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

Abstract

We present electrical resistivity models of the crust and upper mantle from two‐dimensional (2‐D) inversion of magnetotelluric (MT) data collected in the Rio Grande rift, New Mexico, USA. Previous geophysical studies of the lithosphere beneath the rift identified a low‐velocity zone several hundred kilometers wide, suggesting that the upper mantle is characterized by a very broad zone of modified lithosphere. In contrast, the surface expression of the rift (e.g., high‐angle normal faults and synrift sedimentary units) is confined to a narrow region a few tens of kilometers wide about the rift axis. MT data are uniquely suited to probing the depths of the lithosphere that fill the gap between surface geology and body wave seismic tomography, namely the middle to lower crust and uppermost mantle. We model the electrical resistivity structure of the lithosphere along two east‐west trending profiles straddling the rift axis at the latitudes of 36.2 and 32.0°N. We present results from both isotropic and anisotropic 2‐D inversions of MT data along these profiles, with a strong preference for the latter in our interpretation. A key feature of the anisotropic resistivity modeling is a broad (~200‐km wide) zone of enhanced conductivity (<20 Ωm) in the middle to lower crust imaged beneath both profiles. We attribute this lower crustal conductor to the accumulation of free saline fluids and partial melt, a direct result of magmatic activity along the rift. High‐conductivity anomalies in the midcrust and upper mantle are interpreted as fault zone alteration and partial melt, respectively.

Magnetotelluric Evidence for Distributed Lithospheric Modification Beneath the Yinchuan‐Jilantai Rift System and Its Implications for Late Cenozoic Rifting in Western North China

“…Given the estimated temperatures (>700°C) at the uppermost mantle depths beneath the YJRS (Figure 10c; Sun et al., 2013), graphite film can also be excluded since they are suggested to be unstable at such high temperatures according to recent laboratory results (Zhang & Yoshino, 2017). The remaining candidates are water and partial melt, both of which have been widely identified beneath active extensional areas (e.g., Wannamaker et al., 2008; Feucht et al., 2017, 2019).…”

“…Numerical modeling suggests that the thermal effects of repetitive basalt intrusions on the crust would be fully decayed within several millions of years after the cessation of basaltic injections (Annen & Sparks, 2002). Due to the relatively short residence times of fluids in the deep crust, the sources of deep fluids in modern rift zones have been often attributed to recent and/or ongoing tectonic processes that have supplied heat, volatiles, and/or basaltic melts into the crust (e.g., Rippe et al., 2013; Feucht et al., 2017, 2019). Within the YJRS, these fluids are most likely associated with the Late Cenozoic rifting activity as indicated by petrological and geochemical data (H. Zhang, Huang, et al., 2017; L. Zhang, Liu, et al., 2017).…”

“…High electrical conductivities in the middle-lower portion of the crust have been widely reported in active extensional areas, and most plausibly explained by free saline fluids, partial melt or a combination of both (e.g., Feucht et al, 2017Feucht et al, , 2019Meqbel et al, 2014;Rippe et al, 2013;Wannamaker et al, 2008). Given that saline fluids have extremely high conductivity values up to ∼100 S/m (Sakuma & Ichiki, 2016;Sinmyo & Keppler, 2017), or even higher (∼400 S/m, Guo & Keppler, 2019), a small fraction of hypersaline fluids would be sufficient to produce the high conductivities observed in the deep crust (Dong et al, 2020).…”

“…Among various geophysical imaging techniques, the magnetotelluric (MT) method is susceptible to electrical conductivity anomalies associated with the presence of partial melt and free aqueous fluids in the lithosphere and has therefore been extensively used to investigate the deep fluid and magmatic processes beneath continental rift zones (e.g., Dong et al., 2014, 2020; Feucht et al., 2017, 2019; Rippe et al., 2013; Wannamaker et al., 2008). However, for the western NCC, previously reported dense MT observations are largely restricted to the Shanxi rift system (SXRS) on the eastern side of the Ordos Block (Yin et al., 2016, 2017; H. Zhang, Huang, et al., 2017).…”

Crustal Magmatism and Anisotropy Beneath the Arabian Shield—A Cautionary Tale

“…The majority of these studies have been carried out in convergent arc or hot‐spot settings, often at silicic calderas where significant differentiation and fractionation is known to occur. In contrast, relatively few studies have imaged melt in extensional settings outside of mid‐ocean ridges (e.g., Baba et al, ) or continental rifts with little or no Quaternary volcanism (e.g., Feucht et al, ). Those studies of active rift systems on land are primarily in Iceland (e.g., Miensopust et al, ; Rosenkjaer et al, ), within the Taupo Volcanic Zone in New Zealand (e.g., Bertrand et al, ; Heise et al, ) or within the East African Rift system.…”