Magnetotelluric Imaging of Lower Crustal Melt and Lithospheric Hydration in the Rocky Mountain Front Transition Zone, Colorado, USA

Abstract

We present an electrical resistivity model of the crust and upper mantle from two‐dimensional (2‐D) anisotropic inversion of magnetotelluric data collected along a 450 km transect of the Rio Grande rift, southern Rocky Mountains, and High Plains in Colorado, USA. Our model provides a window into the modern‐day lithosphere beneath the Rocky Mountain Front to depths in excess of 150 km. Two key features of the 2‐D resistivity model are (1) a broad zone (~200 km wide) of enhanced electrical conductivity (<20 Ωm) in the midcrust to lower crust that is centered beneath the highest elevations of the southern Rocky Mountains and (2) hydrated lithospheric mantle beneath the Great Plains with water content in excess of 100 ppm. We interpret the high conductivity region of the lower crust as a zone of partially molten basalt and associated deep‐crustal fluids that is the result of recent (less than 10 Ma) tectonic activity in the region. The recent supply of volatiles and/or heat to the base of the crust in the late Cenozoic implies that modern‐day tectonic activity in the western United States extends to at least the western margin of the Great Plains. The transition from conductive to resistive upper mantle is caused by a gradient in lithospheric modification, likely including hydration of nominally anhydrous minerals, with maximum hydration occurring beneath the Rocky Mountain Front. This lithospheric “hydration front” has implications for the tectonic evolution of the continental interior and the mechanisms by which water infiltrates the lithosphere.

Physical State and Structure of the Crust Beneath the Western‐Central United States From Multiobservable Probabilistic Inversion

“…As argued by Chatterjee et al (), Ferri et al (), and others (e.g., Christensen, ; Dawson et al, ; Hacker et al, ; Lin et al, ; Mueller & Massonne, ; Nakajima et al, ; Owens & Zandt, ; Xu et al, ), when melting commences both V p and V s are decreased, but V s is decreased more rapidly than V p , thus increasing the total V p / V s ratio. These observations and interpretation are consistent with the recent MT results by Feucht et al (), who imaged an anisotropic high conductivity region in the middle and lower crust of the SRM, which they interpreted as strike‐parallel fractures/dikes along a north‐south direction containing melts and/or aqueous fluids. Although the resolution of our parameterization does not allow us to capture such small‐scale fracture zones, we note that the regions of significantly high V p / V s values beneath the SRM and the RGR system (latitude ∼39N and longitude ∼106W; e.g., profiles B, C, D, E, F, G, and H in Figure ) coincide well with the regions of high conductivity reported by Feucht et al ().…”

“…These observations and interpretation are consistent with the recent MT results by Feucht et al (), who imaged an anisotropic high conductivity region in the middle and lower crust of the SRM, which they interpreted as strike‐parallel fractures/dikes along a north‐south direction containing melts and/or aqueous fluids. Although the resolution of our parameterization does not allow us to capture such small‐scale fracture zones, we note that the regions of significantly high V p / V s values beneath the SRM and the RGR system (latitude ∼39N and longitude ∼106W; e.g., profiles B, C, D, E, F, G, and H in Figure ) coincide well with the regions of high conductivity reported by Feucht et al (). The spatial correlation between low wavespeeds, high V p / V s , elevated temperatures (see Figure ) and the locations of Cenozoic volcanism in the SRM and margins of the CP, offers compelling support for the occurrence of partial melting within the middle to lower crustal depths in these regions (e.g., beneath the Jemez zone and the SRM).…”

“…The modeling of Ma and Lowry () and Guerri et al () showed that hydration processes can also decreases V p / V s values in the crust, except when melt is present. While a recent magnetotelluric study in the SRM (Feucht et al, ) does not report a significant reduction in bulk resistivity within the upper crust in this region, felsic materials containing unconnected cracks filled with H 2 O‐rich fluids can be an alternative interpretation for the low V p , low V s and low V p / V s values observed at shallow depths (≤ 10–15 km) beneath the SRM and southern margins of the CP. Similar conditions have been reported by Zhang and Lin () beneath the Coso geothermal area in California.…”

Lithospheric Signature of Late Cenozoic Extension in Electrical Resistivity Structure of the Rio Grande Rift, New Mexico, USA

“…Another key feature of our interpretation is that, despite significant variability in inherited lithospheric structure, crustal thickness, width of extension, and surface geology along the axis of the Rio Grande rift, the resistivity structure of the lower crust in this tectonic regime appears to be remarkably uniform (Figure ). The low resistivity zone that we image in this study has been observed previously in regional (Feucht et al, ) and local (Hermance & Pedersen, ; Nettleton, ) studies of the Rio Grande rift conductivity structure. The addition of the two wide‐aperture, deep sensing MT profiles presented here allows us to put these previous observations into a larger tectonic context.…”

“…In summary, it is difficult to differentiate between partial melt and free saline fluids in a tectonic environment in which both are reasonable (e.g., Feucht et al, ; Li et al, ). That the electrical conductivity of the lower crustal anomaly appears to be enhanced in the direction parallel to the rift axis in the anisotropic model (e.g., Figure b) suggests the presence of partial melt within a network of vertical, rift‐parallel lenses.…”