Pseudotachylyte Research Articles

Material mixing in shock-induced pseudotachylites, Vredefort impact structure, South Africa

Impact-generated pseudotachylites (PTs) have been extensively studied in the Vredefort Impact Structure, South Africa, and have been shown to correspond chemically and isotopically to their host rocks. Such melts are frequently described as forming strictly in situ with lithic clasts derived from the immediate wall rocks. Mixed compositions are rare, where PTs are observed directly along a lithological contact. In this study, we document the presence of vast numbers of granitic clasts within dolerite-hosted PT veins within the Vredefort impact structure. The groundmasses of dolerite-hosted PT veins have two distinct geochemical compositions: (1) mafic, and (2) intermediate. The two compositional melt phases have a sharp contact, indicating immiscibility and/or mingling. Shocked zircon and monazite within the granite clasts enclosed in a dolerite-hosted PT vein indicate shock deformation prior to clast transport into the dolerite. In monazite, we report a new shock microtwin geometry 180°/[104].The mechanisms by which granitic clasts were emplaced in the dolerite-hosted veins are complex. Previous experimental work has suggested that varying shock velocity in different lithologies may be responsible for material mixing. In addition, the thermal pulse accompanying the shock wave would have caused quartz in the granite to transform into volumetrically larger high-temperature polymorphs, significantly increasing the granite volume compared to the adjacent dolerite. Together with fluctuation of stresses (post-impact reverberations) after the passage of the main shock wave, this volume adjustment initiated brittle fracturing of the dolerite. A negative pressure gradient was generated within the dolerite that allowed granitic material to be injected into the dolerite-hosted melt veins.

Two genetic types of pseudotachylytes

The study of two varieties of pseudotachylytes (PST) in granitoids of the Riphean complex on the Barents Sea coast of the Kola Peninsula (Rybachii and Srednii peninsulas) and in metapsammite of the Paleoproterozoic complex in the Northern Ladoga region by a few independent analytical methods has made it possible to establish that they belong to different genetic forms, such as mechanically crushed rocks and melting products, respectively. As for the melting differences, we have given a detailed description of the mineral and material transformations of the original rock into the PST glass matrix and obtained evidence for the initial melting out of the micaceous eutectics with its subsequent shift to the granite type. The conclusion has been made on the most likely formation of molten PST due to frictional rock melting under rapid rise of its blocks from a depth of 12–15 km to the crustal surface (less than 3 km) along the faults of presumably seismogenic nature. It is suggested that crushing and frictional melting can be complementary, rather than mutually exclusive processes, and the formation of molten PST is commonly preceded by the mechanical rock crushing stage.

Comparative study of pseudotachylite microstructures in felsic and mafic rocks from the Vredefort impact structure, South Africa. Implications for the experimental studies

Abstract While the formation of tectonic PTs is widely accepted as related to friction melting for all rock types, the mechanism of formation of impact-related pseudotachylites (PTs) is strongly debated. Moreover, the influence of the target lithology on the initial impact PT formation, development and deformation has not been considered in detail, even though impact PTs in the Vredefort structure are found in various lithologies. Here, we investigate PTs that occur in felsic (granite) and mafic (dolerite) rocks that are in contact with one another in the core of the Vredefort impact structure. In this unique outcrop, multiple granitic clasts are observed inside the PTs hosted by dolerite, which indicates material exchange between PTs in the two lithologies, pointing to simultaneous shock deformation in the adjacent rock phases. This outcrop thus provides an unusual opportunity for studying the effect of host lithology on the formation and deformation of PT. Our investigations demonstrate that microstructures and textures in PTs and their chemical composition largely depend on the host rock, suggesting particular rheological syn- and post-formation behaviour of the melt veins of different composition. In granite, melt initiates due to preferential decomposition of mafic and hydrous minerals, and subsequent ductile deformation of quartz and feldspars. In dolerite, melt initiates as a result of comminution of the bulk rock. Due to the different mineral composition of rocks, and corresponding various fracture toughness of different minerals, granitic PTs contain abundant monomineralic clasts composed of more refractory phases, most often quartz, while PT veins in dolerite contain fewer, usually lithic clasts, representing fragments of the host rock. PTs in granite are chemically zoned, which has not been observed for PTs in dolerite. In felsic rocks, PTs are often framed by an ultramylonite and are associated with post-shock ductile features, such as sigmoidal-shaped clasts and dragged edges of the veins, whereas in mafic rocks, PTs are found to be associated with brittle features only, such as (ultra)cataclasites and micro-faults. These differences in initiation and development of PTs based on rock type are consistent with previous experimental studies and must be considered when discussing the formation mechanisms of impact-generated PTs.

The Destiny of a Clast within a Molten Pseudotachylyte Vein

Pseudotachylytes are important markers that can indicate the thermal state during a coseismic slip failure and provide indirect information about the level of stress and sliding velocities attained during that time window. On the other hand, survivor, fragmented clasts embedded in the quenched material are also very impor- tant, in that they provide information about the energy spent to create new fracture surfaces. In this paper, I study the temperature evolution of clasts subjected to the heat dissipated by a just-formed, molten pseudotachylyte (PT) vein. In particular, I find the analytical solutions for the temperature evolution within the PT vein, the surrounding, undamaged host rock, and inside a clast. According to the proposed model, the numerical results show that the clasts tend to preferentially melt in the inner part of the PT vein (i.e., they are completely assimilated by the PT). In contrast, some clasts can survive far from the PT vein center. My solutions, although based upon a simplified model, can provide a theoretical framework to predict the maximum size of the survived clasts at a given distance from the PT center. The distribution of these survivor clasts follows a power-law relation in terms of their radius, and its features generally agree with field and laboratory observations.

Oldest record of brittle deformation along the Median Tectonic Line: fission-track age for pseudotachylyte in the Taki area, Mie Prefecture

Fission-track (FT) dating of pseudotachylyte (PST) associated with the Median Tectonic Line (MTL) in the Taki area, Mie Prefecture, SW Japan, provides new constraints on the timing of movement upon this fault. A PST vein with a thickness of about 5 cm yields a zircon FT age of 60.0 ± 3.5 Ma (1σ). In contrast, a weighted average of zircon FT ages obtained for protolith samples (cataclastic mylonitized Hatai Tonalite) collected 10 cm and 15 m from the PST vein boundary is 69.8 ± 1.2 Ma, which is significantly older than the age of the PST. Decomposition of feldspars in the PST suggests that the temperature exceeded 1100°C, which is sufficient to completely erase previous fission tracks in zircon within several seconds. The distribution of fission-track lengths in zircon from the PST vein also supports the interpretation that the zircon FT age was completely reset during frictional fusion of the PST vein. Considering an apatite FT age of 38.0 ± 1.5 Ma for the host rock, the age of the PST indicates that the frictional fusion was occurred during cooling of the Ryoke granite at temperatures between 250°C (closure temperature of the zircon FT system) and 100°C (closure temperature of the apatite FT system). This PST age is comparable with the oldest K-Ar age obtained for fine fractions of MTL fault gouge derived from both the Sanbagawa pelitic schist and the Izumi Group in Shikoku, indicating that the initiation of brittle fault movement associated with formation of the PST and/or fault gouges along the MTL had occurred by 60 Ma.

Fault reactivation and pseudotachylite generation in the semi-brittle and brittle regimes: examples from the Gavilgarh–Tan Shear Zone, central India

AbstractIn the sheared and foliated granitoids of the Proterozoic Gavilgarh–Tan Shear Zone (GTSZ) in central India, two types of pseudotachylite (Pt-M and Pt-C) are recognized. Pt-M layers are interbanded with mylonite and ultramylonite, show strong internal plastic deformation and buckle folding concurrent with the host rocks, and appear to have formed within the greenschist facies (300–400 °C) in the brittle–plastic transitional (semi-brittle) regime. Pt-C layers show sharp contacts with the host rock, exhibit abundant coeval cataclasis, preserve no evidence of subsequent plastic deformation, and formed at shallower depths, at temperature < 300 °C. Sulphide droplets and embayment of quartz grain margins in the pseudotachylite (Pt-C) matrix indicates a melt origin. Ductile shear sense criteria in the host mylonites are consistently sinistral, while those associated with the deformed pseudotachylite (Pt-M) layers are dextral. It appears therefore that the host mylonite/ultramylonite foliation experienced reactivated slip movement in the ‘semi-brittle’ zone when pseudotachylite was generated and subsequently ductilely deformed. The brittle pseudotachylite (Pt-C) layers were generated later at a shallower level, and at a lower temperature. They are spatially associated with a set of foliation-parallel brittle shears with sinistral-sense displacements. The multiple episodes of frictional melt generation within the Gavilgarh–Tan Shear Zone illustrate that it has a complex history of multiple reactivations. It therefore represents an important new area for the study of seismic behaviour of the upper crust along pre-existing structures and may facilitate a better geological understanding of the present seismic activity in the central Indian Shield.

Generation Depth of the Pseudotachylyte from an Out-of-Sequence Thrust in Accretionary Prism-Geothermobarometric Evidence

Generation Depth of the Pseudotachylyte from an Out-of-Sequence Thrust in Accretionary Prism-Geothermobarometric Evidence

Tektites and pseudotachylytes in Hainan Island, Southern China

テクタイトは地表に産する黒曜石に類似した天然のガラス質物質であり, 中国では, 紀元十世紀の唐の時代にすでに詳しく記載されており, “雷公墨”と名付けられている(李, 1963). テクタイトは, オーストラリア, 北米, 東南アジア, アフリカ北部, チェコスロバキアなどに分布している(Glass, 1990). テクタイトの成因については, 巨大隕石の衝突に伴い地表の岩石が熔融して形成されたという地球成因説(Glass, 1990)と, 月表面の火山物質(または宇宙物質)が隕石の衝突で飛散し, 地球に飛来したという月(または宇宙)成因説(O'keefe, 1976)などがあるが, いずれも成因を衝突熔融に求めている点では共通している. テクタイトの化学組成は地球物質起源のものが多いことから, 前者の説が有力になりつつある. 今回紹介する海南島のテクタイトの産地には, 不規則な割れ目の発達した砂岩中にシュードタキライトも発見されている(第10, 11図). この地点の周辺に明瞭な断層が存在していないことと隕石坑が存在していることから, このシュードタキライトはテクタイトとともに隕石衝突熔融によるものと考えられる.中国海南島に産するテクタイトは Hainanite と命名されている(李, 1963). そのフィッショントラック年代は約70万年であり, 東南アジアおよびオーストラリア産のテクタイトの年代とほぼ一致している(厳ほか, 1979). このテクタイトの産出層は海南島およびその周辺地域の第四系および古地形面の形成時代の鍵層として利用されている.

Micro-Shock Deformation adjacent to the Surface of Shatter Cones from the Beaverhead Impact Structure, Montana

Petrographic and transmission electron microscopy (TEM) study reveals that planar fractures, pseudotachylite (PST) veinlets, and planar deformation features (PDF) in quartz and K-feldspar grains are developed only within ~2 mm of the surface of shatter cones in Proterozoic arkosic sandstones from the Beaverhead (Montana) impact structure. This unique association reinforces the impact-shock origin attibuted to all these shock-metamorphic features. No high-pressure $SiO_{2}$ polymorphs were found in the PST, but possibly this is because these cones formed far from the center of the impact structure, in a shock-pressure zone below the stability field of the high-pressure polymorphs.

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