoids. Otherwise, the ages of MAG and MVG are poorly constrained. A whole-rock Rb-Sr isoch- ron from the MAG gives an apparent age of 300 f 48 Ma with biotite-whole rock pairs giving ages from 209 to 16 Ma (Miller, 1988). K-Ar biotite ages from the MAG range from 25.9 to 24.2 Ma (McBride et al., 1983). In contrast, Rb-Sr data (Miller, 1988) and K-Ar data (Evemden, 1977

181

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.\..\\\.\.

2 KM

-QC6

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Fig. 13a. Geological sketch map of the Quimsa Cruz batholith (see Fig. 1 for location). The boundaries between the?Paleozoic- Mesozoic “Mina Argentina Granite” and the ?Cenozoic “Mina Viloco Granite” are poorly constrained. b. Detailed location map for samples collected from the “Mina Argentina Granite” (MAG). Samples were collected along the road to Choquetanga which cuts through the eastern margin of the MAG, dropping over 1000 m. Samples of granite (QCl-8) were collected for K-Ar and fission track dating.


5000

.s

3 4200 - 4

: 1

a-i h

4 a--I

4000 - ,I 4- In situ altitude of sample QC2 not known h

3800 I , I , I I I, I, I, I, I, I’ 20 22 24 26 28 30 32 34 36 38 40

K-Ar Biotite Age (Ma) Fig. 14. Graph showing K-Ar biotite ages for samples from the MAG (QC1,3-8), plotted against altitude. Sample QC2 is from a loose boulder, about 5 m across, at an altitude of 3970 m. The graph suggests a general increase in age with altitude. A postulated trend is shown by the shaded line.

and McBride et al., 1983) have been used to suggest that the MVG intruded around 25 Ma.

We have dated biotites in a suite of eight samples collected from a ca. 1000 m vertical section from the MAG (Fig. 13b, Table 1). Coarse-grained biotites (OS-1 .O mm size fraction) with K contents between 6.24% and 7.46% gave K-Ar ages between 22.8 f 0.7 Ma and 34.2 f 2.3 Ma. Fig. 14 shows these ages plotted against sample altitude and suggests a slight but general increase in age with altitude. Unfortunately the vertical sampling interval (> 1000 m) is not sufficient to fully substantiate such a trend.

However, some of our samples do show K-Ar ages markedly older than others obtained for the MAG (QC2 and QC7) while others are younger than those previously obtained for the MVG.

Fission track dates on zircons in these samples, and also from metasedimentary rocks outcropping at lower altitude, have also been determined (Lamb and Hurford, in preparation). Zircon ages vary between 38.4 and 20 Ma and also show a trend of age increasing with altitude. However, the K-Ar biotite ages appear to be either the same or even slightly younger than the zircon fission track ages. Only in one case (sample QC7) is the K-Ar age significantly older than the zircon fission track age. These results are puz- zling, as the effective annealing temperature for fission tracks in zircon is usually thought to be lower than the blocking temperature for argon in biotite (Harland ef al., 1990). Despite this, we provisionally suggest that the K-Ar and zircon fission track ages, analyzed on their own, indicate rapid cooling of the MAG between 27 and 22 Ma. If this cooling is due to erosional unroofing, then both the K- Ar and fission track data suggest an average unroofing rate of 0.3-0.4 mm/yr during this period. More detailed studies,

and possibly U-Pb zircon dates, are required to resolve the dating problems in the Quimsa Cruz range. The fission track data will be discussed in more detail elsewhere (Lamb and Hurford, in preparation).

Kholloaqui Kimberlite. Matos (1990) reported the first known kimberlite body, with associated mantle xenoliths, from Bolivia (found by S. Fernandez and R. Matos in 1983). It outcrops near Laguna Kholloaqui (see Table 1 and Figs. 1 and 2 of Matos, 1990 for exact location), northwest of Independencia, in an area well known for alkaline igneous bodies (Ahlfeld, 1965; 1975). The kimberlite forms vertical stocks and dikes which intrude and post- date folding in low-grade Paleozoic strata and contain xenoliths of alkaline syenites. We have dated a large phlogopite crystal (ca. 4 cm across) from the main Kholloaqui dike at 97.7 f 2.8 Ma. In the absence of other data we assume that the crystal does not contain excess 40Ar and that this date represents the age of intrusion.

CONCLUSIONS

In the previous sections, we have presented new age data for Cretaceous and Tertiary rocks at various localities in the Altiplano and Cordillera Oriental of the Bolivian Andes. We have compared our dates with previous work and discussed the implications of these data for the timing of local sedimentation, deformation and erosion. In this section, we discuss the broader implications of these age data for our understanding of the tectonic development of this part of the Central Andes. A picture emerges of a con- stantly shifting locus of deformation within an ever widen- ing zone since the Early Tertiary.


Pre-Cretaceous and Cretaceous

Both the angular unconformity at the base of the Creta- ceous sequences throughout the Cordillera Oriental, and cross-cutting relations of the kimberlite near Independen- cia indicate that there was significant folding prior to ca. 100 Ma. However, the present region of the Andes was at or close to sea level in the latest Cretaceous, when the widespread lacustrine and marginal marine Cretaceous sequences were deposited.

Early Tertiary (Fig. Ha)

In the Altiplano and throughout the Cordillera Oriental, Cretaceous sequences pass up conformably into thick fluvial deposits. Our dates suggest that up to 5 km of red beds were deposited in the central Altiplano during the Early Tertiary. Sedimentological studies (Lamb et al,, in preparation) indicate that much of this fine-medium grained sediment was deposited by rivers flowing from the Cordillera Oriental in the east. Very coarse Eocene conglomerate sequences in the Cordillera Oriental lying conformably immediately above Cretaceous rocks were deposited by river systems flowing from the west (Lamb et al., in preparation). This suggests that in the earliest Tertiary, a narrow zone of uplift, less than 100 km wide, began to develop in what is today the western part of the Cordillera Oriental, shedding sediment both to the west and east.

Middle Tertiary (Fig. 15b)

Our age data from the Corque and Tambillo regions indicate that in the latest Oligocene, at ca. 2.5 Ma, there was widespread development of coarse alluvial sequences in the central Altiplano, again with sediment transport direction radiating from the Cordillera Oriental (Lamb et al., in preparation). The coarse-grained nature of these deposits, compared to the relatively fine-grained early Ter- tiary fluvial deposits, also derived from the east, suggests that relative uplift of the eastern margin of the Cordillera Oriental had become more pronounced by the early Miocene. Seismic sections in the flat Altiplano region to the east of Corque show a marked angular unconformity at the base of the early Miocene sequences. This indicates that folding on the western margin of the proto-Cordillera Oriental had extended ca. 50 km westwards prior to the latest Oligocene. However, the presence of a thick sequence of Miocene and younger sediments above this unconformity suggests that the western front of the deforming Cordillera Oriental subsequently migrated back eastwards to approximately its present position. K-Ar biotite and fission track zircon ages suggest that at the same time, between ca. 27 and 22 Ma, the granitoid plu- tons on the eastern margin of the Cordillera Oriental in the Quimsa Cruz range were undergoing rapid cooling. This was most likely a consequence of rapid erosion in this part of the Cordillera Oriental.

In the latest Oligocene to earliest Miocene, in what is now the Subandes, the lower part of the Petaca Formation (Sanjines and Jimenez, 1976) was deposited disconform- ably on upper Cretaceous strata, east of the eastern margin

of the Eocene foreland basin. This also suggests that the zone of deformation in the Cordillera Oriental had broad- ened and moved further east by the Early Miocene.

Subsequently, throughout the central Altiplano, at ca. 23 to 24 Ma, there was a regional episode of basaltic to dacitic volcanism. This episode seems to coincide with the appearance of frequent and thick tuff units in the Tertiary sequences. The full significance of this episode of volcanism is not clear. However, the eruption of basalts with oce- anic island affinities (Davidson and de Silva, 1992; Hoke et al., 1993) suggests significant mantle melting at depth.

The distribution of Early Miocene sequences indicates that, by then, deposition in the interior of the Cordillera Oriental was localized in small basins. The Early to Mid- dle Miocene Mondragon Formation, and also the Early Miocene Parotani basin-fill, rest unconformably on older folded Cretaceous to Eocene sequences in the Potosi and Cochabamba regions, which deformed during the early Tertiary uplift of the proto-Cordillera Oriental and during the Late Oligocene advance to the east.

In the Early and Middle Miocene there was a marked change in sedimentation patterns in the Altiplano. The deposition of the coarse Tambillo Formation conglomerates in the eastern Altiplano may be related to the continued uplift on the western margin of the Cordillera Oriental, resulting in the folding of the Mondragon Formation further east, prior to the ca. 7 Ma Los Frailes volcanism. In the subsurface beneath Lake Poop0 in the eastern Altipl- ano seismic sections clearly image Middle Miocene synsedimentary, west-verging thrusting and folding. Far- ther west in the southern part of the Corque syncline, gentle folding of the Middle Miocene Totora Formation occurred before the deposition of the Late Miocene Pomata and Crucero Formations.

Late Tertiary (Fig. 15~)

Folding throughout the Cordillera Oriental seems to have ceased by the Late Miocene, when the regional systems of low gradient drainage in the Cordillera Oriental were well established. Also, huge and undeformed ignimbrite sheets, covering a large part of the Cordillera Oriental in the Potosi region, were erupted between ca. 12 Ma and ca. 7 Ma. This ignimbrite activity may be a consequence of crustal melting during thermal re-equilibration, following the Early to Middle Miocene crustal thickening in the Cor- dillera Oriental. Such thermal equilibration characteristi- cally has a 10 to 20 Ma time lag after crustal thickening (England and Thompson, 1986).

Although deformation had largely ceased in the Cordil- lera Oriental by the Late Miocene, intense folding and erosion of the Tertiary sequences occurred in the Corque region of the Altiplano between 9 and 5 Ma, prior to the deposition of the ca. 5 Ma Umala Formation. Much of the deformation may have been compartmentalized by the major fault systems east and west of the Corque Syncline. Farther east, in the Poop0 Basin, synsedimentary deformation had also largely ceased. To the south, in the Tambillo region, strata younger than 10 Ma are undeformed. Subse- quently the locus of significant shortening in the Andes


a.

\ /ARGENTINA j/

c.30-10 Ma

Present day foreiand basins

Late Oligocene to mid- Miocene lnbamontane and foreland basins

B Palaeocene to Oligocene . . . . . . . . . . . . . . Ahiplmo basin

Palaeocene foreland basin

CB MB

ZB

PB

ZSGZ

Generalised flow/drainage

Cochabamba Basins

Mondragon Basin

Zurima Basin

Parotani Basin

Zongo-San Gaban Zone

Main Mountain Fronts or Fault Systems

Fig. 15. Summary paleogeography of Bolivia (without palinspastic restoration). International borders and boundaries of present-day physiographic divisions (see Fig. 1) are shown for reference. 15a. Shows development of proto-Cordillera Oriental. Coarse sediments shed into foreland basin while finer facies accumulated in Altiplano basins. 15b. Note the eastward propagation of the Cordillera Oriental and establishment of new foreland basin. Isolated Miocene basins formed within the Cordillera. 1%. The Subandes foreland thrust belt and basins migrate eastwards. Development of low gradient drainage and erosion surfaces within the Cordillera Oriental occurred, along with strike-slip and basin formation, in the Cochabamba area.


shifted much further east to the Subandean zone, where red bed sediments accumulated since the Late Miocene (Sempere, 1990) were strongly deformed during the Pliocene-Pleistocene. The Altiplano remained an internal drainage basin, accumulating several hundred meters of fine-grained fluvial sediments in local depocenters such as the La Paz Basin.

In the Cochabamba region sinistral strike-slip motion continued on the ESE-trending faults. The Early Miocene Parotani Formation was shortened prior to the development of ca. 3ooO m peneplains in the Cochabamba region which predate the development of the main Sacaba and Cochabamba Basins. The deformation of the Pliocene basin fills indicates that strike-slip related deformation continued in this area until the Late Pliocene to Pleis- tocene.

The development of the Middle-to-Late Miocene basins and associated peneplains in the Cordillera Oriental and their deep dissection in the Plio-Pleistocene is complexly related to significant shortening and uplift in the Suban- dean fold and thrust zone. Initially deformation may have disrupted drainage out of the Cordillera Oriental. Later, shortening in the Subandean zone would have increased the regional topographic gradients on the eastern margin of the Cordillera Oriental. Rivers draining off the Subandean zone may have then cut back through the eastern margin of the perched drainage systems, so that by ca. 1.5 Ma these drainage systems had been captured with the initiation both of the dissection of the Cordillera Oriental and the formation of the present landscape.

Acknowledgements- This work forms part of the Andean Project at the Department of Earth Sciences, Oxford and a convenio with the Servicio Geologic0 de Bolivia (GEOBOL), initiated by John Dewey. Leonore Hoke collaborated in tbe collection of the samples analyzed in this study. This paper has benetitted immensely from discussions with John Dewey, Leonore Hoke, Russ Harmon, Stephen Moorbath, and Eduardo Soria- Escalante. We acknowledge the helpful reviews by A.W. McLaughlin and W. Avila. This work could not have been canied out without the financial support of British Petroleum, the Royal Society, Shell, the British Coun- cil and the Department of E%uth Sciences, Oxford.

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APPENDIX

Outline of Preparation and Analytical Procedure

Faure (1986) has described the K-Ar method of dating. The error quoted for any date is the formal error associated with the analytical procedure (95% confidence limits). The biggest problem with the method is the meaning of any age so measured. In the simplest case, the method deter- mines the age since the rock or mineral last underwent total degassing of all radiogenic argon produced by the decay of 40K. This age is usually taken to date the time elapsed since potassium-bearing minerals in the rock passed through a critical blocking temperature, below which the minerals retain Ar gas. However, the meaning of this age becomes complicated if a rock contains a number of potassium bearing minerals which have widely different blocking temperatures, or if there is a mixed population of the same mineral with different ages. The latter might be the case in a volcaniclastic rock, where for instance newly formed biotites might be mixed with xenocrystic basement biotite. Also, weathering or hydrothermal alteration of minerals might result in argon loss which again complicates the interpretation of the age. At all stages care was taken to overcome these problems.

Multiple samples were taken from most sites to try and average out the effect of variations in age within a single outcrop due to differential weathering. The freshest material containing dark glossy biotites was sampled. Minerals were examined in thin section to determine the presence of weathered rinds, chloritisation, or non-juvenile detrital or xenocrystic mica. Some tuff samples contain muscovite, which is probably juvenile, reflecting the peraluminous nature of the parent magma.

Samples were initially crushed into a coarse gravel, then sieved into 0.125-0.25 mm, 0.25-0.5 mm and 0.5-1.0 mm size fractions. Biotites were separated by paper-shaking, sieving and magnetic separation. A finely-tuned Frantz magnetic separator was used for the latter. In many cases, different size fractions from the same sample were dated. It is unlikely that both juvenile and other biotites would have the same grain- size distribution, so concordant dates from more than one size fraction suggest a single age population of minerals. We interpret such dates to be the age of eruption.

For K determination, a few hundred mg of sample were dissolved in a mix of perchloric and hydrofluoric acids and analyzed on an Instrumenta- tion Laboratory 543 flame photometer, using a lithium internal standard to minimize matrix effects. Replicate analyses of in-house and internal standards indicate that, for pure samples, an overall precision of + 1% ( lo) can be expected. Generally, duplicate determinations are made and the mean is accepted if results are within 2% of each other.

The Ar analyses were carried out on a VG Micromass 1200 gas source mass-spectrometer. All samples were baked overnight at 180°C to minimize atmospheric contamination. Samples were fused using RF induction heating and analyzed using isotope dilution with an enriched 38Ar spike. Ar determination errors were calculated by combining the errors in the isotope ratio measurements assuming 1% error for the spike calibration (based on the statistics of the spike-decay regression analysis) and incor- porating any error enhancement consequent on correcting for the contan- inating atmospheric Ar content. The spike volume was calibrated against international standards and the calculated radiogenic Ar content should be accurate within the quoted limits. Ar analysis quality was also assessed by comparing analyses for related samples.

k-ar dates from the altiplano and cordillera oriental of ... · plegamientos pre-miocenos....

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