Mechanics of Active Salt Diapirism

RI0224. Mechanics of Active Salt Diapirism, by D. D. Schultz-Ela, M. P. A. Jackson, and B. C. Vendeville. 56 p., 40 figs., 1994. Print.


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ABSTRACT
An active diapir forcefully intrudes its overburden, driven by diapir pressure that overcomes the resistance of the overburden strength. Possible causes of the driving pressure are differential loading of the source layer and a density contrast with the overburden. Resisting forces derive from the mass of the roof block and resistance to the faulting and folding that accommodate the intrusion. This report describes the evolution of active diapirs and development of structures in their rock on the basis of analytical, physical, and numerical modeling.

For a typical density contrast between salt and a sedimentary overburden, a simple force balance demonstrates that the diapir height must be more than two-thirds to three-quarters the thickness of the surrounding overburden to initiate substantial active diapirism. Diapirs with widths less than the regional overburden thickness must be even taller to initiate active diapirism. Physical and numerical models show that typical structures of active diapirism are a central crestal graben flanked by relatively unstrained flaps that rotate upward and outward. Curved reverse faults can separate the flaps from the regional overburden. Normal faults in the crestal graben propagate downward, new faults being created farther outward as the roof arches over the rising diapir. Thin, wide roofs develop multiple grabens separating a relatively flat roof from the rotating flaps. In models leading to emergence, the piercing diapir evolves from a pointed crest to a progressively rounding and widening crest as the flaps rotate outward. Our numerical models show that overburden flexure contributes to early formation of active diapir structures, but piercement does not continue to emergence unless the diapir is very wide or tall relative to the overburden thickness. Active diapirism becomes progressively more difficult for diapir crests having the following shapes: rectangle (least difficult), round-cornered rectangle, semicircle, and triangle with a pointed crest (most difficult).

Keywords: active piercement, diapirism, finite-element models, forceful intrusion, halokinesis, physical models, salt domes, salt tectonics

CONTENTS

Abstract

Introduction

Geology of Active Diapirism

     Structural Geometry

     Evolutionary Paths to Active Diapirism

     Evolutionary Paths from Active Diapirism

Conditions for Initiation of Active Diapirism

     Previous Physical Modeling of Active Piercement

     Physical Modeling of Conditions for Initiation of Active Diapirism

     Analytical Model of Conditions for Initiation of Active Diapirism

Structural Evolution of Active Diapirism

     Physical Modeling of Structural Evolution

Finite-Element Modeling of Structural Evolution

     Model Parameters

     Scaling to Nature

     Initial Phase of Active Diapirism

     Evolution of a Piercing Diapir

     Transition to Emergence

   Axisymmetric Model

     Variations in Diapir Geometry

     Asymmetric Models

     Models Simulating Rock Properties

     Effects of Constant-Stress Boundary Condition

     Interlayer Slip in Roof

Comparison of Model Results

Conclusions

Acknowledgments

References

Figures

1. Interpreted seismic section of a salt diapir from Angola showing characteristics of active piercement

2. Schematic cross section showing the anatomy of an active diapir and nomenclature of faults associated with domes

3. Main structural styles associated with natural active diapirs

4. Interpreted seismic profile through the salt stock of Niendorf II, northern Germany

5. Cross section showing the Napoleonville salt dome, southern Louisiana

6. Cross section through the White Castle salt dome, southern Louisiana

7. Cross section through the Heide salt diapir, northern Germany

8. Cross section through the Sottorf and Meckelfeld salt diapirs, northern Germany

9. Meter-scale active diapirs of Neogene marls with gypsum cores piercing weak Quaternary gravels, Ebro Basin, Spain

10. Cross section through Bayou Blue field, southern Louisiana

11. Cross section through the Wathlingen-Hänigsen salt dome, northern Germany

12. Initial geometry of a typical physical model

13. Ridge B of figure 12 after 16 h of active piercement

14. Plot of physical model geometries on axes of diapir height and width

15. Configuration and variables for force-balance analysis

16. Plot of equations 7 and 14 showing conditions for which the major forces driving active diapirism equal those resisting it

17. Plots of conditions for initiation of active diapirism for different values of friction coefficient and density contrast

18. Cross sections in model of forced diapiric piercement showing progressive structural maturity

19. Apparatus for experiments in which the viscous material was forcefully injected up the diapir conduit

20. Evolution of cohesion and friction angle with equivalent plastic strain in finite-element models to simulate strain weakening

21. Basic finite-element mesh used for active diapir simulations and magnified view of top-left corner of the mesh, where all localized deformation occurs

22. Results from a finite-element model with diapir dimensions of hd*= 0.5625 and w* = 1.125

23. Model of figure 22 at a later stage after 136 s

24. Deformation in overburden above the diapir of figure 22 after 7.3 min

25. Contours of equivalent plastic strain in overburden above the diapir in figure 22 at 40 min

26. Outlines of model in diapir area at progressively longer run times and contours of displacement magnitude

27. Further outlines of model in diapir area at progressively longer run times

28. Contours of equivalent plastic strain in overburden above the diapir in figure 22 after 14.3 h

29. Comparison of active diapirism of a plane-strain salt wall with an axisymmetric salt stock with the same material properties and cross-sectional dimensions

30. Round-corner diapir model

31. Round-top diapir model

32. Triangular diapir model

33. Finite-element mesh with 2,376 elements for modeling an asymmetric active diapir with a sloping top surface

34. Plastic strain contours in the overburden surrounding two diapirs with differing degrees of asymmetry

35. Results from the center of the asymmetric active diapir model in figures 33 and 34b

36. Results from model with thin roof and rock properties that included negligible tensional strength

37. Elevations through time of selected points on the overburden top surface of the asymmetric active diapir model shown in figure 35

38. Results from model with constant-stress boundary condition, 0.5-km-thick roof, and rock properties

39. Results from model with constant-stress boundary condition, relatively thick roof, and rock properties

40. Model simulating interlayer slip in the roof above an active diapir with natural rock properties


Citation
Schultz-Ela, D. D., Jackson, M. P. A., and Vendeville, B. C., 1994, Mechanics of Active Salt Diapirism: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 224, 56 p.