Crustal Radial Anisotropy and Linkage to Geodynamic Processes: A Study Based on Seismic Ambient Noise in Southern Madagascar

Abstract

We determined radial anisotropy in the crust of southern Madagascar from the differences between the speeds of vertically and horizontally polarized shear waves (VSV and VSH), which we derived from Rayleigh and Love wave dispersion determined from seismic ambient noise correlations. The amalgamated Precambrian units in the east and the Phanerozoic Morondava basin in the west of southern Madagascar were shaped by different geodynamic processes. The crystalline basement was strongly deformed and metamorphosed to varying degrees during the assembly of Gondwana in the Pan‐African Orogeny, whereas the Morondava basin was completed with the separation of Africa and Madagascar. The different developments are reflected in first‐order differences in the radial anisotropy patterns. In the Precambrian domains, positive anisotropy (VSV<VSH) is found in the upper and lower crust, with a layer of negative anisotropy (VSV>VSH) sandwiched in between. The upper crustal anisotropy may reflect shallowly dipping layering within the Archean and adjacent imbricated nappe stacks, whereas the lower crustal anisotropy likely represents fossilized crustal flow during the postorogenic or synorogenic collapse of the Pan‐African Orogen. The negative anisotropy layer may have preserved vertically oriented large shear zones of late Pan‐African age. Within the Morondava basin, negative anisotropy in the uppermost ∼5 km could have been generated by steep normal faults, jointing, and magmatic dike intrusions. The deeper sediments and underlying crustal basement are characterized by positive anisotropy. This is consistent with horizontal bedding in the sediments and with fabric alignment in the basement created by extension during the basin formation.

Seismic velocity and anisotropy of the uppermost mantle beneath Madagascar from Pn tomography

“…The agreement between Pn and SKS anisotropy patterns supports such an interpretation, and suggests pervasive homogeneous deformation over the whole lithospheric mantle. Our new observations, along with previous studies of crustal radial anisotropy (Dreiling et al 2018) and SKS anisotropy (Reiss et al 2016;Ramirez et al 2018;Scholz et al 2018), suggest that the whole crust and lithospheric mantle have preserved this broad deformation for hundreds of millions of years since the Pan-African orogenies and that both were deformed coherently. This idea is supported by the orientation of the large-scale outcropping tectonic structures (e.g.…”

“…This complex pattern of anisotropy is also confirmed by SKS splitting measurements calculated for stations deployed within southeastern Madagascar (Scholz et al 2018). By analyzing crustal radial anisotropy in southcentral Madagascar, Dreiling et al (2018) found evidence of crustal anisotropy at different vertical levels of the crust. They attributed the anisotropy in the upper crust to shallowly dipping layers, possibly associated with an imbricated nappe stack, and the anisotropy in the middle crust to strongly folded and vertical shear zones; in the lower crust the radial anisotropy was related to the orogenic collapse following the formation of the Pan-African Orogen.…”

Identification of a nascent tectonic boundary in the San-in area, southwest Japan, using a 3D S-wave velocity structure obtained by ambient noise surface wave tomography

“…The slight differences may be caused in part by the different depth resolutions of Rayleigh and Love waves, which are determined by their depth sensitivities (Fig. 9), and by the radial anisotropy of S-waves (e.g., Dreiling et al 2018;Luo et al 2013;Ojo et al 2017).…”

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

“…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).…”