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We were delighted to learn that Karlstrom et al. (2007) had written a Comment on our recent article in Geology; one of our major purposes was to stir debate on the origin of the extensive Paleoproterozoic crust of the southwestern United States. This Reply is intended not so much as a point-by-point defense of our earlier arguments, but as a further attempt to promote debate and study of this question.

Karlstrom et al. state that we have challenged the arc-accretion model for the Yavapai Province and “view the Yavapai province to be underlain by older crust and to be the product of continental rifting.” Although we suggested that older crust, perhaps as enclaves, must be present and have contributed to the formation of ca. 1750 Ma rocks, we did not rule out the accretion of arcs, but rather suggested that there must have been other operative processes as well. That is why the words “an expanded interpretation” were included in our title. Is it possible that southern Laurentia was a “metacraton” (a region of heterogeneous crust that was variably affected by tectonic events) during the Paleoproterozoic (e.g., Abdelsalam et al., 2002; Liegeois et al., 2003)?

With regard to the involvement of older crust, Karlstrom et al. cite the fact that the Indonesian region is a modern analog involving the assembly of a complex collage of terranes, including older crust, onto an older continent. If this model applies to the Southwest United States, key questions remain: what older crust, and where did it come from? Although “where did it come from?” may be a tough question, “what older crust?” is something that can be determined. We are impressed with the fact that, where studied, inherited zircons are mostly ~1840–1870 Ma, while some others are ca. 2500 Ma, suggesting to us that Trans-Hudson–Penokean crust is present. And, in the Grand Canyon, the Elves Chasm pluton has been dated at 1840 Ma, a solid Trans-Hudson age (Hawkins et al., 1996).

Karlstrom et al. also challenge, as overly simplistic, our observation that much of the Paleoproterozoic crust of the Southwest is strongly bimodal, and likely related to continental rifting. Whereas Karlstrom et al. state that their studies of the Yavapai province “suggest that the 1.8–1.7 Ga metavolcanic rocks are dominantly basaltic to basaltic andesite” and that “rhyoliterhyodacite is present but volumetrically minor,” we note that this is certainly not the case in central Colorado where high-silica rhyolite makes up ~40 percent of exposed metavolcanic rocks, and the rest is tholeiitic basalt (e.g., Boardman and Condie, 1986; Bickford and Boardman, 1984), nor is it evidently true in much of New Mexico, where Robertson et al. (1993) described a 1.765–1.72 Ga basalt-dominated bimodal suite that includes rhyolite, but no andesite. Further, Mazatzal age terranes (ca. 1660–1600 Ma) in both Arizona and New Mexico are described as including voluminous assemblages of rhyolite (e.g., Robertson et al., 1993, and references therein). More to the point, where does all this rhyolite come from if it is not derived from remelting of preexisting crust? We are not aware of juvenile island arcs in which this volume of rhyolite is present.

We accept the suggestion of Karlstrom et al. that accretionary complexes and ophiolites may not be preserved at the crustal levels currently exposed in much of the Southwest. However, in central Colorado, where Paleoproterozoic rocks are in upper greenschist to lower amphibolite facies, pillowed basalts and basaltic breccias are beautifully preserved. Indeed, near Salida there is a thick section of basaltic lapillistone that even preserves cross-bedding (Boardman, 1986). To our knowledge, however, there are no preserved ophiolites or accretionary complexes in this region.

Finally, with regard to crustal extension and initiation of bimodal volcanism, our suggestion was part of a proposed model (a “tentative scenario”, as we put it) to stimulate discussion and further research. We think our proposed model is testable through (1) sensitive high-resolution ion microprobe (SHRIMP) dating of inherited components in zircons to determine the age and distribution of older crust; (2) study of the chemistry, particularly the isotopic chemistry, of the extensive bimodal volcanic suites by Sm-Nd whole-rock methods and Hf isotopic methods in zircons; and (3) further structural and geochronological studies of exposed shear zones.

We again wish to express our pleasure that our article has stimulated discussion in Geology, and we encourage others interested in this aspect of the crustal evolution of Laurentia to write additional comments.