DIACHRONOUS COLLISION AT THE LAURENTIAN MARGIN DURING APPALACHIAN-CALEDONIDE OROGENESIS

The northern Appalachians and the Caledonides of the British Isles developed from a Laurentian passive continental margin during episodes of orogenesis that lasted from the Furongian (late Cambrian) until at least the Middle Devonian. The development of the orogen is typically explained as the result of an initial (Taconian-Grampian) arc-continent collision followed by subduction polarity reversal and accretion (Salinian-Acadian) of exotic terranes over the next 100 My. Maps of the orogen typically display considerable along-strike continuity; published interpretations portray events that purportedly occur simultaneously at positions that are widely separated along the length of the orogen.

The closest modern analogue to the Ordovician margin of Laurentia is the northern margin of Australia. Some portions of this margin (Papua – New Guinea) have already undergone arc-continent collision and subduction polarity reversal. At other positions on the margin (Timor), collision is just beginning. North of the margin a complex mosaic of terranes awaits accretion to different parts of the Australian margin during its future evolution. Based on this modern example, simultaneous development along strike is likely to be the exception, rather than the rule, in an orogen like the Appalachians. The present linear character of the orogen is a product of the shortening of accreted belts, not their original geometry.

In the Appalachians, the earliest collision of the Laurentian margin with an encroaching arc occurred on the Newfoundland promontory. Collision propagated diachronously southward, followed by subduction polarity reversal. In contrast, accretion of peri-Gondwanan terranes began in the south in the Early Ordovician, not reaching the British Isles until the mid-Silurian. The Appalachian domain Ganderia contains at least four peri-Gondwanan terranes that were accreted at slightly different times during the Ordovician and Silurian. The recognition of discrete events (e.g. Taconian – Salinian – Acadian), possible in single transects, becomes ambiguous or impossible at the scale of the whole margin.