Diapirs, cryptoexplosive structures, and intrusions
Why include these topics together?
They form at very different strain rates, and by different processes and under different conditions.
However, they all can occur in otherwise undeformed areas, and
can have a central location that the deformation is often associated with (and hence, on a large scale, can be considered point phenomena). However the main reason is simply convenience - these are structural associations of interest that you should be exposed to.
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. /
- Chapt. 19, Fossen, Structural Geology text.
- 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.
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
- salt bulbs
- conditions of formation
- deforming forces:
- differential loading
- thermal convective
- tectonic forces
- central uplifts
- ejecta blankets
- crater lake deposits
- shatter cones
- shock lamellae
- coesite and other high-P phases
- cryptovolcanic vs. impact hypotheses
- Manson structure
- Serpent Mound structure
- Chixclub structure
- laccoliths and loppoliths
- fluid filled crack propagation
Above image from NASA's Visible Earth of a salt
diapir that has pierced an anticline (Kuh-e-namak) and an adjacent
salt diapir to the east (note that radial
drainage). These are in the Zagros mountains of Iran . Image source: http://visibleearth.nasa.gov/view_rec.php?id=17519
Reasons diapirs 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 deformation - halokinesis.
- mechanically intriguing bodies.
Image of a mud diapir off the coast
of California (Santa Monica area). Image source and site of more
- Geometry of salt diapirs 1st discerned in 1862, in U. S.
Gulf coast area, which is now a classic type locality.
- Initially mechanically enigmatic, an early
introduction to the idea the rocks can flow under the right conditions.
- Diapirs can also consist of mud, halite, gypsum
(other salts also), serpentine, possibly gneiss. What do these
all have in common?
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, but shapes highly variable.
- 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 adjacent and 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 and rim sedimentary basins (accommodation space due to underlying salt withdrawal).
- 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.
Virtual Seismic Atlas - While they have their resolution limitations and interpretation challenges, seismic sections of diapirs and other features formed by salt tectonics provide a great deal of insight. Much of this information is proprietary and held by petroleum companies. However, at this web site seismic sections have been collected from industry sources and made freely available for education purposes. There are some 49 seismic sections related to salt tectonics on file, and one can a lot by studying these. This is also a useful resource for other structural assemblages.
This is a reduced resolution copy of a seismic section from the North Sea taken from the Virtual Seismic Atlas site, and which was contributed by Simon Stewart in 2008. Note the internal discordances in the adjacent sediments.
This is a reduced resolution copy of another seismic section from the North Sea, also taken from the Virtual Seismic Atlas site (Dutch graben margin profile 2 - http://www.seismicatlas.org/entity?id=d14186e4-c59a-40be-b8bc-1cc4fb040f0c .
Under what conditions does the material
- change with burial in density of sandstone,
shale, and salt?
- density of shale increases from less than 2 gm/cc to close to 2.5 gm/cc in a variable and non-linear way.
- that of salt basically stays the
same, circa 2.1 gm/cc.
- sand increases due to cementation, but the change is not nearly as dramatic as with shale.
- when does salt become less dense than enclosing sediment? Below that would have
positive buoyancy forces. 3000' reasonable value, but varies highly dependent on diagenetic and loading history and on the ratio of shale to sandstone in the enclosing section.
- 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 initial anticline localization and subsequent
- rates of upward motion: generally mm/year
on average (but faster spurts possible?).
Cartoon image of history of Gulf
Coast salt tectonism. The important thing here is the multiphase
history related to loading and growth of sedimentary wedge on
the continental margin. Source of image: http://capp.water.usgs.gov/gwa/ch_f/F-text1.html
Modeling of salt tectonics:
Mechanics of diapirism
Key in diapirism is the ability to flow in responses to differential
vertical load. Buoyancy - does it have to be positive (density
inversion with depth)? Consider gabbroic magma or serpentine, both mobile but of potentially higher density than enclosing rock.
Jackson and Talbot article provides 6 principal
mechanisms that shape salt tectonics:
- buoyancy halokinesis
- differential loading halokinesis
- gravity spreading halokinesis (for extrusive
- thermal convective halokinesis
- contraction tectonics
- extension tectonics
With differential loading you can easily envision
a positive feedback loop created as the diapir rises.
Dislocation glide map for salt and temperature as critical
factor: experimentally c. 300° C is a critical T at which
strength and work hardening effect are significantly reduced. Petrofabric studies 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 at, causing the transition to a much lower T.
Diapiric rise of magmas (a digression)?
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.
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
- Talbot, C. J. & Jackson, M. P. A., 1987,
Salt Tectonics; Scientific American, v. 257, : 2. A good bit
- Short course on salt tectonics: this site has abundant cross section diagrams from around the world, and a great wealth of information.
Cryptoexplosive structures - geoblemes or astroblemes.
A debate, that is
largely over, provides an interesting framework for this next
section. Cryptoexplosive 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 debate. 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, remove the top.
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.
Such features are worthy of discussion because
- importance of impacts in very early history
- 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:
Image of shock lamallae in quartz
from the K-T boundary. Image source: http://esp.cr.usgs.gov/info/kt/boundary.html
- circular and kms to tens of kms in diameter.
- most apparent in otherwise undeformed rocks.
- central uplift (absent in smaller examples).
- ring depressions (absent in smaller examples).
- shock metamorphic minerals (coesite and stishovite,
- shock deformation lamellae.
- shatter cones.
- suevite (breccia with shock melt matrix).
- ejecta blankets.
- can come in multiple occurrences.
Traits of known impact structures
YouTube slow motion video of small scale impacts.
Where do we get information about impact structures from:
- a) recent unequivocal impact structures on
earth (relatively 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 diameter of bowl shaped
- 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.
- Chesapeake Bay (buried).
Image source: USGS USGS Fact Sheet 049-98, http://pubs.usgs.gov/fs/fs49-98/
Images of Meteor Crater
Modified oblique computer image
of Meteor Crater from NASA's Visible Earth web site.
Image of center of Meteor Crater.
The flat floor is due to infilling lake sediments. The talus cones on the
inner crater walls are of course post impact modifications. The
tilted strata making up the crater rim are evident.
This is a cross section diagram
of the crater from the visitor's center.
Telephoto shot showing steeper tilting
right along the crater rim along with a subvertical radial fault
(can you see where the fault is located?).
The broken jumble of white rock
above is the ejecta blanket, deposited above the tilted red, Triassic
Ejecta breccia near the crater rim
of Meteor or Barringer Crater, Arizona.
Photograph of part of the impactor!
Map and cross section of buried Chesapeake Bay feature, a buried impact. This feature is a bit unusual for its size in the lack of a clearly defined central uplift. Note also how the mechanical difference of overlying sediment versus basement has influenced the geometry. This is one line or research - how layering in the "target" material influences crater development and morphology. Images from USGS report: http://pubs.usgs.gov/pp/p1612/fig2.html .
This is a quarry wall in the Kentland,
Indiana impact site. Note the folded, faulted and subvertical layers
in the Paleozoic limestone. Within the craton and surrounded by
flat lying equivalent rocks, this area stands out as a point locale
of deformation. Shatter cones are also fairly common. This is
a fairly typical example of a crypto-explosive structure. Most
believe this to be an impact structure. The age of formation is
Crater morphology as a function of size
Crater processes and morphology are a function
of the size of the impactor.
King Crater on the far side of the
moon. Note the central peaks related to rebound and the central
peak. Note also the flat floor and the multiple ring structures.
Some of this can be slump modification. This morphology is typical
of larger craters, and distinctly different than the small bowl-shaped
- see the following progression from smaller to larger:
on cryptoexplosive structures and impacts
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.
- Global Impact Crater Database.
Schematic diagram of some intrusion forms and the connection between surface volcanism and the underlying plutonic plumbing. Image source: http://pubs.usgs.gov/of/2004/1007/volcanic.html
- sills - concordant to enclosing layering.
- simple interpretation is that sigma three is vertical.
- role of anisotropy complicates a simple interpretation.
- dikes - discordant to enclosing layering.
- simple interpretation is that sigma three is horizontal.
- commonly associated with crustal extension.
- can form dike swarms, which indicates distributed extension.
- orthogneiss sheets:
The cliffs here are several hundred meters high, and the dark layer is a mafic Cretaceous intrusion that is cutting through Triassic strata along the western shores of Edgeøya, Svalbard. For most of its length the intrusion is sub-parallel to layering, and hence is a sill, but to the right (south), it bends up and is discordant to bedding, and hence is a dike.
Another image along a Cretaceous sill in Triassic strata of western Edgeøya, Svalbard. Note that sill takes discordant steps so that it is at somewhat different stratigraphic positions within the dark, organic-rich Triassic shales (Tschermakfjellet Formation). Note the thin dikes with large alteration zones emanating from the highest part of the sill structure.
Light colored layers are mylonitic orthogneiss sheet within the Modoc zone - a crustal-scale ductile shear zone. The sheet is concordant to the metamorphic/dynamic layering in the enclosing amphibolites. A sheared out pegmatitic zones can be seen in the lower part. These orthogneiss sheets have been dated to circa 310 Ma.
- upwards arching magmatic blister.
- Image to right is a cross section interpretation of a laccolith intrusion in the Henry Mountains as constructed by Gilbert in 1877 who originally worked out their existence. Image source: Jackson, M, Processes of Laccolithic Emplacement in Southeastern Henry Mountains, Utah - pubs.usgs.gov/bul/b2158/B2158-8.pdf .
- extension expected in arched overlying strata.
- can be with or without faults.
- Henry Mountains in Utah a prime example.
- lateral movement of magma?
- saucer or funnel shaped.
- Bushveld in South Africa an example (layered mafic complex)
fluid-filled crack propagation:
- balance at crack tip of tip perpendicular closing stress tractions associated with the host rock body and the tip perpendicular opening stress tractions due to the fluid pressure.
- crack 'blebs' can migrate along a stress gradient.
- interesting implication is that a crack could close behind itself (what trace might be left).
- classic example the Sierra Nevada batholith.
- composite - many intrusions.
- large roof pendants (brittle roof collapse).
- vertical crustal extent??
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