10:00am - 10:30am Invited Session KeynoteTopics: 3.28 Developments and progress in regional geology
Shaping the northern Gondwana margin before the Variscan orogeny: large-scale geodynamic processes and paleogeography
Institute of Geology and Paleontology, Faculty of Science, Charles University, Czech Republic
The Cadomian accretionary orogeny at the end of Neproterozoic to early Cambrian was one of the principal crustal growth events in Europe, as indicated by a number and widespread distribution of Cadomian basement units within the Variscan and Alpine orogenic belts. Although the evolution of the Avalonian–Cadomian active-margin of northern Gondwana has been well established, geodynamic causes of its collapse and processes of its transition to a passive margin are significantly less understood. The particularly intriguing issues, which have a significant impact on interpretations of paleogeography and course of the subsequent Variscan orogeny, include: (1) how and when the Cadomian orogeny ended, (2) what was the provenance of the Avalonian, Cadomian, and other terranes, i.e., where they started their journey from Gondwana to Pangea, (3) how was the Cambro–Ordovician rifting phase expressed along the former active margin and what was its exact timing, duration, style, and magnitude, and (4) how far east reached the Rheic Ocean rift system and what was the nature of its eastern termination. Integrated geochronological and tectonic information suggest that (i) the active-to-passive-margin transition was step-wise, protracted over Cambrian and early Ordovician, (ii) strongly controlled by tectonic inheritance whereby an inherited suture in the Avalonian ribbon terrane facilitated complete rifting and rift–drift transition while the Cadomian terranes remained attached to Gondwana, and (iii) magmatism may have been an important geodynamic driver of rifting during at least the initial stages, before being overridden by the slab pull force of the subducting Iapetus Ocean.
10:30am - 10:45amTopics: 3.28 Developments and progress in regional geology
The Harz Mountains (Germany) – Cadomia meets Avalonia and Baltica: U-Pb ages of detrital and magmatic zircon as a key for the decoding of Pangea´s central suture
1Senckenberg Naturhistorische Sammlungen Dresden, Germany; 2Sächsisches Landesamt für Umwelt, Landwirtschaft und Geologie, Germany; 3Nationale Genossenschaft für die Lagerung radioaktiver Abfälle (Nagra) , Switzerland; 4UNESCO Global Geopark Harz, Germany
The nature of Pangea´s internal suture in the Central European Variscides is evaluated on the example of the Harz Mountains (Germany). U-Pb ages of detrital (n = 2273) and magmatic zircon grains (n = 170) were obtained by LA ICP-MS for provenance studies and absolute age dating. Provenance studies point to a docking of East Avalonia onto southern Baltica at c. 430 Ma and to a closure of the Rheic Ocean at c. 430-420 Ma. In the aftermath, the re-opening of a narrow Rhenish Seaway happened in mid-Devonian time. Devonian aged sedimentary rocks of the Harz Mountains were formed at the north-western (Rheno-Hercynian) and on the south-eastern (Saxo-Thuringian) margin of the Rhenish seaway. Deposits formed on the Rheno-Hercynian margin display sedimentary supply from southern Baltica, while most East Avalonian sources were buried and not available for erosion. Siliciclastic shelf deposits of Saxo-Thuringia were derived from Cadomia and its West African hinterland. Provenance studies and spatial arrangement of the tectonostratigraphic units in the Harz Mountains allow to reconstruct the style of obduction of the Harz Mountains onto the southeastern margin of East Avalonia (Rheno-Hercynian Zone). As a result of the closure of the Rhenish Seaway and the collision with the Mid-German Crystalline Zone, the Harz Mountains show a building plan strongly dominated by thin skinned tectonics. Our new data favour a two-plate model for the origin of Pangea´s internal suture in the Central European Variscides, but a re-opening of a narrow oceanic seaway is necessary after closure of the Rheic Ocean.
10:45am - 11:00amTopics: 3.28 Developments and progress in regional geology
The Ellesmerian orogeny of Laurussia – A far-field effect of the late Devonian collision of Gondwana with North America
1TU Bergakademie Freiberg, Germany; 2Lakehead University, Canada
Convergent tectonics of the Ellesmerian orogeny affected Arctic Canada and Svalbard in late Devonian - early Carboniferous times. The coeval ultra-high pressure metamorphism in NE-Greenland, the formation of the Maritimes Basin in the Canadian Appalachians, and tectonic activity in the Antler orogeny (Nevada, USA), raised a controversy about whether the Ellesmerian orogeny was related to either late Caledonian plate tectonics or renewed, i.e., post-Caledonian convergence of Laurentia with the rest of Laurussia. Here, we argue that the Ellesmerian orogeny represents a far-field effect of the collision of a Gondwanan promontory with the Appalachian segment of Laurussia. This collision caused the decoupling of the North American lithosphere from Laurussia. The northward motion of this lithospheric domain relative to Laurussia explains the tectonic events along its boundaries, namely the Ellesmerian and the Antler orogenies along the frontal and the Panthalassan margins, respectively. Furthermore, it triggered the reactivation of the NE segment of the Greenland Greenland-Scandinavian Caledonides under dextral transpression causing UHP metamorphism. To the southeast, the formation of the Maritimes Basin reflects dextral transtension and associated strike-slip faults parallel to the edge of SE-Greenland. Strain compatibility requires the existence of a dextral lithospheric-scale strike-slip fault that links the tectonic structures and transects the Greenlandian part of the North American Craton. Because this proposed shear zone in SE Greenland follows small-circle trajectories of the motion of Gondwana relative to Laurussia, we interpret the southeastern edge of Greenland as a dextral transform boundary of the decoupled North American lithosphere.