Impact-pressure controlled orientation of shatter cone magnetizations in Sierra Madera, Texas, USA

Abstract

A suite of Sierra Madera Impact deformed rocks was studied and magnetic analyses were performed. We characterized the magnetic signatures of two locations, sites A and B that have different physical characteristics of shock fractured structures as well as the magnetic signatures. Shatter cone at site A has a fine-scale (few to ~10 mm) distributed array of complete shatter cones with sharp apex. Natural remanent magnetization (NRM) of site A shatter cone is distributed within the plane that is perpendicular to the apexes of the cones. Shatter cone at site B shows no apparent cone shape or apex, instead, a relatively larger scale and multiple striated joint set (MSJS) and sinusoidal continuous peak. NRM of site B shatter cone is clustered along the apexes. The difference in magnetization direction is a likely indicator of the shock pressure where parallel to apex indicates pressures larger than 10 GPa and perpendicular to apex indicate pressures less than 10 GPa. Intensities of NRM and saturation isothermal remanent magnetization (SIRM) contrast and fluctuate within a shatter cone as well as in between two sites. We observed a random orientation of magnetic vector directions and amplitudes changing over small scales leading to the absence of coherent macro-scale signature.K e y w o r d s :

A 200 km suspected impact crater Kotuykanskaya near Popigai, Siberia, in the light of new gravity aspects from EIGEN 6C4, and other data

“…The centre of the crater where we see the diffused magnetic boundaries have been demagnetized to a depth of several tens of kilometres and left magnetization only of magnetic sources that are deep-seated. This location correlates well with the location of the suspected crater and leaves an evidence of impact demagnetization process 35,36 .…”

“…When the crust is impacted by meteorite whose mass is such that the atmosphere has negligible effect on the slowing down the impactor, the speed of the impact has tens of km/s. The resulting impact and explosion create a shock pressure exposure of the crust that decays exponentially from the impact epicentre 35,36 . Such pressure exposure necessary demagnetizes the crust.…”

Deformational features and impact-generated breccia from the Sierra Madera impact structure, west Texas

“…This observation was also noted from the Haughton structure, which contains shatter cone fragments within polymict breccia (Osinski and Spray, 2006). Magnetic studies of shatter cones from Sierra Madera indicate that magnetization in the carbonate rocks occurred in two phases corresponding to the initial compressive shock wave and an immediate decompression wave (Adachi and Kletetschka, 2008). These authors suggest that shatter cones at Sierra Madera formed during the second magnetization event, with localized melting along shatter cone surfaces (Adachi and Kletetschka, 2008).…”

“…Magnetic studies of shatter cones from Sierra Madera indicate that magnetization in the carbonate rocks occurred in two phases corresponding to the initial compressive shock wave and an immediate decompression wave (Adachi and Kletetschka, 2008). These authors suggest that shatter cones at Sierra Madera formed during the second magnetization event, with localized melting along shatter cone surfaces (Adachi and Kletetschka, 2008). This event must have occurred early, during the contact/compression stage of crater development, for shatter cone clasts to be incorporated as clasts within polymict breccia.…”