Seismic signature of the Lake Mien impact structure, southern Sweden

Abstract

The EUGENO-S seismic profile IV in southern Sweden crossed Lake Mien, a confirmed heavily eroded 121 Ma impact structure. The Lake Mien structure has a pre-erosion rim diameter of about 9km. It is located within the Transscandinavian Granite Porphyry Blet (TGPB) near the border between Blekinge and Smaland. The compressional- and shear-velocity structure of the upper kilometre of crust in the Lake Mien area was determined using short-period fundamental-model Rayleigh-wave dispersion and independent P- and S-wave data in a simultaneous inversion. A notable result was the low shear velocities of about 2.65 km s-1 in the upper 0.5km of crust of the disruption zone (DZ) surrounding the former, now completely eroded crater rim. The disruption diameter was found to be about 17km along the profile. The normal value of the S-wave velocity in the TGPB and the upper 0.5km is 3.2±0.1 km s-1. Inside the crater rim, the shear velocity was about normal, 3.3±0.1 km s-1. However, the ratio of P velocity to S velocity, α/β ≃1.66±0.04, was significantly lower than normal. In the 0.5-2km layers, the shear velocity was, as expected, lower than normal and α/β was about normal. Known geophysical results were compiled and included in this study (Henkel 1992). The above normal shear velocity and low ratio of α/β were found in the presence of a magnetic anomaly characterized by a central strongly variable field with about normal average, surrounded by a magnetic low. The Bouguer anomaly was characterized by a distinct gravity low of -5 mGal. The source of the gravity low was concentrated close to the centre of the impact within a diameter of about 2km. It had little influence (less than one standard error) on the average shear velocity across the crater. An explanation of the observed normal S velocity and the low value of α/β within the cvreater rim and the upper 0.5km required consideration of the thermal effects of the impact. The results of the geophysical data and interpretations were discussed in the light of a hot blanket model. It was found that the energy release in the blanket was capable of sustianing hydrothermal flow for a few thousand years, which could be sufficient to explain the above observed normal S velocity and low value of α/β. The heat from the blanket was sufficient to produce temperatures above the Curie temperature over a large enough volume to produce a central magnetic anomaly. Knowledge of the depth to the base of this anomaly could be used as a constraint on the impact energy that is independent of the scaling relations.