Acta Geodyn. Geomater., Vol. 5, No. 2 (150), 185–195, 2008 TECTONIC PATTERN OF THE HRONOV-POŘÍČÍ TROUGH AS SEEN FROM POLE- DIPOLE GEOELECTRICAL MEASUREMENTS Jan VALENTA *, Vladimír STEJSKAL and Petra ŠTĚPANČÍKOVÁ Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic, v. v. i., V Holešovičkách 41, 182 09, Praha 8, Czech Republic *Corresponding author‘s e-mail: valenta@irsm.cas.cz (Received January 2008, accepted April 2008) ABSTRACT The Hronov-Poříčí Trough represents the easternmost part of the Trutnov-Náchod Depression. The NW–SE striking structure was formed due to the post-Cretaceous flexural folding and is filled with the Upper Cretaceous sediments. Both the NE and SW margins of the trough are bounded by flexures with the Upper Cretaceous strata dipping 40–60° towards the axis of the trough. The NE flexure is situated close to the parallel Hronov-Poříčí Fault Zone. Up to now, it is not fully known, in what extent the normal faulting was involved in the evolution of the structure. From the geomorphological point of view, the normal fault constraints of the trough seem to be acceptable, as the surface topography of its present margins exhibit many signs typical for fault scarps. However, the existence of a fault system bounding all round the Hronov-Poříčí Trough has not been proved by any geological research. Hence the geophysical research was carried out on both sides of the NW part of the trough to support one of these hypothesis. Five geoelectrical profiles were measured in the area and the fault system was proved on the NE side of the trough. On the SW boundary the fault system was not found. Thus it seems, that the NE boundary is controlled by fault tectonics, whereas the SW boundary is rather formed by a simple flexure. KEYWORDS: Hronov-Poříčí Trough, Hronov-Poříčí Fault Zone, electric resistivity measurements, geomorphology, normal faulting Vrchlabí Fault in the north and by the Nová Paka Fault in the south. Both E–W-striking faults are supposed to be sinistral strike slips (Schenk et al., 1989). The contemporary HPFZ is a result of complicated and long-lasting evolution, which began in the late Paleozoic. Since then, several tectonic phases have taken place. The fault zone has been successively developed from an asymmetric anticline, whose steeply inclined SW arm was axially disrupted due to the regional compression by a reverse fault (Tásler, 1979). Along this fault the NE block was relatively uplifted. The main reverse fault is accompanied by numerous parallel or oblique high- angle dislocations. The relatively frequent local seismic activity is a proof of the present-day mobility of the HPFZ. The strongest historical earthquake of January 10, 1901 reached the magnitude of 4.6 and was felt over an area of 50,000 km 2 (Woldřich, 1901). The isoseists of local earthquakes are elongated mostly NW–SE, parallel to the orientation of the HPFZ. The depth of foci is mostly between 5 and 15 km (Schenk et al., 1989). A possible explanation of the present mobility of the HPFZ was given by Schenk et al. (1989). According to this local geodynamic model, the HPFZ as a reverse fault balances the compression caused by the movements along the Nová Paka and Vrchlabí Faults, bounding the HPFZ in the north and south. This 1. INTRODUCTION The broader area of the Hronov-Poříčí Fault Zone (HPFZ) is characterised by significantly increased tectonic activity. It belongs to a larger seismoactive zone on the NE margin of the Bohemian Massif, which is approximately 40-60 km wide and 150 km long and comprises a number of NW–SE and NNW–SSE-striking faults. This zone forms a SE termination of the important central European tectonic structure – the Elbe Fault system (EFS). In comparison with the well known West Bohemia/Vogtland seismoactive region (see e.g. Bankwitz et al., 2003), the seismoactive region of the SE termination of the EFS is characterized by less frequent occurrence of seismic events. Smaller earthquake swarms in this area were reported by Špaček et al. (2006) from Jeseníky Mts., but these micro-swarms do not number more than 50 weak events (M ≤ 1.3). The strongest earthquakes occurred on the NW margin of this seismoactive region and are connected with movements along the HPFZ (Kárník et al., 1984; Procházková et al., 1986; Schenk et al., 1989). HPFZ is a system of parallel fractures, dividing two important structural units – the Intra-Sudetic Basin and the Krkonoše Piedmont Basin (Fig. 1). The NW–SE-striking fault zone is approximately 40 km long and up to 500 m wide. It is bounded by the