Magnetic Anomaly and Model of the Lonar Meteorite Impact Crater in Maharashtra, India

The ground magnetic field of the Lonar impact crater (Maharashtra State, India) and its surrounding area was measured and studied utilizing 2.5-dimensional potential field modelling. Field data showed the crater depression is associated with a strong circular negative anomaly with an amplitude of more than 1000 nT. The negative anomaly, however, decreases smoothly while moving from south to north. Most of the crater rim exhibits anomalous positive values. Negative anomalies at the rim are seen in the south–southwestern sections and coinciding in the northeastern section with the Dhar valley. Our study shows that most of the anomaly is caused by the topographic effect and a strong SE directed natural remanent magnetization of Deccan Trap basalts, which are the target of the Lonar-creating projectile. The magnetic anomaly of the relatively weakly magnetized impact-produced allochthonous breccia and post-impact sediments is small, being less than 150 nT.

Potential-field modelling of the prospective Chibougamau area (northeastern Abitibi subprovince, Quebec, Canada) using geological, geophysical, and petrophysical constraints

This paper focusses on obtaining a better understanding of the subsurface geology of the Chibougamau area, in the northeast of the Abitibi greenstone belt (Superior craton), using geophysical data collected along a 128 km long traverse with a rough southwest–northeast orientation. We have constructed two-dimensional (2D) models of the study area that are consistent with newly collected gravity data and high-resolution magnetic data sets. The initial models were constrained at depth by an interpretation of a new seismic section and at surface by the bedrock geology and known geometry of lithological units. The attributes of the model were constrained using petrophysical measurements so that the final model is compatible with all available geological and geophysical data. The potential-field data modelling resolved the geometry of plutons and magnetic bodies that are transparent on seismic sections. The new model is consistent with the known structural geology, such as open folding, and provides an improvement in estimating the size, shape, and depth of the Barlow and Chibougamau plutons. The Chibougamau pluton is known to be associated with Cu–Au magmatic-hydrothermal mineralisation and, as the volume and geometry of intrusive bodies is paramount to the exploration of such mineralisation, the modelling presented here provides a scientific foundation to exploration models focused on such mineralisation.

New refraction/wide-angle reflection profile across the Teisseyre-Tornquist Zone offshore Poland

<p>The southern Baltic Sea area is located in the transition zone between the East European Craton (EEC; Baltica) and the West European Platform (Avalonia). The most prominent tectonic feature in the area is the NW–SE trending Tornquist Zone (TZ), crossing the southern Baltic Sea area between Scania in Sweden and Pomerania in Poland. A peculiar feature of the TZ and its southern prolongation (Teisseyre-Tornquist Zone, TTZ) is possibly a crustal keel that was recently postulated for northern Poland based on potential field modelling. A crustal keel was also imaged in the Baltic Sea by BABEL profile A, which crossed the TZ northwest of Bornholm, and by two TTZ’92 profiles crossing the TTZ south of Bornholm. However, the DEKORP-PQ profile shows a flat Moho across the TTZ.</p><p>In order to reconcile those contrasting interpretations of the crustal structure around the TTZ offshore Poland, a 230-km long refraction/wide-angle reflection profile was acquired across the TTZ in the course of RV/MARIA S. MERIAN expedition MSM52 (BalTec) in March 2016. This profile is nearly parallel to the western Polish coast, in half a distance to Bornholm. The data acquisition was conducted with 15 ocean bottom seismometers (OBS) and 3 land stations. The source array consisted of 8 G-guns with the total volume of 32 litres. In total 2227 shot points were recorded. Hydrophone data are of high quality and despite the relatively small source volume, sharp first arrivals of Pg and Pn are observed at over 120 km offsets. Some seismic record sections show clear PmP phases beginning at offsets of 70 km, continuing till the end of the profile.</p><p>Two variants of seismic modelling were performed, which results proved to be similar in terms of P-wave velocities and observed features. Tomographic joint inversion of both first arrivals and Moho reflections was used to extend velocity model depth range. Second was trial-and-error forward modelling technique using all identified seismic phases, paying attention to minimize misfit between calculated and observed P-wave travel times for each individual layer.</p><p>In the area of the TTZ, a complex upper crustal structure deepening towards the southwest is observed. One of the most interesting features is an increase in Vp (>6.5 km/s) at a depth of 16-25 km, offset by ~40 km from the TTZ on the EEC side. Similar feature was observed along the TTZ in SE Poland. Due to the lack of information from refraction, the presented ray-tracing model is the result of testing various possible velocity values for the lower crust in different parts of the model. A layer with Vp>7 km/s with a thickness of ~6 km along the entire model seems to be the best solution The Moho boundary was inferred at 33-38 km depth, deepening towards the EEC, with ~3 km uplift (but not keel) corresponding to the location of the elevated middle-crust velocities. Final velocity models were further verified by forward potential field modelling, testing various Vp – density relations.</p><p>This study was funded by the Polish National Science Centre grant no UMO-2017/27/B/ST10/02316.</p>