Why include diapirs and cryptoexplosive structures together? They form at very different strain rates, and by different processes. However, they both can occur in otherwise undeformed areas, and have a central location that the deformation is often distributed around (and hence can be considered point phenomena). Other than this the reason is simply convenience.

Lecture index: Diapir introduction. / Diapir traits. / Diapir Kinematics. / Mechanics of diapirism. / Cryptoexplosive structures - geoblemes or astroblemes. / Traits of known impact structures. / Images of Meteor Crater. / Crater morphology as a function of size. /

Readings:

• Jackson, M. P. A. & Talbot, C. J., 1986, External shapes, strain rates, and dynamics of salt structures; Geol. Soc. of America Bull., v. 97, p. 305-323.
• Diapirs and circular features in Price, N. J. & Cosgrove, J. W., Analysis of Geological Structures, Cambridge Press. p. 110-121. This provides a good detailed summary.

Terms and concepts:

• salt pillows, salt anticlines, salt diapirs, salt nappes, extrusive domes and salt glaciers
• various materials diapirs form from
• rim syncline and drag anticlines
• rim breccia zone
• radial and concentric fault patterns
• associated local unconformities
• caprock
• salt bulbs
• conditions of formation
• deforming forces:
• differential loading
• thermal convective
• tectonic forces
• central uplifts
• ejecta blankets
• crater lake deposits
• shatter cones
• shock lamellae
• suevite
• coesite and other high-P phases
• tektites
• cryptovolcanic vs. impact hypotheses
• Manson structure
• Serpent Mound structure
• Chixclub structure

Image from NASA's Visible Earth of a salt diapir that has pierced an anticline (Kuh-e-namak) and an adjacent slat diapir in the Zagros mountains of Iran (note that radial drainage). Image source: http://visibleearth.nasa.gov/view_rec.php?id=17519

Why include diapirs and cryptoexplosive structures together? They form at very different strain rates, and by different processes. However, they both can occur in otherwise undeformed areas, and have a central location that the deformation is often distributed around (and hence can be considered point phenomena). Other than this the reason is simply convenience.

Reasons have been studied extensively:

• represent important petroleum traps.
• used to a greater and greater degree as storage site (for gas, petroleum reserves), and considered as a site for radioactive waste disposal.
• locally can cause extensive tectonism - halokinesis.
• mechanically intriguing bodies.

Intro thoughts:

• Geometry 1st discerned in 1862, in U. S. Gulf coast area, is now a classic type locality.
• Initially mechanically enigmatic, an early introduction to the idea the rocks can flow.
• Diapirs can consist of mud, halite, gypsum (other salts also), serpentine, possibly gneiss. What do these all have in common?
• Have been modeled using centrifuges by Ramberg.

Geophysically - usually a marked gravity low (because of the lower density of salt); useful exploration guide.

overall a mushroom shape, that typically originates at depths of 12,000' or greater.

external structures:

• rim breccia zone: solution and fault movement processes contribute to formation of this breccia.
• arching and doming of strata overhead with attendant radial and concentric patterns.
• local unconformities in overlying strata (significance? - multiple movement episodes) which are domed above the diapir
• radial and concentric faults: often normal in character (local extension)
• drag anticlines and rim synclines

internal structures:

• caprock zone.
• upper bulges (lateral flowage).
• multiple bulbs in upper head, multiple lobes of flowage.
• vertical foliation and flow lineation in column, refolded folds.
• source bed pinch off.

Under what conditions does the material become mobile?

• change in burial in density of sandstone, shale, and salt; density of shale increases, that of salt the same. Switch occurs at over at 3000 ', so after that would have appropriate buoyancy forces.
• key is ability to flow in responses to differential vertical load.
• empirically salt diapirs seem to originate from source beds at 15,000 feet depth or greater.
• regional tectonism not necessary, but does alter form - salt walls common, possibly controlled by anticline crest breaching.
• rates of upward motion: generally mm/year on average (but faster spurts possible?)
• solution related collapse perhaps unexplored as a mechanism.

Buoyancy; does it have to be positive (density inversion with depth)? Consider gabbroic magma or serpentine.

Jackson and Talbot article provides 6 principal mechanisms that shape salt tectonics:

• buoyancy halokinesis
• differential loading halokinesis
• gravity spreading halokinesis (for extrusive diapirs)
• thermal convective halokinesis
• contraction tectonics
• extension tectonics

With differential loading you can easily envision a positive feedback loop.

Dislocation glide map and temperature as critical factor: experimentally c. 300° C is critical T at which reduce strength and work hardening effect. Petrofabric studies (describe for the students) indicate preferential gliding on (001) planes, which should operate at 300° C or higher. Yet typical gradient of 30° C/km would yield only 140°C at 15,000 feet. Is there a thermal blanket effect? Perhaps the influence of brine inclusions and other impurities greatly weakens the salt.

Diapiric rise of magmas?

The basic problem; given viscosity of granitic magmas should rise so slowly that cool and crystallize before ever get anywhere.

Fluid migration along a crack provides an alternate model. Basically the idea is that the tips of a fluid filled crack will migrate up a principal stress gradient. At depth vertical fractures will be favored, but at a shallow level topography can create a horizontal gradient.

An aside. My mother-in-law recently asked why the extrusions didn't happen preferentially down in the valley since it was a shorter route and should be easier for the magma to squeeze up there. We were in Death Valley looking at the Ubehebe Maar explosive craters and I had just related the story of the Grand Canyon flows that had dammed the river. Clearly faults can play a role as conduit and in some cases the faults are on the side of the valley. However, there is another possible mechanism. The topography itself actually can generate additional horizontal compressive forces in the rocks in the valley floor and tensile forces in the high flanking areas. It could be much harder to open cracks at a shallow level right beneath the Grand canyon floor than it is up on the shoulder. Thus the magma filled cracks could be following the topographic stress guide.

Animations of development of salt diapirs along with explanatory text and a lot of other material and images from Guglielmo and others at U. Texas.

References on salt diapirism:

• Jackson, M. P. A. & Talbot, C. J., 1986, External shapes, strain rates, and dynamics of salt structures; Geol. Soc. of America Bull., v. 97, p. 305-323. (good detailed summary)
• Talbot, C. J. & Jackson, M. P. A., 1987, Salt Tectonics; Scientific American, v. 257, : 2. A good bit more general.

A debate, that is largely over, provides an interesting framework for this next section. These features stand out as sites of intense deformation within otherwise undeformed rocks. A number of them are well known from the mid continent region. The term cryptoexplosive was coined to try to sty descriptive, and neutral in this debae. The two basic models as to origin of these features are : 1) they are related to a volcanic event, perhaps volcanic gases, in some manner similar to breccia and kimberlite pipes (a geobleme); or, 2) they are related to meteorite impact - some level of exposure of a crater. Most of these features are now recognized as impact or crater structures. The crater is the surface morphology, but what lies below?? What will it look like after the ravages of time modify it.

This is a DEM image of Manicouagan crater in Quebec, Canada. The circular pattern stands out. Note also the central peak It qualifies as a cryptoexplosive structure. Source: http://visibleearth.nasa.gov/view_rec.php?id=16403

Such features are worthy of discussion because of:

• importance of impacts in very early history of earth.
• importance of cratering in understanding other planets surface processes.
• possible role in producing mass extinctions on the earth.
• lesson in material behavior under unusual conditions (shock waves, etc.).
• locally may be oil trap (high fracture permeability).

Typical traits or distinctive features:

• circular and kms to tens of kms in diameter.
• most apparent in otherwise undeformed rocks.
• central uplift.
• ring depressions.
• shock metamorphic minerals (coesite and stishovite, jadeite).
• shock deformation lamellae.
• shatter cones.
• suevite (breccia with shock melt matrix).
• ejecta blankets.
• can come in multiple occurrences.

List of some impact structures on earth and elsewhere:

a) recent unequivocal impact structures on earth (rare).

b) impact structures on other planets/moons.

c) explosion cratering - bombs.

d) laboratory-theoretical data.

Examples of unequivocal impact craters:

• Chubb crater, Canada
• Meteor (Barringer) crater, Arizona:
  • circa 25,000 years old.
  • 200 m deep and 1.1 km diamater of bowl shaped depression.
  • extensive ejecta blanket, 175*10^6 tons, up to 2 km from rim, up to 25 m thick.
  • 30 m of playa fill are underlain by 10 of fallout breccia and then highly shocked and fused Coconino sandstone.
• Steinheim Basin, Germany
• Kentland, Indiana.

Crater processes and morphology are a function of the size of the impactor.

• small and bowl shaped.
• flat-floored with central uplift.
• multi-ring structure with flat floor (flooded by melts?).

Early formulation of debate:

• Bucher, W. H., 1963, Crypto-explosion structures caused from without or within the earth; American Journal of Science, v. 261, # 7, p. 597-649.
• Dietz, R. S., 1963, Crypto-explosion structures: a discussion; American Journal of Science, v. 261, # 7, 650-664.

Other useful references:

• Roddy, D. J., Pepin, R. O. & Merill, R. B. (editors), 1977, Impact and explosion cratering; Pergamon Press, N. Y. 1301 p.
• Silver, L. T. & Schultz, P. H., (editors), 1982, Geological implications of impacts of large asteroids and comets on the earth; Geological Society of America Special Paper # 190, 528 p.
• Hsu, K., 1989, Catastrophic extinctions and the inevitability of the improbable; Journal of the Geological Society of London, vol. 146, p. 749-754.