Flow through a slab tear? Lateral variations in seismic anisotropy beneath the Colombian Andes

Slab tears are observed around the globe with increasing frequency as datasets and imaging methodologies improve, though the interactions between slab tears and the surrounding mantle flow remain enigmatic. Constraints on mantle flow around and through slab tears are crucial to a comprehensive understanding of slab-mantle interactions, as (1) cross-tear flow may allow mixing between upper mantle reservoirs otherwise separated by subducting slabs, and (2) cross-tear flow may impact the strength of nearby corner flow and therefore influence regional dynamic topography. However, the ability to study slab tears and their impact on mantle flow is limited by the number of slabs with clearly observed tearing and sufficient measurement density to capture lateral variations in mantle flow across the tear. On both counts, the Colombian Andes serve as an ideal region to study the interplay between slab tears and mantle dynamics.

The Colombian Andes are shaped by complex interactions between the subducting Nazca and Caribbean plates, as most clearly manifested by the Caldas Tear—a sharp lateral offset in slab seismicity near 5.5°N spanning more than 150 km. Using data collected across this boundary during the recent MUSICA (Modeling, Uplift, Seismicity, and Igneous geochemistry of the Colombian Andes) broadband seismic deployment, we investigate lateral variations in seismic anisotropy across the Colombian Andes by measuring shear-wave splitting of SKS and SKKS phases from teleseismic earthquakes.

Measurements of shear-wave splitting offer direct observational constraints on seismic anisotropy. Seismic anisotropy in the upper mantle forms primarily from the deformation-induced alignment of intrinsically anisotropic olivine crystals. Under various ambient stress and hydration conditions, different olivine petrofabrics develop that relate the bulk anisotropic fast direction to the orientation of maximum extensional strain. By inferring petrofabric type, shear-wave splitting measurements can directly constrain the geometry of deformation in the upper mantle and thus provide insight into the impact of complex slab geometry on mantle dynamics.

Our findings detail a complex regional pattern of mantle flow as the result of three interacting flow components: (1) entrained trench-perpendicular corner flow in the mantle wedges above sinking plates, (2) mantle flow through the Caldas Tear, and (3) trench-parallel flow far east of both subducting plates. Measured splitting delay times far exceed those expected from lithospheric anisotropy alone and thus support a deeper anisotropic source. Additionally, we observe strong back azimuthal variations in splitting measurement quality and quantity that demand further investigation. Future work will explore constraining lateral and vertical anisotropic complexity simultaneously using splitting intensity tomography.