BIRTH OF THE RHEIC OCEAN, A 2-STAGE PROCESS ON THE SOUTHERN (MEXICAN) MARGIN

The birth of the Rheic Ocean has traditionally been attributed to rifting/drifting of Avalonia from the Gondwanan margin in the Early Ordovician based on subsidence curves and Gondwanan faunal affinities in Avalonia. However, recent data from rocks in Mexico inferred to represent part of the Gondwanan (NW Amazonian) margin of the Rheic Ocean, suggest this was a 2-stage process: (1) 546 Ma mafic dike swarm with a plume-like chemistry cutting ~1 Ga basement is inferred to indicate Proterozoic-Cambrian rifting; (2) a major bimodal intrusive suite emplaced between 480 and 440 Ma and consisting of tholeiitic mafic rocks and calcalkaline granitoids of mixed arc-within plate affinity inferred to have been intruded during rifting. Paleogeographic reconstructions suggest that, in the latest Neoproterozoic, Avalonia lay adjacent to Oaxaquia (Mexico) on the NW margin of Amazonia. We infer that both rifting events were connected with the separation of Avalonia. The first event is inferred to have involved a Baja California-like mechanism reflected in Avalonia by: (a) the rapid Early Cambrian subsidence; (b) development of an unique Avalonian Cambrian fauna; and (c) deposition of passive margin sequences on both margins (Gander and Meguma). Movement of Avalonia along the Amazonian margin reintroducted Gondwanan fauna by the end of the Cambrian. The Early Ordovician rifting led to separation of Avalonia and the development of the Rheic Ocean (ss) with the deposition of the first passive margin rocks in Mexico (latest Cambrian-Tremadocian Tiñu Formation). Whereas rifting continued on the southern Mexican margin of the Rheic Ocean throughout the Ordovician, collision of Avalonia with eastern Laurentia produced the late Ordovician-early Silurian Salinian orogeny during closure of Iapetus. In contrast, tectonothermal events in Mexico are associated with closure of the Rheic Ocean in the late Devonian-Mississippian.