Dynamics of Cambro-Ordovician rifting of the northern margin of Gondwana as revealed by the timing of subsidence and magmatism in rift-related basins
Abstract
The Bohemian Massif of Central Europe is a Variscan collage of lithospheric fragments that formed at the northern margin of Gondwana during the late Neoproterozoic. A key geodynamic process that shaped this margin before it became involved in the Variscan orogen was the Cambro-Ordovician rifting that opened the Rheic Ocean.
This rifting event has been studied extensively, yet a number of issues remain unresolved, among which are its geodynamic causes. New U-Pb zircon ages of orthogneisses from the mid-crustal Moldanubian unit, in combination with available information on magmatism and basin subsidence in the upper-crustal Tepla-Barrandian unit of the Bohemian Massif, are here used to reconstruct in detail the mechanism of the Cambro-Ordovician rifting.
We argue that extension occurred in three phases defined by (1) protracted similar to 524-480 Ma intermediate to felsic plutonism (including the dated similar to 490-480 Ma orthogneisses), (2) basaltic submarine volcanism at ca. 470 Ma, and (3) rapid subsidence at ca. 458-452 Ma. This relative timing is interpreted to reflect stretching of the lower lithosphere before upper lithospheric rifting.
In a broader context, these inferences are compatible with contrasting, rheologically controlled modes of northern Gondwana break-up during the early Ordovician, in which the westerly Avalonian-type terranes were rifted away from Gondwana, whereas the easterly Cadomian-type terranes formed a hyperextended Gondwanan shelf.