Anisotropy of magnetic susceptibility (AMS) of impact melt breccia and target rocks from the Dhala impact structure, India

2021 ◽  
Author(s):  
Anuj Kumar Singh ◽  
Jayanta Kumar Pati ◽  
Shiva Kumar Patil ◽  
Wolf Uwe Reimold ◽  
Arun Kumar Rao ◽  
...  
Keyword(s):  
Magnetic Susceptibility ◽  
Anisotropy Of Magnetic Susceptibility ◽  
Impact Structure ◽  
Magnetic Phases ◽  
Impact Melt ◽  
Rock Samples ◽  
Average Value ◽  
Magnetic Fabrics ◽  
Target Rock ◽  
The Impact

ABSTRACT The ~11-km-wide, Paleoproterozoic Dhala impact structure in north-central India comprises voluminous exposures of impact melt breccia. These outcrops are discontinuously spread over a length of ~6 km in a semicircular pattern along the northern, inner limit of the monomict breccia ring around the central elevated area. This study of the magnetic fabrics of impact breccias and target rocks from the Dhala impact structure identified a weak preferred magnetic orientation for pre-impact crystalline target rocks. The pre- and synimpact rocks from Dhala have magnetite and ilmenite as common magnetic phases. The distributions of magnetic vectors are random for most impact melt breccia samples, but some do indicate a preferred orientation. Our anisotropy of magnetic susceptibility (AMS) data demonstrate that the shape of susceptibility ellipsoids for the target rocks varies from prolate to oblate, and most impact melt breccia samples display both shapes, with a slight bias toward the oblate geometry. The average value for the corrected degree of anisotropy of impact melt rock (P′ = 1.009) is lower than that for the target rocks (P′ = 1.091). The present study also shows that both impact melt breccia and target rock samples of the Dhala structure have undergone minor postimpact alteration, and have similar compositions in terms of magnetic phases and high viscosity. Fine-grained iron oxide or hydroxide is the main alteration phase in impact melt rocks. Impact melt rocks gave a narrow range of mean magnetic susceptibility (Km) and P′ values, in contrast to the target rock samples, which gave Km = 0.05–12.9 × 10−3 standard international units (SI) and P′ = 1.036–1.283. This suggests similar viscosity of the source magma, and limited difference in the degrees of recorded deformation. Between Pagra and Maniar villages, the Km value of impact melt breccias gradually decreases in a clockwise direction, with a maximum value observed near Pagra (Km = 1.67 × 10−3 SI). The poor grouping of magnetic fabrics for most impact melt rock samples implies local turbulence in rapidly cooled impact melt at the front of the melt flow immediately after the impact. The mean K1 for most impact melt samples suggests subhorizontal (<5°) flow in various directions. The average value of Km for the target rocks (4.41 × 10−3 SI) is much higher compared to the value for melt breccias (1.09 × 10−3 SI). The results of this study suggest that the melt breccias were likely part of a sheet-like body of sizeable extent. Our magnetic fabric data are also supported by earlier core drilling information from ~70 locations, with coring depths reaching to −500 m. Our extensive field observations combined with available widespread subsurface data imply that the impact melt sheet could have covered as much as 12 km2 in the Dhala structure, with an estimated minimum melt volume of ~2.4 km3.

scholarly journals Ocean resurge-induced impact melt dynamics on the peak-ring of the Chicxulub impact structure, Mexico

2021 ◽  
Author(s):  
Felix M. Schulte ◽  
◽  
Axel Wittmann ◽  
Stefan Jung ◽  
Joanna V. Morgan ◽  
...  
Keyword(s):  
Impact Structure ◽  
Physical Processes ◽  
Hydrothermal Conditions ◽  
Chemical Examination ◽  
Impact Melt ◽  
Target Rock ◽  
Single Process ◽  
Chicxulub Impact ◽  
The Impact

AbstractCore from Hole M0077 from IODP/ICDP Expedition 364 provides unprecedented evidence for the physical processes in effect during the interaction of impact melt with rock-debris-laden seawater, following a large meteorite impact into waters of the Yucatán shelf. Evidence for this interaction is based on petrographic, microstructural and chemical examination of the 46.37-m-thick impact melt rock sequence, which overlies shocked granitoid target rock of the peak ring of the Chicxulub impact structure. The melt rock sequence consists of two visually distinct phases, one is black and the other is green in colour. The black phase is aphanitic and trachyandesitic in composition and similar to melt rock from other sites within the impact structure. The green phase consists chiefly of clay minerals and sparitic calcite, which likely formed from a solidified water–rock debris mixture under hydrothermal conditions. We suggest that the layering and internal structure of the melt rock sequence resulted from a single process, i.e., violent contact of initially superheated silicate impact melt with the ocean resurge-induced water–rock mixture overriding the impact melt. Differences in density, temperature, viscosity, and velocity of this mixture and impact melt triggered Kelvin–Helmholtz and Rayleigh–Taylor instabilities at their phase boundary. As a consequence, shearing at the boundary perturbed and, thus, mingled both immiscible phases, and was accompanied by phreatomagmatic processes. These processes led to the brecciation at the top of the impact melt rock sequence. Quenching of this breccia by the seawater prevented reworking of the solidified breccia layers upon subsequent deposition of suevite. Solid-state deformation, notably in the uppermost brecciated impact melt rock layers, attests to long-term gravitational settling of the peak ring.

Paleomagnetism and rock magnetism of East and West Clearwater Lake impact structures

2019 ◽  
Vol 56 (9) ◽  
pp. 983-993
Author(s):  
Jérôme Gattacceca ◽  
William Zylberman ◽  
Adam B. Coulter ◽  
François Demory ◽  
Yoann Quesnel ◽  
...  
Keyword(s):  
Rock Magnetism ◽  
Drill Hole ◽  
Hydrothermal Activity ◽  
Radiometric Dating ◽  
Impact Structure ◽  
The West ◽  
Impact Melt ◽  
Basement Rocks ◽  
East And West ◽  
The Impact

The East and West Cleawater Lake impact structures (Wiyâshâkimî Lake, Québec), ∼26 and 32 km in diameter, respectively, have been proposed to represent an impact doublet. We investigated their paleomagnetism to contribute to this debate. The paleomagnetic directions of the impact melt rocks and impact melt-bearing breccias from the West Clearwater structure are compatible with the radiometric age of 280–290 Ma previously determined for this structure and indicate that the impact occurred during a reverse polarity interval of the geomagnetic field. A similar remagnetization direction is found in the basement within 10 km of the structure center, whereas basement farther away from the center has escaped remagnetization by the impact. Samples for the East Clearwater structure come from two holes drilled in 1963 and 1964. Unfortunately, the drill hole through the melt rocks is tilted by 30° from the vertical with an unknown azimuth. The paleomagnetic inclination of these melt rocks cannot be constrained to better than between −28° and +32°. This is, however, distinct from the inclination of the melt rocks of the West Clearwater Lake impact structure (−27.8° ± 3.7°), suggesting that the two structures do not represent an impact doublet, in agreement with recent radiometric dating. The basement rocks and the melt rocks within 10 km of the center of the West Clearwater Lake impact structure show a magnetic signature of titanohematite that crystallized during postimpact hydrothermal activity under oxidizing conditions. This is not observed in the basement or the melt rocks from the East Clearwater Lake impact structure.

scholarly journals Design features of the Franz Josef Land volcanic provinces (Arctic Ocean): estimation of lava direction by anisotropy of magnetic susceptibility

2019 ◽  
Vol 486 (2) ◽  
pp. 197-201
Author(s):  
V. V. Abashev ◽  
V. A. Vernikovsky ◽  
A. Yu. Kazansky ◽  
D. V. Metelkin ◽  
N. E. Mikhaltsov ◽  
...  
Keyword(s):  
Magnetic Susceptibility ◽  
Anisotropy Of Magnetic Susceptibility ◽  
Simultaneous Action ◽  
Mechanism Of Formation ◽  
Volcanic Province ◽  
Magnetic Fabrics ◽  
Franz Josef Land ◽  
Multi Stage ◽  
First Results ◽  
History Of

The paper presents the first results of studying the anisotropy of magnetic susceptibility in the basalts from Hooker Island, associated with the direction of the melt movement, the location of the eruption centers and the morphology of magmatic bodies. The established features of the primary magnetic fabrics correspond to the trap mechanism of formation of the volcanic province of the Franz Josef Land Archipelago and are a reflection of the simultaneous action of numerous small eruption centers. Previously obtained conclusions about the long, during the Early Jurassic - Early Cretaceous, multi-stage history of magmatism are not confirmed.

scholarly journals Magnetic properties of pseudotachylytes, Jämtland, central Sweden

2019 ◽  
Author(s):  
Hagen Bender ◽  
Bjarne S. G. Almqvist ◽  
Amanda Bergman ◽  
Uwe Ring
Keyword(s):  
Magnetic Susceptibility ◽  
Shear Zones ◽  
Anisotropy Of Magnetic Susceptibility ◽  
Fault Zones ◽  
Small Sample ◽  
Magnetic Fabric ◽  
Normal Faults ◽  
Magnetic Fabrics ◽  
Nappe Complex ◽  
Fault Kinematics

Abstract. Nappe assembly in the Köli Nappe Complex, Jämtland, Sweden, has been associated with in- and out-of-sequence thrusting. Kinematic data from shear zones bounding the Köli Nappe Complex are compatible with this model, but direct evidence from fault zones internally subdividing the nappe complex does not exist. We studied a series of pseudotachylyte exposures in these fault zones for deciphering the role seismic faulting played in the assembly of the Caledonian nappe pile. To constrain the fault kinematics, microstructural and magnetic fabrics of pseudotachylyte in foliation-parallel fault veins have been investigated. Because the pseudotachylyte veins are thin, we focused on small (c. 0.2 cm3) samples for measuring the anisotropy of magnetic susceptibility. The results show inverse proportionality between specimen size and anisotropy of magnetic susceptibility degree, which is most likely an analytical artifact related to instrument sensitivity and small sample dimensions. This finding implies magnetic anisotropy results acquired from small specimens demand cautious interpretation. However, analysis of structural and magnetic fabric data indicates that seismic faulting occurred during exhumation into the upper crust but yield no kinematic in-formation. Structural field data suggest that seismic faulting was postdated by brittle E–W extensional deformation along steep normal faults. Therefore, it is likely that the pseudotachylytes formed late during out-of-sequence thrusting of the Köli Nappe Complex over the Seve Nappe Complex.

scholarly journals Anisotropy of magnetic susceptibility (AMS) of lava and volcanoclastic flows of the Stephano-Permian Cadi basin (central-eastern Pyrenees)

2020 ◽  
Author(s):  
Ana Simon-Muzas ◽  
Antonio M Casas-Sainz ◽  
Ruth Soto ◽  
Josep Gisbert ◽  
Teresa Román-Berdiel ◽  
...  
Keyword(s):  
Magnetic Susceptibility ◽  
Flow Direction ◽  
Anisotropy Of Magnetic Susceptibility ◽  
Lava Flows ◽  
Magnetic Fabric ◽  
Magnetic Mineralogy ◽  
Thin Sections ◽  
Andesitic Lava ◽  
Magnetic Fabrics ◽  
Standard Specimens

<p>The aim of this work is to apply the anisotropy of magnetic susceptibility (AMS) to determine the primary and tectonic fabrics of lava flows and volcanoclastic materials in one of the Pyrenean Stephano-Permian basins.</p><p>The Pyrenean Range is a double vergence orogen located at the northern end of the Iberian Peninsula. During Carboniferous-Early Permian times the extensional or transtensional regime dominant during the progressive dismantling of the Variscan belt resulted in the development of E-W elongated intra-mountainous basins. This process was coeval with an exceptional episode of magmatic activity, both intrusive and extrusive. The Cadí basin represents a good example of these structures were Stephano-Permian rocks are aligned along an E-W continuous outcrop and reach thickness of several hundreds of meters. The stratigraphy of the study area is characterized by fluviolacustrine sediments changing laterally to volcanoclastic and pyroclastic rocks with interbedded andesitic lava flows.  </p><p>A total of 75 sites (733 standard specimens) were studied and analysed throughout the volcanoclastic, volcanic and intrusive materials of the Stephano-Permian outcrops in the Cadí basin. Samples were drilled in the field along 5 sections with N-S or NW-SE direction in the Grey and Transition Unit. Afterwards, standard specimens were measured in a Kappabridge KLY-3 (AGICO) at the Zaragoza University to characterise the magnetic fabric. The susceptibility bridge combined with a CS-3 furnace (AGICO) was used for the temperature-dependent magnetic susceptibility curves (from 20 to 700 °C) to recognize the magnetic mineralogy. In addition, textural and mineralogical recognition in thin-sections of the samples was carried out in order to recognize the relationship between magnetic and petrographic fabrics.</p><p>The results shows that the bulk magnetic susceptibility of the specimens ranges between 118 and 9060·10<sup>-6</sup> SI but most of the values are bracketed between 160 to 450·10<sup>-6</sup> SI. Taking into account magnetic parameters (Km, Pj and T) there is no correlation between magnetic fabrics and magnetic mineralogy and there is a dominance of triaxial and prolate ellipsoids. Thermomagnetic curves indicate the dominance of paramagnetic behaviour in all the samples and except in one case there is a ferromagnetic contribution due to the generalised presence of magnetite.</p><p>Magnetic ellipsoids can be divided into four main types depending on the orientation of the main axes and associated with the lithologic types: 1) K<sub>max</sub> vertical and K<sub>int </sub>and K<sub>min</sub> horizontal for small intrusive bodies (no restoring); 2) K<sub>max </sub>horizontal or subhorizontal and K<sub>int </sub>and K<sub>min </sub>included in a subvertical plane (before and after restitution) for volcanic breccias; 3) K<sub>min</sub> vertical (after restoring) and three directional maxima for lava flows and 4) non-defined fabric for the explosive materials (probably due to their complex depositional mechanisms). In general, a dominant E-W magnetic lineation is observed in many sites, resulting either from dominant flow direction, or to secondary processes. This is the case for some of the magnetic ellipsoids, that seems to be affected by deformation, K<sub>min</sub> is not normal to bedding and therefore, they do not become vertical after bedding restitution.</p>

scholarly journals Petrographic Investigation of Target Rock Transformation under High Shock Pressures from the Colônia Impact Crater, Brazil

2017 ◽  
Vol 7 (1) ◽  
pp. 13 ◽  
Author(s):  
Victor F. Velázquez ◽  
Rodrigo F. Lucena ◽  
Jose M. Azevedo Sobrinho ◽  
Alethéa E. Martins Sallun ◽  
William Sallun Filho
Keyword(s):  
Shock Waves ◽  
Impact Crater ◽  
Small Scale ◽  
Impact Melt ◽  
Basement Rocks ◽  
High Shock ◽  
Target Rock ◽  
Deformation Phase ◽  
Deformation Features ◽  
The Impact

The Colônia impact crater, developed on crystalline basement rocks, offers an excellent example of one of the most unique features of the impact process: the effects of shock waves on textural and mineralogical changes of the target rock. The impact melt-bearing impactites were derived essentially from the igneous and metamorphic rocks, including granite, mica schist, granitic gneiss, and quartzite. Investigations using optical microscopy indicate that the effect of shock waves on those lithologies caused a wide variety of deformation features and generation of new materials. The most common features include fluidal textures, unusual rearrangement patterns between grains, recrystallization, decomposition and precipitation of new phases, agglutination of glassy and crystalline spherules, and the mobilized melt formed different types of impact melt particles. These transformations cover processes that may involve a new grain growing at the expense of parental grains of the same species, or crystallization of different mineral types from component-providing grains until a complete textural and compositional change of the target rocks occurs. Small-scale structures in deformed rocks are particularly interesting for exploring elastic-plastic deformation, phase transformations, and generation of impact melt products.

Thermo‐mechanical interaction of a large impact melt sheet with adjacent target rock, Sudbury impact structure, Canada

2019 ◽  
Vol 54 (6) ◽  
pp. 1228-1245 ◽  
Author(s):  
Paul L. Göllner ◽  
Torben Wüstemann ◽  
Lisa Bendschneider ◽  
Sebastian Reimers ◽  
Martin D. Clark ◽  
...  
Keyword(s):  
Large Impact ◽  
Impact Structure ◽  
Mechanical Interaction ◽  
Impact Melt ◽  
Target Rock

scholarly journals Structure and internal deformation of thrust sheets in the Sawtooth Range, Montana: insights from anisotropy of magnetic susceptibility

2019 ◽  
Vol 487 (1) ◽  
pp. 189-208 ◽  
Author(s):  
Dave J. McCarthy ◽  
Patrick A. Meere ◽  
Michael S. Petronis
Keyword(s):  
Magnetic Susceptibility ◽  
Strain Analysis ◽  
Anisotropy Of Magnetic Susceptibility ◽  
North American Continent ◽  
The North ◽  
Magnetic Fabrics ◽  
Structural Regime ◽  
Thrust Sheets ◽  
Internal Deformation ◽  
East West

AbstractGeological strain analysis of sedimentary rocks is commonly carried out using clast-based techniques. In the absence of valid strain markers, it can be difficult to identify the presence of an early tectonic fabric development and resulting layer parallel shortening (LPS). In order to identify early LPS, we carried out anisotropy of magnetic susceptibility (AMS) analyses on Mississippian limestones from the Sawtooth Range of Montana. The Sawtooth Range is an arcuate zone of north-trending, closely spaced, west-dipping, imbricate thrust sheets that place Mississippian Madison Group carbonates above Cretaceous shales and sandstones. This structural regime is part of the cordilleran mountain belt of North America, which resulted from accretion of allochthonous terrains to the western edge of the North American continent.Although the region has a general east–west increase in thrust displacement and related brittle deformation, a similar trend in penetrative deformation or the distribution of tectonic fabrics is not observed in the field or in the AMS results. The range of magnetic fabrics identified in each thrust sheet ranges from bedding controlled depositional fabrics to tectonic fabrics at a high angle to bedding.

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