Relationships Between Shatter Cones and Planar Surfaces at Sudbury, Ontario

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay

Abstract

The morphology of shatter cones at Sudbury, Ontario is considerably more varied than most published literature suggests. At least three distinct modes of shatter-cone occurrence have been observed:

  1. "Classic" well-formed large shatter cones sometimes over a meter in length and spanning up to half the circumference of a cone.
  2.  Families of overlapping shatter cones along approximately planar surfaces, each with striations fanning across 45-60 degrees. These cones are typically about 15 cm in length and the planar surfaces are isolated or widely spaced.
  3. Small shatter cones on closely spaced parallel planar surfaces typically a few cm apart. Striations fan across 30 degrees and are typically 10 cm long or less.

Frequently, intersecting planar shatter-coned surfaces combine to create a more complete cone than would occur on either surface alone, however, this mechanism does not seem to account for the largest and most perfect cones. In some cases planar shatter-coned surfaces appear to grade into conventional joints. In other places small Riedel shears splaying off ordinary joints have been observed to show shatter cone striations. These observations suggest there exists a continuum of brittle failure mechanisms ranging from normal jointing to formation of geometrically ideal shatter cones.

A Non-Mystery Concerning Shatter Cones

How can fractures form in the enormous compressional stresses of an impact?

Answer: Once the main shock wave passes, virtually everything that happens during the excavation of an impact crater is tensional. Shatter cone fractures look tensional because they are tensional.

Are Shatter Cones Really Complete Cones?

Although shatter cone lineations do define small circles when plotted on a stereonet, the lineation measurements almost always come from a number of cones in a small area, not a single cone. Although it seems to be widely assumed that cones would be complete if not for interference by other cones or incomplete preservation, field evidence suggests otherwise.

  • At Sudbury, Ontario, there are innumerable road cuts and excavations in shatter-coned rocks, yet complete 360-degree cones are extremely rare.
  • At Kentland, Indiana, shatter-coned limestone is being quarried. The roadbed of the adjacent railroad consists almost entirely of shatter-coned rock. Yet here, also, complete shatter cones are extremely rare.

    Conclusion:

    Both of the localities above should provide ample opportunity to collect complete shatter cones if they really exist. The rarity of complete shatter cones seems to be real and is probably related to their formation mechanism. One possible interpretation is that local strain or strain-rate gradients cause the cone fractures to curve in preferred directions, probably in the direction of less deformation.

    Shatter Cone Morphologies

    At Sudbury, Ontario, shatter cones are frequently observed to lie on approximately planar surfaces. The cones cover a wide range of sizes and completeness, but can be divided into three categories:

    Type I: "Classic" well-formed large shatter cones sometimes over a meter in length and spanning up to half the circumference of a cone.
    Type II: Families of overlapping shatter cones along approximately planar surfaces, each with striations fanning across 45-60 degrees. These cones are typically about 15 cm in length and the planar surfaces are isolated or widely spaced.
    Type III: Small shatter cones on closely spaced parallel planar surfaces typically a few centimeters apart. Striations fan across 30 degrees and are typically 10 cm long or less.

     

    Outcrop along Ramsay Lake Road north of the Laurentian University campus. A very large shatter cone (I) is visible surrounded by shatter-coned planar surfaces (II). This and similar field occurrences suggest that the three types are intergradational and represent a continuum of shatter-cone formation mechanisms.
    An outcrop on Highway 17 just south of Kelly Lake. Although fractures of all orientations are visible, shatter cones are confined to a small number of crudely planar surfaces dipping steeply away from the plane of the picture and striking from right front to left rear.
    A clear example of an approximately planar surface covered with small shatter cone segments.
      This planar surface is mostly covered with negative cones (concave inward). Two cones meet apex-to-apex in the center.
    At this locality, on Ramsay Lake Road, fractures along the deep re-entrant can be seen dipping away from the camera. On the planar joint to the right these fractures can be seen curving smoothly into a bedding-plane joint. On the left, they are visible as closely-spaced subparallel fractures. Although not evident at the scale of the photo, they can be seen on close inspection to have shatter cone lineations.
    Numerous small, subparallel fracture surfaces in these outcrops can be seen, on close inspection in the field, to have shatter cone lineations.
    This outcrop, on the Laurentian University campus, is cut by two sets of subparallel fractures, one oriented vertically in the photo, the other slanting from upper left to lower right. Both fracture sets are shatter-coned.
    Intersection of two fracture sets at the Laurentian University locality. Although lineations, especially on the left surface, clearly radiate, most of the cone appearance results from the juxtaposition of two lineation directions meeting on a corner.
      At a revealing locality along Kontola Road, outcrops along one side of the road show spectacular large shatter cones, whereas outcrops on the other side show almost no shatter cones but an abundance of normal brittle failure structures.
    North side of Kontola Road: Note the regularity of the fracture surfaces. Shatter cones are almost completely absent.
    South side of Kontola Road: Note the irregularity of the fracture surfaces. Shatter cones are abundant and spectacularly developed.

    Questions for Future Research:

  • Can the curvature directions of shatter cones be linked to local or regional structural controls?
  • Can the degree of shatter-coning be mapped to reveal useful information on impact processes?
  • Do more conventional fracture processes like plumose fracturing and conchoidal fracture have anything to tell us about shatter cone formation?
  • Can we identify low-strength materials that might permit shatter coning to be observed in the laboratory at slower speeds and under safer conditions than are now required to generate them?

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    Created 4 August 2004, Last Update 11 January 2020

    Not an official UW-Green Bay Site