Research ArticleGEOLOGY

Shatter cones: (Mis)understood?

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Science Advances  05 Aug 2016:
Vol. 2, no. 8, e1600616
DOI: 10.1126/sciadv.1600616

Figures

  • Fig. 1 Shatter cone morphologies and distribution at the Tunnunik impact structure (Northwest Territories, Canada).

    (A) Shatter cones in dolomite (14-cm-long pencil for scale). (B) Map of shatter cone distribution (red dots) superposed on a Landsat 8 image. Black lines indicate major faults. Locations of images in (A), (C), and (D) are shown. (C) Shatter cones in dolomite from the center of the structure (14-cm-long pencil for scale). (D) Shatter cones in dolomite from the outer edge of the central uplift (person for scale).

  • Fig. 2 Stratigraphic settings of shatter cones in complex impact craters.

    (A) Schematic cross section of a typical complex impact crater in the size range ~10 to 150 km in diameter. (B) Shatter cones in the central uplift of the Tunnunik impact structure (15-cm-long pen for scale). (C) Shatter cones in ejecta from the Haughton structure (35-cm-long rock hammer for scale). (D) Shatter cone clast (arrows) in a breccia dike from the Slate Islands structure (15-cm-long pen for scale). (E) Shatter cone clast in the crater-fill impact melt rock at the Haughton structure (7-cm-diameter lens cap for scale).

  • Fig. 3 Shatter cone morphologies.

    (A to D) Hand specimen images of shatter cones from the Haughton impact structure.

Tables

  • Table 1 Important attributes of the studied sites and stratigraphic and lithologic setting of shatter cones.

    These observations are the result of fieldwork conducted for this study. In many instances, shatter cones have previously been recorded at these structures and references have been provided where appropriate. S, sedimentary; C, crystalline; M, mixed target of sedimentary rocks overlying crystalline basement; CU, central uplift; CFill, crater fill; X, present; np, not preserved (that is, the setting is not preserved or exposed); ?, unknown (that is, the setting is preserved and exposed, but it is unknown whether shatter cones are present); b, bedrock, lb, lithic impact breccia; mb, impact melt-bearing breccia; mr, impact melt rock.

    NameDa*TargetErosional level Stratigraphic and lithologic setting in crater
    CUDikesCFillEjecta
    Haughton23S2Xb [cf., (64)]Xlb,mrXmrXlb,mr
    Mistastin28C2Xb [cf., (66)]?npXlb,mb
    Ries26M2Xb [cf., (65)]XmbXX [cf., (40, 65)]
    Sierra Madera20S6Xb [cf., (30)]Xlb [cf., (30)]npnp
    Slate Islands30C6Xb [cf., (32)]Xlb [cf., (32)]npnp
    Tunnunik28S6Xb [cf., (18)]Xlbnpnp
    West Clearwater36C4Xb [cf., (67)]XmrXlb,mb,mrnp

    *Apparent crater diameter. For Sierra Madera, we have updated the diameter on the basis of this study (see Table 2).

    †Erosional level: 1, ejecta largely preserved; 2, ejecta partly preserved, rim partly preserved; 3, ejecta eroded, rim partly preserved; 4, rim largely eroded, crater-fill breccias/melt rocks preserved; 5, crater-fill breccias/melt rocks partly preserved; 6, crater-fill breccias/melt rocks eroded, isolated breccia dikes; 7, eroded below crater floor.

    • Table 2 New estimates for the apparent crater diameter of selected complex impact structures.
      NameD
      (EID)*
      D
      (literature)
      TargetErosion
      level
      Da (this
      study)§
      Notes
      Charlevoix5228–52M670Initial studies yielded estimates in the order of ~30 km (68, 69). Later studies have
      generally put the diameter at ~52 km, which corresponds to a ring of outer hills (70, 71).
      The outer limit of shatter cones has been mapped at 14-km radius from the crater
      center (27). On the basis of this mapping and using Dsc = 0.4 Da, we estimate a
      minimum apparent crater diameter of this structure at 70 km.
      Gosses Bluff2424S632The diameter of Gosses Bluff is typically assumed to be a subtle topographic
      circular feature at ~24-km diameter (41). Exposure is virtually zero in this region.
      Shatter cones have been accurately mapped in the well-exposed circular ring of
      hills known as Gosses Bluff out to a radius of 6.5 km. On the basis of this
      detailed mapping and using Dsc = 0.4 Da, the apparent crater diameter of this
      structure is estimated fairly robustly at 32 km.
      Keurusselkä3010–30C636The Keurusselkä impact structure is a heavily eroded structure in central Finland
      that was first recognized by the presence of shatter cones (72). There is no ob
      vious topographic form, and estimates of the original crater diameter range from
      10 to 30 km (28, 7274). However, detailed mapping has shown that shatter
      cones are present over an area of 14 km across (73). On the basis of this detailed
      mapping and using Dsc =0.4 Da, the apparent crater diameter of this structure is
      estimated fairly robustly at 36 km.
      Luizi1717S5/615The diameter of the Luizi impact structure was estimated to be about 17 km (75)
      using a combination of satellite images and a digital elevation model. Ferrière et al.
      (75) reported that shatter cones are restricted only to the inner 3.2 km of the
      structure. However, during a second field campaign that included more compre
      hensive mapping, shatter cones were mapped over a region of ~6-km diameter in
      the center of the structure [this study; (76)]. Using Dsc = 0.4 Da, we can estimate the
      apparent crater diameter of this structure at 15 km.
      Presqu’ile2412–24C6/715Little previous work has been done on this structure. Higgins and Tait (17)
      provided size estimates ranging from 12 to 24 km in diameter. Shatter cones
      have been mapped over an area defined by a circle with a 6-km diameter (17).
      Using Dsc = 0.4 Da, we can estimate a minimum apparent crater diameter of this
      structure at 15 km. However, further fieldwork with detailed mapping is re
      quired to better determine the distribution of shatter cones.
      Rochechouart2315–50C532A wide range of estimates have been suggested for the Rochechouart impact
      structure. Initially, a “minimum diameter” of 15 km was proposed (77). Studies in
      the 1970s proposed diameters in the range of 18 to 25 km (78) and 20 to 25 km
      (29). Estimates up to 40 to 50 km have also recently been proposed (79). Shatter
      cones have been mapped over an area defined by a circle with ~12- to 14-km
      diameter (29). This yields an estimate for the apparent crater diameter of this
      structure at 32 km.
      Sierra Madera1313–16S620The widely cited diameter of 13 km comes from early mapping efforts and
      corresponds to the “outer limit of deformation” (30). However, it is notable that there is
      virtually no exposure beyond the area mapped by Wilshire et al. (30). Goldin et al. (80)
      conducted numerical modeling studies with rim (final crater) diameters (D) of 12 and
      16 km. These authors conclude that “models of the 16-km-diameter crater better re
      produce the crater geometry, but fail to agree with previous workers’ interpretations
      and observations of the extent and degree of deformation.” On the basis of the
      mapping by Wilshire et al. (30), Dsc = 8 km, which results in an apparent crater diameter
      of 20 km according to our relationship for Dsc. Given D < Da (35), this is in keeping with
      the numerical modeling studies of Goldin et al. (80).
      Siljan5252–65M675A diameter of 52 km is frequently cited on the basis of the work by Grieve (81). The
      most recent estimates based on the limit of “intense fracturing” is 65 km (82), although
      these authors also state that the diameter could still be larger. Recently, Holm et al. (31)
      conducted the most comprehensive study of shock metamorphism at Siljan to date.
      They describe shatter cones extending to a radius of between 15 and 16 km from the
      crater center. On the basis of this detailed mapping and using Dsc = 0.4 Da, we can
      estimate a minimum apparent crater diameter of this structure fairly robustly at 75 km.

      *Diameter given in the Earth Impact Database (EID) (www.passc.net/EarthImpactDatabase/index.html). Note that no distinction is made in this database between rim (final crater) diameter and apparent crater diameter.

      †Estimates from the literature. Note that typically no distinction is made between rim (final crater) diameter and apparent crater diameter.

      ‡Erosional level: 1, ejecta largely preserved; 2, ejecta partly preserved, rim partly preserved; 3, ejecta eroded, rim partly preserved; 4, rim largely eroded, crater-fill breccias/melt rocks preserved; 5, crater-fill breccias/melt rocks partly preserved; 6, crater-fill breccias/melt rocks eroded, isolated breccia dikes; 7, eroded below crater floor.

      §Apparent crater diameter calculated based on our relationship for shatter cones, 0.4 Da. Given most of the craters in this table are very eroded, this should be taken as a minimum apparent crater diameter.

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