David Buriánek

Palaeostress analysis of a giant Holocene rockslide near Boaco and Santa Lucia (Nicaragua, Central America)

Abstract A giant rockslide occurred on the southern side of an Upper Tertiary shield volcano in central Nicaragua in the Holocene. The failure caused tectonic-like deformation of rock masses and changed the local stress regime. The lower, compressional part of the rockslide produced a stress field with the axis of maximum stress (σ1) parallel to the displacement vector of the main body. The upper part of the rockslide was gravity-driven with σ1 vertical and σ3 horizontal, and oriented SE–NW. The mass tended to move SE. In the crown, the stress field had a subvertical σ1 steeply dipping towards the west. Data at the base of the Santa Lucia Depression, where east- and west-dipping reverse and thrust faults developed, showed that the compressional stress, σ1, was nearly horizontal and east–west oriented, the horizontal σ2 was north–south oriented, and the σ3 was subvertical. These compressional conditions resulted from the collapse of the crown after the main slope failure phase. Simultaneously, along with the gravity relaxation of the main displaced mass, the slopes and mountain slopes along the main scarp depression underwent deep-seated sliding, sagging and flowing.

Neoproterozoic–early Paleozoic peri-Pacific accretionary evolution of the Mongolian collage system: Insights from geochemical and U–Pb zircon data from the Ordovician sedimentary wedge in the Mongolian Altai†

Neoproterozoic to early Paleozoic accretionary processes of the Central Asian Orogenic Belt have been evaluated so far mainly using the geology of ophiolites and/or magmatic arcs. Thus, the knowledge of the nature and evolution of associated sedimentary prisms remains fragmentary. We carried out an integrated geological, geochemical and zircon U–Pb geochronological study on a giant Ordovician metasedimentary succession of the Mongolian Altai Mts. This succession is characterized by dominant terrigenous components mixed with volcanogenic material. It is chemically immature, compositionally analogous to graywacke and marked by significant input of felsic to intermediate arc components, pointing to an active continental margin depositional setting. Detrital zircon U–Pb ages suggest a source dominated by products of early Paleozoic magmatism prevailing during the Cambrian–Ordovician and culminating at ca. 500 Ma. We propose that the Ordovician succession forms an ‘Altai sedimentary wedge’, the evolution of which can be linked to the geodynamics of the margins of the Mongolian Precambrian Zavhan-Baydrag blocks. This involved subduction reversal from southward subduction of a passive continental margin (early Cambrian) to the development of the ‘Ikh-Mongol Magmatic Arc System’ and the giant ‘Altai sedimentary wedge’ above a north-dipping subduction zone (Late Cambrian–Ordovician). Such a dynamic process resembles the tectonic evolution of the peri-Pacific accretionary Terra Australis Orogen. A new model reconciling the Baikalian metamorphic belt along the southern Siberian Craton with peri-Pacific Altai accretionary systems fringing the Mongolian microcontinents is proposed to explain the Cambro–Ordovician geodynamic evolution of the Mongolian collage system.

Geology of the Gobi Altai and Tseel terranes in the central part of the Sagsai River Watershed, SE Mongolian Altai

ABSTRACT A geological map is an indispensable tool for understanding the structure of the Earth’s crust but high-quality geological maps are usually lacking in remote areas of mountainous Central Asia covered by vast deserts. The progress in remote sensing and geographical information system (GIS), as well as the advancement in analytical methods, have generated new challenges in producing modern geological maps in such regions. The presented 1:50,000 geological map along the Sagsai River summarizes new and more accurate geological data from the geologically interesting region at the contact of the supracrustal and deep crustal Tugrug and Tseel units forming the metamorphosed accretionary wedge on the S and SW slopes of the Mongolian Altai. These geological units are formed by the lower Palaeozoic volcano-sedimentary sequences affected by Devonian Barrovian metamorphism ranging from lower greenschist to granulite facies. This metamorphic basement was subsequently intruded by the post-orogenic late Carboniferous Sagsai Pluton. The presented map shows complex relationships between different crustal levels of the metamorphosed accretionary wedge and post-tectonic intrusion evaluated using a GIS, numerical processing of remote sensing data as well as field documentation and laboratory studies.

The sedimentary environment of Devonian siliciclastics in the Babí lom locality (Brunovistulian Platform cover, Southern Moravia, Czech Republic)

Anchimetamorphosed sandstones and conglomerates of the so-called “Basal De vo nian Clastics”, that build a hill (ridge) named Babi lom, are considered to be of Lower to Middle Devonian age. Al though they are strongly tectonically affected, many sedimentary features are well-preserved and distinct. Both the facies and structures typical of alluvial fans and rivers were recognized in the sedimentary association at Babi lom. Among others, there are channel lags with imbricated grains, cross-bedding, and debrites. The way-up direction is clearly seen via repetitive coarsening-upwards sequences. Assuming a lack of rotation of the succession in the X–Y plane, the main palaeodrainage was towards the north and the major supply of material was from the east. The high maturity of the rocks suggests derivation from older deposits and/or highly weathered source rocks. The magmatic rocks of the Brunovistulicum are possibly the primary source of the sediments.

Vývoj půd na opuštěných haldách po těžbě polymetalických rud na Havlíčkobrodsku

Stare haldy po těžbě polymetalických rud představuji prostředi, ve kterem můžeme ve specifických podminkach studovat rana stadia vývoje půd. Studovany byly dva půdni profily vznikajici na haldach různeho staři – lokalita Utin (2. polovina 13. stol. až 1. pol. 14 stol.) a Dlouha Ves (60. leta 20. stol). Z odebraných vzorků jemnozemě byla v obou profilech pozornost věnovana vertikalni distribuci vybraných prvků (Fe, Pb, As, Ag, Cu - ICP-MS) v kontextu s dalsimi půdnimi parametry zahrnujicimi ztratu žihanim (LOI), sekvencni extrakcni analýzu (SEA), půdni reakci potencialni výměnnou (pH/KCl), hmotnostně specifickou a frekvencně zavislou magnetickou susceptibilitu. Půdni profil u Utina vykazuje zvýsene koncentrace Pb (až 4700 ppm), Cu a Ag (oboji do 100 ppm) předevsim při povrchu profilu, kontaminace As je patrna v celem profilu, a to v průměru okolo 3000 ppm. V připadě půdniho profilu u Dlouhe Vsi byly vysoke koncentrace Ag, Cu (cca 500 ppm) a As (cca 13000 ppm; 70 cm) zjistěny ve spodnich castech profilu. Výrazně pozitivni korelace LOI vs. Pb (r2 = 0,96), Cu (r2 = 0,90), Fe (r2 = 0,87), Ag (r2 = 0,94) s přihlednutim k výsledkům SEA v půdnim profilu u Utina ukazuji, že zminěne prvky maji tendenci vazat se vice s organickými slouceninami než se sulfidy. Tim se tyto půdy výrazně odlisuji půdniho profilu u Dlouhe Vsi, kde maji zminěne prvky tendenci vazat se pravě se sulfidy. Zminěný rozdil mezi profily z obou lokalit neni dan pouze předpokladaným vyssim stupněm pedogeneze u výrazně starsi půdy u Utina, výrazný vliv mohou mit i acidobazicke vlastnosti půdniho profilu u Dlouhe Vsi, kde pH/KCl odpovida silně kyselemu prostředi. V tomto prostředi dochazi ke zpomalenemu rozkladu organicke složky. Nižsi stupeň pedogeneze zde podporuji i výsledky frekvencně zavisle magneticke susceptibility (1–5 %).

NOVÝ VÝSKYT SKARNU V GFÖHLSKÉ JEDNOTCE U VEVČIC U JEVIŠOVIC: MINERÁLNÍ ASOCIACE SKARNU A KONTAMINOVANÝCH AMFIBOLICKÝCH PEGMATITŮ

This work provides detailed information about a new occurrence of skarn near Vevcice (near Jevisovice, western Moravia), which is one of the most important outcrops of these rocks in the southeastern part of the Moldanubian Zone (Gfohl Unit). The skarn is characterized by a strong migmatization, which resulted in contaminated amphibole pegmatite veins penetrating the skarn. The mineral assemblage of the skarn is simple, with dominant garnets, both grossular-almandine (Alm 55–57 Grs 31–37 Adr 3–8 ) and grossular-andradite- almandine (Grs 45–53 Adr 20–34 Alm 20–24 ). Garnets clearly predominate over clinopyroxene, hedenbergite with a slightly increased “fassaite component”. The very low amount of quartz contrasts with that in the skarns in the surrounding Gfohl unit. Amphibole (potassium-rich hastingsite) is younger mineral in the skarn, and especially in contaminated pegmatites. Epidote after garnets, and eventually prehnite, belong to the youngest minerals in some types of rocks. Accessory titanite, sometimes rich in Sn, was found frequently, as well as metamict REE-rich epidote. Magnetite is rather exceptional, mainly in assemblages replacing garnet; fluorapatite, ilmenite and zircon are rare. The studied skarn is part of a lithologically varied sequence of the Gfohl Unit, with intercalations of Ca-metasediments, appearing in several non-contiguous horizons around the boundary of so-called Běhařovice-Vemyslice synform, with a granulite–serpentinite complex in its center. This sequence of strongly migmatized biotite paragneiss to leucocratic migmatites also contains diopside and scapolite-diopside gneiss, garnet-pyroxene and phlogopite-diopside skarns containing magnetite and exceptionally Au-Co-Bi and REE mineralization as well as rare occurrences of spinel-forsterite dolomitic marbles.