Impact-generated pseudotachylitic breccia in drill core BH-5 Hättberg, Siljan impact structure, Sweden

Abstract

Pseudotachylitic breccia (PTB) in the form of cm-wide melt breccia veinlets locally occurs on the exposed central uplift of the 380 Ma Siljan impact structure. The host rock to the PTBs is the so-called Järna granite of quartz monzonitic to syenodioritic composition. The nearly 603 m long BH-5 drill core from Hättberg, near the centre of the Siljan central uplift, contains numerous veins and pods of PTB. In particular, two major zones of 60 m combined width contain extensive PTB network breccias (30% actual melt breccia component), with individual melt breccia occurrences up to .1 m in length. Core logging and petrographic and geochemical analysis of the core have been performed, and the data are interpreted to suggest the following. (1) The impact event caused low to moderate (at essentially ,20 GPa) shock deformation in the host rock and in clasts of this lithology within the PTB. (2) Macroscopic deformation of the basement mainly comprises fracturing, with only localised cataclasis.(3) No evidence for shock melting (i.e. compression/decompression melting early in the cratering process) could be observed. (4) Optical and scanning electron microscopy showed that dark PTB contains a definite melt component. (5) Shearing has significantly affected this part of the central uplift, but its effects are limited to very short displacements and likely did not result in extensive melting. (6) A frictional heating component upon melt generation can, however, not be excluded, as many PTB samples contain clasts of a mafic (gabbroic) component, although only in one place along the entire core, a 1.2 cm-wide section through such material in direct contact to host rock was observed. Consequently, we suggest that, upon uplift in the central part of the impact structure, considerable melt volumes were generated locally, especially in areas that had been affected by extensive cataclasis and where grain size comminution favoured melt formation. Rapid decompression related to central uplift formation is the preferred process for the generation of the PTB melt breccias.

The current state of knowledge about shatter cones: Introduction to the special issue

“…Shatter cones occur in supracrustal and/or crystalline basement rocks of impact structures, but also as clasts in ejecta outside of an impact structure (e.g., Gostin et al [1986] for the Acraman impact structure; Masaitis [1999] for the Kara impact structure) or within impact breccia of the crater fill, as illustrated in the case of the pseudotachlitic breccias at the Sudbury impact structure (Thompson and Spray 1994), limestone fragments at the Steinheim basin (Buchner and Schmieder 2010), and recent reports of shatter cones in the polymict breccia from the Vista Alegre structure of Brazil (e.g., Cr osta et al 2010;Pittarello et al 2015), or from drill cores of the El'gygytgyn impact structure, Siberia (Raschke et al 2013) and Siljan (Reimold et al 2015). They have been variably described from crater rim settings and from central uplifts, although the association with central uplifts is more common.…”

“…) and Siljan (Reimold et al. ). They have been variably described from crater rim settings and from central uplifts, although the association with central uplifts is more common.…”

Combining shock barometry with numerical modeling: Insights into complex crater formation—The example of the Siljan impact structure (Sweden)

“…Some samples also contain pseudotachylitic breccia veinlets (e.g., Mohr‐Westheide and Reimold ; Reimold et al. ) and cataclastic shear zones, namely samples H5, H45, H351, H451, H500, H600, and V99. In this study, we refer to PDFs parallel to (0001), i.e., the basal plane of the crystal, as basal PDFs, although these PDFs are multiple mechanical Brazil twin lamellae (e.g., Goltrant et al.…”

“…*This reference to the rock type is taken from the original drawing, but as Reimold et al. () described frequent pseudotachylitic breccia veins throughout the core, it is very likely that these “mylonites” are in fact pseudotachylitic breccia veins.…”

A Late Mesoproterozoic 40Ar/39Ar age for a melt breccia from the Keurusselkä impact structure, Finland

“…They are usually restricted to the central uplift, subcrater, and crater rim domain and typically reflect the modal and geochemical composition and degree of shock metamorphism of the wall rock they were produced from (e.g., Reimold and Gibson 2005;Mohr-Westheide and Reimold 2010). In this context, we also refer to the definition and discussion of "pseudotachylitic breccias" in large terrestrial impact structures, for which the mechanisms of melt formation are probably complex and a matter of ongoing discussion (e.g., Reimold 1995Reimold , 1998Dressler and Reimold 2004;Reimold and Gibson 2005;Riller et al 2010;Reimold et al 2015).…”