Looking for impact related remagnetization in pyrrhotite-bearing target rocks at Haughton impact stucture (Canada)

Shock is one of the major evolution processes of solid matter in the solar system. These shocks have potentially a strong effect on the magnetic remanence of meteorites and planetary/asteroidal surfaces, either through shock demagnetization or magnetization [1,2,3], or by magnetization in a transient magnetic field generated [4] or amplified [5] by the impact. Although shock magnetization or the implication of impact-related transient fields are often invoked to account for meteorite paleomagnetism, neither mechanisms have ever been evidenced demonstratively in terrestrial impact structures [6]. We present here new paleomagnetic data obtained from the Haughton impact structure, a well-preserved Eocene structure 23 km in diameter located in the Canadian Arctic Archipelago and dated at ~39 Ma [7]. In view of the scope of this study, this crater is of particular interest because the target rocks (Paleozoic dolomites and limestones) are magnetically homogeneous at a scale of several crater sizes. Moreover they are are dominated by pyrrhotite, a mineral that is very sensitive to shock magnetization, with a total resetting of its remanence at pressures below 2.8 GPa [8]. We sampled 27 sites in the crater and up to several crater radii. Preliminary results indicate that whereas the rocks outside the crater have preserved their Paleozoic magnetization, the rocks inside the crater have been largely remagnetized in a normal polarity Eocene field after tilting associated to the impact process. This seems to discard both shock-induced remagnetization (that would occur during the compressive phase of the impact before the rocks are tilted), or magnetization in impact-related transient fields. The origin of the remagnetization (chemical, thermal) will be discussed in the light of the comparison between the magnetic properties of the shocked rocks with those of the same rocks outside the crater.