DID SUBDUCTION INITIATE ALONG THE CAMBRIAN ALABAMA PROMONTORY?

Widespread Ordovician plutonism in the Eastern Blue Ridge(EBR) of the southern Appalachians has led some Alabama workers to speculate that these arc magmas formed as a result of subduction beneath the Laurentian margin, in contrast to interpretations involving Paleozoic emplacement of terranes containing an exotic history of Ordovician arc activity. In particular, the Upper Cambrian Elkahatchee Quartz Diorite and Middle-Upper Ordovician Zana and Kowaliga Gneiss in the Ashland-Wedowee belt (EBR) of Alabama represent arc magmatic intrusions influenced by continental crust. The bulk of the middle to upper amphibolite facies Ashland-Wedowee belt consists of metapelitic rocks with minor orthoamphibolite characteristic of continental offshore/slope strata, and is separated from rocks of the Laurentian shelf (lower greenschist facies Talladega belt) across the Hollins Line fault, an Alleghenian post metamorphic thrust system. Interpretation of the Alabama EBR as Laurentian slope-rise deposits dictates that cratonward subduction must have initiated during the Cambrian beneath the Alabama promontory. The mechanisms for transforming a passive continental margin into an active Andean type margin are poorly understood. Geophysical models suggest that spontaneous development of subduction at a passive margin due to sediment loading, differential subsidence, negative buoyancy of oceanic lithosphere, etc…, could result in detachment within or near the ocean-continent transition. However, the lack of spontaneous subduction initiation along well developed Cenozoic passive margins has led many researchers to question the validity of these models. Subduction initiation beneath the Alabama promontory may be the direct result of juxtaposition of old and young oceanic lithosphere associated with the Cambrian passage of the Ouachita rift along the Alabama-Oklahoma Transform south of Laurentia (referenced to the modern orogenic belt). If so, then development of this subduction system has significant tectonic implications for the southeastern Cambrian-Ordovician margin of Laurentia, and integrates a more accepted geophysical model for nucleation of subduction with the observation that development of Andean type subduction should be an integral part of supercontinent assembly.