Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay
Early in the debate over the impact origin of so-called cryptoexplosion structures, critics of impact cited numerous apparently tensional features of shatter cones as evidence they could not be the result of a high-pressure shock wave. Among the features cited were microbrecciation, mineral films on shatter cone surfaces, and shear-related features like steps and en echelon fractures on shatter cone surfaces. At least one author proposes that shatter cones originated by the sudden decompression of diapiric intrusions. Because their overall conclusion of an endogenic origin for cryptoexplosion structures has not been widely accepted, there has been little attention paid to the evidence they presented for a tensional origin for shatter cones. However, a tensional origin for shatter cones can be completely consistent with both an impact origin and the petrographic evidence cited by critics of impact. There are two possible ways tension might operate during an impact event to produce shatter cones. First, immediately following the passage of the compressional front of a shock wave there is a rarefaction. Second, and possibly the more likely mechanism, the highly compressed floor rocks of the transient crater rebound, pulling the floor inward, raising a central uplift, and causing the rapid collapse of the crater walls. The conical geometry of shatter cones seems to demand initiation at some local inhomogeneity, with fracture propagation downward and outward along the surface of the cone. The self-similar �horsetail� texture of shatter cone surfaces suggests that inhomogeneities in the fracture front result in shatter cone initiation at progressively smaller scales. I propose that the deformation front is tensional rather than compressional. Origin of shatter cones by tension during the upheaval of the central uplift accounts for the centripetal orientation of shatter cone axes and their tendency to be most abundant in central uplifts, where rarefaction would presumably be greatest.
Impact paradigm: other evidence for impact is so persuasive that claims of tension features are irrelevant to impact if not actually erroneous.
Endogenic paradigm: extensional features of shatter cones are incompatible with impact origin.
The implicit assumption in both lines of reasoning seems to be that shock and ultra-high-pressure stress are the dominant processes during impact, a natural enough conclusion given the magnitude of the stresses involved.
Reality: After the passage of the main compressional front, almost everything that occurs during impact is tensional in nature.
There is no inherent conflict between impact and a tensional origin for shatter cones.
Looking at shatter cones as tensional rather than compressional structures may provide fresh insights into their formation.
In particular, the writings of advocates for endogenic origin of impact sites contain many useful insights. Virtually every mechanism they propose for shatter cone formation, however, can be interpreted in terms consistent with impact.
Numerical simulation of a 20-ton TNT blast 5.5 msec after detonation, from Ullrich, Roddy and Simmons (1977). Axis scales are in meters, and each axis interval also represents a particle vector velocity of 20 m/sec. Note the following features:
# The main deformation front has traveled 10 m, implying a velocity of 1820 m/sec, but the particle velocity within the compressional wave is only about 20 m/sec.
# Downward (rarefaction) wave near the surface. Apparently a high-velocity shock wave has already passed through.
# Elastic precursor ahead of the main plastic compressional wave.
# Virtually everything behind the deformation front is tensional. The decrease in velocity vectors behind the wave front implies that material near the front is pulling away from material further behind. Closer than 7 m to the blast site, particle motion is actually centripetal.
Reflected Rarefaction Wave
Compressional Wave Front
Region of Rapid Stress Relaxation
Johnson-Talbot (1964): perhaps the most widely-accepted hypothesis. Elastic precursor wave encounters an inhomogeneity and scatters. The scattered and direct waves interfere to stress the rocks in a conical region beyond the Hugoniot elastic limit. The permanently (if slightly) deformed rock separates from the neighboring undeformed rock during the rarefaction phase of the shock. Note that the actual fracturing event is implicitly tensional.
Milton (1977) like many other authors questions whether a shatter cone could survive an intense plastic deformation event, but notes that very strong compressional pulses may have a multiple-wave structure and hypothesizes that a mechanism analogous to the Johnson-Talbot mechanism might operate during the relaxation after peak compression. However, he does not develop this hypothesis in detail.
Gash (1971) suggested that interference between a shock wave and the reflected rarefaction wave from a free surface might result in conical fractures.
The horsetail structure of shattercones bears a resemblance to plumose fracturing, a similarity noted by numerous authors. The similarity probably reflects a real physical mechanism. The self-similar structure of shatter cone surfaces, with small cones on the surface of larger cones, can be explained in terms of fracture initiation by a stress front traveling faster than the fracture propagation speed. Earlier-formed fractures propagate until they encounter fractures farther along, where they may terminate or die out.
The hand specimen from Kentland, Indiana shows that many small horsetail cones have a fracture extending beneath the apex, a continuation of a higher-order cone surface. This geometry suggests that the higher-order cone surface stopped propagating after entering the vicinity of another fracture, possibly because the other fracture had already relieved stresses in the rock.
An alternative explanation for the observed structure in the specimen is that a propagating shatter cone surface occasionally develops small splays, and that once a splay initiates, it becomes the apex of a new cone surface, leaving the old aborted cone surface as a remnant crack under the apex.
Although shatter cones are commonly considered cones, well-formed 360-degree cones are quite rare. It is common to find surfaces covered with numerous shatter cones. If shatter cones were really complete cones, we would expect the cones on a shatter-coned surface to be convex and concave in about equal proportions. Instead, virtually all cones on a shatter-coned surface are convex in the same direction.
Some sort of stress inhomogeneity operates during shatter cone formation that tends to favor development of cones convex in a common direction.
Shatter cones have been reported in rocks displaying shock metamorphic features that indicate shock pressures of 0.5 to 25 Gpa (5 to 250 kb), with pressures around 5 Gpa (50 kb) most often reported.
Shatter cones tend to be found in the central uplifts of small craters (Serpent Mound, Kentland) and in an annular zone surrounding the center of larger basins (Charlevoix, Sudbury).
However, although small complex craters have a central uplift, larger complex craters have a peak-ring structure. The rising central uplift overshoots its stable height and its interior collapses to form a ring of peaks. Clearwater West, Quebec, is perhaps the best terrestrial example.
Thus, the observed zoning of shatter cones is similar to the geometry of central uplifts in craters. The distribution of shatter cones may actually reflect a relationship with the formation of the central uplift rather than shock pressure.
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Created 4 August 2004, Last Update 11 January 2020
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