The recent article by Bartholomew and Tollo (2004) concludes that the enigmatic Goochland terrane is a displaced Laurentian fragment. In their model, the Goochland terrane originated in the New York promontory, rifted away from Laurentia during the formation of Iapetus, and was subsequently reaccreted and translated to the southwest during Alleghanian collisional tectonics. We applaud Bartholomew and Tollo for highlighting the significant role that translational tectonics played in reorganizing the geometry of Appalachian terranes. However, we remain unconvinced that the Goochland terrane necessarily originated in the New York promontory.
In our view, one significant problem with their model is a general misunderstanding of the significance of transpressive structures in the Piedmont. Bartholomew and Tollo “restore” the Goochland terrane to a position in the New York promontory by northward translation along documented dextral transpressive structures in the Piedmont. However, mylonitic rocks in the Spotsylvania zone (the western boundary of the Goochland terrane) experienced general shear that not only translated, but also shortened material. Restoration cannot be accomplished by simply sliding blocks along strike-slip faults; complete restoration requires the consideration of orogen-normal shortening. Kinematic data from the Spotsylvania zone require 40%–70% contraction (Bailey et al., 2004). When these data are integrated across the Piedmont, the Goochland terrane is likely to restore to a position outboard of southern New England (Bailey, 2004).
Secondly, we find the geological evidence cited by Bartholomew and Tollo for a link between the Goochland terrane and the Reading and Manhattan Prongs to be permissive but hardly compelling. The best evidence for this correlation seems to be the similarity in age and composition of A-type granitoids in the Goochland terrane to several felsite dikes in the Reading Prong (Smith, 2003). The age similarities do not require that they be in close proximity at this time. Bartholomew and Tollo bolster their argument by correlating the Sabot Amphibolite in the Goochland terrane with presumed Grenville-age amphibolites in the New Jersey highlands Manhattan Prong. However, the absolute crystallization age of the pro-tolith of the Sabot Amphibolite is unknown at present. Furthermore, the most recent geochronologic evidence (Shirvell et al., 2004; Owens et al., 2004) for Devonian plutonism and metamorphism in the Maidens gneiss (which is interlayered with amphibolite in many places) should raise serious doubts about a Mesoproterozoic age for the Sabot Amphibolite.
The latter observations and data concerning a clear Paleozoic history for what may be the bulk of the Goochland terrane (all rocks currently included under the umbrella of Maidens Gneiss) call into question many long-held assumptions about this block of crust. Admittedly, this information is so new that the implications for the history of the terrane have yet to be fully incorporated into any model. Nonetheless, it is clear that the Mesoproterozoic portions of the Goochland terrane underwent a complex series of events following any detachment from Laurentia (or Gondwana). Most importantly, the terrane evidently experienced high-grade metamorphism in the Acadian (Shirvell et al., 2004), which might also reflect a position closer to New England at that time. Thus, we find the Bartholomew and Tollo model, which pays scant attention to this portion of the terrane's history, to be oversimplified at best.
A major unresolved issue highlighted by Bartholomew and Tollo's work concerns the position of the Goochland terrane during the Mesoproterozoic, particularly during the Grenville orogeny. Was it a part of Laurentia or could it have originated in a peri-Gondwanan setting? Regardless of whether it originated proximal to the New York promontory, Owens cautiously agrees with Bartholomew and Tollo that the terrane is Laurentian. This interpretation is consistent with (although not demanded by) several lines of evidence, including geochronolgical and isotopic data. In addition, the Montpelier Anorthosite in the Goochland terrane and the Roseland Anorthosite in the Blue Ridge are remarkably and anomalously (e.g., Owens et al., 2004) similar in many ways (unique mineralogy, highly alkalic composition, age, etc.). It would be a remarkable coincidence, indeed, if these unusual anorthosites originated at the same time on different continents. On the other hand, Bailey and Shirvell find the evidence for a Laurentian origin for Goochland to be less convincing. In particular, this issue remains unresolved because paleomagnetic and kinematic structural data are not available to uniquely locate the terrane during the Grenville orogeny.
The tectonic model proposed by Bartholomew and Tollo (2004) helps to dismiss simplistic accordion-style tectonic models for the Appalachians. However, we do not find their evidence for a New York origin of the Goochland terrane compelling. Clearly, the Goochland terrane was positioned north of Virginia prior to the late Paleozoic, but its earlier locations, as well as its ancestry as either a Laurentian or peri-Gondwanan fragment, remain unresolved.