LITHOSPHERIC RESISTIVITY STRUCTURE BENEATH THE ROCKY MOUNTAIN FRONT: 2D ANISOTROPIC INVERSION OF MAGNETOTELLURIC DATA

We present two-dimensional anisotropic inversion results from the Deep RIFT Electrical Resistivity (DRIFTER) experiment, a 450 km magnetotelluric transect of the Rio Grande Rift, southern Rocky Mountains and High Plains in central Colorado. Our model reveals a modern-day snapshot of the resistivity structure of the crust and upper mantle beneath the Rocky Mountain – High Plains transition to a depth of 150 km. A key feature of the resistivity model is a broad (~200 km wide) zone of elevated conductivity (10-15 Ω·m) in the mid- to lower-crust beneath the southern Rocky Mountains. Based on previous seismic work, high regional heat flow, and the distribution of late-Cenozoic volcanism in Colorado, we interpret this conductor as a zone of partially molten basalt and associated deep crustal fluids. Given the transient nature of saline fluids and partial melt in the mid- to lower-crust, we conclude that the observed high conductivity is the result of relatively recent (less than 5 Ma) tectonic activity in the region that included the emplacement of basaltic magma into a lower crustal “hot zone”. If we accept existing petrologic models for deep crustal differentiation of mantle-sourced basaltic magmas, this zone of partial melt could serve as the magmatic source region for young, felsic intrusive bodies in the upper crust as well as Quaternary basalt flows in central Colorado. Other features of the resistivity model include an order of magnitude increase in upper mantle resistivity across the Rocky Mountain front and constraints on the location of the lithosphere-asthenosphere boundary beneath the High Plains.