Quaternary neotectonics of the Somogy Hills, Hungary (part I): Evidence from field observations

Quaternary and directly underlying Late Miocene (Pannonian) outcrops were analysed by structural, tectono-morphologic and sedimentologic methods to describe the main fault directions, to separate mass movements from faulting and folding and to separate earthquake-induced sediment deformations from other (e.g. periglacial) effects in the Somogy Hills. This is a gentle hilly area elevated at 200–300 m above sea level, located immediately south of Lake Balaton, Hungary.

Quaternary outcrops showed several consistent directions of faulting, and co-depositional seismic activity. Three different Mohr-sets of faults/joints could be differentiated in Quaternary sediments. The three sets are considered Late Quaternary since all cut young loess sections and have morphological expressions.

On the basis of the microtectonic measurements and morphotectonic investigations, the following sequence of Quaternary events can be proposed:

1. A (W)NW–(E)SE compression and perpendicular extension would create E–W to WNW–ESE oriented right lateral, NNW–SSE to N–S oriented left lateral shear zones, and NW–SE striking normal faults. Some of these can be evidenced in morphology and among the individual fault measurements. Some reactivated faults might suggest that this field is a relatively older one, but fresh topographic elements suggest that this stress field might be operational sub-recently.

2. A second stress field with NNW–SSE extensional and ENE–WSW oriented compressional directions could be separated. This stress field could create NNE–SSW and NW–SE oriented shear fractures and ENE–WSW oriented conjugate normal faults. Flat thrusts giving ENE directed shear may also be active under this field.

3. A third stress field might be proposed with N–S compression and perpendicular extension directions. This would create NE–SW and NW–SE oriented shear fractures, which are observed in the measured fault data. It is remarkable that the NE–SW faults are all steep, subvertical, and give a very well defined fault set. Based on the fresh topographic expression, this stress field is also sub-recent.

The different sub-recent stress fields and related fault patterns might succeed each other or might alternate through time. The first and third deformations have fresh topographic expressions and cannot play synchronously. The observed features suggest a compressionally active neotectonics of the study area.