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Figure 1. Geological sketch map of west-central North America showing the two transverse structural zones and geologically relevant units. Geology from Reed
et al. (2004) with local detail from sources cited in the text; 80 Ma contours of Cretaceous rocks from Roberts and Kirschbaum (1995); Great Basin calderas from
Henry and John (2013); Late Cretaceous-Paleocene Sri isopleths from Armstrong et al. (1977). Inset: The Gulf of Mexico region simplified from Reed et al. (2004)
showing the sinistral separation of Gulf salt (reflected in salt domes) and the southward truncation of the high-amplitude magnetic high that is interpreted to
represent basalts of a volcanic rifted margin (Mickus et al., 2009). The Mexican ridges province is characterized by detachments in shale, whereas elsewhere in
the Gulf, detachments are in salt (Rowan et al., 2004).
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