Uplift of the Colorado Plateau via Lower Crustal Foundering

Abstract:

How the Colorado Plateau reached its current elevation with little internal deformation compared to surrounding regions has perplexed researchers for nearly a century. Hypotheses to explain the two kilometers of uplift since the Late Cretaceous range from delamination of the Farallon plate following flat slab subduction, thermal expansion of upwelling mantle, dynamic topography in response to mantle upwelling, mid-crustal flow from over-thickened crust, and foundering of a dense lower crustal root. Many of these hypotheses are constrained by geodynamic modelling with limited evidence from the rock record. We report here the petrologic and geochemical makeup of lower crustal xenoliths from the Transition Zone in Arizona between the southern Basin and Range Province and the Colorado Plateau. This xenolith suite erupted within a ~25 Ma volcanic host and is dominated by garnet pyroxenite with minor gabbro and amphibolite. Major and trace element geochemistry, petrography, and thermobarometry suggest these rocks represent deep-seated (12–25 kb) cumulates formed during arc magmatism. A preliminary U-Pb sphene age of ~50 Ma suggests that the cumulates formed during the end of the Laramide orogeny. Calculated compositional densities for these cumulates are up to 10% greater than the mantle, suggesting that early to mid-Tertiary arc magmatism generated a dense and unstable lower crustal root. Because these rocks are cold (580–840 °C), thermal contraction may further increase the density contrast. Foundering of this dense root could cause significant uplift. Isostatic calculations show that two kilometers of uplift may be explained by removal of a 20-km-thick root that is 10% denser than the underlying mantle. If lower crustal foundering is indeed responsible for uplift of the Colorado Plateau, the eruption age of the xenolith suite constrains uplift to be younger than ~25 Ma.