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Assembly of lithosphere in southern Laurentia is best interpreted to have taken place along a long-lived (1.8–1.0 Ga) active margin via subduction-accretion processes broadly analogous to present-day convergence between Australia and Indonesia (Karlstrom and Bowring, 1988; Karlstrom et al., 2001, Jessup et al., 2005). Bickford and Hill (2007) challenge this model, and view the Yavapai province to be underlain by older crust and to be the product of continental rifting. This is reminiscent of the arc versus rift debates for Archean greenstone belts in the 1980s. We argue that the four main points in the Bickford and Hill paper are consistent with the arc-accretion model (Bowring and Karlstrom, 1990; Whitmeyer and Karlstrom, 2007).

1) Older Crust: Evidence for older crust and detritus is expected in an accretionary orogen. The Indonesian region is a modern analog involving assembly of a complex collage of terranes onto an older continent. This region has older crustal fragments on which younger island arcs are constructed (e.g., Sumatra but not Java), tectonic imbrication is common, and older detritus can be transported thousands of kilometers in arc-trench systems (Hamilton, 1979). Similarly, in the Proterozoic of the southwestern United States, Nd, Pb, and Hf isotopic data indicate localized older (pre-1.8 Ga) crust. The best-characterized older crust is an isolated exposure of 1.84 Ga Elves Chasm granodiorite, on which part of the 1.75–1.72 Ga arc was built (Hawkins et al., 1996). Bickford and Hill focused on volumetrically minor inherited zircon cores (1.85 Ga, and some Archean) in 1.77 Ga igneous rocks in Colorado and speculated that “Trans-Hudson–Penokean crust, or fragments thereof, underlies much of southern Laurentia” (p. 169). However, without documentation of the extent, age, and tectonic affinities of older crustal components, we caution against assuming that all 1.84 Ga rocks or inherited grains are genetically related, or even originated in the same orogen.

2) Bimodal Volcanism: The assertion that a bimodal (mafic/felsic) composition of volcanic rocks uniquely indicates continental rift zones is overly simplistic. Our studies of the Yavapai province suggest that the 1.8–1.7 Ga metavolcanic rocks are dominantly basaltic to basaltic andesite. Rhyoliterhyodacite is present, but is volumetrically minor, as is true in oceanic arcs such as Krakatowa and Tonga. Metasedimentary rocks are dominantly greywacke turbidites. Most of the volcanic rocks are contemporaneous with calc-alkaline granitoids. The complete volcanic-plutonic-turbidite associations have no modern analogs in continental rift zones, but are typical of arcs.

3) Ophiolites and Accretionary Complexes: Ophiolites, mélanges, and accretionary prisms have low preservation potential in any orogen, and their rarity in the Southwest is not a surprise given the 10–25-km-deep levels of exposure. In addition to the Grand Canyon accretionary complex (Karlstrom and Williams, 2006) and the 1.73 Ga Payson ophiolite (Dann, 1997), there are numerous other examples of ultramafic rocks, pillow basalts, and chert that have been interpreted as some combination of dismembered ophiolite, tectonic mélange, and other oceanic terranes (e.g., Eisele and Isachsen, 2001; Swift and Force, 2001; Tyson et al., 2002; Cavosie and Selverstone, 2003). These occurrences must be incorporated into any alternate tectonic model (c.f. Bickford and Hill's Fig. 3).

4) Extension and Transpression: Indonesia and other modern analogs have complex stress regimes and networks of linked subduction-transform–spreading ridges (Hamilton, 1979). However, intra-arc and backarc extension, slab roll-back in forearc basins, and transpressive pull-apart basins are distinct from continental rifts. In the Southwest, trans-current shear zones are present (Bergh and Karlstrom, 1992), but structural studies suggest early crustal thickening via thrust stacking, followed by progressive horizontal shortening by folding, resulting in mainly dip-slip stretching lineations. We know of no data that support their model that major transcurrent shear zones “caused crustal extension and initiation of bimodal volcanism” (Bickford and Hill, 2007, p. 169).

In summary, Bickford and Hill highlight the need for continued efforts to decipher the accretionary history of southern Laurentia, but they offer no new data that require a different or expanded model.