Rheology of the lower crust controls the polarity of conjugated basins asymmetry on the South Atlantic passive margin

doi:10.13745/j.esf.sf.2020.4.2

Abstract

Abstract:

The passive continental margin basins on both sides of the South Atlantic contain a large amount of oil and gas resources and became an important target for oil and gas exploration in recent years. The conjugated basins of the South Atlantic passive margin has clear asymmetry, such that the width of the Santos Basin (~850 km) on the South American side is four times that of the South Kwanza Basin (~200 km) on the African side. Basin structural asymmetry leads to significant differences in oil and gas distribution and accumulation. However, the dynamics of this asymmetrical structure is still controversial. Here, we carried out a series of 2D thermodynamic numerical modeling experiments, using finite element method to solve the constitutive equation of the visco-elastic-plastic continuous medium. The results showed that the rheology of the lower crust is the key factor controlling the structure of the basin. When the rheology of the lower crust is asymmetrical, basins developed on the craton are narrower than those on the orogenic fold belts. The deep crust of the craton alters the flow of the asthenosphere, thus controls crustal rupture and basin structure. Compared to the previous modeling results, our model can predict not only the formation process of asymmetric basins, but also the polarity of the conjugated basins to better match the geological conditions of the South Atlantic.

Key words: passive continental margin basin of the South Atlantic, asymmetric expansion, lower crust, rheological properties, numerical simulation

CLC Number:

P542
P541

WANG Dianju, LI Jianghai, LI Yihe. Rheology of the lower crust controls the polarity of conjugated basins asymmetry on the South Atlantic passive margin[J]. Earth Science Frontiers, 2020, 27(3): 254-261.

Figures/Tables 6

Fig.1 Geological setting of conjugated basins on the South Atlantic margin. Modified from [5].

Fig.2 Basic setting of numerical modeling

Table 1 The basic model parameter settings

模型层	密度/ (kg·m^-3)	热膨胀系数/ (10^-5K^-1)	热容/ (J·kg^-1·K^-1)	摩擦系数	内聚力/ MPa	幂律指数	活化能/ (kJ·mol^-1)	活化体积/ (cm³·mol^-1)
上地壳	2 700	2.7	750	0.5	5.0	4.0	223	0
下地壳(造山带)	2 850	2.5	750	0.5	5.0	4.2	445	0
下地壳(克拉通)	2 850	2.0	750	0.5	5.0	3.0	345	0
地幔	3 300	3.0	750	0.5	5.0	3.5	530	13

Fig.3 Strain rate evolution of symmetrical spreading thermodynamic model

Fig.4 Strain rate evolution of asymmetric spreading thermodynamic model

Fig.5 Temperature structure evolution of asymmetric spreading thermodynamic model

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