Initiation of Salt Diapirism by Regional Extension: Global Setting, Structural Style, and Mechanical Models

RI0215. Initiation of Salt Diapirism by Regional Extension: Global Setting, Structural Style, and Mechanical Models, by M. P. A. Jackson and B. C. Vendeville. 39 p., 21 figs., 1994. Print.

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ABSTRACT
Initiation of diapirs is one of the least understood aspects of salt tectonics. Sedimentary differential loading and erosion are both effective, but not universal, causes of diapir initiation. A survey of 18 major salt-diapir provinces shows that salt upwelling is closely linked in time and space to regional extension. Extended salt basins typically develop salt structures, whereas nonextended basins do not. Even in salt basins overprinted by inversion or orogenic contraction, diapirs were initiated during extension on divergent continental margins or in intracontinental rifts. 

Regional extension thins brittle overburden by forming grabens and half grabens above flowing salt. These fault structures differentially load the salt by their surface relief and weaken the overburden by fracturing and thinning it. Diapiric walls of pressurized salt rise in reaction to the shifting positions of fault blocks in extending overburdens, regardless of thickness, density, or lithology. If regional extension stops, these reactive diapirs stop rising. Eventually the roof of the reactive diapir can be thinned by extension below a critical thickness. Only then can the diapir break through actively as an independent intrusion.

Diapiric alignments have been ascribed to basement faulting, even where such faults were conjectural or had trivial displacements. Physical modeling shows that extension of the basement has only indirect influence on diapirism by creating space for extension of the overburden, which is the direct cause of diapirism, whether extension is thick or thin skinned and whether the salt was deposited before, during, or after rifting.

Keywords: analog simulation, diapirism, extension tectonics, salt domes, salt tectonics

CONTENTS

Abstract

Part 1: Global Setting and Structural Style

Timing of Salt Tectonics
Sedimentary Differential Loading

Enigmatic Salt Basins without Salt Tectonics

The Missing Factor

Salt Upwelling and Extension Associated in Time

Modern Extension and Salt Diapirism

Postrift Salt

Synrift, Interrift, and Prerift Salt

Superposed Crustal Shortening

Long-Impeded Piercement of Precambrian Salt

Geologic Evidence Summarized

Part 2: Mechanical Modeling

How Extension Triggers Diapirism

Viscous Forces in Salt without Regional Extension

Pressure Forces in Salt without Regional Extension

Physical Modeling

Analytical Modeling

Effect of Density Contrast

Effect of Mechanical Anisotropy

Critical Thickness

Mechanics of Regional Extension

Reactive Diapirism

Regional Causes of Local Extension

Conclusions

Acknowledgments

References

Appendix: Pressure Forces Induced by Differential Loading during Active Piercement

Figures

1. Time relations among salt deposition,salt tectonics, thick- and thin-skinned (rifting) extension, shortening, onset of seafloor spreading, and major unconformities in diapiric provinces

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

3. Extensional salt tectonics in the southeastern Mediterranean Sea

4. Interpreted seismic profiles through large salt sheets building the Sigsbee Escarpment, basal continental slope off Louisiana, northern Gulf of Mexico

5. Two turtle-structure anticlines resulting from extensionally induced sagging of diapirs in the Campos salient, offshore Brazil
6. Extreme extension involving two phases of rafting in the Kwanza Basin. Angola

7. Tracing of a regional seismic profile across the Whale and Horseshoe salt basins in the southern Grand Banks, offshore Newfoundland

8. Tracings of two seismic profiles in the Whale Basin

9. Interpretation of seismic line north of the Frisian Islands, Dutch North Sea

10. Structural maps of the northeast margin of the Sole Pit Basin, United Kingdom southern North Sea

11. Contractional salt tectonics superposed on extensional diapiric salt rollers in the eastern foreland of the Betic Cordilleras near Alicante, southeast Spain

12. A Triassic diapir at Puget-Theniers in the French Maritime Alps
13. Interpretation of seismic line in the United Kingdom southern North Sea

14. Comparison of creep and frictional strengths of dry salt with equivalent strengths of dry and wet sedimentary rocks and sediments

15. Unrealistically high strain rates required for salt strength to equal the strength of overburden between 100 and 7000 m thick

16. Conditions for active piercement of salt overlain by rigid, brittle overburden

17. Experimental data on conditions for active piercement of rectangular ridges of silicone polymer below overburdens of dry sand

18. Schematic cross section showing the symbols for lengths and densities used in the analytical model

19. Tracings of vertical sections through models of reactive diapirism during thin-skinned extension

20. Overhead view of an experimental model of thin-skinned extension above salt

21. Four extensional systems that trigger diapirism


Citation
Jackson, M. P. A., and Vendeville, B. C., 1994, Initiation of Salt Diapirism by Regional Extension: Global Setting, Structural Style, and Mechanical Models: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 215, 39 p.