Midcrustal Deformation in the Central Andes Constrained by Radial Anisotropy

Abstract

The Central Andes are characterized by one of the largest orogenic plateaus worldwide. As a result, they are home to some of the thickest continental crust observed today (up to ~75‐km thick). Understanding the response of the crust to such overthickening provides insights into the ductile behavior of the midcrust and lower crust. One of the best tools for examining crustal‐scale features is ambient noise tomography, which takes advantage of the ambient noise wavefield to sample crustal depths in great detail. We extract Love and Rayleigh wave phase velocities from ambient noise data to invert for Vsh, Vsv, and radial anisotropy throughout the Central Andes. We capture detailed crustal structure, including pronounced along‐strike isotropic velocity heterogeneity and substantial (up to 10%) radial anisotropy that varies with depth. This crustal anisotropy may have several origins, but throughout the majority of the Central Andes, particularly beneath the Altiplano, we interpret radial anisotropy as the result of mineral alignment due to ductile crustal deformation. Only in the strongly volcanic Altiplano‐Puna Volcanic Complex is radial anisotropy likely caused by magmatic intrusions.

Shear Velocity and Radial Anisotropy beneath Southwestern Canada: Evidence for Crustal Extension and Thick‐Skinned Tectonics

“…On the global scale, negative anisotropy has been previously reported in different orogenic foreland belts. For example, radial anisotropy has been reported at 10 km or shallower depths beneath the Central Andean thrust‐and‐fold belt in the Eastern Cordillera of Bolivia (Lynner et al, ). Chen et al () and Guo et al () found Vsv > Vsh with a magnitude of 8–10% beneath the Himalayan fold‐thrust belt.…”

“…Within the continental crust, shear wave anisotropy up to 18% has been reported in western‐central Tibet in connection with rock fabrics induced by crustal thinning and channel flow (e.g., Agius & Lebedev, ; Guo et al, ; Ozacar & Zandt, ; Shapiro et al, ). An analog of the Tibetan Plateau is the central Andes, where strong (12–20%) shear wave radial anisotropy is identified at midcrustal depths through receiver function modeling (Leidig & Zandt, ) and inversion of ambient noise data (Lynner et al, ). Significant crustal‐scale anisotropy belies in many other parts of the world, including the Basin and Range Province of western North America where the amplitudes vary from 2 to 6% (e.g., Moschetti et al, ; Xie et al, ).…”

A Middle Crustal Channel of Radial Anisotropy Beneath the Northeastern Basin and Range

“…Tectonics orientation (CPO; Mainprice & Nicolas, 1989;Weiss et al, 1999). However, there are plausible alternatives or additional contributions such as preferentially oriented fractures in the shallow crust, sedimentary stratigraphy, and organization of partial melt or fluids that may be prevalent in thick orogenic crust or magmatic systems (Almqvist & Mainprice, 2017;Backus, 1962;Hacker et al, 2014;Harmon & Rychert, 2015;Jaxybulatov et al, 2014;Wang et al, 2020;Leary et al, 1990;Lynner et al, 2018;Matharu et al, 2014). The thin crust of the modern Basin and Range makes pervasive midcrustal melting less likely compared to settings such as the Tibetan plateau, which has about double the thickness of radiogenic heat-producing crust (e.g., Hacker et al, 2014).…”

“…Tectonics weakness during MCC formation rather than modern regionally averaged rheology. Sill-like intrusions are interpreted to contribute to strong positive radial anisotropy in active magmatic systems as a result of shape-preferred orientation (SPO) due to large V S contrasts between partially molten and subsolidus crustal rocks (Harmon & Rychert, 2015;Jaxybulatov et al, 2014;Jiang et al, 2018;Lynner et al, 2018). However, crystallized basaltic sills embedded in an intermediate to mafic lower crust may not have large enough velocity contrasts for SPO to cause detectable radial anisotropy (Schmandt et al, 2019).…”

“…Crustal radial anisotropy has been detected in other parts of the North American Cordillera including the southern California transform margin (Wang et al, 2020), the Rio Grande rift (Fu & Li, 2015), the Canadian Rockies (Dalton & Gaherty, 2013), and Alaska (Feng & Ritzwoller, 2019). Globally, crustal radial anisotropy has been identified in many continental areas including tectonically active and cratonic settings (Cheng et al, 2013;Dreiling et al, 2018;Duret et al, 2010;Harmon & Rychert, 2015;Huang et al, 2010;Luo et al, 2013;Lynner et al, 2018;Ojo et al, 2017;Sherrington et al, 2004;Xie et al, 2013). The most conventional interpretation for its origin is the strain-induced alignment of anisotropic crustal minerals forming an aggregate crystallographic preferred (Kreemer et al, 2014).…”