New constraints from detrital zircon ages on prolonged, multiphase transition from the Cadomian accretionary orogen to a passive margin of Gondwana
Abstract
The Avalonian-Cadomian belt formed as a collage of accretionary complexes, island arcs, and sedimentary basins along the northern margin of Gondwana during the Neoproterozoic times. One of the enigmatic issues is how and when this active margin evolved to a passive margin in the southern realms of the Rheic Ocean during late Cambrian to early Ordovician.
This issue is addressed here using U-Pb detrital zircon dating of key stratigraphic sections in the Teplá-Barrandian unit, Bohemian Massif. The new ages constrain the base of the Davle volcanic arc to 608 +- 6 Ma, 591 +- 6 Ma, and 590 +- 4 Ma and its termination to 563 +- 7 Ma.
The lower part of the overlying Příbram-Jince continental basin was deposited between 512 +- 5 and 511 +- 3 Ma. These brackets define a time span of about 50 M.y. for the geodynamic transition from Cadomian subduction, accretion, and arc magmatism to a passive margin and lithospheric extension.
In our model, a major role is assigned to a presumably curved geometry of the adjacent West African continental margin, which consisted of the outboard Armorican-Saxothuringian portion at an angle to the inboard Iberian-Barrandian portion. We suggest that during oblique subduction, ridge-trench collision and slab break-off occurred in the former at ca. 540 Ma, while subduction/accretion continued in the latter and was terminated by a major pulse of sediment accretion at around 527 Ma and ridge subduction at ca. 524-520 Ma.
Finally, the tectonic regime in the Cadomian arc and accreted upper crust switched to extension associated with crustal remelting to produce intraplate magmatism and development of continental basins with short-lived marine transgressions between ca. 512 Ma and ca. 488 Ma. The continued extension led to rifting and basaltic volcanism, directly linked to the diachronous opening of the Rheic Ocean.
We conclude that the prolonged transition from active to passive margin in the Cadomian belt was driven two principal geodynamic causes: first by oblique oceanic subduction and ridge-- trench interactions, bearing many similarities to modern Cordilleran-type active margins, and then by the farfield slab-pull force of the subducting Iapetus Ocean.