Kinematic deformation model of the south-central subandean zone, Bolivia

https://doi.org/10.1016/j.jsames.2020.102768Get rights and content

Highlights

  • The evolution of Subandean structures is related to shortening and mass transfer.

  • The deformed sedimentary column in the Subandean zone can be divided into four main structural levels above the regional detachment that are largely separated or defined by detachment levels.

  • The deformation mechanism of shallower detachments depends on the rheological characteristics of the rock in the studied area.

  • Integration of the MT analysis assisted in identifying the presence of deep structures decoupled from the shallow structuring.

Abstract

The Bolivian Subandean region is characterized as a thin-skinned fold-thrust belt with structures persisting for 50–100 km along strike in a broadly North-South direction. The evolution of the Subandean structures is related to shortening and mass transfer. The stratigraphy comprises ~12 km of Ordovician to Recent sediments, which are dominated by the Paleozoic section including Devonian source and reservoir horizons capped by the Middle Devonian Los Monos regional seal. Detachments occur at top of basement, dipping 2° to the west, within the Silurian and two Devonian levels, including the Los Monos shale.

The fold-belt contains the Mega Gas Fields of Bolivia. This study focused on an exploration target in the Igüembe structure, located nearby the Margarita and Incahuasi Gas Fields. In addition to the standard methodology of integrating seismic lines, surface geology and well data, this study also utilized magnetotelluric (MT) analysis to create regional structural sections that were balanced and restored, recording ~40% shortening between 12Ma and the present. The compression was propagated from west to east along a basal detachment along top basement at ~12 km depth, creating four different and superimposed structural units. The Ordovician to Middle Devonian section deformed as antiformal stacks at ~15 km spacing, with the highly ductile Los Monos horizon overlying it and acting as a major detachment and seal. The Upper Devonian to Cretaceous section was buttressed by over 5 km of overlying Neogene sequence; in such a way, that it was forced to propagate to the surface directly above the antiformal imbricates in a sequence of triangle zones above and along the Los Monos detachment. The 5+ km thick Neogene section is preserved as parallel bedded open synclines between the triangle zone culminations. The Middle Devonian source and reservoir may have been buried 6–8 km prior to deformation, which has reduced porosity, but also generated hydrocarbons. It is plausible that during the Siluro-Devonian Cordilleran compressional phase, gentle regional anticlines or inversion structures might have been formed, perhaps with ~15 km spacing. These may have trapped the early-formed hydrocarbons and preserved porosity. These anticlines would be the nuclei for subsequent contractional deformation in the Neogene, with hydrocarbons re-migration and some hydrocarbons remnants trapped in the original structures.

Introduction

The study area is in the southern portion of the central Subandean zone of Bolivia (Fig. 1). It lies within the sedimentary prism of deformation where the contractional structures were generated by Oligocene to Early Miocene subduction of the Nazca plate under the South American plate. The Bolivian Subandean area is characterized as a thin-skinned thrust fold belt with inverted structures persisting for 50–100 km along strike in a broadly North-South direction (Fig. 1; Dunn et al., 1995; Hernández et al., 2002; Starck et al., 2002; McQuarrie et al., 2005). The ridges typically have elevations between 1300 and 2000 masl, while the intervening synclines lie at 600 to 800 masl.

The objective of this study is to build a regional structural model in the geological province of the south-central Bolivian Subandean region by generating a series of balanced and restored sections across the fold belt. The sections were constructed using the relationships that link the geometry of faults with the folds, all of that based on surface geological data, wells, 2D seismic and magnetotelluric information.

Section snippets

Geological background

The Oligocene to Early Miocene subduction of the Nazca plate produced a long-lived deformation and formed a large volcanic arc along the Pacific coast of South America (Ramos and Aleman, 2000; McQuarrie, 2002; Arriagada et al., 2006 and Ramos, 1999). Thrusting and folding occurred in all sequences causing significant shortening and uplift to create the Andean Mountains. Backarc and transported (piggyback) basins were formed within the high plateau and western sector of the Eastern mountain

Stratigraphy

The stratigraphic framework of the Phanerozoic in the southern Bolivian Subandean region is defined by four tectono-stratigraphic or tectono-sedimentary cycles (Sempéré, 1995). Tacsarian, Cordilleran, Subandean established by Suárez Soruco and López (1983), and the Andean cycle determined by Suárez Soruco (2000). The Andean cycle was subsequently subdivided into Andean I and II by Oller Veramendi (1992) (Fig. 2). The Andean cycle constitutes an important control element in the construction of

Data available

Integration of geological and geophysical information was the foundation for this study. Located within the Central Subandean area, 6 seismic sections (Fig. 4) with PSDM reprocessing and magneto-telluric (MT) data along with their respective interpretations were used as the main input to define the structural model. The MT method consists of passive measurements on the Earth's surface of the natural electromagnetic (EM) field. This EM field, generated by the impact of the solar wind on the

Structural observations, interpretation, restoration and balancing methods

The structural interpretation in the Subandean fold and thrust belts has many uncertainties, especially when the seismic quality is not optimal. Magnetotelluric data provides an independent geophysical tool to aid interpretation. For example, in areas where there is a resistance contrast between the reservoir and the overlying rocks, as it is the case for the Huamampampa and the Los Monos Formations, the MT data can help unraveling some of the complex internal deformation demonstrated by

Discussion

The geometry of the southern Subandean zone has been well-documented, especially in the frontal folds close to the foreland, with images of the predominant structural styles that have been identified on several seismic lines (Baby et al., 1992; Dunn et al., 1995; Blake et al., 1999; Ballard et al., 2018). However, many papers do not present the full dataset that has been used to build their interpretations. This paper displays and explains the raw data, especially the seismic and MT

Conclusions

A plausible structural model has been developed in the geological province of the Bolivian South-Central Subandean, identifying the main structural styles present in the area. The model has been restored and balanced in two dimensions along the structural sections. Although the interpretation has been validated from the geometrical point of view, alternative models could be generated from the available data.

Seismic imaging and geological surface mapping are the principal tools used in the

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors thank YPFB Chaco S.A. who permitted publishing these data.

The authors value the suggestions and comments from four colleagues: Dugmar Mendez, Diego Paredes (from 3D-GEO Pty Ltd), Nabil Chávez (from YPFB Chaco S.A.) and Elvira Pureza Gómez (from CNOOC International). We would also like to take this opportunity to thank Ph. D. Fernando Martinez for the invitation to submit this manuscript and, together with Ph. D. Andrés Folguera and other reviewers, for the effort and expertise that

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      However, through a detailed analysis of the sedimentary infill (Fig. 3), Hernández et al. (2018) show the existence of pre-Andean basement reliefs related to extensional mechanisms that occurred since the Permian–Triassic, phenomena being enhanced during the Upper Triassic-Lower-Jurassic and/or Upper Jurassic-Lower Cretaceous, as well as erosive irregularities within the Carboniferous or the Devonian–Carboniferous boundary. These data suggest that a regular and continuous detachment level following a unique lithology, such as the ones involving the Ordovician or Silurian shales (Hernández et al., 2018; Sánchez et al., 2020), may not be the most realistic situation. In this way, an irregular detachment surface should be considered in regional balanced cross-sections construction involving the entire fold and thrust belt system, especially when integrated three-dimensional analysis is carried out.

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