Seismic Anisotropy and Deep Mantle Flow Beneath the Amazon Craton from Shear Wave Splitting

 The Amazon Craton, one of Earth's largest Archean continental fragments, has remained tectonically stable for over 2 billion years. Yet, the mantle flow patterns beneath it remain poorly constrained due to limited seismic station coverage. We deployed 62 broadband seismometers across the craton for three years (2021–2024) and analyzed shear wave splitting from 327 teleseismic earthquakes (magnitude > 5.8, epicentral distance 85°–135°). Measurements were made for SKS, SKKS, and PKS phases using the rotation-correlation method. Results reveal a complex anisotropic structure with three distinct domains. Domain I (central craton, beneath the Guiana Shield) shows fast polarization directions (FPDs) oriented NE-SW (mean azimuth 52° ± 12°) with delay times (δt) averaging 1.4 s. Domain II (southern craton, beneath the Parecis Basin) exhibits FPDs rotating to NW-SE (mean 128° ± 15°) with larger δt (1.9 s). Domain III (eastern margin, near the Araguaia Belt) displays null splitting or weak anisotropy (δt < 0.5 s). Using a multi-event stacking technique and depth localization via spatial coherency analysis, we attribute the splitting to the lithosphere-asthenosphere boundary (LAB) and not to fossil anisotropy in the cratonic keel. The NE-SW orientation in Domain I aligns with absolute plate motion (APM) from a no-net-rotation reference frame (APM direction 49° at 2.5 cm/yr), suggesting a decoupled asthenosphere flow. However, the NW-SE orientation in Domain II contradicts APM, indicating a possible mantle upwelling branch from the deep-rooted Amazonian mantle plume (located near 5°S, 60°W). Thermochemical modeling shows that a plume with excess temperature of 150 K can deflect asthenospheric flow by up to 90°, matching our observations. Receiver function analysis of P-to-S conversions confirms a LAB depth of 180–220 km, unusually deep for Archean cratons but consistent with a plume-modified lithosphere. No splitting azimuthal variation with backazimuth was observed, ruling out two-layer anisotropy. We propose that the Amazon Craton's deep mantle is currently influenced by a fossil plume conduit from the Cretaceous, not by large-scale mantle convection. This challenges the classical view that all cratons overlie stationary, vertically coherent mantle. Geodynamic implications include revised models of continental drift: the Amazon Craton may have been pinned by a deep mantle anomaly, slowing its northward motion relative to South America. Future seismic tomography is needed to image the plume root. Our dataset is publicly available via the Incorporated Research Institutions for Seismology.