Impactites from the Gardnos impact structure - deformation patterns of different target lithologies

Category Tectonic & Seismotectonic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Kalleson, Elin۱; Jahren, Tom۲; Dypvik, Henning۳
Holding Date 11 October 2008

The target area of the Gardnos impact crater has been described as "mainly crystalline basement", but at a closer look shows the presence of a variety of lithologies. Precambrian granites, granitic gneisses, quartzite and some banded gneisses dominate in the area, appearing with north - south strikes. In addition some darker, amphibolitic rocks (described as meta-gabbros and meta-dolerites at the regional geological map by NGU) occurs as dominant rock bodies of limited extent several places within the crater structure. During impact all of these lithologies were exposed to violent pressure waves penetrating deep into the ground. The lithologies responded differently to the impact. This study presents field and thin section observations of the main impact altered rock types of the crater floor.

The granite/granitic gneiss and most of the quartzites behaved in a general brittle manner; fractured into relatively small pieces (generally < 10 cm) during impact, making up the autochthonous Gardnos Breccia. The Gardnos Breccia consists of light coloured rock fragments in a dark matrix of finely crushed rock material (with some finely dispersed elementary carbon). Thin section observations indicate that minor or no deformation occurs within the rock clasts of the Gardnos Breccia. Banded gneiss fragments are found at a few localities, generally fractured into larger (10-30 cm) clasts. Because of the characteristic banding patterns it offers a rare opportunity to study the clast displacement and rotation due to seismic shaking during impact.

An outcrop of black quartzite is found within the area generally covered by Gardnos Breccia. In field and at hand specimen scale, the quartzite appears to be unbrecciated. Also the amphibolitic rocks generally seem to have avoided brecciation. Being resistant to erosion the amphibolitic rocks often stand out as topographic highs, and they make up a large part of the central uplift. Though the black quartzite and the amphibolitic rocks do not brecciate in the typical large-scale "Gardnos Breccia patterns", they may display brecciation patterns on a microscopic scale. The black quartzite is penetrated by numerous thin fractures, some lined with elementary carbon enrichments. This is probably the explanation of the black colour. The amphibolitic rocks in general seem to be only marginally affected by the impact. The samples from the central peak, however, show microscopic scale brecciation

This preliminary study indicates that most of the deformation resulted in large-scale matrix generation in the granitic and quartzitic rocks, whereas in the amphibolitic rocks brecciation only took place on microscopic scale.