Paper No. 13-3
Presentation Time: 8:45 AM
A COMPARISON OF THE CAMBRIAN PRE-RIFT, SYN-RIFT AND BREAK-UP PULSES THROUGHOUT THE ATLAS‒OSSA-MORENA‒NORTH ARMORICAN RIFT (NW AFRICA AND SW EUROPE) AND WESTERN AVALONIA (ATLANTIC CANADA)
One consensual geodynamic paradigm improved during the last decades suggests that the Rheic Ocean initially formed, in Furongian-Early Ordovician times, as a result of the drift of a handful of peri-Gondwanan terranes, such as Avalonia, from the prolongation of the so-called Atlas‒Ossa-Morena‒North Armorican rift, which remained attached to Gondwana. Models fuelling useful discussions have taken into account the geochemical and isotopic affinity of volcanic products and analyses of relative subsidence, changes in detrital zircon populations, paleomagnetic data, biogeographic shifts and diachronous occurrence of climatically sensitive facies. However, little attention has received the comparison of the Cambrian pre-rift, syn-rift and break-up pulses that should be shared by margins formed on opposing sides of the Rheic Ocean basin. A reappraisal of geodynamic models in the Atlas‒Ossa-Morena‒North Armorican rift and some Cambrian rift shoulders preserved in the Avalon Peninsula of Newfoundland has yielded a common arrangement of major unconformities displaying: (i) angular discordant to paraconformable contacts onlapped by breccia-to-conglomerate lags and unconformity-bounded carbonates, recording (ii) event-concentration levels rich in parautochthonous and allochthonous fossil assemblages punctuated by hiatal stratigraphic diastems, representing (iii) short-term uplift episodes of rift shoulders leading to unroofing of the Ediacaran basement responsible for the occurrence of reworked exotic clasts and allochthonous fossils sourced from underlying Ediacaran and Cambrian volcanosedimentary complexes, and associated with (iv) the onset of coeval hydrothermal polymetallic stockworks throughout synsedimentary fracture networks that controlled the development of localized karstic processes. As a result, the Cambrian rift framework preserved in NW Africa and SW Europe shares with western Avalonia a latest Ediacaran to Fortunian pre-rift unconformity, several Cambrian Epoch 2 to Miaolingian syn-rift episodes, of local to regional distribution, and a final Furongian break-up unconformity that led to passive-margin conditions during Ordovician times.