Roeder 1988 2

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  • TECTONICS, VOL. 7, NO. 1, PAGES 23-39, FEBRUARY 1988

    ANDEAN-AGE STRUCTURE OF EASTERN CORDILLERA

    (PROVINCE OF LA PAZ, BOLIVIA)

    Dietricoh Roeder

    The Anschutz Corporation, Denver, Colorado

    Abstract. A Moho root beneath the

    Bolivian Andes, 40 km deep, is consistent with 230 km of overlap of Neogene age on a single, trenchward dipping transcrustal thrust fault with 10 of finite ramp cutoff (Main Andean Thrust (MAT)). Only 10% of the Andean crustal volume is

    ascribable to magmatic addition. The MAT is located within South American crust of full thickness. It intersects the basement top 450 km inland from the Neogene crustal margin. It is not a collision suture as shown by persistent pre-Neogene facies continuity. Thrusting is not accompanied by terrane accretion. The present bilaterally symmetrical thrust belt responds to elastic line loading and to Coulomb rheology. In the hanging wall of the MAT, a deep high-stress wedge base builds a steep critical slope. In the footwall, the foredeep response is fast subcritical growth by progradation and blind thrusting on a low-stress decollement. Interaction is maintained by out-of-sequence renewal of movement on the MAT.

    Copyright 1988 by the American Geophysical Union.

    Paper number 7T0615. 0278-74 07 / 88 / 007T-0615510. 00

    INTRODUCTION

    In orthern Bolivia, the Andean orogen contains on its cratonic east flank a

    marginal fold-thrust belt of Oligocene to Recent age [Jordan et al., 1983]. In this segment of the Andes, bedrock is not covered by volcanics and in part has potential for commercial hydrocarbons. Therefore, some information is available. Surface mapping and photogeology by the Bolivian state oil company Yacimientos Petroliferos Fiscales Bolivia (YPFB), reflection seismic data and sparse drilling by private industry, potential- field data, and refraction seismic data add quantitative aspects to the puzzle of compressional subduction systems termed Andean by Dewey and Bird [1970].

    Data available for the present study have been condensed into three semibalanced structure sections across the external parts of the eastern Cordillera between 14S and 16S latitude. These sections and their support data show that the retroarc system is a nested, conjugate, bilaterally symmetrical thrust complex, with 230 km of overlap on a single intracrustal detachment. Modeled as a Coulomb plastic wedge, the thrust system supports the view that the Andean topography is upheld largely by crustal compression rather than by underpinning and buoyancy alone. The cross sections are

  • 24 Roeder: Andean-Age Structure of Eastern Cordillera

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    consistent with an elastically loaded and flexed craton; they constrain the wide range of rigidities allowed by the Bouguer gravity data.

    I cannot relate quantitatively recent Andean structure to the bimodally dipping Peru-Chile Benioff zone. Intracrustal architecture is unknown except for a very large Moho root. However, 70 to 90% of the crustal volume including root and inclined slab can be accounted for by the crustal overlap. About 20% may be Mesozoic to Paleogene newly accreted crust, and 10% may be additions by magmatism.

    The present paper is an update of a speculative synthesis prior to the present crop of data [Roeder, 1982a]o It results from an early phase of hydrocarbon exploration. It is an example of what can be done in overthrust belts with modern techniques and a moderate amount of data.

    SETTING

    Figure 1 shows a zonation of the Bolivian Andes into topographically and geologically defined

    parallel belts. Reviews of Bolivian geology are by Ahlfeld and Branisa [1960], Martinez and Tomasi [1978], and Martinez [1980].

    The western Cordillera, largely on Chilean territory, is a plateau below 5000 m with widely spaced cones above 6000 m. It is a Neogene to Recent magmatic arc intruding and covering a poorly known crustal slab which may be a southern extension of the Arequipa basement massif. It is the foreland of west vergent thrusting of Triassic and Tertiary age in the Altiplano and the eastern Cordillera.

    The Altiplano is a Tertiary and Quaternary fluviatile complex of internal drainage at 4000 m elevation. Interbasin ridges and a few oil wells expose a passive-margin series of Paleozoic to late Cretaceous age with minor magmatic constituents, all involved in a west vergent fold-thrust belt of mixed late Paleozoic and Andean kinematic ages. In some areas, enough pre-Tertiary is exposed to demonstrate the structural style. Ages of thrusting, however, are in part based on angular unconformities only and may, in

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  • 26 Roeder: Andean-Age Structure of Eastern Cordillera

    part, refer to precompressional fault- block tilting.

    The eastern Cordillera is a belt of

    folded Paleozoic sediments intruded by groups of late Triassic and Neogene granitoid plutons. It forms a continuous ridge above 5000 m peaking in Illampu (7010 m) and Illimani (6882 m). It contains a 20- to 50-km-wide west slope underlain by a major post-Miocene thrust, and a 100-km-wide east slope down to 1000 m of elevation. Easterly drainage dissects this slope and, in the La Paz river, has retrograded into the Neogene lake beds of the Altiplano.

    The sub-Andean zone is 50 to 100 km

    wide and continues the east-Andean slope eastward and below 1000 m, Bedrock

    consists of Paleozoic sediments largely conformable with overlying Mesozoics and Tertiary. The post-Paleozoic series are preserved in flat-bottomed synclines and on the back limbs of ridge-forming east- vergent thrust fronts.

    The foreland of the sub-Andean zone is

    the Amazon River drainage area at less than 500 m. This area is vaguely specified as Madre de Dios basin and Beni basin; it contains fluviatile Neogene above a gently truncated and SW-dipping pre-Eocene series. A zero edge of pre-Neogene sediments forms the southwest limit of the

    outcropping or subcropping Brazilian shield somewhere 100 to 200 km NE of the sub-Andean east front. There is no

    topographic rise associated with the shield.

    STRATIGRAPHY

    The material involved in Andean thrusting consists of South Anerican crust, a Paleozoic passive-margin prism, thin late Paleozoic to Eocene interarc or cratonic deposits, and an Oligocene to Recent foredeep fill. In the following, the stratigraphic sequence is summarized as a narrative legend to the cross sections (Figures 2 to 5).

    PC, basement with Andean and late Proterozoic ages, is documented on the Brazilian Shield and in the Pampas foreland massifs of Argentina as a thin sliver in the frontal thrust of the study area, in a basement core (1750 m) from an Altiplano well, and in exposures in the western Cordillera.

    COS D, an Ordovician through Devonian turbiditic to deltaic passive-margin series, may reach 15 km in thickness

    [McBride et al., 1983]. Now located in the eastern Cordillera, this depocenter may reflect a Paleozoic continental edge and may suggest that thin crust in the western Cordillera is a pre-Andean carry-over and not exclusively a product of subductive erosion.

    P C, disconformable, 1-km-thick Permo- Carboniferous series, grades upward from Gondwana deposits into tropical-marine carbonates [Helwig, 1972]. The top of the series is gradually truncated eastward to a zero edge within the sub-Andean.

    K, TE, volcanogenic redbeds, shallow marine sandstones and shales, locally intercalated with South American pre- Atlantic basalt flows, spans late Cretaceous to Eocene and thins eastward from 2.5 to less than 1 km between the Altiplano and the sub-Andean.

    TM-QD and TM-C, is a series conformable with an Oligocene to mid-Pliocene fluviatile foredeep fill. The correlatable formations Quendeque and Charqui, preserved in synclines and strike belts of the sub-Andean, are 4 km thick at the frontal thrust and 6 km in the westernmost outcrops. The youngest preserved layers are estimated to be early to mid-Pliocene in age. There is no net foredeep sedimentation at present.

    OROGENY, METAMORPHISM, AND MAGMATISM

    If the Andean orogeny is defined in time to the age of its foredeep fill, it becomes critical to distinguish Andean deformation and pre-Andean deformation.

    In the high core of the eastern Cordillera, the Paleozoic series is folded into 0.01- to 1-km-size kink folds at or past locking position, with axial plane spaced cleavage and with upright axial planes. With few local exceptions, th

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