Joint 118th Annual Cordilleran/72nd Annual Rocky Mountain Section Meeting - 2022

Paper No. 16-3
Presentation Time: 8:50 AM


GANS, Phil, University of California, Santa Barbara, Dept of Earth Science, Webb Hall, Santa Barbara, CA 93106

In the 50 years since Anderson’s (1971) seminal work in the Eldorado Mountains, much has been learned about the Miocene magmatic and structural history of the Colorado River Extensional Corridor (CREC), but many questions remain concerning the nature of magmatic-tectonic interactions first raised by his work. This contribution reviews key observations and unresolved enigmas posed by this failed continental rift.

- The inception of both volcanism and extension migrated northward at ~ 2 cm/yr from the Whipple Mountains at ~22 Ma to the vicinity of Lake Mead at ~12 Ma. What drove this northward migration?

- Voluminous mafic to silicic volcanism predated the inception of local extensional faulting by 1-3 Ma, supporting an active rifting model. Volcanism abruptly waned or shut off during peak extension, only to resume at lower levels as extension slowed or ended. Why was volcanism suppressed during rapid extension?

- Geochemical data indicate mantle-derived mafic magmas fractionated and mixed with crustal melts to form the Miocene basalt to rhyolite compositional suite. What mantle source region(s) were involved and what triggered melting? How much new material was added to the crust and what were the rheological consequences of these magmatic additions?

- Minor amounts of orthogonal (N-S) extension often preceded the inception of rapid, large magnitude NE-SW or E-W extension. Was this driven by along-strike crustal thickness/topographic variations in the propagating rift or something else?

- The initial extensional strain rate at any given location was exceptionally high (≥ 100% in ≤ 1 Ma) suggesting a critical failure threshold followed by a runaway phenomena. Extension then rapidly waned, ending after 3-6 Ma. What threshold needed to be overcome, what were the primary driving forces, and why did extension stop, rather than proceeding to full continental rupture?

- Crustal thickness shows little change from beneath the CREC to much less extended areas on either side, suggesting large-scale flow of lower crust material into the corridor from both the east and the west. How do we reconcile the flat Mojo and present spatial coincidence of supracrustal extension and volcanism, mid-lower crustal plutonism, and mantle melting with unidirectional simple shear advocated by most core complex detachment models?