Prokop Závada

Application of Modern Technologies in Popularization of the Czech Volcanic Geoheritage

Four volcanic geoheritage sites in the Czech Republic were selected to showcase new technologies for communicating the recent results of scientific research to a wider nonprofessional public. The selected volcanic sites are scoria cones with late intrusive plugs, phonolite lava-dome erupted within a maar crater, phonolite laccolith, and a nested group of monogenetic volcanoes of Oligocene to Miocene age. The available scientific results from each volcanic site were summarized and transformed into images (playscreen) used for the 3D animation. The identical sources used for 3D animations were also used for generation of virtual models of augmented reality. The outcomes were tested on school children, and the results indicate that the modern methods applied in popularization of volcanic geoheritage are highly attractive for the youngest generation and have a potential to bring more interest in Earth Sciences. Using such techniques makes the communication of geoheritage to the public more effective.

Detachment folding of partially molten crust in accretionary orogens: A new magma-enhanced vertical mass and heat transfer mechanism

We use structural, petrographic, and geochronological data to examine processes of exhumation of partially molten crust in the Late Devonian–early Carboniferous Chandman dome in the Mongolian tract of the Central Asian Orogenic Belt. The dome is composed of a magmatite-migmatite core and a low-grade metamorphic envelope of early Paleozoic metasediments and Carboniferous clastics. Its tectonic evolution can be divided into three main stages. The oldest fabric is a subhorizontal foliation, S1, in migmatites that is subparallel to the magmatic foliation in granitoids and to the greenschist facies schistosity in the enveloping metasediments. This event is interpreted as a result of horizontal deep crustal flow at depths of 20–25 km. The S1 layering was subsequently transposed into a new foliation, S2, or affected by open to close upright F2 folds that are locally truncated by steep walls of diatexites, suggesting influx of partially molten crust into fold cores. The shallow-dipping magmatic foliation in granitoids is locally reworked by vertical magmatic to gneissic S2 fabrics. Syn-S2 metamorphic assemblages and synkinematic to postkinematic cordierite point to exhumation of the migmatites and granitoids from 20–25 km to ~10 km, and concomitant isobaric heating of the surrounding upper crust. New 40Ar/39Ar ages of 350–340 Ma from both the high-grade core and the metamorphic mantle overlap with previously published crystallization ages of 360–340 Ma, suggesting that magmatism and cooling in the upper crust are partly synchronous. Late syn-D2, S2-parallel leucogranite sheets crosscutting both the magmatic core and the mantling migmatites either exploit S2 or crosscut horizontal S1 fabrics; they are interpreted as brittle expulsion of magma during ongoing syn-D2 exhumation. We suggest that the partially molten crust and magmas rose vertically into the upper crust, along steep planar fabrics that are parallel to the axial fold plane of a crustal detachment folding, without contribution of buoyancy forces. In order to test that crustal-scale detachment folding can exhume partially molten crust, we apply an analogue model with temperature dependent rheology of the lower crust represented by a partially molten wax layer overlain by an upper crustal sand layer. It is shown that the fold core initially filled by low-viscosity partially molten wax rapidly migrates upward during fold lock-up, enhancing upward extrusion of magma and migmatites along the fold axial plane. The exhumation of the lower crust wax is facilitated by erosional unroofing of the upper crustal sand above the hinge of the antiform. In Chandman, localized siliciclastic lower Carboniferous basins rimming the dome attest to this erosional phenomenon. Using a simple geometrical analysis we show that detachment folding can explain magma collection in an orientation perpendicular to the main shortening direction, and episodic emplacement of magmas during amplification of the antiform. In our view, the detachment folding model provides a new model for the exhumation of a weak zone above a rigid floor (basement from which the fold is detached) and its vertical extrusion related to locking of the fold and post-buckle flattening. This model helps elucidate steep retrograde pressure-temperature-time paths along steep fabrics, overlapping ages from different geochronometers, and emplacement of voluminous syntectonic magmas. LITHOSPHERE; v. 9; no. 6; p. 889–909; GSA Data Repository Item 2017331 | Published online 11 October 2017 https://doi.org/10.1130/L670.1

Role of strain localization and melt flow on exhumation of deeply subducted continental crust

A section of anatectic felsic rocks from a high-pressure (>13 kbar) continental crust (Variscan Bohemian Massif) preserves unique evidence for coupled melt flow and heterogeneous deformation during continental subduction. The section reveals layers of migmatitic granofels interlayered with anatectic banded orthogneiss and other rock types within a single deformation fabric related to the prograde metamorphism. Granofels layers represent high strain zones and have traces of localized porous melt flow that infiltrated the host banded orthogneiss and crystallized granitic melt in the grain interstices. This process is inferred from: (1) gradational contacts between orthogneiss and granofels layers; (2) grain size decrease and crystallographic preferred orientation of major phases, compatible with oriented growth of crystals from interstitial melt during granular flow, accommodated by melt-assisted grain boundary diffusion creep mechanisms; and (3) pressuretemperature equilibria modeling showing that the melts were not generated in situ. We further argue that this porous melt flow, focused along the deformation layering, significantly decreases the strength of the crustal section of the subducting continental lithosphere. As a result, detachment folds develop that decouple the shallower parts of the layered anatectic sequence from the underlying and continuously subducting continental plate, which triggers exhumation of this anatectic sequence. LITHOSPHERE GSA Data Repository Item 2017403 https:// doi .org /10 .1130 /L666 .1

Devils Tower (Wyoming, USA): A lava coulée emplaced into a maar-diatreme volcano?

We have investigated the mode of emplacement of iconic Devils Tower, which is a phonolite porphyry monolith in the state of Wyoming in the western United States. Our field survey of this structure and its geological setting, its radiometric dating, and the tectonomagmatic evolution of the region suggest a new genetic interpretation of the volcaniclastic rocks in the area and provide a basis for a new hypothetical emplacement scenario for Devils Tower. This interpretation was inspired by an analogy of the tower with a similar phonolite butte in the Cenozoic volcanic region of the Czech Republic and analogue modeling using plaster of paris combined with finite element thermal numerical modeling. Our results indicate that Devils Tower is a remnant of a coulee or low lava dome that was emplaced into a broad phreatomagmatic crater at the top of a maar-diatreme volcano.