Cornelius Tschegg

Diagenetic control of deformation mechanisms in deformation bands in a carbonate grainstone

Deformation bands are commonly found in porous silicilastic sediments, where strain is accommodated by rotation, translation, and fracturing of individual grains instead of by the formation of a sharp discontinuity. We investigated deformation bands in a high-porosity carbonate rock from the Eisenstadt-Sopron Basin, on the border between Austria and Hungary, using a combination of microstructural and petrophysical methods. We used cathodoluminescence and electron microprobe analyses to assess the distribution and chemical composition of the carbonate particles, deformation bands, and cements. The earliest deformation bands formed before the cementation of the limestone, mainly by rotation of elongated bioclasts to an orientation parallel to the deformation bands. Further movement along the bands after the generation of blocky cement around the bioclasts resulted in cataclastic deformation of both allochems and cement. Moreover, we documented a reduction of porosity from 22 to 35% in the host rock to 2 to 5% in the deformation bands by microcomputed tomography and conventional helium porosimetry. Permeability is reduced as much as three orders of magnitude relative to the host rock, as documented by pressure decay probe permeametry. The observations indicate a change in physical properties of the rock caused by cementation during the generation of deformation bands, which results in a change of deformation mechanism from grain rotation and compaction to cataclastic deformation along a single band. The reduction of porosity and permeability, which is even stronger than observed in most silicilastic rocks, affects the migration of fluids in groundwater or hydrocarbon reservoirs.

Deformation and alteration of a granodiorite during low-angle normal faulting (Serifos, Greece)

On Serifos (Western Cyclades, Greece), a late Miocene I-type granodiorite pluton intruded a low-angle normal fault (LANF) during extension and exhumation of the middle crust. In the studied structural section, the LANF cuts the granodiorite and, due to the minor displacement (

Localization of deformation triggered by chemo-mechanical feedback processes

Fluid-triggered mineral reactions in fault rocks may result in significant fault weakening by reduction of the friction coefficient value, particularly in the case of the formation of phyllosilicate minerals. Here, we document an excellent example of fault weakening controlled by complex chemo-mechanical feedback processes in a low-angle normal fault on the island of Serifos (Western Cyclades, Greece). Within and several tens of meters below the studied fault zone, a reaction front developed between dolomite-calcite mylonites and quartzite mylonite layers, triggered by fluid-assisted nucleation of talc. The talc formation facilitated domino boudinage of these quartzite mylonites, which, under ongoing frictional deformation, increased the permeability of the fault zone and, hence, facilitated additional influx of fluid associated with massive talc formation. Continuing deformation was strongly localized in these talc-rich layers, resulting in the progressive development of a low-angle normal fault at the brittle-ductile transition zone. Associated with another pulse of fluid influx, a late-stage static tremolite growth outlasted the deformation. In this paper, we set up a new chemo-mechanical model for strain softening and address the complex question of relative timing and coupling of mineral reactions and deformation mechanisms between the quartzite boudins and the dolomite-calcite marble host in a well-constrained tectono-metamorphic frame.

Asthenospheric signature in fertile spinel lherzolites from the Viliga Volcanic Field in NE Russia

Abstract Mantle xenolith bearing olivine melanephelinites from the Okhotsk sector of the Okhotsk–Chukotka Volcanic Belt (OCVB), northeastern Russia, occur as small isolated volcanoes emplaced within massive late Early to Late Cretaceous subduction-related calc-alkaline rocks. The xenoliths are typical medium- to fine‐grained anhydrous mainly spinel lherzolites that are strongly to weakly foliated with intensive to minor recrystallization to equigranular texture. The primitive mantle normalized whole-rock REE have flat patterns or patterns with slightly elevated light REE (LREE) ((La/Yb)N=0.48–1.38). The REE in clinopyroxenes have systematically decreasing normalized abundances from Sm to La, implying that the LREE enrichments in the whole‐rock REE patterns are attributed to circulation of minor intergranular fluids or melts. Equilibration temperatures and pressures calculated for the Viliga samples are in the range of 1050–1160 °C and 15–21 kbar, respectively. Ca diffusion rates in olivine reveal a rapid transport to the surface (2–6 days) of these peridotites. Model calculations have shown that the fertile lherzolites can be produced by 2–9% batch melting, whereas the depleted peridotites require 15% batch melting of a primitive source. The cessation of the interaction between the palaeo-Pacific plate and the NE Russian margin at c. 87 Ma apparently caused a ‘piecemeal’ collapse of the former followed by intrusion and ascent of olivine melanephelinitic magma, which entrained xenoliths from the asthenospheric mantle of the subducted plate during the Pliocene through the generated window(s). Moreover, clinopyroxenes that have low 87Sr/86Sr and high 143Nd/144Nd and plot in and above the mid-ocean ridge basalt (MORB) field are consistent with an upwelling asthenospheric mantle through the window(s) created by the ‘piecemeal’ collapse of the palaeo-Pacific plate.