SHATTER CONE AND ITS SURFACE MESHED- STRUCTURE FORMED BY IMPACT MELT-GASIFICATION IN XISHAN TAIHU LAKE CHINA H.N. Wang1, Y. Chen1, X.F. Shen1 L.Y. Zhou1 and Y.W. Wang2

11Department of Earth & Planetary Sciences, Nanjing University, Nanjing, China 210093 ([email protected]) 2 Department of Natural Resources and Environmental Sciences, University of Illinois at Urbana-Champaign, Champaign

Introduction: Meshed--structure shatter cone: Discuss ion: Shatter cones are represented by a range of curved to The height of these cones is generally 50 to 70 cm. The 1. Meshed-structure are related to the melt- curvilinear fractures decorated with divergent striations. striation radiates from the original point at the top of cone, gasification of carbonates target rock: Striations radiate from an apex of a conical feature or from a similar to the longitude meridian. Meanwhile, cone surface Carbonates are brittle, fusible and volatile. Under the narrow apical area. Shatter cones are the only distinct meso shock wave, it is easy to be granulated. Its melting point presents hoopline around the cone. (Fig.D) The grid on is as low as 500-600°C. When the shock wave pass to macroscopic recognition criterion for impact structures. cone surface is generally in centimeter scale (2-4cm x Despite being known for 110 yrs, the formation mechanism through, it melts instantly and forms the atherosclerotic 2-3cm), latitude and longitude lines form a grid meshed- fluid, leaving a meshed structure and imprints after of shatter cone remains unclear. Different hypotheses for structure. This is an important and unique phenomenon cooling. their formation exist in the literature, but none of them never reported in any literature in the past. It could 2. Similar to the digital model: account for all the current field observations of shatter contribute to the discussion and explanation of the theory The conical fracture in Xishan limestone target rock cones. [1], [2], [3], [4] is very similar to the model in article “The formation of of the formation of shatter cone. In addition, there are shatter cones by shock wave interference during It is noticeable that shatter cones may be coated locally complex cone (Fig.E), formed by multiple cones, which has impacting”. [3] The meshed-structure of the conical with thin layers of melt. The microstructures (spherules, the plurality of cone top. It also has a meshed-structure on surface, formed by the longitudinal wave and the melt fibres and splats) were produced by melting- the surface. annular (hoop) shaped wave is presumed to be similar vaporization on the surface of shatter cones. [5], [6], [7], [8] to the shock wave lattice unit. Thus, contributing to the explanation of the formation of the shatter cone. This paper reports our finding of a new type of shatter 3. Formation model of meshed-structure and gas cone in Xishan Island, Taihu (Jiangsu, China), where imprint on conical surface: shatter cone clustered in limestone target rocks. The cone At early stage of meteorite impact. It forms conical fractures and accompanied by granularization → impact surface develops a meshstructure formed by melting- caused high temperature with an instantaneous gasification and a variety of fish-scales, honeycomb decompression → carbonate rock melting-gasification shaped impressions. The discovery of this new type of and formed atherosclerotic fluid → shock wave pass shatter cones could contribute to the discussion of the through and left imprint → instantly quench cooling formation mechanism of shatter cones. D E forms meshed-structure and a variety of imprints (or gas imprints).

Other kinds of imprints (or gas imprints): The characteristics of Xishan shatter cone: In addition to the meshed-structure, the surface of Is Taihu lake a impact structure? Xishan is the largest island in the Taihu Lake. the cone is often marked with ripples, (Fig.F) fish scales, The strata in the island are reversed and honeycomb (Fig.G) and scoop-like imprints. Xishan shatter cone created clusters. The existence of formed as thrust nappe. (Fig.A) shatter cones with both complete and Imprint pit developed on cone surface: Shallow ones develop incomplete development, different size, and different into ripples, fish scales structure; deep pits develop into types indicate that the Taihu Lake experienced a a honeycomb, spoon-shaped structures. meteorite impact event. The remote sensing The occurrence of shatter cones is the only accepted image of Taihu Lake is meso- to macroscopic recognition criterion for impact circular. The Taihu Lake structures (Fig.J). According to the ideal model of has been suspected to McCall 2016 impact crater, (Fig.L) based on the evidence be a meteorite impact obtained in over a decade of research: structure for decades. 1. Impact metamorphic breccia - taihu stone. (Fig.K) 2. Various types of impact ejects (bean, raindrop,(Fig.M) A curved ring,(Fig.N) stick, (Fig.O)) .

The rock of Limestone hills, near the F G lake, almost completely fractured and formed the breccia (the famous "Taihu stone") . In the distribution area of the “Taihu stone”, shatter Petrographic studies: cone clusters were found to have developed Petrographic studies show that shock deformation in amongst them. shatter cone-bearing rocks. They are mainly fine limestones granule. Calcite twice-crystal, planar deformation features(PDFs), planar features (PFs) are visible in fine J k granules.(Fig.H) The melting phenomenon: twice-crystal, PDFs and PFs gradually become shorter and disappear. Granules edge melts and gradually become bright. When multiple granules edge melts and polymerized, it will form irregular veinlets or reticular. Even residual calcite granules are visible L B C embedded in the bright yellow melt.(Fig.I)

There is not only common types of shatter cone that fit in conventional definitions (Fig.B, Fig.C), which conical striation possess bifurcated, but also found M N O new type of shatter cone (Fig.D, Fig.E), which conical It can be concluded that there is not only impact surface with meshed-structures and multiple crater at Taihu Lake, but medium to large ones. impressions (or air imprint). The surface of the cone is Some parts of the impact crater have been found, but often marked with ripples, fish-scales, honeycomb and the specific location, characteristics and formation scoop-like imprints. H I mechanism of the impact crater will need further study.

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