Leah C. Cheek, Kyle J. Grimsley, Erin E. Fenlon, Katie E. Luciano
Department of Geology, College of William and Mary
In Virginia, the Piedmont Province is comprised of a series of terranes, which are typically defined as fault-bounded regions of shared stratigraphic and tectonic character, each with a unique geologic history. The Goochland terrane (Fig. 1) is considered a suspect terrane because its location prior to the Alleghanian orogeny (330-270 Ma) is uncertain and disputed. Farrar (1984) defined the Goochland terrane as a region of Grenvillian rocks that have undergone granulite-facies metamorphism, a higher grade of metamorphism than surrounding terranes. Four major rock units form the Goochland terrane: the State Farm gneiss, the Sabot amphibolite, the Maidens gneiss, and the Montpelier anorthosite (Fig. 2). The Goochland terrane has been variously interpreted as (1) an unrifted fragment of Laurentia (e.g. Glover and Gates, 2002), (2) a Laurentian fragment that was rifted and reaccreted during the Paleozoic (e.g. Bartholomew and Tollo, 2004, Owens and Samson, 2004), (3) and an exotic terrrane (Keppie and others, 1996). Still others claim that insufficient evidence prohibits definite interpretation of the Goochland terrane’s origin (Spears and others, 2004).
One interpretation is that the Goochland is a fragment of Laurentia that never rifted from the North American continent. Glover and Gates (1997) suggested it is a tectonic window (older rocks bounded by reverse faults and exposed by erosion). In light of more recent study revealing kinematic and age relationship data, evidence to support this interpretation is unsatisfactory.
Alternatively, the Goochland has been interpreted as having rifted from Laurentia and later reaccreted and is closely associated with the Blue Ridge Province in particular to its west. It was long thought that the Goochland terrane was related to the Blue Ridge Province, based on similarities between the basement rock and granitic intrusions in both regions. This correlation of the Goochland terrane with the Blue Ridge Province was built upon similarities in geochemistry, geochronology, and isotopic composition (Owens and Tucker, 2003; Owens and Samson, 2004).
Others agree that the terrane is of rifted Laurentian provenance, but suggest it formed near modern-day Manhattan during the assembly of the supercontinent Rodinia over a billion years ago (Bartholomew and Tollo, 2004). Their primary evidence for arguing that the Goochland terrane originated to the northeast is that the age and composition of rift-related granitoids is similar for the two localities, and that these granitoids are not found in the Blue Ridge. In order to account for the ~310 mi (~500 km) translation of the Goochland to its present location Bartholomew and Tollo (2004) used kinematic evidence from Bailey and others (2004) from the bordering Spotsylvania high-strain zone (SHSZ). An analogous example of this interpretation is the kinematic history of Madagascar, which was rifted from Africa during the breakup of Gondwana, and later translated northward along transform faults (Bartholomew and Tollo, 2004).
Controversy arises, however, over the claim that the Goochland terrane originated in Laurentia prior to the Alleghanian orogeny (Owens and Tucker, 2003; Bartholomew and Tollo, 2004; Owens and Samson, 2004). Comparing the Grenvillian age of basement rocks in both the Goochland and South America, Keppie and others (1996) suggest that a peri-Gondwanan origin (close to the ancient supercontinent Gondwana) for the Goochland should be considered. Although this is not the dominant interpretation, Keppie and others (1996) question the validity of discrediting an exotic origin without further research.
Other authors also claim that the prevailing Laurentian origin hypothesis is hardly compelling and somewhat lackluster. Bailey and others (2004) provide a more complete interpretation of the Spotsylvania high-strain zone (SHSZ) kinematics, taking into account the variability of motion along the fault zone. The SHSZ lies between the Goochland terrane and the neighboring Chopawamsic terrane and is key to understanding the kinematics of the Goochland terrane and resolving controversy surrounding its origin and history. The most recent work shows the SHSZ to be a belt approximately 9 miles (15 km) wide, of mylonites , which are highly deformed metamorphic rocks characteristic of fault zones (Spears and others, 2004). Structural direction indicators, including asymmetric porphyroclast tails, boudins, and folded pegmatite dikes, have been used to constrain the kinematics of the Goochland in relation to the SHSZ. Using estimated values for the vorticity and three dimensional strain in the SHSZ, Bailey and others (2004) restored the Goochland terrane to a position 50-186 miles (80-300 km) (Fig. 4) to the northeast of its current location in the Virginia Piedmont (restoration was completed relative to a constant position for the Blue Ridge).
The SHSZ kinematic data (Bailey and others, 2004) supports that the Goochland experienced a significant amount of translation in a southwesterly direction during the Alleghanian orogeny, and this evidence was used by Bartholomew and Tollo (2004) to argue for a Laurentian origin of the terrane. However, Bailey, Owens and Shirvell (2005) assert that evidence for a pre-Alleghanian Laurentian origin of the Goochland terrain is inconclusive. Their work emphasizes the kinematics of the terrane’s movement from New England, but they are unconvinced that there is sufficient evidence to constrain the location of the Goochland prior to its reaccretion during the Alleghanian. Some authors refute a definite Laurentina origin in the absence of paleomagnetic or kinematic data (Keppie and others, 1996, Bailey and others 2005).
Surprising new evidence regarding the age of the Goochland terrane will undoubtedly impact the direction of future research. The above hypotheses include the assumption that the entire Goochland terrane was Precambrian. It was widely believed that the Precambrian Grenvillian orogeny was the event that metamorphosed the Goochland to granulite-facies. However, Shirvell (2004) found that the Goochland’s uppermost unit, the Maidens gneiss, had actually been affected by granulite-facies metamorphism much more recently, about 400 Ma. This discovery implied that much of the Goochland terrane is not in fact Precambrian, and has a more complex metamorphic history than has been previously accounted for in for in previous research into the origin of the Goochland terrane.
Further research will likely continue to rock the Goochland world as the controversy continues to be investigated. This is an active and exciting topic of research that provides interesting insights into the geologic history of Virginia. Currently there is no single compelling hypothesis to explain the enigma that is the Goochland terrane.