Magnetization on impact structures—Constraints from numerical modeling and petrophysics
H.A. Ugalde, N. Artemieva, B. Milkereit, 2005. "Magnetization on impact structures—Constraints from numerical modeling and petrophysics", Large Meteorite Impacts III, Thomas Kenkmann, Friedrich Hörz, Alex Deutsch
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Prominent magnetic anomalies over large impact craters are attributed to remanent magnetization as thermal effects induce extremely high Koenigsberger values (remanent to induced magnetization ratio, Q). Magnetization of impact melt rocks, breccias, and the rocks underneath the crater floor is related to the thermal evolution of large impact craters, from a single heat pulse to long-lived hydrothermal processes and associated alteration and mineral deposits. The magnetic signature observed on large impact structures can be primarily the aggregate of three effects: (1) composition and properties of target rocks, (2) modification of magnetic carriers due to high pressure-temperature (P-T) conditions, and (3) natural remanent magnetization (NRM). Numerical modeling is used to predict the pressure and temperature distribution for varying size craters ranging from a 1.5-km-diameter simple crater to a 90-km-diameter complex crater. We first compare the results of two hydrocodes: the Simplified Arbitrary Lagrangian Eulerian code, version B (SALEB), to produce the final crater shape after a vertical impact; and the Solid, Vapor, Air (SOVA) code to model the initial stage of an oblique impact and to evaluate differences in the volumes of highly shocked materials within the crater created by a vertical impact versus an oblique one. After defining the accuracy of the numerical modeling and reliability of the resulting P-T conditions predicted, numerical modeling is applied for 1–90 km diameter impact structures. The P-T distributions of the target material at its final position obtained with the SALEB code are used for geophysical predictions. The calculated maximum P-T values and their initial and final spatial distributions can be linked with geological and physical and/or chemical processes that lead to different geophysical signatures. For a 5 km diameter crater, brecciation (P > 1 GPa) affects all material in a 2.5 km hemisphere. Brecciation and fluid circulation trigger a hydrothermal system, which introduces chemical remanent magnetization (CRM) and modify magnetic carriers due to alteration. Pressures larger than 30 GPa induce a secondary component of remanent magnetization (shock remanent magnetization, SRM). These P-T conditions are generally restricted to a radius of <0.5 km and depth <0.8 km. Therefore, SRM and melting are confined to small regions, occasionally not even discernible by a regional airborne magnetic survey, and easily removed by erosive processes.