Interpretation of subhorizontal crustal reflections by metamorphic and rheologic effects in the eastern part of the Pannonian Basin

Abstract

S U M M A R YThe geologic origin of subhorizontal reflections, often observed in crustal seismic sections, was investigated by establishing metamorphic facies and strength of rocks in depth, and correlating these properties to seismic reflection sections from eastern Hungary. Estimation of the depths of metamorphic mineral stability zones utilized the principles developed by Fyfe et al. and known geothermal data of the area. The strength versus depth profile was derived by relating local seismic P-wave interval velocities to Meissner et al.'s activation energy. The results show that the series of subhorizontal reflections, observed in the Pannonian Basin, are a consequence of combined metamorphic and rheologic changes in depths. The synthesis of the integrated data set suggests that the retrograde alteration of the pre-Tertiary basement above the percolation threshold was made possible by the softening effect of shear zones and their water-conducting capacity. The subhorizontal reflections of highest energy, of the consolidated crust below the percolation threshold, originate in the depths of greenschist, amphibolite and granulite metamorphic mineral facies, which were formed in geothermal and pressure conditions similar to those existing today. These results imply the overprint of earlier (Variscan) metamorphic sequences of the crust by more recent retrograde metamorphic processes.

Crustal structure of the Pannonian Basin: The AlCaPa and Tisza Terrains and the Mid-Hungarian Zone

“…The collapse phase was followed by a broadly distributed phase of upper-crustal faulting that affected most of the basin but eventually gave way to a failed rift in the SE Pannonian (with greatest extension producing the Makó and Békés basins). Posgay et al (2006) described extensive evidence from core and seismic properties of retrograde metamorphism that has occurred as shallow basement was uplifted from mid-crustal levels A C C E P T E D M A N U S C R I P T…”

“…Apparently absent Moho conversion has also been interpreted at other continental tectonic boundaries, for example in Tibet between the Lhasa and Qiangtang blocks, on either side of a major suture zone, the Banggong-Nujiang Suture(Nábělek et al, 2009). The most reasonable explanation for an apparent lack of P-to-S conversion at the Moho is a gradual transition of shear-wave velocities between the crust and the mantle over a relatively thick zone Posgay et al (2006). described seismic and mineralogical evidence that supports the interpretation of progressive retrograde metamorphism having occurred in the extended basement of the Tisza block.…”

The Alpine-Carpathian-Dinaridic orogenic system: correlation and evolution of tectonic units

“…However, we excluded the basement that crops out in the Moslavačka Gora inselberg in Croatia, previously considered as the westernmost tip of Tisza, from the Mecsek nappe system, and attributed it to the Sava Zone owing to the occurrence of mid-Cretaceous gabbros (109 ± 8 Ma; Balen et al 2003) and Late Cretaceous-age high-temperature metamorphism and magmatism (Starijaš et al 2006). The southern contact between the Bihor and the Codru nappe system is rather poorly constrained by subsurface data, except for the area south of Debrecen, where S-dipping reflectors indicate a shear zone (Posgay et al 2006, their Fig. 2) that we interpret to mark the tectonic boundary between these two nappe systems.…”

Complex evaluation of the Kiskunhalas-NE fractured metamorphic hydrocarbon reservoir

“…Seismic measurements suggest uplifted metamorphic highs interpreted as metamorphic core complexes . A series of subhorizontal reflections were detected in the basement around Szeghalom, related to this rather quick uplift (Posgay et al 2006).…”

“…However, in several cases the basement is characterised morphologically by deep basins and trenches, with uplifted highs among them. Most of the crystalline highs exhumed during the Middle Miocene follow core complex formation mechanisms , Posgay et al 2006) and so reached the surface or a near surface position. In fact, the uplifted highs formed islands in the Pannonian Sea, which usually caused the absence of the impermeable sequence of the Endrőd Formation (clay marl) above the highs ( Fig.…”