The Rise and Fall of Diapirs During Thin-Skinned Extension

RI0209. The Rise and Fall of Diapirs During Thin-Skinned Extension, by B. C. Vendeville and M.P.A. Jackson. 60 p., 51 figs., 1992. Print.


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ABSTRACT
Grabens overlying diapirs have previously been ascribed to intrusion, withdrawal, or dissolution of salt. We propose, however, that many grabens or half grabens above diapirs form by regional thin-skinned extension of a brittle overburden. This regional extension can initiate and promote piercement of diapiric walls through extremely thick overburdens. This Piercement induced by faulting applies regardless of the overburden density. These conclusions cast doubt on some axioms of salt tectonics and are supported by dynamically scaled physical modeling, theoretical reasoning, and observations from seismic sections.

A diapir pierces a thick, brittle overburden in three evolutionary stages: reactive, active, and passive. Diapirs initially grow by reactive piercement, a newly proposed mechanism in which the diapir rises in response to faulting during regional extension. The hanging wall of an initial fault sinks into the source layer until resisted by increasing pressure forces in the source layer and bending resistance in the overburden. New faults form repeatedly nearer the axis of the graben. The dwindling central fault block sinks while the diapir rises below it, regardless of overburden density. Progressively smaller fault blocks are supported by the fluid pressure at progressively higher levels flanking a triangular diapir. Sedimentation keeps diapirs in the reactive stage longer by filling the graben. Reactive diapirism is controlled by the rate of regional extension: whenever regional extension ceases, reactive diapirs stop growing. If the diapir becomes tall enough, its roof sufficiently thinned, and the fault trough deep enough, a diapir can pierce actively by lifting and shouldering aside its roof to emerge rapidly at the surface. During subsequent passive piercement, a diapir widens by regional extension and increases in relief by downbuilding during concurrent sedimentation. Diapirs can also be initiated directly into the passive (thereby bypassing the reactive and active) mode of growth by differential loading if the overburden is thin. Faulting, folding, and thickness changes are negligible around passive diapirs. Rounded stocks can evolve passively from walls initiated by grabens.

The same regional extension that initiates and promotes the rise of diapirs can eventually make diapirs fall. During regional extension, diapirs widen between separating blocks of overburden but begin to subside when salt supply eventually becomes restricted. The formerly rising diapiric crest rapidly transforms into a site of vigorous subsidence and deposition. This site is typically a linear or even circular graben that indents the diapir crest, leaving residual horns of salt, which could be misinterpreted as injections into faults. Potential incompatibilities between deformation in the diapir and in its roof are resolved by local modification of fault geometry or by flow of salt along the diapiric wall from depressions into intervening culminations. Turtle-structure anticlines with keystone grabens form between subsiding walls. During extreme extension, diapirs subside until they are segmented into relics by indenting crestal grabens. Such grabens can eventually ground onto basement and invert to form mock turtle anticlines. Second-cycle diapirs rise from extrusive allochthonous sheets during fall of the parent diapir. Most structures produced by diapir fall during regional extension are conventionally attributed to salt dissolution or forceful intrusion; all three possibilities should be evaluated by the criteria discussed here.

Keywords: diapirism, extension, faulting, grabens, physical modeling, salt domes, salt tectonics

CONTENTS
Abstract

Introduction

Part 1: Diapir Rise

Background

Salt Upwelling and Extension Associated in Time

Why Extension Should Favor Diapirism

Rheologic behavior

Mechanical considerations

Diapirism during Extension after Sedimentation

Kinematics of reactive diapirism

Dynamics of reactive diapirism
Active diapirism

Passive diapirism

Summary of the three stages of diapirism

Diapirism during Extension and Sedimentation

Reactive diapirism

Active diapirism

Passive diapirism

Summary

Diapir Shape Controlled by Rates of Extension and Deposition

Flexure during Diapirism

Part 2: Diapir Fall

Introduction

Inconsistencies and Contradictions in Previous Interpretations

Arching-Related Faulting above Diapirs
Criteria Eliminating the Possibility of Dissolution and Arching

Inapplicability of dissolution to shale diapirs

Inapplicability of dissolution without cap rock

Inapplicability of dissolution to deep salt diapirs encased in shale

Inapplicability of dissolution with rapid sedimentation

Inapplicability of dissolution without extension

Inapplicability of dissolution in palinspastic restorations

Inapplicability of arching without force folding

Inapplicability of arching because of age relations

Inapplicability of arching in thick overburdens

Inapplicability of gentle arching as a cause of major grabens

Experimental Evidence

Why Diapirs Sag While They Widen

Examples from Nature

Examples with moderate extension

Examples with extreme extension

Part 3: Conclusions

Acknowledgments

References

Figures

1. Block diagram illustrating a prevailing double standard for interpreting fault and salt tectonics

2. Map of the northern Red Sea showing linear diapiric walls parallel to and closely associated with grabens and half grabens

3. Pressure distribution in a pressurized fluid overlain by a rigid, brittle overburden

4. Schematic conditions for piercement of nonextensional structures above salt ridges or below topographic troughs

5. Vertical sections through models of reactive diapirism during thin-skinned extension

6. Overhead view and vertical section of a stairstep graben overlying a reactive diapir

7. Overhead view of a stairstep graben overlying a reactive diapir and vertical section through a model of

reactive diapirism during thin-skinned extension

8. A tracing of figure 5C and its restoration showing fault blocks in their preslip positions
9. Kinematics of symmetric reactive diapirism during regional extension

10. Vertical section through an asymmetric reactive diapir overlain by a half graben

11. Kinematics of asymmetric reactive diapirism during regional extension of a prekinematic sequence

12. Vertical cross section of Heidelberg Dome, Mississippi, showing large normal faults associated with the crest of the triangular diapir of Louann Salt

13. Pressures in a fluid layer and displacements and flexural strains in an overburden during reactive diapirism, assuming that the first two faults form sequentially

14. Pressures in a fluid layer and displacements in an overburden during reactive diapirism, assuming that the first two faults formed together as a conjugate pair

15. Effect of extension rate and source-layer viscosity on structural style of a stairstep graben above a pressurized fluid

16. Model section after 6.5 cm extension (6.5 h duration) showing a reactive diapir that has evolved to the active stage of piercement

17. Automatic tracing of a seismic profile of an actively piercing diapir of Argo Salt in the extensional Whale basin off Newfoundland

18. Summary of the three stages of diapiric piercement through prekinematic overburden during thin-skinned extension

19. Effects of aggradation on preventing reactive diapirism

20. Photograph and tracing of a vertical section through a model of a syndepositional reactive diapir produced by regional extension

21. Vertical section through a symmetric reactive diapir that has pierced a prekinematic layer and seven synkinematic layers, each added every 12 h

22. Vertical section through an asymmetric reactive diapir that has pierced an entirely synkinematic overburden containing nine layers, each added every 2 h

23. Cross section through a syndepositional reactive diapir, the South Timbalier Block 54 dome of Louann Salt, offshore Louisiana

24. Serial sections through an elongated diapiric model produced by regional extension

25. Vertical section of Lake Barre dome of Louann Salt, Louisiana Gulf Coast

26. Summary of the three stages of diapiric piercement through synkinematic overburden during thin-skinned extension

27. Restored sections of experimental diapirs that grew during extension and slow aggradation

28. Restored sections of experimental diapirs that grew during extension and deposition
29. Automatic tracing of a seismic profile of Xenophon diapirs of Messinian salt, southeast Mediterranean, and seismic profile of the Sigsbee diapirs of Challenger Salt, abyssal Gulf of Mexico

30. A potential effect of flexural rigidity

31. Vertically exaggerated seismic profile across the Levant platform, southeastern Mediterranean Sea

32. Vertical section through the reactive diapir illustrated in figure 20

33. Tracings from seismic profiles of North Sea elongated salt domes overlain by arched and normally faulted overburden

34. Tracings of seismic time sections from opposite sides of the South Atlantic Ocean, illustrating very different explanations for what are almost twin structures

35. Overhead views of an experimental model of thin-skinned extension above salt

36. Tracing of a vertical section through several structures in the final stage of experiment 25

37. Schematic reversal of rotation direction of rafted fault blocks as they tilt and ground onto basement below the source layer

38. Vertical sections of the final stage of experiment 25, showing progressive fall of a diapir documented by structural variations along strike after the same duration of deformation

39. Vertical sections in the final stage of experiment 25

40. Vertical sections through models with entirely synkinematic overburdens, showing subsiding diapirs with indenting grabens

41. Schematic rise and fall of diapirs during sedimentation

42. Keystone grabens formed by local stretching of the crests of turtle-structure anticlines

43. The rise of second-cycle diapirs from a buried extrusive sheet at the rim of a syncline over a diaper compelled to subside during regional extension and episodic sedimentation

44. Diagram showing changes in diapir area and in the gap between fault blocks

45. Schematic vertical sections showing the rise and fall of a diapir through prekinematic overburden during regional thin-skinned extension

46. Diagrams showing the effect of initial diapir shape on the mass balance of salt during regional extension

47. Three-dimensional linkage between extension of a graben overlying a subsiding depression and adjacent rising culminations of the wall

48. Tracing of a seismic profile from the northwestern Mediterranean Sea
49. Serial sections traced from seismic profiles across a salt wall

50. Restoration of a falling diapir in the Kwanza basin of Angola

51. Schematic summary of the characteristic features of reactive rising diapirs compared with those of falling diapirs


Citation
Vendeville, B. C., and Jackson, M. P. A., 1992, The Rise and Fall of Diapirs During Thin-Skinned Extension: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 209, 60 p.