FROM RIFTED MARGIN GEOMETRY TO ARC SYSTEM COMPLEXITY: REFINING OUR TECTONIC MODELS WITH BILL THOMAS IN MIND

With the exception of the drifting Precordillera terrane (Thomas, 1991), paleogeographic reconstructions of Laurentia in the Ordovician commonly depict an open ocean along the southernmost margin. But the presence of bimodal metavolcanic and metaplutonic rocks of arc and back-arc affinity, preserved today in the Alabama and Georgia Blue Ridge, confirms the existence of an Early to Mid-Ordovician subduction system outboard of that margin. Variations in timing of emplacement of the arc fragments, and subsequent deformation and metamorphism along the arc-continental margin boundary, are not yet fully understood. Studies of modern subduction zones, however, illustrate that the along-strike history of different segments in a single arc system may vary by tens of millions of years, arc systems may be segmented by strike-slip zones, and commonly undergo extension concomitant with contraction. Such complexities likely existed along the southernmost Laurentian margin in Early to Mid-Ordovician time. Factors that may have controlled the observed variations include the geometry of the rifted continental margin, the position of microcontinental blocks relative to the subduction zone and developing arc, and segmentation of the arc along transform faults. Our attempts to refine models of the Early- to Mid-Ordovician southern Laurentian margin and outboard arc should build on the pioneering work of Bill Thomas, who demonstrated that the Late Proterozoic rifted margin of eastern Laurentia was characterized by alternating promontories and embayments that subsequently controlled patterns of syn- and post-rift deposition as well as growth of the Appalachian orogenic belt (e.g., Thomas 1977; 1991; 1993; 2006). Significant features of this irregular margin likely included: 1) oceanward-dipping detachments, comprising wide zones of thinned continental crust, locally punctuated by marginal plateaus or microcontinental blocks (lower-plate geometry), alternating with 2) continentward-dipping detachments, comprising narrow zones of transition between continental and oceanic crust (upper-plate geometry), and 3) transform faults that bounded the oppositely dipping segments. In particular, the Alabama-Oklahoma transform may have marked the southern terminus of subduction outboard of the Laurentian margin in Early to Mid-Ordovician time.