Fluid Distribution in the Central Andes Subduction Zone Imaged With Magnetotellurics

Abstract

We present a model of the electrical resistivity structure of the lithosphere in the Central Andes between 20°and 24°S from 3-D inversion of 56 long-period magnetotelluric sites. Our model shows a complex resistivity structure with significant variability parallel and perpendicular to the trench direction. The continental forearc is characterized mainly by high electrical resistivity (>1,000 Ωm), suggesting overall low volumes of fluids. However, low resistivity zones (LRZs, <5 Ωm) were found in the continental forearc below areas where major trench-parallel faults systems intersect NW-SE transverse faults. Forearc LRZs indicate circulation and accumulation of fluids in highly permeable fault zones. The continental crust along the arc shows three distinctive resistivity domains, which coincide with segmentation in the distribution of volcanoes. The northern domain (20°-20.5°S) is characterized by resistivities >1,000 Ωm and the absence of active volcanism, suggesting the presence of a low-permeability block in the continental crust. The central domain (20.5°-23°S) exhibits a number of LRZs at varying depths, indicating different levels of a magmatic plumbing system. The southern domain (23°-24°S) is characterized by resistivities >1,000 Ωm, suggesting the absence of large magma reservoirs below the volcanic chain at crustal depths. Magma reservoirs located below the base of the crust or in the backarc may fed active volcanism in the southern domain. In the subcontinental mantle, the model exhibits LRZs in the forearc mantle wedge and above clusters of intermediate-depth seismicity, likely related to fluids produced by serpentinization of the mantle and eclogitization of the slab, respectively.

Fluid and Melt Pathways in the Central Chilean Subduction Zone Near the 2010 Maule Earthquake (35–36°S) as Inferred From Magnetotelluric Data

“…The magnetotelluric (MT) method has been used to image a number of subduction zones including Cascadia Wannamaker et al, 2014), Costa Rica (Worzewski et al, 2011), southern Mexico (Jödicke et al, 2006), Japan (Hata et al, 2015), New Zealand (Heise et al, 2017;Wannamaker et al, 2009), and the Central Andes (Brasse & Eydam, 2008;Unsworth et al, 2018;Vargas et al, 2019). This method is useful in these studies of subduction zone processes because the presence of interconnected aqueous fluids and/or partial melts increases the conductivity of both crustal and mantle rocks.…”

Constraining the sub-arc, parental magma composition for the giant Altiplano-Puna Volcanic Complex, northern Chile

“…(average = 6.2‰, n = 39), respectively (see Supplementary Tables S2 and S3). In contrast, typical mantle rocks show very limited variations in their δ 18 O values (e.g. olivine = 5.2 ± 0.3‰; clinopyroxene = 5.6 ± 0.4‰; ref.…”

“…The challenge, therefore, is to constrain the parental melt value, before extensive fractionation or late-stage assimilation has taken place. We aim to achieve this goal by analysing the δ 18 O values of single minerals from rare, weakly differentiated lavas with relatively low silica contents (SiO 2 = 54.6 to 57.2 wt% 12 ) and relatively low Sr and high Nd isotope ratios ( 87 Sr/ 86 Sr = 0.70554 to 0.70669; 143 Nd/ 144 Nd = 0.51234 to 0.51251; ref. 12 ).…”

“…Based on their radiogenic isotope compositions, our samples have experienced limited degrees of crustal modification (e.g. assimilation) and are therefore ideally suited to obtaining the parental δ 18 O values locked in early-formed crystals. The crystal-focused approach we employ here offers critical new insights that have until now been under-explored due to the limitations inherent in whole-rock geochemical approaches (e.g.…”

“…The crystal-focused approach we employ here offers critical new insights that have until now been under-explored due to the limitations inherent in whole-rock geochemical approaches (e.g. the susceptibility of whole-rock samples to secondary alteration and the fact that the δ 18 O values of whole-rock samples represent averages of the various phases that constitute the sample, cf. 1 ).…”