Iapetus became narrower in the Middle Ordovician (figure 3.6), and drift directions changed for all plates. This was the epoch of global activation of tectonic processes (Khain and Seslavinsky 1994). Laurentia was approaching Gondwana, while Baltica continued its counterclockwise rotation and moved from Gondwana to the equator. Siberia drifted northward significantly. By the Middle Ordovician, Avalonia had compensated for the closure of the Iapetus Ocean. Complicated processes of interaction between the Appalachian mobile belt and the Iapetus lithosphere resulted in formation of the volcanic Popologan arc (Van Staal 1994), where the subduction zone dipped southeast toward the oceanic plate.
The Middle Ordovician is characterized by sea level rise and the largest global transgression in the Paleozoic (Algeo and Seslavinsky 1995). Seas covered vast areas along the margins of Gondwana. Prominent carbonate platforms developed in Siberia and Baltica, in the Canadian Cordillera, South America, Spitsbergen, and West Gondwana (Dmitrovskaya 1989; Fritz et al. 1991; Wensink 1991; Acenolaza 1992; Harland et al. 1992; Khain and Seslavinsky 1995). This was a time of extremely widespread turbi-dite deposition.
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