High Temperature Emplacement of Clastic Breccia Dikes and Implications for the Development and Magnetization of Impact Craters

Abstract:

Breccia dikes are a common feature of impact craters on Earth and should also be present within impact structures on other planetary bodies. Within the ~450 Ma, ~30 km diameter Slate Islands impact structure in Ontario, Canada, breccia dikes can be classified into two categories: 1) mm-scale irregular or anastomosing veins composed of a fine-grained to glassy matrix with variable clast content (type A of Lambert, 1981) and 2) thicker (2 cm to >15 m wide) polymict breccia bodies intruding parautochthonous host rock (type B of Lambert, 1981). We have targeted the clasts and matrix from 9 type B breccia dikes throughout the impact structure for paleomagnetic analysis. Preliminary results on one dike show that clasts fail a conglomerate test, indicating that they were completely remagnetized after the breccia dike was emplaced. We interpret this result to indicate that lithic breccia dikes can experience levels of frictional heating capable of fully thermally remagnetizing clasts. Furthermore, breccia bodies from different locales yield similar overprint directions minimally affected by tilting or rotation. This implies that these breccia dikes cooled to blocking temperatures at a rate slower than that of crater modification and obtained their magnetic remanence subsequent to the crater’s final structural state. The magnetic directions of samples yield a virtual geomagnetic pole (VGP) that can serve as a reference direction for constraining the structural dynamics of crater formation/modification and evaluating the mechanism whereby impact-related overprints were imparted into the host rock. Breccia dikes have been interpreted to be present within impact craters on Mars (Head, 2006) and should be expected in other extraterrestrial impact structures where erosion levels have allowed exposure of the crater substructure. While breccia dike material would either be remagnetized or demagnetized by the impact (depending on the presence or absence of an ambient field), the surrounding host rock may retain the bulk of its primary remanence. If this magnetic juxtaposition could be measured on a planetary surface, it has the potential to further constrain the life and death of planetary dynamos.