Cambrian sedimentary basins recorded the initial break-up of northern Gondwana

 

 

 

 

 

Diachronous opening of the Rheic Ocean and separation of Avalonian–Cadomian terranes from Gondwana was a major, but still poorly understood paleogeographic event in the late Ediacaran to early Cambrian. A recent study from the Příbram–Jince basin in the Bohemian Massif revealed a significant provenance and paleocurrent changes in response to dextral transtension that enlarged the basin into a pull-apart structure, suggesting that strike-slip movements along the former Avalonian–Cadomian belt controlled the diachronous opening of the Rheic Ocean. Putting this piece of information into a plate-tectonic picture, it seems that an inherited suture in the Avalonian ribbon terrane facilitated complete rifting and rift–drift transition while the Cadomian terranes, including those now forming the Bohemian Massif, remained attached to Gondwana during this large-scale rifting event.

Syahputra R., Žák J., Nance R.D. (2021): Cambrian sedimentary basins of northern Gondwana as geodynamic markers of incipient opening of the Rheic Ocean. Gondwana Research 105, 492-513. (DOI)

Magmatic pulses during orogeny: when and how fast is magma added to the upper crust?

A new paper by Trubač et al. combines geochronologic data with a GIS-based spatial analysis of plutons in the Bohemian Massif, Variscan belt, to examine the magma fluxes and temporal pattern of plutonism during collisional orogeny. The collisional orogens differ from continental margin arcs, where the plutonic material is added in repeating cycles to the upper crust during protracted oceanic subduction. Instead, it was shown that the collisional plutonism reflects a secular thermotectonic evolution of the orogen, where magma sources change from mantle-dominated to intracrustal, paralleled by increasing significance of crustal recycling at the expense of crustal growth. The analysis also suggests that crustal thickening, commonly regarded as the main cause of plutonism in collisional orogens, may be of relatively lower significance than a late-stage mantle delamination.

Trubač J., Žák J., Kondrová L. (2020): Magmatic tempos in large hot orogens in comparison with continental margin arcs. Journal of Geology 128, 465-475. (DOI)

A new study in Science: the origin of our teeth is among the most primitive jawed fish

In the two extant jawed vertebrate groups, bony fish (and their descendants the land animals) and sharks, new teeth usually develop on the inner side of the old ones. Sharks have no bones and their teeth do not attach to the cartilaginous jaws, whereas in bony fish and land animals, the teeth are always attached to jaw bones. This diversity raises many questions about the origin of teeth. Until now, researchers have focused on fossils of a group of armoured fish,  the arthrodires, the only stem jawed vertebrates in which teeth were known. They struggled to understand how they could have evolved into the teeth of modern vertebrates, as arthrodire teeth are so different in position and mode of tooth addition.

A team of scientists from the Charles University, Uppsala University, the European synchrotron ESRF in Grenoble, Natural History Museum in London, and the National Museum in Prague (Valéria Vaškaninová as a lead author) turned to the acanthothoracids, an early fish group closely related to the common ancestor of jawed vertebrates. The very finest of them come from the Prague Basin, but due to their fragility they have never been investigated in detail. The researchers used synchrotron microtomography at the European synchrotron, the world’s brightest X-ray source, which allowed them to visualise the internal structure of the fossils in 3D, including unexpected well-preserved dentitions. Follow-up scans at higher resolution visualized the growth pattern and perfectly preserved cell spaces inside the dentine.

Like arthrodires, the acanthothoracid dentitions are attached to bones. This indicates that bony fish and land animals retain the ancestral condition, whereas sharks are specialized. In other ways, acanthothoracid dentitions are fundamentally different from those of arthrodires. Like sharks, bony fish and land animals, acanthothoracids only added new teeth on the inside. Even though acanthothoracids are among the most primitive of all jawed vertebrates, their teeth are far more like modern ones than arthrodire dentitions. Their jawbones resemble those of bony fish and seem to be directly ancestral to our own.

Vaškaninová V., Chen D., Tafforeau P., Johanson Z., Ekrt B., Blom, H., Ahlberg P.E. (2020): Marginal dentition and multiple dermal jawbones as the ancestral condition of jawed vertebrates. Science 369, 211-216. (DOI)

Can we compare exceptionally preserved fossils to understand their evolution?

Exceptionally preserved fossils provide deep insight into early animal evolution. For example, the step by step evolution of morphological innovations that ultimately led to the origin of many recent animals was achieved using information from such exceptionally preserved fossils. Each of these sites with exceptional preservation – so-called Lagerstätten – is, however, a subject of specific fossilization bias. This means that at different fossil sites different information is lost during the fossilization process. To understand such information loss is especially important once we want to compare fossils from different sites and infer some general trends. Without knowing fossilization biases, we may end-up comparing apples and oranges. A new study by Saleh et al. (Lukáš Laibl as a co-author) tried to understand such biases. It compared three well known lower Paleozoic Lagerstätten – the Burgess Shale (ca. 505 million years old, Canada), the Chengjiang Biota (ca. 530 million years old, China) and the Fezouata Shale (ca. 479-478 million years old, Morocco). Interestingly, the Fezouata Shale is different from both Burgess and Changjiang. In Fezouata Shale, the soft parts of animals are most commonly associated with partially mineralized and sclerotized tissues. The digestive tract is, for example, preserved mostly in fossils with hard biomineralized exoskeletons (such as in trilobites and echinoderms). For the same reason, the entire soft animals are not preserved in Fezouata Shale. Indeed, most of the animals from Fezouata Shale were either biomineralized (trilobites, echinoderms, gastropods, brachiopods), sclerotized (many types of arthropods) or have had at least external cuticle (various “worms”). On the other hand, Burgess and Changjiang commonly preserve entirely soft animals and also show a higher diversity of different tissue types in particular fossil species. So to sum up, the answer to the question in the title is – yes, but with caution. We have to differentiate between fossilization absence and real absence. Many animals could have lived in the Fezouata Shale environment but were nor preserved because of the nature of the fossilization process. This is likely the case for early chordates that were entirely soft-bodied. On the other hand, the absence of penis worms (priapulids) in the Fezouata Shale is most probably a real aspect of the fauna, since these cuticle-bearing animals would “survive” the fossilization process.

Saleh F., Antcliffe J.B., Lefebvre B., Pitteta B., Laibl L., Peris F.P., Lustri L., Gueriau P., Daley A.C. (2020): Taphonomic bias in exceptionally preserved biotas. Earth and Planetary Science Letters 529, 115873 (DOI)

Zircon ages reveal dynamics of accretionary wedges

The detrital zircon geochronology has been increasingly used to analyze accretionary wedges over the past decade. A new study by Jiří Žák and others, stemming from collaboration of the Center with the Czech Academy of Sciences, explores the exciting potential of this method, with the key outcome being introduction of a new concept of the Age–Distance Curves (ADCs). The analysis of age variations across accretionary wedges may reveal a wealth of information on a variety of dynamic processes that occur above subduction zones: linear ADCs may track continuous accretion whereas variably sloping ADCs may reflect tectonic erosion, increase in the rate of deposition and accretion, or tectonic duplication; stepped ADCs may record the latter two processes, but also out-of-sequence thrusting while sawtooth ADCs may result from emplacement of older units within younger (olistoliths) or, vice versa, from tectonic underplating and exhumation. The new concept also outlined several issues that still remain poorly understood and would deserve attention in future research, for instance, quantifying temporal variations in the rates of accretion, estimating magmatic arc tempos, as well as rates of erosion and sediment supply from arc to trench.

Žák J., Svojtka M., Hajná J., Ackerman L. (2020): Detrital zircon geochronology and processes in accretionary wedges. Earth-Science Reviews 207, 103214 (DOI)

The Štramberk Limestone: Early Cretaceous world under a magnifying glass

A unique exposure of the Štramberk Limestone in the Kotouč quarry was investigated by multidisciplinary approaches to clarify the age and paleoenvironmental conditions. Integrated stratigraphy (magnetostratigraphy, 87Sr/86Sr isotopes from belemnites, δ18O and δ13C stable isotopes from bulk rock, microfossil and ammonite assemblages) revealed an early Berriasian age and refused the presence of the Jurassic/Cretaceous boundary interval. The δ18O isotope signal from bulk rock together with microfossils distribution (calpionellids, calcareous nannofossils) show temperature oscillations throughout the studied successions. Boreal influences were detected by ammonites, supporting also climatic variations within the earliest Cretaceous. The limestone microfacies then revealed that all parts of a carbonate reef have been preserved in the quarry: slope, platform margin reefs, sand shoals, and possibly also patch-reefs in the inner platform.

Vaňková L., Elbra T., Pruner P., Vašíček Z., Skupien P., Reháková D., Schnabl P., Košťák M., Švábenická L., Svobodová A., Bubík M., Mazuch M., Čížková K., Kdýr Š. (2019): Integrated stratigraphy and palaeoenvironment of the Berriasian peri-reefal limestones at Štramberk (Outer Western Carpathians, Czech Republic). Palaeogeography, Palaeoclimatology, Palaeoecology 532, 109256. (DOI)

Mesozoic paleogeography of the northern Bohemian Massif revisited

Since the pioneering works of early 20th century, Late Cretaceous sandstones along the Lusatian Fault were interpreted as sourced from the West Sudetic Island. However, the original concept that basement of the Lusatian Block directly supplied a clastic material to the Late Cretaceous shores and deltas, has been recently challenged. New LA-ICP-MS U–Pb dating of detrital zircons and heavy mineral analysis of Permian, Jurassic, and Upper Cretaceous formations revealed diachronous basin formation and subsequent inversion by reactivation of major faults in the northern Bohemian Massif. In turn, the Middle Jurassic–Early Cretaceous sedimentary cover of the Lusatian Block was redeposited by its later uplift and unroofing. This paper proves that the Bohemian Creatceous Basin is not only an excellent archive of paleoclimate changes, but its record may contribute to deciphering relatively enigmatic post-early Permian tectonosedimentary history of the surrounding basement areas.

Nádaskay R., Žák J., Sláma J., Sidorinová T., Valečka J. (2019): Deciphering the Late Paleozoic to Mesozoic tectonosedimentary evolution of the northern Bohemian Massif from detrital zircon geochronology and heavy mineral provenance. International Journal of Earth Sciences 108, 2653-2681. (DOI)

A “fossil” giant submarine slide discovered in the Bohemian Massif

An intriguing example of a late Precambrian chert-graywacke mélange was explored by Jaroslava Hajná and colleagues in the central Bohemian Massif (mélanges are chaotic rock units that contain mixed exotic blocks and typically form along the active plate margins). A detailed field, geochronologic, and geochemical study revealed two types of chert (pelagic and those with a terrigenous component) that were mixed together during a large-scale submarine slide on an outer slope of oceanic trench. This case turned out as a quite unusual deep-marine succession and led to a definition of a new type of mélange. Published in a special issue of Gondwana Research devoted to the 100th anniversary of the inception of the mélange concept!

Hajná J.Žák J., Ackerman L., Svojtka M., Pašava J. (2019): A giant late Precambrian chert-bearing olistostrome discovered in the Bohemian Massif: a record of Oceanic Plate Stratigraphy (OPS) disrupted by mass-wasting along an outer trench slope. Gondwana Research 74, 173-188. (DOI)

Filip Scheiner successfully defended his Ph.D. thesis

On Friday September, 27th, Filip Scheiner successfully defended his Ph.D. thesis entitled “Geochemical markers from foraminiferal tests as a tool for reconstruction of paleoceanological environments: a case study from the Miocene of the Central Paratethys” (advisor K. Holcová). His Ph.D. research resulted in four papers in highly ranked international journals. Filip will continue as a Junior Researcher affiliated with UNCE. Congratulations!

Laboratory of Experimental Tectonics newly established

Thanks to financial support from the Center, we established a brand new lab focused on analogue tectonic experiments. The lab is equipped with highly sophisticated apparatus purpose-made by the Development Center of the Institute of Organic Chemistry and Biochemistry, Czech Academy of Sciences, which will be able to simulate a wide range of geologic processes, mostly related to active continental margins and subduction of oceanic plates.