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. /

Suggested readings:

Terms and concepts:

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

Diapir introduction

Reasons diapirs have been studied extensively:

Intro thoughts:

Image of a mud diapir off the coast of California (Santa Monica area). Image source and site of more information: http://pubs.usgs.gov/of/1998/of98-518/ .

Diapir traits

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.

External structures:

Internal structures:

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 .

Diapir Kinematics

Under what conditions does the material become mobile?

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:

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:

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. Source: http://visibleearth.nasa.gov/view_rec.php?id=16403

Such features are worthy of discussion because of:

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

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:

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 Moenkopi sandstones.

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 poorly constrained.

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 craters.

Some references on cryptoexplosive structures and impacts

Early formulation of debate:

Other useful references:

Intrusive structures

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

sheet intrusions:

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.



fluid-filled crack propagation:



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